Crack Formation induced by the Post-Growth Alloying of Two-Dimensional Transition-Metal Dichalcogenides

Alloying is a prime method for customizing the opto-electronic properties of two-dimensional (2D) transition-metal dichalcogenides (TMDs). In this method, the tuning of the composition ratio (x) in M_xM^’_1-xX₂ or MX_2xX^’_2(1-x) ternary alloys (M, M^’: transition-metals and X, X^’: chalcogens) allows for engineering the electronic bandgap and obtaining properties that are not intrinsically available in binary TMDs (i.e., MX₂). Ternary compounds can be synthesized via the incorporation of foreign atoms (i.e., M^’ or X^’) into an already-grown MX₂ binary lattice, that is the post-growth alloying of binary crystals.
Here, we show that the post-growth alloying yields strained 2D TMDs with severely disintegrated domains. In a case study (e.g., MoS_2xSe_2(1-x)), we demonstrate that the starting binary crystal (i.e., MoSe₂) fails to adjust its lattice constant as the atoms of the host crystal (i.e., Se) are being replaced by foreign atoms (i.e., S) during the alloying process. Thus, the obtained alloys form in a stretched lattice and experience a larger biaxial strain that relaxes through the formation of cracks. Our calculations demonstrate that pre-existing defects substantially reduce the fracture-inducing strain from 11% (in standard TMD crystals) to a range below 4% in as-synthesized alloys.