Photoluminescence of ZnO Nanowires: A Review

Site-Selective Nanowire Synthesis and Fabrication of Printed Memristor Arrays with Ultralow Switching Voltages on Flexible Substrate

Large area electronics (LAE) with the capability to sense and retain information are crucial for advances in applications such as wearables, digital healthcare, and robotics. The big data generated by these sensor-laden systems need to be scaled down or processed locally. In this regard, brain-inspired computing and in-memory computing have attracted considerable interest. However, suitable architectures have mainly been developed using costly and resource-intensive conventional lithography-based methods. There is a need for the development of innovative, resource-efficient fabrication routes that enable such devices and concepts. Herein, we present ZnO nanowire (NW)-based memristors on a polyimide substrate fabricated by a LAE-compatible and resource-efficient route comprising solution processing and printing technologies. High-resolution "drop-on-demand" and "direct ink write" printers are employed to deposit metallic layers (silver and gold) and a ZnO seed layer, needed for the site-selective growth of ZnO NWs via a low-cost hydrothermal method. The printed memristors show high bipolar resistance switching (ON/OFF ratio >103) between two nonvolatile states and consistent switching at ultralow voltages (all devices showed switching at amplitudes <200 mV), with the best performing device showing consistent cycled resistance switching over 4 orders of magnitude with SET and RESET voltages of about 71 and -57 mV, respectively. Thus, the presented devices offer reliable high resistance switching at the lowest reported voltage for printed memristors and prove to be competitive with many conventional nanofabrication-based devices. The presented results show the potential printed memristors technology holds for large-area, low-voltage sensing applications such as electronic skin.

Principles and Applications of ZnO Nanomaterials in Optical Biosensors and ZnO Nanomaterial-Enhanced Biodetection

Significant research accomplishments have been made so far for the development and application of ZnO nanomaterials in enhanced optical biodetection. The unparalleled optical properties of ZnO nanomaterials and their reduced dimensionality have been successfully exploited to push the limits of conventional optical biosensors and optical biodetection platforms for a wide range of bioanalytes. ZnO nanomaterial-enabled advancements in optical biosensors have been demonstrated to improve key sensor performance characteristics such as the limit of detection and dynamic range. In addition, all nanomaterial forms of ZnO, ranging from 0-dimensional (0D) and 1D to 2D nanostructures, have been proven to be useful, ensuring their versatile fabrication into functional biosensors. The employment of ZnO as an essential biosensing element has been assessed not only for ensembles but also for individual nanomaterials, which is advantageous for the realization of high miniaturization and minimal invasiveness in biosensors and biodevices. Moreover, the nanomaterials' incorporations into biosensors have been shown to be useful and functional for a variety of optical detection modes, such as absorption, colorimetry, fluorescence, near-band-edge emission, deep-level emission, chemiluminescence, surface evanescent wave, whispering gallery mode, lossy-mode resonance, surface plasmon resonance, and surface-enhanced Raman scattering. The detection capabilities of these ZnO nanomaterial-based optical biosensors demonstrated so far are highly encouraging and, in some cases, permit quantitative analyses of ultra-trace level bioanalytes that cannot be measured by other means. Hence, steady research endeavors are expected in this burgeoning field, whose scientific and technological impacts will grow immensely in the future. This review provides a timely and much needed review of the research efforts made in the field of ZnO nanomaterial-based optical biosensors in a comprehensive and systematic manner. The topical discussions in this review are organized by the different modes of optical detection listed above and further grouped by the dimensionality of the ZnO nanostructures used in biosensors. Following an overview of a given optical detection mode, the unique properties of ZnO nanomaterials critical to enhanced biodetection are presented in detail. Subsequently, specific biosensing applications of ZnO nanomaterials are discussed for ~40 different bioanalytes, and the important roles that the ZnO nanomaterials play in bioanalyte detection are also identified.

Single ZnO Nanowire for Electrical and Optical NO2 Gas Sensing: Origin of Reversible and Irreversible Gas Effects Investigated by Photoluminescence Spectroscopy

In this work, the gas sensing properties of a single ZnO nanowire (NW) are investigated, simultaneously in terms of photoluminescence (PL) and photocurrent (PC) response to NO2 gas, with the purpose of giving new insights on the gas sensing mechanism of a single 1D ZnO nanostructure. A single ZnO NW sensing device was fabricated, characterized, and compared with a sample made of bundles of ZnO NWs. UV near-band-edge PL emission spectroscopy was carried out at room temperature and by lowering the temperature down to 77 K, which allows detection of resolved PL peaks related to different excitonic transition regions. Surface effects were observed in PL maps, considering different nano and microstructures. Electrical and optical measurements were acquired at the same time during the NO2 gas exposure, allowing for the comparison of PL and PC response times and signal recovery. During NO2 gas desorption, irreversible behavior in the surface-related and donor-acceptor pair (DAP) regions is interpreted as the effect of an initial transient when electronic transfer from the gas molecules to the bulk occurs through the ZnO NW surface which acts as a channel. To the best of our knowledge, this is the first work which investigates the simultaneous PL optical and PC electrical response signals of a single ZnO NW to gas exposure.

Synergistic Assembly of 1DZnO and Anti-CYFRA 21-1: A Physicochemical Approach to Optical Biosensing

Objective: We conducted a comprehensive physicochemical analysis of one-dimensional ZnO nanowires (1DZnO), incorporating anti-CYFRA 21-1 immobilization to promote fast optical biomarker detection up to 10 ng ml-1. Impact Statement: This study highlights the effectiveness of proof-of-concept 1DZnO nanoplatforms for rapid cancer biomarker detection by examining the nanoscale integration of 1DZnO with these bioreceptors to deliver reliable photoluminescent output signals. Introduction: The urgent need for swift and accurate prognoses in healthcare settings drives the rise of sensitive biosensing nanoplatforms for cancer detection, which has benefited from biomarker identification. CYFRA 21-1 is a reliable target for the early prediction of cancer formation that can be perceptible in blood, saliva, and serum. However, 1DZnO nanostructures have been barely applied for CYFRA 21-1 detection. Methods: We assessed the nanoscale interaction between 1DZnO and anti-CYFRA 21-1 antibodies to develop rapid CYFRA 21-1 detection in two distinct matrices: PhosphateBuffered Saline (PBS) buffer and artificial saliva. The chemical modifications were tracked utilizing Fourier transform infrared spectroscopy, while transmission electron microscopy and energy dispersive spectroscopy confirmed antigen-antibody interplay over nanostructures. Results: Our results show high antibody immobilization efficiencies, affirming the effectiveness of 1DZnO nanoplatforms for rapid CYFRA 21-1 testing within a 5-min detection window in both PBS and artificial saliva. Photoluminescence measurements also revealed distinct optical responses across biomarker concentrations ranging from 10 to 1,000 ng ml-1. Conclusion: Discernible PL signal responses obtained after 5 min affirm the potential of 1DZnO nanoplatforms for further advancement in optical biomarker detection for application in early cancer prognosis.

Transparent and Colorless Luminescent Solar Concentrators Based on ZnO Quantum Dots for Building-Integrated Photovoltaics

Scientific interest in luminescent solar concentrators (LSCs) has reemerged mainly due to the application of semiconductor quantum dots (QDs) as highly efficient luminophores. Recently, LSCs have become attractive proposals for Building-Integrated photovoltaics (BIPV) since they could help conventional photovoltaics to improve sunlight harvesting and reduce production costs. However, most of the modern LSCs rely on heavy-metal QDs which are highly toxic and may cause environmental concerns. Additionally, their absorption spectra give them a characteristic color limiting their potential application in BIPV. Herein, we fabricated transparent and colorless LSCs by embedding nontoxic and cost-effective zinc oxide quantum dots (ZnO QDs) in a PMMA polymer matrix (ZnO-LSC), preserving the QD optical properties and PMMA transparency. The synthesized colloidal ZnO QDs have an average size of 5.5 nm, a hexagonal wurtzite crystalline structure, a broad yellow photoluminescent signal under ultraviolet excitation, and are highly visibly transparent at the employed concentrations (>95% in wavelengths above 400 nm). The optical characterization of the fabricated ZnO-LSCs showed a good visible transparency of 80.3% average visible transmission (AVT), with an LSC concentration factor (C) of 1.02. An optimal device (ZnO-LSC-O) could reach a C value of 2.66 with the combination of optical properties of colloidal ZnO QDs and PMMA. Finally, simulations of the performance of silicon solar cells coupled to the fabricated and optimal LSCs under standard AM 1.5G illumination were performed employing the software COMSOL Multiphysics. The fabricated ZnO-LSC achieved a simulated maximum power conversion efficiency (PCE) of 3.80%, while the optimal ZnO-LSC-O reached 5.45%. Also, the ZnO-LSC generated a maximum power of 15.02 mW and the ZnO-LSC-O generated 40.33 mW, employing the same active area as the simulated solar cell directly illuminated, which generated 14.39 mW. These results indicate that the ZnO QD-based LSCs may be useful as transparent photovoltaic windows for BIPV applications.

