nanomaterials and nanotechnology

Collection of Selected Papers | 2013 | issn 1847-9804 © Vasil Vasilev/Shutterstock © Vasil Editor-in-Chief Paola Prete Institute for Microelectronics and Microsystems, National Research Council, Lecce, Italy

Editorial Board C. N. R. Rao Fellow of the Royal Society, National Research Professor, Linus Pauling Research Professor and President of Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India Toshiaki Enoki Tokyo Institute of Technology, Japan Stephen O’Brien Department of Chemistry, The City College of New York, USA Juan Ramon Morante Catalonia Institute for Energy Research and University of Barcelona, Spain Stephen Pearton Department of Material Science and Engineering, University of Florida, USA Wolfgang Richter University of Rome Tor Vergata, Italy and Technischen Universität Berlin, Germany Federico Rosei Institut National de la Recherche Scientifique, Universite du Quebec, Varennes, Canada Jonathan E. Spanier Department of Materials Science and Engineering, Drexel University, Philadelphia, USA Leander Tapfer Technical Unit of Materials Technologies Brindisi, ENEA, Italy Reshef Tenne Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, Israel Fabrice Vallee CNRS and Université Claude Bernard Lyon, France

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Nanomater. nanotechnol., 2013, Collection of Selected Papers Contents

Wide Bandgap Semiconductor One-Dimensional Nanostructures for Applications in Nanoelectronics and Nanosensors 1 Stephen J. Pearton and Fan Ren

Assembled Nanostructured Architectures Studied by Grazing Incidence X-Ray Scattering 17 Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini

Numerical Techniques for the Analysis of Charge Transport and Electrodynamics in Graphene Nanoribbons 41 Luca Pierantoni and Davide Mencarelli

Synthetic Aspects and Selected Properties of Graphene 49 H. S. S. Ramakrishna Matte, K. S. Subrahmanyam and C. N. R. Rao

Complex Nanostructures by Pulsed Droplet Epitaxy 61 Stefano Sanguinetti, Claudio Somaschini, Sergio Bietti and Noboyuki Koguchi

Magnetic Properties of Fe and Ni Doped SnO2 Nanoparticles 65 Aditya Sharma, Mayora Varshney, Shalendra Kumar, K. D. Verma and Ravi Kumar

ARTICLE

Nanomaterials and Nanotechnology

Wide Bandgap Semiconductor One-Dimensional Nanostructures for Applications in Nanoelectronics and Nanosensors

Invited Review Article

Stephen J. Pearton1,* and Fan Ren2

1 Department of Materials Science and Engineering, University of Florida, Gainesville FL 32611 USA 2 Department of Chemical Engineering, University of Florida, Gainesville FL 32611 USA * Corresponding author E-mail: [email protected]

Received 21 November 2012; Accepted 15 January 2013

© 2013 Pearton and Ren; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract Wide bandgap semiconductor ZnO, GaN and materials for gas detection [1‐5]. One‐dimensional (1D) InN nanowires have displayed the ability to detect many nanostructures, such as nanowires, nanorods and types of gases and biological and chemical species of nanobelts, are particularly suited for chemical sensing interest. In this review, we give some recent examples of due to their large surface‐to‐volume ratio [6‐11]. In terms using these nanowires for applications in pH sensing, of material selection, wide band‐gap semiconductors are glucose detection and hydrogen detection at ppm levels. ideal for gas sensing, having numerous advantageous The wide bandgap materials offer advantages in terms of properties, including an ability to operate at high sensing because of their tolerance to high temperatures, temperatures (or alternatively, a low leakage current at environmental stability and the fact that they are usually room temperature), radiation and environmental piezoelectric. They are also readily integrated with stability, and mechanical robustness. The literature of wireless communication circuitry for data transmission. more than a decade of research indicates an improvement in consistent growth methods and the potential for the Keywords GaN, ZnO, InN immediate industrial use of 1D semiconductor nanostructures. Specific semiconductor materials including III‐nitrides such as GaN [12‐13] and InN [14‐ 1. Introduction 16], metal oxides including ZnO and SnO2 [17‐18], and high‐temperature materials such as SiC [19] have seen the The explosion of interest in nanoscience, coupled with greatest interest for chemical gas sensing applications, growing demand for reliable, low‐power chemical chiefly for the detection of H2, O2, NH3 and ethanol. There sensors for a wide variety of industrial applications, has is also interest in applying these to biomarker detection led to a surge in the development of nanostructured [21‐30]. www.intechopen.com Stephen J. PeartonNanomater. and Fan nanotechnol., Ren: Wide Bandgap 2013, Collection Semiconductor of Selected One-Dimensional Papers, 1-16 1 Nanostructures for Applications in Nanoelectronics and Nanosensors There is a strong need for mobile, accurate, low power There are many potential applications for wide bandgap sensors that can be used in applications such as personal semiconductor nanowire devices because of their health monitoring, security, perimeter defence, food improved carrier confinement over their thin film spoilage, the monitoring of nuclear materials and gas counterparts. For GaN nanowires, there are possible leaks. Currently, many of these applications are applications in low power and high density field‐effect monitored by lab‐based methods, such as enzyme‐linked transistors (FETs), solar cells, terahertz emitters and UV immunosorbent assay (ELISA), particle‐based flow detectors. The high surface‐to‐volume ratio of nanowires cytometric assays or electrochemical measurements. means that if their surfaces are sensitive to external These methods have impressive sensitivity and stimuli or can be functionalized to be sensitive to specific reproducibility. However, many of the applications chemicals or biogens, then they are likely to be attractive mentioned above would benefit from the ability to have for gas and chemical sensor arrays. ZnO is a piezoelectric, sensing capabilities in a broader range of environments. transparent, wide bandgap semiconductor used in surface To move these out of lab environment requires acoustic wave devices. The bandgap can be increased by miniaturization and new approaches to solving power Mg doping. ZnO has been effectively used as a gas sensor consumption and hand‐held capability. In particular, it material based on the near‐surface modification of charge would be good to have a wireless capability for sending distribution with certain surface‐absorbed species. In sensor data to a remote monitoring site and also to addition, it is attractive for biosensors given that Zn and miniaturize the sensors so as to allow for truly mobile or Mg are essential elements for neurotransmitter production long‐term remote monitoring. The techniques mentioned and enzyme functionality. above all have significant limitations in terms of using them outside of controlled lab environments, both in In this review, we discuss the progress of nitride and terms of the size of the components and power oxide semiconductor nanostructures for nanoelectronic requirements. For long‐term monitoring applications, devices and for chemical and gas sensing, specifically for self‐powering techniques must be developed as well as hydrogen. The functionalizing of the surface with oxides, power control methods that minimize power consumption. polymers and nitrides is also useful in enhancing the It is also desirable to minimize the sensor weight and size detection sensitivity for gases and ionic solutions. The requirements and to produce a sensor that is consistent wide bandgaps of these materials make them ideal for with other hand‐held devices. Structures such as solar‐blind UV detection, which can be used for detecting nanowires are attractive candidates because of their ease fluorescence from biotoxins. The use of enzymes or and the low cost of synthesis, low power consumption and adsorbed antibody layers on the semiconductor surface compatibility with existing semiconductor components. leads to the highly specific detection of a broad range of antigens of interest in the medical and homeland security Semiconductor‐based sensors can be fabricated using fields. The use of catalyst metal coatings on GaN, InN and mature techniques from the Si chip industry and/or novel ZnO nanowires has been found to greatly enhance the nanotechnology approaches. Sensors in these harsh detection sensitivity for hydrogen. Pt‐ and Pd‐coated environments must selectively detect hydrogen at room GaN nanowires biased at small voltages show large temperature or below while using minimum power. changes in currents upon exposure to H2 gas at Several groups have already demonstrated the concentrations within the ppm range. Improvements in application of nitride and oxide semiconductor growth techniques for InN nanostructures have produced nanostructures for exclusive H2 sensing using carbon nanobelts and nanorods capable of hydrogen detection nanotubes (CNTs) and ZnO. Several issues must still be down to 20 ppm after catalyst coating. Functionalized addressed, however, including quantifying sensitivity, ZnO nanorods were also investigated for hydrogen improving detection limits at room temperature, detection, but did not generate a relative response as high establishing the reproducibility and stability of the as that for the nitride‐based sensors. sensors and reducing power consumption. Materials such as ZnO show sensitivity to environmental exposure, 2. Gallium Nitride (GaN) nanostructure‐based sensors particularly in terms of forming surface conducting layers in the presence of oxygen or water vapour exposure. GaN’s use as a chemical sensor is well‐documented [26‐ Thus, the issue of surface passivation and encapsulation 34]. GaN has a high breakdown field, can operate at high for the sensors is another area that must be developed. temperatures in excess of 400ºC and has decent thermal This is not expected to be a long‐term impediment, since conductivity in bulk, low‐defect wafers, making it an polymer‐based organic electronic and display materials effective material choice for gas sensing. Schottky diode have a similar issue which has been mitigated to a large devices using thin Pd or Pt contacts have detected extent by employing appropriate methods to limit the hydrogen at concentrations of hundreds of ppm at exposure of the material to ambient conditions. temperatures as low as 100ºC. It is generally accepted that

2 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 1-16 www.intechopen.com H2 is dissociated when adsorbed on Pt and Pd at room structure [12,13]. It turns out to be surprisingly temperature. The reaction is as follows: straightforward to achieve good crystal quality nanowires using a variety of different synthesis methods. H2(ads)  2 H+ + e‐ For systems such as ZnO, it is also possible to grow the nanowires at temperatures around room temperature, Dissociated hydrogen causes a change in the channel and which makes them compatible with a large variety of conductance change of the nanowire by altering the zero‐ substrates, including plastic, tape and even paper. The bias depletion depth. ability to use arbitrary substrates makes this a versatile approach. While reports on nanostructured GaN gas sensors are increasing, the difficulty in achieving reproducible material has limited their number in the literature. The various growth techniques used for nanowire synthesis are still a long way from standardization and, as a result, have produced a wide range of 1D GaN nanostructures of varying crystal quality. The current growth methods of 1D nanostructured GaN include arc‐discharge, laser‐ assisted catalytic growth, the direct reaction of a mixture in a tube furnace and, most recently, chemical vapour deposition (CVD) from Ga‐containing precursors and NH3 on catalyst‐coated substrates.

The use of CVD for 1D nanostructure preparation is based on the vapour‐liquid‐solid (VLS) growth Figure 1. Change in resistance of GaN nanowires, either with or mechanism in which solid GaN nanowires without Pd coatings, as a function of gas ambient switched in nucleate/precipitate on catalyst nanoparticles of and out of the test chamber. (Reprinted with permission from Ref.110, W. Lim, J.S. Wright, B.P. Gila, J.L. Johnson, A. Ural, T. (typically) In, Au, Fe, Ni or Co at the substrate surface. Anderson, F. Ren and S.J. Pearton, Appl. Phys. Lett. 93, 072110 Although a minimal nanowire diameter is directly related (2008). Copyright American Institute of Physics). to device sensing capability and the desired optical characteristics, the diameter is limited by the minimum Often, the addition of a functional catalytic layer to the size of catalytic metal beading at the surface. Previous sensing substrate surface is a crucial step in the reports have demonstrated the growth of GaN nanowires effectiveness of wide band‐gap sensors. Catalytic with minimum diameters of 6 nm using metal complexes functional layers work to dissociate molecular instead of elemental metals as surface catalysts for compounds into atomic components which then bond deposition . GaN nanowire growth by CVD has issues easily onto the sensor interface; for example, the catalytic with the high melting point of Ga‐precursors, which may dissociation of H2 into 2H by Pt metal. Although this encourage Ga oxidation with residual oxygen in the CVD mechanism of dissociation by functional layers for H2 chamber. A hydrogen gas co‐flow with NH3 prevents the sensing is well‐understood, the material choice of the oxidation of GaN nanowires upon deposition. functional layer is still disputed for GaN. Both Pt and Pd have generally been accepted as choice metals for For our work, a growth substrate for GaN nanowire functional coatings on nitride‐based sensors; however, deposition was prepared by e‐beam, evaporating 15 Å there have been few reports comparing the two metal gold onto (100) Si with 100 nm thermally grown oxide. functionalities. We would emphasize that the metal GaN nanowires were deposited by the reaction of liquid catalyst layer is discontinuous and there is no current gallium and NH3 in a 1‐in. atmospheric quartz tube path from the contacts to the metal. This fact rules out a furnace. The substrate was heated to 850C and annealed change in the conductivity of the metal due to the storage for 15 min under Ar ambient in order to form discrete Au of hydrogen as contributing to the sensor signal. nanoparticles on the surface. Growth was performed at 3 850C for 2 ‐5 h under a flow rate of 15‐17 sccm NH and Our GaN nanowires were functionalized for H2 detection 300 sccm H2. The typical length of the produced GaN with thin Pt or Pd films (~100 Å thick) deposited by nanowires was 2‐10 μm. The addition of the metal sputtering. Deposited functional layers were checked for catalyst makes a large difference to the sensitivity of the discontinuity at the surface by SEM. The morphology of sensors, as shown in Figure 1. The inset of Figure 2 (top) functionalized GaN nanowires is shown in the inset of shows the morphology of as‐grown GaN nanowires Figure 2. Al/Pt/Au Ohmic electrodes contacting both ends before functionalization. The characterization of the of multiple nanowires were deposited by sputtering nanowires showed a high‐quality, single‐crystal wurtzite using a shadow mask. Au wires were bonded to the

www.intechopen.com Stephen J. Pearton and Fan Ren: Wide Bandgap Semiconductor One-Dimensional 3 Nanostructures for Applications in Nanoelectronics and Nanosensors contact pad for current‐voltage (I‐V) measurements 2.1 Hydrogen Detection Using Gallium performed at 25‐150C in H2 at varying concentration (10‐ Nitride Nanostructures 3000 ppm) in N2 ambience. Note that no underlying thin film of GaN was observed for the conditions used to Figure 3 shows the time‐dependant relative response grow the tested nanowires. (∆R/R) of the metal‐coated GaN nanowire sensors as the gas ambient is switched from air to varying hydrogen concentrations in N2 ambience and then back to air. As shown from the difference in Figure 3 (top) and 3 (bottom), Pd‐coated GaN nanowires produced a higher relative response over the same nanowires as when functionalized with Pt. Comparatively, Pd‐coated GaN nanowires showed relative responses of ~7.4% at 200 ppm up to ~9.1% at 1500 ppm. The relative responses of Pt‐ coated nanowires were ~1.65% at 200 ppm up to ~1.85% at 2000 ppm. Uncoated nanowires had little or no effective response to changes in measurement ambient, i.e., they showed no significant detection of hydrogen.

It is clear that both Pt and Pd are effective in their catalytic dissociation of molecular H2 into atomic hydrogen. Although it takes >5 min for hydrogen to saturate relative to the response of Pt‐coated nanowires, the initial resistance change is noticed in less than 2 seconds of H2 exposure for both metal‐coated nanowire sensors. This suggests that the diffusion of hydrogen through the metal coating is not the limiting factor in the time response of the sensors, but rather that it is limited by the mass transport of gas into the enclosure. This factor was confirmed by altering the H2 introduction rate into the test chamber. While the resistance change depended on hydrogen gas concentration, variations to change upon hydrogen exposure were small at H2 concentrations above 800 ppm. The rate of resistance

change decreases as the nanowire surface becomes Figure 2. Relative responses of metal coated GaN nanowires to saturated with hydrogen in all cases. In contrast to varying hydrogen concentrations at room temperature. Top) Pt‐ coating. Bottom) Pd‐coating. Insets show GaN nanowires before previous functionalized GaN nanowire sensors, these (as‐grown; top) and after Pt deposition (bottom). devices exhibit near perfect reversibility, as shown by the near complete recovery of the original current response in the air upon the removal of hydrogen. The repeatable 4 N2 detective ability of these sensors over previous one‐use‐ 20 ppm H 2 only functionalized devices is promising for long‐term 50 ppm H 2 applications. 3 100 ppm H 2 200 ppm H2 300 ppm H Although the detection mechanism for hydrogen on GaN 2 2 nanowires is still not fully understood, previous

|dR|/R (%) |dR|/R suggestions for sensing include the desorption of 1 adsorbed hydrogen at the surface and grain boundaries (in polycrystalline material), an exchange of charges 0 between absorbed gas species and the interface leading to a change in the depletion depth, or else changes to the 0 200 400 600 800 1000 1200 conduction at the surface by gas adsorption and Time (sec) desorption. Hydrogen gas sensing on functionalized GaN Figure 3. Relative responses of Pt‐coated InN nanobelts to nano‐surfaces has previously been explained as follows. varying hydrogen concentrations at room temperature. As the sensor surface is exposed to hydrogen gas, hydrogen molecules are catalytically dissociated on the

functional metal surface into atomic hydrogen . These

4 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 1-16 www.intechopen.com hydrogen atoms are absorbed on to the metal surface and indium droplets at the substrate surface. In addition, subsequently diffuse through the nano‐particulate metal gaseous NH3 has only low decomposition at low coating until they are adsorbed at the metal/GaN temperatures, requiring a large NH3/Trimethylindium interface. Hydrogen atoms reaching the GaN surface (TMIn) ratio [60]. However, excess NH3 decomposition form a dipole layer at the interface, causing a measurable produces deleterious H2, which works to reduce InN and change in the electrical field across the depletion region slow the growth rate. directly below the GaN surface. This change in the depletion layer leads to a modulation of the barrier height For the development of InN nanostructured sensors, the and a difference in the measured forward‐ and reverse‐ growth of InN nanobelts was performed by MOCVD on bias currents. Si substrates using TMIn and NH3 as In and N sources, respectively, while nitrogen was used as the carrier gas. The activation energy calculated for both GaN nanowire The nucleation sites for the catalyst‐driven growth of the sensors was higher than expected for a typical H nanobelts were formed by depositing 2 nm of Au film on diffusion process in Pt, Pd or GaN. As such, this energy SiNx‐coated Si substrates by direct current sputtering. was attributed to the chemiadsorption of H to the InN nanobelts were synthesized at 500°C and 5 Torr metal/semiconductor interface. The calculated values pressure for 2 hours. The nanobelts exhibited well‐ were ~7.3 kcal mol‐1 for Pt‐coated GaN and ~2.2 kcal mol‐1 defined side facets with typical lengths of ~1μm. A ~7 for Pd‐coated GaN. Both Pt‐ and Pd‐coated GaN nm‐thick layer of Pd was sputter‐deposited onto some of nanowires showed no change in resistance upon the nanobelts. A shadow mask was used to pattern non‐ exposure to other measurement ambiences, including O2, alloyed, rf‐sputtered Ti/Al/Pt/Au Ohmic contacts on the CO2, NH3 and C2H6. InN nanobelts with a contact separation of ~50 μm. Au

wires were bonded to the contact pads for device packaging. The InN nanobelts sensors were exposed to 10 H in N ambient Air 2 2 different H2 concentrations (100‐1000 ppm H2 in N2 ambient) at 25‐130°C. The sensor response to high 8 concentrations of CO2, C2H6, NH3 and O2 (all in N2 ambient) was also examined. 6

3.1 Hydrogen detection using Indium Nitride nanostructures 4 100 PPM |dR|/R(%) 300 PPM 500 PPM The addition of Pt or Pd metal onto InN nanobelts 2 1000 PPM Base line produced a strong response to H2 as compared with no

0 detectable change in current for uncoated nanobelts 0 200 400 600 800 1000 1200 under the same gas concentration. Pd‐coated InN nanobelts detected hydrogen down to tens of ppm Time (sec) concentration levels with relative responses of ~1.2% at 20 Figure 4. Relative responses of Pd‐coated InN nanobelts to varying hydrogen concentrations at 130ºC. ppm and up to ~4% at 300 ppm at room temperature, as shown in Figure 4. The resistance change increased with 3. Indium Nitride Nanostructure‐based Sensors the measurement temperature from 3.1% at 200 ppm and room temperature to 4.5% and 5.6% and 90°C and 150°C, InN has received significant attention recently due to its respectively. As with GaN nanowires, the increase in narrow direct bandgap (0.7‐0.8 eV) and superior resistance change is a result of the increase in the catalytic electronic transport characteristics. This, combined with dissociation rate of molecular hydrogen. its easily integrated structure with other nitrides, including GaN and AlN, makes it a promising material The relative response of Pd‐coated InN nanobelts was at for high efficiency IR emitters, detectors and solar cells, as least 30% larger for all temperatures in comparison with well as for use in high frequency electronic devices. Pt‐coated InN nanobelt hydrogen sensors. With Pd Though the research is increasing, reports on the growth, functionalization, the InN sensor detected hydrogen properties and applications of InN nanostructures remain down to 100 ppm concentration levels (the equipment in their infancy. limit for this experiment), with a relative response of ~8% at 100 ppm and ~9.5% at 1000 ppm after a 5 minute The growth of InN nanostructures by conventional metal exposure. The initial current response recovered ~50% of organic chemical vapour deposition (MOCVD) has met the initial value within 10 minutes of the removal of the with difficulty because of the low thermal decomposition hydrogen from the ambience. This was slower than temperature of InN (< ~600 ºC) under nitrogen at previous reports using Pt‐coated nanobelts and could be standard pressure, which has led to the formation of due to Pd having a greater affinity for hydrogen.

www.intechopen.com Stephen J. Pearton and Fan Ren: Wide Bandgap Semiconductor One-Dimensional 5 Nanostructures for Applications in Nanoelectronics and Nanosensors The activation energy for H detection on both Pt‐ and Pd‐ reduction of the nanostructure surface area. It is certainly coated InN nanobelts was calculated from the slope of the true that producing higher quality nanostructures Arrhenius plot of natural log (dR/dt) versus 1/T. Pd‐ mitigates the effects of defects in the bulk or on the coated nanobelts were found to have an activation energy surface to the sensing response. It appears that many of of 0.097 eV, whereas Pt‐coated nanobelts had an energy the initial results in the literature concerning the gas of 0.12 eV. These values are low but still larger than those sensing properties of nitride and oxide nanorods have expected for a typical hydrogen diffusion in InN, Pt or been dominated by surface trapping/detrapping Pd. Thus, and as with the results found for functionalized phenomena rather than by the true intrinsic response of GaN nanowire hydrogen sensors, the dominant sensing the material itself. mechanism is likely to be the chemisorption of H to the metal‐nitride interface.

The utility of InN nanorods for hydrogen detection was also investigated. InN nanorods were grown on polished c‐Al2O3 substrates by H‐MOVPE. With this method, TMIn was reacted with HCl to form chlorinated indium. This was then combined with NH3 to form InN on the substrate downstream. The source zone temperature was kept below 300C to prevent TMIn dissociation before reaction with HCl. Growth was completed at atmospheric pressure in N2 ambient in a temperature range between 600‐650C at a N/In mole ratio of 250 and a HCl/TMIn inlet mole ratio of 4 to 5. Finally, InN nanorods were functionalized with a thin, discontinuous Pt‐film (~100Å) by sputtering.

Glancing Incidence X‐Ray Diffraction (GIXD) scans of the InN nanostructures are shown in Figure 5. Since the orientation of each nanostructure was largely random, multiple wurtzite InN peaks were observed for both InN nanobelts and InN nanorods. Of note here are the larger, sharper peaks for the nanorods as against those for the nanobelts. This difference in the XRD profile may be attributed to a thicker, denser nanostructured surface, and is supported by the variation in surface morphology as shown in the SEM inset photos. The nanorods are typically larger, longer and denser over the substrate surface.

The higher quality of these InN nanorods (as compared Figure 5. GIXD 2θ scan of InN nanomaterials. A) InN nanobelts with the previously discussed InN nanobelts) results in a on SiNX‐coated Si. B) InN nanorods on c‐Al2O3. Insets show SEM larger relative response upon exposure to hydrogen. micrographs of the respective deposited InN nanomaterials. Figure 6 shows the relative resistance change of the Pt‐ coated multiple InN nanorods upon hydrogen exposure. 4. Zinc Oxide (ZnO ) nanostructure‐based sensors 100 ppm H2 in N2 ambience produced a relative response of ~10%, while 250 ppm H2 gave a response of ~12%. There is a great deal of literature dedicated to the Unlike the immediate response of the InN nanobelts to synthesis and characterization of various types of hydrogen gas, however, the initial change in the nanostructured ZnO morphologies. A review of the many resistance of InN nanorods upon hydrogen exposure was forms of nano‐sized ZnO, including nanobelts, ‐cages, ‐ quite slow, taking up to five minutes exposure before combs and ‐wires is provided by Wang [11]. This ease of exhibiting a current response. This slower response could nano‐fabrication, combined with the high compatibility be due to the Pt functionalization layer being covered with Si‐based microelectronics afforded to with native oxide, which is removed by exposure to semiconducting metal oxides, makes ZnO a particularly hydrogen. The higher surface density of the nanorods as interesting candidate for solid‐state chemical gas sensing. compared with the previous nanobelts may also inhibit ZnO is a chemically and thermally stable, inherently n‐ an immediate reaction to hydrogen exposure by the type semiconductor with a large exciton binding energy

6 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 1-16 www.intechopen.com and wide bandgap. ZnO‐based gas sensors have already to derive from the desorption of absorbed surface oxygen been developed from thin films, nanoparticles and and grain boundaries in poly‐ZnO or else from changes nanowires. Additionally, ZnO easily forms a in surface or grain boundary conduction by gas heterostructure system with MgO and CdO over a limited adsorption/ desorption. Since the nanowires are typically miscibility range that is promising for blue/UV a single crystal, the latter effect is not likely to contribute. optoelectronics, transparent electronics and sensors. When detecting hydrogen with similar thin film rectifiers, the observed changes in current were consistent with changes in the near surface doping of the ZnO, suggesting the introduction of a shallow donor state via hydrogen adsorption. The nanorods also showed a strong photoresponse above bandgap UV light (366 nm). The quick photoresponse indicated that the changes in conductivity due to the injection of carriers was bulk‐ related and not due to surface effects.

