Nanocrystalline ZnO Thin Films Research Articles

Highly transparent and reproducible nanocrystalline ZnO and AZO thin films grown by room temperature pulsed-laser deposition on flexible Zeonor plastic substrates

Zeonor plastics are highly versatile due to exceptional optical and mechanical properties which make them the choice material in many novel applications. For potential use in flexible transparent optoelectronic applications, we have investigated Zeonor plastics as flexible substrates for the deposition of highly transparent ZnO and AZO thin films. Films were prepared by pulsed laser deposition at room temperature in oxygen ambient pressures of 75, 150 and 300 mTorr. The growth rate, surface morphology, hydrophobicity and the structural, optical and electrical properties of as-grown films with thicknesses ∼65–420 nm were recorded for the three oxygen pressures. The growth rates were found to be highly linear both as a function of film thickness and oxygen pressure, indicating high reproducibility. All the films were optically smooth, hydrophobic and nanostructured with lateral grain shapes of ∼150 nm wide. This was found compatible with the deposition of condensed nanoclusters, formed in the ablation plume, on a cold and amorphous substrate. Films were nanocrystalline (wurtzite structure), c-axis oriented, with average crystallite size ∼22 nm for ZnO and ∼16 nm for AZO. In-plane compressive stress values of 2–3 GPa for ZnO films and 0.5 GPa for AZO films were found. Films also displayed high transmission greater than 95% in some cases, in the 400–800 nm wavelength range. The low temperature photoluminescence spectra of all the ZnO and AZO films showed intense near band edge emission. A considerable spread from semi-insulating to n-type conductive was observed for the films, with resistivity ∼103 Ω cm and Hall mobility in 4–14 cm2 V−1 s−1 range, showing marked dependences on film thickness and oxygen pressure. Applications in the fields of microfluidic devices and flexible electronics for these ZnO and AZO films are suggested.

Effect of oxygen pressure on second harmonic generation in zinc oxide thin films deposited by PLD

Nanocrystalline ZnO thin films with c-axis preferred orientation were deposited on glass substrate at different oxygen pressures by a pulsed laser deposition (PLD) method. The microstructures and linear optical properties of the films are investigated by X-ray diffractometry and spectroscopic ellipsometry, respectively. Second harmonic generation in the films is studied using the rotational Maker fringe technique in the transmission scheme. All the ZnO films show second-order nonlinearity and the largest effective nonlinear coefficient d33, which increases with the grain size, is obtained in samples deposited at the oxygen pressure of 2×10−2Pa. The effects of the oxygen pressure on the microstructure, macrostress and second-order optical nonlinearity are analyzed.

Influence of pH on structural, morphological and optical properties of chemically deposited nanocrystalline ZnO thin films

In the present work, a simple and cost effective successive ionic layer adsorption and reaction method was adopted to grow highly oriented crystalline ZnO nanorods (NRs). The ZnO NRs were grown over glass substrates by varying the pH. The surface morphology studies show that the dimension and orientation of ZnO NRs are influenced by varying the pH conditions. The SEM analysis reveals that the ZnO NRs grow vertically with perfect wurtzite hexagonal shape with a diameter range from 300 nm to 1 μm at optimized pH concentration. The XRD patterns of the ZnO NRs exhibit high crystalline orientation of ZnO wurtzite structure with a preferential (101) plane orientation. Optical spectra recorded using UV–Vis spectrophotometer on ZnO rods show that the optical band gap decreases with increase in pH of the solution. This simple and integrated approach is expected to lead to a cost effective and convenient way towards large scale growth of ZnO rods with subsequent interest in nano-based biosensor applications in future.

The influence of external magnetic field on the structural and optical properties of nanocrystalline ZnO thin films prepared by dip coating method

ZnO thin films were prepared by the sol gel dip coating method. Some samples were prepared under a DC magnetic field DC-MF (B); others were done without it. The sol gel was also continuously stirred. The field orientation in each case of deposition was chosen perpendicular as well as parallel to the growing ZnO-film substrate and the film depositions were carried out at a fixed B intensity value. The influences of magnetic field on the structural, morphological, and optical properties of ZnO thin films were investigated. In the perpendicular case, a cubic structure phase was observed at ambient conditions. It is noteworthy that this phase growth is only possible at high pressure deposition. In the parallel case, the films improved the crystal structure without creating a new phase. The structural properties of the ZnO films such as surface morphology and crystallinity were determined using scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively. The optical properties of the ZnO films were characterized by the ultraviolet–visible (UV–Vis) spectroscopy.

