Flexible ZnO Research Articles

Flexible ZnO nanowire platform by metal-seeded chemical bath deposition: Parametric analysis and predictive modeling

The growth of zinc oxide nanowires (ZnO NWs) on metal-seeded substrates is crucial for photonics, electronics, and sensing applications. Traditionally, NWs are grown using seed sintering on rigid substrates at high-temperature. However, the rise of flexible electronics, which use substrates unable to withstand high temperatures, has shifted focus to metal-assisted synthesis methods that do not require high-temperature sintering. This method has gained increasing attention due to its compatibility with flexible substrates. This article focuses on understanding the underlying growth mechanisms and achieving controlled growth of ZnO NWs on metal seeded flexible substrates. Furthermore, a parametric analysis is carried out to elucidate the correlation among different growth conditions in the chemical bath deposition (CBD) technique. Through a meticulously planned experimental design, the study investigates the influence of different growth conditions on synthesis outcomes. This leads to the formulation of predictive models using advanced machine learning (ML) methods particularly, artificial neural network (ANN). Following validation and training, the ANN model exhibits a remarkable ability to predict synthesis outcomes, yielding R2 values of 0.92 for diameter and 0.96 for length of NWs. Notably, the highest aspect ratio (AR) of ∼24 is attained following the growth conditions: 25 mM precursor concentration, 60 min growth time, and a growth temperature of 95 °C. Additionally, this method of growing ZnO NWs on a metal-seeded substrate offers an alternative approach for fabricating nanodevices for various emerging applications.

3D printing flexible Ga-doped ZnO films for wearable energy harvesting: thermoelectric and piezoelectric nanogenerators

The 3D printing of energy harvesters using earth-abundant and non-toxic elements promotes energy sustainability and market competitiveness. The semiconducting behavior and non-centrosymmetric wurtzite crystal structure of gallium-doped zinc oxide (GZO) films make them attractive for thermoelectric and piezoelectric nanogenerators. This study investigates the thermal, structural, mechanical, thermoelectric, and piezoelectric properties of 3D-printed GZO nanocomposite films. Thermal analysis demonstrates the stability of the nanocomposite film up to 230 °C, making it suitable for wearable energy harvesters. The crystalline structure of the nanocomposite film aligns with the hexagonal wurtzite structure of ZnO and displays a bulk-like microstructure with a uniform distribution of elements. The presence of Ga 2p, Zn 2p, O 1 s, and C 1 s core levels confirms the development of the nanocomposite film, characterized by a fine granular structure and a conductive domain compared to the neat resin film. The inclusion of GZO nanofillers tailors the stress–strain behavior of the nanocomposite film, enhancing flexibility. The 3D-printed GZO nanocomposite films demonstrate a promising thermoelectric power factor and piezoelectric power densities, along with mechanical flexibility and thermal stability. These advancements hold significant potential for wearable and hybrid energy generation technologies.

Asymmetric optical properties and bandgap shift of pre-strained flexible ZnO films

Strain engineering has been extensively explored to modulate the various intrinsic properties of flexible inorganic semiconductor films. However, experimental characterization of tensile and compressive strain-induced modulation of optoelectronic properties and their differences has not been easily implemented in flexible inorganic semiconductor films. Herein, the strain-dependent structural, optical, and optoelectronic properties of flexible ZnO films under pre-tensile and pre-compressive strains are systemically investigated by a Mueller matrix ellipsometry-based quantitative characterization method combined with x-ray diffraction and first-principle calculation. With extended prestress-driven deposition processing under bi-direction bending modes, pre-tensile and pre-compressive strains with symmetric magnitudes can be achieved in flexible ZnO films, which allows precise observation of the strain-driven asymmetric modulation of optoelectronic properties. When the applied prestrain varies approximately equally from 0% (baseline) to −0.99% (compression) and 1.07% (tensility), respectively, the relative changes for the c-axis lattice constant are 0.0133 and 0.0104 Å, respectively. Meanwhile, the dependence factors of the bandgap energy on the pre-compression and pre-tensile strains were determined as −0.0099 and −0.0156 eV/%, respectively, and the complex refractive index also presents an asymmetric varying trend. With the help of the strain–stress analysis and the first-principle calculation, the intriguing asymmetric strain-optical modulation effect could be attributed to the biaxial strain mechanism and the difference in the deformation potential between the two prestrain modes. These systematic investigation consequences are thus promising as a basis for the booming applications of the flexible inorganic semiconductor ensemble.

