Effect of pH of colloidal suspension on crystallization and activation energy of deep levels in SnO2 thin films obtained via sol-gel

Colloidal suspensions of tin dioxide (SnO2) are prepared by the sol-gel method from suspensions with distinct pHs. The particles in solution are embedded by an electrical layer. Decreasing the pH contributes to the destruction of this layer, leading to a high degree of aggregation among particles (clusters) due to the generation of cross-linked bonds (Sn-O-Sn) between them. The aggregation affects the electrical properties of films deposited by dip-coating from these solutions, due to the higher packing produced by acid pH. The X-ray diffractograms of films indicate higher crystallinity for lower pH. The Arrhenius plot leads to activation energies of the deepest level, which was between 67 and 140 meV, for the films prepared from suspensions with pH 6-11. Lower pH films also presented higher electrical conductivity. Obtained activation energies may be related to different types of defects, which could be associated with oxygen vacancies with distinct neighborhoods, influenced by the pH and potential barriers between grains, due to distinct packing caused by cross-linked bonds. TGA/DTA results indicate an easier crystallization process for lower pH, which ends at lower temperature, in good agreement with X-ray diffraction data. (C) 2009 Elsevier Ltd. All rights reserved.Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Growth of Al2O3 thin film by oxidation of resistively evaporated Al on top of SnO2, and electrical properties of the heterojunction SnO2/Al2O3

Aiming for the investigation of insulating properties of aluminum oxide (Al2O3) layers, as well as the combination of this oxide with tin dioxide (SnO2) for application in transparent field effect transistors, Al thin films are deposited by resistive evaporation on top of SnO2 thin films deposited by sol-gel dip-coating process. The oxidation of Al films to Al2O3 are carried out by thermal annealing at 500 A degrees C in room conditions or oxygen atmosphere. X-ray diffraction data indicate that tetragonal Al2O3 is indeed obtained. A simple device and electric circuit is proposed to measure the insulating properties of aluminum oxide and the transport properties of SnO2 as well. Results indicate a fair insulation when four layers or Al2O3 are grown on the tin dioxide film, concomitant with thermal annealing between each layer. The current magnitude through the insulating layer is only 0.2% of the current through the semiconductor film, even though the conductivity of the SnO2 alone is not very high (the average resistivity is 2 Omega cm), because no doping is used. The presented results are a good indication that this combination may be useful for transparent devices.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Eco-friendly luminescent hybrid materials based on EuIII and LiI co-doped chitosan

Biopolymer-based materials have been of particular interest as alternatives do synthetic polymers due to their low toxicity, biodegradability and biocompatibility. Among them, chitosan is one of the most studied ones and has recently been investigated for the application as solid state polymer electrolytes. Furthermore, it can serve as a host for luminescent species such as rare earth ions, giving rise to materials with increased functionality, of particular interest for electrochemical devices. In this study, we investigate chitosan based luminescent materials doped wit Eu3+ and Li+ triflate salts from the structural, photophysical and conductivity points of view. Because the host presents a broad emission band in the blue to green, while Eu3+ emits in the red, fine tuning of emission colour and/or generation of white light is possible by optimizing composition and excitation scheme. Europium lifetimes (5D0) are in the range 270 – 350 µs and quantum yields are as high as 2%. Although Li+ does not interfere with the luminescent properties, it grants ion-conducting properties to the material suggesting that a combination of both properties could be further explored in multifunctional device.We are grateful to the Fundação para a Ciência e Tecnologia and FEDER (PEstC/QUI/UI0686/2013) and program POPH/FSE, grant SFRH/BD/97232/2013 (R. Alves), for financial support of this work. The authors are also grateful to FAPESP (CEPID 2013/07793-6 and PD fellowship 2013/24727-7), and CNPQ (Universal Project 479672/2012-1) and CAPES for the financial support. M. M. Silva acknowledges CNPq (PVE grant 406617/2013-9), for the mobility grant provided by this institution. The help from Dr. Reza Dousti in measuring quantum yields is greatly appreciated

Optical emission and electron capture of rare-earth trivalent ions located at distinct sites in SnO(2) thin films

We present photoluminescence and decay of photo excited conductivity data for sol-gel SnO(2) thin films doped with rare earth ions Eu(3+) and Er(3+), a material with nanoscopic crystallites. Photoluminescence spectra are obtained under excitation with several monochromatic light sources, such as Kr(+) and Ar(+) lasers, Xe lamp plus a selective monochromator with UV grating, and the fourth harmonic of a Nd: YAG laser (4.65eV), which assures band-to-band transition and energy transfer to the ion located at matrix sites, substitutional to Sn(4+). The luminescence structure is rather different depending on the location of the rare-earth doping, at lattice symmetric sites or segregated at grain boundary layer, where it is placed in asymmetric sites. The decay of photo-excited conductivity also shows different trapping rate depending on the rare-earth concentration. For Er-doped films, above the saturation limit, the evaluated capture energy is higher than for films with concentration below the limit, in good agreement with the different behaviour obtained from luminescence data. For Eu-doped films, the difference between capture energy and grain boundary barrier is not so evident, even though the luminescence spectra are rather distinct

Numerical simulation of the liquid phase in SnO(2) thin film deposition by sol-gel-dip-coating

The fluid flow of the liquid phase in the sol-gel-dip-coating process for SnO(2) thin film deposition is numerically simulated. This calculation yields useful information on the velocity distribution close to the substrate, where the film is deposited. The fluid modeling is done by assuming Newtonian behavior, since the linear relation between shear stress and velocity gradient is observed. Besides, very low viscosities are used. The fluid governing equations are the Navier-Stokes in the two dimensional form, discretized by the finite difference technique. Results of optical transmittance and X-ray diffraction on films obtained from colloidal suspensions with regular viscosity, confirm the substrate base as the thickest part of the film, as inferred from the numerical simulation. In addition, as the viscosity increases, the fluid acquires more uniform velocity distribution close to the substrate, leading to more homogenous and uniform films.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)CAPESConselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)CNPqFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESPINCT-MACCINCT-MAC

Characterization of metallic electrical contacts to SnO2 thin films lightly doped with Eu3+ ions, and photo-induced resistivity

Lightly Eu3+-doped (0.05%) SnO2 thin films are deposited by the sol-gel-dip-coating technique, topped by alternative metallic layers of Al, Sn or In, arranged in a parallel layout on the thin film surface, and deposited by the resistive evaporation technique. Electrical characterization results show that the sort of deposited metal strongly modifies the device resistance, besides thermally treated metallic layers decreases the device resistivity, which may be associated with increased tunneling probability. Current as function of applied voltage show a good linear symmetry contacts for a large temperature range (30-320 K). However, this feature is due to the MSM (metal-semiconductor-metal) structure of the device, because the conduction through the reversed-biased junction is the main mechanism of electrical transport at Schottky potential barriers. The barrier height evaluation is also presented, considering that the dominant mechanism is the thermionic emission through the reversed-biased junction, yielding values in the range 124 meV for annealed In contacts to 187 meV for untreated Sn contacts. This paper also shows results of photo-induced electrical characteristics under irradiation with below bandgap (450 nm) as well as above bandgap (266 nm) light on the SnO2 thin films, where the surface is coupled with untreated Sn contacts. (c) 2012 Elsevier B.V. All rights reserved.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP