Photoluminescence Intensity Measurements Research Articles

Photoluminescence measurement of ruby excited by duty ratio-controlled pulse light-emitting diodes (LED) light source.

Photoluminescence (PL) spectra from ruby were obtained using a highly stable LED light source, employing pulse width modulation technique for excitation. The temporal variation in PL intensity caused by the increasing temperature of the LED used for excitation can be mitigated by adjusting the duty ratio (%) of the pulsed LED light to below 10% for cooling the LED. Stable PL spectra measurements were achieved with a duty ratio of less than 10% using a duty ratio-controlled pulsed LED light source, as temperature fluctuations in LED light intensity are minimized at duty ratios less than 10%. Furthermore, fluctuations in the measured PL intensity were diminished by setting the frequency of the pulsed LED light source to greater than 1kHz. This method enables more reliable, cost-effective, and stable PL measurements for material characterization in semiconductors, photonics, and nanotechnology.

Pulsatile therapy for perovskite solar cells

<h2>Summary</h2> The current top challenge for the commercialization of perovskite solar cells is to ensure long-term stability. Herein, we developed pulsatile therapy, which can revive the degraded device and eventually prolong device lifetime by addressing the accumulation of both charges and ions in the middle of maximum power point tracking (MPPT). In this technique, reverse biases are repeatedly applied for a short time to eliminate the charges accumulated and redistribute the ions that have migrated during power harvesting without any pause of operation. Intriguingly, the therapy is not only delaying irreversible degradation but also restoring the degraded power right after a short reverse bias. <i>In situ</i> measurements of photoluminescence intensity and photocurrent dynamics during the therapy confirmed that newly formed defects could be annihilated as a result of discharging and ion redistribution, featuring suppression of defect formation and irreversible degradation. In the long-term test, we observed outstanding improvement of device stability and total harvesting energy.

Fluorescent and thermoresponsive tetraphenylethene-based cross-linked poly(N-isopropylacrylamide)s: Synthesis, thermal/AIE properties, and cell viability

BackgroundThe synthesis, thermal properties, and aggregation-induced emission (AIE)-responsive properties of a series of tetraphenylethene (TPE)-based cross-linked poly(N-isopropylacrylamide)s (PNIPAms) consisting of a hydrophilic/thermosensitive NIPAm monomer and AIE-responsive/hydrophobic TPE derivative through reversible addition-fragmentation chain transfer polymerization was investigated in this study. MethodsThe AIE effect was investigated in a H2O/THF mixture via photoluminescence intensity measurements, and the polymers showed emission (λem = 485 ± 5 nm). When the water fraction was 90%, the PL intensity was enhanced to the highest value. Significant findingsPolymers P1–P3 showed the low critical solution temperature (LCST) at approximately 33.5, 32.4, and 28.1 °C, respectively, and deformed at P4 and P5. From dynamic light scattering, the hydrodynamic diameters of P0 and P1–P2 at a temperature higher than the LCST decreased from 564.0 to 159.6 nm, 417.4 to 126.7 nm, and 469.4 to 134.5 nm (P0, P1, and P2, respectively), indicating the thermoresponsive property. A MTT assay was used for HepG2 liver cancer cells at a concentration of 2 µg/mL, and all the polymers showed cell viability over 80%. P2 and P3 showed emission in the living HepG2 cells for 96 h of incubation. These findings suggest that the synthesized polymers have potential as drug carriers.

Poly(methyl methacrylate) Films with High Concentrations of Silicon Quantum Dots for Visibly Transparent Luminescent Solar Concentrators.

Silicon quantum dots (Si QDs) are attractive, nontoxic luminophores for luminescent solar concentrators (LSCs). Here, we produced Si QD/poly(methyl methacrylate) (PMMA) films on glass by doctor-blading polymer solutions and achieved films with low light scattering at an order of magnitude higher Si QD weight fraction than has been achieved previously in the bulk. We suggest that the fast solidification rate of films as compared to slow bulk polymerization is an enabling factor in avoiding large agglomerates within the nanocomposites. Scanning electron microscopy confirmed that ∼100 nm or larger QD agglomerates exist in light-scattering films, and photoluminescence intensity measurements show that light scattering, if present, significantly reduces waveguiding efficiencies for LSCs. Nonscattering films fabricated in this work exhibit high ultraviolet absorption (>80%) paired with high visible transmission (>87%) and minimal visible haze (∼1%), making them well suited for semitransparent coatings for LSCs realized as solar harvesting windows.

