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Home , Graphene

Surface Science 605 (2011) 1607–1610

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Surface Science

journal homepage: www.elsevier.com/ locate/susc

Graphene — The ultimate surface material

1. Introduction extremely large densities of in computer chips and to produce many other high-power high-frequency electronic devices. Discoveries of allotropes have punctuated scientificand Flexible conductive coatings can be used to manufacture bendable technological advances at the interface of the 2nd and 3rd millennia. touch-screen displays and paints that would change color by One can argue that nanoscale science as a field has emerged at the end application of weak electric currents. can lead to new of the 20th century largely as a result of these discoveries. Every decade generations of solar cells [10], electric batteries [11] and over the past 30 years saw a new form of carbon created in a lab, giving [12]. In these systems the accessible surface area is particularly birth to an exponentially growing number of studies. In 1985 Kroto, important, because it defines the interface for charge separation and Heath, O'Brien, Curl, and Smalley obtained [1].Then,in1991 storage. Graphene is an ideal material in this case, since all are Sumio Iijima clearly identified carbon nanotubes within the prepared located on the surface. material [2]. Finally, in 2004 Novoselov, Geim, Morozov, Jiang, Zhang, Graphene-based can detect . Nanoscale open- Dubonos, Grigorieva and Firsov prepared electrically isolated, single- ings in graphene sheets and nanoribbons may provide new tech- layer graphene [3]. Within extraordinary short periods of time the niques for rapid DNA sequencing [13,14] that will achieve an seminal importance of these works was awarded with two Nobel extremely fine resolution of individual nucleotides due to the single prizes. The 1996 Nobel Prize in Chemistry was awarded jointly to thickness of the detector [15]. Artificial membranes made of Robert F. Curl, Jr., Sir Harold W. Kroto, and Richard E. Smalley “for their graphene [16] can be particularly efficient in separating liquids and discovery of fullerenes”. The 2010 was given to gasses. Plastic containers based on graphene can keep food fresh for and “for groundbreaking experi- weeks. The superb mechanical properties of graphene [17,18] can be ments regarding the two-dimensional material graphene”. utilized to produce longer-lasting medical implants, stronger wind Only one atomic layer thick, graphene is the ultimate surface turbines, lighter aircraft and better sports equipment. The possibilities material. It can be considered as an infinitely large aromatic for these fascinating applications, which can revolutionize our , and the limiting case of the family of flat polycyclic aromatic arise due to the unique chemical structure of graphene, which allows . The term graphene was introduced by Boehm, Setton, maximal surface area per mole of the material. and Stumpp as a combination of and the suffix, -ene[4]. For The experimental technique used by Geim and Novoselov to decades graphene remained solely a topic of theoretical investigation, obtain graphene flakes is amazingly simple [3]. While investigating as experimental characterization was considered difficult if not the electrical properties of graphite, they peeled layers off the surface impossible. In the 1930s Landau [5] and later Mermin [6] argued of graphite with Scotch tape. Eventually, single-atom-thick crystallites that two-dimensional should be thermodynamically unstable. were extracted and transferred onto thin dioxide on a silicon

The theory behind graphene was first explored in detail in 1947 by . The silicon electrode beneath SiO2 could be used to vary the Philip Wallace, who pointed out its unusual electronic properties [7]. graphene charge density over a wide range. At the same time, the SiO2 At that time, the substance was viewed as a starting point for research layer electrically isolated graphene, providing nearly charge-neutral on the more complex, three-dimensional graphite. material. Earlier, single layers of graphite were grown epitaxially on The two-dimensional nature of graphene combined with versatile top of other materials. However, there was a significant charge electronic states afforded by carbon's valence structure gives rise to a transfer from the substrate onto graphene. In many cases, hybridiza- variety of novel physical, chemical and mechanical properties that can tion between the orbitals of the substrate atoms and the π orbitals of revolutionize modern world. Graphene's most renowned properties graphene significantly altered the electronic structure of epitaxial include extremely large electrical and thermal conductivities. Graphene graphene. The electro-neutral graphene sheets produced by Geim and conducts electricity as well as copper. As a conductor of heat, it Novoselov led to the first observation of the anomalous quantum Hall outperforms most known materials. Graphene is so thin that it is almost effect, which proved that in graphene behaved as massless completely transparent, yet at the same time not even helium, the particles. This behavior is characteristic of photons and is extremely smallest gas atom, can permeate it. Graphene, the thinnest material unusual for electrons. The thrilling simplicity of generating high ever produced, is extremely flexible and strong, about 200 times quality graphene created an explosion of research activities. stronger than steel. The special issue of Surface Science devoted to graphene provides Graphene holds a great potential for profoundly transforming a snapshot of current activities in this exciting and rapidly evolving materials and surface science. It can replace silicon in electronics field. The goal is to exemplify the state-of-the-art in fundamental applications [8] and aid in production of electricity conducting plastics research on graphene carried out both theoretically and experimen- [9]. The single atom thickness can allow engineers to achieve tally. Since graphene was first investigated theoretically, many