Bicone nanoflower evolution and multi-peak emission of polymer caped Cu doped ZnO

A low-temperature polymer-assisted wet chemical method was used to synthesise Cu-doped ZnO bicone nanoflowers at three different polyethylene glycol (PEG) concentrations. The effects of PEG concentration on the structural, morphological and optical properties of Cu doped ZnO nanostructures were studied. X-ray diffraction studies revealed that the as-synthesized Cu doped ZnO nanostructures are highly crystalline with a hexagonal wurtzite phase. The scanning electron microscopy analysis showed that the prepared nanostructures have bicone- nanoflower morphology and PEG concentration has strongly influenced the size as well the shape of nanoflowers. The TEM analysis confirmed the nanoflower morphology and the presence of diffraction planes obtained from the XRD data. The compositional analysis was performed by x-ray photoelectron Spectroscopy. The surface passivation effect of PEG on the band gap energies was studied by analysing UV -visible spectra of all the samples. The room-temperature fluorescent spectra of all the nanoflowers showed multiple peak emissions, both in the ultra-violet and visible regions, with varying intensities. These recasted multiple peaks are attributed to the morphological modification caused by the PEG addition.

An Eco-friendly Approach to ZnO NP Synthesis Using Citrus reticulata Blanco Peel/Extract: Characterization and Antibacterial and Photocatalytic Activity

Emission of greenhouse gases and infectious diseases caused by improper agro-waste disposal has gained significant attention in recent years. To overcome these hurdles, agro-waste can be valorized into valuable bioactive compounds that act as reducing or stabilizing agents in the synthesis of nanomaterials. Herein, we report a simple circular approach using Citrus reticulata Blanco (C. reticulata) waste (peel powder/aqueous extract) as green reducing and capping/stabilizing agents and Zn nitrate/acetate precursors to synthesize ZnO nanoparticles (NPs) with efficient antimicrobial and photocatalytic activities. The obtained NPs crystallized in a hexagonal wurtzite structure and differed clearly in their morphology. UV-vis analysis of the nanoparticles showed a characteristic broad absorption band between 330 and 414 nm belonging to ZnO NPs. Fourier transform infrared (FTIR) spectroscopy of ZnO NPs exhibited a Zn-O band close to 450 cm-1. The band gap values were in the range of 2.84-3.14 eV depending on the precursor and agent used. The crystallite size obtained from size-strain plots from measured XRD patterns was between 7 and 26 nm, with strain between 16 and 4%. The highly crystalline nature of obtained ZnO NPs was confirmed by clear ring diffraction patterns and d-spacing values of the observed lattice fringes. ZnNPeelMan_400 and ZnNExtrMan showed good stability, as the zeta potential was found to be around -20 mV, and reduced particle aggregation. Photoluminescence analysis revealed different defects belonging to oxygen vacancies (VO+ and VO+2) and zinc interstitial (Zni) sites. The presence of oxygen vacancies on the surface of ZnAcExtrMan_400 and ZnAcPeelMan_400 increased antimicrobial activity, specifically against Gram-negative bacteria Escherichia coli (E. coli) and Salmonella enteritidis (S. enteritidis). ZnNExtrMan with a minimal inhibitory concentration of 0.156 mg/mL was more effective against Gram-positive bacteria Staphylococcus aureus (S. aureus), revealing a high influence of particle size and shape on antimicrobial activity. In addition, the photocatalytic activity of the ZnO NPs was examined by assessing the degradation of acid green dye in an aqueous solution under UV light irradiation. ZnAcPeelMan_400 exhibited excellent photocatalytic activity (94%) within 90 min after irradiation compared to other obtained ZnO NPs.

Modification of wood lignin and integration with multifunctional polyester nanocomposite

A simple strategy was introduced to develop fluorescent wood with the ability to alter its color when exposed to both visible and ultraviolet lights. Injecting a combination of europium and dysprosium doped aluminate (EDA; 7-12 nm) nanoparticles and polyester resin (PET) into a lignin-modified wood (LMW) produced a translucent smart wooden window with fluorescence and afterglow emission properties. In order to prevent formation of aggregates and improve the preparation process of transparent woods, EDA must be properly disseminated in the polyester matrix. We analyzed the fluorescent wood samples using a variety of spectroscopic and microscopic methods, including energy-dispersive X-ray (EDX), scanning electron microscopy (SEM), photoluminescence spectra, and hardness tests. We found that the photoluminescent woods had an excitation peak at 365 nm and emission peaks at 437 nm and 517 nm. The translucent luminous woods showed rapid and reversible emission response to ultraviolet light. Fluorescence emission was detected for samples with lower EDA content, and afterglow emission was detected for wood samples with higher EDA content. Increases in EDA content were associated with improvements in water resistance and ultraviolet radiation protection in the EDA@PET-infiltrated wood.

Influence of Exposure to a Wet Atmosphere on the UV-Sensing Characteristics of ZnO Nanorod Arrays

Zinc oxide is a promising material for the creation of various types of sensors, in particular UV detectors. In this work, arrays of ordered nanorods were grown by chemical vapor deposition. The effect of environmental humidity on the sensing properties of zinc oxide nanorod arrays was investigated, and a prototype UV sensor using indium as an ohmic contact was developed. UV photoresponses were measured for the samples stored in dry and wet atmospheres. The increase in sensitivity and response of the ZnO nanorod arrays was observed after prolonged exposure to a wet atmosphere. A model was proposed to explain this effect. This is due to the formation of hydroxyl groups on the surface of zinc oxide nanorods, which is confirmed by FTIR spectroscopy data. For the first time, it has been shown that after storage in a wet atmosphere, the sensory properties of the structure remain stable regardless of the ambient humidity.

Control of ZnO nanowires growth in flexible perovskite solar cells: A mini-review

Due to their excellent properties, Zinc oxide nanowires (ZnO NW) have been attractive and considered as a promising electron-transporting layer (ETL) in flexible Perovskite Solar Cells (FPSCs). Since the first report on ZnO NWs-based FPSCs giving 2.6 % power conversion efficiency (in 2013), great improvements have been made, allowing to reach up to∼15 % nowadays. However, some issues still need to be addressed, especially on flexible substrates, to achieve uniform and well-aligned ZnO NWs via low-cost chemical solution techniques. Several parameters, such as the growing method (time, temperature, precursors concentration), addition of seed layer (thickness, roughness, annealing temperature) and substrate (rigid or flexible), play a crucial role in ZnO NWs properties (i.e., length, diameter, density and aspect ratio). In this review, these parameters allowing to control the properties of ZnO NWs, like the growth techniques, utilization of seed layers and the growing method (time or precursors concentration) have been summarized. Then, a particular focus on the ZnO NW's role in FPSCs as well as the use of these results on the development of ZnO NWs-based FPSCs have been highlighted.

