Electronic Functionality in Graphene-Based Nanoarchitectures

PERSPECTIVE pubs.acs.org/JPCL

Electronic Functionality in Graphene-Based Nanoarchitectures: Discovery and Design via First-Principles Modeling Aijun Du and Sean C. Smith* Centre for Computational Molecular Science (CCMS), Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, AIBN Building 75, QLD 4072, Brisbane, Australia

ABSTRACT Graphene has promised many novel applications in nanoscale electronics and sustainable energy due to its novel electronic properties. Computational exploration of electronic functionality and how it varies with architecture and doping presently runs ahead of experimental synthesis yet provides insights into types of structures that may prove profitable for targeted experimental synthesis and characterization. We present here a summary of our understanding on the important aspects of dimension, band gap, defect, and interfacial engineering of graphene based on state-of-the-art ab initio approaches. Some most recent experimental achievements relevant for future theoretical exploration are also covered.

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he 2010 Nobel Prize in Physics was awarded to Andre Geim and Kostya Novoselov for the discovery of graphene, a one-atom-thick sheet of carbon atoms arranged in a honeycomb pattern. Graphene is expected to be important for practical applications in nanoelectronics and sustainable energy due to its unique electronic structure. Currently, a variety of novel synthesis methods including mechanical exfoliation,1,2 reduction of graphene oxide,3,4 chemical vapor deposition5 and thermal decomposition of silicon carbide6 have been developed. Experimentally, low-dimensional nanoscale materials are difficult to manipulate and characterize because of their tiny size, which raises the conundrum of how to proceed forward quickly with exploration and subsequent targeted design of properties. State-of-the-art ab initio calculations are able to address fundamental questions of structure, stability, and electronic properties of graphene-based nanoarchitectures. By shedding light “from the bottom up”, the calculations suggest what types of structures could prove useful for achieving targeted properties. This provides a powerful complement to experimental synthesis and characterization, synergistically unraveling new ways of designing and exploiting electronic functionality in graphene-based materials. In this Perspective, we intend to highlight some important theoretical aspects of engineering graphene through controlling dimensionality, tuning band gaps, creating defects, and forming interface nanocomposites. Advanced theoretical modeling approaches are seen not only to be useful in facilitating the interpretation of important new experimental findings but also in predicting new properties. Graphene nanoribbons (see Figure 1a) have been the focus of intensive research following the discovery of graphene due

r 2010 American Chemical Society

1D graphene nanoribbons have been extensively studied by ab initio approaches, but edge magnetism, charge/spin transport, and excited-state properties still need a greater exploration. to the intriguing properties that accompany the reduced dimensionality of the ribbon morphology. Pioneering studies revealed that zigzag graphene nanoribbons display antiferromagnetic coupling in the ground electronic state, with a band gap that decreases as the ribbon width increases.7,8 Most recently, morphology, synthesis, and application of graphene and graphite nanoribbons have been reviewed by Terrocenes et al.9 These studies have provided a qualitative way of determining the electronic properties of ribbons with widths of practical significance. Additionally, the zigzag edge of graphene nanoribbons has been predicted to have higher chemical reactivity than the armchair edge,10,11 which is consistent with recent experimental observations of the closure of graphite edges and formation of fullerenes directly from graphene fragments.12,13 Metal-free magnetism in graphene nanoribbons has been predicted to be strongly dependent on the crystallographic orientation of their edges.14,15 Received Date: September 28, 2010 Accepted Date: December 20, 2010 Published on Web Date: December 23, 2010

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DOI: 10.1021/jz101347a |J. Phys. Chem. Lett. 2011, 2, 73–80


transport along the nanoribbons, leading to spin-anisotropic transport characteristics.27 Recently, Biel et al. have revealed an anomalous doping effect, that is, resonant backscattering on the charge transport in graphene nanoribbons.28 The effects of edge disorder and roughness on the chargetransport properties have been demonstrated to be significant in graphene naoribbons.29,30 An intriguing study by Cruz-Silva et al. reported formation and quantum transport within graphene loops and scrolls that predominate most experimental observations in multilayer graphene, providing a nice example of the study of graphene-based architectures using a first-principles approach.31 The above studies notwithstanding, spin transport in graphene nanoribbons in the presence of vacancies and/or dopants remains nearly unexplored. Computer packages encoding the nonequilibrium Green function method are available and will facilitate future theoretical explorations.32,33 Graphene nanodots possess discrete energy levels and may have tunable optical properties. Arbitrarily shaped finite graphene nanodots are found to display large net spin in the ground state due to the topological frustration.34 Recently, Yan et al. have successfully synthesized graphene quantum dots with various shapes and tunable size, exhibiting efficient photovoltaic properties.35-37 In this context, the exploration of excited-state properties in graphene nanodots will be an important research direction. Bearing in mind the considerable difficulty of accurate excited-state simulations, this is likewise an important theoretical challenge.