As with to GaN and InN, previous reports have shown that the addition of metallic nanoparticles on the surface increases the detection sensitivity for H2. Accordingly, and as with the GaN nanowires and InN nanobelts, metal catalyst coatings were deposited to ZnO nanorods to

increase the detection efficiency for hydrogen gas. Figure Figure 6. Relative responses of Pt‐coated InN nanorods to 9 shows the time dependence of the relative resistance varying hydrogen concentrations at room temperature. (Reprinted with permission from Ref.111, W. Lim, J.S. Wright, change of metal‐coated multiple ZnO nanorods with B.P. Gila, S.J. Pearton, F. Ren, W. Lai, L.C. Chen, M. Hu and K.H. various metal functionalization layers upon exposure to Chen, Appl. Phys. Lett. 93, 202109 (2008). Copyright American 500 ppm H2. The measured bias voltage was 0.5V. Pt‐ Institute of Physics). coated nanorods exhibited a relative response of up to 8% at room temperature upon exposure to 500 ppm hydrogen 4.1 Hydrogen detection using Zinc Oxide nanostructures concentration in N2 ambience. Unlike the results for the nitride‐based nanostructures, this was a factor of two For our work, the site selective growth of ZnO nanorods larger than obtained with Pd‐coatings. The Pt‐coated ZnO was achieved by Molecular Beam Epitaxy (MBE) on nanorods easily detected hydrogen down to 100 ppm (the Al2O3. The growth time was ~2 h at 600C, producing experimental limit), with relative responses of 4% at this single crystal nanorods with a typical length of 2‐10 μm concentration after a 10 min exposure. All of the other and having a diameter between 30‐150 nm. Al/Ti/Au metalsʹ functional layers showed very little relative electrodes were deposited via e‐beam with a separation responses to hydrogen exposure (Figure 7). distance of ~3 μm. Finally, Au wires were bonded to the contact pads for I‐V measurements performed at Differences in the relative response to hydrogen exposure temperatures between 25‐150ºC in 10% H2 in N2 between Pt‐ and Pd‐coated samples were suggested as ambience. No current was measured through the coming from the catalytic properties of the metals. Pd has discontinuous Au islands and no thin film of ZnO was a higher permeability than Pt, but the solubility of H2 is observed with the grown conditions of nanorods. In some larger in Pd. Additionally, bonding studies of H to Ni, Pt cases, nanorods were functionalized using Pd, Pt, Au, Ni, and Pd surfaces have shown that adsorption energy is Ag or Ti discontinuous thin films (~100 Å thick) lowest on Pt. As shown, however, the calculated deposited by sputtering. activation energy for GaN nanowires and InN nanobelts was lower for Pd‐coated samples rather than Pt. This is Although there was no detectable change in current upon most likely due to the adsorption of hydrogen at an oxide H2 exposure at room temperature, ZnO nanorods did interface between the nitride surface and the metal and reflect changes in current, beginning at ~112C. While not from the adsorption of hydrogen at the metal coating. change in current only approaches 16 nA at 200C, these The existence and importance of this oxide interface for changes are readily detected by conventional ammeters; hydrogen sensing on nitride‐based semiconductors is however, an on‐chip heater would be needed for the well‐documented. Because the hydrogen adsorption on practical application of ZnO nanorods for the detection of the oxide side of a ZnO‐based sensor would contribute to hydrogen. Nonetheless, these results show the possible the creation of a shallow donor state, the sensing capability of ZnO nanorods for use in hydrogen gas mechanism for hydrogen detection on metal coated ZnO sensors without the deposition of an additional functional nanostructures is probably different from that of GaN or layer. In this case, the gas sensing mechanism is believed InN.

www.intechopen.com Stephen J. Pearton and Fan Ren: Wide Bandgap Semiconductor One-Dimensional 7 Nanostructures for Applications in Nanoelectronics and Nanosensors presence of sunlight, such as flame sensors, missile Air 500ppm H2 detectors and aircraft detection. Si detectors have very 8 poor solar‐blind performance while wide bandgap Pt systems offer improved speed and lower dark currents. 6 Pd Au Currently, the most commonly used wide bandgap 4 Ag Ti semiconductor system for UV detection is GaN/AlGaN. Ni There is also interest in developing ZnO/ZnMgO 2 nanowire UV detectors as a complementary technology for UV detection, with the following advantages relative |delta R|/R |deltaR|/R (%) 0 to the nitrides.

0 5 10 15 20 25 30 1. The ZnO‐based materials offer similar band‐gaps to the Time(min) nitrides, but can be grown at much lower temperatures on a wider range of substrates, including large area Si or Figure 7. Relative responses of ZnO nanorods to 500 ppm H2 using various metal functionalizations. (Reprinted with cheap transparent materials, such as glass. The permission from Ref.89, Y.W. Heo, D.P. Norton, L.C. Tien, Y. nanowires can be transferred to any substrate for Kwon, B.S. Kang, F. Ren, S.J. Pearton and J.R. LaRoche, Mat. Sci. integration with other sensors and are compatible with Eng. R 47, 1 (2004). Copyright Elsevier). low temperature materials such as polymers. 2. The nanowire UV detectors operate at very low power Similar to nitride‐based nanostructured sensors is the levels compared with existing nitride UV detectors. rapid, initial response of ZnO multiple nanorod sensors 3. The fabrication approach developed previously for ZnO to hydrogen exposure. Effective nanorod resistance nanowire gas sensors allows for a simple, low‐cost, continues to change for >15 minute of exposure. This single‐step approach to realizing robust UV detectors. suggests that the chemisorption of hydrogen to the 4. ZnO nanowire UV detectors can be readily integrated metal/ZnO interface is the rate‐limiting step in with on‐chip wireless circuits to provide data conductance changes to the ZnO. The recovery of initial transmission to a central monitoring location. Thus, it resistance upon the removal of hydrogen from the is possible to have either single detectors or arrays of ambience was rapid (<20 sec). A calculated activation detectors that operate at very low‐power levels and do energy of 12 kJ/mole originated from the chemisorption not need constant monitoring by humans. of hydrogen onto the metal coating surface. UV detectors have applications in space exploration in 4.2 Other Zinc Oxide nanostructure‐based applications providing imaging and spectroscopic data of nearby galaxies. Because the universe is expanding, UV radiation 4.2.1 UV Photodetectors emitted by distant galaxies is red‐shifted and reaches our galaxy as visible or infrared radiation. Galaxies closer to The development of GaN‐based UV detectors in the the Milky Way can be analysed with UV radiation and spectral range shorter than ~400 nm has attracted much comparisons can be made with visible and infrared interest recently because of their potential application in images to ascertain how the universe formed and changes the detection of biological materials and for the defence with time. industry. In the former case, the UV photons are used to excite fluorescence at UV wavelengths from biological UV Photoresponse of Single ZnO Nanowires materials of interest and this is detected by the ZnO nanowires grown by site‐selective MBE were single photodetectors. Wide bandgap detectors are very useful crystals and typically conducting with a carrier density in bio‐warfare agent detection because some pathogenic within the 1017‐1018 cm‐3 range. These nanowires can be biological molecules fluoresce in the UV spectral region. removed by sonication from their original substrate and then transferred to arbitrary substrates, where they can be The most common UV detectors are based on p‐i‐n Si contacted at both ends by Al//Pt/Au Ohmic electrodes. photodiodes or UV‐filtered photomultiplier tubes. The The current‐voltage and photoresponse characteristics use of nitride semiconductor UV detectors has were obtained both in the dark and with UV (254 or 366 advantages in terms of more precise detection windows, nm) illumination. The current‐voltage (I‐V) characteristics lower background currents due to solar fluxes and a are Ohmic under all conditions, with nanowire wider range of operating temperatures. Photodetectors conductivity under UV exposure of 0.2 Ohm.cm. The that have no response for photons at wavelengths photoresponse showed only a minor component, with >290nm are called solar‐blind and are useful in long decay times (tens of seconds) thought to originate applications that need to detect UV photons in the from surface states. Recent reports have shown the

8 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 1-16 www.intechopen.com sensitivity of ZnO nanowires to the presence of oxygen in the measurement ambient and to UV illumination. The slow photoresponse of the nanowires was suggested as originating in the presence of surface states, which trapped electrons with release time constants from m.sec to hours. In sharp contrast to these results, we have demonstrated that the photoresponse characteristics of single ZnO nanowires grown by site selective Molecular Beam Epitaxy (MBE) have a relatively fast photoresponse and display electrical transport dominated by bulk conduction.

Figure 8 shows the change in current at a fixed bias of the nanowires in the dark and under illumination by 366 nm light. The conductivity is greatly increased as a result of 800 the illumination, as evidenced by the higher current. No effect was observed for illumination for below bandgap light. Transport measurements show that the ideality 600 factor of Pt Schottky diodes formed on those nanowires exhibiting an ideality factor of 1.1, which suggests that 400 there is little recombination occurring in the nanowire. It Current (nA) also exhibits the excellent Ohmicity of the contacts to the 200 nanowire, even at low bias. On blanket films of n‐type ZnO with carrier concentration within the 1016 cm‐3 range, we obtained a contact resistance of 3‐5x10‐5 Ohm.cm‐2 for 0 0 100 200 300 400 these contacts. In the case of ZnO nanowires made by Time (sec) thermal evaporation, the I‐V characteristics were Figure 8. SEM micrograph of ZnO nanowire and time rectifying in the dark and only became Ohmic during dependence of photocurrent as the 366nm light source is above‐bandgap illumination. The conductivity of the modulated. nanowire during illumination with 366 nm light was 0.2 Ohm.cm. 4.2.2 pH measurement

The photoresponse of the single ZnO nanowire at a bias There are a number of applications where an ability to of 0.25 V under pulsed illumination by a 366 nm determine the pH of a solution is desirable in order to wavelength Hg lamp in Figure 8 shows that the understand the level of acidity or alkalinity. These photoresponse is much faster than that reported for ZnO include determining salinity levels in bodies of water, the nanowires grown by thermal evaporation from ball‐ industrial monitoring of sludges or run‐off and, of course, milled ZnO powders and is likely due to the reduced blood in humans. Many oxides are sensitive to changes in influence of the surface states seen in that material. The the pH of solutions touching their surfaces through a generally quoted mechanism for photoconduction is the change in surface conductivity. This makes materials such creation of holes by illumination that discharge the as ZnO, SnO2 and related compounds attractive for pH negatively charged oxygen ions onto the nanowire sensing applications. surface with the de‐trapping of electrons and transit to the electrodes. The recombination times in high quality Of particular interest in determining the ability of ZnO ZnO measured by time‐resolved photoluminescence are nanorods for acting as pH sensors is the need for a short ‐ of the order of tens of ps ‐ while the photoresponse method of introducing the test solution in a controlled measures the electron trapping time. There is also a direct and reproducible fashion to the nanowire surface. This is correlation reported between the photoluminescence usually accomplished by the use of a microchannel lifetime and the defect density in both bulk and epitaxial fabricated in a polymer. Small volumes of the solution ZnO. In our nanowires, the electron trapping times are of can be introduced with a syringe autopipette. A typical the order of tens of seconds, and these trapping effects are geometry for the nanowire sensor and integrated only a small fraction of the total photoresponse recovery microchannel and the conductance of the nanowire is characteristic. Once again, we see an absence of the very shown in Figure 9. The measurements were performed long time constants for recovery seen in nanowires both in the dark or under UV illumination (365 nm prepared by thermal evaporation. wavelength) at room temperature with the nanorod biased at a fixed voltage of 0.5 V

www.intechopen.com Stephen J. Pearton and Fan Ren: Wide Bandgap Semiconductor One-Dimensional 9 Nanostructures for Applications in Nanoelectronics and Nanosensors additional integration of the sensors into a portable, wireless package for remote monitoring applications. Figure 10 shows an optical microscopy image of an integrated pH and glucose sensor chip and cross‐ sectional schematics of the completed pH and glucose device. The gate dimension of the pH sensor device and glucose sensors was 20 × 50 μm2.

For the glucose detection, a highly dense array of 20‐30 nm diameter and 2 μm tall ZnO nanorods was grown on the 20 × 50 μm2 gate area. The lower right inset in Figure 10 shows closer view of the ZnO nanorod arrays grown on the gate area. The total area of the ZnO was increased significantly with the ZnO nanorods. The ZnO nanorod matrix provides a microenvironment for immobilizing negatively charged GOx while retaining its bioactivity, 300 non UV and passes charges produced during the GOx and glucose UV(365nm) 250 interaction to the AlGaN/GaN HEMT. The GOx solution was prepared with a concentration of 10 mg/mL in 10 200 mM phosphate buffer saline (pH value 7.4, Sigma Aldrich). After fabricating the device, a 5 μl GOx (~100

150 U/mg, Sigma Aldrich) solution was precisely introduced 100 to the surface of the HEMT using a pico‐litre plotter. The sensor chip was kept at 4oC in the solution for 48 hours 50 for GOx immobilization on the ZnO nanorod arrays,

Conductance(nS) 0 followed by extensive washing to remove the un‐ immobilized GOx. 2 3 4 5 6 7 8 9101112

pH To take advantage of the quick response (less than 1 sec) Figure 9. The SEM of an integrated microchannel across a ZnO of the HEMT sensor, a real‐time exhaled breath nanorod contacted at both ends by Ohmic contacts. The condensate (EBC) collector is needed. The amount of the conductance of the nanorod as a function of the pH of the solution EBC required to cover the HEMT sensing area is very which flowed across it is shown at the bottom of the figure. small. Each tidal breath contains around 3 l of the EBC.

The contact angle of EBC on Sc2O3 has been measured as The nanorods showed a very strong photoresponse. The being less than 45o, and it is reasonable to assume a conductivity is greatly increased as a result of the perfect half sphere of an EBC droplet formed to cover the illumination, as evidenced by the higher current. No 2 effect was observed for illumination with below bandgap sensing area of the 4 × 50 μm gate area. The volume of a ‐11 light. The adsorption of polar molecules on the surface of half sphere with a diameter of 50 μm is around 3 × 10 ZnO affects the surface potential and device litres. Therefore, 100,000 50 μm diameter droplets of EBC characteristics. can be formed from each tidal breath.

The data in Figure 9 shows that the integrated To condense 3 l of water vapour, only ~ 7 J of energy microchannel/nanorod can indeed provide a sensitive need to be removed for each tidal breath, which can measurement of pH over a range of at least 2 to 12. The easily be achieved with a thermal electric module ‐ a sensitivity was independent of whether the experiments Peltier device. The figure also shows a photograph and were carried out in the dark or under illumination. The schematic of the system for collecting the EBC. The slope of the conductance/pH plot was 8.5 nS/pH in the AlGaN/GaN HEMT sensor is directly mounted on the top dark and 20nS/pH under illumination. The resolution of of the Peltier unit (TB‐8‐0.45‐1.3 HT 232, Kryotherm), the measurements was typically ~0.1 pH, which compares which can be cooled to precise temperatures by applying well with other methods for pH determination. known voltages and currents to the unit. During our measurements, the hotter plate of the Peltier unit was 4.2.3 Biomedical Applications kept at 21oC and the colder plate was kept at 7oC by applying a bias of 0.7 V at 0.2 A. The sensor takes less AlGaN/GaN HEMTs can be used for measurements of than 2 sec to reach thermal equilibrium with the Peltier pH in EBC and glucose, through the integration of the pH unit. This allows the exhaled breath to immediately and glucose sensor onto a single chip and with the condense on the gate region of the HEMT sensor.

10 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 1-16 www.intechopen.com The glucose was sensed by ZnO nanorod functionalized HEMTs with a glucose oxidase enzyme localized on the nanorods, as shown in Figures 11‐13. This catalyzes the reaction of glucose and oxygen to form gluconic acid and hydrogen peroxide. Figure 12 shows the real‐time glucose detection in a PBS buffer solution using the drain current change in the HEMT sensor with a constant bias of 250 mV. No current change can be seen with the addition of a buffer solution at around 200 sec, showing the specificity and stability of the device. By sharp contrast, the current change showed a rapid response of less than 5 seconds when the target glucose was added to the surface. So far, the glucose detection using an Au nano‐particle, ZnO nanorod and nanocomb or carbon nanotube material with GOx immobilization is based on electrochemical

measurement . Since there is a reference electrode Figure 10. SEM image of an integrated pH and glucose sensor. required in the solution, the volume of the sample cannot The insets show a schematic cross‐section of the pH sensor and an SEM of the ZnO nanorods grown in the gate region of the be easily minimized. The current density is measured glucose sensor. when a fixed potential is applied between the nano‐ materials and the reference electrode. This is a first‐order The use of Peltier cooling allows for a compact and low detection and the range of detection limit of these sensors power approach to building the sensor unit. These units is 0.5‐70 μM. Even though the AlGaN/GaN HEMT‐based are inexpensive and very reliable, and can be used to sensor used the same GOx immobilization, the ZnO control the amount of vapour condensation onto the sensor nanorods were used as the gate of the HEMT. The gate area. This might also be used in future to selectively glucose sensing was measured through the drain current control the condensation of particular gases in a mixture, of HEMT, with a change of the charges on the ZnO nano‐ based on their different vapour pressures. An example rods, and the detection signal was amplified through the would be making sure that the exhaled breath condensate HEMT. Although the response of the HEMT‐based sensor is not simply air from the back of the nasal cavities or the is similar to that of an electrochemical‐based sensor, a mouth as opposed to the necessary air from deep in the much lower detection limit of 0.5 nM was achieved for lungs. The simultaneous determination of the pH can help the HEMT‐based sensor due to this amplification effect. in ensuring the test vapour is indeed from the lungs. Since there is no reference electrode required for the HEMT‐based sensor, the amount of the sample only The HEMT sensors only exhibited a response to human depends upon the area of the gate dimension and can be breath and not to calibration injections of N2 gas. The N2 minimized. The sensors do not respond to glucose unless did not cause any change of drain current while the the enzyme is present, as shown in Figures 12 and 13. increase of exhaled breath flow rate decreased the drain current. For every tidal breath, the beginning portion of Although measuring the glucose in the EBC is a non‐ the exhalation is from the physiologic dead space and the invasive and convenient method for the diabetic gases in this space do not participate in CO2 and O2 application, the activity of the immobilized GOx is highly exchange in the lungs. Therefore, the contents in the tidal dependent on the pH value of the solution. The GOx breath are diluted by the gases from this dead space. For activity can be reduced to 80% for pH = 5 to 6. higher flow rate exhalation, this dilution effect is less effective. Once the exhaled breath flow rate is above By way of contrast, when the glucose sensor was used in 1L/min, the sensor current change reaches a limit. As a a pH‐controlled environment, the drain current stayed result, the test subject experiences hyper ventilation and fairly constant. In this experiment, 50 l of PBS solution the dilution becomes insignificant. The sensor is operated was introduced on the glucose sensor to establish the at 50 Hz and a 10% duty cycle, which produces heat baseline of the sensor, as in the previous experiment. during operation. It only takes a few seconds for the EBC Then, glucose at a 10 nM concentration prepared in the to vaporize from the sensing area and causes the spike‐ PBS solution was introduced to the gate area of the like response. The principal component of the EBC is glucose sensor through a micro‐injector. There was no water vapour, which represents nearly all of the volume glucose in the 50 l PBS solution and the PBS solution (>99%) of the fluid collected in the EBC. The measured was added at 20 and 30 min. It took time for the glucose current change of the exhaled breath condensate shows solution to diffuse to the gate area of the sensor through that the pH values are within the range of pH 7‐8. This the blank PBS, and the drain current gradually increased range is the typical pH range of human blood. corresponding to the glucose diffusion process. Since the

www.intechopen.com Stephen J. Pearton and Fan Ren: Wide Bandgap Semiconductor One-Dimensional 11 Nanostructures for Applications in Nanoelectronics and Nanosensors fresh glucose was continuously provided to the sensor surface and the pH value of the glucose was controlled, 20 once the concentration of the glucose reached equilibrium no enzyme at the gate of the glucose sensor, the drain current of the with enzyme 15 glucose remained constant, except in the presence of the A) 

glucose solution, which was taken out from time to time ( ds using a micro‐pipette. Small oscillations of the drain 10 current were observed, which could be eliminated by using a microfluidic device for this experiment.

Change of I 5

0

100 101 102 103 104 105 Concentration (nM)

Figure 13. Change in drain‐source current in HEMT glucose sensors, both with and without the localized enzyme.

The human pH value can vary significantly, depending on health. Since we could not control the pH value of the EBC samples, we needed to measure the pH value while determining the glucose concentration in the EBC. With

the fast response time and low volume of the EBC

required for HEMT‐based sensor, a handheld and real‐

time glucose sensing technology could be realized.

4.2.4 Lactic Acid

Another application for functionalized AlGaN/GaN

sensors is the detection and quantification of lactic acid

content in breath or blood. This is important in sports

training, where the level of exertion and recovery of

swimmers and sprinters is necessary to optimize their

training practices. It is also important in situations like

monitoring of the effectiveness of anaesthetics during

medical procedures or for individuals with conditions Figure 11. Schematic of a ZnO nanorod functionalized HEMT such as diabetes and chronic renal failure. (top) and a SEM of nanorods on the gate area (bottom).

A ZnO nanorod array, which was used to immobilize lactate oxidase (LOx), was selectively grown on the gate area using low temperature hydrothermal decomposition (Figure 14, top). The array of one‐dimensional ZnO nanorods provided a large effective surface area with a high surface‐to‐volume ratio and a favourable environment for the immobilization of LOx. The AlGaN/GaN HEMT drain‐source current showed a rapid response when various concentrations of lactate acid solutions were introduced to the gate area of the HEMT sensor. The HEMT could detect lactate acid concentrations from 167 nM to 139 μM. Figure 14 (bottom) shows a real‐time detection of lactate acid by measuring the HEMT drain current at a constant drain‐

Figure 12. Plot of drain current versus time with successive source bias voltage of 500 mV during the exposure of the exposure of glucose from 500 pM to 125 M in 10 mM phosphate HEMT sensor to solutions with different concentrations buffer saline with a pH value of 7.4, both with and without the of lactate acid. The sensor was first exposed to 20 l of 10 enzyme located on the nanorods. mM PBS and no current change could be detected with

12 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 1-16 www.intechopen.com the addition of 10 l of PBS at approximately 40 seconds, 5. Summary and Outlook showing the specificity and stability of the device. By contrast, a rapid increase in the drain current was Nitride and oxide semiconductor nanostructures are ideal observed when the target lactate acid was introduced to materials for chemical sensing. Material properties, the device surface. including high chemical and thermal stability coupled with a high surface to volume ratio, give these As compared with the amperometric measurement‐based nanostructured sensors high sensitivity with detection lactate acid sensors, our HEMT sensors do not require a capabilities down to the low ppm levels for gases such as fixed reference electrode in the solution to measure the hydrogen. While hydrogen sensing devices have been potential applied between the nano‐materials and the demonstrated on single ZnO nanowires, there are reference electrode. Although the time response of the advantages in the development of wide bandgap sensors HEMT sensors is similar to that of electrochemical‐based involving contacting multiple nanowire sheets. Although sensors, a significant change of drain current was the power requirement for using multiple nanowires is observed in exposing the HEMT to the lactate acid at a higher than that for single nanowire devices, the low concentration of 167 nM due to this amplification simplicity of fabrication, including minimal processing effect. In addition, the amount of the sample, which is steps for multiple nanowire sensors, is highly favourable dependent upon the area of the gate dimension, can be for the future mass production of these sensors. minimized for the HEMT sensor due to the fact that no reference electrode is required. Thus, measuring lactate The selective sensing of hydrogen in nitrogen ambients has acid in the exhaled breath condensate (EBC) can be been shown using nanostructured ZnO, GaN and InN down achieved as a non‐invasive method. to 50 ppm at room temperature. Sensors were functionalized using Pt or Pd in order to facilitate the dissociation of molecular hydrogen and improve sensing efficiency. All of the sensors displayed excellent response characteristics and decent recovery upon exposure to air or pure oxygen.

There is also great promise for using nanowires in conjunction with HEMT sensors to enhance the detection sensitivity for glucose and lactic acid. Once again, the high surface area of nanowires provides an ideal approach for enzymatic detection of biochemically important substances [15‐30].

