ZnO Heterostructures Research Articles

Parity-independent Kondo effect of correlated electrons in electrostatically defined ZnO quantum dots

Quantum devices such as spin qubits have been extensively investigated in electrostatically confined quantum dots using high-quality semiconductor heterostructures like GaAs and Si. Here, we present a demonstration of electrostatically forming the quantum dots in ZnO heterostructures. Through the transport measurement, we uncover the distinctive signature of the Kondo effect independent of the even-odd electron number parity, which contrasts with the typical behavior of the Kondo effect in GaAs. By analyzing temperature and magnetic field dependences, we find that the absence of the even-odd parity in the Kondo effect is not straightforwardly interpreted by the considerations developed for conventional semiconductors. We propose that, based on the unique parameters of ZnO, electron correlation likely plays a fundamental role in this observation. Our study not only clarifies the physics of correlated electrons in the quantum dot but also holds promise for applications in quantum devices, leveraging the unique features of ZnO.

Enhanced electroluminescence from silicon-based light-emitting devices with Mg0.4Zn0.6O/erbium-doped ZnO heterostructures by using ITO/MoO3 combined anode

We have realized the erbium (Er)-related visible and near-infrared (NIR) electroluminescence (EL) from the Mg0.4Zn0.6O/ZnO:Er/n+-Si heterostructured light-emitting device (LED) using the semi-transparent Au film as the anode, where the impact-excitation of Er3+ ions is enabled by hot holes that are generated in the Mg0.4Zn0.6O acceleration layer. However, it remains a challenge to simultaneously achieve more efficient injection of holes into Mg0.4Zn0.6O layer and stronger light emanation for the Mg0.4Zn0.6O/ZnO:Er/n+-Si heterostructured LED. Addressing this issue, we report on the substitution of an ITO/MoO3 combined anode for the Au anode in the aforementioned LED to enhance the EL. Through the optimization of MoO3 film thickness, the substitution of ITO/MoO3 combined anode for Au anode leads to the enhanced EL from the Mg0.4Zn0.6O/ZnO:Er/n+-Si heterostructured LED with a factor of more than 5 in the visible region and with a factor of more than 13 in the NIR region. The higher transmittances and larger refraction indices in both visible and NIR regions and the better hole-injection capability of the ITO/MoO3 combined anode, with respect to those of Au anode, are responsible for the significantly enhanced EL as mentioned above. This work sheds light on the application of ITO/MoO3 combined anode into all-inorganic LEDs.

Green synthesis of Z-scheme N-doped g-C3N4/Nd-doped ZnO heterostructure by pomegranate waste peel with enhanced photocatalytic performance for organic pollutants removal and antibacterial activity

Nowadays, the growing global population and increased industrialization have exacerbated water pollution, posing a significant environmental threat. To tackle this issue, there is an urgent need for effective catalysts to remove pollutants. This study developed a novel N-doped g-C3N4/Nd-doped ZnO (NZ) heterostructure using a green approach by incorporating pomegranate peel waste as a stabilizing and capping agent. Characterization techniques confirmed successful NZ nanohybrid preparation. The synthesized NZ displayed high photocatalytic activity in degrading methylene blue (MB) and tetracycline (TC) pollutants found in wastewater, achieving degradation efficiencies of 95.3 % and 98.3 %, respectively. Meanwhile, it demonstrated satisfactory photostability after five-cycle experiments. The radical trapping experiments revealed that superoxide (O2−) and hydroxyl (OH) are the dominant active species and play an essential role in photocatalytic pollutant deterioration. Additionally, it exhibited suitable antimicrobial activity against Staphylococcus aureus and Vibrio cholerae bacterial strains. The enhanced performance is attributed to the abundant reaction sites of porous N-doped g-C3N4, the photo-redox capability of Nd-doped ZnO, and the efficient charge separation process in the Z-type heterojunction. This work advances sustainable and eco-friendly chemistry for the biosynthesis of organic/inorganic heterojunctions used in pollutant degradation and bacterial disinfection of wastewater.

Cobalt-Doped ZnO Nanocomposits for Efficient Dye Degradation: Charge Transfer.

