A new series of magnetic and luminescent layered hybrid materials obtained from thianthrene phosphonic acid: M(H2O)PO3-S2C12H7 (M = Cu, Zn) and M(H2O)2(PO2OH-S2C12H7)2 (M = Mn, Co).

Four new metallophosphonates with the chemical formulae M(H2O)PO3-S2C12H7 (M = Cu, Zn) and M(H2O)2(PO2OH-S2C12H7)2 (M = Mn, Co) were synthesized using a hydrothermal route from the original bent rigid thianthrene-2-ylphosphonic acid (TPA). This organic precursor crystallizes in a non-centrosymmetric space group P212121 and presents a unique bent geometry due to the presence of two sulfur atoms in its rigid platform architecture. Obtained as single crystal and polycrystalline powders, the structures of the four hybrid materials were solved using X-ray diffraction on single crystals in a monoclinic P21/c space group. These compounds adopt a lamellar structure consisting of one inorganic subnetwork alternating with a 'sawtooth' double organic -S2C12H7 subnetwork. The inorganic layers of these compounds are made of (PO3C) or partially deprotonated (PO2OHC) tetrahedra connected by the apices to isolated ZnO3(H2O) tetrahedra, Cu2O6(H2O)2 copper dimers and cobalt and manganese MO4(H2O)2 octahedra, where the latter two exhibit an isotype structure. Thermogravimetric analysis was performed to confirm the amount of water molecules present in the formula, to track the dehydration process of the structures, and to evaluate their thermal stability. The magnetic properties of the copper, cobalt, and manganese-based materials were investigated from 2 K to 300 K by using a SQUID magnetometer revealing dominant antiferromagnetic interactions with Weiss temperatures of -8.0, -10, and -1 K, respectively. These magnetic behaviors were further corroborated by first-principles simulations based on Density Functional Theory (DFT). Finally, the absorption and photoluminescence properties of both the ligand and hybrid materials were investigated, revealing diverse excitation and recombination mechanisms. The organic moiety based on thianthrene significantly influenced the absorption and emission, with additional peaks attributed to transition metals. Singlet and triplet states recombination were observed, accompanied by an unidentified quenching mechanism affecting the triplet state lifetime.

Artificial Aging of Thin Films of the Indium-Free Transparent Conducting Oxide SrVO3.

SrVO3 (SVO) is a prospective candidate to replace the conventional indium tin oxide (ITO) among the new generation of transparent conducting oxide (TCO) materials. In this study, the structural, electrical, and optical properties of SVO thin films, both epitaxial and polycrystalline, are determined during and after heat treatments in the 150-250 °C range and under ambient environment in order to explore the chemical stability of this material. The use of these relatively low temperatures speeds up the natural aging of the films and allows following the evolution of their related properties. The combination of techniques rather sensitive to the film surface and of techniques sampling the film volume will emphasize the presence of a surface oxidation evolving in time at low annealing temperatures, whereas the perovskite phase is destroyed throughout the film for treatments above 200 °C. The present study is designed to understand the thermal degradation and long-term stability issues of vanadate-based TCOs and to identify technologically viable solutions for the application of this group as new TCOs.

A new approach for microstructure imaging.

A recurring issue with microstructure studies is specimen lighting. In particular, microscope lighting must be deployed in such a way as to highlight biological elements without enhancing caustic effects and diffraction. We describe here a high frequency technique due to address this lighting issue. First, an extensive study is undertaken concerning asymptotic equations in order to identify the most promising algorithm for 3D microstructure analysis. Ultimately, models based on virtual light rays are discarded in favor of a model that considers the joint computation of phase and irradiance. This paper maintains the essential goal of the study concerning biological microstructures but offers several supplementary notes on computational details which provide perspectives on analyses of the arrangements of numerous objects in biological tissues.

Er:KY3F10 laser at 2.80 µm.

We report on the mid-infrared laser operation of a cubic 15 at.% Er3+:KY3F10 crystal. In the quasi-continuous-wave regime, the peak power reaches 255 mW at 2.80 µm (the 4I11/2→4I13/2 transition) with a slope efficiency of 10.9% and a laser threshold of 58 mW. Two pumping schemes (to the 4I11/2 and 4I9/2 states) are compared. The emission properties of the Er3+ ions in KY3F10 are studied, indicating high stimulated-emission cross-section of 0.57×10-20cm2 at 2.80 µm, a large gain bandwidth of 40 nm, and a long 4I11/2 state lifetime of 4.64 ms.

