Molybdenum nitrides have been employed in a variety of applications. For use in catalysis, the cubic γ phase with the nominal stoichiometry Mo 2 N and the space group Fm3̅ m is typically prepared by high-temperature reaction of MoO 3 with NH 3 . The literature presents conflicting reports of the possible presence of residual oxygen from typical ammonolysis reactions and whether such species influence the crystal structure and morphology. With the aim of resolving these open questions, a comprehensive study of the chemistry, crystal structure, and electronic structure of molybdenum nitride materials prepared by ammonolysis has been undertaken here, with particular focus on the role of reaction temperature. Ammonolysis of MoO 3 was carried out at 973 and 1073 K and yielded single-phase cubic products. Using electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis, prompt gamma-ray neutron activation analysis, and combustion analysis, significant concentrations of oxygen and, to a lesser extent, hydrogen were found in both materials. The crystal structure of each phase was refined by Rietveld analysis using combined synchrotron X-ray diffraction and neutron diffraction data. The structures were found to be derivatives of the B1 rock salt (halite) structure, as is often reported for "γ-Mo 2 N." However, both materials adopt the space group Pm3̅ m, as opposed to the typically presumed space group of Fm3̅ m, and both have much higher anion content than implied by the stoichiometry Mo 2 N. Ordering of cation vacancies and of anion species is responsible for the loss of the translational symmetry expected for the space group Fm3̅ m. X-ray absorption spectroscopy studies, along with the EELS and XPS results, showed the Mo oxidation state to be diminished with higher temperature synthesis, consistent with the retention of a lower concentration of anions and in particular oxygen. The difficulty in differentiating oxygen and nitrogen and the impossibility of detecting hydrogen by X-ray and electron diffraction methods, especially in the presence of the heavy element Mo, have likely inhibited accurate identification of Mo 1−x (N 1−y O y )H z as the product of MoO 3 ammonolysis. The findings reported here offer a critical assessment for understanding the properties of molybdenum "nitrides" in electronic and catalytic applications.

Traditional organolead-halide perovskite-based devices have shown rapid improvements in their power conversion efficiency in less than a decade, yet challenges remain for improving stability and film uniformity, as well as the elimination of lead to address toxicity issues. We fabricated lead-free methylammonium bismuth iodide (MBI) perovskite films and studied the effect of solvent annealing with dimethylformamide (DMF) on both (1) the crystallinity and structure of the films with X-ray diffraction and scanning electron microscopy and (2) the local optoelectronic properties of the films as measured via (photo)conductive atomic force microscopy. We found that solvent annealing leads to improved crystallinity and increased grain size in the MBI films as compared to the thermally annealed films. Furthermore, solvent-annealed MBI films show significantly increased electrical conductivity in the out-of-plane direction. Photoconductivity in both solvent-annealed and thermally annealed MBI films was increased in the grain interiors versus the grain boundaries. It was observed that DMF-induced solvent annealing impacts charge transport through the film, which can be a unique design parameter for optimizing local optoelectronic properties. By studying how solvent annealing affects the MBI film structure and changes the ways in which charges are transported through the film, we have developed a better understanding of how local optoelectronic properties are affected by DMF annealing.

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The band bending effect depends on the morphology of the conjugated polymer as studied by Kelvin probe force microscopy.