Structural and Electrical Characterization of Diatom Pinnularia sp. Biosilica Coated with CdS Thin Film
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Structural and Electrical Characterization of Diatom Pinnularia sp. Biosilica Coated with CdS Thin Film Timothy Gutu1, Clayton Jeffryes2, Wei Wang2, Chih-hung Chang2, Gregory L. Rorrer2, Jun Jiao1 1. Physics, Portland State University, Portland, OR, USA. 2. Chemical Engineering, Oregon State University, Corvallis, OR, USA.
ABSTRACT Diatoms are unicellular, photosynthetic microalgae that live in marine and freshwater environments. The cell walls of diatoms are composed of biosilica and have exceedingly hierarchical ornate nanostructures. Consequently, these nanostructures have long been regarded as the paradigm for future silica nanotechnology. We have coated diatom Pinnularia sp. biosilica with a thin film of CdS using a chemical bath deposition technique. Possible uses for these CdS coated diatoms include the development of new nanodevice fabrication techniques and optoelectronic applications. Electron microscopy techniques were utilized to study their morphologies. Their electrical characteristics were investigated using an Agilent 4156C precision semiconductor parameter analyzer and a Cascade probe station. The CdS coating was found to be dense, adherent and nanostructured. The diatoms coated with CdS exhibited both metallic and semiconductor diode behavior.
INTRODUCTION Diatoms are unicellular, photosynthetic microalgae that live in marine and freshwater environments [1-3]. They absorb soluble Si(OH)4 (silicic acid) from their environment to make porous silica (SiO2) shells or frustules with ornately intricate hierarchical architectures at the submicron- and nanoscale [4]. Thus the diatom frustules are composed of hydrated silica, SiO2.nH2O. The shape and size of the frustule is very different among the more than 100,000 known species of diatoms. They are categorized into centric diatoms and pennate diatoms depending on their symmetry. Centric diatoms are radially symmetric whereas pennate diatoms are bilaterally symmetric. Due to their
interesting characteristics, diatoms have recently been touted to be used in nanotechnology to fabricate three-dimensional devices with nanoscale features [5-7]. However, the applications of diatom frustules are limited by the physical and chemical properties of the dielectric silica. The diatom silica has been converted into desirable materials such as BaTiO3, MgO and TiO2 [4]. In this study, we report how the surface chemistry of the biosilica was altered by coating the frustules of diatom Pinnularia sp. with a semiconductor cadmium sulfide (CdS) to introduce new properties.
EXPERIMENTAL DETAILS A chemical bath deposition (CBD) technique was used to coat the diatom biosilica with CdS. The bath solution contained cadmium chloride (CdCl2, 0.004_mol/L) as a source of cadmium ions, ammonium chloride (NH4Cl, 0.04 mol/L) as a buffer, thiourea ((NH2)2CS, 0.04 mol/L) as source of sulfide ions, and ammonium hydroxide (NH4OH, 0.4 mol/L) to maintain the pH at 11. The precursor solutions were prepared separately. The bath temperature was kept at 80 ºC during the depositi
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