Thermally Stable Red-Emitting Oxide Ceramics for Laser Lighting.

Laser-driven phosphor-converted white light sources are highly desirable for their unprecedented energy efficiency and lighting quality. However, important challenges remain due to a lack of efficient and stable red-emitting materials. Here Eu2+ -activated oxide-based double perovskites are explored as red emitters with thermally stable photoluminescence. Sr3 TaO5.5 :Eu2+ ceramics exhibit a red emission band peaking at 620 nm upon blue laser pumping owing to the Eu2+ occupation at highly ordered substitutional lattice sites. A constructed laser-driven white light wheel under an incident power density of 19.2 W mm-2 presents a record luminous flux of 1115 lm with an excellent color rendering index of 90. This study invigorates the development of Eu2+ -activated oxide-based ceramics with thermally stable luminescence for laser-pumped lighting and display applications.

Structural Influence of Lone Pairs in GeP2 N4 , 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 MP2 N4 (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, GeP2 N4 , synthesized from Ge3 N4 and P3 N5 at 6 GPa and 800 °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 MP2 N4 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.

Oxygen Vacancy Induced Structural Distortions in Black Titania: A Unique Approach using Soft X-ray EXAFS at the O-K Edge.

Unknown changes in the crystalline order of regular TiO2 result in the formation of black titania, which has garnered significant interest as a photocatalytic material due to the accompanying electronic changes. Herein, the nature of the lattice distortion caused by an oxygen vacancy was determined that in turn results in the formation of mid-band-gap states found in previous studies of black titania. An innovative technique is introduced using a state-of-the-art silicon drift detector, which can be used in conjunction with extended X-ray absorption fine structure (EXAFS) to measure bulk interatomic distances. Also discussed is how the energy dispersive nature of such a detector can allow for an unimpeded signal, indefinitely in energy space, thereby sidestepping the hurdles of more conventional EXAFS, which is often impeded by other absorption edges.

Measurement of coherence decay in GaMnAs using femtosecond four-wave mixing.

The application of femtosecond four-wave mixing to the study of fundamental properties of diluted magnetic semiconductors ((s,p)-d hybridization, spin-flip scattering) is described, using experiments on GaMnAs as a prototype III-Mn-V system.  Spectrally-resolved and time-resolved experimental configurations are described, including the use of zero-background autocorrelation techniques for pulse optimization.  The etching process used to prepare GaMnAs samples for four-wave mixing experiments is also highlighted.  The high temporal resolution of this technique, afforded by the use of short (20 fsec) optical pulses, permits the rapid spin-flip scattering process in this system to be studied directly in the time domain, providing new insight into the strong exchange coupling responsible for carrier-mediated ferromagnetism.  We also show that spectral resolution of the four-wave mixing signal allows one to extract clear signatures of (s,p)-d hybridization in this system, unlike linear spectroscopy techniques.   This increased sensitivity is due to the nonlinearity of the technique, which suppresses defect-related contributions to the optical response. This method may be used to measure the time scale for coherence decay (tied to the fastest scattering processes) in a wide variety of semiconductor systems of interest for next generation electronics and optoelectronics.