Extended aging of Ge-Se glasses below the glass transition temperature.

Germanium selenide glasses of compositions spanning the whole glass-formation range are aged at room temperature for up to 20 years. A prominent enthalpy relaxation process is observed in all glasses, and its structural origin is analyzed by Raman spectroscopy. The structural relaxation is manifested in the Raman spectra as a decrease in the ratio of edge- to corner-sharing GeSe4/2 tetrahedral units. This structural evolution can be explained in terms of configurational entropy and density changes. Changes in Raman features and enthalpy follow an identical stretched exponential relaxation function characteristic of aging in glasses. The compositional dependence of enthalpy relaxation after 20 years is in agreement with kinetic considerations based on the glass transition temperature of each glass. The relaxation behavior and heat capacity curves are consistent with standard glass relaxation models for all compositions. These results indicate that the non-reversing enthalpy obtained by modulated differential scanning calorimetry (MDSC), which suggests the existence of non-aging glasses, is not a reliable measure of the ability of a glass to relax. Instead, it is suggested that an interpretation of MDSC data in terms of complex heat capacity provides a more complete and reliable assessment of the relaxation properties of glasses.

Structural and chemical homogeneity of chalcogenide glass prepared by melt-rocking.

The chemical and structural homogeneity of selenide glasses produced by mechanical homogenization of the melt in a rocking furnace is investigated by Raman and Energy Dispersive Spectroscopy (EDS). Both techniques demonstrate that the glass is macroscopically homogeneous along the entire length of a 6 cm rod. EDS imaging performed over four orders of magnitude in scale further confirms that the glass is homogeneous down to the sub-micron scale. An estimate of the diffusion coefficient from experimental viscosity data shows that the diffusion length is far larger than the resolution of EDS and therefore confirms that the glass is homogeneous at any length scale. In order to investigate a systematic mismatch in physical properties reported in the literature for glasses produced by extended static homogenization, two germanium selenide samples are produced under the same conditions except for the homogenization step: one in a rocking furnace for 10 h and the other in a static furnace for 192 h. No difference in physical properties is found between the two glasses. The properties of an ultra-high purity glass are also found to be identical. The origin of the systematic deviation reported in the literature for germanium selenide glasses is therefore still unknown, but the present results demonstrate that homogeneity or dryness does not have a significant contribution in contrast to previous suggestions. The implications of glass homogeneity for technological applications and industrial production are discussed.

7 to 8 µm emission from Sm3+ doped selenide fibers.

We report on the observation of the long wave-infrared (LWIR) emission centered at 7.3 µm of Sm3+ doped chalcogenide fibers. The chemical composition of the selenide glass host matrix (Ga5Ge20Sb10Se65) enables the drawing of 500 ppm and 1000 ppm Sm3+ doped fibers. By means of conventional glass elaboration methods, these Sm3+ doped fibered materials exhibit a significant emission band from 6.5 to 8.5 µm with a maximum emission around 7.3 µm whether they are excited at 1.45 µm or at 2.05 µm. Absorption spectra, Judd-Ofelt analysis, NIR, MWIR and LWIR luminescence spectra are presented and discussed.

8 µm luminescence from a Tb3+ GaGeSbSe fiber.

In this Letter, we report for the first time, to the best of our knowledge, on an emission at 8 µm from Tb3+-doped Ga5Ge20Sb10Se65 chalcogenide fibers with doping levels at 1000 ppm and 500 ppm. These fibers were drawn following conventional melt-quenching methods and pumped at 2.05 µm using a Tm3+: YAG laser. The spectroscopic properties of the emitting F47 manifold are investigated to rule out any parasitic signal mimicking the real Tb3+ 8 µm emission. Time-resolved spectroscopic experiments are presented to build a comprehensive study of this 8 µm fluorescence recorded with a clear signal-to-noise ratio.

Effect of the Processing Route on the Thermoelectric Performance of Nanostructured CuPb18SbTe20.

