Photoluminescence Emission Wavelength Research Articles

Colloidal ZnO and Zn(1-x)Co(x)O tetrapod nanocrystals with tunable arm lengths.

Tetrapod-shaped ZnO nanocrystals exhibit exceptional optoelectronic properties, including intense ultraviolet photoluminescence emission, that make them attractive for applications that include lasers, sensors, and photocatalysts. However, synthetic methods that produce ZnO tetrapods typically include high-temperature vapor-deposition approaches that do not readily achieve characteristic dimensions of less than 100 nm or colloidal methods that require added metal dopants, which modify the inherent properties of ZnO. Here, we report a robust, modified solution-phase synthetic protocol for generating colloidal ZnO tetrapods that does not require the use of metal dopants. The ZnO tetrapod arm lengths can be tuned from 10 to 25 nm by adjusting the amount of Zn reagent used in the reaction. Subsequent seeded-growth produced even larger colloidal ZnO tetrapods with 62 nm arms. Photoluminescence (PL) measurements confirm that the tetrapods are of high crystalline quality, and the ultraviolet PL emission wavelengths that are observed fall between those of previously reported metal-doped colloidal ZnO tetrapods, which exhibit dopant-induced red- or blue-shifts. Furthermore, the reaction strategy can be modified to produce cobalt-substituted ZnO, offering a chemical pathway to tetrapod-shaped Zn1-xCoxO nanocrystals.

In-situ photoluminescence measurements during MOVPE growth of GaN and InGaN MQW structures

In this work we report the first quasi-continuous in-situ photoluminescence study of growing InGaN LED structures inside an industrial-grade metal-organic vapor phase epitaxy (MOVPE) reactor at growth temperature. The photoluminescence spectra contain information about temperature, thickness and composition of the epitaxial layers. Furthermore, the in-situ spectra – even at an early stage of the growth of the active region – can be used to predict the photoluminescence emission wavelength of the structure at room temperature. In this study an accuracy of this predicted wavelength in the range of ± 1.3 nm (2σ) is demonstrated. This technique thus appears suitable for closed-loop control of the emission wavelength of InGaN LEDs already during growth.

Hydrophilicity and polarity of cubic silicon carbide nanocrystals identified from photoluminescence emissions in various solvents

Since the surface-structure-related state dominates the photoluminescence (PL) emissions of cubic silicon carbide (3C-SiC) nanocrystals (NCs) with a diameter lower than 3.0 nm, the PL lines of 3C-SiC NCs in deionized water with different pH values, methanol, ethanol, chloroform, and toluene are characterized at the excitation of 300 to 360 nm. The position and intensity of the PL peaks provide experimental evidence that the surface of 3C-SiC is hydrophilic and hydroxyl bonding on the surface decreases the energy gap of the NCs. The PL emission wavelength of 3C-SiC NCs can be adjusted by adjusting the pH value of the solvent. The polar 3C-SiC NCs prefer to precipitate in chloroform and toluene.

Optical and structural properties of nanocrystalline PbS thin film grown by CBD on Si(1 0 0) substrate

PbS nanocrystalline thin film was prepared by chemical bath deposition on Si(1 0 0) substrate at bath temperatures of 25, 45 and 65 °C. Triethanolamine was added to the aqueous solution, which decreased the grain size and increased the luminescence of the nanocrystalline PbS thin film. PbS nanocrystals were identified using XRD, TEM and AFM. The crystalline size of the PbS film deposited at different bath temperatures was estimated by XRD and TEM to be 7–12 nm. The growth mechanism of the PbS crystallites were described at different bath temperatures. The confinement was reflected in the absorption spectra, photoluminescence excitation and photoluminescence spectra. The luminescence of Si(1 0 0) substrate and PbS nanocrystalline film deposited on Si(1 0 0) were compared, and the results revealed that the PbS nanocrystals altered and notably enhanced the emission features of the Si(1 0 0) substrate. The shifting of the maximum photoluminescence emission wavelength of PbS nanocrystals with a change in bath temperature and the variation in photoluminescent intensity of PbS nanocrystals prepared at 25 °C versus deposition time were investigated. A single-peak fit of a Gaussian function was employed to discern the photoluminescence of PbS on Si(1 0 0) substrate.

