Influence of Nanoarchitecture on Charge Donation and the Electrical-Transport Properties in [(SnSe)_1+δ][TiSe₂]_q Heterostructures

A series of [(SnSe)_1+δ]­[TiSe₂]_q heterostructures with systematic changes in the number of TiSe₂ layers in the repeating unit were synthesized, and both the structure and electronic-transport properties were characterized. The c-axis lattice parameter increased linearly as q increased, and the slope was consistent with the thickness of a TiSe₂ layer. In-plane lattice constants for SnSe and TiSe₂ were independent of q. Temperature-dependent resistivity and Hall coefficient data varied systematically as q was increased. The low-temperature electrical data was modeled assuming that only electrons were involved, and the data was fit to a variable range hopping mechanism. The number of carriers involved in this low-temperature transport decreased as q increased, indicating that approximately 1/10th of an electron per SnSe bilayer was transferred to the TiSe₂. Calculations also indicated that there was charge donation from the SnSe layer to the TiSe₂ layer, resulting in an ionic bond between the layers, which aided in stabilizing the heterostructures. The charge donation created a TiSe₂–SnSe–TiSe₂ block with the properties distinct from the constituent bulk properties. At high temperatures in large q samples, the transport data required holes to be activated across a band gap to be successfully modeled. This high-temperature transport scales with the number of TiSe₂ layers that are not adjacent to SnSe. Using a consistent model across all of the samples significantly constrained the adjustable parameters. The charge transfer between the two constituents results in the stabilization of the heterostructure by an ionic interaction and the formation of a conducting TiSe₂–SnSe–TiSe₂ block. This is consistent with prior reports, where interactions between two-dimensional (2D) layers and their surroundings (i.e., adjacent layers, substrate, or atmosphere) have been shown to strongly influence the properties.