Te-Te Bonds Research Articles

Study of SiGeAsTe and SiGeAsSe chalcogenide thin films by Raman spectroscopy and understanding of their OTS properties

In this paper, a range of sputtered ternary and quaternary (Si)(Ge)AsxTe3 layers (x: 2 or 5) and (Si)(Ge)As2Se3 layers are examined using Raman spectroscopy. The results are linked to the Ovonic Threshold Switching properties of these materials when incorporated in selector devices, as observed in separate studies. In case of both the As-rich and As-poor tellurides, a large amount of homopolar bonds are present as the spectra are dominated by peaks associated with As–As and Te–Te bonds. Such homopolar bonds are commonly linked with drift in OTS properties. In the case of the selenides the spectra are dominated by modes associated with heteropolar As–Se bond vibrations. Adding Ge as an alloying element has a significant impact on the bond structure for both material systems. In contrast, Si has a much less pronounced impact and will mostly bond with itself. Time-resolved Raman measurements were also performed to determine the stability of the layers under micro-Raman laser excitation. At the regular exposure settings, no significant changes in the spectra were observed during the measurement. At elevated exposure settings, however, persistent changes in the bond structure can be induced for certain compositions.

Redox dual-responsive diaryl ditelluride-containing nanoparticles as peroxidase mimetics

Polymeric nanoparticles with peroxidase-like enzyme mimetic activity were developed by incorporating catalytically active tellurium (Te) functionalities into cross-linked block copolymers. Bis(2-aminophenyl) ditelluride was used as a cross-linker to react with a diblock copolymer bearing pendent activated ester groups to generate of core-cross-linked nanoparticles (DiTe-NP) containing diaryl ditelluride bonds. DiTe-NP nanoparticles exhibit dual redox-responsiveness inherited from the sensitivity of ditelluride to both oxidants and reductants. When DiTe-NP nanoparticles were treated with H2O2, the oxidation of the ditellurides triggered the cleavage of Te-Te bonds to form Te(IV)-associated species. The oxidation-responsiveness endows DiTe-NP nanoparticles with haloperoxidase (HPO)-mimic activity. With NaBr and hydrogen peroxide, they catalyzed the oxidative bromination of phenol red, bromolactonization of 4-pentenoic acid, and bromination of 1,3,5-trimethoxybenzene due to the in-situ generation of hypobromous acid (HOBr) for halogenations. These ditelluride-containing nanoparticles also exhibited glutathione peroxidase (GPx)-like activity and accelerated the reduction of hydroperoxide by a thiol substrate. The cleavage of ditelluride bonds by a thiol substrate leads to the formation of key intermediates in the catalytic cycle of DiTe-NP nanoparticles. DiTe-NP nanoparticles showed enhanced activity for the reduction of hydrophobic cumene peroxide by 4-nitrothiophenol. These dual redox-responsive ditelluride-containing nanoparticles are promising candidates as peroxidase mimetics to apply as catalysts for green halogenation reactions and decompositions of toxic peroxides.

The Ambiguous Origin of Thermochromism in Molecular Crystals of Dichalcogenides: Chalcogen Bonds versus Dynamic Se─Se/Te─Te Bonds.

We report thermochromism in crystals of diphenyl diselenide (dpdSe) and diphenyl ditelluride (dpdTe), which is at variance with the commonly known mechanisms of thermochromism in molecular crystals. Variable temperature neutron diffraction studies indicated no conformational change, tautomerization or phase transition between 100 K and 295 K. High-pressure crystallography studies indicated no associated piezochromism in dpdSe and dpdTe crystals. The evolution of the crystal structures and their electronic band structure with pressure and temperature reveal the contributions of intramolecular and intermolecular factors towards the origin of thermochromism - especially the intermolecular Se⋅⋅⋅Se and Te⋅⋅⋅Te chalcogen bonds and torsional modes of vibrations around the dynamic Se─Se and Te─Te bonds. Further, a co-crystal of dpdSe with iodine (dpdSe-I2) and an alloy crystal of dpdSe and dpdTe implied a predominantly intramolecular origin of the observed thermochromism associated with vibronic coupling.

