Alexandra Gonçalves

Growth of ZnO:Ga thin films at room temperature on polymeric substrates: thickness dependence

In this paper, we present results concerning the thickness dependence (from 70 to 890 nm) of electrical, structural, morphological and optical properties presented by gallium-doped zinc oxide (GZO) deposited on polyethylene naphthalate (PEN) substrates by r.f. magnetron sputtering at room temperature. For thicknesses higher than 300 nm an independent correlation between the electrical, morphological, structural and optical properties are observed. The lowest resistivity obtained was 5 =10 V cm with y4 a sheet resistance of 15 Vyh and an average optical transmittance in the visible part of the spectra of 80%. It is also shown that by passivating the surface of the polymer by depositing a thin silicon dioxide layer the electrical and structural properties of the films are improved nearly by a factor of two. 2003 Elsevier B.V. All rights reserved.

New challenges on gallium-doped zinc oxide films prepared by r.f. magnetron sputtering

Abstract Gallium-doped zinc oxide films were prepared by r.f. magnetron sputtering at room temperature as a function of the substrate–target distance. The best results were obtained for a distance of 10 cm, where a resistivity as low as 2.7×10 −4 Ω cm, a Hall mobility of 18 cm 2 /Vs and a carrier concentration of 1.3×10 21 cm −3 were achieved. The films are polycrystalline presenting a strong crystallographic c -axis orientation (002) perpendicular to the substrate. The films present an overall transmittance in the visible part of the spectra of approximately 85%, on average.

Office Paper Platform for Bioelectrochromic Detection of Electrochemically Active Bacteria using Tungsten Trioxide Nanoprobes

Electrochemically active bacteria (EAB) have the capability to transfer electrons to cell exterior, a feature that is currently explored for important applications in bioremediation and biotechnology fields. However, the number of isolated and characterized EAB species is still very limited regarding their abundance in nature. Colorimetric detection has emerged recently as an attractive mean for fast identification and characterization of analytes based on the use of electrochromic materials. In this work, WO3 nanoparticles were synthesized by microwave assisted hydrothermal synthesis and used to impregnate non-treated regular office paper substrates. This allowed the production of a paper-based colorimetric sensor able to detect EAB in a simple, rapid, reliable, inexpensive and eco-friendly method. The developed platform was then tested with Geobacter sulfurreducens, as a proof of concept. G. sulfurreducens cells were detected at latent phase with an RGB ratio of 1.10 ± 0.04, and a response time of two hours.

K+-doped poly(ε-caprolactone)/siloxane biohybrid electrolytes for electrochromic devices

Sol-gel derived KCF3SO3-doped di-urethane cross-linked poly(-caprolactone) (PCL(530))/siloxane ormolytes with  > n  0.5 (n indicates the number of (C(=O)(CH2)5O) repeat units per K + ion) have been studied. Xerogel samples with n ≥ 1 are thermally stable up to 235 oC. A minor proportion of PCL crystallites occur in some dilute-to-concentrated samples. At n ≤ 4 a crystalline PCL(530)/siloxaneKCF3SO3 complex with unknown stoichiometry emerges. At n ≤ 2 this complex coexists with pure salt. At temperatures higher than 40 oC the composition which exhibits the highest ionic conductivity is n = 21. “Free” anions, weakly coordinated CF3SO3 ions, contact ion pairs and several higher ionic aggregates emerge in all the * Phone: 00-351-259-350253; Fax: 00-351-259-350480; E-mail: vbermude@utad..pt

Spectroscopic ellipsometry study of amorphous silicon anodically oxidised

In this work, spectroscopic ellipsometry was used to characterise oxide films produced by anodic oxidation of amorphous silicon using an ethylene glycol (0.04 M KNO3) solution. The data obtained show that the growth of the oxide is not only a function of the voltage applied, but also of the current density and of the time process. An empiric model based on a power law is proposed for the growth of the oxide using, as parameters, the voltages and the time process. The oxide produced shows porosity of approximately 12%, which can be reduced down to 6% under well-controlled growth conditions.

Multifunctional Thin Film Zinc Oxide Semiconductors: Application to Electronic Devices

In this paper we report some of the recent advances in transparent thin film oxide semiconductors, specifically zinc oxide (ZnO), produced by rf magnetron sputtering at room temperature with multifunctional properties. By controlling the deposition parameters it is possible to produce undoped material with electronic semiconductor properties or by doping it to get either n-type or p-type semiconductor behavior. In this work we refer our experience in producing n-type doping ZnO as transparent electrode to be used in optoelectronic applications such as solar cells and position sensitive detectors while the undoped ZnO can be used as UV photodetector or ozone gas sensor or even as active layer of fully transparent thin film transistors.

A Study on the Electrical Properties of ZnO Based Transparent TFTs

The purpose of this work is to present in-depth electrical characterization on transparent TFTs, using zinc oxide produced at room temperature as the semiconductor material. Some of the studied aspects were the relation between the output conductance in the post-pinch-off regime and width-to-length ratios, the gate leakage current, the semiconductor/insulator interface traps density and its relation with threshold voltage. The main point of the analysis was focused on channel mobility. Values extracted using different methodologies, like effective, saturation and average mobility, are presented and discussed regarding their significance and validity. The evolution of the different types of mobility with the applied gate voltage was investigated and the obtained results are somehow in disagreement with the typical behavior found on classical silicon based MOSFETs, which is mainly attributed to the completely different structures of the semiconductor materials used in the two situations: while in MOSFETS we have monocrystalline silicon, our transparent TFTs use poly/nanocrystalline zinc oxide with grain sizes of about 10 nm.

Study of Electrochromic Devices Incorporating a Polymer Gel Electrolyte Component

Electrochromic materials have attracted considerable attention during the last two decades as a consequence of their potential application in several different types of optical devices. Examples of these devices include intelligent windows and time labels. In this paper the authors describe results obtained with thin tungsten oxide films produced at room temperature by rf magnetron sputtering under an argon and oxygen atmosphere on transparent conductive oxide coated glass substrates. To protect the surface of the electrochromic film, prevent water absorption and obtain a good memory effect under open circuit voltages, a layer of Ta2O5 was deposited over the WO3 films. In this study, the effect of different electrolyte compositions on the open circuit memory of optical devices has been characterized. The best results were obtained for electrochromic devices with polymer gel p(TMC)3LiClO4 and p(TMC)8LiClO4 electrolytes. These prototype devices present an overall transmittance of ~75% in their bleached state and after coloration 40.5 and 52.5% respectively. These devices also show memory effect and an optical density considered satisfactory for some electrochromic applications.

Role of Hydrogen Plasma on the Electrical and Optical Properties of Indium Zinc Transparent Conductive Oxide

In this work we studied the influence of the power density of hydrogen plasma on electrical and optical properties (Hall mobility, free carrier concentration, sheet resistance, optical transmittance and a.c. impedance) of indium zinc oxide films, aiming to determine their chemical stability. This is an important factor for the optimization of amorphous/nanocrystalline p-i-n hydrogenated silicon (a/nc-Si:H) solar cells, since they should remain chemically highly stable during the p layer deposition. To perform this work the transparent conductive oxide was exposed to hydrogen plasma at substrate temperature of 473 K, 87 Pa of pressure and 20 sccm of hydrogen flow. The results achieved show that IZO films were reduced for all plasma conditions used, which leads mainly to a decrease on films transmittance. For the lowest power density used in the first minute of plasma exposition the transmittance of the IZO films decreases about 29%.