Feasibility of luminescent multilayer sol-gel thermal barrier coating manufacturing for future applications in through-thickness temperature gradient sensing

This paper investigates the feasibility of manufacturing sol-gel multilayer thermal barrier coatings (TBC) functionalized with different lanthanide ions Ln3+ having distinct photo-luminescence emission wavelengths (Ln=Sm, Eu, Dy, Er, Tm) for future applications in temperature gradient sensing. Ln3+ doped 9.75mol% yttria stabilized zirconia (YSZ) powders were produced to study the effect of activator concentration on luminescence intensity and host matrix crystal structure. Self-quenching was found to limit the maximum signal-to-noise ratio achievable with Sm3+, Dy3+, Er3+ and Tm3+ activators, which was not the case for Eu3+ in the 1–10mol% range. The increase in activator was found to affect the crystal structure of YSZ. A solution was proposed that suppressed this effect while significantly increasing the luminescence intensity of all activators. Finally a TBC sensor prototype integrating Eu3+, Er3+ and Dy3+ doped layers distributed throughout the thickness was successfully deposited by a dip-coating sol-gel process and showed promising through-thickness luminescence sensing capabilities.

Simple and Eco-Friendly Hydrothermal Synthesis of Luminescent Carbon Nanoparticles for H<sub>2</sub>O<sub>2</sub> Detection

The fluorescent carbon nanoparticles (average diameter: 10~50 nm) are prepared by using a simple and eco-friendly hydrothermal process, and show a stronge blue emission. The fluorescence of the water soluble carbon nanoparticles could be quenched effectively by H2O2without obvious shift in maximum photoluminescence emission wavelength, which can be used for the detection of H2O2.

Enhanced stability and emission intensity of aqueous CdTe/CdS core–shell quantum dots with widely tunable wavelength

The stability and photoluminescence (PL) emission intensity of quantum dots (QDs) are particularly interesting for various bioapplications. In this paper, monodisperse CdTe/CdS core–shell QDs were synthesized in aqueous phase. The size of CdTe core and the thickness of CdS shell played important roles in the properties of CdTe/CdS QDs. Transmission electron microscopy (TEM) images show that the average diameters of CdTe/CdS core–shell QDs increased to ∼5 nm compared with that of CdTe core QDs (∼3.6 nm). X-ray diffraction patterns show that the CdTe/CdS core–shell QDs possess cubic zinc blende structure. After the overgrowth of CdS shell on CdTe core, a widely tunable PL emission wavelength (485–720 nm) of the core–shell QDs can be obtained, which covers nearly the full visible spectrum. The PL intensity of the CdTe/CdS core–shell QDs shows two-fold increase compared with that of the CdTe core QDs. PL results show that the photostability of the samples is enhanced through the growth of CdS on the CdTe QDs.

Size-controlled synthesis of ZnO quantum dots in microreactors

In this paper, we report on a continuous-flow microreactor process to prepare ZnO quantum dots (QDs) with widely tunable particle size and photoluminescence emission wavelengths. X-ray diffraction, electron diffraction, UV–vis, photoluminescence and transmission electron microscopy measurements were used to characterize the synthesized ZnO QDs. By varying operating conditions (temperature, flow rate) or the capping ligand, ZnO QDs with diameters ranging from 3.6 to 5.2 nm and fluorescence maxima from 500 to 560 nm were prepared. Results obtained show that low reaction temperatures (20 or 35 °C), high flow rates and the use of propionic acid as a stabilizing agent are favorable for the production of ZnO QDs with high photoluminescence quantum yields (up to 30%).

Facile synthesis of Cu–In–Zn–S alloyed nanocrystals with temperature-dependent photoluminescence spectra

A facile strategy has been developed for the synthesis of highly luminescent Cu–In–Zn–S (CIZS) quaternary alloyed nanocrystals. The photoluminescence emission wavelength could be tuned from 520nm to 700nm only by varying the diffusion temperature, and an up to 76% of quantum yield was obtained after coating with the ZnS shell. The results of high resolution transmission electron microscopy and X-ray diffraction demonstrated the small particle size (2.5nm), high crystallinity and alloyed structure of all the CIZS NCs. The photoluminescence mechanism of CIZS NCs before and after ZnS coating was also investigated by analyzing the PL decay curves.

Highly sensitive fluorescent sensor for mercury (II) ion based on layer-by-layer self-assembled films fabricated with water-soluble fluorescent conjugated polymer

In this paper, a simple and sensitive Hg (II) sensor based on the water-soluble fluorescent conjugated polymer multilayer films was developed. The multilayer films were fabricated successfully utilizing layer-by-layer self-assembly technology via the electrostatic interaction between polycation electrolyte PDDA and sulfonic group anion surrounded conjugated polymer PPESO3. In the presence of Hg (II) ion, the photoluminescence (PL) of PDDA/PPESO3 multilayer films quenched effectively without obvious maximum PL emission wavelength shift. The PL intensity ratio I0/I (I0 and I are the fluorescence intensity of PDDA/PPESO3 multilayer films in the absence and presence of Hg(II) ion, respectively) was proportional to the concentration of Hg (II) in the concentration ranges of 2.0×10−7 to 3.0×10−5molL−1. The detection limit (LOD) for Hg (II) was 1.0×10−7molL−1. The proposed film sensor was successfully applied for the determination of Hg (II) in several water samples with satisfactory reproducibility and accuracy. This work may provide a common platform for the further development of various solid-state portable devices.

