Department of Chemistry, University of Houston, Houston, Texas 77204, United States.
Acc Chem Res. 2020 Jun 16;53(6):1139-1150. doi: 10.1021/acs.accounts.0c00057. Epub 2020 May 21.
Surface chemistry is a key area of study in the chemical sciences, and many system properties are dominated by the chemistry at the interface between two bulk media. While the interface may have a large influence on the system behavior, there are relatively few molecules at the interface compared to the bulk; thus, probing their unique properties has become a specialized field in physical chemistry. In addition to the heterogeneous phase chemistry, surfaces also present spatial heterogeneity (Chemistry in Two Dimensions). This 2D chemistry affects the properties as much as the heterogeneous phases. If we consider the Cartesian -axis as defining the dimension across the interface between the two bulk phases, then the - plane is the 2D region of the surface. We might even consider that the majority of surface chemistry has been concerned with this -dimension, i.e., surface structure, partition excess, thermodynamics, etc. relative to the bulk, where the 2D distribution was only considered on average. This treatment is understandable since few techniques provide the spatial and chemical resolution needed to deduce the effects of 2D heterogeneity on the surface properties. It is desirable to use an all-optical technique for interface studies because the optical methods provide the chemical specificity through spectroscopy. Also, the use of second-order spectroscopy is typically surface-sensitive without background subtractions or enhancement mechanisms that could limit the range of systems to be investigated.In this Account, the development and selected results of sum frequency generation microscopy and its contributions to the surface chemistry are presented. Sum frequency generation (SFG) provides a unique probe for studying surface chemistry in ambient conditions with surface specificity. SFG provides image contrast based on multiple-chemically important-mechanisms such as chemical functional groups, molecular orientation, surface concentration, molecular conformation, local electric fields, among others. To understand the spatial distribution of heterogeneous chemistry, multiple microscopy methods have been developed which utilize the SFG process to yield spatial information with chemical sensitivity. These spectroscopic-microscopies come with unique advantages as well as challenges. Multiple solutions have been developed in this field to overcome the challenges and improve the advantages. In this Account, some of the leading SFG surface microscopies for surface studies are introduced. Initially, direct imaging of the SFG signal onto a CCD camera provided spatially and spectrally resolved imaging of monolayers on surfaces. However, to speed up the imaging process, the technique of compressive sensing was applied to SFG imaging. Most recently the use of machine learning methods and target factor analysis have improved the quality and acquisition speed of SFG images.
表面化学是化学科学中的一个重要研究领域，许多系统性质都由两个体相之间界面的化学性质所决定。尽管界面可能对系统行为有很大的影响，但与体相相比，界面处的分子相对较少；因此，探测它们的独特性质已成为物理化学的一个专门领域。除了多相化学之外，表面还呈现出空间异质性（二维化学）。这种二维化学与多相化学一样对性质有影响。如果我们将笛卡尔坐标轴定义为穿过两个体相之间界面的维度，那么 - 平面就是表面的二维区域。我们甚至可以认为，大多数表面化学都与这个维度有关，即表面结构、分配过剩、热力学等，而与体相相比，二维分布只是平均考虑的。这种处理方式是可以理解的，因为很少有技术能够提供空间和化学分辨率，以推断二维异质性对表面性质的影响。在界面研究中使用全光学技术是可取的，因为光学方法通过光谱学提供化学特异性。此外，二阶光谱学的使用通常对表面敏感，不需要背景扣除或增强机制，这些机制可能会限制要研究的系统范围。在本报告中，介绍了和频产生显微镜的发展及其在表面化学中的应用，并展示了其在表面化学中的发展和部分研究成果。和频产生（SFG）提供了一种独特的探针，可在环境条件下对表面化学进行研究，具有表面特异性。SFG 基于多个化学上重要的机制，如化学官能团、分子取向、表面浓度、分子构象、局部电场等，提供了图像对比度。为了理解异质化学的空间分布，已经开发了多种显微镜方法，利用 SFG 过程提供具有化学敏感性的空间信息。这些光谱显微镜具有独特的优势和挑战。为了克服挑战并提高优势，在该领域已经开发了多种解决方案。在本报告中，介绍了一些用于表面研究的领先的 SFG 表面显微镜。最初，将 SFG 信号直接成像到 CCD 相机上，提供了表面单层的空间和光谱分辨成像。然而，为了加快成像过程，将压缩感知技术应用于 SFG 成像。最近，使用机器学习方法和目标因子分析提高了 SFG 图像的质量和采集速度。
