Internal Electrostatic Field Research Articles

Er3+-doping induced formation of orthorhombic/monoclinic Bi5O7I heterostructure with enhanced visible-light photocatalytic activity for removal of contaminants

Bi5O7I is a member of the bismuth oxyiodides family with unique layered structures, generating an internal electrostatic field to promote the separation and transfer of photogenerated charge carriers. To further enhance its visible-light photocatalytic activity, various strategies, including creating the heterojunction, forming the defects, and engineering the bandgap and composition, have been applied. Another promising strategy is to construct a heterostructured photocatalyst. Here, we report a doping-induced formation of orthorhombic/monoclinic-Bi5O7I (o-Bi5O7I/m-Bi5O7I) heterostructure and its visible-light photocatalytic activity for organic dye degradation and NO removal. Under hydrothermal conditions, Bi5O7I is doped with Er3+, upconversion luminescence agent, in a varying content. The XRD results reveal that 3% Er3+ is decisive in commencing the phase transition from orthorhombic to monoclinic. Though an increasing content of Er3+ induces the phase transition, flower-like Bi5O7I microspheres assembled by nanosheets remains unchanged. An obvious redshift in the absorption edge and enhancement of visible-light absorption are observed for Bi5O7I samples doped with >3% Er3+ due to the orthorhombic-to-monoclinic phase transition. The photocatalytic activities of the samples were evaluated by the degradation of three different organic dyes (Rhodamine B, methyl orange and methylene blue) in aqueous solution under white LED light irradiation and the photooxidative removal of gaseous NO under simulated solar light irradiation. The 5% Er3+-doped Bi5O7I shows the highest photocatalytic activity for the photodegradation of Rhodamine B (91.3%), methyl orange (70.0%) and methylene blue (76.0%) and for the photooxidative removal of NO (54.0%) under visible light. A strong luminescence generating extra photons, the formation of an orthorhombic/monoclinic-Bi5O7I heterostructure promoting more effective charge separation, and the emergence of monoclinic-Bi5O7I absorbing more visible light are a collective mechanism leading to the enhancement of photocatalytic activity.

Artificial Second-Order Nonlinear Optics in a Centrosymmetric Optical Material BiVO4: Breaking the Prerequisite for Nonlinear Optical Materials.

Second-order nonlinear optics (NLO) is the foundation of frequency conversion for the generation of coherent light at frequencies where lasers have no emissions or operate poorly. The prerequisite for NLO materials is noncentrosymmetric symmetry that can generate an effectively non-counterbalanced spontaneous electronic polarization. Here, we propose that this material restriction can be broadened by controlling the electron distribution with a local internal electrostatic field (IEF), and we demonstrate artificially created and manipulated second harmonic generation (SHG) in a centrosymmetric optical material, a superimposed Co2+- and Mo6+-doped BiVO4 thin film with 2/m point group symmetry, where a homojunction producing tunable effective polarization is formed. The SHG was characterized and tuned by IEF. This work breaks the structural symmetry constraint on NLO materials. Besides, the phase-matching-like condition was realized for the further improvement of the efficient frequency conversion. Because polarization is also a prerequisite for many other functions besides SHG, we believe that this work should provide some inspiration for the further development of optoelectronic, photonic, and electronic materials.

Fabrication of a Novel p–n Heterojunction BiOCl/Ag6Si2O7 Nanocomposite as a Highly Efficient and Stable Visible Light Driven Photocatalyst

Herein, a visible-light-active BiOCl/Ag6Si2O7 nanocomposite with a strong interfacial interaction p–n heterojunction structure was fabricated via a simple deposition–precipitation method and subsequently investigated as a novel photocatalyst for the first time. The structure, morphology, and optical properties of the prepared samples were thoroughly characterized by field-emission scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, X-ray diffraction, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The photocatalytic performance was evaluated by monitoring the degradation of methyl orange (MO) and phenol and the photocurrent generated under visible-light irradiation. The BiOCl/Ag6Si2O7 photocatalyst increased significantly its photocatalytic performance compared to the pristine BiOCl and Ag6Si2O7 materials. This enhancement could be ascribed to the strong visible light absorption and the effective separation of the photogenerated electrons (e−) and holes (h+) by the internal electrostatic field generated at the junction region. In addition, BiOCl/Ag6Si2O7 showed stable photocurrent over long times and cyclic degradation of MO, thereby demonstrating potential applications in the field of environmental remediation.

