Planar Se multipod crystals with unusual growth directions: thoughts on the spontaneous growth of hexagonal Se

Abstract

Selenium (Se) is an important elemental semiconductor. Its intrinsic crystal structure is composed of hexagonally packed, one-dimensional (1D) spiral chains of Se atoms, which easily results in preferential 1D anisotropic growth along the [001] direction. In this study, we observed that planar Se multipod crystals with unusual growth directions (namely, not along the preferential [001] direction), including tripods, tetrapods, pentapods, hexapods, etc. could be formed spontaneously in water, via the oxidation of Na₂Se by air and subsequent crystallization of Se monomers under ambient conditions. Here, the controlled synthesis of these Se multipods is achieved by using small molecules containing a carboxyl group to stabilize selectively the (001) facets of hexagonal Se (h-Se) crystals. Furthermore, their growth modes were investigated systematically and revealed, using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and X-ray powder diffraction (XRD) characterization methods. Lastly, based on the acquired understanding, we consider that by selecting the type of small molecule and(/or) controlling the transformation rate of Se²⁻ → Se, the spontaneous growth of h-Se crystals could be controlled well.