Uranium-Mediated Peroxide Activation and a Precursor toward an Elusive Uranium cis-Dioxo Fleeting Intermediate.

The activation of chalcogen-chalcogen bonds using organometallic uranium complexes has been well documented for S-S, Se-Se, and Te-Te bonds. In stark contrast, reports concerning the ability of a uranium complex to activate the O-O bond of an organic peroxide are exceedingly rare. Herein, we describe the peroxide O-O bond cleavage of 9,10-diphenylanthracene-9,10-endoperoxide in nonaqueous media, mediated by a uranium(III) precursor [((Me,AdArO)3N)UIII(dme)] to generate a stable uranium(V) bis-alkoxide complex, namely, [((Me,AdArO)3N)UV(DPAP)]. This reaction proceeds via an isolable, alkoxide-bridged diuranium(IV/IV) species, implying that the oxidative addition occurs in two sequential, single-electron oxidations of the metal center, including rebound of a terminal oxygen radical. This uranium(V) bis-alkoxide can then be reduced with KC8 to form a uranium(IV) complex, which upon exposure to UV light, in solution, releases 9,10-diphenylanthracene to generate a cyclic uranyl trimer through formal two-electron photooxidation. Analysis of the mechanism of this photochemical oxidation via density functional theory (DFT) calculations indicates that the formation of this uranyl trimer occurs through a fleeting uranium cis-dioxo intermediate. At room temperature, this cis-configured dioxo species rapidly isomerizes to a more stable trans configuration through the release of one of the alkoxide ligands from the complex, which then goes on to form the isolated uranyl trimer complex.

Conversion between Metavalent and Covalent Bond in Metastable Superlattices Composed of 2D and 3D Sublayers.

Reversible conversion over multimillion times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, an extended category of crystal structures from stable states via redistribution of vacancies, research on kinetic behavior of the vacancies is highly in demand. However, it remains lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies of Sb2Te3 gradually occurs with diverged energy barriers owing to their own activation code: the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Studies on the behavior can be further applied to multiphase superlattices composed of Sb2Te3 (2D) and GeTe (3D) sublayers, which represent superior memory performances, but their operating mechanisms were still under debate due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb2Te3 (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could only be achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies toward vacancy engineering are expected in versatile materials.

Corrosion selectivity and structural evolution in N doped Sb2Te thin films for lithography application

Te-based phase-change resist has lots of advantages such as high-resolution, broad spectral response, and simple preparation process, making it a promising candidate for lithography. Particularly, both positive and negative features can be simultaneously implemented in the resist. However, there are few reports on regulating atom concentration of the resist to realize both positive and negative features. This work has reported N doped Sb 2 Te thin films as both positive and negative resists by adjusting N concentration. The negative resist with the N concentration of lower than 21.7 at% possesses the corrosion selectivity of as high as 4.2 while the positive resist with the N concentration of higher than 24.7 at% has the maximum corrosion selectivity of 10.9. The mechanism of corrosion selectivity has been further investigated. Electrochemical analysis reveals that for the negative resist, the increased annealing temperature results in the gradual increment of charge-transfer resistance and thus the higher corrosion potential. For the positive resist, however, the increased annealing temperature leads to the gradual decreased charge-transfer resistance and thus the lower corrosion potential. Structural analysis indicates that the annealed state for the negative resist can form large amounts of Sb 2 Te or Sb phases containing Sb-Sb and Te-N bonds, which will restrain the corrosion of thin films whereas the formation of Te Te bond in the as-deposited state will promote the corrosion of thin film. However, the annealed state for the positive resist can generate large amounts of Te phase consisting of Te-Te bonds, which is readily oxidized into TeO 2 and further be dissolved whereas the as-deposited state comprising more Sb-N and Te-N bonds will retard the further corrosion of thin film. Results indicate that N doped Sb 2 Te thin film is a promising inorganic resist for lithography applications. • NSb2Te films act as both positive and negative resists by tuning N concentration. • The negative resist possesses the corrosion selectivity of as high as 4.2. • The positive resist has the maximum corrosion selectivity of 10.9. • The mechanism of corrosion selectivity has been investigated in detail.

