Synthesis of highly selective and sensitive Cu-doped ZnO thin film sensor for detection of H2S gas

Copper (Cu) doped zinc oxide (ZnO) thin films were successfully prepared by a simple sol-gel spin coating technique. The effect of Cu doping on the structural, morphology, compositional, microstructural, optical, electrical and H₂S gas sensing properties of the films were investigated by using XRD, FESEM, EDS, FTIR, XPS, Raman, HRTEM, and UV–vis techniques. XRD analysis shows that the films are nanocrystalline zinc oxide with the hexagonal wurtzite structure and FESEM result shows a porous structured morphology. The gas response of Cu-doped ZnO thin films was measured by the variation in the electrical resistance of the film, in the absence and presence of H₂S gas. The gas response in relation to operating temperature, Cu doping concentration, and the H₂S gas concentration has been systematically investigated. The maximum H₂S gas response was achieved for 3 at% Cu-doped ZnO thin film for 50 ppm gas concentration, at 250 °C operating temperature.     

Effect of UV light irradiation on photovoltaic characteristics of inverted polymer solar cells containing sol–gel zinc oxide electron collection layer

An inverted organic bulk-heterojunction solar cell containing a zinc oxide (ZnO) based electron collection layer with a structure of ITO/ZnO/[6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM): regioregular poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxylenethiophene): poly(4-styrene sulfonic acid)/Au (ZnO cell) was fabricated. We examined the relationship between the heating temperature of the ZnO layer and the device performance under irradiation by simulated sunlight while cutting the UV light. The effects of the UV light contained in simulated sunlight were investigated by photocurrent–voltage (I–V) and alternating current impedance spectroscopy (IS) measurements. When the ZnO cells were irradiated with simulated sunlight, they exhibited a maximum power conversion efficiency (PCE) of over 3%, which hardly varied with the heating temperature of ZnO layers treated at 250 °C, 350 °C, and 450 °C. In contrast, when the ZnO cells were irradiated with simulated sunlight without UV content, their photovoltaic characteristics were very different. In the case of the cell with ZnO prepared by heating at 250 °C, PCE of 2.7% was maintained even under continuous irradiation with simulated sunlight without UV. However, for the cells with ZnO prepared by heating at 350 °C and 450 °C, the shapes of the I–V curves changed with the UV-cut light irradiation time, accompanying an increase in their series resistance. Overall, after UV-cut light irradiation for 1 h, the PCE of the cell with ZnO prepared by heating at 350 °C decreased to 1.80%, while that of the cell with ZnO prepared by heating at 450 °C fell to 1.35%. The photo IS investigations suggested that this performance change was responsible for the formation of charge-trapping sites at the ZnO/PCBM:P3HT interface which act as recombination centers for photo-produced charges in the PCBM:P3HT layer.     

Effects of reaction time on the morphological,structural, and gas sensing properties of ZnO nanostructures

Morphological transformation was achieved from ZnO hexagonal needle-like rods to hexagonal flower-like rods by varying the reaction growth time using the hydrothermal method. Optical bandgap energies were calculated from the absorption spectra using UV‐vis spectroscopy. Gas sensing properties of flower-like hexagonal ZnO structures at 50 ppm for ethanol (C₂H₅OH) and nitrogen dioxide (NO₂) at different temperatures were analyzed. The sensor showed a higher response toward C₂H₅OH than NO₂ gas at 350 °C.     

Characterization of AZO/p-Si heterojunction prepared by DC magnetron sputtering

Al-doped ZnO (AZO) film was deposited by direct-current (DC) magnetron sputtering on p-Si (1 0 0) wafer to fabricate Al-doped n-ZnO/p-Si heterojunctions. The microstructural, optical and electrical properties of the AZO film were characterized by XRD, SEM; UV–vis spectrophotometer; four-point probe and Hall effect measurement, respectively. Results show that the AZO film is of good quality. The electrical junction properties were investigated by I–V measurement, which reveals that the heterojunction shows rectifying behavior under a dark condition. The ideality factor and the saturation current of this diode are 20.1 and 1.19×10⁻⁴ A, respectively. The value of I_F/I_R (I_F and I_R stand for forward and reverse current, respectively) at 5 V is found to be as high as 19.7. It shows fairly good rectifying behavior, indicating formation of a diode between AZO and p-Si. High photocurrent is obtained under a reverse bias when the crystalline quality of AZO film is good enough to transmit light into p-Si.     

