Increasing Emission Wavelength Research Articles

Effect of annealing on photoluminescence and optical properties of porous anodic alumina films formed in sulfuric acid for solar energy applications

Photoluminescence and optical properties of porous oxide films formed by two-step aluminum anodization at a fixed current 200mA have been investigated. It was found that the crystallographic structure depend strongly on the annealing temperature. X-ray diffraction (XRD) reveals an amorphisation of the porous oxide films after annealing. This evolution has been confirmed by Raman spectroscopy measurement. Spectroscopic ellipsometry (SE) in the UV–vis and near infra red (IR) spectra shows that refraction index n increases and the extinction coefficient k decreases with annealing temperature. This observation has been confirmed with reflectivity measurements. As a consequence the reflectivity reaches 97% when porous alumina films were annealed at 650°C. Photoluminescence (PL) measurements show two PL peaks in the emission and excitation spectra. The first emission peak is centered at 460nm (α-band) and the second (β-band) shifts from 500 to 525nm, depending on excitation wavelength. For excitation spectra, one spectral peak is located at 271nm and the second shifts to longer wavelengths with increasing emission wavelength. The results indicate the existence of two PL centers. One is associated with oxygen adsorption at the pore wall and oxygen vacancies inside the alumina. The other is related to the adsorption of water and/or OH groups at the surface of the pore wall and to structure defects and sulfur inclusion inside the films.

Electrical and optical characteristics of GaN-based light-emitting diodes fabricated with emission wavelengths of 429–467 nm

The electrical and optical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with emission wavelengths of 429–467nm were investigated. The optical output increased with increasing emission wavelength, which is attributed to the enhanced quantum confinement effect as a result of indium composition fluctuation. With higher indium content, the LEDs exhibited unfavorable performance, including a larger efficiency droop, spectral blueshift, and spectral broadening, due to indium-induced strains. The effect of heterointerfaces associated with the indium content of the active region on the device resistance was negligible.

다층 성장한 InAs/InAlGaAs 양자점의 광학적 특성

자발형성법으로 InP (001) 기판에 성장한 InAs/InAlGaAs 양자점(QDs, quantum dots)의 광학적 특성을 PL (photoluminescence)과 TRPL (time-resolved PL)을 이용하여 분석하였다. InAs 양자점 시료는 single layer InAs/InAlGaAs QDs (QD1)과 7-stacked InAs/InAlGaAs QDs (QD2)를 사용하였다. 저온(10 K)에서 QD1과 QD2 모두 1,320 nm에서 PL 피크가 나타났으며, 온도를 300 K까지 증가하였을 때 각각 178 nm와 264 nm의 적색편이(red-shift)를 보였다. QD1의 PL 소멸시간은 PL 피크인 1,320 nm에서 1.49 ns이고, PL 피크를 중심으로 장파장과 단파장으로 이동하면서 점차 짧아졌다. 그러나 QD2의 PL 소멸시간은 발광파장이 1,130 nm에서 1,600 nm까지 증가할 때 1.83 ns에서 1.22 ns로 점진적으로 짧아졌다. 이러한 QD2의 PL과 TRPL 결과는 평균 양자점의 크기가 InAs/InAlGaAs 층이 증가함에 따라 점차 증가하기 때문으로 single layer인 QD1에 비해 양자점 크기의 변화가 더 크기 때문으로 설명된다. Self-assembled InAs/InAlGaAs quantum dots (QDs) grown on an InP (001) substrate have been investigated by using photoluminescence (PL) and time-resolved PL measurements. The single layer (QD1) and seven stacks (QD2) of InAs/InAlGaAs QDs grown by the conventional S-K growth mode were used. The PL peak at 10 K was 1,320 nm for both QD1 and QD2. As the temperature increases from 10 to 300 K, the PL peaks for QD1 and QD2 were red-shifted in the amount of 178 and 264 nm, respectively. For QD1, the PL decay increased with increasing emission wavelength from 1,216 to 1,320 nm, reaching a maximum decay time of 1.49 ns at 1,320 nm, and then decreased as the emission wavelength was increased further. However, the PL decay time for QD2 decreased continuously from 1.83 to 1.22 ns as the emission wavelength was increased from 1,130 to 1,600 nm, respectively. These PL and TRPL results for QD2 can be explained by the large variation in the QD size with stacking number caused by the phase separation of InAlGaAs.