XPS study of Ge-Se-Te surfaces functionalized with organosilanes

Amorphous Ge25Se55Te20 chalcogenide thin films have been functionalized with three organosilanes: (3-aminopropyl)triethoxysilane, tetraethoxysilane and a mixture of tetraethoxysilane and octyl-trimethoxysilane. The functionalized films have been studied by X-ray photoelectron spectrometry, providing an interesting insight of the surface modification process and chemistry. First, the calculated stoichiometric ratios oxygen/silicon, carbon/silicon and nitrogen/silicon are in good agreement with the theory, demonstrating the effectiveness of the functionalization and relative precursors homocondensation. Secondly, the deconvolution of the 3d5/2 and 3d3/2 tellurium orbitals before and after functionalization show new oxidized environments. More specifically, the simultaneous disappearance of the bands at 573.0 eV and 583.4 eV attributed to TeGe and TeTe bonds (resp.) and appearance of a TeO environment (around 576.5 eV and 587.0 eV) strongly suggest covalent anchorage of the three precursors thanks to the formation of new TeOSi groups. This is the first report on tellurium anchorage of organosilane precursors onto chalcogenide surface.

Crystal structure, XANES and charge distribution investigation of krennerite and sylvanite: analysis of Au-Te and Te-Te bonds in Au1-xAgxTe2 group minerals.

The structure refinement and XANES study of two gold-silver-tellurides [Au1+xAgxTe2, krennerite (x = 0.11-0.13) and sylvanite (x = 0.29-0.31)] are presented and the structures are compared with the prototype structure of calaverite (x = 0.08-0.10). Whereas the latter is well known for being incommensurately modulated at ambient conditions, neither krennerite nor sylvanite present any modulation. This is attributed to the presence of relatively strong Te-Te bonds (bond distances < 2.9 Å) in the two minerals, which are absent in calaverite (bond distances > 3.2 Å). In both tellurides, trivalent gold occurs in slightly distorted square planar coordination, whereas monovalent gold, partly substituted by monovalent silver, presents a 2+2+2 coordination, corresponding to distorted rhombic bipyramids. The differentiation between bonding and non-bonding contacts is obtained by computation of the Effective Coordination Number (ECoN). The CHARge DIstribution (CHARDI) analysis is satisfactory for both tellurides but suggests that the Te-Te bond in the [Te3]2- anion is not entirely homopolar. Both tellurides can therefore be described as Madelung-type compounds, despite the presence of Te-Te in both structures.

Alteration of structural, electronic, and vibrational properties of amorphous GeTe by selenium substitution: An experimentally constrained density functional study

The structural, vibrational and electronic properties of several compositions of amorphous Ge-Se-Te are studied from a combination of x-ray diffraction and density functional-based molecular dynamics. Different structural properties are considered such as structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions. We compare results with experimental findings and a satisfying agreement is found for the structure functions in real and reciprocal spaces. The short range order is found to be more complex than in related binaries that result in mixed geometries ($\ensuremath{\simeq}65%--75%$ tetrahedral, and remaining defect octahedral) for a dominant fourfold Ge (80%). The chalcogen atoms are dominantly twofold, the former having furthermore an important fraction of threefold coordinated atoms (30%--40%). The obtained model structures indicate that Ge-Ge, Ge-Se, and Ge-Te bonds dominate with small fractions of Te-Te bonds remaining from the base system GeTe. The investigation of electronic properties indicates that the addition of Se atoms will lead to Te-related bands that are much more localized so that Ge-Te-Se can be regarded as having an increased covalent character with respect to GeTe.

