Ke San-Huang, Baranger Harold U, Yang Weitao
Department of Chemistry, Duke University, Durham, North Carolina 27708-0354, USA.
J Chem Phys. 2005 Feb 15;122(7):074704. doi: 10.1063/1.1851496.
Using benzene sandwiched between two Au leads as a model system, we investigate from first principles the change in molecular conductance caused by different atomic structures around the metal-molecule contact. Our motivation is the variable situations that may arise in break junction experiments; our approach is a combined density functional theory and Green function technique. We focus on effects caused by (1) the presence of an additional Au atom at the contact and (2) possible changes in the molecule-lead separation. The effects of contact atomic relaxation and two different lead orientations are fully considered. We find that the presence of an additional Au atom at each of the two contacts will increase the equilibrium conductance by up to two orders of magnitude regardless of either the lead orientation or different group-VI anchoring atoms. This is due to a resonance peak near the Fermi energy from the lowest energy unoccupied molecular orbital. In the nonequilibrium properties, the resonance peak manifests itself in a negative differential conductance. We find that the dependence of the equilibrium conductance on the molecule-lead separation can be quite subtle: either very weak or very strong depending on the separation regime.
以夹在两个金电极之间的苯作为模型体系，我们从第一性原理出发，研究了金属 - 分子接触周围不同原子结构引起的分子电导变化。我们的动机源于断结实验中可能出现的各种情况；我们采用的方法是密度泛函理论与格林函数技术相结合。我们关注由以下两种情况引起的效应：（1）接触处存在额外的金原子；（2）分子与电极间距可能发生的变化。充分考虑了接触原子弛豫和两种不同电极取向的影响。我们发现，无论电极取向如何，也无论第 VI 族不同的锚定原子如何，在两个接触处各存在一个额外的金原子都会使平衡电导增加高达两个数量级。这是由于最低能量未占据分子轨道在费米能级附近出现了一个共振峰。在非平衡性质中，共振峰表现为负微分电导。我们发现平衡电导对分子 - 电极间距的依赖性可能非常微妙：根据间距范围，其影响要么非常微弱，要么非常强烈。
