Andrey Krayev1 Thomas Darlington2 P James Schuck3 Nicholas Borys2 Deep Jariwala4

1, Horiba Scientific, Novato, California, United States
2, Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California, United States
3, Columbia University, New York, New York, United States
4, University of Pennsylvania, Philadelphia, Pennsylvania, United States

The optical properties of Van der Waals semiconductors such as transition metal dichalcogenides (TMDC), black phosphorous, SnS etc. are dominated by excitons which are stable at room temperature owing to decreased Coulomb screening which yields large exciton binding energies (> 0.5 eV). Investigation of the photoluminescence (PL) response is one of the most direct methods for probing excitons in these materials.
Nanoscale spectroscopic imaging, such as Tip Enhanced Photoluminescence (TEPL) and Tip enhanced Raman Spectroscopy (TERS), provides new capabilities to probe nanoscale structural and opto-electronic inhomogeneities in novel 2D semiconductors. Owing to the improved spatial resolution of TEPL and TERS compared to conventional confocal microscopy due to the near-field coupling of photons, nanoscale inhomogeneities in charge carrier concentration in CVD grown flakes of MoS2 and WS2 have been directly detected and characterized [1,2]. More recently, brightening of dark excitons has also been observed in monolayer TMDCs when the samples are confined under the gap-mode of TEPL [3].
In this work, we report the importance of sample preparation procedures as well as the nanoscale TEPL and TERS imaging of TMDCs for correct characterization and interpretation of observed TEPL spectra. We report that mono-to-few-layer flakes of WS2 and WSe2 transferred to gold via heat-assisted exfoliation feature a large number of nanoscale bubbles ranging from few tens to about a hundred of nanometers across. Depending on the exact location of the optically active SPM probe relative to the bubble, the spectral response can change dramatically, both in terms of the intensity of the TEPL peak, its spectral position, shape and the ratio between the TERS and TEPL signals. We have observed both red and blue-shifted TEPL peaks in WSe2 ( up to 20nm relative to the position of the far-field PL), which implies broader variety of excitonic behavior in gap-mode near field PL of TMDCs compared to what was reported earlier. The TERS/TEPL peak intensity ratio changes as the probe is positioned away from the center of a nanobubble both for WSe2 and WS2, which may be the consequence of varied distance between the layer of TMDC and underlying gold as well as varied mechanical strain in the vicinity of the bubble. Additional signatures of these effects are also observed in the excitonic emission as the relative intensities of different exciton states varies over the bubble.
The observed nano-PL response demonstrates the importance of nanoscale spectroscopic imaging of 2D semiconductors for comprehensive understanding of possible excitonic resonances, and opens the possibility of rational engineering and manipulation of local nanoscale photon emitters in these materials.

Christoph Kastl 2D Mater. 4 (2017) 021024
Kyoung-Duck Park, arXiv:1706.09085v1