Response time and mechanism of Pd modified TiO2 gas sensor

In this work, gas response properties of Pd modified TiO₂ sensing films are discussed when exposed to H₂ and O₂. TiO₂ films are surface modified in PdCl₂-containing solution by the dipping method and treated for different treatment times to get different surface states. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and Kröger–Vink defect theory are used to characterize the sensing films. The gas response properties indicate that the sensor response time which related to the rate of change of sensor resistance is affected by the activation energy (E). In particular, the sensor treated at 900 °C for 2 h exhibits a response time of about 20–240 ms when exposed to H₂ and 40–130 ms when exposed to O₂ at 500–800 °C.     

Magnetron-sputtered high performance Y-doped ZnO thin film transistors fabricated at room temperature

In this report, sputtered-grown undoped ZnO and Y-doped ZnO (ZnO:Y) thin film transistors (TFTs) are presented. Both undoped ZnO and ZnO:Y thin films exhibited highly preferred c-axis oriented (002) diffraction peaks. The ZnO:Y thin film crystallinity was improved with an increase of (002) peak intensity and grain size. The electrical properties of ZnO:Y TFTs were significantly enhanced relative to undoped ZnO TFTs. ZnO:Y TFTs exhibited excellent performance with high mobility of 38.79 cm² V⁻¹ s⁻¹, small subthreshold swing of 0.15 V/decade, and high I_on/I_off current ratio of the order of 8.17 × 10⁷. The O_1s X-ray photoelectron spectra (XPS) showed oxygen vacancy-related defects present in the ZnO:Y TFTs, which contributed to enhancing the mobility of the TFTs.     

N-doped ZnO based fast response ultraviolet photoconductive detector

We report a study on the fabrication and characterization of ultraviolet photodetectors based on N-doped ZnO films. Highly oriented N-doped ZnO films with 10 at.% N doping are deposited using spray pyrolysis technique onto glass substrates. The photoconductive UV detector based on N-doped ZnO thin films, having a metal–semiconductor–metal (MSM) configuration are fabricated by using Al as a contact metal. I–V characteristic under dark and UV illumination, spectral and transient response of ZnO and N-doped ZnO photodetector are studied. The photocurrent increases linearly with incident power density by more than two orders of magnitude. The photoresponsivity (580 A/W at 365 nm with 5 V bias, light power density 2 μW/cm²) is much higher in the ultraviolet region than in the visible.     

Structural,optical, photoluminescence,electrochemical, and photoelectrochemical properties of Fe doped ZnSe hexagonal nanorods

The present investigation describes that the iron (Fe) doped ZnSe hexagonal nanorods were successfully synthesized via chemical synthesis specifically galvanostatic mode of electrodeposition and addition of Fe in order to improve the PEC performance of ZnSe electrodes using a galvanostatic mode. These crystalline Fe doped ZnSe hexagonal nanorods electrodes are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and optical properties, such as UV–vis spectroscopy, photoluminescence spectroscopy, electrochemical impedance spectroscopy (EIS), Raman spectroscopy and photoelectrochemical properties. It is seen that at 5% Fe doped ZnSe hexagonal nanorods show the open circuit voltage (V_oc) was 100 mV, and short circuit current (I_sc) was 110 μA. The observed fill factor and efficiency were found to be 44% and 0.15%, respectively.     