Identification of (Tb,Eu)9.43(SiO4)6O2-δ Oxy-Apatite Structures as Nanometric Inclusions in Annealed (Eu,Tb)-Doped ZnO/Si Junctions: Combined Electron Diffraction and Chemical Contrast Imaging Studies.

(Tb,Eu)-doped ZnO-annealed films at 1100 °C showed intense photoluminescense (PL) emission from Eu and Tb ions. The high-temperature annealing led to a chemical segregation and a secondary Zn-free phase formation that is suspected to be responsible for the high PL intensity. Large faceted inclusions of rare-earth (RE) silicates of a size of few hundred nanometers were observed. Owing to various advanced electron microscopy techniques, a detailed microstructural study of these nanometric inclusions combining atomic Z contrast imaging (STEM) and precession electron diffraction tomography (PEDT) data was carried out and resulted in the determination of a hexagonal P63/m-type (Tb,Eu)9.43(SiO4)6O2-δ structure related to an oxy-apatite structure. Chemical analyses from spectroscopic data (energy-dispersive X-ray mapping and electron energy loss spectroscopy) at the atomic scale showed that both RE elements sitting on two independent (4f) and (6h) atomic sites have three-fold oxidation states, while refinements of their occupancy sites from PEDT data have evidenced preferential deficiency for the first one. The deduced RE-O distances and their corresponding bond valences are listed and discussed with the efficient energy transfer from Tb3+ toward Eu3+.

Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode.

Driven by the motivation to quantitively control and monitor trace metal ions in water, the development of environmental-friendly electrodes with superior detection sensitivity is extremely important. In this work, we report the design of a stable, ultrasensitive and biocompatible electrode for the detection of trace Ag+ and Cu2+ ions by growing n-type GaN micropillars on conductive p-type GaN substrate. The electrochemical measurement based on cyclic voltammetry indicates that the GaN micropillars exhibit quasi-reversible and mass-controlled reaction in redox probe solution. In the application of trace Ag+ and Cu2+ determination, the GaN micropillars show superior sensitivity and excellent conductivity by presenting a detection limit of 3.3 ppb for Ag+ and 3.3 ppb for Cu2+. Comparative studies on the electrochemical response of GaN micropillars and GaN film in the simultaneous Ag+ and Cu2+ detection reveal that GaN micropillars show three orders of magnitude higher stripping peak current than GaN film. It is assumed that the microarray morphology with large active area and the hydrophilia nature of GaN micropillars are responsible for the excellent sensitivity. This work will open up some opportunities for GaN nanostructure electrodes in the application of trace metal ions detection.

Alignment control and atomically-scaled heteroepitaxial interface study of GaN nanowires.

Well-aligned GaN nanowires are promising candidates for building high-performance optoelectronic nanodevices. In this work, we demonstrate the epitaxial growth of well-aligned GaN nanowires on a [0001]-oriented sapphire substrate in a simple catalyst-assisted chemical vapor deposition process and their alignment control. It is found that the ammonia flux plays a key role in dominating the initial nucleation of GaN nanocrystals and their orientation. Typically, significant improvement of the GaN nanowire alignment can be realized at a low NH3 flow rate. X-ray diffraction and cross-sectional scanning electron microscopy studies further verified the preferential orientation of GaN nanowires along the [0001] direction. The growth mechanism of GaN nanowire arrays is also well studied based on cross-sectional high-resolution transmission electron microscopy (HRTEM) characterization and it is observed that GaN nanowires have good epitaxial growth on the sapphire substrate following the crystallographic relationship between (0001)GaN∥(0001)sapphire and (101[combining macron]0)GaN∥(112[combining macron]0)sapphire. Most importantly, periodic misfit dislocations are also experimentally observed in the interface region due to the large lattice mismatch between the GaN nanowire and the sapphire substrate, and the formation of such dislocations will favor the release of structural strain in GaN nanowires. HRTEM analysis also finds the existence of "type I" stacking faults and voids inside the GaN nanowires. Optical investigation suggests that the GaN nanowire arrays have strong emission in the UV range, suggesting their crystalline nature and chemical purity. The achievement of aligned GaN nanowires will further promote the wide applications of GaN nanostructures toward diverse high-performance optoelectronic nanodevices including nano-LEDs, photovoltaic cells, photodetectors etc.

