U(V) stabilization via aliovalent incorporation of Ln(III) into oxo-salt framework.

Pentavalent uranium compounds are key components of uranium's redox chemistry and play important roles in environmental transport. Despite this, well-characterized U(V) compounds are scarce primarily because of their instability with respect to disproportionation to U(IV) and U(VI). In this work, we provide an alternate route to incorporation of U(V) into a crystalline lattice where different oxidation states of uranium can be stabilized through the incorporation of secondary cations with different sizes and charges. We show that iriginite-based crystalline layers allow for systematically replacing U(VI) with U(V) through aliovalent substitution of 2+ alkaline-earth or 3+ rare-earth cations as dopant ions under high-temperature conditions, specifically Ca(UVIO2)W4O14 and Ln(UVO2)W4O14 (Ln = Nd, Sm, Eu, Gd, Yb). Evidence for the existence of U(V) and U(VI) is supported by single-crystal X-ray diffraction, high energy resolution X-ray absorption near edge structure, X-ray photoelectron spectroscopy, and optical absorption spectroscopy. In contrast with other reported U(V) materials, the U(V) single crystals obtained using this route are relatively large (several centimeters) and easily reproducible, and thus provide a substantial improvement in the facile synthesis and stabilization of U(V).

Cathodoluminescence spectroscopy of monolayer hexagonal boron nitride.

Cathodoluminescence (CL) spectroscopy is a suitable technique for studying the luminescent properties of optoelectronic materials because CL has no limitation on the excitable bandgap energy and eliminates ambiguous signals due to simple light scattering and resonant Raman scattering potentially involved in the photoluminescence spectra. However, direct CL measurements of atomically thin two-dimensional materials have been difficult due to the small excitation volume that interacts with high-energy electron beams. Herein, distinct CL signals from a monolayer hexagonal BN (hBN), namely mBN, epitaxial film grown on a graphite substrate are shown by using a CL system capable of large-area and surface-sensitive excitation. Spatially resolved CL spectra at 13 K exhibited a predominant 5.5-eV emission band, which has been ascribed to originate from multilayered aggregates of hBN, markedly at thicker areas formed on the step edges of the substrate. Conversely, a faint peak at 6.04 ± 0.01 eV was routinely observed from atomically flat areas, which is assigned as being due to the recombination of phonon-assisted direct excitons of mBN. The CL results support the transition from indirect bandgap in bulk hBN to direct bandgap in mBN. The results also encourage one to elucidate emission properties of other low-dimensional materials by using the present CL configuration.

Wafer-Scale Two-Dimensional Semiconductors for Deep UV Sensing.

2D semiconductors (2SEM) can transform many sectors, from information and communication technology to healthcare. To date, top-down approaches to their fabrication, such as exfoliation of bulk crystals by "scotch-tape," are widely used, but have limited prospects for precise engineering of functionalities and scalability. Here, a bottom-up technique based on epitaxy is used to demonstrate high-quality, wafer-scale 2SEM based on the wide band gap gallium selenide (GaSe) compound. GaSe layers of well-defined thickness are developed using a bespoke facility for the epitaxial growth and in situ studies of 2SEM. The dominant centrosymmetry and stacking of the individual van der Waals layers are verified by theory and experiment; their optical anisotropy and resonant absorption in the UV spectrum are exploited for photon sensing in the technological UV-C spectral range, offering a scalable route to deep-UV optoelectronics.

Na4-xSn2-xSbxGe5O16, an Air-Stable Solid-State Na-Ion Conductor.

