ZnO Nanoparticle Films Research Articles

Structural, Optical and Defect State Analyses of ZnO Nanoparticle Films

Synthesis of ZnO nanostructures films by a co-precipitation followed by the deposition processed onto a glass substrate by spin-coating technique was carried out. The effect of annealing temperatures (from 250 to 325 °C for 30 min) on the structural and optical properties of the ZnO films have been investigated. The structural studies reveal that ZnO films are polycrystalline with hexagonal wurtzite structure. The X-ray diffraction (XRD) data show a better crystallinity at (101) crystal plane for the annealed films at 300 °C than the other ZnO films. The average grain size increases (from 31 to 36 nm) with an increase in annealing temperatures. The band gap energy is approximately 3.40 eV for the as-prepared films and varies from 3.25 to 3.18 eV with an increase in annealing temperatures. The photoluminescence (PL) results show a weak ultraviolet and relatively broad visible emissions respect to various defect structures in the ZnO films, in which the interstitial and vacancy oxygen are the main factors influencing the electronic properties in the whole ZnO films.

Mg-Doped ZnO Nanoparticle Films as the Interlayer between the ZnO Electron Transport Layer and InP Quantum Dot Layer for Light-Emitting Diodes

Because of the wide emission spectrum tunability that ranges from the visible region to the near-infrared, InP-based colloidal quantum dots (QDs) show great promise for use in the next-generation f...

Photocatalytic activity of electrophoretically deposited TiO2 and ZnO nanoparticles on fog harvesting meshes

TiO2 and ZnO are photocatalysts that degrade organic chemicals into smaller hydrocarbons in the presence of UV irradiation. Metal meshes have been previously used to harvest fog droplets from air streams. By merging these two observations, we hypothesize that TiO2/ZnO-coated metal meshes installed in the path of a fog-laden wind carrying contaminants, such as volatile organic compounds (VOCs), will degrade these VOCs providing pollution abatement.Stainless steel meshes are coated with a uniform, homogeneous and durable film of TiO2 or ZnO nanoparticles using electrophoretic deposition (EPD) in a phosphate ester solution. The TiO2-coated metal meshes are first activated with UV irradiation and then exposed to a fog-stream in a fog-tunnel, where the fog is pre-seeded with a VOC surrogate, methylene blue (MB), of known concentration. UV–Vis spectroscopy measurements of the water harvested determine the extent of MB degradation.The mechanism by which the phosphate ester develops an electrical charge on a nanoparticle surface leads to a conformal coating over the steel mesh. Maximum MB (0.05 mM) degradation of 36% is observed. Similar photocatalytic degradation of MB by TiO2 and ZnO coated meshes is also observed under direct UV irradiation from sunlight, confirming that the method is viable for outdoor installations.

Optoelectronic properties of hybrid diodes based on vanadyl- phthalocyanine and zinc oxide nanorods thin films

Herein, we report the optoelectronic properties of hybrid diodes fabricated using vanadyl phthalocyanine (VOPc) and zinc oxide nanorods (ZNR) with the configuration: ITO/ZNR/VOPc/MoO3/Al. Vertically aligned ZnO nanorods were grown using a simple aqueous solution (AS) method as a function of growth temperature. The correlation between the morphology of ZNR films and the optoelectronic properties of the ZNR/VOPc hybrid devices was investigated. The results show that the hybrid diodes with ZNR films grown at 120 °C offer the best optoelectronic properties. The higher photocurrent responsivity, Rph, (16.28 A/W) was achieved for devices with ZNR films grown at 120 °C. This value is 25 times higher than the Rph value obtained for the devices made with ZnO nanoparticle films that were reported earlier.

A Parts Per Billion (ppb) Sensor for NO2 with Microwatt (μW) Power Requirements Based on Micro Light Plates.

A film of gas sensitive ZnO nanoparticles has been coupled with a low-power micro light plate (μLP) to achieve a NO2-parts-per-billion conductometric gas sensor operating at room temperature. In this μLP configuration, an InGaN-based LED (emitting at 455 nm) is integrated at a few hundred nanometers distance from the sensor material, leading to sensor photoactivation with well controlled, uniform, and high irradiance conditions, and very low electrical power needs. The response curves to different NO2 concentrations as a function of the irradiance displayed a bell-like shape. Responses of 20% to 25 ppb of NO2 were already observed at irradiances of 5 mWatts·cm-2 (applying an electrical power as low as 30 μW). In the optimum illumination conditions (around 60 mWatts·cm-2, or 200 μW of electric power), responses of 94% to 25 ppb were achieved, corresponding to a lower detection limit of 1 ppb of NO2. Higher irradiance values worsened the sensor response in the parts-per-billion range of NO2 concentrations. The responses to other gases such as NH3, CO, and CH4 were much smaller, showing a certain selectivity toward NO2. The effects of humidity on the sensor response are also discussed.