0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.05.009 1608 decades prior to its experimental discovery, we start with theoretical simulations of self-assembly of graphene fragments in . Here, and computational studies. The focus migrates from fundamental the computational challenges involve both the large number of properties of graphene, to graphene and chemical derivatives, atoms needed to describe the process (tens of thousands) and the to graphene self-assembly and interactions with other materials. Then long timescale of the self-assembly (nanoseconds). Molecular self- we turn our attention to experimental work, covering a number of assembly draws significant attention in , as it techniques used for functionalization and characterization of gra- provides a tool for designing complex materials and biological phene and its derivatives aimed at a variety of applications. structures [22,23]. The authors find that parallel self-assembly of graphene sheets is initiated by entropic contributions to the 2. Theoretical studies of graphene thermodynamic potential. Then, as the sheets approach each other, capillary evaporation of water confined between the sheets The unique physical properties of graphene in combination with takes place. The entropic driving force becomes much stronger and the extremely rich chemistry of the carbon atom create a number of the sheets collapse onto each other. The enthalpic term, including theoretical challenges, making graphene one of the most exciting the potential energy of the interaction, remains weakly repulsive materials for a theoretical research. Theoretical characterization of the throughout the simulation. The results reported by Park and Aluru physical and chemical properties of any material typically starts with establish the dominant mechanism for self-assembly of nanoscale adefinition of an interatomic potential. This electronic potential systems containing graphene and provide a guideline for building ultimately determines and reconciles observed variations in a complex carbon-based . material's structure and associated dynamics and thermodynamics The origin of the attractive forces between graphene and other properties. Reliable interatomic potentials involving carbon are nanoscale objects is investigated in detail in the paper by Dobson[24], difficult to derive given that carbon can exist in electronic states who presents a detailed analytic examination of the ab initio with different orbital hybridizations, including those leading to techniques used to compute such forces. He points out that the extended π- systems. The carbon atoms of graphene are unusual nature of graphene is revealed not only in electronic sp2-hybridized and include easily polarizable π-electrons. In addition, transport, e.g. the anomalous quantum [3], but also in the defects and graphene derivatives can involve both sp and sp3 long-range interactions. Graphene is unusual from this point of view hybridizations. Long-range dispersive effects are particularly impor- due to the gapless semi-metallic nature of its electronic excitations, tant for interlayer coupling and interactions of graphene with resulting in a qualitatively different behavior compared to more substrate and adsorbed species. commonly encountered two-dimensional metals and insulators. Significant theoretical efforts are devoted to the development of Dobson argues that the long-ranged electron correlation physics ab initio and empirical interaction potentials for graphene. Ab initio should be described without assuming pairwise additivity. He derives potentials explicitly include electronic effects and naturally incor- analytic results for the large separation limit and shows that the porate all hybridizations of carbon. At the same time, ab initio random phase approximation [25] can give reliable data for shorter calculations are computationally demanding, especially if one distances. In conclusion, the author suggests definitive experiments, attempts to obtain a reliable description of non-covalent interac- such as graphite exfoliation and cleavage, which test the theoretical tions, such as dispersive effects and long-range π-electron polari- predictions. zation. Empirical potentials are much more computationally The papers by Krepel and Hod[26], and by Ouyang, Sanvito and efficient than ab initio Hamiltonians, and in principle, they can Guo[27] focus on the effects of atoms and edge chemistry on account for all types of interactions. On the other hand, empirical the properties of graphene and graphene nanoribbons [28,29]. potentials require careful parameterization against experimental Krepel and Hod investigate of atoms. Using data and ab initio calculations. An entirely different set of empirical density functional theory (DFT) they find that the interaction of Li parameters is typically needed not only for each chemically distinct atoms with graphene produces significant charge transfer from Li to atom, e.g. sp2 vs. sp3 carbon, but also for different chemical groups, graphene's π-electron system. The amount of the charge transfer is e.g. C_O vs. C_C. near maximum in the equilibrium geometry and decreases at both The work by Karssemeijer and Fasolino[19] employs an advanced long and very short distances. The charge transfer notably modifies class of empirical interaction potentials in order to investigate the the electronic properties of graphene and can be used, for instance, modes for a variety of systems derived from graphene. This to control electron transport through two-dimensional graphene type of potential is known as a bond-order potential [20].Itis and one-dimensional graphene nanoribbons. The adsorption pattern capable of describing not only equilibrium graphitic structures, but changes depending on Li concentration, and at sufficiently large also breaking and formation of bonds in structural phase transitions. densities, Li atoms form conducting bridges, transforming semicon- The authors show that this potential gives good results for graphitic ducting graphene nanoribbons into conductors. The study shows crystals in comparison with many other potentials, and suggest that modifications of the electronic properties of graphene by that it can be improved further by modification of the long-range adsorbents can be used as an accurate sensing mechanism that is interactions. The authors calculate graphene's bending rigidity, capable of detecting not simply the presence of adsorbents, but also which is the key quantity characterizing the mechanical properties adsorbent density. of graphene-based membranes. They rationalize why, contrary to Ouyang, Sanvito and Guo[27] exploit the chemistry of graphene liquid membranes, the bending rigidity of graphene increases with edges in order to achieve controlled n- and p-doping of graphene temperature and identify a specific phonon mode that is responsible nanoribbons and to study the changes in the electronic transport for this behavior. The authors establish that the bending rigidity per induced by doping. Using DFT in combination with the non- layer is the same for graphene and graphite and show how the equilibrium Green's function (NEGF) formalism for the transport graphene property arises in the limit of large diameter carbon calculations [30], they find that both n-doping by atoms and nanotubes (CNT). Karssemeijer and Fasolino predict that multilayer p-doping by atoms limit charge mobility along the quasi-one- graphene is characterized by several low frequency breathing dimensional graphene structure by inducing additional scattering modes that can be uniquely related to the number of layers and, channels in the conduction and valence bands, respectively. The therefore, used for identification of multilayer structures by Raman mobility scales linearly with nanoribbon width and decreases with experiments. increasing density. These findings are particularly Park and Aluru[21] use a simpler version of the empirical important for the electronics applications of graphene that require interatomic interaction potential in order to perform large-scale efficient charge transport. 1609

