Photoluminescence Emission Efficiency Research Articles

Photoluminescence Emission Efficiency Analysis Methodology by Integrating Raman Spectroscopy of the A1(LO) and E2(high) Phonons in a GaInN/GaN Heterostructure

Microscopic lattice vibration images of the E2(high) mode (E2H) and another mode of A1(LO) (A1L) or the higher energy branch of LO‐phonon−plasmon coupling mode (LOPC+) in a Ga0.95In0.05N film on a GaN template are obtained by Raman scattering spectroscopy using a 325 nm laser. The increase in temperature by increasing the laser power is obtained from the decrease in the energy of E2H and the theoretical formula comprising two terms based on the mode energy variation of the bulk material and the thermal strain effect. Using the obtained temperature and the energy shift of the LOPC+, the mapping images of the temperature and electron density in the x–y plane are simultaneously obtained. This image provides the spatial variation of photoluminescence (PL) emission efficiency, given as PL intensity per electron. This method enables the quantitative discussion on photo‐emission efficiency even in the regions of low or high carrier density affected by carrier transport. In the investigated area, a region with a lower PL efficiency is found despite a higher electron density and lower temperature increase than the surrounding region. This imaging analysis is feasible in integrating the carrier and thermal energy transports and recombination processes in carrier dynamics study.

Controlled Emission of Carbon Quantum Dots Derived from Waste Silk Sericin

AbstractRecently, nitrogen rich carbon quantum dots gaining considerable attention due to their extraordinary optoelectronic properties, specifically the enhancement in quantum efficiency of photoluminescence emission. Here the synthesis of multicolor emission of nitrogen rich carbon quantum dots from silk sericin is presented. These quantum dots exhibit blue, green, yellow, and orange emissions with a quantum yield close to 67%, 38%, 16%, and 9.6%, respectively, due to variation in carbonization temperature. The detailed characterization studies show that the multicolor emission of carbon quantum dots is due to both quantum confinement of particles and surface defects generated by surface oxidation. Moreover, these nanoparticles are photostable, biocompatible, non‐toxic, and water soluble, and hence can be the promising materials for optoelectronic devices, bioimaging, biosensing, etc.

Impact of Mg level on lattice relaxation in a p-AlGaN hole source layer and attempting excimer laser annealing on p-AlGaN HSL of UVB emitters

Mg-doped p-type semiconducting aluminium-gallium-nitride hole source layer (p-AlGaN HSL) materials are quite promising as a source of hole ‘p’ carriers for the ultraviolet-B (UVB) light-emitting diodes (LEDs) and laser diodes (LDs). However, the p-AlGaN HSL has a central issue of low hole injection due to poor activation of Mg atoms, and the presence of unwanted impurity contamination and the existence of a localized coherent state. Therefore, first the impact of the Mg level on the crystallinity, Al composition and relaxation conditions in the p-AlGaN HSL were studied. An increasing trend in the lattice-relaxation ratios with increasing Mg concentrations in the p-AlGaN HSL were observed. Ultimately, a 40%–60% relaxed and 1.4 μm thick p-AlGaN HSL structure with total threading dislocation densities (total-TDDs) of approximately ∼8–9 × 108 cm−2 was achieved, which almost matches our previous design of a 4 μm thick and 50% relaxed n-AlGaN electron source layer (ESL) with total-TDDs of approximately ∼7–8 × 108 cm−2. Subsequently, structurally a symmetric p–n junction for UVB emitters was accomplished. Finally, the influence of excimer laser annealing (ELA) on the activation of Mg concentration and on suppression of unwanted impurities as well as on the annihilation of the localized energy state in the p-AlGaN HSL were thoroughly investigated. ELA treatment suggested a reduced Ga–N bonding ratio and increased Ga–O, as well as Ga–Ga bonding ratios in the p-AlGaN HSL. After ELA treatment the localized coherent state was suppressed and, ultimately, the photoluminescence emission efficiency as well as conductivity were drastically improved in the p-AlGaN HSL. By using lightly polarized p-AlGaN HSL assisted by ELA treatment, quite low resistivity in p-type AlGaN HSL at room temperature (hole concentration is ∼2.6 × 1016 cm−3, the hole mobility is ∼9.6 cm2 V1 s−1 and the resistivity is ∼24.39 Ω. cm) were reported. ELA treatment has great potential for localized activation of p-AlGaN HSL as well as n- and p-electrodes on n-AlGaN and p-AlGaN contact layers during the flip-chip (FC) process in low operating UVB emitters, including UVB lasers.