Photophysical Properties of Near-Infrared Phosphorescent π-Extended Platinum Porphyrins

A comprehensive photophysical study is reported on a family of π-extended platinum(II) porphyrin complexes. The platinum(II) complexes are synthesized from the corresponding free base porphyrins by treatment with platinum(II) acetate in hot benzonitrile, affording the complexes in considerably higher yield than by reaction with platinum(II) chloride. A quantitative study of the absorption and luminescence properties of the metalloporphyrins is presented. A series including tetraarylbenzo-, tetraarylnaphtho-, and tetraarylanthroporphyrin exhibits efficient phosphorescence at 773, 890, and 1020 nm in the near-infrared region, with quantum yields of 0.35, 0.15, and 0.08, respectively. The triplet lifetimes and phosphorescence yields decrease with increasing emission wavelength, consistent with energy gap law behavior. A set of six Pt-tetrabenzoporphyrins (TBPs) with different meso-substituents were examined. The Pt-TBPs exhibit efficient phosphorescence with λmax ∼ 770 nm and with a quantum yield ranging from 0.26–0.49, depending on the substitution pattern. The results show that the 5,15-diarylbenzoporphyrins feature 50–60% higher phosphorescence emission yield compared to the 5,10,15,20-tetraarylbenzoporphyrins. The highest phosphorescence quantum efficiency is observed for a platinum(II) 5,15-diarylbenzoporphyin which emits at 770 nm with a quantum yield of 49%.

Failure analysis of InGaN/GaN LEDs with emission wavelength between 440 nm and 550 nm

AbstractWhile longer wavelength emission from InGaN/GaN light‐emitting diodes can be achieved by increasing the Indium (In) content in the qunatum wells, the increased In content gives rise to side‐effects to the material and device performance and reliability. It was found that the induced strain in the wafer and the density of threading dislocations increases with increasing In content. From current‐voltage and 1/f noise measurements, it was observed that the leakage currents, static resistance and noise magnitudes rises monotonically with increasing emission wavelength (In composition), which can be attributed to higher defect concentrations. After undergoing a 1000‐hr reliability test, it was discovered that the optical degradation rates for the longer wavelength green LEDs were significantly higher than those of shorter wavelength. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Wavelength-stable cyan and green light emitting diodes on nonpolar m-plane GaN bulk substrates

We report the development of 480 nm cyan and 520 nm green light emitting diodes (LEDs) with a highly stable emission wavelength. The shift is less than 3 nm when the drive current density is changed from 0.1 to 38 A/cm2. LEDs have been obtained in GaInN-based homoepitaxy on nonpolar m-plane GaN bulk substrates. For increasing emission wavelength we find a large number of additional dislocations generated within the quantum wells (2×108 to ∼1010 cm2) and a decrease in the electroluminescence efficiency. This suggests that the strain induced generation of defects plays a significant role in the performance limitations.

The role of dot height in determining exciton lifetimes in shallow InAs/GaAs quantum dots

The spectral dependence of the photoluminescence recombination lifetime has been measured for individual self-assembled InGaAs/GaAs quantum dots, over the entire emission envelope. The measurements show a rising trend with increasing emission wavelength, increasing from 680 ps at 900 nm to about 1020 ps at 990 nm. Measurements of the out-of-plane diamagnetic coefficients for the dots show almost no correlation with wavelength. As a result, the rising trend in the lifetimes with wavelength is interpreted in terms of the emission energy being predominantly determined by the dot height, with higher dots exhibiting longer lifetimes.

Effect of indium content on performance and reliability of InGaN/GaN light-emitting diodes

While longer wavelength emission from InGaN/GaN light-emitting diodes (LEDs) can be achieved by increasing the indium (In) content in the quantum wells, the increased In content gives rise to side effects to the material and device performance and reliability. It was found that the induced strain in the wafer and the density of threading dislocations increase with increasing In content. From current-voltage and 1/f noise measurements, it was observed that the leakage currents, static resistance, and noise magnitudes rise monotonically with increasing emission wavelength (In composition), which can be attributed to higher defect concentrations. After undergoing a 1000 h reliability test, it was discovered that the optical degradation rates for the longer wavelength green LEDs were significantly higher than those of shorter wavelength.

