Structure and Properties of Modified and Charge-compensated Chalcogenide Glasses in the Na/Ba--Ga--Ge Selenide System PDF Download
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Author: Alvin W. Mao Publisher: ISBN: 9781339824819 Category : Languages : en Pages :
Book Description
Chalcogenide glasses exhibit unique optical properties such as infrared transparency owing to the low-phonon energies, optical non-linearity, and photo-induced effects that have important consequences for a wide range of technological applications. However, to fully utilize these properties, it is necessary to better understand the atomic-scale structure and structure-property relationships in this important class of materials. Of particular interest in this regard are glasses in the stoichiometric system Na2Se/BaSe--Ga2Se3--GeSe2 as they are isoelectronic with the well-studied, oxide glasses of the type M2O(M'O)--Al2O3SiO2 (M = alkali, M' = alkaline earth). This dissertation investigates the structure of stoichiometric Na2Se/BaSe--Ga2Se3--GeSe2 and off-stoichiometric BaSe--Ga2Se3--GeSe2±Se glasses using a combination of Fourier-transform Raman and solid state nuclear magnetic resonance (NMR) spectroscopies. The spectroscopic data is then compared to composition-dependent trends in physical properties such as density, optical band gap, glass transition temperature, and melt fragility to develop predictive structural models of the short- and intermediate-range order in the glass network. These models significantly improve our current understanding of the effects of modifier addition on the structure and properties of chalcogenide glasses, and thus enable a more efficient engineering of these highly functional materials for applications as solid electrolytes in batteries or as optical components in infrared photonics. In general, the underlying stoichiometric Ga2Se3--GeSe2 network consists primarily of corner-sharing (Ga/Ge)Se4 tetrahedra, where the coordination numbers of Ga, Ge, and Se are 4, 4, and 2, respectively. Some edge-sharing exists, but this configuration is relatively unstable and its concentration tends to decrease with any deviation from the GeSe2 composition. Due to the tetrahedral coordination of Ga, the initial addition of Se-deficient Ga2Se3 to GeSe2 results in the preferential formation of Ge-Ge bonds, which are distributed such that the clustering of ethane-like (Se3)Ge-Ge(Se3) units is avoided to the maximum extent. This behavior is entirely consistent with the continuously-alloyed structural scenario of chalcogenide glasses. However, for contents of Ga2Se3 greater than about 25--30 mol%, the avoidance of Ga-Ga and mixed Ga-Ge bonds results in the appearance of three-coordinated Se as an alternate mechanism to accommodate the Se deficiency. The addition of either Na2Se or BaSe to Ga2Se3--GeSe2 glasses introduces an ionic bonding character to an otherwise largely covalently bonded network. As a result, the structure responds by adopting characteristics of the charge-compensated structural scenario of oxide glasses. In the stoichiometric Na2Se/BaSe--Ga2Se3--GeSe2 glasses, the ratio of Na2Se/BaSe:Ga2Se3 = 1 serves as a chemical threshold, where the network consists predominantly of corner-sharing (Ga/Ge)Se4 tetrahedra, and the charge on the Na(Ba) cations is balanced by the GaSe4− tetrahedra. For glasses with Na2Se/BaSe:Ga2Se3 1, the addition of Se-deficient Ga2Se3 induces the formation of Ge-Ge bonds. However, for glasses with Na2Se/BaSe:Ga2Se3 1, the addition of Na2Se/BaSe results in the formation of non-bridging Se atoms, which