The quaternary AgPb18SbTe20 compound (abbreviated as LAST) is a prominent thermoelectric material with good performance. Endotaxially embedded nanoscale Ag-rich precipitates contribute significantly to decreased lattice thermal conductivity (κlatt) in LAST alloys. In this work, Ag in LAST alloys was completely replaced by the more economically available Cu. Herein, we conscientiously investigated the different routes of synthesizing CuPb18SbTe20 after vacuum-sealed-tube melt processing, including (i) slow cooling of the melt, (ii) quenching and annealing, and consolidation by (iii) spark plasma sintering (SPS) and also (iv) by the state-of-the-art flash SPS. Irrespective of the method of synthesis, the electrical (σ) and thermal (κtot) conductivities of the CuPb18SbTe20 samples were akin to those of LAST alloys. Both the flash-SPSed and slow-cooled CuPb18SbTe20 samples with nanoscale dislocations and Cu-rich nanoprecipitates exhibited an ultralow κlatt ∼ 0.58 W/m·K at 723 K, comparable with that of its Ag counterpart, regardless of the differences in the size of the precipitates, type of precipitate-matrix interfaces, and other nanoscopic architectures. The sample processed by flash SPS manifested higher figure of merit ( zT ∼ 0.9 at 723 K) because of better optimization and a trade-off between the transport properties by decreasing the carrier concentration and κlatt without degrading the carrier mobility. In spite of their comparable σ and κtot, zT of the Cu samples is low compared to that of the Ag samples because of their contrasting thermopower values. First-principles calculations attribute this variation in the Seebeck coefficient to dwindling of the energy gap (from 0.1 to 0.02 eV) between the valence and conduction bands in MPb18SbTe20 (M = Cu or Ag) when Cu replaces Ag.

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

Selenium-rich Ge-Te-Se glasses have been synthesized along the GeSe4-GeTe4 pseudo-composition line and acquired by (77)Se Hahn echo magic-angle spinning NMR. The comparison with the GeSe4 spectrum shows a drastic modification of the typical double-resonance lineshape even at low Te concentrations (<10%). In order to rationalize this feature and to understand the effect of Te on the structure of our glasses, first-principles molecular dynamics simulations and gauge including projector augmented wave NMR parameter calculations have been performed. The distribution of the tellurium atoms in the selenium phase was shown to be mainly responsible for the (77)Se lineshape changes. Another possible factor related to the perturbation of the δiso value due to Te proximity appears to be much more limited in the bulk, while the results obtained using molecular models suggest shifts of several hundreds of ppm.

Te-based glass fiber for far-infrared biochemical sensing up to 16 µm.

Chalcogenide glass fibers are very suitable to carry out mid-infrared spectroscopy by Fiber Evanescent Wave Spectroscopy (FEWS). Nowadays, selenide glasses are used for FEWS, but the reachable domain is limited in the infrared to typically 12 µm. Te-rich glasses, due to their heavy atomic weight, are better for far-infrared sensing but they crystallize easily and until now that was difficult to prepare operational optical fibers from such glasses. In this work, Te-Ge-AgI highly purified glasses have been prepared and successfully drawn into optical fiber. The minimum of attenuation is 3 dB/m around 10 µm, which is up to now the lowest value ever measured for Te-based fiber. Overall, such fibers open the sensing window up to 16 µm against 12 µm so far. Then, for the first time, tapered telluride fibers with different diameters at the sensing zone were obtained during the fiber drawing process. Chloroform and butter were used to test the fiber infrared sensing ability, and the sensitivity has been greatly enhanced as the sensing zone fiber diameter decreases. Finally, the new protocol of telluride glass preparation allows shaping them into efficient functional fibers, opening further in the mid-infrared which is essential for chemical spectroscopy.

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.

Phase-change materials based on amorphous equichalcogenides.

Phase-change materials, demonstrating a rapid switching between two distinct states with a sharp contrast in electrical, optical or magnetic properties, are vital for modern photonic and electronic devices. To date, this effect is observed in chalcogenide compounds based on Se, Te or both, and most recently in stoichiometric Sb2S3 composition. Yet, to achieve best integrability into modern photonics and electronics, the mixed S/Se/Te phase change medium is needed, which would allow a wide tuning range for such important physical properties as vitreous phase stability, radiation and photo-sensitivity, optical gap, electrical and thermal conductivity, non-linear optical effects, as well as the possibility of structural modification at nanoscale. In this work, a thermally-induced high-to-low resistivity switching below 200 °C is demonstrated in Sb-rich equichalcogenides (containing S, Se and Te in equal proportions). The nanoscale mechanism is associated with interchange between tetrahedral and octahedral coordination of Ge and Sb atoms, substitution of Te in the nearest Ge environment by S or Se, and Sb-Ge/Sb bonds formation upon further annealing. The material can be integrated into chalcogenide-based multifunctional platforms, neuromorphic computational systems, photonic devices and sensors.

