Strained bubbles in van der Waals heterostructures as local emitters of photoluminescence with adjustable wavelength

TL;DR

This paper demonstrates that hydrocarbon-filled bubbles in monolayer TMDCs on flat substrates act as local, tunable photoluminescent emitters at room temperature, with emission energy controlled by substrate-induced strain.

Contribution

It introduces a universal, substrate-dependent method to create localized, tunable PL emitters in monolayer TMDCs using naturally occurring bubbles.

Findings

PL emission energy can be tuned between 1.72 to 1.81 eV.

PL intensity is enhanced by exciton funneling effect.

PL is localized to bubbles due to substrate-dependent quenching.

Abstract

The possibility to tailor photoluminescence (PL) of monolayer transition metal dichalcogenides (TMDCs) using external factors such as strain, doping and external environment is of significant interest for optoelectronic applications. Strain in particular can be exploited as a means to continuously vary the bandgap. Micrometer-scale strain gradients were proposed for creating 'artificial atoms' that can utilize the so-called exciton funneling effect and work, for example, as exciton condensers. Here we describe room-temperature PL emitters that naturally occur whenever monolayer TMDC is deposited on an atomically flat substrate. These are hydrocarbon-filled bubbles which provide predictable, localized PL from well-separated submicron areas. Their emission energy is determined by the built-in strain controlled only by the substrate material, such that both the maximum strain and the strain profile are universal for all bubbles on a given substrate, i.e., independent of the bubble size. We show that for bubbles formed by monolayer MoS2, PL can be tuned between 1.72 to 1.81 eV by choosing bulk PtSe2, WS2, MoS2 or graphite as a substrate and its intensity is strongly enhanced by the funneling effect. Strong substrate-dependent quenching of the PL in areas of good contact between MoS2 and the substrate ensures localization of the luminescence to bubbles only; by employing optical reflectivity measurements we identify the mechanisms responsible for the quenching. Given the variety of available monolayer TMDCs and atomically flat substrates and the ease of creating such bubbles, our findings open a venue for making and studying the discussed light-emitting 'artificial atoms' that could be used in applications.