Wide Bandgap Semiconductor ZnO Research Articles

Ultrafast Charge Dynamics in Bulk Zinc Oxide under Intense Photoexcitation

The photo-induced charge dynamics of textbook wide-bandgap semiconductor ZnO have been investigated on the picosecond time-scale. We performed optical Pump-THz Probe experiments in order to measure the dielectric constant of the material after high-fluence photo-excitation of charge carriers. The technique allows access to both carrier lifetime and scattering rates, and it provides direct access to the intrinsic dielectric function changes upon excitation. A complex dynamic is unveiled in the high-fluence pumping regime, where the relaxation time is in the hundreds of picoseconds range and increases with increasing Pump fluence, while the onset of photoconductivity takes place in a few picoseconds. The plasma frequency and the relaxation time dependence on the Pump fluence are discussed.

Optical, rheological, dielectric, and electrical properties of multiple oxides nanosuspended glycerol based semiconductor hybrid nanofluids

Semiconductor hybrid nanofluids (SHNFs) of multiple hybrid nano oxides (TiO2 + ZnO, TiO2 + Al2O3, TiO2 + SiO2, ZnO + Al2O3, and ZnO + SiO2) dispersed at fixed concentration (0.01 wt/wt%) in glycerol (Gly) fluid were prepared through solution mixing and probe ultrasonic cavitation process. These formulated SHNFs (i.e., TiO2 + ZnO/Gly, TiO2 + Al2O3/Gly, TiO2 + SiO2/Gly, ZnO + Al2O3/Gly, and ZnO + SiO2/Gly) were characterized by employing an ultraviolet–visible (UV–vis) spectrophotometer (wavelength range 200 – 800 nm), rheometer (shear rate range 1.32 – 5.28 s−1), and precision inductance–capacitance–resistance (LCR) meter (frequency range 50 Hz – 1 MHz). The dependency of UV–vis absorbance on the equal weight amount combinations of semiconductive and insulating nanosuspended hybrid oxides for these SHNFs was explored and analyzed for energy band gaps which showed strongly ruled by the wide band gap semiconductor TiO2 and ZnO materials. Dielectric permittivity, electrical conductivity, impedance, and electric modulus spectra of these SHNF materials were reported at 298.15 K and confirmed the influence of nanosuspended hybrids on the dielectric polarizations and relaxation behaviour. Dynamic viscosity and shear stress study with the shear rate explained the Newtonian behaviour of these SHNF materials. The multi-functionality of these novel SHNFs for the advances in soft condensed matter device/system technologies including optoelectronic, photovoltaic, UV-blocking, sensing, wavelength filtering, nanodielectric, electrical insulator, and thermal energy and mass transporter were demonstrated.

Tuning the Carrier Transfer Behavior of Coaxial ZnO/ZnS/ZnIn2 S4 Nanorods with a Coherent Lattice Heterojunction Structure for Photoelectrochemical Water Oxidation.

Serious degradation and the short photogenerated carrier lifetime for the wide-bandgap semiconductor ZnO have become prominent issues that negatively affect photoelectrochemical (PEC) water splitting. Herein, a novel electron transport pathway was constructed by simple but effective coaxial growth of ZnO/ZnS/ZnIn2 S4 heterostructure nanoarrays to increase the carrier separation efficiency. This new photoanode fulfilled the requirements of both favorable band alignment and stability, achieving a stable photocurrent density of 1.146 mA cm-2 at 1.2 VRHE , which was approximately twice that of pristine ZnO. Detailed experimental studies revealed that the improved PEC activity was due to the lattice-matching interface coherency that activated the carrier transport pathway, giving rise to an optimized interfacial electronic structure for promoted charge separation by the built-in electric field and strengthened water oxidation activity. This design may provide a new approach to fabricating various efficient lattice-matching coherent interface photoanodes for PEC water splitting.

