Tunable axis-dependent conduction polarity in van der Waals materials

Axis-dependent conduction polarity (ADCP), also referred to as goniopolarity, observed in highly anisotropic materials, presents novel opportunities for the development of transverse electronic devices. The ADCP materials exhibit $p$-type conductivity in one direction and $n$-type conductivity in another. In this study, we introduce a tight-binding (TB) Hamiltonian to capture the essence of this intriguing behavior. This single-band model provides a concise description of ADCP, and it allows for the derivation of the relationship between the shape of the Fermi surface and the type of the doped carriers. Utilizing first-principles calculations, we validate our TB ADCP model in an AA-stacked van der Waals $\mathrm{MoS}{\mathrm{i}}_{2}{\mathrm{N}}_{4}$ material, and reveal the origin of the ADCP properties by analyzing its orbital wave function. The observed ADCP is ascribed to the interactions between the $\mathrm{Mo}_d{z}^{2}$ orbitals across the $\mathrm{Mo}{\mathrm{N}}_{2}$ layers, resulting in significantly different Seebeck coefficients with opposite signs when measured along the [100] and [001] directions. Remarkably, we demonstrate that the ADCP in $\mathrm{MoS}{\mathrm{i}}_{2}{\mathrm{N}}_{4}$ can be further enhanced by applying a uniaxial compressive strain along the [001] direction. Our findings not only enhance a simplified TB model to understand this fascinating physical phenomenon in a specific material, but also pave the way for innovative strategies to optimize electric transport in the thermoelectric devices leveraging layered materials. This approach holds promise for significant advancements in the design and functionality of next-generation electronic and thermoelectric devices.