The structure of a Na4Sn2Ge5O16 phase was established via single-crystal X-ray diffraction. Unusually large displacement parameters of Na atoms suggested the possibility of Na+ ionic conductivity. To create Na deficiencies and thus increase the Na+ mobility in Na4Sn2Ge5O16, Sn4+ cations were partially substituted with Sb5+. A series of Na4-xSn2-xSbxGe5O16 samples (x = 0, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, or 0.35) were prepared by solid-state reactions and characterized with electrical impedance spectroscopy in the range of 25-200 °C. The highest ionic conductivity value was achieved in the Na3.8Sn1.8Sb0.2Ge5O16 sample (1.6 mS cm-1 at 200 °C). Na+ migration pathways were calculated using the bond-valence energy landscape approach, and two-dimensional conductivity channels with low energy barriers (≈0.4 eV) were found in the structure. Three-dimensional conductivity can also be achieved in the structure; however, it has a much higher energy barrier. The pristine phase and Na3.8Sn1.8Sb0.2Ge5O16 sample were studied via 23Na and 119Sn solid-state nuclear magnetic resonance. A faster exchange between the Na sites was observed in the doped sample.

Phosphorescence in Mn4+-Doped R+/R2+ Germanates (R+ = Na+ or K+, R2+ = Sr2+).

Single crystals of three new compounds, Na0.36Sr0.82Ge4O9 (1, proposed composition), Na2SrGe6O14 (2), and K2SrGe8O18 (3), were obtained and characterized using single-crystal X-ray diffraction. Their structures contain three-dimensional (3D) anionic frameworks built from GeO4 and GeO6 polyhedra. The presence of octahedral Ge4+ sites makes the new phases suitable for Mn4+ substitution to obtain red-emitting phosphors with a potential application for light conversion. Photoluminescence properties of Mn4+-substituted Na2SrGe6O14 (2) and K2SrGe8O18 (3) samples were studied over a range of temperatures, and red light photoluminescence associated with the electronic transitions of tetravalent manganese was observed. The Na2SrGe6O14 (2) phase was also substituted with Pr3+ on the mixed Na-Sr site similar to the previously studied Na2CaGe6O14:Pr3+. The red emission peak of the Pr3+ activator occurs at a shorter wavelength (610 nm) compared to that of Mn4+ (662-663 nm). Additionally, second harmonic generation (SHG) data were collected for the noncentrosymmetric Na2SrGe6O14 (2) phase, indicating weak SHG activity. Diffuse reflectance spectroscopy and density of states calculations were performed to estimate the band gap values for pristine Na2SrGe6O14 (2) and K2SrGe8O18 (3) phases.

Identifying carbon as the source of visible single-photon emission from hexagonal boron nitride.

Single-photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered increasing attention over the last few years due to their superior optical properties. However, despite the vast range of experimental results and theoretical calculations, the defect structure responsible for the observed emission has remained elusive. Here, by controlling the incorporation of impurities into hBN via various bottom-up synthesis methods and directly through ion implantation, we provide direct evidence that the visible SPEs are carbon related. Room-temperature optically detected magnetic resonance is demonstrated on ensembles of these defects. We perform ion-implantation experiments and confirm that only carbon implantation creates SPEs in the visible spectral range. Computational analysis of the simplest 12 carbon-containing defect species suggest the negatively charged [Formula: see text] defect as a viable candidate and predict that out-of-plane deformations make the defect environmentally sensitive. Our results resolve a long-standing debate about the origin of single emitters at the visible range in hBN and will be key to the deterministic engineering of these defects for quantum photonic devices.

High-Temperature Molecular Beam Epitaxy of Hexagonal Boron Nitride with High Active Nitrogen Fluxes.

Hexagonal boron nitride (hBN) has attracted a great deal of attention as a key component in van der Waals (vdW) heterostructures, and as a wide band gap material for deep-ultraviolet devices. We have recently demonstrated plasma-assisted molecular beam epitaxy (PA-MBE) of hBN layers on substrates of highly oriented pyrolytic graphite at high substrate temperatures of ~1400 °C. The current paper will present data on the high-temperature PA-MBE growth of hBN layers using a high-efficiency radio-frequency (RF) nitrogen plasma source. Despite more than a three-fold increase in nitrogen flux with this new source, we saw no significant increase in the growth rates of the hBN layers, indicating that the growth rate of hBN layers is controlled by the boron arrival rate. The hBN thickness increases to 90 nm with decrease in the growth temperature to 1080 °C. However, the decrease in the MBE temperature led to a deterioration in the optical properties of the hBN. The optical absorption data indicates that an increase in the active nitrogen flux during the PA-MBE process improves the optical properties of hBN and suppresses defect related optical absorption in the energy range 5.0⁻5.5 eV.

