Manipulating thermoelectric properties of 2D MgI2/Bi2S3 heterostructure based on first-principles calculations

The heterostructure has become an effective method to modulate the electrical and thermal properties of 2D materials. In this paper, both the phonon and electron transport properties of the MgI2/Bi2S3 heterostructure are systematically investigated based on first-principles calculations. The results show that the thermal conductivity of the monolayer MgI2/Bi2S3 heterostructure is greatly lowered compared to both monolayer MgI2 and Bi2S3. At 600 K, the thermal conductivity of the MgI2/Bi2S3 heterostructure is only 0.17 W/m.K, which is nearly three and four times lower than that of MgI2 (0.54 W/m.K) and Bi2S3 (0.70 W/m.K), respectively. The results demonstrate that the heterostructure can result in the higher phonon scattering for the acoustic modes and the lower optical mode phonon group velocities. In addition, the extremely small electronic band gap and direct band gap transitions facilitate electron transport, leading to a maximum electrical conductivity of 0.33 x 10(7) S/m at a carrier concentration of 1015 cm(-2) for MgI2/Bi2S3 heterostructure. This value is 83 % and 13 % higher than that of both MgI2 (0.18 x 10(7) S/m) and Bi2S3 (0.29 x 10(7) S/m), respectively. Furthermore, at a carrier concentration of 10(14) cm(-2), the electrical conductivity of the MgI2/Bi2S3 heterostructure remains almost unchanged while the thermal conductivity is drastically reduced. Therefore, a maximum ZT value of 0.54 is obtained, which is about 217 % and 86 % compared to MgI2 (0.17) and Bi2S3 (0.29).