break up the connectivity of the glassy network. The major difference between the modifying elements Na and Ba is that the high field strength of the Ba cation induces a higher degree of chemical disorder in the glass network. This conclusion is evidenced by the presence of some Ge-Ge bonds in BaSe--Ga2Se3--GeSe2 glasses even at the chemical threshold composition of BaSe:Ga2Se3 = 1. The structural duality of the Na2Se/BaSe--Ga2Se3--GeSe2 system is best observed in the off-stoichiometric BaSe--Ga2Se3--GeSe2±Se glasses. Here, the removal of Se from a stoichiometric glass with BaSe:Ga2Se3 > 1 results in Ge-Ge bonds, while its addition in excess of stoichiometry forms Se-Se bonds. Although such behavior is consistent with the continuously-alloyed structural model, it should be contrasted with the response of the network to the removal or addition of BaSe. In the latter case especially, the network responds with the formation of non-bridging Se atoms, which is reminiscent of the charge-compensated structural scenario. The aforementioned structural conclusions are supported by trends in physical properties. Of all the properties measured, the glass transition temperature Tg responds most predictably to changes in glass structure in the sense that the removal of heteropolar (Ga/Ge)-Se bonds from the glassy network consistently results in a decrease in Tg. Indeed, Tg is observed to be maximized around chemical threshold compositions that are expected to have a fully-connected network of (Ga/Ge)Se4 tetrahedra. The formation of homopolar Ge-Ge bonds causes Tg to drop by ~40-80 °C, while the formation of Se-Se and/or non-bridging Se causes Tg to decrease by at least 120 °C. Trends in density reflect both the packing efficiency of the structural units within the glassy network as well as the masses of the constituent elements, and are generally observed to increase or decrease monotonically. As a result, an increase in density is associated with: 1) the removal of inefficiently packed structural units such as edge-sharing tetrahedra, 2) the formation of efficiently packed units such as three-coordinated Se atoms, 3) the removal of lighter elements like Na, and 4) the addition of heavier elements like Ba. Optical band gap is related to the bonding character within the glassy network, and tends to decrease as the bonding character becomes increasingly metallic. Therefore, a decrease in optical band gap is observed with the formation of homopolar Ge-Ge bonds when Ga2Se3 is added to GeSe2. However, the stoichiometric BaSe--Ga2Se3--GeSe2 glasses show an anomaly in this regard because optical band gap decreases with the addition of BaSe, and consequently the removal of Ge-Ge bonds. This observation was ascribed instead to the formation of Ba-Se bonds, which are associated with a lower bandgap compared to the (Ga/Ge)-Se bonds that they replace. Finally, there is no straightforward structural explanation for trends in fragility, because it is related to the number of structural configurations dynamically available to the supercooled liquid. In the binary Ga2Se3--GeSe2 glasses, the fragility tends to increase with the formation of homopolar Ge-Ge bonds, which is consistent with other chalcogenide systems in which fragility increases with the removal of heteropolar bonds within corner-sharing tetrahedra and pyramids. In the stoichiometric BaSe--Ga2Se3--GeSe2 glasses on the other hand, a shift in trend near the compositions where BaSe:Ga2Se3 = 1 coincides with a structural shift between the formation of Ge-Ge bonds and Se-Se/non-bridging Se.
