Wide Range Of Excitation Research Articles

Vibration mitigation of a pipe conveying fluid with a passive geometrically nonlinear absorber: a tuning optimal design

The objective of the presented article is to investigate the nonlinear dynamics and vibration of a simply supported fluid conveying pipe coupled with a geometrically nonlinear absorber, as well as the occurrence of the targeted energy transfer phenomena in the system. In addition, given the sensitivity of passive nonlinear absorbers performance to uncertainties in design (e.g., damping coefficient) and aleatory parameters such as the initial excitation of the primary system, the optimal design of the absorber in presence of uncertainties is also examined in this paper. The absorber consists of two linear springs, a lightweight mass, and one linear damping configured in a particular formation and is coupled to the pipe in grounded and ungrounded configurations. It was indicated that the absorber can perform as a tuned mass damper (TMD), nonlinear energy sink (NES), or a modified NES based on the bifurcation parameter value, and finding the optimal type of the absorber is left to the optimization solver without any preceding knowledge of the physics of the system. It is illustrated that the stability of the pipe and the absorber effectiveness is reduced by increasing the inside flow velocity. Moreover, the efficiency of the absorber slightly dependents on the density of the inside fluid. The results show that the stochastic optimally designed absorber can nonlinearly resonate with the pipe over extensive frequency ranges which leads to a synchronized motion of the pipe and absorber. In this situation a significant irreversible energy pumping from the pipe to the absorber may occur that make the motion localizes to the absorber and reduce the amplitude of the pipe for a wide range of initial excitation and fluid velocity. Also, comparing the present work to other similar works yields a great result.

Albumin-stabilized fluorescent metal nanoclusters: fabrication, physico-chemical properties and cytotoxicity

Fluorophores with a wide excitation range and the possibility of multicolor fluorescence are required for different applications such as sensing or multiplex analysis. Here we report about fluorescent Au, Cd and bimetallic Au-Cd nanoclusters (NCs) with sizes less than 1.6 nm fabricated and stabilized with bovine serum albumin (BSA). Based on mass spectrometry (MS) and energy-dispersive X-ray spectroscopy (EDX) data the average number of metal atoms per each BSA molecule was estimated to be 15 for BSA-Au NCs; 40 for BSA-Cd NCs; 9 atoms of Au and 26 of Cd for bi-metallic Au-Cd NCs. The NCs demonstrated large Stokes shifts of more than 130 nm. Fluorescence emission maxima for Au NCs at excitation wavelength of 325 nm were at 410 nm and 650 nm; for Cd NCs excitation at 370 nm resulted in the maximal emission at 500 nm, whereas for mixed Au-Cd NCs two peaks of emission were observed at 440 and 640 nm after excitation at 365–380 nm. For human cell lines HEp-2 and L41 both Au and Cd NCs in complex with BSA were less toxic compared to highly toxic original CdCl2 and HAuCl4 solutions. Protein nanoparticles (NPs) fabricated from BSA with metal NCs retained fluorescent properties.

Transcranial photoacoustic imaging of NMDA-evoked focal circuit dynamics in the rat hippocampus

Objective. We report the transcranial functional photoacoustic (fPA) neuroimaging of N-methyl-D-aspartate (NMDA) evoked neural activity in the rat hippocampus. Concurrent quantitative electroencephalography (qEEG) and microdialysis were used to record real-time circuit dynamics and excitatory neurotransmitter concentrations, respectively. Approach. We hypothesized that location-specific fPA voltage-sensitive dye (VSD) contrast would identify neural activity changes in the hippocampus which correlate with NMDA-evoked excitatory neurotransmission. Main results. Transcranial fPA VSD imaging at the contralateral side of the microdialysis probe provided NMDA-evoked VSD responses with positive correlation to extracellular glutamate concentration changes. qEEG validated a wide range of glutamatergic excitation, which culminated in focal seizure activity after a high NMDA dose. We conclude that transcranial fPA VSD imaging can distinguish focal glutamate loads in the rat hippocampus, based on the VSD redistribution mechanism which is sensitive to the electrophysiologic membrane potential. Significance. Our results suggest the future utility of this emerging technology in both laboratory and clinical sciences as an innovative functional neuroimaging modality.