200-mm GaN-on-Si Based Blue Light-Emitting Diode Wafer with High Emission Uniformity

We investigated the emission wavelength uniformity of 200-mm GaN-on-Si based blue light-emitting diode (LED) wafer grown by metalorganic vapor phase epitaxy (MOVPE). The larger the Si substrate diameter becomes, the more difficult to obtain uniform distribution of the emission wavelength because of the larger bow during growth, resulting in larger on-wafer inhomogeneity in growth temperature. Owing to the GaN-on-Si buffer strain management, optimized gas flow condition, and precise control of temperature balance in a reactor, we have achieved high thickness and crystal quality uniformity over the 200-mm GaN-on-Si based blue LED wafer. As a result, excellent blue photoluminescence emission wavelength uniformity from the InGaN-multi-quantum wells can be demonstrated on a 200-mm wafer with a standard deviation of 2.53 nm (0.57%). Less wavelengths binning with these highly uniform emission over the 200-mm wafer show the capability of sustainable cost reduction in LED fabrication based on GaN-on-Si technology.

Sonoelectrochemical synthesis of water-soluble CdTe quantum dots

A facile and fast one-pot method has been developed for the synthesis of CdTe quantum dots (QDs) in aqueous phase by a sonoelectrochemical route without the protection of N2. The morphology, structure and composition of the as-prepared products were investigated by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and energy dispersive X-ray spectrometer (EDS). The influences of current intensity, current pulse width, and reaction temperature on the photoluminescence (PL) and quantum yield (QY) of the products were studied. The experimental results showed that the water-soluble CdTe QDs with high PL qualities can be conveniently synthesized without precursor preparation and N2 protection, and the PL emission wavelength and QY can be effectively controlled by adjusting some parameters. This method can be expected to prepare other QDs as promising building blocks in solar cell, photocatalysis and sensors.

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%.

Synthesis of photoluminescent carbon nanoparticles from graphite

Photoluminescent carbon nanoparticles (CNPs) with diameters ranging from 1.5 to 6.5 nm were synthesized from raw graphite without surface passivation. The photoluminescent (PL) emission spectra illustrate that both excitation wavelength and solution pH can significantly influence the maximum emission wavelength and PL intensity of the CNP solution. As the excitation wavelength decreases and solution pH increases, a blue shift in the maximum PL emission wavelength occurs.

Facile synthesis of ZnS–CdIn2S4-alloyed nanocrystals with tunable band gap and its photocatalytic activity

High-quality colloidal ZnS–CdIn2S4 (ZCIS) quaternary alloyed nanocrystals (NCs) have been synthesized via the reaction between acetate salts of the corresponding metals and elemental sulfur in the presence of dodecanethiol in octadecene media at 230°C. The photoluminescence (PL) emission wavelength can be conveniently tuned from 498 to 635nm via the variation of stoichiometric ratio of corresponding components. The influence of various experimental variables, such as reaction temperature, amounts of sulfur and dodecanethiol, and Zn:CdIn2 ratio, on the optical properties of the obtained ZCIS NCs have been systematically investigated. The plain ZCIS core NCs did show PL but with quite low quantum yield (QY, typically below 5%). In order to improve the PL QY, ZnS shell was subsequently overcoated around the ZCIS core NCs. With the overgrowth of ZnS shell, the PL QY can be up to 48%. Furthermore, the obtained ZCIS/ZnS NCs show a promising photocatalytic activity in the degradation of rhodamine B.

Facile Synthesis and Luminescence Characteristics of High-Quality CdS: Eu/ZnS Core/Shell Nanocrystals with Biocompatibility

In this paper, we report a facile method to synthesize high quality CdS: Eu nanocrystals (NCs) and CdS: Eu/ZnS NCs with strong photoluminescence (PL). The influence of various experimental variables including the concentration of Eu3+ ions, the reaction time and the reaction temperature were investigated systematically. In addition, the PL properties of CdS: Eu NCs exhibited pH sensitive. Under the acid condition, pH value of the CdS: Eu NCs solution played an important role in determining PL emission intensity. However, under the alkaline condition, the obtained CdS: Eu NCs exhibited a tunable PL emission wavelength (from 490 nm to 610 nm) when pH value was adjusted from pH 7 to 10. After coating with ZnS shell, the CdS: Eu/ZnS NCs showed enhanced PL intensity compare with one of the CdS: Eu NCs. The CdS: Eu NCs and CdS: Eu/ZnS NCs were characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). In addition, the biocompatibility of these NCs was measured by hemolytic test, which indicated that CdS: Eu/ZnS NCs were more biocompatible than CdS: Eu NCs at the same conditions. It can be expected that CdS: Eu/ZnS NCs are promising biolabeling materials.