Author: Alvin W. Mao Publisher: ISBN: 9781339824819 Category : Languages : en Pages :
Book Description
Chalcogenide glasses exhibit unique optical properties such as infrared transparency owing to the low-phonon energies, optical non-linearity, and photo-induced effects that have important consequences for a wide range of technological applications. However, to fully utilize these properties, it is necessary to better understand the atomic-scale structure and structure-property relationships in this important class of materials. Of particular interest in this regard are glasses in the stoichiometric system Na2Se/BaSe--Ga2Se3--GeSe2 as they are isoelectronic with the well-studied, oxide glasses of the type M2O(M'O)--Al2O3SiO2 (M = alkali, M' = alkaline earth). This dissertation investigates the structure of stoichiometric Na2Se/BaSe--Ga2Se3--GeSe2 and off-stoichiometric BaSe--Ga2Se3--GeSe2±Se glasses using a combination of Fourier-transform Raman and solid state nuclear magnetic resonance (NMR) spectroscopies. The spectroscopic data is then compared to composition-dependent trends in physical properties such as density, optical band gap, glass transition temperature, and melt fragility to develop predictive structural models of the short- and intermediate-range order in the glass network. These models significantly improve our current understanding of the effects of modifier addition on the structure and properties of chalcogenide glasses, and thus enable a more efficient engineering of these highly functional materials for applications as solid electrolytes in batteries or as optical components in infrared photonics. In general, the underlying stoichiometric Ga2Se3--GeSe2 network consists primarily of corner-sharing (Ga/Ge)Se4 tetrahedra, where the coordination numbers of Ga, Ge, and Se are 4, 4, and 2, respectively. Some edge-sharing exists, but this configuration is relatively unstable and its concentration tends to decrease with any deviation from the GeSe2 composition. Due to the tetrahedral coordination of Ga, the initial addition of Se-deficient Ga2Se3 to GeSe2 results in the preferential formation of Ge-Ge bonds, which are distributed such that the clustering of ethane-like (Se3)Ge-Ge(Se3) units is avoided to the maximum extent. This behavior is entirely consistent with the continuously-alloyed structural scenario of chalcogenide glasses. However, for contents of Ga2Se3 greater than about 25--30 mol%, the avoidance of Ga-Ga and mixed Ga-Ge bonds results in the appearance of three-coordinated Se as an alternate mechanism to accommodate the Se deficiency. The addition of either Na2Se or BaSe to Ga2Se3--GeSe2 glasses introduces an ionic bonding character to an otherwise largely covalently bonded network. As a result, the structure responds by adopting characteristics of the charge-compensated structural scenario of oxide glasses. In the stoichiometric Na2Se/BaSe--Ga2Se3--GeSe2 glasses, the ratio of Na2Se/BaSe:Ga2Se3 = 1 serves as a chemical threshold, where the network consists predominantly of corner-sharing (Ga/Ge)Se4 tetrahedra, and the charge on the Na(Ba) cations is balanced by the GaSe4− tetrahedra. For glasses with Na2Se/BaSe:Ga2Se3 1, the addition of Se-deficient Ga2Se3 induces the formation of Ge-Ge bonds. However, for glasses with Na2Se/BaSe:Ga2Se3 1, the addition of Na2Se/BaSe results in the formation of non-bridging Se atoms, which break up the connectivity of the glassy network. The major difference between the modifying elements Na and Ba is that the high field strength of the Ba cation induces a higher degree of chemical disorder in the glass network. This conclusion is evidenced by the presence of some Ge-Ge bonds in BaSe--Ga2Se3--GeSe2 glasses even at the chemical threshold composition of BaSe:Ga2Se3 = 1. The structural duality of the Na2Se/BaSe--Ga2Se3--GeSe2 system is best observed in the off-stoichiometric BaSe--Ga2Se3--GeSe2±Se glasses. Here, the removal of Se from a stoichiometric glass with BaSe:Ga2Se3 > 1 results in Ge-Ge bonds, while its addition in excess of stoichiometry forms Se-Se bonds. Although such behavior is consistent with the continuously-alloyed structural model, it should be contrasted with the response of the network to the removal or addition of BaSe. In the latter case especially, the network responds with the formation of non-bridging Se atoms, which is reminiscent of the charge-compensated structural scenario. The aforementioned structural conclusions are supported by trends in physical properties. Of all the properties measured, the glass transition temperature Tg responds most predictably to changes in glass structure in the sense that the removal of heteropolar (Ga/Ge)-Se bonds from the glassy network consistently results in a decrease in Tg. Indeed, Tg is observed to be maximized around chemical threshold compositions that are expected to have a fully-connected network of (Ga/Ge)Se4 tetrahedra. The formation of homopolar Ge-Ge bonds causes Tg to