ZnO Band Edge Research Articles

Ultralow–biased solar photoelectrochemical hydrogen generation by Ag2S/ZnO heterojunction with high efficiency

BackgroundDue to its narrow band gap comparable to that of toxic PbS, Ag2S is a promising lead–free alternative for photosensitizing a variety of relatively wide band gap semiconductors, between which the staggered band alignment, however, severely limits their photoelectrochemical activity at low bias. MethodsTo address this issue, a quasi–type–II heterojunction is put forward, consisting of Ag2S deposited on the defective ZnO nanorods (NRs) flooded with numerous lattice imperfections by means of successive ionic layer adsorption and reaction (SILAR). Significant findingsThose defects serve as important relays in view of the characteristic shallow energy states increasing the density of states close to the conduction band edge of ZnO, via which is the electron injection from Ag2S to ZnO largely accelerated. This allows the photoexcited electrons efficiently separated from the geminate holes to in turn contribute to the hydrogen evolution reaction, as evidently manifested in the outstanding photocurrent density amounted to 6.5 mA cm−2 delivered by the Ag2S/ZnO heterojunction under one sun illumination at an external bias of -0.3 VAg/AgCl, well excelling those of additional Ag2S–based photoelectrodes reported in the literature.

Fabrication of hierarchical hybrid ZnO/Au micro-/nanostructures for efficient dye degradation: role of gold nanostructures in photophysical process

In this work, colloidal solutions of gold nanoparticles (AuNPs) with an average diameter of 16 nm and gold nanorods (AuNRs) with an average aspect ratio of 6.8 were synthesized. These gold nanostructures were decorated on nanosheets assembled 3D ZnO microflower (ZnO-MF) synthesized via facile and cost-effective co-precipitation method. Plasmonic gold nanostructures act as optical concentrators and a reservoir of electrons under LSPR excitation. They extend the light harvesting capability of ZnO from UV–VIS–NIR region, promotes interfacial charge transfer process and helps in electron-hole pair separation. We report the occurrence of FRET (Fluorescence Resonance Energy Transfer) between defect rich ZnO-MF and gold nanostructures under UV light excitation as evident from micro-PL spectra. This triggers surface plasmon resonance in gold nanostructures which causes direct transfer of hot electrons to ZnO-MF. Time-resolved Photoluminescence of the hybrid samples monitored at near band edge of ZnO shows elongated decay kinetics as compared to bare samples which could be attributed to direct transfer of hot electrons from gold to conduction band of ZnO-MF after LSPR excitation. Thereafter, the photocatalytic activity of the hybrids is evaluated by degradation of four different dyes under sunlight. ZnO-MF/AuNPs exhibited superior photocatalytic activity compared to ZnO-MF/AuNRs. This could be attributed to numerous reasons like better energy transfer efficiency, effective near-field enhancement, small size of AuNPs and enhanced electron-hole pair separation which makes ZnO-MF/AuNPs a promising photocatalyst.

Calculation of Band Offsets of Mg(OH)2-Based Heterostructures

The band alignment of Mg(OH)2-based heterostructures is investigated based on first-principles calculation. (111)-MgO/(0001)-Mg(OH)2 and (0001)-wurtzite ZnO/(0001)-Mg(OH)2 heterostructures are considered. The O 2s level energy is obtained for each O atom in the heterostructure supercell, and the band edge energies are evaluated following the procedure of the core-level spectroscopy. The calculation is based on the generalized gradient approximation with the on-site Coulomb interaction parameter U considered for Zn. For MgO/Mg(OH)2, the band alignment is of type II, and the valence band edge of MgO is higher by 1.6 eV than that of Mg(OH)2. For ZnO/Mg(OH)2, the band alignment is of type I, and the valence band edge of ZnO is higher by 0.5 eV than that of Mg(OH)2. Assuming the transitivity rule, it is expected that Mg(OH)2 can be used for certain types of heterostructure solar cells and dye-sensitized solar cells to improve the performance.

The effect of iron content on properties of ZnO nanoparticles prepared by microwave hydrothermal method

Iron doped (in the range from 1 to 10 mol %) ZnO nanoparticles (NPs) were obtained by microwave hydrothermal method. The effects of iron content on morphology, crystal structure, optical and magnetic properties of ZnO nanoparticles were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), cathodoluminescence (CL), photoluminescence (PL) and magnetic measurements. XRD studies revealed presence of ZnFe2O4 foreign phase in NPs for iron doping concentrations above 5% placing the solubility limit of Fe in ZnO NPs near this value. The photoluminescence and cathodoluminescence spectra show an increase of the intensity ratio between near band edge (NBE) and deep level emission band (DLE) of ZnO, which is tentatively related by us with intrinsic cracking of ZnO crystals. The blue-shift of NBE band maxima was observed as a concentration of Fe dopant increases. The results of magnetic measurements indicate superparamagnetic behavior of nanoparticles.

