Large area chemical vapour deposition grown transition metal dichalcogenide monolayers automatically characterized through photoluminescence imaging

Toby Severs Millard,A-Rang Jang,Sam Randerson,Armando Genco,Alexander I Tartakovskii,Seongjoon Ahn,Evgeny M Alexeev,Hyeon Suk Shin

doi:10.1038/s41699-020-0146-y

Abstract

Chemical vapour deposition (CVD) growth is capable of producing multiple single-crystal islands of atomically thin transition metal dichalcogenides (TMDs) over large areas. Subsequent merging of perfectly epitaxial domains can lead to single-crystal monolayer sheets, a step towards scalable production of high quality TMDs. For CVD growth to be effectively harnessed for such production it is necessary to be able to rapidly assess the quality of material across entire large area substrates. To date, characterisation has been limited to sub-0.1-mm2 areas, where the properties measured are not necessarily representative of an entire sample. Here, we apply photoluminescence (PL) imaging and computer vision techniques to create an automated analysis for large area samples of monolayer TMDs, measuring the properties of island size, density of islands, relative PL intensity and homogeneity, and orientation of triangular domains. The analysis is applied to ×20 magnification optical microscopy images that completely map samples of WSe2 on hBN, 5.0 mm × 5.0 mm in size, and MoSe2–WS2 on SiO2/Si, 11.2 mm × 5.8 mm in size. Two prevailing orientations of epitaxial growth were observed in WSe2 grown on hBN and four predominant orientations were observed in MoSe2, initially grown on c-plane sapphire. The proposed analysis will greatly reduce the time needed to study freshly synthesised material over large area substrates and provide feedback to optimise growth conditions, advancing techniques to produce high quality TMD monolayer sheets for commercial applications.

Highlights

Van der Waals layered crystals such as transition metal dichalcogenides (TMDs), hexagonal boron nitride and graphene are defined by their strong covalent bonds in-plane and weak interlayer forces[1]
The extreme thinness and mechanical stability of these materials provide the potential for flexible and transparent devices[15]. To realise such devices, the goal of scalable and controlled production of mono- and multilayers must first be achieved, an objective which has been of great interest for commercial application ever since TMDs were rediscovered as atomically thin materials[4,5]
The monolayer TMD islands were identified through their PL as the only luminescent objects on the studied substrates, and their size and shape were extracted using MATLAB shape recognition functions

Summary

Introduction

Van der Waals layered crystals such as TMDs, hexagonal boron nitride (hBN) and graphene are defined by their strong covalent bonds in-plane and weak interlayer forces[1] These characteristics allow for individual atomically thin layers to be removed from the bulk crystal, and for single layers to be brought together to build vertical heterostructures that display promising new properties[1,2,3]. MoSe2, WSe2, MoS2 and WS2 are documented as the most promising and widely studied TMD monolayers, emitting bright PL while remaining stable in air, at room temperature[6] Such traits lend these materials to applications in optoelectronic devices[7] such as LEDs3,8, photovoltaic cells[9,10], photodetectors[11] and single photo-emitters[12,13]. To realise such devices, the goal of scalable and controlled production of mono- and multilayers must first be achieved, an objective which has been of great interest for commercial application ever since TMDs were rediscovered as atomically thin materials[4,5]

Methods

Results

Conclusion