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.
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