Figure 1. (a) Two-dimensional graphene nanosheet with a onedimensional nanoribbon and zero-dimensional nanodot. (b) Schematic illustration showing the zigzag graphene nanoribbon (green) connected to two ferromagnetic electrodes (blue). Both electrodes are subjected to applied magnetic fields (red arrows), which results in magnetization on both electrodes. The difference in the electrostatic potentials between the electrodes leads to a bias voltage, which generates a current.25 (b) Reproduced with permission of Nature Publishing Group.

However, as of yet, there is no direct experimental evidence demonstrating such edge magnetism in graphene nanoribbons. An in-depth understanding of magnetic correlation and edge roughness has to be developed. Spintronics seeks to exploit the spin degree of freedom in addition to the charge of electrons. A zigzag graphene nanoribbon has been predicted to carry a spin-polarized current under the response of an external electric field.16 Such halfmetallicity can be significantly enhanced by chemical doping at the zigzag edge even in the absence of an electric field.17-21 Tunable half-metallicity has been recently predicted and attributed to a strong dipole moment at the interface between graphene nanoribbons and polyvinyl-lidene fluoride.22 However, the predicted half-metallicity has never been reported experimentally due to the difficulties in controlling the edge configuration with atomic precision and significant edge reconstruction.23 Up to now, most studies have been focused on zigzag graphene nanoribbons. However, it is expected that armchair nanoribbons may also become half metallic upon appropriate functionalization. Additionally, a variety of polyaromatic hydrocarbons are found to display an antiferromagnetic ground state as that in zigzag graphene nanoribbons, thus offering an interesting study as the size limit for graphene nanoribbons.24 Looking toward building realistic electronic devices, charge and spin transport in graphene nanoribbons are crucial issues. A very large value of magnetoresistance in a zigzag graphene nanoribbon has been predicted by modeling a ribbon between two ferromagnetic contacts, forming a magnetoresistive junction,25 as shown in Figure 1b. Graphenenanoribbon-based field effect transistors with high on/off ratios have been predicted by first-principles transport calculations using the nonequilibrium Green function method.26 Substitutional B atoms in graphene nanoribbons have been shown to act as scattering centers for electronic


Opening a gap in graphene for building field effect transistors remains a challenge and will be informed by an in-depth understanding from theoretical modeling. Graphene is a promising candidate for making a truly smaller and faster transistor because its electron mobility is an order of magnitude higher than that of silicon. Experimentally, a prototype graphene field effect transistor was first fabricated with mechanically exfoliated graphene on silicon dioxide.38 Recently, significant progress has been made toward building “real world” graphene devices.39,40 However, they displayed ambipolar conduction with a small-current on/ off ratio. A formidable problem is the lack of an obvious “band gap”,41 resulting in difficulties controlling the carrier type. Currently, several strategies including exerting strain, creating a potential difference in bilayer graphene, patterned hydrogenation, and sculpting into nanoribbons have been proposed to open a gap in graphene. Initially, uniaxial strain was predicted to open a small gap in graphene; however, subsequent studies indicated that this will not work until the strain is as large as 26.5%, which is currently unreachable

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connection graphene and BN nanoribbons.62 Importantly, the most recent experiments demonstrated a similar role (gap opening) in a hybridized graphene and BN sheet, as shown in Figure 2d.63 Theoretical explorations are urgently needed to address how the gap is opened for hybrid C/BN superlattices with different sizes and shapes. Most recently, the coronene molecule has been used to fabricate a field effect transistor with high performance due to an improved electronic band gap compared to that of graphene.64 The current study can be enlarged into the exploration of new types of graphene molecules, opening many possibilities for the next generation of ultra-high-speed electronics.

Theoretical explorations of defect chemistry in relation to the most recent experimental findings are needed.

Figure 2. (a) The optimized geometry for a graphene with patterned hydrogenation.45 (b) Band structure for graphene at different coverages of hydrogen atoms.45 (c) Porous graphene opens a gap (Reprinted from ref 57). (d) C/BN hybridization in graphene.63 (a), (b), and (d) reproduced with permission of Nature Publishing Group.