Compliant substrate epitaxial MgZnO films using fluorphlogopite mica approaching homoepitaxy quality

Based on the success of high-quality epitaxial grown ZnO film on the compliant Fluorphlogopite Mica (F-Mica) substrate, the growth of MgZnO films on F-Mica substrate is studied to promote the development of flexible ZnO quantum well structures and devices. These flexible epitaxial MgZnO films show high luminous quality approaching homoepitaxial films. The phenomenon of Mg compositional fluctuations is confirmed by the research on band tail state. The effect of the compositional fluctuations on epitaxial growth dynamical and thermodynamic processes is also revealed by the comprehensively investigation of the crystal quality, optical properties, morphology, and growth rate. The study provides guidance for energy band engineering of ZnO on flexible substrates, and lays the foundation for the bendable optoelectronic devices on F-Mica substrate.

Ternary flexible ZnO (NWs)/PPy-silver composites film for ppb-level ammonia detection

Ternary flexible ZnO (NWs)/PPy-silver composites film for ppb-level ammonia detection

Low‐Temperature Water‐Assisted Oxidation of Sponge‐Like Zn Nanostructures for Environmental and Energy Harvesting Applications

AbstractIn this work, the oxidation of sponge‐like nanostructured Zn films exploiting their interaction with water in liquid and vapor phases is investigated. By simply exposing the sputtered porous metal layers to a water vapor atmosphere or incubating them in water at different temperatures, the full conversion to ZnO is obtained. Depending on the kind of treatment, the oxidized ZnO layers exhibit different morphologies and physico‐chemical properties. When in combination with low heating of the surface, a better crystallinity and the growth of hexagonal nanocrystals (nanoprisms and nanoflowers) from the nanobranched Zn structure is observed, preserving the sponge‐like morphology of the starting material. Good photocatalytic activities for the degradation of an organic dye are measured under simulated sunlight irradiation. The piezoelectric and semiconducting response of the oxidized ZnO layers is also examined, revealing appealing performance in both cases and envisaging their use as nanogenerators and photoanode material in dye‐sensitized solar cells. The multifunctional properties of the oxidized ZnO film are discussed in terms of the selected water oxidation approach that tune the corresponding morphology and crystallinity. The low‐temperature oxidation approaches here proposed allow the fabrication of flexible semiconductive ZnO films highly desirable in different fields of nanoelectronics.

Performance of flexible ZnO nanowire piezoelectric generators enhanced by bifacial superposition

High performance piezoelectric nanogenerators have important applications in many fields. In this work, we report a bifacial superposed zinc oxide piezoelectric nanogenerator that shows better performance than a single unit. A stainless steel substrate with both front and back zinc oxide nanowire arrays is an elementary unit. Then we engage in an alternating superposition of two elementary units with a carbon-contained polyurethane foil. Multiple units can be similarly assembled by adding alternating structures to further enhance the piezoelectric output performance. Nanogenerators obtained by the above bifacial method are flexible and stability with high output. The method can be used to increase the overall piezoelectric performance and is expected to have a wide application in self-powered device fields.

