[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
Applied Physics A 04/2013; 111(1):251-259. · 1.69 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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; · 2.06 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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; · 1.55 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate an InP nanowire based photodetector laterally integrated between two (111)-oriented vertical silicon surfaces. The nanowires are grown through a simple single step chemical vapor deposition (CVD) process using gold nanoparticles as catalyst with in-situ p-doping and have been heteroepitaxially bridged between a pair of prefabricated p-doped Si electrodes. Nonlinear current-voltage characteristics are observed. Although this nonlinearity resembles a back-to-back rectifying profile it originates from space-charge limited conductivity of the nanowires. DC photoelectric characteristics of the device were measured under optical illumination (lambda=630 nm) above the bandgap energy (1.34 eV or ~925 nm at room temperature) of InP. The variation in photoconductance with varying input optical power demonstrates high sensitivity of the device to optical illumination.
[Show abstract][Hide abstract] ABSTRACT: The concept of randomly-oriented semiconductor nanowires formed on non-single-crystal substrates is introduced and compared with semiconductor nanowires synthesized on single-crystal-substrates in the framework of epitaxial growth. In principle, epitaxial growth of semiconductor nanowires with the presence of metal-catalysts requires no single-crystal substrates owing to the small size of nanowires. A segment on a substrate from which crystallographic information is transferred to a single nanowire would only need to be as larger as the cross-section of a nanowire if a specific geometrical alignment for a group of nanowires is not required, suggesting that randomly-oriented semiconductor nanowires be formed on a surface that is characterized with short-range atomic order in contrast to long-range atomic order that exists on the surface of single-crystal substrates. The surfaces exhibiting short-range atomic order can be prepared on non-single-crystal substrates, further suggesting functional devices that utilize randomly-oriented semiconductor nanowires be fabricated on non-single-crystal substrates. Design, fabrication and characteristics of a photoconductor that utilizes an ensemble of randomly-oriented indium phosphide nanowires are described.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we present an integrated fabrication process for realizing a switching/modulation mechanism for negative index materials (NIMs) based on photoconductive coupling. The metamaterial element chosen is an array of regular copper split-ring resonator (SRR) that was fabricated on two different substrates: high-resistivity silicon (HRS) and fused silica glass. The switching mechanism proposed can be achieved through tuning the SRR gap and/or substrate conductivity. The photosensitive material of the SRR structure (amorphous silicon for the glass substrate samples and intrinsic silicon for the HRS substrate samples) upon illumination generates excess carriers that essentially shunt the gap capacitance thus diminishing the resonance response significantly. The response in terms of S-parameters is simulated using HFSS under varying magnitude of optical illumination. Our simulation with a single SRR to demonstrate total suppression of resonance amplitude with a high extinction ratio is applicable to NIMs comprising of both negative permeability and negative permittivity without any loss of generality. This method may provide a basis for long-sought practical applications and devices based on NIM in the fields of ultra-fast communications at RF and optical frequencies, sensing and imaging promising a potential of dramatically improving the performance of existing phased array antennas, optical beam-forming networks, antenna remoting and transportation of RF power through fiber-radio.