Suppressing a Charge Density Wave by Changing Dimensionality in the Ferecrystalline Compounds ([SnSe]_1.15)₁(VSe₂)_<i>n</i> with <i>n</i> = 1, 2, 3, 4

The compounds, ([SnSe]_1.15)₁(VSe₂)_<i>n</i> with <i>n</i> = 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 <i>n</i> = 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 (<i>n</i> = 1) to 172 K (<i>n</i> = 3). For <i>n</i> = 1, this temperature correlates with the onset of the charge density wave transition. The Hall-coefficient changes sign when <i>n</i> is greater than 1, and the temperature dependence of the Hall coefficient of the <i>n</i> = 2 and 3 compounds is very similar to the bulk, slowly decreasing as the temperature is decreased, while for the <i>n</i> = 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

Synthesis and Systematic Trends in Structure and Electrical Properties of [(SnSe)_1.15]_<i>m</i>(VSe₂)₁, <i>m</i> = 1, 2, 3, and 4

Four compounds [(SnSe)_1.15]_<i>m</i>(VSe₂)₁, where <i>m</i> = 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 00<i>l</i> diffraction data are used to determine the position of the Sn, Se, and V planes along the <i>c</i>-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 <i>hk</i>0 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 <i>m</i> increases. Below 150 K the transport data strongly indicates a charge density wave transition whose onset temperature systematically increases as <i>m</i> 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