Kinetics of the Topochemical Transformation of (PbSe) m(TiSe2) n(SnSe2) m(TiSe2) n to (Pb0.5Sn0.5Se) m(TiSe2) n.

Solid-state reaction kinetics on atomic length scales have not been heavily investigated due to the long times, high reaction temperatures, and small reaction volumes at interfaces in solid-state reactions. All of these conditions present significant analytical challenges in following reaction pathways. Herein we use in situ and ex situ X-ray diffraction, in situ X-ray reflectivity, high-angle annular dark field scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy to investigate the mechanistic pathways for the formation of a layered (Pb0.5Sn0.5Se)1+δ(TiSe2) m heterostructure, where m is the varying number of TiSe2 layers in the repeating structure. Thin film precursors were vapor deposited as elemental-modulated layers into an artificial superlattice with Pb and Sn in independent layers, creating a repeating unit with twice the size of the final structure. At low temperatures, the precursor undergoes only a crystallization event to form an intermediate (SnSe2)1+γ(TiSe2) m(PbSe)1+δ(TiSe2) m superstructure. At higher temperatures, this superstructure transforms into a (Pb0.5Sn0.5Se)1+δ(TiSe2) m alloyed structure. The rate of decay of superlattice reflections of the (SnSe2)1+γ(TiSe2) m(PbSe)1+δ(TiSe2) m superstructure was used as the indicator of the progress of the reaction. We show that increasing the number of TiSe2 layers does not decrease the rate at which the SnSe2 and PbSe layers alloy, suggesting that at these temperatures it is reduction of the SnSe2 to SnSe and Se that is rate limiting in the formation of the alloy and not the associated diffusion of Sn and Pb through the TiSe2 layers.

Interfacial Defect Vibrations Enhance Thermal Transport in Amorphous Multilayers with Ultrahigh Thermal Boundary Conductance.

The role of interfacial nonidealities and disorder on thermal transport across interfaces is traditionally assumed to add resistance to heat transfer, decreasing the thermal boundary conductance (TBC). However, recent computational studies have suggested that interfacial defects can enhance this thermal boundary conductance through the emergence of unique vibrational modes intrinsic to the material interface and defect atoms, a finding that contradicts traditional theory and conventional understanding. By manipulating the local heat flux of atomic vibrations that comprise these interfacial modes, in principle, the TBC can be increased. In this work, experimental evidence is provided that interfacial defects can enhance the TBC across interfaces through the emergence of unique high-frequency vibrational modes that arise from atomic mass defects at the interface with relatively small masses. Ultrahigh TBC is demonstrated at amorphous SiOC:H/SiC:H interfaces, approaching 1 GW m-2 K-1 and are further increased through the introduction of nitrogen defects. The fact that disordered interfaces can exhibit such high conductances, which can be further increased with additional defects, offers a unique direction to manipulate heat transfer across materials with high densities of interfaces by controlling and enhancing interfacial thermal transport.

Kinetically Controlled Formation and Decomposition of Metastable [(BiSe)1+δ] m[TiSe2] m Compounds.

Preparing homologous series of compounds allows chemists to rapidly discover new compounds with predictable structure and properties. Synthesizing compounds within such a series involves navigating a free energy landscape defined by the interactions within and between constituent atoms. Historically, synthesis approaches are typically limited to forming only the most thermodynamically stable compound under the reaction conditions. Presented here is the synthesis, via self-assembly of designed precursors, of isocompositional incommensurate layered compounds [(BiSe)1+δ] m[TiSe2] m with m = 1, 2, and 3. The structure of the BiSe bilayer in the m = 1 compound is not that of the binary compound, and this is the first example of compounds where a BiSe layer thicker than a bilayer in heterostructures has been prepared. Specular and in-plane X-ray diffraction combined with high-resolution electron microscopy data was used to follow the formation of the compounds during low-temperature annealing and the subsequent decomposition of the m = 2 and 3 compounds into [(BiSe)1+δ]1[TiSe2]1 at elevated temperatures. These results show that the structure of the precursor can be used to control reaction kinetics, enabling the synthesis of kinetically stable compounds that are not accessible via traditional techniques. The data collected as a function of temperature and time enabled us to schematically construct the topology of the free energy landscape about the local free energy minima for each of the products.

Structural Changes as a Function of Thickness in [(SnSe)1+δ]mTiSe2 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]mTiSe2 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)m(TiSe2)n 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.

Interface-Driven Structural Distortions and Composition Segregation in Two-Dimensional Heterostructures.

The discovery of emergent phenomena in 2D materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drive both structure and properties. While no segregation for single bilayers was observed, segregation of Pb to the surface of three bilayer thick PbSe-SnSe alloy layers was discovered within [(Pbx Sn1-x Se)1+δ ]n (TiSe2 )1 heterostructures using electron microscopy. This segregation is thermodynamically favored to occur when Pbx Sn1-x Se layers are interdigitated with TiSe2 monolayers. DFT calculations indicate that the observed segregation depends on what is adjacent to the Pbx Sn1-x Se layers. The interplay between interface- and volume-free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.

Long-Range Order in [(SnSe)1.2]1[TiSe2]1 Prepared from Designed Precursors.

