V. J. Logeeswaran

University of California, Davis, Davis, California, United States

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Publications (48)45.68 Total impact

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    ABSTRACT: We synthesized catalyst-free �-Ga2O3 nanorods with terminated ultra-sharp tips by heat treating single crystalline GaAs in a chemical vapor deposition (CVD) chamber without introducing a precursor. The unique, straight-forward, synthetic route and a possible growth mechanism are discussed to explain the different morphology of the grown nanorods and the ultra-sharp nanostructures. The morphology and structure of the nanorods were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Raman-spectroscopy. The ultra-sharp tips were found to have radii of ∼3–5 nm and were utilized to achieve enhanced field emission. The field emission characteristics demonstrated a turn-on field of 2.1 V�m−1, a threshold electric field of 5.6 V�m−1, and a geometrical field enhancement factor of 3786, making them comparable to nanostructured diamond and highly oriented single wall carbon nanotubes.
    Science of Advanced Materials 02/2015; 7:211-218. · 2.91 Impact Factor
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    ABSTRACT: A process of heterogeneously integrating organically modified siliceous aerogel (Ormosil) films onto microstructured substrates is presented. These substrates are architecturally designed to mimic photon detectors for remote sensing applications. Here, ultrasonically homogenized Ormosil sols are drop-cast onto silicon micropyramidal arrays then dried in the ambient to produce highly porous low-density siliceous films with excellent uniformity. The highly facile process yields films endowed with high optical transmittance, high static contact angle of 168°, excellent thermal stability up to 400 °C and, to some extent, excellent adhesion to the microstructured substrates on which they sit. Additional planarization benefits are easily afforded by controlling the substrate arraignment during the ambient drying process which the sol undergoes. In contrast, only conformal films were obtained when sols were spin coated over similar microstructured substrates. In correlating the resultant macroporous films’ structural integrity with the underlying substrate topography, this study established that the weak physical bond between the facets of the microstructures and gel acts as crack nucleation points that induce and exacerbate crack propagation within the film. This phenomenon does not manifest itself when thinner films are prepared even on the same microstructured substrates as well as films of similar thickness on planar substrates. Initial studies establish that the non-homogenized sols can yield macroscopic aerogel monoliths with properties akin to those exhibited by supercritically dried monoliths. It is our belief that this study can enlighten the intricacies and pitfalls encountered when fabricating macroscopically monolithic Ormosil films over topographically structured surfaces
    Acta Materialia 06/2014; 72:159. · 3.94 Impact Factor
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    Hakan Karaagac, V. J. Logeeswaran, M. Saif Islam
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    ABSTRACT: In this study, regularly patterned and hierarchically structured silicon (Si) micro-scale pillars and walls with high aspect ratio were fabricated using the deep reactive ion etching (DRIE) process. Dense arrays of ZnO nanowires were hydrothermally grown on the surface of the Si structures subsequent to the deposition of Aluminum–ZnO (AZO) thin films onto the vertically oriented p- and n-type Si micro-scale pillars and walls – resulting in three-dimensional (3D) heterostructures. Electrical and optical measurements of the fabricated p–n nano-heterojunctions demonstrate strong capabilities for detecting ultraviolet (UV)–visible (VIS) photons with drastically reduced reflection loss. We also demonstrate low-voltage sensing of gases using these structures through the field ionization process.
    Physica Status Solidi (A) Applications and Materials 07/2013; 210(7). · 1.53 Impact Factor
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    Matthew Monari Ombaba, V. J. Logeeswaran, M. Saif Islam
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    ABSTRACT: In this paper we report a novel application of electrically conductive film (ECF) of Ag sub-micron particles that includes both isotropic and anisotropic film technologies in providing simultaneous electrical contact and mechanical anchor between fracture transfer-printed (1-D) single crystal semiconductor micro- and nano-pillars and a carrier substrate. We assembled silver sub-micron particles (AgSP) monolayers with varying particle diameters and investigated their optical and electrical characteristics prior to their incorporation into thermoplastic polymers. It was found that transfer-printing of the Si micropillar arrays, into electrically conductive thermoplastic receiver substrates, made of films of AgSP/PMMA blends atop metallic substrates could be effectively achieved to yield electrically interfaced 1-D Si micropillar arrays with retention of their orientation and integrity according to the SEM images. The carrier substrate can potentially be reused to generate additional Si micropillar arrays that can be similarly harvested.