Optical Properties of Annealed ZnO Thin Films Fabricated by Pulsed Laser Deposition

The annealing effects on the structural and optical properties of grown ZnO thin films fabricated by pulsed laser deposition were investigated by using different techniques. The XRD reveals the presence of hexagonal wurtzite structure of ZnO with preferred orientation (002). The average grain size (D) of annealed ZnO nanocrystalline thin films is found to be in the range 7.77–15.91 nm. SEM of the thin film consisted of many grains distributed uniformly throughout the surface. TEM shows a relatively smooth surface with an average nanocrystalline size of 13 nm in close agreement with the XRD result. The transmittance spectra demonstrate the highly transparent nature of the films in the visible region (>70 %). The calculation of optical band gap energy (Eg) is found to be in the range 2.95-3.23 eV. The PL spectra show that the amorphous film gives a UV emission only and the annealed nanocrystalline films produce UV, violet, blue and green emissions which indicates that the point defects increased as the amorphous film was annealed. It is concluded that the structural, optical and emission properties of ZnO films grown on glass substrates by PLD strongly depend on the annealing times. The results demonstrate that these annealed films are promising materials for optical component applications.

Photo-induced magnetoresistance of nanocrystalline ZnO thin film

Nanocrystalline zinc oxide thin film has been prepared by conventional spray method. The optical energy gap is about 3.39 eV, which is greater than the energy gap of bulk ZnO. The prepared thin film has semiconductor behavior in dark measurements, while it has metallic behavior under ultraviolet illumination. There is a negative magnetoresistance effect for the prepared film under ultraviolet radiation. Furthermore, there is a magnetic transition from diamagnetic to paramagnetic state under ultraviolet radiation. The results has been explained in terms of the band conduction theory and spin polaron model.

The Effect of Mn Incorporation on the Structural, Morphological, Optical, and Electrical Features of Nanocrystalline ZnO Thin Films Prepared by Chemical Spray Pyrolysis Technique

Un-doped and Mn-doped ZnO nanocrystalline thin films and n-ZnO/n-Si heterojunction have been prepared by chemical spray pyrolysis technique. The microstructure, morphology, optical, and electrical properties have been studied. The X-ray analyses have revealed that all films are in single phase and have wurtzite structure. Besides, it has been indicated that there are not any secondary phases. The optical properties have been evaluated by UV–Vis measurement. It has shown that band gap decreases with Mn incorporation from 3.29 to 3.19 eV. Schottky diode applications of the films have been performed by evaporation of Au on pure and Mn-doped ZnO films. Current–voltage (I–V) and capacitance–voltage (C–V) measurements of the n-ZnO/ n-Si heterojunction indicate good diode characteristic and the barrier heights have been calculated as 0.89 and 0.79 eV for un-doped and Mn 1 pct-doped ZnO films. Besides, schematic cross section of the Au/n-ZnO/n-Si/Al device and energy band diagram of n-ZnO/n-Si heterojunction has been illustrated to clarify the transport mechanism. All results suggest that the characteristic properties of the ZnO thin films can be adjustable with the Mn doping and Al/n-Si/n-ZnO/Au diode can be used for UV detection application in photonic devices.

Optical and Sensing Properties of Cu Doped ZnO Nanocrystalline Thin Films

Undoped and Cu doped ZnO films of two different molarities deposited by spray pyrolysis using zinc nitrate and cupric chloride as precursors show polycrystalline nature and hexagonal wurtzite structure of ZnO. The crystallite size varies between 10 and 21 nm. Doping increases the transmittance of the films whereas the optical band gap of ZnO is reduced from 3.28 to 3.18 eV. With increment in doping the surface morphology changes from irregular shaped grains to netted structure with holes and then to net making needle-like structures which lends gas sensing characteristics to the films. Undoped ZnO shows maximum sensitivity at 400°C for higher concentration of CO2. The sensitivity of Cu doped sample is maximum at 200°C for all CO2concentrations from 500 to 4000 ppm.

Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

Enhancement of gas sensing performance of indium doped ZnO thin film at optimal indium concentration as compared to its undoped counterpart.