3. Experimental characterization of graphene and related surfaces SiC surface becomes fully covered with bi-layer graphene for a sufficiently large exposure to atomic . Further, the authors The surfaces of graphene and graphene derivatives are being discover that hydrogen intercalation is reversible. It can be used to thoroughly investigated by multiple physical–chemical experimental tune electron and hole mobilities and to generate p-doped graphene. techniques. They include transmission electron microscopy (TEM), In combination with Li-doping investigated in the theoretical work of scanning tunneling microscopy (STM) and (STS), atomic Krepel and Hod[26] introduced above, hydrogen intercalation can be force microscopy (AFM), X-ray diffraction (XRD), reflectivity (XRR) exploited to produce p–n junctions. and standing wave (XSW) analyses, X-ray photoelectron emission The other experimental papers features in this special issue focus microscopy (XPEEM), core-level photoelectron spectroscopy (PES), on graphene derivatives, such as graphene oxide, and use the angle resolved PES (ARPES), low-energy electron diffraction (LEED), graphene surface as a template for creating well defined arrays of , and ultrafast experiments. These tech- adsorbents, including metallic clusters and organic . niques developed and tested with other types of materials prove The work of Pandey et al. [36] investigates one of the many possible extremely valuable for characterization of various species and chemical derivatives of graphene. In this study, Raman spectroscopy structures derived from graphene. Many measurements are accom- in combination with STM and AFM is used to investigate the panied by theoretical calculations that assist in the interpretation of morphology of graphene oxide (GO) deposited from solution on the experimental data. highly-oriented pyrolitic graphene. They observe both tearing and Yeh and co-authors [31] use STM and STS in order to investigate folding of GO, occurring along high symmetry directions of the GO with atomic resolution the structure, strain and boundary effects of sheet. They argue that the extent of folding depends on the deposition graphene grown by chemical vapor deposition on a copper substrate rate, which can be reduced by a highly controlled deposition. Bent GO and graphene transferred onto . They discover large resembles CNTs, leading the authors to introduce the term “GO differences in the graphene structure depending on the substrate. nanotubes” (GONT). Compared to CNTs, GONTs are less stiff and tend carbon structures deposited onto copper are strongly to crack. This work is particularly interesting, since it anticipates a strained and exhibit ripples and regions of varying lattice structures. failure mode for graphene electronic devices when exposed to oxygen The boundaries between the structures are charged. The strain and other chemical reagents [37]. induced by the strong interaction of graphene with copper signifi- Sutter and colleagues [38] use monolayer graphene on a ruthenium cantly distorts the carbon lattices, producing strong pseudo-magnetic surface as a template to create arrays of Ru nanoclusters with a

