Forming glasses from Se and Te

High-Conductivity Chalcogenide Glasses in Ag-Ga2Te3-SnTe Systems and Their Suitability as Thermoelectric Materials

A novel high-conductivity Ag_x[(Ga₂Te₃)₃₄(SnTe)₆₆]_100-x tellurium-based glassy system was fabricated via melt spinning with the glass formation area in the range of x = 0-15 mol %. A bulk Ag₁₀[(Ga₂Te₃)₃₄(SnTe)₆₆]₉₀ glass (A10) was obtained via spark plasma sintering at 450 K using a 5 min dwell time and 400 MPa pressure. The fabricated A10 glass exhibited higher room-temperature conductivity (σ_300 K = 46 S m^-1), larger glass transition temperature (T_g = 482 K), and ultralower thermal conductivity (∼0.19 W m^-1 K^-1) compared to those of previously reported Cu-Ge-Te, Cu-As-Te, Cu-Ge-As-Te, and Cu-As-Se-Te glassy systems with the approximate doping concentrations of 5-20%, demonstrating that this distinctive Ag-Ga₂Te₃-SnTe system is interesting materials for thermoelectric applications. The high-conductivity Ag-Ga₂Te₃-SnTe glassy system will extend investigations into similar glassy semiconductors and also can be used for preparing glass ceramics with potential applications in other fields.

High Refractive Index GRIN Lens for IR Optics

Infrared gradient refractive index (GRIN) material lenses have attracted much attention due to their continuously varying refractive index as a function of spatial coordinates in the medium. Herein, a glass accumulation thermal diffusion method was used to fabricate a high refractive index GRIN lens. Six Ge_17.2As_17.2Se_xTe_(65-x) (x = 10.5-16) glasses with good thermal stability and high refractive index (n_{@10 μm} > 3.1) were selected for thermal diffusion. The refractive index span (∆n) of 0.12 was achieved in this GRIN lens. After thermal diffusion, the lens still had good transmittance (45%) in the range of 8-12 μm. Thermal imaging confirmed that this lens can be molded into the designed shape. The refractive index profile was indirectly characterized by the structure and composition changes. The structure and composition variation became linear with the increase in temperature from 260 °C to 270 °C for 12 h, indicating that the refractive index changed linearly along the axis. The GRIN lens with a high refractive index could find applications in infrared optical systems and infrared lenses for thermal imaging.

Effect of Se on Structure and Electrical Properties of Ge-As-Te Glass

The Ge-As-Te glass has a wide infrared transmission window range of 3–18 μm, but its crystallization tendency is severe due to the metallicity of the Te atom, which limits its development in the mid- and far-infrared fields. In this work, the Se element was introduced to stabilize the Ge-As-Te glass. Some glasses with ΔT ≥ 150 °C have excellent thermal stability, indicating these glasses can be prepared in large sizes for industrialization. The Ge-As-Se-Te (GAST) glasses still have a wide infrared transmission window (3–18 μm) and a high linear refractive index (3.2–3.6), indicating that the GAST glass is an ideal material for infrared optics. Raman spectra show that the main structural units for GAST glass are [GeTe₄] tetrahedra, [AsTe₃] pyramids, and [GeTe₄Se_4−x] tetrahedra, and with the decrease of Te content (≤50 mol%), As-As and Ge-Ge homopolar bonds appear in the glass due to the non-stoichiometric ratio. The conductivity σ of the studied GAST glasses decreases with the decrease of the Te content. The highest σ value of 1.55 × 10⁻⁵ S/cm is obtained in the glass with a high Te content. The activation energy E_a of the glass increases with the decrease of the Te content, indicating that the glass with a high Te content is more sensitive to temperature. This work provides a foundation for widening the application of GAST glass materials in the field of infrared optics.

Structural and optical properties of amorphous Si-Ge-Te thin films prepared by combinatorial sputtering

The lack of order in amorphous chalcogenides offers them novel properties but also adds increased challenges in the discovery and design of advanced functional materials. The amorphous compositions in the Si–Ge–Te system are of interest for many applications such as optical data storage, optical sensors and Ovonic threshold switches. But an extended exploration of this system is still missing. In this study, magnetron co-sputtering is used for the combinatorial synthesis of thin film libraries, outside the glass formation domain. Compositional, structural and optical properties are investigated and discussed in the framework of topological constraint theory. The materials in the library are classified as stressed-rigid amorphous networks. The bandgap is heavily influenced by the Te content while the near-IR refractive index dependence on Ge concentration shows a minimum, which could be exploited in applications. A transition from a disordered to a more ordered amorphous network at 60 at% Te, is observed. The thermal stability study shows that the formed crystalline phases are dictated by the concentration of Ge and Te. New amorphous compositions in the Si–Ge–Te system were found and their properties explored, thus enabling an informed and rapid material selection and design for applications.

