Irnaging Silver Nanowire using Near-field Scanning Optical Microscope
Seung Yun Yang
A thesis submitted in conforrnity with the requirements for the degree of Master of Science - Graduate Department of Chemistry University of Toronto
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Seung Yun Yang The degree of Master of Science, 2001 Graduate Department of Chemistry University of Toronto
Abstract
Near-field scanning optical microscopy (NSOM) has become a widespread technique due to its promising capability to image with sub-micron resolution-
The first use of a new tapping-mode NSOM instrument developed in our group is presented here. The system is employed to image silver nanowires fabricated by a non-
Iithographic, electrochemical templating technique.
The technical problems of NSOM relating to the fabrication of the tip were investigated and solved. Appropriate combinations of the tip parameters were found to provide high transmittance of light and good resolution for both topographic and optical images obtained simultaneously. The improved spatial resolution and low detection let capabilities of the tapping-mode probe design are demonstrated through the topographic and optical NSOM images of nanowires.
The optical image reveals subwavelength opticai information on the nanowires and also indicates the possible distribution of optical fields in the vicinity of the nanowire. Acknowledgements
1 wodd like to thank Professor Martin Moskovits for his enthusiasm and guidance during my studies. 1wodd also like to thank the members of our group for their encouragement: Li-lin Tay, Anita Osika, Dr. Diyaa Mdowii, Dr. Dimitri Davydov, Ken
Bosnick and Tom Haslea. Finaliy, 1would like to thank my parents and my sister for their support and understanding, Table of Contents
page *. Abstract 11 Acknowledgements luS.. Table of Contents iv List of Tables M List of Figures vii
1. Introduction 1.1. Near-field Scanning Optical Microscopy (NSOM) 1.1.1. NOSM vs. conventional optical microscopy 1.1.2. Near-field optics 1.1.3. DZEaction limit 1.1.4. Feedback mechanism 1.1 S. Tapping mode NSOM - quality factor 1.1.6. Light source 1.2. Anodic aliuninum oxide films 1.2.1. Electropolishing of AIuminum 1.2.2. Mechanism of pore formation 1.2.3. Chemistry of pore formation 1.2.4. Electrodeposition of metal References
2. Experimental 2.1. Near-field spectroscopic Optical Microscope (NSOM) 2.1.1. NSOM tip preparation 2.1 -2. NSOM tip installation 2.1 -3. NSOM optical instrumentation 2.1.4. NSOM instrumentation 2.1 -5. NSOM operation 2.2. AAO template preparation 2.2.1. Ai Electropolishing 2.2.2. AI anodization 2.3. Silver deposition 2.4. Preparation of the SEM sample 2.5. Preparation of confocal microscopy sample
3. Results & Discussion 3.1. NSOM topography 3.1.1. ~i-ppreparation 3.1.2. Quality factor 3.1 -3. Gain and time constant 3.2. NSOM optical image 3.3. Extemal vibration isolation 3.4. NSOM imaging of siiver nanowire 3 -4.1. Control experiment 3 -42. NSOM imaging of silver nanowire 3 A.2.l. Silver nanowire topography 3A2.2. Dimension of silver nanowire 3.4.2.3. Silver nanowire opticai image combined with topography 3.5. SEM images of silver nanowires 3 .S. 1. AAO opthkation 3 52.Dimensions of silver nanowire 3.6. Confocal images of silver nanowires 3.6.1. Dimensions of silver nanowire References
Conclusions 63 List of Tables
Table 1. NSOM expriment condition Table 2. Pulling-Heating condition (tip preparation) Table 3. miiiing-Heating condition (near disconnecting point) Table 4. Additional etching Table 5. Measured dimensions of AFM grating Table 6. Quaiity factor variation Table 7. Measured dimensions of AFM grating with different quality factor Table 8. Measured dimensions of skewed image of AFM grating Table 9. Gain and constant variation Table 10. Measured dimension of fluorescent bail Table 1 1. Optical signal procession conditions Table 12. Summary: Dimension of Ag nanowires List of Fisrures
Figure 1. Schematic representation of achieving subdifEaction limit spatial resolution Figure 2. Six common NSOM configurations Figure 3. Types of NSOM probes a) shear force b) tapping mode Figure 4. Schematic diagram of Aliimiaum oxide film Figure 5. SEM micrograph of AAO template top and side view Figure 6. Cyclic voltammogmm response during the metai deposition Figure 7. Tip preparation a) Stripping the fiber b) etching process Figure 8. Tip installation Figure 9. Schematic view of NSOM Figure 10. Schematic diagram of anodization appratus Figure 11. SEM micrographs of silver deposited AAO a) side view b) top view Figure 12. Examples of SEM micrographs of fiee Ag nanowires Figure 13. Thinned fiber during the pdling process Figure 14. Tip produced using pulling-heating process Figure 15. Topographic images of AFM calibration grathg using tip A (a, b) and tip B (c, d) Figure 16. a) blunt tip before additional etching (x50) b) additional etching for 10 sec. (x50) c), d) after additional etching for 20 sec. (XIO) and (x50), respective1y Figure 17. Topographic images of AFM calibration grating Figure 18. Topographic profile of AFM grating image using figure 17.a) Figure 19. Topographic images of AFM grating with different quality factors
vii Figure 20. Topographic Mages of skewed image of AFM grating Figure 2 1. Topographic (a, b) and optical (c, d) images of fluorescence bds Figure 22. Improved optical 2-D (a, b) and 3-D (c) images of fluorescence balls Figure 23. Optical profile of fluorescence balls using figure 22.a) Figure 24. Topographic images with extemai vibration Figure 25. Images of blank cover glas Figure 26. NSOM topographie images of nanowires and profiles across nanowires Figure 27. Diameter of Ag nanowire measured with NSOM Figure 28. Length of Ag nanowire measured using NSOM Figure 29. Topographic, optical images and profiles of Ag nanowires Figure 30. Interference patterns in NSOM opticai image Figure 3 1. SEM micrographs of AAO Figure 32. Diameter and length of nanowires measured using SEM micrographs Figure 33. SEM micrographs of nanowires Figure 34. Length of silver nanowire using confocal microscope Figure 35. Confocal images of nanowires
viii 1. Introduction
The area of nanometre-scale science and technology has ken growing recently, due in part to the miniaturization of electronic devices."-" Nano-science provides new insights into the understanding of properties of structures with reduced dimensions and introduces new terms such as quantum wells (12), wires ('*') and dots.('") It has been proposed, for example, that nanowires with diameters of a few atoms may be used as single-atom digital switches in nanoelectronic It has ken observed that the conductance quantkation is sensitive to the adsorption of a molecule onto the nanowire, which may lead to applications in chernical ~ensors.('-~)Well-ordered nanowire arrays can be used as field emission displays and data storage.('-') For these and other applications, there is a growing need for imaging at higher spatial resoluti~n.(~-~)
Near-field scanning optical microscopy (NSOM) has attracted considerable interest as a technique for imaging surface details of samples with a resolution below conventional difhction limit~.('~'NSOM combines the benefits of optical microxopy with the high resolution of scanned probe imaging meth~ds.('-'~)It also retains some attractive features of optical techniques, such as non-destnictiveness, relative low cos& versatility and high contrast!-") Today, NSOM is still under development and commercial instniments have only just begun to appear.
