Change In Photoluminescence Intensity Research Articles

Photoluminescence Characterization of Nonradiative Recombination Centers in Light Emitting Materials by Utilizing Below-Gap Excitation: A Mini Review of Two-Wavelength Excited Photoluminescence

We have developed an optical method of detecting and characterizing nonradiative recombination (NRR) centers without electrical contact. The method combines a below-gap excitation (BGE) light with a conventional above-gap excitation light in photoluminescence (PL) measurement, and discriminates the PL intensity change due to switching on and off the BGE. A quantitative analysis of the detected NRR centers became possible by utilizing the saturating tendency of the PL intensity change with increasing the BGE density due to trap filling effect. Some experimental results of AlGaAs, InGaN, and AlGaN quantum wells were shown to allocate the development and present status as well as to exemplify their interpretations.

A Label-Free Photoluminescence Genosensor Using Nanostructured Magnesium Oxide for Cholera Detection.

Nanomaterial-based photoluminescence (PL) diagnostic devices offer fast and highly sensitive detection of pesticides, DNA, and toxic agents. Here we report a label-free PL genosensor for sensitive detection of Vibrio cholerae that is based on a DNA hybridization strategy utilizing nanostructured magnesium oxide (nMgO; size >30 nm) particles. The morphology and size of the synthesized nMgO were determined by transmission electron microscopic (TEM) studies. The probe DNA (pDNA) was conjugated with nMgO and characterized by X-ray photoelectron and Fourier transform infrared spectroscopic techniques. The target complementary genomic DNA (cDNA) isolated from clinical samples of V. cholerae was subjected to DNA hybridization studies using the pDNA-nMgO complex and detection of the cDNA was accomplished by measuring changes in PL intensity. The PL peak intensity measured at 700 nm (red emission) increases with the increase in cDNA concentration. A linear range of response in the developed PL genosensor was observed from 100 to 500 ng/μL with a sensitivity of 1.306 emi/ng, detection limit of 3.133 ng/μL and a regression coefficient (R2) of 0.987. These results show that this ultrasensitive PL genosensor has the potential for applications in the clinical diagnosis of cholera.

Control of photoluminescence of CdSe/ZnS quantum dots in a nematic liquid crystal by an electric field

Photoluminescence (PL) spectra of a nematic liquid crystal suspension doped with 10 wt % CdSe/ZnS semiconductor quantum dots (QDs) with a core size of 3.5 nm in a 20-µm thick plane-parallel cell have been investigated. The spectrum of a homogeneously oriented suspension layer exhibits a significant shift of the PL peak by ~83 nm to a wavelength of 676 nm as compared with the QD spectrum in toluene. The application of a dc electric field with a strength of 0.25 V/µm to the cell leads to a slight hypsochromic shift of this peak, the position of which does not change with a further increase in the field strength up to 2 V/µm. The PL intensity changes exponentially and decreases by a factor of two at the field strength of 1 V/µm. The mechanism of PL quenching of QDs in a homogeneously oriented layer of nematic liquid crystal is discussed.

Metal Structural Integrity Monitoring via Optical Response of Quantum Dots-epoxy Resin

Abstract To avoid the loss and accident caused by energy equipment failure, structural integrity monitoring requires precise sensors for measurement of stress and strain. Quantum dots (QDs)-epoxy resin composite shows variation of photoluminescence (PL) intensity under stretching. This paper presents three different phenomena of PL intensity during cyclic stretching. QDs-Epoxy Resin blank stretching and ANSYS calculation provide PL intensity tendency with strain transforming. Continuous cyclic loading after the PL intensity change becomes stable, illustrating a non-synchronized strain between metal and coating. The synchronous strain can be achieved in a wide application with the development of sensitive optical stress-strain sensors via optimization and characterization of QDs-Epoxy Resin.

