Investigation of the pressure dependence of band gaps for silver halides within a first-principles method

The pressure dependence of the direct and indirect band gaps in rocksalt silver halides has been studied using the full-potential linearized augmented plane wave method within the generalized gradient approximation for the exchange-correlation potential. It is found that indirect band gaps (L→X and L→Γ) exhibit different responses to application of pressure. Similar trends are found for the indirect band gaps of AgCl and AgBr while the trend in AgI (L→Γ) band gap is different. In all the compounds, the effect of pressure on the direct band gaps (Γ→Γ, X→X and L→L) show qualitatively similar results. The fundamental indirect band gap (L→Γ) pressure coefficients are −4.19 meV (GPa)⁻¹ and −3.81 meV (GPa)⁻¹ for AgCl and AgBr while for AgI (L→X) it is −61.50 meV (GPa)⁻¹. The band gap pressure coefficient as well as the volume deformation potential for the various band gaps of the compounds have also been investigated.     

Room temperature luminescence in CuI/AgI quantum dots

Room temperature luminescence in a CuI/AgI glass system is investigated by irradiating the system at 410 nm (3.02 eV). The spectrum peaks at 635 nm (1.95 eV) and 700 nm (1.77 eV), while the intensity is significantly enhanced (centered at 635 nm) by increasing the amount of AgI. We propose a model based on an increase in the AgI:Cu⁺ species at higher AgI concentration at which the red emission is attributed to the radiative recombination from carriers trapped at the donor sites (e.g., interstitial silver ions) and the acceptor sites (e.g., a vacancy-compensated divalent cation). The PL efficiency is also estimated by comparison with a standard rhodamine B solution.     

Intrinsic paramagnetic defects probe superionic phase transition in Ag1−xCuxI (x=0, 0.05, 0.15 and 0.50) nanocrystals

EPR probed the zincblende (γ) to cubic (α) AgI structural phase transition in AgI at 423 K through two intrinsic paramagnetic centers: an Ag²⁺-based hole center (signal ‘A’) and an Ag⁰-based conduction electron center (signal ‘B’) associated with AgI nanocrystallites. Sudden drops in intensity (I_PP), <g>, and ΔH_PP observed at 423 K for pure AgI nanocrystals. Addition of Cu in AgI increases the thermal stability of the cation sublattice as seen from the increase in the transition temperature from 423 K (undoped AgI) to 453 K. Abrupt jumps in the number of spins (N) and reciprocal susceptibility (1/χ) observed at increased phase transition temperatures in Cu-substituted AgI relative to that in undoped AgI reflects progressively strengthened local bonding configuration of γ-AgI structure induced by Cu.     

Fine structure on the excitonic emission in AgI nanoparticles embedded in silica glass

Stable photoluminescence (PL) from AgI nanoparticles embedded in silica glass was investigated at room temperature. The Z_1,2 excitonic emission of AgI exhibits fine structure with spacing of ∼0.20 eV (1610 cm⁻¹), which is assigned to the frequency of vibration in interfacial water species. The PL excitation spectrum displays two newly observed bands at 3.45 and 4.35 eV associated with AgI-silica interaction. We suggest that the excitons in AgI are localized in the AgI/SiO₂ interface region before radiative recombination.     

Solution calorimetric investigation of AgCl-AgI ionic conductor composites at 298 K: observation of metastable AgI modifications

The enthalpies of solution of pure silver halides AgCl and AgI and a composite material with molar composition 0.5 AgCl-0.5 AgI were measured at 298 K in a mixture of Na₂S₂O₃ (1 M) and NH₄OH (1 M). X-ray diffraction patterns showed that the composite material contained the metastable γ-AgI phase; different mechanisms for its stabilization were discussed. The phase transition enthalpies of AgI modifications and the enthalpy of formation of the composite material were deduced from the measurements. The latter could be related to a change of interfacial enthalpies.     

Optical properties of nanocrystalline silver halides

A review of quantum confinement effects in nanocrystals of silver bromide (AgBr) and silver iodide (AgI) is presented. AgBr is an indirect gap semiconductor while AgI has a direct band-to-band lowest energy transition. An examination of the low-temperature optical properties of quantum confined AgBr grown using a variety of synthetic techniques will be made. The dynamics of some of the involved excitonic processes will be measured and discussed in reference to a possible breakdown in the momentum selection rules as the nanocrystals are made smaller. Other explanations for this behavior such as impurity exclusion and surface effects will also be considered, as will the dynamics associated with the trapping of excitons at intrinsic iodide impurities in AgBr. Absorption measurements on AgI nanocrystals will be discussed and compared with the exciton photophysics in AgBr. Both AgBr and AgI display an increasing blue shift of their luminescence, arising from the recombination of excitons, as the crystallite size decreases. The luminescence intensity arising from this process increases with decreasing size in AgBr but it disappears in small crystals of AgI. This leads to the conclusion that in the latter material nonradiative decay channels are opening up as the size decreases.     

