Wide Band Gap ZnO Research Articles

Decoration of ZnO surface with tiny sulfide-based nanoparticles for improve photocatalytic degradation efficiency

Modifying wide band gap ZnO nanoparticles surface by combine narrow bandgap semiconductors is a novel route to promote the ZnO to diverse applications. Herein, different metal sulfides (CdS, Ag2S and Bi2S3) were decorated on ZnO surface using facile a chemical route for photocatalytic application. Crystal structure, surface morphology and optical changes for the surface modified ZnO were studied by using various characterization techniques. The XRD spectra exhibited mixed phase of decorated metal sulfide nanoparticles along with strong pattens of hexagonal structure ZnO. The SEM images were confirmed that tiny CdS, Ag2S and Bi2S3 sulfide nanoparticles are well decorated on ZnO hexagonal rods surface. Band gap of the ZnO was tuned into visible region by modifying the surface by the sulfide nanoparticles. Textile industry-based crystal violet (CV) dye was used as a model pollutant to evaluate the photocatalytic activity of sulfides decorated well-crystalline ZnO photocatalysts under natural sunlight. Among the three catalysts, the Ag2S decorated ZnO achieved greatest photodegradation efficiency of 94.1% for degradation of the CV dye with rate constant value of 0.050. The highest catalytic activity may be related to Ag2S acting a significant part in reducing bandgap and boosting hole, superoxide radical, and hydroxyl radical formation, which inhibits recombination, hence enhancing the photocatalyst's efficacy, activity, and also stability.

Electrochemically Determining Electronic Structure of ZnO Quantum Dots with Different Surface Ligands.

The electronic structure of quantum dots (QDs) including band edges and possible trap states is an important physical property for optoelectronic applications. The reliable determination of the energy levels of QDs remains a big challenge. Herein we employ cyclic voltammetry (CV) to determine the energy levels of three types of ZnO QDs with different surface ligands. Coupled with spectroscopic techniques, it is found that the onset potential of the first reductive wave is likely related to the conduction band edges while the first oxidative wave originates from the trap states. The determined specific energy levels in CV further demonstrates that the ZnO QDs without surface ligands mainly have oxygen interstitial defects whilst the ZnO QDs covered with ligands contain oxygen vacancies. The present electrochemical method offers a powerful and effective way to determine the energy levels of wide bandgap ZnO QDs, which will boost their device performance.

Regulation of charge transfer in ZnO/Bi2Sn2O7 heterojunction for enhanced photocatalytic performance towards antibiotic degradation

Regulating the migration behavior of photo-generated carriers is an effective strategy to design high-performance photocatalysts. Herein, ZnO/Bi2Sn2O7 type II heterojunction was fabricated through the composite of wide-bandgap ZnO and narrow-bandgap Bi2Sn2O7, and the charge transfer mechanism was changed from type II into Z-scheme by photo-depositing Ag nanoparticles on ZnO/Bi2Sn2O7 composite. The loaded Ag nanoparticles not only expanded absorption of ZnO to visible-light range through surface plasmon resonance effect, but also were served as bridge for photo-generated carriers, thereby changing transfer behavior of charge carriers in ZnO/Bi2Sn2O7 composite system. Compared with ZnO/Bi2Sn2O7 type II heterojunction, Ag@ZnO/Bi2Sn2O7 Z-Scheme heterostructure promoted separation efficiency of photo-generated carriers, and the average carrier lifetime was increased by 2.0 times. The mineralization rate of ciprofloxacin by Ag@ZnO/Bi2Sn2O7 composite was 2.6 times higher than that of ZnO/Bi2Sn2O7 composite, which reached 16.9 times that of pure Bi2Sn2O7 photocatalyst, exhibiting stronger redox ability.

