Presence Of Internal Electric Field Research Articles

Nitrogen-rich carbon nitride (C3N5) coupled with oxygen vacancy TiO2 arrays for efficient photocatalytic H2O2 production

Developing efficient and facilitated recycling photocatalysts for H2O2 formation is an ideal strategy for solar-to-chemical energy conversion. In this work, we synthesized ultrathin C3N5 nanosheets through the process of thermal polymerization and polyvinylpyrrolidone (PVP)-assisted solvent exfoliation. Subsequently, the obtained ultrathin C3N5 nanosheets were tightly attached to the surface of TiO2-x arrays, resulting in an enhanced photocatalytic H2O2 production rate. The density functional theory (DFT) calculations demonstrate that an internal electric field (IEF) is generated between the TiO2-x array and the ultrathin C3N5 due to the different work functions. The presence of IEF provides an additional driving force for carrier separation and transfer in the heterointerface. Benefitting from this unique strategy, the optimal heterojunction obtains the highest H2O2 formation rate (2.93 μmol/L/min), which is about 4.1 times than that of TiO2-x arrays. The rotating disk electrode (RDE) analysis manifests H2O2 formation through 2e--dominated oxygen reduction reaction (ORR). This research provides an innovative strategy for assembling a type-II heterojunction with a useful IEF for efficient photocatalytic H2O2 production.

Mechanistic study of B-TiO2/BiVO4 S-scheme heterojunction photocatalyst for tetracycline hydrochloride removal and H2 production

S-schemed photocatalysts have been employed extensively to address energy issues and environmental contaminants because of their high redox ability and effective separation of photocharges. In this study, a novel S-scheme B-TiO2/BiVO4 photocatalyst was successfully fabricated using a simple in-situ hydrothermal method to enhance tetracycline hydrochloride (TCH) degradation and H2 evolution. The unique composition of B-TiO2 and BiVO4, as well as their synergistic effect, increased the number of multidimensional active reaction sites and enhanced the efficiency of photogenerated electron pair separation. In particular, when exposed to visible light, the rate of H2 evolution increased to 561.99 μmol·g−1·h−1, and the rate of TCH breakdown increased to 89.30 % in 120 min. The effects of initial pH, catalyst dosage, TCH concentration, and various coexisting anions were studied. It was discovered that pH and various anions had the greatest impact on the photocatalytic performance of B-TiO2/BiVO4 for the breakdown of TCH. In addition, free radical capture experiments and Electron paramagnetic resonance (EPR) revealed that ·O2− and h+ were the most important reactive species that accelerated the photocatalytic activities of B-TiO2/BiVO4. Work function calculations demonstrated that the presence of internal electric field (IEF) in the special B-TiO2/BiVO4 heterojunction, indicating the formation of an S-scheme heterojunction in the B-TiO2/BiVO4 system, which greatly facilitated the separation of electron-hole (e−-h+) pairs to promote efficient TCH degradation and hydrogen production. This study provides a potential strategy for developing high-performance photocatalysts that can produce clean energy without negatively impacting the environment.

Influence of Internal Electric Field on the Spectral Characteristics of Blue GaN-Based Superluminescent Light-Emitting Diodes

The effects of internal electric field on the spectral characteristics of InxGa1−xN/GaN superluminescent light-emitting diodes are studied theoretically. The Schrodinger and Poisson equations and the rate and optical propagating equations are solved in the presence of the internal electric field. By increasing the indium mole fraction in the quantum well, the internal electric field increases linearly. The injection current affects the intensity, bandwidth and wavelength of the spectrum. The results show that the internal electric field shifts the spectral radiations to the red region. Also, in the presence of internal electric field, the peak intensity of spectra increases, and the optical gain reduces.

Construction of highly ordered ZnO microrod@SnO2 nanowire heterojunction hybrid with a test-tube brush-like structure for high performance lithium-ion batteries: experimental and theoretical study

ZnO@SnO2 heterojunction hybrid with a novel test-tube brush-like structure is fabricated successfully through a facile hydrothermal approach. Sixfold symmetric epitaxial growth of SnO2 nanowires on non-polarized planes of ZnO microrods is realized via the approach. The synthesized ZnO@SnO2 with the unique structure as a lithium-ion battery anode material exhibits enhanced electrochemical properties when compared with the single-phase ZnO microrod and SnO2 nanowire electrode. The existence of heterostructure on the influence of lithium storage performance is studied by using first-principles calculations through density functional theory method. Experimental and theoretical calculation results demonstrate that the excellent electrochemical performance of the heterojunction hybrid is ascribed to the synergistic effect of the respective components. The construction of ZnO@SnO2 heterojunctions can not only provide facile Li+ kinetics and enhanced ion diffusion, but also improve the mobility rate of electrons and holes by means of the presence of internal electric field.