experimentally.42,43 Nevertheless, the effect of shear and multiaxial strain could be more promising for future theoretical exploration. Second, opening a gap in bilayer graphene under response to an external electric field could, in principle, be achievable;44 however, no direct experimental evidence has been reported as of yet. Third, patterned hydrogenation of graphene has recently been predicted to be a very promising approach to a tune the band gap by modifying the coverage of atomic hydrogen on graphene.45-49 Figure 2a and b presents graphene with patterned hydrogenation and the corresponding band structure for graphene at different coverages of hydrogen atoms, respectively. Finally, the most obvious and promising solution could be cutting graphene down to nanometer-sized graphene nanoribbons. Metal nanoparticle etching methods have been reported,50 but the most viable large-scale routes could be the unzipping of multiwalled carbon nanotubes or graphene directly from the edge into graphene nanoribbons.51-54 The mechanism for unwrapping remains, however, fully unclear, and in-depth understanding of the underlying electronic functionality is urgently needed. Such a simulation has to be carried out in a large system and for a lengthy time scale, far beyond the current limits of popular density functional methods. The self-consistent charge density functional tight-binding (SCCDFTB) molecular dynamics approach may prove to be more practical.55 Most recently, some new paths for opening a gap in graphene have been proposed. We have recently performed electronic structure calculations for porous graphene, that is, graphene with uniformly distributed pores.56 State-of-the-art hybrid functional calculations predicted that such porosity in graphene will open a gap,57-59 as shown in Figure 2c. Additionally, the band gap in hybrid C/BN materials has been reported to be tunable by changing the atomic ratio in the C/BN configuration60,61 Here, the C-B and C-N interfaces play a critical role, as has been illustrated by modeling the formation of hybrid C/BN zigzag nanotubes via the hybrid


Vacancies are a typical defect in graphene, particularly when it is produced from the reduction of graphene oxide via high-temperature thermal annealing. The chemical, electronic, and magnetic properties of graphene may potentially be tailored through vacancy-controlled engineering. Theoretically, vacancies containing large holes with various shapes in graphene have been reported to display room-temperature ferromagnetism with potential applications in novel magnetic devices.65 A new type of defect, a carbon spiral helix, derived from monovacancies in graphene was recently predicted to be the most stable configuration.66 Systematic theoretical work has recently predicted how vacancy defects might be controlled in graphene during the reduction of graphene oxide via a thermal annealing strategy.67 Most recently, vacancy clusters in graphene have been predicted to serve as quantum dots, which opens up tantalizing new possibilities for 2D nanoelectronics.68 Experimentally, vacancies have been revealed to be a source of ferromagnetism in graphene.69 An extended linear defect in graphene sheets is found to act as a quasi-one-dimensional metallic nanowire, which may form building blocks for atomic-scale carbon nanoelectronics.70 In Figure 3a and b, we present, respectively, the optimized geometry and density of state plots for a new type of one-dimensional defect on graphene. Despite the above theoretical and experimental studies, an in-depth understanding on vacancy-based defect chemistry, for example, how vacancies affect the electronic structure, transport, and collective magnetism, is urgently needed. Adding heteroatoms or molecules onto graphene is another important strategy to modify the electronic structure of graphene. For example, typical dopants (N and B) will respectively inject electron and hole defects into graphene-based materials. Thus, transport and gas adsorption properties are expected to be significantly enhanced, as demonstrated in recent experiments.71,72 The adsorption of gold or bismuth atoms onto graphene will greatly shift the Fermi level, leading

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Figure 3. (a) The optimized geometry and (b) density of state plots for a new type of one-dimensional defect on graphene.70 (c) Doping of hydrogen in graphene (semihydrogenation) shows room-temperature ferromagnetism (reprinted from ref 74). (a) Reproduced with permission of Nature Publishing Group.

to efficient hole doping in graphene.73 Doping graphene with atomic hydrogen yields semihydrogenated graphene, which has been predicted to be ferromagnetic close to room temperature,74 as shown in Figure 3c. Additionally, gas molecules, for example, NO2, doping on graphene has been reported to be an efficient gas sensor, and a microscopic understanding of the general relation between the doping strength and whether adsorbates are open- or closed-shell systems has been demonstrated.75 Most interestingly, an electron acceptor, tetrafluorotetracyanoquinodimethane, on graphene will display significant charge transfer from graphene to the doped molecule, leading to efficient hole accumulation in the graphene layer,76 highlighting another interesting route for tuning the electronic structure of graphene.

