Department of Chemistry, University of Wisconsin─Madison, Madison, Wisconsin 53706, United States.
University of Nova Gorica, Materials Research Laboratory, Vipavska 13, SI-5000 Nova Gorica, Slovenia.
J Phys Chem B. 2023 Mar 9;127(9):2083-2091. doi: 10.1021/acs.jpcb.2c08431. Epub 2023 Feb 23.
Strong electric fields exist between the electric double layer and charged surfaces. These fields impact molecular structures and chemistry at interfaces. We have developed a transparent electrode with infrared plasmonic enhancement sufficient to measure FTIR and two-dimensional infrared spectra at submonolayer coverages on the surface to which a voltage can be applied. Our device consists of an infrared transparent substrate, a 10-20 nm layer of conductive indium tin oxide (ITO), an electrically resistive layer of 3-5 nm AlO, and a 3 nm layer of nonconductive plasmonic gold. The materials and thicknesses are set to maximize the surface number density of the monolayer molecules, electrical conductivity, and plasmonic enhancement while minimizing background signals and avoiding Fano line shape distortions. The design was optimized by iteratively characterizing the material roughness and thickness with atomic force microscopy and electron microscopy and by monitoring the plasmon resonance enhancement with spectroscopy. The design is robust to repeated fabrication. This new electrode is tested on nitrile functional groups using a monolayer of 4-mercaptobenzonitrile as well as on CO and CC stretching modes using 4-mercaptobenzoic acid methyl ester. A voltage-dependent Stark shift is observed on both monolayers. We also observe that the transition dipole strength of the CN mode scales linearly with the applied voltage, providing a second way of measuring the surface electric field strength. We anticipate that this cell will enable many new voltage-dependent infrared experiments under applied voltages.
强电场存在于双电层和带电表面之间。这些电场会影响界面处的分子结构和化学。我们开发了一种具有足够的红外等离子体增强的透明电极，可在施加电压的表面上以亚单层覆盖率测量 FTIR 和二维红外光谱。我们的器件由红外透明基底、10-20nm 厚的导电氧化铟锡（ITO）层、3-5nm 厚的电阻 AlO 层和 3nm 厚的非导电等离子体金层组成。材料和厚度的选择是为了最大限度地提高单层分子的表面数密度、电导率和等离子体增强，同时最小化背景信号并避免 Fano 线形状失真。通过原子力显微镜和电子显微镜迭代表征材料粗糙度和厚度，并通过光谱监测等离子体共振增强，对设计进行了优化。该设计具有很强的可重复性。我们使用单层 4-巯基苯甲腈和 4-巯基苯甲酸甲酯的 CO 和 CC 伸缩模式对腈官能团进行了测试。在这两种单层上都观察到了与电压有关的 Stark 位移。我们还观察到 CN 模式的跃迁偶极子强度与施加的电压呈线性关系，这为测量表面电场强度提供了第二种方法。我们预计这种电池将在施加电压下实现许多新的与电压相关的红外实验。
