Modulation of nearly free electron states in hydroxyl-functionalized MXenes: a first-principles study

Abstract

Two-dimensional transition metal carbides and nitrides (named as MXenes) and their functionalized ones exhibit various physical and chemical characteristics. For example, it has been reported that the nearly free electron (NFE) states can be energetically found near the Fermi levels in hydroxyl functionalized MXenes. Most of these OH-terminated MXene are metallic, but some of them, e.g. Sc₂C(OH)₂, are semiconductors with NFE state conduction bands. In a variety of low dimensional materials, such as graphene, BN nanotubes and fullerenes, NFE states have been theoretically predicted and/or observed experimentally. In these systems, NFE states play less important roles in chemical reactions or electronic device applications because they appear at energies several electron-volts above the Fermi level. Here, based on the density functional theory (DFT) calculations and the image-potential well model, we show that the wave functions of these NFE states are spatially extensive outside the surface. We propose that the energy gap width is affected by the interlayer distance because of the significant overlap and the hybridization between the wave functions of NFE states from the neighboring layers. We also demonstrated that the energetics of the NFE bands can be engineered by external electric fields. This results in semiconducting to metallic transition in Sc₂C(OH)₂. The band-gap manipulation makes Sc₂C(OH)₂ an excellent candidate for electronic switch applications. Finally, by performing a set of electron transport calculations, the I–V characteristics of the Sc₂C(OH)₂ device are investigated at various gate voltages. It is illustrated that the NFE states in Sc₂C(OH)₂ contribute to the transport properties significantly.