Ionic Conductivity and Structure of Glasses Synthesized by Mechanical-Milling Methods in the x[Na2S]-(100 - x) [0.5GeS2-0.5Ga2S3] System.

Chalcogenide glasses in the Na2S-GeS2-Ga2S3 pseudoternary system were synthesized using a combination route of melt-quenching and mechanical-milling methods. First, a glass rich in germanium (90GeS2-10Ga2S3) is synthesized by melt-quenching synthesis in a silica tube sealed under vacuum. This glass is used as a precursor for the second step of mechanochemistry to explore the Na2S-GeS2-Ga2S3 pseudoternary system. By using this synthesis route, the glass-forming ability is improved as the vitreous domain is enlarged, especially for Na- and Ga-rich compositions. The as-obtained amorphous powders are characterized by Raman spectroscopy, differential scanning calorimetry, X-ray total scattering, and pair distribution function (PDF) analysis. The evolution of the Raman features observed is reproduced using density functional theory calculations. Impedance spectroscopy was performed to determine the conductivity of the new glasses. The addition of germanium sulfide to the Na2S-Ga2S3 pseudobinary system enables one to increase the conductivity by 1 order of magnitude. The highest room-temperature ionic conductivity, as measured by impedance spectroscopy, is 1.8 × 10-5 S·cm-1.

Mechanochemical Synthesis and Study of the Local Structure of NaGaS2 Glass and Glass-Ceramics.

NaGaS2 is a newly discovered compound that has already shown great promise for a variety of applications because of its layered structure and ion exchange properties. In this work, crystalline NaGaS2 has been synthesized by an alternative method to what has been previously published, namely, by mechanochemistry, either by a direct one-step process or by a two-step process. In the one-step process, crystalline NaGaS2 is directly formed by milling sodium sulfide Na2S and gallium(III) sulfide Ga2S3. However, an amorphous material is present in majority together with the crystalline phase. In the two-step process, amorphous NaGaS2 is first obtained by mechanical milling and then heated above its glass transition temperature to obtain a glass-ceramic mainly composed of crystalline NaGaS2. For the two-step process, changes of the local atomic-level structure in amorphous NaGaS2 and after crystallization were analyzed by high-field solid-state nuclear magnetic resonance (NMR) spectroscopy as well as by X-ray total scattering and pair distribution function (PDF) analysis. Based on quantitative analysis on the 23Na NMR spectra, modifying the annealing treatment can promote the formation of the crystalline phase up to a molar fraction of 83.8%.

Glassy GaS: transparent and unusually rigid thin films for visible to mid-IR memory applications.

Phase-change materials based on tellurides are widely used for optical storage (DVD and Blu-ray disks), non-volatile random access memories and for development of neuromorphic computing. Narrow-gap tellurides are intrinsically limited in the telecom spectral window, where materials having a wider gap are needed. Here we show that gallium sulfide GaS thin films prepared by pulsed laser deposition reveal good transparency from the visible to the mid-IR spectral range with optical gap Eg = 2.34 eV, high refractive index nR = 2.50 over the 0.8 ≤ λ ≤ 2.5 µm range and, unlike canonical chalcogenide glasses, the absence of photo-structural transformations with a laser-induced peak power density damage threshold above 1.4 TW cm-2 at 780 nm. The origin of the excellent damage threshold under a high-power laser and UV light irradiation resides in the rigid tetrahedral structure of vitreous GaS studied by high-energy X-ray diffraction and Raman spectroscopy and supported by first-principles simulations. The average local coordination number appears to be m = 3.44, well above the optimal connectivity, 2.4 ≤ m ≤ 2.7, and the total volume of microscopic voids and cavities is 34.4%, that is, lower than for the vast majority of binary sulfide glasses. The glass-crystal phase transition in gallium sulfide thin films may be accompanied by a drastic change in the nonlinear optical properties, opening up a new dimension for memory applications in the visible to mid-IR spectral ranges.

Experimental and Theoretical Insights into the Structure of Tellurium Chloride Glasses.

The structure of the binary chalcohalide glasses Te1- xCl x (0.35 ≤ x ≤ 0.65) is considered by combining experimental and theoretical results. The structural network properties are influenced by a competition between ionic and covalent bonding in such glasses. At first, a focus is placed on the detailed information available by using the complementary high-energy X-ray and the neutron diffractions in both the reciprocal and real spaces. The main characteristic suggested by the structure factors S( Q) concerns the presence of three length scales in the intermediate range order. The total correlation function T( r) lets us also suppose that the structure of these glasses is more complicated than Te-chain fragments with terminal Cl as demonstrated in crystalline Te3Cl2. Molecular dynamics simulations were subsequently performed on Te3Cl2 and Te2Cl3, and coupled with the experimental data, a highly reticulated network of chalcogen atoms, with a fair amount of chlorine atoms bonded in a bridging mode, is proposed. The simulations clearly lead to a glass description that differs markedly from the simple structural model based on only Te atom chains and terminal Cl atoms. Solid-state NMR experiments and NMR parameters calculations allowed validation of the presence of Te highly coordinated with chlorine in these glasses.