drop by ~40-80 °C, while the formation of Se-Se and/or non-bridging Se causes Tg to decrease by at least 120 °C. Trends in density reflect both the packing efficiency of the structural units within the glassy network as well as the masses of the constituent elements, and are generally observed to increase or decrease monotonically. As a result, an increase in density is associated with: 1) the removal of inefficiently packed structural units such as edge-sharing tetrahedra, 2) the formation of efficiently packed units such as three-coordinated Se atoms, 3) the removal of lighter elements like Na, and 4) the addition of heavier elements like Ba. Optical band gap is related to the bonding character within the glassy network, and tends to decrease as the bonding character becomes increasingly metallic. Therefore, a decrease in optical band gap is observed with the formation of homopolar Ge-Ge bonds when Ga2Se3 is added to GeSe2. However, the stoichiometric BaSe--Ga2Se3--GeSe2 glasses show an anomaly in this regard because optical band gap decreases with the addition of BaSe, and consequently the removal of Ge-Ge bonds. This observation was ascribed instead to the formation of Ba-Se bonds, which are associated with a lower bandgap compared to the (Ga/Ge)-Se bonds that they replace. Finally, there is no straightforward structural explanation for trends in fragility, because it is related to the number of structural configurations dynamically available to the supercooled liquid. In the binary Ga2Se3--GeSe2 glasses, the fragility tends to increase with the formation of homopolar Ge-Ge bonds, which is consistent with other chalcogenide systems in which fragility increases with the removal of heteropolar bonds within corner-sharing tetrahedra and pyramids. In the stoichiometric BaSe--Ga2Se3--GeSe2 glasses on the other hand, a shift in trend near the compositions where BaSe:Ga2Se3 = 1 coincides with a structural shift between the formation of Ge-Ge bonds and Se-Se/non-bridging Se.
Author: Maxwell Adam Thomas Marple Publisher: ISBN: 9780438628403 Category : Languages : en Pages :
Book Description
Solid state batteries are a safer alternative to the current liquid battery technology, although for efficient performance the solid electrolyte must have high room temperature ionic conductivity. Glassy solid electrolytes are superior alternatives to their crystalline counterparts owing to their lack of grain boundaries that can act as a source of resistance for ionic current and as potential pathways for Li dendrite growth. Chalcogenide glasses are favorable materials for solid electrolytes as they have higher ionic conductivity compared to oxides, which can be ascribed to the greater polarizability of the chalcogens and are therefore, the most likely materials to satisfy the requirements for solid state battery applications. Chalcogenide glasses are an important class of materials that are sulfides, selenides or tellurides of group IV and/or V elements, namely Ge, As, P and Si with minor concentrations of other elements such as Ga, Sb, In. These glasses have found a variety of technological applications in the fields of optoelectronics, remote sensing, memory and energy storage. The unique compositional flexibility of chalcogenide glasses in the form of continuous alloying enables tuning of their optical, electronic, thermo-mechanical and other properties. The structure of these glasses are characterized by their covalently bonded networks that largely obey the 8–N coordination rule, violation of chemical order, and structural peculiarities such as the formation of homopolar bonds, molecular and other low-dimensional structural units. All of these are expected to control a wide range of physical properties relevant to various technological applications hence complete knowledge of the atomic structure of these materials is therefore of key importance in understanding and formulating accurate predictive models. The first chapter of this dissertation details the application of high-resolution two-dimensional nuclear magnetic resonance (2D NMR) and Raman spectroscopy to investigate the structure of Si[subscript x]Se[subscript 1-x] glasses as this glass-forming system forms the basis for the classic glassy Li-ion conducting chalcogenides via modification of the network by incorporation of Li2S. The results indicate that the structure of these glasses consists of a network with nearly perfect short-range chemical order, but with strong intermediate-range clustering. Initial addition of Si to Se results in cross-linking of Se chain segments with nanoclusters of corner- and edge-shared SiSe[subscript 4/2] tetrahedra. These clusters percolate via coalescence near x ≥0.2 to finally form a low-dimensional network with high molar volume, at the stoichiometric composition (x=0.33) that is composed of chains of edge-sharing tetrahedra cross-linked by corner-shared tetrahedra. This structural evolution can explain the compositional