Progress towards blue emitting MgO-ZnO-Ga2O3 nanocomposites synthesized by bio mediated route

MgO-ZnO-Ga2O3 nanocomposites are synthesized by solution combustion method using Aloe Vera gel as a reducing agent to increase the efficiency of blue emission. The appearance of Bragg reflections corresponding to MgO, ZnO and Ga2O3 clearly indicates the formation of nanocomposites. The surface morphology consists irregular shape and sized NPs. The Energy dispersive X-ray analysis confirms the purity of the sample. The band energy gap was tuned to 3.1 eV. The Photoluminescence excitation and emission spectra was discussed and compared it with emission spectra of individual oxides as well as with other reported blue emitted nanophosphors. Further, the chromaticity coordinates and Color correlated temperature coordinates clearly confirms their warm blue emission. Further, the powder dusting method was employed to collect the latent fingerprints on the pores and non-pores surfaces. The synthesized MgO-ZnO-Ga2O3 nanocomposites exhibits well-resolved ridge patterns that can be used to identify latent finger prints with clarity. From all these results, the present synthesized MgO-ZnO-Ga2O3 nanocomposite might find an application in display technology as a blue nanophosphor material and for latent finger print detection in crime investigation.

Antibacterial Efficacy of ZnO/Bentonite (Clay) Nanocomposites against Multidrug-Resistant Escherichia coli

The emergence of multidrug-resistant (MDR) bacteria has spurred the exploration of therapeutic nanomaterials such as ZnO nanoparticles. However, the inherent nonspecific toxicity of ZnO has posed a significant obstacle to their clinical utilization. In this research, we propose a novel approach to improve the selectivity of the toxicity of ZnO nanoparticles by impregnating them onto a less toxic clay mineral, Bentonite, resulting in ZB nanocomposites (ZB NCs). We hypothesize that these ZB NCs not only reduce toxicity toward both normal and carcinogenic cell lines but also retain the antibacterial properties of pure ZnO nanoparticles. To test this hypothesis, we synthesized ZB NCs by using a precipitation technique and confirmed their structural characteristics through X-ray diffraction and Raman spectroscopy. Electron microscopy revealed composite particles in the size range of 20-50 nm. The BET surface area of ZB NCs, within a relative pressure (P/P0) range of 0.407-0.985, was estimated to be 31.182 m2/g. Notably, 50 mg/mL ZB NCs demonstrated biocompatibility with HCT 116 and HEK 293 cell lines, supported by flow cytometry and fluorescence microscopy analysis. In vitro experiments further confirmed a remarkable five-log reduction in the population of MDR Escherichia coli in the presence of 50 mg/mL of ZB NCs. Antibacterial activity of the nanocomposites was also validated in the HEK293 and HCT 116 cell lines. These findings substantiate our hypothesis and underscore the effectiveness of ZB NCs against MDR E. coli while minimizing nonspecific toxicity toward healthy cells.

Green Synthesis of Er-Doped ZnO Nanoparticles: An Investigation on the Methylene Blue, Eosin, and Ibuprofen Removal by Photodegradation

We present a study on the green synthesis of undoped and Er-doped ZnO compounds using Mangifera indica gum (MI). A set of tests were conducted to assess the structure of the material. The tests included X-ray diffraction, Raman, and Fourier-transform infrared spectroscopy. Optical properties were studied using diffuse reflectance and photoluminescence. Morphological and textural investigations were done using SEM images and N2 adsorption/desorption. Furthermore, photocatalytic tests were performed with methylene blue (MB), yellow eosin (EY), and the pharmaceutical drug ibuprofen (IBU) under UV irradiation. The study demonstrated that replacing the stabilizing agent with Mangifera indica gum is an effective method for obtaining ZnO nanoparticles. Additionally, the energy gap of the nanoparticles exhibits a slight reduction in value. Photoluminescence studies showed the presence of zinc vacancies and other defects in both samples. In the photocatalytic test, the sample containing Er3+ exhibited a degradation of 99.7% for methylene blue, 81.2% for yellow eosin, and 52.3% for ibuprofen over 120 min. In the presence of methyl alcohol, the degradation of MB and EY dyes is 16.7% and 55.7%, respectively. This suggests that hydroxyl radicals are responsible for the direct degradation of both dyes. In addition, after the second reuse, the degradation rate for MB was 94.08%, and for EY, it was 82.35%. For the third reuse, the degradation rate for MB was 97.15%, and for EY, it was 17%. These results indicate the significant potential of the new semiconductor in environmental remediation applications from an ecological synthesis.

Rapid Growth of Metal-Metal Oxide Core-Shell Structures through Joule Resistive Heating: Morphological, Structural, and Luminescence Characterization

The aim of this study is to prove that resistive heating enables the synthesis of metal/metal oxide composites in the form of core-shell structures. The thickness and morphology of the oxide layer depends strongly on the nature of the metal, but the influences of parameters such as the time and current profiles and the presence of an external field have also been investigated. The systems chosen for the present study are Zn/ZnO, Ti/TiO2, and Ni/NiO. The characterization of the samples was performed using techniques based on scanning electron microscopy (SEM). The thicknesses of the oxide layers varied from 10 μm (Zn/ZnO) to 50 μm (Ni/NiO). In the case of Zn- and Ti-based composites, the growth of nanostructures on the oxide layer was observed. Micro- and nanoneedles formed on the ZnO layer while prism-like structures appeared on the TiO2. In the case of the NiO layer, micro- and nanocrystals were observed. Applying an external electric field seemed to align the ZnO needles, whereas its effect on TiO2 and NiO was less appreciable, principally affecting the shape of their grain boundaries. The chemical compositions were analysed using X-ray spectroscopy (EDX), which confirmed the existence of an oxide layer. Structural information was obtained by means of X-ray diffraction (XRD) and was later checked using Raman spectroscopy. The oxide layers seemed to be crystalline and, although some non-stoichiometric phases appeared, the stoichiometric phases were predominant; these were wurtzite, rutile, and cubic for Zn, Ti, and Ni oxides, respectively. The photoluminescence technique was used to study the distribution of defects on the shell, and mainly visible bands (2-2.5 eV), attributed to oxygen vacancies, were present. The near-band edges of ZnO and TiO2 were also observed around 3.2-3.3 eV.

Surface modification of ZnO nanowires using single walled carbon nanotubes for efficient UV-visible broadband photodetection

A UV-visible broadband photodetector (PD) based on single walled carbon nanotube (SWCNT)/Zinc oxide nanowire (ZnO NW) hybrid is being reported. This work focuses on designing a stable, fast, efficient and reliable hybrid broadband PD by surface modification of ZnO NWs using SWCNT. The study shows that spectral response of the hybrid heterostructure (HS) spans beyond the UV spectrum and into the visible region which is due to the integration of SWCNTs. Photoluminescence (PL) study reveals surface plasmon (SP) mediated resonance phenomenon resulting in an increase in PL intensity. High nanotube charge carrier mobility and conductivity allows the hybrid HS to attain high values of spectral responsivity (Rλ= 187.77 A W-1), external quantum efficiency (EQE = 5.82 × 104%), specific detectivity (D* = 7.04 × 1011Jones) and small noise equivalent power (NEP = 4.77 × 10-12W) values for the SWCNT/ZnO NW hybrid HS. The device also gives quick rise (trise= 0.43 s) and fall (tfall= 0.60 s) time values.

Nanostructured Channel for Improving Emission Efficiency of Hybrid Light-Emitting Field-Effect Transistors

We report on the mechanism of enhancing the luminance and external quantum efficiency (EQE) by developing nanostructured channels in hybrid (organic/inorganic) light-emitting transistors (HLETs) that combine a solution-processed oxide and a polymer heterostructure. The heterostructure comprised two parts: (i) the zinc tin oxide/zinc oxide (ZTO/ZnO), with and without ZnO nanowires (NWs) grown on the top of the ZTO/ZnO stack, as the charge transport layer and (ii) a polymer Super Yellow (SY, also known as PDY-132) layer as the light-emitting layer. Device characterization shows that using NWs significantly improves luminance and EQE (≈1.1% @ 5000 cd m-2) compared to previously reported similar HLET devices that show EQE < 1%. The size and shape of the NWs were controlled through solution concentration and growth time, which also render NWs to have higher crystallinity. Notably, the size of the NWs was found to provide higher escape efficiency for emitted photons while offering lower contact resistance for charge injection, which resulted in the improved optical performance of HLETs. These results represent a significant step forward in enabling efficient and all-solution-processed HLET technology for lighting and display applications.