(Invited) Advanced Characterization of III-Nitrides: From 2-D Epi-Layers to Bulk Defect Analysis

Electron Microscopy is a powerful tool for aid in the development of new compound semiconductors because it has the ability to characterize materials from the atom (aberration-corrected Transmission Electron Microscopy) to the millimeter length scale (montaged scanning electron microscopy - SEM). By including important aspects of the electron-sample interaction in a microscopy, we can couple imaging with crystallography (diffraction), chemistry (energy-dispersive spectroscopy (EDS), electron energy loss spectroscopy (EELS)) and chemical bonding (electron energy loss spectroscopy) at ultrahigh resolution. All of these techniques allow us to probe defect structure, the fine details of interfaces, and the growth mechanism in electronic materials.Recently, the concept of remote epitaxy has been introduced, with the idea of growing semiconductors through a “veil” of 2-D material – demonstrated first in GaAs/Graphene/GaAs. This facile method demonstrates the new methods for creating defect-reduced but transferable thin films, where the deposited atoms line up “through the veil” across from their substrate counterparts. This exciting development has spurred great interest in similar types of epitaxy and understanding their details at the atomic scale. It also allows for easy transfer of near-perfect films to other substrates.In this talk, I will present how EM can elucidate materials problems through several examples different types of heteroepitaxy: the first example is epitaxy of 2-dimensional forms of III-N and III-O and group III metals on SiC mediated through epitaxial graphene (EG), a technique similar to but not exactly remote epitaxy. This technique, known as confinement heteroepitaxy (CHet) is a fabrication technique for 2D heterostructure in which atoms are intercalated between defective epitaxial graphene and substrates of silicon carbide in a CVD process typically at 800 °C. The confined layers can be designed with the merit of intercalation in several combinations and has the potential to create wafer-scale van der Waals heterostructures. I will discuss the details of the SiC/EG/III-V/Graphene growth, as elucidated using TEM imaging, SEM imaging, and atomic resolution EELS.A second example is the idea of a self-assembled heteroepitaxy of CdTe on sapphire. Grown by pulsed laser deposition, we discovered using advanced EM that the CdTe spontaneously forms a van der Waals interface of Te-Te bonds, thus making these films easy to peel/transfer from the substrate. We used atomic resolution imaging, EELS spectroscopy both of composition and of the interface plasmon to demonstrate this spontaneous and unexpected coordination at the interface. This exciting example of 3D-3D epitaxy with van der Waals interface could be developed for novel semiconductor devices or in other materials systems, and shows that advanced EM techniques are breakthrough enabling tool for compound semiconductor development.

Multiple singularities of optical properties in GexTe100-x films

We deposited germanium telluride films and investigated their optical properties. We found multiple singularities in compositional dependent cut-off absorption edge, optical band gap, and refractive index. We further measured Raman spectra of the films and decomposed the spectra into different structural units, and found that, for the structural origins in the changes of optical properties, homopolar Te-Te bonds in anti-symmetric stretching of Te3 pyramid and heteropolar Ge-Te bonds in corner sharing structure are dominated in Te-rich GexTe100-x (x ≤ 33) films, while Ge-Ge bonds in v3(F2) mode and GeGe4 tetrahedron are dominated in Ge-rich GexTe100-x (x ≥ 33) films.

Understanding the complicated crystallization behaviors in Germanium-Tellurides

Crystallization behaviors and configurations of germanium-tellurides (GexTe100-x) at a wide compositional range (12.5 ≤ x ≤ 85) have been investigated. It is found GexTe100-x film exhibits one- or two-step crystallization behavior with a complicated competition between GeTe and Te (or Ge) phases. The composition dependences of crystallization temperature and activation energy have been revealed. They exhibit four singularities at x = 20.4 (GeTe4), x = 33 (GeTe2), x = 49 (GeTe), and x = 22 (Ge22Te78). Based on the method of X-ray photoelectron spectroscopy, the structural sketches for the GexTe100-x glasses that is helpful to understand the evolution of crystallization behavior are built up. It is evident that, the presence of homopolar TeTe and heteropolar GeTe bonds is the main reason for the structural and thermal stability of Ge-deficient films, and GeGe bond in GeGe4 tetrahedron appears when x > 50 and effectively enhances the stability of Ge-rich films.