variation of the glass transition temperature and the molar volume of these glasses. The structure-property relationship for ionic conductivity in chalcogenide glasses is explored next, in a Ag-ion conducting system that has technological application in conductive bridge random access memory. Novel homogeneous glasses in the ternary system Ag2Se-Ga2Se3-GeSe2 (AGGS) are synthesized and studied using Raman, 77Se, [superscript 71/69]Ga, and 109Ag NMR spectroscopy. The structure of these glasses consists primarily of a network of corner sharing (Ga/Ge)Se[subcript 4/2] tetrahedra with a small fraction of homopolar Se-Se bonds. Compositional modification of the atomic structure follows the charge compensated network model with Ag2Se acting as a network modifier, forming non-bridging Se in glasses with Ag/Ga >1, while Ga2Se3 plays the role of an intermediate glass former. Electrical Impedance Spectroscopy (EIS) reveals the ionic conductivity of the AGGS glasses to be quite high at ambient temperature, reaching up to 10−4 S/cm for glasses with the highest Ag content. Transference number measurements using the electromotive force (EMF) method as well as variable temperature 109Ag NMR line shape studies indicate that the conductivity is predominantly ionic in nature. The high ionic conductivity can be related to a heavily modified structural network that results in a potential energy landscape with many suitable hopping sites for the Ag ions. The structural characteristics and electrical properties of the AGGS glasses are used to guide the development of an analogous Li containing system. Two glass systems are investigated, the first is the stoichiometric Li2S-Ga2Se3-GeSe2 system where the Li2S content is varied to study the influence of Li concentration on ionic conductivity. The structure is characterized using Raman and one– and two– dimensional [superscript 6/7]Li, 77Se, and 71Ga NMR spectroscopy and can be described as a charge-compensated network consisting of corner sharing (Ga/Ge)(Se,S)[subscript 4/2] tetrahedra with Li acting as a network modifier, charge compensating the non-bridging Se and S. These non-bridging units are found to have the greatest influence towards maximizing ionic conductivity as they depolymerize the network and alter the Li-ion dynamics. The dc conductivity data indicate a rapid rise in Li-ion mobility with increasing temperature and, more interestingly, with Li concentration. The compositional variation of E[subscript dc] indicates the formation of a low-energy barrier (~0.35 eV) percolation pathway for Li-ion hopping through the glass network. The highest room temperature ionic conductivity of ~ 10−4 S/cm is found in the glass with the highest Li2S content, suggesting that the concentration of the Li ions, rather than their mobility, is the limiting factor for achieving high ionic conductivity in this system. Besides Li concentration, the nature of the chalcogen atoms in the network is found to have an important influence on Li mobility. This phenomenon is investigated in 40%Li2S-60%Ge(S,Se)2 glasses, as a function of the S/(S+Se) ratio. The network structure of these glasses consists of corner-sharing GeS[subscript x/2]Se[subscript (4-x)/2] tetrahedra, with S and Se being randomly distributed over all bridging and non-bridging environments. While these glasses are found to have comparable ionic conductivity that varies little with composition, the activation energy and the pre-exponential factor display a nonlinear variation with the S/(S+Se) ratio and are shown to be related to the progressive phonon softening, as Se replaces S in the mixed-chalcogen network. When taken together, these results suggest that the phonon softening of the structural network of solid electrolytes, induced via compositional modification, can be used to tune their electrical properties.
Author: Robert Fairman Publisher: Academic Press ISBN: 0080525261 Category : Science Languages : en Pages : 307
Book Description
Chalcogenide glass is made up of many elements from the Chalcogenide group. The glass is transparent to infrared light and is useful as a semiconductor in many electronic devices. For example, chalcogenide glass fibers are a component of devices used to perform laser surgery. This book is a comprehensive survey of the current state of science and technology in the field of chalcogenide semiconductor glasses. While the majority of the book deals with properties of chalcogenide glass, chapters also deal with industrial applications, synthesis and purification of chalcogenide glass, and glass structural modification. The first individual or collective monograph written by Eastern European scientists known to Western readers regarding structural and chemical changes in chalcogenide vitreous semiconductors(CVS)Chapters written by B.G. Kolomiets who discovered the properties of chalcogenide glass in 1955Provides evidence and discussion for problems discussed by authors from opposing positions.