Investigation of Photoluminescence and Optoelectronics Properties of Transition Metal-Doped ZnO Thin Films

Thin films of zinc oxide (ZnO) doped with transition metals have recently gained significant attention due to their potential applications in a wide range of optoelectronic devices. This study focuses on ZnO thin films doped with the transition metals Co, Fe, and Zr, exploring various aspects of their structural, morphological, optical, electrical, and photoluminescence properties. The thin films were produced using RF and DC co-sputtering techniques. The X-ray diffraction (XRD) analysis revealed that all the doped ZnO thin films exhibited a stable wurtzite crystal structure, showcasing a higher structural stability compared to the undoped ZnO, while the atomic force microscopy (AFM) imaging highlighted a distinctive granular arrangement. Energy-dispersive X-ray spectroscopy was employed to confirm the presence of transition metals in the thin films, and Fourier-transform infrared spectroscopy (FTIR) was utilized to investigate the presence of chemical bonding. The optical characterizations indicated that doping induced changes in the optical properties of the thin films. Specifically, the doped ZnO thin film's bandgap experienced a significant reduction, decreasing from 3.34 to 3.30 eV. The photoluminescence (PL) analysis revealed distinguishable emission peaks within the optical spectrum, attributed to electronic transitions occurring between different bands or between a band and an impurity. Furthermore, the introduction of these transition metals resulted in decreased resistivity and increased conductivity, indicating their positive influence on the electrical conductivity of the thin films. This suggests potential applications in solar cells and light-emitting devices.

Facile Formation of Sulfurized Nanorod-Like ZnO/Zn(OH)2 and Hierarchical Flower-Like γ-Zn(OH)2 /ϵ-Zn(OH)2 from a Green Synthesis and Application as Luminescent Solar Concentrator

This research endeavors to overcome the significant challenge of developing materials that simultaneously possess photostability and photosensitivity to UV-visible irradiation. Sulfurized nanorod (NR)-like ZnO/Zn(OH)2 and hierarchical flower-like γ-Zn(OH)2 /ϵ-Zn(OH)2 were identified from XRD diffraction patterns and Raman vibrational modes. The sulfurized material, observed by FEG-SEM and TEM, showed diameters ranging from 10 and 40 nm and lengths exceeding 200 nm. The S2- ions intercalated Zn2+ , modulating NRs to dumbbell-like microrods. SAED and HRTEM illustrated the atomic structure in (101) crystal plane. Its direct band gap of 3.0 eV was attributed to the oxygen vacancies, which also contribute to the deep-level emissions at 422 and 485 nm. BET indicated specific surface area of 4.4 m2  g-1 and pore size as mesoporosity, which are higher compared to the non-sulfurized analogue. These findings were consistent with the observed photocurrent, photostability and photoluminescence (PL), further supporting the suitability of sulfurized NR-like ZnO/Zn(OH)2 as a promising candidate for Luminescent solar concentrators (LSC)-photovoltaic (PV) system.

A Molecular Dynamics Simulation Study of In- and Cross-Plane Thermal Conductivity of Bilayer Graphene

Efficient thermal management of modern electronics requires the use of thin films with highly anisotropic thermal conductivity. Such films enable the effective dissipation of excess heat along one direction while simultaneously providing thermal insulation along the perpendicular direction. This study employs non-equilibrium molecular dynamics to investigate the thermal conductivity of bilayer graphene (BLG) sheets, examining both in-plane and cross-plane thermal conductivities. The in-plane thermal conductivity of 10 nm × 10 nm BLG with zigzag and armchair edges at room temperature is found to be around 204 W/m·K and 124 W/m·K, respectively. The in-plane thermal conductivity of BLG increases with sheet length. BLG with zigzag edges consistently exhibits 30-40% higher thermal conductivity than BLG with armchair edges. In addition, increasing temperature from 300 K to 600 K decreases the in-plane thermal conductivity of a 10 nm × 10 nm zigzag BLG by about 34%. Similarly, the application of a 12.5% tensile strain induces a 51% reduction in its thermal conductivity compared to the strain-free values. Armchair configurations exhibit similar responses to variations in temperature and strain, but with less sensitivity. Furthermore, the cross-plane thermal conductivity of BLG at 300 K is estimated to be 0.05 W/m·K, significantly lower than the in-plane results. The cross-plane thermal conductance of BLG decreases with increasing temperatures, specifically, at 600 K, its value is almost 16% of that observed at 300 K.

Improvement of the optical, photocatalytic and antibacterial properties of ZnO semiconductor nanoparticles using different pepper aqueous extracts

Peppers are fruits that grow on plants of the genus Capsicum and are popular for their use in gastronomy as a condiment and for their anti-inflammatory and anti-cancer properties due to their phytocompounds such as flavonoids, polyphenols, or alkaloids. Semiconductor zinc oxide (ZnO) nanoparticles (NPs) were synthesized using a green approach employing natural aqueous extracts of several varieties of peppers (jalapeño, morita, and ghost). The obtained NPs were characterized by different techniques, and their photocatalytic and antibacterial activity was studied. The signal at 620 cm-1 in the FTIR spectra belonging to the Zn-O bond, the appearance of the main peaks of a hexagonal wurtzite structure in the XRD pattern, and the characteristic signals in the UV-Vis spectra confirm the correct formation of ZnO NPs. The photocatalytic activity was analyzed against Methylene Blue (MB), Rhodamine B (RB), and Methyl Orange (MO) under UV and sunlight. All syntheses were able to degrade more than 93% of the pollutants under UV light. Antibacterial assays were performed against gram-positive and gram-negative bacteria. All syntheses exhibited antibacterial activity against all bacteria and maximum growth inhibition against Bacillus subtilis. The prominent results demonstrate that natural aqueous extracts obtained from peppers can be used to synthesize ZnO NPs with photocatalytic and biomedical applications.

Architecture design of TiO2 with Co-doped CdS quantum dots photoelectrode for water splitting

Photoelectrochemical hydrogen production is a critical key to solving the carbon-zero goal of countries due to renewable sources of solar light and combustion products of hydrogen-only water. Here, an architecture design for an n-type nano rosettes-rod TiO2 (RT) surface using CdS and Co-doped CdS quantum dots (QDs) is carried out utilizing the SILAR (simple ionic layer adsorption and reaction) method. Furthermore, the photocatalytic behaviour of Co-doped CdS QDs SILAR cycles deposition is investigated in various cycles, including 5, 8, 10, and 12. The FESEM, Raman XRD, Uv-Vis spectrometer, and vibration modes are used to evaluate the photoelectrode surface structure, crystal structure, and solar light absorption, respectively. FESEM images and XRD pattern revealed successive CdS QDS and Co-doped CdS QDs deposition on the RT boundary and rising SILAR cycles of Co-doped CdS QDs lead to further coverage of RT surface. UV-vis spectrometer indicated shifting solar light absorption to the visible region by applying more SILAR cycles of Co-doped CdS QDs deposition. The electrochemical parameters obtained from EIS showed total polarization resistance (Rp) of the RT electrode dramatically decreased with 10 SILAR cycle Co-doped CdS QDs deposition (5093 Ω cm2 and 617 Ω cm2). Linear sweep voltammetry (LSV) and chronoamperometric photocatalytic performance measurements indicated Co-doped CdS QDs on RT extremely enhanced photoresponse under solar irradiation and 10 SILAR cycle Co-doped CdS QDs improved photocurrent density about fourfold according to blank RT electrode.

Engineering the defect distribution in ZnO nanorods through laser irradiation

In recent years, defect engineering has shown great potential to improve the properties of metal oxide nanomaterials for various applications thus received extensive investigations. While traditional techniques mostly focus on controlling the defects during the synthesis of the material, laser irradiation has emerged as a promising post-deposition technique to further modulate the properties of defects yet there is still limited information. In this article, defects such as oxygen vacancies are tailored in ZnO nanorods through nanosecond (ns) laser irradiation. The relation between laser parameters and the temperature rise in the ZnO due to laser heating was established based on the observation in the SEM and the simulation. Raman spectra indicated that the concentration of the oxygen vacancies in the ZnO is temperature-dependent and can be controlled by changing the laser fluence and exposure time. This is also supported by the absorption spectra and the photoluminescence spectra of ZnO NRs irradiated under these conditions. On the other hand, the distribution of the oxygen vacancies was studied by XPS depth profiling, and it was confirmed that the surface-to-bulk ratio of the oxygen vacancies can be modulated by varying the laser fluence and exposure time. Based on these results, four distinctive regimes containing different ratios of surface-to-bulk oxygen vacancies have been identified. Laser-processed ZnO nanorods were also used as the catalyst for the photocatalytic degradation of rhodamine B (RhB) dye to demonstrate the efficacy of this laser engineering technique.

