Article pubs.acs.org/JPCC
Big Bandgap in Highly Reduced Graphene Oxides Ke-Yan Lian,†,‡ Yong-Fei Ji,‡ Xiao-Fei Li,‡,¶ Ming-Xing Jin,† Da-Jun Ding,† and Yi Luo*,‡,§ †
Institute of Atomic and Molecular Physics, Jilin University, Changchun, 130012, People’s Republic of China Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden ¶ School of Physics and Microelectronics Science, Hunan University, Changsha 410082, People’s Republic of China § Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: It is generally believed that the bandgap of the graphene oxide is proportional to the concentration of the oxygen atoms and a highly reduced graphene oxide (rGO) without vacancy defects should be gapless. We show here from first principles calculations that the bandgap can be effectively opened even in low oxidation level with the absorption of oxygen atoms either symmetrically or asymmetrically. The properly arranged absorption can induce a bandgap up to 1.19 eV for a C/O ratio of 16/1 in a symmetric system and a bandgap up to 1.58 eV for a C/O ratio of 32/3 in an asymmetric system, at generalized gradient approximation (GGA) level. The hybridization between the in-plane pxy orbitals of oxygen atoms and the out-of-plane pz frontier orbital of graphene is responsible for the opening of the bandgap. This finding sheds new light on the bandgap engineering of graphene.
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oxidation time was confirmed by experiments.31−33 Their electronic structures were further studied via first principles calculations, revealing a significant effect of the oxidation level.34 It was shown that the bandgap of GO opened at a 50% coverage of oxygen and with the increase of oxidation level, the bandgap expanded to 2.8 eV.35 The opening of the bandgap is attributed to the presence of a range of oxygen-containing functional groups, such as epoxy and hydroxyl groups on the basal plane and carboxylic, hydroxyl, and carbonyl groups at the edge. However, at the same time, these moieties generate abundant disruptive sp3 hybridized local states, resulting in an impaired conductive network of π-electrons and inferior electrical properties compared to pristine graphene. Reduction of GO by various chemical or thermal methods was widely applied to recover the structure framework of pristine graphene, leading to a pronounced improvement of the electrical conductivity.33,36 It is unfortunate that the enhanced conductivity has always been achieved at the expense of an obvious decrease in bandgap (without considering the involved vacancy defect throughout the reduction procedure). For example, it was shown recently in experiment that with a concurrent increase in the C/O ratio, the optical gap decreased from 3.5 eV to 1 eV.37 The lack of a good balance between the bandgap and the conductivity seems to be the bottleneck that limits the ultimate utility of (reduced) GO in high-performance nanoelectronics. Interestingly, it was found that symmetric doping may open a bandgap in graphene at low concentrations.38−40
INTRODUCTION The fascinating electronic properties of graphene have been at the center of attention in recent years.1−4 Graphene is a zero bandgap material with very high electron mobility and has been considered to be an active candidate for many important nanoelectronic devices, such as field-effect transistors,5 ultrasensitive sensors,6 and organic photovoltaic cells.7 However, at the moment graphene mainly serves as the reinforcement to enhance the electrical properties of its composite materials.8,9 The direct use of graphene for electronic devices requires the opening of its bandgap without loss of its high conductivity. Many different attempts have been made both theoretically and experimentally, following three main routes to disrupt the connectivity of π-electron network of pristine graphene.10−12 The first way is to cut it into ribbons