Synthesis and characterisation of hexagonal molybdenum nitrides

Four hexagonal molybdenum nitrides—three modifications of δ-MoN and Mo₅N₆—were prepared by the plasma-enhanced chemical vapour deposition (PECVD) method and ammonolysis of MoCl₅ and MoS₂. The nitrides were structurally characterised by X-ray diffraction, high-resolution transmission electron microscopy, and selected area electron diffraction. δ₁-MoN is best described by the WC-type structure with stacking faults due to nitrogen atom disorder. Ordering of nitrogen atoms results in δ₂-MoN with the NiAs-type structure. Formation of trigonal molybdenum clusters in δ₃-MoN is responsible for the doubling of the unit cell in a and b directions compared to δ₂-MoN. Mo₅N₆ can be viewed as an intergrowth structure of the WC- and NiAs-type building blocks, accompanied by vacancies on Mo sites. Influence of reaction conditions on the formation of the four nitrides is discussed; their magnetic properties are presented.     

The influence of laser power and repetition rate on oxygen and nitrogen insertion into titanium using pulsed Nd:YAG laser irradiation

Oxygen and nitrogen insertion in a titanium substrate is performed in air using a Q-switched Nd-YAG laser. This process modifies the surface by the formation of specific layers on the substrate. These layers show different properties, largely influenced by the insertion of elements in the layers. The treatment conditions, especially the laser parameters (fluence and repetition rate), must be known and controlled. Using nuclear analysis, we demonstrate that oxygen insertion is mainly influenced by repetition rate, and that nitrogen insertion is controlled by laser fluence. The physical phenomena involved in the oxygen and nitrogen insertion are discussed.     

Polaron binding energy and effective mass in nitride single semiconductor heterostructures

Polaronic corrections in the conduction band of III–V nitride-based single heterostructures (SHS) are calculated within second-order Rayleigh–Schrödinger perturbation theory, using the dielectric continuum model for the interface and half-space polar optical phonon modes. The particular case of the AlN/GaN system is considered for illustration. The formation of a polarization-induced sheet density charge at the interface between both materials allows to study the single-electron states with the use of an approximate analytical expression for the potential energy function. The band-bending profile is then described in a way that includes many-body effects through a one-dimensional Hartree potential. Comparison of the calculated polaron effective mass in this material with recent experimental results in nitride SHS shows that higher perturbative orders in the interaction should be included in order to obtain better agreement.     

Supersonic cluster beam deposition of nanostructured titania

Nanostructured titanium dioxide films have been deposited by supersonic cluster beam deposition (CBD). Nanoparticles are produced by a pulsed microplasma cluster source (PMCS) and selected by aerodynamic separation effects. The as-deposited film is a complex mixture where amorphous material coexists, at the nanoscale, with anatase and rutile crystal phases. The nanocrystalline fraction of the film is characterized by crystal size ranging from 100 nm to less than 5 nm. We have characterized the film structure by transmission electron microscopy, Raman spectromicroscopy, X-ray diffraction, and UV-visible spectroscopy showing that correlations exist between cluster size and film properties. In particular if very small clusters are deposited, the film shows a predominant rutile phase whereas larger clusters form films with mainly anatase structure. Our observations suggest that phonon confinement effects are responsible for a significant shift and broadening observed for the Raman peaks. In addition, optical gap tuning is provided by mass selection: large clusters assembling generates a film with 3.22 eV optical gap, while smallest clusters 3.52 eV.     

Luminescent Nitridophosphates CaP2N4:Eu2+, SrP2N4:Eu2+, BaP2N4:Eu2+, and BaSr2P6N12:Eu2+

Nitridophosphates MP₂N₄:Eu²⁺ (M=Ca, Sr, Ba) and BaSr₂P₆N₁₂:Eu²⁺ have been synthesized at elevated pressures and 1100–1300 °C starting from the corresponding azides and P₃N₅ with EuCl₂ as dopant. Addition of NH₄Cl as mineralizer allowed for the growth of single crystals. This led to the successful structure elucidation of a highly condensed nitridophosphate from single‐crystal X‐ray diffraction data (CaP₂N₄:Eu²⁺ (P6₃, no. 173), a=16.847(2), c=7.8592(16) Å, V=1931.7(6) ų, Z=24, 2033 observed reflections, 176 refined parameters, wR₂=0.096). Upon excitation by UV light, luminescence due to parity‐allowed 4f⁶(⁷F)5d¹→4f⁷(⁸S_7/2) transition was observed in the orange (CaP₂N₄:Eu²⁺, λ_max=575 nm), green (SrP₂N₄:Eu²⁺, λ_max=529 nm), and blue regions of the visible spectrum (BaSr₂P₆N₁₂:Eu²⁺ and BaP₂N₄:Eu²⁺, λ_max=450 and 460 nm, respectively). Thus, the emission wavelength decreases with increasing ionic radius of the alkaline‐earth ions. The corresponding full width at half maximum values (2240–2460 cm^?1) are comparable to those of other known Eu²⁺‐doped (oxo)nitrides emitting in the same region of the visible spectrum. Following recently described quaternary Ba₃P₅N₁₀Br:Eu²⁺, this investigation represents the first report on the luminescence of Eu²⁺‐doped ternary nitridophosphates. Similarly to nitridosilicates and related oxonitrides, Eu²⁺‐doped nitridophosphates may have the potential to be further developed into efficient light‐emitting diode phosphors.     

Photochemically Induced Intramolecular Six‐Electron Reductive Elimination and Oxidative Addition of Nitric Oxide by the Nitridoosmate(VIII) Anion

UV photolysis of the nitridoosmate(VIII) anion, OsO₃N^?, in low‐temperature frozen matrices results in nitrogen–oxygen bond formation to give the Os^II nitrosyl complex OsO₂(NO)^?. Photolysis of the Os^II nitrosyl product with visible wavelengths results in reversion to the parent Os^VIII complex. Formally a six‐electron reductive elimination and oxidative addition, respectively, this represents the first reported example of such an intramolecular transformation. DFT modelling of this reaction proceeds through a step‐wise mechanism taking place through a side‐on nitroxyl Os^VI intermediate, OsO₂(η²‐NO)^?.