Microstructure and optical properties of Pr3+-doped hafnium silicate films.

In this study, we report on the evolution of the microstructure and photoluminescence properties of Pr3+-doped hafnium silicate thin films as a function of annealing temperature (TA). The composition and microstructure of the films were characterized by means of Rutherford backscattering spectrometry, spectroscopic ellipsometry, Fourier transform infrared absorption, and X-ray diffraction, while the emission properties have been studied by means of photoluminescence (PL) and PL excitation (PLE) spectroscopies. It was observed that a post-annealing treatment favors the phase separation in hafnium silicate matrix being more evident at 950°C. The HfO2 phase demonstrates a pronounced crystallization in tetragonal phase upon 950°C annealing. Pr3+ emission appeared at TA = 950°C, and the highest efficiency of Pr3+ ion emission was detected upon a thermal treatment at 1,000°C. Analysis of the PLE spectra reveals an efficient energy transfer from matrix defects towards Pr3+ ions. It is considered that oxygen vacancies act as effective Pr3+ sensitizer. Finally, a PL study of undoped HfO2 and HfSiOx matrices is performed to evidence the energy transfer.

Structural factors impacting carrier transport and electroluminescence from Si nanocluster-sensitized Er ions.

We present an analysis of factors influencing carrier transport and electroluminescence (EL) at 1.5 µm from erbium-doped silicon-rich silica (SiOx) layers. The effects of both the active layer thickness and the Si-excess content on the electrical excitation of erbium are studied. We demonstrate that when the thickness is decreased from a few hundred to tens of nanometers the conductivity is greatly enhanced. Carrier transport is well described in all cases by a Poole-Frenkel mechanism, while the thickness-dependent current density suggests an evolution of both density and distribution of trapping states induced by Si nanoinclusions. We ascribe this observation to stress-induced effects prevailing in thin films, which inhibit the agglomeration of Si atoms, resulting in a high density of sub-nm Si inclusions that induce traps much shallower than those generated by Si nanoclusters (Si-ncs) formed in thicker films. There is no direct correlation between high conductivity and optimized EL intensity at 1.5 µm. Our results suggest that the main excitation mechanism governing the EL signal is impact excitation, which gradually becomes more efficient as film thickness increases, thanks to the increased segregation of Si-ncs, which in turn allows more efficient injection of hot electrons into the oxide matrix. Optimization of the EL signal is thus found to be a compromise between conductivity and both number and degree of segregation of Si-ncs, all of which are governed by a combination of excess Si content and sample thickness. This material study has strong implications for many electrically-driven devices using Si-ncs or Si-excess mediated EL.

Electrically tailored resistance switching in silicon oxide.

Resistive switching in a metal-free silicon-based material offers a compelling alternative to existing metal oxide-based resistive RAM (ReRAM) devices, both in terms of ease of fabrication and of enhanced device performance. We report a study of resistive switching in devices consisting of non-stoichiometric silicon-rich silicon dioxide thin films. Our devices exhibit multi-level switching and analogue modulation of resistance as well as standard two-level switching. We demonstrate different operational modes that make it possible to dynamically adjust device properties, in particular two highly desirable properties: nonlinearity and self-rectification. This can potentially enable high levels of device integration in passive crossbar arrays without causing the problem of leakage currents in common line semi-selected devices. Aspects of conduction and switching mechanisms are discussed, and scanning tunnelling microscopy (STM) measurements provide a more detailed insight into both the location and the dimensions of the conductive filaments.

Thickness-dependent optimization of Er3+ light emission from silicon-rich silicon oxide thin films.

This study investigates the influence of the film thickness on the silicon-excess-mediated sensitization of Erbium ions in Si-rich silica. The Er3+ photoluminescence at 1.5 µm, normalized to the film thickness, was found five times larger for films 1 µm-thick than that from 50-nm-thick films intended for electrically driven devices. The origin of this difference is shared by changes in the local density of optical states and depth-dependent interferences, and by limited formation of Si-based sensitizers in "thin" films, probably because of the prevailing high stress. More Si excess has significantly increased the emission from "thin" films, up to ten times. This paves the way to the realization of highly efficient electrically excited devices.