Non-layered InSe nanocrystalline bulk materials with ultra-low thermal conductivity

Exploring new prototypes for a given chemical composition is of great importance and interest to several disciplines. As a famous semiconducting binary compound, InSe usually exhibits a two-dimensional layered structure with decent physical and mechanical properties. However, it is less noticed that InSe can also adopt a monoclinic structure, denoted as mcl-InSe. The synthesis of such a phase usually requires high-pressure conditions, and the knowledge is quite scarce on its chemical bonding, lattice dynamics, and thermal transport. Here in this work, by developing a facile method combining mechanical alloying and spark plasma sintering, we successfully synthesize mcl-InSe bulks with well-crystallized nanograins. The chemical bonding of mcl-InSe is understood as compared with layered InSe via charge analysis. Low cut-off frequencies of acoustic phonons and several low-lying optical modes are demonstrated. Noticeably, mcl-InSe exhibits a low room-temperature thermal conductivity of 0.6 W·m⁻¹·K⁻¹, which is smaller than that of other materials in the In–Se system and many other selenides. Low-temperature thermal analyses corroborate the role of nanograin boundaries and low-frequency optical phonons in scattering acoustic phonons. This work provides new insights into the non-common prototype of the InSe binary compound with potential applications in thermoelectrics or thermal insulation.