Enhanced photoluminescence through Forster resonance energy transfer in Polypyrrole-PMMA blends for application in optoelectronic devices

Polypyrrole-PMMA (PPy-PMMA) blends have been prepared by chemical oxidative polymerization method. The band gap of all the blends lies in semiconductive range though PMMA is insulating in nature. Spherical agglomerated structure of the blends is observed from microstructural images. High photoluminescence (PL) intensity is obtained in PPy-PMMA blends even if PPy has low PL emission efficiency. The enhancement of PL emission intensity of the blends is due to Forster resonance energy transfer. Here an energy transfer occurs from donor PMMA to acceptor PPy which enhances the emission intensity of PPy. PL spectra show the increase in emission intensity with increase in concentration of PMMA in the blend up to a certain limit. Donor-donor interaction is also observed in the blend due to which the emission intensity of the blend having excess amount of PMMA suddenly falls down. Thus by tuning the amount of PMMA added in PPy matrix the PL intensity of the blend can be tuned and the blend can be used as a promising material for optoelectronic devices.

Improvement of emission efficiency with a sputtered AlN buffer layer in GaInN-based green light-emitting diodes

We examined a sputtered AlN (sp-AlN) buffer layer to improve the emission efficiency of GaInN-based green LEDs. Both the electroluminescence and photoluminescence emission efficiencies are improved in a LED with a sp-AlN buffer layer compared to that with a conventional low-temperature GaN buffer layer. To identify the origin of the improvements, we carried out the analysis including X-ray diffraction, surface morphology, initial nucleation growth, and temperature-dependent PL. Through analysis and consideration, we showed that the improvements are mainly caused by less strain and a lower degree of carrier localization in quantum wells (QWs), that is highly influenced by the grain diameter of the GaN island at initial growth. These results indicate that a sp-AlN buffer layer induces less compositional pulling that results in less strain and alloy fluctuation in QWs, simultaneously.

Photonic properties of novel Yb3+ doped germanium-lead oxyfluoride glass-ceramics for laser cooling applications

In recent years, our research group has developed and studied new rare-earth doped materials for the promising technology of solid-state laser cooling, which is based on anti-stokes fluorescence. To the best of our knowledge, our group is the only one in Canada leading the research into the properties of nanoparticles, glasses and glass-ceramics for optical refrigeration applications. In the present work, optical properties of 50GeO2-30PbF2-18PbO-2YbF3 glass-ceramics for laser cooling are presented and discussed as a function of crystallization temperature. Spectroscopic results show that samples have near infrared photoluminescence emission due to the 2F5/2–2F7/2 Yb3+ transition, centered at ~1016 nm with an excitation wavelength of 920 nm or 1011 nm, and the highest photoluminescence emission efficiency occurs for heat-treatment for 5 h at 350°C. The internal photoluminescence quantum yield varies between 99% and 80%, depending on the temperature of heat-treatment, being the most efficient under 1011 nm excitation. The 2F5/2 lifetime increases from 1.472 to 1.970 ms for heat treatments at 330°C to 350°C, respectively, due to energy trapping and the low phonon energy of the nanocrystals. The sample temperature dependence was measured with a fiber Bragg grating sensor, as a function of input pump laser wavelength and processing temperature. These measurements show that the heating process approaches near zero for an excitation wavelength between 1020 and 1030 nm, which is an indication that phonons are removed effectivelly from the glass-ceramic materials, and they can be used for optical laser cooling applications. On the other hand, the temperature increase as a function of input laser power into samples remains constant between 920 and 980 nm wavelength excitation, a temperature variation of 36 K/W (temperature of 58°C/W) was attained under excitation at 950 nm, showing a possible use for biomedical applications to be explored.1)

Radio- and photoluminescence properties of Ce/Tb co-doped glasses with huntite-like composition

Optical properties of yttria-aluminoborate (YAB) glasses with general composition 10(CexTbyY(1-x-y))-30Al2O3-60B2O3 are investigated and compared with data available on YAB crystals with huntite-like structure. Ce doped samples show optical features ascribable to preferential location of rare earth ions in sites with specific geometry similar to that observed in crystalline structures. Samples prepared with Tb ions as emission activator and Ce ions as sensitizer have been studied within the framework of non-radiative energy transfer. The resulting Förster radius is of 4.6 ± 0.5 Å comparable with that observed in Ce/Tb co-doped YAl3(BO3)4 crystals. The investigated materials possess radio- and photoluminescence emission efficiencies and performances comparable to that of crystalline counterparts with the advantage of having easiness of preparation and workability typical of glassy systems.

