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Spurred by recent discoveries of high-temperature superconductivity in Fe–Se-based materials, the magnetic, electronic, and catalytic properties of iron chalcogenides have drawn significant attenti...

We explore the effect of charge density wave (CDW) on the in-plane thermoelectric transport properties of (PbSe)1+δ(VSe2)1 and (PbSe)1+δ(VSe2)2 heterostructures. In (PbSe)1+δ(VSe2)1 we observe an abrupt 86% increase in the Seebeck coefficient, 245% increase in the power factor, and a slight decrease in resistivity over the CDW transition. This behavior is not observed in (PbSe)1+δ(VSe2)2 and is rather unusual compared to the general trend observed in other materials. The abrupt transition causes a deviation from the Mott relationship through correlated electron states. Raman spectra of the (PbSe)1+δ(VSe2)1 material show the emergence of additional peaks below the CDW transition temperature associated with VSe2 material. Temperature-dependent in-plane X-ray diffraction (XRD) spectra show a change in the in-plane thermal expansion of VSe2 in (PbSe)1+δ(VSe2)1 due to lattice distortion. The increase in the power factor and decrease in the resistivity due to CDW suggest a potential mechanism for enhancing the thermoelectric performance at the low temperature region.

The reaction of ultrathin layers of Mo and Ti with Se was investigated, and significantly different reaction pathways were found. However, in both systems postdeposition annealing results in smooth...

This work presents the preparation of a series of [(PbSe)1+δ]4[TiSe2]4 isomers via a low temperature synthesis approach that exploits precursor nanoarchitecture to direct formation of specific isomers. The targeted isomers formed even when the precursors did not have the correct amount of each element to make a unit cell from each repeating sequence of elemental layers deposited. This suggests that the exact composition of the precursors is less important than the nanoarchitecture in directing the formation of the compounds. The as-deposited diffraction data show that the isomers begin to form during the deposition, and Ti2Se, in addition to PbSe and TiSe2, are present in the specular diffraction patterns. HAADF-STEM images reveal impurity layers above and below an integer number of targeted isomer unit cells. The structural data suggest that Ti2Se forms as Se is deposited on the initial Ti layers and remains throughout isomer self-assembly. During growth, the isomers deplete the local supply of Ti and Pb, creating diffusion gradients that drive additional cations toward the growth front, which leaves surface impurity layers of TiSe2 and TiO2 after the supply of Pb is exhausted. The deposited stacking sequences direct formation of the targeted isomers, but fewer repeating units form than intended due to the lack of material per layer in the precursor and formation of impurity layers. All isomers have negative Hall and Seebeck coefficients, indicating that electrons are the majority carrier. The carrier concentration and conductivity of the isomers increase with the number of interfaces in the unit cell, resulting from charge donation between adjacent layers. The opposite variation of the carrier concentration and mobility with temperature result in minima in the resistivity between 50 and 100 K. The very weak temperature dependence of the carrier concentration likely results from changes in the amount of charge transfer between the layers with temperature.

A series of [(SnSe)1+δ]m[TiSe2]2 heterostructure thin films built up from repeating units of m bilayers of SnSe and two layers of TiSe2 were synthesized from designed precursors. The electronic structure of the films was investigated using X-ray photoelectron spectroscopy for samples with m = 1, 2, 3, and 7 and compared to binary samples of TiSe2 and SnSe. The observed binding energies of core levels and valence bands of the heterostructures are largely independent of m. For the SnSe layers, we can observe a rigid band shift in the heterostructures compared to the binary, which can be explained by electron transfer from SnSe to TiSe2. The electronic structure of the TiSe2 layers shows a more complicated behavior, as a small shift can be observed in the valence band and Se3d spectra, but the Ti2p core level remains at a constant energy. Complementary UV photoemission spectroscopy measurements confirm a charge transfer mechanism where the SnSe layers donate electrons into empty Ti3d states at the Fermi energy.

We explore the effect of charge density wave (CDW) on the in-plane thermoelectric transport properties of (PbSe)

The reaction of ultrathin layers of Mo and Ti with Se was investigated, and significantly different reaction pathways were found. However, in both systems postdeposition annealing results in smooth dichalcogenide films with specific thicknesses determined by the precursor. X-ray diffraction (XRD) patterns of as-deposited Mo|Se films around a 1:2 ratio of Mo to Se contain weak, broad reflections from small and isolated MoSe

While a linear growth behavior is one of the fingerprints of textbook atomic layer deposition processes, the growth often deviates from that behavior in the initial regime, i.e. the first few cycles of a process. To properly understand the growth behavior in the initial regime is particularly important for applications that rely on the exact thickness of very thin films. The determination of the thicknesses of the initial regime, however, often requires special equipment and techniques that are not always available. We propose a thickness determination method that is based on X-ray reflectivity (XRR) measurements on double layer structures, i.e. substrate/base layer/top layer. XRR is a standard thin film characterization method. Utilizing the inherent properties of fast Fourier transformation in combination with a multi-Gaussian fitting routine permits the determination of thicknesses down to $t \approx 2$ nm. We evaluate the boundaries of our model, which are given by the separation and full width at half maximum of the individual Gaussians. Finally, we compare our results from two layer stacks with data from X-ray fluorescence spectroscopy, which is a standard method for measuring ultra thin films.