Glass formation and properties of GeTe4-Ga2Te3-AgX (X = I/Br/Cl) far infrared transmitting chalcohalide glasses

A new family of Tellurium-based glasses GeTe₄-Ga₂Te₃-AgX (X = I/Br/Cl) has been investigated and the glass-forming region was determined. Properties measurements include XRD, DTA, vis-NIR, and IR transmission spectra. The amorphous nature of the glasses has been proved by X-ray diffraction. Among the three systems, the GeTe₄-Ga₂Te₃-AgI glass system shows superior glass-forming ability and thermal stability. The maximum value of ΔT (= T_x − T_g) lies at about 110 °C for the glass composition 60GeTe₄-2-20Ga₂Te₃-20AgI, while for 60GeTe₄-20Ga₂Te₃-20AgBr and 60GeTe₄-20Ga₂Te₃-20AgCl, the ΔT values are both only 88 °C. Most of the studied glasses have a wide optical transmission window from 1.8 to 25 μm.     

Preparation, characterization and visible-light photocatalytic activity of AgI/AgCl/TiO2

In this paper, a novel composite photocatalyst AgI/AgCl/TiO₂ was prepared by ion exchange method and characterized by XRD, SEM and UV-Vis spectrometry. The as-prepared AgI/AgCl/TiO₂ composites show much higher photocatalytic activity than AgCl/TiO₂ and AgI/TiO₂ under visible-light irradiation (λ > 400 nm) in the process of methyl orange (MO) degradation. When the molar percentage of AgI to initial AgCl is 20% (sample SE-20%), the maximal degradation efficiency of MO has reached 85.8% after irradiation for 120 min. The enhancement of photocatalytic activity of the composite photocatalyst AgI/AgCl/TiO₂ will be attributed to its good absorption in the visible-light region, especially low recombination rate of the electron-hole pairs based on the photoluminescence (PL) spectra investigation of AgI/AgCl/TiO₂ and the matching band structures of AgI, AgCl and TiO₂. The detection of reactive species by radical scavengers displays that O₂⁻ and H₂O₂ are the main reactive species for the degradation of MO under visible-light irradiation. Moreover, PL analysis by using terephthalic acid (TA) as a probe molecule further reveals that OH can be negligible for the degradation of MO.     

Preparation and microwave absorption properties of loose nanoscale Fe3O4 spheres

Magnetite nanoparticles are found to assemble into randomly dispersed loose nanoscale spheres with diameters ∼300 nm in ethylene glycol in the presence of polyethylene and a small quantity of polyethyleneimine. Modern analysis methods are employed to provide structure information of the magnetic loose spheres. The ferromagnetic saturation magnetization is ∼80.0 emu g⁻¹, and the coercive force is 209 Oe. The microwave electromagnetic parameters are measured by a vector network analyzer. The synthesized loose spheres exhibit novel microwave properties compared with the conventional Fe₃O₄ nanoparticles. An additional microwave loss peak appears in the Ku band, which is attributed to the loose structure.     

Effects of carbonization temperature on microstructure and electrochemical performances of phenolic resin-based carbon spheres

Activated phenol resin-based carbon spheres (APCS) electrodes with high double layer capacitance and good rate capability were prepared from phenol resin-based spheres (PS) at different carbonization temperatures prior to KOH activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the APCS in 6 M KOH electrolyte. APCS carbonized at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the APCS in 6 M KOH aqueous solution can be as high as 282 F g⁻¹. It remains 252 F g⁻¹ as the current density increases to 1000 mA g⁻¹.     

Characterization of pulsed laser deposited chalcogenide thin layers

In this work we report on pulsed laser deposition (PLD) of chalcogenide thin films from the systems (AsSe)_100−xAgI_x and (AsSe)_100−xAg_x for sensing applications. A KrF* excimer laser (λ = 248 nm; τ_FWHM = 25 ns) was used to ablate the targets that had been prepared from the synthesised chalcogenide materials. The films were deposited in either vacuum (4 × 10⁻⁴ Pa) or argon (5 Pa) on silicon and glass substrates kept at room temperature. The basic properties of the films, including their morphology, topography, structure, and composition were characterised by complementary techniques. Investigations by X-ray diffraction (XRD) confirmed the amorphous nature of the films, as no strong diffraction reflections were found. The film composition was studied by energy dispersive X-ray (EDX) spectroscopy. The morphology of the films investigated by scanning electron microscopy (SEM), revealed a particulate-covered homogeneous surface, typical of PLD. Topographical analyses by atomic force microscopy (AFM) showed that the particulate size was slightly larger in Ar than in vacuum. The uniform surface areas were rather smooth, with root mean square (rms) roughness increasing up to several nanometers with the AgI or Ag doping. Based upon the results from the comprehensive investigation of the basic properties of the chalcogenide films prepared by PLD and their dependence on the process parameters, samples with appropriate sorption properties can be selected for possible applications in cantilever gas sensors.     

