Structure, Stability, and Properties of the Intergrowth Compounds ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>, where <i>m</i> = <i>n</i> = 1–20

Intergrowth compounds of ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>, where 1 ≤ <i>m = n ≤ </i>20, with the same atomic composition but different <i>c</i>-axis lattice parameters and number of interfaces per volume were synthesized using the modulated elemental reactant technique. A <i>c</i>-axis lattice parameter change of 1.217(6) nm as a function of one unit of <i>m</i> = <i>n</i> was observed. In-plane X-ray diffraction shows an increase in distortion of the rock salt layer as a function of <i>m</i> and a broadening of the NbSe₂ reflections as <i>n</i> increases, indicating the presence of different coordination environments for Nb (trigonal prismatic and octahedral) and smaller crystallite size, which were confirmed via scanning transmission electron microscopy investigations. The electrical resistivities of all 12 compounds exhibit metallic temperature dependence and are similar in magnitude as would be expected for isocompositional compounds. Carrier concentration and mobility of the compounds vary within a narrow range of 2–6 × 10²¹ cm^–3 and 2–6 cm² V^–1 s^–1, respectively. Even at a thickness of 12 nm for the SnSe and NbSe₂ blocks, the properties of the intergrowth compounds cannot be explained as composite behavior, due to significant charge transfer between them. Upon being annealed at 500 °C, the higher order <i>m</i> = <i>n</i> compounds were found to convert to the thermodynamically stable phase, the (1,1) compound. This suggests that the capacitive energy of the interfaces stabilizes these intergrowth compounds

Effect of Local Structure of NbSe₂ on the Transport Properties of ([SnSe]_1.16)₁(NbSe₂)_<i>n</i> Ferecrystals

([SnSe]_1.16)₁(NbSe₂)_<i>n</i> ferecrystals were synthesized through the modulated elemental reactants technique by increasing the number of Nb|Se layers in the precursor from 1 to 4. The <i>c</i>-lattice parameter of the intergrowth was observed to change as a function of <i>n</i> by 0.635(2) nm. The <i>c</i>-lattice parameter of SnSe was observed to be 0.588(8) nm and independent of <i>n</i>. The electrical resistivity does not decrease as <i>n</i> increases as expected from simple models, but instead the trend in the resistivity is (1,3) > (1,4) ≥ (1,1) > (1,2). The carrier concentration increases with <i>n</i> as expected, so the unusual trend in resistivity is a result of the carrier mobility decreasing with increasing <i>n.</i> In-plane X-ray diffraction line widths and STEM images of the (1,4) compound show that it has small in-plane grain sizes and a large diversity of stacking sequences respectively, providing a potential explanation for the reduced carrier mobility

The Influence of Interfaces on Properties of Thin-Film Inorganic Structural Isomers Containing SnSe–NbSe₂ Subunits

Inorganic isomers ([SnSe]_1+δ)_<i>m</i>(NbSe₂)_<i>n</i>([SnSe]_1+δ)_<i>p</i>(NbSe₂)_<i>q</i>([SnSe]_1+δ)_<i>r</i>(NbSe₂)_<i>s</i> where <i>m</i>, <i>n</i>, <i>p</i>, <i>q</i>, <i>r</i>, and <i>s</i> are integers and <i>m</i> + <i>p</i> + <i>r</i> = <i>n</i> + <i>q</i> + <i>s</i> = 4 were prepared using the modulated elemental reactant technique. This series of all six possible isomers provides an opportunity to study the influence of interface density on properties while maintaining the same unit cell size and composition. As expected, all six compounds were observed to have the same atomic compositions and an almost constant <i>c</i>-axis lattice parameter of ≈4.90(5) nm, with a slight trend in the <i>c</i>-axis lattice parameter correlated with the different number of interfaces in the isomers: two, four and six. The structures of the constituents in the <i>ab</i>-plane were independent of one another, confirming the nonepitaxial relationship between them. The temperature dependent electrical resistivities revealed metallic behavior for all the six compounds. Surprisingly, the electrical resistivity at room temperature decreases with increasing number of interfaces. Hall measurements suggest this results from changes in carrier concentration, which increases with increasing thickness of the thickest SnSe block in the isomer. Carrier mobility scales with the thickness of the thickest NbSe₂ block due to increased interfacial scattering as the NbSe₂ blocks become thinner. The observed behavior suggests that the two constituents serve different purposes with respect to electrical transport. SnSe acts as a charge donor and NbSe₂ acts as the charge transport layer. This separation of function suggests that such heterostructures can be designed to optimize performance through choice of constituent, layer thickness, and layer sequence. A simplistic model, which predicts the properties of the complex isomers from a weighted sum of the properties of building blocks, was developed. A theoretical model is needed to predict the optimal compound for specific properties among the many potential compounds that can be prepared

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

Charge Density Wave Transition in (PbSe)_1+δ(VSe₂)_<i>n</i> Compounds with <i>n</i> = 1, 2, and 3

A series of (PbSe)_1+δ(VSe₂)_<i>n</i> heterostructures with extensive turbostratic disorder were synthesized with <i>n</i> = 1–3 through low temperature annealing of appropriately designed layered precursors. The crystal structures consist of alternating layers of CdI₂ type structured VSe₂ and distorted NaCl type structured PbSe. The <i>n</i> = 1 compound has a positive Hall coefficient and a charge density wave like transition at 100 K, during which the resistivity increases by a factor of 3.5 and the Hall coefficient increases by a factor of 8. The <i>n</i> = 2 and 3 compounds have negative Hall coefficients and significantly smaller changes in the slope of the resistivity and Hall coefficient as a function of temperature at similar temperatures. The distinctly different transport properties of the compound containing a monolayer of VSe₂ compared to compounds with thicker VSe₂ layers highlights the complexity of the electronic structure of these stacked systems. The differences cannot be simply explained by charge transfer between VSe₂ and PbSe within a rigid band model. More sophisticated interactions between the constituent layers, electron–phonon interactions, and/or correlation between electrons need to be considered to explain the change in carrier type and the charge density wave (CDW) transition

