Scanning Tunneling Microscopic Observation of Adatom-Mediated Motifs on Gold−Thiol Self-Assembled Monolayers at High Coverage

Self-assembled monolayers (SAMs) formed by chemisorption of a branched-chain alkanethiol, 2-methyl-1-propanethiol, on Au(111) surfaces were studied by in situ scanning tunneling microscopy (STM) under electrochemical potential control and analyzed using extensive density functional theory (DFT) calculations. The SAM forms in the unusual (8 × √3)-4 superlattice, producing a very complex STM image. Seventy possible structures were considered for the SAM, with the calculated lowest-energy configuration in fact predicting the details of the unusual observed STM image. The most stable structure involves two R−S−Au−S−R adatom-mediated motifs per surface cell, with steric-induced variations in the adsorbate alignment inducing the observed STM image contrasts. Observed pits covering 5.6 ± 0.5% of the SAM surface are consistent with this structure. These results provide the missing link from the structural motifs observed on surfaces at low coverage and on gold nanoparticles to the observed spectroscopic properties of high-coverage SAMs formed by methanethiol. However, the significant role attributed to intermolecular steric packing effects suggests a lack of generality for the adatom-mediated motif at high coverage.

本研究针对在Au(111)表面通过化学吸附支链烷硫醇2-甲基-1-丙硫醇所形成的自组装单分子层（Self-assembled monolayers, SAMs）展开系统研究：采用电化学电势调控下的原位扫描隧道显微镜（scanning tunneling microscopy, STM）进行原位表征，并结合大规模密度泛函理论（density functional theory, DFT）计算开展分析。该SAM形成了罕见的(8 × √3)-4超晶格结构，对应极为复杂的STM图像。研究共考量70种可能的SAM构型，其中计算得到的最低能量构型精准重现了实验观测到的非典型STM图像细节。最稳定的构型在每个表面原胞中包含两个R−S−Au−S−R型吸附原子介导的基元，吸附质排列因空间位阻产生的变化正是实验观测到的STM图像衬度的来源。观测到的覆盖SAM表面5.6 ± 0.5%的凹坑与该结构模型高度吻合。本研究填补了从低覆盖度下表面及金纳米颗粒上观测到的结构基元，到甲硫醇形成的高覆盖度SAMs的光谱特性之间的缺失环节。不过，分子间空间位阻堆积效应所起到的关键作用表明，吸附原子介导的基元在高覆盖度条件下并不具有普适性。