Moiré-Modulated Conductance of Hexagonal Boron Nitride Tunnel Barriers.

Monolayer hexagonal boron nitride (hBN) tunnel barriers investigated using conductive atomic force microscopy reveal moiré patterns in the spatial maps of their tunnel conductance consistent with the formation of a moiré superlattice between the hBN and an underlying highly ordered pyrolytic graphite (HOPG) substrate. This variation is attributed to a periodc modulation of the local density of states and occurs for both exfoliated hBN barriers and epitaxially grown layers. The epitaxial barriers also exhibit enhanced conductance at localized subnanometer regions which are attributed to exposure of the substrate to a nitrogen plasma source during the high temperature growth process. Our results show clearly a spatial periodicity of tunnel current due to the formation of a moiré superlattice and we argue that this can provide a mechanism for elastic scattering of charge carriers for similar interfaces embedded in graphene/hBN resonant tunnel diodes.

Lattice-Matched Epitaxial Graphene Grown on Boron Nitride.

Lattice-matched graphene on hexagonal boron nitride is expected to lead to the formation of a band gap but requires the formation of highly strained material and has not hitherto been realized. We demonstrate that aligned, lattice-matched graphene can be grown by molecular beam epitaxy using substrate temperatures in the range 1600-1710 °C and coexists with a topologically modified moiré pattern with regions of strained graphene which have giant moiré periods up to ∼80 nm. Raman spectra reveal narrow red-shifted peaks due to isotropic strain, while the giant moiré patterns result in complex splitting of Raman peaks due to strain variations across the moiré unit cell. The lattice-matched graphene has a lower conductance than both the Frenkel-Kontorova-type domain walls and also the topological defects where they terminate. We relate these results to theoretical models of band gap formation in graphene/boron nitride heterostructures.

Alternative ways to optimize treatment for retinal vein occlusion with peripheral capillary non-perfusion: a pilot study.

PURPOSE:: We compared the efficacy and safety of ranibizumab versus ranibizumab plus scatter laser photocoagulation (SLP) in patients with chronic post-central retinal vein occlusion (CRVO) macular edema (ME). METHODS:: This prospective non-randomized pilot study included 250 patients with peripheral retinal ischemia and CRVO-related ME. The mean follow-up period was 24.5 ± 6.5 months. The clinical assessments conducted included best corrected visual acuity, optical coherence tomography, and multi-field fluorescein angiography with measurement of the ischemic area. The study population comprised two comparable patient groups with peripheral retinal ischemia that received different treatments for post-CRVO ME: ranibizumab with peripheral SLP of capillary non-perfusion areas (Group 1); and Lucentis® monotherapy (Group 2). Data analyses were performed using Statistica 7 software suite and included the estimation of х ± δ values and their dispersion and covariation coefficients at different stages of the study. RESULTS:: Clinically significant retinal ischemia was detected in 175 (70%) patients, occupying an average of 435.12 ± 225.13 mm2, i.e., 167.15 ± 45.16 optic disc areas. Peripheral ischemia was found in 125 patients, representing 50% of all patients with CRVO and 71.4% of all patients with ischemic CRVO. The mean number of ranibizumab injections in patients who underwent SLP was 3.5 ± 1.6. Patients treated with ranibizumab monotherapy for 24 months received 10.6 ± 2.5 injections. Functional and anatomic results were comparable in the two groups. CONCLUSIONS:: The combination of ranibizumab injections and peripheral SLP in capillary non-perfusion areas can significantly decrease the number of injections and reduce neovascular complications.