Tuning Electrical Properties through Control of TiSe₂ Thickness in (BiSe)_1+δ(TiSe₂)_<i>n</i> Compounds

A series of (BiSe)_1+δ(TiSe₂)_<i>n</i> compounds where <i>n</i> was varied from two to four were synthesized and electrically characterized to explore the extent of charge transfer from the BiSe layer to the TiSe₂ layers. These kinetically stable heterostructures were prepared using the modulated elemental reactants (MER) method, in which thin amorphous elemental layers are deposited in an order that mimics the nanostructure of the desired product. X-ray diffraction (XRD), X-ray area diffraction, and scanning transmission electron microscopy (STEM) data show that the precursors formed the desired products. Specular diffraction scans contain only 00<i>l</i> reflections, indicating that the compounds are crystallographically aligned with the <i>c</i>-axis perpendicular to the substrate. The <i>c</i>-axis lattice parameter increases by 0.604(3) nm with each additional TiSe₂ layer. In-plane diffraction scans contain reflections that can be indexed as the (<i>hk</i>0) of the BiSe and TiSe₂ constituents. Area diffraction scans are also consistent with the samples containing only BiSe and TiSe₂ constituents. Rietveld refinement of the 00<i>l</i> XRD data was used to determine the positions of atomic planes along the <i>c</i>-axis. STEM data supports the structures suggested by the diffraction data and associated refinements but also shows that antiphase boundaries occur approximately 1/3 of the time in the BiSe layers. All samples showed metallic behavior for the temperature-dependent electrical resistivity between 20 K and room temperature. Electrical measurements indicated that charge is transferred from the BiSe layer to the TiSe₂ layer. The measured Hall coefficients were all negative indicating that electrons are the majority carrier and are systematically decreased as <i>n</i> was increased. Assuming a single parabolic band model, carrier concentration decreased when the number of TiSe₂ layers is increased, suggesting that the amount of charge donated by the BiSe layer to the TiSe₂ layers is constant. Seebeck coefficients were negative for all of the (BiSe)_1+δ(TiSe₂)_<i>n</i> compounds studied, indicating that electrons are the majority carrier, and decreased as <i>n</i> increased. The effective mass of the carriers was calculated to be 5–6 m_e for the series of compounds

Expanding the Concept of van der Waals Heterostructures to Interwoven 3D Structures

Several members of a new family of heterostructures [(LaSe)_1.17]₁V_{<i>n</i>(1+<i>y</i>)+1}Se_2<i>n</i>+2 with <i>n</i> = 1, 2, and 3 were prepared using a diffusion constrained, kinetically controlled synthesis approach. Specular diffraction patterns collected as a function of annealing temperature show the evolution of designed precursors into highly ordered heterostructures. Scanning transmission electron microscopy (STEM) images reveal that the structure of <i>n</i> = 3 consists of rock salt structured LaSe bilayers alternating with vanadium selenide layers of varying thickness, which are structurally closely related to V₃Se₄. Interplanar distances obtained from the STEM images were successfully used as the starting point for Rietveld refinements of the specular diffraction patterns of these crystallographically aligned compounds. Utilizing this unorthodox combined approach to extract detailed structural information unambiguously, we demonstrated that these thin film compounds are the first examples of chalcogenide-based heterostructures, where the bulk structures of both building blocks lack a van der Waals gap, yet a nonepitaxial incommensurate interface forms. Moreover, the refinement results of the <i>n</i> = 2 and 3 heterostructures suggest that the structure of the V–Se layers can be varied ranging from VSe₂ to VSe depending on the film composition. The electrical resistivity of the [(LaSe)_1.17]₁V_{<i>n</i>(1+<i>y</i>)+1}Se_2<i>n</i>+2 heterostructures changes systematically from semiconducting toward metallic behavior with increasing <i>n</i>, showcasing the ability to tune physical properties by precisely controlling the layer sequence in these heterostructures

BaTiO₃ Thin Films from Atomic Layer Deposition: A Superlattice Approach

A superlattice approach for the atomic layer deposition of polycrystalline BaTiO₃ thin films is presented as an example for an effective route to produce high-quality complex oxide films with excellent thickness and compositional control. This method effectively mitigates any undesirable reactions between the different precursors and allows an individual optimization of the reaction conditions for the Ba–O and the Ti–O subcycles. By growth of nanometer thick alternating Ba­(OH)₂ and TiO₂ layers, the advantages of binary oxide atomic layer deposition are transferred into the synthesis of ternary compounds, permitting extremely high control of the cation ratio and superior uniformity. Whereas the Ba­(OH)₂ layers are partially crystalline after the deposition, the TiO₂ layers remain mostly amorphous. The layers react to polycrystalline, polymorph BaTiO₃ above 500 °C, releasing H₂O. This solid-state reaction is accompanied by an abrupt decrease in film thickness. Transmission electron microscopy and Raman spectroscopy reveal the presence of hexagonal BaTiO₃ in addition to the perovskite phase in the annealed films. The microstructure with relatively small grains of ∼70 Å and different phases is a direct consequence of the abrupt formation reaction. The electrical properties transition from the initially highly insulating dielectric semiamorphous superlattice into a polycrystalline BaTiO₃ thin film with a dielectric constant of 117 and a dielectric loss of 0.001 at 1 MHz after annealing at 600 °C in air, which, together with the suppression of ferroelectricity at room temperature, are very appealing properties for voltage tunable devices