Doping enhances the optical properties of high-band gap zinc oxide nanoparticles (ZnO NPs), essential for their photocatalytic activity. We used the combustion approach to synthesize cobalt-doped ZnO heterostructure (CDZO). By creating a mid-edge level, it was possible to tune the indirect band gap of the ZnO NPs from 3.1 eV to 1.8 eV. The red shift and reduction in the intensity of the photoluminescence (PL) spectra resulted from hindrances in electron-hole recombination and sp-d exchange interactions. These improved optical properties expanded the absorption of solar light and enhanced charge transfer. The field emission scanning electron microscopy (FESEM) image and elemental mapping analysis confirmed the CDZO's porous nature and the dopant's uniform distribution. The porosity, nanoscale size (25-55 nm), and crystallinity of the CDZO were further verified by high-resolution transmission electron microscopy (HRTEM) and selected area electron image analysis. The photocatalytic activity of the CDZO exhibited much greater efficiency (k=0.131 min-1) than that of ZnO NPs (k=0.017 min-1). Therefore, doped heterostructures show great promise for industrial-scale environmental remediation applications.

Ultrafast Hole Trapping in Te-MoTe2-MoSe2/ZnO S-Scheme Heterojunctions for Photochemical and Photo-/Electrochemical Hydrogen Production.

Te-MoTe2-MoSe2/ZnO S-scheme heterojunctions are engineered to ascertain the advanced redox ability in sustainable HER operations. Photo-physical studies have established the steady state transfer of photo-induced charge carriers whereas an improved transfer dynamics realized by state-of-art ultrafast transient absorption and irradiated-XPS analysis of optimized 5wt% Te-MoTe2-MoSe2/ZnO heterostructure. 2.5, 5, and 7.5wt% Te-MoTe2-MoSe2/ZnO photocatalysts (2.5MTMZ, 5MTMZ and 7.5MTMZ) exhibited 2.8, 3.3, and 3.1-fold higher HER performance than pristine ZnO with marvelous apparent quantum efficiency of 35.09%, 41.42% and 38.79% at HER rate of 4.45, 5.25, and 4.92 mmol/gcat/h, respectively. Electrochemical water splitting experiments manifest subdued 583 and 566 mV overpotential values of 2.5MTMZ and 5MTMZ heterostructures to achieve 10 mAcm-2 current density for HER, and 961 and 793 mV for OER, respectively. For optimized 5MTMZ photocatalyst, lifetime kinetic decay of interfacial charge transfer step is evaluated to be 138.67 ps as compared to 52.92 ps for bare ZnO.

Enhanced photoelectrochemical sensor for dopamine detection based on MOFs-derived ZnO and TpPa-1 heterostructure composite

This study used ZIF-8 metal–organic framework as a precursor to create a N-doped ZnO. Subsequently, covalent organic framework named TpPa-1 (Tp, 1,3,5-triformylphloroglucinol; Pa-1, p-phenylenediamine) with a ZnO heterostructure composite (ZnO/TpPa-1) was created through an in-situ synthesis approach. Due to its superior band structure and outstanding photocatalytic performance, the ZnO/TpPa-1 composite is a good candidate for the development of a new type of photoelectrochemical (PEC) sensor for the detection of dopamine (DA). According to experimental data, ZnO/TpPa-1/ITO had a photocurrent that was roughly 1.5 times higher than TpPa-1/ITO and 4.5 times higher than ZnO/ITO. The primary reason for this improvement was the synergistic effects brought about by effective light usage and charge transfer at the interface between the TpPa-1 π-conjugated backbone and ZnO porous structure. The determination ranges for DA demonstrated two linear ranges, 0.005–0.25 μmol/L and 0.25–1.00 μmol/L, and the detection limit was as low as 1.3 nmol/L. Moreover, the PEC sensor exhibited good stability, selectivity, repeatability, and suitability for real-world samples. This work provides important new understandings for covalent organic framework (COF)-based heterostructure design, which can be used to improve PEC sensor performance.

