Notingher Ioan, Elfick Alistair
University of Edinburgh, School of Engineering and Electronics, The King's Buildings, Edinburgh EH9 3JL, The United Kingdom.
J Phys Chem B. 2005 Aug 25;109(33):15699-706. doi: 10.1021/jp0523120.
Finite element (FE) models were built to define the optimal experimental conditions for tip-enhanced Raman spectroscopy (TERS) of thin samples. TERS experimental conditions were mimicked by including in the FE models dielectric or metallic substrates with thin dielectric samples and by considering the wavelength dependence of the dielectric properties for the metallic materials. Electromagnetic coupling between the substrate/sample and the SPM tips led to dramatic changes of both the spatial distribution and magnitude of the scattered electric field which depended on the substrate dielectric permittivity and excitation wavelength. Raman scattering as high as 10(8) with a spatial resolution of approximately 8 nm was estimated for gold SPM tips and gold substrate when excitation is performed at 532 nm (near-resonance wavelength). For dielectric samples (approximately 4 nm thick), the enhancement of Raman scattering intensity is estimated at approximately 10(5); this does not depend significantly on the sample dielectric permittivity for dielectric samples. These results suggest that TERS experimental conditions should be estimated and optimized for every individual application considering the geometric factors and electric properties of the materials involved. Such optimizations could enlarge the range of applications for TERS to samples eliciting weaker intrinsic Raman scattering, such as biological samples.
构建了有限元（FE）模型，以确定薄样品的针尖增强拉曼光谱（TERS）的最佳实验条件。通过在有限元模型中纳入带有薄介电样品的介电或金属基底，并考虑金属材料介电特性的波长依赖性，来模拟TERS实验条件。基底/样品与扫描探针显微镜（SPM）针尖之间的电磁耦合导致散射电场的空间分布和大小都发生显著变化，这取决于基底的介电常数和激发波长。当在532nm（近共振波长）进行激发时，对于金SPM针尖和金基底，估计拉曼散射高达10^8，空间分辨率约为8nm。对于介电样品（约4nm厚），拉曼散射强度的增强估计约为10^5；对于介电样品，这并不显著依赖于样品的介电常数。这些结果表明，应考虑所涉及材料的几何因素和电学性质，针对每个具体应用估计和优化TERS实验条件。这种优化可以将TERS的应用范围扩大到诸如生物样品等固有拉曼散射较弱的样品。