Flexible ZnO:PVDF based free standing piezoelectric nanogenerator for vibrational energy harvesting and wearable shoe insole pedometer sensor

The increasing demand of energy harvesting and wearable electronics applications requires the development of flexible and highly durable piezoelectric nanogenerators (PENG). In this work, We have fabricated flexible ZnO:PVDF based PENGs for vibrational energy harvesting and shoe insole pedometer applications. ZnO nanorods were grown by hydrothermal method and incorporated into PVDF matrixes in the concentration range of 5–15 wt%. These homogeneous ZnO:PVDF composite films were sandwiched between Al electrodes to realize flexible free standing PENGs. The open circuit voltage (Voc), short circuit current (Isc) and instantaneous power density of PENGs increased from ∼ 4 V, 0.4 µA and 1.6 µW/cm2 to ∼ 14.6 V, 0.6 µA and 21.3 µW/cm2 respectively when ZnO nanorods concentration in PVDF increased from 0 to 10 wt% and decreased with further increase in ZnO nanorods concentration. The enhancement in output characteristics of ZnO:PVDF PENG with increasing ZnO concentration was attributed to β-phase crystallization of PVDF, which was confirmed by XRD and FTIR measurements. The deterioration in output characteristics beyond 10 wt% of ZnO nanorods concentration was explained considering the variation of dielectric constant and remnant polarization. The rectified output of the PENGs was effectively stored in various commercially available capacitors and used to power LEDs and stopwatch displays. The PENG with 10 wt% ZnO concentration was used to develop shoe insole pedometer which was interfaced with android based mobile application via Bluetooth. The shoe insole pedometer was successfully tested for 5000 steps at different walking frequencies ranging from 0.5 Hz to 2 Hz in the speed range of ∼ 1.4 km/h to 5.5 km/h. This study not only gives an insight into the mechanism of ZnO:PVDF based PENGs but also opens up their potential in vibration energy harvesting and shoe insole pedometer applications.

Optical and Field Emission Properties of Transparent Flexible ZnO Nanowire Films

Optical and Field Emission Properties of Transparent Flexible ZnO Nanowire Films

Flexible ZnO Nanosheet‐Based Artificial Synapses Prepared by Low‐Temperature Process for High Recognition Accuracy Neuromorphic Computing

AbstractIn neuromorphic computing networks, a flexible synaptic memristor with high recognition accuracy is highly desired. In this study, ZnO nanosheets (ZnO NS) embedded within a polymethyl methacrylate host material are used as the intermediate layer to prepare flexible synaptic memristor at a low‐temperature of 80 °C. The device shows excellent switching characteristics with low SET/RESET voltages (−0.4 V/0.4 V) and stable retention characteristic (104 s). By modulating the conductance continuously, the flexible synaptic memristor simulates typical synaptic plasticities, including excitation post‐synaptic current, paired‐pulse facilitation, and spike‐timing dependent plasticity. Especially, the neuromorphic system built from flexible ZnO NS‐based memristors achieves a high recognition accuracy up to 97.7% for handwriting digit. Under the influence of 5% Uniform noise and 5% Gaussian noise, recognition accuracies are maintained at 94.6% and 93.7%, respectively. These properties are well maintained even when bending 1000 times at a radius of 5 mm. The flexible ZnO NS‐based memristor shows great prospects in wearable devices and neural morphology calculation.

Epitaxy and bonding of peelable ZnO film on graphene/ZnO substrate

High-quality flexible ZnO films are commonly used to prepare ZnO-based flexible devices. However, the existing methods for preparing flexible films have some bottlenecks, such as poor quality, high cost, and complex steps in the flexible production process. The present study presents results on peelable ZnO film grown on graphene/ZnO substrate by plasma-assisted molecular beam epitaxy (PAMBE). Double-layer graphene (DLG) can be used as a release layer to convert the contact between ZnO film and substrate from chemical bonding to van der Waals force interactions. Hence, the ZnO film grown on graphene/ZnO substrate can be peeled off by a thermal release tape. The O-plasma irradiation introduced many dangling bonds on the surface of graphene. These dangling bonds combined with Zn atoms to form COZn bonds in the nucleation of the ZnO film. This study proposes a bonding mechanism of ZnO on graphene, verified by experiments. The flexible ZnO film obtained by this method provides a simple and feasible way for potential applications in ZnO-based flexible optoelectronic devices.