Fourier transform second harmonic generation for high-resolution nonlinear spectroscopy

In this communication we demonstrate a new characterization technique combining Fourier transform (FT) spectroscopy and second harmonic generation (SHG) that enables high spectral resolution with broadband femtosecond laser pulses. The strong and narrow exciton resonances of the wide band gap semiconductor ZnO were chosen for demonstrating the capabilities of the method. FT-SHG offers high reproducibility and high spectral resolution within the bandwidth of the input pulse.

ZnO1−x coatings deposited by atmospheric plasma spraying for room temperature ppb-level NO2 detection

NO2 is one of the main pollutants that affect air quality, and the use of ZnO as a gas-sensitive material to prepare sensors for NO2 detection has broad prospects. Wide bandgap semiconductor ZnO requires a high-temperature or ultraviolet-light illumination condition for excitation, and the high temperature or ultraviolet light will increase energy consumption and accelerate component aging. This paper reports the method of deposition of ZnO1−x gas-sensitive coatings by atmospheric plasma spraying. With the optimization of hydrogen flow rate in plasma forming gas, the ZnO1−x coatings can be directly deposited by a typical atmospheric plasma spraying process. Under the illumination of visible light, the as-sprayed ZnO1−x gas sensors achieved room temperature responses to ppb-level NO2. The best hydrogen flow rate for atmospheric plasma sprayed ZnO1−x coatings was found to be 7 L/min, and the H-7 ZnO1−x sensor exhibited the best gas sensing performance under blue light illumination. In addition, the enhanced effects of oxygen vacancies on optical and gas sensing properties were also discussed.

Эффекты магнитного поля в экситонной генерации оптических гармоник

Mechanisms responsible for the optical harmonics generation in several classes of materials at excitonic transitions are considered. In the cubic semiconductor GaAs, magnetically induced second harmonic generation (SHG) was observed in the region of orbital quantization for the valence and conduction bands. An unusually strong third harmonic generation was detected in the region of the 1s exciton in an external magnetic field due to the exciton-polariton resonance. Magnetic field induced SHG was revealed in the hexagonal wide bandgap semiconductor ZnO at exciton resonances 1s, 2s, and 2p. Depending on the symmetry of the exciton states, the SHG mechanisms are determined by the spin and orbital Zeeman effects, and the magneto-Stark effect. Magnetically induced contribution to SHG was studied in the antiferromagnet Cr2O3 at exciton transitions in an external magnetic field.

Synergy of nano-ZnO and 3D-graphene foam electrodes for asymmetric supercapacitor devices.

Two kinds of electrode materials were produced to fabricate asymmetric supercapacitor devices: (i) Highly defective, n-type wide bandgap semiconductor ZnO nanocrystalline electrodes below 50 nm were synthesized with the aid of the high energy ball milling technique. (ii) Flexible 3D-graphene foams were synthesized via the chemical vapor deposition technique. Extensive defect structure analysis was performed via enhanced characterization techniques mainly the spectroscopy ones: electron paramagnetic resonance (EPR), Raman, and photoluminescence (PL). Compared to bulk ZnO electrodes the nanoscale ZnO electrodes revealed a dramatic increase of defect concentration. The surface defect plays a crucial role in the electrochemical performance of supercapacitor devices. Strong decreases in charge transfer resistance were observed for the smallest crystallite size which is 15 nm. This work also shows that synthesis, controlling the defect structures, electronic and electrical characterization and the device production are extremely important to obtain high performance faradaic asymmetric supercapacitors.

Phonon Evidence of Kohn Anomalies in Nanogenerator ZnO

The origin of unique piezotronic properties within low dimensional nanomaterial systems will enable an in-depth understanding of nanogenerators for broad applications in the future. Notably, the low dimensional ZnO exhibits stronger temperature sensitivity than the bulk ZnO, which will be proved by the extreme phonon instability at room temperature 300 K. The temperature dependence shows a nearly Fermi-Dirac δ-function. We have proposed the selection criteria for the nanogenerator material screening based on the theoretical derivation of elastic perturbation entropy (EP-S). The resulted phonon conservation behaviors induced by the discontinuity in phonon dispersion dominates the highly sensitive response to the local electrical field change. The Kohn Anomalies (KA) has been identified in the wide bandgap semiconductor ZnO, in which discontinuous energy conversions induced by such KA or boundary discontinuity will not only minimize the degeneration effect but also induce the high piezotronic response. Moreover, we have carefully examined the ZnO surface on both structural and electronic structures, which identified the surface metallic properties. Thus, these theoretical results have supplied sound phonon evidence for the unique piezotronic properties in ZnO, which will facilitate the new insight in the future research of nanogenerators.