Electrical Activity and Extremes of Individual Suspended ZnO Nanowires for 3D Nanoelectronic Applications

We explored the electrical activity and extremes inside individual suspended zinc oxide (ZnO) nanowires (NWs) (diameter: 50-550 nm, length: 5-50 μm) subjected to high forward bias-induced Joule heating using two-terminal current-voltage measurements. NWs were isolated using a reproducible nanometrology technique, employing a nanomanipulator inside a scanning electron microscope. Schottky behavior is observed between installed tips and ZnO NW. The suspended ZnO NWs exhibited an average electrical resistivity ρ (approximately 2.3 × 10-2 Ω cm) and a high electron density n (exceeding 1.89 × 1018 cm-3), comparable to that of InP NWs, GaN NWs, and InAs NWs (1018∼1019 cm-3), suggesting the potential to drive advancements in high-performance NW devices. A maximum breakdown current density (JBD) of ∼0.14 MA/cm2 and a maximum breakdown power density (PBD) of 6.93 mW/μm3 were obtained, both of which are higher than substrate-bound ZnO NWs and consistent with previously reported results obtained from probed ZnO NWs grown vertically on the substrate. Moreover, we discovered that NWs experienced thermal breakdown due to Joule heating and exploited this breakdown mechanism to further investigate the temperature distribution along the ZnO NWs, as well as its dependence on the electrical properties and thermal conductance of contact electrodes. Thermal conductance was determined to be ∼0.4 nW K-1 and ∼1.66 pW K-1 at the tungsten(W)-ZnO NW and platinum(Pt)-ZnO NW contacts, respectively. In addition, we measured the elastic modulus (130-171 GPa), which closely approximated bulk values. We also estimated the nanoindentation hardness to be between 5 and 10 GPa. This work provides valuable insights into the electrical activity and extreme mechanisms, thus providing a better understanding of the potentials and limitations associated with utilizing suspended NWs in 3D nanodevices.

ZnO-decorated green-synthesized multi-doped carbon dots from Chlorella pyrenoidosa for sustainable photocatalytic carbamazepine degradation

The promising green synthesis of carbon dots (CDs) from microalga Chlorella pyrenoidosa was achieved using simple hydrothermal and microwave-assisted methods. Doping of nanomaterials by nonmetals (N, S, and P) was confirmed by X-ray photoelectron spectroscopy (XPS), while the existence of metals in the CDs was confirmed by inductively coupled plasma optical emission spectroscopy (ICP-OES) and transmission electron microscopy (TEM), and Mg, Ca, K, and Na were found as the dominant doped metals. The novel nanomaterials with excellent photoluminescence (PL) properties were used for the modification of ZnO obtained by a simple hydrothermal process. In this regard, a series of ZnO decorated with multi-doped carbon dots (xCDs) was prepared and their photocatalytic properties were evaluated. The ZnO-xCD photocatalysts were characterized by various advanced techniques including X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), XPS, Brunauer-Emmett-Teller (BET), PL, ultraviolet-visible (UV-vis) spectroscopy and electrochemical impedance spectroscopy (EIS) analysis. The photocatalytic behaviour of the obtained materials was investigated in the degradation of carbamazepine (CBZ). The influence of the synthesis method of xCDs and their content on the activity of the photocatalyst was examined. The photocatalyst ZnO modified with 3% xCDs obtained by the microwave-assisted method revealed the highest effectiveness for CBZ degradation and allowed for a first-order degradation rate of 2.85 times in comparison with non-modified ZnO. The improvement of the photocatalytic process was achieved by support with peroxymonosulphate resulting in up to 3.18 times a first order kinetic rate constant compared with that of simple photocatalysis in the presence of ZnO-xCDs. Taken together, our synthesized multi-doped CDs and their nanohybrids with ZnO, can be considered as promising candidates for photocatalytic applications.

Facile synthesis of robust Ag/ZnO composites by sol-gel autocombustion and ion-impregnation for the photocatalytic degradation of sucrose

Metallic Ag nanoparticles decorated on ZnO photocatalysts were prepared by facile sol-gel autocombustion followed by ion-impregnation. Electron microscopy studies revealed the presence of impregnated Ag as nanoparticles on ZnO surfaces, which affected the microstructure of ZnO particles. XRD patterns of Ag/ZnO composites confirmed the metallic phase of Ag. No peak shift for ZnO phase peaks suggests that the impregnated Ag was barely incorporated into the ZnO lattice. Consequently, DRS spectra of Ag/ZnO composites revealed the same absorption edges and E_g for pure and Ag/ZnO. The photocatalytic activity of Ag/ZnO composites for sucrose degradation under UV light was 40% higher than that of pure ZnO. Metallic Ag nanoparticles on the ZnO surface suppressed the surface defects and the recombination of photoexcited electrons and holes. The highest activity with 100% degradation of 100 ppm sucrose (1200 µg of carbon) within 105 min was achieved using ZnO with 10% w/w Ag (10% Ag/ZnO). Ag L3-edge XANES spectra of fresh and spent Ag/ZnO catalysts confirmed the stability of metallic Ag after the usage. The Ag/ZnO catalyst could be used for 5 cycles without losing photocatalytic activity. The Ag/ZnO catalyst was utilized to degrade sugar-contaminated condensate from the sugar mill. 10% Ag/ZnO revealed the highest photocatalytic performance, capable of degrading 90% of sugar in the condensate within 90 min.

Comparison of the Degradation Effect of Methylene Blue for ZnO Nanorods Synthesized on Silicon and Indium Tin Oxide Substrates

In the context of ZnO nanorods (NRs) grown on Si and indium tin oxide (ITO) substrates, this study aimed to compare their degradation effect on methylene blue (MB) at different concentrations. The synthesis process was carried out at a temperature of 100 °C for 3 h. After the synthesis of ZnO NRs, their crystallization was analyzed using X-ray diffraction (XRD) patterns. The XRD patterns and top-view SEM observations demonstrate variations in synthesized ZnO NRs when different substrates were used. Furthermore, cross-sectional observations reveal that ZnO NRs synthesized on an ITO substrate exhibited a slower growth rate compared to those synthesized on a Si substrate. The as-grown ZnO NRs synthesized on the Si and ITO substrates exhibited average diameters of 110 ± 40 nm and 120 ± 32 nm and average lengths of 1210 ± 55 nm and 960 ± 58 nm, respectively. The reasons behind this discrepancy are investigated and discussed. Finally, synthesized ZnO NRs on both substrates were utilized to assess their degradation effect on methylene blue (MB). Photoluminescence spectra and X-ray photoelectron spectroscopy were employed to analyze the quantities of various defects of synthesized ZnO NRs. The effect of MB degradation after 325 nm UV irradiation for different durations can be evaluated using the Beer-Lambert law, specifically by analyzing the 665 nm peak in the transmittance spectrum of MB solutions with different concentrations. Our findings reveal that ZnO NRs synthesized on an ITO substrate exhibited a higher degradation effect on MB, with a rate of 59.5%, compared to NRs synthesized on a Si substrate, which had a rate of 73.7%. The reasons behind this outcome, elucidating the factors contributing to the enhanced degradation effect are discussed and proposed.

Various Applications of ZnO Thin Films Obtained by Chemical Routes in the Last Decade

This review addresses the importance of Zn for obtaining multifunctional materials with interesting properties by following certain preparation strategies: choosing the appropriate synthesis route, doping and co-doping of ZnO films to achieve conductive oxide materials with p- or n-type conductivity, and finally adding polymers in the oxide systems for piezoelectricity enhancement. We mainly followed the results of studies of the last ten years through chemical routes, especially by sol-gel and hydrothermal synthesis. Zinc is an essential element that has a special importance for developing multifunctional materials with various applications. ZnO can be used for the deposition of thin films or for obtaining mixed layers by combining ZnO with other oxides (ZnO-SnO2, ZnO-CuO). Also, composite films can be achieved by mixing ZnO with polymers. It can be doped with metals (Li, Na, Mg, Al) or non-metals (B, N, P). Zn is easily incorporated in a matrix and therefore it can be used as a dopant for other oxidic materials, such as: ITO, CuO, BiFeO3, and NiO. ZnO can be very useful as a seed layer, for good adherence of the main layer to the substrate, generating nucleation sites for nanowires growth. Thanks to its interesting properties, ZnO is a material with multiple applications in various fields: sensing technology, piezoelectric devices, transparent conductive oxides, solar cells, and photoluminescence applications. Its versatility is the main message of this review.

