Kristin M. Poduska

Decoupling Local Disorder and Optical Effects in Infrared Spectra: Differentiating Between Calcites with Different Origins

Infrared spectral peak broadening due to atomic disorder and narrowing due to particle-size-related optical absorption effects can be decoupled experimentally and theoretically. Applied to different sources of polycrystalline calcite, the method provides a powerful diagnostic tool for archaeology, geology, and materials/biomaterials science.

Roughness effects on contact angle measurements

We have developed a simple and economical procedure to demonstrate the effects of roughness and wetting fraction on the equilibrium contact angles of liquid droplets on solid surfaces. Contact angles for droplets placed on a rough surface, which wet only a portion of the surface, are larger than the contact angles of droplets formed by condensation of steam, which wet the surface more completely. These contact angle data facilitate assessments of changes in true surface area, due to surface roughening, as well as changes in the fractional contact areas of the water droplets, due to the formation of air pockets between the rough surface and the droplet.

Structure determination of La3CuGeS7 and La3CuGeSe7

Abstract We have synthesized La 3 CuGeS 7 and La 3 CuGeSe 7 and characterized their structures from single crystal X-ray diffraction data. From lattice constant data alone, Guittard et al. [C.R. Acad. Sci. Paris 267 (1968) 767; Bull. Soc. Chim. Fr. 7 (1970) 2467] had assumed that both of these phases were members of a large family of hexagonal compounds with the general formula Ln 6 MS 6 (TX 4 ) 2 or Ln 6 M 2 ′S 6 (TX 4 ) 2 , where Ln=lanthanide element, M=divalent element (Mg, Ni, Mn), M′=monovalent element (Cu, Ag), T=(Ge, Si, Al or Fe), and X=(S, Se); our data confirm this assumption. La 3 CuGeS 7 and La 3 CuGeSe 7 both contain isolated [Ge(S,Se)] 4− tetrahedra, trigonal planar Cu(S,Se) 3 units, and La counter-cations. Consistent with its assignment to a non-centrosymmetric space group ( P 6 3 ), La 3 CuGeSe 7 shows a weak second harmonic generation response when irradiated with 1064-nm laser light.

Dynamics, crystallization and structures in colloid spin coating

Spin coating is an out-of-equilibrium technique for producing polymer films and colloidal crystals quickly and reproducibly. In this review, we present an overview of theoretical and experimental studies of the spin coating of colloidal suspensions. The dynamics of the spin coating process is discussed first, and we present insights from both theory and experiment. A key difference between spin coating with polymer solutions and with monodisperse colloidal suspensions is the emergence of long range (centimeter scale) orientational correlations in the latter. We discuss experiments in different physical regimes that shed light on the many unusual partially ordered structures that have long-range orientational order, but no long-range translational order. The nature of these structures can be tailored by adding electric or magnetic fields during the spin coating procedure. These partially ordered structures can be considered as model systems for studying the fundamentals of poorly crystalline and defect-rich solids, and they can also serve as templates for patterned and/or porous optical and magnetic materials.

Electrochemically controlled growth and positioning of suspended collagen membranes.

Two independently recognized in vitro polymer aggregation variables, electric field and pH, can be used in concert to produce suspended membranes from solutions of type I collagen monomers, without need of a supporting substrate. A collagen network film can form at the alkaline-acidic pH interface created during the normal course of water electrolysis with parallel plate electrodes, and the anchoring location can be controlled by adjusting the bulk electrolyte pH. Electrosynthesized films remain intact upon drying and rehydration and function as ion-separation membranes even in submillimeter channels. This approach could benefit lab-on-a-chip technologies for the rational placement of membranes in microfluidic devices.

Structural and thermopower studies of CeNiAl4- and CeNiIn4-related compounds

Abstract The CeNi1−xCoxAl4 (x=0–0.5), CeNiIn4−xGax (x=0–0.75), and CeNiAl4−xInx (x=0–0.25) solid solutions are isotypic to CeNiAl4, a known intermediate valent (IV) material. We present the lattice parameters and temperature dependences of the Seebeck coefficients for a range of alloys from each of these solid solutions. In several cases, including CeNi0.75Co0.25Al4 and CeNiGaIn3, the thermopowers of these alloys are significantly higher than those for the parent compounds (x=0). We also report strong evidence for stacking faults in single crystals of some of these materials.

Selective formation of Ohmic junctions and Schottky barriers with electrodeposited ZnO

Constant-potential electrochemical synthesis of ZnO on metal substrates enables selective formation of either Ohmic or Schottky-barrier contacts. Using a mildly acidic nitrate-based aqueous electrolyte, there is a substrate-dependent deposition potential below which electrodeposited ZnO heterojunctions display Schottky response with high contact resistances (∼105Ω) and above which Ohmic behavior and low contact resistances (∼1Ω) occur. Voltammetric evidence for Zn metal deposition, in conjunction with Schottky-barrier heights that are consistent with values expected for a ZnO–Zn junction, suggests that more negative deposition potentials create a Zn-based interface between the substrate and ZnO that leads to rectifying behavior.

Correlating mechanical properties with aggregation processes in electrochemically fabricated collagen membranes.

We show that mechanical stiffness is a useful metric for characterizing complex collagen assemblies, providing insight about aggregation products and pathways in collagen-based materials. This study focuses on mechanically robust collagenous membranes produced by an electrochemical synthesis process. Changing the duration of the applied electric field, or adjusting the electrolyte composition (by adding Ca(2+), K(+), or Na(+) or by changing pH), produces membranes with a range of Youngs moduli as determined from force-displacement measurements with an atomic force microscope. The structural organization, characterized by UV-visible spectroscopy, Raman spectroscopy, optical microscopy, and atomic force microscopy, correlates with the mechanical stiffness. These data provide insights into the relative importance of different aggregation pathways enabled by our multiparameter electrochemically induced collagen assembly process.

Tuning magnetic hysteresis of electrodeposited Fe3O4

We demonstrate that changes in electrolyte composition and applied potential during aqueous electrodeposition can be used to tune the magnetic hysteresis response of thin-film Fe3O4 (magnetite) on polycrystalline metal substrates. X-ray diffraction data confirmed that magnetite formation in electrolytes containing KCH3COO (0.04–2.0M) and Fe(SO4)2(NH4)2 (0.01M) required temperatures between 60 and 85°C, and deposition potentials between −0.300 and −0.575V or galvanostatic current densities between 50 and 88μA∕cm2. Scanning electron microscopy studies show that magnetite crystallites tend to adopt different habits depending on the electrolyte composition. Room-temperature magnetic hysteresis responses (squareness and coercivity) are dependent upon the crystal habit of deposits, implying that the electrolyte’s acetate concentration influences the magnetic domain structure of the resulting magnetite deposits. Magnetite crystallites grown from electrolytes with low acetate concentrations showed pseudo-single-d...

Structure and physical properties of CeSbTe

Abstract The structure of CeSbTe was determined by the Rietveld refinement method using powder X-ray diffraction ( R p =6.22% and wR p =8.74%). CeSbTe adopts the PbFCl structure type and crystallizes in the space group Pnma (No. 62) of the orthorhombic system with Z =4 and a =19.0404(6) A, b =4.3520(1) A, c =4.3660(2) A, V =361.78(2) A 3 . It can be viewed as a layered compound which contains an Sb −1 layer distorted from a perfect square net into zig-zag chains. Both its crystallographic and electronic structure indicate a 2-D character. Based on transport properties and band structure calculations, CeSbTe is a metal with a low carrier concentration. Additionally, it exhibits paramagnetic behavior typical of Ce 3+ .