Direct laser writing of buried waveguide in As2S3 glass using a helical sample translation.

We report the fabrication and the characterization of buried waveguide in As(2)S(3) glass. It is well known that the interaction of femtosecond pulses with this material at high laser repetition rates results in a mainly negative refractive index variation, due to heat accumulation effect. However, we show here that a helical translation of the sample parallel to the laser beam, allows the inscription of a core of positive refractive variation, with full control over its magnitude and diameter. An example demonstrating the high symmetry of the guided mode is given.

Free carrier accumulation during direct laser writing in chalcogenide glass by light filamentation.

We present direct laser writing of channels in chalcogenide glass under light filamentation conditions. Because of the intrinsic properties of the filament, the positive refractive index profile of the channels exhibits a cylindrical symmetry of high quality. The role of the repetition rate is also investigated. It is shown that if the time separation between pulses is shorter than the lifetime of the plasma, the free carriers accumulate and induce a larger variation of the refractive index.

Sensitive and selective detection of OH radicals using Faraday rotation spectroscopy at 2.8 µm.

We report on the development of a Faraday rotation spectroscopy (FRS) instrument using a DFB diode laser operating at 2.8 µm for the hydroxyl (OH) free radical detection. The highest absorption line intensity and the largest gJ value make the Q (1.5) double lines of the 2Π3/2 state (υ = 1 ← 0) at 2.8 µm clearly the best choice for sensitive detection in the infrared region by FRS. The prototype instrument shows shot-noise dominated performance and, with an active optical pathlength of only 25 cm and a lock-in time constant of 100 ms, achieves a 1σ detection limit of 8.2 × 10(8) OH radicals/cm3.

Lung cell fiber evanescent wave spectroscopic biosensing of inhalation health hazards.

Health risks associated with the inhalation of biological materials have been a topic of great concern; however, there are no rapid and automatable methods available to evaluate the potential health impact of inhaled materials. Here we describe a novel approach to evaluate the potential toxic effects of materials evaluated through cell-based spectroscopic analysis. Anchorage-dependent cells are grown on the surface of optical fibers transparent to infrared light. The probe system is composed of a single chalcogenide fiber (composed of Te, As, and Se) acting as both the sensor and transmission line for infrared optical signals. The cells are exposed to potential toxins and alterations of cellular composition are monitored through their impact on cellular spectral features. The signal is collected via evanescent wave absorption along the tapered sensing zone of the fiber through spectral changes between 3,000 and 600 cm(-1) (3,333-16,666 nm). Cell physiology, composition, and function are non-invasively tracked through monitoring infrared light absorption by the cell layer. This approach is demonstrated with an immortalized lung cell culture (A549, human lung carcinoma epithelia) in response to a variety of inhalation hazards including gliotoxin (a fungal metabolite), etoposide (a genotoxin), and methyl methansesulfonate (MMS, an alkylating agent). Gliotoxin impacts cell metabolism, etoposide impacts nucleic acids and the cell cycle, and MMS impacts nucleic acids and induces an immune response. This spectroscopic method is sensitive, non-invasive, and provides information on a wide range of cellular damage and response mechanisms and could prove useful for cell response screening of pharmaceuticals or for toxicological evaluations.

Biologically inspired sensing: infrared spectroscopic analysis of cell responses to an inhalation health hazard.

This work describes the development of a biologically based sensing technique to quantify chemical agents that pose inhalation health hazards. The approach utilizes cultured epithelial cells (A549 human type II pneumocytes) of the lung, exposed to potential toxins and monitored through the noninvasive means of infrared spectroscopy to quantify changes to cell physiology and function. Cell response to Streptolysin O, a cholesterol-binding cytolysin, is investigated here. Infrared spectra display changes in cell physiology indicative of membrane damage, altered proteins, and some nucleic acid damage. Methods to improve cell adhesion through modification of support surface properties are detailed. This spectroscopic approach not only provides a robust means to detect potential toxins but also provides information on modes of damage and mechanisms of cellular response.

Evaluation of toxic agent effects on lung cells by fiber evanescent wave spectroscopy.

Biochemical changes in living cells are detected using a fiber probe system composed of a single chalcogenide fiber acting as both the sensor and transmission line for infrared optical signals. The signal is collected via evanescent wave absorption along the tapered sensing zone of the fiber. We spectroscopically monitored the effects of the surfactant Triton X-100, which serves as a toxic agent simulant on a transformed human lung carcinoma type II epithelial cell line (A549). We observe spectral changes between 2800-3000 cm(-1) in four absorptions bands, which are assigned to hydrocarbon vibrations of methylene and methyl groups in membrane lipids. Comparison of fiber and transmission spectra shows that the present technique allows one to locally probe the cell plasma membrane in the lipid spectral region. These optical responses are correlated with cellular metabolic activity measurements and LDH (lactate dehydrogenase) release assays that indicate a loss of cellular function and membrane integrity as would be expected in response to the membrane solubilizing Triton. The spectroscopic technique shows a significantly greater detection resolution in time and concentration.