Large stark tuning of InAs/InP quantum dots

InAs/InP quantum dots are excellent sources of telecom single-photon emission and are among the most promising candidates for scalable quantum photonic circuits. However, geometric differences in each quantum dot lead to slightly different emission wavelengths and hinder the possibility of generating multiple identical quantum emitters on the same chip. Stark tuning is an efficient technique to overcome this issue as it can control the emission energy of individual quantum dots through the quantum-confined Stark effect. Realizing this technique in InAs/InP quantum dots has previously been limited to shifts of less than 0.8 meV due to jumps in the emission energy because of additional charges at high electric field intensities. We demonstrate up to 5.1 meV of Stark tuning in the emission wavelength of InAs/InP quantum dots. To eliminate undesirable jumps to the charged state, we use a thin oxide insulator to prevent carrier injection from the contacts, thereby significantly improving the tuning range of the Stark effect. Moreover, the single-photon nature and narrow linewidth of the quantum dot emission are preserved under a wide range of applied electric fields. Using photoluminescence intensity measurements and time-resolved lifetime spectroscopy, we confirmed that this Stark tuning range is limited by carrier tunneling at high electric fields. This result is an important step toward integrating multiple identical quantum emitters at telecom wavelengths on a chip, which is crucial for realizing complex quantum photonic circuits for quantum information processing.

Advanced synthesis of carbon dots novel insights into temperature effect on fluorescent performance

As a new generation of semiconductor reinforcements, fluorescent carbon dots have unpredictable and potential applications in bioimaging, photocatalysis, and optoelectronic devices. This work shows an environmentally friendly process for carbon dots preparation using citric acid monohydrate and urea. The influence of different parameters on luminous property was carefully studied based on the photoluminescence intensity measurement, including synthesis temperature, time and the concentration of carbon sources and deactivator. Then the synthesized products were further characterized by TEM, FT-IR, XPS, UV-vis, and XRD. Highest fluorescence intensity could be achieved under the carbon sources and deactivator concentration of 0.3 mol/L and 1 mol/L, respectively, in a total reaction system of 40 ml at 190?C for 18 hours. The screening analysis revealed that synthesis temperature has a strong correlation with the photoluminescence intensity. This study provides further insights into designing high fluorescent performance for carbon dots preparation.

Low Temperature Photoluminescence Spectroscopy of Defect and Interband Transitions in CdSexTe1-x Thin Films

We present the defect analysis by photoluminescence (PL) spectroscopy of CdSexTe1-x thin films, grown with varying Se content by a co-sputtered deposition method. We observe a peak at 1.203 eV in the CdSexTe1-x film for x = 0.21, which shifts towards higher energies with increase in laser power. This peak was assigned to a donor-to-acceptor (DAP) transition, with a measured j-shift of ∼4.7 meV/decade. Temperature dependent PL intensity measurements confirm that the observed DAP peak involves a shallow defect state of binding energy ∼34.7 meV. In contrast, a free-to-bound (FB) peak at 1.294 eV involving a shallow defect of binding energy ∼18.3 meV was observed in the CdSexTe1-x film for x = 0.14. Additionally, we observe band edge emission at 1.452 eV and 1.448 eV in CdSexTe1-x films for x = 0.14 and x = 0.21 respectively. Our analysis shows that the Se concentration not only changes the band gap energy of the resulting CdSexTe1-x alloy thin film, but also modifies the nature of the dominant observed defect emission.

Study of chemical physics on energy transfer phenomenon between quantum dots and a designed diporphyrin in solution

The present paper reports a case study of energy transfer (ET) phenomenon from the excited CdS/ZnS core-shell type quantum dots (QDs) having particle size of ~2 nm to a designed diporphyrin (1) molecule in solution. Photoluminescence intensity measurements and lifetime quenching experiments establish the process of Förster resonance energy transfer (FRET) from QDs to 1 in toluene. Magnitude of bimolecular quenching constant of QDs/1 system (2.185 × 10−13 s−1) rules out the possibility of diffusion controlled mechanism in present work. It is proposed that QDs/1 system may be employed as an efficient energy transfer unit for possible application in photosensitization.

