Violet phosphorus transmission and photoconductance spectroscopy.

Violet phosphorus is a semiconducting allotrope of phosphorus with a layered crystalline structure consisting of orthogonaly oriented layers of phosphorus chains composed of P2[P8]P2[P9] repeating units. Here, we report optical transmission spectroscopy and photoconductivity measurements of exfoliated flakes of violet phosphorus in the thin-film bulk limit. The violet phosphorus was synthesized by chemical vapour transport, and subsequnetly protected from oxidation with an inert gas environment. A peak photoconductive responsivity of R = 7 mA/W at photon energy 2.8 eV was observed. The spectral dependence of optical transmission and photoconductivity of violet phosphorus leads us to identify optical transitions at van Hove singularities corresponding to energies E 1 =1.80±0.05 eV and E 2 =1.95±0.05 eV. Density functional theory was applied to the calculation of vP bandstructure, and a dipole transition analysis shows that optical transitions at the Z and A 0 points of the Brillouin zone are in agreement with experimental observations. Exposure to ambient environmental conditions for several minutes is sufficient to significantly reduce vP photoconductivity, while longer exposure leads to blistering due to oxidation and thus a locally inert chemical environment is essential to accessing violet phosphorus intrinsic optoelectronic properties.