Book Description

With the advent of two-dimensional (2D) materials came a library of new systems that not only individually host novel states of matter but are naturally suited to being stacked into heterostructures – a virtually limitless space of possible geometries for study. The incorporation of electrical contacts transforms these heterostructures into 2D electrical devices wherein electronic behavior can be simultaneously probed and tuned by locally applied electric fields. This thesis presents pioneering studies of the electronic band structure in such devices in-operando using sub-micrometer angle-resolved photoemission spectroscopy (micro-ARPES), a surface-sensitive technique already perfectly suited to probe the electronic states in these 2D systems made more powerful by the addition of in-situ control of carrier densities and local electric fields.Following a brief background review of 2D materials and ARPES, I will show how with careful sample design and preparation micro-ARPES was applied for the first time to artificially stacked structures of prototypical 2D materials, graphene and atomically thin semiconducting transition metal dichalcogenides (TMD). From there, I will demonstrate how in-situ control of an electrostatic back gate voltage alleviates a fundamental limitation of ARPES as a probe of only occupied states and study the electronic landscape in two-terminal TMD field-effect transistor devices as a function of applied electric field and doping. I will then take a detour to discuss the possible effects of relative crystallographic alignments in heterostructures on the micro-ARPES spectra, which ultimately stem from a combination of moiré effects and electron diffraction. I will then turn to gated devices with monolayer WTe2 and study the bands as a function of doping and temperature to shed light on the of its unusual insulating state below 100K which interplays with topologically insulating and superconducting behavior recently shown in transport measurement. Finally, I will conclude with a brief discussion of the outlook for micro-ARPES of 2D materials in devices under operation.