Optical Floating-Zone Furnace Single-Crystal Synthesis of van der Waals Material InSe

Investigations of crystal structures and intrinsic properties of advanced materials require synthesis of high-quality single crystals. In case of incongruently melting solids, specialized growth methods must be applied, but these can be particularly challenging for two-dimensional (2D) van der Waals materials prone to twinning and defects. Here, a very rare synthesis of a large single crystal of an incongruently melting van der Waals material is reported using the traveling solvent floating zone method in an optical mirror furnace. Use of a melt zone with a delicately balanced stoichiometry yielded a ∼3 × 1 cm3 single crystal of InSe, which is a widely studied flexible semiconductor. The average crystal structure of InSe determined from single crystal X-ray diffraction reveals stacking disorder along the c-axis, which can be modeled by a major- (∼75%) and minor component (∼25%). The In–In and In–Se bond lengths are 2.775(3) and 2.632(3) Å, respectively, while the In–In–Se and Se–In–Se angles are 118.5(1) and 99.1(2)°, respectively. The van der Waals interlayer distance was found to be 3.08(3) Å. The easy layer-slippage appears to govern the mechanical flexibility demonstrated macroscopically with a bending test on the single crystal.