ZnO Nanofilms Research Articles

Study on the growth and optical properties of ZnO thin films fabricated by ultrasonic spray pyrolysis

In this paper, ZnO nanofilms are prepared on quartz substrate by ultrasonic spray pyrolysis (USP) technology. The effects of substrate temperature and deposition time on the micro-structural, morphological and optical properties of ZnO thin films are investigated in detail. Measurements indicate that the growth process of the film is as follows: Micro grains accumulation [Formula: see text] grains’ expansion [Formula: see text] grow into certain nanostructures under different temperature. Nanostructures of the film are as follows: Irregular broken hexagonal-like structure ([Formula: see text]C); coexisting of broken hexagonal- and sheet-like structure (450–500[Formula: see text]C); regular closed packed sheets-like structure ([Formula: see text]C). In addition, all results show that RMS roughness of the films is less than 20 nm. Moreover, the visible transmittance of these samples is higher than 90%. Which display that ZnO films with low roughness and excellent optical properties can be also fabricated by low-cost USP method.

The effect of strontium doping on structural and morphological properties of ZnO nanofilms synthesized by ultrasonic spray pyrolysis method

Abstract Pristine and strontium doped ZnO nanometric films were successfully synthesized on heated glass substrates by the ultrasonic spray pyrolysis technique. The samples were characterized by means of X-ray diffraction (XRD), Atomic Force Microscope (AFM), UV–visible spectroscopy and photoluminescence (PL). X-ray diffraction patterns confirmed the hexagonal (wurtzite) structure, where the most pronounced (002) peak indicates the preferential orientation along the c-axis perpendicular to the sample surface. The intensity of this peak was increased rapidly from the first doping of 1% and its position was shifted toward higher angles under Sr-doping effect. For the used doping range of 1–5%, the Sr-doping at 3% attracted an especial attention. At this concentration, the particular transformation in the surface morphology of doped ZnO films was observed. The surface became granular and rough by expanding the crystallites' size. From optical measurements, transmittance and PL spectra were found to be sensitive to Sr-doping, where two different behaviors were observed before and after 3% of Sr-doping.

Tailoring nucleation process to gain highly oriented ZnO thin film with excellent UV luminescent and electrical performances

We present a novel approach for tuning orientation in ALD deposited ZnO nanofilm by inserting interfacial buffer layer (IBL) at interface between film and substrate, which is found to induce strong orientation and significant improvement in crystalline quality by largely decreasing grain boundary (GB) area through tailoring nucleation process. As a result, the UV emission in ZnO film with IBL is enhanced by over an order of magnitude due to the reduction of surface quenching at GB. Meanwhile, the decrease of electron scattering at GB also helps improve the electrical mobility of the film, leading to great reduction in resistivity.

ZnS coating for enhanced environmental stability and improved properties of ZnO thin films.

Low environmental stability of ZnO nanostructures in hydrophilic systems is a crucial factor limiting their practical applications. ZnO nanomaterials need surface passivation with different water-insoluble compounds. This study describes a one-step passivation process of polycrystalline ZnO films with ZnS as a facile method of ZnO surface coating. A simple sulfidation reaction was carried out in gas-phase H2S and it resulted in formation of a ZnS thin layer on the ZnO surface. The ZnS layer not only inhibited the ZnO dissolving process in water but additionally improved its mechanical and electrical properties. After the passivation process, ZnO/ZnS films remained stable in water for over seven days. The electrical conductivity of the ZnO films increased about 500-fold as a result of surface defect passivation and the removal of oxygen molecules which can trap free carriers. The nanohardness and Young's modulus of the samples increased about 64% and 14%, respectively after the ZnS coating formation. Nanowear tests performed using nanoindentation methods revealed reduced values of surface displacements for the ZnO/ZnS system. Moreover, both ZnO and ZnO/ZnS films showed antimicrobial properties against Escherichia coli.

