van der Waals Stacking-Determined Polymorphs of Quasi-One-Dimensional BiSCl Grown by Chemical Vapor Deposition.

We report chemical vapor deposition (CVD) synthesis of two quasi-one-dimensional (quasi-1D) polymorphs of BiSCl, denoted by y-BiSCl and r-BiSCl. The length of the CVD samples can reach about 0.4 mm. Such quasi-1D samples of the two polymorphs can be readily separated into individual pieces for either characterization or application. The two polymorphs can be clearly differentiated by Raman spectroscopy. First-principles calculations and group analysis are used to assign each Raman peak to the corresponding vibrational mode. Ultraviolet-visible measurements on solution grown thin-film samples reveal that the two polymorphs exhibit significantly different band gaps of 2.08 eV (y-BiSCl) and 1.81 eV (r-BiSCl). First-principles calculation further shows that the interatomic chain binding energy is 18.1 meV/Å, confirming that the van der Waals stacking determines the difference in their band gaps. Our findings highlight the possibility of realizing the desired functionalities in quasi-1D materials by controlling stacking orientation.