N-type ZnO Research Articles

Sensitivity Enhancement of Resistive Ethanol Gas Sensor by Optimized Sputtered-Assisted CuO Decoration of ZnO Nanorods.

In this study, sputtered-assisted CuO-decorated ZnO nanorod (NR) gas sensors were fabricated for ethanol gas sensing studies. CuO nanoparticles have been successfully formed on ZnO nanorods by means of a physical process as the decorative metallic element. The amount of decoration affecting the sensor's performance has been optimized. Cu layers with different thicknesses of 5, 10, and 20 nm were deposited on hydrothermally grown ZnO NRs using the sputtering technique. Upon subsequent annealing, Cu was oxidized to CuO. The gas sensing studies revealed that the sensor with an initial Cu layer of 5 nm had the highest response to ethanol at 350 °C. The sensor also showed good selectivity, repeatability, and long-term stability. The enhanced ethanol sensing response of the optimized gas sensor is related to the formation of p-n heterojunction between p-type CuO and n-type ZnO and the presence of the optimal amount of CuO on the surface of ZnO NRs. The results presented in this study highlight the need for optimizing the amount of Cu deposition on the surface of ZnO NRs in order to achieve the highest response to ethanol gas.

An individual ZnO microwire homojunction LED with ultraviolet electroluminescence spectrally purified using Pt nanoparticles cladding

Low-dimensional ultraviolet light sources are enormous significance due to their wide range of potential applications, ranging from bioscience and high-resolution biological imaging, environmental conservation to public hygiene. Nevertheless, developing energy-efficient and cost-efficient light sources still remains serious challenges. In this study, p-type ZnO microwires with Sb-incorporation (ZnO:Sb MWs) were synthesized individually. The p-type conductivity was proofed using an individual MW based back-gated field-effect transistor; while the ZnO:Sb MWs with quadrilateral cross-section exhibited stimulated emission resonant in well-defined Fabry-Perot mode on the benefit of single-crystalline quality and atomically smooth side facets. The newly synthesized ZnO:Sb MW was further used to fabricate homojunction light-emitting device by employing n-type ZnO film as electron-transporting layer. A strong ultraviolet emissions peaking at around 378.5 nm was achieved upon forward biased electrically, but illustrating much broader electroluminescence (EL) linewidth. Introducing Pt nanoparticles (PtNPs) with desired plasmons, the light-out intensity is significantly enhanced. Particularly, the broad EL profiles narrowed to about 16.6 nm at high injection regime, achieving electron-holes radiative recombination in the ZnO:Sb MWs active region. The results exhibit a prominent progress achieving high-quality ZnO nano/microstructures with genuine p-type conductivity, which has great hopes for its future applications and development in ZnO homojunction optoelectronic devices.

Tailoring the optical and electrical behavior of Cu2O/ZnO heterojunction by varying the Zn2+ ion concentration for solar-cell applications

Heterojunctions made from metal oxide semiconductors are of great interest due to their harmless non-toxic property, enormous availability, and chemically stable nature. In this work, we report the fabrication of Cu2O/ZnO heterojunction by deposition of p-type Cu2O followed by n-type ZnO via an inexpensive spin coating method at ambient conditions. This report focuses on the study of the properties of Cu2O/ZnO heterojunction thin films with different precursor concentrations of ZnO (0.05 M, 0.1 M, 0.15 M, 0.2 M, and 0.25 M). The structural, morphological, optical, and electrical properties are performed for different compositions. The band gap of all Cu2O/ZnO heterojunction thin films ranges between 1.86 eV and 2.02 eV. The IV characteristics confirm p-n photoactive heterojunction behavior for all the samples with increased current density for lower Zn2+ ion concentration.

Spectral Engineering of Hybrid Biotemplated Photonic/Photocatalytic Nanoarchitectures.

