Ruhr-Universität Bochum, D-44780 Bochum, Germany.
Phys Chem Chem Phys. 2013 Oct 14;15(38):16001-11. doi: 10.1039/c3cp52181h. Epub 2013 Aug 20.
Density-functional theory calculations of propanethiolate self-assembled monolayers at the Au(111) surface have been carried out to study the response properties of organic adlayers at metal surfaces to externally applied mechanical forces. "Force maps" are introduced in order to systematically quantify the system's nanomechanical response to both the magnitude and direction of the external forces applied here in an isotensional setup. Depending on the direction and magnitude of the forces we find that there are two dominant phenomena for the most stable c(4 × 2) structure containing two propanethiolate gold adatom adsorbate complexes C3S-Auad-SC3 per unit cell: a sliding of the complete topmost metal layer including adsorbates or the detachment of the thiolate-adatom complexes only. Investigations of the atomistic mechanisms show that the initial configuration of the bulky C3S-Auad-SC3 complexes is responsible for sizable variations in the effective barrier heights for different force directions. A lateral force along the [11[combining macron]0] direction with magnitude between 0.6 nN and 1.4 nN induces a plane sliding of the complete outermost Au(111) layer. In contrast, a force normal to the surface induces a detachment of the thiolate complexes if the force exceeds a threshold value of about 1.4 nN, which is in excellent agreement with experimental results. The use of tilted pulling directions, i.e., applying forces with nonzero normal and lateral components, facilitates the removal of the adsorbate complexes even at notably smaller force magnitudes. This mechanochemical promotion is traced back to a destabilization of the molecule-metal interface as a result of a (slight) lateral displacement to energetically unfavorable sites, thereby weakening chemical bonding of adsorbate complexes to the substrate. These observations are vastly different from what has been found for single molecule pulling normal to the surface, where nanowire formation is typically observed. Overall, a rich spectrum of nanomechanical response scenarios is revealed by these explorative calculations, which is expected to significantly grow upon varying further parameters such as the chain length, surface plane, and substrate metal.
已对金（111）表面的丙硫醇自组装单层进行了密度泛函理论计算，以研究有机覆盖层对金属表面外部施加机械力的响应特性。为了系统地量化系统对本文中在等张设置下施加的外力的大小和方向的纳米力学响应，引入了“力图”。根据力的方向和大小，我们发现对于最稳定的 c(4 × 2)结构，存在两种主要现象，该结构包含每个单元中的两个丙硫醇金吸附原子吸附物络合物 C3S-Auad-SC3：整个最顶层金属层的滑动，包括吸附物，或仅硫醇-吸附原子络合物的脱离。对原子机制的研究表明，大体积 C3S-Auad-SC3 络合物的初始构型负责不同力方向下有效势垒高度的显着变化。大小在 0.6 nN 和 1.4 nN 之间的沿 [11[combining macron]0] 方向的侧向力会引起最外层 Au(111)层的平面滑动。相比之下，如果力超过约 1.4 nN 的阈值，则垂直于表面的力会导致硫醇络合物的脱离，这与实验结果非常吻合。使用倾斜的拉动方向，即施加具有非零法向和侧向分量的力，即使在明显较小的力大小下也有利于去除吸附物络合物。这种机械化学促进作用可追溯到分子-金属界面的不稳定性，这是由于（轻微）侧向位移到能量不利的位置，从而削弱了吸附物络合物与衬底的化学结合。与沿表面垂直施加的单个分子拉动相比，这些观察结果有很大的不同，在这种情况下，通常会观察到纳米线的形成。总体而言，这些探索性计算揭示了丰富的纳米力学响应场景，预计随着进一步改变参数（例如链长、表面平面和衬底金属），这种情况会大大增加。