Across barrier fission analysis of At⁎ isotopes formed in 3,4,6,8He+209Bi reactions

The fission decay mechanism of various 212,213,215,217At⁎ isotopes formed in tightly (3,4He) and loosely bound (6,8He) projectile induced reactions on 209Bi target, is studied within the framework of dynamical cluster-decay model (DCM), over a wide range of excitation energies spread across the Coulomb barrier. By optimizing appropriate neck-length parameter ΔR, the fission cross-sections are calculated for 212,213,215At⁎ compound nuclei (CN) at above barrier energies, where some experimental data is available. The DCM calculations are extended at below barrier region for these nuclei, and for one more reaction 8He+209Bi→217At⁎, using the systematics of 212,213,215At⁎ isotopes. The magnitude of fission cross-sections increase with the addition of neutrons in the He-projectile (or say in the At compound nucleus since the target nucleus of each reaction is same). To obtain better description of fission dynamics involved for neutron-deficient and neutron-rich He-induced reactions, the fragmentation potential and prefromation probability P0 of decaying fragments are analyzed along with corresponding barrier tunneling probability P. The fission fragment mass distributions are explored for all ‘At’ isotopes, and most probable fission fragments are identified. It has been observed that asymmetric fission forms the predominant decay mode for all isotopes, although a minor hump with very small preformation factor appears around symmetric fragments for lighter 212,213At⁎ nuclei. Finally, the N/Z dependence of fission cross-sections and most probable decaying fragments is explored in view of fragmentation structure and related cross-sectional yields.

Generalized Dynamic Hysteresis Model for Improved Iron Loss Estimation of Complex Flux Waveforms

This paper presents a generalized dynamic hysteresis model for estimating the iron loss of complex flux waveforms. A versatile measurement system is developed to experimentally investigate the magnetic and iron loss properties of the studied soft magnetic material over a wide range of ac and dc excitation. With the measured hysteresis loop data available, an improved dynamic hysteresis model is proposed to properly include the impact of the flux density changing rate, instantaneous magnetization state, and time history of magnetic flux on dynamic field modeling. Experimental results have confirmed the desired accuracy of iron loss predictions over broad operating ranges, including conditions of pure sinusoidal fields, non-sinusoidal flux waveforms with superimposed harmonics, and flux waveforms in the presence of pre-magnetized dc-bias fields. The model parameters identified from symmetrical hysteresis loops can be utilized to predict the iron loss over a variety of conditions, making it a generalized tool for the iron loss estimation of complex flux waveforms.

Multi-excitation and single color emission carbon dots doped with silicon and nitrogen: Synthesis, emission mechanism, Fe3+ probe and cell imaging

Fluorescent carbon dots (CDs) are drawing increasing attention due to their excellent properties, however, CDs usually show excitation-dependent emission property. Herein, we prepared a series of multi-excitation and single color emission carbon dots doped with nitrogen and silicon (SiN-CDs) by one-pot hydrothermal method from a hyperbranched polysiloxane. Interestingly, the synthesized SiN-CDs have three excitation bands at around 240, 300 and 360 nm, and only one emission band at around 440 nm. Meanwhile, SiN-CDs emit single color light in a wide excitation range and different concentrations. Structural and optical property characterizations show that the multi-excitation and single color emission property of SiN-CDs is caused by the cooperation of the sp2 carbon core with the abundant fluorescent groups on the surface. In particular, SiN-CDs have multiple functions, not only can be internalized into live cells with obvious accumulation in cell nuclei, but also possess high selectivity, sensitivity and anti-interference ability in detecting of Fe3+.

Investigation of vibration data-based human load monitoring system

Structural design of flexible footbridges requires a thorough understanding of pedestrian-induced vibration such that their dynamic behavior is accurately predicted. Human-induced vibration creates complicated ground reaction forces that contribute to human–structure interaction in the footbridges. It, therefore, becomes a significant challenge to the bridge designers to accurately estimate the moving pedestrian load on the footbridges during the design phase. This article examines the issues of human–structure interaction in slender pedestrian bridges and aims to analyze the walking pattern of the pedestrian from human-induced vibration data of the bridge. A wavelet-based time–frequency decomposition technique is adopted to extract the walking pattern of the pedestrian followed by time-series analysis of the walking pattern to develop a statistical model of pedestrian-induced vibration. Full-scale testing is conducted on a footbridge to validate the proposed technique under a wide range of pedestrian excitation. An experimental study is conducted to demonstrate the proposed method using the pedestrian’s walking on a force plate monitored by video cameras and vibration sensors. Identified walking patterns are then compared with the actual walking patterns measured by motion sensors attached to the test subject.