The effect of annealing on the photoluminescent and optical properties of porous anodic alumina films formed in sulfamic acid

Photoluminescent and optical properties of porous oxide films formed by two-step aluminum anodization at a constant potential of 30 V in sulfamic acid have been investigated after their annealing, ranging from room temperature up to 600 °C. X-ray diffraction reveals the amorphous nature of porous oxide films. Infrared and energy dispersive spectroscopy indicates the presence of sulfuric species incorporated in oxide films during the anodization. Photoluminescence (PL) measurements show PL bands in the range from 320 to 600 nm. There are two peaks in emission and excitation spectra. One emission peak is at constant wavelength centered at 460 nm and the other shifts from 390 to 475 nm, depending on excitation wavelength. For excitation spectra, one spectral peak is at constant wavelength at 270 nm and the other also shifts to longer wavelengths while increasing emission wavelength. Upon annealing of the as-prepared oxide films PL increases reaching maximum value at about 300 °C and then decreases. The results indicate the existence of two PL centers, one placed at surface of the pore wall, while the other positioned inside the oxide films.

An unusual red-edge excitation and time-dependent Stokes shift in the single tryptophan mutant protein DD-carboxypeptidase from Streptomyces: the role of dynamics and tryptophan rotamers.

The fluorescence emission of the single tryptophan (W233) of the mutant protein DD-carboxypeptidase from streptomyces is characterized by a red-edge excitation shift (REES), i.e., the phenomenon that the wavelength of maximum emission depends on the excitation wavelength. This phenomenon is an indication for a strongly reduced dynamic environment of the single tryptophan, which has a very low accessibility to the solvent. The REES shows, however, an unusual temperature and time dependence. This, together with the fluorescence lifetime analysis, showing three resolvable lifetimes, can be explained by the presence of three rotameric states that can be identified using the Dead-End Elimination method. The three individual lifetimes increase with increasing emission wavelength, indicating the presence of restricted protein dynamics within the rotameric states. This is confirmed by time-resolved anisotropy measurements that show dynamics within the rotamers but not among the rotamers. The global picture is that of a protein with a single buried tryptophan showing strongly restricted dynamics within three distinct rotameric states with different emission spectra and an anisotropic environment.

The Influence of Surface Trapping and Dark States on the Fluorescence Emission Efficiency and Lifetime of CdSe and CdSe/ZnS Quantum Dots

To investigate the influence of surface trapping and dark states on CdSe and CdSe/ZnS quantum dots (QDs), we studied the absorption, fluorescence intensity and lifetime by using one-and two-photon excitation, respectively. Experimental results show that both one- and two-photon fluorescence emission efficiencies of the QDs enhance greatly and the lifetime increase after capping CdSe with ZnS due to the effective surface passivation. The lifetime of one-photon fluorescence of CdSe and CdSe/ZnS QDs increase with increasing emission wavelength in a supralinear way, which is attributed to the energy transfer of dark excitons. On the contrary, the lifetime of two-photon fluorescence of bare and core-shell QDs decrease with increasing emission wavelength, and this indicates that the surface trapping is the dominant decay mechanism in this case.

Time‐Resolved and Steady‐State Fluorescence Spectroscopy of Eumelanin and Indolic Polymers

Eumelanin plays a variety of important physiological roles in human skin. However, its structure and fundamental properties still remain poorly understood. Although the absorbance of eumelanin is broad and reveals little about its structure, a variety of techniques have revealed the presence of a disordered array of chromophores within the melanin compound. In order to examine the fluorescence decay dynamics of these chromophores, time-resolved spectroscopy was applied to solutions of synthetic eumelanin and a melanin-like polymer of N-methyl,5-hydroxy,6-methoxyindole (N-Me-5H6MI). Solutions were excited with 80 fs laser pulses at 355, 370, 390 and 400 nm, and decay time courses were acquired at 20 nm intervals between 400 and 600 nm for each excitation wavelength. Decay profiles for both eumelanin and the polymer exhibited a characteristic multiexponential behavior with decay times between 0.5 and 15 ns, although steady-state spectra for the polymer exhibited only two peaks. The long-decay component in the polymer showed a significant decrease in both amplitude (30-5%) and decay time (14-6 ns) with increasing emission wavelength. In contrast, the amplitude and decay time in melanin increased slightly (10-15% and 7-10 ns, respectively) from 400 to 520 nm emission, at which point they leveled off. These trends were consistent for all excitation wavelengths. These results suggest that the multiexponential behavior of melanin fluorescence is characteristic of each oligomer within the eumelanin compound, and is consistent with the assertion that the diversity of constituents within eumelanin provides it with a robustness in spectral properties.

