Showing papers on "Photoluminescence published in 2001"

Nearly 100% internal phosphorescence efficiency in an organic light emitting device

Abstract: We demonstrate very high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent molecule doped into a wide energy gap host. Using bis(2-phenylpyridine)iridium(III) acetylacetonate [(ppy)2Ir(acac)] doped into 3-phenyl-4(1′-naphthyl)-5-phenyl-1,2,4-triazole, a maximum external quantum efficiency of (19.0±1.0)% and luminous power efficiency of (60±5) lm/W are achieved. The calculated internal quantum efficiency of (87±7)% is supported by the observed absence of thermally activated nonradiative loss in the photoluminescent efficiency of (ppy)2Ir(acac). Thus, very high external quantum efficiencies are due to the nearly 100% internal phosphorescence efficiency of (ppy)2Ir(acac) coupled with balanced hole and electron injection, and triplet exciton confinement within the light-emitting layer.

Green luminescent center in undoped zinc oxide films deposited on silicon substrates

Abstract: The photoluminescence (PL) spectra of the undoped ZnO films deposited on Si substrates by dc reactive sputtering have been studied. There are two emission peaks, centered at 3.18 eV (UV) and 2.38 eV (green). The variation of these peak intensities and that of the I–V properties of the ZnO/Si heterojunctions were investigated at different annealing temperatures and atmospheres. The defect levels in ZnO films were also calculated using the method of full-potential linear muffin-tin orbital. It is concluded that the green emission corresponds to the local level composed by oxide antisite defect OZn rather than oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni, and interstitial oxygen Oi.

Highly polarized photoluminescence and photodetection from single indium phosphide nanowires.

Abstract: We have characterized the fundamental photoluminescence (PL) properties of individual, isolated indium phosphide (InP) nanowires to define their potential for optoelectronics. Polarization-sensitive measurements reveal a striking anisotropy in the PL intensity recorded parallel and perpendicular to the long axis of a nanowire. The order-of-magnitude polarization anisotropy was quantitatively explained in terms of the large dielectric contrast between these free-standing nanowires and surrounding environment, as opposed to quantum confinement effects. This intrinsic anisotropy was used to create polarization-sensitive nanoscale photodetectors that may prove useful in integrated photonic circuits, optical switches and interconnects, near-field imaging, and high-resolution detectors.

Ultraviolet-emitting ZnO nanowires synthesized by a physical vapor deposition approach

Abstract: ZnO nanowires were mass produced using a physical vapor deposition approach. The ZnO nanowire monocrystallites have an average diameter around 60 nm and length up to a few micrometers. The unidirectional growth of the ZnO nanowires was controlled by the conventional vapor-liquid-solid mechanism. Intensive UV light emission peaked around 3.27 eV was observed at room temperature, which was assigned to emission from free exciton under low excitation intensity. The observed room temperature UV emission was ascribed to the decrease in structure defects as compared to bulk ZnO materials, and in particularly to the size effect in the ZnO wires.

Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films

Abstract: Photoluminescence and cathodoluminescence (CL) spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green (yellow) emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy Vo+ (the single negatively charged interstitial oxygen ion Oi−) center in the particle.

Study of Au/Au3+-TiO2 Photocatalysts toward Visible Photooxidation for Water and Wastewater Treatment

Abstract: With an attempt to extend light absorption of TiO2-based photocatalyst toward the visible light range and eliminate the rapid recombination of excited electrons/holes during photoreaction, a new type of photocatalysts (Au/Au3+-TiO2) powder was prepared by a photoreduction/sol−gel process. The crystal phase composition, surface structure, and light absorption of the new photocatalysts were comprehensively examined by X-ray differential detection (XRD), UV−visible absorption spectra, X-ray photoelectron emission spectroscopy (XPS), and photoluminescence (PL) spectra. The photooxidation efficiencies of the photocatalysts were also evaluated in the photodegradation of methylene blue (MB) in aqueous solutions under visible light irradiation from a high-pressure sodium lamp (λ > 400 nm). The results of PL analyses in this study indicated that the gold/gold ion-doping on the surface of TiO2 could eliminate the electron/holes recombination and also increase the light absorption in the visible range. The analytica...

Preparation and photoluminescence of highly ordered TiO2 nanowire arrays

Abstract: Highly ordered TiO2 nanowire (TN) arrays were prepared in anodic alumina membranes (AAMs) by a sol-gel method. The TNs are single crystalline anatase phase with uniform diameters around 60 nm. At room temperature, photoluminescence (PL) measurements of the TN arrays show a visible broadband with three peaks, which are located at about 425, 465, and 525 nm that are attributed to self-trapped excitons, F, and F+ centers, respectively. A model is also presented to explain the PL intensity drop-down of the TN arrays embedded in AAMs: the blue PL band of AAMs arises from the F+ centers on the pore walls, and the TNs first form in the center area of the pores and then extend to the pore walls.