Transforming sunlight through ultrasonically engineered ZnO and g-C₃N₄ Z-scheme heterostructure for superior photocatalysis: Experimental and theoretical study

In this study, we enhanced ZnO photocatalytic activity by synthesizing nanostructures with high aspect ratio of exposed polar facets. Post-synthesis ultrasonication induced morphological reconfiguration, improving optoelectronic properties, charge carrier separation, photocurrent, and shifted the valence band edge potential to higher positive values, providing a heightened overpotential for hydroxyl radical formation. The resulting overpotential for hydroxyl radical formation significantly boosted phenol degradation under UV light. To extend photoresponse, we employed ultrasonication to create a direct Z-scheme heterostructure with gCN, preventing particle aggregation. The S-ZnO/gCN photocatalyst, exhibiting a highly ordered structure, demonstrated superior photocatalytic activity under solar light for phenol degradation. Experimental results showed that increased dissolved oxygen accelerated hydroquinone and catechol formation, enhancing phenol degradation. Experimental results were supported by theoretical calculations. This innovative approach not only improves ZnO photocatalytic activity but also broadens its application to solar light-driven reactions.

Rietveld refined structural, optical and temperature dependent impedance spectroscopy of NiO–ZnO heterostructure composite: Synthesized through solid state method for high-frequency devices

Multifunctional NiO–ZnO heterostructure composite powder is economically synthesized through a solid-state route having a crystallite size of 23 (±2) nm. Chemical composition is identified through FTIR, the peak associated with Zn–O is detected at 1049 cm−1. However, the peaks detected at 870 cm−1 and 590 cm−1 show Ni–O stretching vibrations. UV-DRS technique is employed to estimate the energy band gap (Eg) around 3.18 eV. The impedance studies confirmed the presence of a non-Debye-type relaxation mechanism, electrical parameters confirm the contribution of electroactive regions i.e. grains and interfaces. An equivalent circuit model (R-CPE) is employed to measure various electrical parameters. Adiabatic small polaron hopping model is applied to estimate activation energies for Ni+2 (0.30 eV), Zn+2 (0.35 eV), and bulk (0.32 eV) respectively. The hopping length at the interface is estimated at around 0.8 Å by employing Mott's variable range hopping model. Jonscher's power law (JPL) is used to fit ac conductivity data, and conclude activation energy around 0.3 eV. The conduction mechanism through the whole frequency range (1–107 Hz) is governed by correlated barrier hopping. Various physical parameters i.e. binding energy (Wm), hopping length, and barrier height compared throughout the whole temperature range (313–393 K). The rate of increase in real permittivity at low frequency is related to the disorder of the cation at sublattices. These results deduced potential for NiO–ZnO heterostructure composite for high dielectric permittivity with low tangent loss for high energy storage applications.

Porous Ag-ZnO/Ag heterostructure: Microscopic and electrochemical investigation

Porous Ag-ZnO/Ag heterostructures have been synthesized by the combustion approach. The independent silver crystal formation was confirmed by the appearance of an independent peak on the XRD pattern and HRTEM image d-spacing analysis. The low-angle shift on the XRD pattern also confirms the inclusion of silver in the ZnO lattice. Thus, the XRD and HRTEM analyses confirm the formation of the Ag-ZnO/Ag (za) heterostructure. Silver inclusion in the ZnO lattice was also confirmed by the DRS-UV–vis indirect bandgap change from 3.03 to 2.86 eV. The porous nature of the materials is confirmed by the FESEM image. The PL spectra intensity reduction for za indicates the presence of charger transfer. Silver dopant surface distribution and composition were confirmed from the EDS-elemental mapping analysis. The porous ZnO heterostructure also showed better lead detection capacity compared to ZnO. Thus, porous materials have future outlooks for pollutant detection and removal applications.