Microfluidic-enhanced 3-D photoanodes with free interfacial energy barrier for photoelectrochemical applications

The sluggish reaction kinetics and poor interfacial mass transfer seriously limit the industrial applications of planar photoelectrocatalytic devices. Here, the principle of 3-D flow-through photoanodes with free interfacial barrier for electron transfer and microfluidic channels for reactant transportation was demonstrated. Owing to the epitaxial growth of anisotropic ZnO nanorods with internal electrostatic field onto carbon cloth (CC), the spatial separation of photo-induced charge carriers was realized. Experimental characterizations confirmed the formation of Schottky-barrier-free interface with low electrical resistance for electron transfer, resulting in the significantly decreased onset potential. Compared to traditional planar FTO/ZnO photoanodes, flow-through CC/ZnO photoanodes exhibited 4 times (for Rhodamine B) and 3 times (for bisphenol A) higher degradation kinetics. Fluid dynamics simulation suggested that the flow-through mode greatly enhanced the microscale velocity magnitude and the mass transfer of reactants. Thus, this work presents an ideal platform for the design of 3-D microfluidic-enhanced system for photoelectrochemical applications.

Surface potential stability of large-area Teflon PTFE electret dosimeters of different thicknesses

The effects of charge polarity and thickness on the surface charge potential stability responses of large-area Teflon polytetrafluoroethylene (PTFE) discs electrostatically charged by a new “modified single point-to-plane corona poling rotating system” were studied. Large-area Teflon discs (6 cm diameter) with 0.2, 0.5 and 1 mm thicknesses were used to produce both negatively or positively charged Teflon discs, at the same charging conditions. The deposited charge potential stability of the Teflon discs of different thicknesses monitored by a non-contacting electrostatic voltmeter for an extended time were monitored up to 77 days. Some inter-related parameters such as surface charge potential and external electrostatic field, internal electrostatic field, surface charge density, and the storage duration were also studied. The thick large-area Teflon disc electrets produced provided a steady-state charge stability on either negative (0.2, 0.5 and 1 mm thick) or positive (1 mm thick) surface charge potentials for the long period studied. The Teflon electret discs produced demonstrated having the highest charge potential stability in thicker electrets (1 mm), either negative or positive polarity, with provision of high potential for different applications in general and radiation protection dosimetry in particular.

Highly efficient photocatalytic degradation of naphthalene by Co3O4/Bi2O2CO3 under visible light: A novel p–n heterojunction nanocomposite with nanocrystals/lotus-leaf-like nanosheets structure

Exploring visible-light-driven nanomaterials to substitute crucial bulk composite under UV-light is of great significance in photodegradation of hardly-decomposed persistent pollutants in the environment. Herein, the novel p–n heterojunction photocatalyst p-Co3O4/n-Bi2O2CO3 with zero dimensional(0D)-two dimensional(2D)nanocrystals/lotus-leaf-like nanosheets structure was successfully obtained for the first time via a facile hydrothermal method, in which the density of Co3O4 loaded on the Bi2O2CO3 was easily tuned by the concentration of Co3O4 precursor in the solution. The outstanding feature of the photocatalyst is that the resultant hybrid nanocomposite exemplified the p–n heterojunction visible-light materials for degrading the naphthalene efficiently and effectively. As the presence of Co3O4 and the synergistic interactions of p–n heterojunction, the recombination of photogenerated charge carriers has been suppressed, and the visible light absorbance has been improved. Consequently, the visible light photocatalytic performance has been enhanced. The p-Co3O4/n-Bi2O2CO3 sample with Co3O4/Bi2O2CO3 mass ratio 1:6 exhibited the highest photocatalytic activities among all the as-prepared samples with the degradation efficiency of about 91.02% naphthalene in 150 min. The photocatalytic mechanism of p-Co3O4/n- Bi2O2CO3 for degrading naphthalene was analyzed based on the semiconductor energy band theory and the formation of an internal electrostatic field. The probable degradation intermediates and products of naphthalene were identified by GCMS. The proposed photodegradation pathways of naphthalene were described by combining the frontier electron density calculation and GCMS results.