In the present work, a home-built NSOM is used as a near-field instrument for visualization of silver nanowires. It was cotismicted by D. Davydov in this group previously. It employs a new configuration for the reading tip assembly. The shape of the probe tip is optimized to produce reasonable topographie and optical images. Topographie and opticd images of nanowires are recorded using this newly configured neôr-field scanning optical microscope. The measured dimension of nanowires determinecl by NSOM is wmpared to that of conventional SEM and confocal images. 1.1. Near-field Scanning Optical Microscopy (NSOM)
The abiüty to view samples under high magnincation is enomously important in many areas such as biology and material science.('*12)Approximately one hundred years ago, Lord Rayleigh dehed a universally accepted criterion for the maximum optical resolution of a Lens. Applying the classical Rayleigh criterion leads to the conclusion that a conventional optical microscope is capable of a resolution no better than approximately one haif wavelength (U2)of the light source used.(l-13)Using visible radiation, the theoretical resolution attainable is 200-300 nm, which is restrictive in many applications!1-14) In order to obtain an adequaîe understanding of the morphology of nano-size Structure, it is necessary to use a direct imaging technique that possesses chernical contrast and spatial resolution on the nanometer ~cale.('-~~)This motivated the development of near-field scanning optical microscope, which overcomes the diffraction limit of the resolution by scanning an optical probe in close pronmity to the ~arn~le.('-'~
In this method, the light is transmitted through a submicron aperture in an opaque screen and forms a subwavelength-size spot. This is schematicdly iliustxated in figure 1. This spot is scanned over to produce a high-resolution image. (1.14) incident radidion
t I one wavelength
Figure 1. Schematic representation of achieving subdiffiaction lirnit spatial resolution.
Ln an NSOM experiment, the sample placed in the x, y plane is raster scanned, while the sample/probe distance is controlled by the feedback mechanism. During a scanning process, near-field topographie and optical images are sirnultaneously recorded.
1.1.1, NSOM vs. conventionai o~ticalmicrosco~v
When an eleciromagnetic wave hits an object, the wave is difbcted into two cornponents: a propagating component with low spatial fkequency Vfi2h)and an evanescent component with high spatial fkequency fi>~h).('-'"The evanescent component decays exponentiaily with distance and is confhed to distances of the order of a subwavelength fiom the ~bject?'~)Conventional optical rnicroscopy employs the low spatial fiequency component to image an object. Subwavelength spatial resolution idormation can be obtained by NSOM, when the distance between the sensor and the surface is a few nanometres. There are six comrnon NSOM configurations (figure 2) that dominate most of the
research. In collection mode NSOM (A), the sample is iiluminated by a fa-field device
and the NSOM probe is used to collect Light. In illumination mode NSOM (e), light is
iiluminated with the tip and collected fiom beneath the sample in the fa-field. In
illiImination/collection mode (c),the sample is iliuminated with light through the tip and
light fiom the sample is coliected though the same tip. In oblique coilection mode NSOM
@), the sample is illuminated obliquely with a far-field device, and light is coiiected
through the tip aperture. In oblique iliumination mode NSOM (E), the sample is
illuminated with light fiom the tip aperture, and the signal is cokted obliquely with a
fa-field device. In dark-field mode NSOM CF), incident light is totally interndy reflected
fiom the substrate surface. In this way, an optical signal only from the region of the
sample in the near-field region is collected through the tip aperture in the near-field. In this study, the dark-field mode NSOM configuration is employed to collect the evanescent wave produced by a sample using total intemal reflection. Figure 2. Six common NSOM configuration^.('-'^) 1.1 -3.DBkaction limit
One of the assumptions used in formulating the Rayleigh difktction liniit is that one is operating in the far field, that is that the light has traveled a gr& distance fiom the
source as compared to its wavelength and to the size of the source. By placing a sub- wavelength-sized light source or detector (Le. "nano-objective") in the immediate vicinity of the sample, the interacting light will not be allowed to propagate to a distance where the diffraction Limit is applicab~e.'~-'~This rather complex concept may better be appreciated by considering the difnaction of light fkom a common difhction grating.
When a grating is illuminated by an incident beam, diEerent diffraction orders will propagate in different directions (a sample is illuminated by a monochromatic light, i.e. the incident Lght has only one wavelength). If the period of the grating is g-radually decreased, the difnsicted orders will propagate at a still larger angle. Eventually, as the penod becomes smaller than half the wavelength of the light, the Macted orders will propagate along the plane of the grating. Thus in this simple demonstration the difhction orders can no longer be observed at a distance; i.e. it looks as if it were smooth and homogeneous. These mctionorders that propagate along the grating plane belong to a certain type of waves classified as evanescent, and their amplitudes decay exponentialiy perpendicular to the plane of the grating."-'6)These evanescent waves, however, still contain information about the gratiag penod and they can be detected by perturbing them with the probe of a NSOM.
This phenomenon can be denved from Maxwell equations indicating that any form of electromagnetic radiation enclosed within a boundary will produce waves outside that boundary that decay at an exponential rate. If an object is close enough to that boundary
(within a fi-action of a wavelength), it will receive some of the evanescent radiation
Due to its size, a NSOM probe can be described as a spatial delta-fiinction for the emitted light intensity. Therefore, the Fourier spectnim contains a wide variety of spatial fiequencies. That is why a NSOM probe is a source not only for propagating light but also for non-propagating (evanescent) light. The forbidden light with its high spatial muencies contains most of the information on sample structures that are smaller thaa half of the wavelength.
When the tip approaches a sample surface and comes closer than the decay length for evanescent waves, the evanescent light is converted into propagating light. This light is radiated under supercritical angles, i.e. angles that are greater than the angle of total internai reflection for the dielectric sample. Collection of this forbidden light ailows obtaining additional information on the optical properties of a sample. Detection of the high spatial fiequencies is favorable not only because of the resulting higher resolution, but dso since these contain the k-vectors that can excite surface plasmons in thin metal films and excitons in semiconductors and i~olatorî.(l-'~)
1.1 -4. Feedback mechanism
A reliable distance detection scheme is an essentiai feature of any operational
NSOM ('-12). The NSOM tip must be positioned and held within nanometres of the sample surface during a scanning process. A common version of NSOM regdates the tip- sample distance using "shear force control".('-") In this mode, the tip is dithered at its resonance frequency in a direction parallel to the sample As the tip approaches to the sample surface, shear forces between the sample and the tip dampen the amplitude of the tip vibration.('-'*) This results in a decrease of the resonance ftequency. The amplitude is monitored and used to generate a feedback signal to control the distance between the tip and the sample surface. The achievable lateral resolution is determked by the amplitude of the vibration and the size of the aperture.(1.18)
In an alternative mode of operation, "tapping mode", the fiber tip vibrates perpendicular to the sample This mode takes advantage of a higher force gradient and, therefore, produces better resolution. However, in order to generate stable operation, relatively high quality factors of the tuning forkEber assemblies are required compared to those tolerable with "shear force mode9, .(120) (The quaiity factor, Q, is a measure of the sharpness of the resonance of a resonant, oscillating system. It is defïned as the ratio of the fiequency at resonance divided by the eequency-width of the amplitude versus fiequency function measured about the resonance fiequency.) Tapping mode and shear force mode of NSOM probes are shown in figure 3.