Low temperature fabrication of a ZnO nanoparticle thin-film transistor suitable for flexible electronics

A bottom-gate/top-drain/source contact ZnO nanoparticle thin-film transistor was fabricated using a low temperature annealing process (150 °C) suitable for flexible electronics. Additionally, a high-k resin filled with TiO₂ nanoparticles was used as gate dielectric. After fabrication, the transistors presented almost no hysteresis in the I–V curve, a threshold voltage (V_T) of 2.2 V, a field-effect mobility on the order of 0.1 cm²/V s and an I_ON/I_OFF ratio of about 10⁴. However, the transistor is sensitive to aging effects due to interactions with the ambient air, resulting in current level reduction caused by trapped oxygen at the nanoparticle surface, and an anti-clockwise hysteresis in the transfer curve. It was demonstrated, conjointly, the possible desorption of oxygen by voltage stress and UV light exposure.     

The optical properties of rare earth Gd doped ZnO nanocrystals

We present a study of the light emission properties, from UV to blue spectral region, of Gd doped ZnO nanocrystals fabricated by means of a thermal evaporation vapor phase deposition process. The samples were grown from a mixed Zn/Gd source, with a molar percentage of Gd ranging from 0% (pure ZnO) to 5%, 10%, or 15%, in a constant O₂/Ar gas mixture flowing at 500° C. The pure ZnO nanocrystals exhibited a strong and predominant UV emission peaking at 375 nm. Besides the UV emission of ZnO nanocrystals, two strong blue emissions, located at 432 and 397 nm, are observed for the sample doped with 5% Gd. The strong blue emissions are mainly induced by the impurity levels of Gd introduced into the band gap of the ZnO nanocrystals. The UV emission of ZnO decreases as the doping concentration of Gd increases, and the blue emission is replaced by a broad defect emission due to the greater number of defects and impurities, as well as Gd₂O₃ on the surface. The results show that the optical properties of ZnO can be tuned by the doping concentration of Gd.     

ZnO/p-Si heterojunction photodiode by sol–gel deposition of nanostructure n-ZnO film on p-Si substrate

The electrical and photovoltaic properties of the nanostructure ZnO/p-Si diode have been investigated. The nanostructure ZnO/p-Si diode was fabricated using sol–gel spin coating method. The ideality factor and barrier height of the diode were found to be 3.18 and 0.78 eV, respectively. The obtained n ideality factor is higher than 2, indicating that the diode exhibits a non-ideal behavior due to the oxide layer and the presence of surface states. The nanostructure of the ZnO improves the quality of ZnO/p-Si interface. The diode shows a photovoltaic behavior with a maximum open circuit voltage V_oc of 0.26 V and short-circuits current I_sc of 1.87×10^?8 A under 100 mW/cm². It is evaluated that the nanostructure ZnO/p-Si diode is a photodiode with the obtained electronic parameters.     

Enhancement of the hydrogen gas sensitivity by large distribution of c-axis preferred orientation in highly Ga-doped ZnO polycrystalline thin films

The hydrogen gas sensing properties of highly Ga-doped ZnO (GZO) polycrystalline thin films deposited by radio-frequency magnetron sputtering have been studied. The relationship between the microstructural properties of preferred c-axis oriented thin films and the hydrogen gas sensing properties is described. The crystallite size and the preferred orientation distribution were characterized by X-ray diffraction. The crystallite size increased and the preferred orientation distribution decreased with increasing film thickness. In order to control the crystallite size and the c-axis orientation separately, a highly oriented ZnO template layer with different thickness was employed for deposition of 30-nm-thick GZO films. The c-axis orientation of these films were nearly comparable each other, while the crystallite size increased significantly with increasing thickness of the ZnO templates. The hydrogen gas sensitivity at an operating temperature of 330 °C increased slightly with decreasing crystallite size, while the sensitivity was dramatically enhanced by increasing the preferred orientation distribution. It is therefore proposed that the c-axis orientation plays an important role in determining the sensitivity of the hydrogen gas sensor.     

