Transition metal dichalcogenides (TMDs) have become a hotbed of research due to their inherent optical properties and tunability. These 2D layered materials have tunable bandgaps by transitioning from bulk to monolayer, exhibit intralayer and interlayer excitons with strong binding energies at room temperature, possess valley pseudospin, and demonstrate properties that can be affected and tuned according to moiré physics. The optical properties in TMDs are heavily influenced by sample defects such as grain boundaries, charge transfer effects, site vacancies as well as sample strain. Properly characterising defects and strain is essential for implementing these materials in optical devices. This is however difficult to do with diffraction limited techniques due to the fact that the defects and strain, that convolve to form a collective dielectric disorder, exist at the nanoscale. Here we show that the scattering type scanning near-field optical microscopy (s-SNOM) can be used to characterise this disorder by extracting the dielectric function from near field optical images. Using a pre-established inversion method we extracted dielectric values of monolayer WS2 on an Au substrate from different harmonics (2nd harmonic - Fig1(f), 4th harmonic – Fig1(g)) and compared them with spectroscopic ellipsometry (Fig1(h, i)). The data points from the 2nd harmonic reveal values consistent with the ellipsometry measurements while the 4th harmonic data points diverge more towards the dielectric function of WS2. This highlights the surface sensitivity achievable with the s-SNOM which can be done on the nanoscale [1]. We also show a correlation between tip-enhanced photoluminescence intensity and s-SNOM near-field contrast, being important to identify and image the distribution of areas of strain, doping and defects. This work showcases a method of characterising local, nanoscale optical properties by extracting the dielectric function and thus offering the opportunity to carefully choose optimal sample areas to launch, for example, strongly coupled exciton-polaritons [2]. This could act as the foundation for utilising these quasi-particle states for nanoscale applications such simple logic based polaritonic devices.

References

[1] O. Garrity et.al, App. Surface Science, 2022, Vol. 574, (2022)

[2] P. Kusch et.al, Phys. Rev. B, 2021, 103, 235409