Comparison of effects of ZnO and TiO2 compact layer on performance of perovskite solar cells

In this work, organic−inorganic perovskite solar cells using compact layers of TiO2 and ZnO have been constructed. The ZnO compact layer was synthesized through the chemical hydrolysis process. The TiO2 compact layer was formed by spin-coating C16H28O6Ti in n-butanol. AFM and light absorption studies showed that the surface roughness and optical property of TiO2 and ZnO are nearly identical. However, according to the energy level diagram, the conduction band edge of ZnO is lower than that of TiO2, which is beneficial for electron transfer from the light absorption material to the FTO substrate and thus reducing electron-hole recombination. As a result, employing ZnO in the cell improved both the photocurrent and photovoltaic performance in comparison to TiO2. The best conversion efficiency obtained from the device using ZnO as the compact layer is 17.1%, which is attributed to facilitated photo-generated electron transport in the perovskite solar cell.

Theoretical model investigating the magnetic properties of cobalt-doped ZnO

We propose a theoretical model to investigate the magnetic properties of cobalt-doped ZnO (ZnO:Co) thin films qualitatively. The model was built on the dilute Co dopants in the host of ZnO forming the magnetic Co+2 ions and the energy level of the magnetic ions crossing the band edge of ZnO resulting in a magnetic interaction between the Co+2 spins and the spins of the electrons from the conduction band of ZnO. The mechanism of the ferromagnetism revealed in the studied system is proposed here to be induced not only by the mediated conducting electrons via spin interactions but also by the Coulomb excitations, arising from the electrons localized by the oxygen vacancies. This approach of including Coulomb excitation in the modified carrier-mediated model could explain well the magnetic properties of ZnO:Co and solves the drawback of the carrier-mediated model in interpreting the appearance of ferromagnetism in the insulating ZnO:Co. We propose that the Coulomb excitations induced by the electrons captured by the oxygen vacancies are an essential element in the magnetic ZnO, which reveals the fact that the bound magnetic polaron model without considering the Coulomb excitation is insufficient to describe the magnetic properties of ZnO.

Tailoring the optical properties of zinc/copper–incorporated onto eggshell synthesized via electrochemical method

Zinc/copper–incorporated onto eggshell was prepared by electrochemical technique and catalysts was subjected UV–vis DRS analysis for optical properties evaluation. The reflectance band edge of ZnO was slightly shifted from 375 to 371 nm after coupling with CuO. Different ZnO and CuO ratio content influence the intensity of ZnO–CuO characteristic reflectance band. Increased CuO ratio content narrower the band energy with 1 wt% ZnO–5 wt% CuO/ES catalyst established lowest value 1.67 eV.

Preparation and photocatalytic hydrogen evolution of g-C3N4/ZnO composite

The photocatalytic composite of g-C3N4/ZnO with different g-C3N4 content have been prepared by thermopolymerization method. The catalysts are characterized and analyzed by SEM, XRD, UV-Vis, BET and other analytical methods. The results show that g-C3N4/ZnO composites caused the red shift of the absorption band edge of ZnO, which increased the absorption of visible light and the separation rate of photogenerated electron-hole pairs. It is found that the H2 production rate of 25% g-C3N4/ZnO sample is best with 306.25 μmol∙h-1∙g-1, which is 4.6 times higher than of g-C3N4.

Conduction mechanisms in ZnO nanowires based Schottky diode grown under an electric field