Alternative ways to optimize treatment for retinal vein occlusion with peripheral capillary non-perfusion: a pilot study / Opções alternativas para otimizar o tratamento da oclusão da veia retiniana sem perfusão capilar periférica (um estudo piloto)

ABSTRACT Purpose: We compared the efficacy and safety of ranibizumab versus ranibizumab plus scatter laser photocoagulation (SLP) in patients with chronic post-central retinal vein occlusion (CRVO) macular edema (ME). Methods: This prospective non-randomized pilot study included 250 patients with peripheral retinal ischemia and CRVO-related ME. The mean follow-up period was 24.5 ± 6.5 months. The clinical assessments conducted included best corrected visual acuity, optical coherence tomography, and multi-field fluorescein angiography with measurement of the ischemic area. The study population comprised two comparable patient groups with peripheral retinal ischemia that received different treatments for post-CRVO ME: ranibizumab with peripheral SLP of capillary non-perfusion areas (Group 1); and Lucentis® monotherapy (Group 2). Data analyses were performed using Statistica 7 software suite and included the estimation of х ± δ values and their dispersion and covariation coefficients at different stages of the study. Results: Clinically significant retinal ischemia was detected in 175 (70%) patients, occupying an average of 435.12 ± 225.13 mm2, i.e., 167.15 ± 45.16 optic disc areas. Peripheral ischemia was found in 125 patients, representing 50% of all patients with CRVO and 71.4% of all patients with ischemic CRVO. The mean number of ranibizumab injections in patients who underwent SLP was 3.5 ± 1.6. Patients treated with ranibizumab monotherapy for 24 months received 10.6 ± 2.5 injections. Functional and anatomic results were comparable in the two groups. Conclusions: The combination of ranibizumab injections and peripheral SLP in capillary non-perfusion areas can significantly decrease the number of injections and reduce neovascular complications.

RESUMO Objetivo: A investigação centra-se na terapia de edema macular pós-oclusão da veia retiniana central (OVCR) em casos com isquemia retiniana periférica. O objetivo foi comparar a eficácia e a segurança do tratamento com ranibizumab vs ranibizumab + fotocoagulação com laser de dispersão (SLP) em pacientes com edema macular crônico secundário a oclusão da veia retiniana central isquêmica. Métodos: O estudo prospectivo não-randomizado incluiu 250 pacientes com isquemia retiniana periférica e edema macular relacionados a oclusão da veia retiniana central. O tempo médio de seguimento foi de 24,5 ± 6,5 meses. A avaliação clínica incluiu acuidade visual melhor corrigida, tomografia de coerência óptica (OCT) e angiografia por fluoresceína multi-campo com a medição da área de isquemia. A população estudada foi constituída por dois grupos de pacientes comparáveis com o oclusão da veia retiniana central isquêmica, que receberam tratamento diferente. Em nossa prática anterior, utilizamos ranibizumab (Lucentis®) em monoterapia (de acordo com a licença do medicamento) para edema macular pós-oclusão da veia retiniana central com isquemia retiniana periférica (Grupo 2). Mais recentemente, começamos a combinar ranibizumab com SLP periférica de áreas não perfusão capilar (Grupo 1). As análises de dados foram realizadas com o software Statistica 7 e incluíram a estimação dos valores de х ± δ e seus coeficientes de dispersão e covariân cia em diferentes estágios do estudo. Resultados: Identificou-se isquemia retiniana clinicamente significativa em 175 (70%) pacientes, atingindo uma média de 435,12 ± 225,13 mm2, ou seja, 167,15 ± 45,16 áreas de disco óptico. Isquemia periférica foi encontrada em 125 casos, representando 50% de todos os pacientes com oclusão da veia retiniana central e 71,4% de todos os pacientes com oclusão da veia retiniana central isquêmica. O número médio de injeções de rani bizumab em pacientes com SLP foi de 3,5 ± 1,6. Os pacientes tratados com ranibizu mab em monoterapia durante 24 meses receberam 10,6 ± 2,5 injeções. Os resultados funcionais e anatômicos foram comparáveis nos dois grupos. Conclusões: A combinação de injeções de ranibizumab com SLP periférica em áreas de não-perfusão capilar pode diminuir significativamente o número de injeções e reduzir as complicações neovasculares.