Production of acceptor complexes in sol-gel ZnO thin films by Sb doping

Sb doped ZnO nanofilms were successfully deposited on glass substrates via economical sol-gel method. X-ray diffraction (XRD) patterns revealed the hexagonal wurtzite structure of the elaborated films and showed that the crystal growth of the doped samples is (002) preferentially oriented. X-ray photoelectron analysis suggested the successful substitution of antimony in Zn sites. According to XPS analysis, the presence of an important antimony ions states in fivefold coordination suggested the formation of the complex (SbZn−2VZn) level. It was found that low Sb doping (less than 1%) leads to slight broadening of the band gap and blue shift of the near band edge (NBE) emission of ZnO. It indicates the reduction of the intrinsic defects density for this doping regime. However, for Sb doping more than 1%, the optical results revealed the creation of an important defects density such as stacking faults and zinc vacancies (VZn) that lead to the formation of acceptors levels. From photoluminescence (PL) at low temperature, the presence of acceptor-bound exciton A0X (152meV) has been demonstrated. Stacking faults in ZnO seems to contribute particularly to the PL signal through the 3.316eV emission attributed to a free electron-to-neutral acceptor (e, A°) transition with binding energy of about 137meV. The p-type conductivity of ZnO thin films is confirmed by Hall measurement.

Size-Controlled Graphene Nanodot Arrays/ZnO Hybrids for High-Performance UV Photodetectors.

Graphene nanodots (GNDs) are one of the most attractive graphene nanostructures due to their tunable optoelectronic properties. Fabricated by polystyrene‐nanosphere lithography, uniformly sized graphene nanodots array (GNDA) is constructed as an ultraviolet photodetector (PD) with ZnO nanofilm spin coated on it. The size of GNDA can be well controlled from 45 to 20 nm varying the etching time. It is revealed in the study that the photoelectric properties of ZnO/GNDA PD are highly GNDA size‐dependent. The highest responsivity (R) and external quantum efficiency of ZnO/GNDA (20 nm) PD are 22.55 mA W−1 and 9.32%, almost twofold of that of ZnO PD. Both ZnO/GNDA (20 nm) PD and ZnO/GNDA (30 nm) PD exhibit much faster response speed under on/off switching light and have shorter rise/decay time compared with ZnO PD. However, as the size of GNDA increase to 45 nm, the PD appears poor performance. The size‐dependent phenomenon can be explained by the energy band alignments in ZnO/GNDA hybrids. These efforts reveal the enhancement of GNDs on traditional photodetectors with tunable optoelectronic properties and hold great potential to pave a new way to explore the various remarkable photodetection performances by controlling the size of the nanostructures.

Structure, morphology and photoluminescence attributes of Al/Ga co-doped ZnO nanofilms: Role of annealing time

The influence of annealing time on the structure, morphology and photoluminescence behavior (Al)/(Ga) co-doped ZnO (AGZO). nanofilms are grown on the p-type Si(100) substrate via combined sol-gel, spin coating annealed in air at 500°C at 0–3h. Samples are characterized using XRD, TEM, AFM, FESEM, EDX, (PL) and Raman measurements. XRD pattern confirmed the growth of highly poly-crystalline hexagonal wurtzite structure of ZnO with preferred orientation along (101) direction. At (3h) is found to cause lattice contraction and strain relaxation. TEM images revealed the nucleation of nanoparticles (NPs) and SAED pattern identified the lattice parameter. Raman spectra of AGZO exhibited optical and acoustic modes. FESEM displayed an increase in the particles size and number of nanoflakes with increasing annealing time. EDX detected right elemental traces. PL revealed an intense emission peak centered at 3.23eV, which is continuously shifted toward lower frequency with increasing time.

Photocatalytic hollow TiO2 and ZnO nanospheres prepared by atomic layer deposition

Carbon nanospheres (CNSs) were prepared by hydrothermal synthesis, and coated with TiO2 and ZnO nanofilms by atomic layer deposition. Subsequently, through burning out the carbon core templates hollow metal oxide nanospheres were obtained. The substrates, the carbon-metal oxide composites and the hollow nanospheres were characterized with TG/DTA-MS, FTIR, Raman, XRD, SEM-EDX, TEM-SAED and their photocatalytic activity was also investigated. The results indicate that CNSs are not beneficial for photocatalysis, but the crystalline hollow metal oxide nanospheres have considerable photocatalytic activity.