Double dye cubic-sensitized solar cell based on Förster resonant energy transfer

To extend the spectral response range of dye-sensitized solar cells through Förster resonant energy transfer, eosin Y and rhodamine B were chosen as an donor and a acceptor to cubic-sensitize nanocrystalline ZnO thin film.

Gas Sensing Studies on Nanocrystalline ZnO Thin Films Prepared by Dip Coating

Gas Sensing Studies on Nanocrystalline ZnO Thin Films Prepared by Dip Coating

Synthesis of sol–gel derived nano-crystalline ZnO thin films as TCO window layer: effect of sol aging and boron

The present paper reports synthesized nano-sized ZnO thin films for use as front window electrodes in PV applications, and their behavior after different sol aging times (1 h, 24 h, 3 days and 21 days) for undoped and 1% B amount doping. The highly nanocrystalline ZnO thin films were synthesized by the sol–gel method and B was used as a guest material to examine its role in structural, optical, electrical and morphological properties in this process. X-ray diffraction studies revealed that all the prepared films exhibited the ZnO hexagonal wurtzite structure with preferential orientation along the (0 0 2) crystal plane. Analyses of X-ray diffraction patterns demonstrated that the sol aging can improve the degree of preferred crystal orientation along the c-axis, and it did not change the crystal growth orientation. Average crystallite sizes of the undoped and B doped ZnO films were increased from 16 to 31 nm and from 19 nm to 32 nm with increasing sol aging from 1 h to 21 days. Moreover, AFM results showed that the RMS values increased from 5 nm to 12 nm as an indication of a good film formation on the surface. The minimum value of resistivity was found to be 7.24 × 10−3 Ω m with B and a high optical transmittance of ∼98% in the visible region was observed. The films produced at optimum conditions are suitable for optical and electrical applications owing to their low resistivity, high optical transmittance, optimum band gap, good crystallization and surface formation.

Dye-sensitized solar cell using sprayed ZnO nanocrystalline thin films on ITO as photoanode

ZnO thin films had been successfully prepared by spray pyrolysis (SP) technique on ITO/Glass substrates at different substrate temperature in the range 250–400°C using Zinc acetylacetonate as precursor. The X-ray diffraction studies confirmed the hexagonal wurtzite structure with preferred orientation along (002) plane at substrate temperature 350°C and the crystallite size was found to vary from 18 to 47nm. The morphology of the films revealed the porous nature with the roughness value of 8–13nm. The transmittance value was found to vary from 60% to 85% in the visible region depending upon the substrate temperature and the band gap value for the film deposited at 350°C was 3.2eV. The obtained results revealed that the structures and properties of the films were greatly affected by substrate temperature. The near band edge emission observed at 398nm in PL spectra showed better crystallinity. The measured electrical resistivity for ZnO film was ∼3.5×10−4Ωcm at the optimized temperature 350°C and was of n-type semiconductor. The obtained porous nature with increased surface roughness of the film and good light absorbing nature of the dye paved way for implementation of quality ZnO in DSSCs fabrication. DSSC were assembled using the prepared ZnO film on ITO coated glass substrate as photoanode and its photocurrent – voltage performance was investigated.

Nanocrystalline ZnO thin film deposition on flexible substrate by low-temperature sputtering process for plastic displays.

A low temperature sputter deposition process is adopted to fabricate nanocrystalline ZnO thin films on plastic (polyethylene terepthalate) substrate. Very good crystalline films are synthesized at a substrate temperature around 120 degrees C. Structural and microstructural analyses confirm the proper phase formation of the nanomaterial with an average nanoparticle size around 5-10 nm. Optical transmission analysis of the film deposited on plastic substrate depicts nearly 90% visible transmittance with a direct bandgap around 3.56 eV. This cost-effective, low-temperature fabrication of nanocrystalline thin film with very good structural and optical properties will find important applications in plastic display technology. Also the process is a vacuum-based clean process, which is compatible to CMOS-IC fabrication techniques and therefore, can easily be integrated with modern solid state device fabrication processes for diverse device applications.

