Synthesis and electrical characterization of ZnO and TiO2 thin films and their use as sensitive layer of pH field effect
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Synthesis and electrical characterization of ZnO and TiO2 thin films and their use as sensitive layer of pH field effect transistor sensors Jessica C. Fernandes and Marcelo Mulato Department of Physics, FFCLRP/University of São Paulo, Av. Bandeirantes, 3900 – Ribeirão Preto/São Paulo – Brazil ABSTRACT Oxide thin films of zinc and titanium materials were deposited by different deposition techniques, to be applied as sensitive layers of pH sensor – EGFET device. The deposition techniques tested were dip-coat, spin-coat, electrodeposition and spray-pyrolysis. The routine and the parameters of each technique were changed aiming optimized the procedures. The pHs buffer solutions tested ranged from 2 to 12. ZnO thin film shows sensitivity about 23 mV/pH, while TiO2 thin films shows only 13.8 mV/pH. The final purpose of this study is to optimize the parameters for each deposition technique for both oxide materials. INTRODUCTION The interest to investigate metal oxide nanostructures thin films has been increasing recently, due to their many unique properties linked to the nanometer size of the particles. Besides, deposited oxides had a large and variety applications in sensors devices, fuel cells and microelectronic circuits fabrication [1,2]. The pH sensor can be applied in medical and industrial fields, so the development of alternative pH electrodes, as pH sensor based on field effect transistor [3], solid-state [4], and microelectrode among others, spurred considerable advantage [5]. Both zinc (ZnO) and titanium oxides (TiO2) deposited as thin films has been largely applied as pH sensor due to their larger ranging in pH selectivity. The utilization of these materials as pH sensor has great interesting currently. The main purpose of the development of field effect transistor sensors and biosensors to be used in medicine field is the possibility of miniaturizing these devices. Different deposition techniques can change the structural properties of thin film, as the grain sizes, the roughness, and the porosity. These characteristics can directly influences on the sensitivity of the film when it is applied as a sensing layer of an ionic sensor. The extended-gate field effect transistor (EGFET) device possibilities the electrical characterization of ionic sensors. EXPERIMENTAL DETAILS For ZnO thin films, four different deposition techniques (as dip-coat, spin-coat, spraypyrolysis, and electrodeposition) were tested, while for TiO2 thin films only dip and spin-coat deposition techniques were used. Both spray-pyrolysis and electrodeposition were not applied to TiO2 thin films due to the viscosity of the precursor solution. Precursor solution for both ZnO and TiO2 were an optimization of different sol-gels routines found in the literature.
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The final ZnO sol-gel routine used in all depositions techniques, except electrodeposition, was composed by 13.7 g of dihydrate zinc acetate diluted in 100 ml of ethanol, with four portion of 175 μL of lactic acid 85%, added in a range of 30 min each one. The solution was left in a refl
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