Chen Yuhong, Bâldea Ioan, Yu Yongxin, Liang Zining, Li Ming-De, Koren Elad, Xie Zuoti
Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa 3200003, Israel.
Department of Materials Science and Engineering, Guangdong Provincial Key Laboratory of Materials and Technologies for Energy Conversion (MATEC), Guangdong Technion-Israel Institute of Technology, 241 Daxue Road, Shantou, Guangdong 515063, China.
Langmuir. 2024 Feb 27;40(8):4410-4423. doi: 10.1021/acs.langmuir.3c03759. Epub 2024 Feb 13.
In this paper, we report results on the electronic structure and transport properties of molecular junctions fabricated via conducting probe atomic force microscopy (CP-AFM) using self-assembled monolayers (SAMs) of -alkyl chains anchored with acetylene groups (CA; = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage (-) characteristics of CA CP-AFM junctions can be very accurately reproduced by the same off-resonant single-level model (orSLM) successfully utilized previously for many other junctions. We demonstrate that important insight into the energy-level alignment can be gained from experimental data of transport (processed via the orSLM) and ultraviolet photoelectron spectroscopy combined with ab initio quantum chemical information based on the many-body outer valence Green's function method. Measured conductance < < is found to follow the same ordering as the metal work function Φ < Φ < Φ, a fact that points toward a transport mediated by an occupied molecular orbital (MO). Still, careful data analysis surprisingly revealed that transport is not dominated by the ubiquitous HOMO but rather by the HOMO-1. This is an important difference from other molecular tunnel junctions with p-type HOMO-mediated conduction investigated in the past, including the alkyl thiols (CT) to which we refer in view of some similarities. Furthermore, unlike in CT and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in CA an additional MO shift, whose independence of size () rules out significant image charge effects. Along with the prevalence of the HOMO-1 over the HOMO, the impact of the "second" (tip) electrode on the energy level alignment is another important finding that makes the CA and CT junctions different. What ultimately makes CA unique at the microscopic level is a salient difference never reported previously, namely, that CA's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present CA from the CT, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.
在本文中，我们报告了通过导电探针原子力显微镜（CP-AFM）制备的分子结的电子结构和输运性质的结果，该分子结使用在银、金和铂电极上由乙炔基（CA；= 8、9、10和12）锚定的-烷基链的自组装单分子层（SAMs）。我们发现，CA CP-AFM结的电流-电压（-）特性可以通过之前成功用于许多其他结的相同非共振单能级模型（orSLM）非常准确地重现。我们证明，基于多体外层价格林函数方法，结合从头算量子化学信息，通过输运实验数据（经orSLM处理）和紫外光电子能谱，可以深入了解能级排列。测得的电导< < 遵循与金属功函数Φ < Φ < Φ相同的顺序，这一事实表明输运是由占据分子轨道（MO）介导的。然而，仔细的数据分析令人惊讶地发现，输运并非由普遍存在的最高占据分子轨道（HOMO）主导，而是由HOMO-1主导。这与过去研究的其他具有p型HOMO介导传导的分子隧道结有重要区别，鉴于一些相似性，我们在此提及烷基硫醇（CT）。此外，与CT和过去研究的其他硫醇基锚定结不同，AFM针尖在CA中会引起额外的MO位移，其与尺寸（）无关排除了显著的镜像电荷效应。除了HOMO-1比HOMO普遍外，“第二”（针尖）电极对能级排列的影响是另一个重要发现，这使得CA和CT结有所不同。最终使CA在微观层面独一无二的是一个此前从未报道过的显著差异，即CA的炔基官能团产生了两个能量相近的（HOMO和HOMO-1）轨道。这将目前的CA与CT区分开来，CT的源于其硫醇基的HOMO在能量上与其他MOs有很好的分离。