Experimental and theoretical study for InAs quantum dashes-in-a-step-well structure on (001)-oriented InP substrate

An InAs/In0.73Ga0.27As/In0.53Ga0.47As/In0.53Al0.235Ga0.235As quantum dashes-in-a-step-well (QDSWELL) structure grown via molecular beam epitaxy has been studied. It is observed that the photoluminescence (PL) emission wavelength of such a structure can be as long as 2.12 μm at room temperature (RT). This is the longest emission wavelength of InAs QDashes to be realized at RT. The electron and hole energy levels of the InAs/In0.73Ga0.27As/In0.53Ga0.47As/In0.53Al0.235Ga0.235As QDSWELL structure have been calculated using effective-mass envelope-function theory. The calculated transition energy EE1-HH1 (from the first electron energy level E1 to the first heavy-hole energy level HH1) agrees with the measured PL emission peak position quite well. It is found that QDSWELL is an alternative structure for realizing lasers with wavelengths beyond 2 μm at RT.

Synthesis, Characterization and Properties of Electron Donor-Acceptor Complexes Based on 9,9-Diarylfluorene

A series of fluorene-triphenylamine derivatives containing an electron donor-acceptor (D-A) structure based on 9,9-diarylfluorene was designed and synthesized. Their optical properties were investigated by UV-Vis spectroscopy and photoluminescence (PL) techniques in solution as well as in the solid state. The maximum PL emission wavelengths of the compounds ranged from 430 to 530 nm. A dual fluorescence phenomenon was observed in particular polar solvents and the relationship between emission properties and molecular structures was studied. The results reveal the existence of a charge transfer (CT) excited state in the molecules and the PL properties of these compounds depend on the structure of the compound and also on the polarity of the solvent. The molecular constitution of the compounds improves the hole-injection issues for fluorene-based materials because of the introduction of a triphenylamine group. Cyclic voltammetry (CV) shows that the highest occupied molecular orbital (HOMO) energy level of the compounds is located between -5.24 and -5.50 eV and it can be tailored by changing the electronegativity of the substituent group. Simultaneously, the spiro-skeleton molecular structure leads to an excellent glass transition temperature (192-206 °C) and it retains good morphological stability. The thermogravimetric (TG) curves of the compounds show a thermal-decomposition temperature of higher than 400 °C.

New method to isolate and distribute photoluminescence emissions from InAs quantum dots over a wide-wavelength range

We propose a new method to isolate and distribute the photoluminescence emission wavelengths of InAs quantum dots (QDs) over a wide-wavelength range by the growth of the partial GaAs capping layer at low temperature and annealing processes. When the partial capping layer was grown at 485°C, the emission wavelengths of the QDs were distributed within a small range around the mean wavelength. On the other hand, when the partial capping layer was grown at a lower temperature of 423°C, the emission wavelengths of the individual QDs were isolated and distributed over a wide range. Thus fabricated QDs are effective for the QD based devices such as single photon sources.

2 – 3 μ m mid infrared light sources using InGaAs/GaAsSb “W” type quantum wells on InP substrates

We have investigated the InGaAs/GaAsSb/InAlGaAs/InAlAs type-II “W” quantum wells (QWs) grown on InP substrates by molecular beam epitaxy. The photoluminescence (PL) emission wavelength longer than 2.56 μm at room temperature (RT) is demonstrated for the first time in this material system. The PL emission peaks of our designed samples can cover a wide range from 2 to 2.5 μm at cryogenic temperature. The samples show good optical quality that the reduction in integrated PL intensity is only around one order of magnitude from 35 K to RT. We found that the integrated PL intensity decreased as the emission wavelength increased, which is due to the reduction in the electron-hole wave function overlap. This is consistent with the calculated result. In the power dependent PL measurements, the emission peak of “W” type QWs show blue shifts with the excitation power (Pex) but does not follow the Pex1/3 rule as predicted by type-II band bending model. The localized states filling effect gives reasonable explanations for the observed phenomena.