Analysis of Near-Surface Metal Contamination by Photoluminescence Measurements

In this work, molybdenum and tungsten-implanted wafers are analyzed by an innovative technique based upon photoluminescence measurements, with the aim to assess the ability of this technique to detect metal contamination in the near-surface region. Surface Photovoltage (SPV) measurements of carrier diffusion length are compared to the results of photoluminescence measurements. It is shown that molybdenum and tungsten contamination are easily detected by photoluminescence intensity measurements down to about 1010cm−2 contaminant dose. Vice versa, SPV has limited sensitivity to these elements (≥5·1010cm−2), because of their low diffusivity. Therefore, photoluminescence intensity measurement can be a valid alternative to conventional carrier diffusion length measurements for monitoring slow diffuser contamination.

Photoluminescence Characterization of Cadmium Sulphide (CdS) Nanowires for Polarization Studies

In this paper, the polarizing properties for CdS nanowire arrays were explored for their potential use in the design of nanowire based polarizers and optical switches. These free standing cadmium sulphide (CdS) nanowires were grown in anodized aluminum oxide (AAO) template via dc electrodeposition. Raman and photoluminescence (PL) measurements were investigated for parallel and perpendicular polarization with two orientations of the sample having light propagating parallel to the nanowire axis in one orientation and light propagating perpendicular to the nanowire axis in other orientation. Polarization-sensitive measurements show strong polarization anisotropy in the photoluminescence (PL) intensity measurements observed in parallel and perpendicular orientation to the long axis of a nanowire. The measured PL ratio, ρ, for parallel to perpendicular orientation was around 0.80- 0.85 which shows strong polarization anisotropy for the grown CdS nanowires. Strong peaks of A1 (TO) at 235 cm-1, E2 mode at 255 cm-1 along with 1 LO (longitudinal optical) at 303 cm-1, 2 LO peak at 604 cm-1 of the CdS nanowires were seen with different polarizations for Raman spectral studies. These polarization studies show that these dc electrodeposited grown CdS nanowire arrays are well suited for uses in polarization-based nanoscale devices such as in optical switches, and high performance photodetectors.

Sensitive optical bio-sensing of p-type WSe2 hybridized with fluorescent dye attached DNA by doping and de-doping effects

Layered transition metal dichalcogenides, such as MoS2, WSe2 and WS2, are exciting two-dimensional (2D) materials because they possess tunable optical and electrical properties that depend on the number of layers. In this study, the nanoscale photoluminescence (PL) characteristics of the p-type WSe2 monolayer, and WSe2 layers hybridized with the fluorescent dye Cy3 attached to probe-DNA (Cy3/p-DNA), have been investigated as a function of the concentration of Cy3/DNA by using high-resolution laser confocal microscopy. With increasing concentration of Cy3/p-DNA, the measured PL intensity decreases and its peak is red-shifted, suggesting that the WSe2 layer has been p-type doped with Cy3/p-DNA. Then, the PL intensity of the WSe2/Cy3/p-DNA hybrid system increases and the peak is blue-shifted through hybridization with relatively small amounts of target-DNA (t-DNA) (50–100 nM). This effect originates from charge and energy transfer from the Cy3/DNA to the WSe2. For t-DNA detection, our systems using p-type WSe2 have the merit in terms of the increase of PL intensity. The p-type WSe2 monolayers can be a promising nanoscale 2D material for sensitive optical bio-sensing based on the doping and de-doping responses to biomaterials.

Co-adsorption of water and oxygen on GaN: Effects of charge transfer and formation of electron depletion layer

Species from ambient atmosphere such as water and oxygen are known to affect electronic and optical properties of GaN, but the underlying mechanism is not clearly known. In this work, we show through careful measurement of electrical resistivity and photoluminescence intensity under various adsorbates that the presence of oxygen or water vapor alone is not sufficient to induce electron transfer to these species. Rather, the presence of both water and oxygen is necessary to induce electron transfer from GaN that leads to the formation of an electron depletion region on the surface. Exposure to acidic gases decreases n-type conductivity due to increased electron transfer from GaN, while basic gases increase n-type conductivity and PL intensity due to reduced charge transfer from GaN. These changes in the electrical and optical properties, as explained using a new electrochemical framework based on the phenomenon of surface transfer doping, suggest that gases interact with the semiconductor surface through electrochemical reactions occurring in an adsorbed water layer present on the surface.

