Virgil C. Solomon

University of Connecticut, Storrs, Connecticut, United States

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Publications (14)81.15 Total impact

  • Poonam Singh, Virgil C. Solomon, Daniel A. Buttry
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    ABSTRACT: Colloidal silver nanoparticles (Ag NPs) capped with adenosine triphosphate (ATP) were prepared using borohydride reduction of Ag+ in the presence of ATP. Subsequent characterization was done using transmission electron microscopy/high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and non-contact atomic force microscopy (NcAFM) confirming the size and composition of the Ag NPs. This report focuses on two topics: (1) the change in NP size and properties as a function of molar ratios of Ag+ to ATP capping ligand to BH4 − reductant, and (2) the electrochemical behavior of the NPs in layer-by-layer (LbL) multilayer films. On the basis of electrostatic interaction between negatively charged phosphate groups on Ag NPs and positively charged poly(diallyldimethylammonium) hydrochloride, NPs were immobilized on 3-mercaptopropionic acid (MCP)-functionalized gold electrodes using LbL assembly method followed by characterization of the film using NcAFM. Furthermore, the redox chemistry for phase transformations of immobilized Ag NPs to AgCl or Ag2O in multilayer films was examined using cyclic voltammetry (CV) in NaOH and NaCl solutions. A non-linear increase of charge with an increase in the number of bilayers in the film was observed up to five layers. Underpotential deposition of Pb on multilayer film of Ag NPs confirmed the presence of Ag in multilayer films. The stability of the LbL film toward electrochemical cycling to higher potentials (i.e., +0.8 V) in NaOH solutions was evaluated.
    Journal of Nanoparticle Research 07/2014; 16(7). DOI:10.1007/s11051-014-2496-x · 2.28 Impact Factor
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    ABSTRACT: Triphenylphosphine (PPh3)−capped 1.8 nm diameter gold nanoparticles (AuNPs) are characterized by a combination of 1H, 2H, and 31P solution− and solid−state NMR. The 31P{1H} NMR resonance associated with the surface−bound PPh3 is clearly identified and is present as a broad peak centered at 56 ppm. 31P and 1H hole burning NMR experiments show that the line broadening associated with the surface−bound PPh3 is primarily due to a variety of different chemical shift environments at the surface of the nanoparticles. The surface bound PPh3 can be displaced with either d15−PPh3 or Au(d15−PPh3)Cl in CD2Cl2 solution. In both cases, exchange results in loss of Au(PPh3)Cl from the nanoparticle surface, with no evidence for loss of the PPh3 ligand alone. Solution−state NMR was used to determine the room temperature rate constants for these exchange processes, with values of 0.17 and 0.20 min−1, respectively. Thus, essentially the same rate is observed for displacement of Au(PPh3)Cl from the surface with either d15−PPh3 or Au(d15−PPh3)Cl. The observed 31P chemical shift of surface−bound PPh3 is consistent with mixed valence Au(0) and Au(I) at the nanoparticle surfaces, suggesting the presence of surface−bound Au complexes.
    The Journal of Physical Chemistry C 09/2009; 113(37). DOI:10.1021/jp905141h · 4.84 Impact Factor
  • Microscopy and Microanalysis 07/2009; 15. DOI:10.1017/S1431927609097992 · 1.76 Impact Factor
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    Microscopy and Microanalysis 07/2008; 14:1138 - 1139. DOI:10.1017/S1431927608087205 · 1.76 Impact Factor
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    ABSTRACT: By spark-eroding Fe75Si15B10 in water/ethanol mixtures, spherical particles with nanostructured cores consisting of mixed amorphous and crystalline phases were produced. The relative volume fractions of the amorphous and crystalline phases were dependent on the water/ethanol ratio. In the same process, continuous oxide layers were formed on the particle surfaces. The basic mechanisms responsible for the formation of the surface oxide layers and the core nanostructures were modeled. At frequencies ranging from 1 to 100 MHz, the combination of the core nanostructures and the insulating oxide shells yielded exceptionally low-loss magnetic behavior.
    05/2008; 23(06):1758 - 1763. DOI:10.1557/JMR.2008.0199
