Molecular-Network Transformations in Tetra-Arsenic Triselenide Glassy Alloys Tuned within Nanomilling Platform.

Polyamorphic transformations driven by high-energy mechanical ball milling (nanomilling) are recognized in a melt-quenched glassy alloy of tetra-arsenic triselenide (As4Se3). We employed XRPD analysis complemented by thermophysical heat-transfer and micro-Raman spectroscopy studies. A straightforward interpretation of the medium-range structural response to milling-driven reamorphization is developed within a modified microcrystalline model by treating diffuse peak-halos in the XRPD patterns of this alloy as a superposition of the Bragg-diffraction contribution from inter-planar correlations, which are supplemented by the Ehrenfest-diffraction contribution from inter-atomic and/or inter-molecular correlations related to derivatives of thioarsenide As4Sen molecules, mainly dimorphite-type As4Se3 ones. These cage molecules are merely destroyed under milling, facilitating the formation of a polymerized network with enhanced calorimetric heat-transfer responses. Disruption of intermediate-range ordering, due to weakening of the FSDP (the first sharp diffraction peak), accompanied by an enhancement of extended-range ordering, due to fragmentation of structural entities responsible for the SSDP (the second sharp diffraction peak), occurs as an interplay between medium-range structural levels in the reamorphized As4Se3 glass alloy. Nanomilling-driven destruction of thioarsenide As4Sen molecules followed by incorporation of their remnants into a glassy network is proved by micro-Raman spectroscopy. Microstructure scenarios of the molecular-to-network polyamorphic transformations caused by the decomposition of the As4Se3 molecules and their direct destruction under grinding are recognized by an ab initio quantum-chemical cluster-modeling algorithm.

Radiation-induced modification effects in covalent-network glass formers: Phenomenological description within unified configuration-enthalpy model.

Externally-induced modification activated by high-energy excitation such as γ-irradiation from 60Co source is analyzed in chalcogenide glasses in terms of radiation-structural and glass-relaxation metastability, at the example of sulphides (including stoichiometric arsenic trisulphide, As2S3) and selenides. Unified configuration-enthalpy model evolving conjugated configuration-coordinate (free energy in dependence on configuration coordinate) and thermodynamic enthalpy (temperature deviations in enthalpy, configurational entropy or free volume) diagrams is proposed to describe metastability in these glasses under external influences, such as (1) physical ageing, (2) irradiation, (3) thermal annealing, (4) rejuvenation, and their combinations. The model predicts glass stabilization in the ground state composed of partial sub-states related to (i) irradiation, (ii) rejuvenation, (iii) annealing, as well as (iv) ideal-glass deep states, connected by over-barrier jumping and through-barrier tunneling transitions. This model allows an adequate parameterization of optical responses related to metastability in these glasses defined through blue (bleaching) or red (darkening) shift in their optical transmission spectra.

Equimolar As4S4/Fe3O4 Nanocomposites Fabricated by Dry and Wet Mechanochemistry: Some Insights on the Magnetic-Fluorescent Functionalization of an Old Drug.

Multifunctional nanocomposites from an equimolar As4S4/Fe3O4 cut section have been successfully fabricated from coarse-grained bulky counterparts, employing two-step mechanochemical processing in a high-energy mill operational in dry- and wet-milling modes (in an aqueous solution of Poloxamer 407 acting as a surfactant). As was inferred from the X-ray diffraction analysis, these surfactant-free and surfactant-capped nanocomposites are ß-As4S4-bearing nanocrystalline-amorphous substances supplemented by an iso-compositional amorphous phase (a-AsS), both principal constituents (monoclinic ß-As4S4 and cubic Fe3O4) being core-shell structured and enriched after wet milling by contamination products (such as nanocrystalline-amorphous zirconia), suppressing their nanocrystalline behavior. The fluorescence and magnetic properties of these nanocomposites are intricate, being tuned by the sizes of the nanoparticles and their interfaces, dependent on storage after nanocomposite fabrication. A specific core-shell arrangement consisted of inner and outer shell interfaces around quantum-confined nm-sized ß-As4S4 crystallites hosting a-AsS, and the capping agent is responsible for the blue-cyan fluorescence in as-fabricated Poloxamer capped nanocomposites peaking at ~417 nm and ~442 nm, while fluorescence quenching in one-year-aged nanocomposites is explained in terms of their destroyed core-shell architectures. The magnetic co-functionalization of these nanocomposites is defined by size-extended heterogeneous shells around homogeneous nanocrystalline Fe3O4 cores, composed by an admixture of amorphous phase (a-AsS), nanocrystalline-amorphous zirconia as products of contamination in the wet-milling mode, and surfactant.

Direct investigation of silver photodissolution dynamics and reversibility in arsenic trisulphide thin films by atomic force microscopy.

The dynamics and reversibility of Ag nanoparticle photodissolution into As2S3 chalcogenide thin films have uniquely been directly measured by continuous AFM imaging during patterned optical illumination. The surface morphology, roughness and particle size distribution have thus been spatially and statistically monitored as a function of time, both during and following optical exposure. Photodissolution was observed to proceed via two mechanisms. In one case, nanoparticles abruptly and nearly completely disappeared along a sharp dissolution front traveling laterally at ∼0.19 µm s(-1). Following illumination, similarly sized nanoparticles uniformly reformed on the surface. A more inhomogeneous photodissolution process was separately recorded, clearing irregular ∼1-2 µm patches that grew with time until most of the surface was free of nanoparticles. Post-illumination, surface nanoparticle development and coverage were similarly inhomogeneous, with larger but 50% fewer particles in the final distribution. In every experiment, an initial roughening was detected before the nanoparticle surface coverage visually diminished, indicating the onset of photodissolution at widely distributed energetically and kinetically favored sites which temporarily enhances nanoscale roughness. Such direct studies of surface dynamics are crucial to understanding and ultimately optimizing chalcogenide film applications such as photomasks, optoelectronics media and bio-chemical sensors.

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.

A study of reversible gamma-induced structural transformations in vitreous Ge23.5Sb11.8S64.7 by high-resolution X-ray photoelectron spectroscopy.

The structural origin of reversible gamma-induced effects in vitreous Ge(23.5)Sb(11.8)S(64.7) has been investigated by high-resolution X-ray photoelectron spectroscopy (XPS). The changes in valence band spectrum from gamma-irradiation suggest a decrease of sulfur lone pair electron concentration accompanied by changes in bonding states of S and Ge. The appearance of additional doublets in the core-level XPS spectra of Ge, Sb, and S atoms for gamma-irradiated sample is described by the formation of over- and under-coordinated charged defect pairs (Ge(3)(-)-S(3)(+)) as a result of radiation treatment. The results verify the switching of Ge-S covalent bonds into S-S bonds as the main microstructural mechanism for gamma-induced optical effects in this glass.