Pristine ZnO Thin Films Research Articles

(002) oriented ZnO and ZnO:S thin films by direct ultrasonic spray pyrolysis: A comparative analysis of structure, morphology and physical properties

In this study, we deposit sulfur incorporated ZnO thin films (ZnO:S) by ultrasonic deposition and analyze changes in the structure, morphology, optical and electrical properties of the films in comparison with the pristine ZnO thin films. Further, electronic band structures of ZnO and (ZnO:S) were theoretically calculated using DFT-VASP method. ZnO and (ZnO:S) thin films formed at different conditions showed hexagonal crystal structure with crystallites of 30–40 nm in size fully oriented along [002] direction perpendicular to substrate. The optical band gap of ZnO was 3.2 eV which was reduced to 2.8 eV due to sulfur incorporation. Vacuum annealed ZnO showed a very low sheet resistance of 67 Ω/□ and the value was increased to 470 Ω/□ in ZnO:S films, nearly metallic behavior. Hall effect measurements confirmed the n-type conductivity with a maximum carrier concentration of 5.6 × 1020 cm−3. These films can find potential applications as window layers with improved charge extraction contacts for solar cell applications.

Growth, properties and photocatalytic degradation of congo red using Gd:ZnO thin films under visible light

Pristine ZnO thin films, as well as 1, 2, 3 and 4 at.% Gd:ZnO thin films, were synthesized via a spray pyrolysis approach on glass substrates and different analysis methods were used to explore the effect of Gd ion doping on the structural, morphological, chemical, and optical characteristics. The presence of functional groups and chemical bonds ascribed to ZnO were confirmed by Fourier transform infrared spectroscopy (FTIR) measurement. The presence of Zn, O, and Gd in the formed pristine and Gd:ZnO thin films was confirmed using EDX analysis and elemental mapping investigations, which was further validated using X-ray photoelectron spectroscopy (XPS) analysis that indicated the existence of Gd in the layers with Gd in the + 3 state. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis revealed that the films are polycrystalline with a hexagonal wurtzite structure, and surface morphology changed with increasing Gd content. The ultraviolet–visible (UV–Vis) spectrophotometer for all films shows good visible transmittance averaging 60–80% and their band gap energy (Eg) ranging between 3.24 and 3.26 eV. The photocatalytic proprieties of the films were carried out by assessing the degradation efficiency of the dye Congo red (CR) under visible light. The results show that Gd:ZnO films with different Gd contents have better catalytic degradation efficiencies than pristine ZnO.

Increased visible light absorption and charge separation in 2D–3D In2S3-ZnO heterojunctions for enhanced photoelectrochemical water splitting

A 2D–3D (ZnO/In2S3 heterojunction) photoanodes have been grown via a two-step physical vapor deposition (PVD) technique for ZnO, followed by chemical vapor deposition (CVD) for In2S3 2D nanolayers to study the photoelectrochemical (PEC) properties in the present study. 2D nanolayers of In2S3 deposited on ZnO thin film provide increased surface area and efficient electron-hole separation at the heterojunction interface which results in a significant reinforcement of photocurrent density and incident photon-to-current conversion efficiency (IPCE). The ZnO/In2S3 heterojunction shows a photocurrent density of 2.4 mAcm−2 at 1.23 V vs. RHE and IPCE value of 43% at 0.5 V with the stability of more than 100 min in comparison to both pristine ZnO and In2S3 samples. Measurements of photocurrent density at different wavelengths for the heterojunction sample confirm that the enhancement in photoelectrical response at the interface is due to higher absorption in the visible range as compared to pristine ZnO thin films and increased carrier separation at the interface in comparison to the pristine 2D In2S3 nanolayers. The experimental determination of the work function (KPFM), valence band offset (XPS), and bandgap values (absorption spectra) confirm favorable type II band alignment resulting in efficient charge transfer within the heterojunction sample. The present approach can be extended to other 2D heterojunction systems having unique optical and electronic properties.

Tuning the optical and electrical properties of magnetron-sputtered Cu–ZnO thin films using low energy Ar ion irradiation

Cu doped ZnO (Cu–ZnO) nanocomposite thin films were successfully synthesized at room temperature by RF magnetron sputtering. The signature of Cu in pristine ZnO thin films was confirmed by Rutherford Backscattering Spectroscopy (RBS) with atomic concentration of ~10%. The Cu–ZnO thin films were irradiated with 800 keV Ar ion beam at three different ion fluences viz. 5 × 1015, 7 × 1015 and 9 × 1015 ions/cm2. As characterized by X-ray diffraction, the pristine and irradiated films show a hexagonal wurtzite structure with remarkable intensities of (100), (002) and (101) orientations. The surface morphology of the pristine and irradiated films was recorded using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The images clearly show the grain growth near the surface of films with increasing the ion fluences. The surface plasmon absorption (380 nm onwards) indicating the formation of metal nanoparticles in oxide matrices was confirmed by UV–visible spectroscopy of ion irradiated films. The electrical properties of the pristine and irradiated films were deduced by current voltage (I–V) and Hall measurements. The conductivity of the film increases as irradiation proceeds, achieves a maxima and then decreases with further processing of the ion beam. This study explores ion beam irradiation as a versatile tool to tailor the physical properties of nanocomposite thin films. Further, the properties can be precisely controlled by optimizing ion fluences and incident ion energy as well.