Highly C-oriented (002) plane ZnO nanowires synthesis

Nanowires are widely used for energy harvesting, sensors, and solar cells. We report a study on the role of buffer layer in the growth of zinc oxide (ZnO) nanowires (NWs) synthesised by a chemical bath deposition (CBD) method. To control the thickness of the buffer layer, multilayer coatings corresponding to one layer (100 nm thick), three layers (300 nm thick), and six layers (600 nm thick) of ZnO sol-gel thin-films were used. The evolution of the morphology and structure of ZnO NWs was characterized by scanning electron microscopy, X-ray diffraction, photoluminescence, and Raman spectroscopy. Highly C-oriented ZnO (002)-oriented NWs were obtained on both substrates, silicon and ITO, when the thickness of the buffer layer was increased. The role of ZnO sol-gel thin films used as a buffer layer for the growth of ZnO NWs with (002)-oriented grains also resulted in a significant change in surface morphology on both substrates. The successful deposition of ZnO NWs on a variety of substrates, as well as the promising results, open up a wide range of applications.

UV-shielding properties of a cost-effective hybrid PMMA-based thin film coatings using TiO2 and ZnO nanoparticles: a comprehensive evaluation

This study aimed to assess the UV-shielding features of the PMMA-based thin film coatings with the addition of TiO₂ and ZnO nanoparticles as nanofillers considering different contents. Furthermore, the effect of TiO₂/ZnO nanohybrids at different ratios and concentrations was examined. The XRD, FTIR, SEM, and EDX analyses characterized the prepared films' functional groups, structure, and morphology. Meanwhile, the coatings' optical properties and UV-protecting capability were investigated by ultraviolet-visible (UV-Vis) spectroscopy. The UV-Vis spectroscopic study revealed that as the concentration of nanoparticles increased in the hybrid-coated PMMA, the absorption in the UVA region increased. Overall, it can be concluded that the optimal coatings for PMMA were 0.1 wt% TiO₂, 0.1 wt% ZnO, and 0.025:0.025 wt% TiO₂: ZnO nanohybrid. Considering the acquired FT-IR of PMMA with different content of nanoparticles before and after exposure to the UV irradiation, for some films, it was confirmed that the polymer-based thin films degraded after 720 h, with either decreasing or increasing intensity of the degraded polymer, peak shifting, and band broadening. Notably, the FTIR results were in good agreement with UV-Vis outcomes. In addition, XRD diffraction peaks demonstrated that the pure PMMA matrix and PMMA coating films did not show any characteristic peaks indicating the presence of nanoparticles. All diffraction patterns were similar with and without any nanoparticles. Therefore, it depicted the amorphous nature of polymer thin film.

Highly Active Nanocrystalline ZnO and Its Photo-Oxidative Properties towards Acetone Vapor

Zinc oxide is one of the well-known photocatalysts, the potential applications of which are of great importance in photoactivated gas sensing, water and air purification, photocatalytic synthesis, among others. However, the photocatalytic performance of ZnO strongly depends on its morphology, composition of impurities, defect structure, and other parameters. In this paper, we present a route for the synthesis of highly active nanocrystalline ZnO using commercial ZnO micropowder and ammonium bicarbonate as starting precursors in aqueous solutions under mild conditions. As an intermediate product, hydrozincite is formed with a unique morphology of nanoplates with a thickness of about 14-15 nm, the thermal decomposition of which leads to the formation of uniform ZnO nanocrystals with an average size of 10-16 nm. The synthesized highly active ZnO powder has a mesoporous structure with a BET surface area of 79.5 ± 4.0 m2/g, an average pore size of 20 ± 2 nm, and a cumulative pore volume of 0.507 ± 0.051 cm3/g. The defect-related PL of the synthesized ZnO is represented by a broad band with a maximum at 575 nm. The crystal structure, Raman spectra, morphology, atomic charge state, and optical and photoluminescence properties of the synthesized compounds are also discussed. The photo-oxidation of acetone vapor over ZnO is studied by in situ mass spectrometry at room temperature and UV irradiation (λmax = 365 nm). The main products of the acetone photo-oxidation reaction, water and carbon dioxide, are detected by mass spectrometry, and the kinetics of their release under irradiation are studied. The effect of morphology and microstructure on the photo-oxidative activity of ZnO samples is demonstrated.

Laser-Induced Au Catalyst Generation for Tailored ZnO Nanostructure Growth

ZnO nanostructures, semiconductors with attractive optical properties, are typically grown by thermal chemical vapor deposition for optimal growth control. Their growth is well investigated, but commonly results in the entire substrate being covered with identical ZnO nanostructures. At best a limited, binary growth control is achieved with masks or lithographic processes. We demonstrate nanosecond laser-induced Au catalyst generation on Si(100) wafers, resulting in controlled ZnO nanostructure growth. Scanning electron and atomic force microscopy measurements reveal the laser pulse's influence on the substrate's and catalyst's properties, e.g., nanoparticle size and distribution. The laser-induced formation of a thin SiO₂-layer on the catalysts plays a key role in the subsequent ZnO growth mechanism. By tuning the irradiation parameters, the width, density, and morphology of ZnO nanostructures, i.e., nanorods, nanowires, and nanobelts, were controlled. Our method allows for maskless ZnO nanostructure designs locally controlled on Si-wafers.

Photoresponsive Activity of the Zn0.94Er0.02Cr0.04O Compound with Hemisphere-like Structure Obtained by Co-Precipitation

In this work, a ZnO hemisphere-like structure co-doped with Er and Cr was obtained by the co-precipitation method for photocatalytic applications. The dopant's effect on the ZnO lattice was investigated using X-ray diffraction, Raman, photoluminescence, UV-Vis and scanning electron microscopy/energy dispersive spectroscopy techniques. The photocatalytic response of the material was analyzed using methylene blue (MB) as the model pollutant under UV irradiation. The wurtzite structure of the Zn_0.94Er_0.02Cr_0.04O compound presented distortions in the lattice due to the difference between the ionic radii of the Cr³⁺, Er³⁺ and Zn²⁺ cations. Oxygen vacancy defects were predominant, and the energy competition of the dopants interfered in the band gap energy of the material. In the photocatalytic test, the MB degradation rate was 42.3%. However, using optimized H₂O₂ concentration, the dye removal capacity reached 90.1%. Inhibitor tests showed that ^•OH radicals were the main species involved in MB degradation that occurred without the formation of toxic intermediates, as demonstrated in the ecotoxicity assays in Artemia salina. In short, the co-doping with Er and Cr proved to be an efficient strategy to obtain new materials for environmental remediation.

Effect of (Ce, Dy) co-doping on the microstructural, optical, and photoluminescence characteristics of solution combustion synthesized ZnO nanoparticles

Solution combustion synthesized ZnO nanoparticles that were Ce doped, Dy doped or co-doped at varying dopant concentrations were characterized for their microstructural, optical, and photoluminescence (PL) characteristics. The synthesized nanoparticles matched the standard hexagonal wurtzite structure of ZnO. The lattice fringes in the high-resolution transmission electron micrographs and the bright spotty rings in the selected area electron diffraction patterns authenticated the high crystallinity of the nanoparticles. The diffuse reflectance spectroscopy resolved the energy bandgap for the undoped ZnO as 3.18 eV, which decreased upon doping and co-doping. A sharp narrow ultraviolet emission peak at ~398 nm that originated from excitonic recombination was found in the PL spectra of the nanoparticles. The visible emission peaks in the PL spectra were assigned to the f-d and f-f electron transitions of Ce³⁺ and Dy³⁺ ions, respectively, in addition to different native defects in ZnO. The visible emissions (blue, yellow, and red) improved upon (Ce, Dy) co-doping, therefore (Ce, Dy) co-doped ZnO nanoparticles can be considered a promising luminescent material for the development of energy-saving light sources.