We present a cathodoluminescence (CL) and electrical study of aligned ZnO nanowires based Schottky diodes synthesized by applying an AC electric field between two Au microcontacts. Our results reveal that the applied electric field aligns the ZnO nanowires between the electrodes and inhibits the formation of ZnO oxygen vacancies (VO). Local CL measurements of ZnO nanowires grown at different zones of the device show that the applied electric field inhibited the formation of oxygen vacancies (VO). Furthermore, CL spectra display an energy shift of the ZnO band edge emission, generated by changes in the relative intensity of two CL bands centered at 3.23 and 3.27 eV that correspond to the donor-acceptor pair and free electrons-acceptor transitions, respectively. We propose the formation of zinc vacancies (VZn) in nanowires that act as acceptor centers in the generation of these two electronic transitions. I-V curves acquired at room temperature reveal the photoresponse of the ZnO nanowires based Schottky diode exposed under UV (365 nm) illumination, exhibiting photocurrent intensities several times higher than that observed under dark conditions for applied bias lower than 1 V. The electrical conduction mechanisms in aligned ZnO nanowires of the device were tunneling and thermionic-emission for applied bias lower than 400 and 700 mV under dark and UV (365 nm) illumination conditions, respectively. For higher bias values, the device showed a conduction mechanism type field-emission.

Ultra-Violet Electroluminescence of ZnO Nanorods/MEH-PPV Heterojunctions by Optimizing Their Thickness and Using AZO as a Transparent Conductive Electrode

In this paper, a series of ITO/ZnO/ZnO nanorods/MEH-PPV/Al were prepared with different thicknesses of MEH-PPV that were changed from 15, 10 to 7 nm. The electric field in the devices was analyzed. An increase in the electric field on ZnO made hole injection easy and the electrons tunnel fast through thinner MEH-PPV to ZnO. This made the carriers prefer to recombine inside the ZnO layer, and the emission of ZnO was predominant under direct current (DC) bias. Furthermore, another device was fabricated with the structure of AZO (Al-doped ZnO)/ZnO/ZnO nanorods/MEH-PPV/Al. Ultra-violet (UV) electroluminescence (EL) at 387 nm from ZnO band edge emission was realized under DC bias. The turn-on voltage of the devices having AZO as the electrode is lower than that of ITO, and the EL power is enhanced. This work also studies the effect of inserting LiF underneath the Al electrode and above the layer of MEH-PPV. The LiF film inserted caused an obvious decrease in turn-on voltage of the devices and a pronounced increase in the EL power. The mechanism of electroluminescence enhancement is also discussed.

Catalyst-Free Vertical ZnO-Nanotube Array Grown on p-GaN for UV-Light-Emitting Devices.

One-dimensional (1D) structures-based UV-light-emitting diode (LED) has immense potential for next-generation applications. However, several issues related to such devices must be resolved first, such as expensive material and growth methods, complicated fabrication process, efficiency droop, and unavoidable metal contamination due to metal catalyst that reduces device efficiency. To overcome these obstacles, we have developed a novel growth method for obtaining a high-quality hexagonal, well-defined, and vertical 1D Gd-doped n-ZnO nanotube (NT) array deposited on p-GaN films and other substrates by pulsed laser deposition. By adopting this approach, the desired high optical and structural quality is achieved without utilizing metal catalyst. Transmission electron microscopy measurements confirm that gadolinium dopants in the target form a transparent in situ interface layer to assist in vertical NT formation. Microphotoluminescence (PL) measurements of the NTs reveal an intense ZnO band edge emission without a defect band, indicating high quality. Carrier dynamic analysis via time-resolved PL confirms that the emission of n-ZnO NTs/p-GaN LED structure is dominated significantly by the radiative recombination process without efficiency droop when high carrier density is injected optically. We developed an electrically pumped UV Gd-doped ZnO NTs/GaN LED as a proof of concept, demonstrating its high internal quantum efficiency (>65%). The demonstrated performance of this cost-effective UV LED suggests its potential application in large-scale device production.

Unusual bandgap bowing in highly mismatched ZnOS alloys: Atomistic tight-binding band anti-crossing model

An atomistic band anticrossing (BAC) model is developed and used to study “unusual bowing” in energy bandgap and its dependence on the material composition in minority O anion-alloyed ZnS (ZnS1−xOx) and minority S anion-alloyed ZnO (ZnO1−xSx) highly mismatched alloys. For dilute O in ZnS1−xOx, it is found that the bandgap decreases as the O composition is increased. A “down-shift” in the conduction band edge (CBE) of host ZnS, which arises from an interaction between the CBE and the localized O defect state, is identified as the root cause. However, the reduction in bandgap as a function of dilute S composition in the ZnO1−xSx alloy follows an “up-shift” in the valence band edge (VBE) of host ZnO, which arises from an interaction between the VBE and the localized S defect state. The BAC model captures the E+ and E− splitting in the sub-bands, which are found to be an admixture of the extended CBE (VBE) of ZnS (ZnO) and the localized O (S) state. A fully atomistic 8-band sp3-spin tight-binding basis set is used to construct the Hamiltonian for the wurtzite host materials as well as their alloy supercells. For alloy supercells, a strain is computed via the valence force-field formalism using Keating potentials. The O and S energy states are found to be approximately 199 meV below the CBE of ZnS and 190 meV above the VBE of ZnO, respectively. Overall, the calculated energy bandgaps using the BAC model are in good agreement with corrected local density approximation (LDA+U) calculations and experimental results.