Figure 3. Types of NSOM probes a) shear force b) tapping mode. 1.1 -5. Tamina mode NSOM - auality factor
The performance of the tapping mode using standard tuning fork depends on the resonance quality factor of the tuning fork / optical fiber a~sernbl~.<'-'~)In our assemblies a hining fork with a (unloaded) resomt fiequency of 32,768 Hz is employed as a resooator in a quartz crystal osciilator. The quality factor is given by the peak fkquency divided by the width of the resonance at half of its maximum value (Q =f v~ /f hh)
''2? In airythe hining fork has a very high quality factor, -15000. The principal antisymmetric vibrational mode of the fork does not involve any motion of the tuniog fork's base in the direction parallel to the motion of the tine.('20) When the fiber tip is glued to the hining fork, the mas and the eessof one of the tines is changed causing the syrnmetry to break.(l2') This results in a drop in the quality factor, Q, by a factor of 2
- 100."~~)The degree of asymmeîry cm be chamcterized by the fkequency mismatch, &l
The fiequency difference 4f between the upper and the lower tines, is given by
Af/f =%(AK/K-AMfM), where M is the mass of the the; LU( and AM are changes in the effective force constant and the effective mass of the the due to the presence of the fiber, respectively, and K =
P;EI/L' is the effective force constant of a the with Young's modulus E, cross-sectional moment of inertia 1, length L, and vibrational overtone nurnber, n. Q is found to depend on &-as
l/Q = l/Qo + a~f2, where Qois the Q factor of the undisturbed tuning fork. The fiber tip should be attached in a way to minimize the fiequency mismatch between the two tines of the tuning fork, since the quality factor decreases as Afincreases. It has also been found empincally that it is crucial to maintain this façtor higher than 3000 to get stable The high Q factor makes the effective cornpliance of the oscillating fork small. It cmbe shown by the relation k = R /(d3~),where k is the static cornpliance and k e~ is the effective cornpliance when oscillated. The high Q makes the system very sensitive to small drag forces. Also, the minimum force detectable is proportional to Q? Therefore, the Q factor must be large if high sensitivity is to be achie~ed.('~)
1.1.6. Light source
The NSOM utilizes an argon laser as the light source. The wavelengths availabIe with an Ar laser typically consists of 10 discrete lines between 457 and 5 14.5 m. The laser light is coupled into an optical fiber which illuminates the prism. For dark-field mode NSOM, laser sources have two ad~anta~es.(l-'~)The low angular divergence of the laser results in a small range of incident angles and better control of the evanescent decay length!1-13) Also, a large laser flux can be incident on the wimple.('-") Typical laser powers that can be coupled into the fiber are dependent on the wavelength used, the coupling efficiency, and the effective aperture size of the reading tip.('-13) Increasing input power can result in the lem heating effects. 1.2, Anodic aluminum oxide films
Aluminm anodic oxidation has been shown to be capable of building an extremely highly ordered periodic structure of nanoholes by the anodization process. (1.23)
The pores have been found to be uniform and parallel, making anodic duminum oxide
(AAO) films ideal templates for nanometre-scale particles.('23)
Anodized alumülum has received much attention due to its self-ordered hexagonal array of cells with cylindrical pore.(124)These structures are a desirable material for microfabricated fluidic devices, quantum-dot arrays, polarizers, magnetic memory amiys, and photonic crystal. (125)
There are two types of anodic aliiminum oxides, the nonporous barrier oxide and the porous oxide. The barrier oxïde forms when Al is anodized in neutral or basic electrolyte (pH > s)!'.~) This barrier layer can act as a diode or rectifier aiter the oxide layer has go~n."~@Application of a positive (anodic) potential to Al in an acidic electrolyte results in the growth of an ordered porous film. Several acidic electrolytes can be use& bcluding sulfuric, phosphonc, oxalic, and chromic acid.(lm Parameters such as the pore length and pore diameter are dependent on the electrolyte used and the voltage applied.(13n The bottom of the pores consists of a thùi barrier layer over the metallic Al surface.(127)The geometry of anodic porous aluminum is schematically represented, as shown in figure 4. 1Aluminum owde
Figure 4. Schematic diagram of Aluminum oxide film.
The bottom of the pores consists of close-packed hexagonal cells and the pores grow perpendicular to the duminum s~bstrate.('-~An example of AAO template SEM image is presented in figure 5.
Figure 5. SEM micrograph of AAO template top and side view.
13 1.2.1. Electrowlishina of Aliuninurn
It is important to polish the daceof the metal before an anodin'ng, since the pore grows perpendicular to the surface. This can accomplished by an electropolishing technique, which reduces the roughness of the aluminum foi1 in the range fiom 5 pm to
3Onm t~~icall~.('-~)
1.2.2. Mechanism of Pore formation
Thompson and Wood (128) proposed the following mechanism for the formation of porous anodic alumina. After electropolishing, the Al has an almost flat dacewith small etch pits and bumps. The barrïer oxide film grows over the flat surface. As the Ai is anodized, pores are initiateci Çom the imperfection of the surface. This concentrates the electric field where the oxide film is thinner, which ai& the local dissolution of the oxide through a field-assisted etching process. The pore bottom deepens and a major pore forms. These coupled phenornena remove the oxide at the bottom of the pores and leave the pore walls intact. The barrier film growth rate is detennined at the meWoxide interface, while the oxide dissolution rate is deterrnined at the oxide/electrolyte interface.
Pores grow as a result of a steady state between field-assisted oxide dissolution at the oxide/electroIyte interface and oxide growth at the metaVoxide interface. The resulting structure is an ordered hexagonal array of celis with cylindrical pores with alumina cell wall.
1.2.3. Cherni- of wre formation
Al3+ions form at the metdoxide interface and migrate to the oxide layer. A~(s)+ AJ~+(~~~~,+ 3e- (1-1) The water-splitting reaction occurs at the oxide/electrolyte interface.
3/2 H20(l) i 3c(aq) + 3/202-(,,ide) (1-2) The 02-(oxide) ions migrate fiom the oxidelsolution interface to the metaVoxide interface
in order to form A1203. The protons generated by the water-splitting reaction cm locally
dissolve more oxide.
10 4203(s) + 3w(aq) AL^+(^^) + 3/2H20(Z)
Hydronium ions also migrate to the cathode releasing H2 gas and complete the circuit.
