Flexible ZnO Research Articles

Flexible lateral-bridged ZnO nanowire UV sensor for focal-plane image sensor application

We report a large-format flexible UV image-sensing array consisting of 32 × 32 sensors based on lateral-bridged ZnO nanowires (NWs). Systematic investigations for the array have shown that 100% of the sensors showed a response with high uniformity (the nonuniformity of photocurrent was 44%) and high average light–dark current ratio (∼1.1 × 105), average responsivity (∼923 A/W), and average detectivity (∼2.5 × 1014 Jones). The pixel size is 60 × 125 µm2 and pixel pitch is 150 µm. The array also exhibited excellent flexibility and electrical stability under the bending state. The results indicate that the sensor array offers exacting possibilities for fabricating flexible UV focal-plane image sensors.

Origin of Prestress-Driven Optical Modulations of Flexible ZnO Thin Films Processed in Stretching Mode.

Experimental verification of optical modulation with external stress has not been easily available in flexible systems. Here, we intentionally induced extra stress in wide band gap ZnO thin films by a unique prestress-driven deposition processing that utilizes a stretching mode. The stretching mode provides homogeneous but biaxial stresses in the hexagonal wurtzite structure, leading to the extension of the c-axis and the contraction of the a-axis. As a result, the reduction of the optical band gap by ∼150 meV was observed for the strain of ∼4.87%. The band gap narrowing was found to occur from the respective downward and upward shifts of the conduction band minimum and valence band maximum under the applied stress. The experimental evidence of optical modulations was supported by the theoretical calculations using density functional theory. The reduced strong interactions between Zn d and O p orbitals were assumed to be responsible for the band gap narrowing.

Flexible ZnO Thin-Film Transistors on Thin Copper Substrate

In this paper, we report the fabrication of flexible ZnO thin-film transistors (TFTs) on the 25- $\mu \text{m}$ -thick copper substrate. The Cu substrate compatible with high-temperature processes was deposited by electroplating, which led to a smooth surface. The ZnO layer was deposited at room temperature, and a thermal annealing process was conducted at 300 °C in the air after the fabrication of TFTs. The threshold voltage, hysteresis voltage, and contact resistance are improved after annealing, whereas the mobility, subthreshold swing, and ON/OFF ratio are barely changed. The stabilities of ZnO TFTs are also enhanced, evidenced by the smaller threshold voltage shifts induced by positive and negative gate bias stresses. The field-effect mobility, threshold voltage, and subthreshold swing are stable even when exposed to a mechanical tensile strain up to 1.25% and 10 000 bending cycles.

Enhancing electrical stability for flexible ZnO nanowire UV sensor by textured substrates

The major problem in the fabrication of electronic devices on plastic substrate arises from the mismatch and weak physical bonding between inorganic semiconductor crystal and the organic plastics, so the electrical performance stability under mechanical stress is an essential factor affecting the practical application of flexible electronic sensors. In this paper, a flexible ZnO nanowire (NW) UV sensor is presented as an exemplary verification for investigating the effects of substrate morphology on reliability of flexible electronic devices. Sensors on ordinary smooth polyimide (PI) substrate have cracked during the device fabrication process due to the residual stress caused by temperature, humidity, etc. These cracks were short pieces and randomly distributed, so they may not significantly affect the device performance, but after mechanical bending, cracks that penetrating the electrodes were generated and caused electric contact failure. Although improving the ZnO seed layer quality could reduce the cracking and buckling that formed during device fabrication, it would not prevent cracking during mechanical bending. In a 3 × 3 sensor array, only one sensor survived after bending. Device failed when the bending angle larger than − 40° or 60°. On the contrary, device fabricated on textured PI substrates exhibited much better electrical stability. All the sensors remained their original performances after mechanical bending in a 3 × 3 sensor array. The light–dark current ratios kept in the level of 105 under the 365 nm UV light of 15 mW/cm2. The bending angle varied from − 80° to 80°. The enhancement of electrical stability was because that textured substrate could tolerate more stress than smooth substrate to prevent the film from cracking and at the same time it could increase the contact area between ZnO film and PI substrate, which improved the interfacial bonding strength and delayed the local strain of the film.

Highly performance of flexible transparent conductive Cu-based ZnO multilayer thin films via BS barrier layer

Highly performance of flexible transparent conductive Cu-based ZnO multilayer thin films are achieved by barrier layer of amorphous thin films based on barium stannate. The flexible multilayers exhibit high figure of merit (FoM) of 20.38 × 10−3 Ω−1 with the sheet resistance of 6.19 Ω/sq. and resistivity of 5.28 × 10−5 Ω cm, while the average optical transmittance is about 81.3% in the visible range. The FoM value is the highest among all the reported Cu-based transparent conductive thin films. The influence mechanism of BS barrier layers on electrical and optical properties of the multilayers is also proposed. Our results demonstrate that BS barrier layer is an effective way in tuning/improving the optical and electrical performances in the Cu-based ZnO multilayer thin films.

