Keywords: Layered compounds / Thin films / Electronic structure / Thermoelectric materials / Charge transfer (SnSe) 1.2 TiSe 2 was found to self-assemble from a precursor containing modulated layers of Sn-Se and Ti-Se over a surprisingly large range of layer thicknesses and compositions. The constituent lattices form an alternating layer superstructure with rotational disorder present between the layers. This compound was found to have the highest Seebeck coefficient measured for analogous TiX 2 containing misfit layered compounds to date, suggesting potential for [a]83 low-temperature thermoelectric applications. Electrical characterization suggests that electrons transferred from SnSe to TiSe 2 are responsible for the higher carrier concentration observed relative to bulk TiSe 2 . The transfer of charge from one constituent to the other may provide a mechanism for doping layered dichalcogenides for various applications without negatively affecting carrier mobility.

The compounds, ([SnSe]1.15)1(VSe2)n with n = 1, 2, 3, and 4, were prepared using designed precursors in order to investigate the influence of the thickness of the VSe2 constituent on the charge density wave transition. The structure of each of the compounds was determined using X-ray diffraction and scanning transmission electron microscopy. The charge density wave transition observed in the resistivity of ([SnSe]1.15)1(VSe2)1 was confirmed. The electrical properties of the n = 2 and 3 compounds are distinctly different. The magnitude of the resistivity change at the transition temperature is dramatically lowered and the temperature of the resistivity minimum systematically increases from 118 K (n = 1) to 172 K (n = 3). For n = 1, this temperature correlates with the onset of the charge density wave transition. The Hall-coefficient changes sign when n is greater than 1, and the temperature dependence of the Hall coefficient of the n = 2 and 3 compounds is very similar to the bulk, slowly decreasing as the temperature is decreased, while for the n = 1 compound the Hall coefficient increases dramatically starting at the onset of the charge density wave. The transport properties suggest an abrupt change in electronic properties on increasing the thickness of the VSe2 layer beyond a single layer.

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The synthesis and characterization of turbostratically disordered (BiSe) 1.15 TiSe 2 is reported. Specular and in-plane x-ray diffraction studies indicate an alternating structure containing two planes of a distorted rock salt structured BiSe and a Se-Ti-Se trilayer of TiSe 2 with independent lattices. The title compound was found to be turbostratically (rotationally) disordered about the c-axis, and the BiSe layer displays an orthorhombic in-plane structure with a = 4.562(2) Å and b = 4.242(1) Å. Temperature dependent electrical resistivity reveals that the disordered compound is metallic, but with less temperature dependence than may be expected for a 3D crystal, which is attributed to the lack of coherent vibrations due to the turbostratic disorder. The room temperature resistivity was found to be ρ = 5.0 × 10 −6 m with a carrier concentration of n = 5 × 10 21 cm −3 . Comparing the carrier concentration to (PbSe) 1.16 TiSe 2 suggests that the bismuth is trivalent and donates an electron to the conduction band of the TiSe 2 constituent.

The compounds ([SnSe]1+δ) m (NbSe2)1, where 1 ≤ m ≤ 10, were prepared from a series of designed precursors. The c-axis lattice parameter systematically increases by 0.577(5) nm as the value of m is increased, which indicates that an additional bilayer of rock salt structured SnSe is inserted for each unit of m. The in-plane structure of both constituents systematically changes as the thickness of SnSe increases. Both X-ray diffraction and electron microscopy studies show the presence of turbostratic disorder between the different constituent layers. The electrical resistivity and Hall coefficient increase systematically as a function of m stronger than would be expected for noninteracting metallic NbSe2 and semiconducting SnSe layers, suggesting the presence of charge transfer between the layers. The temperature dependence of the resistivity changes from metallic behavior for m < 4 to weakly increasing, for higher m, as temperature decreases. Compounds with m > 3 show an upturn in the resistivity below 50 K and a corresponding increase in the Hall coefficient, which both become more pronounced as m increases. This suggests localization of carriers, which is expected in two-dimensional crystals. The extent of charge transfer in ([SnSe]1+δ) m (NbSe2)1 can be tuned as a function of SnSe thickness and spans over the same range as reported in the literature for various NbX2 based intercalated and misfit layer compounds.