(Invited) Naked Eye Blue Emission in Ce3+ Codoped SiOxNy: Toward Si-Based Light-Emitting Devices

Rare earth (RE) doped silicon host matrices has been broadly investigated to procure light emitting sources for integrated optoelectronics devices using the RE inter-4f transitions. Classically, those transitions are partially allowed and result in a very low absorption cross section inducing a non-efficient excitation. Due to its 5d-4f transitions, Ce3+ ion is quite different from other RE3+ ions due to a large absorption cross section (10-19 cm-2) as compared to the other RE3+ ions (10-21 cm-2).[1] Furthermore, due to its single valence d-orbital electron, the 5d band is strongly dependent on the local environment, resulting typically in a large Stokes shift depending on the host matrix composition.[2] Ce-doped SiOxNy films have been deposited by magnetron reactive sputtering from Si and CeO2 targets under nitrogen reactive gas atmosphere, with a typical thickness of 120 nm. Three investigation approaches were explored. In the first one samples grown with nitrogen highly rich plasma and a low Ce concentration (0.3 at.%) were tested. Photoluminescence (PL) experiments show a wide blue emission band ranging from 400 to 650 nm under UV photons excitation. Reference samples grown with lower nitrogen content did not show any visible emission. To explain the blue emission origin, we have studied extensively the role of different RE ions emitting centers in Ce-doped SiOxNy films (e.g. band tails, CeO2, Ce clusters, Ce3+ ions), with different activation scenarios. The results confirmed that blue emission is mainly due to the Ce3+ ion. In addition, based on refractive index measurements, the Ce-doped SiOxNy films compositions were deduced from a Bruggeman effective medium model, confirming the presence of Si3N4 and SiO2 phases. Furthermore, the presence of those phases was confirmed independently by their bonding signatures identified by infrared spectroscopy (FTIR) analysis. By means of photoluminescence excitation spectroscopy (PLE), a wide excitation range from 250 to 400 nm was evidenced and various excitation channels of Ce3+ ions involving direct or indirect mechanisms were proposed. In the second approach, we focused on samples grown at high nitrogen flow, where the effect of Ce3+ concentration variation was investigated. Under UV excitation, a strong blue emission is visible to the naked eyes for SiOxNy sample doped with high Ce3+ concentration (6 at. % as determined by RBS measurements). The external quantum efficiency was measured for the selected best emitting samples with help of integrating sphere. No saturation of the PL intensity was observed, demonstrating the absence of Ce clusters and/or silicate phase formation due do the nitrogen content.[3] We believe that this result is very promising for considering silicon based emitting applications. Finally, in the third approach we measured electroluminescence (EL) from Ce-doped SiOxNy prototype devices. Signal evolution was investigated as the function of the N flow, the Ce concentration and the inclusion of Al dopants with the aim to improve the electrical conductivity. The influence of these factors on observed EL was studied through the conduction mechanisms. [1] J.M. Ramírez, A. Ruiz-Caridad, J. Wojcik, A.M. Gutierrez, S. Estradé, F. Peiró, P. Sanchís, P. Mascher, B. Garrido, "Luminescence properties of Ce3+ and Tb3+ co-doped SiOxNy thin films: Prospects for color tunability in silicon-based hosts", J. Appl. Phys., 119 (2016) 113108. [2] J. Li, O.H.Y. Zalloum, T. Roschuk, C.L. Heng, J. Wojcik, P. Mascher, "Light Emission from Rare-Earth Doped Silicon Nanostructures", Advances in Optical Technologies, 2008 (2008) 10. [3] C. Labbé, Y.T. An, G. Zatryb, X. Portier, A. Podhorodecki, P. Marie, C. Frilay, J. Cardin, F. Gourbilleau, "Structural and emission properties of Tb3+-doped nitrogen-rich silicon oxynitride films", Nanotech., 28 (2017) 115710 (115714pp).

