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 Na 2 Se 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 Se 2− → Se, the spontaneous growth of h-Se crystals could be controlled well.

Shaping highly regular glass architectures: A lesson from nature

“…Our study uncovers a half-a-billion-year-old fabrication concept that only recently has been used by mankind to produce novel materials for the use in solar cells, plasmonics, optoelectronics, and sensing ( 20 – 25 ). The described growth mechanism uses similar branching principles that are used in the synthesis of abiotic inorganic nanocrystals and thus provides insights into the origins of morphological symmetry in these complex biological organisms.…”

“…In these structures, the branching occurs on specific crystallographic planes. For example, gold (face-centered cubic) ( 21 ) and selenium (hexagonal) ( 20 ) can form a variety of planar perfectly symmetric multipods while maintaining a single crystal-like nature. CdSe and CdTe have been shown to form either single-crystalline rods or highly regular tetrapods, consisting of cubic (zinc-blende) nuclei with {111} facets, each projecting a hexagonal (wurtzite-type) arm, or hyperbranched spherical structures, depending on growth conditions ( 24 , 25 ).…”

“…Ultimately, this guides the symmetry of the spicules. A similar synthesis approach of highly regular man-made inorganic nanocrystals is currently under fast development (20)(21)(22)(23)(24)(25). Rod-like, regular multipods and hyperbranched crystals are synthesized from a large variety of materials, such as metals, metal oxides, and semiconductors.…”