The Art of Positronics in Contemporary Nanomaterials Science: A Case Study of Sub-Nanometer Scaled Glassy Arsenoselenides.

The possibilities surrounding positronics, a versatile noninvasive tool employing annihilating positrons to probe atomic-deficient sub-nanometric imperfections in a condensed matter, are analyzed in application to glassy arsenoselenides g-AsxSe100-x (0 < x < 65), subjected to dry and wet (in 0.5% PVP water solution) nanomilling. A preliminary analysis was performed within a modified two-state simple trapping model (STM), assuming slight contributions from bound positron-electron (Ps, positronium) states. Positron trapping in g-AsxSe100-x/PVP nanocomposites was modified by an enriched population of Ps-decay sites in PVP. This was proven within a three-state STM, assuming two additive inputs in an overall trapping arising from distinct positron and Ps-related states. Formalism of x3-x2-CDA (coupling decomposition algorithm), describing the conversion of Ps-decay sites into positron traps, was applied to identify volumetric nanostructurization in wet-milled g-As-Se, with respect to dry-milled ones. Under wet nanomilling, the Ps-decay sites stabilized in inter-particle triple junctions filled with PVP replaced positron traps in dry-milled substances, the latter corresponding to multi-atomic vacancies in mostly negative environments of Se atoms. With increased Se content, these traps were agglomerated due to an abundant amount of Se-Se bonds. Three-component lifetime spectra with nanostructurally- and compositionally-tuned Ps-decay inputs and average lifetimes serve as a basis to correctly understand the specific "rainbow" effects observed in the row from pelletized PVP to wet-milled, dry-milled, and unmilled samples.

High-Energy Mechanical Milling-Driven Reamorphization in Glassy Arsenic Monoselenide: On the Path of Tailoring Special Molecular-Network Glasses.

The impact of high-energy milling on glassy arsenic monoselenide g-AsSe is studied with X-ray diffraction applied to diffuse peak-halos proper to intermediate- and extended-range ordering revealed in first and second sharp diffraction peaks (FSDP and SSDP). A straightforward interpretation of this effect is developed within the modified microcrystalline approach, treating "amorphous" halos as a superposition of the broadened Bragg diffraction reflexes from remnants of some inter-planar correlations, supplemented by the Ehrenfest diffraction reflexes from most prominent inter-molecular and inter-atomic correlations belonging to these quasi-crystalline remnants. Under nanomilling, the cage-like As4Se4 molecules are merely destroyed in g-AsSe, facilitating a more polymerized chain-like network. The effect of nanomilling-driven molecular-to-network reamorphization results in a fragmentation impact on the correlation length of FSDP-responsible entities (due to an increase in the FSDP width and position). A breakdown in intermediate-range ordering is accompanied by changes in extended-range ordering due to the high-angular shift and broadening of the SSDP. A breakdown in the intermediate-range order is revealed in the destruction of most distant inter-atomic correlations, which belong to remnants of some quasi-crystalline planes, whereas the longer correlations dominate in the extended-range order. The microstructure scenarios of milling-driven reamorphization originated from the As4Se4 molecule, and its network derivatives are identified with an ab initio quantum-chemical cluster modeling code (CINCA).

Chalcogenide glass optical waveguides for infrared biosensing.