3H' (uq) + 3e- + 1/2H2(g)
Oxide produced through equation (1-2) builds the sidewalls of the AAO. The oxide is a
mixture of y-Ai203,y'-AlzOs, a-&O3, and amorphous w3-(1 28)
1-2.4. Electrodewsition of metai
Metal electrodeposition into porous aluminum oxide films has been used for
coloring aluminum, since this produces an aesthetically pleasing finish.('29) Metal can be
electrochemically deposited into the pores by imrnersing the anodized aluminum to an
appropriate electrolyte followed by AC electrolysis. As the electrodeposition proceeds,
metal fills the pore fiom the bottom, reproducing the shape of the pores. (1.29)
The barrier layer of AAO acts an electrical rectifier."") Current passes during the
cathodic direction and not the anodic direction. This rectifying property makes it possible to deposit metal into MO.Figure 6 shows a typical cyciic voltarmnogram (CV) ohtained
during metal deposition. According to this CV, the metal is deposited into the pore during the cathodic cycle and the deposited metal can be re-oxidized and removed during the anodic sweep. However, the re-oxidation is prevented in the pore due to the recwing property. The rate of metal deposition is reduced as the thickness of the barrier layer is increased.'l2' Also, the rate is substantially Iowered when the pore size is smaii.(12)
l
Voltage
Figure 6. Cyclic voitamrnogram response during the metal deposition- References
1.1. C. A. Huber, T. E. Huber, M. Sadoqi, J. A. Lubin, S. Manalis, and C. B. Prater, Science 263,800 (1994). 1.2. R. E. Ricker, A. E. Miller, D. F. Yue, and G. Bane jee, J. Electron. Mater. 25, 1585 (1 996). 1.3. D. AlMawiawi, N. Coombs, and M. Moskovits, J. Appl. Phys. 70,4421 (1 99 1). 1.4. D. E. Jesson, K. M. Chen, and D. J. Pemycook, MRS Bull 2 1-4:3 1 (1 996). 1.5. D. P. E. Smith, Science 269,371 (1995). 1.6. C. 2. Li, H. Sha, andN. J. Tao, Phys. Rev. B. 58,6775 (1998). 1.7. L. Chico, V. H. Crespi, L. X. Benedicf S. G. Louie, and M. L. Cohen, Phys. Rev. Lett. 76,971 (1996). 1-8. F. E. Scire anci E. C. Teague, Rev. Sci. Instmm. 49, 1735 (1978). 19. W. De& and D. W. Pohl, J. Vac. Sci. Technol. B9,5 10 (1991). 1.10. E. Betzig, P. L. Finn, and J. S. Weiner, Appl. Phys. Lett. 60,2484 (1992). 1.1 1. E. Betig, J. K. Trautman, T. D. Harris, J. S. Weiner, and R L. Kostelak, Science 25 1, 1468 (1991). 1.12. M. A. Paesler and P. J. Moyer, Near field Optics, John Wiley & Sons, New York (1 996). 1.13. E. Abbe, Archiv. f. Mikroskop. 9,413 (1 873). 1.14. E. H. Synge, Philos. Mag. 6,3556 (1 928). 1-15. E. H. Synge, Philos. Mag. 13,297 (1 932). 1.16. H. Heinzelrnann and D. W. Pohl, Appl. Phys. A59,89 (1994). 1.17. E. Betzig, P. L. Finn, and J. S. Weiner, Appl. Phys. Lett. 60,2484 (1992). 1.18. M. F. Garcia-Pmjo, J. A. Veerman, A. G. T. Ruiter, and N. F. Van Hulst, Ultramicroscopy 71,3 11 (1998). 1-19. C.Talley, M. A. Lee, and R. C. Dunn, Appl. Phys. Lem 72,2954 (1998). 1.20. K. B. Shelimov, D. Davydove, and M. Moskovits, Rev. Sci. Instnim. 71,437 (2000). 1.21. Y. Martin, C. C. Williams, and H. K. Wickramashinghe, J. Appl. Phys. 61,4723 (1987). 1.22. D. N. Davydov, K. B. Shelimov, T. L. Haslett, and M. Moskovits, Appl. Phys. Lett. 75,1796 (1999). 1.23. D. Al-Mawlawi, C. 2. Liu, and M. Moskovits, J. Mater. Res. 9, 1014 (1994). 1.24. F. Keller, M. S. Hunter, and D. L. Robinson, J. Electrochem. Soc. 100,411 (1953). 1.25. M. Saito, M. Kirihara, T. Taniguchi, and M. Miyagi, Appl. Phys. Lett. 55,607 (1 989). 1.26. M. M. Lohrengel, Mater. Sci. Eng. R11,243 (1993). 1.27. A. R. Despic, J. Electroanal. Chem. Interfacial Electrochem. 191,417 (1985). 1.28. G. E. Thompson, R. C. Furneaux, and G. C. Wood, Trans. Inst. Metal Finishing 56, 159 (1978). 1.29. S. Kawai and R. Ueda, Electrochem. Sci. Technol. 122,32 (1975). 2.1. Near-field Spectroscopic Optical Microscope (NSOM)
2.1.1. NSOM tir, pre~aration
Singie mode fibers (Newport, F-SV)with 4 Fm core and 125 pm cladding diameter were used to make the NSOM tip. A 1-1.5 cm length of the plastic jacket of the fiber was removed with a sharp blade at one end of the fiber. The fiber was protected firom etchant (HF) penetration under the plastic by depositing a protective layer of polymethylmethacrylate over the length of the fiber. The stripped end of the fiber was immersed into a 40% aqueous HF solution for approximately 20 minutes. This reduced the fiber thickness to approximately 60 p.It was then mounted in the groove on the commercial spark fiber splicer. The position of the fiber was adjusted to be coaxial with the splicer, in order to produce a straight tip. The tip was formed by heating its thin portion with an arc discharge. The tips were pulled apart by the contraction of the fiber itself. The end of the pded tip was protected with a layer of polymethylmethacrylate and immersed into HF solution until the desired thickness was obtauied. The protective layer was removed with acetone. This reduced the fiber thickness merto 2-3 pm with a tip radius less than 1 Pm. When the tip was not sharp enough, additional etching in HF for up to 20 seconds was performed. The tip preparation procedure is schematically illustrateci in figure 7. Figure 7. Tip preparation a) stripping the fiber, b) etching process.
2-1.2.NSOM ti~installation
Quartz microtuning fork crystals (Raltron, R3 8) with resonance frequency of
32,768 Hz and Q factors of approxhateïy 15,000 in air were used. The dimensions of the tuning fork tines are 3.5 x 0.6 x 0.25 mm. The metal cap was removed fiom the tuning fork using a small fiie. The tuning fork was installed into a tunllig fork holder and fixed with a set screw. The fiber was epoxy glued to the lower tine of the tuning fork (figure 8.
Point A) and at two spots, to the tuning fork holder (figure 8. Point B and point C). The fiber was placed perpendicular to the fork tines and the tip protnided 0.4-0.6 mm. The fiber tip was then installed onto the piezo-tube scanner. Two electrodes of the tuning fork were soldered to side pins.
Figure 8. Tip installation.
2.1.3. NSOM o~ticalinstrumentation
A laser beam was directed onto the entrance face of a total internal reflection prism. The light collected by the tip was directed toward a detector. A photomultiplier tube (PMT) was used as an optical signal detector. The PMT is very sensitive even at room temperature and ideal for hi&-sensitivïty optical detection.''-lQ The PMT output is a monotonie hction of the strength of the optical signal in the proximity of the sample. A pre-amplifier is used to convert the low level tunnelhg current between the tip and the sample to a high level, low impedance voltage sigoal, which can then be transmitted to appropriate electmnic The pre-amplifier was kept close to the tunnel junction to minimirre noise and to maximize signal-to-noise ratio.('24)There is a trade-off in circuit performance between bandwidth and As the bandwidth increases, noise increases. Reduced noise improves the resolution, yet it slows down the scanning rate. Longer acquisition times potentiaiiy increase image distortion due to sample drift.
