Daniel G. Georgiev

Ageing, fragility and the reversibility window in bulk alloy glasses

Non-reversing relaxation enthalpies (ΔH(nr)) at glass transitions T(g)(x) in the P(x)Ge(x)Se(1-2x) ternary display wide, sharp and deep global minima ([Formula: see text]) in the 0.09<x<0.145 range, within which T(g) s become thermally reversing. In this reversibility window, glasses are found not to age, in contrast to ageing observed for fragile glass compositions outside the window. Thermal reversibility and lack of ageing seem to be paradigms of self-organization which molecular glasses share with protein structures which repetitively and reversibly change conformation near T(g) and the folding temperature respectively.

Intrinsic nanoscale phase separation of bulk As2S3 glass

Raman scattering on bulk As x S1− x glasses shows that vibrational modes of As4S4 monomer first appear near x = 0.38, and their concentration increases precipitously with increasing x, suggesting that the stoichiometric glass (x = 0.40) is intrinsically phase separated into small As-rich (As4S4) and large S-rich clusters. Support for the Raman-active vibrational modes of the orpiment-like and realgar-like nanophases is provided by ab-initio density functional theory calculations on appropriate clusters. Nanoscale phase separation provides a basis for understanding the global maximum in the glass transition temperature T g near x = 0.40, and the departure from Arrhenius temperature activation of As2S3 melt viscosities.

Reversible Gelation of II–VI Nanocrystals: The Nature of Interparticle Bonding and the Origin of Nanocrystal Photochemical Instability

Semiconducting nanocrystals (NCs) with dimensions smaller than the bulk exciton Bohr radius exhibit unique, size-tunable opto-electronic properties due to quantum confinement effects. Accordingly, there has been tremendous interest in the synthesis and characterization of colloidal semiconductor NCs, and they have been investigated for a variety of applications ranging from biological labeling and diagnostics to photovoltaics, photodetectors, sensors, and catalysts. However, the stability of colloidal NCs is a major issue in many of these applications. The most common method of stabilization is by chemically attaching ligands to the surface atoms of the NCs. Consequently, semiconducting NCs are generally synthesized in the presence of coordinating surfactant ligands, for example, trioctylphosphine oxide, which confers dispersibility in non-polar media. For biological applications, dispersibility in polar media (i.e. water) is needed and this is often achieved by substitution of nonpolar surfactant ligands with carboxylate-terminated thiolates by treatment with mercaptoundecanoic acid, mercaptoacetic acid, dihydrolipoic acid, etc., in base. However, the stability of thiolate coated II–VI semiconductor NCs, such as CdSe, is typically poor, often leading to precipitation. In 2001, Peng and co-workers reported a detailed study of the photochemical instability of thiolate-capped CdSe NCs. The photooxidation of the thiolate ligands on the NCs can be catalyzed by the CdSe NCs in the presence of light and O2, producing disulfides and thus, effectively decomplexing the particle en route to aggregation. However, if free thiols are present in the solution, they can replace the thiolates lost as disulfide, dispersing aggregates as they form and prolonging the stability of the sol. We and others have exploited oxidative removal of the thiolates as a means to link particles together into three-dimensional architectures (gels), in which metal chalcogenide NCs are assembled into porous network structures. Importantly, analyses of oxidized CdS sols have shown that the thiols and their oxidized products (disulfides and sulfonates) can be completely removed from the gel network, and are therefore not participating in interparticle bonding, which leads to the conclusion that CdS NCs are physically connected to each other without any organic linkers. The extent of particle interaction in the network (and the related extent of quantum confinement) is found to be a direct function of the dimensionality of the network, itself controlled by the density. Such architectures are of interest for applications requiring maximal transport of charge (through the gel network) and small molecules (through the interconnected pore network), such as sensing and photocatalysis. Here we show that, analogous to the work of Peng et al., thiolates can be employed to break up the gel network into its constituent NCs. Additionally, for the first time, we probe the properties of the interparticle bonding in CdSe gels and aggregates. Dispersion studies were performed principally with CdSe gels, aerogels and xerogels prepared from oxidation of high temperature prepared NCs that were capped with 11-mercaptoundecanoic acid (MUA) in the presence of base (tetramethylammonium hydroxide (TMAH)). Treatment of CdSe wet gels with fresh methanolic solutions of MUA and TMAH (pH 12.0) results in formation of a sol that is visually identical to the precursor NCs (Figure 1) in 2–3 min. Like-