6. Acknowledgments

The work at UF was partially supported by NSF (J.M. Zavada).

7. References

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14 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 1-16 www.intechopen.com [27]. Chu B H, Lin H, Gwo S, Wang Y L, Pearton S J, [29] Chu B H, Chang C Y, Kroll K , Denslow N, Wang Johnson J W, Rajagopal P, Roberts J, Piner E L, Yu‐Lin, Pearton S J, Dabiran A M, Wowchak A Linthicum K J and Ren F (2010) Chloride ion M, Cui B, Chow P P and Ren F (2010) Detection of an detection by InN gated AlGaN/GaN high electron endocrine disrupter biomarker, vitellogenin, in mobility transistors. J. Vac. Sci. Technol. B 28, L5‐8. largemouth bass serum using AlGaN/GaN high [28] Chu B H, Kang B S, Chang C Y, Ren F, Goh A, Sciullo electron mobility transistors. Appl. Phys. Lett. 96: A, Wu W, Lin J, Gila B P, Pearton S J, Johnson J W, 013701‐013703. Piner E L and Linthicum K J (2010) Wireless [30] Wang Y‐L, Chu B H, Chang C Y, Chen K H, Zhang Y, detection system for glucose and pH sensing in Sun Q, Han J, Pearton S J and Ren F (2009) Hydrogen exhaled breath condensate using AlGaN/GaN high sensing of N‐polar and Ga‐polar GaN Schottky electron mobility transistors, IEEE Sensors Journal. diodes, sensors and actuators. B: Chemical. 142: 175‐ 10: 64‐69. 180.

www.intechopen.com Stephen J. Pearton and Fan Ren: Wide Bandgap Semiconductor One-Dimensional 15 Nanostructures for Applications in Nanoelectronics and Nanosensors

ARTICLE

Nanomaterials and Nanotechnology

Assembled Nanostructured Architectures Studied by Grazing Incidence X-Ray Scattering

Invited Review Article

Davide Altamura1, Teresa Sibillano1, Dritan Siliqi1, Liberato De Caro1 and Cinzia Giannini1,*

1 Istituto di Cristallografia, Sede di Bari, Bari, Italy * Corresponding author: [email protected]

Received 15 October 2012; Accepted 12 December 2012

© 2012 Altamura et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract In this chapter, we will focus on a specific X-ray- employed table-top set-up is described in a dedicated based technique among those employed in surface paragraph. Further examples of grazing incidence studies science and which is especially suitable for the study of as performed by the authors with a table-top set-up are self-assembled nanocrystals: Grazing Incidence Small Angle reported: a GISAXS study of 3D iron oxide nanocrystal X-ray Scattering (GISAXS). We will first introduce the superlattices, showing the importance of modelling in main field of investigation considered herein, with basic order to obtain structural information from data; a notions of X-ray scattering from surfaces, and then combined small/wide angle scattering (GISAXS/GIWAXS) address basic concepts about GISAXS. Finally, we will study of 3D PbS nanocrystal superlattices; and a GIWAXS describe a few relevant examples of studies, of study of P3HT nanofibres, showing how the ordering at nanostructured architectures, through ex situ and in situ the molecular and atomic length scales can be obtained by experiments of grazing incidence X-ray scattering. This exploring different angular ranges in the same grazing manuscript is focused on the former, showing that they incidence geometry. Finally, selected examples of in situ can be performed by using laboratory instruments. In situ GISAXS studies, performed with synchrotron radiation investigations still need synchrotron radiation sources in sources, are described. most cases; therefore, only a few examples selected from the literature are reported here, for the sake of Keywords GISAXS, X-ray Imaging, Nanomaterials, completeness. The experiments described are mainly Self-assembly performed in the small angle range, providing information on the size and shape of nanocrystals, together with their spatial arrangement. Both 2D and 3D 1. Towards the Self-assembly architectures are considered. In particular, GISAXS of Nanostructured Architectures measurements of 2D superlattices of nano-octapods, performed both at a third generation synchrotron As of today, the synthesis of nanoparticles, nanocrystals beamline and with a table-top set-up, are compared; the and nanostructured architectures can be realized, on the

www.intechopen.com Davide Altamura,Nanomater. Teresa Sibillano, nanotechnol., Dritan Siliqi, 2013, Liberato Collection De Caroof Selected and Cinzia Papers, Giannini: 17-40 17 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering one hand, by physical approaches, such as Molecular Last generation breeds of so-called hybrid NCs (HNCs) Beam Epitaxy, Laser Ablation, Sputtering and Metal are structurally elaborated multi-material colloidal Organic Vapour Phase Epitaxy; and on the other hand, by nanostructures, consisting of two or more different chemical routes, such as Chemical Vapour Deposition material domains interconnected through permanent (the reaction of metal-organic species in the gas phase) or chemical bonding, possibly forming heteroepitaxial colloidal synthesis (the formation of free-standing interfaces. nanoparticles in liquid media in the presence of organic stabilizers), the latter being the main approach considered For example, Au-Fe3O4, Au-FePt and Ag-Fe3O4 here. A relatively large range of colloidal nanomaterial heterodimers have been exploited as dual functional platforms have been successfully fabricated.1-5 probes upon site-selective functionalization with different biomolecules. The processed HNCs have been made Independently of the specific synthesis approach, the simultaneously hydrophilic, fluorescent, responsive to physical and chemical properties of nanostructured magnetic forces and capable of binding to specific materials are indeed distinctly different from those of receptors.12, 13 bulk matter with the same chemical composition. This difference is related to the reduced size, which leads to The creation of asymmetrically functionalized material quantum confinement and/or to structural phase changes. sections has been also envisioned as a strategy for promoting the self-assembly of HNCs into functional As a consequence, novel electronic configurations (and mesoscopic NC-based “superstructures”. 7-10 thus magnetic and optoelectronic responses) and different chemical reactivity (e.g., catalytic properties) are Self-assembling is among the most innovative and obtained for nanostructures, compared to their bulk creative concepts of modern nanotechnology: carefully material counterparts. A material made of small designed building blocks, either separated or linked, nanocrystals is expected to be more reactive than the spontaneously form complex ordered aggregates,14 their same mass of material made up of larger particles, as the interactions usually being non-covalent (e.g., electrostatic smaller the nanocrystals, the larger the overall exposed interactions, hydrogen bonds, van der Waals’ forces, surface area. The fraction of atoms at the surface of a coordination interactions, etc.).15 The assembled nanometre-sized domain significantly increases with the superstructures typically behave as more than the sum of surface area per unit volume, changing - for instance - their individual parts or else exhibit completely new from ~100% for nanocrystals as small as 1 nm in diameter, types of behaviour.16 Self-assembling is centrally to about 15% for 10 nm nanocrystals.6 important in life science: cells contain a large range of self-assembled complex structures (lipid membranes, A variety of intriguing solid-state properties coupled folded proteins, structured nucleic acids, protein with facile post-synthesis processability make NCs a aggregates, molecular machines).17 Self-assemblies are at major class of attractive “man-made” materials, aimed at the base of novel smart materials with regular structures, achieving specific functionalities. In particular, colloidal such as molecular crystals,18 liquid crystals19 and NCs are suitable vehicles to bring about the functions of semicrystalline and phase-separated polymers.20 Its great crystals in a solution phase. They are composed of an potential in materials and condensed matter science21,22 is inorganic crystalline core and a surface shell of surfactant mainly due to the particular behaviour of assembled or ligand molecules that coordinate to unsaturated superstructures, which typically consist of more than the surface atoms. Due to such organic surface capping, NCs sum of their individual nanostructures' contributions or can be solubilized in a variety of solvents, embedded in a else exhibit completely new types of behavior.16 polymeric matrix, immobilized on substrates, integrated into electrical circuits, or have their surface functionalized In colloidal synthesis, a diverse range of sizes and shapes with biological molecules or with another inorganic of building blocks are accessible today, leading to, e.g., material. spheres, rods, cubes, wires, tetrapods and octapods,23 whose self-assembling allows them to fabricate new The advantages arising from the peculiar behaviour of hierarchically-ordered materials (‘nanocrystal nano-sized matter can be combined - and, hence, further solids’).24,25,26 In the 1890s, three mathematicians (Federov, extended - by fusing various single-component NCs into Schoenflies and Barlow) independently discovered the a unique multifunctional nano-object, thanks to the number of ways that exist to periodically distribute association of material sections with, e.g., magnetic, identical objects of an arbitrary shape in 3D space. Thanks optical or catalytic properties.7-10 “Smart” platforms can to their work, it is well known in crystallography that then be engineered so that they are able to accomplish there exist 230 different space groups for three- multiple actions (e.g., in biomedicine, environmental dimensional crystal lattices. A similar question is under clean-up, catalysis, sensing).10, 11 discussion today in view of predicting how polyhedra of

18 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 17-40 www.intechopen.com nanometric size can self-assemble into complex beamline of the ESRF (Grenoble) while using table-top structures. Very recently, Damasceno27 published instrumentation are compared and the possible theoretical predictions about 145 convex polyhedra, improvement of the laboratory data quality is whose assembly arises solely from their anisotropic demonstrated. Experiments performed by the authors of shape. Depending on the “coordination number” in the this manuscript, using table-top instruments are reported, fluid phase (the number of nearest neighbours showing the importance of modelling for GISAXS data surrounding each polyhedron) and the isoperimetric interpretation: the application of grazing incidence quotient (the deviation of the actual polyhedron shape techniques to derive the spatial arrangement of from the sphere), the assembly of hard polyhedra can nanoparticles together with their crystalline structure result in crystals (periodic arrays with long range (GISAXS and GIWAXS respectively); the application of positional and orientational order), plastic crystals grazing incidence techniques to the study of molecular (periodically ordered structures with positionally blocked architectures (GIWAXS). Finally, a few examples of in situ sites but with building units which are free to rotate), GISAXS studies selected from the literature - performed liquid crystals (structures with positional disorder but a with synchrotron radiation sources - are described. strong orientational order) and fully disordered structures (amorphous). 2. X-ray scattering from surfaces

NC properties are strongly influenced by their size and X-ray techniques as applied in grazing incidence shape with respect to both the inner core and the surface geometry are highly sensitive to surface structure and ligands, and several tools have to be combined to morphology. This high sensitivity to surfaces can be characterize both parts in detail. Meanwhile, synthesis easily understood by bearing in mind the well known proceeds from single-material nanocrystals (NCs) to Lambert-Beer law which, for a monochromatic and hybrid NCs (HNCs) and finally to self-assembled NCs; in collimated radiation beam impinging perpendicular to a addition, the techniques typically adopted for the material surface, can be written as: structural-compositional investigation of the inorganic part - such as transmission electron microscopy (TEM), −µz I= I0 e (1) high-resolution TEM (HRTEM), electron diffraction (ED),

Small and Wide Angle X-ray Scattering (SAXS and where I and I0 are the transmitted and incident intensity, WAXS), solid-state Raman spectroscopy, steady-state and and µ and z are the linear attenuation coefficient and time-resolved optical spectroscopy, Mössbauer thickness of the material traversed by the beam, spectroscopy, X-ray absorption and fluorescence respectively. The beam will therefore penetrate into the spectroscopy (XAS and XFS respectively) - improved as material for a depth roughly equal to 1/µ. If the beam well. A comprehensive description of these techniques is impinges on the surface at an arbitrary angle αi, the actual beyond the scope of this chapter and can be found in penetration depth will be reduced to (1/µ)sinαi. Therefore, many excellent books and reviews. the smaller the incidence angle, the larger the surface

sensitivity. X-rays can interact in different ways when In what follows, the theoretical basis and the application impinging on a material surface or more generally on an of grazing incidence X-ray scattering in the investigation interface, and can be scattered in different angular ranges. of nanostructured architectures is presented. Depending on which angular range is investigated in an

X-ray scattering/diffraction experiment, the information In Section 2, we describe the basic interaction properties carried by the scattered X-ray beam is either related to the of X-rays with matter - in particular, reflection and atomic structure or to the morphology of the surface (and, refraction at an interface and how they affect a scattering possibly, other structures deposited above or buried experiment. The theoretical and experimental basis of below it). The larger the scattering angle, the smaller the grazing incidence small angle X-ray scattering (GISAXS) length scale probed in the experiment. This can be better is illustrated and linked to the wide angle X-ray scattering understood by introducing the scattering vector Q, defined investigation performed in the same grazing incidence as the difference between the incident and the scattered geometry (GIWAXS). wave vectors ki and kf, both having a modulus equal to

|ki,f| =2π/λ, and directed along the propagation direction of In Section 3, we describe a table-top experimental set-up the incident and scattered beams, respectively. If the allowing us to perform GISAXS and GIWAXS experiments. scattering angle (between ki and kf ) is defined as 2θ, then

the modulus of the scattering vector is expressed as a Section 4 is dedicated to selected examples of GISAXS function of half the scattering angle such that: and GIWAXS experimental studies. Ex situ experiments are first described. Data collected on the same assembly |Q| = Q = |ki - kf | = (4π/λ)sinθ (2) of octapod-shaped nanoparticles at the ID01 synchrotron

www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 19 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering Moreover, the measured X-ray scattering pattern is Assume a parallel beam (linearly polarized plane wave) related to the Fourier transform of the electron density travelling in vacuum or in air (n0 ≈ 1) and impinging on distribution in the irradiated sample,28 so that the length the material flat surface (n ≠ 1) with an incidence angle αi, scale d of the electron density variations (i.e., the size of an amplitude Ei and a wave vector ki; the reflected beam the “objects”) probed in the experiment is related to the leaves with an angle αf, an amplitude Ef and a wave inverse of Q: vector kf ; the transmitted beam makes an angle αt with the surface and has an amplitude Et and a wave vector kt. d = 2π/Q (3) The Snell-Descartes’ law gives: The space of the Q vectors is called the 'reciprocal space' cos(α )= n cos( α ) and α= α (4) (being the direct space related to the d-spacing). Since the i t i f

λ scattering angle is also related to the X-ray wavelength As long as n > 1, total reflection cannot occur when an used in the experiment, and because of eq. (3), it is often electromagnetic wave travels from the vacuum to the more convenient to refer to the scattering vector material (even if αi = 0). Fortunately, unlike visible light, (modulus) rather than to the scattering angle. Moreover, when hard X-rays (i.e., energy above ~ 5 keV) are Q it is worth noting that the direction of corresponds considered, the refractive index of materials is generally with that actually probed in the material sample by the less than unity (n < 1) - see Figure 1b - and can be scattering experiment. In order to decode the expressed as: structural/morphological information contained in the λ2 2 ρ ′ δ = scattering pattern, different phenomena related to the X- 2π 2 𝑒𝑒𝑒𝑒 𝑁𝑁𝑁𝑁𝐴𝐴𝐴𝐴 ∑𝑗𝑗𝑗𝑗 �𝑍𝑍𝑍𝑍𝑗𝑗𝑗𝑗 −𝑓𝑓𝑓𝑓𝑗𝑗𝑗𝑗 � = 1 δ β = 2 ′′ (5) ray interaction with surfaces/interfaces - especially when λ 2 ρ 𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗 λµ β = 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 ∑ 𝐴𝐴𝐴𝐴= 2π 2 4π dealing with grazing-incidence experiments - have to be 𝑛𝑛𝑛𝑛 − − 𝑖𝑖𝑖𝑖 � 𝑒𝑒𝑒𝑒 𝑁𝑁𝑁𝑁𝐴𝐴𝐴𝐴 ∑𝑗𝑗𝑗𝑗 �𝑓𝑓𝑓𝑓𝑗𝑗𝑗𝑗 � taken into account and correctly described in the 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 ∑ 𝐴𝐴𝐴𝐴𝑗𝑗𝑗𝑗𝑗𝑗𝑗𝑗 theoretical model used for data analysis. The most where the summation is over all of the atomic species j important are reflection and refraction, which will briefly present in the molecular unit, Zj, fj' and fj'' are the atomic be described in the following section. number and the anomalous dispersion corrections (Z + f' is the real and f'' is the imaginary part of the atomic 2.1 X-ray reflection and refraction at surfaces/interfaces scattering factor), Aj, ρ and µ are the atomic weight of species j, the mass density and the attenuation coefficient Consider an ideally flat interface between a vacuum and of the material (basically related to photoelectric a material bulk (with an index of refraction n, depending absorption), respectively, NA is Avogadro’s number, e upon the wavelength): the reflection/refraction geometry and m are the electron charge and mass, and λ is the X- is shown in Figure 1. ray wavelength.

Figure 1. a) Reflection and refraction of a plane wave with an amplitude Ei incident upon the interface between a vacuum and a material of refractive index n; b) Refractive index n versus the frequency of the electromagnetic wave.

20 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 17-40 www.intechopen.com

Figure 2. Fresnel quantities as a function of the incident angle αi normalized by the critical angle of the substrate αc = 2 δ for absorption

β/δ = 0:001; 0:005; 0:01; 0:05; 0:1: (a) the reflection coefficient , (b) the transmission coefficient , (c) the normalized penetration depth, and (d) the phase shift at reflection. [Reproduced with the permission of the Elsevier, from G. Renaud, R. Lazzari, and F. Leroy, “Probing surface 𝑅𝑅𝑅𝑅 𝑇𝑇𝑇𝑇 and interface morphology with Grazing Incidence Small Angle X-Ray Scattering”, Surf. Sci. Reports, vol. 64, pp. 255–380, 2009].

When n < 1, total external reflection occurs on a function of the refractive index and the incidence angle, the vacuum side, although for very grazing angles since by the Fresnel’s formulas: δ and β are usually within the 10-5 and 10-6 ranges, 2 respectively, leading to a critical angle for total external 2 2 ( ) = = reflection ~ 2 within the 0.1°− 0.5° range. This 2 2 𝑓𝑓𝑓𝑓 𝑖𝑖𝑖𝑖 + 𝑖𝑖𝑖𝑖 condition is fully exploited in the X-ray Specular 𝐼𝐼𝐼𝐼 𝑠𝑠𝑠𝑠𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝛼𝛼𝛼𝛼 − �𝑛𝑛𝑛𝑛 − 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑠𝑠𝑠𝑠 𝛼𝛼𝛼𝛼 𝑐𝑐𝑐𝑐 𝑅𝑅𝑅𝑅 𝛼𝛼𝛼𝛼𝑖𝑖𝑖𝑖 � � 𝛼𝛼𝛼𝛼 √ 𝛿𝛿𝛿𝛿 𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖 Reflectivity (αf =αi) technique:28 the value of the critical 𝐼𝐼𝐼𝐼 𝑠𝑠𝑠𝑠𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝛼𝛼𝛼𝛼 � 𝑛𝑛𝑛𝑛 − 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑠𝑠𝑠𝑠 𝛼𝛼𝛼𝛼 (7) angle is indeed strongly dependent upon the density of 2 𝑎𝑎𝑎𝑎𝑛𝑛𝑛𝑛𝑎𝑎𝑎𝑎 2 the material layers close to the surface (see also eq. 5) ( ) = = + 2 2 and allows surface investigations with hard X-rays (e.g., 𝐼𝐼𝐼𝐼𝑡𝑡𝑡𝑡 𝑠𝑠𝑠𝑠𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝛼𝛼𝛼𝛼𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖 𝑖𝑖𝑖𝑖 layer deposition, molecular adsorption/desorption, 𝑇𝑇𝑇𝑇 𝛼𝛼𝛼𝛼 𝐼𝐼𝐼𝐼 �𝑠𝑠𝑠𝑠𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝛼𝛼𝛼𝛼 �𝑛𝑛𝑛𝑛 −𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑠𝑠𝑠𝑠 𝛼𝛼𝛼𝛼 � Figure 2 reproduces the calculated Λ, R and T, for some oxidation, etc.).28 typical situations, together with the phase relation

between the reflected waves and the incident waves. For When α i < αc, the component of the transmitted wave- αi < αc, R = 1 and total external reflection occurs; as vector normal to the surface becomes imaginary and the expected, the penetration depth is minimum and within refracted wave is exponentially damped as a function of the nanometre range. When αi >>αc, the reflectivity falls the distance below the surface, resulting in an evanescent off rapidly to the asymptotic behaviour ∼1/Q4. Moreover, wave travelling parallel to the surface. The penetration slight variations of the incident angle allow for tuneable depth of the X-rays can be calculated as:29 depth analysis and the investigation of buried interfaces

λ (up to a micrometer in depth). The behaviour explained Λ = (6) so far is exploited by X-Ray Reflectivity so as to gain a 2 2 4π 2 different kind of valuable information, especially in the 𝐼𝐼𝐼𝐼𝑚𝑚𝑚𝑚 �𝛼𝛼𝛼𝛼𝑖𝑖𝑖𝑖 −𝛼𝛼𝛼𝛼𝑐𝑐𝑐𝑐 − 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 case of multilayered structures. In particular, X-ray which decreases with any increase of the incidence angle. specular reflectivity (where αi = αf and Q is always The reflection (R) and the transmission (T) coefficients of perpendicular to the surface) is sensitive to the electron the surface are critically dependent on αi and are given as density profile in the direction perpendicular to the

www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 21 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering surface and can provide measurements of density and the T(α) function (Figure 2). Working at 2αc or 3αc allows thickness of material layers, as well as possible for more comfortable measurement conditions, if the periodicity (in the case of superlattices). On the other background remains acceptable. Regardless, grazing hand, off-specular X-ray reflectivity (where the condition incidence often remains as the mandatory condition in αi = αf is not fulfilled and Q can also have a component investigating surfaces with hard X-rays. This principle is parallel to the surface) is produced by surface (and any used for Grazing Incidence Wide Angle X-ray Scattering interface) inhomogeneity, which can be due to artificial (GIWAXS)30-34 as well as for Grazing Incidence Small modifications (periodic or not) or to the intrinsic Angle X-ray Scattering (GISAXS)35-42 in studying - roughness of the surface (or interface). Off-specular XRR respectively - the structure and the morphology of the can, therefore, provide in-plane correlation lengths surface (as well as of nano-objects above or below it). (characteristic lengths over which a given electron density profile is repeated) for the surface and the other interfaces 2.2 Grazing Incidence Small Angle X-ray in a multilayer and, if performed for different Q values, Scattering (GISAXS) can also provide out of plane correlation lengths28. The periodicity at the atomic level - i.e., with typical The exit beam (scattered and coming out from the material) periods of a few ångströms - can be characterized by experiences the same refraction effects as the incident measuring the scattered intensity far away from the beam, and transmission across the interface is again origin of the reciprocal space (i.e., large scattering angles). enhanced when the exit angle from the surface is equal to If objects of much larger size (particles/islands, typically αc (it can be explained on the basis of the reciprocity between a few nanometres and several tens of principle). These two regions of increased transmittivity nanometres) are present in the sample, additional (which leads to maxima in the measured intensity in off- scattering can be measured close to the origin of the specular reflection geometry) are known as Yoneda wings. reciprocal space (i.e., small scattering angles). Its A comprehensive discussion of the refraction effects on the measurement and analysis is the object of a well known outgoing beam can be found in refs.28-29. In general, and old method: Small Angle X-ray Scattering (SAXS),45 working in grazing incidence is important whenever the where measurements are usually performed in properties of the surface - or of nanostructures lying above transmission geometry. This method has been extended or below it - are to be investigated. Indeed, the limited to analyse the morphology of nanometre scale particles penetration depth of X-rays in grazing incidence geometry deposited on or embedded below the surface of a sample strongly reduces the absorption and the background from by performing SAXS experiments in grazing-incidence the bulk material, while the signal from the investigated geometry (Grazing Incidence SAXS), which makes them material layers - or particles - is enhanced. In the case of surface sensitive.46 Grazing Incidence Small Angle X-ray particles on a surface, scattering from the bulk material Scattering (GISAXS) has been developed and fully (substrate) can be avoided by keeping the incidence angle applied to ex situ as well as in situ studies. The latter can just below the critical angle of the substrate αc. However, be performed in UHV, in real time, in order to follow - for data needs to be analysed by taking into account the example - the formation of 3D islands growing on a reflected intensity from the surface. substrate and to study the evolution of their morphology, which is one of the most exciting possibilities of GISAXS It is worth noting that when αi = αc, great care must be and a very important step in the control of nanometre- taken in order to keep αi strictly constant during the sized objects (nano-objects) during their fabrication. whole process of data collection: indeed, even very small variations of the incidence angle lead to large intensity The experimental geometry of GISAXS is schematically variations, since αi = αc corresponds to the maximum of represented in Figure 3.

Figure 3. Typical geometry for a GISAXS experiment.