Antenna-enhanced high harmonic generation in a wide-bandgap semiconductor ZnO

We demonstrate high harmonic generation (HHG) into deep-UV range in a ZnO single crystal with resonant nanoantennas. Non-perturbative HHG is successfully induced by optical excitation of as low as 20 GW/cm2. The spectral selection rule is found to reflect crystal symmetry, suggesting the possibility of nano-scaled EUV sources and band-structure reconstruction.

Construction of heterojunction ZnFe2O4/ZnO/Ag by using ZnO and Ag nanoparticles to modify ZnFe2O4 and its photocatalytic properties under visible light

In this study, a narrow band gap semiconductor ZnFe2O4 is prepared first and then a wide band gap semiconductor ZnO is used to modify it by using hydrothermal method under the participation of the chelating agent nitrilotriacetic acid (NTA), which will be removed in the process of high temperature baking. The structure and composition of ZnFe2O4/ZnO heterojunction were characterized by using powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and UV–Vis diffuse reflectance spectra (DRS). In order to improve the photocatalytic activity of ZnFe2O4/ZnO heterojunction under visible light irradiation Ag nanoparticles were loaded on its surface by using a very simple and special method namely silver mirror reaction at room temperature without agitation. The methyl orange (MO) was taken as model dyes to illustrate the photocatalytic performance of synthesized products on visible light irradiation. Maximum degradation of 84% for MO was achieved when one 85 W ordinary household energy-saving lamp was used as light source and ZnFe2O4/ZnO/Ag as multicomponent photocatalyst.The photocatalytic degradation reaction kinetics of methyl orange by ZnFe2O4/ZnO and ZnFe2O4/ZnO/Ag were investigated. Better photoelectrochemical performance of ZnFe2O4/ZnO and ZnFe2O4/ZnO/Ag composites provides substantial evidence for its excellent photocatalytic properties. Furthermore, the photocatalytic mechanisms of ZnFe2O4/ZnO and ZnFe2O4/ZnO/Ag composites for methyl orange degradation under visible light were proposed.

Antenna-enhanced high harmonic generation in a wide-bandgap semiconductor ZnO.

High harmonic generation (HHG) in solids has great potential for coherent extreme ultraviolet (EUV) sources, all-optical band-structure reconstruction, and electron dynamics metrology. Solid HHG driven by plasmonic near-fields will open a new paradigm, enabling high repetition-rate HHG with a compact laser, HHG manipulation with meta-surfaces, and precise control over carrier trajectory. In this paper, we demonstrate antenna-enhanced HHG in a wide-bandgap semiconductor ZnO. By exploiting gold nano-antennas resonating at the driver wavelength of 2 μm, we successfully trigger HHG at input intensity of ~0.02 TW/cm2 and observe harmonic radiations up to 9th-order. Orders-of-magnitude enhanced conversion efficiency at the hot-spots brings about ten-fold enhancement in the total yield. The spectral selection rule is found to reflect crystal symmetry, suggesting the possibility of nano-scaled EUV sources and band-structure reconstruction.