Incorporation of Au nanoparticles into thermoelectric mesoporous ZnO using a reverse triblock copolymer to enhance electrical conductivity

Since the thermoelectric properties are proportional to the electrical conductivity and inversely proportional to the thermal conductivity, independent control of thermal conductivity and electrical conductivity is essential. Therefore, in this study, mesoporous structure was applied to decrease thermal conductivity. And, Au NPs were incorporated to increase the electrical conductivity. Reverse triblock copolymer and cosolvent system were introduced for uniform dispersion of Au NPs. The collapse of the pore structure was minimized due to the uniform dispersion of Au NPs and the porosity increased by 10% from 20% to 30% when compared with pristine ZnO thin films. A mesoporous ZnO composite thin film containing 1 at.% Au NPs exhibited an electrical conductivity that was five times greater than that of a pristine mesoporous ZnO thin film. In addition, the power factor increased by about 3.5 times, from 9.53 ± 1.50 μW/mK2 to 32.72 ± 5.16 μW/mK2 at 503 K.

Effect of mesopore-induced strain/stress on the thermoelectric properties of mesoporous ZnO thin films

In present study, induced strain and stress effect of mesoporous structure on the thermoelectric properties of ZnO thin films were systematically investigated. In this work, mesoporous ZnO thin films were synthesized by evaporation-induced self-assembly and sol-gel process. As the pore structure is formed, the grain growth of ZnO is inhibited and lattice distortion is induced. In this paper, strain/stress induction according to surfactant (Brij-S10) concentration was analyzed through Williamson-Hall analysis. And the relationship between strain induction and thermoelectric properties was studied. 0.07 M ratio of Brij-S10 to Zn induces the enhanced thermoelectric properties as compared with pristine ZnO thin films. Hence, the induced strain and stress could play an important role in enhancing the thermoelectric properties of mesoporous ZnO thin films.

Flexible Thermoelectric ZnO–Organic Superlattices on Cotton Textile Substrates by ALD/MLD

The thermoelectric properties of both pristine ZnO and ZnO–organic superlattice thin films deposited on a cotton textile are investigated. The thin films are fabricated by atomic layer deposition/molecular layer deposition, using hydroquinone as the organic precursor for the superlattices. The resulting thin‐film coatings are crystalline, in particular when deposited on a textile substrate with a thin predeposited Al2O3 seed layer. The thermoelectric properties of the ZnO and ZnO–organic superlattice coatings are comparable to those for thin films deposited on conventional inorganic substrates; Al doping can be employed to further improve the thermoelectric properties. The ZnO–organic superlattice thin films moreover show enhanced resistance to mechanical strain. Due to the higher flexibility and lower thermal conductivity in comparison to pristine ZnO thin films, the ZnO–organic superlattice thin films are a possible material platform for flexible thermoelectrics that can be integrated in textiles and applied in wearable electronics.

Effect of excimer laser annealing on the properties of ZnO thin film prepared by sol-gel method

Pristine ZnO thin films have been deposited with zinc acetate [Zn(CH 3COO) 2], mono-ethanolamine (stabilizer), and isopropanol solutions by sol-gel method. After deposition, pristine ZnO thin films have been irradiated by excimer laser ( λ = 248, KrF) source with energy density of 50 mJ/cm 2 for 30 sec. The effect of excimer laser annealing on the optical and structural properties of ZnO thin films are investigated by photoluminescence and field emission scanning electron microscope. As-grown ZnO thin films show a huge peak of visible region and a wide full width at half maximum (FWHM) of UV region due to low quality with amorphous ZnO thin films. After KrF excimer laser annealing, ZnO thin films show intense near-band-edge (NBE) emission and weak deep-level emission. The optically improved pristine ZnO thin films have demonstrated that excimer laser annealing is novel treatment process at room temperature.

Aluminum-Doped Zinc Oxide as Transparent Electrode Materials

Pristine and Al-doped zinc oxide nanopowders were synthesized via a surfactant-assisted complex sol-gel method, possessing a pure ZnO phase structure and controllable grain size which was characterized by X-ray diffraction and scanning electron microscopy. Using these nanopowders, the pristine and Al-doped ZnO magnetron sputtering targets were prepared following a mold-press, cold isostatical-press and schedule sintering temperature procedure. The relative density of these as-prepared targets was tested by Archimedes’ method on densitometer. All of the results were above 95 theory density percents, and the resistivity was tested on four-probe system at a magnitude of 10-2Ω cm. Related pristine ZnO thin films and Al-doped ZnO thin films were fabricated by magnetron sputtering method, respectively. The pristine and Al-doped ZnO films deposited on the quartz glass by dc sputtering owned a (002) orientation with a thickness of 350 nm at a deposition power of 100 W for two hours under an argon plasma. A good optical transparency above 80% and low resistivity of 1.60×10-3Ω cm were obtained with a deposition temperature of 573 K. The optical energy bandgap could be tailored by Al doping at 4 at.% Al.

Ferromagnetism in laser ablated ZnO and Mn-doped ZnO thin films: A comparative study from magnetization and Hall effect measurements

Room temperature FM was observed in pristine ZnO thin films grown by pulsed laser deposition on Al2O3 substrates. It seems to originate from other defects but not oxygen vacancies. Magnetization of thinner films is much larger than that of the thicker films, indicating that defects are mostly located at the surface and/or the interface between the film and the substrate. Data on the Fe:ZnO and Mn:ZnO films show that a transition-metal doping does not play any essential role in introducing the magnetism into ZnO. In the case of Mn doping, the magnetic moment could be very slightly enhanced. Hall effect measurements reveal that an incorporation of Mn does not change the carrier type, but decreases the carrier concentration, and increases the Hall mobility, resulting in more resistive Mn:ZnO films. Since no anomalous Hall effect was observed, it is understood that the observed FM is not due to the interaction between the free-carrier and the Mn impurity.