PbSe) 1.14 ] m (NbSe 2 ) n compounds with 1 ≤ m ≤ 6 and n = 1 were synthesized using the modulated elemental reactants (MER) method. X-ray diffraction patterns (XRD) showed that the desired compounds self-assembled during annealing of the precursors with their c-axis crystallographically aligned normal to the substrate. The c-axis lattice parameter increased by 0.612 nm as m, the number of PbSe bilayers, increased by one. Analysis of the in-plane diffraction patterns indicated that the a-lattice parameters remained constant as m was varied. Reciprocal space maps along hkl (h, k ≠ 0; l ≠0) indicate very short coherence lengths in mixed-index directions, consistent with the rotational disorder between layers observed in electron microscopy cross sections. The in-plane electrical resistivity and Hall coefficients were measured for each ferecrystal from 22 to 295 K. The resistivity systematically increased as m increased, but the magnitude of the increase is greater than predicted assuming independent layers. Assuming the metallic conduction results from a single band in the NbSe 2 layer, the carrier concentrations determined from the Hall coefficients decreases as m increases, suggesting increased charge transfer from PbSe to NbSe 2 with increasing values of m. First-principles electronic-structure calculations based on the generalized gradient approximation to density functional theory suggest that the PbSe valence band overlaps the empty bands in NbSe 2 , supporting the idea of interlayer charge transfer from PbSe to NbSe 2 .

Thin films with tunable and homogeneous composition are required for many applications. We report the synthesis and characterization of a new class of compositionally homogeneous thin films that are amorphous solid solutions of AlO and transition metal oxides (TMO) including VO, CrO, MnO, FeO, CoO, NiO, CuO, and ZnO. The synthesis is enabled by the rapid decomposition of molecular transition-metal nitrates TM(NO) at low temperature along with precondensed oligomeric Al(OH)(NO) cluster species, both of which can be processed from aq solution. The films are dense, ultrasmooth (R < 1 nm, near 0.1 nm in many cases), and atomically mixed amorphous metal-oxide alloys over a large composition range. We assess the chemical principles that favor the formation of amorphous homogeneous films over rougher phase-segregated nanocrystalline films. The synthesis is easily extended to other compositions of transition and main-group metal oxides. To demonstrate versatility, we synthesized amorphous VCrMnFeZnAlO and VCrFeAlO with R ≈ 0.1 nm and uniform composition. The combination of ideal physical properties (dense, smooth, uniform) and broad composition tunability provides a platform for film synthesis that can be used to study fundamental phenomena when the effects of transition metal cation identity, solid-state concentration of d-electrons or d-states, and/or crystallinity need to be controlled. The new platform has broad potential use in controlling interfacial phenomena such as electron transfer in solar-cell contacts or surface reactivity in heterogeneous catalysis.

(BiSe)(1+δ)NbSe2 ferecrystals were synthesized in order to determine whether structural modulation in BiSe layers, characterized by periodic antiphase boundaries and Bi-Bi bonding, occurs. Specular X-ray diffraction revealed the formation of the desired compound with a c-axis lattice parameter of 1.21 nm from precursors with a range of initial compositions and initial periodicities. In-plane X-ray diffraction scans could be indexed as hk0 reflections of the constituents, with a rectangular basal BiSe lattice and a trigonal basal NbSe2 lattice. Electron micrographs showed extensive turbostratic disorder in the samples and the presence of periodic antiphase boundaries (approximately 1.5 nm periodicity) in BiSe layers oriented with the [110] direction parallel to the zone axis of the microscope. This indicates that the structural modulation in the BiSe layers is not due to coherency strain resulting from commensurate in-plane lattices. Electrical transport measurements indicate that holes are the dominant charge carrying species, that there is a weak decrease in resistivity as temperature decreases, and that minimal charge transfer occurs from the BiSe to NbSe2 layers. This is consistent with the lack of charge transfer from the BiX to the TX2 layers reported in misfit layer compounds where antiphase boundaries were observed. This suggests that electronic considerations, i.e., localization of electrons in the Bi-Bi pairs at the antiphase boundaries, play a dominant role in stabilizing the structural modulation.

