Research

 

  • Synthesis of Free Standing Silica Thin Films with Highly Ordered Nanopores
  • The synthesis of mesoporous silica thin films on nontraditional conductive substrates using an electro-assisted self-assembly (EASA) technique is described.  This work extends prior demonstrations of EASA by exploring the effects of precursor sol pH, temperature, and substrate roughness and describes a new approach to synthesizing a mesoporous silica film that is detachable from the substrate. The latter uses a conductive polymer as a planarization layer for successful EASA on indium-tin oxide coated glass (ITO) and as a sacrificial layer that can be subsequently dispersed in water to release the silica film.  This is a particularly important development because it opens up possibilities for synthesizing perpendicularly aligned nanoporous silica on a broad range of surfaces and non-conductive substrates and producing free standing nanostructured thin films. The silica films that are produced have well ordered hexagonally packed mesopores that are vertical to the substrate surface. The thicknesses of these mesoporous silica films were examined by scanning electron microscopy (SEM) and determined to be controllably variable between ~100 – 200 nm. Transmission electron microscopy (TEM) showed organized porous structural features that were approximately 3 nm in diameter. Grazing-incidence small angle X-ray scattering (GISAXS) analysis yielded an ~4.2 nm pore-to-pore distance and confirmed that a p6mm orientation persisted throughout the 6 cm2 mesoporous silica film samples.

    Publication:

    J. Cheng, S. Rathi, P. Stradins, G. Frey, R. T. Collins, S. K. R. Williams, RSC Adv., 4 (15), 7627 - 7633 (2014)

     

  • Field-flow Fractionation

    One of the grand challenges in the photoelectrochemical splitting of water is the energetic mismatch between the semiconductor band edges and the water redox reactions at the semiconductor /electrolyte interface. Successfully controlling the interfacial energetics would be a significant advance towards an efficient commercial solar hydrogen process. Our group designs and prepares materials that can be deposited on semiconductor electrodes and tune their workfunction over a 2 eV range. In this project we collaborate with Dr. Dana Olson's group at NREL.

    Publication:

    S. K. R. Williams, K. D. Caldwell, Anal. Bioanal. Chem., 406, 1577-1578 (2014)

  •  

  • Photon Correlation Spectroscopy Coupled with Field-Flow Fractionation for Polymer Analysis
  • The effects of analyte concentration, flow rate, scattering angle, and MW on the accuracy of D measurements from online PCS were evaluated by comparison with values obtained in batch mode. Analyte concentrations greater than 0.1 mg mL-1 at the detector and measurement at small scattering angles are desired for accurate online D determinations. While the absolute numbers reported in this study are instrument specific, the general observations are expected to be applicable across different instruments.

    A S/N ratio > 2.5 was established as the threshold for maintaining errors in D below 10%. This criterion was applied when obtaining average D results for ThFFF/PCS of polymers in organic solvent and FlFFF/PCS of proteins in aqueous solution. Excellent agreement was obtained between FFF/PCS and batch mode D values. The use of an optimized instrument configuration was crucial to successful ThFFF/PCS determinations of D and DT. The combination of a separation stage with online PCS has enabled the successful characterization of subpopulations present in mixtures or high polydispersity samples which would otherwise remain hidden in traditional batch mode measurements. The guidelines presented for online PCS are shown to be valid for both lipophilic and hydrophilic analytes, and are expected to be applicable to other separation techniques.

    Publication:

    J. R. Runyon, S. K. R. Williams,  in Handbook of Spectroscopy, 2nd Edition, G. Gauglitz, D.S. Moore, Eds., Chapter 32, pp 1201-1228, Wiley-VCH Verlag GmbH & Co. KgaA, Weinheim Germany, 2014.  Print ISBN: 978-3-527-32150-6

  • Shear-Induced Structures and Thickening in Fumed Silica Slurries

  • Chemical mechanical polishing (CMP) is an essential technology used in the semiconductor industry to polish and planarize a variety of materials for the fabrication of microelectronic devices (e.g., computer chips). During the high shear (∼1,000,000 s−1) CMP process, it is hypothesized that individual slurry particles are driven together to form large agglomerates (≥0.5 mm), triggering a shear thickening effect. These shear-induced agglomerates are believed to cause defects during polishing. In this study, we examined the shear thickening of a 25 wt % fumed silica slurry with 0.17 M added KCl using in situ small-angle light scattering during rheological characterization (rheo-SALS). The salt-adjusted slurry displays a ∼3-fold increase in viscosity at a critical shear rate of 20,000 s−1 during a stepped shear rate ramp from 100 to 25,000 s−1. As the shear rate is reduced back to 100 s−1, the slurry displays irreversible thickening behavior with afinal viscosity that is 100-times greater than the initial viscosity. Corresponding rheo-SALS images indicate the formation of micrometer scale structures (2−3μm) that directly correlate with the discontinuous and irreversible shear thickening behavior of the fumed silica slurry; these micrometer scale structures are 10-times the nominal particle diameter (∼0.2 mm). The scattering patterns from the 25 wt % slurry were corroborated through rheo-SALS examination of 27 and 29 wt % slurries (CKCl = 0.1 M). All slurries, regardless of ionic strength and solids loading, display scattering patterns that are directly associated with the observed thickening behavior. Scattering was only observable during and after thickening (i.e., no scattering was detected in the absence of thickening). This work serves as the first in situ observation of micrometer scale structures within the fumed silica CMP slurry while under shear.

    Publication:

    N. C. Crawford, S. K. R. Williams, D. Boldridge, M. L. Liberatore, Langmuir, 29, 12915-12923 (2013).