Honeycomb type ZnO nanostructures for sensitive and selective CO detection

Excellent sensing performance for CO gas is demonstrated using inexpensive sensor devices based on honeycomb type ZnO nanostructures, fabricated by colloidal lithography and lift-off process. This newly proposed method for gas sensors is cost effective and provides significant enhancement of both sensitivity and selectivity of CO detection. Honeycomb type ZnO nano films developed in ∼21nm ZnO layer consisting of 1μm period hexagonal lattice of air-holes with diameter varying from ∼600–900nm are investigated for CO sensing. These structures are fabricated by a combination of self-assembly of polystyrene (PS) spheres, their size reduction by oxygen plasma and magnetron-sputtering of ZnO followed by PS mask removal. The hole diameter and hence the width of ZnO honeycomb walls are determined by size reduction of PS spheres. Fabricated ZnO honeycomb type sensors show superior sensing performance compared to planar ZnO films, and response as high as ∼81.2% at 300°C for a 3ppm CO with a detection resolution of 500ppb and response and recovery times of ∼180 and ∼210s, respectively, were obtained. The repeatability of the observed results is confirmed and in addition, the CO selectivity is shown for gas mixtures consisting of CH4, H2S, CO2, NO2, SO2 and H2. The developed nanostructuring method is generic and can be adapted for improving performance of other metal-oxide based gas sensors.

Electromechanical scale-bridging model for piezoelectric nanostructures

In past experimental studies, piezoelectric nanostructures have exhibited extraordinary behavior and unusual deformations. In order to establish the corresponding theoretical framework, a scale-bridging model, which takes into account surface piezoelectricity and the wave nature of electrons in ultra-narrow media by reflecting lattice distortions of atomic structures, has been proposed in this work. After applying this model to ZnO nanofilms with thicknesses ranging between 0.3 nm and 2.8 nm, asymmetric lattice distortions of the Zn- and O-terminated surfaces were observed and subsequently quantified using a lattice distortion factor. The material characteristics evaluated by using this model were found to be in good agreement with the results of first-principles calculations and corresponding experiments, and thus can be used for predicting the properties of thicker nanofilms. After bridging to the continuum scale, the data produced via finite element analysis significantly differed from the results obtained by the conventional model, owing to the unusual deformations caused by the nano-sized material properties and asymmetric surface characteristics. The obtained results help to achieve a better understanding of the properties of piezoelectric nanostructures with extraordinary characteristics and, therefore, can be used for the nanostructural design.

Investigation of some physical properties of ZnO nanofilms synthesized by micro-droplet technique

In this paper, ZnO nanocrystals were synthesized using a simple micro-droplets technique from a solution prepared by dissolving zinc acetate di-hydrate [Zn(CH3COO)2, 2H2O] in methanol. Microdroplets were deposited on glass substrates heated at 100°C, the obtained samples of ZnO films were investigated by XRD, AES, AFM, ellipsometry and PL. XRD patterns reveal the wurtzite structure of ZnO where the lattice parameters a and c, calculated from XRD signals, show a nanometric character of ZnO nanoparticles. The chemical composition of ZnO film surfaces was verified by Auger electron spectroscopy (AES). From Auger signals, oxygen (O-KLL) and zinc (Zn-LMM) Auger transitions indicate well the presence of Zn-O bonding. The surface topography of the samples was measured by atomic force microscopy (AFM) where ZnO nanoparticles of average size ranging between 20 and 80nm were determined. Some optical properties as dielectric constants, refractive index, extinction coefficient as well as the optical band gap were determined from ellipsometry analysis. The dispersion of the refractive index was discussed in terms of both Cauchy parameters and Wemple & Di-Dominico single oscillator model. The photoluminescence (PL) measurements exhibited two emission peaks. The first at 338nm, corresponding to the band gap of ZnO, is due to excitonic emission while the second around 400nm, is attributed to the single ionized oxygen vacancies.

Morphologies, field-emission and ultrafast nonlinear optical behavior of pure and Ag-doped ZnO nanostructures

Pure and Ag-doped ZnO nanofilms with four morphologies, namely nanocone, nanocrystal, nanochips and nanoleaves, have been successfully synthesized by magnetron sputtering method. The composition of the Ag-doped ZnO (AZO) nanostructures was explored by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The XRD patterns revealed that the samples exhibited the hexagonal wurtzite structure with preferential orientation along the (002) crystal plane, although the c-axis as preferred orientation decreased with Ag doping. Moreover, we report the photoluminescence, field-emission (FE) and ultrafast nonlinear optical behavior of AZO films. A blue-shift was demonstrated in the UV region of photoluminescence results for the AZO nanostructures. The resulting FE showed that the superior FE properties were attributed to the better morphologies. The ultrafast time resolved optical Kerr gate experiment results showed that AZO was a good candidate for an optical Kerr gate medium due to its ultrafast response and large third-order nonlinear susceptibility. The result of time-resolved photoluminescence showed that the first decay time was about several picoseconds for films, and the slow relaxation process had the lifetime longer than hundred picoseconds. By analyzing the experimental results of the time-resolved photoluminescence experiment, we found the fast decay process could be attributed to the exciton migration and the slow decay to the recombination process of the relaxed excitons.