Single- and double-layer graphene on a silicon carbide substrate was first studied using ab initio approaches.77 It was found that the first carbon layer to be covalently bonded to the substrate and the second graphene layer exhibits a weak van der Waals bonding character, as in a freestanding graphene. Additionally, a novel quasi-periodic 6 6 domain pattern as a result of interfacial reconstruction has been reported to resolve the long-standing experimental controversy on the periodicity of interfacial superstructures.78 Most recently, interface modification of epitaxial graphene on a silicon carbide substrate has been demonstrated theoretically, potentially leading to fine-tuning of band alignment.79 Metal-graphene contacts are expected to have novel applications in graphene-based electrode materials. The shift of the Fermi level in the graphene/metal contact can be described very well by a simple analytical model which characterizes the metal solely in terms of the work functions.80 Additionally, the graphene/Ru(0001) interface has been systematically studied, and two superstructures (see Figure 4a) have been revealed to be in good agreement with the experimental findings.81 Importantly, the general gradient approximation is found to predict a very weak interaction in the graphene/Ni(111) interface, whereas local density approximation is shown to predict a strong binding between carbon and nickel atoms,82 which is in agreement with the experimental findings.83 This indicates that the van der Waals interaction is extremely important in simulating the graphene/metal interface. The recent study of the graphene/Ir(111) interface using a combined density functional theory and London dispersion approach also clearly demonstrated this.84 A van der Waals functional has been used to the study graphene/metal interface, but currently,

A wide open field that demands theoretical exploration is interfacial engineering. Graphene have been successfully synthesized on, or transferred onto, semiconductors, metals, and dielectric substrates. The interfaces so formed possess novel electronic characteristics and have important implications for achieving new functionality in catalysis, electronics, and sustainable energy applications. The understanding of energetics, adhesion, dynamics, and charge-transfer properties at graphene/ substrate interfaces is crucial to the development of new functionalized electronic devices.


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ahead of experimental synthesis but provides insights into types of structures that may prove profitable for future experimental synthesis and characterization.

Looking forward, state-of-the-art ab initio approaches will furnish a clear interpretive picture of groundstate, excited-state, and dynamical properties in graphene-based nanoarchitectures, illuminating mechanisms that underlie existing experimental data as well as suggesting new avenues for future materials design.

Figure 4. (a) The optimized geometry for graphene on a Ru(0001) surface showing the corrugation effect.81 (b) The formation of an interface charge-transfer complex in the graphene/TiO2 nanocomposite. (a) Reproduced with permission of the American Institute of Physics.

great caution must to be exercised because it is strongly system-dependent.85,86 Graphene/metal oxide nanocomposites have demonstrated novel electronic, electrochemical, and enhanced photocatalytic properties and currently are the subject of intensive research. A Perspective has been published mainly focusing on the experimental side by Kamat.87 Currently, theoretical understanding of such novel nanocomposites is almost wholly lacking. We have recently demonstrated that the graphene/titania interface forms a charge-transfer complex in the ground state, as shown in Figure 4b, leading to efficient hole accumulation in graphene (unpublished results). Importantly, electrons in the upper valence band states located mainly in graphene may be directly excited into titania, yielding a well-separated electron-hole pair with potentially enhanced photocatalytic activity. It is worth noting that the simulation of graphene/metal oxide interfaces remains a significant challenge because a large supercell must be utilized due to the large lattice mismatch. Excited-state methods such as the quasi-particle approximation and timedependent density functional theory will also be very important for graphene/metal oxide nanocomposite modeling but are, as of yet, unreachable for such large interface models. Additionally, van der Waals interactions must be explicitly considered in simulating graphene/metal oxide interfaces. To conclude, numerous fundamental breakthroughs in developing graphene-based nanoarchitectures for practical applications in nanoelectronics and sustainable energy have been achieved since the discovery of graphene in 2004. Due to space limitations, this Perspective only covers a few selected topics on dimensionality, defects, band gaps, and interfacial engineering of graphene. However, it is worth mentioning in conclusion that biological engineering utilizing graphene has also become increasingly popular, particularly the applications related to DNA detection and sensing.88-92 Computational exploration of electronic functionality and how it varies with architecture and doping presently runs


AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Fax: 617-33463992. E-mail: [email protected].

Biographies Dr. Aijun Du received his Ph.D. from Fudan University in China in 2002. He is currently a research fellow in the centre for computational molecular science at the University of Queensland. His current research interests are first-principles engineering of nanoscale materials for energy, environmental, and nanoelectronics application. For more details, see http://web.aibn.uq.edu.au/cbn/ index.htm Prof. Sean Smith received his Ph.D. from the University of Canterbury, New Zealand, in 1989. He is Director of the Centre for Computational Molecular Science at UQ and Deputy Director of the Australian Research Council Centre of Excellence for Functional Nanomaterials. Augmenting his long-time passion for kinetics and reaction dynamics, he currently explores structure, complexation, dynamics, and transport phenomena within nanomaterials, proteins, and hybrid nanobio systems. For more details, see http:// web.aibn.uq.edu.au/cbn/index.htm

ACKNOWLEDGMENT We acknowledge generous grants of highperformance computer time from both the CCMS/AIBN cluster computing facility at The University of Queensland and the Australian NCI facility.

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