Strong Localization Effects in the Photoluminescence of Transition Metal Dichalcogenide Heterobilayers

TL;DR

This study uses high-resolution tip-enhanced photoluminescence to investigate local excitonic effects in TMDC heterobilayers, revealing that nanobubbles and strain can produce signals similar to interlayer excitons, complicating their identification.

Contribution

It demonstrates that local strain and nanobubbles significantly influence PL signals in TMDC heterobilayers, challenging previous interpretations of interlayer exciton signatures.

Findings

Nanobubbles as small as 60 nm can alter PL signals.

Strain-induced effects can mimic interlayer exciton signatures.

Certain heterobilayers show no interlayer exciton features in expected spectral regions.

Abstract

The emergence of various exciton-related effects in transition metal dichalcogenides (TMDC) and their heterostructures has inspired a significant number of studies and brought forth several possible applications. Often, standard photoluminescence (PL) with microscale lateral resolution is utilized to identify and characterize these excitonic phenomena, including interlayer excitons (IEXs). We studied the local PL signatures of van der Waals heterobilayers composed of exfoliated monolayers of the (Mo,W)(S,Se)$_2$ TMDC family with high spatial resolution (down to 30 nm) using tip-enhanced photoluminescence (TEPL) with different orders (top/bottom) and on different substrates. We evidence that other PL signals may appear near the reported energy of the IEX transitions, possibly interfering in the interpretation of the results. While we can distinguish and confirm the presence of IEX-related PL in MoS$_2$-WS$_2$ and MoSe$_2$-WSe$_2$, we find no such feature in the MoS$_2$-WSe$_2$ heterobilayer in the spectral region of 1.7-1.4 eV, where the IEXs of this heterobilayer is often reported. We assign the extra signals to the PL of the individual monolayers, in which the exciton energy is altered by the local strains caused by the formation of blisters and nanobubbles, and the PL is extremely enhanced due to the decoupling of the layers. We prove that even a single nanobubble as small as 60 nm---hence not optically visible---can induce such a suspicious PL feature in the micro-PL spectrum of an otherwise flat heterobilayer.