Lock-in detection is used to improve the signal-to-noise ratio of the optical signais. A lock-in amplifier compares the fmt resonance fiequency of the unhindered fiber to the tapping signal The RHK unit compares the feedback signal to a reference value and varies the tip height to maintain a constant feedback signal, which roughiy corresponds to a constant tip height above the sample surface.
2.1.4. NSOM instrumentation
A hullng fork / optical fiber assembly is attached to a piezoelectric tube scanner.
In a piezoelectric materid, an induced stress causes an internai electric field in the material?-'6) Reversibly, an applied electric field induces a stress in the material. This stress manifests itseif as material deformation. This defonnation can be used to scan a sample with high precision. The scanner is coaxially mounted in an inchworm linear motor. An inchworm motor is used for coarse approach of the fiber tip to the sample surface. The =pie stage and the huiing fork assembly are mounted on top of an inverted microscope, which allows easy sample viewing and positioning. The probe is set manually to a spot on the sample that is illuminateci by the laser. A stabilized sinusoidal signal of about 4V peak-to-peak amplitude is applied to the input of the phase dete~tor."~)The phase detector consists of the multiplier and phase sensitive circuits.
Phase sensitive circuits utilinng two LC tmps to obtain the same resonance fiequency as that of the tuning fork wd. Such a circuit has a flat amplitude characteristic and a very steep phase characteristic around the resonant fkequency (fo 2 40 The phase sensitive detection is tuned in such a way that the phase shift of the signal £3 is 90 degrees with respect to the signal A (figure 9) when the tip is far fiom the SUrfaçe.
C=AxB=&sin(ot)xBocos(ot+~(f)) -+(f)
This makes the output signal of the multiplier zero. When the tip cornes to close proximity with the sample surface, the resonant fiequency of the tuning fork changes resulting in additional phase shift in signal B.('^) The semitivity of the phase detector is adjusted to 100 mV/Hz. The output voltage signal which is proportional to the tuning fork fiequency shift is then applied to the input of the RHK feedback control electronics. Figure 9. Schematic view of NSOM
2.1 .S. NSOM operation
The silver nanorod sample was deposited on a piece of a Pyrex slide and mounted on the surface of an attenuated-total-internai refiection (ATR) pnsm with a drop of an index-matching fluid. Laser light (514 nm) was chopped using a rotating wheel chopper before it was coupled into the fiber and focused into ATR prism. The experiments were performed under arnbient conditions. The sample was excited by the evanescent field of the totally intemally reflected iight. An input light power into the sample of -2 mW was used. Scanning areas 1 x 1 pn to 30 x 30 pm were scanned with a scanning speed of 700
ms per he.NSOM experiment condition is listed in table 1.
Table 1. NSOM experiment condition.
Images were processed with StiMage 386 software which includes linear and parabolic baseline subtraction. 2.2. AAO tempIate preparatïon
2.2.1. Al Electro~olishine;
Aluminum sheets with the purity of 99.99 % and 0.007" thickness were used.
Aluminum sheets were cleaned with acetone, ethanol, and deionized water for 10 minutes each in an ultrasonic bath. Samples were electropolished in a 5: 1 solution of
EtOH(95%)/HC104(60%) at 25V and 0-5 C for 15-30 seconds until a mirror-like surface was obtained. The polished samples were electrochemicdy anodized in a cell containing acid electrolyte and graphite counter electrode, while stirring vigorously.
2.2-2. Al anodization
Oxalic acid of 0.4 M was used as an acid electrolyte.
Method 1)
The Al was anodized at 0-5 C for 2 hours at a voltage of 4OV. The aluminum oxide film was sûipped in a mixture of 0.2M H2CrO4 and 0.4M of &PO4 for approximately an hour at 60-70 C and anodized again for nearly 20 hours. The applied voltage was gradually decreased to 8- 10 V at the end of the anodization to reduce the barrier layer thickness. AAO was placed in a solution of H3P04(0.1M) for 2 hours at 30C to widen oxide pores.
Method 2)
A voltage of 40 V was applied at 0-5 C for 20 hours for anodization. Pore widening was performed at 30 C with fiPo4 (O. 1M) for 2 hours. The aluminum oxide was re-anodized at 5-8 C for 1 hour using 20 V. The voltage was gradually reduced to 10
V when the desired anodization thewas reached. The sample was washed in distilled water for one hour. A schematic diagram of anodization apparatus is presented in figure
10.
Figure 10. Sc hematic diagram of anodization apparatus.
23. Siiver deposition
Silver was deposited by AC electrolysis using a solution of 1.5 g/L AgN03 and 40 g/L bonc acid. This was carried out at room temperature in an electrochemical cell containing sheet graphite counter electrodes. An AC potential of 12-15V with sine wave was applied, depending on the deposition rate. This was roughly deterxnined by the rate at which the sarnple darkened. SEM micrographs of silver deposited AAO are shown in figure 11. Figure 1 1. SEM micrographs of silver deposited AAO.
a) side view b) top view
Silver nanowires were liberated fiom AAO by etching the oxide in a mixture of CrOd
H3P04solution. This procedure only dissolved the oxide, and did not affect the aluminm or the silver particle. The aluminum piece with the oanowires then washed with distilled water few times. A drop or two of methanol soiution was placed on top of the duminum piece. As the solvent evaporated, the particles were dispersed on the surface. Figure 12 displays fkee silver nanowires in SEM images.
Figure 12. Examples of SEM micrographs of fiee Ag nanowires. 2.4. Preparation of the SEM sample
AAO sample was bent with tweezers to expose a kshsection of the porous film.
Top and side view images were obtained by attaching the bent sample to an aluminum stage. Free nanowires were sprinkled ont0 an aluminum microscope stub directly.
2.5. Prepamtion of codocal microscopy sample
Zeiss LSM410 Laser Scanning Confocal microscope, equipped with a Krypton-
Argon, mixed gas laser was used to image silver nanowire. Free nanowires were dispersed on a glas cover slide out of one drop of methanol solution. The cover slip was placed and the sample was inserted to a confocal microscope inverted. A drop of distillecl water was used as an index fluid between the cover slide and the objective lem (x68). A wavelength of 488 nm was used to excite nanowires 3. Results & Discussion
3.1. NSOM topography
3.1.1. T~Dpreparation
Proper NSOM tip preparation was essential for good images to be produced.
Various tip preparation conditions were attempted to get an optimal tip configuration.
The fiber tips were produced using commercial spark splicer. The pulse duration and
applied spark power were varied. This is summarized in tables 2 and 3.
Table 2. Puliing -Heating condition (tip preparation).
A thinned fiber (trial number 3) during the pdling process is depicted in figure 13. Figure 13. Thinned fiber during the pulling process.
Extra care was taken when the thinned portion of the fiber, which was almost invisible under the optical microscope, was handled. Reduced spark power and pulse duration
(table 3) was used to produce the sharpest tips possible.
Table 3. PuIling - Heating condition (near disconnecting point).
An applied spark power of below 10, bends the fiber rather than melting it. Perfect alignment of the fiber before pdling is very important in order to obtain sharp and straight tips. The fiber thickness and the tip sharpness were checked using the optical microscopy. The tip fadius was Iess than 1 pm and could not be resolved well.