Hydroelectric generator from transparent flexible zinc oxide nanofilms

Harvesting wave energy based on waving potential, a newly found electrokinetic effect, is attractive but limited mainly to monolayer graphene. Here we demonstrate that moving a transparent flexible ZnO nanofilm across the surface of ionic solutions can generate electricity. The generated electricity increases linearly with the moving velocity with an open-circuit voltage up to tens of millivolt and a short-circuit current at the order of microampere. The harvested electricity can be efficiently scaled up through series and parallel connections. Theoretical simulations show that it is the proper electrical property that endows the ZnO nanofilm with the outstanding capacity in harvesting the wave energy.

Influence of sintering temperatures of ceramic targets on microstructures and photoelectric properties of titanium-doped ZnO nano-films

Thin films of Ti-doped ZnO (TZO) were prepared by RF magnetron sputtering using targets prepared with sintering temperatures in the range 1100–1500 °C; the microstructures and optoelectronic properties of the TZO targets and films were characterized by SEM, XRD, Hall Effect analysis, UV–VIS spectrophotometry and physical property measurement system. Results indicated that the target sintering temperature affected both the TZO targets and films. The Ti/Zn atomic ratios in the targets decreased progressively with increasing sintering temperature, but by a smaller amount in films prepared from them. XRD patterns showed that all films were preferentially oriented along the c axis at 2θ ~ 34° in their XRD patterns. The films sputtered with targets sintered at above 1300 °C were relatively smooth, and had larger average grain size. The target sintered at 1450 °C had the highest density. The best optoelectronic properties were found with the film sputtered from the target sintered at 1300 °C; this sample had superior crystal properties, high average optical transmittance (88.9%), and the lowest resistivity (8.47 × 10−4 Ω cm). Furthermore, the resistivity of all the films changed with temperature between 10 and 350 K, they experienced an initial decrease followed by an increase as the temperature was raised.

Inorganic Nano Light-Emitting Transistor: p-Type Porous Silicon Nanowire/n-Type ZnO Nanofilm.

An inorganic nano light-emitting transistor (INLET) consisting of p-type porous Si nanowires (PoSiNWs) and an n-type ZnO nanofilm was integrated on a heavily doped p-type Si substrate with a thermally grown SiO2 layer. To verify that modulation of the Fermi level of the PoSiNWs is key for switchable light emitting, I-V and electroluminescent characteristics of the INLET are investigated as a function of gate bias (V g ). As the V g is changed from 0 V to -20 V, the current level and light-emission intensity in the orange-red range increase by three and two times, respectively, with a forward bias of 20 V in the p-n junction, compared to those at a V g of 0 V. On the other hand, as the V g approaches 10 V, the current level decreases and the emission intensity is reduced and then finally switched off. This result arises from the modulation of the Fermi level of the PoSiNWs and the built-in potential at the p-n junction by the applied gate electric field.

Solvothermal synthesis of ZnO nanoparticles at low temperatures as cathode buffer layers for polymer solar cells with an inverted device structure

Inverted polymer solar cells of the conventional poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61butyric acid methyl ester (PC61BM) blend on indium tin oxide substrates were fabricated. By using mixed-solvent dispersed ZnO nanoparticles (NPs) as cathode buffer layer, device performances are improved obviously. ZnO NPs with a size of 3–5 nm synthesized by solvothermal synthesis at 65 °C show relatively wide photoluminescence peak of 300–650 nm. Based on Hansen solubility parameter theory, a mathematical method was applied to calculate the Hansen solubility parameters of bi-solvent system to disperse ZnO NPs and the proportion of each component in the mixed solvent. This excellent dispersion for ZnO NPs in the bi-solvent system has a important influence on the performance of the device. Compared to other methods of ZnO nanofilm fabrication, this method reveals a simple, convenient, moderate and effective way to manufacture the favorable buffer layer in organic solar cells.

