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

• Published in: NPJ 2D materials and applications 2018-04, Vol.2 (1), Article 10
• Main Authors: Taghinejad, Hossein, Eftekhar, Ali A., Campbell, Philip M., Beatty, Brian, Taghinejad, Mohammad, Zhou, Yao, Perini, Christopher J., Moradinejad, Hesam, Henderson, Walter E., Woods, Eric V., Zhang, Xiang, Ajayan, Pulickel, Reed, Evan J., Vogel, Eric M., Adibi, Ali
• Format: Article
• Language: English
• Subjects: 639/301/1023/1026; 639/301/357/1018; Alloys; Chalcogenides; Chemistry and Materials Science; Composition; Crystal defects; Crystal lattices; Crystals; Domains; Heterostructures; Lattice parameters; Materials Science; Modulation; Molybdenum compounds; Monolayers; Nanotechnology; Optoelectronics; Selenium; Semiconductor materials; Strain relaxation; Sulfur; Surfaces and Interfaces; Synthesis; Tensile strain; Ternary alloys; Thin Films; Transition metal compounds; Two dimensional materials
• ISSN: 2397-7132; 2397-7132
• DOI: 10.1038/s41699-018-0056-4

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., MoS 2 x Se 2(1 -  x ) ) and inflicts fracture on the crystals. We find that the starting crystal (MoSe 2 ) 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. 2D alloys: intrinsic strain in MoS 2x Se 2(1-x) ternary crystals Composition modulation synthesis of ternary alloys of atomically thin transition metal dichalcogenides gives rise to intrinsic biaxial strain. A team led by Ali Adibi at Georgia Institute of Technology reported the onset of a substantial biaxial strain in monolayer MoS 2x Se 2(1-x) that is intrinsically linked to the two-step composition modulation synthesis used to grow the ternary alloy. As the S atoms replace the Se atoms of the starting MoSe 2 host crystal, the resulting alloy forms a stretched lattice and develops a large biaxial tensile strain. Morphological and spectroscopic characterisations suggest that such strain results in the onset of fracture in the crystal, and further relaxes via formation of cracks within the crystal domains. Theoretical modelling indicates that pre-existing cracks give a substantial contribution in weakening the strength of the synthesized van der Waals alloy.