Negative thermal expansion of group III-Nitride monolayers

Materials with a negative thermal expansion coefficient have diverse potential applications in electronic engineering. For instance, mixing two materials with negative and positive thermal expansion coefficients can avoid changing volume with temperature. In this study, we investigate the variation of linear thermal expansion coefficients (LTECs) of group III-Nitride monolayers (h-XN, where X = B, Al, Ga, In) with temperature using quasi-harmonic approximation. We also explore phonon thermal properties of h-XN monolayers, including specific heat, entropy, and free energy. These systems are revealed to exhibit considerably high negative LTEC values below the room temperature. To understand the origin of negative thermal expansion, we analyze the contribution of individual phonon branches to the LTEC, and it is found that the highest contribution is originating from ZA (out-of-plane acoustic) phonon mode. While h-BN and h-AlN monolayers exhibit negative LTEC values in the studied temperature range (0-800 K), unlike their bulk counterparts, the negative LTEC values converge to the zero for h-GaN and h-InN monolayers above room temperatures. These findings can be crucial in designing h-XN based nanoscale heat devices.