Oscar Resto

Single-crystal γ-MnS nanowires conformally coated with carbon.

We report for the first time the fabrication of single-crystal metastable manganese sulfide nanowires (γ-MnS NWs) conformally coated with graphitic carbon via chemical vapor deposition technique using a single-step route. Advanced spectroscopy and electron microscopy techniques were applied to elucidate the composition and structure of these NWs at the nanoscale, including Raman, XRD, SEM, HRTEM, EELS, EDS, and SAED. No evidence of α-MnS and β-MnS allotropes was found. The γ-MnS/C NWs have hexagonal cross-section and high aspect ratio (∼1000) on a large scale. The mechanical properties of individual γ-MnS/C NWs were examined via in situ uniaxial compression tests in a TEM-AFM. The results show that γ-MnS/C NWs are brittle with a Youngs modulus of 65 GPa. The growth mechanism proposed suggests that the bottom-up fabrication of γ-MnS/C NWs is governed by vapor-liquid-solid mechanism catalyzed by bimetallic Au-Ni nanoparticles. The electrochemical performance of γ-MnS/C NWs as an anode material in lithium-ion batteries indicates that they outperform the cycling stability of stable micro-sized α-MnS, with an initial capacity of 1036 mAh g(-1) and a reversible capacity exceeding 503 mAh g(-1) after 25 cycles. This research advances the integration of carbon materials and metal sulfide nanostructures, bringing forth new avenues for potential miniaturization strategies to fabricate 1D core/shell heterostructures with intriguing bifunctional properties that can be used as building blocks in nanodevices.

Compositional engineering of BaTiO3/(Ba,Sr)TiO3 ferroelectric superlattices

Epitaxial strain is one of the major factors influencing physical properties of artificial superlattice (SL) structures. One way to control the local stress in epitaxial films is altering the lattice parameters by doping. Superlattices of BT/Ba(1−x)SrxTiO3 (BT/BST) with x = (0, 0.3, 0.4, 0.5, 0.6, 0.7, 1) with a modulation period of about 80–90 A were grown on La0.67Sr0.33MnO3 coated (100) MgO substrates by pulsed laser deposition technique. The modulated structure of the thin films was confirmed by x-ray diffraction, Raman spectroscopy, and transmission electron microscopy. The increase in Sr concentration in BST layers of the SLs results in an increase in in-plane (a) and out-plane (c) compressive/tensile misfit strains in the BT/BST layers, respectively. The highest value of the dielectric constant was obtained for BT/Ba0.3Sr0.7TiO3 (BT/BST3070) and BT/Ba0.7Sr0.3TiO3 (BT/BST7030) SLs. Slim, asymmetric, but well-saturated ferroelectric hysteresis loops were observed in all SLs. Additionally, BT/BST SLs ...

Reactively sputtered a‐SixGe1−x:H alloys with compositional gradient in plane of film

A reactive sputtering technique has been utilized which enables one to deposit in a single run the alloy system a‐SixGe1−x :H over the range x=0.05–0.7 and survey IR, optical, and electrical transport properties. The technique can readily be generalized to other binary and ternary semiconductor systems.

Highly-crystalline γ-MnS nanosaws

We report here a straightforward method to fabricate single-crystal wurtzite-structured manganese sulfide nanosaws (γ-MnS NSs) via a chemical vapor deposition technique, which can be extended to different types of binary chalcogenide-based 1D building blocks. Advanced spectroscopy and electron microscopy techniques were applied to reveal the structure, morphology and composition of these nanostructures. Our studies indicate that γ-MnS NSs are of high crystalline quality and purity, exhibit high aspect ratio and have extrusive teeth with average tip size of approximately a couple of atoms. These NSs are structurally stable and do not present divergence in surface energy. The formation of the backsaw is governed by a vapor–liquid–solid growth mechanism, whereas the teeth are a result of a self-catalyzed growth process induced by the Mn-terminated (0001) surface. Our optical and electrochemical analyses indicate that γ-MnS NSs can be used as efficient visible light emitters with an activation energy of ∼40 meV, and as anode materials with improved cycling stability for LIBs. This bottom-up approach represents a step ahead to provide promising and potential Mn-based materials for high-performance LIB technology and optoelectronics, and adds a new member to the family of saw-like wurtzite-structured nanoribbons, such as CdSe and ZnS.