Ultra-selective hydrogen sensors based on CuO - ZnO hetero-structures grown by surface conversion

Synergistic effects of mixed oxides have the potential to improve sensing performances of environmental, domestic and industrial monitoring devices. However, mixed oxides often come in the form of separate particles and are thus addressed separately by the environment, instead of capitalizing on the interface between the metal oxides. This paper describes a new core@shell gas sensing material of tetrapodal zinc oxide with a surface coating of crystalline copper oxide(t-ZnO@CuO). The special surface conversion strategy yields a unique, self-assembled and pinhole-free coating of CuO nanoplatelets. The morphologies, structural, chemical and gas sensing properties of the heterostructure were investigated. To evaluate the sensing properties of the heterostructure, t-ZnO@CuO was fabricated as nanosensors, consisting of one core-shell rod of CuO-coated crystalline ZnO. The single core@shell rod showed high selectivity towards hydrogen already at comparatively low operation temperatures of 150 °C. Computational calculations based on the density functional theory (DFT) have been carried out to understand the interaction of the H2 gas molecule with the surface of the CuO nanostructures. The surface conversion was done wet chemically and is a novel method for generating heterostructures that can be potentially transferred to heterojunctions with unique properties for chemosensors.

Improving photocatalytic efficiency of ZnO nanoflowers through gold incorporation for Rhodamine B photodegradation

In this study, we reported the synthesis and characterization of Au-doped ZnO hybrid nanostructures via a wet chemical approach and assessed their photocatalytic efficiency for the removal of Rhodamine (Rh–B). X-ray powder diffraction (XRD) has been done to verify the crystal structure, i.e. phase identification, crystallite size, texture, stress, and strain. The XRD patterns demonstrate that the wurtzite structure has been retained and that Au nanoparticles were effectively ingested into the ZnO nanoflowers lattice. This demonstrates that Au nanoparticles are effectively absorbed into the ZnO nanoflowers lattice, considering that peaks shifted in the direction of smaller Bragg's angles. As a result, the size of the crystallites increases from (20.7–25.3) nm, and their microstrain decreases from (0.92–0.72) x 10−3 A0 for pure-ZnO to Au-doped ZnO heterostructures, respectively. Scanning Electron Microscopy (SEM) investigation reveals flower-like structures of pure ZnO while a further closed look indicates that these flowers are made up of a variety of oriented, thorn-like branched NWs with diameters ranging from (50–90) nm and lengths ranging from a few microns. The corresponding EDS evaluation of pure ZnO nanoflowers and Au-doped ZnO heterostructures clearly shows that the noble metal Au NPs are deposited, and enormously safeguard the surface of ZnO nanoflowers. Reducing the Au-doped ZnO band gap which generated a red shift in light absorption, was confirmed by the PL spectra which in turn increased the photocatalytic efficiency and reduced the dye degradation time of Rh–B. The photocatalytic ability of Au-doped ZnO heterostructures considerably increased the photodegradation efficiency of Rh–B molecules from 28 min for pure ZnO to 10 min for Au-doped ZnO heterostructures. Due to their wide surface area and distinct morphology as compared to pure ZnO nanoflowers.

Aloe-inspired eco-friendly synthesis of Ag/ZnO heterostructures: boosting photocatalytic potential

The current research focuses on the development of Ag–ZnO heterostructures through a “bottom-up” approach involving the assembly and extraction of Aloe barbadensis Miller gel. These heterostructures composed of metals/semiconductor oxide display distinct and notable optical, electrical, magnetic, and chemical properties that are not found in single constituents and also exhibit photocatalytic applications. These synthesized heterostructures were characterized by XRD, FTIR, SEM, and UV–visible spectroscopy. The high peak intensity of the Ag/ZnO composite shows the high crystallinity. The presence of Ag–O, Zn–O, and O–H bonding is verified using FTIR analysis. SEM analysis indicated the formation of spherical shapes of Ag/ZnO heterostructures. The Zn, O, and Ag elements are further confirmed by EDX analysis. Ag–ZnO heterostructures exhibited excellent photocatalytic activity and stability against the degradation of tubantin red 8BL dye under visible light irradiation.

