Synthesis and characterization of ultrathin black phosphorus

Abstract

Two-dimensional materials attract much attention from material scientists due to their unique physical and optical properties. Graphene and transition metal dichalcogenides are examples of widely studied 2D materials. Black Phosphorus (BP), namely phosphorene, is an emerging candidate in this field. It has a thickness-dependent energy bandgap ranging from 0.3 eV to 2.0 eV in bulk to monolayer and high carrier mobility, making it an excellent choice for optoelectronics applications. Furthermore, the unique A17 orthorhombic crystal structure of BP with weak interlayer van der Waals force also provides the anisotropic behaviour in BP along with the zigzag (ZZ) and armchair (AC) directions. This anisotropy makes it a great thermoelectric and piezoelectric candidate. In this study, BP crystals were synthesized by the chemical vapour transportation method. Red phosphorus, tin, and iodine were used as precursors and heated in a vacuum-sealed quartz tube to produce BP. It was then mechanically exfoliated for characterization. Small Raman shift in bulk and exfoliated BP was observed, and anisotropy polarised Raman was measured. Single crystal quality and high purity of BP were confirmed by selected area electron diffraction and energy dispersive x-ray in the transmission electron microscope observation. Molecular beam epitaxy BP synthesis was attempted by using GaP source and mica substrate with a fast cooling after growth. Raman spectroscopy, atomic force microscopy, and x-ray diffraction characterized the large area thin film uniformity and crystalline structure. The strain-induced electrical conductance of BP in ZZ and AC orientations was also investigated. Conductance (G) of 4.43μS and 1.01 μS were observed in the AC and ZZ directions, with a ratio Gac/Gzz of 4.4 without strain. Conductance along ZZ and AC were also measured with applied 0 -1.4% uniaxial strain in either ZZ or AC direction. In most cases, the conductance drops exponentially to about 20% of the original value at 1.4% strain except the conductance of ZZ orientation when applying strain in the same direction, which is half of its original. Results suggested that the unique conductance behaviour in ZZ direction with uniaxial ZZ strain could be attributed to the spatial preference transition of electrical conductance in BP.