GaxSe10-x based solar cells: Some alternatives for the improvement in their performance parameters

We report on strategies that improve Se-derivative based solar cells performance. With this aim, a compact thin film based on ZnO nanoparticles is deposited onto fluorine doped tin oxide (FTO) as an electron-transport layer, in thermally evaporated GaxSe10-x based solar cells. ZnO nanoparticles films are synthesized by sol-gel process whereas GaxSe10-x material is obtained by mechanical alloying. Using current-voltage measurements, impedance spectroscopy, and capacitance-voltage profiling, device characteristics and performance limiting factors are revealed and discussed. Particularly, the use of ZnO nanoparticles results in improved device performance as well as long-term stability. In comparison to Se-only devices with the structure FTO/Se/Au (power conversion efficiency of 0.98%), under 100 mW/cm2 AM 1.5 G illumination the devices achieved a power conversion efficiency of 2.7% with the structure FTO/ZnO/GaSe9/Au (open circuit voltage of 0.71 V, short-circuit current of 7.9 mA/cm2). Hence, an increase of around 175% in the power conversion efficiency is obtained in comparison to Se-only devices. In addition, the effect of others parameters, like thickness of the active layer as well as the gallium contents in the alloy, are discussed.

Comparison of ZnO buffer layers prepared by spin coating or RF magnetron sputtering for application in inverted organic solar cells

We compared the electrical, optical, structural, and morphological properties of radio-frequency (RF) magnetron-sputtered ZnO and solution-processed ZnO nanoparticle (NP) buffer layers on ITO cathodes for use in inverted polymer solar cells (IPSCs). Continuous sputtering resulted in integration of the ZnO buffer layer in the ITO cathodes, which were then used as transparent cathodes for IPSCs. Although the electrical, optical, and morphological properties as well as work function of RF-sputtered ZnO film were similar to those of solution-processed ZnO NP film, the power conversion efficiency (PCE) of IPSCs with an RF-sputtered ZnO buffer layer was much lower than that of IPSCs with a solution-processed ZnO NP buffer layer due to vertical phase segregation of the organic active layer. However, intentional bias sweeping of IPSCs with an RF-sputtered ZnO buffer layer improved performance due to diffusion of PC70BM through the PV-D4610 donor layer and formation of a suitable heterojunction structure. Based on transmission electron microscope examination and dark current-voltage curves, we suggest a possible mechanism to explain the difference in behavior of RF-sputtered ZnO and solution-processed ZnO NP buffer layers in IPSCs.

Magnetic response of random lasing modes in a ZnO nanoparticle film deposited on a NiFe thin film

This study experimentally demonstrates lasing mode switching within a ZnO nanoparticle film coated onto a magnetic thin film of NiFe alloy. When a neodymium magnet is brought close to or moved away from the film, switching behavior is observed in the lasing modes, although such change is not induced in a ZnO nanoparticle film on a glass substrate. Our results suggest that the observed changes in lasing modes are because of a magneto-optical effect at the surface of the NiFe thin film. The magneto-optical effect would be enhanced by localized fields near the surface, inducing suppression or enhancement of the lasing modes in response to the surrounding environments, and accounting for the lasing mode switching.

Spectrum Selectivity and Responsivity of ZnO Nanoparticles Coated Ag/ZnO QDs/Ag UV Photodetectors

Photoresponsivity and spectrum selectivity of ZnO nanoparticles (NPs)-coated colloidal quantum dots (CQDs)-based Ag/ZnO QDs/Ag interdigitated metal-semiconductor-metal (MSM) photodetectors under front and back illuminations have been investigated in this letter. It is shown that the ZnO NPs film coated over the interdigitated MSM photodetector acts as a filter layer to enhance the spectrum selectivity at the cost of reduced responsivity of the detector under front illumination. Whereas, the same NPs layer acts as a scattering medium to enhance the responsivity at the cost of reduced selectivity of the detector under back illumination condition. The measured responsivity under front illumination is 6 A/W with a full width at half maxima (FWHM) of ~25 nm, whereas it is increased to 20 A/W with an FWHM of ~49 nm under back illumination at an applied bias of 2 V. Thus, the ZnO NPs layer can be used to optimize the photoresponse characteristics of the ZnO CQDs-based MSM photodetector by changing the direction of illumination on the device.

