The Influence of Interfaces on Properties of Thin-Film Inorganic Structural Isomers Containing SnSe–NbSe₂ Subunits

Abstract

Inorganic isomers ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>([SnSe]_1+δ)_<i>p</i>(NbSe₂)_<i>q</i>([SnSe]_1+δ)_<i>r</i>(NbSe₂)_<i>s</i> where <i>m</i>, <i>n</i>, <i>p</i>, <i>q</i>, <i>r</i>, and <i>s</i> are integers and <i>m</i> + <i>p</i> + <i>r</i> = <i>n</i> + <i>q</i> + <i>s</i> = 4 were prepared using the modulated elemental reactant technique. This series of all six possible isomers provides an opportunity to study the influence of interface density on properties while maintaining the same unit cell size and composition. As expected, all six compounds were observed to have the same atomic compositions and an almost constant <i>c</i>-axis lattice parameter of ≈4.90(5) nm, with a slight trend in the <i>c</i>-axis lattice parameter correlated with the different number of interfaces in the isomers: two, four and six. The structures of the constituents in the <i>ab</i>-plane were independent of one another, confirming the nonepitaxial relationship between them. The temperature dependent electrical resistivities revealed metallic behavior for all the six compounds. Surprisingly, the electrical resistivity at room temperature decreases with increasing number of interfaces. Hall measurements suggest this results from changes in carrier concentration, which increases with increasing thickness of the thickest SnSe block in the isomer. Carrier mobility scales with the thickness of the thickest NbSe₂ block due to increased interfacial scattering as the NbSe₂ blocks become thinner. The observed behavior suggests that the two constituents serve different purposes with respect to electrical transport. SnSe acts as a charge donor and NbSe₂ acts as the charge transport layer. This separation of function suggests that such heterostructures can be designed to optimize performance through choice of constituent, layer thickness, and layer sequence. A simplistic model, which predicts the properties of the complex isomers from a weighted sum of the properties of building blocks, was developed. A theoretical model is needed to predict the optimal compound for specific properties among the many potential compounds that can be prepared