Probing the influence of dielectric environment on excitons in monolayer WSe2: Insight from high magnetic fields

TL;DR

This study reveals how the dielectric environment influences exciton size and binding energy in monolayer WSe2, using high magnetic fields to measure diamagnetic shifts and compare with theoretical models.

Contribution

It introduces a systematic method to probe exciton size changes due to dielectric screening in 2D materials via high magnetic field magneto-absorption measurements.

Findings

Exciton size increases with dielectric screening.

Binding energy decreases as dielectric environment is tuned.

Experimental results align with theoretical predictions.

Abstract

Excitons in atomically-thin semiconductors necessarily lie close to a surface, and therefore their properties are expected to be strongly influenced by the surrounding dielectric environment. However, systematic studies exploring this role are challenging, in part because the most readily accessible exciton parameter -- the exciton's optical transition energy -- is largely \textit{un}affected by the surrounding medium. Here we show that the role of the dielectric environment is revealed through its systematic influence on the \textit{size} of the exciton, which can be directly measured via the diamagnetic shift of the exciton transition in high magnetic fields. Using exfoliated WSe$_2$ monolayers affixed to single-mode optical fibers, we tune the surrounding dielectric environment by encapsulating the flakes with different materials, and perform polarized low-temperature magneto-absorption studies to 65~T. The systematic increase of the exciton's size with dielectric screening, and concurrent reduction in binding energy (also inferred from these measurements), is quantitatively compared with leading theoretical models. These results demonstrate how exciton properties can be tuned in future 2D optoelectronic devices.