Study of energy transfer phenomenon between quantum dots and zinc porphyrin in solution

The mechanism of energy transfer (ET) phenomenon from the excited CdSxSe1−x/ZnS alloyed quantum dots (QDs) to 2,3,7,8,12,13,17,18-Octaethyl-21H,23H-porphine zinc(II) (1) molecule is investigated in solution through Förster resonance energy transfer (FRET) calculations, photoluminescence intensity measurements and lifetime quenching experiments. The process of ET from the excited QDs to 1 in toluene is established by FRET calculations. ET efficiency in QDs/1 complex is regulated by surface passivation and ultrafast quenching mechanism. We propose that CdSxSe1−x/ZnS alloyed quantum dots may be suitably employed as an efficient energy transfer donor for zinc-porphyrin for possible application in light harvesting devices.

Yellow-light emitting Tb3Al5O12:Ce3+ phosphor properties sensitized by Bi3+ ions

A new yellow-light emitting garnet phosphor Tb3Al5O12:Ce3+, Bi3+ (TAG:Ce3+, Bi3+) was synthesized by the metal organic decomposition method. The phosphor properties with different Ce3+/Bi3+ dopant concentrations were studied in detail using X-ray diffraction (XRD) analysis, photoluminescence (PL) and PL excitation spectroscopies, and diffuse reflectance measurements. The optimum concentrations of Ce3+/Bi3+ deduced from the PL intensity measurements by excitation at λex = 325nm were ∼ 0.2mol% (Ce3+ and Bi3+). The addition of Bi3+ ions into the Ce3+ singly doped phosphor increased ∼ 43% of the PL intensity. The XRD data further suggested an improvement in the crystalline quality of the Ce3+ singly doped phosphor by codoping with Bi3+. An increase in the PL intensity was attributed to an energy transfer from Tb3+ to Bi3+ and Ce3+ together with an improved crystalline quality of the (Ce3+, Bi3+)-codoped phosphor. Temperature dependence of the luminescence intensities was also measured at T = 20–500K in 10-K increments and analyzed using the conventional model with thermal quenching energies of Eq1 = 35meV and Eq2 = 0.40eV. The experimental data for the Ce3+ singly doped phosphor, however, required an additional term of trap states with energy of Et = 3meV, in agreement with poor crystalline quality of the TAG:Ce3+ phosphor.

Two-step migration of particles in evaporating bimodal suspension films at high Peclet numbers

During the drying of bimodal colloidal suspensions containing particles of various sizes, smaller particles preferentially migrate to the top surface under particular drying conditions, leading to undesirable drying defects in batteries and in other coating applications. Despite extensive previous studies, the migration mechanism is far from being understood because few in situ observations are available to support the hypotheses. To remedy this, we use real-time photoluminescence (PL) microscopy to investigate the migration of small fluorescent latex particles co-dispersing with large nonemissive latex particles. Comparing the measured PL intensity with that predicted by a model allows us to determine the quantity of small particles near the evaporating surface. The results reveal that the fluorescent particles segregate in two steps: The primary segregation occurs early in the evaporation stage, whereas the secondary stepwise migration occurs when the air–liquid interface invades the particle consolidation layer. The latter migration is attributed to the flow-induced motion of small particles that move through interstitial spaces between large particles.

Avoidance of instable photoluminescence intensity from AlGaN bulk layers

The emission intensity from c‐plane AlGaN bulk layers changes strongly on time scales from seconds to hours during above band gap illumination in photoluminescence experiments. Responsible for this effect is the accumulation of photo‐generated charge carriers at the semiconductor surface, modifying surface states and surface recombination. The effect has a negative impact on the exactness and comparability of photoluminescence intensity measurements, especially for measurements with long exposure times or repeated excitation on the same spot. We realized temporally stable photoluminescence intensity by using an AlN cap layer separating the studied AlGaN bulk layer from the crystal surface. We verified the temporal stability in temperature‐dependent photoluminescence measurements in vacuum and in ambient air. Surface recombination was suppressed for the capped AlGaN layer, resulting in significantly higher emission intensity at room temperature.