Controlled crystallization of BiOCl/BiF3 on ZnO–Bi2O3–B2O3 glass surfaces for photocatalytic and self-cleaning applications

Transparent ZnO–Bi2O3–B2O3 glass was fabricated by using melt-quenching technique. The controlled surface crystallization of glasses was achieved via etching using HCl/HF. Four different concentrations (3, 6, 9, and 12 M) of each acid were used to get different amount of surface crystallization. X-ray diffraction (XRD) confirms BiOCl and BiF3 surface crystallization in glass. The band gaps of BiF3 crystallized glasses were found to be more than band gap of BiOCl crystallized glasses, which was mainly due to higher band gap of BiF3 than that of BiOCl. Glass sample having maximum BiOCl crystals showed more hydrophobicity (contact angle of 143.4°), than that in glass sample having maximum BiF3 crystals (contact angle of 116.1°). BiOCl crystallized glasses showed superior photocatalytic activities than that of BiF3 crystallized glasses. It is due to the narrow band gap values of BiOCl than that of BiF3 and also due to presence of internal electric field in BiOCl layered structure, which prevents electron-hole pair recombinations. Methylene Blue was fully degraded by one of BiOCl crystallized samples within 225 min under ultraviolet irradiation (UV), however maximum 90% degradation was achieved in one of BiF3 crystallized samples even after 300 min under UV irradiation.

Internal-field-dependent low-frequency piezoelectric energy harvesting characteristics of in situ processed Nb-doped Pb(Zr,Ti)O3 thin-film cantilevers

Piezoelectric thin-film-based cantilevers have been investigated for higher energy-harvesting performance with simplified processing steps. Here, a simple in situ film deposition process of heavily Nb-doped lead zirconate titanate (PZT) films, which does not require annealing and poling, has been demonstrated to verify the possibility of use of the resultant films as energy-harvesters specifically for low frequency vibrating sources. The in situ domain formation of the films during the deposition was demonstrated from the apparent shifts of the capacitance-electric field curves, indicating the presence of internal electric fields. The so-called imprint behavior was found to be directly related to the performance of piezoelectric energy harvesting. As an optimal example, 12 mol% Nb-doped cantilever harvesters that showed the largest imprint behavior exhibited the best values of ∼19.1 GPa figure-of-merit and ∼1436 μWcm−3g−2 power density, which are competitive compared to other reported values.

Nonlinear optical properties in the laser-dressed two-level AlxGa1−xN/GaN single quantum well

The electronic states in the laser-dressed hexagonal and cubic Al[Formula: see text]Ga[Formula: see text]N/GaN single quantum wells are calculated using the effective mass equation. The hexagonal single quantum well contains an internal electric field due to spontaneous and piezoelectric polarizations. The effective mass equation is solved by the finite difference method. The energy levels in both cubic and hexagonal laser-dressed wells are found to increase with increase in laser dressing as the effective well widths in both the wells increase. The intersubband energy spacing between first excited state and ground state increases in the cubic quantum well, whereas it decreases in the hexagonal well due to the presence of internal electric field in it. Using the compact density matrix method with iterative procedure, first-, second- and third-order nonlinear optical susceptibilities in the laser-dressed quantum well are calculated taking only two levels. While the susceptibilities in the hexagonal well are found to get red shifted, the susceptibilities in the cubic well are blue shifted.

Tunable intersubband transitions in ZnO/ZnMgO multiple quantum wells in the mid infrared spectral range

We report on controllable tuning of intersubband transitions in ZnO/Zn0.60Mg0.40O multiple quantum well structures grown by molecular beam epitaxy on sapphire. The transitions from the first to the second electronic energy state within the conduction band are directly observed by infrared spectroscopy. By variation of the quantum well width, the intersubband transition energies are tuned from 290 to 370 meV. The experimental results are in good agreement with theoretical calculations assuming the presence of internal electric fields of 2 MV·cm−1.