CdSe ve CdSe/ZnO Filmlerin Elektrokimyasal Sentezi: Morfolojik, Yapısal ve Elektronik Özellikler

CdSe films were electrochemically prepared on ITO electrode in aqueous solution applying a constant potential. Structural, morphological and optical features of CdSe thin films were examined with FE-SEM, XRD and UV-visible spectrophotometry techniques. XRD results revealed CdSe films were deposited, in the form of cubic crystals from aqueous solution of Cd2+ and Se4+, in presence of Na2SO4 supporting electrolyte. Band gap values of CdSe films were found between 1.88 and 2.0 eV. Wide band gap ZnO was electrochemically deposited on narrow band gap CdSe to improve its optoelectronic properties. Band gap value of CdSe/ZnO nanonorods was determined as 2.7 eV. Mott-Schottky equation was utilized to calculate flat band potential (EFB), as well as charge carrier density (ND) of materials. ND values were found as 1.623×1020 and 9.186×1020 cm-3 for CdSe and CdSe/ZnO, respectively. ZnO offers higher stability and lower band gap is promising material to be utilized in solar cell applications.

Quantum plasmonics of few electrons in strongly confined doped semiconducting oxide: A DFT + U study of ZnGaO

It has been reported in photodoping experiments that localized surface plasmonic resonances can be sustained with electrons as few as 3. We performed first principles calculations of density functional theory, with the Hubbard U correction, to see if localized surface plasmonic resonances can also be sustained by doping a wide bandgap ZnO with few shallow donors of Ga. We distributed 3–6 dopants approximately uniformly, due to quasi-spherical geometry of the quantum dot, in the dilute doping limit. The uniform distribution of dopants in quantum dots has been reported experimentally. Although the dopant configurations are limited due to computational cost, our findings shed light on absorption trends. Results for quantum dots of 1.4 nm, passivated with pseudo-hydrogens, show that localized surface plasmonic resonances can be generated in the near infrared range. The absorption linewidths for such small-sized quantum dots are broad. We find that the resonance linewidth depends on the orientation of surfaces and the number of secondary peaks on the concentration of dopants. The absorption coefficients, as functions of the principal values of the dielectric tensor, indicate that an electric field with orientation parallel to that of the most symmetric surface will produce localized surface plasmonic resonances with high quality factors.

Hot-phonon effects in photo-excited wide-bandgap semiconductors

Carrier and lattice relaxation after optical excitation is simulated for the prototypical wide-bandgap semiconductors CuI and ZnO. Transient temperature dynamics of electrons, holes as well as longitudinal-optic (LO), transverse-optic (TO) and acoustic phonons are distinguished. Carrier-LO-phonon interaction constitutes the dominant energy-loss channel as expected for polar semiconductors and hot-phonon effects are observed for strong optical excitation. Our results support the findings of recent time-resolved optical spectroscopy experiments.

Terrestrial solar radiation driven photodecomposition of ciprofloxacin in clinical wastewater applying mesostructured iron(III) oxide

Cationic cylindrical polymer brushes based on polybutadiene-block-poly(2-vinylpyridine) were applied as structure-directing agent for mesostructuring Fe2O3 nanoparticles into nanotubes. After temperature-controlled template removal, the obtained non-woven catalysts were tested for the photodegradation of ciprofloxacin under terrestrial solar radiation. At a slightly basic pH value, as typically encountered in clinical wastewaters, the mesostructured Fe2O3 shows a 4.5 times faster degradation of ciprofloxacin than commercial Aeroxide® TiO2 P25. Even wide-bandgap ZnO, mesostructured in the same way, is 1.6 times slower. Moreover, the non-woven-like structure of the catalyst allows for easy recovery of the catalyst and operation in a continuous flow reactor.Graphical abstract

Highly efficient inverted polymer solar cells by using solution processed MgO/ZnO composite interfacial layers