22 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 17-40 www.intechopen.com

The incident beam impinges upon the sample under the where ρ is the electron density and the integral is over the grazing incidence (close to αc) and a 2D detector is placed volume V of the particle/island. The square modulus of downstream, recording the intensity in the (Qy,Qz) plane the amplitude F(Q) gives the X-ray intensity scattered by of the reciprocal space (orthogonal to the primary beam a single nanoparticle. In this description, the particles are direction). Since only a very small fraction of the beam generally treated as a continuum, being their scattering intensity is scattered by the sample, the transmitted and power related to their average electron density (and, reflected beams (as well as any residual direct beam) are hence, refractive index), because the length scales probed completely stopped by a beam-stop in front of the by the small angle X-ray scattering are large compared to detector so as to avoid saturation. The scattering signal inter-atomic distances. contains information on the nanoparticle shape, height (H) and lateral size (2R), as well as on the spatial The mutual arrangement of islands is described through organization of the nanoparticles with respect to one an interference function S(Q) which, for periodic another. Nanoparticle morphology is described through a structures, is analogous to the case of atomic crystal form factor F(Q), which is the Fourier transform of its lattices; meanwhile, in the case of non-periodic structures shape function (being the last equal to 1 within the it expresses the distribution of inter-particle (centre-to- nanoparticle volume, and 0 outside): centre) distances though a pair correlation function. The interference function is usually expressed by:

( )= ρ( ) (8) S(Q) = 1 + ( ( ) 1) (9) −𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖푖𝑖𝑖𝑖𝑖 −𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖푖𝑖𝑖𝑖𝑖 𝐹𝐹𝐹𝐹 𝐐𝐐𝐐𝐐 𝐫𝐫𝐫𝐫 𝑒𝑒𝑒𝑒 𝑎𝑎𝑎𝑎𝒓𝒓𝒓𝒓 𝜌𝜌𝜌𝜌 ∫ 𝑔𝑔𝑔𝑔 𝐫𝐫𝐫𝐫 − 𝑒𝑒𝑒𝑒 𝑎𝑎𝑎𝑎𝐫𝐫𝐫𝐫 ∫ FULL SPHERE HALF SPHERE

CYLINDER PYRAMID

CUBOCTAHEDRON TETRAHEDRON

Figure 4. Form factor calculations for different shapes.

www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 23 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering LATTICE OF FULL SPHERES LATTICE OF HALF SPHERES

LATTICE OF CYLINDERS LATTICE OF PYRAMIDS

LATTICE OF CUBOCTAHEDRONS LATTICE OF TETRAHEDRONS

Figure 5. GISAXS intensity of 2D square lattice assemblies, for different form factors.”

where ρ is the density of nanoparticles and the statistical The resulting intensities have characteristic profiles with function g(r) is the particle-particle pair correlation a series of well-defined intensity minima. The positions of function describing how the particles are distributed with the minima, together with the intensity profile, allow the respect to each other. In the case of very diluted particle determining of the particle shape as well as its average ensembles, the waves scattered by different particles will vertical and lateral size. not interfere, so that the scattering pattern will be related just to the particle form factor (see Fig. 4). On the other If the particles are arranged in a 2D assembly and their hand, as long as the inter-particle distance is reduced, the scattering signals are correlated, the main resulting scattered waves will interfere so that the scattering pattern feature is one or more interference peaks (see Figure 5) will also be affected by the spatial distribution of the along 2θf (which means that the Q vector component is particles (i.e., by the interference function). Therefore, the parallel to the sample surface) whose position Qp directly intensity I(Q) scattered by a system of Np identical particles yields a rough estimation of the average centre-to-centre will in general be expressed by: inter-particle distances d, according to d = 2π/Qp.37-51

2 I (Q) ∝ Np × | ( )|2×| ( )| (10) The distributions of size parameters can yield large variations in fringe visibility and can be derived through a 𝐹𝐹𝐹𝐹 𝐐𝐐𝐐𝐐 𝑆𝑆𝑆𝑆 𝐐𝐐𝐐𝐐 In practice, the form factor F(Q) can be analytically fitting procedure. A detailed analysis allows us to fully calculated for many simple shapes, usually taken by reproduce the 2D GISAXS patterns. The form factor can be islands growing on a surface (e.g. truncated cylinders, calculated in the Born approximation, just as with the ellipsoids or pyramids), as shown in Figure 4. Fourier transform of the shape function of the object, as

24 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 17-40 www.intechopen.com previously stated (eq. 8). However, for incidence angles close molecular systems (e.g., polymers), down to atomic to the critical angle of the substrate, refraction-reflection crystals (see also the Applications section below). effects at the surface of the substrate have to be taken into account. The full X-ray scattering process can then be The proper GISAXS experimental set-up has to be chosen schematically represented (Figure 6)51 by the sum of four depending upon the specific scientific need. Until a few processes involving the direct scattering from the particle years ago, GISAXS experiments were basically performed (as in the Born approximation), a reflection on the substrate with synchrotron radiation sources, as their brilliance was surface followed by scattering by the particle (and vice a key factor for the success of the experiment. versa), reflection on the substrate surface followed by scattering by the particle and subsequent further reflection Today, with the advent of novel high brilliance laboratory on the substrate. The four processes are mathematically sources, at the very least ex situ GISAXS experiments can be described within the Distorted Wave Born Approximation realized with laboratory instrumentation. In situ GISAXS (DWBA) so that the scattered intensity is finally given by the experiments as well as sub-micrometric spatial resolution sum of four terms,51 where those involving reflections are GISAXS experiments remain fully within the domain of weighted by the corresponding Fresnel reflection synchrotron sub-micro-focus/nano-focus beamlines.53-55 coefficients. It is worth noting that only the wave vector component (kz) perpendicular to the substrate surface is In the next paragraph, we will describe a laboratory set-up involved in this approach, so that multiple scattering effects which can be used for ex situ GISAXS experiments, while for can usually be clearly recognized in the GISAXS patterns other, more sophisticated and complex experiments, the along the direction (z) perpendicular to the surface (see, e.g., reader can find all of the detailed information in the relevant the sharp intensity variation in Figures 4 and 5 at αf ∼ 0.2°). publications (see the Applications section).

The same grazing incidence geometry can be used to obtain 3. GISAXS & GIWAXS Instrumentation information on the atomic structure of the investigated samples instead of the morphology. This is accomplished by As an example of a laboratory instrument allowing for the collecting data for much larger values of the scattering angle simultaneous collection of both GISAXS and GIWAXS data, (and, hence, of the scattering vector length), as is typical for the X-ray Micro-Imaging Laboratory (XMI-LAB),52 recently standard X-ray diffraction. This approach is referred to as installed at the CNR-IC in Bari, will be described (Figure 7). Grazing Incidence Wide Angle X-ray Scattering (GIWAXS). As in GISAXS, the incidence angle can still be varied, The XMI-LAB schematic layout is shown in Figure 7a. changing the penetration depth of the X-ray beam and, The laboratory is equipped with a Fr-E+ SuperBright consequently, probing different sample thicknesses below rotating copper anode micro-source (45 kV/55 mA; Cu- the surface. Both GIWAXS and GISAXS can be collected at Kα, λ = 0.154 nm, 2475 W), shown in Figure 7b, and a the same time52 on two detectors at different distances from SAXS/WAXS (SWAXS) three pinhole camera (Figure 7c). the sample, respectively, giving morphological/structural The X-ray beam is focused through a multilayer focusing information both at the nano-scale and the atomic scale. optics (Confocal Max-Flux − CMF® 15-105) and collimated Such simultaneous acquisition, without repositioning the by three pinholes, with diameters of 150µm/50µm/200µm sample, ensures that all of the derived information is (high spatial resolution configuration) or actually related to the same region of the sample. 300µm/150µm/500µm (high flux configuration). The flux measured at the sample position is ~9x106 photons/s in Finally, and naturally, all of the intermediate length the high flux configuration and the ratio between the flux scales between the GISAXS and GIWAXS regimes can be values in the two configurations is around 34. The system explored by collecting data in the suitable q-range, which is equipped with two distinct detectors: a Triton™20 gas- is normally obtained experimentally by simply moving filled proportional counter (1024x1024 array, 195μm pixel the (2D) detector the required distance from the sample. size) for (GI)SAXS acquisition and an image plate (IP) In this way, a wide range of length scales can be probed detector (250x160 mm2 in size, with 50 or 100μm effective and many different systems can be studied, ranging from pixel size depending upon binning, and an off-line assemblies of (even very small) nanoparticles to ordered RAXIA reader) to collect (GI)WAXS.

Figure 6. Refraction-reflection effects at the surface of the substrate taken into account in the DWBA. www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 25 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering

Figure 7. (a) Scheme of the XMI-LAB; (b) SuperBright rotating copper anode micro-source (45 kV/55 mA; Cu-Kα, λ = 0.15405 nm); (c) SAXS/WAXS (SWAXS) three pinhole camera [Reproduced with the permission of the International Union of Crystallography (http://journals.iucr.org/) from D. Altamura, R. Lassandro, F. A. Vittoria, L. De Caro, D. Siliqi, M. Ladisa and C. Giannini, “X-ray micro- imaging laboratory (XMI-LAB)”, Journal of Applied Crystallography, vol. 45 (4), 869-873, 2012].

In order to simultaneously measure the SAXS and WAXS 4. Applications - ex situ studies images, a 6mm hole is left open in the centre of the Image Plate IP detector. Depending upon the pinhole set, a beam 4.1 GISAXS - 2D Self-assembly of CdSe/CdS nano-octapods stopper (Ø= 4.0mm; 5.5 mm) is mounted on a holder in front of the SAXS detector, which contains a pin-diode to The accuracy in the determination of size and shape of monitor the beam transmitted through the sample. the building blocks assembled in 1D, 2D or 3D architectures - along with their mutual positions - by The sample detector distance is ~2200 mm for the GISAXS, is strongly dependent upon the visibility of the (GI)SAXS detector and ~28 mm for the (GI)WAXS interference fringes and, hence, on the signal-to-noise and detector. These distances give access to Q - ranges of signal-to-background ratios in the X-ray scattering about 0.006÷0.2Å−1 for (GI)SAXS and 0.7÷ 4.7Å−1 for pattern. The use of synchrotron radiation is often (GI)WAXS. preferable (or even mandatory) because of the weak scattering signal due to the small amount of material in The specimen can be mounted in SAXS/WAXS the investigated nanostructured sample. Unfortunately, (transmission) or GISAXS/GIWAXS (reflection) geometry, access to synchrotron radiation instrumentation is the latter using a remote-controlled goniometer. The traditionally difficult and characterized by long waiting goniometer has a 125x125 mm2 stage with the rotation times, and definitely not suitable for in-line routine axes parallel and perpendicular to the primary beam monitoring, as often required by the synthesis of novel direction, as well as the vertical translation movable by materials. As shown by the research group working at the means of three independent stepper piezoelectric motors. XMI-Lab of the CNR-IC (Bari), laboratory data quality The accessible angular range is between -1 and +6 degrees can be further improved by applying a suitable with sub-arcsecond precision. mathematical treatment (restoration algorithm). If a high brilliance laboratory micro-source (e.g., Rigaku FR-E+

26 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 17-40 www.intechopen.com

Superbright) is employed for data collection in Such a superlattice structure has been analysed through combination with the algorithm, a final data quality GISAXS for the first time and was chosen as the case comparable to that of a third generation synchrotron can study because of the complex shape of the building be achieved.56 The application of the algorithm allows the blocks, which leads to detail-rich characteristic X-ray deconvolution of the source function (describing the scattering patterns (see Figure 9). primary beam with its size and divergence) from the measurement and the restoration, at least partially, of GISAXS data were collected at the ID01 beamline of the missing data cut away by the beam stopper, making European Synchrotron Radiation Facility (ESRF) and at hidden features in the GISAXS pattern become visible. the XMI-LAB (IC-CNR): the raw data are shown in For this reason the algorithm has been named 'RESTORE- Figures 9a and 9b, respectively. The angle of incidence DATA'. A suitable test sample was chosen for this study, was 0.13°. Reported in Figure 9c is the 2D GISAXS comprised by octapod-shaped nanocrystals57,58 which pattern shown in Figure 9b, after applying the RESTORE- have been self-assembled in 2D square lattices on the DATA algorithm. A selected area, marked with a red surface of a silicon substrate. The octapods consist of a square region for each figure, is zoomed in upon and CdSe core from which eight CdS pods depart according reported in Figure 9d-f in false colour output, allowing a to an octahedral symmetry. TEM and SEM pictures, better appreciation of the differences between the two as together with schemes of the octapods and their collected GISAXS maps and the improved visibility of the assemblies, are reported in Figure 8. features after data restoration (Figure 9c).

Figure 8. (a) TEM) image of the “unassembled” octapods, deposited on a standard TEM grid. Sketch of an octapod as viewed in two different projections. (b) Sketch of a 2D square lattice of side-to-side aligned octapods, each of them touching the substrate with four pods, as clearly shown by the tilted SEM, inset. In (c) and (d) can be seen SEM images of the actual 2D square lattices of octapods, from the same sample as that shown in (b), at two different magnifications [Reproduced with the permission of the International Union of Crystallography (http://journals.iucr.org/) from L. De Caro, D. Altamura, F. A. Vittoria, G. Carbone, F. Qiao, L. Manna & C. Giannini, “A superbright X-ray laboratory micro-source empowered by a novel restoration algorithm”, J. Appl. Cryst. vol. 45, pp. 869−873, 2012].

www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 27 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering

Figure 9. (a) GISAXS data collected at the ID01 beamline (ESRF) and (b) at the XMI-LAB (IC-CNR); (c) Data in (b) after applying the RESTORE-DATA algorithm. The red square regions marked in panels (a), (b) and (c) are reported in false colours in panels (d), (e) and (f), respectively [Reproduced with the permission of the International Union of Crystallography (http://journals.iucr.org/) from L. De Caro, D. Altamura, F. A. Vittoria, G. Carbone, F. Qiao, L. Manna & C. Giannini, “A superbright X-ray laboratory micro-source empowered by a novel restoration algorithm”, J. Appl. Cryst. vol. 45, pp. 869−873, 2012].

Figure 10. Horizontal cuts extracted along the red arrows in panels d-f of Fig. 9: XMI-LAB raw data (blue line), ESRF-ID01 raw data (red line), restored XMI-LAB data (black line) [Reproduced with the permission of the International Union of Crystallography (http://journals.iucr.org/) from L. De Caro, D. Altamura, F. A. Vittoria, G. Carbone, F. Qiao, L. Manna & C. Giannini, “A superbright X- ray laboratory micro-source empowered by a novel restoration algorithm”, J. Appl. Cryst. vol. 45, pp. 869−873, 2012].

Essentially, the RESTORE-DATA algorithm recovers the Ibk(k) basically represents the background contribution coherent part Icoh(k) from the scattered intensity I(k) due to electronic noise (which is not influenced by the measured using an (intrinsically) incoherent source: convolution), n(k) denotes the intrinsic statistical noise

affecting the measurement (shot-noise) and B(k) describes = ⊗ + + I()()k[] Icoh () k Is k B()()() k Ibk k n k (11) the beam-stopper shape. Icoh(k) is, of course, assumed to Here k is the scattering vector component perpendicular be the only unknown quantity of the experiment, and is to the incident beam propagation direction, Is(k) describes derived through the maximization of the likelihood the X-ray source, “⊗” denotes the convolution product, probability.

28 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 17-40 www.intechopen.com

As a result, the RESTORE-DATA algorithm leads to a superlattice organization was revealed by inspecting the virtual enhancement of the source brilliance, which is island across the sharp ridges visible at such stepped responsible for the improvements in Figures 9c and 9f. regions and/or in correspondence with induced cracks: Such improvements can also be appreciated in the 1D regularly stacked NC layers were formed, compatible profiles reported in Figure 10, representing the linear cuts with cubic- and hexagonal-type structures viewed down taken along the red arrows in panels d-f of Figure 9, the [111] and [001] directions, respectively. which are the kind of dataset generally used in the actual fitting procedure for GISAXS data. GISAXS data were collected at a very small incidence angle (αi = 0.05deg), as shown in Figure 12a. Indeed, the 4.2 GISAXS – 3D Self-assembly of iron oxide NCs59 particular morphology of the sample allowed the measurement of the X-ray intensity scattered from 3D The bright-field TEM image of ~10 nm iron oxide NCs, regions of the islands, even with the beam impinging at a shown in Figure 11a, demonstrates the high highly grazing incidence angle on the substrate. monodispersity of the NCs, which were used to make millimetre-scale supercrystals, organized as 3D ordered The data were analysed against a new theoretical model59 NCs. A guided procedure was used, driving the (see the calculated map in Figure 12b), as the magnetically responsive NCs organized during slow applicability of software available for the fitting of the solvent evaporation from corresponding colloidal whole GISAXS intensity, up to recently, has been either solutions under the action of an external applied restricted to 2D in-plane assemblies of particles magnetic field.60 SEM images, collected at various (IsGISAXS),61 limited to the calculation and indexing of magnifications, are displayed in Figures 11b-d and show the expected diffraction spot positions (NANOCELL)62 or the NCs assembled in a compact film on a substrate, from devoted to particular types of samples - e.g., with thin which several well-separated, distinct 3D conically film morphology and flat interfaces (NANODIFT).63 shaped islands protruded outwards. The inner

Figure 11. (a) TEM image of the 2D monolayer of NCs on a Cu-supported carbon film of a TEM grid, which had self-assembled upon solvent evaporation; (b-d) Low-resolution SEM images at different magnifications, showing the island-like features on the surface of the NC-built superlattice films; (e-f) High-resolution SEM images of regions within cracks of the islands, where the 3D ordered NC packing can be clearly seen [Reprinted with permission from D. Altamura, V. Holý, D. Siliqi , C.I. Lekshimi , C. Nobile, G. Maruccio, P.D. Cozzoli, L. Fan, F. Gozzo and C. Giannini, “Exploiting GISAXS for the Study of a 3D Ordered Superlattice of Self- Assembled Colloidal Iron Oxide Nanocrystals” , Cryst. Growth Des., vol. 12, pp. 5505−5512, 2012, Copyright (2012) American Chemical Society].

www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 29 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering

Figure 12. (a) Experimental and (b) simulated GISAXS maps for 0.05° incidence. Linear cuts along (c) the vertical and (d, e) horizontal directions, through the lines indicated by the white ticks in the maps. The data is simulated using the Born approximation in the LMA description [Reprinted with permission from D. Altamura, V. Holý, D. Siliqi , C.I. Lekshimi , C. Nobile, G. Maruccio, P.D. Cozzoli, L. Fan, F. Gozzo and C. Giannini, “Exploiting GISAXS for the Study of a 3D Ordered Superlattice of Self- Assembled Colloidal Iron Oxide Nanocrystals” , Cryst. Growth Des., vol. 12, pp. 5505−5512, 2012 Copyright (2012) American Chemical Society].

For this study, a new program has been implemented for emission in the near-infrared region and the strong simulating and fitting GISAXS data collected from compact confinement of charge carriers. In this research field, PbS 3D assemblies. The program is based on a model colloidal nanocrystal assemblies with monomodal and previously published by Buljan, et al.,64 describing diluted bimodal size distribution have recently been fabricated.65 assemblies where the sizes and positions of the quantum dots could be assumed to be statistically uncorrelated. An example of GISAXS/GIWAXS studies on the assembly That model was then improved and applied to the GISAXS of three-dimensional lead chalcogenide (PbS) study of a 3D ordered non-diluted assembly of colloidal iron nanocrystals (see also the works by Altamura et al., 201266 oxide NCs. Fitting the experimental data (see the calculated and Corricelli et al., 201165 and references therein for and experimental profiles in Figures 12c, d, e) allowed us details on the chemical synthesis), formed by the slow to determine the fcc-like packing of the superlattice as evaporation of solvent on a Si substrate, is reported well as to probe the morphological and size-statistical below. properties of the assembled NCs. The results were found to be in excellent agreement with information derived by Figure 13 shows GIWAXS (upper part) and GISAXS other experimental techniques (SEM and TEM), thus (lower part) data from the same sample region, related to proving that the proposed model was indeed successfully the PbS nanocrystal lattice and the self-assembling applicable to the study of 3D closed-packed NC superlattice symmetry, respectively. The 1D profiles assemblies. reported in Figures 13b and 13d are obtained through the azimuthal integration of the corresponding Figures 13a 4.3 GISAXS/GIWAXS – 3D self-assembly of PbS NCs52 and 13c. These profiles reveal the rock-salt (CsSl-type) cubic structure of the PbS nanocrystals (about 2 nm in Among the different IV-VI semiconductor NCs, lead diameter, as determined by the red superimposed fitted chalcogenide NCs are very appealing because of their line) and a body-centred cubic superlattice.67

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Figure 13. Data collected from an assembly of 2 nm PbS nanocrystals: (a) GIWAXS; (b) 1D pattern extracted from (a); (c) 2D GISAXS image; (d) 1D GISAXS profile [Reproduced with the permission of the International Union of Crystallography (http://journals.iucr.org/) from D. Altamura, R. Lassandro, F. A. Vittoria, L. De Caro, D. Siliqi, M. Ladisa and C. Giannini, “X-ray micro-imaging laboratory (XMI- LAB)”, Journal of Applied Crystallography, vol. 45 (4), 869-873, 2012].

Figure 14. GIWAXS analysis of the realized P3HT nanostructures. 2D patterns of 270 nm wide features obtained by a PDMS mould (a) and a PFPE mould (b), with 80 nm wide features realized by a PFPE mould (c) [Reproduced by permission of The Royal Society of Chemistry from E. Mele, F. Lezzi, A. Polini, D. Altamura, C. Giannini, and D. Pisignano, “Enhanced charge-carrier mobility in polymer nanofibers realized by solvent-resistant soft nanolithography” J. Mater. Chem., vol. 22, pp. 18051-18056, 2012 (http://pubs.rsc.org/En/content/articlelanding/2012/jm/c2jm33611a)]; geometry of the measurement (d) [Reprinted from http://repository.lib.ncsu.edu/ir/handle/1840.16/7766]; orientation of the lamellar structures with respect to the substrate surface (e) [Reprinted with permission from D. J. Herman, J. E. Goldberger, S. Chao, D. T. Martin and S. I. Stupp, “Orienting Periodic Organic/Inorganic Nano-scale Domains Through One-Step Electrodeposition” ACS Nano, 2011, 5, 565–573.Copyrigth (2011) American Chemical Society].

www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 31 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering 4.4 GIWAXS – Organic field effect transistors based on poly(3- to analyse several evaporation stages, performing hexylthiophene)68 GISAXS experiments with a suitable time resolution. A few selected examples are described in what follows, Nanofibres of regioregular poly(3-hexylthiophene) although many others can be found in the literature (see, (P3HT) were fabricated by solvent-resistant e.g., refs.71-73). nanolithography and used as the active medium in organic field effect transistors (OFET).68 This process 5.1 In situ GISAXS – Dynamic investigation of gold favoured a remarkable improvement of the device nanocrystal assembly74 performance because of the nanofluidic flow in perfluoropolyether capillaries and the slow solvent In this work, stable aqueous solutions of monodisperse evaporation rate in the mould cavities, which induced the gold NCs were synthesized according to the preparation reorganization of the P3HT chains and allowed us to route developed by Brust et al.75 and followed by heat obtain a charge carrier mobility 60 times higher than in treatment in order to narrow the size distribution ( 7%). the corresponding homogenous films. GIWAXS 2D A micro-emulsion process driven by interfacial tension ∼ patterns were collected and are shown in Figure 14 for was used to encapsulate NCs within micelles of OFET with 270 nm wide features, obtained by cetyltrimethylammonium bromide (CTAB). The final polydimethylsiloxane (PDMS) (a), by perfluoropolyether solution was highly stable, with a concentration of Au (PFPE) moulds (b), and for 80 nm wide features by PFPE NCs in water of 50 mg/mL. For self-assembly into a moulds (c). Self-organization in P3HT resulted in a silica matrix, 0.08 mL of tetraethyl orthosilicate was ∼ lamellar structure with two-dimensional conjugated added to 1 mL of Au NC solution, along with 0.05 mL of sheets formed by inter-chain stacking, leading to (100) 0.07N HCl catalyst. For the in situ scattering studies reflections due to the layer lamellar structure and (010) presented here, this solution was further diluted by a reflections due to π–π inter-chain stacking. Orientations of factor of 10. P3HT crystallites with respect to the substrate were identified from the different intensity distributions of The dynamic self-assembly of pathway of ordered gold (100) and (010) reflections. The sequence of schematics nanocrystal arrays, during the self assembly of gold from left to right in Figure 14(e) represents GIWAXS 2D nanocrystal micelles, has been investigated by patterns together with the relevant lamellar structures performing in situ GISAXS at a synchrotron beamline oriented mostly parallel to the substrate surface (face-on), with and without the presence of colloidal silica predominantly perpendicular to the substrate surface precursors. Scattering data were obtained every 120 s (edge-on) and randomly with respect to the substrate with an integration time of 30s. Figure 15 shows GISAXS surface, respectively. Therefore, Figures 14a-c show that data from the self-assembly of Au NC micelles with the preferential orientation of the P3HT ordered domains (A,B), the corresponding evolution of unit cell parameters is with the (100)-axis normal to the film and the (010)- over time (C), and without (D, E,F) the presence of silica axis in the plane of the film (edge-on orientation).69 precursors at different times.

