Cobalt-doped ZnO Thin Films Research Articles

Impact of oxygen vacancies on enhanced NO2 gas sensing performance of lithium doped Co:ZnO thin films

This article explored the influence of lithium on cobalt-doped ZnO thin films fabricated via the sol–gel spin coating technique for NO2 gas sensing applications. The abundance of oxygen vacancies in ZnO can be proven by scanning electron microscopy, four-probe Hall measurements, photoluminescence spectroscopy, UV-visible spectroscopy, and x-ray photoelectron spectroscopy. The presence of lithium plays a crucial role in generating more oxygen vacancies in Co-doped ZnO was discussed. Among the fabricated samples, (Li-Co) co-doped ZnO exhibits better sensitivity (2940.17%), selectivity, repeatability, and stability (after 90 days) toward 75 ppm NO2 gas.

Sensitivity Enhancement of CO2 Sensors at Room Temperature Based on the CZO Nanorod Architecture.

Cobalt-doped ZnO (CZO) thin films were deposited on glass substrates at room temperature by radio frequency (RF) magnetron sputtering of a single target prepared with ZnO and Co3O4 powders. Changes in the crystallinity, morphology, optical properties, and chemical composition of the CZO thin films were investigated at various sputtering powers of 45, 60, and 75 W. All samples presented a hexagonal wurtzite-type structure with a preferential c-axis at the (002) plane, along with a distinct change in the strain values through X-ray diffraction patterns. Scanning electron and atomic force microscopy revealed uniform and dense deposition of nanorod CZO samples with a high surface roughness (RMS). The Hall mobility and carrier concentration increased with the introduction of Co+ ions into the ZnO matrix, as seen from the Hall effect study. The gradual increase of the power applied on the target source significantly affected the morphology of the CZO thin film, which is reflected in the CO2-sensing performance. The best gas response to CO2 was recorded for CZO-60 W with a response of 1.45 for 500 ppm CO2, and the response/recovery times were 72 and 35 s, respectively. The distinguishing feature of the CZO sensor is its ability to effectively and rapidly detect the CO2 target gas at room temperature (∼27 °C, RT).

Sol-gel synthesis of porous cobalt-doped ZnO thin films leading to rapid and large scale Orange-II photocatalysis

Sol-gel synthesis of porous cobalt-doped ZnO thin films leading to rapid and large scale Orange-II photocatalysis

Investigation of substrate effect on Co-doped ZnO thin films prepared by thermionic vacuum arc technique

• The substrate effect was investigted fort h Co-ZnOthin film. • The average crystallite size is 20 and 25 nm. • The band gap value of the Co-doped ZnO thin film was calculated as 3.22 eV. In this paper, the substrate effect on cobalt-doped ZnO thin films and crystal defects on optical properties of the sample have been investigated. The structural, elemental, surface and optical properties of the films were characterized using X-ray diffraction (XRD), atomic force microscopy, field emission scanning electron microscopy and UV-Vis spectrophotometry. The XRD results confirm the polycrystalline nature and formation of ZnO (100) plane for both substrates was detected. Metal oxide and bimetal oxide structures were determined using by XRD analyses. Average crystallite size of the films has been computed as 20 and 25 nm on the glass and Si substrates, respectively. The thicknesses of the films were recorded as 85 and 90 nm on the glass and Si substrates, respectively. The mean transmittance value was measured as 71%. The band gap value of Co-doped ZnO thin film was calculated as 3.22 eV. The Urbach energy was achieved as 227 meV depending on the lattice distortions and structural defects. According to obtain results, TVA is a promising method for Co doped ZnO thin film production.

Enriched optoelectronic properties of cobalt-doped ZnO thin films for photodetector applications

Enriched optoelectronic properties of cobalt-doped ZnO thin films for photodetector applications

Studies on structural and optical properties of Cobalt doped ZnO thin films prepared by Sol-gel spin coating technique

Abstract The Cobalt doped Zinc oxide (CZO) thin films were first prepared by sol–gel method. CZO transparent conductive thin films were deposited on glass substrates using spin coating technique. From XRD graph, the (0 0 2) diffraction peak intensity of CZO film decreased with increased doping concentration from 3 to 8 wt%. No extra peaks related to cobalt metal and other oxide phases were detected which illustrates that the obtained films are of single phase. Crystallite size (D) decreases gradually with increase in the Co doping level. The transmission curves of the Co doped ZnO thin films were taken in the wavelength area 300–800 nm. The average value was determined to be 80%. SEM images indicate the uniform distribution of spherical particles without any cracks or pore and exhibits good crystalline quality. It clearly shows that the crystalline decreases as cobalt concentration increases.