Solar radiation is a cheap and abundant energy for water remediation, hydrogen generation by water splitting, and CO2 reduction. Supported photocatalysts have to be tuned to the pollutants to be eliminated. Spectral engineering may be a handy tool to increase the efficiency or the selectivity of these. Photonic nanoarchitectures of biological origin with hierarchical organization from nanometers to centimeters are candidates for such applications. We used the blue wing surface of laboratory-reared male Polyommatus icarus butterflies in combination with atomic layer deposition (ALD) of conformal ZnO coating and octahedral Cu2O nanoparticles (NP) to explore the possibilities of engineering the optical and catalytic properties of hybrid photonic nanoarchitectures. The samples were characterized by UV-Vis spectroscopy and optical and scanning electron microscopy. Their photocatalytic performance was benchmarked by comparing the initial decomposition rates of rhodamine B. Cu2O NPs alone or on the butterfly wings, covered by a 5 nm thick layer of ZnO, showed poor performance. Butterfly wings, or ZnO coated butterfly wings with 15 nm ALD layer showed a 3 to 3.5 times enhancement as compared to bare glass. The best performance of almost 4.3 times increase was obtained for the wings conformally coated with 15 nm ZnO, deposited with Cu2O NPs, followed by conformal coating with an additional 5 nm of ZnO by ALD. This enhanced efficiency is associated with slow light effects on the red edge of the reflectance maximum of the photonic nanoarchitectures and with enhanced carrier separation through the n-type ZnO and the p-type Cu2O heterojunction. Properly chosen biologic photonic nanoarchitectures in combination with carefully selected photocatalyst(s) can significantly increase the photodegradation of pollutants in water under visible light illumination.

High performance self-powered UV photodetection by ZnO/Cu3N core-shell nanorod heterostructures via p-n junction formation

Self-powered ultraviolet photodetectors (UVPDs) have a wide range of contributions in distributing the wireless sensor networks and internet of things (IoT) systems, helping in energy conservation as it requires no external power source. This study proposes a new p-n junction hybrid structure based on a p-type Cu3N (shell layer) over n-type ZnO NR (core) arrays. The ZnO/Cu3N core-shell heterostructures based self-powered UV photodetector fabricated by depositing the p-type Cu3N layer using high power impulse magnetron sputtering (HiPIMS) over vertically aligned n-type ZnO NR arrays grown by the hydrothermal method. The ZnO/Cu3N core-shell UV photodetector offers enhanced photocurrent (∼1 mA) and excellent photosensitivity of 2.5 × 105% at − 5 V. While the UVPD is in a self-powered state, it shows an outstanding performance by exhibiting an impressive photocurrent and Ion/Ioff ratio of ∼0.06 μA and 500, respectively. The device demonstrates fast photo response and recovery time. Furthermore, the device outshines because of significant repeatability for around 75 days. As a proof-of-concept, these ZnO/Cu3N based results can promote the fabrication next generation self-powered UV photodetectors.

Color-tunable transparent photovoltaics for onsite power production under sunlight and indoor light

Transparent photovoltaics (TPVs) are energy-generating devices made of transparent material and incorporating solar cell technology. The ultimate goal of using such devices is to reach a level of sustainability by harnessing energy from common structures such as windows on buildings and vehicles. Applications of this concept can be widened by incorporating a color-tuning scheme into TPVs. In this sense, light can be managed; here, we introduce a method for this using a ZnO-based back reflection method. Large-scale (16 cm2) TPVs are achieved by solid-state sputtering of metal oxide layers. They exhibited high power conversion efficiency (η) (3.8% for indoor light-emitting diode lamp) and long-term stability (>100 days). The abrupt heterojunction between n-type ZnO and p-type NiO spontaneously forms a space charge region with a strong electric field. An additional thin Si film in the heterojunction effectively improves solar cell performance, making it useful for power generation, even under indoor illumination. Substantial efficiency of 3.02% was achieved by an Al-doped ZnO for the Sun spectrum. This implies the ability to produce TPV power from the Sun or artificial light in versatile ways. TPVs and color-tuned photovoltaics can be used in windows and other such objects to realize an onsite power supply.

