Structural Changes as a Function of Thickness in [(SnSe)_1+δ]_mTiSe₂ Heterostructures

Abstract

Single- and few-layer metal chalcogenide compounds are of significant interest due to structural changes and emergent electronic properties on reducing dimensionality from three to two dimensions. To explore dimensionality effects in SnSe, a series of [(SnSe)]TiSe intergrowth structures with increasing SnSe layer thickness (m = 1-4) were prepared from designed thin-film precursors. In-plane diffraction patterns indicated that significant structural changes occurred in the basal plane of the SnSe constituent as m is increased. Scanning transmission electron microscopy cross-sectional images of the m = 1 compound indicate long-range coherence between layers, whereas the m ≥ 2 compounds show extensive rotational disorder between the constituent layers. For m ≥ 2, the images of the SnSe constituent contain a variety of stacking sequences of SnSe bilayers. Density functional theory calculations suggest that the formation energy is similar for several different SnSe stacking sequences. The compounds show unexpected transport properties as m is increased, including the first p-type behavior observed in (MSe)(TiSe) compounds. The resistivity of the m ≥ 2 compounds is larger than for m = 1, with m = 2 being the largest. At room temperature, the Hall coefficient is positive for m = 1 and negative for m = 2-4. The Hall coefficient of the m = 2 compound changes sign as temperature is decreased. The room-temperature Seebeck coefficient, however, switches from negative to positive at m = 3. These properties are incompatible with single band transport indicating that the compounds are not simple composites.

Investigating the Formation of MoSe2 and TiSe2 Films from Artificially Layered Precursors

“…Key goals include developing an approach that is scalable to wafer-scale synthesis, capable of controlling thickness to a precise number of unit cells, and able to control defect levels. Thin-film transition-metal dichalcogenides (TMDs) and other layered chalcogenides, due to their diverse and exotic physical properties that can be manipulated by varying the thickness, substrate, or adjacent layers in heterostructures, have attracted significant attention from the two-dimensional (2D) research community. − While thickness-dependent properties were initially discovered by cleaving bulk samples, subsequent synthesis efforts have focused on developing wafer-scale preparation techniques, such as chemical vapor deposition (CVD). − More recently, atomic layer deposition, − metal–organic-CVD, − and direct deposition methods (sputtering, pulsed laser deposition, e-beam) − have been used to make high-quality layered TMDs . These approaches use elevated temperatures or light to increase reaction rates, and the quality of the product depends on the temperature and the photon energy utilized (when applicable).…”

Synthesis of Tunable SnS-TaS2 Nanoscale Superlattices

“…This is smaller than but still on the order of grains reported for similar selenides made from thermally evaporated precursors. Grain size has played a role in electronic properties of these selenide superlattices, particularly as it relates to thermal resistance and in-plane carrier mobility. ,, Unlike some superlattices containing SnSe, we do not observe size-dependent structural distortions in the SnS layers. Full diffraction patterns can be seen in Figure S1a.…”

“…While misfit compounds display a number of exotic properties as a result of their construction, they are thermodynamic in nature and limited to architectures in which n = 1, 2 and m = 1, thus precluding investigation of property tunability via a full series of variably stacked materials. However, as self-assembled analogues to misfit compounds, chalcogenide thin film superlattices formed from designed precursors remove thermodynamic constraints, allowing the study of systematic changes in structure and properties as a function of precise changes to the stacking sequences (i.e., m , n = 1, 2, 3, 4, ...) and strategic control of dopants. − Reports have revealed the tunability of properties relevant to a number of technologically promising applications, including surveying architectural effects on thermoelectric performance, , charge density wave transitions, − and superconductivity onset …”

Superlattices based on van der Waals 2D materials

“…The dimensionality (m layers of material 1 and n layers of material 2 per unit cell, i.e. their ratio m/n) of these materials is taken into account to control the properties of both materials: the electrical transport [39][40][41] , the charge transfer between the layers 42,43 or the presence/strength of a charge density wave state 44 . Another strategy followed to modulate the electrical properties is by substitution or doping in specific sites of the layers [45][46][47] .…”