Quantum Mechanical Effects in High-Resolution Tip-Enhanced Raman Imaging
Quantum Mechanical Effects in High-Resolution Tip-Enhanced Raman Imaging
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DOI：
10.1021/acs.jpcc.2c03309
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发表时间：
2022-07
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通讯作者：
Rebecca L. M. Gieseking
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作者：
Rebecca L. M. Gieseking
针尖增强拉曼光谱（TERS）由于等离子体激元电场的限制，具有原子级尖锐针尖可以实现亚纳米空间分辨率。虽然局部电磁场的增强通常是主要的增强机制，但化学相互作用可以在亚纳米尺度上大幅修改TERS强度。模拟这些化学相互作用需要对分子和金属尖端进行量子力学处理。半经验的INDO/CIS模型再现的TD-DFT光谱的银纳米团簇，并允许直接分解的TERS增强到电磁和化学的贡献。在一个典型的Agnanosire-CO2系统中，我们发现，在亚纳米尺度上的化学相互作用大大修改TERS强度，抑制拉曼活性伸缩模式的信号，增强拉曼非活性模式的信号。TERS图像中的增强轮廓是由三个不同因素的变化引起的：（1）纳米线上的有效电场，（2）基态电荷转移的程度，以及（3）由于破缺而导致的σ型和π型分子轨道之间的混合程度。这些结果表明，几种量子力学效应之间的微妙相互作用对于理解亚纳米尺度上TERS图像的起源至关重要。
Tip-enhanced Raman spectroscopy (TERS) with atomically sharp tips can achieve subnanometer spatial resolution due to confinement of the plasmonic electric field. Although enhancement of the local electromagnetic field is generally the dominant enhancement mechanism, chemical interactions may substantially modify the TERS intensity on the subnanometer scale. Modeling these chemical interactions requires a quantum mechanical treatment of both the molecule and the metal tip. The semiempirical INDO/CIS model reproduces the TD-DFT optical spectra of Ag nanoclusters and allows straightforward decomposition of the TERS enhancement into electromagnetic and chemical contributions. In a prototypical Agnnanowire–CO2system, we show that chemical interactions on a subnanometer scale substantially modify the TERS intensity, suppressing the signal of the Raman-active stretching mode and enhancing the signal of the Raman-inactive mode. The enhancement profile in the TERS images results from changes in three distinct factors: (1) the effective electric field on the nanowire, (2) the extent of ground-state charge transfer, and (3) the extent of mixing between σ-type and π-type molecular orbitals due to symmetry-breaking. These results suggest that a subtle interplay between several quantum mechanical effects are critical to understand the origins of TERS images on the subnanometer scale.