This work introduces the electrospray technique as a suitable option to fabricate large-scale colloidal nanostructures, including colloidal crystals, in just a few minutes. It is shown that by changing the deposition conditions, different metamaterials can be fabricated: from scattered monolayers of polystyrene nanospheres to self-assembled three-dimensional ordered nanolayers having colloidal crystal properties. The electrospray technique overcomes the main problems encountered by top-down fabrication approaches, largely simplifying the experimental setup. Polystyrene nanospheres, with 360-nm diameter, were typically electrosprayed using off-the-shelf nanofluids. Several parameters of the setup and deposition conditions were explored, namely the distance between electrodes, nanofluid conductivity, applied voltage, and deposition rate. Layers thicker than 20 μm and area of 1 cm2 were typically produced, showing several domains of tens of microns wide with dislocations in between, but no cracks. The applied voltage was in the range of 10 kV, and the conductivity of the colloidal solution was in the range of 3 to 4 mS. Besides the morphology of the layers, the quality was also assessed by means of optical reflectance measurements showing an 80% reflectivity peak in the vicinity of 950-nm wavelength.
We present spatially resolved and injection dependent excess carrier lifetime measurements on silicon. At low level injection conditions an anomalous increase often interferes in such measurements. The origin of the anomalous increase is discussed. Assuming trapping as the origin, highly resolved images of trap parameters together with low level injection recombination lifetimes with strongly reduced trapping effects have been obtained. A theoretical model for infrared lifetime imaging based on the Hornbeck and Haynes model [J. A. Hornbeck and J. R. Haynes, Phys. Rev. 97, 311 (1955)] is presented which describes the trapping effect on the measured lifetime. This model is fitted to experimental spatially resolved data to extract trapping parameters, particularly trap density and the trap-escape ratio, i.e., the ratio between escape rate of minority carriers from the trap level into the minority band (detrapping) and trap rate from the minority band into the trap level (trapping). An upper bound for the low level injection recombination lifetimes is determined. Lifetime and trapping parameters are compared with dislocation density maps. A strong correlation is found between the total trap density and the crystal defect density
This work describes the design, simulation, fabrication process, and characterization of high voltage photovoltaic mini‐modules using silicon on insulator (SOI) wafers. The mini‐modules are made of a number of small area photovoltaic cells (<1 mm2) monolithically connected in series. Isolation between cells is performed by means of anisotropic etching of the active layer of the SOI wafer. Measurements using standard sunlight (AM1·5 100 mW/cm2) confirm the viability of this technology to fabricate small area arrays showing open circuit voltages, Voc, between 620 mV and 660 mV and photocurrent densities up to 22·3 mA/cm2 for single cells of 0·225 mm2 area and 10 µm active film thickness. Series connection scales up Voc and the maximum power, Pm, from 625 mV and 21·2 µW, respectively, in a single cell to 103 V and 3·2 mW when 169 cells are connected in series in a 0·42 cm2 module total area. Copyright © 2008 John Wiley & Sons, Ltd.
The wettability of a surface has been shown, for many years now, to increase by the application of a voltage difference between the liquid droplet and the substrate, 1À3 which, most often, is a conductor covered by a dielectric (electrowetting on the dielectric, EWOD). There are basically two explanations of the phenomenon. The first one considers that the solidÀliquid surface tension is modulated by the electrostatic energy stored by the unit area in the effective capacitance created by the liquid and the substrate. 4 The second explanation considers that there is a net force acting on the electric charge that accumulates at the triple line (TPL) formed among the air, the solid substrate, and the liquid. 5,6 In fact, irrespectively of the explanation given, the contact angle decrease is independent of the polarity of the applied voltage, and the LippmannÀYoung 7 equation for the static contact angle change,has been experimentally demonstrated 8 for a wide range of voltage values prior to a saturation regime. In eq 1, θ 0 is the contact angle before the voltage is applied, θ V is the contact angle after a voltage V is applied, ε 0 is the vacuum permittivity, ε r and d are the relative permittivity and the thickness of the dielectric layer, respectively, and γ LV is the surface tension of the liquidÀgas interface. This increase in wettability contrasts with the decrease in wettability observed after electron bombardment. 9 In a recent paper, 10 we preliminarily discussed a contactless method to increase the wettability by creating the charging conditions of the TPL by air ionization using a corona charge instrument. We are providing in this article a more detailed discussion and systematic measurements of the observations we have made using this technique.Corona ionizers are based on the ionization of molecules of the surrounding air by the application of a sufficiently high potential between specific geometry electrodes (e.g., pin to plane) creating a large electric field gradient. 11 The control of static charge on insulating materials is a widespread use of this technique in the semiconductor industry to avoid undesired electrostatic discharge (ESD). In fact, this is the only practical way to neutralize static charge because grounding has no effect on the level of charge in insulators.In this article, we have used corona ionization to build electrostatic charge on a EWOD structure and analyze the effects this may have on the contact angle between a drop and the surface. Our preliminary experiments reported in ref 10 did show that the contact angle decreased after exposure to corona ionization, and this observation motivated the detailed study of the phenomenon that we report here. There are few works relating electrowetting to air ionization. Vallet et al. 12 attributed to air ionization the saturation phenomena of the contact angle, Blake 13 used a corona charging procedure to assist the study of contact angle change for a constant speed moving substrate, and Arifin et al. 14 investigated the effect of the e...
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