Due to the remarkable properties of chalcogenide (Chg) glasses, Chg optical waveguides should play a significant role in the development of optical biosensors. This paper describes the fabrication and properties of chalcogenide fibres and planar waveguides. Using optical fibre transparent in the mid-infrared spectral range we have developed a biosensor that can collect information on whole metabolism alterations, rapidly and in situ. Thanks to this sensor it is possible to collect infrared spectra by remote spectroscopy, by simple contact with the sample. In this way, we tried to determine spectral modifications due, on the one hand, to cerebral metabolism alterations caused by a transient focal ischemia in the rat brain and, in the other hand, starvation in the mouse liver. We also applied a microdialysis method, a well known technique for in vivo brain metabolism studies, as reference. In the field of integrated microsensors, reactive ion etching was used to pattern rib waveguides between 2 and 300 µm wide. This technique was used to fabricate Y optical junctions for optical interconnections on chalcogenide amorphous films, which can potentially increase the sensitivity and stability of an optical micro-sensor. The first tests were also carried out to functionalise the Chg planar waveguides with the aim of using them as (bio)sensors.

Forming glasses from Se and Te.

Despite being close neighbors on the Periodic Table, selenium and tellurium present a totally different abilities to form glasses. Se is a very good glass former, and gives rise to numerous glass compositions which are popular for their transparency in the infrared range and their stability against crystallization. These glasses can be shaped into sophisticated optical devices such as optical fibers, planar guides or lenses. Nevertheless, their transparencies are limited at about 12 microm (depending on the thickness of the optical systems) due to the relatively small mass of the Se element. On the other hand, tellurium is heavier and its use in substitution for Se permits to shift the IR cutoff beyond 20 microm. However, the semimetallic nature of Te limits its glass formation ability and this glass family is known to be unstable and consequently has found application as phase change material in the Digital Versatile Disk (DVD) technology. In this paper, after a review of selenide glasses and their applications, it will be shown how, in a recent past, it has been possible to stabilize tellurium glasses by introducing new elements like Ga or I in their compositions.

Ultraflexible Glassy Semiconductor Fibers for Thermal Sensing and Positioning.

Flexible, large-area, and low-cost thermal sensing networks with high spatial and temporal resolution are of profound importance in addressing the increasing needs for industrial processing, medical diagnosis, and military defense. Here, a thermoelectric (TE) fiber is fabricated by thermally codrawing a macroscopic preform containing a semiconducting glass core and a polymer cladding to deliver thermal sensor functionalities at fiber-optic length scales, flexibility, and uniformity. The resulting TE fiber sensor operates in a wide temperature range with high thermal detection sensitivity and accuracy, while offering ultraflexibility with the bending curvature radius below 2.5 mm. Additionally, a single TE fiber can either sense the spot temperature variation or locate the heat/cold spot on the fiber. As a proof of concept, a two-dimensional 3 × 3 fiber array is woven into a textile to simultaneously detect the temperature distribution and the position of heat/cold source with the spatial resolution of millimeter. Achieving this may lead to the realization of large-area, flexible, and wearable temperature sensing fabrics for wearable electronics and advanced artificial intelligence applications.

Planar waveguide obtained by burying a Ge22As(20Se58 fiber in As2S3 glass.

We demonstrate the possibility of fabricating an infrared transmitting waveguide by burying fiber in chalcogenide glasses. Two highly mature chalcogenide glasses are used for these experiments. GASIR glass from Umicore IR Glass, Olen, Belgium, with the composition of Ge(22)As(20)Se(58) is used to draw fibers that are then buried in an As(2)S(3) glass substrate. The glasses we used are compatible, and we obtained a high quality interface. We performed a transmission test with a CO(2) laser at 9.3 microm. The potential for extremely low loss planar waveguides is discussed.

Detrimental Effects of Doping Al and Ba on the Thermoelectric Performance of GeTe.

GeTe-based materials are emerging as viable alternatives to toxic PbTe-based thermoelectric materials. In order to evaluate the suitability of Al as dopant in thermoelectric GeTe, a systematic study of thermoelectric properties of Ge1-xAlxTe (x = 0⁻0.08) alloys processed by Spark Plasma Sintering are presented here. Being isoelectronic to Ge1-xInxTe and Ge1-xGaxTe, which were reported with improved thermoelectric performances in the past, the Ge1-xAlxTe system is particularly focused (studied both experimentally and theoretically). Our results indicate that doping of Al to GeTe causes multiple effects: (i) increase in p-type charge carrier concentration; (ii) decrease in carrier mobility; (iii) reduction in thermopower and power factor; and (iv) suppression of thermal conductivity only at room temperature and not much significant change at higher temperature. First principles calculations reveal that Al-doping increases the energy separation between the two valence bands (loss of band convergence) in GeTe. These factors contribute for Ge1-xAlxTe to exhibit a reduced thermoelectric figure of merit, unlike its In and Ga congeners. Additionally, divalent Ba-doping [Ge1-xBaxTe (x = 0⁻0.06)] is also studied.