Large enhancement of UV luminescence emission of ZnO nanoparticles by coupling excitons with Ag surface plasmons

For an emitter based on bandgap emission, defect mediated emission has always been considered as the most important loss. Here, a novel approach which can overcome such emission loss is proposed using films of ZnO nanoparticles (NPs) on Ag NPs embedded in silica. The effects of the size of Ag NPs on the enhancement of ultra-violet (UV) photoluminescence (PL) of ZnO NPs for such a system have been studied. For the ZnO NPs without Ag NPs, two emission bands have been seen: one in the UV region and the other one in the visible region. This UV PL emission intensity has been seen to increase significantly with a drastic reduction of the visible PL emission intensity in the case of the sample containing ZnO NPs on silica embedded Ag NPs. A linear increase in UV emission with increase in the size of Ag NPs has been found. For the largest size of Ag NPs (∼10 nm, considered in the present study), the PL emission enhancement becomes about 4 times higher than that of sample without Ag NPs. The observed enhancement of the UV PL emission was caused by coupling between spontaneous emission in ZnO and surface plasmons of Ag. The larger Ag NPs provided a larger scattering cross section in coupling surface plasmons to light leading to an increase in UV emission. Thus, it is possible to convert the useless defect emission to the useful excitonic emission with a large enhancement factor.

White light induced photo-thermal switching in a graphene-flake-mixed ZnO nanoparticle random laser

We experimentally demonstrate lasing mode switching within a graphene-flake-mixed ZnO nanoparticle film, in which the lasing suppression is observed when white light is illuminated on the film. The similar changes are also observed by changing the temperature of the same sample (about 30 °C increase form room temperature), while no lasing suppression is observed in a ZnO nanoparticle film without graphene flakes. In addition, we also observe that a thin glass substrate coated by a graphene-flake-mixed ZnO nanoparticle film shows a bend toward the film-coated side with increasing the white light intensity, due to the negative thermal expansion of graphene flakes. From these results, we consider that, beside the temperature dependent ZnO emission property, the photo-thermal response of graphene flakes is the dominant key for our observed lasing mode changes, in which the local structural change in a graphene-flake-mixed ZnO nanoparticle film would be induced by the white light absorption of graphene flakes. This study suggests the possibility to provide a method to remotely and non-invasively control and tune the lasing modes by external white light illumination.

Highly Efficient and Stable Flexible Perovskite Solar Cells with Metal Oxides Nanoparticle Charge Extraction Layers.

In this study, the fabrication of highly efficient and durable flexible inverted perovskite solar cells (PSCs) is reported. Presynthesized, solution-derived NiOx and ZnO nanoparticles films are employed at room temperature as a hole transport layer (HTL) and electron transport layer (ETL), respectively. The triple cation perovskite films are produced in a single step and for the sake of comparison, ultrasmooth and pinhole-free absorbing layers are also fabricated using MAPbI3 perovskite. The triple cation perovskite cells exhibit champion power conversion efficiencies (PCEs) of 18.6% with high stabilized power conversion efficiency of 17.7% on rigid glass/indium tin oxide (ITO) substrates (comparing with 16.6% PCE with 16.1% stabilized output efficiency for the flexible polyethylene naphthalate (PEN)/thin film barrier/ITO substrates). More interestingly, the durability of flexible PSC under simulation of operative condition is proved. Over 85% of the maximum stabilized output efficiency is retained after 1000 h aging employing a thin MAPbI3 perovskite (over 90% after 500 h with a thick triple cation perovskite). This result is comparable to a similar state of the art rigid PSC and represents a breakthrough in the stability of flexible PSC using ETLs and HTLs compatible with roll to roll production speed, thanks to their room temperature processing.

Ultrasonic vibration imposed on nanoparticle-based ZnO film improves the performance of the ensuing perovskite solar cell

This work focuses on the development of nearly annealing-free ZnO-based perovskite solar cells (PSCs), suitable for low-cost manufacturing of PSCs on flexible substrates. To this end, thin film of ZnO nanoparticles is employed as the electron transporting layer (ETL), because of its low-temperature solution-processability and high electron mobility. In order to remove the structural and surface defects, ultrasonic vibration is imposed on the substrate of the as-spun wet ZnO films for a short duration of 3 min. It is shown that the ultrasonic excitation bridges the ZnO nanoparticles (cold sintering), and brings about significant improvement in the ZnO film nanostructure and functionality. In addition, ethyl acetate (EA), as an emerging volatile anti-solvent, is employed to deposit the methylammonium (MA) lead halide perovskite thin film atop the ZnO ETL, in order to prepare perovskite layers that only need an annealing time of 30 s. The ZnO-based PSCs, with a simple structure and free of additional treatments, except for the ultrasonic vibration, exhibit a promising performance with a power conversion efficiency (PCE) of over 11%, 40% higher than that of the control device. The ultrasonic vibration treatment is facile, low-cost, environmentally friendly, and compatible with the scalable coating and printing techniques, such as spray and blade coating.

