Probing 2D Black Phosphorus by Quantum Capacitance Measurements

TL;DR

This study uses quantum capacitance measurements to explore the density of states in black phosphorus, revealing localized states and anisotropic electronic properties that are not accessible through conductance measurements.

Contribution

First experimental quantum capacitance measurements of few-layer black phosphorus, comparing results with DFT and revealing localized states near the band edge.

Findings

Transport gap from quantum capacitance is smaller than from conductance.

Localized states near the band edge are confirmed by temperature-dependent conductivity.

Asymmetry between electron and hole sides due to anisotropic band dispersion.

Abstract

Two-dimensional materials and their heterostructures have emerged as a new class of materials for not only fundamental physics but also for electronic and optoelectronic applications. Black phosphorus (BP) is a relatively new addition to this class of materials. Its strong in plane anisotropy makes BP a unique material to make conceptually new type of electronic devices. However, the global density of states (DOS) of BP in device geometry has not been measured experimentally. Here we report the quantum capacitance measurements together with conductance measurements on a hBN protected few layer BP ($\sim$ 6 layer) in a dual gated field effect transistor (FET) geometry. The measured DOS from our quantum capacitance is compared with the density functional theory (DFT). Our results reveal that the transport gap for quantum capacitance is smaller than that in conductance measurements due to the presence of localized states near the band edge. The presence of localized states is confirmed by the variable range hopping seen in our temperature-dependence conductivity. A large asymmetry is observed between the electron and hole side. The asymmetric nature is attributed to the anisotropic band dispersion of BP. Our measurements establish the uniqueness of quantum capacitance in probing the localized states near the band edge, hitherto not seen in the conductance measurements.