Engineering of TeO2-ZnO-BaO-Based Glasses for Mid-Infrared Transmitting Optics

In this paper, the glass systems, TeO2-ZnO-BaO (TZB), TeO2-ZnO-BaO-Nb2O5 (TZB-Nb) and TeO2-ZnO-BaO-MoO3 (TZB-Mo), were fabricated by the traditional melt-quench protocol for use as mid-infrared (mid-IR) transmitting optical material. The effect of Nb2O5 and MoO3 on the key glass material properties was studied through various techniques. From the Raman analysis, it was found that the structural modification was clear with the addition of both Nb2O5 and MoO3 in the TZB system. The transmittance of studied glasses was measured and found that the optical window covered a region from 0.4 to 6 μm. The larger linear refractive index was obtained for the Nb2O5-doped TZB glass system than that of other studied systems. High glass transition temperature, low thermo-mechanical coefficient and high Knoop hardness were noticed in the Nb2O5-doped TZB glass system due to the increase in cross-linking density and rigidity in the tellurite network. The results suggest that the Nb2O5-doped TZB optical glasses could be a promising material for mid-infrared transmitting optics.

Combining Ultrafast Calorimetry and Electron Microscopy: Reversible Phase Transformations in SeTeAs Alloys

Reversible amorphous-crystalline phase transitions are studied using complementary ultrafast differential scanning calorimetry and transmission electron microscopy techniques, which together allow a wealth of thermal and structural properties to be determined. The SeTe(As) system is investigated because these chalcogenide based materials have favorable properties as a phase-change memory material and in optical systems. Using calorimetry, we find that the addition of 10 at. % As to SeTe alloys strongly increases their glass forming ability, increasing both glass transition and crystallization temperatures while reducing critical quench rate. Ex situ investigation of Se _x Te_90-x As₁₀ using electron microscopy and elemental mapping reveals a two-phase lamellar segregation mechanism, where a trigonal SeTe-phase and an amorphous As-rich phase are formed. These findings demonstrate the power of combining thermal and structural analysis techniques.

Tracing Tellurium and Its Nanostructures in Biology

Tellurium (Te) is a semimetal rare element in nature. Together with oxygen, sulfur (S), and selenium (Se), Te is considered a member of chalcogen group. Over recent decades, Te applications continued to emerge in different fields including metallurgy, glass industry, electronics, and applied chemical industries. Along these lines, Te has recently attracted research attention in various fields. Though Te exists in biologic organisms such as microbes, yeast, and human body, its importance and role and some of its potential implications have long been ignored. Some promising applications of Te using its inorganic and organic derivatives including novel Te nanostructures are being introduced. Before discovery and straightforward availability of antibiotics, Te had considered and had been used as an antibacterial element. Antilishmaniasis, antiinflammatory, antiatherosclerotic, and immuno-modulating properties of Te have been described for many years, while the innovative applications of Te have started to emerge along with nanotechnological advances over the recent years. Te quantum dots (QDs) and related nanostructures have proposed novel applications in the biological detection systems such as biosensors. In addition, Te nanostructures are used in labeling, imaging, and targeted drug delivery systems and are tested for antibacterial or antifungal properties. In addition, Te nanoparticles show novel lipid-lowering, antioxidant, and free radical scavenging properties. This review presents an overview on the novel forms of Te, their potential applications, as well as related toxicity profiles.

Thickness and optical constants calculation for chalcogenide-alkali metal Se80Te8(NaCl)12 thin film

Chalcogenide-alkali metal semiconducting thin films of four different thicknesses of Se₈₀Te₈(NaCl)₁₂ are deposited from bulk by thermal evaporation technique. The crystallinity of the film improves with increasing of thickness as indicated by the recorded X-ray diffraction patterns. The transmission and reflection spectra are measured in the wavelength range of the incident photons from 250 to 2500nm. The thickness and optical constants of the films are calculated based on Swanepeol method using the interference patterns appeared in the transmission spectra. It is found that the films have absorption mechanism which is an indirect allowed transition. The effect of the film thickness on the refractive index and the high-frequency dielectric constant are studied. With increasing the film thickness, both the absorption coefficient and high-frequency dielectric constant increase while the single-oscillator energy, optical band gap and extinction coefficient decrease.