Efficient green luminescence of terbium oxalate crystals: A case study with Judd-Ofelt theory and single crystal structure analysis and the effect of dehydration on luminescence

Design and synthesis of Lanthanide based metal organic framework is a frontier area of research owing to their structural diversity enabling specific applications. The luminescence properties of rare earths, tuned by the structural features of Ln-MOFs are investigated extensively. Rare earth oxalates which can be synthesized in a facile method, ensuring the structural features of MOFs with excellent photoluminescence characteristics deserves much attention. This work is the first time report on the single crystal structure and Judd-Ofelt (JO) theoretical analysis – their correlation with the intense and sharp green luminescence of Terbium oxalate crystals. The intense green luminescence observed for Terbium oxalate crystals for a wide range of excitation from DUV to visible region despite the luminescence limiting factors are discussed. The absence of concentration quenching and lifting up of forbidden nature of f-f transitions, allowing direct excitation of Terbium ions is analysed with the help of JO theory and single crystal structure analysis. The JO analysis predicted the asymmetry of Terbium sites, allowing the electric dipole transitions and from the JO intensity parameters, promising spectroscopic parameters – emission cross section, branching ratio, gain band width and gain coefficient of the material were calculated. The single crystal structure analysis revealed the asymmetry of Tb sites and structure of Terbium oxalate is formed by the hydrogen bonded stacking of overlapped six Terbium membered rings connected by the oxalate ligands. The molecularly thick layers thus formed on the crystal surface are imaged by the atomic force microscopy. The presence of water channels in the structure and the effect of lattice water molecules on the luminescence intensity are also investigated.

Thermal nanoimprint lithography for drift correction in super-resolution fluorescence microscopy.

Localization-based super-resolution microscopy enables imaging of biological structures with sub-diffraction-limited accuracy, but generally requires extended acquisition time. Consequently, stage drift often limits the spatial precision. Previously, we reported a simple method to correct for this by creating an array of 1 μm3 fiducial markers, every ~8 μm, on the coverslip, using UV-nanoimprint lithography (UV-NIL). While this allowed reliable and accurate 3D drift correction, it suffered high autofluorescence background with shorter wavelength illumination, unstable adsorption to the substrate glass surface, and suboptimal biocompatibility. Here, we present an improved fiducial micro-pattern prepared by thermal nanoimprint lithography (T-NIL). The new pattern is made of a thermal plastic material with low fluorescence backgrounds across the wide excitation range, particularly in the blue-region; robust structural stability under cell culturing condition; and a high bio-compatibility in terms of cell viability and adhesion. We demonstrate drift precision to 1.5 nm for lateral (x, y) and 6.1 nm axial (z) axes every 0.2 seconds for a total of 1 min long image acquisition. As a proof of principle, we acquired 4-color wide-field fluorescence images of live mammalian cells; we also acquired super-resolution images of fixed hippocampal neurons, and super-resolution images of live glutamate receptors and postsynaptic density proteins.

The nitrogen concentration effect on Ce doped SiOxNy emission: towards optimized Ce3+ for LED applications.

Ce-Doped SiOxNy films are deposited by magnetron reactive sputtering from a CeO2 target under a nitrogen reactive gas atmosphere. Visible photoluminescence measurements regarding the nitrogen gas flow reveal a large emission band centered at 450 nm for a sample deposited under a 2 sccm flow. Special attention is paid to the origin of such an emission at high nitrogen concentration. Different emitting centers are suggested in Ce doped SiOxNy films (e.g. band tails, CeO2, Ce clusters, Ce3+ ions), with different activation scenarios to explain the luminescence. X-ray photoelectron spectroscopy (XPS) reveals the exclusive presence of Ce3+ ions whatever the nitrogen or Ce concentrations, while transmission electron microscopy (TEM) shows no clusters or silicates upon high temperature annealing. With the help of photoluminescence excitation spectroscopy (PLE), a wide excitation range from 250 nm up to 400 nm is revealed and various excitations of Ce3+ ions are proposed involving direct or indirect mechanisms. Nitrogen concentration plays an important role in Ce3+ emission by modifying Ce surroundings, reducing the Si phase volume in SiOxNy and causing a nephelauxetic effect. Taking into account the optimized nitrogen growth parameters, the Ce concentration is analyzed as a new parameter. Under UV excitation, a strong emission is visible to the naked eye with high Ce3+ concentration (6 at%). No saturation of the photoluminescence intensity is observed, confirming again the lack of Ce cluster or silicate phase formation due to the nitrogen presence.

