Topological quantum materials with layered heterostructure hold great promise for exhibiting low thermal conductivity. Homologous (Bi-2)(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 BiBi 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 (kappa(lat)) and good carrier mobility (mu). Herein, we report a low-temperature simple solution phase synthesis of ultrathin 2D nanosheets of BiSe and Bi4Se3 from the (Bi-2)(m)(Bi2Se3)(n) homologous series. While the 2D nanosheets exhibit ultralow.lat in the range of ca. 0.24-0.27 W/mK, the nanosheets also show good mu. BiSe and Bi4Se3 (both, layered heterostructure) nanosheets exhibit lower.lat compared to the Bi2Se3 (kappa(lat) of similar to 0.35 W/ mK) nanosheets (simple layered structure) because of significant phonon scattering at various interfaces of heterostructured BiSe and Bi4Se3.
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