Effect of precursor preoxidation on the structure of phenolic resin-based activated carbon spheres

Activated carbon spheres with 3D hierarchical porous structure were prepared from phenol-formaldehyde resins with oxidation treatment in air and physical activation in an inert atmosphere. Based on the results of thermogravimetric analysis, infrared spectrometer (IR), scanning electron microscopy (SEM) and nitrogen adsorption/desorption, the effect of preoxidation on the morphology and structure of activated carbon spheres was investigated. The results show that decomposition and crosslinking reactions occur during the preoxidation and the structural changes of precursor generated by the preoxidation lead to differences in the pore structure of activated carbon spheres. The carbon spheres exhibit the unique 3D hierarchical porous structure, high specific surface area of 1897 m²/g and high pore volume of 2.22 cm³/g.     

Synthesis and optimum luminescence of monodispersed spheres for BaWO4-based green phosphors with doping of Tb

Monodispersed spheres for Tb³⁺-doped BaWO₄ (BWO:Tb) phosphors were prepared by a hydrothermal method. X-ray diffraction (XRD) and field-emission scanning electron microscopy were used to characterize the resulting samples. Emission and excitation spectra were studied using xenon excited spectroscopic experiments at room temperature. Because 12 at% BWO:Tb phosphor exhibits intensive green emission under 254 nm excitation in comparison with the commercial green fluorescent lamp phosphor (LaPO₄:Ce,Tb), it is considered to be a new promising green phosphor for fluorescent lamps application.     

Magnetic composites based on hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides

Materials containing hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides were obtained from a chemical precursor prepared by admixing chitosan and iron and aluminum hydroxides. The oxides were first characterized with scanning electron microscopy, X-ray diffraction, and Mössbauer spectroscopy. Scanning electron microscopy micrographs showed the size distribution of the resulting spheres to be highly homogeneous. The occurrence of nano-composites containing aluminum oxides and iron oxides was confirmed from powder X-ray diffraction patterns; except for the sample with no aluminum, the superparamagnetic relaxation due to iron oxide particles were observed from Mössbauer spectra obtained at 298 and 110 K; the onset six line-spectrum collected at 20 K indicates a magnetic ordering related to the blocking relaxation effect for significant portion of small spheres in the sample with a molar ratio Al:Fe of 2:1.     

Anomalous ionic conduction in AgBrAgI mixed crystals and multiphase systems

Ionic conductivity, σ, of the AgBrAgI system has been studied as a function of composition and temperature. The maximum conductivity of $\text{3 × 10}{}^{-4}\text{Ω}{}^{\mathrm{?1}}\text{cm}{}^{\mathrm{?1}}$ at 25°C is obtained for a AgI-20 mole% AgBr two-phase mixture which is $\text{\$}\text{?}$3 orders of magnitude larger than that predicted by the classical theories of Lord Rayleigh and Maxwell. On the other hand, the substitution of so-called homovalent ions, e.g. Br^? in AgI and I^? in AgBr one phase solid solutions leads to anomalously large increase in the ionic conductivity that cannot be explained in terms of the charge compensation (doping) mechanism, and is attributed to purely elastic displacements (lattice distortion) due to the very “wrong” size of the substituted ions. A quadratic dependence of conductivity on the concentration of substituents is substantiated. An important consequence of the latter anomaly is that AgBr + 30 mole% AgI exhibits σ $\text{\$}\text{?}$7 Ω^?1 cm^?1 at 380°C which is $\text{\$}\text{?}$170% higher than that of α-AgI, the best known superionic conductor, at its melting point (557°C).     