XPS investigation of the photon degradation of Znq2 green organic phosphor

By substituting Al with Zn to form bis-(8-hydroxyquinoline) zinc (Znq2), the device performance of organic light emitting diodes (OLED) can be improved. Znq2 also has a more closed packed crystal structure that makes it less vulnerable to reactions with atmospheric oxygen and moisture leading to more stable and longer lasting devices. In this work the effect of photon degradation of Znq2 in air was investigated. Znq2 powder was synthesized using a co-precipitation method and recrystallized in acetone. The structure of the sample was confirmed to be Znq2·2H2O by X-ray diffraction. The photoluminescence (PL) emission data also confirmed that the Znq2·2H2O crystal form of Znq2 was present. To study the photon degradation, the sample was irradiated with a UV lamp for 400h. The emission data was collected and the change in PL intensity with time was monitored. X-ray photoelectron spectroscopy was performed on the as prepared and photon-degraded samples. The Zn2p and N1s peaks showed no change after degradation. The O1s and C1s peaks confirmed that the phenoxide ring ruptured and that C=O and C–O species had formed.

A New Nasicon-Structure Phosphor As a Blue Component for White LED

White light-emitting diodes (WLEDs) continue to gain scientific and commercial important in the present age of solid-state lighting. High power WLED components are required strong thermal stability because its generate heat during LED operation. Thus, the eventual performance of the WLEDs based on phosphors strongly depends on the luminescence properties of the phosphors. Therefore, it is highly desirable to find new phosphors with high efficiency and thermal stability. In this study, we report a new blue-emitting phosphor, the Eu2+-doped NASICON, which can be used to forbid thermal quenching of optical characteristic for WLEDs. The influence of isomorphic displacements on phase transition temperature and the change of photoluminescence intensity have been explored, and established their correlation. By fabricating of WLEDs, we explored for its potential suitability in WLED applications. This suggests that the phosphor displays great potential application as near-ultraviolet LEDs for white light generation, with high quality. Figure 1

Synthesis, photoluminescence, and photostability of Y2O3:Bi3+,Eu3+ nanosheets

Y2O3:Bi3+,Eu3+ nanosheets are produced by calcinating hydrothermally synthesized precursor nanosheets. Transmission electron microscope observation confirms that the sheet-like morphology remains unchanged after calcination at either 600 or 800°C for 2h. The excitation spectrum monitored at 612nm emission corresponding to the 5D0→7F2 transition of Eu3+ has a broad band at 331nm that is attributed to the 6s2→6s6p transition of Bi3+. The Y2O3:Bi3+,Eu3+ nanosheets therefore exhibit the red emission of Eu3+ through energy transfer from Bi3+ to Eu3+ following excitation of Bi3+. The photostability of Y2O3:Bi3+,Eu3+ nanosheets is evaluated from their change in photoluminescence intensity during continuous excitation. The photoluminescence intensity of Y2O3:Bi3+,Eu3+ nanosheets decreases to more than 90% of the initial intensity soon after irradiation begins, and then the photoluminescence intensity gradually recovers. In contrast, the photoluminescence intensity of YVO4:Bi3+,Eu3+ nanoparticles decreases to 53.6% of the initial intensity during excitation. These results indicate that Y2O3:Bi3+,Eu3+ nanosheets have higher photostability than YVO4:Bi3+,Eu3+ nanoparticles.

Dual emissive manganese and copper Co-doped Zn-In-S quantum dots as a single color-converter for high color rendering white-light-emitting diodes.

Novel white light emitting diodes (LEDs) with environmentally friendly dual emissive quantum dots (QDs) as single color-converters are one of the most promising high-quality solid-state lighting sources for meeting the growing global demand for resource sustainability. A facile method was developed for the synthesis of the bright green-red-emitting Mn and Cu codoped Zn-In-S QDs with an absorption bangdgap of 2.56 eV (485 nm), a large Stokes shift of 150 nm, and high emission quantum yield up to 75%, which were suitable for warm white LEDs based on blue GaN chips. The wide photoluminescence (PL) spectra composed of Cu-related green and Mn-related red emissions in the codoped QDs could be controlled by varying the doping concentrations of Mn and Cu ions. The energy transfer processes in Mn and Cu codoped QDs were proposed on the basis of the changes in PL intensity and lifetime measured by means of steady-state and time-resolved PL spectra. By integrating these bicolor QDs with commercial GaN-based blue LEDs, the as-fabricated tricolor white LEDs showed bright natural white light with a color rendering index of 95, luminous efficacy of 73.2 lm/W, and color temperature of 5092 K. These results indicated that (Mn,Cu):Zn-In-S/ZnS QDs could be used as a single color-converting material for the next generation of solid-state lighting.