Influence of cobalt doping on the structural, optical, topographical and sensing properties of spray deposited zinc oxide thin films

Structural, optical and gas sensing properties of spray deposited undoped and cobalt doped ZnO thin films have been carried out and reported. Hexagonal wurtzite structure of the investigated thin films is confirmed using X-ray diffraction technique. Interestingly, crystallite sizes are found to decrease with increasing Co concentration with 0.003 M having least size of 40 nm. The growth orientation plane of ZnO thin films are found to vary with Co doping. Atomic force micrographs reveal spherical grains with compact structure for both the films. The optical band gap values at higher doping concentrations were found to be increased. The Co doped (0.003 M) film gained less band gap (3.36 eV) which is beneficial for ammonia sensing application. The sensing characteristics of doped (Co-0.003 M) ZnO (0.1 M) film at various ammonia vapour concentrations at room temperature were studied.

Structural and magnetic properties of cobalt doped ZnO thin films deposited by cost effective nebulizer spray pyrolysis technique

In this work, we report the structural, optical and magnetic properties of Co doped ZnO thin films fabricated by nebulizer spray pyrolysis technique. Five different thin films were fabricated with various doping concentration of cobalt. These thin films posses hexagonal wurtzite structure with polycrystalline nature and strongly oriented along with (002) and (101) direction. From x-ray diffraction pattern, average crystallite sizes were calculated and are found to be decreases with Co doping concentration. The dislocation density and microstrain values of the coated films increased gradually with doping concentration. The SEM/AFM studies reveal the change from spherical shape to petal shaped grains and it is also found that the size of grains and roughness decreases with Co increasing doping concentration. The lowering of the optical band gap energy from ∼ 3.24 eV to 3.05 eV shows doping by Co in ZnO gives rise to band gap bowing in ZnO:Co thin films. The M-H curve confirms the god ferromagnetism in Co doped ZnO thin films.

STRUCTURAL AND OPTICAL PROPERTIES OF UNDOPED AND SILVER, LITHIUM AND COBALT-DOPED ZnO THIN FILMS

Undoped and silver, lithium and cobalt-doped ZnO thin films have been successfully deposited on glass by chemical bath deposition (CBD). The reaction temperature was 50∘C and the films were annealed at 400∘C for 4[Formula: see text]h in a high temperature furnace. UV/VIS spectrum was used to determine optical transmittance, optical band gap ([Formula: see text] and absorbance values of Ag:ZnO, Co:ZnO, Li:ZnO and undoped ZnO thin films. Optical band gap ([Formula: see text] and absorbance values of undoped ZnO, Ag:ZnO, Co:ZnO and Li:ZnO thin films were found as 0.0158, 0.0064, 0.2638, 0.0956 and 3.24, 3.13, 3.27, 2.96 eV, respectively. Extinction coefficients and refraction indexes of the films were found to be 0.0096, 0.0038, 0.0068, 0.019 (extinction coefficient) and 1.26, 1.14, 1.66, 2.33 (refraction index), respectively. X-ray patterns of undoped ZnO, Ag:ZnO, Co:ZnO and Li:ZnO thin films were confirmed as amorphous.

Influence of pulse repetition rate on the growth of cobalt-doped ZnO thin films by pulsed electron beam ablation

Abstract

Preparing transparent cobalt-doped ZnO thin films by DC magnetron sputtering

Thin films of cobalt-doped ZnO were prepared by DC magnetron sputtering on glasses. The cobalt dopant concentration was at 3%wt. The sputtering power was set at 200 watts and sputtered for 20 mins. All films are transparent. The films were then annealed at 100-500 °C under air atmosphere. Crystal structural and optical properties of the films were characterized by XRD and UV-Vis, respectively. The XRD patterns show peaks corresponding to (002) plane indicate thin films obtain hexagonal wurtzite structure. The effects of annealing temperature on crystal structure and optical properties were discussed.

