Modulation of Nearly Free Electron States in Hydroxyl-Functionalized MXenes: A First-Principles Study

TL;DR

This study uses first-principles calculations to explore how nearly free electron states in hydroxyl-functionalized MXenes can be modulated by external electric fields, affecting their electronic properties and potential device applications.

Contribution

It introduces an image-potential well model to analyze NFE states in MXenes and demonstrates how external electric fields can tune their electronic properties, including band-gap manipulation.

Findings

NFE states are near the Fermi level in hydroxyl MXenes.

External electric fields can convert semiconducting MXenes into metals.

Interlayer distances influence the energy gap widths.

Abstract

The transition metal carbides (namely MXenes) and their functionalized derivatives exhibit various physical and chemical characteristics and offer many potential applications in electronic devices and sensors. Using density functional theory (DFT), it is revealed that the nearly free electron (NFE) states are near the Fermi levels in hydroxyl (OH) functionalized MXenes. Most of the OH-terminated MXene are metallic, but some of them, e.g. Sc2C(OH)2, are semiconductors and the NFE states are conduction bands. In this paper, to investigate the NFE states in MXenes, an attractive image-potential well model is adopted. Compared the solutions of this model with the DFT calculations, it is found that due to the overlap of spatially extensive wave functions of NFE states and their hybridization between the artificial neighboring layers imposed by the periodical boundary conditions (PBCs), the DFT results represent the properties of multiple layers, intrinsically. Based on the DFT calculations, it is found that the energy gap widths are affected by the interlayer distances. We address that the energetics of the NFE states can be modulated by the external electric fields and it is possible to convert semiconducting MXenes into metals. This band-gap manipulation makes the OH-terminated semiconducting MXenes an excellent candidate for electronic switch applications. Finally, using a set of electron transport calculations, I-V characteristics of Sc2C(OH)2 devices are investigated with the gate voltages.