Gayatri Keskar

Publications (10) View all

• Article: Directed self-assembly of hybrid oxide/polymer core/shell nanowires with transport optimized morphology for photovoltaics.

  Shanju Zhang, Candice I Pelligra, Gayatri Keskar, Jie Jiang, Pawel W Majewski, André D Taylor, Sohrab Ismail-Beigi, Lisa D Pfefferle, Chinedum O Osuji
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  ABSTRACT: An entirely bottom-up approach for the preparation of liquid crystalline suspensions of core-shell nanowires for ordered bulk heterojunction photovoltaics is demonstrated. Side-on attachment of polythiophene derivatives to ZnO nanowires promotes a co-axial polymer backbone-nanowire arrangement which favors high hole mobility. This strategy offers structural control over multiple length scales and a viable means of fabricating ordered films over large areas.
  Advanced Materials 11/2011; 24(1):82-7. · 13.88 Impact Factor
• Article: Liquid crystalline order and magnetocrystalline anisotropy in magnetically doped semiconducting ZnO nanowires.

  Shanju Zhang, Candice I Pelligra, Gayatri Keskar, Pawel W Majewski, Fang Ren, Lisa D Pfefferle, Chinedum O Osuji
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  ABSTRACT: Controlled alignment of nanomaterials over large length scales (>1 cm) presents a challenge in the utilization of low-cost solution processing techniques in emerging nanotechnologies. Here, we report on the lyotropic liquid crystalline behavior of transition-metal-doped zinc oxide nanowires and their facile alignment over large length scales under external fields. High aspect ratio Co- and Mn-doped ZnO nanowires were prepared by solvothermal synthesis with uniform incorporation of dopant ions into the ZnO wurtzite crystal lattice. The resulting nanowires exhibited characteristic paramagnetic behavior. Suspensions of surface-functionalized doped nanowires spontaneously formed stable homogeneous nematic liquid crystalline phases in organic solvent above a critical concentration. Large-area uniaxially aligned thin films of doped nanowires were obtained from the lyotropic phase by applying mechanical shear and, in the case of Co-doped nanowires, magnetic fields. Application of shear produced thin films in which the nanowire long axes were aligned parallel to the flow direction. Conversely, the nanowires were found to orient perpendicular to the direction of the applied magnetic fields. This indicates that the doped ZnO possesses magnetocrystalline anisotropy sufficient in magnitude to overcome the parallel alignment which would be predicted based solely on the anisotropic demagnetizing field associated with the high aspect ratio of the nanowires. We use a combination of magnetic property measurements and basic magnetostatics to provide a lower-bound estimate for the magnetocrystalline anisotropy.
  ACS Nano 09/2011; 5(10):8357-64. · 10.77 Impact Factor
• Article: Lyotropic self-assembly of high-aspect-ratio semiconductor nanowires of single-crystal ZnO.

  Shanju Zhang, Pawel W Majewski, Gayatri Keskar, Lisa D Pfefferle, Chinedum O Osuji
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  ABSTRACT: Lyotropic nanowire dispersions are attractive precursors for semiconductor device fabrication because they permit the alignment control of active nanomaterials. The reliable production of nanowire-based mesophases, however, is very challenging in practice. We show that appropriately functionalized high-aspect-ratio nanowires of single-crystal ZnO spontaneously form nematic phases in organic and aqueous media. These systems show isotropic, biphasic, and nematic phases on increasing concentration, in reasonable agreement with Onsager's theory for rigid rods interacting via excluded volume. Suspensions were readily processed to produce films with large-area monodomains of aligned nanowires. Imprints of the director field in quiescently dried films display a propensity for bend deformation in the organic mesophase versus splay deformation in the aqueous case, suggesting that system elasticity may be tuned via surface functionalization. These results provide critical insight for the utilization of semiconductor nanowires as novel mesogens and further enable the use of solution-based routes for fabricating optoelectronic devices.
  Langmuir 07/2011; 27(18):11616-21. · 4.19 Impact Factor
• Article: Deconvolution of damping forces with a nonlinear microresonator.

  Bevan Elliott, Herbert W Behlow, Doyl Dickel, Malcolm J Skove, Apparao M Rao, Gayatri Keskar
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  ABSTRACT: We report a fully electrical microcantilever device that utilizes capacitance for both actuation and detection and show that it can characterize various gases with a bare silicon microcantilever. We find the motion of the cantilever as it rings down when the oscillating force is removed, by measuring the voltage induced by the oscillating capacitance in the microcantilever∕counterelectrode system. The ringdown waveform was analyzed using an iterative numerical algorithm to calculate the oscillator motion, modeling the cantilever∕electrode capacitance to calculate the electrostatic force. We find that nonlinearity in the motion of the cantilever is not necessarily a disadvantage. After calibration, we simultaneously measure viscosity and density of several gaseous mixtures, yielding viscosities within ±2% and densities within ±6% of NIST values.
  The Review of scientific instruments 05/2011; 82(5):055103. · 1.52 Impact Factor
• Article: Determination of carbon nanotube density by gradient sedimentation.

  Qi Lu, Gayatri Keskar, Razvan Ciocan, Rahul Rao, Rakesh B Mathur, Apparao M Rao, Lyndon L Larcom
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  ABSTRACT: Density gradient centrifugation is a high-resolution technique for the separation and characterization of large molecules and stable complexes. We have analyzed various nanotube structures by preparative centrifugation in sodium metatungstate-water solutions. Bundled, isolated and acid-treated single-walled nanotubes (SWNTs) and multiwall nanotubes (MWNTs) formed sharp bands at well-defined densities. The structure of the material in each band was confirmed by transmission electron microscopy and Raman spectroscopy. Our data suggest respective densities of 1.87, 2.13, 1.74, and 2.1 g/cm(3) for bundled, isolated, and acid-treated SWNTs and MWNTs. These measured results compare well with their calculated densities.
  The Journal of Physical Chemistry B 01/2007; 110(48):24371-6. · 3.70 Impact Factor