The Effect of Preparation Conditions on Raman and Photoluminescence of Monolayer WS2

TL;DR

This study investigates how different preparation methods and substrates affect the optical properties of monolayer WS2, revealing significant shifts in exciton energy and controllability, which are crucial for device reproducibility.

Contribution

It provides a detailed comparison of optical properties between as-grown and transferred monolayer WS2, highlighting the impact of preparation conditions on excitonic behavior and strain effects.

Findings

Transferred WS2 shows a ~50 meV exciton energy shift.

Optical control of excitonic states is possible in transferred WS2.

Preparation conditions significantly influence laser power-dependent behavior.

Abstract

We report on preparation dependent properties observed in monolayer WS2 samples synthesized via chemical vapor deposition (CVD) on a variety of common substrates (Si/SiO2, sapphire, fused silica) as well as samples that were transferred from the growth substrate onto a new substrate. The as-grown CVD materials (as-WS2) exhibit distinctly different optical properties than transferred WS2 (x-WS2). In the case of CVD growth on Si/SiO2, following transfer to fresh Si/SiO2 there is a ~50 meV shift of the ground state exciton to higher emission energy in both photoluminescence emission and optical reflection. This shift is indicative of a reduction in tensile strain by ~0.25%. Additionally, the excitonic state in x-WS2 is easily modulated between neutral and charged exciton by exposure to moderate laser power, while such optical control is absent in as-WS2 for all growth substrates investigated. Finally, we observe dramatically different laser power-dependent behavior for as-grown and transferred WS2. These results demonstrate a strong sensitivity to sample preparation that is important for both a fundamental understanding of these novel materials as well as reliable reproduction of device properties.