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

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 (<i>ZT</i>) on the Seebeck coefficient, electrical conductivities, and thermal conductivities as a result of this crossover and gate-controlling. We identify that maximizing <i>ZT</i> 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.