A highly efficient inverted polymer solar cell (PSC) has been successfully demonstrated by utilizing a wide bandgap magnesium oxide (MgO) film and ZnO stacked structure as an effective cathode interfacial layer. The MgO/ZnO bilayer structure is designed to combine the superiorities of both ZnO ETL and MgO film, based on the efficiency comparison of the PSCs without and with MgO interlayer. The ZnO film can serve as an efficient electron transport layer (ETL), while the MgO layer can reduce the surface defects of FTO and block the holes effectively, leading to an elevated electron collection and suppressed carrier recombination at the interface. With the excellent dual functions interface characteristics induced by the MgO/ZnO stacked films, the corresponding inverted PSC device based on the PTB7-Th:PC71BM photoactive layer system presents a superior power conversion efficiency (PCE) of 11.02%, which is higher than that of the PSC without MgO (8.79%). We believe that the MgO/ZnO bilayer structure is a superior interfacial contender for the organic photovoltaics and other photoelectric devices requiring cathode interfacial layers.

Magnetic Field Effects in Optical Harmonics Generation by Excitons

Mechanisms that are responsible for the optical harmonics generation at exciton transitions are analyzed in several classes of materials. In the cubic GaAs semiconductor, the magneto-induced optical second-harmonic generation second optical harmonic is observed in the region of orbital quantization of the valence and conduction bands. An unusually strong amplitude of the optical third-harmonic generation in an external magnetic field in the region of 1s exciton due to exciton–polariton resonance is found. The optical second-harmonic generation due to 1s, 2s, and 2p exciton resonances in a magnetic field is revealed in a hexagonal wide band-gap ZnO semiconductor. Depending on the symmetry of the exciton states, the mechanisms of the optical second-harmonic generation involve the spin and orbital Zeeman effects, and the magneto-Stark effect. The magneto-induced contribution to second harmonic generation (SHG) in the region of exciton transitions in the Cr2O3 antiferromagnet placed in an external magnetic field is studied.

The future and challenges of wide bandgap ZnO semiconductor materials

The future and challenges of wide bandgap ZnO semiconductor materials

Atomic Layer Deposition Process-Enabled Carrier Mobility Boosting in Field-Effect Transistors through a Nanoscale ZnO/IGO Heterojunction.

Low-temperature (≤400 °C), stackable oxide semiconductors are promising as an upper transistor ingredient for monolithic three-dimensional integration. The atomic layer deposition (ALD) route provides a low-defect, high-quality semiconducting oxide channel layer and enables accurate controllability of the chemical composition and physical thickness as well as excellent step coverage on nanoscale trench structures. Here, we report a high-mobility heterojunction transistor in a ternary indium gallium zinc oxide system using the ALD technique. The heterojunction channel structure consists of a 10 nm thick indium gallium oxide (IGO) layer as an effective transporting layer and a 3 nm thick, wide band gap ZnO layer. The formation of a two-dimensional electron gas was suggested by controlling the band gap of the IGO quantum well through In/Ga ratio tailoring and reducing the physical thickness of the ZnO film. A field-effect transistor (FET) with a ZnO/In0.83Ga0.17O1.5 heterojunction channel exhibited the highest field-effect mobility of 63.2 ± 0.26 cm2/V s, a low subthreshold gate swing of 0.26 ± 0.03 V/dec, a threshold voltage of -0.84 ± 0.85 V, and an ION/OFF ratio of 9 × 108. This surpasses the performance (carrier mobility of ∼41.7 ± 1.43 cm2/V s) of an FET with a single In0.83Ga0.17O1.5 channel. Furthermore, the gate bias stressing test results indicate that FETs with a ZnO/In1-xGaxO1.5 (x = 0.25 and 0.17) heterojunction channel are much more stable than those with a single In1-xGaxO1.5 (x = 0.35, 0.25, and 0.17) channel. Relevant discussion is given in detail on the basis of chemical characterization and technological computer-aided design simulation.

