Anomalous Dependence of Seebeck Coefficient and <i>ZT</i> Value on Van Hove Singularity: A Case Study of Janus 2H-WSH Monolayer

A focus of recent research has been the improvement of the ZT value (figure of merit) of materials, with particular emphasis on optimizing the Seebeck coefficient, which plays a more critical role in determining ZT than electrical or thermal conductivity. Previous studies have shown that the Seebeck coefficient can be effectively enhanced by increasing the density of states (DOS) at the Fermi level. As a result, significant efforts have been devoted to constructing van Hove singularities (VHS) at the Fermi level through energy band engineering, aiming to regulate the ZT value by increasing the Seebeck coefficient. In this work, we conduct systematic first-principles calculations on the thermoelectric performance of the Janus 2H-WSH monolayer as a case study. We find that by adjusting the Fermi level to the band edge corresponding to the VHS, the Seebeck coefficient of 2H-WSH approaches zero, leading to a sharp decline in the ZT value. The reason is that the Seebeck coefficient is primarily determined by the gradient of the transport distribution function, of which the DOS is only one component. Therefore, relying solely on increasing the DOS is insufficient to enhance the Seebeck coefficient and, consequently, the ZT value. Furthermore, by tuning the Fermi level, the ZT value of Janus 2H-WSH monolayer can be significantly modulated. With/without spin–orbit coupling (SOC), the ZT value can be adjusted from 1.72 × 10–6/2.88 × 10–5 to 0.023/0.045, corresponding to an improvement by a factor of 13182/1547. Such a case is anticipated to occur frequently in realistic materials, extending beyond the 2H-WSH monolayer. Consequently, this issue merits significant attention.