or form quantum dots so that a gap is introduced through the quantum confinement.13−15 The second way is to break the equivalence of two sublattices of graphene by physical treatments, such as making use of bilayer graphene sheets, interacting the graphene layer with the substrate, using the influence of defects and dislocations, applying an external electrical field, designing heterojunction, etc.16−22 As the third way, chemical absorption and doping are effective methods in bandgap engineering of graphene.23−26 It was reviewed that random absorption can induce the opening of bandgap only in high concentration,27 for example, the gap induced by hydrogenation approaches a size of zero after the H coverage decreases to 67%.28 Among many chemically modified graphene composites, the (reduced) graphene oxide (GO) is the most intensively investigated one.29,30 A controllable gap opening of GO by different © XXXX American Chemical Society
Received: November 30, 2012 Revised: March 6, 2013
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In this paper, we show that a bandgap can be opened in highly reduced GO at low oxidation level for both symmetric and asymmetric adsorption. With a C/O ratio of 16/1, the bandgap can be tuned from zero to 1.19 eV in symmetric adsorption, and with a C/O ratio of 32/3 we obtain a bandgap as large as 1.58 eV in asymmetric adsorption. The hybridizations between p-orbitals of oxygen atoms and the frontier orbitals of graphene play decisive roles in determining the bandgap of highly reduced GO. Such a theoretical prediction of the widely opened bandgap in graphene at very low oxygen concentration with low symmetry is very useful for graphene bandgap engineering, and can in principle be verified by a recently developed fabrication technique utilizing the homogeneous oxidation of epitaxial graphene with atomic oxygen prepared in ultrahigh vacuum.41
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Figure 1. (a) Adsorption positions for the second oxygen atom with respect to the first fixed oxygen atom (O), indicated by Pn (n = 1 to 6) and Tn (n = 1 to 5), respectively. (b) Periodic structure of the configuration P1. (c) The relationship between the average energy per atom and the corresponding bandgap for all configurations.
COMPUTATIONAL DETAILS All results presented here were obtained from density functional theory (DFT) calculations with the Vienna Ab initio Simulation Package (VASP).42,43 Core electrons were taken into account by projector augmented wave (PAW) pseudopotentials,44 while a 400 eV plane wave cutoff was used for valence electrons. The revised Perdew−burke−Emzerhof (PBE) generalized gradient approximation (GGA) was used for the exchange correlation potential.45 It is well-known that the band gaps calculated by GGA functionals may be underestimated, while the predicted trends remain reasonable. The Γ-centered Monkhorst−Pack scheme in the full Brillouin zone (BZ) was used to generate k-points for the BZ integration. An equivalent Monkhorst−Pack k-grid of 16 × 16 × 1 for a 1 × 1 graphene unit cell was used in structural relaxation. An equivalent Monkhorst−Pack k-grid of 32 × 32 × 1 for a 1 × 1 graphene unit cell was used in the calculation of the density of states (DOS). A Gaussian smearing of 0.1 eV was used for DOS calculation shown in this paper. Full relaxation of each configuration was performed until the force on each atom was less than 0.01 eV/Å. To eliminate the interaction between graphene layers, a slab with vacuum layer larger than 12 Å along the direction normal to the GO plane was used. In the highly reduced GO, it is known that only epoxy groups remain.46 In other words, all the oxygen atoms are assumed to be adsorbed on the bridge sites in low oxidation level. Moreover, only one side of the graphene is oxidized in our calculation, while the other side is supposed to be protected by the substrate.