Improved Photocatalytic Activity via n-Type ZnO/p-Type NiO Heterojunctions

The design and construct pn heterojunction to reduce the recombination rate of photogenerated electron-hole pairs can effectively improve photocatalytic activity. In this study, ZnO/NiO heterojunctions were fabricated by annealing a Zn/Ni metal organic framework precursor synthesized via coprecipitation. The effects of the precursor annealing temperature on the microstructure, morphology, and optical properties of the ZnO/NiO nanocomposites were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV-vis absorption spectroscopy. The results showed that the nanocomposite was composed of hexagonal wurtzite ZnO and cubic NiO, with the former being the dominant phase. Large ZnO nanoparticles were attached to small NiO nanoparticles, and a pn heterojunction interface was formed. The photodegradation performance of the nanomaterials was evaluated by monitoring the degradation of RhB under irradiation by ultraviolet light. The ZnO/NiO nanocomposites exhibited excellent photocatalytic activity when the annealing temperature was 550 °C. The photodegradation mechanism was also analyzed in detail, revealing that the heterojunction between the n-type ZnO and the p-type NiO played an important role in impeding the recombination of photogenerated electron-hole pairs and improving the photocatalytic efficiency.

Fabrication of transparent ZnO/Cu-Mg(OH)2 heterojunction diodes by electrochemical deposition

The pn heterojunction diodes were fabricated using p-type Cu-doped Mg(OH)2 and n-type ZnO. The Cu-Mg(OH)2 thin films were fabricated via electrochemical deposition (ECD) using an aqueous solution of Mg(NO3)2 and Cu(NO3)2. The ZnO films were deposited on the Cu-Mg(OH)2 film via ECD from a solution of Zn(NO3)2. The Cu-Mg(OH)2 films were transparent in the visible range and showed a clear p-type response during photoelectrochemical measurement. The ZnO/Cu-Mg(OH)2 diodes exhibited appreciable rectification properties. Thus, Mg(OH)2 can be a promising ultra-wide bandgap semiconductor for transparent electronics applications.

X-ray Diffraction, Micro-Raman and X-ray Photoemission Spectroscopic Investigations for Hydrothermally Obtained Hybrid Compounds of Delafossite CuGaO2 and Wurtzite ZnO

P-type delafossite CuGaO2 is a wide-bandgap semiconductor for optoelectronic applications, and its lattice parameters are very similar to those of n-type semiconductor wurtzite ZnO. Accordingly, the investigation of crystalline heterostructures of CuGaO2 and ZnO has attracted significant attention. In this study, interfacial CuGaO2/ZnO hetero-compounds were examined through X-ray diffraction (XRD) analysis, confocal micro-Raman spectroscopy, and X-ray photo-electron spectroscopy (XPS). XRD and Raman analysis revealed that the hydrothermal deposition of ZnO on hexagonal platelet CuGaO2 base crystals was successful, and the subsequent reduction process could induce a unique, unprecedented reaction between CuGaO2 and ZnO, depending on the deposition parameters. XPS allowed the comparison of the binding energies (peak position and width) of the core level electrons of the constituents (Cu, Ga, Zn, and O) of the pristine CuGaO2 single crystallites and interfacial CuGaO2/ZnO hybrids. The presences of Cu2+ ions and strained GaO6 octahedra were the main characteristics of the CuGaO2/ZnO hybrid interface. The XPS and modified Auger parameter analysis gave an insight into a specific polarization of the interface, promising for further development of CuGaO2/ZnO hybrids.

Design of SrZr0.1Mn0.4Mo0.4Y0.1O3-δ heterostructured with ZnO as electrolyte material: Structural, optical and electrochemical behavior at low temperatures

