Evidence of van Hove Singularities in Ordered Grain Boundaries of Graphene

Chuanxu Ma, Haifeng Sun, Yeliang Zhao, Bin Li, Qunxiang Li, Aidi Zhao, Xiaoping Wang, Yi Luo, Jinlong Yang, Bing Wang, and J. G. Hou

Phys. Rev. Lett. 112, 226802 – Published 6 June 2014

Abstract

It has long been under debate whether the electron transport performance of graphene could be enhanced by the possible occurrence of van Hove singularities in grain boundaries. Here, we provide direct experimental evidence to confirm the existence of van Hove singularity states close to the Fermi energy in certain ordered grain boundaries using scanning tunneling microscopy. The intrinsic atomic and electronic structures of two ordered grain boundaries, one with alternative pentagon and heptagon rings and the other with alternative pentagon pair and octagon rings, are determined. It is firmly verified that the carrier concentration and, thus, the conductance around ordered grain boundaries can be significantly enhanced by the van Hove singularity states. This finding strongly suggests that a graphene nanoribbon with a properly embedded ordered grain boundary can be a promising structure to improve the performance of graphene-based electronic devices.

Received 27 November 2013

DOI:https://doi.org/10.1103/PhysRevLett.112.226802

Authors & Affiliations

Chuanxu Ma, Haifeng Sun, Yeliang Zhao, Bin Li, Qunxiang Li, Aidi Zhao, Xiaoping Wang, Yi Luo, Jinlong Yang, Bing Wang^*, and J. G. Hou^†

Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

^*bwang@ustc.edu.cn
^†jghou@ustc.edu.cn

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Vol. 112, Iss. 22 — 6 June 2014

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Images

Figure 1
(a) STM topographic image ( $11.5 \times 11.5 {nm}^{2}$ ), showing a GB in graphene and (b) magnified image of the marked area ( $5.8 \times 5.2 {nm}^{2}$ , $- 0.8 V$ , and 0.5 nA). (c) Optimized structural model of the $(3, 1) ∣ (1, 3)$ GB. The notation of the GB is shown. (d) Filled-state STM images ( $- 0.8 V$ ) and (e) empty-state STM images ( $+ 0.8 V$ ) from experimental (upper, $1.8 \times 1.7 {nm}^{2}$ ) and simulated results (lower).
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Figure 2
(a) STM current image and (b) $d I / d V$ map within the same area ( $11.2 \times 10.4 {nm}^{2}$ , $+ 0.4 V$ , and 0.5 nA). The dashed curves correspondingly mark the GB. (c) Representative $d I / d V$ curves ( $- 0.8 V$ and 0.5 nA) acquired along the GB (sites 1–7) and at sheet (site 8), marked by circles in (a). (d) $(d I / d V) / (I / V)$ curves acquired across the GB along the arrowed line in (a), shifted vertically for clarity. (e) Lower panel: calculated local DOSs. Upper panel: experimental data from (d) for comparison. Middle panel: model for calculations of the averaged local DOSs, which are obtained by summing the partial DOSs of the colored atoms and then averaging over the atom numbers. The dashed lines in (c)–(e) are eye guides only.
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Figure 3
(a) STM topographic image ( $4.2 \times 3.3 {nm}^{2}$ , $+ 0.8 V$ , and 0.5 nA) of a $(2, 0) ∣ (2, 0)$ GB. The line profile shows a period of 0.5 nm along the GB (blue line). (b) Filled-state STM images ( $- 0.8 V$ ) and (c) empty-state STM images ( $+ 0.8 V$ ) from experimental (upper, $2.9 \times 1.4 {nm}^{2}$ ) and simulated results (lower). The structural model of the $(2, 0) ∣ (2, 0)$ GB is superposed. (d) Upper panel: an averaged $(d I / d V) / (I / V)$ curve at the GB and an individual $(d I / d V) / (I / V)$ curve at a site 5 nm from the GB. Lower panel: calculated DOSs at the labeled sites. Middle panel: model used in the calculations of the averaged local DOSs. (e) Upper panel: $(d I / d V) / (I / V)$ curves acquired at different sites (marked in upper-left inset), shifted for clarity. Lower panel: theoretically calculated partial DOSs of the four nonequivalent carbon atoms at GB core (marked in lower-left inset). The dashed lines in (d) and (e) are eyes guides only.
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Figure 4
[(a),(b)] Structural models, [(c),(d)] band structures (solid lines), and [(e),(f)] spatial distribution of GB states of (3,1)-GNR and (2,0)-GNR, respectively. The $x$ direction is defined perpendicular to the GBs, and the $y$ direction is parallel to the GBs. The (3,1)-GNR has partially zigzag edges, while (2,0)-GNR fully zigzag edges, with width of about 3.4 and 3.1 nm, respectively. The dashed lines in (c) and (d) give the band structures of graphene sheet containing $(3, 1) ∣ (1, 3)$ GB and $(2, 0) ∣ (2, 0)$ GB, respectively, for comparison. The open circles in (c) and (d) denote the contributions from the GB states (red: empty states, orange: filled states) and the edge states (green). The isosurface value is $0.01 e / Å^{3}$ used in (e) and (f).
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