The lattice parameters a (the distance between the In situ experiments suggest a mechanism of self-assembly backbones) and b (π-stacking distance) ≅ c/2 (separation whereby lattice formation is driven by the bulk between the side chains) were derived from the out-of- concentration of Au-NCs during solvent evaporation. The plane (along Qz) and in-plane (along Qr) radial cuts preferential orientation is probably induced by extracted from the 2D GIWAXS maps, and are confinement between the liquid/solid and liquid/air approximately (1.67 ± 0.03) nm and (0.38 ± 0.02) nm, interfaces. Different behaviour in relation to the domain respectively, as is typical of ordered P3HT.70 orientations has been observed during the slow evaporation of the solvent and rapid kinetics in spin- 5. Applications - in situ studies coated films. Moreover, the self-assembly of Au-NCs is observed without the presence of silica, confirming that GISAXS can also be applied in situ for the real-time this process is driven by a long-range (non-specific) force. monitoring of the nanoparticles assembly and growth, Indeed, the addition of silica precursors (along with acid allowing for the study of the kinetic processes and the catalyst) modifies the self-assembly pathway in a manner temporal correlations of structural parameters such as consistent with Coulomb screening rather than specific array size, shape and spatial distribution, which are close-range interactions. Without silica, the Au NC lattice fundamental for tuning the physical properties of the collapses upon the completion of the solvent's systems. The use of the synchrotron radiation is needed evaporation.

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Figure 15. GISAXS data from the self-assembly of Au NC micelles with (A-C) and without (E,F) silica precursors. (C) Evolution of unit cell parameters (space group R m) over time. (D) An intermediate layer observed in the Au-NC-micelle (no silica). The incidence angle is 0.2° [Reprinted with permission from D. Dunphy, H. Fan, X. Li, J. Wang, and C.J. Brinker, “Dynamic Investigation of Gold Nanocrystal Assembly Using In Situ Grazing-Incidence Small-Angle X-ray Scattering”, Langmuir, vol. 24, pp. 10575-10578, 2008, Copyright (2008), American Chemical Society].

Figure 16. A t-qy map of a drying colloidal drop forming ordered (a) monolayer, (c) sub-monolayer arrays of nanoparticles measured in the substrate mode and in (e) drop mode; the corresponding (b-d-f) partial integrated scattering (PIS) plot [Reprinted with permission from P. Siffalovic, E. Majkova, L. Chitu, M. Jergel, S. Luby, A. Satka, S.V. Roth, “Self assembly of iron oxide nanoparticles studied by time resolved grazing incidence small angle X ray scattering”, Physical Review B, vol. 76, pp. 195432-8, 2007 Copyright (2007) by The American Physical Society].

5.2 In situ GISAXS – Self-assembly solution were deposited onto silicon substrates of 1cm2 of iron oxide nanoparticles76 covered with a native SiO2 layer. Monolayers of the iron oxide nanoparticles were prepared through solvent The iron oxide nanoparticles were synthesized through a evaporation from the drop of the colloidal solution, high temperature solution phase reaction of metal applied on the substrate at room temperature, following acetylacetonates (Fe(acac)3) with 1,2-hexadecanediol, oleic three different stages of drying. acid and oleylamine in phenyl ether using toluene as a solvent.77 The nanoparticles are of a single domain and The self-assembly of the colloidal nanoparticles was behave as single bipoles due to the weak dipole-dipole studied through time-resolved GISAXS measurements interaction of magnetic iron oxide nanoparticles. They also using both synchrotron radiation and a conventional X- present a crystalline structure and superparamagnetic ray rotating anode source, both in the substrate and drop properties at room temperature. The drops of colloidal modes. www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 33 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering Moreover, in order to quantify the temporal evolution, the scattered intensity has also been integrated over the qy interval to obtain a partial integrated scattering (PIS) as a function of time.

During the synchrotron measurements, the contributions of the volume and surface X-ray scattering were separated during self-assembly. The focused X-ray beam provided the temporal evolution of the volume and surface scattering from the drop, being undisturbed by the substrate scattering and, thus, giving an insight into possible nanoparticle self-assemblies forming inside or on top of the drop surface. At a distance from the surface larger than 80 µm, the colloidal solution shows the absence of self-assembled clusters in the drop volume or Figure 17. Partial integrated scattering plot of a drying colloidal self-assembled domains on the drop surface, such that we drop in the substrate mode, measured with an X-ray anode can assume that the self-assembly takes place near the source [Reprinted with permission from P. Siffalovic, E. three-phase (solid/liquid/vapour) drop contact line as the Majkova, L. Chitu, M. Jergel, S. Luby, A. Satka, S.V. Roth, “Self solvent evaporates. assembly of iron oxide nanoparticles studied by time resolved grazing incidence small angle X ray scattering”, Physical Review B, vol. 76, pp. 195432-8, 2007 Copyright (2007) by The American Physical Similar measurements with a better temporal resolution Society]. have been performed with an x-ray rotating anode, enabling to also monitor the early stage of the self- Three typical stages of the temporal evolution in the assembling process. Moreover, fast transients of the substrate mode and the drop mode are shown in Figure GISAXS in the final evaporation stage have been 16 (a-c-e) using the t-qy intensity maps, in which the observed - as shown in Figure 17 - indicating the highly intensity corresponding to a particular (t, qy) point is nonlinear behaviour of the volume and surface X-ray obtained by an integration of the measured intensity over scattering, due to the evaporation-driven surface tension the qz interval at a constant qy in the GISAXS pattern. instabilities of the drying drop.

Figure 18. GISAXS patterns of the superstructures of medium-length NRs (L = 22 nm) at the liquid/air interface, at different stages after the beginning of solvent evaporation: (a) 12 min, (b) 14 min, and (c) 16 min. Colour scales are logarithmic. Scale bars are 0.5 nm-1. (d, e) Schematics of the NRs' structures as present (d) at the beginning and (e) at the end of the self-assembly process [Reprinted with permission from F. Pietra, F.T. Rabouw, W.H. Evers, D.V. Byelov, A. Petoukhov, C. de Mello Donegá, and D. Vanmaekelbergh, “Semiconductor nanorod self-assembly at the liquid/air interface studied by in-situ GISAXS and ex-situ TEM”, Nano Lett., Just Accepted Manuscript, 2012, Publication Date (Web): 05 Oct 2012 Copyright (2012) American Chemical Society].

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5.3 In situ GISAXS – Semiconductor nanorod obtained by controlling solvent evaporation, as shown in self-assembly at the liquid/interface78 Figure 18 for medium-length NRs.

The self-assembly of colloidal CdSe/CdS nanorods (NRs) By this method, it is possible to prove that the NRs' at the liquid/air interface was studied by combining time- superstructure occurs at the liquid/air interface and partly resolved in situ GISAXS and ex situ Fast Fourier follow the dynamics of self-organization. The NR length and Transform TEM (FFT-TEM). The method79 used for the the initial concentration in the NR dispersion are able to tune chemical synthesis of CdSe/CdS dot core/rod shell NRs, the orientation of the NRs in the final superstructure, thus allows for control over the length of the NRs by tuning allowing the construction of a model of hierarchical self- the CdSe seed concentration and by adjusting the organization that accounts for the NR length and temperature and reaction time. The process of the NRs' concentration dependence of the superstructures formed. self-assembly was studied by in situ GISAXS on three different bathes of NRs with variable narrow size 5.4 In situ GISAXS – Flat-top silver nanocrystals on the two distribution so as to follows the dynamics of NR self- polar faces of ZnO80 assembly (long NRs with length L = (48 ± 4) nm and diameter 2R = (4.1 ± 0.4) nm, (ii) medium-length NR with In this work, an in situ GISAXS technique was used to L = (22.2 ± 2.3) nm and 2R = (6.3 ± 1.0) nm, and (iii) short study the growth of silver at room temperature on two NRs with L = (16.3 ± 2.4) nm and 2R = (3.4 ± 0.3) nm). The polar (0001)-O and (0001)-Zn surfaces of ZnO. The ZnO experiment was performed at a synchrotron beamline substrates were hydrothermally grown as single crystals � with an incident X-ray energy of 13.3 KeV at a grazing with (0001) and (0001) orientations. The clean surfaces incidence angle of αi = 0.061° so that the beam only probes were obtained after several cycles of sputtering with Ar+ � the toluene/air interface at most 10/20 nm deep into the and annealing at high temperature in UHV, followed by NRs dispersion. cooling down under O2 pressure. The silver was evaporated using a heated Knudsen cell, the substrate Through in situ GISAXS patterns, it is possible to follow being kept at room temperature. The amount of Ag/ZnO the dynamics of the self-assembly and study the films deposited, expressed in nanometres, can be formation of large areas of vertically- and horizontally- determined by using a calibrated flux of silver, measured aligned nanorods (NRs) at the liquid/air interface with a quartz microbalance.

Figure 19. In situ GISAXS measurements during Ag depositing on the ZnO(0001) Zu-terminated surface (left column) and on the ZnO(0001) O-terminated one (right column) [Reprinted with permission from N. Jedrecy, G. Renaud, R. Lazzari, and J. Pupille, “Flat-top silver nanocrystals on the two polar faces of ZnO: an all angle x-ray scattering investigation”, Physical Review B, vol. 72, pp. 045430, 2005 � Copyright (2005) by The American Physical Society].

www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 35 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering Figure 19 shows some experimental GISAXS patterns target molecules' quantification, improving devices' recorded during the growth of Ag on the Zn and O sensitivity and capacity for the high throughput surfaces, indicating a decrease of the distance between screening of biological samples. the two intensity lobes with increasing coverage. 7. Acknowledgements The results show that on both polar surfaces of ZnO, silver forms nanoclusters assembled with well-defined The authors would like to thank the SEED project “X-ray top facets, in (111) epitaxy with [101]Ag||[100]ZnO. synchrotron class rotating anode micro-source for the structural micro imaging of nanomaterials and � In situ and quasi-real-time GISAXS allows a precise engineered bio-tissues (XMI-LAB)”- IIT Protocol n.21537 understanding of the metal/oxide interface structure, both of 23/12/2009 and the FIRB 2009/2010 project “Rete in the nanoscopic and mesoscopic scale, with significant integrata per la Nano Medicina (RINAME)” – differences in the morphology of the films. In particular, RBAP114AMK_006. Rocco Lassandro is acknowledged the GISAXS data shows a triangular development of Ag for the technical support given in the XMI-LAB. nanoclusters on the Zn face, whereas the O face leads to a hexagonal one. 8. References

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www.intechopen.com Davide Altamura, Teresa Sibillano, Dritan Siliqi, Liberato De Caro and Cinzia Giannini: 39 Assembled Nanostructured Architectures Studied By Grazing Incidence X-Ray Scattering

ARTICLE

Nanomaterials and Nanotechnology

Numerical Techniques for the Analysis of Charge Transport and Electrodynamics in Graphene Nanoribbons

Invited Feature Article

Luca Pierantoni1,2,* and Davide Mencarelli1

1 Università Politecnica delle Marche, Ancona, Italy 2 INFN-Laboratori Nazionali di Frascat, Frascati, Italy * Corresponding author: [email protected]

Received 18 Oct 2012; Accepted 14 Nov 2012

© 2012 Pierantoni and Mencarelli; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract In this paper, we report on multiphysics full‐ 1. Introduction wave techniques in the frequency (energy)‐domain and the time‐domain, aimed at the investigation of the The theoretical, scientific and technological relevance of combined electromagnetic‐coherent transport problem in carbon‐based materials (carbon nanotubes, graphene) carbon based on nano‐structured materials and devices, have been highlighted in a variety of works, both e.g., graphene nanoribbons. experimental and theoretical [1‐11]. They are fated to become competitive and compatible with the established The frequency‐domain approach is introduced in order to silicon technology for applications to electronics. The describe a Poisson/Schrödinger system in a quasi static analysis of charge transport in carbon nano‐structures can framework. An example of the self‐consistent solution of be carried out by discrete models, such as tight binding laterally coupled graphene nanoribbons is shown. (TB), and continuous models, such as effective mass and k∙p approximations, which stem from the approximation The time‐domain approach deals with the solution of the of TB around particular points of the dispersion curves. combined Maxwell/Schrödinger system of equations. The These techniques are suited for the analysis of propagation of a charge wavepacket is reported, showing CNT/graphene/GNR in a variety of problems such as the effect of the self‐generated electromagnetic field that bending [17‐18], lattice defects and discontinuities [14], affects the dynamics of the charge wavepacket. and edge terminations [19‐20]. However the latter methods require high computational resources, and can Keywords Dirac Equation, Graphene Nanoribbon, hardly include the effect of i) the self‐generated Quantum Electrodynamics, Transmission Line Matrix electromagnetic field, ii) impinging external EM fields. Recently, we have introduced full‐wave techniques (fig. 1) both in the frequency (energy)‐domain [21‐26], and the time‐domain [28‐36] for the investigation of new devices

www.intechopen.com Luca PierantoniNanomater. and nanotechnol.,Davide Mencarelli: 2013, Numerical Collection Techniques of Selected for Papers, the Analysis 41-48 41 of Charge Transport and Electrodynamics in Graphene Nanoribbons based on carbon materials, namely carbon nanotube hopping elements of the Hamiltonian from a unit cell to (CNT), multiwall (MW) CNT, graphene and graphene the previous one from the left (right), and E is the nanoribbon (GNR). injection energy.

For both the approaches, the quantum transport is In [24], we showed that fundamental physical constraints described by the Schrödinger equation or its Dirac‐like and consistence relations in quantum transport, such as counterpart, for small energies. The electromagnetic field reciprocity and charge conservation, correspond provides sources terms for the quantum transport respectively to familiar reciprocity and power equations that, in turn, provide charges and currents for conservation in a microwave field. We emphasized that the electromagnetic field. the proposed approach allows handling multiport graphene systems, where carriers can get into (and out of) In this contribution, we report some new examples of many different physical ports, each characterized by their self‐consistent quasi‐static calculations, where charges’ own chirality and possibly by a large number of virtual transport is affected by the self‐generated potential, in ports, i.e., electronic channels or sub‐bands. Interesting addition to the electrostatic potential applied by external results involve new concept‐devices, such as GNR nano‐ electrodes, in a typical FET configuration [25,26]. transistors and multipath/multilayer GNR circuits, where Regarding the time‐domain technique, we show the charges are ballistically scattered among different ports dynamics of a charge wavepacket from source to drain under external electrostatic control. We developed a in‐ electrodes in a GNR realistic transistor environment. house solver for simulating CNT short‐channel transistors, with a user friendly interface. The software, written in Matlab, has been, in particular, focused on the simulation of GNR short‐channel transistors, as shown in fig. 1. In modelling the graphene‐metal contact, we introduce a sort of metal doping of GNR, coherently with experimental observation; in fact, graphene over metal seems to preserve its unique electronic structure, and the metal just shifts the graphene Fermi level with respect to Figure 1. Frequency‐ and time‐domain techniques. the conical point, by a fraction of eV [27]. Possibly, the metal contact opens just a small (tens to hundreds eV) 2.1 Frequency‐domain: Poisson‐coherent transport bandgap.

We perform the analysis of self‐consistent charge 2.2 Time‐domain: Maxwell‐coherent transport transport by using a scattering matrix technique [24], which is physically equivalent to the Green’s function In the time‐domain, a full‐wave approach has been approach, usually referred to as non‐equilibrium Green’s introduced: the Maxwell equations, discretized by the function (NEGF) method. In synthesis, each GNR port, transmission line matrix (TLM) method, are self‐ seen as the termination of a semi‐infinite waveguide, is consistently coupled to the Schrödinger/Dirac equations, described by means of a basis of electronic discretized by a proper finite‐difference time‐domain or a eigenfunctions, that, in turns, are solution of the GNR TLM scheme [28‐29]. unit‐cell under periodic condition. The analysis is fully self‐consistent since the solution of the transport The goal is to develop a method that accounts for equation, and the solution of the Poisson equation for the deterministic electromagnetic eld dynamics, together electrostatic potential generated by the GNR charge with the quantum coherent transport in the nanoscale density, are obtained by using an iterative approach. In environment. In [29‐30], we introduced exact boundary the scattering‐matrix approach, a multimode conditions that rigorously model absorption and injection transmission matrix model of quantum transport allows of charge at the terminal planes, in a realistic field effect easy simulation of very large structures, despite the transistor environment. possibly high number of electronic channels involved. In order to characterize a GNR, periodic along the z‐ Several examples of the electromagnetics/transport direction, the Hamiltonian of the unit cell is appropriately dynamics are shown in [28‐29]. It is highlighted that the rearranged by selecting three consecutive unit cells self‐generated electromagnetic field may affect the dynamics (group velocity, kinetic energy, etc.) of the l 0 r H  l  H   H  r  E (1) quantum transport. This is particularly important in the analysis of time transients and in describing the where ψl, ψr, ψ, are the wavefunctions of three behaviour of high energy carrier bands, as well as the consecutive unit cells and matrix Hl(Hr) denotes the onset of non‐linear phenomena due to external impinging

42 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 41-48 www.intechopen.com electromagnetic fields. For graphene/GNR, in the optics. FDTD is a more general technique, suited for presence of an EM field, the Dirac equation reads: discretizing different kinds of equations, e.g., parabolic, hyperbolic, etc. With respect to FDTD, TLM is directly  ie  related to the discretization of, mainly, hyperbolic i   σ pˆ  q A  c  equations (Maxell, Dirac), but it has the advantages that t   (2) each portion of the segmented space has an equivalent  ie  local electric circuit [38]. Moreover, TLM can easily i   σ pˆ  q A  c  t   incorporate external sources as equivalent voltage/current local generators. The solution of the Dirac/graphene equation (2) is the four component spinor complex wavefunction ψ(r,t): In TLM, that is considered as the implementation of the Huygens principle, propagation and the scattering of the wave amplitudes are expressed by operator equations     T    T (3) ψ(,)r t  1 2 3 4      [38]. The latter property is well illustrated in the Symmetrical Condensed Node (SCN) formulation [38]. where A and ϕ are vector and scalar potentials, directly related to the EM field through the appropriate gauge, e.g., the “Lorentz” gauge, and q is the electron charge; Vp is the static potential profile. In eq. (2),  are the Pauli matrices, p is the canonical (linear) momentum, k is the kinematic momentum, that, includes the EM field contribution:

pˆ i kˆ  pˆ q A r, t  (4)

The computational scheme develops as follows: i) the EM field is discretized by the Transmission Line Matrix method using the Symmetrical Condensed Node (SCN) approach. ii) Quantum phenomena are introduced in a subregion of the 3D‐domain, e.g., a 1D‐2D dimensional CNT region, described by the Schrödinger equation, Figure 2. Concept of the full‐wave time‐domain technique. The and/or a 2D graphene/nanoribbon region, described by electromagnetic field provides sources for the quantum device that, in turn, provides (quantum‐mechanical) current sources for the Dirac equation. iii) At each time step, the the electromagnetic field. Schrödinger/Dirac equation is solved by accounting for the quantum device boundary conditions, initial conditions (e.g., injected charge), and additional source In [31‐32], we explored the correlation between Dirac and terms constituted by the EM field, sampled in the domain Maxwell equations, in the time domain; transmission‐line of the quantum device(s). iv) From the wavefunction equations, valid for both EM and quantum current are (charge) solution of the Schrödinger/Dirac equation, we derived. This is a step forward toward an effective derive the quantum mechanical (QM) current over the integration of the Dirac theory in the numerical device domain. This current is a distribution of local simulation of EM field problems. sources for the EM field that is injected into the TLM nodes, located only on the grid points of the In [33], we presented, for the first time, a TLM condensed Schrödinger/Dirac equation domain. v) At the next time node scheme for solving the Dirac equation in 2D step t+1, the TLM method provides a new updated graphene. This scheme satisfies the standard charge distribution of field values that are, again, sampled over conservation requirement and allows adopting boundary the device domain, and so on, iteratively. In fig. 2, the conditions for graphene circuits. scheme of the method is depicted in the case of graphene. The correlation between the graphene/Dirac equation and The reason for choosing TLM for the discretization of its self‐consistent symmetrical condensed node ‐ Maxwell equations has to be highlighted. Space‐ transmission line matrix formulation is highlighted. This discretizing methods, like nite‐difference time‐do‐main concept, in turn, is related to the generalized Huygens (FDTD) and transmission line matrix (TLM) [38], are well‐ principle for the Dirac equations. known techniques that allow the EM full‐wave modelling of 3D structures with nearly arbitrary geometry for a The above technique has been already used for the wide range of applications from EM compatibility to investigation of realistic and intriguing applications in www.intechopen.com Luca Pierantoni and Davide Mencarelli: Numerical Techniques for the Analysis 43 of Charge Transport and Electrodynamics in Graphene Nanoribbons novel areas, bridging nanoscience and engineering can still imply a strong effect when wider GNR, i.e., applications. We could define this research area as smaller band gaps, are considered. “radio‐frequency nanoelectronic engineering”, [39‐40]. In [34], we analyse the idea of realizing a harmonic radio‐ It is noted that the self‐consistent potential of fig. 4b is frequency identification (RFID), based on “tag on paper” strongly different from the potential of fig 4a; as largely with embedded graphene as a frequency multiplier. expected, changing the distance between the GNR does not simply imply a potential “composition” following a In [35‐36], we introduce a model for the metal‐carbon superposition of effects ‐ the iterative process develops contact. The metal‐carbon transition is one of the most very differently in the two cases and the final results are challenging and not completely understood problems not easily predictable. that limits production and reproducibility of nanodevices, arising due to the difficulty of engineering the contact resistance between metal and nano‐structures.

3. Results

3.1 Frequency‐domain: Schrödinger‐Poisson

In order to show the potentialities of our approaches, in the following we show the comparison between the potential distributions in a region occupied by two laterally coupled GNR. The coupling takes place by means of the Coulomb interaction. The schematic view of the device under study is shown in fig. 3: two semiconducting GNRs connect the source and drain of a a)

FET‐like device.

A potential difference of 0.1 V is applied between drain and source; the source is assumed at 0 V, equipotential with the lateral gate (G). The nanoribbons are about 2.2 nm wide and the area of the square “window” delimited by the electrodes is 20x20 nm2.

Graphene nanoribbons V z y

d x drain b) Figure 4. Self‐consistent potential for different distances of the two coupled GNR channels: a) d=2.4 nm b) d=0.15 nm. G source G

3.2 Time‐domain: Dirac‐Maxwell

Figure 3. A two‐channel GNR‐FET; d is the distance between the We analyse the space‐time evolution of a Gaussian charge 2, two GNR channels. wavepacket || with a broad energy band (up to 1 eV), propagating on a “metallic” GNR (150x5 nm), as shown In the following, we report the numerical result in fig. 5. We consider the GNR in a realistic FET obtained after numerical convergence, expressing the environment, with two metallic source‐drain electrode self‐consistent potential in the plane of the nanoribbons. contacts. In order to model the injection‐absorption of We somehow exaggerated the effect of the metal doping charge, we apply absorbing boundary conditions as in by assuming a 2.9 eV shift of the Dirac point, in order to [29]. In fig. 6 (a), we show the charge wavepacket place the Fermi level about 0.7eV above the band gap, evolution after t=0, t=20, t=50, t=100 fs, respectively. The and to have appreciable charge injection from the metal correspondent transversal and longitudinal current to nanoribbon “bridges”. In practice, a smaller doping components are reported in Fig. 6 (b), for t=20, t=50 fs.

44 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 41-48 www.intechopen.com t=20 fs 5nm 6 fs

x z 2 

t=0 t=2 t=4

Figure 5. Propagation of a charge wavepacket in the presence of a static potential barrier, with E=0.45 eV.

2 Fig (6a) Propagation of 

t=0 fs

(a) t=20 fs 3 nm  t=50 fs 6nm 6 fs

t=0 2 150 nm t=2 t=100 fs  t=4 Fig (6b) 8

t=20 fs t=50 fs (b) Figure 7. Spatial distribution of a charge wavepacket at t=0, t=2, t=4 fs. (a): only the Dirac equation is solved. (b): the coupled Dirac‐Maxwell system is computed.