Growth and Properties of Cl− Incorporated ZnO Nanofilms Grown by Ultrasonic Spray-Assisted Chemical Vapor Deposition

Pure and Cl- incorporated ZnO nanofilms were grown by the ultrasonic spray-assisted chemical vapor deposition (CVD) method. The properties of the nanofilms were investigated. The effects of growth temperature and Cl- concentration on the crystal structure, morphology, and optical properties of the nanofilms were studied. Temperature plays an important role in the growth mode and morphology of the pure nanofilms. Preferential growth along the c-axis occurs only at modulating temperature. Lower temperature suppresses the preferential growth, and higher temperature suppresses the growth of the nanofilms. The morphologies of the nanofilms change from lamellar and spherical structures into hexagonal platelets, then into separated nanoparticles with an increase in the temperature. Incorporating Cl- results in the lattice contracting gradually along with c-axis. Grains composing the nanofilms refine, and the optical gap broadens with increasing of Cl- concentration in growth precursor. Incorporating Cl- could reduce oxygen vacancies and passivate the non-irradiated centers, thus enhancing the UV emission and suppressing the visible emission of ZnO nanofilms.

Physical study on Cobalt–Indium Co-doped ZnO nanofilms as hydrophobic surfaces

The present work reports some physical investigations on (Co,In) codoped zinc oxide nanofilms deposited on glass substrates at 460 °C by the spray pyrolysis technique. The effect of Co and In concentration on the structural, morphological, optical and surface wettability properties have been investigated using X-ray diffraction (XRD) patterns, Raman spectroscopy, SEM, optical measurement, photoluminescence spectroscopy as well as the measurement of hydrophobicity in terms of water contact angle. It is found that all films crystallized in würtzite ZnO phase, with a preferentially orientation towards (002) direction parallel to c-axis. The Raman spectra of the samples exhibit the presence of E2high characteristic mode of würtzite structure with high crystallinity as well as two dominant bands 1LO and 2LO. Also, no additional modes introduced by codopoing have been found. SEM micrographs show the uniform deposition of fine grains on surface films. Thicknesses of films are less than 100 nm. In addition, optical investigations indicate that the band gap narrowing of (Co,In) codoped ZnO thin films is due to the increase in the band tail width. Indeed, PL study indicates that (Co,In) codoped ZnO nanofilms exhibit a large decrease of the UV luminescence, which is assigned to the trapping of photo-generated electrons by both In3+ and Co2+ ions as well as an improvement of charge separation in the ZnO thin films. Finally, the (Co,In) codoping influences the surface wettability property and transform the ZnO character from hydrophilic (θ<90°) for pure ZnO nanofilm to hydrophobic (θ>90°) for (Co,In) codoped ZnO ones.

Carrier transport behaviors depending on the two orthogonally directional energy bands in the ZnO nanofilm affected by oxygen plasma

An oxygen plasma treatment of ZnO nanostructures has frequently been used for obtaining a desired optoelectrical property. Nevertheless, a detailed study regarding carrier transport behaviors affected by the plasma has scarcely been managed, especially in the thin film structure, owing to its more complex physics than those of a one-dimensional nanostructure. Herein, we demonstrate an analysis of carrier transport behaviors on an oxygen plasma-treated ZnO nanofilm (50 nm thick) on a SiO2/Si substrate. By comparison with the as-grown sample, we observed drastic changes in carrier transport behavior according to the short exposure times of 30 s and 60 s. The plasma effect leading to the distinction was confirmed to originate from the bombardment of energetic ions near the surface and the diffusion of various oxygen ions and radicals into the host. The mechanism of the resulting carrier transport was comprehended through the revelation of two orthogonally directional energy band structures (surface band bending in the surface layer and localized energy bending at the grain boundary). Furthermore, we experimentally observed that the increased electrical barrier of the grain boundary, due to negatively absorbed oxygen ions, could be helpful in impeding persistent photoconductivity and in reducing dark current.

Study on Field Emission Characteristics of Ti-based Chrysanthemum-like Nano-ZnO Cathode by Electrophoresis Deposition

Abstract Chrysanthemum-like ZnO nanowire clusters were prepared by hydrothermal process, then were transferred to Ti substrate by electrophoresis method, and uniform ZnO nano-film with certain thickness was formed. Finally ZnO field emission cathode sample was fabricated after vacuum heat treatment. The microstructure and the morphology of chrysanthemum-like ZnO nanowire clusters were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Field emission tests were conducted for cathode samples after heat treatment, results reveal that the low turn-on field is 2.0 V/μm for the chrysanthemum-like ZnO, and the threshold field is 5.5 V/μm. Moreover, the field emission current density is 200 μA/cm2 under the condition of electric field of 7.5 V/μm, and the field emission current density and its stability can be effectively improved by appropriate enlarging the thickness of the ZnO coating. The field emission mechanism was also explored.