Probing Electronic Properties of Atomically Thin Film Black Phosphorus

Abstract

Atomically thin black phosphorus, which possesses high theoretical mobility and a tunable bandgap structure, has attracted much attention since its rediscovery in 2014. However, degradation of the quality under atmospheric conditions limits its applications in nanoelectronic and optoelectronic devices. To solve this problem, I take advantage of hexagonal boron nitride thin flakes to obtain stable black phosphorus and investigate the electronic properties of monolayer and few-layer black phosphorus encapsulated between hexagonal boron nitrides thin flakes. This thesis mainly involves of three projects I have been devoted to. My first project measures the transport properties of this sandwiched structure. It achieves a high FET mobility ( $1350 \ \text{cm}^2 \text{V}^{-1} \text{s}^{-1}$ at room temperature) in few-layer black phosphorus-based heterostructure and quantum oscillations at cryogenic temperatures are observed at magnetic field $\sim 6 \ \text{T}$. This sandwiched heterostructure ensures that the quality of black phosphorus remains high even when exposed in the air for one week. My second project investigates the electron states of monolayer and few-layer black phosphorus at temperatures down to $2 \ \text{K}$ through capacitance spectroscopy with a vertical heterostructure. Electron states in conduction and valence bands are accessible within a wide range of temperature and frequencies. We have observed the giant temperature-dependence of the electron states in few-layer black phosphorus. Combined with the firstprinciple calculations, we conclude that thermal excitation of charge trap states and oxidation-induced effect are the main reasons for this phenomenon. My third project manages to observe the negative compressibility in atomically thin black phosphorus at low carrier density region. The encapsulation of black phosphorus by hexagonal boron nitrides with few-layer graphene as a terminal ensures the ultraclean interfaces. This observed the negative compressibility is explained based on the Coulomb correlation among in-plane charges and their image charges in a gate electrode in the framework of Debye screening.