Transparent Co3O4/ZnO photovoltaic broadband photodetector

Based on the requirements of transparent optoelectronic devices, herein we report a highly transparent broadband photodetector, which is based on spinel structured p-Co3O4 and n-ZnO heterojunction. The XRD and XPS analysis confirmed the wurtzite ZnO phase along with the formation of spinel structured Co3O4. The structural property shows the good crystalline properties of the heterostructure thin films. The optical properties reveal the dual absorption states of Co3O4 corresponding to the Co2+ state (Eg = 1.93 eV) and Co3+ state (Eg = 1.42 eV) along with wide band gap ZnO (3.2 eV). Moreover, the photodetector shows an impressive transmittance of ~76% in the visible window. Further, the photoelectrical performance shows the prominent photovoltaic effect (self-powered operation) with a significant photosensitivity of 4.57 × 104. Moreover, the device is also capable of absorbing broad wavelengths from ultraviolet (UV) to infrared (IR) range. Importantly the device exhibits a fast response speed of 81.7 μs and a decay time of 178.8 μs which are found to be the fastest speed reported for transparent metal oxide-based p-n heterojunction devices under white light and self-powered mode. This work thus provides a comprehensive way of utilizing the robust metal oxide layers for developing a fast-high performing transparent self-powered device. The dual bandgap configuration of spinel Co3O4 demonstrates the wide range photo-responses, which is a promising feature for broadband transparent photodetectors and enhanced transparent photovoltaics.

In-situ ionothermal precipitation of well-dispersed ZnO nanoparticles onto 2-dimension neat graphene sheets with excellent photocatalytic activity

Neat graphene (NG) is rarely selected as a support to improve the photocatalytic performance of metal oxides due to its extremely poor processability in aqueous solutions. In this study, an in-situ ionothermal precipitation was developed to fabricate ZnO nanoparticle and few-layered NG sheet nanohybrids (ZnO/NG) in one-pot. Choline chloride/diethylene glycol-based deep eutectic solvent was used as reaction medium to dissolve the zinc salt precursor and disperse graphene. XRD, Raman, XPS and TEM analyses showed that ZnO particles with sizes of 15–25 nm were uniformly dispersed on the graphene sheets and a bond of ZnOC was formed between the interfaces. The UV–vis diffuse reflectance spectra measurement exhibited that the incorporation of graphene slightly narrowed the bandgap of ZnO nanoparticles, but significantly enhanced the light absorption of the nanohybrids in the visible range. Photocatalytic degradation of methylene blue dye evidenced that ZnO/NG offered excellent photocatalytic performance, regardless of UV or visible light irradiation. Under UV light, graphene as an electron transporter effectively promoted the separation of photogenerated carriers, while under visible light graphene was sensitized from the ground-state to the excited-state. The resulting electrons were injected into the conduction band of ZnO to transform wide bandgap ZnO into a photocatalyst driven by visible light. This work provided a green and simple method to prepare ZnO/NG nanohybrids with efficient and stable photocatalytic performance for water pollution remediation.

Single-electron pumping in a ZnO single-nanobelt quantum dot transistor

Diluted magnetic semiconductors (DMSs) have traditionally been employed to implement spin-based quantum computing and quantum information processing. However, their low Curie temperature is a major hurdle in their use in this field, which creates the necessity for wide bandgap DMSs operating at room temperature. In view of this, a single-electron transistor (SET) with a global back-gate was built using a wide bandgap ZnO nanobelt (NB). Clear Coulomb oscillations were observed at 4.2 K. The periodicity of the Coulomb diamonds indicates that the Coulomb oscillations arise from single quantum dots of uniform size, whereas quasi-periodic Coulomb diamonds correspond to the contribution of multi-dots present in the ZnO NB. By applying an AC signal to the global back-gate across a Coulomb peak with varying frequencies, single-electron pumping was observed; the increase in current was equal to the production of electron charge and frequency. The current accuracy of about 1% for both single- and double-electron pumping was achieved at a high frequency of 25 MHz. This accurate single-electron pumping makes the ZnO NB SET suitable for single-spin injection and detection, which has great potential for applications in quantum information technology.