fields. In contrast, transfer of graphene from copper onto SiO2 largely narrow size distribution. By controlling the graphene–substrate removes the distortions, reducing the average strain. The associated coupling with oxygen intercalation, they are able to establish whether charging and pseudo-magnetic fields are mostly eliminated, with the array assembly is determined by graphene alone or if the assembly residuals due to stable chemical defects. The study shows that the relies on the presence of a substrate. The results show large substrate nature of the substrate can have a very profound influence on effects on the deposition process as well as cluster growth. More graphene properties and, furthermore, illustrates why graphene precise cluster distributions can be obtained by coupling to the transfer onto SiO2 performed by Geim and Novoselov was essential substrate. Deposition on graphene decoupled from the substrate for their observation of the unique transport properties of pristine by intercalation produced large clusters with variable heights. By graphene [3]. coupling graphene to the Ru surface, the authors are able to obtain The time-resolved spectroscopic studies of Huang and collabora- clusters that are exclusively one monolayer high and contain 40–50 tors [32] further emphasize the importance of the substrate. They atoms. DFT calculations help the authors to establish the mechanism investigate the energy flow from hot charge carriers to phonon modes of the cluster nucleation. These results clearly show the advantages of and show that the rates of the electron–phonon relaxation depend the graphene surface. Being one monolayer thick, graphene provides strongly on the substrate. By comparing carrier dynamics in few- and an ideal assembly template [39]. It separates adsorbents from the multi-layer epitaxial graphene, graphite, and monolayer graphene on substrate without turning of the necessary and favorable adsorbent– glass, the authors find that the so-called hot-phonon effect is absent in substrate interactions. the few-layer epitaxial graphene on , while it is present The lateral and vertical structures of 0, 1, and 2 of a in the other samples. Generally, charge carriers lose their excess perylene derivative on the graphene/SiC substrate are investigated by energy within 0.2 ps due to strong coupling to graphene optical Emery et al. [40] using STM, STS, and XRR. Perylene is a typical organic [33]. In turn, optical phonons relax through anharmonic , which is used in electronics, and other coupling to acoustic phonons and substrate [34]. At high excitation applications. When deposited on graphene, perylene presents a densities a significant population of hot optical phonons is created. As chemically stable and electronically decoupled film. STM a result, the carrier cooling rate is slowed down, given that phonons reveals the highly ordered lateral structure of the perylene derivative, can no longer accept the electronic energy. By measuring the rates of while XRR confirms the characteristic π–π stacking bond lengths electron–phonon relaxation Huang et al. are able to characterize the indicative of weak perylene–graphene interactions. This work pro- strength of the graphene–substrate coupling and compare it to the vides valuable guidelines for organic functionalization of graphene interlayer interactions. The knowledge gained in this study is surfaces. Such functionalization is often needed in the design and particularly relevant for high-field electrical transport through fabrication of graphene-based devices [41]. graphene. Watcharinyanon and co-workers [35] expose graphene grown on 4. Conclusions silicon carbide to atomic hydrogen in order to decouple the two materials. As demonstrated by the time-domain study of Huang et al. The papers highlighted in the special issue provide a surface discussed above [32], this substrate strongly interacts with graphene. science perspective of the ongoing graphene research. The majority of Watcharinyanon and co-authors show that hydrogen atoms penetrate the theoretical and experimental studies presented here focus on through graphene and the carbon