2D Nanosheets of Topological Quantum Materials from Homologous (Bi2)m(Bi2Se3)nHeterostructures: Synthesis and Ultralow Thermal Conductivity

Topological quantum materials with layered heterostructure hold great promise for exhibiting low thermal conductivity. Homologous (Bi2)m(Bi2Se3)n(m, n: integers) series hosts different layered topological quantum materials such as Bi4Se3(m= 1, n= 1; a topological semimetal), BiSe (m= 1, n= 2; a weak topological insulator), and well-known Bi2Se3(m= 0, n= 1; a strong topological insulator). In BiSe, the Bi–Bi bilayer is sandwiched between the Se–Bi–Se–Bi–Se quintuple layers via weak van der Waals (vdWs) interactions, while in Bi4Se3, the Bi–Bi bilayer and the Se–Bi–Se–Bi–Se quintuple layer stack alternatively via weak vdWs interactions, thereby forming natural vdWs heterostructure. Synthesis of ultrathin two-dimensional (2D) nanosheets of these quantum materials with a natural heterostructure is of high significance in terms of low lattice thermal conductivity (κlat) and good carrier mobility (μ). Herein, we report a low-temperature simple solution phase synthesis of ultrathin 2D nanosheets of BiSe and Bi4Se3from the (Bi2)m(Bi2Se3)nhomologous series. While the 2D nanosheets exhibit ultralow κlatin the range of ca. 0.24–0.27 W/mK, the nanosheets also show good μ. BiSe and Bi4Se3(both, layered heterostructure) nanosheets exhibit lower κlatcompared to the Bi2Se3(κlatof ∼0.35 W/mK) nanosheets (simple layered structure) because of significant phonon scattering at various interfaces of heterostructured BiSe and Bi4Se3.