Structural and magnetic properties of Co-doped ZnO thin films grown by ultrasonic spray pyrolysis method

Cobalt-doped ZnO thin films with several different percentage of Co from 0 up to 15 at% were synthesized via a cheap, simple and versatile method i.e. ultrasonic spray pyrolysis at atmospheric pressure and a substrate temperature of 350 °C. The structure of the as-prepared samples was characterized by X-ray diffraction (XRD), Raman spectroscopy and FTIR. The Co-doping effect is revealed by the presence of three additional peaks around 235, 470 and 538 cm−1 respect to the Raman spectra of the unsubstituted film. Fourier transform infrared spectroscopy (FTIR) put in evidence the decrease of the bond force constant f with increasing Co-doping. By ultra-violet visible near infrared (UV–Vis–NIR) spectroscopy on Co-doped samples it was possible to show the presence of additional absorption bands at approximately 570, 620 and 660 nm suggesting that Co2+ ions do not change their oxidation when substituted to zinc and the ZnO lattice does not change its wurtzite structure as well. Finally, all our samples exhibit a paramagnetic behavior without any trace of intrinsic room temperature ferromagnetism.

Studies on visible light photocatalytic and antibacterial activities of nanostructured cobalt doped ZnO thin films prepared by sol–gel spin coating method

Nanostructured cobalt doped ZnO thin films were deposited on glass substrate by sol–gel spin coating technique and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy and UV–Vis spectroscopy. The XRD results showed that the thin films were well crystalline with hexagonal wurtzite structure. The results of EDAX and XPS revealed that Co was doped into ZnO structure. FESEM images revealed that the films possess granular morphology without any crack and confirm that Co doping decreases the grain size. UV–Vis transmission spectra show that the substitution of Co in ZnO leads to band gap narrowing. The Co doped ZnO films were found to exhibit improved photocatalytic activity for the degradation of methylene blue dye under visible light in comparison with the undoped ZnO film. The decrease in grain size and extending light absorption towards the visible region by Co doping in ZnO film contribute equally to the improved photocatalytic activity. The bactericidal efficiency of Co doped ZnO films were investigated against a Gram negative (Escherichia coli) and a Gram positive (Staphylococcus aureus) bacteria. The optical density (OD) measurement showed better bactericidal activity at higher level of Co doping in ZnO.

Features of the hopping conductivity in gallium and cobalt doped ZnO thin films

The effect of Ga and Co doping on the hopping transport of electrons in ZnO films was studied. All investigated films were deposited on monocrystalline substrates by water assisted metal organic chemical vapour deposition method. Temperature dependencies of the resistivity of all films obeyed the Mott's law at low temperatures. Both positive and negative magnetoresistance were observed in the temperature range of the Mott's law. The values of the localization length obtained from magneto transport data for ZnO and ZnO:Ga films are larger than the effective Bohr radii of a shallow donor state and are independent on the Ga content. These observations point to the localization of electrons in the random potential originating from the structural disorder. In Co doped ZnO films large positive magnetoresistance was observed in low magnetic fields at low temperatures. The value of positive magnetoresistance increases with an increase of Co content and decrease of temperature. Observed magneto transport properties of Co-doped films were interpreted by the decrease of electron hopping probability due to decrease of the density of localized states in magnetic field.

Effect of band gap energy on the electrical conductivity in doped ZnO thin film

The transparent conductive pure and doped zinc oxide thin films with aluminum, cobalt and indium were deposited by ultrasonic spray technique on glass substrate at 350 °C. This paper is to present a new approach to the description of correlation between electrical conductivity and optical gap energy with dopants' concentration of Al, Co and In. The correlation between the electrical and optical properties with doping level suggests that the electrical conductivity of the films is predominantly estimated by the band gap energy and the concentrations of Al, Co and In. The measurement in the electrical conductivity of doped films with correlation is equal to the experimental value, the error of this correlation is smaller than 13%. The minimum error value was estimated in the cobalt-doped ZnO thin films. This result indicates that such Co-doped ZnO thin films are chemically purer and have far fewer defects and less disorder owing to an almost complete chemical decomposition.

Structure, X-ray photoelectron spectroscopy and photoluminescence investigations of the spray deposited cobalt doped ZnO thin films

A simple and cost-effective pneumatic spray pyrolysis technique (SPT) was employed to synthesize cobalt-doped zinc oxide (Co-ZnO) thin films onto the glass substrates at 450°C. The effects of Co doping on the structural, optical, compositional, morphological and photoluminescence properties of ZnO thin films were investigated. XRD analysis confirmed the polycrystalline nature of the films having hexagonal crystal structure as well as successful incorporation of Co2+ ions into the lattice position of Zn2+ ions in the ZnO host. The traces of Co incorporation were confirmed by the shift of peak position of (002) plane, change in lattice parameters, the shift of optical absorption edge towards higher wavelength and observations of three bands related to d–d transitions. The observed band gap energy of Co-ZnO films decreases from 3.22 to 2.76eV. The transmittance of the ZnO thin films decreases after Co doping. The X-ray photoemission spectra of doped films indicate the Co substitutes for Zn2+ and exist in +2 state. All the deposited pure and Co-ZnO thin films exhibit room temperature photoluminescence (PL).

