Laser‐irradiated zinc oxide thin‐film transistors fabricated by solution processing

Abstract— This study investigates the effects of subjecting zinc oxide (ZnO) thin films to laser irradiation. The optical, structural, and electrical properties of the as‐deposited and laser‐irradiated films at different laser energies were studied. The transmittances without/with laser irradiation showed a net increase from 85 to 92% (@550 nm) for 250‐nm ZnO films, indicating an improvement in sample crystal linity. In addition, laser treatment decreased the ZnO band gap. Composition structure analysis shows that the crystallinity increased when the laser energy increased. Thin‐film transistors (TFTs) with a ZnO active layer were fabricated. The mobility of as‐deposited ZnO TFT devices (0.19 cm²/V‐sec) increased more than 2.5 times for ZnO of unirradiated laser treatment (0.49 cm²/V‐sec).     

Electrochemical deposition of polypyrrole/graphene oxide composite on microelectrodes towards tuning the electrochemical properties of neural probes

By exploiting the electrostatic interaction between positively charged pyrrole cation radicals and negatively charged graphene oxide (GO) sheets, we prepared polypyrrole/graphene oxide (PPy/GO) composite films by a one-step electrochemical process. We studied the effects of the polymerization current density and the GO content in electrolyte on the formation of PPy/GO coatings onto platinum neural microelectrode sites. As compared with pure PPy film, PPy/GO coatings show a rougher surface feature with micrometer-scale bulges. The impedance of the PPy/GO coated Pt electrode is only about 10% of the bare Pt electrode at the biological relevant 1 kHz, while the charge capacity density is more than two orders of the magnitude of the bare Pt electrode. Moreover, the PPy/GO coated Pt electrodes show higher performance than the PPy coated electrodes for the application of neural probe.     

Fabrication and gas-sensing properties of hierarchically porous ZnO architectures

Hierarchically three-dimensional (3D) porous ZnO architectures are synthesized by a template-free, economical aqueous solution method combined with subsequent calcination. First, the precursors of interlaced and monodisperse basic zinc nitrate (BZN) nanosheets are prepared. Then calcination of the precursors produces hierarchically 3D porous ZnO architectures composed of interlaced ZnO nanosheets with high porosity resulting from the thermal decomposition of the precursors. The products are characterized by X-ray diffraction, thermogravimetric-differential thermalgravimetric analysis, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller N₂ adsorption-desorption analyses. The BET surface area of the hierarchically porous ZnO nanostructures was calculated to be 12.8 m² g⁻¹. Compared with ZnO rods, the as-prepared porous ZnO nanosheets exhibit a good response and reversibility to some organic gases, such as ethanol and acetone. The responses to 100 ppm ethanol and acetone are 24.3 and 31.6, respectively, at a working temperature of 320 °C. These results show that the porous ZnO architectures are highly promising for gas sensor applications, as the gas diffusion and mass transportation in sensing materials are significantly enhanced by their unique structures. Moreover, it is believed that this solution-based approach can be extended to fabricate other porous metal oxide materials with a unique morphology or shape.     

Effect of aluminium doping on structural and gas sensing properties of zinc oxide thin films deposited by spray pyrolysis

A facile spray pyrolysis route is used to deposit aluminium doped ZnO (AZO) thin films on to the glass substrates. It is observed that on aluminium doping the particle size of ZnO reduces significantly; moreover, uniformity of particle also gets enhanced. Their XRD study reveals that intensity ratio of crystal planes depend on the aluminium doping concentration. The gas response studies of; ∼800 nm thick Al-doped ZnO films at different operating temperatures show that 5 at% Al-doped ZnO thin film exhibits highest response towards H₂S gas at 200 °C. The results suggest that the gas response strongly depends on the particle size and aluminium doping in the ZnO.     

Synthesis and characterisation of nanosized TiO2-ZrO2 binary system prepared by an aqueous sol-gel process: Physical and sensing properties

Nanostructured TiO₂-ZrO₂ thin films and powders were prepared by a straightforward aqueous particulate sol-gel route. Titanium (IV) isopropoxide and zirconium (IV) acetate hydrate were used as precursors, and hydroxypropyl cellulose was used as a polymeric fugitive agent in order to increase the specific surface area. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy revealed that the powder were crystallised at the low temperature of 500 °C, containing anatase-TiO₂ and tetragonal-ZrO₂ phases. Furthermore, it was found that ZrO₂ retarded the anatase-to-rutile transformation up to 900 °C. The activation energies for crystallite growth of TiO₂ and ZrO₂ components in the binary system were calculated 10.16 and 3.12 kJ/mol, respectively. Transmission electron microscope (TEM) image showed that one of the smallest crystallite sizes was obtained for TiO₂-ZrO₂ binary mixed oxide, being 5 nm at 500 °C. Field emission scanning electron microscope (FESEM) analysis revealed that the deposited thin films had nanostructured morphology with the average grain size of 20 nm at 500 °C and 36 nm at 900 °C. Thin films produced under optimised conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO and NO₂ gases at low operating temperature of 150 °C, resulted in an increase of thermal stability of sensing films as well as a decrease in the power consumption. Furthermore, calibration curves revealed that TiO₂-ZrO₂ sensor follows the power law, S = A[gas]^B (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two types of gases, and it has excellent capability for the detection of low gas concentrations.     