Halide‐Modulated Functionality of Wide Band Gap Zinc Oxide Semiconductor Nanoparticle

AbstractSurface modification of inorganic nanomaterials using different ions is well‐known to induce significant modulation in functionality of the nanoparticles (NP), which improves tuning their relevant properties for suitable applications in the field of nanotechnology. Here, we report synthesis and surface modification of a model inorganic wide bandgap semiconductor ZnO NP by halide ions by simple precipitation method and demonstrate that proximity of various halide ions to the NP modifies their electronic properties. It is interesting to note that such surface modulation has little impact on its morphology, while significantly influencing their applications in photocatalytic activity or magnetism. Indeed, the halide attached ZnO NP displays a large reduction of defect state emission. Various microscopic and spectroscopic tools (including picosecond resolved technique) were used to characterize and understand the key role of the halide ions to modulate the light induced charge separation in ZnO NP. Picosecond resolved fluorescence spectra unveil a significant quenching, implying an electronic cross‐talking between the NP and the attached halide ions. Magnetic measurements indicate that attachment of a suitable halide ion introduces room temperature ferromagnetism in this system. Finally, the photocatalytic activity of the ZnO NP for the photodegradation of a model pollutant, methyl orange was evaluated under UV light irradiation. Halide ZnO is anticipated to be a very promising photocatalytic agent; hence it can be used to cleanup environmental pollution. First principles analysis has been performed to understand the modulation of electronic properties like photocatalysis and magnetism in halide attached nanosystem. The experimental behavior resembles nicely with theoretical results.

Effect of the magnetic order on the room-temperature band-gap of Mn-doped ZnO thin films

Exchange interaction between localized magnetic moments mediated by free charge carriers is responsible for a non-monotonic dependence of the low-temperature energy band-gap in dilute magnetic semiconductors. We found that in weakly doped Mn-ZnO films, increasing the exchange interaction by increasing the concentration of free charge carriers results in a red-shift of the near-band-edge emission peak at room temperature. An increase of Mn concentration widens the band gap, and a blue-shift prevails. Exchange interaction can be used to tune the room-temperature optical properties of the wide-band gap semiconductor ZnO.

Wide Bandgap Semiconductor One-Dimensional Nanostructures for Applications in Nanoelectronics and Nanosensors

Wide bandgap semiconductor ZnO, GaN and InN nanowires have displayed the ability to detect many types of gases and biological and chemical species of interest. In this review, we give some recent examples of using these nanowires for applications in pH sensing, glucose detection and hydrogen detection at ppm levels. The wide bandgap materials offer advantages in terms of sensing because of their tolerance to high temperatures, environmental stability and the fact that they are usually piezoelectric. They are also readily integrated with wireless communication circuitry for data transmission.

Spin transport and spin dephasing in zinc oxide

The wide bandgap semiconductor ZnO is interesting for spintronic applications because of its small spin-orbit coupling implying a large spin coherence length. Utilizing vertical spin valve devices with ferromagnetic electrodes (TiN/Co/ZnO/Ni/Au), we study the spin-polarized transport across ZnO in all-electrical experiments. The measured magnetoresistance agrees well with the prediction of a two spin channel model with spin-dependent interface resistance. Fitting the data yields spin diffusion lengths of 10.8nm (2K), 10.7nm (10K), and 6.2nm (200K) in ZnO, corresponding to spin lifetimes of 2.6ns (2K), 2.0ns (10K), and 31ps (200K).

Ideal Energy‐Level Alignment at the ZnO/P3HT Photovoltaic Interface

AbstractHybrid semiconductor‐polymer nanostructured solar cells hold the promise of photovoltaic energy conversion based on abundant and nontoxic materials and scalable manufacturing processes. After a decade of intense research activity, hybrid solar cells still exhibit low short‐circuit currents and moderate open‐circuit voltages. These bottlenecks call for a detailed understanding of the physics underlying the device operation at the nanoscale. Using first‐principles calculations the ideal energy‐level alignment of hybrid solar cell interfaces based on the wide bandgap semiconductor ZnO and the polymer poly(3‐hexylthiophene) (P3HT) is investigated. The interfacial charge transfer is quantified and it is shown that this effect increases the ideal open‐circuit voltage with respect to the electron‐affinity rule by as much as 0.5 V. The results of this work suggests that there is significant room for optimizing this class of excitonic solar cells by tailoring the semiconductor/polymer interface at the nanoscale.

