Article pubs.acs.org/JPCC
Band Gap Tuning of Graphene by Adsorption of Aromatic Molecules Chung-Huai Chang,†,‡ Xiaofeng Fan,§ Lain-Jong Li,‡ and Jer-Lai Kuo*,‡ †
Department of Physics, National Taiwan University, Taipei, Taiwan Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan § College of Materials Science and Engineering, Jilin University, Changchun, People's Republic of China ‡
S Supporting Information *
ABSTRACT: The effects of adsorbing simple aromatic molecules on the electronic structure of graphene were systematically examined by first-principles calculations. Adsorptions of different aromatic molecules borazine (B3N3H6), triazine (C3N3H3), and benzene (C6H6) on graphene have been investigated, and we found that molecular adsorptions often lead to band gap opening. While the magnitude of band gap depends on the adsorption site, in the case of C3N3H3, the value of the band gap is found to be up to 62.9 meV under local density approximationwhich is known to underestimate the gap. A couple of general trends were noted: (1) heterocyclic molecules are more effective than moncyclic ones and (2) the most stable configuration of a given molecule always leads to the largest band gap. We further analyzed the charge redistribution patterns at different adsorption sites and found that they play an important role in controling the on/off switching of the gapthat is, the energy gap is opened if the charge redistributes to between the C−C bond when the molecule is adsorbing on graphene. These trends suggest that the different ionic ability of two atoms in heterocyclic molecules can be used to control the charge redistribution on graphene and thus to tune the gap using different adsorption conditions. bons12 and graphene quantum dots,8,13 have been proposed. These predictions have been verified experimentally by, for example, cleaving carbon nanotubes along the axial direction14,15 or e-beam lithography of graphene.16 Nevertheless, these approaches face the drawback that the process is not scalable. Another approach to open the band gap of AB stacked bilayer graphene is to add on an additional electrical field provided by a top-gate.11,17−20 Other methods by disrupting the conjugated carbon structures such as introducing StonWales defects21 and hydrogenation22have also been developed. It is also possible to open the gap of graphene by interacting graphene with SiC substrates to break its symmetry.23−25 For electronic or optical applications, it would be beneficial to be able to lay the gap-opened graphene on arbitrary substrates. Therefore, it is beneficial to search for alternative approaches to open the band gap at around Dirac point26−30 but without introducing too large disturbance (like chemical bonds).
1. INTRODUCTION Graphene, a single-layer hexagonal sp2 carbon lattice, has attracted intensive interest since its first isolation from graphite.1 Due to the symmetry equivalence of two atoms in its primitive unit cell, graphene exhibits linear energymomentum dispersion near the Fermi level, which results in mass-less electrons. The ultrahigh electron mobility2−5 obtained from its unique electronic structure promises a variety of novel applications in electronics.6−10 Although graphene is of high potential for future electronics, many issues have yet to be solved before it can be used for electronic applications. One of the most notorious issues is the “lack of an energy gap”5 in its electronic structure, which directly results in a low on−off ratio for graphene-based transistors (3.5 Å, seen in Figure 5(c)). However, for hetercyclic molecules (borazine and triazine), the “cross” configuration is more stable than others at long distances, which possess a lower formation energy than the other four configurations. This notion is in line with what we have
graphene. It should be noticed that the charge distribution pattern does not change much in the range of 0.9∼1.2 Å, although we just give the pattern by choosing a single plane. 3.3. Distance Dependence of Formation Energy and Band Gap. Dispersion force is known to make an important contribution to the binding in the weakly bound systems. The lack of dispersion in the LDA method we use would underestimate the binding energy. However, it is known in the literature that LDA gives a reasonable distance between the absorbed molecule and graphene. Furthermore, as a result of weak binding, the absorbed molecule is expected to be 13792
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discussed previously in section 1; furthermore, the details of the electrostatic interaction for the “cross” configuration plays an important role in its stabilitzation. For short distances, the coulomb repulsive force increases, which causes the formation energy of all configurations to increase rapidly. For borazine, the more stable configrations are “B-top” and “bridge”. For triazine, the more stable configrations are “C-top” and “bridge”. For benzene, the more stable configrations are “AB” and “bridge”. At the same time, the most unstable configuration is the “AA” or “all-top”. In addition, it is noticed that most charges are distributed on N atoms for borazine and triazine. Therefore, the reason that “B-top” (or “C-top” and “AB”) and “bridge” configurations are more stable in short distances should be that the charge from the molecule is separated from the graphene plane to a larger extent, and therefore reduces the repulsive force. The changes of band gap with respect to the adsorption distance for all of the configurations are shown in Figure 5, in order to detect the effect of interaction distance to the electronic properties of graphene. Actually, the symmetry has a very important effect on the opening of band gap at the Dirac point. For example, the band gap cannot be open (or slightly open) for benzene with “AB” configuration or triazine with an “N-top” configuration. Moreover, with the breaking of graphene electrons due to the absorption of molecules, the decrease of the distance can augment the band gap, such as “AA” (or “All-top”) and “cross” configurations. Therefore, we can predict that the opening of graphene with the absorped polarized aromatic molecules can be modulated with the pressure.
may open new routes for tailoring the device characteristics from semiconducting to metallic.
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ASSOCIATED CONTENT
S Supporting Information *
Role of dispersion on the stable configurations, DFT calculations based on dispersion corrected GGA. Additional calculations using a more time-consuming hybrid functional to estimate the bandgap of the most stably adsorbed configurations. 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 This work was financially supported by the Academia Sinica Research Program on NanoScience and Nano Technology and the National Science Council (NSC98-2113-M-001-029-MY3 and NSC-99-2112-M-001-021-MY3) of Taiwan. Computational resources are supported in part by the National Center for High Performance Computing.
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REFERENCES
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4. CONCLUSIONS In this study, we perform a systematic ab initio study on the adsorptions of simple aromatic molecules on graphene using density-functional theory with the local-density approximation (LDA). The adsorption of borazine (B3N3H6), triazine (C3N3H3), and benzene (C6H6) on all distinct high symmetry sites of graphene has been investigated. Our results show that the most stable configuration is the “cross” configuration for heterocyclic aromatic molecules, and “AB” configuration for benzene. In the case of heterocyclic aromatic molecules, the most stable configurations, with adsorption energies of −0.398 eV (B3N3H6) and −0.340 eV (C3N3H3), always lead to the largest band gaps (45 meV for B3N3H6 and 62.9 meV for C3N3H3). The well-known limitation of LDA to underestimate the magnitude of the band gap suggests that our calculated values serve as a lower boundary. Dependence of binding energy on adsorption distance has also been investigated. It is shown that a band gap of more than 100 meV can be achieved by pushing molecules toward graphene within the region the adsorption is still favorable (that is with negative formation energy). To further investigate the mechanism for band gap opening, we analyzed the charge redistribution patterns at different adsorption sites and found that the energy gap is widened if the charge redistribution between C−C bond is enhanced. The simplicity and cost-effectiveness of molecular decoration of graphene makes it a promising approach for band gap engineering. Our calculations demonstrated that the generated gap is tunable by adsorption of different aromatic molecules. Further manipulations on these organic molecules, for example, adsorbing and desorbing by solvent extractions or oxidation, 13793
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