High-Performance Quantum Dot Light-Emitting Diodes Based on Al-Doped ZnO Nanoparticles Electron Transport Layer.

ZnO nanoparticles (NPs) are widely used as the electron transport layer (ETL) in quantum dot light-emitting diodes (QLEDs) owing to their suitable electrical properties. However, because of the well-aligned conduction band levels, electrons in QDs can be spontaneously transferred to adjacent ZnO NPs, leading to severe exciton dissociation, which reduces the proportion of radiative recombination and deteriorates the device efficiency. In this work, Al-doped ZnO NPs are thoroughly investigated as a replacement of ZnO for QLEDs. The energy band structures of Al-doped ZnO are modified by adjusting the concentration of Al dopants. With the increasing Al content, the work function and the conduction band edge of ZnO are gradually raised, and thus the charge transfer at the interface of QDs/ETL is effectively suppressed. Consequently, the green QLEDs with 10% Al-doped ZnO NPs exhibit maximum current efficiency and external quantum efficiency of 59.7 cd/A and 14.1%, which are about 1.8-fold higher than 33.3 cd/A and 7.9% of the devices with undoped ZnO NPs. Our work suggests that Al-doped ZnO NPs can serve as a good electron transport/injection material in QLEDs and other optoelectronic devices.

Pure ultraviolet emission from ZnO quantum dots-based/GaN heterojunction diodes by MgO interlayer

We demonstrate the fabrication and characterization of ZnO/GaN-based heterojunction light-emitting diodes (LEDs) by using air-stable and solution-processable ZnO quantum dots (QDs) with a thin MgO interlayer acting as an electron blocking layer (EBL). The ZnO QDs/MgO/p-GaN heterojunction can only display electroluminescence (EL) characteristic in reverse bias regime. Under sufficient reverse bias, a fairly pure ultraviolet EL emission located at 370 nm deriving from near band edge of ZnO with a full width at half maximum (FWHM) of 8.3 nm had been obtained, while the deep-level emission had been almost totally suppressed. The EL origination and corresponding carrier transport mechanisms were investigated qualitatively in terms of photoluminescence (PL) results and energy band diagram. Open image in new window

Gold nanoparticle-embedded silk protein-ZnO nanorod hybrids for flexible bio-photonic devices

Silk protein has been used as a biopolymer substrate for flexible photonic devices. Here, we demonstrate ZnO nanorod array hybrid photodetectors on Au nanoparticle-embedded silk protein for flexible optoelectronics. Hybrid samples exhibit optical absorption at the band edge of ZnO as well as plasmonic energy due to Au nanoparticles, making them attractive for selective UV and visible wavelength detection. The device prepared on Au-silk protein shows a much lower dark current and a higher photo to dark-current ratio of ∼105 as compared to the control sample without Au nanoparticles. The hybrid device also exhibits a higher specific detectivity due to higher responsivity arising from the photo-generated hole trapping by Au nanoparticles. Sharp pulses in the transient photocurrent have been observed in devices prepared on glass and Au-silk protein substrates due to the light induced pyroelectric effect of ZnO, enabling the demonstration of self-powered photodetectors at zero bias. Flexible hybrid detectors have been demonstrated on Au-silk/polyethylene terephthalate substrates, exhibiting characteristics similar to those fabricated on rigid glass substrates. A study of the performance of photodetectors with different bending angles indicates very good mechanical stability of silk protein based flexible devices. This novel concept of ZnO nanorod array photodetectors on a natural silk protein platform provides an opportunity to realize integrated flexible and self-powered bio-photonic devices for medical applications in near future.

