Charge transfer vs. dimensionality: what affects the transport properties of ferecrystals?

Abstract

A series of ([SnSe]_1+δ)_m(NbSe₂)₂ compounds with two layers of NbSe₂ separated by m bilayers of SnSe, where 1 ≤ m ≤ 20, were prepared from modulated precursors by systematically changing the number of SnSe layers in the repeating unit. A change in the c-lattice parameter of 0.579(3) nm per SnSe bilayer was observed. The thickness of the NbSe₂ layer was determined to be 1.281(4) nm: twice the value of a single NbSe₂ layer. HAADF-STEM images revealed the presence of extensive rotational disorder and the lack of any epitaxial relationship among the constituent layers. Two different coordination environments for the Nb in NbSe₂ (trigonal prismatic and octahedral) were observed. The electrical resistivity increases and the carrier concentration decreases in the ([SnSe]_1+δ)_m(NbSe₂)₂ compounds with increasing number of SnSe bilayers. The temperature dependence of the resistivity suggests localization of carriers for higher m values. The decline in carrier concentration as a function of m implies the presence of charge transfer from SnSe to NbSe₂. The transport properties of the ([SnSe]_1+δ)_m(NbSe₂)₂ compounds and the previously reported ([SnSe]_1+δ)_m(NbSe₂)₁ compounds both have unusually temperature independent resistivity compared to bulk NbSe₂. Compounds with similar m/n ratios exhibit similar transport properties. Consequently, the dominant effect on the transport properties of ([SnSe]_1+δ)_m(NbSe₂)₂ is charge transfer, and there are only subtle differences between a monolayer and a bilayer of NbSe₂.