In the same figure, (c), we show the propagation of two pulses launched through the source and drain electrodes, Fig (6c) propagation of two pulses for t=0, t=20, t=50, t=100 fs. t=20 fs t=0 fs We then consider the presence of a potential barrier of 0.45 eV with respect to bounding materials (e.g., metal contacts). In fig. 7, we plot the spatial, longitudinal distributions of the charge wavepacket in three different time‐steps, t=2, 4, 8 fs, respectively.

t=50 fs The core point is that in one case (fig.7, a), we solve only the Dirac equation and do not consider

t=100 fs the self‐induced EM field, whereas in the other case (fig.7, b), we consider the coupled Dirac‐Maxwell system. Figure 6. Time‐evolution of the wavepacket (a). Transversal and We observe that, depending on the initial energy of the longitudinal currents (b). Two launched pulses (c) from the charge wavepacket, the self‐induced electromagnetic field source and drain terminals. affects the propagation characteristics.

www.intechopen.com Luca Pierantoni and Davide Mencarelli: Numerical Techniques for the Analysis 45 of Charge Transport and Electrodynamics in Graphene Nanoribbons This is evident by following the dynamics of the [10] Guo J., et al. M. (2004), A numerical study of scaling (squared) wavefunction with and without the “self‐ issues of Schottky‐barrier of carbon nanotube generated” electromagnetic field. For example, the transistors, IEEE Transaction on Electron Devices, vol. distribution of the peaks (points of maxima) is different in 51, n. 2, pages: 172‐177. the former and in the latter cases. Physically, the [11] A. Kand et al., (2005), Leakage and performance of kinematic momentum, k, provides the EM field zero‐Schottky‐barrier of carbon nanotube transistors, contribution to the kinetic energy (3) of the Dirac Jour. of App. Physics, vol. 98. equation. The quantum‐mechanical current, in turn, [12] L. Brei, H. A. Fertig, Electronic States of Graphene provides current sources for the electromagnetic field. Nanoribbons, cond‐matt. 0603107, pp. 1‐5, Mar 2006. [13] G. Giovannetti et al., Substrate induced band‐gap on This effect, as a result of this phenomenon, would be even hexagonal boron nitride: Ab initio density functional more evident, and also enhanced in the presence of an calculations, Phys. Rev. B 76, 073103’ pp. 1‐4, (2007). additional external impinging EM field. [14] G. Fiori, Negative Differential Resistance in mono and bilayer graphene p‐n junctions IEEE Trans. Elec. 4. Conclusions devices, vol. 55, n. 9, Sept. 2008. [15] Z. F. Wang, R. Xiang, Q. W. Shi, J. Yang, X. Wang, J. We reported on multiphysics full‐wave techniques in the G. Hou and J. Chen, Phys. Re B 74, 125417, 2006. frequency (energy)‐domain and the time‐domain, aimed [16] T. Ando, Physical Review B 44, 8017, 1991. at the investigation of the combined electromagnetic‐ [17] E. Castro, N. M. R. Peres and J. M. B. Lopes dos coherent transport problem in graphene nanoribbons. Santos, Phys. Stat. Sol. (b) 244, , 2311–2316, 2007.

[18] A. Rycerz, Nonequilibrium valley polarization in In the frequency‐domain, we describe a Poisson/Schrödinger graphene nanoconstrictions, Cond‐mat., 0710.2859v2, system in a quasi static framework. pp.1‐10, 2007.

[19] A. Akhmerov, C. W. J. Beenakker, Boundary In the time‐domain, we deal with the solution of the conditions for Dirac fermions on a terminated combined Maxwell/Schrödinger coupled equations. honeycomb lattice, Cond‐mat., 0710.2723v1, pp. 1‐10,

In the frequency‐domain, we analyse the field coupling of 2008. graphene nanoribbons in an FET d configuration [20] G. Lee and K. Cho, Phys. Rev. B, Apr. 10, 2009. [21] D. Mencarelli, L. Pierantoni, T. Rozzi, Optical In the time‐domain, we present the charge wavepacket Absorption of Carbon Nanotube Diodes: Strength of propagation, showing the effect of the self‐generated the Electronic Transitions and Sensitivity to the electromagnetic field, that affects the dynamics of the Electric Field Polarization, Journal of Applied Physics, charge wavepacket. vol. 103, Issue 6, pp.0631‐03, March 2008. [22] D. Mencarelli, T. Rozzi, C. Camilloni, L. Maccari, A. 5. References Di Donato, L. Pierantoni, Modeling of Multi‐wall CNT Devices by Self‐consistent Analysis of Multi‐ [1] Z. Chen, Y.‐M. Lin, M. J. Rooks and P. Avouris, vol. channel Transport, IOP Science Nanotechnology, vol. 40, Issue 2, pages 228‐232, Dec. 2007. 19, Number 16, April 2008. [2] M. Y. Han et al., Phys. Rev. Lett. 98, 206805, 2007. [23] D. Mencarelli, T. Rozzi, L. Pierantoni, Coherent [3] X. Li, et al., Science 319, 1229, 2008. Carrier Transport and Scattering by Lattice Defects in [4] C. Stampfer, J. Güttinger, S. Hellmüller, F. Molitor, K. Single‐ and Multi‐Branch Carbon Nanoribbons, Ensslin, and T. Ihn, Phys. Rev. Lett. 102, 056403, 2009. Physical Review B, vol.77, pp.1954351‐11, May 2008. [5] X. Liu, J. B. Oostinga, A. F. Morpurgo, L. M. K. [24] D. Mencarelli, T. Rozzi, L. Pierantoni, Scattering Vandersypen, Phys. Rev. B 80, 121407, 2009. matrix approach to multichannel transport in many [4] C. Stampfer, et al., Phys. Re Lett. 102, 056403, 2009. lead graphene nanoribbons, IOP Science [6] K. Wakabayashi and M. Sigrist, Phys. Rev. Lett. 84, Nanotechnology, vol. 21, no.15, March 2010, pp. 5570‐ 3390–93, 2000. 15580. [7] S. Souma, M. Ogawa, T. Yamamoto and K. Watanabe, [25] D. Mencarelli, L. Pierantoni, A. Di Donato, T. Rozzi, J. of Computational Electronics, vol. 7, n.3, 390‐393, Self‐consistent simulation of multi‐walled CNT 2008. nanotransistors, Int. Journal of Micr. and Wireless [8] Guo J., Datta S. and Lundstrom M. (2004), A Technologies, vol. 2, no. 5, pp 453‐456, Dec. 2010. numerical study of scaling issues of Schottky‐barrier [26] D. Mencarelli, L. Pierantoni, M. Farina, A. Di Donato, of carbon nanotube transistors, IEEE Transaction on T. Rozzi, A multi‐channel model for the self‐ Electron Devices, vol. 51, n. 2, pages: 172‐177 consistent analysis of coherent transport in graphene [9] Lin Y.‐M., et al., High Performance Dual‐Gate Carbon nanoribbons, ACS Nano, vol. 5, Issue 8, August 2011. Nanotube FETs with 40‐nm Gate Length, IEEE Electr. pp. 6109‐6128. Dev. Letters, vol. 26.

46 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 41-48 www.intechopen.com [27] A.L. Walter, et al., Electronic structure of graphene [34] L. Pierantoni, D. Mencarelli, T. Rozzi, F. Alimenti, L. on single‐crystal copper substrates, Phys. Rev. B 84, Roselli, P. Lugli, Multiphysics analysis of harmonic 195443, 2011. RFID tag on paper with embedded nanoscale [28] L. Pierantoni, D. Mencarelli and T. Rozzi, A new 3D‐ material, Proceedings of the 5th European Conference on Transmission Line Matrix Scheme for the Combined Antennas and Prop., Rome, Italy, April 11‐15, 2011. Schrödinger‐Maxwell Problem in the [35] L. Pierantoni D. Mencarelli, T. Rozzi, Full‐Wave Electronic/Electromagnetic Characterization of Techniques for the Multiphysics Modeling of the Nanodevices, IEEE Trans. on Microwave Theory and Electromagnetic/Coherent‐Transport Problem Techniques, vol. 56, no. 3, March 2008, pp.654‐662 Graphene Nanodevices, Proceedings of the IEEE [29] L. Pierantoni, D. Mencarelli, and T. Rozzi, Boundary International Symposium on Antennas and Propagation Immittance Operators for the Schrödinger‐Maxwell (AP‐S) and USNC‐URSI National Radio Science Problem of Carrier Dynamics in Nanodevices, IEEE Meeting, Chicago, IL, USA, July. 8‐14, 2012. Trans. Microw. Theory Tech., vol. 57, issue 5, pp. 1147‐ [36] L. Pierantoni D. Mencarelli, T. Rozzi, Advanced 1155, May 2009. Techniques for the Investigation of the Combined [30] L. Pierantoni D. Mencarelli, T. Rozzi, Modeling of the Electromagnetic‐Quantum Transport Phenomena in Electromagnetic/Coherent Transport Problem in Carbon Nanodevices, Proc. of the Int. Conference on Nano‐structured Materials, Devices and Systems Electromagnetics in Advanced Applications (ICEAA) Using Combined TLM‐FDTD techniques, Microwave 2012‐IEEE APWC 2012‐EEIS 2012, Cape Town, South Symposium Digest, 2011 Int. Microwave Symposium, Africa, Sept. 2‐7, 2012, pp. 873–876. Baltimore, MA, USA, June 5‐10, 2011, pp. 1‐4. [37] G. Vincenzi, G. Deligeorgis, F. Coccetti, M. [31] T. Rozzi, D. Mencarelli, L. Pierantoni, Deriving Dragoman, L. Pierantoni, D. Mencarelli, R. Plana, Transmission Line Models and E.M. Fields from Extending ballistic graphene FET lumped element Dirac Spinor in Time Domain, IEEE Trans. models to diffusive devices, Solid‐State Electronics, Microwave Theory Techn., Special Issue on RF vol. 76, Oct. 2012, pp. 8–12. Nanoelectronics, vol. 59, no.10, Oct. 2011, pp. 2587‐ [38] L. Pierantoni, A. Massaro, T. Rozzi, Accurate 2594. Modeling of TE/TM Propagation and Losses of [32] T. Rozzi, D. Mencarelli, L. Pierantoni, Towards a Integrated Optical Polarizer, IEEE Trans. Microw. Unified Approach to Electromagnetic Fields and Theory Tech., vol. 53, no.6, June 2005, pp. 1856‐1862. Quantum Currents From Dirac Spinors, IEEE [39] L. Pierantoni, RF Nanotechnology ‐ Concept, Birth, Transactions on Microwave Theory and Techniques, Mission and Perspectives, IEEE Microwave Magazine, Special Issue on RF Nanoelectronics, vol. 59, no.10, Oct. vol. 11, no. 4, pp. 130‐137, June 2010. 2011, pp. 2587‐2594. [40] L. Pierantoni, F. Coccetti, P. Lugli, S. Goodnick, [33] D. Mencarelli, L. Pierantoni T. Rozzi, Graphene Guest Editorial, IEEE Transactions on Microwave Modeling by TLM approach, Microwave Symposium Theory and Techniques, Special Issue on RF Digest, 2012 Int. Microwave Symposium, Montreal, QC, Nanoelectronics,vol. 59, no.10, Oct. 2011, pp. 2566‐ Canada, June 17‐22, 2012, pp. 1‐3. 2567, vol. 59, no.10, Oct. 2011, pp. 2566‐2567.

www.intechopen.com Luca Pierantoni and Davide Mencarelli: Numerical Techniques for the Analysis 47 of Charge Transport and Electrodynamics in Graphene Nanoribbons

ARTICLE

Synthetic Aspects and Selected Properties of Graphene

Invited Feature Article

H. S. S. Ramakrishna Matte1, K. S. Subrahmanyam1 and C. N. R. Rao1,*

1 Chemistry and Physics of Materials Unit, International Centre for Materials Science, New Chemistry Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre for Advanced Scientific Research, India Corresponding author E-mail: [email protected]

Received 30 May 2011; Accepted 10 June 2011

© 2011 Matte et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract Graphene has generated great sensation owing 1. Introduction to its fascinating properties with possible potential applications. This two‐dimensional material exhibits half‐ Graphene, a single sheet of sp2 hybridized carbon atoms integer quantum Hall effect and an ambipolar electric densely packed in a honeycomb lattice, has received a great field effect, along with ballistic conduction of charge attention due to its fascinating properties.[1‐3] A remarkable carriers. In this article, we provide a overview on some feature of graphene is that the energy of electrons is linearly aspects of graphene devoting the special attention to dependent on the wave vector near the crossing points in the synthesis, functionalization, self‐assembly, surface Brillouin zone. The charge carriers mimic relativistic properties, gas adsorption and fluorescence quenching particles which can be are satisfactorily on the basis of the ability of graphene. Graphenes with varying number of Dirac equation rather than the Schrcodinger equation.[1, 4, 5] layers can be synthesized by using different strategies. Graphene, exhibits exceptional electronic, optical, magnetic, Graphene can be functionalized by different means in thermal and mechanical properties, including high values of order to disperse it in various solvents. We also present its Young’s modulus ( 1100 GPa),[6] fracture strength (125 the self‐assembly of graphene at the liquid‐liquid GPa),[6] thermal conductivity (5000Wm‐1K‐1),[7] mobility of interface besides its surface properties including charge carriers (200000 cm2 V‐1 s‐1),[8] specific surface area adsorption of hydrogen, carbon dioxide and methane. (calculated value, 2630 m2g‐1),[2] high chemical stability and The remarkable property of graphene of quenching high optical transmittance, quantum Hall effect at room fluorescence of aromatic molecules is shown to be temperature[9‐11] and a tunable band gap.[12] These exceptional associated with photo‐induced electron transfer. properties render graphene suitable for potential applications in designing field‐effect transistor (FET), sensors and Keywords Graphene, Nanostructures, Synthesis, supercapacitors. Although graphene is expected to be flat, Hydrogen Storage, Self Assembly ripples occur due to thermal fluctuations.[1] Single‐layer and bi‐layer graphene were prepared by micro‐mechanical cleavage[5], several strategies have since been developed.[3, 13, 14]

www.intechopen.com Nanomater.H.S.S. Ramakrishna nanotechnol., Matte, 2013, K. CollectionS. Subrahmanyam of Selected and Papers, C. N. R. 49-60 Rao: 49 Synthetic Aspects and Selected Properties of Graphene Graphene has been characterized by microscopic and other (a) (b) 1(c) physical techniques including atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning tunneling microscopy (STM) and Raman spectroscopy.[2] In this article we discuss synthetic aspects and some of the selected properties of graphene. 100 nm 2. Synthetic aspects 100 nm

(c) The method involving micromechanical cleavage from 1450 2800 (d) [5] 1400 highly ordered pyrolyitc graphite (HOPG) , a layer is 2450 peeled off the HOPG crystal by using scotch tape and then 1350 2100 transferred on to a silicon substrate. Single‐layer graphene 1300 1750 Intensity(a.u) (SG) was also prepared by epitaxial growth on silicon (a.u) Intensity 1250 1400 carbide (SiC), in which single crystal SiC substrates or 1200 1050 commercial polycrystalline SiC granules heated in vacuum 1250 1500 1750 2000 2250 2500 2750 1250 1500 1750 2000 2250 2500 2750 -1 to high temperatures in the range of 1200–1600 °C.[15] As the Raman Shift (cm ) Raman Shift (cm-1)

sublimation of silicon is higher than that of carbon, excess Figure 1. TEM images of graphene prepared by the thermal carbon is left behind on the surface, which rearranges to decomposition of (a, b) methane (70 sccm) on nickel at 1000 ºC and form graphene. Large area and high quality SG films were Raman spectra of graphene prepared by the thermal prepared more conveniently employing chemical vapor decomposition of hydrocarbons on a nickel sheet: (c) methane (70 deposition (CVD) by decomposing hydrocarbons. Different sccm) at 1000 ºC, (d) ethylene (4 sccm) at 900 ºC. (From reference 3) hydrocarbons such as methane, ethylene, acetylene and benzene were decomposed on various transition metal (a) (b) substrates like Ni, Cu, Co, Au and Ru.[16] In our experiments, nickel (Ni) and cobalt (Co) foils with thickness of 0.5 mm and 2 mm respectively were used as catalysts. These foils were cut into 5x5 mm2 pieces and polished mechanically and the CVD process carried out by decomposing hydrocarbons around 800–1000 ºC. By employing a nickel foil, CVD was 20 nm carried out by passing methane (60‐70 sccm) or ethylene (4‐8 (c) sccm) along with a high flow of hydrogen around 500 sccm (d) at 1000 ºC for 5‐10 minutes. With benzene as the 1.6 hydrocarbon source, benzene vapor diluted with argon and 1.2 hydrogen was decomposed at 1000 ºC for 5 minutes. On a 0.8

cobalt foil, acetylene (4 sccm) and methane (65 sccm) were 0.4

decomposed at 800 and 1000 ºC respectively. Figures 1(a) (nm) Height 5 nm 0.0 and (b) shows high‐resolution TEM images of graphene 0 150 300 450 600 sheets obtained by thermal decomposition of methane on a Distance (nm) nickel foil.[3] The inset in Figures 1 (a) shows selected area Figure 2. TEM images of (a) HG, (c) EG, (d) DG and (b) AFM electron diffraction (SAED) pattern. image of HG along with height profile. (From reference 3 and 14)

A radio frequency plasma enhanced chemical vapor Raman spectroscopy is an important tool to characterize deposition (PECVD) system was used to synthesize graphene and provides information about the quality and graphene on a variety of substrates such as Si, W, Mo, Zr, number of layers in a given sample. The G‐band (around Ti, Hf, Nb, Ta, Cr, 304 stainless steel, SiO2 and Al2O3. This 1580 cm‐1) in the Raman is sensitive to doping and other method reduces the energy consumption and prevents the effects. While the 2D band (around 2670 cm‐1) effects the formation of amorphous carbon or other types of number of layers. The D band (around 1340 cm‐1), the unwanted products.[17‐19] Graphene has been prepared in defect related band, also a signature of the quality of the gas phase employing a substrate free atmospheric pressure graphene film. The Raman spectra of graphene obtained microwave plasma reactors. Preparation of SG by chemical on a nickel sheet by the thermal decomposition of method involves reduction of single‐layer graphene oxide methane and ethylene show an intense 2D band relative (SGO) dispersion in dimethlyformamide with hydrazine to the G band with hardly any D band (see Figures 1 (c) hydrate.[20] The obtained reduced graphene oxide (RGO) and (d)), clearly indicative of SGs.[16] High cooling rate may yet contain some residual oxygen functionalities and enables the formation of minimum number of layers and readily soluble in dimethyl formamide (DMF). Exfoliation efficient transfer on to substrate. of graphite in N‐methyl pyrrolidone or surfactant/water

50 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 49-60 www.intechopen.com solution employing ultrasonication also yields stable SG easily dispersible in liquids, it is necessary to physically or dispersions.[21, 22] Gram quantities of single‐layer graphene chemically attach certain molecules or functional groups to has been obtained by a solvothermal procedure using graphene without significantly changing its desirable sodium and ethanol.[23] properties. Functionalization of graphene has been carried out by employing different strategies.[2, 37] For example, Few‐layer graphene (FG) has been prepared by thermal Haddon and co‐workers have achieved functionalization of reduction of GO (EG) at high temperatures in an inert graphene by employing covalent modification. To solubilize atmosphere.[24‐26] FG is also obtained by chemical reduction of graphene in non polar solvent amidation of graphene has SGO with hydrazine hydrate, sodium borohydrate and been carried out. In this procedure acid‐treated graphene ethyleneglycol.[2] It is shown that FG can also be obtained containing surface –OH and –COOH groups was first [27] under mild conditions using ascorbic acid. In another reacted with SOCl2 to create ‐COCl groups, followed by approach FG is obtained using sugars such as glucose, reaction with a long chain aliphatic amines.[38] Another fructose and sucrose as reducing agents.[28] Microwave method employed by these workers is by grafting aryl irradiation has been used for the quick and scalable chemical groups through diazotization reaction.[39] Soluble graphene reduction for the synthesis of graphene.[2] Hydrothermal layers in THF can be generated by the covalent attachment route was employed to prepare graphene from graphene of alkyl chains to graphene layers via reduction of graphite oxide. In this procedure water behaves as supercritical water fluoride with alkyl lithium reagents.[40] Such covalent (SC) and play the role of reducing agent.[29] GO is also functionalization enables solubilization in various solvents reduced photochemically employing UV‐irradiated to such as CCl4, THF and CH2Cl2 (Figure 3(a)).[26] Similar prepare graphene in which H3PW12O40 is used as procedures have been employed by Subrahmanyam et al as photocatalyst.[30] It is also demonstrated that environmental well.[41] The reaction of graphene with a mixture of bacteria was employed as an electron donor to reduce concentrated H2SO4 and HNO3 gives water‐soluble graphene.[31] Graphene can be prepared by heating graphene which is stable for several months. Graphene is [26, 32, 33] nanodiamond in an inert atmosphere. In this solubilized in CCl4 by interaction with organosilane and preparation, pristine nanodiamond powder (particle size 4‐6 organotin reagents such as hexadecyltrimethoxysilane nm, Tokyo Diamond Tools, Tokyo, Japan) placed in a graphite (HDTMS) (see Figure 3 (b)) and dibutyldimethoxytin container was heated in a graphite furnace in a helium (DBDT).[41] To functionalize graphene some typical organic atmosphere at different temperatures (1650, 1850, 2050 and reactions such as diazonium,[39, 42, 43] nucleophilic ring‐ 2200°C) for 1 hr. We observed that there is a slight increase in opening,[44] carbodiimide‐activated esterification,[45] 1,3‐ the number of layers and decrease in lateral dimensions in the dipolar cycloaddition,[46] amide bond formation,[47] in situ samples heated at 2200 ºC in comparison to 1650 º C. We show living free‐radical polymerization,[48] etc. have been typical TEM images of EG and DG in 2 (a) and (b) respectively employed. Graphene sheets that are covalently functionalized with 1‐(3‐aminopropyl)‐ 3‐methylimidazolium We have recently found that graphene can be prepared by bromide are dispersed in water, DMF, and DMSO.[44] arc evaporation of graphite in the presence of hydrogen. This procedure yields graphene (HG) sheets with 2‐3 layers having flake size of 100‐200 nm.[34] This makes use of the knowledge that the presence of H2 during arc‐discharge process terminates the dangling carbon bonds with hydrogen and prevents the formation of closed structures. The conditions that are favorable for obtaining graphene in the inner walls are the high current (above 100 A), the high voltage (>50 V), and the high pressure of hydrogen (above 200 torr). In Figure 2 (c) and (d) we show TEM and AFM images of HG sample respectively. This method has been conveniently employed to dope graphene with boron and nitrogen.[35] To prepare boron and nitrogen doped graphene (B‐HG and N‐HG) the discharge is carried out in the presence of of H2+diborane and H2+ (pyridine or ammonia) respectively. Cheng et al. employed hydrogen arc discharge process as a rapid heating method to prepare graphene from GO.[36]

Figure 3. Photographs of (a) dispersion of the amide‐ 3. Functionalization and Solubilization functionalized EG in dichloromethane (b) dispersion of HDTMS

treated EG in CCl4, (c) dispersion of PYBS treated EG in DMF Pristine graphene is insoluble in liquids such as water, and (d,e and f) water dispersions of EG treated with IGP, SDS polymer resins and most solvents. To make graphene more and CTAB. (From reference 14)

www.intechopen.com H.S.S. Ramakrishna Matte, K. S. Subrahmanyam and C. N. R. Rao: 51 Synthetic Aspects and Selected Properties of Graphene

Figure 4. Illustration of the exfoliation of few‐layer graphene with CS to yield monolayer graphene– CS composites. (From reference 49)

Without disrupting the electronic structure, graphene can be functionalized through non‐covalent modification by wrapping with surfactants or through π‐π interaction with aromatic molecules such as 1‐pyrenebutanoic acid succinimidyl ester (PYBS) (Figure 3(c)) and the potassium salt of coronene tetracarboxylic acid (CS).[49] Non‐ Figure 5. (a) FESEM image a SWNT film (3.3 μg in 10 ml) formed covalent interaction of graphene with surfactants such as at the interface after12 hours. Inset is the image obtained after complete evaporation. (b) TEM image of a graphene film (3.3 μg Igepal CO‐890 (polyoxyethylene (40) nonylphenylether, in 10 ml) formed at the interface after 2.5 hours. (c) and (d) show IGP), sodium dodecylsulfate (SDS) and AFM the height profiles of graphene films (3.3 μg in 10 ml) cetyltrimethylammoniumbromide (CTAB) gives water‐ assembled at interface after 2.5 hours and after complete soluble graphene. Figure 3 (d, e and f) shows evaporation respectively. (From reference 63) photographs of water‐soluble graphene obtained with IGP, SDS and CTAB respectively.[41] Interaction of CS solvents are reported to give rise to interfacial with few‐layer graphene causes exfoliation and assemblies.[57] Nanosheets of graphene can be generated selectively solubilizing single‐and double‐layer at the liquid‐liquid interface.[58, 59] Membranes of graphene graphenes in water through molecular charge‐transfer oxide (GO) can be obtained at the liquid‐air interface by interaction (see Figure 4). Some stabilizers such as evaporating the hydrosol of GO.[60] Ropes and bundles of sodium dodecylbenzene sulfonate (SDBS),[22] perylene‐ carbon nanotubes have been formed along with the based bolaamphiphile detergent,[50] sodium cholate,[51] 1‐ graphene by the reduction of GO admixed with the pyrenemethylamine hydrochloride[52] and 7,7,8,8‐ nanotubes.[61] Layer‐by‐layer assembly has been tetracyanoquinodimethane,[53] etc. have been used as employed for the formation of nanofilms of reduced promoting agents to obtain graphene dispersions. Water‐ graphene oxide with multi‐walled carbon nanotubes.[62] soluble graphene can also be prepared by PEGylation method in which, acidified graphene is treated with In the Figure 5(a) shows a FESEM image of a film of excess of polyethylene glycol (PEG) and conc. HCl under formed at the interface with a dispersion of a 3.3 μg of solvothermal conditions.[26] Using DDAB as a SWNTs in 10 ml of toluene after 12 hours of assembly. phasetransfer agent graphene has been dispersed in These films show a dense and homogeneous network of chloroform.[54] SWNTs. In the Figure 5(b) we show a TEM image of the graphene film formed at the interface with 3.3 μg of 4. Assembly at the Liquid‐liquid interface graphene in 10 ml of toluene after 2.5 hours. We have also obtained films of few‐layer graphene at the interface after Self‐assembly of nanocarbons of different dimensionalities different durations of assembly and after complete is of interest because of its possible use in designing in evaporation of the organic layer. The lateral dimensions transparent conducting electrodes, solar cells and other of films formed at the interface are generally around 15‐ devices. Self‐assembly of C60 nanosheets comprising 25 μm after 2.5 hours of assembly.[63] Figure 5(c) and (d) hexagonal, rhombohedral and mixed polygonal respectively show the height profiles of films formed after aggregates were prepared by solvent engineering,[55] 2.5 hours of assembly and after the complete evaporation. while size‐tunable hexagonal nanosheets have been The average height profiles of these two films are 4‐7 nm generated at the liquid‐liquid interface.[56] In the case of and 35‐40 nm respectively. There is an increase in the film single‐walled carbon nanotubes (SWNTs), water thickness with increase in time duration of assembly as dispersions containing surfactants mixed with non‐polar expected.