The Transfer Direction of Photogenerated Electrons in BaTiO3/TiO2 and CaTiO3/TiO2

BaTiO3/TiO2 and CaTiO3/TiO2 bilayer-type photocatalyst films have been prepared and characterised by X-ray diffraction, Raman, X-ray photoelectron spectroscopy, UV-vis, and scanning electron microscopy techniques. The photodeposition of silver was done to confirm the reduction positions of the titanate/TiO2 films. Silver deposited preferentially on the side of TiO2 for BaTiO3/TiO2 whereas on the side of CaTiO3 for CaTiO3/TiO2. These results imply that the direction of photogenerated electron transfer is coincident with the semiconductor physical principles. Upon exposure to light, electron–hole pairs are generated and subsequently separated by an internal electrostatic field in the titanate–TiO2 heterojunction.

Enhanced Photocatalytic Activities of g-C3N4 via Hybridization with a Bi-Fe-Nb-Containing Ferroelectric Pyrochlore.

Ferroelectricity may promote photocatalytic performance because the carrier-separation efficiency can be effectively improved by the internal electrostatic field caused by spontaneous polarization. Heterostructures that combine ferroelectric materials with other semiconductor materials can be further advantageous to the photocatalysis process. In this work, Bi1.65Fe1.16Nb1.12O7 was hybridized with g-C3N4 via a facile low-temperature method. The results of high-resolution transmission electron microscopy confirmed that a tight interface was formed between g-C3N4 and Bi1.65Fe1.16Nb1.12O7, which gave the (g-C3N4)-(Bi1.65Fe1.16Nb1.12O7) heterojunction a more superior visible light photocatalytic performance. The degradation of rhodamine B by optimized (g-C3N4)0.5-(Bi1.65Fe1.16Nb1.12O7)0.5 under visible light was almost 3.3 times higher than that by monomer Bi1.65Fe1.16Nb1.12O7 and 7.4 times higher than that by g-C3N4. The (g-C3N4)0.5-(Bi1.65Fe1.16Nb1.12O7)0.5 sample also showed the highest photocurrent in the photoelectrochemical tests. To further verify the benefit of the built-in electric field in terms of the photocatalytic performance, Bi2FeNbO7, with a higher spontaneous polarization, was also synthesized and hybridized with g-C3N4. Both Bi2FeNbO7 and (g-C3N4)0.5-(Bi2FeNbO7)0.5 exhibited better photocatalytic activities than those of Bi1.65Fe1.16Nb1.12O7 and (g-C3N4)0.5-(Bi1.65Fe1.16Nb1.12O7)0.5, although the latter ones had a stronger visible-light absorbance. This implies the very promising prospects of applying ferroelectric materials for solar energy harvest.