Preliminary survey of matrix effects in the Microwave-sustained, Inductively Coupled Atmospheric-pressure Plasma (MICAP)

Matrix effects caused by Na and Al in the nitrogen Microwave-sustained, Inductively Coupled, Atmospheric-pressure Plasma (MICAP) were investigated. Easily ionizable elements, such as Na, can suppress or enhance the analyte signal; Al is shown here to produce a similar effect. The influence of these matrices was examined for 18 emission lines of 8 analyte atoms and ions having a wide range of excitation and ionization energies. The plasma operating conditions were fixed during all experiments at a total nitrogen flow of 19.4Lmin−1 and a microwave power of 1.5kW. An Fe solution was used to determine the excitation temperature of the plasma by the Boltzmann plot method at selected matrix concentrations. In addition, vertical emission profiles of the plasma were measured. The matrix effect becomes worse at higher concentrations of an easily ionizable element. The effect is caused not only by a shift in ionization equilibrium but also by a possible change in plasma ionization temperature. Correction methods to reduce the matrix effects were tested and are discussed.

Preparation and Luminescence of Cluster Compounds [W6Br8L6]2– with L = CF3COO and C7H7SO3

Optically active cluster compounds [W6Br8L6]2– are prepared from Cs2[W6Br14], which is obtained via reduction of WBr6 with elemental antimony in the presence of CsBr. Cesium ions and six apical bromide ligands of Cs2[W6Br14] are successively replaced for n‐tetrabutyl ammonium (TBA)+ and ligands (L). Light yellow single‐crystals of (TBA)2[W6Br8L6] with L = CF3COO and C7H7SO3 were isolated from dichloromethane solution and structurally characterized by means of X‐ray single‐crystal diffraction. Photoluminescence studies reveal a wide excitation range within 250 to 500 nm for both compounds and a broad emission centered at λem,max = 735 nm (L = CF3COO) and 730 nm (L = C7H7SO3). Oxygen dependencies of emission intensities are reported.

Mercaptosuccinic acid-coated NIR-emitting gold nanoparticles for the sensitive and selective detection of Hg2 +

A sensitive fluorescent detection platform for Hg2+ was constructed based on mercaptosuccinic acid (MSA) coated near-infrared (NIR)-emitting gold nanoparticles (AuNPs). The thiolated mercaptosuccinic acid was employed as both reducing agent and surface coating ligand in a one-step synthesis of NIR-emitting AuNPs (MSA-AuNPs), which exhibited stable fluorescence with the maximum wavelength at 800nm and a wide range of excitation (220–650nm) with the maxima at 413nm. The MSA coated NIR-emitting AuNPs showed a rapid fluorescence quenching toward Hg2+ over other metal ions with a limit of detection (LOD, 3δ) as low as 4.8nM. The sensing mechanism investigation revealed that the AuNPs formed aggregation due to the “recognition” of Hg2+ from the MSA, and the resultant strong coupling interaction between Hg2+ and Au (I) to further quench the fluorescence of the AuNPs, which synergistically resulted in a highly sensitive and selective fluorescence response toward Hg2+. This proposed strategy was also demonstrated the possibility to be used for Hg2+ detection in water samples.

Advances in quantum dot-mediated siRNA delivery

Nano-sized quantum dots (QDs) exhibit uniquely optical properties that are tunable with different sizes and shapes. QDs can emit narrow symmetric bands under a wide excitation range, possess anti-photobleaching stability, and be bio-functionalized on the large surface area. Therefore, QDs are attractive vectors for imaging-guided therapy. Small-interfering RNA (siRNAs)-based therapeutics hold great potential to target a large part of the currently undruggable genes, but overcoming the lipid bilayer to deliver siRNA into cells has remained a major challenge to solve for widespread development of siRNA therapeutics. In this mini-review, we focus on theranostic QD/siRNA assembles for enhancing delivery of siRNA and facilitating evaluation of therapeutic efficacy via imaging of QDs, with special attention to carbonaceous QDs for delivery of siRNA.

The behavior of a carbonate sand subjected to a wide strain range of medium-frequency flexural excitation

Carbonate sands comprise typical foundation soils for offshore engineering structures such as oil platforms, found in natural deposits. These sands have a high shear strength but also very high compressibility which characteristics are due to the shape and the low tensile strength of the grains. For the seismic design of offshore structures, two key properties comprise the stiffness and damping of the subsoil which depend on the porosity, the level of the confining pressure and the strain amplitude. In this article, the small-strain Young’s modulus (Emax), small-strain damping (Df,min) and the strain-dependent Young’s modulus and damping of a carbonate sand were studied in the laboratory through resonant column tests in flexural mode of vibration. This mode applies a medium-frequency excitation to the sample, typically in the range of 30–80 Hz. The samples were prepared in a dry state and tested under isotropic conditions of the confining pressure with (p′) varying from about 25 to 800 kPa. The small-strain dynamic properties were analysed by means of power law type formulae and the sensitivity of Emax and Df,min to pressure was quantified and discussed. The results indicated that the flexural damping values were much greater in comparison to corresponding results in the literature derived from torsional resonant column tests on sands. This observation is in alignment with a recent study on the flexural response of quartz sands. Overall, the sensitivity of stiffness to pressure was less pronounced than that of reported data in the literature. The medium-strain tests showed a similar trend with respect to the stiffness degradation and damping increase as in the case of torsional resonant column tests reported in the literature for granular soils with a slight dependency of the medium-strain properties to the confining pressure level.

