Crystal Structure Identification and Application of Type-II Red Phosphorus

Red phosphorus (RP) is one of the allotropes of phosphorus and has many structural phases. Type-I RP is amorphous, and other types (type II, III, IV, and V) are known to be crystalline phases. The structures of Type-IV (fibrous RP) and Type-V (Hittorf’s phosphorus) have been identified through previous experiments, but the structures of Type-II and Type-III have yet to be reported. Here we unravel the crystal structure of type-II RP by complementary structure characterization techniques via powder X-ray diffraction, 3D electron diffraction, and atomic-resolution scanning transmission electron microscopy (STEM) imaging. The type-II RP crystals were synthesized by the chemical vapor transport method and identified through powder XRD and Raman spectroscopy in comparison with previous literature. From our study, we successfully identify that type-II RP has approximately 250 phosphorus atoms in a triclinic unit cell. Moreover, STEM imaging clearly confirms the helical tubular structure is the building block of type-II RP. The electrical properties of type II RP toward optoelectric device applications are also experimentally verified.