Fabrication of novel ZnO nanoporous films for efficient photocatalytic applications

The ZnO nanoparticle films have been grown on the glass substrates with ZnO seed layer by an aqueous chemical growth method at the low reaction temperature. The concentration of folic acid plays a crucial role for controlling the morphology of ZnO nanostructures. The ZnO nanoporous films can be easily fabricated by annealing nanoparticle films under ambient condition. The photocatalytic activity under 10 W UV light to photodegrade rhodamine B were evaluated with ZnO nanoporous films at the different annealing temperature. The ZnO nanoporous films exhibited much higher photodegradation efficiency than commercial TiO2 or ZnO nanopowders. Furthermore, the resultant ZnO nanoporous films on the glass substrates show a superior stability according to the reusability test. More important, the ZnO nanoporous films can provide a facile, low cost, high surface-to-volume ratio, high photocatalytic efficiency, and high reusability for practical wastewater treatment applications.

CO Sensor based on Thin Film of ZnO Nanoparticles

ABSTRACTIn this research, zinc chloride has been used as precursor and zinc oxide nanostructures have been synthesized by Sol-Gel process, using deionized water and 2-propanol as solvents in order to evaluate their influence on the final materials and their properties. Thin films of synthesized samples were deposited on glass substrates by the dipping method. The structure and morphology of crystals were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) respectively. The electrical response of the samples to CO was investigated at different operating temperatures and sensitivity curves are presented for samples synthesized in water and 2-propanol (IsOH) solvents. The SEM analysis revealed that ZnO thin films have yielded to different morphologies depending on the solvent, and material was found on the non-immersed side of the substrate attributable to migration during the dip-coating process. XRD analysis shows that the samples present the ZnO wurtzite structure. In EDS analysis it was found the presence of chlorine on the sample, opening the possibility the presence of zinc oxychloride.

Low-voltage resistance switching of solution-processed ZnO nanoparticle films

Resistive random access memory (ReRAM) has been considered as one of the most promising next-generation nonvolatile memory devices. ZnO is one kind of direct-band-gap semiconductors exhibiting good thermal stability, abundant raw materials for synthesis and low preparation cost. It shows extensive application potential in light-emitting diodes, photocatalysis and perovskite solar cells. Compared to ZnO bulk, nano-sized ZnO exhibits many special characteristics, such as non-toxic, non-migratory, fluorescence, piezoelectricity and conductivity. Moreover, ZnO is also one kind of transition metal oxides exhibiting resistive switching properties. There are many methods to synthesize nano-ZnO, such as chemical vapor deposition, sol-gel method and hydrothermal synthesis method. The common method to synthesize ZnO nanoparticles is the solution method using methanol as the reaction solvent. This method needs to heat methanol to 80°C, while the boiling point of methanol is only 64.7°C. Thus, this method is not safe. In this paper, ZnO nanoparticles were prepared by solution method using ethanol as the reaction solvent, which make the reaction temperature decrease to 40°C. Then, ZnO nanoparticles were dissolved in anhydrous ethanol to prepare a smooth, transparent and dense ZnO nanoparticle film by means of spin-coating under optimized preparation conditions. X-ray diffraction spectrum and scanning electron microscope image demonstrate polycrystalline ZnO nanoparticles with an average particle size of 7–10 nm were obtained. Ultraviolet-visible absorption spectra reveal the optical band gap of the n-type ZnO nanoparticle film is about 3.34 eV. Current-voltage curves of the ITO/ZnO/Al capacitor structure exhibit excellent bipolar resistive switching characteristics: the Set/Reset voltage is as low as ± 0.2 V, and the high/low resistance ratio is more than 100 at the reading voltage of 0.18 V, which is expected to be applied in the next generation of low-power-consumption nonvolatile memory devices. The formation/rupture of conductive Zn-atom filaments due to the electrochemical redox reaction of ZnO molecules induced by the external electric field is responsible for the resistive switching.

Enhancing the Performance of Quantum Dot Light-Emitting Diodes Using Room-Temperature-Processed Ga-Doped ZnO Nanoparticles as the Electron Transport Layer.