Thermoelectric Properties of Highly-Crystallized Ge-Te-Se Glasses Doped with Cu/Bi.

Chalcogenide semiconducting systems are of growing interest for mid-temperature range (~500 K) thermoelectric applications. In this work, Ge20Te77Se3 glasses were intentionally crystallized by doping with Cu and Bi. These effectively-crystallized materials of composition (Ge20Te77Se3)100-xMx (M = Cu or Bi; x = 5, 10, 15), obtained by vacuum-melting and quenching techniques, were found to have multiple crystalline phases and exhibit increased electrical conductivity due to excess hole concentration. These materials also have ultra-low thermal conductivity, especially the heavily-doped (Ge20Te77Se3)100-xBix (x = 10, 15) samples, which possess lattice thermal conductivity of ~0.7 Wm-1 K-1 at 525 K due to the assumable formation of nano-precipitates rich in Bi, which are effective phonon scatterers. Owing to their high metallic behavior, Cu-doped samples did not manifest as low thermal conductivity as Bi-doped samples. The exceptionally low thermal conductivity of the Bi-doped materials did not, alone, significantly enhance the thermoelectric figure of merit, zT. The attempt to improve the thermoelectric properties by crystallizing the chalcogenide glass compositions by excess doping did not yield power factors comparable with the state of the art thermoelectric materials, as these highly electrically conductive crystallized materials could not retain the characteristic high Seebeck coefficient values of semiconducting telluride glasses.

Mid-infrared spectroscopy of serum, a promising non-invasive method to assess prognosis in patients with ascites and cirrhosis.

BACKGROUND & AIMS: Prognostic tests are critical in the management of patients with cirrhosis and ascites. Biological tests or scores perform poorly in that situation. Mid-infrared fibre evanescent wave spectroscopy (MIR-FEWS) which allows for global serum metabolic profiling may provide more relevant information by measuring a wider range of metabolic parameters in serum. Here we present the accuracy of a MIR-FEWS based predictive model for the prognosis of 6 months survival in patients with ascites and cirrhosis. METHODS: Patients with ascites were prospectively included and followed up for 6 months. MIR-FEWS spectra were measured in serum samples. The most informative spectral variables obtained by MIR-FEWS were selected by FADA algorithm and then used to build the MIR model. Accuracy of this model was assessed by ROC curves and 90%/10% Monte Carlo cross-validation. MIR model accuracy for 6 months survival was compared to that of the Child-Pugh and MELD scores. RESULTS: 119 patients were included. The mean age was 57.36±13.70, the MELD score was 16.32±6.26, and the Child-Pugh score was 9.5±1.83. During follow-up, 23 patients died (20%). The MIR model had an AUROC for 6 months mortality of 0.90 (CI95: 0.88-0.91), the MELD 0.77 (CI95: 0.66-0.89) and Child-Pugh 0.76 (CI95: 0.66-0.88). MELD and Child-Pugh AUROCs were significantly lower than that of the MIR model (p = 0.02 and p = 0.02 respectively). Multivariate logistic regression analysis showed that MELD (p<0.05, OR:0.86;CI95:0.76-0.97), Beta blockers (p = 0.036;OR:0.20;CI95:0.04-0.90), and the MIR model (p<0.001; OR:0.50; CI95:0.37-0.66), were significantly associated with 6 months mortality. CONCLUSIONS: In this pilot study MIR-FEWS more accurately assess the 6-month prognosis of patients with ascites and cirrhosis than the MELD or Child-Pugh scores. These promising results, if confirmed by a larger study, suggest that mid infrared spectroscopy could be helpful in the management of these patients.

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.