P-type semiconductor SrZr0.1Mn0.4Mo0.4Y0.1O3-δ (SZMMY) is for the first time composited with n-type ZnO to prepare a solid oxide electrolyte used in fuel cell operable at low temperature. Prepared nanocomposite electrolyte material is considered as a novel material owing to the results obtained in terms of improved ionic conductivity, power density, and current density at lower operational temperature. The material has been analyzed as well crystalline material with a dual-phase as confirmed by X-Ray Diffraction (XRD). The structural morphology of designed electrolyte materials was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), including high-resolution TEM (HR-TEM). The electrochemical impedance spectra (EIS) showed a remarkably lower charge transfer resistance than conventional electrolyte materials. Obtained results illustrated that ionic conductivity increased, which lead to the acceleration of the electrode reactions. Heterostructure nanocomposite SZMMY-ZnO is beneficial to attain a high ionic conductivity due to the suppression of electronic conduction through the p-n junction. The maximum power density was noted as 841 mW cm−2 at 550 °C with a maximum current density of 2287 mA cm−2. Based on optical properties through the p-n junction, the internal electronic current was blocked, which further reduced the short circuit problem in the heterostructure. In addition, the performance and lifetime test indicated good stability of the cell at 550 °C with a very small degradation loss. The present study suggests that the SZMMY-ZnO is a promising electrolyte for low-temperature-SOFCs development.

Effects of Group-I Elements on Output Voltage Generation of ZnO Nanowires Based Nanogenerator; Degradation of Screening Effects by Oxidation of Nanowires.

Here, we report the successful incorporation of group I elements (K, Na, Li) to ZnO nanowires. Three distinct (2, 4, and 6 wt.%) doping concentrations of group I elements have been used to generate high piezoelectric voltage by employing a vertically integrated nanowire generator (VING) structure. X-ray photoelectron spectra (XPS) indicated the seepage of dopants in ZnO nanowires by substitution of Zn. Shallow acceptor levels (LiZn, NaZn, KZn) worked as electron trapping centers for intrinsically n-type ZnO nanowires. Free moving electrons caused a leakage current through the nanowires and depleted their piezoelectric potential. Reverse leakage current is a negative factor for piezoelectric nanogenerators. A reduction in reverse leakage current signifies the rise in output voltage. A gradual rise in output voltage has been witnessed which was in accordance with various doping concentrations. K-doped ZnO nanowires have generated voltages of 0.85 V, 1.48 V, and 1.95 V. For Na-doped ZnO nanowires, the voltages were 1.23 V, 1.73 V, and 2.34 V and the voltages yeilded for Li-doped ZnO nanowires were 1.87 V, 2.63 V, and 3.54 V, respectively. Maximum voltage range has been further enhanced by the surface enrichment (oxidized with O2 molecules) of ZnO nanowires. Technique has been opted to mitigate the screening effect during an external stress. After 5 h of oxidation in a sealed chamber at 100 ppm, maximum voltage peaks were pronounced to 2.48 V, 3.19 V, and 4.57 V for K, Na, and Li, respectively. A low-cost, high performance mechanical transducer is proposed for self-powered devices.

Polaron induced ferromagnetism in ZnO

The odd phenomenon of the ferromagnetic moment of n-type ZnO increasing with temperature has been observed in experiments. We proposed an impurity model accompanying the polaron theory to explain this result because the electron-phonon interaction plays a critical role in inducing ferromagnetism in n-type ZnOs. Thus, we propose that the ferromagnetism of ZnOs comes from the polarons in the impurity states, and the strong Coulomb interaction in these states causes the spin of electrons to split. In addition, the proper hybridization interaction between the conduction band and the impurity states promotes the increase of electron density in the impurity states as well as the ferromagnetism. In contrast, a too strong hybridization interaction will delocalize the electrons in the impurity states to suppress the ferromagnetism. The polarons induce the ferromagnetism by the Coulomb excitation in the impurity states. The electron thermal excitation produces the polarons in the impurity states near the ZnO conduction band edge coupling with the phonon. Based on the model, the ferromagnetism increases with the temperature consistent with the experimental results.