A light emitting device with TiO2:Er3+/ZnO heterostructure for enhanced near-infrared electroluminescence

Near-infrared (NIR) electroluminescence (EL) devices based on Er3+ ions doped TiO2 emitting layer have been fabricated by employing a facile sol–gel method. The effect of Er3+ ion doping concentration on the EL performance of TiO2:Er3+ thin film devices was investigated. Moreover, a novel device with the core of TiO2:1%Er3+/ZnO heterostructure was designed and fabricated. The EL performance of the device with optimized Er3+ ion doping concentration and improved structure was significantly improved. The NIR EL-enabling voltage of the improved device is as low as ∼5 V. The attenuated concentration quenching effect and the ZnO film as an electron blocking layer should contribute to the improved EL performance of the optimized device.

Giant Dielectric Constant and Fast Adsorption–Sunlight Photocatalytic Properties of Al-Doped CuO–ZnO Heterostructures

Giant Dielectric Constant and Fast Adsorption–Sunlight Photocatalytic Properties of Al-Doped CuO–ZnO Heterostructures

Multi-vacancy synergistic effect in a NiSe0.4S1.6/ZnO heterostructure for promoting photocatalytic hydrogen production

Defect engineering is one of the methods to improve catalytic activity of photocatalysts, but there are few reports on multi-vacancy systems. In this paper, S doping NiSe2 (NiSe0.4S1.6, NSS) was synthesized and further the NSS/xZnO heterostructures containing Ni, Se and O vacancies were constructed. The hydrogen and active oxygen ions are in situ generated at oxygen vacancy (VO) in ZnO, which greatly reduce the recombination of photocarriers, and the generated active oxygen ions and sacrificial agent anions (S2−, SO32−) involve in the regeneration of oxygen vacancy, which may be one of the reasons for the high activity and stability of the composite catalysts. The polarization electric field is formed by regulating the selenium and nickel double vacancy (VSe, VNi) generated by doping NiSe2 with S, which further improves the charge separation leading to the promotion of photocatalytic hydrogen production. The synergistic effect of the three types of vacancies and the redox properties of the composite catalyst lead to a high H2 production rate of 17.04 mmol·g−1·h−1, 54 and 76 times higher than NSS and ZnO, and a high apparent quantum yield of 15.30 % at 400 nm. The hydrogen production mechanism was explored by EPR, Mott-Schottky, VB-XPS, in suit XPS, DFT, etc. This work proves that the combination of transition metal oxides with rich oxygen defects and metal chalcogenides with metal and chalcogen di-vacancies containing local polarized electric fields is an effective strategy to design high active photocatalyts for water splitting.

Neuromorphic Computing of Optoelectronic Artificial BFCO/AZO Heterostructure Memristors Synapses.

Compared with purely electrical neuromorphic devices, those stimulated by optical signals have gained increasing attention due to their realistic sensory simulation. In this work, an optoelectronic neuromorphic device based on a photoelectric memristor with a Bi2FeCrO6/Al-doped ZnO (BFCO/AZO) heterostructure is fabricated that can respond to both electrical and optical signals and successfully simulate a variety of synaptic behaviors, such as STP, LTP, and PPF. In addition, the photomemory mechanism was identified by analyzing the energy band structures of AZO and BFCO. A convolutional neural network (CNN) architecture for pattern classification at the Mixed National Institute of Standards and Technology (MNIST) was used and improved the recognition accuracy of the MNIST and Fashion-MNIST datasets to 95.21% and 74.19%, respectively, by implementing an improved stochastic adaptive algorithm. These results provide a feasible approach for future implementation of optoelectronic synapses.

Spatially Indirect Excitons in GaN–ZnO Heterostructures with Long and Electrically Controllable Lifetime

Spatially Indirect Excitons in GaN–ZnO Heterostructures with Long and Electrically Controllable Lifetime

Micro-nano hierarchical urchin-like ZnO/Ag hollow sphere for SERS detection and photodegradation of antibiotics

Abstract Hollow urchin-like substrates have been widely interested in the field of surface-enhanced Raman scattering (SERS) and photocatalysis. However, most reported studies are simple nanoscale urchin-like substrate with limited light trapping range and complicated preparation process. In this paper, a simple and effective controllable synthesis strategy based on micro-nano hierarchical urchin-like ZnO/Ag hollow spheres was prepared. Compared with the 2D structure and solid spheres, the 3D urchin-like ZnO/Ag hollow sphere has higher laser utilization and more exposed specific surface area due to its special hollow structure, which resulted in excellent SERS and photocatalytic performance, and successfully realize the detection and photodegradation of antibiotics. The limited of detection of metronidazole can reach as low as 10−9 M, and degradation rate achieve 89 % within 120 min. The experimental and theoretical results confirm that the ZnO/Ag hollow spheres can be used in the development of ZnO heterostructure for the detection and degradation of antibiotics, which open new avenues for the development of novel ZnO-based substrate in SERS sensing and catalytic application to address environmental challenges.