Effects of Buffer-layer Thickness and Active-layer Growth Temperature on ZnO Nanocrystalline Thin Films Grown by Molecular Beam Epitaxy

Effects of Buffer-layer Thickness and Active-layer Growth Temperature on ZnO Nanocrystalline Thin Films Grown by Molecular Beam Epitaxy

Effect of γ-irradiation on Structural and Optical Ellipsometry Parameters of ZnO Nanocrystalline Thin Films

Effect of γ-irradiation on Structural and Optical Ellipsometry Parameters of ZnO Nanocrystalline Thin Films

Effect of different sol concentrations on the properties of nanocrystalline ZnO thin films grown on FTO substrates by sol-gel spin-coating

Nanocrystalline ZnO thin films grown on fluorine-doped tinoxide (FTO) substrates were fabricated using the spin-coating method. The structural and the optical properties of the ZnO thin films prepared using different sol concentrations were investigated by using field-emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD), photoluminescence (PL) measurements, and ultraviolet-visible (UV-vis) spectrometry. The surface morphology of the ZnO thin films, as observed in the SEM images, exhibited a mountain-chain structure. XRD results indicated that the thin films were preferentially orientated along the direction of the c-axis and that the grain size of the ZnO thin films increased with increasing sol concentration. The PL spectra showed a strong ultraviolet emission peak at 3.22 eV and a broad orange emission peak at 2.0 eV. The intensities of deep-level emission (DLE) gradually increased with increasing sol concentration from 0.4 to 1.0 M. The transmittance spectra of the ZnO thin films showed that the ZnO thin films were transparent (~85%) in the visible region and exhibited sharp absorption edges at 375 nm. Thus, The Urbach energy of ZnO thin films decreased with increasing sol concentration.

Effect of Co doping on structural, morphological and LPG sensing properties of nanocrystalline ZnO thin films

Nanocrystalline cobalt doped zinc oxide (CZO) thin films were deposited on to the corning glass substrates by spray pyrolysis technique using zinc acetate and cobaltous nitrate as precursors. Structural, morphological, photoluminescence and gas sensing properties of the films with various Co doping concentrations were investigated. XRD patterns confirm that, films are polycrystalline with hexagonal (wurtzite) crystal structure. XPS reveals films are sub-stoichiometric in nature with Co present in two chemical states. SEM images show the films are compact, densely packed with hexagonal flakes and spherical grains on the surface. The direct band-gap energy increases from 3.27 to 3.33eV with cobalt doping concentration. LPG response of the film increased with Co concentration up to 2at% and decreased thereafter. Pd sensitized 2at% CZO thin film shows 47% gas response as compared with 33% for un-sensitized CZO thin film. Film shows stable gas response even for 20min in presence of LPG.

Microstructural and Electrical Conductivity of Annealed ZnO Thin Films

Undoped nanocrystalline ZnO thin films were deposited onto the glass substrates via the low cost sol-gel dip coating method. The as-grown ZnO films were annealed at the temperatures ranging from 400 °C to 550 °C. The X-ray diffraction (XRD) pattern revealed that the annealed ZnO films were polycrystalline with hexagonal wurtzite structure and majority preferentially grow along (002) c-axis orientation. Atomic force microscopy (AFM) micrographs showed the improvement of RMS roughness and grain size as annealing temperature increased. The ZnO films that annealed at 500 oC exhibited the lowest resistivity value.

Enhancement of gas sensor response of nanocrystalline zinc oxide for ammonia by plasma treatment

Abstract The effect of oxygen plasma treatment on nanocrystalline ZnO thin film based gas sensor was investigated. ZnO thin films were synthesized on alkali-free glass substrates by a sol–gel process. ZnO thin films were treated with oxygen plasma to change the number of vacancies/defects in ZnO. The effect of oxygen plasma on the structural, electrical, optical and gas sensing properties was investigated as a function of plasma treatment time. The results suggest that the microstructure and the surface morphology can be tuned by oxygen plasma treatment. The optical transmission in the visible range varies after the oxygen plasma treatment. Moreover, it is found that the oxygen plasma has significant impact on the electrical properties of ZnO thin films indicating a variation of resistivity. The oxygen plasma treated ZnO thin film exhibits an enhanced sensing response towards NH 3 in comparison with that of the as-deposited ZnO sensor. When compared with the as-deposited ZnO film, the sensing response was improved by 50% for the optimum oxygen plasma treatment time of 8 min. The selectivity of 8 min plasma treated ZnO sensor was also examined for an important industrial gas mixture of H 2 , CH 4 and NH 3 .