Sensing characteristics of zinc oxide thin films deposited by spray pyrolysis onto tubular Pyrex substrate

This paper presents NO₂ sensing properties of ZnO thin films grown onto tubular Pyrex substrate using the spray pyrolysis method. The sensor response was found to depend essentially on four parameters: chemical composition, structure, morphology and operating temperature. The crystallinity and morphology of the as-preapred films were analyzed using X-Ray Diffraction (XRD) and Atomic Force Microscopy (AFM). The sensing properties of ZnO toward NO₂ were investigated at different operating temperatures and NO₂ concentrations. Optimization of the preparation conditions show that ZnO thin film deposited during 15 min exhibit the highest sensitivity with fast response and recovery time.     

High photosensitivity with enhanced photoelectrical contribution in hybrid nanocomposite flexible UV photodetector

To devise a reliable strategy to develop an ultraviolet (UV) sensitive hybrid photodetector, plasma process is utilized as a single step method for production of large area nanocomposite films based on plasma polymerized aniline–titanium dioxide (PPani–TiO₂). The synthesis of PPani–TiO₂ nanocomposite films are made using reactive magnetron sputtering in combination with plasma polymerization. The deposited PPani–TiO₂ nanocomposite films are characterized and discussed in terms of structural, optical and electrochemical properties. A hybrid flexible nanostructured UV photodetector is constructed from PPani–TiO₂ nanocomposite and its optoelectronic properties are evaluated which exhibits a greatly enhanced photosensitivity resulting in high photoconductive gain (G = 4.56 × 10⁴) and high responsivity (R = 9.36 × 10³ AW⁻¹) under UV illumination of 254 nm. The flexible devices are successfully operated under bending up to 170° (bending radius, R = 8 mm) and showed a good folding strength and stability. The proposed plasma based method provides a green technology where the self-assembly of molecules, that is, the spontaneous association of atomic or molecular building blocks under plasma environment, emerge as a successful strategy to form well-defined structural and morphological units of nanometer dimensions.     

Fabrication of Ag nanoparticle/ZnO thin films using dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with photoreduction method and its application

We report a novel method to grow silver nanoparticle/zinc oxide (Ag NP/ZnO) thin films using a dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with a photoreduction method. The crystalline quality, optical properties, and electrical characteristics of Ag NP/ZnO thin films depend on the AgNO₃ concentration or Ag content and annealing temperature. Optimal Ag NP/ZnO thin films have been grown with a AgNO₃ concentration of 0.12 M or 2.54 at%- Ag content and 500 °C- rapid thermal annealing (RTA); these films show orientation peaks of hexagonal-wurtzite-structured ZnO (002) and face-center-cubic-crystalline Ag (111), respectively. The transmittance and resistivity for optimal Ag NP/ZnO thin films are 85% and 6.9×10⁻⁴ Ω cm. Some Ag NP/ZnO transparent conducting oxide (TCO) films were applied to InGaN/GaN LEDs as transparent conductive layers. The InGaN/GaN LEDs with optimal Ag NP/ZnO TCO films showed electric and optical performance levels similar to those of devices fabricated with indium tin oxide.     

Photoluminescence from c-axis oriented ZnO films synthesized by sol-gel with diethanolamine as chelating agent

ZnO films were synthesized on SiO₂/Si substrates through the sol-gel technique using diethanolamine as chelating agent and annealed in Ar+O₂ atmospheres with different O₂ flow-rates in the 10–100 sccm range. Samples were studied by scanning electron microscopy and X-ray diffraction, evidencing a nanostructured morphology with a preferential orientation along the (0 0 2) direction (c-axis orientation), which is uncommon when diethanolamine is used as the chelating agent. The room temperature photoluminescence spectra show strong UV emissions at around 375 and 384 nm from near band-edge transitions and phonon replica, and a broad defect-related band extending from the visible to near infrared (∼500–800 nm). The analysis of the defect-related emission band and its various components as a function of the O₂ flow-rate is discussed in terms of contributions from specific luminescent point defect centers established during annealing.     