Evidence for nanoscale phase separation of stressed–rigid glasses

Ternary (Ge2X3)x(As2X3)1−x glasses with X = S or Se are of interest because they span a mean coordination number r in the 2.40 < r < 2.8 range that is characteristic of stressed–rigid glasses. We have examined X = S glasses in Raman scattering and T-modulated differential scanning calorimetry measurements over the 0 < x < 1.0 range. Glass transition temperatures, Tg(x), increase monotonically in the 0 < x < 0.8 range and decrease thereafter (0.8 < x < 1) to display a global maximum near x = 0.8. Raman scattering provides evidence of sharp modes of As4S4 and As4S3 monomers, with scattering strength of these modes showing a global maximum near x = 0.3 and 0.5 respectively. The results suggest that at low x (0 < x < 1/2), addition of Ge2S3 to the As2S3 base glass results in insertion of Ge(S1/2)4 tetrahedra in the As(S1/2)3-based backbone as compensating As-rich monomers segregate from the backbone to deliver the requisite S. At higher x (0.4 < x < 0.8), the Ge2S3 additive continues to enter the glass in a majority (As2S3)(GeS2) backbone and several minority nanophases including an ethane-like Ge2(S1/2)6 and a distorted rock-salt-like GeS. In the 0.8 < x < 1 range, the nanophases grow qualitatively at the expense of the backbone as Tg values decrease and the end-member composition (x = 1) is realized. Heterogeneity of glasses near x = 1/2 or mean coordination, r = 2.60 derives intrinsically from the presence of several minority nanophases and a majority backbone showing that stressed–rigid networks usually phase separate on a nanoscale.

Solid state chemistry of bulk mixed metal oxide catalysts for the selective oxidation of propane to acrylic acid

Abstract Syntheses of Mo–V–Sb–Nb–O bulk materials, which are candidate catalyst systems for the selective oxidation of propane to acrolein and acrylic acid, were made using soluble precursor materials. The products were characterized by X-ray powder diffraction and Raman spectroscopic studies. The objectives of this work were to explore the utility of liquid phase automated synthesis for the preparation of bulk mixed metal oxides, and the identification of the oxide phases present in the system. This is the first published study of the phase composition for these materials. After calcination of these bulk oxides under flowing nitrogen at 600°C, and using stoichiometric ratios of Mo–V–Sb–Nb (1:1:0.4:0.4) and Mo–V–Sb–Nb (3.3:1:0.4:0.4) it was demonstrated that a mixture of phases were obtained for the syntheses. X-ray powder diffraction studies distinguished SbVO 4 , Mo 6 V 9 O 40 , MoO 3 , and a niobium-stabilized defect phase of a vanadium-rich molybdate, Mo 0.61–0.77 V 0.31–0.19 Nb 0.08–0.04 O x , as the major phases present. Complementary data were provided by the Raman spectroscopic studies, which illustrated the heterogeneity of the phases present in the mixture. Raman also indicated bands attributable to the presence of phases containing terminal MO bonds as well as M–O–M polycrystalline phases. Previous studies on this system have identified SbVO 4 and niobium-stabilized vanadium molybdate species as the active phases necessary for the selective oxidation of alkanes.

Intermediate phase, network demixing, boson and floppy modes, and compositional trends in glass transition temperatures of binary As x S 1 − x system

The structure of binary

Controllable excimer-laser fabrication of conical nano-tips on silicon thin films

{\text{As}}_{x}{\text{S}}_{1\ensuremath{-}x}

Sharp Rigid to Floppy Phase Transition Induced by Dangling Ends in a Network Glass

glasses is elucidated using modulated differential scanning calorimetry, Raman scattering, IR reflectance, and molar volume experiments over a wide range

Single pulse excimer laser nanostructuring of thin silicon films: Nanosharp cones formation and a heat transfer problem

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Macroscopic phase separation of Se-rich ( x< 1/3) ternary Agy(GexSe1−x)1−y glasses

of compositions. We observe a reversibility window in the calorimetric experiments, which permits fixing the three elastic phases: flexible at