Real time optical observation and control of atomically thin transition metal dichalcogenide synthesis

TL;DR

This paper presents a real-time optical monitoring system for CVD synthesis of atomically thin TMDCs, revealing reaction mechanisms and enabling controlled growth of layered crystals.

Contribution

It introduces a custom CVD chamber with real-time optical observation, elucidating reaction pathways and demonstrating controlled crystal formation in salt-assisted TMDC synthesis.

Findings

Revealed salt-metal reactions forming intermediates

Identified vapour-solid-solid and vapour-liquid-solid growth routes

Demonstrated guiding crystal growth via pre-patterned channels

Abstract

Understanding the mechanisms involved in chemical vapour deposition (CVD) synthesis of atomically thin transition metal dichalcogenides (TMDCs) requires the precise control of numerous growth parameters. All the proposed mechanisms and their relation to the growth conditions are inferred from characterising intermediate formations obtained by stopping the growth blindly. To fully understand the reaction routes that lead to the monolayer formation, real time observation and control of the growth are needed. Here, we demonstrate how a custom-made CVD chamber that allows real time optical monitoring can be employed to study the reaction routes that are critical to the production of the desired layered thin crystals in salt assisted TMDC synthesis. Our real time observations reveal the reaction between the salt and the metallic precursor to form intermediate compounds which lead to the layered crystal formation. We identified that both the vapour-solid-solid and vapour-liquid-solid growth routes are in an interplay. Furthermore, we demonstrate the role H$_{2}$ plays in the salt-assisted WSe$_{2}$ synthesis. Finally, we guided the crystal formation by directing the liquid intermediate compound through pre-patterned channels. The methods presented in this article can be extended to other materials that can be synthesized via CVD.