Effect of annealing on the sub-bandgap, defects and trapping states of ZnO nanostructures

Annealing treatment was applied to different mesoporous ZnO nanostructures prepared by wet chemical synthesis, i.e. nanoflowers (NFs), spherical aggregates (SPs), and nanorods (NRs). The sub-bandgap, defect properties as well as the trapping state characteristics after annealing were characterized spectroscopically, including ultrasensitive photothermal deflection spectroscopy (PDS), photoluminescence and photo-electrochemical methods. The comprehensive experimental analysis reveals that annealing alters both the bandgap and the sub-bandgap. The defect concentration and the density of surface traps in the ZnO nanostructures are suppressed upon annealing as deduced from photoluminescence and open-circuit voltage decay analysis. The photo-electrochemical investigations reveal that the surface traps dominate the near conduction band edge of ZnO and, hence, lead to high recombination rates when used in DSSCs. The density of bulk traps in ZnO SPs is higher than that in ZnO NFs and ZnO NRs and promote lower recombination loss between photoinjected electrons with the electrolyte-oxidized species on the surface. The highest power conversion efficiency of ZnO NFs-, ZnO SPs-, and ZnO NRs-based DSSC obtained in our system is 2.0, 4.5, and 1.8%, respectively.

Pristine and cadmium-doped zinc oxide: chemical synthesis and characterizations

The chemical synthesis of pristine and cadmium-doped ZnO powders using a simple, cost-effective at 65 °C is reported and characterized for their structures, optical and morphological studies using X-ray diffraction, UV–visible–Near Infra-Red (UV–Vis–NIR) spectroscopy, scanning electron microscopy measurement techniques where XRD spectra confirm the formation of ZnO and Cd-doped ZnO with hexagonal crystal structure. The particle size of ZnO is reduced on Cd-doping from 16 to 14 nm. Plane-view surface morphology analysis supported for spherical-type crystallites and UV–Vis–NIR spectra reveal shift in the band edge of ZnO after Cd-doping. Photo-degradation study of Methylene Blue dye shows Pristine ZnO degrades dye faster than Cd-doped ZnO.

Optical spectroscopy characterization of Cu doped ZnO nano- and microstructures grown by vapor-solid method

Cu doped ZnO micro- and nanostructures have been grown by a vapor-solid method. The influence on the morphology of the structures of the precursor used for both host and dopant has been investigated. The density and uniformity of the structures is higher when ZnS is used for host precursor. Regarding the dopant incorporation, different degree of segregation is observed depending on the precursor used (metallic Cu or Copper oxides). The dopant distribution has been studied by X-ray Microanalysis, (EDS). The luminescence properties have been studied by cathode- and photoluminescence techniques. The ZnO band edge emission is quenched when Cu is incorporated to the host while defect and Cu related bands are still present. Moreover, a modification of the spatial distribution of the luminescence centres due to segregation is also observed. Raman spectroscopy and X ray diffraction have been also used to assess the properties of the samples grown.

Core-shell ZnO@CuInS2 hexagonal nanopyramids with improved photo-conversion efficiency

A soft chemical route has been employed to synthesize ZnO@CuInS2 (ZnO@CIS) core–shell hexagonal nanopyramids using 2-mercaptopropionic acid as a surface-functionalized agent. The core–shell formation was confirmed by the X-ray diffraction, high resolution transmission electron microscope and X-ray photoelectron spectroscopy characterizations. UV–visible absorption spectrum of the core–shell nanopyramids shows a red shift of the band edge of pure ZnO in the visible region along with broadening of the absorption spectrum. ZnO@CIS nanopyramids can be used as an antireflection coating on Silicon solar cell, exhibit lower reflectance and better light trapping ability than the Silicon solar cell with ZnO nanopyramids. Moreover higher photocurrent is obtained for the n-ZnO/n-CIS core–shell junction due to favorable band bending. The efficiency of the p-Si/n-ZnO solar cell (6.6%) is improved to 10% for p-Si/n-ZnO@n-CIS configuration.

Understanding the Effect of MgO Interfacial Layer on ZnO/High-K/FTO Transparent Thin Film Transistors for Large-Area Transparent Electronics Applications

We have fabricated transparent ZnO thin film transistors (TFTs) with fluorine doped tin oxide (FTO) as a gate electrode and HfO2 as a gate dielectric. To suppress the high interface state density (Dit), a series of thickness of MgO interfacial layer between ZnO and HfO2 has been investigated. ZnO TFTs with 10 nm MgO interfacial layer displayed improved performance with field effect mobility of 0.3 cm2V-1s-1, threshold voltage of 3.7 V and on/off ratio of 106 compared to 5 nm, 15 nm and 20 nm MgO interfacial layer. Dit and its energy level distribution below 0.25-0.32 eV below the conduction band edge of ZnO were quantified using the conductance method.