ARTICLE pubs.acs.org/JPCC
Transport Properties of Zigzag Graphene Nanoribbons Decorated by Carboxyl Group Chains C. X. Zhang, Chaoyu He, Zhizhou Yu, K. W. Zhang, L. Z. Sun,* and Jianxin Zhong* Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Xiangtan University, Xiangtan 411105, China
bS Supporting Information ABSTRACT: The transport properties of zigzag graphene nanoribbons (ZGNRs) decorated by carboxyl group (OH) chains are systematically investigated using the density functional theory in combination with the nonequilibrium Green’s function method. ZGNRs with nine zigzag carbon chains (9ZGNR) decorated by mOH (m is the number of oxidized carbon chains) are taken as typical systems. We find that the OH chains can effectively modulate the electronic structures and transport properties of the 9ZGNR. The systems behave as metal when m e 4, and a transmission plateau up to 6G0 is found around the Fermi level when m = 3. However, when m > 4, the 9ZGNR-mOH systems become semiconductors. Interestingly, 9ZGNR-7OH and 9ZGNR-8OH behave as n-type semiconductors. It is found that such modulation depends on the edge states as well as the oxygen atoms at the interface. When the width of undecorated carbon regions is 4. 9ZGNR-5OH shows a transmission gap of 0.2 eV, and 9ZGNR-6OH exhibits a transmission dip around the Fermi level. Interestingly, 9ZGNR-7OH and 9ZGNR-8OH behave as n-type semiconductors. To clarify the reason of the transmission enhancement of 9ZGNR-mOH for m e 4, we analyze the average local density of states (LDOS) within the energy window of [ 0.1 eV, 0.1 eV]. Figure 3 presents the average LDOS of 9ZGNR-H, 9ZGNR-1OH, 9ZGNR-3OH, and 9ZGNR-4OH in the real space. The LDOS of 9ZGNR-H mainly concentrates along the edge carbon atoms. Besides the edge carbon atoms, the LDOS of 9ZGNR-1OH also concentrates on the oxygen atoms as well as interfacial carbon atoms between sp2 and sp3 carbon regions. This indicates that extra effective transport channels are introduced to the OH-decorated system, which induces the transmission enhancement of the system. As for 9ZGNR-3OH, the LDOS almost distributes on all carbon atoms of sp2 region as well as the oxygen atoms at the interface. As a result, the effective transport channels are further increased, which even induce a transmission plateau up to 6G0 around the Fermi level. The LDOS of 9ZGNR-4OH only distributes on the carbon atoms in the sp2 region whose width is 3 and the oxygen atoms at the interface near this region. Hence, its effective transport channels 21895
dx.doi.org/10.1021/jp204888q |J. Phys. Chem. C 2011, 115, 21893–21898
The Journal of Physical Chemistry C
Figure 4. PDOS of specific carbon and oxygen atoms for 9ZGNR-1OH (a) and 9ZGNR-4OH (b). The scripts of w and n represent the wide (narrow) graphene regions corresponding to the signs in Figure 1b. For 9ZGNR-1OH, w and n represent the graphene regions with the same width of 4. As for 9ZGNR-4OH, w (n) corresponds to the graphene region with width of 3 (2), respectively.
are less than those of 9ZGNR-3OH and its transmission enhancement is depressed. However, the effective transport channels of 9ZGNR-4OH are still more than those of 9ZGNR-H. To investigate the mechanism of the modulation effect of OH chains on the transport of GNRs, we analyze the electronic structures of 9ZGNR-mOH. Figure 4 represents the projected density of states (PDOS) of carbon atoms at the edge of the ribbon Cedge, carbon atoms at the interface Cinf, and oxygen atoms at the edge of the oxidized regions Oinf for 9ZGNR-1OH and 9ZGNR-4OH. As for 9ZGNR-mOH, the states around the Fermi level mainly derive from the edge and interfacial carbon atoms of the sp2 graphene regions, whose planar structure is wellremained, which is similar to the hybrid graphane graphene nanoribbons.16,35,36 However, because of the charge transfer from the Oinf to Cinf, the states of lone-pair electrons of the Oinf
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also contribute to the states around the Fermi level. From Figure 4a, one can obviously see that the peak of PDOS of 9ZGNR-1OH around the Fermi level derives from the pz of Cedge and Cinf as well as the py of Oinf due to the coupling between the Oinf and its adjacent Cinf. Whereas the valence band maximum (VBM) and conduction band minimum (CBM) of 9ZGNR-4OH derive from the pz of Cedge and Cinf as well as the py of Oinf, as shown in Figure 4b. In comparison with 9ZGNR-1OH, the states of 9ZGNR-4OH show two apparent peaks around the Fermi level. Such peaks come from Cedge and Cinf in the same sp2 graphene regions and show similar characteristics around the Fermi level. The two peaks have wider separation for the narrower sp2 graphene region. Such phenomenon derives from the Peierls distortion34 of narrow graphene nanoribbons, which is similar to the report of Tozzini et al. in graphene nanoribbons sculpted in graphane.35 As for 9ZGNR-5OH, 9ZGNR-6OH, 9ZGNR-7OH, and 9ZGNR-8OH, the Peierls distortion even induces the transition of the system from metals to semiconductors. Figure 5 shows the band structures and the charge density distributions corresponding to the VBM and the CBM for 9ZGNR-mOH. The band structures and charge density of 9ZGNR-H are also included for comparison purpose. The VBM and CBM of 9ZGNR-H, as shown in Figure 5, are degenerate at the Fermi level forming the flat band within 2π/ 3 e k e π, whereas the 9ZGNR-mOH presents two pairs of bands near the Fermi level when m e 7. When m e 4, both the VBM and the CBM pass through the Fermi level, and the valence band overlaps with the conduction band, making the ribbons behave as metals. The degeneration and overlap of the two pairs of bands around the Fermi level introduce more effective transport channels and enhance their transport properties. When m g 5, the width of both graphene regions is