This work presents the preparation of a series of [(PbSe)

We report cross-plane thermoelectric measurements of SnSe and SnSe2 films grown by the modulated element reactant (MER) approach. These materials exhibit ultralow cross-plane thermal conductivities, which are advantageous for thermoelectric energy conversion. The initially grown SnSe films have relatively low cross-plane Seebeck coefficients (−38.6 μV/K) due to significant unintentional doping originating from Se vacancies when annealed in nitrogen, as a result of the relatively high vapor pressure of Se. By performing postgrowth annealing at a fixed Se partial pressure (300 °C for 30 min using SnSe2 as the Se source in a sealed tube), a transition from SnSe to SnSe2 is induced, which is evidenced by clear changes in the X-ray diffraction patterns of the films. This results in a 16-fold increase in the cross-plane Seebeck coefficient (from −38.6 to −631 μV/K) after Se annealing due to both the SnSe-to-SnSe2 transition and the mitigation of unintentional doping by Se vacancies. We also observe a correspond...

The composition and thickness of thin films determine their physical properties, making the ability to measure the number of atoms of different elements in films both technologically and scientifically important. For thin films, below a certain thickness, the X-ray fluorescence intensity of an element is proportional to the number of atoms. Converting this intensity to the number of atoms per unit area is challenging due to experimental geometries and other correction factors. Hence, the ratio of intensities is more commonly used to determine the composition in terms of element ratios using standards or a model. Here, the number of atoms per unit area was determined using X-ray structure information for over 20 different crystallographically aligned samples with integral unit cell thicknesses. The proportionality constant between intensity and the number of atoms per unit area was determined from linear fits of the background subtracted X-ray fluorescence intensity plotted versus the calculated number of ...

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.

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 un...

Miniaturization of microelectronic devices has reached a fundamental scaling limit; parasitic electron tunneling through the ultrathin gate dielectric has become a major obstacle to continued device performance. One method for overcoming this limitation is to replace SiO2 gate dielectrics with thicker high-κ metal oxides. La2O3 and ZrO2 are two such materials that have received significant interest, but low stability to post-anneal water absorption and low-crystallization temperatures, respectively, have limited their widespread use. We recently reported an aqueous, all-inorganic route to high-κ lanthanum zirconium oxide dielectric films (1/1 La/Zr), which mitigates the disadvantages of the binary oxides but maintains their high-κ properties. In this contribution, we vary the La/Zr ratio of the aqueous precursor to optimize the properties of the resulting films. We find that the La0.20Zr0.80Oy composition is optimal for providing a high dielectric constant (∼18.2 at 600 °C) while maintaining excellent film morphology and stability. 20% La was necessary to prevent crystallization up to 600 °C, but films with higher La content displayed diminished dielectric constants and decreased stability towards post-anneal water absorption.

Several members of a new family of heterostructures [(LaSe)1.17]1Vn(1+y)+1Se2n+2 with n = 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 n = 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, ...

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 SnSe and the TiSe2 directions as √3 × aTiSe2 equals aSnSe. Fast Fourier transform analysis of the STEM images exhibits sharp maxima in hkl families where h,...

We report cross-plane thermoelectric measurements of misfit layered compounds (SnSe)n(TiSe2)n (n = 1,3,4,5), approximately 50 nm thick. Metal resistance thermometers are fabricated on the top and bottom of the (SnSe)n(TiSe2)n material to measure the temperature difference and heat transport through the material directly. By varying the number of layers in a supercell, n, we vary the interface density while maintaining a constant global stoichiometry. The Seebeck coefficient measured across the (SnSe)n(TiSe2)n samples was found to depend strongly on the number of layers in the supercell (n). When n decreases from 5 to 1, the cross-plane Seebeck coefficient decreases from −31 to −2.5 μV/K, while the cross-plane effective thermal conductivity decreases by a factor of 2, due to increased interfacial phonon scattering. The cross-plane Seebeck coefficients of the (SnSe)n(TiSe2)n are very different from the in-plane Seebeck coefficients, which are higher in magnitude and less sensitive to the number of layers in...

Controlling carrier concentration is critical in many device applications, and both chemical substitution and modulation doping have been used in industry. For most inorganic materials, very low doping efficiencies are observed as site occupancies depend on both thermodynamic and kinetic factors. We demonstrate that we can make kinetically controlled site-specific substitutions in a series of (BixSn1–xSe)1+δTiSe2 compounds using the modulated elemental reactants method. These compounds were characterized using a combination of X-ray diffraction, resistivity and Hall coefficient measurements, and high angle annular dark field scanning transmission electron microscopy (HAADF STEM). For small x, the doping efficiency is 0.7, close to that observed for B in silicon. For higher x values, a structural distortion is observed in X-ray diffraction data in which the symmetry of the in-plane unit cell decreases. HAADF STEM data reveals the presence of antiphase boundaries (Bi–Bi pairs) in the BixSn1–xSe layers, whic...

We report cross-plane thermoelectric measurements of SnSe and SnSe

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)

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)

Synthesis and characterization of Mg x Zr2−x N2 (0.5 ≤ x ≤ 1.8) thin films deposited by reactive magnetron co-sputtering in nitrogen plasma is reported. Composition measurements show that nitrides with low oxygen content (less than 1%) can be formed up to x = 1.0, at which point an increase in oxygen content is observed. Up to composition of x = 1.6 the Mg x Zr2−x N2 thin films form in a rocksalt-derived crystal structure, as revealed by X-ray diffraction measurements. At x > 1.6 the films rapidly oxidize. The lattice constant of the stoichiometric MgZrN2 composition is a = 4.537 A, and only small changes in lattice parameter are observed with changing composition. Electrical conductivity decreases by several orders of magnitude with increasing Mg-content. The conductivity of Mg-rich (x ≥ 1) films increases with increasing measurement temperature, indicating semiconducting character of Mg-rich Mg x Zr2−x N2. Optical absorption measurements of these Mg-rich samples show a clear absorption onset at 1.8 eV, also indicative of semiconducting behavior.

Self-assembly of designed precursors has enabled the synthesis of novel heterostructures that exhibit extensive rotational disorder between constituents. In (SnSe)