Performance enhancement of Cu-based AZO multilayer thin films via graphene fence engineering for organic solar cells

Performance enhancement of flexible transparent conductive Cu-based Al-doped ZnO (AZO) multilayer thin films has been achieved by fence engineering of graphene film. The multilayers show the excellent performance (figure of merit of 3.38 × 10−2 Ω−1, commutation quality factor of 430): sheet resistance of 4.37 Ω/sq., resistivity of 3.98 × 10−5 Ω cm, and average transmittance of above 82% in the visible range, which are the highest values obtained up to now in the flexible transparent conductive films. The commutation quality factor is about four times larger than that of indium tin oxide, the most widely used flexible transparent conductive films today. To assess the application potentials, flexible organic solar cells (FOSCs) are achieved, whose efficiency is 4.63%, which is increased by 95% and 26% compared with the reference FOSCs used a conventional ITO and Cu-based AZO multilayer electrodes, respectively.

Scale-Up of the Electrodeposition of ZnO/Eosin Y Hybrid Thin Films for the Fabrication of Flexible Dye-Sensitized Solar Cell Modules.

The low-temperature fabrication of flexible ZnO photo-anodes for dye-sensitized solar cells (DSSCs) by templated electrochemical deposition of films was performed in an enlarged and technical simplified deposition setup to demonstrate the feasibility of the scale-up of the deposition process. After extraction of eosin Y (EY) from the initially deposited ZnO/EY hybrid films, mesoporous ZnO films with an area of about 40 cm2 were reproducibly obtained on fluorine doped tin oxide (FTO)-glass as well as flexible indium tin oxide (ITO)–polyethylenterephthalate (PET) substrates. With a film thickness of up to 9 µm and a high specific surface area of up to about 77 m2·cm−3 the ZnO films on the flexible substrates show suitable properties for DSSCs. Operative flexible DSSC modules proved the suitability of the ZnO films for use as DSSC photo-anodes. Under a low light intensity of about 0.007 sun these modules achieved decent performance parameters with conversion efficiencies of up to 2.58%. With rising light intensity the performance parameters deteriorated, leading to conversion efficiencies below 1% at light intensities above 0.5 sun. The poor performance of the modules under high light intensities can be attributed to their high series resistances.

UV-activated ZnO films on a flexible substrate for room temperature O2 and H2O sensing

We demonstrate that UV-light activation of polycrystalline ZnO films on flexible polyimide (Kapton) substrates can be used to detect and differentiate between environmental changes in oxygen and water vapor. The in-plane resistive and impedance properties of ZnO films, fabricated from bacteria-derived ZnS nanoparticles, exhibit unique resistive and capacitive responses to changes in O2 and H2O. We propose that the distinctive responses to O2 and H2O adsorption on ZnO could be utilized to statistically discriminate between the two analytes. Molecular dynamic simulations (MD) of O2 and H2O adsorption energy on ZnO surfaces were performed using the large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) with a reactive force-field (ReaxFF). These simulations suggest that the adsorption mechanisms differ for O2 and H2O adsorption on ZnO, and are governed by the surface termination and the extent of surface hydroxylation. Electrical response measurements, using DC resistance, AC impedance spectroscopy, and Kelvin Probe Force Microscopy (KPFM), demonstrate differences in response to O2 and H2O, confirming that different adsorption mechanisms are involved. Statistical and machine learning approaches were applied to demonstrate that by integrating the electrical and kinetic responses the flexible ZnO sensor can be used for detection and discrimination between O2 and H2O at low temperature.

Low‐temperature synthesis of strain sensor based on flexible ZnO nanowire‐cellulose paper composite

ZnO nanowire was synthesised chemically at low temperature on a flexible three-dimensional and porous cellulose paper. The morphology and crystallography of the composite was characterised by field emission scanning electron microscope and X-ray diffraction. Performance of the developed ZnO nanowire-cellulose paper composite as strain sensor demonstrated good stability, high gauge factor and good repeatability. The results indicate the possible use of this sensitive and robust strain sensor in the fields of biomedical sciences, MEMS devices and structural health monitoring and other fields.