Prediction of Stable and High-Performance Charge Transport in Zigzag Tellurene Nanoribbons

The 2-D materials possess application perspective in semiconductor logic devices owing to their bonding free surface carrier transport. However, the scaling transistor width-induced edge issues have significantly limited their surface transport within the lateral direction, leading to effective mass increasing, and bandgap fluctuations. Therefore, it is important to find a novel 2-D material with both excellent surface transport and immunity to edge effect. In this paper, inspired by the recently discovered tellurene and its unique Te-Te bonds, the edge effect on the transport characteristics of tellurene is studied theoretically in the form of tellurene nanoribbons (TNRs). Among the four edge types, three of them exhibit semiconducting characteristic owing to the unique bonding property of Te. Simulation results demonstrate that the tetragonal edges (TEs) in zigzag TNRs (ZTNRs) almost have no influence on surface transport, resulting in minimized bandgap and effective mass variations, compared to that of graphene nanoribbons, and Si and Ge nanowires. Transport behaviors of the TE-ZTNRs are investigated using a metal–oxide–semiconductor transistor model and a multiscale simulation flow, with the calculated current densities exceeding $\textsf {1}$ mA/ $\mu \text{m}$ and the on–off ratios over $10^{\textsf {11}}$ . Based on the results, an explicit prediction that the TE-ZTNRs are competitive candidates for nanoscale transistor materials is proposed.

Radical Anions, Radical-Anion Salts, and Anionic Complexes of 2,1,3-Benzochalcogenadiazoles.

By means of cyclic voltammetry (CV) and DFT calculations, it was found that the electron-acceptor ability of 2,1,3-benzochalcogenadiazoles 1-3 (chalcogen: S, Se, and Te, respectively) increases with increasing atomic number of the chalcogen. This trend is nontrivial, since it contradicts the electronegativity and atomic electron affinity of the chalcogens. In contrast to radical anions (RAs) [1].- and [2].- , RA [3].- was not detected by EPR spectroscopy under CV conditions. Chemical reduction of 1-3 was performed and new thermally stable RA salts [K(THF)]+ [2].- (8) and [K(18-crown-6)]+ [2].- (9) were isolated in addition to known salt [K(THF)]+ [1].- (7). On contact with air, RAs [1].- and [2].- underwent fast decomposition in solution with the formation of anions [ECN]- , which were isolated in the form of salts [K(18-crown-6)]+ [ECN]- (10, E=S; 11, E=Se). In the case of 3, RA [3].- was detected by EPR spectroscopy as the first representative of tellurium-nitrogen π-heterocyclic RAs but not isolated. Instead, salt [K(18-crown-6)]+ 2 [3-Te2 ]2- (12) featuring a new anionic complex with coordinate Te-Te bond was obtained. On contact with air, salt 12 transformed into salt [K(18-crown-6)]+ 2 [3-Te4 -3]2- (13) containing an anionic complex with two coordinate Te-Te bonds. The structures of 8-13 were confirmed by XRD, and the nature of the Te-Te coordinate bond in [3-Te2 ]2- and [3-Te4 -3]2- was studied by DFT calculations and QTAIM analysis.

Mechanochemical synthesis of binary phosphorus telluride: Short range structure and thermal properties

A binary phosphorus telluride sample (20 at% Te) was synthesized by mechanochemical solid state reaction from elemental powders. The X-ray diffraction pattern and neutron total structure factor show no detectable Bragg peaks, indicating that the sample is amorphous with respect to X-ray and neutron diffraction. The neutron and X-ray total pair distribution functions show correlation peak arising from PTe heteropolar bonds in addition to those arising from PP and TeTe bonds. The TeTe bond length is slightly shorter than that in the crystal structure of trigonal tellurium, suggesting a different environment from a polymeric chain structure. The thermal analysis results show that the binary phosphorus telluride is thermally unstable, that is, it decomposes above 200 °C into trigonal tellurium crystals and gaseous phosphorus, where phosphors begins to gasify at a lower temperature than the sublimation temperature of red phosphors.