Facile synthesis and characterization of glass/cobalt core/shell composite spheres with tunable shell morphologies

Polyhedral cobalt microcrystals assembled on hollow glass spheres are successfully synthesized by a facile and easy-control hydrothermal reduction process, and thus hierarchical glass/cobalt core/shell composite hollow spheres are fabricated with low-density (0.96 g cm⁻³). By properly tuning the process conditions and the component of the reaction solution, a series of composite spheres with gradient in morphologies of the shell layer can be prepared. Based on a series of contrast experiments, the probable formation mechanism of the core/shell hierarchical structures is proposed. The magnetic properties of the products are studied and the results demonstrate that the composite spheres present ferromagnetic properties related to the special shell morphologies. The composite hollow spheres thus obtained may have some promising applications in the fields of low-density magnetic materials, conduction, and catalysis, etc. This work provides an additional strategy to prepared core/shell composite spheres with tailored shell morphology and magnetic properties.     

Determination of high-purity polarization state of X-rays

We report on the measurement of the highest purity of polarization of X-rays to date. The measurements are performed by combining a brilliant undulator source with an X-ray polarimeter. The polarimeter is composed of a polarizer and an analyzer, each based on four reflections at channel-cut crystals with a Bragg angle very close to 45°. Experiments were performed at three different X-ray energies, using different Bragg reflections: Si(400) at 6457.0 eV, Si(444) at 11,183.8 eV, and Si(800) at 12,914.0 eV. At 6 keV a polarization purity of 1.5 × 10^− 9 is achieved. This is an improvement by more than two orders of magnitude as compared to previously reported values. The polarization purity decreases slightly for shorter X-ray wavelengths. The sensitivity of the polarimeter is discussed with respect to a proposed experiment that aims at the detection of the birefringence of vacuum induced by super-strong laser fields.     

Synthesis and characterization of the efficient visible-light-induced photocatalyst AgBr and its photodegradation activity

AgBr photocatalysts were prepared with the 1-hexadecyl-3-methylimidazolium bromide ([C₁₆mim]Br) reactable ionic liquid at different temperatures by one-step hydrothermal method, in which the ionic liquid acted as a precursor and a template to control the size of AgBr crystal. The photocatalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), the Brunauer–Emmett–Teller (BET) surface area and diffuse reflectance spectroscopy (DRS). Methyl orange (MO) dye was chosen as a model pollutant to investigate the photocatalytic activity and the stability of the samples under visible light irradiation. The results indicated that the AgBr photocatalysts showed high efficiency in the degradation of MO under visible light irradiation. The kinetic property of the reaction followed the first-order reaction model. During the photocatalytic degradation reaction, AgBr was transformed to the Ag⁰/AgBr composite. However, the photocatalytic efficiency was still high and the photocatalytic activity was stable. The possible photocatalytic mechanism of the photocatalysts was also eventually proposed.     

Electron beam induced green luminescence and degradation study of CaS:Ce nanocrystalline phosphors for FED applications

Green luminescence and degradation of Ce³⁺ doped CaS nanocrystalline phosphors were studied with a 2 keV, 10 μA electron beam in an O₂ environment. The nanophosphors were synthesized by the co-precipitation method. The samples were characterized using X-ray diffraction, Transmission electron microscopy, Scanning electron microscopy/electron dispersive X-ray spectroscopy and Photoluminescence (PL) spectroscopy. Cubic CaS with an average particle size of 42 ± 2 nm was obtained. PL emission was observed at 507 nm and a shoulder at 560 nm with an excitation wavelength of 460 nm. Auger electron spectroscopy and Cathodoluminescence (CL) were used to monitor the changes in the surface composition of the CaS:Ce³⁺ nanocrystalline phosphors during electron bombardment in an O₂ environment. The effect of different oxygen pressures ranging from 1 × 10⁻⁸ to 1 × 10⁻⁶ Torr on the CL intensity was also investigated. A CaSO₄ layer was observed on the surface after the electron beam degradation. The CL intensity was found to decrease up to 30% of its original intensity at 1 × 10⁻⁶ Torr oxygen pressure after an electron dose of 50 C/cm². The formation of oxygen defects during electron bombardment may also be responsible for the decrease in CL intensity.     

Phase formation by ion beam mixing in the Ti/Si multilayer system

The irradiation effect of 350 MeV Au⁺ ions on Ti/Si multilayers has been studied using Rutherford backscattering spectroscopy, X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXRD). Intermixing effects have been studied as a function of fluences of 0.46 × 10¹⁴, 1.82 × 10¹⁴ and 4.62 × 10¹⁴ cm⁻². Rutherford backscattering spectra (RBS) confirm mixing at the interface. X-ray reflectivity patterns show damage at the interfaces with the absence of a continuous fringe pattern at high fluence doses in comparison to the pristine interface. Mixing leads to titanium di-silicide (TiSi₂) phase formation as a shown by grazing incidence X-ray diffraction patterns. The observed intermixing is attributed to energy deposited by the incident ions in the electronic system of the target. Swift heavy ion irradiation induced intermixing increases with fluence.