石墨烯纳米片的电子性质：能隙、永久偶极子、终止效应和拉曼光谱
Electronic properties of graphene nano-flakes: Energy gap, permanent dipole, termination effect and Raman spectroscopy
摘要：
采用密度泛函理论研究了不同边缘钝化处理的石墨烯纳米片的电子性质。考虑用F和H原子钝化：C $_{N_c}$ X $_{N_x}$（X=F或H）。我们用$10<N_c<56$研究了GNF，并将我们自己限制在最低能量配置。我们发现：i）最高占据分子轨道（HOMO）和最低未占据分子轨道（LUMO）之间的能量差$Δ$随着$N_c$而减小，ii）拓扑缺陷（五边形和七边形）打破了GNF的对称性并增强了电极化的强度，iii）双层GNF的相互作用可以通过偶极-偶极相互作用来理解，发现偶极-偶极相互作用对GNF的相对取向敏感，iv）永久偶极子取决于边缘终止原子，而能隙与之无关，以及v）GNF中七边形和五边形缺陷的存在导致自旋上电子和自旋下电子的能量之间的最大差异，与F钝化的GNF相比，H钝化的GNF的差异更大。我们的研究清楚地显示了几何形状、尺寸、终止方式和双层结构对小石墨烯纳米纤维电子性质的影响，揭示了石墨烯纳米薄片的重要特征，这些特征可以用拉曼光谱来检测。
英文摘要：
The electronic properties of graphene nano-flakes (GNFs) with different edge passivation is investigated by using density functional theory. Passivation with F and H atoms are considered: C$_{N_c}$ X$_{N_x}$ (X=F or H). We studied GNFs with $10<N_c<56$ and limit ourselves to the lowest energy configurations. We found that: i) the energy difference $Δ$ between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases with $N_c$, ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, iii) the mutual interaction of bilayer GNFs can be understood by dipole-dipole interaction which were found sensitive to the relative orientation of the GNFs, iv) the permanent dipoles depend on the edge terminated atom, while the energy gap is independent of it, and v) the presence of heptagon and pentagon defects in the GNFs results in the largest difference between the energy of the spin-up and spin-down electrons which is larger for the H-passivated GNFs as compared to F-passivated GNFs. Our study shows clearly the effect of geometry, size, termination and bilayer on the electronic properties of small GNFs.This study reveals important features of graphene nano-flakes which can be detected using Raman spectroscopy.
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