Decay Lifetimes of Para-Substituted Polystyrene in Solid Films and in Dichloromethane

The fluorescence decay times of polystyrene, poly(4-bromostyrene), poly(4-chlorostyrene), poly(4-methylstyrene), poly(α-methylstyrene), poly(4-methoxystyrene), and poly(4-tert butylstyrene) were measured in solid films and in dichloromethane solution. A detailed analysis of the emission profile performed by nanosecond time resolved fluorescence spectroscopy confirmed the presence of monomer fluorescence as well as excimer fluorescence in both media. Monomer fluorescence decay times gave shorter times compared with that of excimer decay times in dichloromethane solution. Fluorescence from substituted polystyrene was mainly excimer fluorescence with shift of maximum emission to longer wavelength. The ratio of monomer to excimer contributions was found to be dependent on the emission wavelength, but was not affected by polymer concentration. Both monomer and excimer fluorescence lifetimes as well as excimer intensity increase with increasing emission wavelength. An accompanying decrease in monomer contribution is also observed is solution in comparison with that in solid films.

Molecular Mobility and Oxygen Permeability in Amorphous Bovine Serum Albumin Films

We have used phosphorescence from the xanthene probe erythrosin B to characterize the molecular mobility and oxygen permeability as a function of temperature in amorphous solid bovine serum albumin (BSA) films. Analysis of the emission spectrum using a lognormal fitting function provided information on how temperature modulates the emission peak frequency and bandwidth (full width at half maximum). The peak frequency decreased gradually at low and more steeply at high temperature, whereas the bandwidth increased gradually at low and more steeply at high temperature, both changes indicating a softening of the protein matrix at ∼60°C. Phosphorescence intensity decay transients were well fit using a stretched exponential decay function at all temperatures. Lifetimes decreased gradually at low and more steeply at high temperature; Arrhenius analysis of the rate constant for nonradiative collisional quenching indicated an increase in quenching indicative of matrix softening at ∼70°C. The oxygen quenching rate was calculated from a comparison of emission lifetimes in the presence and absence of oxygen. This rate varied linearly with the collisional quenching rate over nearly three orders of magnitude, suggesting that the more global motions that control oxygen translational diffusion are modulated by more local motions that influence collisional quenching of erythrosin. The emission spectrum shifted to higher energy as a function of time following excitation, whereas the phosphorescence lifetime decreased with increasing emission wavelength; both behaviors provided strong evidence for distinct sites within the protein matrix varying in molecular mobility. These results enrich our molecular understanding of the intrinsic mobility of proteins within the amorphous solid phase, provide evidence for a dynamic transition within solid BSA, and provide insight into the molecular mechanisms controlling oxygen diffusion.

On-chain defect emission in conjugated polymers – Comment on ‘Exciton dissociation dynamics in a conjugated polymer containing aggregate states’ [A. Haugeneder, U. Lemmer, U. Scherf, Chem. Phys. Lett. 351 (2002) 354

On-chain chemical defects have recently been identified as the source of broad, low-energy emission features in polyfluorenes. By considering the evolution of the time-resolved luminescence spectrum of a dilute solution of a ladder-type poly( p-phenylene) it is shown that on-chain defects also play an important role in the emission from these materials. A broad band centred at 560 nm, which has previously been attributed to aggregate emission, is identified in both solution and film, suggesting an incompatibility with the aggregate picture. This provides an explanation for the recent observation of a decrease of field-induced exciton dissociation with increasing emission wavelength as excitons are spatially more localised on chemical defects than on the polymer backbone.

Spatial inhomogeneity investigation of QW emission in InGaN MQW LEDs

The luminescence properties of InGaN multiquantum well (MQW) light emitting diodes (LEDs) with various emission wavelengths were investigated using electroluminescence (EL) microscopy (ELM) and micro-EL (μ-EL) spectroscopy. Spatial inhomogeneity of the QW emission was observed as a function of the emission wavelength in spectrally resolved ELM images. The results show that the emission from diodes with shorter emission wavelengths exhibit less spatial inhomogeneity, while bright features were observed on the surface of samples with longer emission wavelengths. μ-EL spectra obtained from 5×5 μm 2 regions show an increase in the full width at half maximum (FWHM) of the spectra with increasing injection current and emission wavelength. With increasing emission wavelength, there is a shift in the main emission area from the p-contact area to the n-contact. The μ-EL spectra obtained from different bright spots in the ELM images demonstrate the variation of main emission peak position and FWHM of the EL spectra. The results suggest that the band-tailing effect due to small self-organised In-rich regions play a key role in the emission of the InGaN MQWs.

