2D van der Waals Molecular Crystal β-HgI2 : Economical, Rapid, and Substrate-Free Liquid-Phase Synthesis and Strong In-Plane Optical Anisotropy.

2D materials have a great potential for wide-range applications due to their adjustable bandgap characteristics and special crystal structures. β-HgI2 is a new 2D van der Waals inorganic molecular crystal material with a wide bandgap of 4.03 eV, on whose preparation and properties there are few relevant reports due to the feature of instability of molecular crystals. Here, an economical method to control the synthesis of large-size 2D β-HgI2 single crystal by using a mineralizer-assisted solution is reported. According to angle-resolved polarization Raman spectroscopy and first-principles optical absorption calculation, 2D β-HgI2 flake has a strong in-plane anisotropic light scattering characteristic and high optical absorption dichroism (az /ay  = 3.4), which is due to a low in-plane symmetry of the orthorhombic structure of β-HgI2 . More importantly, due to the molecular crystal structure of β-HgI2 , its sensitivity to temperature is less than that of 2D materials such as MoS2 , which has been confirmed by temperature-dependent Raman spectroscopy. In the work, more 2D inorganic molecular crystals are studied in the aspect of growth, which provides a theoretical basis for 2D molecular crystal optoelectronic devices' potential applications.