Strain relaxation via formation of cracks in compositionally modulated two-dimensional semiconductor alloys

Ali A Eftekhar,Pulickel Ajayan,Yao Zhou,Xiang Zhang,Ali Adibi,Philip M Campbell,Walter E Henderson,Hossein Taghinejad,Eric M Vogel,Hesam Moradinejad,Evan J Reed,Brian Beatty,Christopher J Perini,Eric V Woods,Mohammad Taghinejad

doi:10.1038/s41699-018-0056-4

Abstract

Composition modulation of two-dimensional transition-metal dichalcogenides (TMDs) has introduced an enticing prospect for the synthesis of Van der Waals alloys and lateral heterostructures with tunable optoelectronic properties. Phenomenologically, the optoelectronic properties of alloys are entangled to a strain that is intrinsic to synthesis processes. Here, we report an unprecedented biaxial strain that stems from the composition modulation of monolayer TMD alloys (e.g., MoS2xSe2(1 - x)) and inflicts fracture on the crystals. We find that the starting crystal (MoSe2) fails to adjust its lattice constant as the atoms of the host crystal (selenium) are replaced by foreign atoms (sulfur) during the alloying process. Thus, the resulting alloy forms a stretched lattice and experiences a large biaxial tensile strain. Our experiments show that the biaxial strain relaxes via formation of cracks in interior crystal domains or through less constraint bounds at the edge of the monolayer alloys. Griffith’s criterion suggests that defects combined with a sulfur-rich environment have the potential to significantly reduce the critical strain at which cracking occurs. Our calculations demonstrate a substantial reduction in fracture-inducing critical strain from 11% (in standard TMD crystals) to a range below 4% in as-synthesized alloys.

Highlights

Transition-metal dichalcogenides (TMDs) are the most extensively explored family of two-dimensional (2D) materials beyond graphene
Using a monolayer MoS2xSe2(1 - x) material platform, we show that the indirect nature of the two-step composition–modulation technique inherently yields a synthesis of strained TMD alloys with disintegrated crystal structures
In the second step, we proceed with the sulfurization of MoxW1 - xSe2 between 1.5 eV (MoSe2) crystals using a high-temperature annealing process under a sulfur vapor ambient that replaces selenium (Se) atoms with sulfur (S) atoms

Summary

Introduction

Transition-metal dichalcogenides (TMDs) are the most extensively explored family of two-dimensional (2D) materials beyond graphene. We found that after sulfurization at ~1000 °C for 10 min, the pristine MoSe2 monolayer crystals (Fig. 1a, b) are fully converted to MoS2, as verified via PL mappings before (Fig. 1c) and after (Fig. 1d) conversion.

Results

Conclusion