The calculated average energies (per atom) and bandgaps for all considered configurations are plotted in Figure 1c. The average energy is calculated by Etotal/Natom, where Etotal is the total energy of the supercell and Natom is the atom number included in the supercell. From this chart one can find that the energy differences among different configurations are quite small. For example, the differences among P1, P3, P4, and T5 are less than 6 meV, making them capable of existing in the same condition. At the same time, their bandgaps vary to a wide energy range from 0.1 to 1.19 eV. On the basis of these two points, one can conclude that a big bandgap can be opened in highly reduced GO. It should be pointed out that the bandgap does not have a simple relationship with the energy of the configuration. It is interesting to see that the configurations with the lowest and the highest energies both give almost zero bandgaps, consistent with what has been normally known in low oxidation condition. On the other hand, the bandgap values are quite sensitive to the adsorption positions of oxygen atoms. For instance, the oxygen atoms adsorbed on two parallel edges are inclined to open a wider bandgap, such as the configurations P1, P2, and P3, shown in Figure 1c. It is very surprising to see that at the adsorption site of P1, oxidation results in a bandgap as large as 1.19 eV. By inspecting its periodic structure in Figure 1b, one can observe that in this configuration all oxygen atoms are aligned in rows, arranged in a rectangular pattern. The first impression is that such a maximum match between sublattices formed by oxygen atoms and graphene sheet might favor the hybridization between the oxygen atoms and carbon atoms, inducing the large bandgap of configuration P1. We calculated the band structures of all these systems under investigation. The results for configurations P1, P6, and T4 are presented in parts a, b, and c of Figure 2, respectively. We can determine that although the geometries of all configurations are not affected strongly by the oxygen adsorption, the resulting band structures are quite different from each other. For the configuration P1, with the oxygen atoms arranged in a rectangle sublattice, the whole system possesses a C2v symmetry. It can be seen that a wide bandgap is opened right at the Dirac point, showing an obvious effect of oxidation on the electronic structure. For configurations P6 and T4, the bandgaps are hardly opened and the Dirac cone seems to be intact. Actually, P6 also belongs to C2v symmetry, while its oxygen atoms, which
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RESULTS AND DISCUSSION A highly reduced GO model with the oxygen concentration of 6.25% is used at the first stage of our study. The calculation models are confined within a 4 × 4 graphene supercell, as marked in Figure 1, including 32 carbon atoms and 2 oxygen atoms for each configuration. After fixing the adsorption position of one oxygen atom to a vertex, labeled with O, one can find 12 inequivalent adsorption positions for the other oxygen atom. Except the site marked with I in Figure 1a, all other 11 sites have been fully optimized to respective stable geometries. Depending on the adsorption positions, these configurations can be divided into two categories roughly. In the first case, the two edges of honeycomb lattice, which the oxygen atoms are adsorbed on, are parallel to each other, labeled with Pn (n = 1 to 6) in Figure 1a. For the second group marked by Tn (n = 1 to 5), the two edges are present with a mutual angle. B
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Figure 3. The partial density of states of p-electrons for (a) the configuration P1 and (b) the configuration P6. The black dashed lines are for carbon atoms and the red solid lines are for oxygen atoms, respectively.
Figure 2. The band structures of (a) the configuration P1, (b) the configuration P6, and (c) the configuration T4.
Dirac cone and lead to the opening of the bandgap up to 1.19 eV. In Figure 3b, the PDOS for the configuration P6 are given. For oxygen atoms, the same main peak in PDOS of the y component as for the configuration P1 occurs. However, the four low-energy side peaks observed for the configuration P1 are totally missing. Instead, new peaks at higher energy appear, which implies that the p orbital of the oxygen atom is hybridized with a higher energy band of the graphene. In this case, the Dirac cone is not affected and the bandgap remains zero. It is interesting to see that although both P1 and P6 configurations fall into the same category, i.e. two oxygen atoms are located on two parallel carbon bonds and even possess the same symmetry, they have evolved into two very different hybridization schemes since the adsorbed oxygen atoms take different arrangements. We can say that it is the oxygen arrangements that result in very different bandgap values. Among all these 11 configurations shown in Figure 1, we obtained only one with the bandgap larger than 1 eV. This configuration turns out to be a comparably high symmetric one. To further examine the possibility to open a large bandgap for asymmetrical configuration under a low oxidation level, we continued the oxidation study based on configuration P1. Taking P1 as the starting point of modeling, we added the third oxygen atom to the remaining possible adsorption positions within the 4 × 4 supercell. The ratio of C/O at this