Cui Jie, Zhang Yao, Dong Zhen-Chao
Hefei National Research Center for Physical Sciences at the Microscale and Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei 230026, China.
School of Physics and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China.
Nanophotonics. 2023 Oct 20;12(21):4031-4042. doi: 10.1515/nanoph-2023-0403. eCollection 2023 Oct.
Tip-enhanced Raman spectroscopy (TERS) has recently been developed to push the spatial resolution down to single-chemical-bond scale. The morphology of the scanning tip, especially the atomistic protrusion at the tip apex, plays an essential role in obtaining both high spatial resolution and large field enhancement at the Ångström level. Although it is very difficult to directly characterize the atomistic structures of the tip, the Raman scattering from the apex's own vibrations of the metal tip can provide valuable information about the stacking of atoms at the tip apex. However, conventional quantum chemistry packages can only simulate the Raman scattering of small metal clusters with few atoms due to huge computational cost, which is not enough since the shaft of the tip behind the apex also makes significant contributions to the polarizabilities of the whole tip. Here we propose an atomistic polarization model to simulate the Raman spectra of large metal tips at subwavelength scales based on the combination of the atomistic discrete dipole approximation model and the density functional theory. The atomistic tip with different sizes and stacking structures is considered in its entirety during the calculation of polarizabilities, and only the vibrational contributions from the tip apex are taken into account to simulate the Raman spectra of the tip. The Raman spectral features are found to be very sensitive to the local constituent element at the tip apex, atomic stacking modes, and shape of the tip apex, which can thus be used as a fingerprint to identify different atomistic structures of the tip apex. Moreover, our approaches can be extended to the metal tips with sub-wavelength sizes, making it possible to consider both the large scale and the atomistic detail of the tip simultaneously. The method presented here can be used as a basic tool to simulate the Raman scattering process of the metal tips and other nanostructures in an economic way, which is beneficial for understanding the roles of atomistic structures in tip- and surface-enhanced spectroscopies.
尖端增强拉曼光谱（TERS）最近得到了发展，可将空间分辨率降低到单化学键尺度。扫描尖端的形态，特别是尖端顶点处的原子级突出部分，对于在埃级获得高空间分辨率和大场增强起着至关重要的作用。尽管直接表征尖端的原子结构非常困难，但金属尖端顶点自身振动产生的拉曼散射可以提供有关尖端顶点处原子堆积的有价值信息。然而，由于计算成本巨大，传统的量子化学软件包只能模拟少数原子组成的小金属簇的拉曼散射，这是不够的，因为顶点后面的尖端轴对整个尖端的极化率也有显著贡献。在此，我们基于原子离散偶极近似模型和密度泛函理论的结合，提出了一种原子极化模型，用于模拟亚波长尺度下大金属尖端的拉曼光谱。在计算极化率时，会全面考虑具有不同尺寸和堆积结构的原子级尖端，并且仅考虑尖端顶点的振动贡献来模拟尖端的拉曼光谱。发现拉曼光谱特征对尖端顶点处的局部组成元素、原子堆积模式和尖端顶点形状非常敏感，因此可以用作识别尖端顶点不同原子结构的指纹。此外，我们的方法可以扩展到亚波长尺寸的金属尖端，从而能够同时考虑尖端的大尺度和原子细节。这里提出的方法可以作为一种基本工具，以经济的方式模拟金属尖端和其他纳米结构的拉曼散射过程，这有助于理解原子结构在尖端增强和表面增强光谱学中的作用。
