Structural Influence of Lone Pairs in GeP2N4, a Germanium(II) Nitridophosphate

Owing to their widespread properties, nitridophosphates are of high interest in current research. Explorative high-pressure high-temperature investigations yielded various compounds with stoichiometry MP2N4 (M=Be, Ca, Sr, Ba, Mn, Cd), which are discussed as ultra-hard or luminescent materials, when doped with Eu2+. Herein, we report the first germanium nitridophosphate, GeP2N4, synthesized from Ge3N4 and P3N5 at 6 GPa and 800 degrees C. The structure was determined by single-crystal X-ray diffraction and further characterized by energy-dispersive X-ray spectroscopy, density functional theory calculations, IR and NMR spectroscopy. The highly condensed network of PN4-tetrahedra shows a strong structural divergence to other MP2N4 compounds, which is attributed to the stereochemical influence of the lone pair of Ge2+. Thus, the formal exchange of alkaline earth cations with Ge2+ may open access to various compounds with literature-known stoichiometry, however, new structures and properties

Inverse-Tunable Red Luminescence and Electronic Properties of Nitridoberylloaluminates Sr2-xBax[BeAl3N5]:Eu2+ (x=0-2)

The nitridoberylloaluminate Ba-2[BeAl3N5]:Eu2+ and solid solutions Sr2-xBax[BeAl3N5]:Eu2+ (x=0.5, 1.0, 1.5) were synthesized in a hot isostatic press (HIP) under 50 MPa N-2 atmosphere at 1200 degrees C. Ba-2[BeAl3N5]:Eu2+ crystallizes in tri-clinic space group P (1) over bar (no. 2) (Z=2, a=6.1869(10), b= 7.1736(13), c= 8.0391(14) angstrom, alpha = 102.754(8), beta = 112.032(6), gamma = 104.765(7)degrees), which was determined from single-crystal X-ray diffraction data. The lattice parameters of the solid solution series have been obtained from Rietveld refinements and show a nearly linear dependence on the atomic ratio Sr:Ba. The electronic properties and the band gaps of M-2[BeAl3N5](M=Sr, Ba) have been investigated by a combination of soft X-ray spectroscopy and density functional theory (DFT) calculations. Upon irradiation with blue light (440-450 nm), the nitridoberylloaluminates exhibit intense orange to red luminescence, which can be tuned between 610 and 656 nm (fwhm =1922-2025 cm(-1) (72-87 nm)). In contrast to the usual trend, the substitution of the smaller Sr2+= by larger Ba leads to an inverse-tunable luminescence to higher wavelengths. Low-temperature luminescence measurements have been performed to exclude anomalous emission

Selective Area Band Engineering of Graphene using Cobalt-Mediated Oxidation

This study reports a scalable and economical method to open a band gap in single layer graphene by deposition of cobalt metal on its surface using physical vapor deposition in high vacuum. At low cobalt thickness, clusters form at impurity sites on the graphene without etching or damaging the graphene. When exposed to oxygen at room temperature, oxygen functional groups form in proportion to the cobalt thickness that modify the graphene band structure. Cobalt/Graphene resulting from this treatment can support a band gap of 0.30 eV, while remaining largely undamaged to preserve its structural and electrical properties. A mechanism of cobalt-mediated band opening is proposed as a two-step process starting with charge transfer from metal to graphene, followed by formation of oxides where cobalt has been deposited. Contributions from the formation of both CoO and oxygen functional groups on graphene affect the electronic structure to open a band gap. This study demonstrates that cobalt-mediated oxidation is a viable method to introduce a band gap into graphene at room temperature that could be applicable in electronics applications

Tuning the Electronic Band Gap of Oxygen-Bearing Cubic Zirconium Nitride: c-Zr3–x(N1–xOx)4

International audienceResearch has shown that group IVB nitrides have a narrow direct electronic band gap, but there have been few investigations into the dependence of the electronic band gap on cation defects and whether tuning the electronic band gap is possible by controlling the level of substitutional oxygen in c-Zr3–x(N1–xOx)4. We use a combination of soft X-ray spectroscopy and density functional theory to study the electronic structure and determine the electronic band gap as well as the exciton binding energy of an oxygen-bearing defect zirconium nitride, c-Zr2.86(N0.88O0.12)4, and oxygen-free hafnium nitride, c-Hf3N4. Moreover, we extend our structural model to consider the dependence of the electronic band gap on oxygen substitution in c-Zr3–x(N1–xOx)4. The results suggest that the electronic band gap can be precisely controlled between 1.47 and 1.84 eV (674–844 nm) by adjusting the stoichiometry, without adversely affecting the electronic structure. The exciton binding energy of c-Zr2.86(N0.88O0.12)4 is estimated to be 37 meV, much larger than current materials being used (GaAs). This larger exciton binding energy combined with both the direct narrow electronic band gap and stable electronic structure demonstrates that this material is an ideal candidate to replace currently used materials, such as GaAs, in infrared light-emitting diode application