Thermoelectric Optimization and Quantum-to-Classical Crossover in Gate-Controlled Two-Dimensional Semiconducting...

We investigate the thermoelectric performance of Pt-WSe₂-Pt nanojunctions with gate-tunable architectures and varying channel lengths from 3 to 12 nm using a combination of first-principles simulations, including density functional theory (DFT) (Vienna Ab initio Simulation Package (VASP)), DFT with nonequilibrium Green's function (NEGF) formalism (NanoDCAL), and nonequilibrium molecular dynamics simulations (NEMD) (Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)). Our study reveals a gate- and temperature-controlled quantum-to-classical crossover in electron transport, transitioning from quantum tunneling in short junctions to thermionic emission in longer ones. We observe nontrivial dependencies of the thermoelectric figure of merit (ZT) on the Seebeck coefficient, electrical conductivities, and thermal conductivities as a result of this crossover and gate-controlling. We identify that maximizing ZT requires tuning the chemical potential just outside the band gap, where the system lies at the transition between insulating and conducting regimes. While enormous Seebeck coefficients (>5000 μV/K) are observed in the insulating state, they do not yield high ZT due to suppressed electrical conductivity and dominant phononic thermal transport. The optimal ZT (>2.3) is achieved in the shortest (3 nm) junction at elevated temperatures (500 K), where quantum tunneling and thermionic emission coexist.