Preparation of a p-n heterojunction BiFeO3@TiO2 photocatalyst with a core–shell structure for visible-light photocatalytic degradation

Magnetically separable bismuth ferrite (BiFeO3) nanoparticles were fabricated by a citrate self-combustion method and coated with titanium dioxide (TiO2) by hydrolysis of titanium butoxide (Ti(OBu)4) to form BiFeO3@TiO2 core–shell nanocomposites with different mass ratios of TiO2 to BiFeO3. The photocatalytic performance of the catalysts was comprehensively investigated via photocatalytic oxidation of methyl violet (MV) under both ultraviolet and visible-light irradiation. The BiFeO3@TiO2 samples exhibited better photocatalytic performance than either BiFeO3 or TiO2 alone, and a BiFeO3@TiO2 sample with a mass ratio of 1:1 and TiO2 shell thickness of 50–100 nm showed the highest photo-oxidation activity of the catalysts. The enhanced photocatalytic activity was ascribed to the formation of a p-n junction of BiFeO3 and TiO2 with high charge separation efficiency as well as strong light absorption ability. Photoelectrochemical Mott–Schottky (MS) measurements revealed that both the charge carrier transportation and donor density of BiFeO3 were markedly enhanced after introduction of TiO2. The mechanism of MV degradation is mainly attributed to hydroxyl radicals and photogenerated electrons based on energy band theory and the formation of an internal electrostatic field. In addition, the unique core–shell structure of BiFeO3@TiO2 also promotes charge transfer at the BiFeO3/TiO2 interface by increasing the contact area between BiFeO3 and TiO2. Finally, the photocatalytic activity of BiFeO3@TiO2 was further confirmed by degradation of other industrial dyes under visible-light irradiation.

Novel TiO 2 /Ag 2 CrO 4 nanocomposites: Efficient visible-light-driven photocatalysts with n–n heterojunctions

Abstract In this study, novel TiO 2 /Ag 2 CrO 4 nanocomposites were fabricated by a facile refluxing method using commercial TiO 2 , silver nitrate, and sodium chromate at low temperature of 96 °C. The as-prepared photocatalysts were characterized by XRD, EDX, SEM, TEM, UV–vis DRS, FT–IR, BET, and PL techniques. Compared with the TiO 2 sample, TiO 2 /Ag 2 CrO 4 nanocomposites exhibited highly enhanced photocatalytic activity in degradations of rhodamine B (RhB), methylene blue (MB), and fuchsine dyes under visible-light irradiation. Among the prepared photocatalysts, the TiO 2 /Ag 2 CrO 4 (50%) nanocomposite showed the superior activity of 8.8, 5.4, and 44-folds relative to the TiO 2 sample in degradations of RhB, MB, and fuchsine, respectively. The trapping experiments demonstrated that holes and superoxide anion radicals were the main reactive species in the degradation reaction. Based on the obtained results, the photocatalytic activity enhancement was attributed to strong visible-light absorption, effective separation of photogenerated charge carriers by internal electrostatic field produced through n–n heterojunction formed between counterparts of the nanocomposite, and enhanced surface area.

Anisotropic Photoelectrochemical (PEC) Performances of ZnO Single-Crystalline Photoanode: Effect of Internal Electrostatic Fields on the Separation of Photogenerated Charge Carriers during PEC Water Splitting

This work investigates the anisotropic PEC performances of ZnO single-crystalline (SC) photoanodes and the effect of internal electrostatic fields on the separation of photogenerated charge carriers during PEC water splitting. It was found that the internal electrostatic field can greatly influence the bulk charge separation efficiencies during PEC water splitting depending on its orientations, which can only be promoted as the internal electrostatic field in accordance with the direction of the holes’ transportation. Due to the surface stabilization of ZnO polar surfaces, the internal electrostatic field would be gradually decreased to zero near the surface of ZnO SC photoanodes. Therefore, the interfacial charge separation would be mainly determined by the interfacial electric fields in the space charge region formed by the equilibration of the Fermi levels between ZnO and the electrolyte solution. However, the differences on the bulk charge separation efficiencies of ZnO SC photoanodes are much larger ...

Bare Cd1-xZnxS ZB/WZ Heterophase Nanojunctions for Visible Light Photocatalytic Hydrogen Production with High Efficiency.