Author: J-L Adam Publisher: Woodhead Publishing ISBN: 0857093568 Category : Technology & Engineering Languages : en Pages : 719
Book Description
The unique properties and functionalities of chalcogenide glasses make them promising materials for photonic applications. Chalcogenide glasses are transparent from the visible to the near infrared region and can be moulded into lenses or drawn into fibres. They have useful commercial applications as components for lenses for infrared cameras, and chalcogenide glass fibres and optical components are used in waveguides for use with lasers, for optical switching, chemical and temperature sensing and phase change memories. Chalcogenide glasses comprehensively reviews the latest technological advances in this field and the industrial applications of the technology.Part one outlines the preparation methods and properties of chalcogenide glasses, including the thermal properties, structure, and optical properties, before going on to discuss mean coordination and topological constraints in chalcogenide network glasses, and the photo-induced phenomena in chalcogenide glasses. This section also covers the ionic conductivity and physical aging of chalcogenide glasses, deposition techniques for chalcogenide thin films, and transparent chalcogenide glass-ceramics. Part two explores the applications of chalcogenide glasses. Topics discussed include rare-earth-doped chalcogenide glass for lasers and amplifiers, the applications of chalcogenide glasses for infrared sensing, microstructured optical fibres for infrared applications, and chalcogenide glass waveguide devices for all-optical signal processing. This section also discusses the control of light on the nanoscale with chalcogenide thin films, chalcogenide glass resists for lithography, and chalcogenide for phase change optical and electrical memories. The book concludes with an overview of chalcogenide glasses as electrolytes for batteries.Chalcogenide glasses comprehensively reviews the latest technological advances and applications of chalcogenide glasses, and is an essential text for academics, materials scientists and electrical engineers working in the photonics and optoelectronics industry. Outlines preparation methods and properties, and explores applications of chalcogenide glasses. Covers the ionic conductivity and physical aging of chalcogenide glasses, deposition techniques for chalcogenide thin films, and transparent chalcogenide glass-ceramics Discusses the control of light on the nanoscale with chalcogenide thin films, chalcogenide glass resists for lithography, and chalcogenide for phase change optical and electrical memories
Author: Robert Fairman Publisher: Elsevier ISBN: 0080541054 Category : Science Languages : en Pages : 323
Book Description
Chalcogenide glass is made up of many elements from the Chalcogenide group. The glass is transparent to infrared light and is useful as a semiconductor in many electronic devices. For example, chalcogenide glass fibers are a component of devices used to perform laser surgery. The properties of chalcogenide glass result not only from their chemical composition and atomic structure, but also from the impact of numerous external factors. A comprehensive survey is presented of the properties of chalcogenide glass under various external impacts. Practical recommendations are presented for a wide range of applications. Part II is the second part of a three-volume work within the Semiconductors and Semimetals series. * The first collective monograph written by Eastern European scientists on the electrical and optical properties of chalcogenide vitreous semiconductors (CVS).* Contributions by B.G. Kolomiets, who discovered the properties of chalcogenide glass in 1955!* Provides objective evidence and discussion by authors from opposing positions.