52 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 49-60 www.intechopen.com a 240 adsorption (a) desorption -1 180

1500 1550 1600 1650 1700 120 (STP) g (STP)

3

C (46  ) : G (3.3 g in 10 ml) 60 60

V/cm EG

0 Intensity (a.u.) Intensity   C 60(23 : G (3.3 g in 10 ml) 0.0 0.2 0.4 0.6 0.8 1.0

P/Po

40 (b) adsorption G (3.3 g in 10 ml) desorption 32 1000 1500 2000 2500 3000 -1 -1 Raman shift (cm ) 24 b

(STP) g (STP) 3 16 C60 ( 23  EG C60 ( 23  + Graphene (3.3 g in 10 ml)

V/cm 8

0 0 1000 2000 3000 4000 5000

Absorbance (a.u.) P (kPa)

200 400 600 800 Figure 7. (a) Carbon dioxide and methane adsorption and Wavelength (nm) desorption isotherms of EG measured at 195 K, 1 atm and 298 K, Figure 6. (a) Raman spectra and (b) electronic absorption spectra 50 bar respectively. (From reference 69). of composite films of C60 and few‐layer graphene film. Inset in (a) shows stiffening of Raman G‐band of few‐layer graphene film in the presence of C60. (From reference 63) 70 6 (a)

CH4 uptake 60 In the Figure 6 (a) we show the Raman G‐band of 5 CO2 uptake 50 graphene film along with the bands in the composites of 4 40 graphene with C60. Pure graphene shows the G‐band at

3 1590 cm‐1 while composites containing C60 exhibit 30 ‐1 2 stiffening of the G‐band. The G‐band occurs at 1596 cm (wt uptake %) 4

20 (wt %) uptake when the concentration of C60 is 23 μM and shifts 1601 1 2

CH 10 ‐1 cm when the C60 concentration is 46 μM. These results 0 CO 0 suggest the occurrence of charge‐transfer interaction 0 200 400 600 800 1000 1200 1400 2 between C60 and graphene, similar to that found between Surface area (m /g) graphene and electron‐acceptor molecules like TCNE and nitrobenzene.[64] UV–visible absorption spectra of the 6 (b) Activated charcoal composite films shown in the in Figure 6 (b) has 5 characteristic electronic absorption bands of C60 with only changes in the intensities.[63] 4

3 5. Surface Properties EG 2 %) (wt uptake 4 RGO Theoretical calculations predict to show a large surface 1 CH area by Single‐layer graphene close to 2600 m2/g.[65] Surface SGO 0 HG areas of few‐layer graphene samples (EG, DG, HG and 10 20 30 40 50 60 70 RGO) prepared by different methods have been measured CO2 uptake (wt %) employing Brunauer‐Emmett‐Teller (BET) method. [66, 67] The surface areas are in the range of 270‐1550 m2/g Figure 8. (a) Plot of the BET surface area and the weight following the trend EG > DG > RGO > HG. Thus, few‐layer percentage of methane uptake (at 298 K and 50 bar) and carbon graphenes show large surface areas, some of them dioxide uptake (at 195 K and 1 atm); b) Plot of weight percentage approaching the value of single layer graphene. of methane uptake and weight percentage of carbon dioxide uptake. (From reference 69).

www.intechopen.com H.S.S. Ramakrishna Matte, K. S. Subrahmanyam and C. N. R. Rao: 53 Synthetic Aspects and Selected Properties of Graphene Uptake of CO2 by the graphene samples was measured at show that the H2 molecule sits alternatively in parallel 195 K and 1 atm.[66] The uptake values vary between 5 and perpendicular orientations on the six‐membered and 45 wt % with EG exhibiting the highest uptake. rings of graphene layer and that single‐layer graphene Figure 7(a) shows typical CO2 adsorption and desorption can accommodate up to 7.7 wt% of hydrogen.[66] curves of the EG sample. The uptake of CO2 by EG at 298 K and 50 bar is 51 %. First‐principles calculations show that CO2 molecules sit alternatively in a parallel fashion on the six‐membered rings.[66] Employing first‐principles calculations, adsorption of different gas molecules (CO, NO, NO2, O2, N2, CO2, and NH3) on graphene nano‐ ribbons has been studied.[68] It is shown that NH3 can modify the conductance of the nano‐ribbons, while other gas molecules have little effect. This property can be used to detect NH3 out of other gases.

Adsorption of methane on the graphene samples was measured at 273 K and 298 K.[69] Typical adsorption data of EG at 298 K, 50 bar shown in Figure 7(b). The weight uptake of CH4 by graphene samples varies between 0 and 3 wt % showing EG highest value. In Figure 8(a), we show the uptake of CO2 and CH4 against the surface area. The uptake values of CH4 and CO2 vary linearly with the surface area. A plot of the CH4 uptake versus CO2 uptake is nearly linear (Figure 8(b)). EG and RGO with relatively high CO2 and CH4 uptakes contain oxygen functionalities on the surface. Interestingly, In the case of HG with little or no uptake of these gases, the surface of HG was clean with negligible oxygen functionalities. EG and RGO with relatively high CO2 and CH4 uptakes contain oxygen functionalities on the surface. Interestingly, in the case of Figure 9. (a) Hydrogen adsorption and desorption isotherms of HG with little or no uptake of these gases, the surface of EG at 1 atm and 77 K. (b) Linear relationship between the BET HG was clean with negligible oxygen functionalities. surface area and the weight percentage of hydrogen uptake at 1 atm of pressure and 77 K. (From reference 66).

6. Hydrogen storage

Hydrogen uptake data of different graphene samples have been reported.[66] In Figure 9 (a) H2 adsorption and desorption curves of the EG sample are shown. H2 adsorption measurements at 1 atm and 77 K show that DG, EG and HG can absorb 1.2, 1.7 and 1.0 wt% of H2. These samples show higher uptakes at 100 bar and 300 K, the values being 2.5, 3.1 and 2.0 wt% for DG, EG and HG respectively. The adsorption is completely reversible and comparable to that of carbon nanotubes[70] and porous open framework materials.[71] The values of the H2 uptake at 1 atm and 77K by the various graphene samples vary linearly with the surface area (see Figure 9 (b)). By extrapolation of the linear plot to the surface area of Figure 10. Change in the weight percentage of hydrogen of EGH single‐layer graphene, we estimate its H2 uptake to be and HGH with temperature. (Inset) The evolution of hydrogen around 3 wt% at 1 atm and 77 K. Though the H2 uptake of as recorded by a gas chromatograph. (From reference 72) graphenes are low compared to the 6.0 wt% target of the US Department of Energy, there is scope for significant Birch reduction of few‐layer graphene samples enables [72] improvement, by producing samples with a smaller chemisorption of hydrogen up to 5 wt %. Birch number of layers and higher surface areas. It is possible reduction of EG and HG has been carried out with lithium in liquid ammonia at −33 °C [73] Spectroscopic that single layer graphene will exhibit 5‐6 wt% of H2 3 uptake at 100 atm and 300 K. First‐principles calculations studies reveal the presence of sp C‐H bonds in the hydrogenated graphenes. Elemental analysis of reduced

54 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 49-60 www.intechopen.com EG (EGH) and HG (HGH) samples showed the hydrogen on the addition of EGA. The intensity of the fluorescence content to be around 5 wt % in the samples obtained with bands decrease markedly with the increase in EGA the use of excess Li. The hydrogenated graphene concentration as illustrated in the Figure 11 (c) and (d). Fluorescence decay measurements on I monitored at 395 containing ~5 wt % hydrogen is stable and can be stored nm could be fitted to a three‐exponential decay [79] with over long periods. We have examined its thermal stability lifetimes of 1.8, 5.7 and 38.7 ns. Addition of EGA causes a in detail. In Figure 10 we show the change in weight significant decrease in all the three lifetimes with values percentage of hydrogen in the EGH and HGH samples on 1.2, 4.6 and 29.1 ns respectively for the addition of 0.3 mg heating to different temperatures (as obtained from of EGA. elemental analysis). All the hydrogen gets released 1.0 -5 (a) I (10 M in DMF) (c) I around 500 °C. Gas chromatography shows that the 0.2 mg EG + I 0.2 mg EGA + I 0.8 0.3 mg EG + I 0.3 mg EGA + I evolution of hydrogen starts around 200 °C and is 0.4 mg EG + I 0.4 mg EGA + I complete at 500 °C (see inset Figure 10) We also find that 0.6 irradiation of the hydrogenated samples with UV 0.4 Intensity (a.u.)Intensity absorbance (a. u.) absorbance radiation or with a KrF excimer laser results to 0.2

dehydrogenation. In the case of UV irradiation, 0.0 dehydrogenation occurs over a few hours, whereas the 300 350 400 nm 350 400 450 500 550 1.6 nm same result is obtained within 2 min using the laser. -5 (b) II (10 M in Chloroform) (d) II 0.2 mg EGA + II 0.2 mg EGA + II 0.3 mg EGA + II 1.2 0.3 mg EGA + II 0.4 mg EGA + II 0.4 mg EGA + II

7. Fluorescence quenching 0.8

0.4 Intensity (a. u.) (a. Intensity Fluorescence quenching property of graphene has been absorbance made use for the selective detection of biomolecules[74] and 0.0 other purposes.[75, 76] Quenching of the fluorescence of -0.4 450 500 550 600 650 porphyrin by graphene and photophysical properties of 300 400 500 600 nm nm porphyrin‐graphene complexes have been reported .[47, 77] Theoretical studies show that long‐range energy transfer Figure 11. Electronic absorption spectra and of (a) PyBS, I, (10‐5 is operative in the fluorescence quenching of a dye M in DMF), (b) OPV ester, II, (10‐5 M in chloroform) and molecule in the presence of graphene . The quenching of fluorescence spectra of (a) PyBS, I, (10‐5 M in DMF) , (b) OPV ‐5 the green emission of ZnO nanoparticles accompanying ester, II, (10 M in chloroform) with increasing concentration of graphene (EGA). (From reference 78) the photoreduction of graphene oxide is, however caused by electron transfer from ZnO. Electron transfer has been In Figure 12 (a), we compare the transient absorption similarly invoked in the case of TiO2‐ graphene oxide.[76] spectrum of the pure I with that of I on addition of 0.3 mg The interaction of graphene with pyrene‐butanaoic acid of graphene. The spectrum of PyBS shows an absorption succinimidyl ester, (PyBS), I , and oligo(p‐ maximum around 430 nm together with a broad band in phenylenevinylene) methyl ester (OPV‐ester), II , with a the 450‐ 530 nm range due to the triplet state.[80] Upon graphene derivative, EGA, soluble in chloroform and addition of EGA, new bands emerge around 470 and 520 dimethylformamide (DMF).[78] nm in the transient absorption spectrum at 500

nanoseconds. The 470 nm band can be assigned to the Absorption spectra of PyBS , I, in DMF and OPV ester, II , pyrenyl radical cation as reported in the literature[79], in chlororform solution (10‐5 M) are shown in Figure 11 suggesting the occurrence of photo‐induced electron (a) and (b) respectively in the presence of varying transfer from the PyBS to the graphene. Accordingly, we concentrations of graphene, EGA. observe the transient absorption around 520 nm which These spectra show characteristic absorption bands of I we assign to the graphene radical anion. The decay of the and II. The increase in intensities of these bands with the radical cation formed in the presence of graphene was graphene concentration is entirely accounted for the fast, as evidenced from the appearance of a short‐lived increasing intensity of the graphene absorption band component (900 ns) in the decay profile (Figure 12(b)). around 270 nm. Thus, electronic absorption spectra of I + However, the decay of the transient absorption of pure EGA and II + EGA show no evidence of interaction PyBS monitored at 470 nm (see inset of Figure 12(b)) between the two molecules in the ground state. We also shows a long‐lived triplet with a lifetime of 6.17 do not see of new absorption bands attributable to microseconds. The transient absorption at 520 nm decays charge‐transfer. Unlike the absorption spectra, simultaneously with that of the pyrene radical cation fluorescence spectra of I and II show remarkable changes indicating that it is due to the graphene radical anion.

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www.intechopen.com H.S.S. Ramakrishna Matte, K. S. Subrahmanyam and C. N. R. Rao: 59 Synthetic Aspects and Selected Properties of Graphene

ARTICLE

Complex Nanostructures by Pulsed Droplet Epitaxy

Invited Feature Article

Stefano Sanguinetti1,*, Claudio Somaschini1, Sergio Bietti1 and Noboyuki Koguchi1

1 L-NESS and Dipartimento di Scienza dei Materiali, Università di Milano Bicocca, Italy *Corresponding author E-mail: [email protected]

Received 8 April 2011; Accepted 31 May 2011

© 2011 Sanguinetti et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract What makes three dimensional semiconductor 1. Introduction quantum nanostructures so attractive is the possibility to tune their electronic properties by careful design of their The principle of nanotechnology is the manipulation of size and composition. These parameters set the the matter at the nanoscale in order to take advantage of confinement potential of electrons and holes, thus the different physical properties of materials via size and determining the electronic and optical properties of the shape fine tuning. Amongst the more relevant nanostructure. An often overlooked parameter, which has nanoscience advancements an important place is taken by an even more relevant effect on the electronic properties quantum confinement effects that take place in three of the nanostructure, is shape. Gaining a strong control dimensional semiconductor nanostructures. Because of over the electronic properties via shape tuning is the key them, these quantum nanostructures (QN) can be to access subtle electronic design possibilities. The Pulsed considered as artificial atoms and like the natural atoms Dropled Epitaxy is an innovative growth method for the show a discrete spectrum of energy levels [1]. More than fabrication of quantum nanostructures with highly natural atoms, QNs electronic properties can be finely designable shapes and complex morphologies. With tuned, on demand, adjusting structural parameters, such Pulsed Dropled Epitaxy it is possible to combine different as size, composition and morphology. The latter nanostructures, namely quantum dots, quantum rings parameter is the most relevant for the control of the QN and quantum disks, with tunable sizes and densities, into electronic properties, as tiny variations in morphology a single multi‐function nanostructure, thus allowing an can cause dramatic changes on the electronic properties unprecedented control over electronic properties. [2]. One of the most pursued method for the fabrication of QNs is the molecular beam epitaxy (MBE) growth of Keywords Quantum Nanostructures, III‐V Semiconductors, lattice‐mismatched III‐V semiconductor materials via the Droplet Epitaxy Stranki‐Krastanov (SK) mode [3]. This technique exploits

www.intechopen.com Stefano Nanomater.Sanguinetti, nanotechnol.,Claudio Somaschini, 2013, CollectionSergio Bietti of and Selected Noboyuki Papers, Koguchi: 61-64 61 Complex Nanostructures by Pulsed Droplet Epitaxy

Figure 1. Atomic force microscope (AFM) images of dot/ring (a), dot/disk (b), ring/ring (c) and ring/disk (d) GaAs/AlGaAs QNs

the self‐assembly of pyramidal‐like QNs, driven by the ring at the center and an outer region made by a ring or a relaxation of strain accumulated in the epilayer. Despite disk. The fabrication of the reported composite QNs is the high success of the technique, which led to made in a multiple step procedure which include first the fundamental physical understandings and to a variety of Ga droplet deposition and then several As supply steps at applications [3‐5], the available design degrees of different temperatures and fluxes, as detailed in Refs. freedom remain limited. The precise engineering of size [10,13,14,18]. Let us follow in more detail the PDE and shape of QNs via SK self‐assembly remains fabrication. When Ga is deposited on AlGaAs (100) group problematic [6], thus limiting the possibilities of a real on III terminated surface (4x6 reconstruction [19]), it self‐ demand design of the electronic properties. It is worth assemble in form of small droplets, whose size and mentioning that the possibility to control QN shape density can be finely tuned by substrate temperature, Ga allows to access fundamental quantum design parameters flux and coverage. When droplets with the required that include geometrical quantum phase [7], spin‐spin density and size are formed, PDE fabrication of QNs interaction [8] and quantum state couplings [9]. proceeds via the supply of predetermined As quantities at controlled fluxes and substrate temperatures. This 2. The Pulsed Droplet Epitaxy allows for the fine control of the three phenomena that are occurring during the As supply: i) the To overcome the SK growth limitations, a kinetic limited thermodynamically driven diffusion of Ga atoms from growth procedure, the Pulsed Droplet Epitaxy (PDE), a the droplets to form a two dimensional (2D) layer on the variant of molecular beam epitaxy (MBE), was introduced substrate; ii) The incorporation of As in the liquid Ga at [10–14]. Unlike the SK self‐assembly technique, PDE does the droplet original position, thus developing a 3D not rely on strain for the formation of three‐dimensional nanocrystal; and iii) the kinetic of the change in the (3D) crystals. PDE is based on the pulsed deposition of III surface reconstruction around the droplets from Ga‐rich and V column elements at controlled temperatures and to As‐rich, caused by the adsorption of As on the flat fluxes. In this respect, PDE can be considered as a variant surface. The interplay between these phenomena sets the of the well established droplet epitaxy growth technique final configuration of the GaAs nanostructure, between [15,16], which demonstrated the possibility to grow QNs the limit cases of a total lateral growth of GaAs around with shapes ranging from dots [17] to rings [10,18]. The the droplet edges (strong Ga diffusion) and of a complete first step of PDE is the formation, in an MBE crystallization of Ga within the original droplets (very environment, of nanoscale reservoirs of metal atoms on efficient As incorporation). It is possible to switch the growth surface in forms of nanometer size droplets between the 2D (ring and disk shapes) and 3D (dot with small size dispersion. This is achieved in a group V shapes) growth modes and set Ga diffusion pinning sites free environment. The metallic droplet on the surface will by changing the As flux and substrate temperature constitute the group III localized sources from which the conditions during the PDE fabrication process. Indeed a QNs will evolve. Second, and more relevant for the QN strong diffusion of Ga is found at lower As flux and/or shape control, is the pulsed supply of group V elements higher temperatures and therefore under these conditions at different temperatures and fluxes, for the the lateral 2D growth will be predominant (ring and transformation of the metallic droplet into the QN. The disk); for higher As flux and lower substrate possibility to finely control, through flux and temperatures, instead, the development of a 3D structure temperature, the transformation kinetics of the metal is favored (dots). Finally the change in the surface droplets in to III‐V nanocrystals allows for the formation reconstruction which appears due to the As adsorption of QNs with complex and controlled shapes. In particular on to the substrate surface during the crystallization can it is possible to combine quantum dots, rings and disks in be used to finely tune the shape of the laterally grown a single, multi‐function QN. We show here (Fig. 1) some GaAs between a flat disk and a ring [12]. As rule of examples of complex GaAs QNs grown on Al0.3Ga0.7As thumb, disks are obtained at temperatures above 350 °C, buffer layers. The presented QNs are made by a dot or a while lower temperatures give rise to ring shapes. In both

62 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 61-64 www.intechopen.com electronic states we performed theoretical calculations based on effective mass approximation [20]. The results for a typical dot/ring QN are reported in Fig. 2. Both dot and ring are capable of quantum confinement, giving rise to the well know ladder of quantum states belonging to carriers confined in the dot and the ring. More details on the electronic states and the comparison with the observed photoluminescence emission can be found in Ref. [14]. Many‐body interaction should be strongly influenced as well. The shape design possibilities introduced by PDE permit to fabricate complex QN made by quantum units of different geometry and Figure 2. Isosurface plots of the electronic probability density at dimensionality. This permits to finely tune the electronic 50% of the maxima in the dot/ring. From left to right, these property of the QN and, more relevant to applications, panels show the wave function of the ground state of the the QN response to external fields. In fact, the coupling nanostructure, which corresponds to a wavefunction totally between states belonging to different parts of the QN can localized in the dot, and that of the an excited radial state, fully localized in the ring. The AFM profile of a dot/ring structure is be switched on or off due to selection rule breaking due reported as well. to external fields [9]. Recent theoretical investigations indicated that changes in QN shape are accompanied by cases, As pressure is maintained in the 10‐6‐10‐7 Torr range an alteration of the ground state total spin [8,23], thus [12]. Dots require low growth temperatures (below 200 making possible to devise tunneling spin switches. In °C) and high As fluxes (higher than 10‐5 Torr) [17]. PDE addition, the dimensionality of the QN, namely zero, one allows to switch between different growth modes during or two dimensions, plays a fundamental role, changing a single QN fabrication, thus achieving complex, the way the QN responds to external fields. Infrared composite, structures. This is the key factor introduced by detectors made by quantum dot embedded in a quantum PDE growth protocol, that is multiple crystallization well show increased performances, due to better coupling steps, performed at different temperatures. With short between zero dimensional dot and two dimensional well time As pulses (on the order of seconds) we can electronic states respect to bulk states. Via PDE we can selectively crystallize only a fraction of the Ga atoms devise single QN dot in a well photodetector where dot stored in the droplets reservoir on the surface and and well are both included in a single QN. As a further subsequently change the conditions for next step, thus demonstration of the potential of PDE approach we combining different building blocks within a single QN. fabricated even more complex systems, as shown in Fig. The PDE nanostructures electronic and emission 3. Again the multi‐step synthesis was applied, selectively properties were extensively studied showing high optical forming each block of the QN at different temperatures. quality [20], peculiar carrier dynamics [21] and single Here two As shots at low pressure and relatively high photon emitter properties [22]. temperature (thus in the 2D growth mode conditions) were applied to the Ga droplet ensemble to obtain the The introduction of the high shape flexibility in the QN ring/ring/ring sample [10]. The dot/ring/ring required as design, allowed by PDE, leads to a deep change in the well two As shots in 2D growth mode followed by a low way nanostructure based devices are devised. For the temperature and high As flux. The latter is needed to first time it is possible to fabricate QNs where the freeze the metallic Ga which remains in the central part of electronic properties are designed on demand for a the nanostructure after the two initial As shots, into the specific device function via shape engineering, that is the central dot [14]. form follows function approach. PDE allows for the fabrication of QNs where optical properties, intersublevel energy spacing, level dimensionality and even the interaction between nearby QNs are freely accessible for engineering. In fact, the change in shape of a QN often leads to a change in electronic state symmetries and characteristics, thus the relevant phenomena are much richer. At its basic level, the change in electronic structure is the change in single‐particle electronic states. This includes the state energy, the overall shape of the wave function, the symmetry, the polarization, and the Figure 3. Atomic force microscope images of dot/ring/ring (a), localization. In order to gain better insight on the effective ring/ring/ring (b) GaAs/AlGaAs QNs. ability of our QNs to effectively tune single‐particle

