Katherine E. Plass

Repeated epitaxial growth and transfer of arrays of patterned, vertically aligned, crystalline Si wires from a single Si(111) substrate

Multiple arrays of Si wires were sequentially grown and transferred into a flexible polymer film from a single Si(111) wafer. After growth from a patterned, oxide-coated substrate, the wires were embedded in a polymer and then mechanically separated from the substrate, preserving the array structure in the film. The wire stubs that remained were selectively etched from the Si(111) surface to regenerate the patterned substrate. Then the growth catalyst was electrodeposited into the holes in the patterned oxide. Cycling through this set of steps allowed regrowth and polymer film transfer of several wire arrays from a single Si wafer.

Radial PN junction, wire array solar cells

Radial pn junctions are of interest in photovoltaics because of their potential to reduce the materials costs associated with cell fabrication. However, devices fabricated to date based on Au-catalyzed vapor-liquid-solid growth have suffered from low open-circuit voltages (to our knowledge the highest reports are 260 mV in the solid state and 389 mV in solid-liquid junctions). Herein we report on the potential of low-cost catalysts such as Cu and Ni to fabricate Si wire arrays with potentially higher minority-carrier lifetimes than is possible with a Au catalyst, as well as on the use of reactive ion etching to fabricate high-purity analogs to vapor-liquid-solid grown arrays.

Phase-selective synthesis of bornite nanoparticles

Nanoparticles of the copper iron sulfide phase bornite (ideally Cu5FeS4) have been synthesized phase selectively. Either the low or high bornite phase can be obtained through alteration of reactant ratios or reaction temperature, revealing a phase-selectivity that results from distinct rates of formation. The phase, shape, size, and composition of these novel nanomaterials are characterized by powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDS). The light absorption behaviour was investigated using ultra-violet/visible/near-infrared spectroscopy (UV/vis/NIR), revealing direct band gaps that are phase-dependent (low bornite, Eg = 0.86 eV and high bornite, Eg = 1.25 eV). The band gap exhibited by high bornite nanoparticles lies in the range of optimal solar energy conversion efficiency for a single-junction photovoltaic, making it a potentially useful light absorber consisting of inexpensive, abundant elements. Lastly, the selective formation of bornite nanoparticles, as opposed to the copper sulphides, chalcocite (Cu2S) and digenite (Cu1.80S), or chalcopyrite (CuFeS2) is demonstrated, suggesting solid solution formation between bornite and digenite nanoparticles.

Self-assembly of isomeric monofunctionalized thiophenes.

Controlling the self-assembly of thiophene-containing molecules and polymers requires a strong fundamental understanding of the relationship between molecular features and structure-directing forces. Here, the effects of ring-substitution position on the two-dimensional self-assembly of monosubstituted thiophenes at the phenyloctane/HOPG interface are studied using scanning tunneling microscopy (STM). The influence of π···π-stacking, hydrogen-bonding, and alkyl-chain interactions are explored computationally. Alteration of the amide attachment point from the 2- to the 3-position induces transformation from head-to-tail packing to head-to-head packing. This may be attributed to canceling of lateral dipoles.