Substrate-Induced Variances in Morphological and Structural Properties of MoS2 Grown by Chemical Vapor Deposition on Epitaxial Graphene and SiO2.

Jakub Sitek,Clifford Mcaleese,Janusz Plocharski,Arkadiusz P Gertych,Ben R Conran,Wlodek Strupinski,Iwona Pasternak,Mariusz Zdrojek

doi:10.1021/acsami.0c06173

Abstract

In this work, we report the impact of substrate type on the morphological and structural properties of molybdenum disulfide (MoS2) grown by chemical vapor deposition (CVD). MoS2 synthesized on a three-dimensional (3D) substrate, that is, SiO2, in response to the change of the thermodynamic conditions yielded different grain morphologies, including triangles, truncated triangles, and circles. Simultaneously, MoS2 on graphene is highly immune to the modifications of the growth conditions, forming triangular crystals only. We explain the differences between MoS2 on SiO2 and graphene by the different surface diffusion mechanisms, namely, hopping and gas-molecule-collision-like mechanisms, respectively. As a result, we observe the formation of thermodynamically favorable nuclei shapes on graphene, while on SiO2, a full spectrum of domain shapes can be achieved. Additionally, graphene withstands the growth process well, with only slight changes in strain and doping. Furthermore, by the application of graphene as a growth substrate, we realize van der Waals epitaxy and achieve strain-free growth, as suggested by the photoluminescence (PL) studies. We indicate that PL, contrary to Raman spectroscopy, enables us to arbitrarily determine the strain levels in MoS2.

Highlights

Two-dimensional (2D) semiconductors are currently of interest to the research community as they are foreseen to be important building blocks for beyond-silicon electronics.MoS2, a transition-metal dichalcogenide (TMDC), is the most researched representative of 2D semiconductors due to the presence of a direct band gap, which changes to indirect with an increasing number of layers,[1] and due to widespread material availability
We indicate PL as a method that enables us to arbitrarily determine the strain levels in MoS2, and we show that MoS2 grown on graphene follows van der Waals epitaxy, resulting in unstrained growth
atomic force microscope (AFM) images of MoS2 grown on SiO2, sapphire, and graphene are shown in Figure 2, accompanied by height profiles

Summary

■ INTRODUCTION

Two-dimensional (2D) semiconductors are currently of interest to the research community as they are foreseen to be important building blocks for beyond-silicon electronics. Numerous works are showing the impact of strain on the photoluminescence peak position.[55,57,63,65,66] Interestingly, in the case of MoS2 on SiO2, the reported A exciton energy of the unstrained MoS2 is varying from 1.855 to 1.9 eV,[66] and the theoretical value for the band gap in MoS2 is predicted to be 1.9 eV.[62] in the case of MoS2 on graphene, the reported energy of A exciton is in a much narrower range, between 1.8621 and 1.88 eV.[60] Similar values of A exciton position were measured for freestanding MoS2 monolayers, in which the influence of substrate is negligible.[67]. It proves that the PL-based determination of strain is a viable and accessible method to establish strain levels in MoS2

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES