Chemical exfoliation of MAX phases into two-dimensional (2D) MXenes can be considered as a major breakthrough in the synthesis of novel 2D systems. To gain insight into the exfoliation possibility of MAX phases and to identify which MAX phases are promising candidates for successful exfoliation into 2D MXenes, we perform extensive electronic structure and phonon calculations, and determine the force constants, bond strengths, and static exfoliation energies of MAX phases to MXenes for 82 different experimentally synthesized crystalline MAX phases. Our results show a clear correlation between the force constants and the bond strengths. As the total force constant of an “A” atom contributed from the neighboring atoms is smaller, the exfoliation energy becomes smaller, thus making exfoliation easier. We propose 37 MAX phases for successful exfoliation into 2D Ti₂C, Ti₃C₂, Ti₄C₃, Ti₅C₄, Ti₂N, Zr₂C, Hf₂C, V₂C, V₃C₂, V₄C₃, Nb₂C, Nb₅C₄, Ta₂C, Ta₅C₄, Cr₂C, Cr₂N, and Mo₂C MXenes. In addition, we explore the effect of charge injection on MAX phases. We find that the injected charges, both electrons and holes, are mainly received by the transition metals. This is due to the electronic property of MAX phases that the states near the Fermi energy are mainly dominated by d orbitals of the transition metals. For negatively charged MAX phases, the electrons injected cause swelling of the structure and elongation of the bond distances along the c axis, which hence weakens the binding. For positively charged MAX phases, on the other hand, the bonds become shorter and stronger. Therefore, we predict that the electron injection by electrochemistry or gating techniques can significantly facilitate the exfoliation possibility of MAX phases to 2D MXenes.

Two-dimensional (2D) van der Waals heterostructures (vdWHs) have shown multiple functionalities with great potential in electronics and photovoltaics. Here, we show their potential for solid-state thermionic energy conversion and demonstrate a designing strategy towards high-performance devices. We propose two promising thermionic devices, namely, the p-type Pt-G-WSe₂-G-Pt and n-type Sc-WSe₂-MoSe₂-WSe₂-Sc. We characterize the thermionic energy conversion performance of the latter using first-principles GW calculations combined with real space Green’s function (GF) formalism. The optimal barrier height and high thermal resistance lead to an excellent performance. The proposed device is found to have a room temperature equivalent figure of merit of 1.2 which increases to 3 above 600 K. A high performance with cooling efficiency over 30% of the Carnot efficiency above 450 K is achieved. Our designing and characterization method can be used to pursue other potential thermionic devices based on vdWHs.

Using first-principles density functional theory (DFT) calculations combined with the Boltzmann transport theory, we investigate the effect of strain on the electronic and thermoelectric transport properties of the 1T-TiSe ₂ monolayer, a two-dimensional (2D) material, and compare it with the bulk phase within the PBE, LDA+ U and HSE exchange–correlation functionals.

Analytical solutions based on the Boltzmann transport equation within the relaxation time approximation are developed to relate the Nernst coefficient to materials band structure and relaxation times parameters in simple conductors. The dependence of the Nernst coefficient on the effective mass, anisotropy of bands, energy bandgap, and scattering parameters are investigated. The obtained relations are compared to the existing solutions presented in the past using different approximations.

MAX phase is a family of ceramic compounds, typically known for their metallic properties. However, we show here that some of them may be narrow bandgap semiconductors. Using a series of first-principles calculations, we have investigated the electronic structures of 861 dynamically stable MAX phases. Notably, Sc₂SC, Y₂SC, Y₂SeC, Sc₃AuC₂, and Y₃AuC₂ have been identified as semiconductors with band gaps ranging from 0.2 to 0.5 eV. Furthermore, we have assessed the thermodynamic stability of these systems by generating ternary phase diagrams utilizing evolutionary algorithm techniques. Their dynamic stabilities are confirmed by phonon calculations. Additionally, we have explored the potential thermoelectric efficiencies of these materials by combining Boltzmann transport theory with first-principles calculations. The relaxation times are estimated using scattering theory. The zT coefficients for the aforementioned systems fall within the range of 0.5 to 2.5 at temperatures spanning from 300 to 700 K, indicating their suitability for high-temperature thermoelectric applications.