Low-Temperature Single Carbon Nanotube Spectroscopy of sp3 Quantum Defects.

Aiming to unravel the relationship between chemical configuration and electronic structure of sp3 defects of aryl-functionalized (6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature single nanotube photoluminescence (PL) spectroscopy studies and correlate our observations with quantum chemistry simulations. We observe sharp emission peaks from individual defect sites that are spread over an extremely broad, 1000-1350 nm, spectral range. Our simulations allow us to attribute this spectral diversity to the occurrence of six chemically and energetically distinct defect states resulting from topological variation in the chemical binding configuration of the monovalent aryl groups. Both PL emission efficiency and spectral line width of the defect states are strongly influenced by the local dielectric environment. Wrapping the SWCNT with a polyfluorene polymer provides the best isolation from the environment and yields the brightest emission with near-resolution limited spectral line width of 270 μeV, as well as spectrally resolved emission wings associated with localized acoustic phonons. Pump-dependent studies further revealed that the defect states are capable of emitting single, sharp, isolated PL peaks over 3 orders of magnitude increase in pump power, a key characteristic of two-level systems and an important prerequisite for single-photon emission with high purity. These findings point to the tremendous potential of sp3 defects in development of room temperature quantum light sources capable of operating at telecommunication wavelengths as the emission of the defect states can readily be extended to this range via use of larger diameter SWCNTs.

Drug-Derived Bright and Color-Tunable N-Doped Carbon Dots for Cell Imaging and Sensitive Detection of Fe3+ in Living Cells

Inspired by the diverse drug compounds with various heteroatoms (such as N, S, and P) in the drug library, facile synthesis of a new kind of bright and color-tunable N-doped carbon dots (NCDs) has been reported by using a popular antibiotic-aminosalicylic acid-as precursor. The N doping of CDs not only enable great improvement of photoluminescence (PL) efficiency and tunability of PL emission, but also enrich surface functional groups to broaden its application. The as-prepared NCDs possess tunable PL and show a quantum yield of 16.4%, which is the result of PL improvement effect of introduced nitrogen atoms among CDs. The cellular toxicity on H1299 cancer cells indicates that the NCDs have negligible cytotoxicity, excellent biocompatibility, and great resistance to photobleaching. Moreover, the drug-derived NCDs showed excellent sensitivity in detection of Fe3+ in living cells, which indicates the potential application in diagnosis and related biological study.

Improving Photoluminescence Emission Efficiency of Nanocluster-Based Materials by in Situ Doping Synthetic Strategy

Solid-state red phosphors of Mn2+-doped nanocrystals usually suffer from poor intensity. While the d–d emission of Mn2+ in yellow window has been extensively studied, shift toward lower energy remains challenging. Typically, intrinsic surface defects and self-purification of dopants are two obstacles for enhancing the intensity of red emission. Moreover, for red phosphors Mn2+ ions also need an appropriate host matrix and environment. Through an in situ doping strategy and optimization of the Mn2+ doping level, intense red-emitting Mn2+ dopant emission is reported here for MnCdInS@InS host. The doping strategy allows doping of Mn2+ at the core and/or surface sites of supertetrahedral “core–shell” nanocluster (Mn@MnCdInS@InS), leading to the red emission (at 643 nm) with over 40% quantum yield. Moreover, systematic control of doping level results in a series of crystalline Mn2+-doped materials with tunable photoluminescence quantum yield. In addition to the synthesis of an important class of red-emitting m...

Plasmon Enhanced Internal Quantum Efficiency of CdSe/ZnS Quantum Dots

The effects of surface plasmon oscillations on the radiative recombination rate and internal quantum efficiency of silver coated CdSe/ZnS quantum dots imbedded in quartz matrix is studied theoretically and numerically. The analysis is carried out by introducing the local field enhancement factor for the description of the dielectric function of the quantum dots together with the modified Drude model for the metal-coat that takes into account the effects of interband transitions and the size dependent damping parameter. It is found out that the internal quantum efficiency of photoluminescence emission of the metalcoated CdSe/ZnS quantum dots is enhanced by about 5-folds compared with the quantum dots without metal-coat. Moreover, increasing the metal fraction of the coated CdSe/ZnS quantum dots from 0.80 to 0.92 results in a significant increase of the intensity that is accompanied by a blue-shift of the quantum efficiency curves. The results obtained can be utilized in the design and fabrication of optoelectronic devices that require high intensity of photoluminescence emission.

