Self-assembled isolated monodisperse NiO1+γ nanoparticles as catalytic templates for nanomaterials synthesis

We present the self-organization of isolated monodisperse nickel oxide (NiO_1+γ) nanoparticles on surfaces of arbitrary area sizes. Ni films deposited on titanium oxynitride support films are annealed in a nitrogen/air environment at atmospheric pressure for various annealing times. After the annealing treatments, randomly distributed spatially isolated NiO_1+γ nanoparticles that are anchored to the support film are observed with a site-density of 11 ± 1 μm⁻² and with dimensions of 16 ± 2 nm in height and 82 ± 10 nm in diameter. The anchored nanoparticles, once formed, are immobile during further annealing, even for annealing times of 40 min or annealing temperatures of 800 °C, making the nanoparticle formation a well-controlled process that yields templates suitable for further processing at elevated temperatures. We demonstrate the utilization of these NiO_1+γ nanoparticle templates as nucleation sites for carbon nanotubes at temperatures of around 680 °C.     

Amorphous and ‘micromorph’ silicon tandem cells with high open-circuit voltage

For amorphous and ‘micromorph’ silicon multi-junction solar cells, we have developed tunnel recombination junctions consisting of two microcrystalline doped layers with a defect-rich interface. While the solar cells performed reasonably well under AM 1.5 light, we found in spectral response measurements that the first deposited cell of tandem structures in nip and pin configuration was apparently leaking under low light conditions. Insertion of a thin protection layer of n-type amorphous silicon solved this issue, and led to an increase in open-circuit voltage. Voltages as high as 1.76 V have been obtained for a-Si/a-Si pinpin tandem cells.     

Effect of ammonia on Ta filaments in the hot wire CVD process

The exposure of Ta filaments to a pure NH₃ ambient in a hot wire chemical vapour deposition (HWCVD) reactor affects the resistance of the wires. For filament temperatures below 1950 °C the resistance increases over time, which is probably caused by in-diffusion of N atoms. Using the filaments in a mixed SiH₄ and NH₃ atmosphere (under SiN_x deposition conditions) the filaments are hardly affected. Only at the “cold” parts near the electrical contact SiN_x deposition on the Ta filaments is observed. X-ray diffraction patterns and cross-section microscope images reveal that in a CH₄, H₂ and NH₃ ambient the TaC_0.275N_0.218 phase is formed on the surface of the filament. Annealing of these filaments at 2000 °C causes the TaC0.275N0.218 structure to separate into Ta and Ta₂C phases.     

Influence of visual feedback on human task performance in ITER remote handling

In ITER, maintenance operations will be largely performed by remote handling (RH). Before ITER can be put into operation, safety regulations and licensing authorities require proof of maintainability for critical components. Part of the proof will come from using standard components and procedures. Additional verification and validation is based on simulation and hardware tests in 1:1 scale mockups.The Master Slave manipulator system (MS2) Benchmark Product was designed to implement a reference set of maintenance tasks representative for ITER remote handling. Experiments were performed with two versions of the Benchmark Product. In both experiments, the quality of visual feedback varied by exchanging direct view with indirect view (using video cameras) in order to measure and analyze its impact on human task performance.The first experiment showed that both experienced and novice RH operators perform a simple task significantly better with direct visual feedback than with camera feedback. A more complex task showed a large variation in results and could not be completed by many novice operators. Experienced operators commented on both the mechanical design and visual feedback. In a second experiment, a more elaborate task was tested on an improved Benchmark product. Again, the task was performed significantly faster with direct visual feedback than with camera feedback. In post-test interviews, operators indicated that they regarded the lack of 3D perception as the primary factor hindering their performance.     

