Grain-growth engineering and mechanical properties of physical-vapour-deposited InSe platelets

The present work demonstrates a novel use of physical vapour deposition for grain-growth engineering by optimizing supersaturation, which led to the evolution of stoichiometric indium monoselenide crystals, employing a custom-fabricated dual-zone furnace. The growth zone was kept at a constant temperature for different experimental runs (673–883 K), while the source zone was kept at a stable temperature of 1123 K. In this way, the temperature difference ΔT = 240–450 K resulted in a significant increase of the mass transport between the zones so as to accomplish bulk crystallization. At comparatively low supersaturation (ΔT = 240 K), the presence of nodules and flakes was observed. When ΔT = 250 K, multiple grains were formed owing to temperature asymmetry at the rough vapour–solid interface. A further increase in supersaturation (ΔT = 330 K) facilitated polyhedral grain growth, with distinct grain boundaries. A subsequent increment in ΔT (400 K) led to evolution of the polycrystalline morphology to well developed hexagonal platelets owing to adsorption of atoms on surface steps and kinks in accordance with the leading-edge growth mechanism. Energy-dispersive analysis by X-rays and X-ray diffraction experiments were carried out to confirm the structure and phase of crystals. Microindentation studies were done to assess the hardness and mechanical stability of the as-grown crystals in response to external loads in order to explore their suitability for solar cell applications. The investigations of bulk vapour phase transport, morphology and strengthening of InSe platelets provide pathways for the production of crystalline textures with versatile properties.