Room-temperature single-photon generation from solitary dopants of carbon nanotubes

On-demand single-photon sources capable of operating at room temperature and the telecom wavelength range of 1,300-1,500 nm hold the key to the realization of novel technologies that span from sub-diffraction imaging to quantum key distribution and photonic quantum information processing. Here, we show that incorporation of undoped (6,5) single-walled carbon nanotubes into a SiO2 matrix can lead to the creation of solitary oxygen dopant states capable of fluctuation-free, room-temperature single-photon emission in the 1,100-1,300 nm wavelength range. We investigated the effects of temperature on photoluminescence emission efficiencies, fluctuations and decay dynamics of the dopant states and determined the conditions most suitable for the observation of single-photon emission. This emission can in principle be extended to 1,500 nm by doping of smaller-bandgap single-walled carbon nanotubes. This easy tunability presents a distinct advantage over existing defect centre single-photon emitters (for example, diamond defect centres). Our SiO2-encapsulated sample also presents exciting opportunities to apply Si/SiO2-based micro/nano-device fabrication techniques in the development of electrically driven single-photon sources and integration of these sources into quantum photonic devices and networks.

In(AsN) mid-infrared emission enhanced by rapid thermal annealing

We report a substantial increase in the quality and photoluminescence (PL) emission efficiency of In(AsN) dilute nitride alloys grown on both p-type InAs and semi-insulting GaAs substrates, in response to rapid thermal annealing. At 4K the PL emission efficiency increases by 25 times due to a reduction in non-radiative Shockley–Read–Hall recombination originating from elimination of point defects. For annealing temperatures up to 500°C the activation energy for thermal quenching increases by a factor of three, with no change in the residual electron concentration and mobility. Temperature dependent PL, together with X-ray diffraction measurements, reveals an improvement in compositional uniformity. Our results are significant for photonic device applications and particularly for the development of cryogenic mid-infrared photodiodes, monolithic detectors and focal plane arrays.

Enhancing optical characteristics of InAs/InGaAsSb quantum dot structures with long-excited state emission at 1.31 μm.

In this study, the optical properties of InAs quantum dots (QDs) with various strain-reducing layers (SRLs) of GaAsSb and InGaAsSb are characterized using photoluminescence (PL) and time-resolved PL (TRPL) measurements. The room-temperature PL results for the InAs/InGaAsSb QDs revealed stronger emission intensities than InAs QDs capped with an GaAs(1-x)Sb(x) (x = 20%)SRL, although both samples were grown under the same Sb flux during the molecular beam epitaxy process. The InAs/InGaAsSb QDs showed a significant elongation of emission wavelengths to 1450 and 1310 nm for the ground and first-excited state at room temperature. The energy band alignment of the InAs QD heterostructures was found tailoring from type II to type I as the GaAsSb SRL was replaced by InGaAsSb layer, which improved the radiative efficiency and was verified by power-dependent PL and TRPL measurements. Post-growth rapid thermal annealing was applied on the InAs/InGaAsSb QDs to further enhance the QD quality and PL emission efficiency. The greatly improved PL intensity, reduced linewidth, shortened radiative lifetime, with increasing annealing temperature were demonstrated, and InAs/InGaAsSb QDs exhibited enhanced optical characteristics for long-wavelength emission applications.

Synthesis and photoluminescence properties of novel copolymers containing salenAlQ-complex moieties and N-vinylcarbazole segments

Two novel salenAlQ-complex monomers and the corresponding copolymers are synthesized and characterized by 1H NMR, 13C NMR, FT-IR spectroscopy, and elemental analysis. The properties of the copolymers are investigated by gel permeation chromatography (GPC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), UV–vis absorption and photoluminescence (PL) spectra. The results indicate that the copolymers exhibit appropriate molecular weight, good solubility-processability, perfect thermal stability, and improved hole-transporting performance. Particularly, since two ligands (salen and quinoline) coordinate with aluminum ions simultaneously, the copolymers exhibit exciting photoluminescence phenomenons. The solution emission is mainly salen-centered, while the solid emission is mainly quinoline-centered. In addition, the PL emission intensities, efficiencies and decay lifetimes of the copolymers are higher than those of the corresponding monomers. These results demonstrate that the incorporated NVK segments are used as multiple roles (hole-transport group, light-harvesting groups and luminescence protection) in the copolymers.