Colloidal ZnO nanoparticle (NP) films are recognized as efficient electron transport layers (ETLs) for quantum dot light-emitting diodes (QD-LEDs) with good stability and high efficiency. However, because of the inherently high work function of such films, spontaneous charge transfer occurs at the QD/ZnO interface in such a QD-LED, thus leading to reduced performance. Here, to improve the QD-LED performance, we prepared Ga-doped ZnO NPs with low work functions and tailored band structures via a room-temperature (RT) solution process without the use of bulky organic ligands. We found that the charge transfer at the interface between the CdSe/ZnS QDs and the doped ZnO NPs was significantly weakened because of the incorporated Ga dopants. Remarkably, the as-assembled QD-LEDs, with Ga-doped ZnO NPs as the ETLs, exhibited superior luminances of up to 44 000 cd/m2 and efficiencies of up to 15 cd/A, placing them among the most efficient red-light QD-LEDs ever reported. This discovery provides a new strategy for fabricating high-performance QD-LEDs by using RT-processed Ga-doped ZnO NPs as the ETLs, which could be generalized to improve the efficiency of other optoelectronic devices.

Observation of oxygen vacancy migration in memory devices based on ZnO nanoparticles

We investigate the mechanism of resistive switching in non-volatile memory devices based on an ITO/ZnO nanoparticles/Al structure using electroabsorption (EA) spectroscopy and X-ray photoelectron spectroscopy (XPS). By incorporating a small amount of low-bandgap organic semiconductor, poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), as a probe molecule for EA characterization, we study the change in the built-in potential during the switching process under different ambient conditions. We compare the concentrations of oxygen vacancies between the Al/ZnO interface and the bulk of the ZnO nanoparticle film by XPS. We also investigate the effect of an external electrical field on the concentration of oxygen vacancies at the Al/ZnO interface. We find that the resistive switching can be attributed to the migration of oxygen vacancies driven by the electrical field, accompanied by adsorption/desorption of oxygen molecules at the Al/ZnO interface. This process gives rise to the formation of a dipole layer, which modulates the injection barrier, and is responsible for switching the resistance state of the memory device.

Coating ZnO nanoparticle films with DNA nanolayers for enhancing the electron extracting properties and performance of polymer solar cells.

Here we present for the first time polymer solar cells that incorporate biological material that show state of the art efficiencies in excess of 8%. The performance of inverted polymer solar cells was improved significantly after deposition of ZnO nanoparticles (ZnO-NPs) together with a thin deoxyribonucleic acid nanolayer and used as an electron extraction layer (EEL). The ZnO-NPs/DNA double layer improved the rectifying ratio, shunt resistance of the cells as well as lowering the work function of the electron-collecting contact. Importantly, the ZnO-NPs/DNA bilayer enhanced the power conversion efficiency of cells considerably compared to cells with EELs made of only DNA (improvement of 56% in relative terms) or only ZnO-NPs (improvement of 19% in relative terms) reaching a best power conversion efficiency of 8.5%. The ZnO-NPs/DNA double layer cells also outperformed ones made with one of the most efficient previous synthetic composite EELs (i.e. ZnO/PEIE(poly(ethyleneimine)-ethoxylated)). Since all fabrication procedures were carried out at low (<150 °C) or room temperature, we have applied the findings to flexible substrates as well as on glass obtaining a high PCE of 7.2%. The solar cells with the biological/metal-oxide composite EELs also delivered an improvement in the stability (∼20% in relative term) compared to that with ZnO-NPs only. All these findings show that natural materials, in this case DNA, the premium biological material, can be incorporated in organic semiconductor devices in tandem with inorganic devices delivering uncompromising levels of performance as well as significant improvements.

Enhancing Quantum Dot LED Efficiency by Tuning Electron Mobility in the ZnO Electron Transport Layer

Quantum‐dot (QD) light‐emitting diodes (QLEDs) are an important new class of optoelectronic device. Despite the ubiquity of ZnO as the electron‐transport material in QLEDs, little is known about how its properties influence QLED performance. Here, it is demonstrated that the defect density and electron mobility of the ZnO nanoparticle electron‐transport layer strongly affect QLED device efficiency and can be used to balance electron and hole injection into the QD layer. Films of ZnO nanoparticles exhibiting electron mobilities tuneable over an order of magnitude are made by annealing out defect states in suspensions of ZnO nanoparticles prior to deposition. By incorporating these ZnO films into a typical QLED device, it is demonstrated that a clear maximum in QLED external quantum efficiency can be found at ZnO mobilities around 2–4 × 10−4 cm2 V−1 s−1, yielding over 10 000 cd m−2 at low operating voltage. The work demonstrates a simple method for enhancing QLED performance without modification of the device architecture and provides valuable insights into the physics of QLED operation.