moment changes from 16/1 to 32/3. Labels used in Figure 1 are combined to denote our new configurations. There exists a total of 7 unequivalent structures, including P1−P2, P1−P3, P1− P4/P5, P1−P6, P1−T1/T4, P1−T2/T5, and P1−T3, respectively. After full optimization, their band structures were calculated, as shown in Figure 4. The bandgap energies for configurations mentioned above are 1.58, 0.48, 1.20, 0.57, 1.24, 0.48, and 0.94 eV, respectively. Among all 7 configurations, P1−P2 has the largest bandgap opening with energy as large as 1.58 eV. This system happens to be the combination of both structures with the two largest bandgap energies for a C/O ratio equal to 16/1. In this configuration, the third adsorption position falls into the middle of two existing oxygen atoms in P1 alternatively. This kind of alternate adsorption breaks the specific rectangular arrangement in P1, while the symmetry of the whole system
adsorbed on different rows of graphene surface, form into a staggered arrangement instead of a homogeneous rectangle in line. Although the configuration P6 has the same symmetry as P1, the oxidation generates quite different effects on each electronic structure, stating that symmetry is not a unique factor to affect the bandgap opening and the oxygen arrangement pattern is quite important, if not decisive. Different adsorption positions of oxygen atoms are expected to availably affect the hybridization between oxygen and carbon atoms and lead to different bandgap opening. In general, a low oxidation does not largely reduce the carrier density of the graphene sheet and the high electron mobility of the graphene remains unchanged. One can thus conclude that the highly reduced GO can possess two attractive properties, large bandgap and high electron mobility. Since the orbitals of oxygen and carbon atoms hybridize can determine what the bandgap value should be, we have plotted the partial density of states (PDOS) for these configurations to understand the hybridization. Results for configurations P1 and P6 with a wide and a narrow bandgap are presented respectively in parts a and b of Figure 3. We assume that the xy is within the plane of the graphene sheet, while z is normal to the plane. As shown in the inserts of Figure 3a, the x and y directions are perpendicular to and along the C−C bond, respectively. From the top down, the PDOS of three components of p orbitals, px, py, and pz, for both carbon and oxygen atoms are sequentially displayed, in which the black dashed lines are for carbon atoms and the red solid lines are for oxygen atoms, respectively. It can be seen that for the configuration P1, the p-electrons of the oxygen atoms near the Fermi level have mainly the in-plane y component, with negligible x and z components. The main peak of the oxygen py orbital is located around −2 eV below the Fermi level and is accompanied by four extended small peaks of lower energy, labeled as A, B, C, and D, respectively. The hybridization between the oxygen and the carbon atoms is clearly demonstrated in the PDOS of their corresponding y and z components. These low-energy features also indicate that the oxygen orbitals interact with the orbitals of the carbon atoms around the Fermi level, or say that it will directly affect the C
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Figure 5. The partial density of states of p-electrons for (a) the configuration P 1 −P 2 , (b) the configuration P 1 −P 4 , (c) the configuration P1−P3, and (d) the configuration P1−T2. The black dashed lines are for carbon atoms and the red solid lines are for oxygen atoms, respectively.
Figure 4. The band structures of (a) the configuration P1−P2, (b) the configuration P 1 −P 4 , (c) the configuration P 1 −P 6 , (d) the configuration P1−P3, (e) the configuration P1−T1, and (f) the configuration P1−T2.
remains the same. It is interesting to note that P1−P2 possesses an indirect bandgap. If the wide bandgap is not unexpected for P1−P2, the comparable large bandgaps of configuration P1−P4, especially for configuration P1−T1, are exciting. In the configuration P1−T1, the third oxygen atom locates on the edge of the honeycomb lattice not parallel to the adsorption sites of the first two oxygen atoms. This kind of combination of adsorption sites from different categories reduces the symmetry of the whole system to C1. The bandgap structure for P1−T1 reveals that it is possible to open a large bandgap for asymmetric structures under a low oxidation condition. The tunable bandgap from 0.48 to 1.58 eV by moving only one oxygen atom demonstrates the importance of oxygen atom arrangement. The PDOS for these new configurations were calculated to further understand the hybridization. As presented in Figure 5, the xy and z components of p orbitals for both carbon and oxygen atoms are sequentially plotted for configurations P1−P2, P1−P4, P1−P3, and P1−T1 from the top down. The xy is still within the plane of the graphene sheet and the z is normal to the plane. The black dashed lines are for carbon atoms and the red solid lines are for oxygen atoms, respectively. From top to bottom, the main peaks of the xy component of oxygen atoms become narrower and narrower and the peak positions become further and further from the Fermi level. This indicates that the weakening of the hybridizations between the p-orbital of oxygen atoms and the frontier orbitals of graphene results in the decreasing of the bandgap.
