The Fabrication Of Thin Black Phosphorus And Its Optical And Electronic Characterization

Atomically-thin two dimensional materials have gained intensive research for promising applications in electronics, energy storage and catalysis. This is due to its unique micro-structure and physical properties contrast to their counterpart bulk forms. Among them, graphene and transition metal dichalcogenides (TMDCs) are two representative members. However, their application as channel materials seems to be less feasible due to the zero-bandgap for graphene and limited carrier mobility attainable for monolayer TMDCs. Black phosphorus (BP), as a new role in the two dimensional layered material, is recently attacting great interest from researches, because of its direct electronic bandgap from ～0.3 eV of bulk form to 2.0 eV of monolayer BP, and because of its high carrier mobility. What’s more, black phosphorus is anisotropic in the atomic structure and electronic properties, which presents a wealth of opportunities in applications of novel anisotropic optics and electronics. Here, we tried to fabricate thin black phosphorus with various layers through micro-mechanical exfoliation and plasma-assisted etching, and then to study the optical and electronic properties of black phosphorus with the investigation on the atomic structure. The main results are summarized as following:We studied the approaches to fabricate thin black phosphorus from single layer to five layers, and characterized its optical properties. Thin black phosphorus was fabricated via mechanical exfoliation and refined by the plasma etching under proper parameters. Layer-dependent Raman behaviors included with peak positions and intensities were summarized to be the fingerprint for thickness determination, and the reflection contrast spectrum of black phosphorus were collected to develop a fast and reliable method to identify the thickness of thin film. For the Photoluminescence spectrum in the as-prepared mono-and bi-layer black phosphorus, two strong photoluminescence peaks located at 1.65 eV and 2.15 eV were recognized respectively, which proves the broadband property of black phosphorus.The anisotropic optical and electronic properties were investigated. We combined the transmission electron microscope and Raman spectroscope to correlate the angle-dependent Raman behaviors to the crystallographic orientation, which means that it could be unambiguous to identify the crystallographic direction with the angle-dependent Raman spectrum. Multi-electrodes were patterned with inter-step of fixed angle to measure the electronic conductance after the crystallographic direction identified, we proposed that the zigzag direction are superior in both electronic conductance and optical response.In order to study the atomic crystal structure of black phosphorus, we explored the possible methods to transfer the thin BP film for TEM characterization. After compared the alkali solution method, the PMMA method without the involvement of aqueous solution and the dry transfer method with PDMS, we proposed the combination of dry transfer method and in-situ annealing could get clean and thin areas in TEM for atomic characterization and electronic studies.