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Raman microscopy proved to be an extremely useful technique for characterization of 2D materials such as graphene, transition metal dichalcogenides (TMDs), black phosphorous, etc. Unfortunately, natural spatial resolution of confocal Raman microscopy, which is limited by the wavelength of the laser used (400-800 nm), is not sufficient for mapping heterogeneities and defects in these materials with characteristic dimensions of few – to few tens of nanometers.
Tip Enhanced Raman Spectroscopy (TERS) provides dramatically improved spatial resolution of Raman maps, down to few nanometers, and in addition provides dramatic enhancement of the Raman signal. Since TERS is a relatively new technique, peculiarities of the near-field Raman response of many 2D materials still remain to be discovered and explained.
Interesting unexpected effect was observed in the course of TERS characterization of WSe2 and MoSe2 exfoliated to gold and chromium. It is well known that TERS produces the strongest enhancement in so-called gap mode, when a thin sample is sandwiched between a plasmonic tip and a plasmonic substrate, usually silver or gold. To our surprise, TERS spectra of both WSe2 and MoSe2 crystals exfoliated to chromium showed very similar intensities of characteristic Raman bands compared to samples exfoliated to gold, although the background spectra obtained from the bare metal areas were much weaker for chromium compared to gold. Calculations of the optical field intensity of a TERS probe over gold and chromium surface confirmed that we indeed observed a gap mode TERS response on non-plasmonic chromium substrate. This important observation expands the choice of substrates suitable for high quality TERS characterization of 2D materials.
Another interesting phenomenon discovered in the course of TERS imaging of WSe2 and MoSe2 deposited on metallic substrates was the appearance of new Raman peak in WSe2 deposited on silver at 295-297 cm-1 and an intense peak in TERS spectra of MoSe2 at 335cm-1, which is either absent or much less pronounced in conventional confocal Raman spectra of this material. We’ll discuss possible nature of these unexpected peaks.
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