Nanotube Structure of AsPS_4-Se (x = 0, 1)

Single-wall nanotubes of isostructural AsPS_4-Se (x = 0, 1) are grown from solid-state reaction of stoichiometric amounts of the elements. The structure of AsPS₄ was determined using single-crystal X-ray diffraction and refined in space group $$. The infinite, single-walled AsPS₄ nanotubes have an outer diameter of ≈1.1 nm and are built of corner-sharing PS₄ tetrahedra and AsS₃ trigonal pyramids. Each nanotube is nearly hexagonal, but the ≈3.4 Å distance between S atoms on adjacent nanotubes allows them to easily slide past one another, resulting in the loss of long-range order. Substituting S with Se disrupted the crystallization of the nanotubes, resulting in amorphous products that precluded the determination of the structure for AsPS₃Se. ³¹P solid-state NMR spectroscopy indicated a single unique tetrahedral P environment in AsPS₄ and five different P environments all with different degrees of Se substitution in AsPS₃Se. Optical absorption spectroscopy revealed an energy band gap of 2.7 to 2.4 eV for AsPS₄ and AsPS₃Se, respectively. Individual AsPS₄ microfibers showed a bulk conductivity of 3.2 × 10^-6 S/cm and a negative photoconductivity effect under the illumination of light (3.06 eV) in ambient conditions. Thus, intrinsic conductivity originates from hopping through empty trap states along the length of the AsPS₄ nanotubes.