Ferroelectric-carbon nanotube memory devices

One-dimensional ferroelectric nanostructures, carbon nanotubes (CNT) and CNT-inorganic oxides have recently been studied due to their potential applications for microelectronics. Here, we report coating of a registered array of aligned multi-wall carbon nanotubes (MWCNT) grown on silicon substrates by functional ferroelectric Pb(Zr,Ti)O3 (PZT) which produces structures suitable for commercial prototype memories. Microstructural analysis reveals the crystalline nature of PZT with small nanocrystals aligned in different directions. First-order Raman modes of MWCNT and PZT/MWCNT/n-Si show the high structural quality of CNT before and after PZT deposition at elevated temperature. PZT exists mostly in the monoclinic Cc/Cm phase, which is the origin of the high piezoelectric response in the system. Low-loss square piezoelectric hysteresis obtained for the 3D bottom-up structure confirms the switchability of the device. Current-voltage mapping of the device by conducting atomic force microscopy (c-AFM) indicates very low transient current. Fabrication and functional properties of these hybrid ferroelectric-carbon nanotubes is the first step towards miniaturization for future nanotechnology sensors, actuators, transducers and memory devices.

Ultrananocrystalline Diamond-Decorated Silicon Nanowire Field Emitters

Silicon nanowires (SiNWs) were uniformly decorated with ultrananocrystalline diamond (UNCD) by a novel route using paraffin wax as the seeding source, which is more efficient in the creation of diamond nuclei than traditional methods. These one-dimensional ultrananocrystalline diamond-decorated SiNWs (UNCD/SiNWs) exhibit uniform diameters ranging from 100 to 200 nm with a bulbous catalytic tip of ∼250 nm in diameter and an UNCD grain size of ∼5 nm. UNCD/SiNW nanostructures demonstrated enhanced electron field emission (EFE) properties with a turn-on field of about 3.7 V/μm. Current densities around 2 mA/cm(2) were achieved at 25 V/μm, which is significantly enhanced as compared to bare SiNWs.

Temperature‐Activated Reverse Sensing Behavior of Pd Nanowire Hydrogen Sensors

Hydrogen sensors built with individual palladium nanowires (Pd NWs) have been achieved by integrating Pd NWs across microelectromechanical system (MEMS) electrodes, followed by assembling and bonding them to a chip carrier platform. The sensing measurements reveal that the sensors with individual Pd NWs show reverse sensing behaviors between the temperature zones of (370-263 K) and (263-120 K).

Field Emission and Radial Distribution Function Studies of Fractal-like Amorphous Carbon Nanotips

The short-range order of individual fractal-like amorphous carbon nanotips was investigated by means of energy-filtered electron diffraction in a transmission electron microscope (TEM). The nanostructures were grown in porous silicon substrates in situ within the TEM by the electron beam-induced deposition method. The structure factorS(k) and the reduced radial distribution functionG(r) were calculated. From these calculations a bond angle of 124° was obtained which suggests a distorted graphitic structure. Field emission was obtained from individual nanostructures using two micromanipulators with sub-nanometer positioning resolution. A theoretical three-stage model that accounts for the geometry of the nanostructures provides a value for the field enhancement factor close to the one obtained experimentally from the Fowler-Nordheim law.

Enhancement of the photoluminescence properties of porous silicon by silica gel coating

We have spin coated silica gel films of ∼10μm onto porous silicon (PS) substrates with photoluminescence (PL) emissions peaks in the 600–700nm spectral range, producing a 20-fold enhancement of the original intensity in the shorter wavelength end. We attribute this enhancement to the reduced nonradiative recombination that follows the interface passivation of the PS surfaces by an oxygen enriched SiOx (x→2) layer of silica gel. The PL stability of enhanced substrates was significantly improved by sputtering the samples with SiO2 after the silica gel spin coating, which resulted in a final blueshift of the PL. The technique described herein is a cost effective method for producing passivated photoluminescent enhanced silicon structures that can be used for optoelectronic applications.

Effect of nanocrystallinity on the electrochemical performance of LiMn2O4 cathode

In the present work, we have synthesized nanocrystalline LiMn2O4 of different grain sizes, for application to Li ion rechargeable batteries. LiMn2O4 cathode powder was synthesized by sol-gel method and was high-energy ball milled for 8 and 16h, respectively. The particle sizes of pure, 8h, and 16h ball-milled samples were determined from TEM and measured as ∼225, 40, and 10nm, respectively. The initial discharge capacity of the pure LiMn2O4 cathode material was found to be about 128.75mAh∕g, which was found to decrease on reducing the particle size.