Revealing the nature of low-temperature photoluminescence peaks by laser treatment in van der Waals epitaxially grown WS₂ monolayers

Abstract

Monolayers of transition metal dichalcogenides (TMD) are promising materials for optoelectronics devices. However, one of the challenges is to fabricate large-scale growth of high quality TMD monolayers with the desired properties in order to expand their use in potential applications. Here, we demonstrate large-scale tungsten disulfide (WS) monolayers grown by van der Waals Epitaxy (VdWE). We show that, in addition to the large structural uniformity and homogeneity of these samples, their optical properties are very sensitive to laser irradiation. We observe a time instability in the photoluminescence (PL) emission at low temperatures in the scale of seconds to minutes. Interestingly, this change of the PL spectra with time, which is due to laser induced carrier doping, is employed to successfully distinguish the emission of two negatively charged bright excitons. Furthermore, we also detect blinking sharp bound exciton emissions which are usually attractive for single photon sources. Our findings contribute to a deeper understanding of this complex carrier dynamics induced by laser irradiation which is very important for future optoelectronic devices based on large scale TMD monolayers.

Tailoring of Bound Exciton Photoluminescence Emission in WS2 Monolayers

“…This effect is rather interesting, suggesting that band can be sensitive also to other forms of radiation, being useful for some applications. Similar effects have been observed also by other groups …”

“…In addition to the charge‐neutral exciton peak, a charged trion peak is also often found at about 43 meV lower energies . At low temperatures ( T ≈ 10 K), a broad defect‐bound exciton band () with a peak position of about 1.9 eV has been found to dominate the PL spectra . This spectral region includes also a single‐photon emission (a series of sharp peaks in the defect band region) that has been detected in the low‐temperature PL spectra of WS 2 monolayers and in other 2D compounds .…”

Enhancing and quantifying spatial homogeneity in monolayer WS2

“…It can be shown that the parameter carries a spatial dependence in addition to the macroscopic time-dependence, by applying spatial statistics on PL maps that accounts for the spatial inhomogeneity. In this measurement, a separate monolayer flake is chosen and the laser power is increased to , from in the previous measurement, which is expected to increase the rate of change of the spectral weightings 44 , 51 . Rapid spectral tuning is crucial for scalability as this will reduce the overall treatment time of large-area flakes.…”

Excited Rydberg states in MoSe2/WSe2 heterostructures

“…The intensities of the Raman peaks were also converted to absolute scattering probability using the Si peak intensity as a reference 28 with corrections for Fabry-Perot effects modelled using the reflectivity spectra as were performed in our previous paper 8 . The laser power incident on the sample was kept below 100 µW to avoid photo-doping and laser heating effects 45,46 .…”