Plasmon charge transfer dynamics in layered Au–ZnO nanocomposites

Understanding the charge transfer dynamics at the interface of metals and semiconductors has received much attention in efficient plasmonic induced photonic devices. Here, we present ultrafast charge transfer dynamics in Au–ZnO nanocomposite systems by exciting them in interband and intraband levels of Au with pump energies higher (2.48 eV) and lower (1.96 eV) than the threshold energy for interband transition (2.4 eV), using the femtosecond time-resolved pump-probe technique. The spectral responses for both the excitations exhibit different behaviors, and these variations are quantitatively interpreted in terms of pump-induced changes in the dielectric constant of Au. It is found from the temporal dynamics that the electron–phonon component in the Au–ZnO system decays relatively faster (∼3 ps in the multilayer Au–ZnO) than that observed in the Au sample (∼7 ps in Au) for both the pump energies. The transfer of highly energetic hot electrons from Au nanoparticles to ZnO across the Schottky barrier results in an accessible optical response for wide bandgap ZnO in the visible to infrared range via plasmon charge collection. The calculated charge transfer rate in the Au–ZnO system is found to be (>1011 s−1). Our results demonstrate the pump excitation dependent ultrafast plasmon charge behavior in an optically active Au–ZnO system that can be attractive for efficient plasmonic-based hybrid photonic devices.

Rapid visible and solar photocatalytic Cr(VI) reduction and electrochemical sensing of dopamine using solution combustion synthesized ZnO–Fe2O3 nano heterojunctions: Mechanism Elucidation

In recent times cost-effective advanced materials with dual applications in photocatalytic water treatment and electrochemical sensing have been explored and developed. Herein we report facile solution combustion synthesis of ZnO/Fe2O3 (ZF) type-II heterojunction for electrochemical sensing of dopamine and visible assisted photocatalytic reduction of carcinogenic Cr(VI) into Cr(III). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used to evaluate the compatibility of the electrodes for DA sensing. Furthermore, DPV curves revealed that ZF2 (ZnO: Fe2O3- 3:0.5) modified GCEs possessed 0.27 μM and 0.7 μM of LOD and LOQ with good linear range from 3 μM to 30 μM. Overall, reproducibility and interference studies confirm the efficient use of material for DA sensing. The junction ZF2 shows best performance with 88% Cr(VI) photo-reduction under visible light in 90 min and 100% reduction with tartaric acid as sacrificial agent (utilizing holes) in just 45 min. The effect of sacrificial agents and scavengers suggest the photogenerated electrons were major active species followed by ●O2−. The CB edges have enough potential for rapid reduction of hexavalent chromium under visible light and solar light. The photoluminescence and electrochemical impedance suggest lowered recombination, high charge separation and charge transfer capacity in the heterojunction. The transfer of electrons from conduction band of Fe2O3 to that of ZnO having high enough negative potential to reduce Cr(VI), thus utilizing the CB potential of wide band gap ZnO for rapid visible photocatalysis. In addition shortcomings as low conductivity Fe2O3 and high band gap of ZnO are both minimized in the junction. This study confirmed that popular semiconductors as ZnO and Fe2O3 have enough potential in dual role as electrochemical sensors and visible photocatalysts with best performance by optimizing the conditions and ratios.

Kaolin-supported ZnO nanoparticle catalysts in self-sensitized tetracycline photodegradation: Zero-point charge and pH effects