First-principles study of the liquid and amorphous phases ofIn2Te3

Structural, dynamical, and electronic properties of the liquid and amorphous phase of the ${\mathrm{In}}_{2}{\mathrm{Te}}_{3}$ compound have been studied by means of density functional molecular dynamics simulations. This system is of interest as a phase change material, undergoing a fast and reversible change between the crystalline and amorphous phases upon heating. It can be seen as a constituent of ternary InSbTe alloys which are receiving attention for application in electronic phase change memories. Amorphous models of ${\mathrm{In}}_{2}{\mathrm{Te}}_{3}300$-atom large have been generated by quenching from the melt by using different exchange and correlation functionals and different descriptions of the van der Waals interaction. It turns out the local bonding geometry of the amorphous phase is mostly tetrahedral with corner and edge sharing tetrahedra similar to those found in the crystalline phases of the InTe, ${\mathrm{In}}_{2}{\mathrm{Te}}_{3}$, and ${\mathrm{In}}_{2}{\mathrm{Te}}_{5}$ compounds. Benchmark calculations on the crystalline $\ensuremath{\alpha}$ phase of ${\mathrm{In}}_{2}{\mathrm{Te}}_{3}$ in the defective zincblend geometry have also been performed. The calculations reveal that the high symmetric $F\overline{4}3m$ structure inferred experimentally from x-ray diffraction for the $\ensuremath{\alpha}$ phase must actually result from a random distribution of Te-Te bonds in different octahedral cages formed by the coalescence of vacancies in the In sublattice.

Nanotwin Detection and Domain Polarity Determination via Optical Second Harmonic Generation Polarimetry.

We demonstrate that optical second harmonic generation (SHG) can be utilized to determine the exact nature of nanotwins in noncentrosymmetric crystals, which is challenging to resolve via conventional transmission electron or scanned probe microscopies. Using single-crystalline nanotwinned CdTe nanobelts and nanowires as a model system, we show that SHG polarimetry can distinguish between upright (Cd-Te bonds) and inverted (Cd-Cd or Te-Te bonds) twin boundaries in the system. Inverted twin boundaries are generally not reported in nanowires due to the lack of techniques and complexity associated with the study of the nature of such defects. Precise characterization of the nature of defects in nanocrystals is required for deeper understanding of their growth and physical properties to enable their application in future devices.

Bonding nature of the amorphous structure studied by a combination of cutoff and electronic localization function

The analysis of the local structure of covalent glass is one of the major challenges for analyzing the amorphous structure. Usually, people use a cutoff distance to determine the coordinated atoms and relevant structural information, such as coordination number and bond angles. Recently, the electron localization function (ELF) has been used to analyze the local structure of amorphous Ge2Sb2Te5. But how to determine the EFL threshold and cutoff distance has not been reported. Here, according to the ab-initio calculations, we systematically investigate the relationship between the bond number and the ELF threshold, and also the cutoff distance in amorphous GeTe. The reasonable value of the ELF threshold and the cutoff distance are determined according to the inflection point and slope change of the bond number with ELF value respectively. Furthermore, the minimal ELF value distributions of Ge-Ge, Ge-Te and Te-Te bonds are presented. The comparison shows that the majority of removed bonds in structural analysis are weak Ge-Te bonds due to the low localization degree of electron. In contrast, the stronger Ge-Ge bonds are almost unchanged when changing the ELF threshold value from 0.58 to 0.63 because of the high localization degree of electron. The average minimal ELF value of Ge-Te bonds in crystalline GeTe is calculated, and it is close to the ELF threshold that is determined by the inflection point. t is easy to find that the Ge-Te bonds which are removed by increasing the ELF threshold are relatively weak. Therefore, these weaker bonds should be removed in structure analysis, which also means that the ELF threshold determined by the inflection point are reasonable value. Finally, based on the EFL threshold value, the coordination number and bond angle distribution of Ge in amorphous GeTe are obtained. The analysis of the coordination number of the Ge atoms shows that as the ELF threshold increases from 0.58 to 0.63, the 5- fold Ge atoms almost disappear because they are against the (8-N) rule. Furthermore, when the ELF threshold value is 0.58, the bond angle distribution analysis of Ge atoms shows that the local structure is a configuration that is mainly defectively octahedral (3-fold Ge) and distorted tetrahedral (4-fold Ge), but it remains unchanged when the threshold value increases to 0.63. It further demonstrates that all the removed chemical bonds are weaker ones as the ELF threshold increases. This approach is useful to improve the accuracy of amorphous structure analysis by obtaining the more reasonable inter-atomic bonding information. And it should be applied to the structural analyses of other systems generally.

