Thermoelectric transport properties of two-dimensional materials <i>X</i>Te<sub>2</sub> (<i>X</i> = Pd, Pt) via first-principles calculations

<sec>Developing efficient thermoelectric materials has never lost the attraction due to their promising performances in the energy conversion. The different mechanisms of phonon scattering lead to the various outstanding performances of layered materials in thermoelectric properties. So we investigate the structure, electronic and thermoelectric transport properties of Penta-<i>X</i>Te₂ (<i>X</i> = Pd, Pt) layers based on the density functional theory and Boltzmann transport theory. Those monolayers have a beautiful penta-graphene-like buckled structure with a space group of <i>P</i>2_1/<i>c</i> (No.14). The values of optimized lattice constant <i>a</i> (<i>b</i>) are 6.437 Å (6.145 Å) and 6.423 Å (6.12 Å) for PdTe₂ and PtTe₂ monolayers, respectively. In order to assess the stability, we calculate the phonon dispersion along the high symmetry lines in the Brillouin zone. The second-order harmonic and third-order anharmonic interatomic force constants (IFCs) are calculated by using 5 × 5 × 1 supercell and 4 × 4 × 1 supercell based on the relaxed unit cell. All these results indicate that those monolayers are thermodynamically stable. Energy band structure is essential in obtaining reliable transport properties. So we calculate the band structures of penta-<i>X</i>Te₂. Both PdTe₂ and PtTe₂ are semiconductors with indirect band gaps of 1.24 eV and 1.38 eV, respectively, which are in good agreement with previous experimental and theoretical results.</sec><sec>The lattice thermal conductivity of <i>X</i>Te₂ decreases with temperature increasing, but the electronic thermal conductivity varies with temperature in the opposite way exactly. It is found that the thermal conductivity comes from the contribution of the lattice thermal conductivity at low temperature. The room-temperature total thermal conductivities in the <i>x</i> (<i>y</i>) direction of the PdTe₂ and PtTe₂ monolayers are 3.95 W/(m·K) (2.7 W/(m·K)) and 3.27 W/(m·K)(1.04 W/(m·K)), respectively. The contribution of low thermal conductivity indicates that the thermoelectric properties of PtTe₂ monolayer may be better than those of PdTe₂ monolayer.</sec><sec>The relaxation time (<i>τ</i>) and carrier mobility (<i>μ</i>) are obtained based on the Bardeen-Shockley deformation potential (DP) theory in two-dimensional materials. Remarkably, they have the higher hole mobility than the electron mobility. The anisotropic electronic transport properties of <i>X</i>Te₂ are obtained by solving Boltzmann transport equation. The electrical conductivity over relaxation time (<i>σ</i>/<i>τ</i>) and Seebeck coefficient (<i>S</i>) contribute to the figure of merit <i>ZT</i>. High Seebeck coefficient (<i>S</i>) with the value larger than 400 μV/K can be found in both p-type and n-type cases, suggesting that the TE performance of <i>X</i>Te₂ may be considerable. The room-temperature largest <i>ZT</i> values of penta-<i>X</i>Te₂ (<i>X</i> = Pd, Pt) at p-type are 0.83 and 2.75 respectively. The monolayer PtTe₂ is a potential thermoelectric material.</sec>