Figure 14. Tip produced using pulling-heating process
Two different types of probe tips were produced using pulling-heating process and shown in figure 14. Tip A was pulied using the condition listed as trial number 3 of table 2. Tip
B was pulled using a same condition as triai number 7 of table 2.
An AFM calibration grating (607-AFM, Ted Pella) was used to confirm the spatial resolution obtainable with the NSOM tip. The replica grating has 2160 lines/mm (463 nm per line) arranged as a 2-dimensional lattice. 128 x 128 line images were collected.
Figure 15 a) and b) show topographic images scanned using tip A in the forward and reverse directions, respectively. Figure 15 c) and d) show topographic images scaaned using tip B in the forward and reverse directions. a) tip A forward motion b) tip A reverse motion
c) tip B forward motion d) tip B reverse motion
Figure 15. Topographie images of AFM calibration grating using tip A (a, b) and tip B (c, d).
Figure 15 shows four equivalent quality pictures of MMgrathg. The geornetry of the tip
did not seem to afIèct topographie resolution.
The quality of the tip was improved by additional tip etching in 40 % HF
solutions. The etching time was increased up to 20 seconds and the results were compared. DifYerent etching conditions are summarized in table 4. Also, probe tips after additional etching are shown in figure 16. Improvement of the sharpness is shown between tîps before additional etching (figure 16 a) and additional etching for 10 seconds
(figure 16 b). Etching time should be adjusted depending on the initial condition of the tip in order to produce the sharpest tip as possible.
Table 4. Additional tip etching Figure 16, a) blunt tip before additional etching (x5O) b) additional etching for 10 sec. (x50) c), d) after additional etching for 20 sec. (x10) and (xSO), respectively.
The topography of the AFM calibration grating was scanned using a blunt tip (figure 17 a, b) and a sharp one (figure 17 c, d). As expected, the resoiution of the topography was poor with a blunt tip. Using a sharp tip, each grating line was well resolved even in the enlarged images (figure 17 e, f). a) blunt tip forward motion b) blunt tip reverse motion
c) sharp tip forward motion d) sharp tip reverse motion
e) enlarged image of c) f) enlarged image of d)
Figure 17. Topographie images of AFM calibration grating. Dimensions of the AFM grating were measured by generating a profile across the sample surface. Figure 18 shows a cut through such an image. A dimension of the grating is determined by measuring a width of the peak at half of its maximum height in figure 18.
An average value of the dimension is listed in table 5.
Figure 18. Topographie profile of AFM grating image using figure 17. a).
Table 5. Measured dimensions of AFM grating.
The measuted dimensions of the AFM grating agreed well with the reported values. The sharpness of the probe tip was an important factor for good topographic resolution.
3.1.2. Oualitv factor
DifTerent quality factors were obtained with different tips. Depending on the tip, the tuning fork position was adjusteci and the fiber was bent to different degrees. DBerent quality factors were tried in imaging with a reasonably good tip. Varied quality factors are displayed in table 6.
Table 6. Quality factor variation.
The AFM grating was scanned using tips with Q at 2000 and with QS000. These images are shown in figure 19. Figure 19 a) and c) were generated during tip forward motion; figure 19 b) and d) were generated during tip reverse motion. A contrast of the resolution is shown depending on the quality factor used with a same tip. The grating features are
&ter resolved in images with Q>5000.
a)tip forward motion at Q = 2000 b) tip reverse motion at Q = 2000 c) tip forward motion at Q>5000 d) tip reverse motion at Q> 5000
Figure 19. Topographic images of AFM grating with different quality factors.
The dimension of the grating was measured by generating a topographic profile and the average values were presented in table 7. Again, a dimension was determined by measuring a width of the peak at half of its maximum height in the topographic profile.
Table 7. Measured dimensions of AFM grating with difEerent quality factors.
High qudity factor made the systern more sensitive to the feedback, which resulted in high spatial resolution. A quality factor greater than 3000 was required to resolve the grating pattem.
The position at which the tip was giued on the tuning fork also afEected the spatial resolution crucially. When the tip was not positioned perpendicuiar to the surfâce, a skewed image was obtained. Also, the image of the forward motion was sbifted fiom the image of the reverse motion. DBerent parts of the tip other than the sharpest tip end may have been used as a reading probe due to incorrect tip/tuning fork geornetry. This is illustratecl in figure 20 and the measured dimensions are listed in table 8.
a)tip forward motion b) tip reverse motion
Figure 20. Topographie images of skewed image ofAFM grating.
Table 8. Measured dimensions of skewed image of AFM grating.
3.1 -3. Gain and time constant
The gain and time constant of the feedback circuit were adjusted to ailow the tip to oscillate with a measUrable amplitude. Gain and time constant variation conditions are listed in table 9. Table 9. Gain and thne constant variation.
A high value of the gain results in a faster feedback response, but it also caused undatnped tip oscillations, if too much noise is present in the signal. The total time response time of the feedback loop depends on both the gain and time constant settings.
Low gain (O) with high tune constant (10 ms) combination produced better images than high gain with low time constant combinations This setting was stable throughout the image acquisition.
3.2. NSOM optical image
Proper coupling of the laser light into the single fiber was a crucial adjustment in obtaining good optical signais?-" Single fiber coupiing efficiency up to 5 % was attainable under good circumsfances. Laser power of approximately 2 mW after coupling was used to excite the nanowires. The NSOM tip was placed directly above where the laser light was illuminated as seen through the inverted optical microscope. Misplacing
NSOM tip would not produce any optical signal, since the laser light was very focused and the spot was small.
The optical resolution was tested by ninning control samples. The sample consisted of a closed-packed array of 477 nm polystyrene spheres doped with a fluorescent dye (Fluorebrite, Polysciences, Inc.). Tip A geometry (figure 14) was used. Topographic and optical images of control sample are shown in figure 21. Topographic images (figure 21 a, b) provided excellent resolution, showing the interstices between spheres clearly. In the optical images (figure 2 1 c, d), the lateral resolution was poor, since the optical signal originated fiom several adjacent spheres.
a) topography forward motion b) topography reverse motion
c) optical forward motion d) optical reverse motion
Figure 2 1. Topographic (a,b) and optical (c,d) images of fluorescence balls. A dimension of the sample was detennined by measuring the width of the peak at haif of
its maximum height in figure 22. A topography in figure 21 a) was used to determine the
size of fluorescence ball. This resuit is presented in table 1 1.
Table 10, Measured dimension of fluorescence ball.
Sensitivities of pre-amplifier and lock-in amplifier were varied to improve the optical
signal. This is surnmarized in table 10. Also, the prism was cleaned with HF solution
before the sample was placed in order to prevent light scattering fiom other sources. Tip
B geometry (figure 14) was used.
Table 11. Optical signai processing conditions
5 -O 10 25 10 40 20 5 20 40 50 100 90 70 Overload 500 450 200 Overload 100 Overload
Combination of 500 jAN and 50 mV produced near-field optical images with most clearly resolved fluorescence balls. Also, tip B geometry was favorable in producing good optical images. Optical images (fluorescence bail) produced using tip B geometry is presented in figure 22. The features are better recognized in 3-D images (figure 22 c).
a) optical forward motion b) optical reverse motion
c) optical 3-D reverse motion
Figure 22. Improved optical 2-D (a,b) and 3-D(c) images of fluorescence balls. The optical profile of the sample, generated fiom figure 22 a), is illustrated in figure 23.
