Porous Three Dimensional Arrays of Plasmonic Nanoparticles

Plasmonic interactions in a well-defined array of metallic nanoparticles can lead to interesting optical effects, such as local electric field enhancement and shifts in the extinction spectra, which are of interest in diverse technological applications, including those pertaining to biochemical sensing and photonic circuitry. Here, we report on a single-step wafer scale fabrication of a three-dimensional array of metallic nanoparticles whose sizes and separations can be easily controlled to be anywhere between fifty to a few hundred nanometers, allowing the optical response of the system to be tailored with great control in the visible region of the spectrum. The substrates, apart from having a large surface area, are inherently porous and therefore suitable for optical sensing applications, such as surface enhanced Raman scattering, containing a high density of spots with enhanced local electric fields arising from plasmonic couplings.

Probing ultrafast carrier dynamics, nonlinear absorption and refraction in core-shell silicon nanowires

We investigate the relaxation dynamics of photogenerated carriers in silicon nanowires consisting of a crystalline core and a surrounding amorphous shell, using femtosecond time-resolved differential reflectivity and transmission spectroscopy at 3.15 eV and 1.57 eV photon energies. The complex behaviour of the differential transmission and reflectivity transients is the mixed contributions from the crystalline core and the amorphous silicon on the nanowire surface and the substrate where competing effects of state-filling and photoinduced absorption govern the carrier dynamics. Faster relaxation rates are observed on increasing the photogenerated carrier density. Independent experimental results on crystalline silicon-on-sapphire (SOS) help us in separating the contributions from the carrier dynamics in crystalline core and the amorphous regions in the nanowire samples. Further, single-beam z-scan nonlinear transmission experiments at 1.57 eV in both open- and close-aperture configurations yield two-photon absorption coefficient beta (similar to 3 cm/GW) and nonlinear refraction coefficient gamma (-2.5 x 10 (-aEuro parts per thousand 4) cm(2)/GW).

Development of experimental setup for study of adsorption characteristics of porous materials down to 4.2 K

The knowledge of adsorption characteristics of activated carbon (porous material) in the temperature range from 5 to 20 K is essential when used in cryosorption pumps for nuclear fusion applications. However, such experimental data are very scarce in the literature, especially below 77 K. So, an experimental system is designed and fabricated to measure the adsorption characteristics of porous materials under variable cryogenic temperatures (from 5 K to 100 K). This is based on the commercially available micropore-analyser coupled to a closed helium cycle two-stage Gifford McMahon (GM) Cryocooler, which allows the sample to be cooled to 4.2 K. The sample port is coupled to the Cryocooler through a heat switch, which isolates this port from the cold head of the Cryocooler. By this, the sample temperature can now be varied without affecting the Cryocooler. The setup enables adsorption studies in the pressure range from atmospheric down to 10(-4) Pa. The paper describes the details of the experimental setup and presents the results of adsorption studies at 77 K for activated carbon with nitrogen as adsorbate. The system integration is now completed to enable adsorption studies at 4.2 K.

Morphology Controlled Growth of Meso-Porous Co3O4 Nanostructures and Study of Their Electrochemical Capacitive Behavior

The morphology of nanocrystalline Co3O4 synthesized through microwave irradiation of a solution of a cobalt complex is found to depend reproducibly on the conditions of synthesis and, in particular, on the composition of the solvent used. Despite the rapidity of the process, oriented aggregation occurs under certain conditions, depending on solvent composition. Annealing the oriented samples leads to microstructures with significant porosity, rendering the material suitable as electrodes for electrochemical capacitors. Electrochemical analysis of the oxide samples was carried out in 0.1M Na2SO4 electrolyte vs. Ag/AgCl electrode. A stable specific capacitance of 221 F/g was measured for a meso-porous sample displaying oriented aggregation. Stability of these oxide materials were checked for longer charge-discharge cycling. (C) 2012 The Electrochemical Society. DOI: 10.1149/2.002210jes] All rights reserved.

Fabrication of Highly Dense Nanoholes by Self-Assembled Gallium Droplet on Silicon Surface

We report the fabrication of nanoholes on silicon surface by exploiting the solubility of silicon in gallium by local droplet etching. Nanometer-sized gallium droplets yield nanoholes when annealed in ultra-high vacuum at moderate temperatures (similar to 500 degrees C) without affecting the other regions. High vacuum and moderate annealing temperatures are key parameters to obtain well-defined nanoholes with diameter comparable to that of Ga droplets. Self-assembly of Ga droplet leads to a nanohole density of 4-8 x 10(10)/cm(2).

