Li Hang, Zhang Yu-Fan, Zhang Xian-Biao, Farrukh Aftab, Zhang Yang, Zhang Yao, Dong Zhen-Chao
Hefei National Laboratory for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
J Chem Phys. 2020 Dec 28;153(24):244201. doi: 10.1063/5.0033383.
[n]Cycloparaphenylene ([n]CPP) molecules have attracted broad interests due to their unique properties resulting from the distorted and strained aromatic hoop structures. In this work, we apply sub-nanometer resolved tip-enhanced Raman spectroscopy (TERS) to investigate the adsorption configurations and structural deformations of [12]CPP molecules on metal substrates with different crystallographic orientations. The TERS spectra for a [12]CPP molecule adsorbed on the isotropic Cu(100) surface are found to be essentially the same over the whole nanohoop, indicating an alternately twisted structure that is similar to the [12]CPP molecule in free space. However, when the [12]CPP molecules are adsorbed on the anisotropic Ag(110) surface, the molecular shape is found to be severely deformed into two types of adsorption configurations: one showing an interesting "Möbius-like" feature and the other showing a symmetric bending structure. Their TERS spectral features are found to be site-dependent over the hoop and even show peak splitting for the out-of-plane C-H bending vibrations. The deformed structural models gain strong support from the spatial distribution of "symmetric" TERS spectra at different positions on the hoop. Further TERS imaging, with a spatial resolution down to ∼2 Å, provides a panoramic view on the local structural deformations caused by different tilting of the benzene units in real space, which offers insights into the subtle changes in the aromatic properties over the deformed hoop owing to inhomogeneous molecule-substrate interactions. The ability of TERS to probe the molecular structure and local deformation at the sub-molecular level, as demonstrated here, is important for understanding surface science as well as molecular electronics and optoelectronics at the nanoscale.
[n]环对亚苯基（[n]CPP）分子因其扭曲且应变的芳香环结构所产生的独特性质而引起了广泛关注。在这项工作中，我们应用亚纳米分辨的针尖增强拉曼光谱（TERS）来研究[12]CPP分子在具有不同晶体取向的金属基底上的吸附构型和结构变形。发现吸附在各向同性的Cu(100)表面上的[12]CPP分子的TERS光谱在整个纳米环上基本相同，表明其具有类似于自由空间中[12]CPP分子的交替扭曲结构。然而，当[12]CPP分子吸附在各向异性的Ag(110)表面上时，发现分子形状严重变形为两种吸附构型：一种呈现出有趣的“类莫比乌斯”特征，另一种呈现出对称弯曲结构。发现它们的TERS光谱特征在环上依赖于吸附位点，甚至对于面外C-H弯曲振动显示出峰分裂。变形的结构模型从环上不同位置的“对称”TERS光谱的空间分布中获得了有力支持。进一步的TERS成像，空间分辨率低至约2 Å，提供了关于苯单元在实空间中不同倾斜导致的局部结构变形的全景视图，这有助于深入了解由于分子 - 基底相互作用不均匀而导致的变形环上芳香性质的细微变化。如本文所示，TERS在亚分子水平探测分子结构和局部变形的能力对于理解表面科学以及纳米尺度的分子电子学和光电子学非常重要。