Laser-localized hydrothermal synthesis of flexible ZnO gas sensor for room-temperature detection of nitrogen dioxide

Laser-localized hydrothermal synthesis of flexible ZnO gas sensor for room-temperature detection of nitrogen dioxide

Flexible superhydrophobic ZnO coating harvesting antibacterial and washable properties

The problem of bacterial proliferation on medical or food processing devices has been a challenge for decades. A facile and environmental-friendly fabrication of superhydrophobic coating has been reported. Modified ZnO nano particles can self-assemble on the water surface and construct into a membrane, which is subsequently “print” on a tape, significantly simplify the preparation process and time. The fabricated coating showed excellent superhydrophobicity and water drop impact resistance. Moreover, the coating had long-time underwater stability and strong antibacterial activity against Staphylococcus aureus. Importantly, its flexibility and self-cleaning property make the coating can be applied to various complex shapes, satisfying actual health care needs on a large scale.

Evaluation of IDE-based flexible thin film ZnO sensor for VOC sensing in a custom designed gas chamber at room temperature

Evaluation of IDE-based flexible thin film ZnO sensor for VOC sensing in a custom designed gas chamber at room temperature

Characteristics of flexible ZnO nanorod UV photodetectors processed by using a direct silicon etching transfer method

A flexible UV photodetector based on ZnO NRs with good photoelectrical properties was fabricated by applying DSET to achieve superior UV-sensing properties.

Synthesis and characterization of ZnO nanorods-Zn2SiO4 nanoparticles-PMMA nanocomposites for UV-C protection

Nowadays, UV-C is used in a variety of applications, such as disinfection and sterilization of air, surface, and liquids. In particular, its application against the pandemic COVID-19 virus is of great interest. On the other hand, it must be remarked that UV-C is harmful to the eyes, face, head, ears, and skin of engaged employees or personnel but studies on the UV-C range are rare. To the best of our knowledge, studies on zinc silicate as UV-absorber are very rare. In this research, flexible and transparent ZnO nanorods-Zn2SiO4 nanoparticles-PMMA nanocomposite thin films were made for protecting against UV-C radiation. To find the most suitable morphology, the effect of pH variation on the morphology and structural properties of the prepared ZnO–Zn2SiO4 composites were studied. Here we show that the most regular morphology of composites consisting of ZnO nanorods decorated with Zn2SiO4 nanoparticles was obtained at pH 12.5. The UV–vis spectra show that ZnO–Zn2SiO4-PMMA nanocomposite thin films prepared at pH 12.5 exhibit optical transparency in the visible-wavelength region and a prominent UV-C absorbing capability.

TiO2 thin films derived by facile sol-gel method: Influence of spin rate and Al-doping on the optical and electronic properties

Titanium dioxide (TiO2) thin films with enhanced properties have been synthesized by sol-gel method and they can be used for various applications as in field-effect transistors (FETs), sensors, photovoltaics etc. The properties of TiO2 thin films were optimized by changing spin speed (1000 and 3000 rpm) and doping Al in the TiO2 lattice. The influence of spin speed and Al doping on the grain orientation and surface morphology of titania were studied using x-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Uniform, smooth and compact TiO2 thin films having tetragonal anatase phase and a band gap of 3.54 eV along with an optimum thickness and high dielectric constant of ~28 were obtained at a spin speed of 3000 rpm. These films have been previously reported as a gate dielectric layer in the fabrication of flexible ZnO:Al thin film transistor with a mobility of 4.96 cm2/Vs, therefore, the samples prepared at different spin speed could be useful for the fabrication of organic and inorganic FETs. Detailed study was performed on Al-doped TiO2 thin films. XRD and various vibrational modes noticed in the Raman spectra confirmed the formation of crystalline TiO2 films with a pure anatase tetragonal phase. A systematic shift in (101) plane in XRD and E1 g mode in Raman spectra confirms the substitutional doping of Al in the TiO2 matrix. A red shift in the band gap energy with increased transmittance in visible region was observed for the Al-doped TiO2 samples. The higher value of Urbach energy for the Al-doped samples reveals the presence of defects related to oxygen which were further studied by the photoluminescence spectra (PL). AFM images revealed compact, void free and uniform deposition of both TiO2 and TiO2: Al films. The particle size was found to be in the range of 30–40 nm and 40–100 nm for TiO2 and TiO2: Al, respectively. Enhanced electrical conductivity was observed for the Al-doped TiO2 thin films. The current Al-doped TiO2 system with enhanced optical, morphological and electrical properties could be a potential candidate in optoelectronic, sensing and photovoltaics applications.