Tellurium speciation, connectivity, and chemical order in As(x)Te(100-x) glasses: results from two-dimensional 125Te NMR spectroscopy.

The short-range structure, connectivity, and chemical order in As(x)Te(100-x) (25 ≤ x ≤ 65) glasses are studied using high-resolution two-dimensional projection magic-angle-turning (pjMAT) (125)Te nuclear magnetic resonance (NMR) spectroscopy. The (125)Te pjMAT NMR results indicate that the coordination of Te atoms obeys the 8-N coordination rule over the entire composition range. However, in strong contrast with the analogous glass-forming As-S and As-Se chalcogenides, significant violation of chemical order is observed in As-Te glasses over the entire composition range in the form of homopolar As-As (Te-Te) bonds, even in severely As (Te)-deficient glasses. The speciation of the Te coordination environments can be explained with the dissociation reaction model As2Te3 → 2As + 3Te(II), characterized by a dissociation constant that is independent of glass composition. These structural characteristics can be attributed to the high metallicity of Te and the strong energetic similarity between the Te-Te, Te-As, and As-As bonds, and they are consistent with the monotonic and often nearly linear variation of physical properties observed in telluride glasses as a function of the Te content.

Coherent phonon modes of crystalline and amorphous Ge2Sb2Te5 thin films: A fingerprint of structure and bonding

Femtosecond optical pump-probe measurements have been made upon epitaxial, polycrystalline, and amorphous thin films of Ge2Sb2Te5 (GST). A dominant coherent optical phonon mode of 3.4 THz frequency is observed in time-resolved anisotropic reflectance (AR) measurements of epitaxial films, and is inferred to have 3-dimensional T2-like character based upon the dependence of its amplitude and phase on pump and probe polarization. In contrast, the polycrystalline and amorphous phases exhibit a comparatively weak mode of about 4.5 THz frequency in both reflectivity (R) and AR measurements. Raman microscope measurements confirm the presence of the modes observed in pump-probe measurements, and reveal additional modes. While the Raman spectra are qualitatively similar for all three phases of GST, the mode frequencies are found to be different within experimental error, ranging from 3.2 to 3.6 THz and 4.3 to 4.7 THz, indicating that the detailed crystallographic structure has a significant effect upon the phonon frequency. While the lower frequency (3.6 THz) mode of amorphous GST is most likely associated with GeTe4 tetrahedra, modes in epitaxial (3.4 THz) and polycrystalline (3.2 THz) GST could be associated with either GeTe6 octahedra or Sb-Te bonds within defective octahedra. The more polarizable Sb-Te bonds are the most likely origin of the higher frequency (4.3–4.7 THz) mode, although the influence of Te-Te bonds cannot be excluded. The effect of high pump fluence, which leads to irreversible structural changes, has been explored. New modes with frequency of 3.5/3.6 THz in polycrystalline/amorphous GST may be associated with Sb2Te3 or GeTe4 tetrahedra, while a 4.2 THz mode observed in epitaxial GST may be related to segregation of Sb.