Towards a GC-based microsystem for benzene and 1,3 butadiene detection: Pre-concentrator characterization

The performance of a gas pre-concentrator/gas chromatographic column/gas sensor system was studied step by step towards benzene vapor in presence of 1,3 butadiene vapor. The pre-concentrator, based on an activated carbon of high adsorption capacity, was first characterized using a GC/FID system. By this way, the adsorption capacity of each pollutant was evaluated, first separately, then mixed with different proportions in air. It was shown that the affinity of the pre-concentrator for benzene was higher than for butadiene. Once the optimal pre-concentration parameters were adjusted, we replaced the FID detector by a metal oxide gas sensor. The use of such gas pre-concentrator allowed decreasing the detection limit of the system towards benzene in presence of butadiene by at least ten times.     

CuO/SnO2-In2O3 sensor for monitoring CO concentration in a reducing atmosphere

The CO sensing property of CuO-loaded SnO₂-In₂O₃ sensor was investigated in a reducing atmosphere. The sensor response to CO for CuO/SnO₂-In₂O₃ (8/2) was much higher than that for CuO/SnO₂ in the range of 200-1000 ppm of CO concentration. Such a high sensor response of CuO/SnO₂-In₂O₃ may originate from the high dispersion of CuO playing a role as sensing site.     

Gas sensing properties of p-type hollow NiO hemispheres prepared by polymeric colloidal templating method

This work presents a simple and versatile route to produce macroporous p-type metal oxide thin films. Two-dimensional arrays of p-type NiO films with a hollow hemisphere structure were fabricated by colloidal templating and RF-sputtering followed by a subsequent heat treatment. The diameter and shell thickness of the NiO hemisphere were 800 nm and 20 nm, respectively. X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the pure NiO phase with grain size of 10 nm was obtained at calcination temperatures that exceeded 450 °C. Close-packed arrays of hollow NiO hemispheres were found to exhibit p-type gas sensing properties against (CO, H₂, C₃H₈, CH₄, NO₂, and C₂H₅OH), leading to significantly enhanced responses to C₂H₅OH (R_gas/R_air = 5.0 at 200 ppm).     

Effect of micro-electrode geometry on NO2 gas-sensing characteristics of one-dimensional tin dioxide nanostructure microsensors

Electrodes with micro-gaps are fabricated by using dc-sputtering and FIB techniques. SnO₂ nanowires are deposited on the micro-gap (1-30 μm) by suspension dropping method to fabricate a micro-gas sensor. The sensing ability of various SnO₂ micro-gap sensors is measured. A comparison between sensors reveals that the short-gap electrode has numerous advantages in terms of reliability, high sensitivity and detection of low concentrations of NO₂, while the large-gap electrode is relatively sensitive for high concentrations. Conductance measurements are carried out at different surface temperatures and NO₂ concentrations in order to investigate the effects that the gap size has on the overall sensor conductance. The results suggest that the interface between the electrode and sensitive layer has a very important role for the sensing mechanism of tin dioxide gas sensors.     

Structural and properties of nanocrystalline WO3/TiO2-based humidity sensors elements prepared by high energy activation

Nanocrystalline WO₃/TiO₂-based powders have been prepared by the high energy activation method with WO₃ concentration ranging from 1 to 10 mol%. The samples were thermal treated in a microwave oven at 600 °C for 20 min and their structural and micro-structural characteristics were evaluated by X-ray diffraction, Raman spectroscopy, EXAFS measurements at the Ti K-edge, and transmission electron microscopy. Nitrogen adsorption isotherms and H₂ Temperature Programmed Reduction were also carried out for physical characterization. The crystallite and particle mean sizes ranged from 30 to 40 nm and from 100 to 190 nm, respectively. Good sensor response was obtained for samples with at least 5 mol% WO₃ activated for at least 80 min. Ceramics heat-treated in microwave oven for 20 min have shown similar sensor response as those prepared in conventional oven for 120 min, which is highly cost effective. These results indicate that WO₃/TiO₂ ceramics can be used as a humidity sensor element.