Silicon surface texturing with a combination of potassium hydroxide and tetra-methyl ammonium hydroxide etching

A two step silicon surface texturing, consisting of potassium hydroxide (KOH) etching followed by tetra-methyl ammonium hydroxide etching is presented. This combined texturing results in 13.8% reflectivity at 600 nm compared to 16.1% reflectivity for KOH etching due to the modification of microstructure of etched pyramids. This combined etching also results in significantly lower flat-band voltage (V-FB) (-0.19V compared to -1.3 V) and interface trap density (D-it) (2.13 x 10(12) cm(-2) eV(-1) compared to 3.2 x 10(12) cm(-2) eV(-1)). (C) 2013 American Institute of Physics. http://dx.doi.org/10.1063/1.4776733]

Wave propagation in a porous composite beam: Porosity determination, location and quantification

A wave-based method is developed to quantify the defect due to porosity and also to locate the porous regions, in a composite beam-type structure. Wave propagation problem for a porous laminated composite beam is modeled using spectral finite element method (SFEM), based on the modified rule of mixture approach, which is used to include the effect of porosity on the stiffness and density of the composite beam structure. The material properties are obtained from the modified rule of mixture model, which are used in a conventional SFEM to develop a new model for solving wave propagation problems in porous laminated composite beam. The influence of the porosity content on the group speed and also the effect of variation in theses parameters on the time responses are studied first, in the forward problem. The change in the time responses with the change in the porosity of the structure is used as a parameter to find the porosity content in a composite beam. The actual measured response from a structure and the numerically obtained time responses are used for the estimation of porosity, by solving a nonlinear optimization problem. The effect of the length of the porous region (in the propagation direction), on the time responses, is studied. The damage force indicator technique is used to locate the porous region in a beam and also to find its length, using the measured wave propagation responses. (C) 2012 Elsevier Ltd. All rights reserved.

Proton Conduction in Metal-Organic Frameworks and Related Modularly Built Porous Solids

Proton-conducting materials are an important component of fuel cells. Development of new types of proton-conducting materials is one of the most important issues in fuel-cell technology. Herein, we present newly developed proton-conducting materials, modularly built porous solids, including coordination polymers (CPs) or metalorganic frameworks (MOFs). The designable and tunable nature of the porous materials allows for fast development in this research field. Design and synthesis of the new types of proton-conducting materials and their unique proton-conduction properties are discussed.

k.p based closed form energy band gap and transport electron effective mass model for 100] and 110] relaxed and strained Silicon nanowire

In this paper, we address a physics based closed form model for the energy band gap (E-g) and the transport electron effective mass in relaxed and strained 100] and 110] oriented rectangular Silicon Nanowire (SiNW). Our proposed analytical model along 100] and 110] directions are based on the k.p formalism of the conduction band energy dispersion relation through an appropriate rotation of the Hamiltonian of the electrons in the bulk crystal along 001] direction followed by the inclusion of a 4 x 4 Luttinger Hamiltonian for the description of the valance band structure. Using this, we demonstrate the variation in Eg and the transport electron effective mass as function of the cross-sectional dimensions in a relaxed 100] and 110] oriented SiNW. The behaviour of these two parameters in 100] oriented SiNW has further been studied with the inclusion of a uniaxial strain along the transport direction and a biaxial strain, which is assumed to be decomposed from a hydrostatic deformation along 001] with the former one. In addition, the energy band gap and the effective mass of a strained 110] oriented SiNW has also been formulated. Using this, we compare our analytical model with that of the extracted data using the nearest neighbour empirical tight binding sp(3)d(5)s* method based simulations and has been found to agree well over a wide range of device dimensions and applied strain. (C) 2012 Elsevier Ltd. All rights reserved.

Synthesis and characterization of porous flowerlike alpha-Fe2O3 nanostructures for supercapacitor application

Porous flower-like alpha-Fe2O3 nanostructures synthesized by an ethylene glycol mediated self-assembly process are crystalline and porous with BET surface area of 64.6 m(2) g(-1). The discharge capacitance is 127 F g(-1) when the electrodes are cycled in 0.5 M Na2SO3 at a current density of 1 A g(-1). Capacitance retention after 1000 cycles is about 80% of the initial capacitance. The high discharge capacitance and its retention are attributed to high surface area and porosity of the iron oxide. As the iron oxides are inexpensive, the nano alpha-Fe2O3 is expected to be of potential use for supercapacitor application.