Light scattering coefficients for textured SnO

Transparent electrodes that are used in amorphous silicon solar cells are textured to provide light scattering. We studied the light scattering behavior in transmission and reflection and found that in order to describe the measured spectra thickness variations of 50–60 nm over several micrometers have to be assumed. This is in qualitative agreement with measured rms roughnesses as determined with atomic force microscopy (AFM). It is important to include these thickness variations in the modeling of amorphous silicon solar cells. The wavelength dependence of the light scattering in transmission at the TCO–air interface was found to be λ⁻³ for light incident from both sides. Scattering of the weakly absorbed long wavelength light at the back contact is therefore essential in order to obtain high solar cell efficiencies.     

A "smart needle" to facilitate difficult vascular access in pediatric patients

A new Doppler probe inside a needle was evaluated in a group of infants and children to assess its suitability for subclavian venous access. While the efficient use of this device required learning a new technique, we found the device useful to accurately locate the subclavian vein and differentiate it from the artery in all patients weighing more than 3 kg. This device will be most helpful in cases of difficult access, eg, scar from previous access, obesity, or edema.     

Low-temperature deposition of polycrystalline silicon thin films by hot-wire CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430°C. The material properties have been optimized for photovoltaic applications by varying the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction and an average grain size of 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, equal to crystalline silicon. An activation energy of 0.54 eV for the electrical transport confirmed the intrinsic nature of the films. The material has a low dangling bond-defect density of 10¹⁷ cm³. A photo conductivity of 1.9 × 10⁻⁵ Ω⁻¹cm⁻¹ and a photoresponse (σ_ph/σ_d) of 1.4 × 10² were achieved. A high minority-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mobility-lifetime (μτ) product of 7.1 × 10⁻⁷cm²V⁻¹ from steady-state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction n---i---p cell made in the configuration n⁺-c-Si/i-poly-Si: H/p-μc-Si : H/ITO yielded 3.15% efficiency under 100 mW/cm² AM 1.5 illumination.     

Post-deposition thermal annealing studies of hydrogenated microcrystalline silicon deposited at 40 °C

Post-deposition thermal annealing studies, including gas effusion measurements, measurements of infrared absorption versus annealing state, cross-sectional transmission electron microscopy (X-TEM) and atomic force microscopy (AFM), are used for structural characterization of hydrogenated amorphous and microcrystalline silicon films, prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) at low substrate temperature (T_S). Such films are of interest for application in thin semiconductor devices deposited on cheap plastics. For T_S ∼ 40 °C, H-evolution shows rather complicated spectra for (near-) microcrystalline material, with hydrogen effusion maxima seen at ∼ 200-250 °C, 380 °C and ∼ 450-500 °C, while for the amorphous material typical spectra for good-quality dense material are found. Effusion experiments of implanted He demonstrate for the microcrystalline material the presence of a rather open (void-rich) structure. A similar tendency can be concluded from Ne effusion experiments. Fourier Transform infrared (FTIR) spectra of stepwise annealed samples show Si-H bond rupture already at annealing temperatures of 150 °C. Combined AFM/X-TEM studies reveal a columnar microstructure for all of these (near-) microcrystalline materials, of which the open structure is the most probable explanation of the shift of the H-effusion maximum in (near-) microcrystalline material to lower temperature.     

Utilization of geometric light trapping in thin film silicon solar cells: simulations and experiments

In this study, we present a new light absorption enhancement method for p‐i‐n thin film silicon solar cells using pyramidal surface structures, larger than the wavelength of visible light. Calculations show a maximum possible current enhancement of 45% compared with cells on a flat substrate. We deposited amorphous silicon (a‐Si) thin film solar cells directly onto periodically pyramidal‐structured polycarbonate (PC) substrates, which show a significant increase (30%) in short‐circuit current over reference cells deposited on flat glass substrates. The current of the cells on our pyramidal structures on PC is only slightly lower than that of cells on Asahi U‐type TCO glass (Asahi Glass Co., Tokyo, Japan), but suffer from a somewhat lower open circuit voltage and fill factor. Because the used substrates have a locally flat surface area due to the fabrication process, we believe that the current enhancement in the cells on structured PC can be increased using larger or more closely spaced pyramids, which can have a smaller flat surface area. Copyright © 2012 John Wiley & Sons, Ltd.