Figure 23. Optical profile of fluorescence balls using figure 22. a).
Fluorescence contrast is one of the most important mechanisms in optical microscopy
since it is less susceptible to an optical artifa~t."~33' Therefore, it gives a good indication
that hi&-resolution image redts fiom the optical signal. This contrast mechanism was
successful showing the periodicity of fluorescence balIs. Optical contrast of 4V is shown
in figure 23.
33. External vibration isolation
The NSOM instrument should be shielded Fom external vibration when assessing the resolution and sensitivity of NSOM?" This is especially me, since nano-scaie
images at slow scan rates are our primary interest here. Vibration isolation was done by
suspending the instrument on four springs. When the instrument is on the table, only extemai vibration was obtained in the topography regardless of sample used. In figure 24, a typical NSOM topography with extenial vibration is shown. a) tip forward motion b) tip reverse motion
Figure 24. Topographic images with extemal vibration.
3.4. NSOM imaging of silver nanowire
3.4.1. Control emerirnent
Topographic and optical images of blank cover glas were obtained using NSOM (figure
25). Images a), c), and e)are obtained during the tip forward motions; images b), d), and f) are obtained during tip reverse motions.
a) topography tip forward motion b) topography tip reverse motion c) opticai tip forward motion d) optical tip reverse motion
e) 3-D topography f) 3D opticai image
Figure 25. Images of blank cover glas
The height changes associated with the suface was less than 12 nm in topography.
Optical intensity ciifferences over the surface was 1 V in optical image. This displays only random noise with an intensity of four orders of magnitude weaker in the optical image. The surface plots emphasize the flatness of the cover glas. 3.4.2. NSOM im&g of silver nanowire
3 -4.2.1. Silver nanowire to~ogra~hy
Images of silver nanowires were obtained with NSOM. Various configurations of nanowires were capîured with different magnifïcation. Image acquisition rate slower than
700 ms/line was not desirable since sample drift was fast.
Figure 25 a) shows topographic images in 30 x 30 parea Figure 26. b) and c) show zoomed images of some of the interesthg features in a). A few overiaid nanowires are observed. Also, topographic profiles across the surface were generated.
a. 1) topography tip forward motion a2) topography tip reverse motion
a3) topographic profile of the surface (indicated as A) b.a) topography forward motion b.2) topography reverse motion
b3) topographie profile of the surface (indicated as A) c.1) topography forward motion c2)topography reverse motion
c.3) topographic profile of the surface (indicated as A)
Figure 26. NSOM topographic images of nanowires and profiles across nanowires.
3.4.2.2. Dimension of silver nanowire
Dimensions of nanowires were measured using the NSOM topographic profiles
(figure 26) and iliustrated in figure 27. The diameter of nanowires was determined using
two rnethods. In lateral resolution, a width of the peak at half of its maximum height
represented a diameter of the nanowire in profile. In verîid resolution, a height of the
peak represented a diameter of the nanowire in profile. The background light scattering
fiom the cover glas was considered before setting the baseline. The distribution of the diameters is presented in figure 27. The average diameter of nanowire obtained was 300 nm in lateral and 25 nm in vertical resolution. A sample size of 12 nanowires was used for calculating the average values
-- a) lateral resolution b) vertical resohtion
Figure 27. Diameter of Ag nanowire measured with NSOM.
Measurement using vertical resolution produced more reliable data, when considering the size of the aluminum anodic pores (28 MI). Measurement dong x-y direction depends on the size of the probe radius. The base-width of the image is the sum of the outer diameter of the tip and the particle or nanowire diameter. However, z-direction movement is controlled by the feedback mechanism, which represents the height of the nanowire cross-section more accurately. The topographic features have base-widths > 300 nm, The width of the features suggests that the topographic image can be interpreted as an image of the tip recorded by significantly smaller Ag nanowires. Deconvolving the expected particle diameter, the topographic image implies that the effective tip radius is nominally about 300 nm wide. The topographic features were nominally 15-3 0 nm hi& in verticai resolution, consistent with the expected diameter of the nanowires. Some height ciifferences are twice or three times the height difference of others at places where nanowires are overlaid, an indication that single nanowire can be recognized. The length of the nanowire is determined in lateral resolution and the length distribution is presented in figure 28. The average length of the nanowire was 13 W.
O 5 10 15 20 Length of nanowire (micron)
Figure 28. Length of Ag nanowire measured ushg NSOM.
3.4.2.3. Silver nanowire optical image combined with towm~hy
Near-field opticaï and topographic images of siiver nanowires are show in figure
29. The two lefi-panels (a, b) are the topographic images; the two right-panels (c, d) are optical images. The images were obtained with a fiber tip, which produced reasonable topographic and optical images. Often a compromise was required in choosing a tip so as to obtain reasonable topographic and optical images atthe same time. This almost aiways meant that the topographic images obtained were somewhat poorer than the optimum obtaiaable (e.g. figure 26).
A nanowire of length 1Spm, was captured in figure 29 b). This same wire appeared shifted in figure 29 a). A shifi between topographies seems to occur due to following reasons. The geometry of the probing tiphning fork may not be perfectly symmetrical. Therefore, the aperture effective in the forward motion may be different
hmthe aperture operating in the reverse motion. There can be sample shift during the
image acquisition.
There was aiso a shift in the topography during the scan. This might be associated with
the change of the probe geometry due to the adsorption of a foreign particle onto the fiber
tip during the scan. It was already observed with the AFM calïbration grating images.
The change of the tip geometry could lead to a change in the topography.
The topographic profile determined across a nanowire (figure 29. b) is shown in
figure 29. e). The diameter of the nanowire using lateral resolution is 2 pm due to the
large tip radius. From the vertical measwement, the diameter of the same nanowire is
detenniEled to be 25 m.
The topography and the optical images of the nanowire are compared. Nearly
one-to-one correspondence between the optical and topographic resolution has been
obtained through two very different interactions, namely the &tical contrast and the tip
sample shear force.
The optical profile generated across a nanowire (fiom figure 29 d) is shown in
figure 29. f). Optical contrast of 4V was shown between the nanowire and the glas
surface.
An incompatibility was found between the characteristics of a tip that produced
good topographic and optical images. #en the tip aperture is very sharp, good topographic images were obtained. Tips that produced opfimal topographic images, however, possessed effective opticai apertures too small to generate good optical signals. ContrariWise, when the probe tip was blunt, the topographic resolution is poor, but the optical signais were good. A long etching process irnproved the sharpness, but decreased the optical near-field signal down to a level unacceptable for spectroscopy by opening up the aperture?) Tip configuration should be optimized such a way that the probing tip is srnail enough to generate good topographic image, at the same time big enough to feed optical signal through, a) topography forward b) topography reverse c) optical forward d) opticd reverse
e) topographie profile of the surface b)
f) optical profile across ananowire of the surface d)
g) optical profile alona a nanowire of the surface d)
Figure 29. Topographie, optical images and profiles of Ag nanowires.