    Applied Physics A 04/2013; 111(1):251-259. · 1.69 Impact Factor
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    Hakan Karaagac, V. J. Logeeswaran, M. Saif Islam
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    ABSTRACT: In this paper, high aspect ratio vertically oriented p-silicon (100) micropillars and microwalls were fabricated using the deep reactive ion etching (DRIE) process with the BOSCH recipe of cyclical passivation and etching. Two different patterns were etched; uniform pillar arrays of dimensions ~15µm (height) x 2µm (diameter) and wall arrays of dimensions ~1.5µm (width) x 25µm (height). Three-dimensional (3D) heterostructures of n-ZnO/p-Si heterostructures were fabricated from growing hydrothermally dense arrays of ZnO nanowires (290-400 nm in length and 48-80 nm in diameter) and depositing Aluminum-ZnO (AZO) thin film onto the high aspect ratio vertically oriented p-silicon micropillars and microwalls. The performances of the fabricated heterostructure optoelectronic devices were characterized for different applications including solar cells, photodetectors and field ionization gas sensors.
    Proc SPIE 10/2012;
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    ABSTRACT: In this paper, we present a method to realize a three dimensional (3D) homogeneous and isotropic negative index materials (3D-NIMs) fabricated using a low cost and massively parallel manufacturable microfabrication and microassembly technique. The construction of self- assembled 3D-NIM array was realized through two dimensional (2-D) planar microfabrication techniques exploiting the as-deposited residual stress imbalance between a bi-layer consisting of e-beam evaporated metal (650nm of chromium) and a structural layer of 500nm of low stress silicon nitride deposited by LPCVD on a silicon substrate. A periodic continuation of a single rectangular unit cell consisting of split-ring resonators (SRR) and wires were fabricated to generate a 3D assembly by orienting them along all three Cartesian axes. The thin chromium and silicon nitride bi-layer is formed as hinges. The strain mismatch between the two layers curls the structural layer (flap) containing the SRR upwards. The self-assembled out-of-plane angular position depends on the thickness and material composing the bi-layer. This built-in stress-actuated assembly method is suitable for applications requiring a thin dielectric layer for the SRR. The split-ring resonators and other structures are created on the membrane which is then assembled into the 3-D configuration.
    MRS Online Proceeding Library 01/2011; 919.
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    V.J. Logeeswaran, A.M. Katzenmeyer, M.S. Islam
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    ABSTRACT: Development of devices that can be fabricated on amorphous substrates using multiple single-crystal semiconductors with different physical, electrical, and optical characteristics is important for highly efficient portable and flexible electronics, optoelectronics, and energy conversion devices. Reducing the use of single-crystal substrates can contribute to low-cost and environmentally benign devices covering a large area. We demonstrate a technique to harvest and transfer vertically aligned single-crystal semiconductor micro- and nanopillars from a single-crystal substrate to a low-cost carrier substrate while simultaneously preserving the integrity, order, shape, and fidelity of the transferred pillar arrays. The transfer technique facilitates multilayer process integration by exploiting a vertical embossing and lateral fracturing method using a spin-coated polymer layer on a carrier substrate. Electrical contacts are formed using a bilayer of metal and conducting polymer such as gold (Au) and polyaniline (PAni). In this method, the original single-crystal substrate can be repeatedly used for generating more devices and is minimally consumed, whereas in conventional fabrication methods, the substrate is employed solely as a mechanical support. This heterogeneous integration technique potentially offers devices with low physical fill factor contributing to lower leakage current and noise, reduced parasitic capacitance, and enhanced photon-semiconductor interactions, and enables heterogeneous multimaterial integration such as silicon with compound semiconductors for rapidly expanding large-scale applications, including low-cost and flexible electronics, displays, tactile sensors, and energy conversion systems.
