Significant enhancement of the thermoelectric properties of CaP3 through reducing the dimensionality

Abstract

Through first-principles calculations and using the Boltzmann transport equation, we explore the thermoelectric (TE) properties of CaP₃ from the three-dimensional (3D) bulk form to a two-dimensional (2D) monolayer. Herein, our results prove that reducing the dimensionality not only can avoid the poor TE performance along certain crystal orientations, but also can effectively reduce the thermal conductivity. More remarkably, the Seebeck coefficients of the monolayer exhibit a dramatic enhancement compared to its bulk form. Some inherent phonon properties, such as the low acoustic phonon group velocity of ∼1.41 km s⁻¹, large Grüneisen parameters of ∼30, and short phonon relaxation time, can greatly hinder its heat transport ability, leading to an ultralow lattice thermal conductivity of ∼0.65 W m⁻¹ K⁻¹ for the monolayer at room temperature. The size effect is much less sensitive due to the short intrinsic phonon mean free path (MFP). The maximum figure of merit (ZT) for n-type doping at 700 K can reach 6.39 in the nanosheet along the a direction, which is enhanced nearly two times compared to its bulk form (2.83), due to the quantum confinement effect. Collectively, this work shows that low-dimensional nanostructure technology can effectively improve the thermoelectric conversion of this class of materials.