No abstract

A modification of the modulated elemental reactants synthetic technique was developed and used to synthesize eleven members of the [(SnSe) 1.15 ] m (TaSe 2 ) n family of compounds, with m and n equal to integer values between 1 and 6. Each of the intergrowth compounds contained highly oriented intergrowths of SnSe bilayers and TaSe 2 monolayers with abrupt interfaces perpendicular to the c-axis. The c-lattice parameter increased 0.579(1) nm per SnSe bilayer and 0.649(1) nm per Se−Ta−Se trilayer (TaSe 2 ) as m and n were varied. ab-plane X-ray diffraction patterns and transmission electron microscope images revealed a square in-plane structure of the SnSe constituent, a hexagonal in-plane structure for the TaSe 2 constituent, and rotational disorder between the constituent layers. Temperature dependent electrical resistivity, measured on several specimens, revealed metallic behavior, and a simple model is presented to explain the differences in resistivity as a function of m and n.

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.

A new polytype of the misfit layer compound ([SnSe] 1.16) 1 (NbSe 2) 1 with extensive rotational disorder was prepared from designed modulated elemental reactants. This polytype, previously referred to as a ferecrystal due to the extensive rotational disorder, formed over a range of compositions and precursor thicknesses and the resulting c-axis lattice parameters ranged from 1.2210(4) to 1.2360(4) nm. These values bracket the value published for the crystalline misfit compound prepared at high temperature. The a-and b-axis in-plane lattice parameters of both the SnSe and NbSe 2 constituents were incommensurate, which differs from the misfit layered compound formed via high temperature reaction that has a common b-axis lattice parameter for the two constituents. The in-plane area per unit cell of the ferecrystal is 1-2% larger than the compound formed at high temperature. The ferecrystalline ([SnSe] 1.16) 1 (NbSe 2) 1 compound is 1.6 times more conductive than the misfit layer compound. Hall effect measurements indicate that the ferecrystal is a p-type metal and that the higher conductivity is a consequence of higher mobility of carriers in the ferecrystalline compound.

Four compounds [(SnSe)1.15] m (VSe2)1, where m = 1–4, were synthesized to explore the effect of increasing the distance between Se–V–Se dichalcogenide layers on electrical transport properties. These kinetically stable compounds were prepared using designed precursors that contained a repeating pattern of elemental layers with the nanoarchitecture of the desired product. XRD and STEM data revealed that the precursors self-assembled into the desired compounds containing a Se–V–Se dichalcogenide layer precisely separated by a SnSe layer. The 00l diffraction data are used to determine the position of the Sn, Se, and V planes along the c-axis, confirming that the average structure is similar to that observed in the STEM images, and the resulting data agrees well with results obtained from calculations based on density functional theory and a semiempirical description of van der Waals interactions. The in-plane diffraction data contains reflections that can be indexed as hk0 reflections coming from the two independent constituents. The SnSe layers diffract independently from one another and are distorted from the bulk structure to lower the surface free energy. All of the samples showed metallic-like behavior in temperature-dependent resistivity between room temperature and about 150 K. The electrical resistivity systematically increases as m increases. Below 150 K the transport data strongly indicates a charge density wave transition whose onset temperature systematically increases as m increases. This suggests increasing quasi-two-dimensional behavior as increasingly thick layers of SnSe separate the Se–V–Se layers. This is supported by electronic structure calculations.