Self-Organization Effects of Thin ZnO Layers on the Surface of Porous Silicon by Formation of Energetically Stable Nanostructures

The formation of complex surface morphology of a multilayer structure, the processes of which are based on quantum phenomena, is a promising domain of the research. A hierarchy of pore of various sizes was determined in the initial sample of porous silicon by the atomic force microscopy. After film deposition by spray pyrolysis, ZnO nanoclusters regularly distributed over the sample surface were formed. Using the electron paramagnetic resonance (EPR) method it was determined that the localization of paramagnetic centers occurs more efficiently as a result of the ZnO deposition. An increase in the number of deposited layers, leads to a decrease in the paramagnetic center relaxation time, which is probably connected with the formation of ZnO nanocrystals with energetically stable properties. The nucleation and formation of nanocrystals is associated with the interaction of particles with an uncompensated charge. There is no single approach to determine the mechanism of this process. By the EPR method supplemented with the signal cyclic saturation, spectral manifestations from individual centers were effectively separated. Based on electron paramagnetic resonance and photoluminescence studies it was revealed that the main transitions between energy levels are due to oxygen vacancies and excitons.

Deep-Level Emission Tailoring in ZnO Nanostructures Grown via Hydrothermal Synthesis

Zinc oxide (ZnO) nanostructures are widely used in various fields of science and technology due to their properties and ease of fabrication. To achieve the desired characteristics for subsequent device application, it is necessary to develop growth methods allowing for control over the nanostructures' morphology and crystallinity governing their optical and electronic properties. In this work, we grow ZnO nanostructures via hydrothermal synthesis using surfactants that significantly affect the growth kinetics. Nanostructures with geometry from nanowires to hexapods are obtained and studied with photoluminescence (PL) spectroscopy. Analysis of the photoluminescence spectra demonstrates pronounced exciton on a neutral donor UV emission in all of the samples. Changing the growth medium chemical composition affects the emission characteristics sufficiently. Apart the UV emission, nanostructures synthesized without the surfactants demonstrate deep-level emission in the visible range with a peak near 620 nm. Structures synthesized with the use of sodium citrate exhibit emission peak near 520 nm, and those with polyethylenimine do not exhibit the deep-level emission. Thus, we demonstrate the correlation between the hydrothermal growth conditions and the obtained ZnO nanostructures' optical properties, opening up new possibilities for their precise control and application in nanophotonics, UV-Vis and white light sources.

AC/DC Electric-Field-Assisted Growth of ZnO Nanowires for Gas Discharge

Using ZnO nanowires as needle anodes in gas discharge is helpful for maintaining continuous discharge with a relatively low voltage. It is necessary that the ZnO nanowires are far enough apart to guarantee no electric field weakening and that the nanowire anodes are easy to assemble together with the discharging devices. An AC/DC electric-field-assisted wet chemical method is proposed in this paper. It was used to grow ZnO nanowires directly on discharging devices. The nanowires covered the whole electrode in the case in which only a DC field was applied. Moreover, the tips of the nanowires were scattered, similar to the results observed under the application of AC fields. The average distance between the tips of the highest nanowires was approximately equal to 4 μm, which almost meets the requirement of gas discharge. The research concerning growing ZnO nanowires directly on PCBs shown that, at the current time, ZnO nanowires on PCBs did not meet the requirements of gas discharge; however, in this study, the parameters regarding ZnO nanowire growth were established.

Structural, Morphological, Electronic Structural, Optical, and Magnetic Properties of ZnO Nanostructures

ZnO nanostructures were grown on a Si(111) substrate using a vapor-liquid-solid (VLS) growth procedure (pristine ZnO) and annealed via a rapid thermal-annealing process in an argon atmosphere at 1100 °C (Ar-ZnO). The synthesized ZnO nanostructures were investigated through structural, electronic structural, morphological, optical, and magnetic characterizations. X-ray diffraction and selective area electron diffraction (SAED) measurements revealed that both samples exhibited the hexagonal wurtzite phase of nanocrystalline ZnO. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy carried out at the O K-edge inferred the presence of the intrinsic-defect states. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy images displayed the formation of ZnO nanostructures. The photoluminescence (PL) spectra demonstrated an emission band in the UV region along with an additional defect band in the visible region. PL spectral analysis confirmed the presence of intrinsic defects in Ar-ZnO nanowires, contributing to the enhanced emission in the visible region. The Raman spectra showed the characteristic band (434 cm^-1) corresponding to the vibrational modes of hexagonal wurtzite ZnO, with an additional band attributable to intrinsic defects. DC magnetization measurements showed a ferromagnetic response in both samples with enhanced coercivity in Ar-ZnO (~280 Oe). In brief, both samples exhibited the presence of intrinsic defects, which are found to be further enhanced in the case of Ar-ZnO. Therefore, it is suggested that intrinsic defects have played an important role in modifying the optical and magnetic properties of ZnO with enhanced results for Ar-ZnO.

Modulation of ZnO Nanostructure for Efficient Photocatalytic Performance

Structure has been considered to play an important role in photocatalytic performance of the semiconductors, but the intrinsic factors were rarely revealed. Herein, ZnO nanomaterials in the structures of thin film, nanowire array and nanosheet array were synthesized, and their structural characteristics, optical properties, photocurrent response and photocatalytic efficiency were compared with each other for illustrating the issue. The photoluminescence intensity decreased in the order of nanosheets, thin film and nanowires for improved lifetime of the photoexcited charges. The absorption of the nanosheets and nanowires improved obviously in the visible range with a redshift of the absorption edge than that of the thin film. The nanowires possessed the highest response current of 82.65 μA at a response time of 2.0 ms in a sensitivity of 87.93 at the light frequency of 1 Hz, and gained the largest catalytic efficiency of 2.45 μg/cm² h for the methylene blue degradation in UV light. Nevertheless, the improvement of catalytic efficiency of the nanosheets (up to 42.4%) was much larger than that of nanowires (5.7%) and thin film (2.6%) for the Au coating. The analysis revealed that the photocatalytic efficiency of the ZnO nanomaterials was modulated by the structure as it contained different surface area, roughness, defect and doping states, vacancies, polar and non-polar crystalline faces, which would provide structural design of semiconductor nanomaterials for the photoelectric and photocatalytic applications.

Effect of h-BN Support on Photoluminescence of ZnO Nanoparticles: Experimental and Theoretical Insight

Herein we report a simple and easily scalable method for fabricating ZnO/h-BN composites with tunable photoluminescence (PL) characteristics. The h-BN support significantly enhances the ultraviolet (UV) emission of ZnO nanoparticles (NPs), which is explained by the ZnO/h-BN interaction and the change in the electronic structure of the ZnO surface. When h-BN NPs are replaced with h-BN microparticles, the PL in the UV region increases, which is accompanied by a decrease in visible light emission. The dependence of the PL properties of ZnO NPs on the thickness of h-BN carriers, observed for the first time, is explained by a change in the dielectric constant of the support. A quantum chemical analysis of the influence of the h-BN thickness on the electron density redistribution at the wZnO/h-BN interface and on the optical properties of the wZnO/h-BN composites was carried out. Density functional theory (DFT) calculations show the appearance of hybridization at the h-BN/wZnO interface and an increase in the intensity of absorption peaks with an increase in the number of h-BN layers. The obtained results open new possibilities for controlling the properties of ZnO/h-BN heterostructures for various optical applications.