Quaternary Al xIn yGa 1− x− yN layers deposited by pulsed metal-organic vapor-phase epitaxy for high efficiency light emission

Quaternary AlInGaN quantum wells in GaN barriers were grown by metal-organic vapor-phase epitaxy. Changing to a growth sequence with pulsed metal-organic supply leads to structures with enhanced photoluminescence efficiencies. The amount of material was varied, resulting in AlInGaN layer thicknesses between nominally 1.5 and 10 nm. We have analyzed the material properties by X-ray diffraction (XRD) as well as photoluminescence (PL) spectroscopy. The observed XRD-spectra and the PL intensity show the high quality of the deposited material. By analyzing the PL spectra, we have found an energy shift of the resonance lines from 2.65 to 3.33 eV with decreasing well thickness. We attribute this shift mainly to the presence of internal electric fields in the AlInGaN/GaN heterostructures. Power-dependent and time-resolved PL experiments confirm this observation. By properly adjusting the material composition, we could achieve polarization field compensation of the quaternary QW structures. Also, first luminescence experiments on ternary InGaN QW embedded in quaternary barrier material were performed.

Pulsed layer growth of AlInGaN nanostructures

AbstractA pulsed layer growth mode in the metal‐organic vapor phase epitaxy (MOVPE) was used to fabricate excellent quality AlInGaN nanostructures. The amount of material was varied, resulting in AlInGaN layer thicknesses between nominally 1.5 nm and 6 nm, respectively. We have analyzed the material properties by X‐ray diffraction (XRD) as well as photoluminescence (PL) spectroscopy. The observed XRD‐spectra and the PL intensity show the high quality of the deposited material.By analyzing the PL spectra we have found an energetic shift of the resonance lines from 2.65 eV to 3.33 eV with decreasing well thickness. We attribute this shift mainly to the presence of internal electric fields at the AlIn‐GaN/GaN interface. Power‐dependent and time resolved PL experiments confirm this observation. Comparing the luminescence at elevated temperatures, the pulsed layer epitaxy structures reveal a much higher intensity as the conventional grown samples. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Internal electric fields in vacuum-evaporated organic films as studied by electroabsorption spectroscopy

Electroabsorption (EA) spectroscopy (Stark effect spectroscopy) has been applied to provide information on the internal electric fields occurring in thin films of organic compounds sandwiched between two semitransparent aluminium electrodes (Al/organic film/Al/glass substrate) under ambient conditions. A wide range of molecular materials, commonly used in organic light-emitting diodes, has been investigated. The analysis of the (1 ω)EA signals measured on the first harmonic of the applied sinusoidal electric field frequency ( ω) indicates a sample asymmetry and the presence of internal electric fields ( F i) in organic films with estimated values of F i between 4 × 10 4 and 3 × 10 5 V/cm. The possible origin of internal electric fields is discussed. The observed features of F i can be consistently explained by the asymmetrical distribution of space charge in the organic film associated with different defect (of structural and/or impurity origin) characteristics on the substrate and non-substrate side of the sample.

Observation of long-lived oblique excitons in GaN-AlGaN multiple quantum wells

The 2-K recombination dynamics of coupled GaN-AlGaN multiple quantum wells reveals a composite nature in terms of the joint contributions of short-lived direct excitons and long-lived oblique ones. The possibility to observe these oblique excitons, which produce a unusual blueshift of the photoluminescence at long decay times is found to be in straightforward correlation with the presence of internal electric fields together with the existence of a slight disorder at the monolayer scale. [S0163-1829(99)09815-X].

Evidence of internal electric fields inGaInP2by scanning capacitance and near-field scanning optical microscopy

GaInP{sub 2} is studied in cross section with the scanning capacitance and near-field scanning optical microscope. Our study shows significant differences in the electronic and optical properties between ordered single- and two-variant GaInP{sub 2}. In single-variant samples, spatially uniform capacitance signal, photoluminescence intensity, and band gap are observed. In contrast, a spatially nonuniform capacitance signal, photoluminescence intensity, and band gap are observed in samples with nominally uniform doping. Imaging of the same regions by scanning capacitance and near-field scanning optical microscopes demonstrates that the photoluminescence (observed by the near-field scanning optical microscope) comes only from the n-type-like regions (observed by the scanning capacitance microscope) in lightly doped (n-type) two-variant GaInP{sub 2}. The local capacitance and photoluminescence measurements can be explained by the presence of internal electric fields in two-variant GaInP{sub 2}. {copyright} {ital 1997} {ital The American Physical Society}

Simultaneous diffusion of two or more ions in the presence of internal electric fields

The theory of diffusion in the presence of internal electric fields developed in the preceding paper is generalized to the case of simultaneous diffusion of more than one species. It is shown that for the case of two chemically similar impurities, an experiment can be devised which virtually eliminates the effects of these internal fields, making possible accurate measurement of diffusion parameters. The theory is applied to H−D diffusion in rutile and it is shown that the main effect of the internal fields is to reduce the difference between the two diffusion coefficients while keeping the average nearly constant. Detailed calculations for the system are presented.