Characterization and performance of Schottky diode based on wide band gap semiconductor ZnO using a low-cost and simplified sol–gel spin coating technique

In this study, a Schottky diode based on wide band gap semiconductor ZnO was fabricated on p-type Si substrate using sol–gel spin coating method. Al/ZnO/p-Si diode indicates a rectification behavior. At lower voltages, the forward current of the diode was found to obey the intrinsic thermally generated charge carriers and at relatively higher voltages, the current mechanism of the diode is controlled by a space charge limited conduction mechanism. Under reverse bias conditions, the current–voltage characteristics of the diode exhibit the lower current as compared under forward bias, indicating the existence of a current limitation mechanism induced by two field lowering mechanisms which are Poole–Frenkel and Schottky mechanisms. The parameters, series resistance R S, the ideality factor n and the barrier height φ B0 of the diode were determined by performing different plots obtained from the experimental forward bias current–voltage. The capacitance measurements show that the values of capacitance were almost independent of the forward bias under various frequencies. The higher values of the capacitance at low frequencies were attributed to the excess capacitance resulting from the interface states in equilibrium with the ZnO.

Irradiation Effect of 8MeV Protons on Single-Crystalline Zinc Oxide Films

Wide-bandgap semiconductor ZnO potentially exhibits high radiation hardness since large displacement threshold energy of constituent atoms can be expected due to the small lattice constant and large bandgap energy. To study the radiation hardness, the effect of proton irradiation on single-crystalline n-type ZnO films was examined. These films were grown by molecular beam epitaxy, and irradiated by 8MeV protons with fluences of 1.4 × 1015, 2.8 × 1015, 5.6 × 1015 and 1.4 × 1016p/cm2. A rapid increase of electrical resistance by a decrease of carrier density was observed with a threshold fluence of about 1 × 1015p/cm2. This change in electrical properties was associated with a steep deterioration of the near-bandedge emission intensity in cathodoluminescence. These radiation damages were found to recover after a thermal annealing over 600°C. Such high radiation hardness of ZnO exceeds that of GaN, indicating promising application of this material to space- and nuclear-electronics.

Magnetic Resonance of Impurities, Intrinsic Defects and Dopants in ZnO

In the last years an increasing technological interest in the wide band gap semiconductor ZnO has widely stimulated research on this material. The electrical and optical properties of ZnO are strongly affected by the presence of defects and dopant. For instance, to obtain p-type conductivity it requires not only to find a suitable doping element but also to suppress counteracting intrinsic defects. Magnetic resonance experiments can contribute considerable knowledge to this subject. We review the current knowledge on impurities, intrinsic defects and doping elements in ZnO as obtained by magnetic resonance experiments in this paper.

Study of ferromagnetism in Mn doped ZnO dilute semiconductor system

The wide band gap semiconductor ZnO when doped with a very low percent of some transition metal ions can exhibit above room temperature ferromagnetism, transforming it into a unique compound for spin-electronic applications. In the present work we have compared the electronic structure of two polycrystalline Zn1-xMnxO pellets (for x=0.02 and 0.04), prepared by low temperature processing, and carefully characterized. The Rietveld refinement of the XRD patters established that the samples have the ZnO lattice with ZnS type Wurtzite hexagonal symmetry and no detectable impurities. The samples exhibit distinctly different magnetic properties. The pure ZnO pellet shows a diamagnetic behaviour, the 2% sample displayed a clear FM ordering at 300 K while the 4% sample did not show any ordering even upon cooling. Their electronic structure has been investigated using x-ray absorption and x-ray photoemission spectroscopy with an aim to find out how the changes in the electronic structure can correlate to the magnetic properties in such diluted magnetic semiconductor materials. The results show that most of the Mn ions of the ferromagnetic sample are in the divalent state. For the higher Mn percent nonmagnetic sample, a larger contribution of higher oxidation Mn states are dominant and the oxygen content also increases. The two factors can be correlated to the suppressed ferromagnetism, though it is hard to exactly predict that which of these two factors weighs more.