Flexible cellulose and ZnO hybrid nanocomposite and its UV sensing characteristics

This paper reports the synthesis and UV sensing characteristics of a cellulose and ZnO hybrid nanocomposite (CEZOHN) prepared by exploiting the synergetic effects of ZnO functionality and the renewability of cellulose. Vertically aligned ZnO nanorods were grown well on a flexible cellulose film by direct ZnO seeding and hydrothermal growing processes. The ZnO nanorods have the wurtzite structure and an aspect ratio of 9 ~ 11. Photoresponse of the prepared CEZOHN was evaluated by measuring photocurrent under UV illumination. CEZOHN shows bi-directional, linear and fast photoresponse as a function of UV intensity. Electrode materials, light sources, repeatability, durability and flexibility of the prepared CEZOHN were tested and the photocurrent generation mechanism is discussed. The silver nanowire coating used for electrodes on CEZOHN is compatible with a transparent UV sensor. The prepared CEZOHN is flexible, transparent and biocompatible, and hence can be used for flexible and wearable UV sensors.

Light-Emission Enhancement in a Flexible and Size-Controllable ZnO Nanowire/Organic Light-Emitting Diode Array by the Piezotronic Effect

Enhancing the light emission of LED arrays by the piezotronic effect of ZnO NWs has recently attracted wide attention for the visual mapping of pressure. Here, to overcome deficiencies of ZnO LEDs, such as uncontrollable emission and poor emission efficiency, we designed a flexible, patterned ZnO/organic light-emitting device by combining the advantages of organic light-emitting materials and the piezoelectric effect of ZnO nanowires (NWs). The spatial resolution of the device can be tuned by the pattern size of the ZnO NW array. In addition, the light-emission character is solely determined by the organic light-emitting layer. The light-emission performance of the organic device is enhanced by the piezotronic transistors of the ZnO NW array through the modification of the energy band and reducing the Schottky barrier at the interface of the electrode and the semiconductor. By combining the advantages of the organic material and the piezotronic effect of ZnO NWs, this device shows potential for many appli...

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.

All-inkjet-printed flexible ZnO micro photodetector for a wearable UV monitoring device

Fabrication of small-sized patterns of inorganic semiconductor onto flexible substrates is a major concern when manufacturing wearable devices for measuring either biometric or environmental parameters. In this study, micro-sized flexible ZnO UV photodetectors have been thoroughly prepared by a facile inkjet printing technology and followed with heat treatments. A simple ink recipe of zinc acetate precursor solution was investigated. It is found that the substrate temperature during zinc precursor ink depositing has significant effects on ZnO pattern shape, film morphology, and crystallization. The device fabricated from the additive manufacturing approach has good bendability, Ohmic contact, short response time as low as 0.3 s, and high on/off ratio of 3525. We observed the sensor’s dependence of response/decay time by the illuminating UV light intensity. The whole process is based on additive manufacturing which has many benefits such as rapid prototyping, saving material, being environmentally friendly, and being capable of creating high-resolution patterns. In addition, this method can be applied to flexible substrates, which makes the device more applicable for applications requiring flexibility such as wearable devices. The proposed all-inkjet-printing approach for a micro-sized ZnO UV photodetector would significantly simplify the fabrication process of micro-sized inorganic semiconductor-based devices. A potential application is real-time monitoring of UV light exposure to warn users about unsafe direct sunlight to implement suitable avoidance solutions.

Flexible chitosan-nano ZnO antimicrobial pouches as a new material for extending the shelf life of raw meat

As a breakthrough to open up the industrial use of novel environmentally benign packaging material, we propose the first report on portable chitosan-ZnO nano-composite pouches that will serve as elite entrants in smart packaging. A facile, one pot procedure was adopted for the preparation of the C-ZnC films. In order to tune the property of C-ZnC films, four different composite films were prepared by varying the concentration of ZnO. The prepared films were found to be much superior when compared to bare chitosan and other conventional films. Two bacterial strains that commonly contaminate in packed meat were selected as target microbes to elucidate the antimicrobial activity of the prepared C-ZnO film. Detailed investigations revealed that the antimicrobial efficiency is linearly related to the amount of ZnO nano-particles in the composite. The C-2 films exhibited excellent antimicrobial activity and was fabricated into packaging pouches for raw meat. The prepared pouches showed significant action against the microbes in raw meat owing to its complete inhibition of microbial growth on the sixth day of storage at 4°C. The C-2 pouches stand as a top-notch material when compared to polyethylene bag in extending the shelf life of raw meat.

Room-temperature deposition of flexible transparent conductive Ga-doped ZnO thin films by magnetron sputtering on polymer substrates

Flexible Ga doped ZnO transparent conductive films were prepared on polymer substrates at room temperature by RF magnetron sputtering. The effect of the Ar flow rate is more important than that of r.f. power and sputtering pressure. The dependence of the structural, electrical, optical and adhesive properties of flexible Ga doped ZnO transparent conductive thin films on the Ar flow rate was investigated for the first time. When the Ar flow rate is 40 sccm, the residual stress is changed from compressive stress to tensile stress, indicating the crystallinity of the film is best. And the lowest square resistance of 7.9 Ω/sq. is obtained at the Ar flow rate of 40 sccm. Regardless of the Ar flow rate, the optical transmittance in the visible light of all the films is about 90%. The best adhesive behavior is obtained at the Ar flow rate of 40 sccm.

