The recent chemical exfoliation of layered MAX phase compounds to novel two-dimensional transition metal carbides and nitrides, the so-called MXenes, has brought a new opportunity to materials science and technology.

A newly synthesized family of two-dimensional transition metal carbides and nitrides, so-called MXenes, exhibit metallic or semiconducting properties upon appropriate surface functionalization. Owing to their intrinsic ceramic nature, MXenes may be suitable for energy conversion applications at high temperature. Using the Boltzmann theory and first-principles electronic structure calculations, we explore the thermoelectric properties of monolayer and multilayer M₂C (M = Sc, Ti, V, Zr, Nb, Mo, Hf, and Ta) and M₂N (M = Ti, Zr, and Hf) MXenes functionalized with F, OH, and O groups. From our calculations, it turns out that monolayer and multilayer nanosheets of Mo₂C acquire superior power factors to other MXenes upon any type of functionalization. We therefore propose the functionalized Mo₂C nanosheets as potential thermoelectric materials of the MXene family. The exceptional thermoelectric properties of the functionalized Mo₂C nanosheets are attributed to the peculiar t_2g band shapes, which are a combination of flat and dispersive portions. These types of band shapes allow Mo₂C to gain a large Seebeck coefficient and simultaneously a good electrical conductivity at low carrier concentrations.

MXenes are a set of two-dimensional transition metal carbides and nitrides that offer many potential applications in energy storage and electronic devices. As an important parameter to design new electronic devices, we investigate the work functions of bare MXenes and their functionalized ones with F, OH, and O chemical groups using first-principles calculations. From our calculations, it turns out that the OH-terminated MXenes attain ultralow work functions between 1.6 and 2.8 eV. Moreover, depending on the type of the transition metal, the F or O functionalization affects increasing or decreasing the work functions. We show that the changes in the work functions upon functionalizations are linearly correlated with the changes in the surface dipole moments. It is shown that the work functions of the F- or O-terminated MXenes are controlled by two factors: the induced dipole moments by the charge transfers between F/O and the substrate, and the changes in the total surface dipole moments caused by surface relaxation upon the functionalization. However, in the cases of the OH-terminated MXenes, in addition to these two factors, the intrinsic dipole moments of the OH groups play an important role in determining the total dipole moments and consequently justify their ultralow work functions.

Two-dimensional (2D) topological insulators (TIs) have been recognized as a new class of quantum state of matter. They are distinguished from normal 2D insulators with their nontrivial band-structure topology identified by the $Z_2$ number as protected by time-reversal symmetry (TRS). Two-dimensional TIs have intriguing spin-velocity locked conducting edge states and insulating properties in the bulk. In the edge states, the electrons with opposite spins propagate in opposite directions and the backscattering is fully prohibited when the TRS is conserved. This leads to a quantized dissipationless “two-lane highway” for charge and spin transportation and promises potential applications. Up to now, only very few 2D systems have been discovered to possess this property. The lack of suitable material obstructs further study and application. Here, by using first-principles calculations, we propose that functionalized MXenes with oxygen, ${\mathrm{M}}_2{\mathrm{CO}}_2$ (M=W, Mo, and Cr), are 2D TIs with the largest gap of 0.194 eV in the W case. They are dynamically stable and natively antioxidant. Most importantly, they are very likely to be easily synthesized by recently developed selective chemical etching of transition-metal carbides (the ${\mathrm{M}}_{n+1}{\mathrm{AX}}_n$ phase). This will pave the way to tremendous applications of 2D TIs, such as “ideal” conducting wire, multifunctional spintronic devices, and the realization of topological superconductivity and Majorana modes for quantum computing.