Synthesis and characterization of violet phosphorus

Abstract

Two-dimensional (2D) layered materials including graphene, transition metal dichalcogenide (TMDC), and black phosphorus (BP), have been attracting investigation because of their unique physical and optical properties. BP is a semiconductor with high carrier mobility and the band gap can be up to 2.0 eV for monolayer. In addition, phosphorus also exhibits other common allotropes, which include white, red phosphorus and a relatively rare allotrope called violet phosphorus (VP). VP exhibits a monoclinic with double tubes layered structure. The tubes are perpendicularly connected, and the double tubes layers are stacked along c-direction via van der Waals forces. It has been predicted theoretically that monolayer VP has a high anisotropic hole mobility (3000 - 7000 cm²/Vs) and a wide direct band gap of 2.5 eV. [1] In our work, VP crystals were synthesized by chemical vapour transport (CVT) method. The synthesis process mainly employed red phosphorus, tin powder and granular size iodine (I2) as precursors and annealed in a sealed tube. The VP crystals can be exfoliated to thin flakes by mechanical and solution-based ultrasonic methods. The VP flakes exhibit prominent thickness dependence photoluminescence (PL). The PL emission of the VP flakes can be tuned from 670 to 630 nm as the VP thickness decreases from 100 nm to 15 nm. We also study the anisotropic property of VP under angle-resolved polarized Raman and PL spectroscopy. The Raman scattering and PL of VP were investigated in parallel and perpendicular polarization measurements. The intensities of the Raman peaks are significantly changed by different angles of the polarized incident light. Two different modes (Ag and Bg) are included in the Raman spectrum. The polar plots of Ag modes exhibit 4-lobed shape with the local maximum intensities at θ = 0°, 90°, 180°, and 270°. The Bg modes exhibit 4-lobed shape with the local maximum at θ = 45°, 135°, 225°, and 315°. The intensities of the PL emission along x-direction are slightly larger than y-direction under the polarized PL measurement.