Active-ion-gated room temperature acetone gas sensing of ZnO nanowires array

Reducing the high operation temperature of gas sensor to room temperature (RT) have attracted intense interests for its distinct preponderances, including energy-saving and super stability, which presents great prospects in commercial application. The exciting strategies for RT gas sensing, such as unique materials with activated surface or light activation, do not directly modulate the active ions for gas sensing, limiting the RT gas sensing performances. Here, an active-ion-gated strategy has been proposed for RT gas sensing with high performance and low power consumption, in which gas ions in triboelectric plasma are introduced into metal oxide semiconductor (MOS) film to act as both floating gate and active sensing ions. The active-ion-gated ZnO nanowires (NWs) array shows a sensitivity of 38.3% to 10 ppm acetone gas at RT, and the maximum power consumption is only 4.5 mW. At the same time, the gas sensor exhibits excellent selectivity to acetone. More importantly, the response (recovery) time of this sensor is as low as 11 s (25 s). It is found that OH-(H2O)4 ions in plasma are the key for realizing RT gas sensing ability, and an accompanied resistive switch is also observed. It is considered that the electron transfer between OH-(H2O)4 and ZnO NWs will forms a hydroxyl-like intermediate state (OH*) on the top of Zn2+, leading to the band bending of ZnO and activating the reactive O2 - ions on the oxygen vacancies. The active-ion-gated strategy proposed here present a novel exploration to achieving RT gas sensing performance of MOS by activating sensing properties at the scale of ions or atoms.

Tuning the Quantum Properties of ZnO Devices by Modulating Bulk Length and Doping

The quantum transport properties of ZnO devices with five different bulk configurations are investigated with numerical methods. The calculation results reveal that the transport property at a higher energy range can be tuned by changing the length of central scattering. By substituting some Zn atoms with Cu atoms, it is found that the doped Cu atoms have an obvious effect on the quantum properties at the entire energy range investigated, and could result in different transmission. The properties of ZnO devices are also influenced by the doping positions of Cu atoms. The tuning mechanism relies on the shifting of carrier distributions in the scattering center of the device.

Spatially Resolved Dynamics of Cobalt Color Centers in ZnO Nanowires

The dynamics of color centers, being a promising quantum technology, is strongly dependent on the local environment. A synergistic approach of X-ray fluorescence analysis and X-ray excited optical luminescence (XEOL) using a hard X-ray nanoprobe is applied. The simultaneous acquisition provides insights into compositional and functional variations at the nanoscale demonstrating the extraordinary capabilities of these combined techniques. The findings on cobalt doped zinc oxide nanowires show an anticorrelation between the band edge emission of the zinc oxide host and the intra-3d cobalt luminescence, indicating two competing recombination paths. Moreover, time-resolved XEOL measurements reveal two exponential decays of the cobalt luminescence. The fast and newly observed one can be attributed to a recombination cascade within the cobalt atom, resulting from direct excitation. Thus, this opens a new fast timescale for potential devices based on cobalt color centers in ZnO nanowires in photonic circuits.

Syntheses of APTMS-Coated ZnO: An Investigation towards Penconazole Detection

Extrinsic chemiluminescence can be an efficient tool for determining pesticides and fungicides, which do not possess any intrinsic fluorescent signal. On this basis, (3-aminopropyl) trimethoxysilane (APTMS)-coated ZnO (APTMS@ZnO) was synthesized and tested as an extrinsic probe for the fungicide penconazole. Several synthetic routes were probed using either a one-pot or two-steps method, in order to ensure both a green synthetic pathway and a good signal variation for the penconazole concentration. The synthesized samples were characterized using X-ray diffraction (XRD), infrared (IR), Raman and ultraviolet-visible (UV-Vis) spectroscopy, scanning electron microscopy (SEM) imaging and associated energy-dispersive X-ray (EDX) analysis. The average size of the synthesized ZnO nanoparticles (NPs) is 54 ± 10 nm, in line with previous preparations. Of all the samples, those synthesized in two steps, at temperatures ranging from room temperature (RT) to a maximum of 40 °C, using water solvent (G-APTMG@ZnO), appeared to be composed of nanoparticles, homogeneously coated with APTMS. Chemiluminescence tests of G-APTMG@ZnO, in the penconazole concentration range 0.7-1.7 ppm resulted in a quenching of the native signal between 6% and 19% with a good linear response, thus indicating a green pathway for detecting the contaminant. The estimated detection limit (LOD) is 0.1 ± 0.01 ppm.

pH Controlled Nanostructure and Optical Properties of ZnO and Al-Doped ZnO Nanorod Arrays Grown by Microwave-Assisted Hydrothermal Method

The low-temperature microwave-assisted hydrothermal method was used to successfully grow pure and Al-doped ZnO (AZO) nanorod (NR) arrays on glass substrates. The combined effects of doping and pH on the structural properties, surface chemistry, and optical properties of all samples were investigated. Thermodynamic-based simulations of the growth solution were performed and a growth mechanism, that considers the effects of both the pH and Al-doping, is proposed, and discussed. Tuning the solution pH is key parameter to grow well-aligned, single crystal, highly packed, and high aspect ratio nanorod arrays. Moreover, the optical absorption in the visible range is enhanced by controlling the pH value. The PL spectra reveal a shift of the main radiative emission from the band-to-band into a transition involving deep defect levels of Zinc interstitial Zni. This shift is caused by an enhancement of the non-radiative components (phonon relaxation) at high pH values. The production of well-ordered ZnO and AZO nanorod arrays with visible-active absorption/emission centers would increase their potential use in various applications.

Spin-Coating and Aerosol Spray Pyrolysis Processed Zn1-xMgxO Films for UV Detector Applications

A series of Zn_1-xMg_xO thin films with x ranging from 0 to 0.8 were prepared by spin coating and aerosol spray pyrolysis deposition on Si and quartz substrates. The morphology, composition, nano-crystalline structure, and optical and vibration properties of the prepared films were studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), and optical and Raman scattering spectroscopy. The optimum conditions of the thermal treatment of samples prepared by spin coating were determined from the point of view of film crystallinity. The content of crystalline phases in films and values of the optical band gap of these phases were determined as a function of the chemical composition. We developed heterostructure photodetectors based on the prepared films and demonstrated their operation in the injection photodiode mode at forward biases. A device design based on two Zn_1-xMg_xO thin films with different x values was proposed for extending the operational forward bias range and improving its responsivity, detectivity, and selectivity to UV radiation.

A Review of the Impact of Zinc Oxide Nanostructure Morphology on Perovskite Solar Cell Performance

Zinc oxide (ZnO) has been widely studied over the last decade for its remarkable properties in optoelectronic and photovoltaic devices because of its high electron mobility and excitonic properties. It has probably the broadest range of nanostructured forms that are also easy and cheap to synthesize using a wide variety of methods. The volume of recent work on ZnO nanostructures and their devices can potentially overshadow significant developments in the field. Therefore, there is a need for a concise description of the most recent advances in the field. In this review, we focus on the effect of ZnO nanostructure morphologies on the performance of ZnO-based solar cells sensitized using methylammonium lead iodide perovskite. We present an exhaustive discussion of the synthesis routes for different morphologies of the ZnO nanostructure, ways of controlling the morphology, and the impact of morphology on the photoconversion efficiency of a given perovskite solar cell (PSC). We find that although the ZnO nanostructures are empirically similar, one-dimensional structures appear to offer the most promise to increasing photoconversion efficiency (PCE) by their proclivity to align and form vertically stacked layers. This is thought to favor electron hopping, charge mobility, and conductivity by allowing multiple charge conduction pathways and increasing the effective junction cross-sectional area. The combined effect is a net increase in PCE due to the reduced surface reflection, and improved light absorption.

MOF-derived nanocrystalline ZnO with controlled orientation and photocatalytic activity

We show here that MOF-5, a sample Zn-based MOF, can uniquely transform into distinct zinc oxide nanostructures. Inspired by the interconversion synthesis of zeolites, we converted MOF-5 into nanocrystalline ZnO. We found the conversion of MOF-5 into ZnO to be tunable and straightforward simply by controlling the treatment temperature and choosing an appropriate structure-directing agent (SDA). Refined X-ray diffraction (XRD) patterns showed that a synthesis temperature of 180 °C (sample ZnO-180) was optimal for achieving high crystallinity. We examined ZnO-180 with high-resolution transmission electron microscopy (HRTEM), which confirmed that the samples were made of individual crystallites grown along the c-axis, or the (001) direction, thus exposing lower energy surfaces and corroborating the XRD pattern and the molecular dynamics calculations. Further investigations revealed that the obtained ZnO at 180 °C has a superior photocatalytic activity in degrading methylene blue to other ZnO nanostructures obtained at lower temperatures.