Structural and magnetic properties of cobalt-doped ZnO thin films on sapphire (0 0 0 1) substrate deposited by pulsed laser deposition

Co-doped ZnO thin films on sapphire (0 0 0 1) substrates were deposited by PLD at various substrate temperatures in a one Pa oxygen ambient condition. The structural and magnetic properties of as-grown thin films were characterized by using XRD, UV–Visible absorption spectrophotometer, XPS and AGM. All samples possessed wurtzite hexagonal structure of ZnO. Co atoms incorporated into ZnO matrix and substituted for Zn in ZnO lattice, and Zn interstitials existed in as-grown thin films. The c-axis lattice constants of as-deposited thin films were larger than the standard data. Crystallization of as-prepared thin films increased and then decreased with substrate temperatures enhancing. All samples exhibited room-temperature ferromagnetism. The average moment per Co atom was much smaller than that of Co2+ (3d7), due to the different distribution of Co2+ ions and defects, as well as the different defect concentrations in thin films.

Ultrasonically sprayed ZnO:Co thin films: Growth and characterization

In this work, undoped and cobalt-doped ZnO thin films were deposited at 275±5°C on glass substrates by the ultrasonic spray pyrolysis technique. The structural, electrical, optical and surface characterization of the films as a function of the cobalt concentration in the spraying solution were studied by means of x-ray diffractometer, current–voltage characteristics, UV–vis spectrophotometer and atomic force microscope, respectively. X-ray diffraction reveals that the films are polycrystalline in nature with preferred orientations of (002) for the ZnO:Co (0, 2, 4at.%) and (100) for the ZnO:Co (6at.%). The optical transmittance of all films was studied as a function of wavelength in the range of 300–1100nm. They exhibit high transparency in the visible wavelength region with some interference fringes and sharp ultraviolet absorption edges. The optical band gap and Urbach energy values of the films were found in the range of 3.250–3.301eV and 90–230meV, respectively. The electrical studies for all films were carried out by using conductivity-temperature measurements and it was seen that the electrical conductivity of ZnO films decreases slightly depending on the increasing of Co doping. Also, Co doping increases both energies of donor-like traps and activation energy for ZnO films. The surface morphology was analyzed by atomic force microscope and a strong dependence on the cobalt incorporation was found.

Solution-based synthesis of cobalt-doped ZnO thin films

Undoped and cobalt-doped (1–4wt.%) ZnO polycrystalline, thin films have been fabricated on quartz substrates using sequential spin-casting and annealing of simple salt solutions. X-ray diffraction (XRD) reveals a wurzite ZnO crystalline structure with high-resolution transmission electron microscopy showing lattice planes of separation 0.26nm, characteristic of (002) planes. The Co appears to be tetrahedrally co-ordinated in the lattice on the Zn sites (XRD) and has a charge of +2 in a high-spin electronic state (X-ray photoelectron spectroscopy). Co-doping does not alter the wurzite structure and there is no evidence of the precipitation of cobalt oxide phases within the limits of detection of Raman and XRD analysis. Lattice defects and chemisorbed oxygen are probed using photoluminescence and Raman spectroscopy — crucially, however, this transparent semiconductor material retains a bandgap in the ultraviolet (3.30–3.48eV) and high transparency (throughout the visible spectral regime) across the doping range.

Effect of substrate temperature on the stability of transparent conducting cobalt doped ZnO thin films

Transparent conducting Co doped ZnO thin films have been fabricated by Ultrasonic spray. The thin films were deposited at three different substrate temperatures of 300, 350 and 400 °C. The obtained films had a hexagonal wurtzite structure with a strong (002) preferred orientation. The maximum crystallite size value of the film deposited at 350 °C is 55.46 run. Spectrophotometer (UV-vis) of a Co doped ZnO film deposited at 350 °C shows an average transmittance of about 90%. The band gap energy increased from 3.351 to 3.362 eV when the substrate temperature increased from 300 to 350 °C. The electrical conductivity of the films deposited at 300, 350 and 400 °C were 7.424, 7.547 and 6.743 (Ω·cm)−1 respectively. The maximum activation energy value of the films at 350 °C was 1.28 eV, indicating that the films exhibit a n-type semiconducting nature.