54 Often, the optical images were shifted with respect to the cornpanion topographic image. It is suspected that different parts of the tip are involved in generatiag the topographic and the optical images. Aiso, adsorption of foreign particle on the probe may deflect the optical path. The optical image can give information about the structure of silver nanowire, It is assumed that nanowire reproduces structure of AAO template pore.
However, the optical profile almg a single nanowire shows the inhomogenity of the silver particles. The pronle of the optical images dong the nanowire (figure 29 g) show intensity differences of 2V. However, the optical image is to a large extent detemiined by the pattern of optical fields about the wire which will depend critically on location on or near the wire, as weU as on the polarhtion of the illuminahg light and the disposition of the wire with respect to the polarization direction.
One also needs to consider the possibility that the optical image results fiom optical artifacts, since the Mages produced resemble to some extent the topographic images shown in figure 29. The issue optid image tip artifact in NSOM has been discussed by several gro~ps.(3-6)According to these studies, the interferometric dependence of the rdected signal on the tip-sarnple separation can lead to the modulation of the optical intensity as the tip follows the nanoscopic sample top~gra~h~?.~Sandoghdar et al. found that there was a dependence of the optical resolution on the wavelength of the laser used?.') One might suppose that near-field rnicroscopy should be insensitive to the wavelength sioce it is the size of the aperture or scattering center which determines the resolution. The z-motion artifact generates feahues in the optical images that are highIy correlated with the structures in the topographic image. Stiii, this issue ca.only be resolved by testing with a sample with opticai but no topographic ~ontrast.(~-')
Some opticd images demonstrate periodic patterns dong the same direction
(figure 30). Such effects have been observed in near-field measurements of optical channel waveguides and directional co~~lers."~~*") Since the sample is king excited by a coherent source, scattered radiation could lead to an interference pattem. (3.10)
Operating NSOM in constant-gap mode (shear-force feedback control over the tip surface distance) can cause the other, non-periodical optical artifacts that often swamp any tme optical ~ontnist.(~-")It leads to the difEerence between the topographic and optical images of the same sample such that the optical image contains more features than the topography .
a) optical forward motion b) optical reverse motion
Figure 30. Interference patterns in NSOM optical image. Clearly more experiments mut be performed to gather sub-wavelength optical information on silver nanowires. Focusing laser Iight into smaiier spots is necessary to illuminate only one end of nanowire. Then, plasmon propagation dong the nanowire caa be studied. This result WUbe useful developing optoelectronic components that control the directionai flow of optical idonnation. Preliminary results of this sort have, in fact ken reported. (3.12)
3.5. SEM images of silver nanowires
3.5.1. AAO o~timization
AAO prepared by two different methods (page 23) are shown in figure 3 1. Method 1
(page 23) which produces more periodic AAO was used to produce the AAO employed in this experiment. The average size of AAO pores in SEM image was 28 nm.
a) using method 1 b) using method Figure 3 1. SEM rnicrographs of AAO. 3 -5.2. Dimensions of silver nanowire
The dimensions of nanowires were measured fiom the SEM micrographs. The average dimensions of the Ag nanowires was found to be 23 nm x 13 Pm- The distributions of the length and the diameter are shown in figure 32. A sample size of 20 nanowires was used to generate these distributions.
Figure 32.Diameter and length of nanowires measured using SEM micrographs.
SEM images of silver nanowires were dso used as a cornparison with the NSOM images.
Figure 33 shows SEM Mages of Ag nanowires. Figure 33. SEM micrographs of nanowires.
3.6. Confocal images of silver nanowires
3.6.1. Dimensions of silver nanowire
Dimensions of nanowires were obtained using confocal microscopy and illustrated in figure 34. A sample size of 14 nanowires was used to generate the distribution. The average length of nanowire was found to be 13 Fm. This value agreed well with the SEM resdt.
I Length of nanowire (micron) I
Figure 34. Length of silver nanowire using confocal microscope. Confocal images of nanowires were used for cornparison with NSOM images.
Figure 35 shows four typical confocal images of nanowires using 488 nm wavelen*
Figure 35. Confocal images of nanowires.
Width of nanowire cannot be measwed using confocal microscope, since confocal microscopy cannot surpass the diffrziction Mt. The apparent size of silver nanowKe was compared over three different methods and sbowed a good agreement. This is siimmarized in table 12. Table 12. Summary: Dimension of Ag nanowues, References
3.1. D. Courjan and C. Bainier, Rep. Prog. Phys. 57,989 (1984). 3.2. E. Betzig, R J. Chichester, Science 262, 1422 (1993). 3.3. E. Betzig, J. Trautman, Science 257,189 (1992). 39. E. L. Buckland, P. J. Moyer, and M. A. Paesler, J. Appl. Phys. 73, 1O 18 (1993). 3.5. L. Novotny, Appl. Phys. Lett. 69,3806 (1996). 3.6. S. 1. Bozhevolnyi, 1.1. Smolyaninov, and 0. Keller, Appl. Opt 34,3793 (1995). 3.7. V. Sandoghdar, S. Wegscheider, G. Krausch, and J. Mlynek, J. Appl. Phys. 8 1, 2499 (1997). 3.8. L. No~~toy,B. Hecht, and D. W. Pohl Ultramicroscopy 7 1,341 (1 998). 3.9 H. Bielefeldt, 1. Horsch, G. Krausch, M. Lux-Steiner, J. C. Meiners, and 0. Marti, Appl. Phys. A59, 103 (1994). 3.10. D. Coujion, I. M. Vigoureux, M. Spajer, K. Sarayeddine, and S. Leblanc, Appl. Opt. 29,3734 (1990). 3.1 1. S. 1. Bozhevolnyi, M. Xiao, and 0. Keller, Appl. Opt. 33,876 (1994). 3.12. Y. Chen and A. Pepin, Electrophoresis 22, 187 (2001). Conclusions
NSOM is a viable technique for elucidating structural and optical properties of
matends at the nanometric scale. It provides a powerful tool combining high spatial
resoiution in the optical images with sirnultaneous topographical information.
Several parameters of the probe tip were varied to produce optimal images using
NSOM. The reproducible and controllable fabrication of the aperture is one of the most
crucial issues for NSOM. Topography largely depends on the sharpness of the tip as weli
as the quality factor of the tuning forWfiber assembly. Optical resolution depends on the
shape and the size of the tip. The above four parameters should be balanced to produce
both topographic and optical images of sufficient qualitty.
The ability of the tapping-mode NSOM probes to track small topography features
in ambient environment has been demonstrated. Images of nanowires illustrate the abilr'ty
of the probe tip to track small height changes (15 nm). When imaging well-resoived
srnail features, the lateral resoiution aîtained by our NSOM in topographic images is 300
. nm, essentidy reflecting the dimensions of the probe tip. Topographic images of
nanowires are compatible to that of SEM and confocal microscopy. As expected, there
was no simple correlation between the topographic and optical images of silver
nanowires.
Despite king developed over more than a decade ago, NSOM is still a non-
routine technique. The improvement of the instrumental configuration of NSOM is a
significant goal whenever it serves to improve the Lateral resolution of the images, or
profits fiom the simultaneous recording of optical and topographic images.