The metastable heterostructure, (BiSe) 0.97 MoSe 2 , containing alternating bilayers of BiSe and MoSe 2 trilayers was synthesized using the modulated elemental reactant method to determine if charge transfer from BiSe to MoSe 2 would stabilize the metallic 1T polymorph of MoSe 2 . Optimum synthesis conditions were determined by following the structural evolution as a function of temperature. The structure of the product contained distorted rock salt-structured BiSe layers alternating with hexagonal MoSe 2 layers. High-angle annular dark field scanning transmission electron microscopy images revealed that two different polymorphs of MoSe 2 coexisted in (BiSe) 0.97 MoSe 2 . Raman spectroscopy confirmed the presence of 1T MoSe 2 layers. X-ray photoelectron spectroscopy (XPS) indicated that there were two different electronic states for both Mo and Bi. The Mo states are consistent with having octahedral and trigonal prismatic coordination of molybdenum as found in the 1T and 2H polymorphs of MoSe 2 . The two different electronic states for Bi are consistent with the presence of antiphase boundaries in the BiSe layers. Estimating the relative amount of each electronic state from the XPS spectra indicates that the percentage of 1T MoSe 2 is about 40%, whereas the amount of Bi 3+ in the BiSe is approximately 60%. The measured resistivity increases as temperature is decreased, consistent with an activated conduction mechanism with a small activation energy (∼0.05 eV). The temperature stability and low resistivity of (BiSe) 0.97 MoSe 2 make it potentially interesting as a means of improving electrical contacts to MoSe 2 .

(BiSe)(NbSe) heterostructures with n = 1-4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with n = 1 to a square basal plane for n = 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 n = 1 compound, is dramatically reduced for the n = 2-4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d, 5d doublet peaks narrowed toward higher binding energies as n 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 n. 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.

Films containing 8, 16, 24, 32 and 64 MoSe2 layers were synthesized using the modulated elemental reactants method. X-ray reflectivity patterns showed that the annealed films were the targeted number of MoSe2 layers thick with atomically smooth interfaces. In-plane x-ray diffraction (XRD) scans contained only hk0 reflections for crystalline MoSe2 monolayers. Specular XRD patterns contained only 00l reflections, also indicating that the hk0 plane of the MoSe2 layers are parallel to the substrate. Both XRD and electron microscopy techniques indicated that the hk0 planes are rotationally disordered with respect to one another, with all orientations equally probable for large areas. The rotational disorder between MoSe2 layers is present even when analyzed spots are within 10 nm of one another. Cross-plane thermal conductivities of 0.07–0.09 W m−1 K−1 were measured by time domain thermoreflectance, with the thinnest films exhibiting the lowest conductivity. The structural analysis suggests that the ultralow thermal conductivity is a consequence of rotational disorder, which increases the separation between MoSe2 layers. The bonding environment of the Se atoms also becomes significantly distorted from C3v symmetry due to the rotational disorder between layers. This structural disorder efficiently reduces the group velocity of the transverse phonon modes but not that of longitudinal modes. Since rotational disorder between adjacent layers in heterostructures is expected if the constituents have incommensurate lattices, this study indicates that these heterostructures will have very low cross-plane thermal conductivity.

Relative humidity during the spin-processing of thin film solution precursors is often not controlled or measured, and its effect on film thickness is generally unappreciated. Herein, we report that the relative humidity during spin-processing has a marked impact on the film thickness of amorphous metal oxide (aluminum oxide and lanthanum zirconium oxide) and hafnium oxide-sulfate (HafSOx) thin films deposited from aqueous precursors. In the humidity range studied [20–95% relative humidity (RH)], film thicknesses varied by a factor of nearly 3, and this effect is independent of the metal precursor identity. Our data suggest that film thickness depends linearly on evaporation rate (100 – RH) for all systems studied, suggesting this effect is predominantly due to the unique characteristics of water as a solvent. In situ X-ray reflectivity studies of HafSOx films deposited under different humidities reveal that, while the thickness varies significantly with humidity, the density of the as-deposited films is similar, suggesting that humidity primarily affects the relative amount of material deposited. Because reproducible film thickness is critical for many applications, our data highlight the importance of controlling humidity during spin-processing.

Self-assembly of designed precursors has enabled the synthesis of novel heterostructures that exhibit extensive rotational disorder between constituents. In (SnSe)TiSe 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 TiSe 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> TiSe directions as √3 × a equals a. 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 TiSe 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 TiSe 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 TiSe. The calculations suggest that the long-range order in (SnSe)TiSe results from an accidental coincidence in the lattice parameters of SnSe and TiSe. 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.