Optical Probing of Local pH and Temperature in Complex Fluids with Covalently Functionalized, Semiconducting Carbon Nanotubes

We show that local pH can be optically probed through defect photoluminescence from semiconducting carbon nanotubes covalently functionalized with aminoaryl groups. Switching between protonated and deprotonated forms of the amino moiety produces an energy shift in the defect state of the functionalized nanotube by as much as 33 meV in the near-infrared region. This unexpected observation enables a new optical pH sensor that features ultrabright near-infrared II (1.1–1.4 μm) photoluminescence, a sensitivity for pH changes as small as 0.2 pH units over a wide working window that covers the entire physiologic pH range, and potentially molecular resolution. Independent of pH, this nanoprobe can simultaneously act as a nanothermometer by monitoring temperature-modulated changes in photoluminescence intensity, which follows the van’t Hoff equation. This work opens new opportunities for quantitative probing of local pH and temperature changes in complex biological systems.

Electric field changes on Au nanoparticles on semiconductor supports--the molecular voltmeter and other methods to observe adsorbate-induced charge-transfer effects in Au/TiO2 nanocatalysts.

Infrared (IR) studies of Au/TiO2 catalyst particles indicate that charge transfer from van der Waals-bound donor or acceptor molecules on TiO2 to or from Au occurs via transport of charge carriers in the semiconductor TiO2 support. The ΔνCO on Au is shown to be proportional to the polarizability of the TiO2 support fully covered with donor or acceptor molecules, producing a proportional frequency shift in νCO. Charge transfer through TiO2 is associated with the population of electron trap sites in the bandgap of TiO2 and can be independently followed by changes in photoluminescence intensity and by shifts in the broad IR absorbance region for electron trap sites, which is also proportional to the polarizability of donors by IR excitation. Density functional theory calculations show that electron transfer from the donor molecules to TiO2 and to supported Au particles produces a negative charge on the Au, whereas the transfer from the Au particles to the TiO2 support into acceptor molecules results in a positive charge on the Au. These changes along with the magnitudes of the shifts are consistent with the Stark effect. A number of experiments show that the ∼3 nm Au particles act as "molecular voltmeters" in influencing ΔνCO. Insulator particles, such as SiO2, do not display electron-transfer effects to Au particles on their surface. These studies are preliminary to doping studies of semiconductor-oxide particles by metal ions which modify Lewis acid/base oxide properties and possibly strongly modify the electron-transfer and catalytic activity of supported metal catalyst particles.

Photoinduced variation of the luminescent properties of PbS nanoparticle suspensions stabilized by polyvinylpyrrolidone

We have observed a significant increase in the intensity of photoluminescence (PL) from polyvinylpyrrolidone-stabilized suspensions of PbS quantum dots upon irradiation in a broad spectral range and subsequent storage in the dark at room temperature. It is also established that analogous changes are observed in dense polymer coatings deposited on glass substrates from these suspensions. It is suggested that the observed changes in PL intensity are related to the evolution of the clusters of PbS quantum dots.

In-Situ Strain Gauge: Based on Photoluminescence of Quantum Dots

Due to that the spectra of CdSe/ZnS core/shell nanocrystals showed a wavelength shift when under external pressures, researches are conducted to design a wavelength shift-based strain or stress gauge. However, the spectrum's shift can only be subtly sensitive when the pressure applied to nanocrystals reaches GPa, which makes the gauge hard to be applied to practice. In this paper, it was found that the photoluminescence(PL) intensity of CdSe/ZnS core/shell quantum dots (QDs)epoxy resin composites changes significantly with strain. Meanwhile, the changes remains stable after 3-5 cycles. We coated the QDs-epoxy resin composites on the surface of metal tensile sample, the PL intensity and strain changes synchronously. In addition, it has been successfully proved that the variation of QDs’ concentrations along with the strain is the main reason for the photoluminescence intensity changing. This research has shown the possibility that this kind of nanocomposite can be designed as a new strain gauge to quantitatively detect stress or strain in-situ by being coated on the structure surface.