Förster Resonance Energy Transfer Mediated Photoluminescence Quenching in Stoichiometrically Assembled CdSe/ZnS Quantum Dot-Peptide Labeled Black Hole Quencher Conjugates for Matrix Metalloproteinase-2 Sensing

The steady state and time-resolved photoluminescence quenching of streptavidin modified CdSe/ZnS quantum dots (QDs) instigated by biotin-peptide-BHQ-1 (biotin-pep-BHQ-1) molecule was investigated. Here, we have achieved an efficient photoluminescence (PL) quenching of QDs with the conjugation of dark quencher (black hole quencher-BHQ) molecules intermediated with the GPLGVRGK peptide. The luminescence of streptavidin-QDs585 was decreased upon titration with a nano molar concentration of the biotin-GPLGVRGK-BHQ-1 molecule. It has been suggested that the decrease of QDs PL occurred through a Förster resonance energy transfer (FRET) mechanism from the analysis of steady state photoluminescence intensity measurements as well as time resolved lifetime measurements of streptavidin-QDs and QDs-(pep-BHQ-1)n conjugates. The sequence of intermediate peptide GPLG↓VRGK can act as a target material for matrix metalloproteinases-2 (MMP-2) produced by cancer cells at its Gly and Val region, shown by the down-headed arrow. Interestingly, here the reported self-assembled QDs-(pep-BHQ-1)n conjugates could detect the presence MMP-2 at a detection limit of 1 ng/mL with a clear luminescence recovery.

Carbon Dot Nanothermometry: Intracellular Photoluminescence Lifetime Thermal Sensing.

Nanoscale biocompatible photoluminescence (PL) thermometers that can be used to accurately and reliably monitor intracellular temperatures have many potential applications in biology and medicine. Ideally, such nanothermometers should be functional at physiological pH across a wide range of ionic strengths, probe concentrations, and local environments. Here, we show that water-soluble N,S-co-doped carbon dots (CDs) exhibit temperature-dependent photoluminescence lifetimes and can serve as highly sensitive and reliable intracellular nanothermometers. PL intensity measurements indicate that these CDs have many advantages over alternative semiconductor- and CD-based nanoscale temperature sensors. Importantly, their PL lifetimes remain constant over wide ranges of pH values (5-12), CD concentrations (1.5 × 10-5 to 0.5 mg/mL), and environmental ionic strengths (up to 0.7 mol·L-1 NaCl). Moreover, they are biocompatible and nontoxic, as demonstrated by cell viability and flow cytometry analyses using NIH/3T3 and HeLa cell lines. N,S-CD thermal sensors also exhibit good water dispersibility, superior photo- and thermostability, extraordinary environment and concentration independence, high storage stability, and reusability-their PL decay curves at temperatures between 15 and 45 °C remained unchanged over seven sequential experiments. In vitro PL lifetime-based temperature sensing performed with human cervical cancer HeLa cells demonstrated the great potential of these nanosensors in biomedicine. Overall, N,S-doped CDs exhibit excitation-independent emission with strongly temperature-dependent monoexponential decay, making them suitable for both in vitro and in vivo luminescence lifetime thermometry.

Photoluminescence quantum efficiency of Er optical centers in GaN epilayers

We report the quantum efficiency of photoluminescence processes of Er optical centers as well as the thermal quenching mechanism in GaN epilayers prepared by metal-organic chemical vapor deposition. High resolution infrared spectroscopy and temperature dependence measurements of photoluminescence intensity from Er ions in GaN under resonant excitation excitations were performed. Data provide a picture of the thermal quenching processes and activation energy levels. By comparing the photoluminescence from Er ions in the epilayer with a reference sample of Er-doped SiO2, we find that the fraction of Er ions that emits photon at 1.54 μm upon a resonant optical excitation is approximately 68%. This result presents a significant step in the realization of GaN:Er epilayers as an optical gain medium at 1.54 μm.

Temperature dependence studies of Er optical centers in GaN epilayers grown by MOCVD

ABSTRACTWe report the temperature dependence of Er optical centers in GaN epilayers prepared by metal-organic chemical vapor deposition under the resonant excitation (4I15/2 → 4I9/2) excitation using a Ti:Sapphire laser (λexc = 809 nm). High resolution infrared spectroscopy and temperature dependence measurements of photoluminescence intensity from Er ions in GaN have been performed to identify the crystal filed splitting of the first excited state, 4I13/2. Here, we have employed a simple approach to determine activation energies which are related to the thermal population of electrons from the lowest level to the higher level of the crystal field splitting of the first excited state.