Self-assembly of designed precursors has enabled the synthesis of novel heterostructures that exhibit extensive rotational disorder between constituents. In (SnSe)1.2TiSe2 nanoscale regions of long-range order were observed in scanning transmission electron microscopy (STEM) cross sectional images. Here a combination of techniques are used to determine the structure of this compound, and the information is used to infer the origin of the order. In-plane X-ray diffraction indicates that the SnSe basal plane distorts to match TiSe2. This results in a rectangular unit cell that deviates from both the bulk structure and the square in-plane unit cell previously observed in heterostructures containing SnSe bilayers separated by layers of dichalcogenides. The distortion results from lattice matching of the two constituents, which occurs along the <100> SnSe and the <110> TiSe2 directions as √3 × aTiSe2 equals aSnSe. Fast Fourier transform analysis of the STEM images exhibits sharp maxima in hkl families where h,k ≠ 0. The period is the same as that observed for 00l reflections, indicating regions of long-range superlattice order in all directions. X-ray reciprocal space maps contain broad maxima in hkl families of TiSe2 and SnSe based reflections consistent with the superlattice period, indicating that long-range order is present throughout a significant fraction of the film. The STEM images show that <110> planes of TiSe2 are adjacent to <100> planes of SnSe. Density functional theory suggests the preferred orientation is due to favored directions of nucleation with significant energy differences between islands of SnSe with different orientation relative to TiSe2. The calculations suggest that the long-range order in (SnSe)1.2TiSe2 results from an accidental coincidence in the lattice parameters of SnSe and TiSe2. These findings support a layer by layer nucleation process for the self-assembly of heterostructures from designed precursors, which rationalizes how designed precursors enable compounds with different constituents, defined thicknesses, and specific layer sequences to be prepared.

The synthesis of [(PbSe)1+δ]m(TiSe2)n[(SnSe2)1+γ]m(TiSe2)n heterostructures with designed nanoarchitectures by self assembly of amorphous precursors.

Targeted heterostructures containing intergrown two dimensional (2D) layers of 3 different constituent layers, SnSe2, PbSe and TiSe2, were prepared by controlling the composition and sequence of elemental bilayers within a designed precursor. Varying the structure of the precursor enabled the number of structural units of each constituent and the sequence of crystalline 2D layers to be precisely controlled. The stacking of the 2D layers, their structures, and the segregation of the elements between them were determined using X-ray diffraction and electron microscopy techniques, with the observed sequence of the 2D layers consistent with the targeted intergrowth. This ability to prepare targeted heterostructures is critical, since the number of possible configurations in the final compound increases rapidly as the number of constituents increases, from almost 60 000 with two constituents to over 130 million with three constituents and to over 35 billion with four constituents for 20 or fewer distinct layers in the unit cell. This general route for synthesizing specific multiple component heterostructures will accelerate the feedback loop in this growing research area, permitting theorists to assume specific structures in the search for enhanced properties and providing experimentalists with crystallographically aligned samples to test these predictions.

Non-uniform Composition Profiles in Inorganic Thin Films from Aqueous Solutions.

A variety of metal oxide films (InGaOx, AlOx, "HafSOx") prepared from aqueous solutions were found to have non-uniform electron density profiles using X-ray reflectivity. The inhomogeneity in HafSOx films (Hf(OH)4-2x-2y(O2)x(SO4)y·zH2O), which are currently under investigation as inorganic resists, were studied in more detail by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and medium-energy ion scattering (MEIS). The HAADF-STEM images show a greater concentration of heavy atoms near the surface of a single-layer film. MEIS data confirm the aggregation of Hf at the film surface. The denser "crust" layer in HafSOx films may directly impact patterning resolution. More generally, the phenomenon of surface-layer inhomogeneity in solution-deposited films likely influences film properties and may have consequences in other thin-film systems under investigation as resists, dielectrics, and thin-film transistor components.

Misfit Layer Compounds and Ferecrystals: Model Systems for Thermoelectric Nanocomposites.

A basic summary of thermoelectric principles is presented in a historical context, following the evolution of the field from initial discovery to modern day high-zT materials. A specific focus is placed on nanocomposite materials as a means to solve the challenges presented by the contradictory material requirements necessary for efficient thermal energy harvest. Misfit layer compounds are highlighted as an example of a highly ordered anisotropic nanocomposite system. Their layered structure provides the opportunity to use multiple constituents for improved thermoelectric performance, through both enhanced phonon scattering at interfaces and through electronic interactions between the constituents. Recently, a class of metastable, turbostratically-disordered misfit layer compounds has been synthesized using a kinetically controlled approach with low reaction temperatures. The kinetically stabilized structures can be prepared with a variety of constituent ratios and layering schemes, providing an avenue to systematically understand structure-function relationships not possible in the thermodynamic compounds. We summarize the work that has been done to date on these materials. The observed turbostratic disorder has been shown to result in extremely low cross plane thermal conductivity and in plane thermal conductivities that are also very small, suggesting the structural motif could be attractive as thermoelectric materials if the power factor could be improved. The first 10 compounds in the [(PbSe)1+δ]m(TiSe2)n family (m, n ≤ 3) are reported as a case study. As n increases, the magnitude of the Seebeck coefficient is significantly increased without a simultaneous decrease in the in-plane electrical conductivity, resulting in an improved thermoelectric power factor.