Nonradiative centers in deep‐UV AlGaN‐based quantum wells revealed by two‐wavelength excited photoluminescence

AbstractWe have succeeded in detecting nonradiative recombination (NRR) centers in InAlGaN multiple quantum wells (MQWs) for the sterilization wavelength at around 265 nm by our scheme of two‐wavelength excited photoluminescence (PL). Samples studied are InAlGaN multiple quantum well structures with InAlGaN electron blocking layer grown on sapphire (0001) substrates by metal‐organic chemical vapor deposition (MOCVD) technique at the growth temperature of 880 °C (sample A) and 920 °C (sample B). The MQW consists of three InAlGaN wells of 2 nm sandwiched by 7 nm InAlGaN barrier layers. With the addition of the below‐gap excitation (BGE) light of 1.17 eV, the PL intensity decreased for the sample A but increased for the sample B. Both change in the PL intensity implies the existence of NRR centers, which were activated by the BGE. We attribute both intensity change to two‐levels model and one level model, respectively. Based on rate equation analysis, a set of NRR parameters of sample B was determined by utilizing a saturating tendency of the PL intensity change. Spectroscopic and quantitative advantages of the method enable us to clarify energy distribution of NRR centers without providing electrode.

Temperature-dependent photoluminescence of CuAlO2 single crystals fabricated by using a flux self-removal method

The temperature-dependent behavior of p-type transparent semiconducting oxide CuAlO2 single crystals prepared by using a flux self-removal method in alumina crucibles was investigated through transmittance and photoluminescence (PL) measurements at temperatures from 12 K to room temperature. The low-temperature (12 K) PL spectrum shows two weak, broad emission peaks, one at 3.52 eV and the other at 3.08 eV, which we assign to excitonic emission and to defectrelated emission originating from copper vacancies. The positions of the PL peaks as functions of temperature exhibit a normal behavior satisfying the standard Varshini law, and the Debye temperature is found to be θD = 610 ± 80 K. The exciton-binding energy of the CuAlO2 single crystal is estimated to be 49 meV from the PL intensity change with temperature.

Fabrication of the optical fiber pH sensor based on CdSe/ZnS quantum dot.

During the past decade, there has been a lot of research on the application of quantum dots (QDs) in photonics and biotechnology. QDs are semiconductor nanocrystals which show many peculiar optical and electrical properties due to their quantum confinement effects. In this work, the correlation of the fluorescence intensity of QDs and the pH under ambient temperature was studied. An optical fiber pH sensor was prepared using CdSe/ZnS carboxyl quantum dots as sensing media. QDs with different sizes and emission colors (green, yellow, and red) were immobilized on the surface of the optical fiber through EDC/sulfo-NHS coupling reaction. The optical fiber was immersed in water, and the photoluminescence (PL) spectra of QDs were obtained at ambient temperature under various pH conditions. The photoluminescence (PL) intensity of QDs increased linearly with the increase of pH, and among the QDs, the red emitting QDs show the largest change in PL intensity with pH. This shows that QDs can be used as pH sensing media and optical fiber pH probe can be prepared.

Photoluminescence properties of Cu–Mn–In–S/ZnS core/shell quantum dots

The effect of Mn ions on the optical properties of Cu–Mn–In–S/ZnS core/shell quantum dots (QDs) was studied by steady-state and time-resolved photoluminescence (PL) spectroscopy. It was found that introduction of Mn ions into CuInS2/ZnS core/shell QDs resulted in a reduction of the PL intensity and lifetime. Further no significant PL quenching was observed in Cu–Mn–In–S/ZnS core/shell QDs while the PL intensity of CuInS2/ZnS core/shell QDs significantly decreased with increasing the temperature from 93 to 270K. The recombination process of PL in Cu–Mn–In–S/ZnS core/shell QDs was discussed based on the change in PL intensity and lifetime.