Aqueous route to color-tunable Mn-doped ZnS quantum dots

Mn-doped zinc sulfide (Mn:ZnS) quantum dots stabilized by 3-mercaptopropionic acid (MPA) were synthesized at 100 °C in basic aqueous solution using the nucleation-doping strategy. The optical properties and structure of the obtained Mn:ZnS QDs have been characterized by UV–vis, photoluminescence (PL) and time-resolved PL spectroscopies, transmission electron microscopy (TEM), X-ray diffraction (XRD), and electron spin resonance (ESR). The obtained nearly monodisperse Mn:ZnS@MPA QDs have an average diameter of ca. 2.5 nm and a zinc-blende crystal structure. By varying the base (LiOH, NaOH, KOH, CsOH) used to deprotonate the MPA ligand and adjust the pH of the aqueous solution to 11, the PL emission wavelengths can be tuned within a relatively large optical window, from 567 to 594 nm. The variations of charge density near the surface of the QDs obtained by changing the cation associated to the MPA ligand and of the dopant location in the ZnS host are at the origin of the PL shifts observed. In order to improve the PL emission efficiency, a ZnS shell was subsequently overcoated around the Mn:ZnS core nanocrystals. With ZnS shell growth, the PL emission wavelength was restricted between 570 and 583 nm but PL quantum efficiency of Mn:ZnS/ZnS core/shell QDs increased up to 18.4%.

Strong photoluminescence from diameter-modulated single-walled carbon nanotubes

Employing the photoluminescence (PL) properties of the semiconducting single-walled carbon nanotubes (SWNTs) in applications is limited by their low quantum yield. Here, we report a simple method for enhancing the efficiency of PL emission by modulating the diameter and hence the bandgap of semiconducting SWNTs along their axes. The PL enhancement is explained by the exciton energy transfer via exciting SWNT segments of large bandgaps which induces emission from the neighboring segments of small bandgaps. The diameter-modulated SWNTs were grown by periodic modulation of the laser beam, and hence, the temperature in a laser assisted chemical vapor deposition process.

Annealing structured Au nanoparticles enhanced light emission from CdSe quantum dots

Enhancement of the light emission of CdSe quantum dots was observed by coupling through localized surface plasmons from Au nanoparticles. The enhancement was found to be relative to the shapes and sizes of Au nanoparticles. Au nanoparticles of different sizes were synthesized by a citrate-seeded method. By varying the annealing temperature, worm-like Au nanoparticles of different aspect ratios from 1 to 5 were obtained. Samples of the CdSe coupled with Au with an aspect ratio of 2 and annealed at 500 °C exhibited the best photoluminescence emission efficiency. Furthermore, a stronger photoluminescence enhancement was observed with increasing the size of Au nanoparticles. It was also found that when the localized surface plasmons resonance absorption wavelength of Au nanopartiles was just a little smaller than the emission peak of CdSe, the CdSe quantum dots exhibited the strongest photoluminescence intensity, with an enhancement of 6 times.

Interdot carrier transfer in semimagnetic Pb 1− xMn xSe nanocrystals embedded in oxide glass

Temperature- and excitation-intensity-dependent photoluminescence (PL) spectra of semimagnetic Pb 1− x Mn x Se nanocrystals embedded in glass matrix have been studied. Two types of dot families with different sizes and dispersions were identified by spectral deconvolution in Gaussian components with different full widths at half maxima values. Temperature induced carrier-transfer interdots are responsible for the sigmoidal temperature dependence of the higher PL peak energy and for anomalous enhanced photoluminescence emission efficiency, at low temperatures. The activation energy of nonradiative channel responsible for a strong thermal quenching, at T>80 K, is deduced from an Arrhenius plot of integrated PL intensity.

Photoluminescence Characteristics of InAs Quantum Dots with GaInP Cover Layer Grown by Metalorganic Chemical Vapor Deposition

The photoluminescence (PL) characteristics of InAs quantum dots (QDs) with a GaInP cover layer grown by metalorganic chemical vapor deposition were investigated to clarify the effect of the cover layer. By comparing the PL peak wavelength between the GaInP and GaInAs covered structures, the strain of the cover layer was identified as the main mechanism responsible for the emission wavelength shift of the InAs QDs. The PL emission efficiency and the thermal annealing effect of the GaInP-covered structure were also investigated.