    IEEE Transactions on Electron Devices 09/2010; · 2.36 Impact Factor
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    M.S. Islam, V.J. Logeeswaran
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    ABSTRACT: New discoveries in materials on the nanometer- length scale are expected to play an important role in addressing ongoing and future challenges in the field of communication. Devices and systems for ultra-high-speed short- and long-range communication links, portable and power-efficient computing devices, high-density memory and logics, ultra-fast interconnects, and autonomous and robust energy scavenging devices for accessing ambient intelligence and needed information will critically depend on the success of next-generation emerging nanomaterials and devices. This article presents some exciting recent developments in nanomaterials that have the potential to play a critical role in the development and transformation of future intelligent communication networks.
    IEEE Communications Magazine 07/2010; · 4.46 Impact Factor
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    ABSTRACT: We demonstrate a smooth and low loss silver (Ag) optical superlens capable of resolving features at 1/12th of the illumination wavelength with high fidelity. This is made possible by utilizing state-of-the-art nanoimprint technology and intermediate wetting layer of germanium (Ge) for the growth of flat silver films with surface roughness at subnanometer scales. Our measurement of the resolved lines of 30 nm half-pitch shows a full-width at half-maximum better than 37 nm, in excellent agreement with theoretical predictions. The development of this unique optical superlens leads promise to parallel imaging and nanofabrication in a single snapshot.
    Applied Physics Letters 01/2010; 96(4):043102-043102-3. · 3.52 Impact Factor
  • Proceedings of SPIE; 01/2010
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    ABSTRACT: We report a novel method to fabricating single crystal and highly oriented 1-D Silicon micropillars and nanowires and then transferring them to coat a target surface of any topology using an innovative harvest/lift-off process. This method enables highly crystalline micro- and nano- pillars of different materials with diverse bandgaps and physical properties to be fabricated on appropriate mother substrates and transferred to form multilayered 3D stacks for multifunctional devices. This approach not only ensures the incorporation of any kind of material (with the best device characteristics) on a single substrate facilitating substrate-free device fabrications on any topology, but also allows the repeated use of a mother substrate for continual production of new devices. This capability of fabricating substrate-less devices will offer a universal platform for material integration and allow solar active devices to be coated on various surface topologies that would be suitable for solar hydrogen generation.
    Proc SPIE 08/2009;
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    ABSTRACT: We demonstrate a smooth and low loss silver (Ag) optical superlens capable of resolving features at 1/12th of the illumination wavelength with high fidelity. This is made possible by utilizing state-of-the-art nanoimprint technology and intermediate wetting layer of germanium (Ge) for the growth of flat silver films with surface roughness at sub-nanometer scales. Our measurement of the resolved lines of 30nm half-pitch shows a full-width at half-maximum better than 37nm, in excellent agreement with theoretical predictions. The development of this unique optical superlens lead promise to parallel imaging and nanofabrication in a single snapshot, a feat that are not yet available with other nanoscale imaging techniques such as atomic force microscope or scanning electron microscope.
    06/2009;
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    ABSTRACT: In many computer and server communications copper cables and wires are currently being used for data transmission and interconnects. However, due to significant shortcomings, such as long transmission time, high noise level, unstable electrical properties, and high power consumption for cooling, researchers are increasingly turning their research interests toward alternatives, such as fiber optic interconnects and free-space optical communication technologies. In this paper, we present design considerations for an integrated receiver for high-speed free-space line-of-sight optical interconnects for distortion-free data transmission in an environment with mechanical vibrations and air turbulences. The receiver consists of an array of high-speed photodiodes for data communication and an array of quadrant photodiodes for real-time beam tracking in order to compensate for the beam misalignment caused by vibrations in servers. Different configurations for spatially positioning the quadrant and data photodiodes are discussed for 4×4 and 9×9 multielement optical detector arrays. We also introduce a new beam tracking device, termed the strip quadrant photodiodes, in order to accurately track highly focused optical beams with very small beam diameter.
    Applied Physics A 06/2009; 95(4):1079-1088. · 1.69 Impact Factor
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    ABSTRACT: A new route to grow an ensemble of indium phosphide single-crystal semiconductor nanowires is described. Unlike conventional epitaxial growth of single-crystal semiconductor films, the proposed route for growing semiconductor nanowires does not require a single-crystal semiconductor substrate. In the proposed route, instead of using single-crystal semiconductor substrates that are characterized by their long-range atomic ordering, a template layer that possesses short-range atomic ordering prepared on a non-single-crystal substrate is employed. On the template layer, epitaxial information associated with its short-range atomic ordering is available within an area that is comparable to that of a nanowire root. Thus the template layer locally provides epitaxial information required for the growth of semiconductor nanowires. In the particular demonstration described in this paper, hydrogenated silicon was used as a template layer for epitaxial growth of indium phosphide nanowires. The indium phosphide nanowires grown on the hydrogenerated silicon template layer were found to be single crystal and optically active. Simple photoconductors and pin-diodes were fabricated and tested with the view towards various optoelectronic device applications where group III–V compound semiconductors are functionally integrated onto non-single-crystal platforms.