Composite cyclodextrin-calcium carbonate porous microparticles and modified multilayer capsules: novel carriers for encapsulation of hydrophobic drugs

Novel composite cyclodextrin (CD)-CaCO3 spherical porous microparticles have been synthesized through Ca2+-CD complex formation, which influences the crystal growth of CaCO3. The CDs are entrapped and distributed uniformly in the matrix of CaCO3 microparticles during crystallization. The hydrophobic fluorescent molecules coumarin and Nile red (NR) are efficiently encapsulated into these composite CD-CaCO3 porous particles through supramolecular inclusion complexation between entrapped CDs and hydrophobic molecules. Thermogravimetric (TGA) and infrared spectroscopy (IR) analysis of composite CD-CaCO3 particles reveals the presence of large CDs and their strong interaction with calcium carbonate nanoparticles. The resulting composite CD-CaCO3 microparticles are utilized as sacrificial templates for preparation of CD-modified layer-by-layer (LbL) capsules. After dissolution of the carbonate core, CDs are retained in the interior of the capsules in a network fashion and assist in the encapsulation of hydrophobic molecules. The efficient encapsulation of the hydrophobic fluorescent dye, coumarin, was successfully demonstrated using CD-modified capsules. In vitro release of the encapsulated coumarin from the CD-CaCO3 and CD-modified capsules has been demonstrated.

Ethylenediamine assisted synthesis of wurtzite zinc sulphide nanosheets and porous zinc oxide nanostructures: near white light photoluminescence emission and photocatalytic activity under visible light irradiation

Porous fungus-like ZnO nanostructures have been synthesized by simple thermal annealing of the hydrothermally synthesized sheet-like ZnS(en)(0.5) complex precursor in air at 600 degrees C. Structural and morphological changes occurring during ZnS(en)(0.5) -> ZnS -> ZnO transformations have been observed closely by annealing the as-synthesized precursor at 100-600 degrees C. Wurtzite ZnS nanosheets and ZnS-ZnO composites are obtained at temperatures of 400 degrees C and 500 degrees C, respectively. Thermal decomposition and oxidation of the ZnS(en) 0.5 nanosheets have been confirmed by differential scanning calorimetry and thermo-gravimetric analysis. The visible light driven photocatalytic degradation of methylene blue dye has been demonstrated in the synthesized samples. ZnS-ZnO composite shows the highest dye degradation efficiency of 74% due to the formation of surface complex as well as higher visible light absorption as a result of band-gap narrowing effect. The porous ZnO nanostructures show efficient visible photoluminescence (PL) emission with a colour coordinate of (0.29, 0.35), which is close to that of white light (0.33, 0.33). The efficient visible PL emission as well as visible light driven photocatalytic activity of the materials synthesized in the present work might be very attractive for their applications in future optoelectronic devices, including in white light emitting devices.

Indium Assisted Growth of Silicon Nanowires by Electron beam evaporation

Silicon nanowires were grown on Si substrates by electron beam evaporation (EBE) was demonstrated using Indium as an alternate catalyst to gold. We have studied the effect of substrate (growth) temperature, deposition time on the growth of nanowires. It was observed that a narrow temperature window from 300 degrees C to 400 degrees C for the nanowires growth. At growth temperature >= 400 degrees C suppression of nanowires growth was observed due to evaporation of catalyst particle. It is also observed that higher deposition times also leading to the absence of nanowire growth as well as uncatalyzed deposition on the nanowires side walls due to limited surface diffusion of ad atoms and catalyst evaporation.

Critical investigation on hydrogen bonding by Fourier transform infrared spectroscopy in hydrogenated amorphous silicon thin films

Fourier Transform Infrared (FTIR) spectroscopic analysis has been carried out on the hydrogenated amorphous silicon (a-Si:H) thin films deposited by DC, pulsed DC (PDC) and RF sputtering process to get insight regarding the total hydrogen concentration (C-H) in the films, configuration of hydrogen bonding, density of the films (decided by the vacancy and void incorporation) and the microstructure factor (R*) which varies with the type of sputtering carried out at the same processing conditions. The hydrogen incorporation is found to be more in RF sputter deposited films as compared to PDC and DC sputter deposited films. All the films were broadly divided into two regions namely vacancy dominated and void dominated regions. At low hydrogen dilutions the films are vacancy dominated and at high hydrogen dilutions they are void dominated. This demarcation is at C-H = 23 at.% H for RF, C-H = 18 at.% H for PDC and C-H = 14 at.% H for DC sputter deposited films. The microstructure structure factor R* is found to be as low as 0.029 for DC sputter deposited films at low C-H. For a given C-H, DC sputter deposited films have low R* as compared to PDC and RF sputter deposited films. Signature of dihydride incorporation is found to be more in DC sputter deposited films at low C-H.