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

The transition metal carbides (namely MXenes) and their functionalized derivatives exhibit various physical and chemical characteristics. For example, it is revealed that the nearly free electron (NFE) states are near the Fermi levels in hydroxyl (OH) functionalized MXenes. Most of these OH-terminated MXene are metallic, but some of them, e.g. Sc2C(OH)2, are semiconductors and the NFE states become conduction bands. In a variety of low dimensional materials, NFE states have been observed but they are less important because their energies are several electron-volts higher than the Fermi levels. In this paper, we address that the NFE states in Sc2C(OH)2 are crucial in the determination of the transport properties. Based on the density functional theory (DFT) calculations and the image-potential well model, it is revealed that the wave functions of these NFE states are spatially extensive. 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. It is also demonstrated that these NFE bands can be engineered by the external electric fields. It is possible to convert semiconducting Sc2C(OH)2 into metallic one. The band-gap manipulation makes Sc2C(OH)2 an excellent candidate for electronic switch applications. Finally, using a set of electron transport calculations, I-V characteristics of Sc2C(OH)2 device is investigated with the gate fields.