Gas sensing performance of carbon monoxide sensor based on rod-shaped tin diselenide/MOFs derived zinc oxide polyhedron at room temperature

This paper introduces a high-efficiency CO gas sensor based on ZnO/SnSe2 composite film. The rod-shaped SnSe2 and polyhedral ZnO composite nanostructures were prepared by template sacrificial method and solvothermal method. The developed composite material was characterized by scanning X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and I-V testing methods, and its microstructure and composition were explored. Through comparative experiments, it was found that the response of ZnO/SnSe2 was the highest when the loading rate of SnSe2 is 25%, which was superior to pure ZnO and SnSe2 sensors. The ZnO/SnSe2 composite sensor showed excellent selectivity and good dynamic characteristics to CO at room temperature (RT). To further improve the performance of the prepared CO sensor, we studied the effect of ultraviolet (UV) light on the sensor, and the research concluded that UV light can improve the gas-sensing characteristics of the sensors. The possible CO sensing mechanism is related to the heterostructure between n-type SnSe2 and n-type ZnO nanomaterials and the photoelectrons excited by UV light.

Adsorption-Mediated n-Type ZnO Surface Reconstruction for Optically Enhanced Volatile Organic Compound Sensing

Adsorption-Mediated n-Type ZnO Surface Reconstruction for Optically Enhanced Volatile Organic Compound Sensing

Drop-Dry Deposition of Ni(OH)2 Precursors for Fabrication of NiO Thin Films.

Drop–dry deposition (DDD) is a method of depositing thin films by heating and drying the deposition solution dropped on a substrate. We prepared Ni(OH)2 precursor thin films by DDD and annealed them in air to prepare NiO thin films. The appropriate deposition conditions were found by changing the number of drop–dry cycles and the concentrations of chemicals in the solution, and the Ni(OH)2 precursor film with a thickness of 0.3 μm and optical transmittance of more than 95% was successfully deposited. Raman and X-ray diffraction measurements were performed, and it was found that the NiO film was successfully fabricated after annealing at 400 °C. The p-type conductivity of the annealed film was confirmed by photoelectrochemical measurements. In addition, we prepared n-type ZnO by electrochemical deposition on NiO thin films. The current–voltage measurement results show that the ZnO/NiO heterojunction had rectification properties.

Excellent photo-detection properties of cerium doped ZnO device fabricated by spray pyrolysis technique

Recently, n-type ZnO is identified as a promising candidate for ultraviolet (UV) photodetectors (PDs) due to their suitable optoelectronics properties. Hence, in this work we fabricated the ZnO and Ce doped ZnO (ZnO:Ce) based UV Photo detectors by spray pyrolysis technique. Prior to the photodetection studies, the wurtzite structure of ZnO:Ce samples is confirmed by X-ray diffraction analysis. The 2D and 3D topography and morphology of the films are probed by AFM study. Further, the elemental compositions are confirmed with EDX spectrum. The optical studies such as UV–visible absorbance and photoluminescence emission spectra are respectively recorded to find the bandgap and the defect states in ZnO:Ce. Finally, the UV photodetection properties of fabricated thin film devices are studied under the irradiation of 365 nm laser. Photo detector sensing parameters such as responsivity, detectivity and the external quantum efficiency of the ZnO:Ce(2.0%) sample exhibited maximum values of 0.27 AW−1, 63% and 2.18 × 1010 Jones and it proved to be a potential sensor application as UV Photodetector.

Improved performance of near UV-blue n-ZnO/p-GaN heterostructure LED with an AlN electron blocking layer

High quality single crystalline n-type ZnO films, with a thickness of 200 nm, were grown on top of AlN/GaN layers under 25 W, 50 W and 100 W sputtering powers. The AlN and p-type GaN layers were deposited by metalorganic chemical vapor deposition (MOCVD). From X-ray diffraction measurements, the samples exhibited the (000l) peaks corresponding to both ZnO, GaN and AlN monocrystalline layers. As the sputtering power was increased, the ZnO grain size increased and the dislocation density decreased. This result is supported by a reduction of the rms values obtained on the ZnO layers from Atomic Force Microscopy (AFM) results. In addition, photoluminescence peaks of ZnO at 372 nm, 375 nm and 380 nm were seen as dependent on the sputtering power. We have used an AlN electron blocking layer between ZnO and GaN films to improve the electroluminescence from the n-ZnO side. Room temperature electroluminescence (EL) of the LEDs demonstrated near UV-blue emission consisting of predominating peaks centred at 405 nm, 390 nm and 380 nm for the device with ZnO deposited at 25 W, 50 W and 100 W sputtering powers, respectively. Moreover, the I-V curves of the LEDs showed a rectifying behavior with 6.8 V, 6.4 V, 5.2 V threshold voltages for 25 W, 50 W and 100 W values.