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    ABSTRACT: Although the majority of glasses in use in technology are complex mixtures of oxides or chalcogenides, there are numerous examples of pure substances-'glassformers'-that also fail to crystallize during cooling. Most glassformers are organic molecular systems, but there are important inorganic examples too, such as silicon dioxide and elemental selenium (the latter being polymeric). Bulk metallic glasses can now be made; but, with the exception of Zr50Cu50 (ref. 4), they require multiple components to avoid crystallization during normal liquid cooling. Two-component 'metglasses' can often be achieved by hyperquenching, but this has not hitherto been achieved with a single-component system. Glasses form when crystal nucleation rates are slow, although the factors that create the slow nucleation conditions are not well understood. Here we apply the insights gained in a recent molecular dynamics simulation study to create conditions for successful vitrification of metallic liquid germanium. Our results also provide micrographic evidence for a rare polyamorphic transition preceding crystallization of the diamond cubic phase.
    Nature 09/2007; 448(7155):787-90. DOI:10.1038/nature06044 · 42.35 Impact Factor
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    ABSTRACT: Phosphonic acid capped SnO2 nanoparticles with diameters less than 5 nm were synthesized and characterized with multinuclear solution and solid-state magic angle spinning (MAS) NMR. Two types of phosphonic acid ligands were used to derivatize the SnO2 surface, producing (i) water soluble SnO2 nanoparticles capped with 2-carboxyethanephosphonic acid (CEPA) and (ii) insoluble SnO2 nanoparticles capped with phenylphosphonic acid (PPA). Multiple surface environments were observed with 31P solution and solid-state MAS NMR for both capping agents. The 31P resonances of derivatized SnO2 nanoparticles display isotropic chemical shifts that are more shielded compared to the native phosphonic acids. This observation is indicative of a strong interaction between the phosphonic acid group and the SnO2 surface. 1H MAS NMR spectra display a complete absence of the acidic protons of the phosphonic acid groups, strongly supporting the formation of P−O−Sn linkages. 1H → 31P cross polarization (CP) build-up behavior confirms the absence of the vast majority of phosphonic acid protons. Some of the build-up curves displayed oscillations that could be fit to extract the magnitude of the 1H−31P dipolar coupling constant. The dipolar coupling can then be used to calculate the distance between phosphorus and the close proximity protons. The results presented herein indicate primarily bi- and tridentate phosphonic acid bonding configuration at the SnO2 surface, in both CEPA and PPA capped nanoparticles.
    Chemistry of Materials 04/2007; 19(10). DOI:10.1021/cm062821u · 8.54 Impact Factor
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    ABSTRACT: The microstructural and chemical characteristics of Ni–Mn–Ga particles produced by spark-erosion using liquid argon or liquid nitrogen dielectrics were investigated by electron microscopy techniques. The particles were predominantly micron-sized spheres; those obtained in argon were solid, whereas those obtained in nitrogen were primarily hollow. The investigation of annealed particles revealed twinned microstructure and provided evidence supporting the cryogen solubility model of hollow particle formation, as well as possibly explaining the non-homogeneous structure of particles formed by spark-erosion in liquid nitrogen.
    Scripta Materialia 04/2007; 56(7-56):593-596. DOI:10.1016/j.scriptamat.2006.12.034 · 2.97 Impact Factor
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    ABSTRACT: Spherical Fe–Ga particles were prepared by spark erosion in liquid Ar, which directly incorporated the desirable rapid quench from high temperatures. The compositions of the particles investigated were 15.0, 16.3, and 18.9 at. % Ga, respectively, as determined from electron-probe microanalysis, x-ray diffraction, and Mössbauer spectra. Composites for magnetostriction measurements were prepared by mixing particles with epoxy at the volume fraction of 48% and curing in a magnetic field. Magnetostriction values of the composites were comparable to those of polycrystalline chill-cast alloys of the same compositions. Composites with particles having Ga concentrations of 18.9 at. % had the highest magnetostriction, similar to results reported for bulk Fe–Ga alloys.
    Applied Physics Letters 10/2006; 89(14). DOI:10.1063/1.2358825 · 3.52 Impact Factor