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.

ZnO nanofiber (NFs) growth from ZnO nanowires (NWs) by controlling growth temperature on flexible Teflon substrate by CBD technique for UV photodetector

In this study, ZnO nanofibers (ZnO NFs) were successfully grown for the first time on Teflon substrates using CBD technique. The well-aligned ZnO nanorods (ZnO NRs) were transformed to ZnO nanofibers (NFs) by varying growth temperature and growth time. The high intensity and distinct growth orientation of peaks observed in the XRD spectra of the NFs indicate high crystal quality. The field emission scanning electron microscopy (FESEM) revealed high density of small diameter sized and long ZnO nanofibers (NFs) that are distributed in random directions. Raman analyses revealed a high E2 (high) peak at 436 nm, which indicates the wurtzite structure of ZnO. A flexible ZnO nanofiber (NFs)-based metal-semiconductor-metal UV detector was fabricated and analyzed for photo response and sensitivity under low power illumination (375 nm, 1.5 mW/cm2). The results showed a sensitivity of 4045% which can be considered a relatively high response and baseline recovery for UV detection.

Performance Boosting of Flexible ZnO UV Sensors with Rational Designed Absorbing Antireflection Layer and Humectant Encapsulation.

Flexible ZnO thin film UV sensors with 3 orders of magnitude improvement in sensitivity and 2 orders of magnitude acceleration in speed are realized via light absorption efficiency enhancement and surface encapsulation. Devices are constructed on polyethylene substrate incorporating morphology controlled ZnO nanorod arrays (NRAs) as absorbing antireflection layers. By adjusting the morphology of ZnO NRAs, the light absorptance exceeds 99% through effectively trapping incident photons. As a result, the sensitivity of the UV sensor reaches 109 000. Moreover, a mechanism of competitive chemisorption between O2 and H2O at oxygen vacancy sites is proposed to explain the phenomenon of the speed acceleration in moist environment. A new approach of humectant encapsulation is used to make H2O participant rapid processes dominant for speed acceleration. Two orders of magnitude speed enhancement in reset time is achieved by polyethylene glycol encapsulation. After a total 3000 cycles bending test, the decay in the responsivity of UV sensor is within 20%, indicating good mechanical stability. All these results not only demonstrate a simple, effective and scalable approach to fabricate high sensitive and fast response flexible ZnO UV sensors, but also provide meaningful references for performance boosting of photoelectronic devices based on other oxide semiconductors.

Effect of annealing in hydrogen atmosphere on ZnO films for field emission display

Surface morphology, crystallinity, conductivity and optical transmittance of ZnO films can be modified by annealing process. Hydrogen is one of the popular annealing gases as well as nitrogen, argon, oxygen and air which are commonly used for thin film cleaning or the removal of native oxide. In general, annealing is done at high temperatures (> 600degC) to improve the film properties. From a view point of environment, however, lower annealing temperature is preferable. In this work, low annealing process was challenged to understand the effect of annealing temperature on properties of ZnO thin films and nanostructured film grown on glass substrates for transparent field emission device applications. The annealing temperature employed was 100, 200 and 450°C at 100 sccm hydrogen flow rate. ZnO thin films were deposited by RF magnetron sputtering. The ZnO thin films were characterized by X-ray diffraction analysis (XRD), Atomic Force Microscopy (AFM), UV-VIS and Raman spectroscopy. The sheet resistances reduced about 15 kohm/sq at low annealing temperature. By contrast, the optical transmittance did not show any significant changes after annealing. The FE current density increased after the ZnO nanostructures film was annealed in 100°C. The results obtained could motivate a surface treatment for flexible ZnO thin film since the substrate is always suffered by heat.

Fabrication of flexible Al-doped ZnO films via sol–gel method

1% Al-doped ZnO transparent and conducting thin films on flexible polyimide (PI) substrates were successfully synthesized by the conventional sol–gel spin coating method. All the films with different number of layers (9, 10 and 11) retained a hexagonal wurtzite structure of ZnO and showed preferential c-axis orientation. Good adhesion was evidenced by dense microstructure and homogeneously distributed grains of the films on flexible PI substrates. The average transmittances reached 70% for all the films above a cut-off wavelength of ∼500nm originated from PI substrates. Comparing to the resistance of films annealed in air, the electrical conductivity could be enhanced by further vacuum (P∼10−2Pa) post-annealing, and lowest resistivity of 2.52×10−1Ωcm was obtained in the film with 10 layers.