buffer layer. Core-level PES proves the interactions of graphene with substrates and adsorbents. Here, the that these bond to the substrate's Si atoms. STM images in theoretical challenges reside in the accurate calculation of the combination with changes in the LEED pattern reveal that the carbon interaction potentials, which involve a very diverse set of atoms, buffer transforms into a second graphene layer forming bi-layer molecules, and solid state materials and are important precursors for graphene. The APRES and XPEEM measurements uncover distinct the exploration of a variety of physical mechanisms. Experimentally, it features in graphene's electronic band structure, confirming that the is particularly important to achieve precise control, stability and 1610 reproducibility of the surface structures involving graphene. These [14] S.K. Min, W.Y. Kim, Y. Cho, K.S. Kim, Nature Nanotechnology 6 (2011) 162. [15] T. Nelson, B. Zhang, O.V. Prezhdo, Nano Letters 10 (2010) 3237. factors are essential for the future progress in the development of [16] S. Garaj, W. Hubbard, A. Reina, J. Kong, D. Branton, J.A. Golovchenko, Nature 467 graphene-based materials and devices. Given well-defined surface (2010) 190. structures, one will be able to take the next step and consistently [17] C.A. Marianetti, H.G. Yevick, Physical Review Letters 105 (2010). [18] E. Cadelano, P.L. Palla, S. Giordano, L. Colombo, Physical Review Letters 102 characterize charge transport, atom , energy dissipation, and (2009). other dynamical processes that form the physical basis for the new [19] L.J. Karssemeijer, A. Fasolino, Surface Science 605 (2011) 1611. technologies. [20] M. Aoki, Physical Review Letters 71 (1993) 3842. The flat geometry of graphene provides multiple opportunities for [21] J.H. Park, N.R. Aluru, Surface Science 605 (2011) 1616. [22] P. Maksymovych, D.B. Dougherty, Surface Science 602 (2008) 2017. designing two-dimensional assemblies composed of various catalysts [23] C.E. Castro, F. Kilchherr, D.N. Kim, E.L. Shiao, T. Wauer, P. Wortmann, M. Bathe, H. and molecules with specific functionalities. Fundamental studies of Dietz, Nature Methods 8 (2011) 221. surface interactions, charge transfer, charge transport, and other [24] J.F. Dobson, Surface Science 605 (2011) 1621. [25] S. Tretiak, S. Mukamel, Chemical Reviews 102 (2002) 3171. processes will be instrumental to the development of novel catalyst [26] D. Krepel, O. Hod, Surface Science 605 (2011) 1633. systems and sensing devices. Other types of graphene-based systems [27] Y. Ouyang, S. Sanvito, J. Guo, Surface Science 605 (2011) 1643. will emerge as scientists continue to explore two-dimensional carbon [28] B.F. Habenicht, O.N. Kalugin, O.V. Prezhdo, Nano Letters 8 (2008) 2510. [29] B. Biel, X. Blase, F. Triozon, S. Roche, Physical Review Letters 102 (2009). nanoscale structures [42]. Concepts resulting from the combination of [30] M. Galperin, M.A. Ratner, A. Nitzan, Journal of Chemical Physics 121 (2004) 11965. advanced experimental and theoretical efforts will provide intellec- [31] N.-C. Yeh, M.L. Teague, S. Yeom, B.L. Standley, R.T.-P. Wu, D.A. Boyd, M.W. tual cohesiveness to the burgeoning field of fundamental research on Bockrath, Surface Science 605 (2011) 1649. [32] L. Huang, B. Gao, G. Hartland, M. Kelly, H.L. Xing, Surface Science 605 (2011) 1657. graphene and its derivatives [43]. [33] B.F. Habenicht, C.F. Craig, O.V. Prezhdo, Physical Review Letters 96 (2006). [34] V.V. Chaban, T.I. Savchenko, S.M. Kovalenko, O.V. Prezhdo, Journal of Physical Acknowledgments Chemistry B 114 (2010) 13481. [35] S. Watcharinyanon, C. Virojanadara, J.R. Osiecki, A.A. Zakharov, R. Yakimova, R.I.G. Uhrberg, L.I. Johansson, Surface Science 605 (2011) 1662. Financial support of NSF grants CHE-1050405 and DMR-1035196 [36] D.K. Pandey, T.F. Chung, G. Prakash, R. Piner, Y.P. Chen, R. Reifenberger, Surface and DOE grant DE-FG02-05ER15755 is gratefully acknowledged. Science 605 (2011) 1669. [37] P.Y. Brisson, H. Darmstadt, M. Fafard, A. Adnot, G. Servant, G. Soucy, Carbon 44 (2006) 1438. References [38] E. Sutter, P. 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