Department of Energy Systems Engineering, DGIST , Daegu 42988, Korea.
GREMAN, UMR 7347, CNRS, Université François Rabelais , 37200 Tours, France.
Nano Lett. 2017 Jul 12;17(7):4061-4066. doi: 10.1021/acs.nanolett.7b00596. Epub 2017 Jun 7.
A fullerene derivative with five hydroxyphenyl groups attached around a pentagon, (4-HOCH)HC (1), has shown an asymmetric current-voltage (I-V) curve in a conducting atomic force microscopy experiment on gold. Such molecular rectification has been ascribed to the asymmetric distribution of frontier molecular orbitals over its shuttlecock-shaped structure. Our nonequilibrium Green's function (NEGF) calculations based on density functional theory (DFT) indeed exhibit an asymmetric I-V curve for 1 standing up between two Au(111) electrodes, but the resulting rectification ratio (RR ∼ 3) is insufficient to explain the wide range of RR observed in experiments performed under a high bias voltage. Therefore, we formulate a hypothesis that high RR (>10) may come from molecular orientation switching induced by a strong electric field applied between two electrodes. Indeed, molecular dynamics simulations of a self-assembled monolayer of 1 on Au(111) show that the orientation of 1 can be switched between standing-up and lying-on-the-side configurations in a manner to align its molecular dipole moment with the direction of the applied electric field. The DFT-NEGF calculations taking into account such field-induced reorientation between up and side configurations indeed yield RR of ∼13, which agrees well with the experimental value obtained under a high bias voltage.
一个在五元环上连接了五个羟苯基的富勒烯衍生物(4-HOCH)HC(1),在金的导电原子力显微镜实验中表现出不对称的电流-电压(I-V)曲线。这种分子整流归因于其羽毛球形状结构上的前线分子轨道的不对称分布。我们基于密度泛函理论(DFT)的非平衡格林函数(NEGF)计算确实显示了 1 在两个 Au(111)电极之间直立时的不对称 I-V 曲线,但得到的整流比(RR∼3)不足以解释在高偏压下进行的实验中观察到的广泛的 RR。因此,我们提出了一个假设,即高 RR(>10)可能来自于两个电极之间施加的强电场引起的分子取向切换。事实上,在 Au(111)上的 1 自组装单层的分子动力学模拟表明,1 的取向可以在直立和侧躺两种构型之间切换,从而使其分子偶极矩与施加电场的方向对齐。考虑到这种在直立和侧躺构型之间的电场诱导重取向的 DFT-NEGF 计算确实产生了约 13 的 RR,这与在高偏压下获得的实验值非常吻合。