Half-metals and spin gapless semiconductors are promising candidates for spintronic applications due to the complete (100%) spin polarization of electrons around the Fermi level. Based on recent experimental and theoretical findings of graphene-like monolayer transition metal carbides and nitrides (also known as MXenes), we demonstrate using first-principles calculations that monolayers Ti₂C and Ti₂N exhibit nearly half-metallic ferromagnetism with the magnetic moments of 1.91 and 1.00μ_B per formula unit, respectively, while monolayer V₂C is a metal with unstable antiferromagnetism, and monolayer V₂N is a nonmagnetic metal. Interestingly, under a biaxial strain, there is a phase transition from a nearly half-metal to truly half-metal, spin gapless semiconductor, and metal for monolayer Ti₂C. Monolayer Ti₂N is still a nearly half-metal under a suitable biaxial strain. Large magnetic moments can be induced by the biaxial tensile and compressive strains for monolayer V₂C and V₂N, respectively. We also show that the structures of these four monolayer MXenes are stable according to the calculated formation energy and phonon spectrum. Our investigations suggest that, unlike monolayer graphene, monolayer MXenes Ti₂C and Ti₂N without vacancy, doping or external electric field exhibit intrinsic magnetism, especially the half-metallic ferromagnetism and spin gapless semiconductivity, which will stimulate further studies on possible spintronic applications for new two-dimensional materials of MXenes.

Using the first-principles full-potential linearized augmented plane-wave method based on density functional theory, we have investigated the electronic structure and magnetism of hypothetical $M$C ($M=\mathrm{Mg}$, Ca, Sr, and Ba) compounds with the zinc-blende (ZB) crystal structure. It is shown that ZB $\mathrm{CaC}$, $\mathrm{SrC}$, and $\mathrm{BaC}$ are half-metallic ferromagnets with large half-metallic gaps (up to $0.83\mathrm{eV}$). The half metallicity is found to be robust with respect to the lattice compression and is maintained up to the lattice-constant contraction of 14%, 13%, and 9% for $\mathrm{CaC}$, $\mathrm{SrC}$, and $\mathrm{BaC}$, respectively. The exchange interactions in these compounds are studied using the augmented spherical wave method in conjunction with the frozen-magnon approach. The Curie temperature is estimated within both the mean field approximation and the random phase approximation. The predicted Curie temperatures of all three half-metallic compounds considerably exceed the room temperature. The large half-metallic gaps, the robustness of the half metallicity with respect to the lattice contraction, and the high Curie temperatures make these systems interesting candidates for applications in spintronic devices. The absence of the transition-metal atoms makes these compounds important model systems for the study of the origin and properties of the half-metallic ferromagnetism of $s\text{−}p$ electron systems.

Improving the thermoelectric efficiency is one of the greatest challenges in materials science. The recent discovery of excellent thermoelectric performance in simple orthorhombic SnSe crystal offers new promise in this prospect [Zhao et al. Nature 508, 373 (2014)]. By calculating the thermoelectric properties of orthorhombic IV-VI compounds GeS,GeSe,SnS and SnSe based on the first-principles combined with the Boltzmann transport theory, we show that the Seebeck coefficient, electrical conductivity and thermal conductivity of orthorhombic SnSe are in agreement with the recent experiment. Importantly, GeS, GeSe and SnS exhibit comparative thermoelectric performance compared to SnSe. Especially, the Seebeck coefficients of GeS, GeSe and SnS are even larger than that of SnSe under the studied carrier concentration and temperature region. We also use the Cahill's model to estimate the lattice thermal conductivities at the room temperature. The large Seebeck coefficients, high power factors and low thermal conductivities make these four orthorhombic IV-VI compounds promising candidates for high-efficient thermoelectric materials.

Monolayer transition-metal dichalcogenides (TMDCs) M X ₂ ( M = Mo, W, Zr, Hf, etc; X = S, Se, Te) have become well-known in recent times for their promising applications in thermoelectrics and field effect transistors. In this work, we perform a systematic study on the thermoelectric properties of monolayer ZrSe ₂ and HfSe ₂ using first-principles calculations combined with Boltzmann transport equations. Our results point to a competitive thermoelectric figure of merit (close to 1 at optimal doping) in both monolayer ZrSe ₂ and HfSe ₂ , which is markedly higher than previous explored monolayer TMDCs such as MoS ₂ and MoSe ₂ . We also reveal that the higher figure of merits arise mainly from their low lattice thermal conductivity, and this is partly due to the strong coupling of acoustic modes with low frequency optical modes. It is found that the figure of merits can be better optimized in n -type than in p -type. In particular, the performance of HfSe ₂ is superior to ZrSe ₂ at a higher temperature. Our results suggest that monolayer ZrSe ₂ and HfSe ₂ with lower lattice thermal conductivity than usual monolayer TMDCs are promising candidates for thermoelectric applications.