carbon ring structure determines the frontier orbital hybridization between the oxygen and carbon atoms. In the cases that the oxygen atoms strongly interact with the energy bands near the Fermi level of the graphene, even low oxidation can lead to the opening of the bandgap up to 1.19 eV for a C/O ratio of 16/1 and 1.58 eV for a C/O ratio of 32/3. In other cases where the p-orbital of oxygen atoms hybridize with the frontier orbitals of carbon atoms through the higher energy bands, rGO still leaves the Dirac cone and zero bandgap intact.
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ASSOCIATED CONTENT
S Supporting Information *
The coordinates of the supercells for all considered structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the support from the Swedish Research Council and the Göran Gustafsson Foundation for Research in Natural Sciences and Medicine. The simulations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at PDC, NSC, and HPC2N.
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CONCLUSION Our calculations have demonstrated that it is possible to open a large bandgap in graphene with both symmetric and asymmetric oxidation under a low oxidation level. It is revealed that the local oxygen atom arrangement with respect to the
REFERENCES
(1) Novoselov, K.; Geim, A.; Morozov, S.; Jiang, D.; Grigorieva, M.; Dubonos, S.; Firsov, A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200.
D
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(2) Zhang, Y.; Tan, Y.; Stormer, H.; Kim, P. Experimental Observation of The Quantum Hall Effect and Berry’s Phase in Graphene. Nature 2005, 438, 201−204. (3) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109−162. (4) Schwierz, F. Graphene Transistors. Nat. Nanotechnol. 2010, 5, 487−496. (5) Novoselov, K.; Geim, A.; Morozov, S.; Jiang, D.; Zhang, Y.; Dubonos, S.; Grigorieva, I.; Firsov, A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666. (6) Schedin, F.; Geim, A.; Morozov, S.; Hill, E.; Blake, P.; Katsnelson, M.; Novoselov, K. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652−655. (7) Eda, G.; Lin, Y.; Miller, S.; Chen, C.; Su, W.; Chhowalla, M. Transparent and Conducting Electrodes for Organic Electronics From Reduced Graphene Oxide. Appl. Phys. Lett. 2008, 92, 233305−233305. (8) Yang, N.; Zhai, J.; Wang, D.; Chen, Y.; Jiang, L. TwoDimensional Graphene Bridges Enhanced Photoinduced Charge Transport in Dye-Sensitized Solar Cells. ACS Nano 2010, 4, 887−894. (9) Tang, Y.-B.; Lee, C.-S.; Xu, J.; Liu, Z.-T.; Chen, Z.-H.; He, Z.; Cao, Y.