Author: Ting Wang Publisher: ISBN: Category : Languages : en Pages : 0
Book Description
Germanium-Arsenic-Selenium chalcogenide glasses are considered as good candidates for photonic applications due to their excellent transparency in the infrared range and high optical nonlinearities. A deep understanding of composition-structure-property relationship in Ge-As-Se ternary system is thus becoming increasingly important, which can serve as a guideline for materials selection. In this work, the structure and various physical properties of GexAsySe100-x-y bulk glasses have been systematically investigated. Raman spectra and EXAFS measurements reveal that chemically ordered network model can be applied to Selenium-rich glasses, but fails to explain bonding characterization of Selenium-poor compositions. Atomic arrangements are more sensitive to the changes in chemical compositions. A tight association between the fragility and the deviation from stoichiometry has been found in the Ge-As-Se system by differential scanning calorimetry. It is shown that chemical compositions with the lowest values of fragility index are far less likely to incur structural relaxation. Those strong glasses are normally chosen as ideal materials for fabrication of stable photonic devices. The variation of density and elastic modulus as a function of mean coordination number both show two transition thresholds, which correlate with floppy-to-rigid phase transition and 2D-to-3D structure transition respectively. The results provide clear evidence that some physical properties of Ge-As-Se chalcogenide glasses are significantly determined by their mean coordination numbers, but could be further tuned by the chemical compositions. The detailed optical investigation shows that the generalized Miller's rule is a simple but effective approach to estimate the nonlinearities of a broad variety of chalcogenide glasses. Nonlinear properties of these materials exhibit strong dependence upon their optical bandgap in the near infrared. It seems that the highest nonlinearity at telecommunications wavelengths is predictable in chalcogenide glasses.
Author: Derrick Charles Kaseman Publisher: ISBN: 9781369343144 Category : Languages : en Pages :
Book Description
Chalcogenide glasses constitute an important class of materials that are sulfides, selenides or tellurides of group IV and/or V elements, namely Ge, As, P and Si with minor concentrations of other elements such as Ga, Sb, In. Because of their infrared transparency that can be tuned by changing chemistry and can be actively altered by exposure to band gap irradiation, chalcogenide glasses find use in passive and active optical devices for applications in the areas of photonics, remote sensing and memory technology. Therefore, it is important to establish predictive models of structure-property relationships for these materials for optimization of their physical properties for various applications. Structural elucidation of chalcogenide glasses is experimentally challenging and in order to make predictive structural models, structural units at both short and intermediate -range length scales must be identified and quantified. Nuclear Magnetic Resonance (NMR) spectroscopy is an element-specific structural probe that is uniquely suited for this task, but resolution and sensitivity issues have severely limited the applications of such techniques in the past. The recent development of multi-dimensional solid-state NMR techniques, such as Phase Adjusted Spinning Sidebands (PASS) and Magic Angle Turning (MAT) can potentially alleviate such issues. In this study novel two-dimensional, high-resolution 77Se and 125Te MATPASS NMR spectroscopic techniques are utilized to elucidate quantitatively the compositional evolution of the short- and intermediate- range atomic structure in three binary chalcogenide glass-forming systems, namely: Ge[subscript x]Se[subscript100-x], As[subscript x]Se[subscript 100-x], and As[subscript x]Te[subscript 100-x]. The spectroscopic results provide unambiguous site speciation and quantification for short- and intermediate-range structural motifs present in these glasses. In turn, for all systems, robust structural models and the corresponding structure-property relationships are successfully established as a function of composition. The results indicate that the physical properties are intimately tied to the topology and chemical order present in each system. Finally, a dynamic version of the two-dimensional 31P PASS NMR spectroscopy is used to study the molecular motion in a supercooled chalcogenide liquid of composition P5Se3. The results clearly display the presence of isotropic rotational reorientation of the constituent molecules at timescales significantly decoupled from that of the structural relaxation near and above T[subscript g]. This behavior is atypical of conventional molecular glasses in organic systems in which rotational and translational dynamics remain coupled near T[subscript g]. When taken together with previous reports on the dynamics of other globular inorganic molecules, the results support the existence of a “plastic glass” phase where the molecules perform rapid rotation without significant translation.