36.3: Flexible ZnO Thin‐Film Transistors Fabricated on PEN Substrate by Atomic Layer Deposition at Low Temperature

Flexible ZnO thin‐film transistors (TFTs) have been successfully fabricated by atomic layer deposition (ALD) on polyethylene naphthalate (PEN) substrate with a maximum processing temperature of 200 °C. The flexible ZnO TFTs show competitive electrical properties with high saturation mobility (μsat) of 17.5 cm2/Vs, small subthreshold slope (SS) of 91 mV/dec, high Ion/Ioff of 3.8×109 and good uniformity. The larger characteristic trapping time τ for flexible ZnO TFT under NBS is the indication of less trapped charges compared with the device under PBS. In addition, we also investigate the mechanical bending test of flexible ZnO TFTs. The flexible ZnO TFTs exhibit a negative turn‐on voltage (Von) shift and degraded SS with increasing tensile strains because the mechanical strain at the bending part of TFT channel generates donor‐like and acceptor‐like defects. Our results demonstrate that ALD process flexible ZnO TFTs exhibiting tremendous potential to be applicable in flexible electronics.

Flexible zinc oxide photoelectrode for photo electrochemical energy conversion

Photoelectrochemical properties have been investigated for flexible photoelectrodes containing 310 nm thick ZnO film on spin-coated ITO/PET. The high crystalline structure of ZnO was studied using x-ray diffraction pattern. A value of 3.4 eV has been estimated for optical band gap from its absorption spectra. The flexible ZnO photoelectrode was demonstrated to generate photoelectrochemical current. The photocurrents are enhanced by 4% whereas flat-band potential is shifted by 8 V due to the illumination. Values of 1.022 and 0.714 AW−1 were found to be for photo switching and photoresponsivity, respectively. ZnO/ITO/PET can be used as a substrate for making flexible hybrid PEC devices to generate solar power and solar fuels.

Transparent and flexible ZnO nanorods induced by thermal dissipation annealing without polymer substrate deformation for next-generation wearable devices.

The fabrication of a transparent and flexible ultraviolet photodetector based on hydrothermally grown ZnO nanorods requires an annealing step to render the sol–gel spin-coated ZnO seed layer crystalline. As high-temperature annealing deforms low-melting-point polymer substrates, we herein devised a thermal dissipation annealing (TDA) method in which heat transfer to ZnO thin films is synchronized with heat release from the polymer substrate to crystallize the ZnO seed layer without polymer substrate deformation and melting. ZnO nanorods (NRs) were hydrothermally grown on non-annealed and annealed ZnO seed layers, and NR density and diameter were shown to be higher in the latter case, as the crystallized ZnO seed layer provided heterogeneous nucleation sites for NR growth. In addition, the larger density and diameter of ZnO NRs grown on the annealed ZnO seed layer were confirmed by analysis of O 1s signal intensities. A transparent and flexible UV photodetector based on ZnO NRs grown on the annealed ZnO seed layer exhibited a higher photocurrent/dark current ratio, photosensitivity, and photoresponsivity than that fabricated using a non-annealed seed layer. Taken together, the above results suggest that the TDA method is an effective way of fabricating transparent and flexible UV photodetectors with high photosensitivity, photoresponsivity, and photocurrent stability and it means that the next generation wearable devices can be easily realized by using the TDA method.