Edge-Termination and Core-Modification Effects of Hexagonal Nanosheet Graphene

Jin-Pei Deng,Chih-Hsun Lin,Wen-Hao Chen,Shou-Po Chiu,Bo-Cheng Wang

doi:10.3390/molecules19022361

Abstract

Optimized geometries and electronic structures of two different hexagonal grapheme nanosheets (HGNSs), with armchair (n-A-HGNS, n = 3–11) and zigzag (n-Z-HGNS, n = 1–8) edges have been calculated by using the GGA/PBE method implemented in the SIESTA package, with the DZP basis set, where n represents the number of peripheral rings. The computed HOMO-LUMO energy gap (Eg = ELUMO − EHOMO) decreases for fully H-terminated A- and Z-HGNSs with increasing n, i.e., with increasing nanosheet size and pπ-orbitals being widely delocalized over the sheet surface. The full terminations, calculated with various functional groups, including the electron-withdrawing (F-, Cl-, and CN-) and -donating (OH-, and SH-) substitutions, were addressed. Significant lowering of EHOMO and ELUMO was obtained for CN-terminated HGNS as compared to those for H-terminated ones due to the mesomeric effect. The calculated Eg value decreases with increasing n for all terminations, whereby for the SH-termination in HGNS, the termination effect becomes less significant with increasing n. Further, the calculation results for stabilities of HGNS oxides support the tendency toward the oxidative reactivity at the edge site of the sheet, which shows most pronounced C-C bond length alternation, by chemical modification. Physical properties of HGNSs with various numbers of the core-defects, which can be obtained by strong oxidation, were also investigated. Their structures can change drastically from planar to saddle-like shapes. These conformations could be used as stationary phases with controlled interaction in the separation methods such as HPLC and the other chemical analysis techniques.

Highlights

The recent discovery of grapheme [1], a single atomic sheet of graphite, has ignited intense research activities to elucidate the electronic properties of this novel two-dimensional pπ-conjugated extended system
These calculation results should provide more information on nano-scale phenomena, including structural and electronic properties, for both A-hexagonal grapheme nanosheets (HGNSs) and Z-HGNS that should be helpful for the development of new technologies with applications ranging from nano-electronics to nano-mechanical devices and chemical analysis techniques
Fused hexagonal rings expand along the C2 symmetry axis as C6 extension within the D6h point group (Figure 1); ; for n-A-HGNS; n- is the number of peripheral rings around the nanosheet

Summary

Introduction

The recent discovery of grapheme [1], a single atomic sheet of graphite, has ignited intense research activities to elucidate the electronic properties of this novel two-dimensional pπ-conjugated extended system. The physical and chemical properties of these graphene nanosheets correspond to 2-D quantum dot features due to their size, edged shape and termination which could make them behave as conductors and semiconductors. Properties suitable for potential applications, chemically modified HGNS with edge-termination at the periphery of graphene were investigated. According to the optimized geometries, the presence of a vacancy defect in HGNS changes its structure from planar (normal HGNS) to saddle- or twisted-like These calculation results should provide more information on nano-scale phenomena, including structural and electronic properties, for both A-HGNS and Z-HGNS that should be helpful for the development of new technologies with applications ranging from nano-electronics to nano-mechanical devices and chemical analysis techniques

Results and Discussion

Size-Dependent Effect

Edge-Termination Effect

Experimental

Conclusions