CBE and MOCVD growth of GaInNAs

We have investigated growth of GaInNAs by MOCVD using dimethylhydrazine (DMHy) and by CBE using RF-radical nitrogen. It was found that there was a large difference in nitrogen incorporation characteristics between the two growth methods. Nitrogen composition of MOCVD increased with a superlinear characteristic against nitrogen source flow and indium composition enhanced such a characteristic. CBE showed a linear incorporation characteristic of nitrogen composition due to the strong reactivity of nitrogen atoms. Photoluminescence intensity was decreased by increasing emission wavelength for both growth techniques, however, the degradation behavior seemed to depend on the nitrogen incorporation characteristic. The hydrogen and carbon concentration of MOCVD grown sample was more than ten times larger than that of CBE grown samples. The comparison of growth characteristics and material qualities may provide useful information for improving the GaInNAs crystal quality.

Fluorescence heterogeneity of tryptophans in Na,K-ATPase: evidences for temperature-dependent energy transfer

The intrinsic fluorescence emission kinetics of Na,K-ATPase, a large membrane protein containing 16 tryptophan residues, was studied by time-resolved techniques. The lifetime distributions recovered by the Maximum Entropy Method exhibit a strong dependence on the emission wavelength at temperatures between 37°C and −70°C. From the `blue' edge of the fluorescence emission spectrum up to the maximum of emission, the lifetime distribution at room temperature is the result of four broad peaks which cover the time range 0.3–7 ns. With increasing emission wavelength, these peaks move to longer lifetimes and the peak at shorter times are suppressed at the red edge, while the longest component (6–7 ns) becomes dominant. With decreasing temperature, the number of lifetime components is reduced for the benefit of the long one. At cryogenic temperatures, the emission decay in the red-edge of the fluorescence spectrum consists of one major slow component (6–7 ns) and a fast one (0.5 ns) associated with a negative pre-exponential term. This is a characteristic feature of an excited-state reaction. The temperature dependence of this fast component and the fluorescence anisotropy decay at low temperature in the red-edge, indicate that this excited state reaction may be accounted for a unidirectional inter-tryptophan fluorescence energy transfer from `blue' populations of donors to `red' populations of acceptors. This is also illustrated by the time-resolved emission spectra. In the blue edge of the fluorescence emission spectrum, moreover, the time course of the anisotropy decay suggests the existence of homo-transfer of excitation energy involving `blue' tryptophan residues. The steady-state anisotropy excitation spectrum in vitrified solvent agrees with this suggestion. These different energy transfer mechanisms may be used as structural probes to detect more accurately conformational changes of the protein elicited by effectors and ion binding or release.

New fluorescent dyes in the red region for biodiagnostics

The increased sensitivity together with the advent of low-cost optical sources and detectors in the visible-near IR region has led us to current efforts to develop new efficient fluorescent labels for biodiagnostics with absorption and emission beyond 600 nm. In view of the general fluorescence decrease with increasing emission wavelength, we investigated the possibility to shift the absorption of rhodamine dyes toward the region 620-670 nm. The hydrophobic nature of all known long-wavelength dyes results in the tendency to form intra- and intermolecular aggregates in hydrophilic solvents, especially in aqueous environment. Due to the aggregation with biological materials, fluorescence quenching of the dyes is often observed. New strategies for prevention of these processes are considered.

Polarization retention in the visible photoluminescence of porous silicon.

The polarization state of the visible photoluminescence of porous silicon excited with linearly polarized light has been examined. The photoluminescence was linearly polarized in the same direction as the excitation light, independent of the crystallographic orientation of the sample. The degree of linear polarization decreased significantly with increasing emission wavelength. However, no definite trend of its excitation-wavelength dependence was observed. An interesting correlation was found to exist between the photoluminescence efficiency and the degree of linear polarization, where polarized photoluminescence was more prominent in photoluminescence-inefficient samples. Furthermore, the polarization retention phenomenon was found to be independent of temperature. These experimental results can be explained by the radiative recombination of excitons localized within optically anisotropic and randomly distributed Si nanocrystallites.