Benchmarking TERS and TEPL probes: towards a reference sample for quantification of near-field enhancement factors in...

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-WSe₂) 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_1g and 2LA(M) peaks in the Raman spectra of WSe₂ 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.