Ionic Gate Spectroscopy of 2D Semiconductors

TL;DR

Ionic gate spectroscopy offers a straightforward, transport-based method to accurately measure band edge alignments in 2D semiconductors, overcoming limitations of traditional optical and tunneling techniques.

Contribution

This paper introduces and reviews ionic gate spectroscopy as a novel technique for determining band edge positions in 2D semiconductors using ionic gate field-effect transistors.

Findings

Enables direct measurement of band edge shifts from transport data

Applicable to various 2D transition metal dichalcogenides and heterostructures

Circumvents issues caused by exciton binding energy and small crystal size

Abstract

Reliable and precise measurements of the relative energy of band edges in semiconductors are needed to determine band gaps and band offsets, as well as to establish the band diagram of devices and heterostructures. These measurements are particularly important in the field of two-dimensional materials, in which many new semiconducting systems are becoming available through exfoliation of bulk crystals. For two-dimensional semiconductors, however, commonly employed techniques suffer from difficulties rooted either in the physics of these systems, or of technical nature. The very large exciton binding energy, for instance, prevents the band gap to be determined from a simple spectral analysis of photoluminescence, and the limited lateral size of atomically thin crystals makes the use of conventional scanning tunneling spectroscopy cumbersome. Ionic gate spectroscopy is a newly developed technique that exploits ionic gate field-effect transistors to determine quantitatively the relative alignment of band edges of two-dimensional semiconductors in a straightforward way, directly from transport measurements (i.e., from the transistor electrical characteristics). The technique relies on the extremely large geometrical capacitance of ionic gated devices that -- under suitable conditions -- enables a change in gate voltage to be directly related to a shift in chemical potential. Here we present an overview of ionic gate spectroscopy, and illustrate its relevance with applications to different two-dimensional semiconducting transition metal dichalcogenides and van der Waals heterostructures.