Author: Xiaolin Zheng Publisher: ISBN: Category : Languages : en Pages : 224
Book Description
Being compared with oxide glasses, chalcogenide glasses have fine infrared transmissivity and higher optical nonlinearity, and also could be drawn into optical fibers. So chalcogenide glasses and fibers have potential wide applications in the fields of all-optical information processing, infrared lasers, nonlinear optical devices, and so on, the studies of their optical nonlinearity are one of the attractive subjects in the area of optoelectronics at present. The main purpose of this paper is to improve the stability and enhance the intensity of nonlinearity in chalcogenide glasses and fibers by means of exploring new glass compositions, optimizing the external field poling method, designing and fabricating fibers with special structures, all of these will promote their real applications. The main results are concluded as follows . The glass-forming region of GeS2-GA2S3-AgX (X=Cl, Br, I) and GeS2-Ga (In)2S3-CuI systems were determined , the maximal content of the additive halides are 70% and 12% respectively. In both two systems glasses, with the increasing addition of halides, the thermal stability reduce, density and linear refractive index increase, the ultraviolet cut-off edges shift to longer wavelength, while the infrared cut-off edges keep almost the same. 30GeS2 35Ga2S3 35AgCl and 47.5GeS2 17.5Ga2S3 35AgCl surface- and bulk-crystallized glasses that contain AgGaGeS4 nonlinear optical crystallites were prepared. Obvious second harmonic generation (SHG) could be observed in these crystallized glasses, and their intensity relate to the distribution and size of the precipitated AgGaGeS4 crystals, the maximal second-order nonlinearity coefficients is as high as 12.4pm/V. These crystallized glasses have good chemical and SHG stability. For GeS2-Ga (In)2S3-CuI systems glasses, due to their small glass-forming region, they are not suit for the preparation of crystallized glasses that contain CuGaS2 or CuInS2 nonlinear optical crystals. According to the structural studies of two system glasses, the main structural units of theses glasses are [YS4-xXx] (Y=Ge, Ga, In. X=Cl, Br, I) mixed anion tetrahedrons, they form a three-dimensional glassy network through bridging sulphur bonds. When the contents of halides MX(M=Ag, Cu. X=Cl, Br, I) are low, some [XxS3-xGe(Ga)S3-xXx] (X=Cl, Br, I) mixed ethane-like structural units exist in the glass network, and they will gradually transform to [YS4-xXx] (Y=Ge, Ga, In. X=Cl, Br, I) mixed anion tetrahedrons with the increasing content of halides, till totally disappear. Both two system glasses have ultrafast (~150fs) third-order optical nonlinearity and reverse saturation absorption, they belong to self-focusing medium. The third-order optical nonlinearity mainly originate from the distortion of electron cloud of Y-X (Y=Ge, Ga, In, X=Cl, Br, I, S) bonds in the structural units. For GeS2-GA2S3-AgX (X=Cl, Br, I) system glasses, the largest nonlinear susceptibility n2 is 10.50x10-18 m/W, the smallest figure of merit (FOM) is 0.606. In addition, the relation of n2 with n0 do not obey Miller's rule, but in accordance with the structural variation. Among the glass compositions with different additive halogens, Br-containing glasses have relatively best third-order nonlinearities. For GeS2-Ga (In)2S3-CuI system glasses, the largest nonlinear susceptibility n2 is 9.37x10-18 m/W, the smallest figure of merit (FOM) is 2.237. High purity AS2S3 glass performs and low loss single index fibers with diameter of 100~400μm that drawn form these performs were prepared, the transmission losses between 2~6 μm is only 0.5dB/m. AS2S3 tapered fibers have a uniform diameter of taper wasit, fine surface smoothness, and sharp taper transition part.
Author: Alvin W. Mao
Author: Alvin W. Mao
Author: Maxwell Adam Thomas Marple
Author: Robert Fairman
Author: J-L Adam
Author: Robert Fairman
Author: Ting Wang
Author: Derrick Charles Kaseman
Author: Yogesh Mehrotra
Author: Xiaolin Zheng
Author: F. Sava