Synthesis of ZnO and Fe2O3 nanoparticles by sol–gel method and their application in dye-sensitized solar cells

ZnO and Fe₂O₃ nanoparticles have been formed in a silica matrix, through the sol–gel method and were used as a photoanode to fabricate dye-sensitized solar cells (DSCs). The obtained oxides were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscope and UV–visible absorption spectroscopy. The results indicate that ZnO and Fe₂O₃ prepared by this method may be used as photoanodes in photo-electro-chemical energy conversion systems. DSSCs have been built using eosin Y as photosensitizer and their photocurrent, open-circuit voltage, fill factor and efficiency have been measured under direct sunlight illumination (1000 Wcm^?2). A ZnO-film solar cell had the best performance with an open-circuit voltage of V_oc=0.7 V and short-circuit current density of I_sc=490 μA/cm². This was attributed to high optical gap energy and transparency of ZnO compared to Fe₂O₃. The effects of annealing temperature and concentration of Fe₂O₃ on conversion efficiency of the Fe₂O₃ based solar cell were also studied.     

Au nanoparticle modified flower-like ZnO structures with their enhanced properties for gas sensing

The flower-like ZnO (F-ZnO) synthesized by a solution approach was hydrothermally functionalized with Au nanoparticles (Au NPs). The Au coverage on the surface of F-ZnO was controllable by adjusting the Au concentration of the precursor. The gas sensing performance of the formed hybrid was systematically investigated. A Au-functionalized F-ZnO hybrid structure, combining excellent catalytic activity of Au NPs and efficient charge-transfer layer at the Au/ZnO interface, was demonstrated to possess the superior response to pristine ZnO. The optimal Au loading is 6 wt%, and its gas response is nearly 17 times higher than that of pristine ZnO and ~2.5 times higher than that of commercial ZnO functionalized with 6 wt% Au. Such a hybrid structure exhibits a great potential for gas sensing applications.     

Rapid synthesis of ZnO dandelion-like nanostructures and their applications in humidity sensing and photocatalysis

In this study, ZnO dandelion-like nanostructures were rapidly synthesized on Si substrates using a two-step thermal oxidation approach. The ZnO nanostructures were grown at various thermal oxidation temperatures ranging from 400 °C to 700 °C. These nanostructures were then applied to humidity sensing and photocatalysis. The ratio of measured resistances in the humidity sensors for relative humidity (RH) levels of 11% and 95% at room temperature (RT) were found to rise from 10² to 10⁵ times for humidity sensors constructed with the nanostructures grown at temperatures from 400 °C to 700 °C, respectively, and sensor response time decreased from 15 s to 5 s. These results show that the proposed ZnO dandelion-like nanomaterial shows promise as a candidate for fabricating high-performance humidity sensors when the nanostructures are grown at 700 °C. In addition, the photocatalytic effect of the nanostructures was tested with a decomposition of methyl orange (MO) dye under UV illumination. Experimental results show that the ZnO dandelion-like nanomaterial grown at a thermal oxidation temperature of 700 °C exhibits an excellent photocatalytic effect, which degrades to almost 90% of the MO activity over 120 min.     

Effect of Bi doping on structural,morphological, optical and ethanol vapor response properties of SnO2 nanoparticles

The gas sensing behavior of thick films of Bi doped SnO₂ has been investigated towards ethanol vapor. The screen printing technique was used to prepare the thick films. The films were sintered at 650 °C for 2 h. The structural, surface morphological, optical and gas sensing properties of undoped and Bi doped SnO₂ thick films have been studied. X-ray diffraction and Raman spectroscopy confirmed that the films consisted exclusively of tetragonal tin oxide, without any impurity phases. FE-SEM studies revealed the formation of highly porous microstructure with grain size in few tens of nanometers. From the optical studies, the band gap was found to be decreased with bismuth doping (3.96 eV for undoped, 3.83 eV, 3.71 eV and 3.6 eV for 1 mol%, 2 mol% and 3 mol% Bi, respectively). The 3 mol % Bi doped SnO₂ thick films exhibited the highest sensitivity to 100 ppm of ethanol vapor at 300 °C. The effect of microstructure on sensitivity, response time and recovery time of the sensor was studied and discussed.