In this work, we report the synthesis of Cd1-xZnxS zinc blende/wurtzite (ZB/WZ) heterophase nanojunctions with highly efficient charge separation by a solvothermal method in a mixed solution of diethylenetriamine (DETA) and distilled water. l-Cysteine was selected as a sulfur source and a protecting ligand for stabilization of the ZB/WZ homojunction. The optimal ternary chalcogenide Cd0.7Zn0.3S elongated nanocrystals (NCs) without any cocatalyst loading show very high visible light photocatalytic activity with H2 production efficiency of 3.13 mmol h(-1) and an apparent quantum efficiency of 65.7% at 420 nm. This is one of the best visible light photocatalysts ever reported for photocatalytic hydrogen production without any cocatalysts. The charge separation efficiency, having a critical role in enhancing photocatalytic activity for hydrogen production, was significantly improved. Highly efficient charge separation with a prolonged carrier lifetime is driven by the internal electrostatic field originating from the type-II staggered band alignment at the ZB/WZ junctions, as confirmed by steady and time-resolved photoluminescence spectra. Further, the strong binding between the l-cysteine ligand and Cd1-xZnxS elongated nanocrystals protects and stabilizes NCs; the l-cysteine ligand at the interface could trap holes from Cd1-xZnxS NCs, while photogenerated electrons transfer to Cd1-xZnxS catalytic sites for proton reduction. Our results demonstrate that Cd1-xZnxS ZB/WZ heterophase junctions stabilized by l-cysteine molecules can effectively separate charge carriers and achieve highly visible light photocatalytic hydrogen production. The present study provides a new insight into the design and fabrication of advanced materials with homojunction structures for photocatalytic applications and optoelectronic devices.

Nucleation and growth of (10͞11) semi-polar AlN on (0001) AlN by Hydride Vapor Phase Epitaxy.

Wurtzite AlN is widely used for deep ultraviolet optoelectronic devices (DUV), which are generally grown along the [0001]-direction of the wurtzite structure on currently available substrates. However, huge internal electrostatic fields are presented within the material along [0001] axis induced by piezoelectric and spontaneous polarization, which has limited the internal quantum efficiency of AlN based DUV LEDs dramatically. The internal fields can be strongly reduced by changing the epitaxial growth direction from the conventional polar c-direction into less polar crystal directions. Twinned crystal is a crystal consisting of two or more domains with the same crystal lattice and composition but different crystal orientations. In other words, twins can be induced to change crystal directions. In this work we demonstrated that the epitaxial growth of () semi-polar AlN on (0001) AlN by constructing () and () twin structures. This new method is relative feasible than conventional methods and it has huge prospect to develop high-quality semi-polar AlN.

Novel Ag3PO4/CeO2 p-n Hierarchical Heterojunction with Enhanced Photocatalytic Performance

The composite Ag3PO4/CeO2 photocatlyst, a novel p-n type heterojunction, has been successfully fabricated through a facile hydrothermal process combined with a successive in situ precipitation technique. The X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and UV-visible diffuse reflectance spectra (DRS) were used to characterize the as-obtained products. The SEM and TEM image show that CeO2 particles have been successfully loaded and well distributed in the surface of Ag3PO4. The photocatalytic activities of the p-Ag3PO4/n-CeO2 heterojunctions were investigated for their efficiency on the degradation of Rhodamine B (RhB) under ultra-violet light and visible light irradiation, and the results showed that the p-Ag3PO4/n-CeO2 heterojunctions possessed remarkable photocatalytic activities. The enhanced photocatalytic activity can be attributed to the extended absorption in the visible light region resulting from the Ag3PO4 and the effective separation of photogenerated carriers driven by the internal electrostatic field in the junction region.

