Kerfoot James, Legge Elizabeth J, Collins Amy, Chauhan Jasbinder, Rossnagel Kai, Beton Peter H, Mellor Christopher J, Pollard Andrew J, Rance Graham A, George Michael W
Nanoscale and Microscale Research Centre (nmRC), University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
National Physical Laboratory (NPL), Hampton Road, Teddington, TW11 0LW, UK.
Analyst. 2025 Jul 7;150(14):3077-3088. doi: 10.1039/d5an00456j.
Benchmarking the near-field signal enhancement attained using plasmonic metal-coated atomic force microscopy (AFM) probes for tip-enhanced Raman spectroscopy (TERS) and tip-enhanced photoluminescence (TEPL) measurements is challenging given the absence of a suitable reference sample that is simple to prepare, easy to use and compatible with different instrument configurations. To this end, in this study, we have fabricated a flake of monolayer tungsten diselenide (1L-WSe) stamped across the interface of gold and silver thin films on silicon dioxide and glass. We have demonstrated these samples to be effective for the facile determination of near-field Raman and photoluminescence contrast factors in both gap and non-gap mode, respectively. We show that the near-degenerate E12g + A and 2LA(M) peaks in the Raman spectra of WSe enable quantification of Raman contrast factors, with a ∼1.6-fold increase in TERS signal enhancement in gap mode, relative to non-gap mode, observed for a typical probe. Similar differences in the photoluminescence contrast factors were observed comparing in-contact and out-of-contact signal intensity ratios from gap and non-gap mode TEPL measurements. Moreover, in developing a reference methodology we found that the line shape of the TEPL profile was dependent upon the magnitude of the signal enhancement, with a disproportionate increase in the longer wavelength shoulder of the emission observed in gap mode. As this contribution to the asymmetric line shape is tentatively assigned to a dark exciton, which possesses an out-of-plane transition dipole moment, our TEPL measurements indicate that the directionality of the near-field enhancement provides a further handle enabling quantification of probe performance. Using samples prepared on glass, and comparing results obtained from two different instruments, each with a different excitation laser wavelength and optical access, we demonstrate the universal applicability of our reference material for sensitivity benchmarking of metallised AFM probes in both gap and non-gap mode, suitable for both reflection and transmission geometries, and across the range of laser wavelengths typically used for TERS and TEPL.
对于使用等离子体金属涂层原子力显微镜（AFM）探针进行针尖增强拉曼光谱（TERS）和针尖增强光致发光（TEPL）测量所获得的近场信号增强进行基准测试具有挑战性，因为缺乏一种易于制备、易于使用且与不同仪器配置兼容的合适参考样品。为此，在本研究中，我们制备了一片单层二硒化钨（1L-WSe）薄片，该薄片压印在二氧化硅和玻璃上的金和银薄膜界面上。我们已经证明这些样品分别对于在间隙模式和非间隙模式下轻松测定近场拉曼和光致发光对比度因子是有效的。我们表明，WSe拉曼光谱中近简并的E12g + A和2LA(M)峰能够对拉曼对比度因子进行量化，对于典型探针，在间隙模式下TERS信号增强相对于非间隙模式增加了约1.6倍。通过比较间隙模式和非间隙模式TEPL测量的接触和非接触信号强度比，观察到了光致发光对比度因子的类似差异。此外，在开发一种参考方法时，我们发现TEPL谱线的形状取决于信号增强的幅度，在间隙模式下发射的较长波长肩部观察到不成比例的增加。由于这种对不对称谱线形状的贡献暂定为具有面外跃迁偶极矩的暗激子，我们的TEPL测量表明近场增强的方向性提供了另一种手段，能够对探针性能进行量化。使用在玻璃上制备的样品，并比较从两台不同仪器获得的结果，每台仪器具有不同的激发激光波长和光学通路，我们证明了我们的参考材料对于金属化AFM探针在间隙和非间隙模式下的灵敏度基准测试具有普遍适用性，适用于反射和透射几何结构，以及通常用于TERS和TEPL的激光波长范围。