Yttrium (Y) doped ZnO nanowire/p-Si heterojunction devices for efficient self-powered UV-sensing applications

In the current work, n-type Y-doped ZnO nanowires (n-Y:ZnO) and p-type Si heterojunctions are fabricated by employing two-step chemical bath deposition (CBD) process. The work demonstrates the systematic incorporation of Y in ZnO nanowires for achieving the low power UV response. The crystalline nature, morphology, chemical compositions of doped nanowires are studied extensively by employing XRD, FESEM, HRTEM, elemental mapping, EDS and XPS. The change in defect states due to Y incorporation has been studied in detail by using deconvoluated PL spectra. The results indicate that ZnO nanowires exhibit Zn interstitials dependent huge UV/blue luminescence and oxygen vacancy related lowest green luminescence simultaneously for selective 1% Y-doping. The best self-powered photoresponse has been achieved for 1% Y-doped ZnO nanowires/p-Si heterojunction under relatively low power UV illumination (374 nm @5 mW/cm2). More significantly, at self-powered mode, such heterojunction has been delivered a stable and fast (<1 s) photoresponse with a maximum responsivity of 225 mA/W and high On/Off ratio of ∼104. The work provides an insight into the fabrication of heterojunction UV detectors for next-generation self-powered, large area optoelectronic devices.

Effects of As, P and Sb on the output voltage generation of ZnO nanowires based nanogenerator: Mitigation of screening effect by using surface modified ZnO nanowires

Here, we report high output voltage generation by employing p-type ZnO nanowires as an integral part of vertically integrated nanowire generators (VING). Study has been carried out to generate high piezoelectric voltage by introducing impurities to ZnO nanowires from group V (P, As, Sb) elements which worked as acceptor impurities for intrinsically n-type ZnO nanowires by which reverse leakage current through nanowires has been minimized. Three distinct doping concentrations (2, 4 and 6 wt %) of (P, As, and Sb) have been incorporated in ZnO nanowires at room temperature. X-ray photoelectron spectra (XPS) has indicated the presence of SbZn–2VZn, PZn + 2VZn, AsZn–2VZn complexes acceptors for Sb, P, As doping respectively. Gradual rise in piezoelectric output voltage has been observed. P/ZnO nanowires generated output voltages of 0.9 V, 1.45 V and 1.85 V respectively. For As/ZnO nanowires, output voltages are 1.25 V, 1.51 V and 1.92 V and with Sb doping recorded voltage values are 1.78 V 2.1 V and 2.5 V respectively. To Acquire optimal output voltage doped ZnO nanowires have been further oxidized (with O2) to mitigate the screening effect and maximum voltage generated by oxidized ZnO are 2.38 V, 2.86 V, and 3.45 V respectively.

Photocatalytic study of metal oxide enhanced ZnO synthesized by a one-step cyclic-microwave method: The role of the p-n heterostructure

In this work, the role of the p-n junction in the photocatalytic degradation of various dyes in contaminated wastewater was studied. The n-type ZnO was synthesized by the cyclic-microwave method and was enhanced by various metal oxides, e.g., CuO, NiO, and Fe2O3. The materials were characterized by various methods. The degradation of Methylene blue (MB), Methyl orange (MO), and Rhodamine B (RhB) under the photocatalysts and UV light irradiation was examined. From the results, the performance of the dye degradation by the p-n CuO/ZnO and NiO/ZnO materials was better than that of the pure ZnO. On the other hand, the n-n heterostructure Fe2O3/ZnO showed a decrease in photocatalytic performance. The photocatalyst with the highest performance was Ni(5)-ZnO, which can degrade dyes contaminating water by greater than 99.71%. The physical principle behind the increase in the photocatalytic performance is due to the formation of the p-n junction by CuO and NiO to the n-type ZnO, resulting in the efficient generation of electron-hole pairs.