Enhanced photoelectrochemical activity and photocatalytic water oxidation of NiO nanoparticle-decorated SrTiO3 nanocube heterostructures: Interaction, interfacial charge transfer and enhanced mechanism

Here we demonstrate the enhanced photoelectrochemical (PEC) performance and photocatalytic activity towards water oxidation of n-type SrTiO3 (STO) nanocubes coupled with p-type NiO nanoparticles (denoted as p-NiO@n-STO nanoparticle@nanocube), fabricated by calcining Ni(NO3)2-loaded STO nanocubes. High resolution transmission electron microscope studies show that intimate p–n junctions are formed between n-type STO and p-type NiO interfaces. As a p–n junction photoanode, the p-NiO@n-STO nanoparticle@nanocube nanostructures exhibit the enhanced PEC activity and photocatalytic water oxidation. The photocurrent density of the p-NiO@n-STO nanostructure electrode reaches 3.5µAcm−2, which is 7 times higher than that (0.5µAcm−2) for pure STO nanocube electrode at 0V versus Ag/AgCl. The p-NiO@n-STO nanoparticle@nanocube nanostructures also exhibit pronounced photoresponse to generate O2. The O2 generation is about 100µmol after irradiation 3h with a 300-W Xenon lamp, which is about 1.7 times higher than that (60µmol) of pure STO nanocubes. The intimate p–n junctions between p-NiO and n-STO interfaces accelerate holes migration from valence band of n-STO to that of p-NiO and reduction of the recombination of electron–hole pairs by the internal electrostatic field in the interface, which are believed to be the main reasons for enhanced PEC response and water-splitting performance of the as-prepared p-NiO@n-STO nanoparticle@nanocube nanostructures. The present work demonstrates that the p–n junction p-NiO@n-STO heterostructures have great potential applications in the field of photoelectrical devices and photocatalysis for water splitting.

A simple and effective method for fabricating novel p–n heterojunction photocatalyst g-C3N4/Bi4Ti3O12 and its photocatalytic performances

A novel g-C3N4/Bi4Ti3O12 photocatalyst with the p–n heterojunction structure was prepared through ball milling. The morphologies, structures, and optical properties of the photocatalyst were comprehensively characterized. The transmission electron microscopy (TEM) images and the X-ray photoelectron spectroscopy (XPS) showed the interfacial interaction between g-C3N4 and Bi4Ti3O12, indicating that a heterojunction between g-C3N4 and Bi4Ti3O12 was formed during ball milling. In addition, the optimum activity of the coupling semiconductor is higher than that of individual g-C3N4 and Bi4Ti3O12 for the degradation of acid orange-II(AO-7). The enhanced photocatalytic activity could be ascribed to the p–n junction of g-C3N4/Bi4Ti3O12 with strong oxidative ability and efficient charge separation. The highest activity was obtained in the g-C3N4/Bi4Ti3O12p–n heterojunction using a composite of 10wt.% g-C3N4 and 90wt.% Bi4Ti3O12 by ball milling in 16h. The removal of AO-7 was not only mainly initiated by valence-band holes (h+), but also affected by O2−, which was verified by the effects of scavengers. The mechanism underlying the observed photocatalytic activity was also described based on the semiconductor energy band theory and the formation of an internal electrostatic field. The probable degradation pathways were proposed and discussed according to the primary degradation products identified by GC/MS.

Mapping the electrostatic potential of Au nanoparticles using hybrid electron holography

Electron holography is a powerful technique for characterizing electrostatic potentials, charge distributions, electric and magnetic fields, strain distributions and semiconductor dopant distributions with sub-nm spatial resolution. Mapping internal electrostatic and magnetic fields within nanoparticles and other low-dimensional materials by TEM requires both high spatial resolution and high phase sensitivity. Carrying out such an analysis fully quantitatively is even more challenging, since artefacts such as dynamical electron scattering may strongly affect the measurement. In-line electron holography, one of the variants of electron holography, features high phase sensitivity at high spatial frequencies, but suffers from inefficient phase recovery at low spatial frequencies. Off-axis electron holography, in contrast, can recover low spatial frequency phase information much more reliably, but is less effective in retrieving phase information at high spatial frequencies when compared to in-line holography. We investigate gold nanoparticles using hybrid electron holography at both atomic-resolution and intermediate magnification. Hybrid electron holography is a novel technique that synergistically combines off-axis and in-line electron holography, allowing the measurement of the complex wave function describing the scattered electrons with excellent signal-to-noise properties at both high and low spatial frequencies. The effect of dynamical electron scattering is minimized by beam tilt averaging.