A novel aptamer functionalized CuInS2 quantum dots probe for daunorubicin sensing and near infrared imaging of prostate cancer cells

In this paper, a novel daunorubicin (DNR)-loaded MUC1 aptamer-near infrared (NIR) CuInS2 quantum dot (DNR–MUC1–QDs) conjugates were developed, which can be used as a targeted cancer imaging and sensing system. After the NIR CuInS2 QDs conjugated with the MUC1 aptamer–(CGA)7, DNR can intercalate into the double-stranded CG sequence of the MUC1–QDs. The incorporation of multiple CG sequences within the stem of the aptamers may further increase the loading efficiency of DNR on these conjugates. DNR–MUC1–QDs can be used to target prostate cancer cells. We evaluated the capacity of MUC1–CuInS2 QDs for delivering DNR to cancer cells in vitro, and its binding affinity to MUC1-positive and MUC1-negative cells. This novel aptamer functionalized QDs bio-nano-system can not only deliver DNR to the targeted prostate cancer cells, but also can sense DNR by the change of photoluminescence intensity of CuInS2 QDs, which concurrently images the cancer cells. The quenched fluorescence intensity of MUC1–QDs was proportional to the concentration of DNR in the concentration ranges of 33–88nmolL−1. The detection limit (LOD) for DNR was 19nmolL−1. We demonstrate the specificity and sensitivity of this DNR–MUC1–QDs probe as a cancer cell imaging, therapy and sensing system in vitro.

Structural and Optical Properties of Nickel-Doped and Undoped Zinc Oxide Thin Films Deposited by Sol-Gel Method

ZnO thin films with and without Ni-doping were successfully deposited by sol-gel method with zinc acetate dihydrate as inorganic precursor, and nickel (II) acetate tetrahydrate as dopant. The solutions were prepared by dissolving zinc acetate and nickel (II) acetate in ethanol and diethanolamine (DEA) as its chelating agent. Thin films were fabricated by using spin-coating method on glass substrates. ZnO films were obtained by pre-heating and post-heating at 300 °C for 10 minutes and 500 °C for 1 h respectively. The films were analyzed by X-ray diffraction (XRD), UV-Vis transmittance and photoluminescence (PL). All samples exhibit high transparency in visible. Ni dopant does not alter so much ZnO structure, which due to the ion substitution between Ni and Zn. However, the Ni tends to create a dopant energy interlayer in ZnO energy band gap which cause significant change in PL intensity.

Optical detection of nonradiative recombination centers in AlGaN quantum wells for deep UV region

AbstractBy superposing a chopped below‐gap excitation (BGE) light on a cw above‐gap excitation and observing the intensity change of photoluminescence (PL) due to the BGE, we clarified a distribution of nonradiative recombination (NRR) centers in AlGaN multi‐quantum well (MQW) structures for the wavelength region of deep ultraviolet (UV). The decrease in band‐edge PL peak intensity at 10 K exemplified the presence of a pair of NRR centers in AlGaN well layers whose transition energy corresponds to that of the BGE. The BGE power dependence of the normalized PL intensity showed a quenching saturation due to trap‐filling effect of electrons in one of the centers which were activated by the BGE. The BGE energy dependence of the normalized PL intensity revealed that the optical activation energy of the dominant NRR process via these trap states in the AlGaN MQW is around 1.17 eV. The purely optical scheme of detection enables us to obtain clues for identifying defect levels in DUV region and eliminating them during growth process without any preparation of electrodes. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimal Oxide Passivation of Ge for Optoelectronics

Oxide passivation is an active field of investigation for reducing surface state densities on Ge surfaces, namely below high-K gate stacks. However, the quality of Ge oxide passivation has not been as widely investigated in the context of performance in minority carrier photonic devices. p-Ge substrates with a foundry-applied oxide passivation are compared with oxide-stripped substrates passivated by dry thermal oxides grown between 400°C-650°C in O2. By using p-Ge, only the behavior of minority conduction electrons and their associated traps become the relevant focus of this study. Passivation quality, as evaluated by photoluminescence (PL), is found to vary as a function of temperature between 400–650°C. Changes in PL intensity are attributed to differences in surface termination of the p-Ge substrate. X-ray photoelectron spectroscopy (XPS) studies find that the presence of suboxides (GeOx) increase with oxidation temperature, with GeO2 dominant at 400°C and mixed suboxides at higher temperatures. Nonuniform desorption of Ge oxide is confirmed above 550°C. Thermal oxidation is also shown to improve the quality of ion-damaged Ge surfaces. We conclude that suboxides confer better passivation than a GeO2 interface, as determined by PL intensity.