Tip-Enhanced Raman Scattering on Both Sides of the Schrödinger Equation.
Tip-Enhanced Raman Scattering on Both Sides of the Schrödinger Equation.
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DOI：
10.1021/acs.accounts.1c00597
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发表时间：
2021-12
影响因子：
18.3
通讯作者：
P. El-Khoury
中科院分区：
文献类型：
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作者：
P. El-Khoury
概述历史上，分子光谱学家将注意力集中在薛定谔方程的右侧。我们的主要目标曾经并且仍然与确定（生物）分子系统的哈密顿算符有关。从理论光谱学家的角度来看，这需要改变哈密顿量模型的参数，直到预测的可观测值与其实验类似物一致。在这种情况下，不太重视等式的左侧，其中描述了系统与其直接本地环境之间的相互作用。后者在光学显微镜和光谱学的现代应用中特别有意义和信息丰富，这些应用利用表面等离子体来增强分子散射截面并提高通常受衍射限制的可达到的空间分辨率。事实上，对金属纳米尖端顶端附近的光进行操纵已经使得单分子检测、识别和成像成为可能。所谓的尖端增强光学纳米光谱/纳米成像方法的独特优势是不言而喻的：超高空间分辨率（纳米或更高）和终极灵敏度（低至几摩尔）都是可以实现的，同时保留一次对一个分子进行化学指纹识别的能力（例如，通过拉曼散射）。同一方法的一个同样有趣的方面源于使用单个分子的特性来表征其所在的局部环境。薛定谔方程左侧的单分子光谱学概念当然并不新颖，并且在开创性的单分子研究中得到了讨论，这些研究最终导致了诺贝尔化学奖。也就是说，当使用尖端增强拉曼散射 (TERS) 执行时，通过超灵敏光谱进行局部环境测绘会获得独特的风味。这是本帐户的主题。在最近的一系列报告中，我们的小组利用 TERS 来表征纳米局域和增强光场的不同属性。用于此目的的平台由化学功能化的等离子体纳米结构和纳米粒子组成，使用原子力显微镜的可见光照射的金或银涂层探针进行成像。通过对记录的光谱纳米图像的详细分析，我们发现分子拉曼光谱可用于跟踪环境条件下具有纳米空间分辨率的光场的幅度、共振、时空梯度，甚至矢量分量。另一方面，了解空间变化的光场如何调制分子纳米拉曼光谱对于纳米光子学的新兴领域至关重要。例如，通过 TERS 跟踪等离激元增强的化学转变需要对分子重定向和多极拉曼散射的光学特征有更深入的基本了解，这两者都可能由 TERS 中起作用的局部光场梯度驱动。我们阐述了这些概念，并向读者介绍了薛定谔方程左侧通常不太受重视但同样令人兴奋的 TERS 世界。
ConspectusHistorically, molecular spectroscopists have focused their attention to the right-hand side of the Schrödinger equation. Our major goal had and still has to do with determining a (bio)molecular system's Hamiltonian operator. From a theoretical spectroscopist's perspective, this entails varying the parameters of a model Hamiltonian until the predicted observables agree with their experimental analogues. In this context, less emphasis has been put on the left-hand side of the equation, where the interplay between a system and its immediate local environment is described. The latter is particularly meaningful and informative in modern applications of optical microscopy and spectroscopy that take advantage of surface plasmons to enhance molecular scattering cross-sections and to increase the attainable spatial resolution that is classically limited by diffraction. Indeed, the manipulation of light near the apex of a metallic nanotip has enabled single molecule detection, identification, and imaging. The distinct advantages of the so-called tip-enhanced optical nanospectroscopy/nanoimaging approaches are self-evident: ultrahigh spatial resolution (nanometer or better) and ultimate sensitivity (down to yoctomolar) are both attainable, all while retaining the ability to chemically fingerprint one molecule at a time (e.g., through Raman scattering). An equally interesting aspect of the same approach stems from using the properties of a single molecule to characterize the local environment in which it resides. This concept of single molecule spectroscopy on the left-hand side of the Schrödinger equation is certainly not novel and has been discussed in pioneering single molecule studies that ultimately led to a Nobel prize in chemistry. That said, local environment mapping through ultrasensitive optical spectroscopy acquires a unique flavor when executed using tip-enhanced Raman scattering (TERS). This is the subject of this Account.In a series of recent reports, our group utilized TERS to characterize different properties of nanolocalized and enhanced optical fields. The platforms that were used to this end consist of chemically functionalized plasmonic nanostructures and nanoparticles imaged using visible-light-irradiated gold- or silver-coated probes of an atomic force microscope. Through a detailed analysis of the recorded spectral nanoimages, we found that molecular Raman spectra may be used to track the magnitudes, resonances, spatiotemporal gradients, and even vector components of optical fields with nanometer spatial resolution under ambient conditions. On the other side of the equation, understanding how spatially varying optical fields modulate molecular nano-Raman spectra is of utmost importance to emerging areas of nanophotonics. For instance, tracking plasmon-enhanced chemical transformations via TERS necessitates a deeper fundamental understanding of the optical signatures of molecular reorientation and multipolar Raman scattering, both of which may be driven by local optical field gradients that are operative in TERS. We illustrate these concepts and introduce the readers to the generally less appreciated and equally exciting world of TERS on the left-hand side of the Schrödinger equation.