Insights into the Charge-Transfer Stabilization of Heterostructure Components with Unstable Bulk Analogs

Solid state chemists have yet to find a targeted approach based on simple rules to predict new materials with desired physical properties. Recent advances in computational high-throughput methods have led to the creation of large databases with predicted new compounds. While many of these compounds are unstable, some may be stabilized inside heterostructures. BiSe is an example for such a compound where the rock-salt structure is unstable in bulk but can be found in misfit layer compounds and ferecrystals. In some of these heterostructures, BiSe also exhibits antiphase boundaries (APBs), periodic Bi–Bi pairings that interrupt the alternating pattern of the rock-salt structure. Understanding the behavior of BiSe may aid in the discovery of new heterostructure components where no stable bulk analog exists. We used density functional theory (DFT) and crystal orbital Hamilton populations (COHPs) to explain the different stabilities of rock-salt structured BiSe. COHPs show that rock-salt structured BiSe has occupied antibonding states at the Fermi level, which destabilize the structure. In heterostructures, these states can be depopulated by donating electrons into an adjacent layer or by forming APBs to localize electrons into a Bi–Bi bond. The results suggest that the depopulation of antibonding states is crucial to stabilizing rock-salt structured BiSe, and that BiSe needs to be paired with a suitable electron acceptor. We predict that this is a general principle that can be applied to other compounds with unstable polytypes and suggest that COHPs should play a larger role in the discovery of new heterostructure components

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

Structural Changes as a Function of Thickness in [(SnSe)_1+δ]_<i>m</i>TiSe₂ Heterostructures

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)_1+δ]_<i>m</i>TiSe₂ intergrowth structures with increasing SnSe layer thickness (<i>m</i> = 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 <i>m</i> is increased. Scanning transmission electron microscopy cross-sectional images of the <i>m</i> = 1 compound indicate long-range coherence between layers, whereas the <i>m</i> ≥ 2 compounds show extensive rotational disorder between the constituent layers. For <i>m</i> ≥ 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 <i>m</i> is increased, including the first p-type behavior observed in (MSe)_<i>m</i>(TiSe₂)_<i>n</i> compounds. The resistivity of the <i>m</i> ≥ 2 compounds is larger than for <i>m</i> = 1, with <i>m</i> = 2 being the largest. At room temperature, the Hall coefficient is positive for <i>m</i> = 1 and negative for <i>m</i> = 2–4. The Hall coefficient of the <i>m</i> = 2 compound changes sign as temperature is decreased. The room-temperature Seebeck coefficient, however, switches from negative to positive at <i>m</i> = 3. These properties are incompatible with single band transport indicating that the compounds are not simple composites

Structural Changes in 2D BiSe Bilayers as <i>n</i> Increases in (BiSe)_1+δ(NbSe₂)_<i>n</i> (<i>n</i> = 1–4) Heterostructures

(BiSe)_1+δ(NbSe₂)_<i>n</i> heterostructures with <i>n</i> = 1–4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with <i>n</i> = 1 to a square basal plane for <i>n</i> = 2–4. The BiSe in-plane structure was also influenced by small changes in the structure of the precursor, without significantly changing the out-of-plane diffraction pattern or value of the misfit parameter, δ. Density functional theory calculations on isolated BiSe bilayers showed that its lattice is very flexible, which may explain its readiness to adjust shape and size depending on the environment. Correlated with the changes in the BiSe basal plane structure, analysis of scanning transmission electron microscope images revealed that the occurrence of antiphase boundaries, found throughout the <i>n</i> = 1 compound, is dramatically reduced for the <i>n</i> = 2–4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d_3/2, 5d_5/2 doublet peaks narrowed toward higher binding energies as <i>n</i> increased from 1 to 2, also consistent with a reduction in the number of antiphase boundaries. Temperature-dependent electrical resistivity and Hall coefficient measurements of nominally stoichiometric samples in conjunction with structural refinements and XPS data suggest a constant amount of interlayer charge transfer independent of <i>n</i>. Constant interlayer charge transfer is surprising given the changes in the BiSe in-plane structure. The structural flexibility of the BiSe layer may be useful in designing multiple constituent heterostructures as an interlayer between structurally dissimilar constituents