Electronic structure and thermoelectric properties of Mo-based dichalcogenide monolayers locally and randomly modified by substitutional atoms.

Density functional theory and Boltzmann transport equations are used to investigate electronic band structure and thermoelectric (TE) properties of different two-dimensional (2D) materials containing Mo, S, Nb, Se, and Te. In MoS-based monolayers (MLs) the substitution of S atoms by Te atoms up to the concentration of 12.5 at% leads to a more significant change of the band structure than in the corresponding case with Se atoms. In particular, the bandgap is reduced. At a high concentration of Se or Te the electronic structure becomes more similar to that of the SeMoS or TeMoS Janus layers, and the MoSe or MoTe MLs. It is found that local and random introduction of substitutional Se or Te atoms yields not very different results. The substitution of Mo by Nb, at the concentration of 2.1 at% leads to hole levels. The thermoelectric properties of the considered 2D materials are quantified by the Seebeck coefficient and thermoelectric figure of merit. The two characteristics are determined for different levels of p- or n-doping of the MLs and for different temperatures. Compared to the pristine MoS ML, Te substitutional atoms cause more changes of the thermoelectric properties than Se atoms. However, MLs with Se substitutional atoms show a high thermoelectric figure of merit in a broader range of possible p- or n-doping levels. In most cases, the maximum thermoelectric figure of merit is about one, both in p- and n-type materials, and for temperatures between 300 and 1200 K. This is not only found for MoS-based MLs with substitutional atoms but also for the Janus layers and for MoSe or MoTe MLs. Interestingly, for MLs with one Nb as well as two or four Te substitutional atoms the highest values of the TE figure of merit of 1.2 and 1.40, respectively, are obtained at a temperature of 1200 K.