www.intechopen.com Stefano Sanguinetti, Claudio Somaschini, Sergio Bietti and Noboyuki Koguchi: 63 Complex Nanostructures by Pulsed Droplet Epitaxy 3. Conclusions [11] C. Somaschini, S. Bietti, A. Fedorov, N. Koguchi, and S. Sanguinetti, “Concentric Multiple Rings by In conclusion, with PDE, it is possible to grow Droplet Epitaxy: Fabrication and Study of the semiconductor coupled quantum systems, fabricated with Morphological Anisotropy,” Nanoscale Research a pure self‐assembly technique, based on multiple, partial Letters, vol. 5, 2010, p. 1865. crystallization steps of the Ga. PDE allows for the [12] C. Somaschini, S. Bietti, N. Koguchi, and S. realization of complex QNs where single building blocks, Sanguinetti, “Shape control via surface reconstruction such as quantum dot and rings, can be combined together kinetics of droplet epitaxy nanostructures,” Appl. Phys. with a high shape flexibility. PDE can be therefore used as Lett., vol. 97, 2010, p. 203109. basis for the conception of novel devices in optoelectronics [13] C. Somaschini, S. Bietti, S. Sanguinetti, N. Koguchi, and quantum information fields, as well as for the and a Fedorov, “Self‐assembled GaAs/AlGaAs investigation of phenomena in fundamental physics. coupled quantum ring‐disk structures by droplet epitaxy.,” Nanotechnology, vol. 21, 2010, p. 125601. 4. Acknowledgments [14] C. Somaschini, S. Bietti, N. Koguchi, and S. Sanguinetti, “Coupled quantum dot–ring structures This work was supported by the CARIPLO Foundation by droplet epitaxy,” Nanotechnology, vol. 22, 2011, p. (prj. QUADIS2 ‐ no. 2008‐3186). 185602. [15] N. Koguchi, S. Takahashi, and T. Chikyow, “New 5. References MBE growth method for InSb quantum well boxes,” J. Cryst. Growth, vol. 111, 1991, p. 688. [1] M.A. Kastner, “Artificial Atoms,” Physics Today, vol. [16] N. Koguchi and K. Ishige, “New selective molecular 46, 1993, p. 24. beam epitaxial growth method for direct formation [2] J. Li and L.‐W. Wang, “Shape Effects on Electronic States of GaAs quantum dots,” J. Vac. Sci. Technol. B, vol. 11, of Nanocrystals,” Nano Lett., vol. 3, 2003, p. 1357. 1993, p. 787. [3] A.D. Yoffe, “Semiconductor quantum dots and related [17] K. Watanabe, N. Koguchi, and Y. Gotoh, “Fabrication systems: electronic, optical, luminescence and related of GaAs Quantum Dots by Modified Droplet properties of low dimensional systems,” Advances in Epitaxy,” Jap. J. Appl. Phys., vol. 39, 2000, p. L79. Physics, vol. 50, 2001, p. 1 [18] T. Mano, T. Kuroda, S. Sanguinetti, T. Ochiai, T. Tateno, [4] X. Li, Y. Wu, D. Steel, D. Gammon, T.H. Stievater, D.S. J. Kim, T. Noda, M. Kawabe, K. Sakoda, G. Kido, and N. Katzer, D. Park, C. Piermarocchi, and L.J. Sham, “An Koguchi, “Self‐Assembly of Concentric Quantum All‐Optical Quantum Gate in a Semiconductor Double Rings,” Nano Lett., vol. 5, 2005, p. 425. Quantum Dot,” Science, vol. 301, 2003, p. 809. [19] A. Ohtake, P. Kocán, K. Seino, W. Schmidt, and N. [5] C.L. Salter, R.M. Stevenson, I. Farrer, C. a Nicoll, D. a Koguchi, “Ga‐Rich Limit of Surface Reconstructions Ritchie, and a J. Shields, “An entangled‐light‐emitting on GaAs(001): Atomic Structure of the (4×6) Phase,” diode,” Nature, vol. 465, Jun. 2010, pp. 594‐597. Phys. Rev. Lett., vol. 93, 2004, p. 266101. [6] S. Kiravittaya, A. Rastelli, and O.G. Schmidt, [20] T. Kuroda, T. Mano, T. Ochiai, S. Sanguinetti, K. “Advanced quantum dot configurations,” Rep. Prog. Sakoda, G. Kido, and N. Koguchi, “Optical Phys., vol. 72, 2009, p. 046502. transitions in quantum ring complexes,” Phys. Rev. B, [7] R. Capozza, D. Giuliano, P. Lucignano, and A. vol. 72, 2005, p. 205301. Tagliacozzo, “Quantum Interference of Electrons in a [21] S. Sanguinetti, M. Abbarchi, A. Vinattieri, M. Ring: Tuning of the Geometrical Phase,” Phys. Rev. Zamfirescu, M. Gurioli, T. Mano, T. Kuroda, and N. Lett., vol. 95, 2005, p. 226803. Koguchi, “Carrier dynamics in individual concentric [8] Y. Saiga, D. Hirashima, and J. Usukura, “Ground‐state quantum rings: Photoluminescence measurements,” properties of quantum rings with a few electrons: Phys. Rev. B, vol. 77, 2008, p. 125404. Magnetization, persistent current, and spin [22] M. Abbarchi, C. Mastrandrea, A. Vinattieri, S. chirality,” Phys. Rev. B, vol. 75, 2007, p. 045343. Sanguinetti, T. Mano, T. Kuroda, N. Koguchi, K. [9] L. Dias da Silva, J. Villas‐Bôas, and S. Ulloa, Sakoda, and M. Gurioli, “Photon antibunching in “Tunneling and optical control in quantum ring double quantum ring structures,” Phys. Rev. B, vol. molecules,” Phys. Rev. B, vol. 76, 2007, p. 155306. 79, 2009, p. 085308. [10] C. Somaschini, S. Bietti, N. Koguchi, and S. [23] N. Yang, J.‐L. Zhu, and Z. Dai, “Rotating Wigner Sanguinetti, “Fabrication of multiple concentric molecules and spin‐related behaviors in quantum nanoring structures.,” Nano Lett., vol. 9, 2009, p. 3419 rings,” J. Phys.: Condens. Matter, vol. 20, 2008, p. 29520

64 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 61-64 www.intechopen.com ARTICLE

Magnetic Properties of Fe and Ni

Doped SnO2 Nanoparticles

Regular paper

Aditya Sharma1,*, Mayora Varshney1, Shalendra Kumar2, K. D. Verma1 and Ravi Kumar3

1 Material Science Research Laboratory, Department of Physics, S.V.College, India 2 School of Nano & Advance Materials Engineering, Changwon National University, Republic of Korea 3 Centre for Materials Science and Engineering, National Institute of Technology, India *Corresponding author E-mail: [email protected]

Received 8 May, 2011; Accepted 30 June, 2011

© 2011 Sharma et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract In this work, we report the room temperature semiconductors, the ferromagnetic semiconductors are ferromagnetism in Sn1‐xFexO2 and Sn1‐xNixO2 (x = 0.00, 0.03 emerged as attracting materials and driven a considerable and 0.05) nano‐crystalline powders. All the samples were attention in the recent years [1‐2]. To have a magnetic prepared using co‐precipitation method. X‐Ray semiconductor, a non‐magnetic semiconductor is doped Diffraction (XRD), transmission electron microscopy with transition metal ions to achieve magnetism from it. It (TEM), energy dispersive x‐ray analysis (EDAX), UV‐ is further found that the transition metal impurity is of visible absorption spectroscopy and room temperature only few percent of the cations, i.e., it is very dilute. Due to magnetization measurements were performed to study this reason such material are known as Diluted Magnetic the crystal structure, morphology, elemental analysis, Semiconductor (DMS). Such materials promise to provide a optical band gap and magnetic properties of Fe and Ni wide range of applications in various fields like logic, doped SnO2. TEM results depict the formation of storage, communications, quantum computation and spherically shaped and small sized nanoparticles of the diameter of ~ 3 nm. The band gap energy of the Fe and Ni multi‐functionality on the same chip [3]. For realizing this doped samples found to decrease with increasing their goal it is very necessary that the magnetism in these DMS concentrations. The higher saturation magnetization was systems should be carrier‐mediated. It is known that the observed in low concentration Fe and Ni doped tin oxide. number of charge carriers in a semiconductor can be tuned with the application of electric voltages, so if the Keywords Nanoparticles, XRD, DMS, Ferromagnetism. magnetism is charge carriers mediated then this will give a handle to control the magnetism in the system with the help of applied electrical signals and will increase the 1. Introduction multi‐functionality of the material. The study of DMS materials is being carried out since last few decades. Apart Among the various important categories of the functional from the present day applications in the field of materials like; polymers, magnetic, dielectric and Spintronics, they are interesting to the physics community

www.intechopen.com Aditya Sharma,Nanomater. Mayora nanotechnol.,Varshney, Shalendra 2013, CollectionKumar, K.D.Verma of Selected and Papers, Ravi Kumar: 65-69 65

Magnetic Properties of Fe and Ni Doped SnO2 Nanoparticles due to the rich variety of phenomena and physics they system, the magnetic moments per Mn site were found to exhibit [4]. Preparation of single phase DMS, devoid of any decrease with increasing Mn content, x, indicating an anti‐ secondary phases or metallic clusters, is a big challenge. ferromagnetic (AFM) nature of Mn‐O‐Mn interaction, More often it turns out that the system is having impurity reminiscent of AFM in rutile MnO2 [10]. It was also phases when the system is characterized using various suggested that the exchange interaction mechanism of the complimentary techniques. Even if the material is single observed RTFM in Sn1‐xMxO2 (M = Mn & Fe) powders phases, the reports on the same system, prepared under involve an electron trapped at an oxygen vacancy same conditions by various groups are very contradictory. adjacent to the transition metal ion (F‐centre exchange) Apart from the issue of reproducibility, the magnetism in [11]. Till date the origin of ferromagnetism still remains these systems has always been controversial and the indistinct in so far proposed DMS systems because of their debate on the origin and the mechanism of ferromagnetism multi phase nature etc. In this study we report the room is far from over. One of the most widely studied system temperature ferromagnetism in chemically synthesized, and the one in which carrier‐mediated ferromagnetism has single phase, Fe and Ni doped tin oxide nanoparticles. been established is the (Ga,Mn)As system. But the problem is it low Curie temperature (Tc = 150 K) [5]. To use these 2. Experimental Details materials in industry for the device applications, their Curie temperature should be well above room temperature All the reagents used were of analytical grade without (> 400 K). One of the most important predictions on further purification. 0.01M solutions of SnCl4.5H2O, preparation of DMS showing above room temperature NiCl2.5H2O and FeCl2.5H2O were prepared with a molar ferromagnetism (RTFM) was made based on the mean‐ ratio of x= Fe/(Fe+Sn) and Ni/(Ni+Sn) by proper dissolving field theoretical treatments using the Zener exchange into de‐ionized water. Ammonium Hydroxide (NH4OH) mechanism [6]. According to their calculations 5 % Mn was added into the solution (drop by drop), with stirring, doped ZnO and GaN were expected to show RTFM for a until the white precipitates were obtained. After 30 hole concentration of 3.5 × 1020 cm‐3. The holes in the minutes of stirring the resultant mixtures were rinsed, valence band have a strong exchange interaction with the several times, with de‐ionized water to remove chlorine magnetic impurity. Due to this reason these holes can and other ionic impurities, which may formed during the mediate an indirect exchange coupling among the doped synthesis process. These washed precipitates were dried in transition metal ions, which lead to the long range ordering air at 40 0C for 20 hours followed by natural cooling up to and to RTFM, as well as spin‐splitting of the electronic the room temperature and then final powder products states proportional to the magnetization of the doped ions. were collected carefully. A detailed characterization of the Following these predictions there were flurry of reports samples was carried out using XRD, TEM, EDAX, UV‐ from all over the globe on the transition metal doped ZnO visible spectroscopy and dc‐magnetization measurements. and other oxide semiconductor materials. The main XRD patterns were obtained using Bruker D8 advanced difficulty in preparing p‐type oxides semiconductor diffractometer using CuKα radiation (λ=1.540Å) operated materials [7] is the large n‐type background that is most of at voltage 40 kV and current of 40 mA. TEM measurements the time present in the oxide systems. This brings to were performed using FEI‐Tecnai‐20 transmission electron another problem in the oxides. This n‐type background is microscope operated at 200 kV. Optical absorption spectra conventionally attributed to the defects like O vacancies were recorded with the conventional two beam method and metal interstitials. Therefore, it is difficult to establish a using Hitachi UV‐3300, UV‐VIS spectrometer. Room mechanism to explain origin of room temperature temperature magnetization measurements were performed ferromagnetism in the oxide systems. using commercial quantum Design physical properties measurement system (PPMS). Among the other known oxide semiconductors, tin oxide (SnO2) is a very interesting oxide material with a wide 3. Results and Discussion band gap of 3.6 eV. Its higher optical transparency, chemical stability, and electrical conductivity make it a Fig. 1 (a) – (e) show the XRD patterns of Sn1‐xFexO2 (x= 0.0, very attractive material for solar cells, catalysis and gas 0.03, and 0.05) Sn1‐xNixO2 (x = 0.03 and 0.05) nanoparticles, sensing applications. In the nano‐scale form TM‐doped respectively. It is clearly evident from Fig.1 that the peaks SnO2 is reported to demonstrate more interesting structural corresponding to the rutile–type SnO2 are detected for all and magnetic properties. For instance, chemically compositions, indicating Fe and Ni doping does not affect synthesized Co‐doped SnO2 exhibit different size/shape the original tetragonal unit cell of SnO2. However, the features with increasing Co content. Moreover, very low observed XRD peaks are quite wider and having magnetic moments (0.133μB/Co) with a high coercivity sufficient background. This kind of broad XRD peaks, (~630 Oe) were also observed [8]. In the thin film form, Co with high background, have also been reported in case of doped SnO2 shows high magnetic moment (~7.5μB /Co) very small sized SnO2 quantum dot/nanoparticles [12‐13]. with low coercivity (~50 Oe) [9]. In the Mn‐doped SnO2 The peak broadening and high background may arise

66 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 65-69 www.intechopen.com by chemical coating method [14]. The average grain sizes were calculated from XRD patterns using the Scherrer relation; (D=0.9λ/βcosθ, where β is FWHM of the XRD peaks and λ is the wavelength of incident x‐rays). Thus calculated average grain size falls in the small size regime of ~3.5 nm in all the samples, prepared with different concentrations of Fe and Ni.

To further confirm the particle size, morphology and elemental concentration of as‐prepared samples, systematic TEM and EDAX measurements were performed. Fig. 2 (a ‐ c) shows the TEM images of (a) SnO2, (b) Sn0.95Fe0.05O2 and (c) Sn0.95Ni0.05O2 samples, respectively. It clear from the figure that spherical shaped nanoparticles have been formed in un‐doped and TM ion doped SnO2. However, some aggregation of nanoparticles has been observed in all TEM micrographs. This aggregation in wet chemically synthesized nanoparticles is expected due to the presence of substantial OH‐ ions in samples [15]. This aggregation make difficult to determine crystal size accurately. The average size, which is estimated from individual spherical nanocrystals, is ~3.1 nm, 3.6 nm and 3.4 nm for SnO2, Sn0.95Fe0.05O2 and Sn0.95Ni0.05O2 samples, respectively. Thus calculated particle sizes are in very good agreement to Figure 1. (Color online) XRD patterns of (a) SnO2, Sn0.97Fe0.03O2 and Sn0.95Fe0.05O2 (b) Sn0.97Ni0.03O2 and Sn0.95Ni0.05O2 nanoparticles, those calculated from XRD data. Fig. 3 (a‐e) shows the respectively. EDAX spectra, collected from the average scanned area, of un‐doped SnO2, Sn0.97Fe0.03O2, Sn0.95Fe0.05O2, Sn0.97Ni0.03O2 and due to the (i) poor performance of used diffractometer Sn0.95Ni0.05O2 samples, respectively. The self generated and (ii) very narrow size distribution of the so formed elemental composition (wt. %) details are also presented in nanoparticles/quantum dots. In the present case, the the Fig. 3. It is clear from the Fig. 3 that Sn, and O are only small size of the as‐synthesized nanoparticles may the main elemental species in pure SnO2 sample while, responsible for the XRD peak broadening. Moreover, an additionally, Fe and Ni peaks were observed in Fe and Ni extra peak is present in un‐doped and doped samples doped samples. However, C peaks, very closer to the O (marked by asterisk). This peak, appeared at ~22, does peaks, were also appeared in all the samples. These C not match with any phase of SnO, Sn2O3, Sn3O4 or even peaks were eliminated from the EDAX spectra, during the phases of Fe2O3 and Fe3O4, as verified from the data analysis, to better examine the O peaks in the spectra. corresponding JCPDS files. This peak may belong to some Moreover, the weight percentage of the doped transition organic phase formed during the growth of nanoparticles metal elements was found to little higher than that of by the chemical method. Similar kind of organic phases nominal concentration, used during the calculations and have also been reported in the TiO2 thin films, deposited sample preparation.

Figure 2. TEM images of (a) SnO2, (b) Sn0.95Fe0.05O2 and (c) Sn0.95Ni0.05O2 nanoparticles, respectively.

www.intechopen.com Aditya Sharma, Mayora Varshney, Shalendra Kumar, K.D.Verma and Ravi Kumar: 67

Magnetic Properties of Fe and Ni Doped SnO2 Nanoparticles To study the optical properties and the effect of Fe and Ni doping on the optical band gap of SnO2 nanoparticles, systematic, UV‐visible absorption spectra were recorded in the incident photon wavelength of 200 nm to 600 nm. The Fig. 4 shows the absorption spectra of SnO2, Sn0.95Fe0.05O2, and Sn0.95Ni0.05O2 nanoparticles, respectively. The inset of the figure shows the Tauc’s plot for determining the band gap energy of nanoparticles. The estimated band gap energy of un‐doped SnO2 is ~4.1 eV, while, the band gap energy of the Fe and Ni doped compounds (for x = 0.05) found to almost same and is ~ 3.87 eV. The observed band gap energy of un‐doped SnO2 nanoparticles is quite higher than the band gap energy of bulk SnO2 (3.6 eV). The appearance of such larger band gap energy is expected in SnO2 nanoparticles because of their small size of ~3 nm. This observation is consistence with the previously reported large band gap energy in Figure 4. (Color online) UV‐visible absorption spectra of SnO2, 2 1‐x x 2 SnO nanoparticles and nano‐rods [15]. Sn Fe O and Sn0.95Fe0.05O2 and Sn0.95Ni0.05O2 nanoparticles, respectively. The Sn1‐xNixO2, samples show slightly less band gap energy inset of the figure shows the Tauc’s plot of the same samples. than the un‐doped SnO2. The decrease in the band gap energy may be due to the accumulation of donor energy levels of TM ions in the actual band gap of SnO2. Such band gap narrowing is also observed in case of Co doped ZnO thin films [16] and is attributed to the presence of Co at the cation site in the host matrix. In present case also, Fe and Ni are expected to present at Sn site of SnO2 lattice, which is leading to the observed band‐gap narrowing. It is clear from the above discussion that Fe and Ni ions have been incorporated in the SnO2 lattice and affect the semiconducting properties of the material by tailoring the band gap energy.

Figure 5. (Color online) Room temperature hysteresis curves of Sn0.97Fe0.03O2, Sn0.95Fe0.05O2, Sn0.97Ni0.03O2 and Sn0.95Ni0.05O2 nanoparticles. Inset of the figure shows hysteresis curve of un‐ doped SnO2 nanoparticles.

To probe the magnetic properties of SnO2, Sn0.97Fe0.03O2, Sn0.95Fe0.05O2, Sn0.97Ni0.03O2 and Sn0.95Ni0.05O2 nanoparticles, room temperature magnetization measurements were performed on the samples and are shown in Fig. 5. Inset of the Fig. 5 shows hysteresis loop for un‐doped SnO2 nanoparticles. It is clear from the inset of the Fig. 5 that un‐doped SnO2 show diamagnetic behaviour. This behaviour of un‐doped SnO2 arises due to the 4+ valance state of tin (Sn4+) which favours 4d10 electronic configuration of Sn in SnO2 and, hence, there is no un‐ paired d electrons in the materials for any kind of ferromagnetic ordering. Ferromagnetic hysteresis loops were observed in Fe and Ni doped SnO2 nanoparticles. The saturation magnetization and magnetic moments per

Figure 3. EDAX spectra of (a) SnO2, (b) Sn0.97Fe0.03O2 (c) Fe/Ni ions were found to decrease with increasing in the Sn0.95Fe0.05O2 (d) Sn0.97Ni0.03O2 and (e) Sn0.95Ni0.05O2, respectively. Fe and Ni concentration. The observed magnetic

68 Nanomater. nanotechnol., 2013, Collection of Selected Papers, 65-69 www.intechopen.com moments are 0.029 μB/Fe, 0.014 μB/Fe, 0.028 μB/Ni, and 6. References 0.015 μB/Ni for Sn0.97Fe0.03O2, Sn0.95Fe0.05O2, Sn0.97Ni0.03O2 and Sn0.95Ni0.05O2 nanoparticles, respectively. The higher [1] G. A. Prinz, Science 282, (1998) 1660. magnetic moments, in case of x = 0.03 Fe and Ni doped [2] S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. samples, are may be due to the indirect exchange Daughton, S. V. Molnar, M. L. Roukes, A. Y. interaction among TM ions, mediated by O ions. As the Chtcheljanova and D. M. Treger, Science 294, (2001) TM ion concentration increases, the nearby two or more 1488. TM ions are expected to come closer sufficiently. If such [3] David D. Awschalom and Michael E. Flatté, Nat. Phys. TM atom pairs are present in the SnO2 lattice, the well 3, 153 (2007). known super‐exchange interaction is expected between [4] J. K. Furdyna, J. Appl. Phys. 64, R29 (1988). them. The super‐exchange interaction may lead to the [5] Sanghoon Lee, J.‐H. Chung, Xinyu Liu, Jacek K. anti‐ferromagnetic type interaction among neighbouring Furdyna, and Brian J. Kirby, Mat. Today 12, 14 (2009). TM‐ions, leading to the observed decrease in magnetic [6] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. moment with increasing TM concentration. The decrease Ferrand, Science 287, 1019 (2000). in the magnetic moments of Fe and Ni ions, with [7] D. C. Look and B. Claflin, Phys. Stat. Sol. (b) 241, 624 increasing their doping concentrations, is consistent with (2004). our previously reported results of Co‐doped SnO2 [8] A. Punnoose, J. Hays, V.Gopal and V. Shutthanandan, nanoparticles [17], where we have observed a little Appl. Phys. Lett 85 (2004) 1559. reduction in the magnetic moments of Co ions, with [9] S. B. Ogale, R. J. Choudhary, J. P. Bhuban, S. E. increasing their doping concentrations. Therefore, the Lofland, S. R. Shinde, S. N. Kale, V. N. Kulkarni, J. observed room temperature ferromagnetism in Higgins, C. Lanci, J. R. Simpson, N. D. Browning, S. chemically synthesized Fe and Ni doped SnO2 samples is D. Sarma, H. D. Drew, R. L. Greene and T. intrinsic to the material and confirm the formation of Venkatesan, Phys. Rev. Lett. 91, (2003) 077205. SnO2 based DMS systems. [10] Y. Xiao, S. Ge, Li Xi, Y. Zuo, X. Zhou, B. M. Zhang, Li Zhang, C. Li, X. Han, and Z. C. Wen, Appl. Sur. Sci. 4. Conclusions 254, (2008) 7459. [11] J. M. D. Coey, A. P. Douvalis, C. B. Fitzgerald and M. Fe and Ni doped SnO2 nanoparticles have been Venkatesan, Appl. Phys. Lett. 84, (2004) 1332. successfully synthesized using simple wet chemical [12] K. Anandan, S. Gnanam, J. Gajendiran, and V. method. Experimental findings indicate that Fe and Ni Rajendran, J. Non‐oxide Galsses, 2 (2010) 83. are incorporated into SnO2 lattice without forming any [13] L. Jiang, G. Sun, Z. Zhou, S. Sun, Q. Wang, S. Yan, H. TM cluster and/or oxide phases. TEM results indicate that Li, J. Tian, J. Guo, B. Zhou, and Q. Xin, J. Phys. Chem. very narrow distribution of spherically shaped B 109 (2005), 8774. nanoparticles (~3 nm) can be achieved by using co‐ [14] H. Rath, P. Das, T. Som, P.V. Satyam, U. P. Singh, P. precipitation method. The optical band gap energy found K. Kularia, D. Kanjilal, D. K. Avasthi, and N. C. to decrease with increasing TM concentration, which may Mishra, J. Apll. Phys. 105 (2009) 074311. arise due to the formation of donor energy levels in the [15] S. Das, S. Kar and S. Choudhary, J. Appl. Phys. 99, actual band gap of SnO2. Remarkably, room temperature (2006) 114303. ferromagnetism has been observed in Fe and Ni doped [16] R. Kumar, F. Singh, B. Angadi, W. K. Choi, K. Jeong, SnO2. It is reasonable to state that the, single phase, TM J‐H. Song, M. W. Khan, J. P. Srivastava, A. Kumar metal doped SnO2 based DMS materials can be and R. P. Tandon, J. Appl. Phys. 100, (2006) 113708. synthesized using the wet chemical method. [17] A. Sharma, A. P. Singh, P. Thakur, N.B. Brookes, S. Kumar, C. G. Lee, R. J. Choudhary, K. D. Verma and 5. Acknowledgements R. Kumar, J. Appl. Phys. 107 (2010) 093918.

Authors (Aditya Sharma, Mayora Varshney and K. D. Verma) are thankful to Inter University Accelerator

Center, New Delhi, India for providing financial assistance and experimental facilities under the UFUP projects (code‐41304 and 46301).

www.intechopen.com Aditya Sharma, Mayora Varshney, Shalendra Kumar, K.D.Verma and Ravi Kumar: 69

Magnetic Properties of Fe and Ni Doped SnO2 Nanoparticles