-L.; Yuan, G.; Song, H.; Chen, L.; et al. Incorporation of Graphenes in Nanostructured TiO2 Films via Molecular Grafting for Dye-Sensitized Solar Cell Application. ACS Nano 2010, 4, 3482−3488. (10) Abergel, D.; Apalkov, V.; Berashevich, J.; Ziegler, K.; Chakraborty, T. Properties of Graphene: a Theoretical Perspective. Adv. Phys. 2010, 59, 261−482. (11) Liu, L.; Shen, Z. Bandgap Engineering of Graphene: A Density Functional Theory Study. Appl. Phys. Lett. 2009, 95, 252104−252104. (12) Li, X.; Wang, L.; Chen, K.; Luo, Y. Strong Current Polarization and Negative Differential Resistance in Chiral Graphene Nanoribbons with Reconstructed (2, 1)-Edges. Appl. Phys. Lett. 2012, 101, 073101− 073101. (13) Han, M.; Ö zyilmaz, B.; Zhang, Y.; Kim, P. Energy Band-Gap Engineering of Graphene Nanoribbons. Phys. Rev. Lett. 2007, 98, 206805. (14) Son, Y.-W.; Cohen, M. L.; Louie, S. G. Energy Gaps in Graphene Nanoribbons. Phys. Rev. Lett. 2006, 97, 216803. (15) Li, X.-F.; Wang, L.-L.; Chen, K.-Q.; Luo, Y. Tuning the Electronic Transport Properties of Zigzag Graphene Nanoribbons via Hydrogenation Separators. J. Phys. Chem. C 2011, 115, 24366−24372. (16) Giovannetti, G.; Khomyakov, P.; Brocks, G.; Kelly, P.; Van Den Brink, J. Substrate-Induced Band Gap in Graphene on Hexagonal Boron Nitride: Ab Initio Density Functional Calculations. Phys. Rev. B 2007, 76, 073103. (17) Ni, Z.; Yu, T.; Lu, Y.; Wang, Y.; Feng, Y.; Shen, Z. Uniaxial Strain on Graphene: Raman Spectroscopy Study and Band-Gap Opening. ACS Nano 2008, 2, 2301−2305. (18) Casolo, S.; Martinazzo, R.; Tantardini, G. Band Engineering in Graphene with Superlattices of Substitutional Defects. J. Phys. Chem. C 2011, 115, 3250−3256. (19) Zhou, S. Y.; Gweon, G.-H.; Fedorov, A. V.; First, P. N.; De Heer, W. A.; Lee, D.-H.; Guinea, F.; Neto, A. H.; Lanzara, A. Substrate-Induced Bandgap Opening in Epitaxial Graphene. Nat. Mater. 2007, 6, 770−775. (20) Castro, E. V.; Novoselov, K. S.; Morozov, S. V.; Peres, N. M. R.; Lopes dos Santos, J. M. B.; Nilsson, J.; Guinea, F.; Geim, A. K.; Castro Neto, A. H. Biased Bilayer Graphene: Semiconductor with a Gap Tunable by the Electric Field Effect. Phys. Rev. Lett. 2007, 99, 216802. (21) Gui, G.; Li, J.; Zhong, J. Band Structure Engineering of Graphene by Strain: First-Principles Calculations. Phys. Rev. B 2008, 78, 075435. (22) Li, X.-F.; Wang, L.-L.; Chen, K.-Q.; Luo, Y. Design of Graphene-Nanoribbon Heterojunctions from First Principles. J. Phys. Chem. C 2011, 115, 12616−12624. (23) Zanella, I.; Guerini, S.; Fagan, S. B.; Mendes Filho, J.; Souza Filho, A. G. Chemical Doping-Induced Gap Opening and Spin Polarization in Graphene. Phys. Rev. B 2008, 77, 073404.