Photoluminescence and photocatalytic properties of rhombohedral CuGaO2 nanoplates.

Rhombohedral phase CuGaO2 nanoplates with a diameter of about 10 μm were synthesized via low temperature hydrothermal method. Room temperature and low temperature photoluminescence of the obtained CuGaO2 nanoplates were characterized. CuGaO2 nanoplates exhibited blue emission at room temperature and free exciton emission were appeared at low temperature. The blue emission is originated from defects such as Cu vacancies, which is the possible origin of p-type conductivity. The appearance of free exciton emission can demonstrate the direct bandgap transition behavior of CuGaO2 nanoplates. The as-prepared p-type CuGaO2 nanoplates were further decorated by n-type ZnO nanoparticles via calcination method to fabricate p-n junction nanocomposites. The nanocomposites exhibited enhanced photocatalytic activity which can be ascribed to the effective separation of photogenerated carriers by the internal electrostatic field in the p-n junction region, and the enhanced light absorption properties resulted from sub-bandgap absorption effect of p-n junction. This work has offered a new insight into the design of p-n junction devices using p-type CuGaO2 nanoplates.

Stacking faults as quantum wells in nanowires: Density of states, oscillator strength, and radiative efficiency

We investigate the nature of excitons bound to ${I}_{1}$ basal-plane stacking faults $[({I}_{1},X)]$ in GaN nanowire ensembles by continuous-wave and time-resolved photoluminescence spectroscopy. Based on the linear increase of the radiative lifetime of these excitons with temperature, they are demonstrated to exhibit a two-dimensional density of states, i.e., a basal-plane stacking fault acts as a quantum well. From the slope of the linear increase, we determine the oscillator strength of the $({I}_{1},X)$ and show that the value obtained reflects the presence of large internal electrostatic fields across the stacking fault. While the recombination of donor-bound and free excitons in the GaN nanowire ensemble is dominated by nonradiative phenonema already at 10 K, we observe that the $({I}_{1},X)$ recombines purely radiatively up to 60 K. This finding provides important insight into the nonradiative recombination processes in GaN nanowires. First, the radiative lifetime of about 6 ns measured at 60 K sets an upper limit for the surface recombination velocity of $210\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}\phantom{\rule{0.16em}{0ex}}\mathrm{s}}^{\ensuremath{-}1}$ considering the nanowires mean diameter of 50 nm. Second, the density of nonradiative centers responsible for the fast decay of donor-bound and free excitons cannot be higher than $6\ifmmode\times\else\texttimes\fi{}{10}^{16}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. As a consequence, the nonradiative decay of donor-bound excitons in these GaN nanowire ensembles has to occur indirectly via the free exciton state.

Design of anti-ring back reflectors for thin-film solar cells based on three-dimensional optical and electrical modeling

The optical and electrical properties of a photonic-plasmonic nanostructure on the back contact of thin-film solar cells were investigated numerically through the three-dimensional (3D) finite-difference time-domain method and the 3D Poisson and drift-diffusion solver. The focusing effect and the Fabry-Perot resonances are identified as the main mechanisms for the enhancement of the optical generation rate as well as the short circuit current density. However, the surface topography of certain nanopattern structures is found to reduce the internal electrostatic field of the device, thus limiting charge collection. The optimized conditions for both optics and electronics have been analyzed in this paper.