Study on interface engineering and chemical bonding of the ReS2@ZnO heterointerface for efficient charge transfer and nonlinear optical conversion efficiency.

Rhenium sulfide (ReS2) has emerged as a promising two-dimensional material, demonstrating broad-spectrum visible absorption properties that make it highly relevant for diverse optoelectronic applications. Manipulating and optimizing the pathway of photogenerated carriers play a pivotal role in enhancing the efficiency of charge separation and transfer in novel semiconductor composites. This study focuses on the strategic construction of a semiconductor heterostructure by synthesizing ZnO on vacancy-containing ReS2 (VRe-ReS2) through chemical bonding processes. The ingeniously engineered built-in electric field within the heterostructure effectively suppresses the recombination of photogenerated electron-hole pairs. A direct and well-established interfacial connection between VRe-ReS2 and ZnO is achieved through a robust Zn-S bond. This distinctive bond configuration leads to enhanced nonlinear optical conversion efficiency, attributed to shortened carrier migration distances and accelerated charge transfer rates. Furthermore, theoretical calculations unveil the superior chemical interactions between Re vacancies and sulfide moieties, facilitating the formation of Zn-S bonds. The photoluminescence (PL) intensity is increased by the formation of VRe-ReS2 and ZnO heterostructure and the PL quantum yield of VRe-ReS2 is improved. The intricate impact of the Zn-S bond on the nonlinear absorption behavior of the VRe-ReS2@ZnO heterostructure is systematically investigated using femtosecond Z-scan techniques. The charge transfer from ZnO to ReS2 defect levels induces a transition from saturable absorption to reverse saturable absorption in the VRe-ReS2@ZnO heterostructure. Transient absorption measurements further confirm the presence of the Zn-S bond between the interfaces, as evidenced by the prolonged relaxation time (τ3) in the VRe-ReS2@ZnO heterostructure. This study offers valuable insights into the rational construction of heterojunctions through tailored interfacial bonding and surface/interface charge transfer pathways. These endeavors facilitate the modulation of electron transfer dynamics, ultimately yielding superior nonlinear optical conversion efficiency and effective charge regulation in optoelectronic functional materials.

Development of highly sensitive SnO2@ZnO based chemiresistor for Ammonia sensing

A room temperature (28°C) functional SnO2 supported ZnO (SnO2@ZnO) heterostructure based chemiresistor has been fabricated for sensing low-trace of ammonia (5–35 ppm). The decoration of SnO2 nanospheres on ZnO has been done via low-cost hydrothermal technique. The surface morphological examination done by FE-SEM reiterate the decoration of SnO2 nanospheres on the surface of ZnO. The Rietveld refinement for structural analysis of SnO2@ZnO heterostructure confirmed that both ZnO and SnO2 phases structures are present in prepared SnO2@ZnO. At lowermost concentration of 5 ppm, the sensor response of SnO2@ZnO heterostructure was 25.48 and for the maximum concentration of 35 ppm it was 159.16. The quick response and recovery time at 5 ppm were observed to be 4.05 s and 6.74 s, respectively. The response heightening supported the efficient fabrication of n-n heterojunction between the surface of ZnO and SnO2 spheres, resulting in the improved sensing performance. Also, the SnO2@ZnO chemiresistor verified high long-term stability and selectivity to ammonia as compared to other interfering gases such as methanol, ethanol, aniline, and toluene. This work opens a novel window for the development of devices that are room temperature operatable, highly sensitive and selective for quick detection of ammonia gas for its commercialization.