(24) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Graphane: A TwoDimensional Hydrocarbon. Phys. Rev. B 2007, 75, 153401. (25) Leenaerts, O.; Peelaers, H.; Hernández-Nieves, A. D.; Partoens, B.; Peeters, F. M. First-Principles Investigation of Graphene Fluoride and Graphane. Phys. Rev. B 2010, 82, 195436. (26) Karlickỳ, F.; Zbořil, R.; Otyepka, M. Band Gaps and Structural Properties of Graphene Halides and Their Derivates: A Hybrid Functional Study with Localized Orbital Basis Sets. J. Chem. Phys. 2012, 137, 034709. (27) Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.; Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of Graphene: Covalent and Non-Covalent Approaches, Derivatives and Applications. Chem. Rev. 2012, 112, 6156− 6214. (28) Gao, H.; Wang, L.; Zhao, J.; Ding, F.; Lu, J. Band Gap Tuning of Hydrogenated Graphene: H Coverage and Configuration Dependence. J. Phys. Chem. C 2011, 115, 3236−3242. (29) Saxena, S.; Tyson, T.; Shukla, S.; Negusse, E.; Chen, H.; Bai, J. Investigation of Structural and Electronic Properties of Graphene Oxide. Appl. Phys. Lett. 2011, 99, 013104−013104. (30) Wang, G.; Sun, X.; Liu, C.; Lian, J. Tailoring Oxidation Degrees of Graphene Oxide by Simple Chemical Reactions. Appl. Phys. Lett. 2011, 99, 053114−053114. (31) Nourbakhsh, A.; Cantoro, M.; Vosch, T.; Pourtois, G.; Clemente, F.; Van Der Veen, M.; Hofkens, J.; Heyns, M.; De Gendt, S.; Sels, B. Bandgap Opening in Oxygen Plasma-Treated Graphene. Nanotechnology 2010, 21, 435203. (32) Luo, Z.; Vora, P. M.; Mele, E. J.; Johnson, A. T. C.; Kikkawa, J. M. Photoluminescence and Band Gap Modulation in Graphene Oxide. Appl. Phys. Lett. 2009, 94, 111909. (33) Guo, L.; Shao, R.-Q.; Zhang, Y.-L.; Jiang, H.-B.; Li, X.-B.; Xie, S.Y.; Xu, B.-B.; Chen, Q.-D.; Song, J.-F.; Sun, H.-B. Bandgap Tailoring and Synchronous Microdevices Patterning of Graphene Oxides. J. Phys. Chem. C 2012, 116, 3594−3599. (34) Yan, J.; Xian, L.; Chou, M. Structural and Electronic Properties of Oxidized Graphene. Phys. Rev. Lett. 2009, 103, 86802. (35) Boukhvalov, D. W.; Katsnelson, M. I. Modeling of Graphite Oxide. J. Am. Chem. Soc. 2008, 130, 10697−10701. (36) Gómez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett. 2007, 7, 3499−3503. (37) Mathkar, A.; Tozier, D.; Cox, P.; Ong, P.; Galande, C.; Balakrishnan, K.; Leela Mohana Reddy, A.; Ajayan, P. M. Controlled, Stepwise Reduction and Band Gap Manipulation of Graphene Oxide. J. Phys. Chem. Lett. 2012, 3, 986−991. (38) Dong, X.; Shi, Y.; Zhao, Y.; Chen, D.; Ye, J.; Yao, Y.; Gao, F.; Ni, Z.; Yu, T.; Shen, Z.; et al. Symmetry Breaking of Graphene Monolayers by Molecular Decoration. Phys. Rev. Lett. 2009, 102, 135501. (39) García-Lastra, J. M. Strong Dependence of Band-Gap Opening at the Dirac Point of Graphene upon Hydrogen Adsorption Periodicity. Phys. Rev. B 2010, 82, 235418. (40) Gao, H.; Wang, L.; Zhao, J.; Ding, F.; Lu, J. Band Gap Tuning of Hydrogenated Graphene: H Coverage and Configuration Dependence. J. Phys. Chem. C 2011, 115, 3236−3242. (41) Hossain, M.; Johns, J.; Bevan, K.; Karmel, H.; Liang, Y.; Yoshimoto, S.; Mukai, K.; Koitaya, T.; Yoshinobu, J.; Kawai, M. Chemically Homogeneous and Thermally Reversible Oxidation of Epitaxial Graphene. Nat. Chem. 2012, 4, 305−309. (42) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558. (43) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15. (44) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. (45) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. B 1996, 77, 3865. E
dx.doi.org/10.1021/jp3118067 | J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Article
(46) Compton, O.; Jain, B.; Dikin, D.; Abouimrane, A.; Amine, K.; Nguyen, S. Chemically Active Reduced Graphene Oxide with Tunable C/O Ratios. ACS Nano 2011, 5, 4380−4391.
F
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