Effect of Dy3+ Amount on the Structural and Luminescence Properties of LaNbO4:Dy3+ Phosphor Obtained by One-Step Spray Pyrolysis Process

In this study, Dy3+-doped LaNbO4 phosphors with a blue to white emission that can be tuned by varying the activator concentration were prepared by one-step spray pyrolysis process. According to the powder X-ray diffraction results, increasing Dy3+ concentration favored the tetragonal phase. Slight changes were observed in the recorded Raman spectra as a function of Dy3+ concentrations while SEM (scanning electron microscopy) micrographs presented similar profiles regardless the Dy3+ amount. Investigation of Dy3+ emission data in the visible range showed that rising the Dy3+ concentration progressively shifted the emission towards the white color. All samples presented a wide excitation range from 254 to 475 nm.

Rotational state-dependent attachment of He atoms to cold molecular ions: An action spectroscopic scheme for rotational spectroscopy

We present a kinetics model description of a newly developed action spectroscopic method for rotational spectroscopy based on rotational state-dependent three-body attachment of He atoms to cold molecular ions stored in a cryogenic 22-pole ion trap. The model results from numerical simulations and an approximate analytical expression are compared to measurements of the J=1–0 rotational transition of CD+, for which we obtain a refined transition frequency of 453.5218509(7)GHz. From the analysis of the spectroscopic data recorded at varying experimental conditions, e.g. over a wide range of He number densities and excitation powers, we deduce that the ternary rate coefficient in the first excited rotational state of CD+ is reduced to (55±5)% of the rotational ground state value. This decrease in the rate coefficient can be rationalized as an increase of the redissociation probability in the ternary collision process. A summary of rotational spectroscopy measurements of other molecular ions using the new method will be given, and its general applicability is discussed.

Inverse design of nonlinearity in energy harvesters for optimum damping

This paper presents the inverse design method for the nonlinearity in an energy harvester in order to achieve an optimum damping. A single degree-of-freedom electromechanical oscillator is considered as an energy harvester, which is subjected to a harmonic base excitation. The harvester has a limited throw due to the physical constraint of the device, which means that the amplitude of the relative displacement between the mass of the harvester and the base cannot exceed a threshold when the device is driven at resonance and beyond a particular amplitude. This physical constraint requires the damping of the harvester to be adjusted for different excitation amplitudes, such that the relative displacement is controlled and maintained below the limit. For example, the damping can be increased to reduce the amplitude of the relative displacement. For high excitation amplitudes, the optimum damping is, therefore, dependent on the amplitude of the base excitation, and can be synthesised by a nonlinear function. In this paper, a nonlinear function in the form of a bilinear is considered to represent the damping model of the device. A numerical optimisation using Matlab is carried out to fit a curve to the amplitude-dependent damping in order to determine the optimum bilinear model. The nonlinear damping is then used in the time-domain simulations and the relative displacement and the average harvested power are obtained. It is demonstrated that the proposed nonlinear damping can maintain the relative displacement of the harvester at its maximum level for a wide range of excitation, therefore providing the optimum condition for power harvesting.

Efficiency studies on semipolar GaInN-GaN quantum well structures

In order to clarify the reasons for the fairly poor electroluminescence (EL) performance of semipolar LED structures grown on patterned sapphire wafers, we have analyzed both, pure photoluminescence (PL) test structures without doping only containing 5 GaInN quantum wells and full EL test structures, all emitting at a wavelength of about 510 nm. Evaluating the PL intensity over a wide range of temperatures and excitation powers, we conclude that such quantum wells possess a fairly large internal quantum efficiency of about 20%. However, on EL test structures containing nominally the same quantum wells, we obtained an optical output power of only about 150μW at an applied current of 20 mA. This may be due partly to some thermal destruction of the quantum wells by the overgrowth with p-GaN. Even more important seems to be the not yet finally optimized p-doping of these structures.