Synthesis and Thermal Properties of Solid-State Structural Isomers: Ordered Intergrowths of SnSe and MoSe₂

Abstract

A family of structural isomers [(SnSe)1.05]m(MoSe2)n were prepared using the modulated elemental reactant method by varying the layer sequence and layer thicknesses in the precursor. By varying the sequence of Sn-Se and Mo-Se layer pairs deposited and annealing the precursors to self-assemble the targeted compound, all six possible isomers [(SnSe)1.05]4(MoSe2)4, [(SnSe)1.05]3(MoSe2)3[(SnSe)1.05]1(MoSe2)1, [(SnSe)1.05]3(MoSe2)2[(SnSe)1.05]1(MoSe2)2, [(SnSe)1.05]2(MoSe2)3[(SnSe)1.05]2(MoSe2)1, [(SnSe)1.05]2(MoSe2)1[(SnSe)1.05]1(MoSe2)2[(SnSe)1.05]1(MoSe2)1, and [(SnSe)1.05]2(MoSe2)2[(SnSe)1.05]1(MoSe2)1[(SnSe)1.05]1(MoSe2)1 were prepared. The structures were characterized by X-ray diffraction and electron microscopy which showed that all of the compounds have very similar c-axis lattice parameters and in-plane constituent lattice parameters yet distinct isomeric structures. These studies confirm that the structure, order, and thickness of the constituent layers match that of the precursors. The cross-plane thermal conductivity is found to be very low (∼0.08 Wm(-1) K(-1)) and independent of the number of SnSe-MoSe2 interfaces within uncertainty. The poor thermal transport in these layered isomers is attributed to a large cross-plane thermal resistance created by SnSe-MoSe2 and MoSe2-MoSe2 turbostratically disordered van der Waals interfaces, the density of which has less variation among the different compounds than the SnSe-MoSe2 interface density alone.

Inorganic Crystals with Glass‐Like and Ultralow Thermal Conductivities†

“…These values are well below the calculated minimum thermal conductivities for the PbSe, SnSe, MoSe 2 , or WSe 2 constituents, and also significantly lower than the values measured thus far for crystalline misfit layered compounds. Interestingly, the thermal conductivity values are higher than those obtained for turbostratically disordered WSe 2 alone, and found to be essentially independent of the MX‐TX 2 interface density, a result which is in stark contrast to conventional semiconducting superlattices. The in‐plane thermal conductivity of free standing [(PbSe) 1+δ ] m [WSe 2 ] m films was found to be significantly higher than the cross‐plane values but still very low (∼ 0.4 W/m‐K) and nearly temperature independent .…”

“…The cross‐plane thermal conductivity of semiconducting [(MSe) 1+δ ] m [TSe 2 ] n thin films with M = Pb or Sn, T = Mo or W, and 1 ≤ m, n ≤ 5 has been measured by time domain thermoreflectance, revealing ultralow values between approximately 0.06 W/m‐K and 0.2 W/m‐K . These values are well below the calculated minimum thermal conductivities for the PbSe, SnSe, MoSe 2 , or WSe 2 constituents, and also significantly lower than the values measured thus far for crystalline misfit layered compounds.…”

Anomalously Low Heat Conduction in Single‐Crystal Superlattice Ceramics Lower Than Randomly Oriented Polycrystals

“…This material system consists of disordered layered basal plane crystals and exhibits κ values less than 0.1 W m −1 K −1 , which is attributed to strongly localized lattice vibrations. [ 14–18 ] There were also computational efforts with ab initio calculations to quantify the heat conduction across the interfaces, and the results indicated that the boundary conductance enhances with increasing interface density. [ 19 ]…”

Formation of a Selenide‐Based Heterostructure From a Designed Precursor†

“…While thermodynamically stable misfit layer compounds can be prepared typically only when n = m = 1, ferecrystals can be synthesized with a wide range of m and n values . One can even prepare ferecrystals that are structural isomers, only distinguishable from one another by the number of interfaces between the different building blocks …”