Study of Ga incorporation in glassy arsenic selenides by high-resolution XPS and EXAFS

Effect of Ga addition on the structure of vitreous As2Se3 is studied using high-resolution X-ray photoelectron spectroscopy and extended X-ray absorption fine structure techniques. The "8-N" rule is shown to be violated for Ga atoms and, possibly, for certain number of As atoms. On the contrary, Se keeps its 2-fold coordination according to "8-N" rule in the amorphous phase throughout all the compositions. Crystalline inclusions appear in the amorphous structure of the investigated glasses at Ga concentrations greater than 3 at. %. These inclusions are presumably associated with Ga2Se3 crystallites and transition phases/defects formed at the boundaries of these crystallites and host amorphous matrix. The existence of Ga-As and Se-Se bonds in the samples with higher Ga content is supported by present studies.

Impact of Te on the structure and 77Se NMR spectra of Se-rich Ge–Te–Se glasses: a combined experimental and computational investigation

⁷⁷Se NMR experiments and Car–Parrinello simulations have been done on glasses in the Ge–Te–Se system to understand their structure focusing on the effect of tellurium atoms.

Shaping of looped miniaturized chalcogenide fiber sensing heads for mid-infrared sensing

Chalcogenide glass fibers are promising photonic tools to develop Fiber Evanescent Wave Spectroscopy (FEWS) optical sensors working in the mid-infrared region. Numerous pioneering works have already been carried out showing their efficiency, especially for bio-medical applications. Nevertheless, this technology remains confined to academic studies at the laboratory scale because chalcogenide glass fibers are difficult to shape to produce reliable, sensitive and compact sensors. In this paper, a new method for designing and fabricating a compact and robust sensing head with a selenide glass fiber is described. Compact looped sensing heads with diameter equal to 2 mm were thus shaped. This represents an outstanding achievement considering the brittleness of such uncoated fibers. FEWS experiments were implemented using alcoholic solutions as target samples showing that the sensitivity is higher than with the routinely used classical fiber. It is also shown that the best compromise in term of sensitivity is to fabricate a sensing head including two full loops. From a mechanical point of view, the breaking loads of the loop shaped head are also much higher than with classical fiber. Finally, this achievement paves the way for the use of mid-infrared technology during in situ and even in vivo medical operations. Indeed, is is now possible to slide a chalcogenide glass fiber in the operating channel of a standard 2.8 mm diameter catheter.

From selenium- to tellurium-based glass optical fibers for infrared spectroscopies

Chalcogenide glasses are based on sulfur, selenium and tellurium elements, and have been studied for several decades regarding different applications. Among them, selenide glasses exhibit excellent infrared transmission in the 1 to 15 µm region. Due to their good thermo-mechanical properties, these glasses could be easily shaped into optical devices such as lenses and optical fibers. During the past decade of research, selenide glass fibers have been proved to be suitable for infrared sensing in an original spectroscopic method named Fiber Evanescent Wave Spectroscopy (FEWS). FEWS has provided very nice and promising results, for example for medical diagnosis. Then, some sophisticated fibers, also based on selenide glasses, were developed: rare-earth doped fibers and microstructured fibers. In parallel, the study of telluride glasses, which can have transmission up to 28 µm due to its atom heaviness, has been intensified thanks to the DARWIN mission led by the European Space Agency (ESA). The development of telluride glass fiber enables a successful observation of CO₂ absorption band located around 15 µm. In this paper we review recent results obtained in the Glass and Ceramics Laboratory at Rennes on the development of selenide to telluride glass optical fibers, and their use for spectroscopy from the mid to the far infrared ranges.

Tellurium: an element with great biological potency and potential

Tellurium has long appeared as a nearly 'forgotten' element in Biology, with most studies focusing on tellurite, tellurate and a handful of organic tellurides. During the last decade, several discoveries have fuelled a renewed interest in this element. Bioincorporation of telluromethionine provides a new approach to add heavy atoms to selected sites in proteins. Cadmium telluride (CdTe) nanoparticles are fluorescent and may be used as quantum dots in imaging and diagnosis. The antibiotic properties of tellurite, long known yet almost forgotten, have attracted renewed interest, especially since the biochemical mechanisms of tellurium cytotoxicity are beginning to emerge. The close chemical relationship between tellurium and sulfur also transcends into in vitro and in vivo situations and provides new impetus for the development of enzyme inhibitors and redox modulators, some of which may be of interest in the field of antibiotics and anticancer drug design.