Intermixing studies in GaN1-xSbx highly mismatched alloys.

GaN1-xSbx with x∼5%-7% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photoelectrochemical cell for solar water splitting. In this study, we grew GaN1-xSbx under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN1-xAsx, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb all suggested that GaN1-xSbx alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

Hexagonal Boron Nitride Tunnel Barriers Grown on Graphite by High Temperature Molecular Beam Epitaxy.

We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moiré patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate.

Surface-Enhanced Raman Scattering of Silicon Nanocrystals in a Silica Film.

Surface-enhanced Raman scattering (SERS) is an intriguing effect, efficiency of which depends on many factors and whose applicability to a given system is not obvious before the experiment. The motivation of the present work is to demonstrate the SERS effect on silicon nanocrystals (Si-nc) embedded in silica, the material of high technological importance. Using the Ag overlayer method, we have found the SERS effect for this material. The best result is obtained for Ag layers of a weight thickness of 12 nm, whose surface plasmons are in a resonance with the laser wavelength (488 nm). The enhancement obtained for the Raman signal from 3-4-nm Si-nc in a 40-nm SiOx film is above 100. The SERS effect is about twice stronger for ultra-small Si-nc (~1 nm) and/or disordered silicon compared to Si-nc with sizes of 3-4 nm. The SERS measurements with an Ag overlayer allow detecting silicon crystallization for ultra-thin SiOx films and/or for very low Si excess and suppress the Raman signal from the substrate and the photoluminescence of the film.

Strain-Engineered Graphene Grown on Hexagonal Boron Nitride by Molecular Beam Epitaxy.

Graphene grown by high temperature molecular beam epitaxy on hexagonal boron nitride (hBN) forms continuous domains with dimensions of order 20 µm, and exhibits moiré patterns with large periodicities, up to ~30 nm, indicating that the layers are highly strained. Topological defects in the moiré patterns are observed and attributed to the relaxation of graphene islands which nucleate at different sites and subsequently coalesce. In addition, cracks are formed leading to strain relaxation, highly anisotropic strain fields, and abrupt boundaries between regions with different moiré periods. These cracks can also be formed by modification of the layers with a local probe resulting in the contraction and physical displacement of graphene layers. The Raman spectra of regions with a large moiré period reveal split and shifted G and 2D peaks confirming the presence of strain. Our work demonstrates a new approach to the growth of epitaxial graphene and a means of generating and modifying strain in graphene.

Giant Raman gain in silicon nanocrystals.

Nanostructured silicon has generated a lot of interest in the past decades as a key material for silicon-based photonics. The low absorption coefficient makes silicon nanocrystals attractive as an active medium in waveguide structures, and their third-order nonlinear optical properties are crucial for the development of next generation nonlinear photonic devices. Here we report the first observation of stimulated Raman scattering in silicon nanocrystals embedded in a silica matrix under non-resonant excitation at infrared wavelengths (~1.5 µm). Raman gain is directly measured as a function of the silicon content. A giant Raman gain from the silicon nanocrystals is obtained that is up to four orders of magnitude greater than in crystalline silicon. These results demonstrate the first Raman amplifier based on silicon nanocrystals in a silica matrix, thus opening new perspectives for the realization of more efficient Raman lasers with ultra-small sizes, which would increase the synergy between electronic and photonic devices.

Directions in QSAR modeling for regulatory uses in OECD member countries, EU and in Russia.

The aim of this article is to show the main aspects of quantitative structure activity relationship (QSAR) modeling for regulatory purposes. We try to answer the question; what makes QSAR models suitable for regulatory uses. The article focuses on directions in QSAR modeling in European Union (EU) and Russia. Difficulties in validation models have been discussed.