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    ABSTRACT: Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005.
    Microscopy and Microanalysis 08/2005; 11(S02):1832-1833. DOI:10.1017/S1431927605507098 · 1.76 Impact Factor
  • YJ Tang, Virgil C. Solomon, D. J. Smith, H. Harper, AE Berkowitz
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    ABSTRACT: The magnetic entropy change of tetragonal and orthorhombic NiMnGa fine particles made by spark erosion was investigated in this paper. It was found that the structure and crystalline phase transformation temperatures can be strongly affected by the compositions of the particles, while Curie temperature is less sensitive to the compositions. Due to the possible distribution of the particle size and compositions in these particles, the magnetic entropy changes observed are much broader and smaller than those of bulk NiMnGa alloys. The maximum absolute value of entropy change ΔS = 2 J Kg−1 K−1 was observed for tetragonal structure NiMnGa particles at 95 °C in a field of 2 T.
    Journal of Applied Physics 05/2005; 97(10):10M309-10M309-3. DOI:10.1063/1.1852451 · 2.19 Impact Factor
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    ABSTRACT: Spherical particles of the ferromagnetic shape memory material Ni51Mn29Ga20 obtained by spark erosion transform during cooling from the high-temperature Heusler L21 cubic phase into tetragonal martensite. Using the Fresnel (i.e., Lorentz) imaging mode, magnetic domains with an average width of 100 nm are observed in the modulated martensitic phase in the absence of a magnetic field. The magnetization distribution within individual particles is determined using electron holography. The magnetic flux lines change direction when crossing boundaries between crystallographic twin variants. These boundaries, where the easy c-axis of the crystallographic variants rotate by 86.5°, coincide with quasi-90° magnetic domain walls, with thickness of approximately 17 nm. The magnetization saturation determined by electron holography is about 0.57 T.
    Applied Physics Letters 05/2005; 86(19). DOI:10.1063/1.1925319 · 3.52 Impact Factor
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    ABSTRACT: Hollow spherical particles of Ni, CoFe, the ferromagnetic shape memory alloy Ni49Mn30Ga21, and the giant magnetostrictive alloy Fe83Ga17, with diameters up to several tens of microns were produced by spark erosion, using liquid nitrogen as the dielectric liquid. In contrast, the particles were primarily solid when the dielectric liquid was argon. The wall thicknesses of the hollow particles depended on the elemental composition. Different models are considered to account for the formation of the spark-eroded hollow spheres, and some of the potential benefits to be derived from their use are described.
    Applied Physics Letters 08/2004; 85(6):940-942. DOI:10.1063/1.1779962 · 3.52 Impact Factor
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    ABSTRACT: Powders of Ni-Mn-Ga ferromagnetic shape memory alloy (FSMA) were produced by spark erosion. Powders quenched in liquid nitrogen are primarily hollow spherical particles, whereas those quenched in liquid argon are mostly solid spheres. Electron microscopy observations of powders as-sparked in nitrogen show highly disordered nanocrystalline grains with an average size of less than 100 nm whereas those sparked in liquid argon display extensive crystallinity. Magnetic measurements indicate that the powders do not become fully ferromagnetic until after annealing at 973 K for 5 h. Investigations of annealed powders using differential scanning calorimetry reveal thermoelastic martensitic transition behavior. The annealed powders have a microtwinned martensite with many large grains showing a five-layer period modulation. These Ni-Mn-Ga powders should be suitable for the development of magnetic-field-driven FSMA powder/polymer microcomposites. © 2004 American Institute of Physics.
    Journal of Applied Physics 05/2004; 95(11):6954-6956. DOI:10.1063/1.1687204 · 2.19 Impact Factor

Publication Stats

200 Citations
81.15 Total Impact Points

Institutions

  • 2008–2009
    • University of Connecticut
      • Institute of Materials Science
      Storrs, Connecticut, United States
  • 2004–2009
    • Arizona State University
      • • Department of Chemistry and Biochemistry
      • • Department of Physics
      Phoenix, Arizona, United States