    Applied Physics A 06/2009; 95(4):1005-1013. · 1.69 Impact Factor
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    ABSTRACT: Carbon-based electronic materials have received much attention since the discovery and elucidation of the properties of the nanotube and fullerene allotropes and conducting polymers. Amorphous carbon, graphite, graphene, and diamond have also been the topics of intensive research. In accordance with this interest we herein provide the details of a novel and facile method for synthesis of poly(hydridocarbyne) (PHC), a pre-ceramic carbon polymer reported to undergo a conversion to diamond-like carbon (DLC) upon pyrolysis and also provide electrical characterization after low-temperature processing and pyrolysis of this material. The results indicate that the strongly insulating polymer becomes notably conductive in bulk form upon heating and contains interspersed micro and nanostructures which are the subject of ongoing research.
    Nanotechnology, 2008. NANO '08. 8th IEEE Conference on; 09/2008
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    ABSTRACT: Not Available
    Nanotechnology, 2008. NANO '08. 8th IEEE Conference on; 09/2008
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    ABSTRACT: We demonstrate a high-speed polarization-insensitive photoconductor based on intersecting InP nanowires synthesized between a pair of hydrogenated silicon electrodes deposited on amorphous SiO2 surfaces prepared on silicon substrates. A 14-ps full width at half maximum de-embedded impulse response is measured, which is the fastest reported response for a photodetector fabricated using nanowires. The high-speed electrical signal measurements from the photoconductor are performed by an integrated coplanar waveguide transmission line. The demonstrated ability to grow intersecting InP nanowires on hydrogenated microcrystalline Si surfaces will facilitate the construction of ultra-fast photodetectors on a wide range of substrates.
    Applied Physics A 03/2008; 91(1):1-5. · 1.69 Impact Factor
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    ABSTRACT: Carbon-based electronic materials have received much attention since the discovery and elucidation of the properties of the nanotube, fullerene allotropes, and conducting polymers. Amorphous carbon, graphite, graphene, and diamond have also been the topics of intensive research. In accordance with this interest, we herein provide the details of a novel and facile method for synthesis of poly(hydridocarbyne) (PHC), a preceramic carbon polymer reported to undergo a conversion to diamond-like carbon (DLC) upon pyrolysis and also provide electrical characterization after low-temperature processing and pyrolysis of this material. The results indicate that the strongly insulating polymer becomes notably conductive in bulk form upon heating and contains interspersed micro- and nanostructures, which are the subject of ongoing research.
    Journal of Nanomaterials 01/2008; · 1.61 Impact Factor
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    ABSTRACT: We successfully synthesized beta-Ga2O3 nanorods with ultra-sharp tips without use of a catalyst. The nanorods were produced by heating a GaAs wafer in a CVD chamber. The morphology and structure of the nanorods were characterized by scanning electron microscopy(SEM), Energy Dispersive X-ray Spectroscopy (EDS) and Raman-Scattering Spectroscopy. The field emission characteristics demonstrated a turn-on field of about 2.1 V mum-1 and the threshold electric field of 5.6 V mum-1.
    Nanotechnology, 2008. NANO '08. 8th IEEE Conference on; 01/2008
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    ABSTRACT: We describe the fabrication and characterization of photoconductors in which an ensemble of indium phosphide nanoneedles was utilized. DC electrical transport properties of the fabricated photoconductors were characterized under illumination with monochromatic light at 633 nm. This is the first demonstration of III-V compound semiconductor nanometerscale structures monolithically integrated on non-single crystalline silicon-based materials as an optoelectronic device that can be fabricated by a simple and flexible process not limited by single crystal substrates.
    Nanotechnology, 2007. IEEE-NANO 2007. 7th IEEE Conference on; 09/2007