Natural waters are prone to pollution with organic contaminants, waste pharmaceuticals being an example. Pharmaceutical compounds are widely detected in different surface waters, such as lakes and rivers. Removal of such contaminants from water is therefore imperative. Different strategies are commonly followed such as adsorption, chlorination, ozonation, peroxidation and others. Each method has its shortcomings. Photodegradation of water organic contaminants by semiconductors is a safe and non-costly method. In photodegradation, the organic contaminant is completely mineralized with radiation and oxygen using stable semiconductor nanoparticles as photocatalysts. ZnO nanopowder is widely described for photodegradation processes. Due to its wide band gap (~3.2 eV) ZnO excitation is limited to UV radiation. Moreover, the nano-particle nature for the ZnO catalyst makes it difficult to recover and re-use after process completion. Supporting the ZnO nanoparticles onto stable solid material surfaces is one possible way out, and is studied here. Raw clay (kaolin) is described to support ZnO particles in photodegradation of a widely encountered pharmaceutical contaminant, tetracycline (TC). The results show the possibility of annealed ZnO/Kaolin to remove TC from water, firstly by adsorbing the contaminant then by photodegrading it into mineral species. The study shows that TC molecules pre-adsorbed onto ZnO/Kaolin were mineralized during photodegradation experiments. Complete mineralization of reacted contaminant molecules was confirmed by spectrophotometry, high performance liquid chromatography (HPLC) and total organic carbon (TOC) study. Control experiments with a cut-off filter (400 nm and shorter) confirmed the ability of the catalyst to function in the visible region, where contaminant molecules behave as sensitizers in the photodegradation process. Optimizing the efficiency of the ZnO catalyst in TC photodegradation processes by studying the effect of pH using the point of zero charge (pHPZC) concepts is achieved. Collectively, the results show the possibility to prepare an efficient recoverable ZnO/Kaolin catalyst, which can be sensitized with TC molecules themselves with optimal working pH range 6–9.

Prestress-Driven Optical Modulations of Flexible Oxide Thin Films Processed in Stretching Mode

The effects of stress in inorganic thin films have been long studied to minimize the negative influences of thermally-mismatched substrates mainly in terms of crystal growth and mechanical failure. Although it is well known that extra stress in thin films can affect optical properties, like bandgap, via the theoretical calculations of electronic structure, however, experimental verification of the optical modulation has not been available in the flexible systems. Here, we are dealing with a flexible semiconducting inorganic thin film which can be a fairly good representative for any semiconductor-involved flexible devices or components. We intentionally induced extra compressive stress in wide bandgap ZnO thin films by unique in situ stress-driven deposition processing that utilizes a uniaxial stretching mode of polymer substrate. The stretching mode provides homogeneous but biaxial stresses in the hexagonal wurtzite structure, leading to the extension of c-axis and the contraction of a-axis. The asymmetric consequences affect unexpectedly the energy levels of band structure, which is the origin of the in situ stress-driven optical modulations. We verify such changes successfully by using both the experimental and theoretical methods. The in situ deposition process of flexible thin films under a stretching mode provides compressive strain up to ~4.87%, which corresponds to an in-plane stress of less than ~9 GPa. The existence of the compressive stress induces distortion of the hexagonal unit cell as verified by high-resolution TEM images. We experimentally demonstrate the reduction of the optical bandgap by ~150 meV with the maximum compressive strain of ~4.87%. The experimental evidences of optical modulations were supported by the theoretical calculations using density functional theory. The reduced strong interactions between Zn 3d and O 2p orbitals are assumed to be responsible for the bandgap narrowing.

Narrowband Ultraviolet Photodetectors Based on Nanocomposite Thin Films with High Gain and Low Driving Voltage.

Narrowband ultraviolet (UV) photodetectors are highly desired in multiple areas. Photodetectors based on organic-inorganic nanocomposites offer high sensitivity, widely adjustable response range, light weight, and low-temperature solution processibility. However, the broad absorption range of organic and inorganic semiconductor materials makes it difficult to achieve a narrowband detection feature for nanocomposite photodetectors. In this work, nanocomposite thin films containing the wide band gap conjugated polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)- alt- co-(bithiophene)] (F8T2) blended with wide band gap ZnO nanoparticles (NPs) serve as the active layers of the photodetectors. Narrowband UV photodetectors with high gain and low driving voltage are demonstrated by adopting a symmetric device structure, controlling the active layer composition and microstructure, and manipulating the light penetration depth in the active layer. The fabricated photodetector exhibits a high external quantum efficiency of 782% at 358 nm under a low forward bias of 3 V with the full-width at half-maximum of 16 nm. Combined with a low dark current, a high specific detectivity of 8.45 × 1012 Jones is achieved. The impacts of the F8T2:ZnO NPs weight ratio and the device structure on the UV-selectivity and the device performance are investigated and discussed. Our method offers a pathway to design and fabricate narrowband UV photodetectors.

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

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