Efficient blue emission from siloles

Abstract: 2,3,4,5-Tetraphenylsiloles with different 1,1-substituents on the ring silicon atoms, i.e., 1,1-dimethyl-2,3,4,5-tetraphenylsilole (1), 1-methyl-1-(3-chloropropyl)-2,3,4,5-tetraphenylsilole (2), 1-methyl-1,2,3,4,5-pentaphenylsilole (3) and hexaphenylsilole (4), are synthesized and characterized. While all the siloles emit intense blue light readily observable by naked eyes under normal room illumination conditions, the film of their acyclic cousin without silicon, namely 1,2,3,4-tetraphenylbutadiene (5), does not fluoresce, revealing the vital role of the planar and rigid silacyclopentadiene ring in the solid-state photoluminescence process. The electronic transitions of the siloles can be tuned by varying the 1,1-substituents, and the inductive and conjugating effects of the aromatic rings confer low LUMO energy levels and high emission efficiencies on the phenyl-substituted siloles. The electroluminescence device of the 1-phenylsilole 3 shows a high brightness (4538 cd m−2 at 18 V) and an excellent external quantum efficiency (0.65% at 17 V and 94 mA cm−2).

Catalytic growth and characterization of gallium nitride nanowires.

Abstract: The preparation of high-purity and -quality gallium nitride nanowires is accomplished by a catalytic growth using gallium and ammonium. A series of catalysts and different reaction parameters were applied to systematically optimize and control the vapor−liquid−solid (VLS) growth of the nanowires. The resulting nanowires show predominantly wurtzite phase; they were up to several micrometers in length, typically with diameters of 10−50 nm. A minimum nanowire diameter of 6 nm has been achieved. Temperature dependence of photoluminescence spectra of the nanowires revealed that the emission mainly comes from wurtzite GaN with little contribution from the cubic phase. Moreover, the thermal quenching of photoluminescence was much reduced in the GaN nanowires. The Raman spectra showed five first-order phonon modes. The frequencies of these peaks were close to those of the bulk GaN, but the modes were significantly broadened, which is indicative of the phonon confinement effects associated with the nanoscale dimen...

Electrochromic Nanocrystal Quantum Dots

Abstract: Incorporating nanocrystals into future electronic or optoelectronic devices will require a means of controlling charge-injection processes and an understanding of how the injected charges affect the properties of nanocrystals. We show that the optical properties of colloidal semiconductor nanocrystal quantum dots can be tuned by an electrochemical potential. The injection of electrons into the quantum-confined states of the nanocrystal leads to an electrochromic response, including a strong, size-tunable, midinfrared absorption corresponding to an intraband transition, a bleach of the visible interband exciton transitions, and a quench of the narrow band-edge photoluminescence.

Spin Relaxation Quenching in Semiconductor Quantum Dots

Abstract: We have studied the spin dynamics in self-organized InAs/GaAs quantum dots by time-resolved photoluminescence performed under strictly resonant excitation. At low temperature, we observe strictly no decay of both the linear and the circular luminescence polarization. This demonstrates that the carrier spins are totally frozen on the exciton lifetime scale.

Highly efficient electroluminescent materials based on fluorinated organometallic iridium compounds

Abstract: We report a class of highly efficient electroluminescent materials based on fluorinated iridium compounds. Using aluminum as the cathode, a device, using fac-tris[5-fluoro-2(5-trifluoromethyl-2-pyridinyl)phenyl-C,N]iridium (Ir-2h) as the luminescent layer, displayed intense electroluminescence at 525 nm with an efficiency of 20 cd/A and a maximum radiance of 4800 cd/m2. Differing from the previously reported Ir(ppy)3, Ir-2h can be used in the undiluted form without the use of a charge-transporting host. This indicates that Ir-2h by itself has good enough charge-transporting properties. Photoluminescence studies at room temperature and 77 K revealed that electroluminescence originates from the metal-to-ligand charge transfer state with a quantum yield of 0.56 for Ir-2h and 0.5 for Ir(ppy)3 in toluene at room temperature. In the thin-film form, photoluminescence quantum yield of Ir-2h is a factor of 10 greater than that of Ir(ppy)3 due to the larger self-quenching effect of Ir(ppy)3.

Liquid-Phase Synthesis of Colloids and Redispersible Powders of Strongly Luminescing LaPO4:Ce,Tb Nanocrystals

Abstract: Nanoparticles with high photoluminescence quantum yield have been recently considered as possible biolabels and as emitters in optoelectronic devices. Now gram amounts of nontoxic, chemically stable LaPO4 :Ce,Tb nanocrystals have been obtained in a coordinating solvent. These nanoparticles can be easily redispersed in polar solvents to give scatter-free colloids that exhibit quantum yields of up to 61 %.

Synthesis and Photoluminescence of Nanocrystalline ZnS:Mn^(2+)

Abstract: The influence of the synthesis conditions on the properties of nanocrystalline ZnS:Mn2+ is discussed. Different Mn2+ precursors and different ratios of the precursor concentrations [S2-]/[Zn2+] were used. The type of Mn2+ precursor does not have an effect on the luminescence properties in the synthesis method described. On going from an excess of [Zn2+] to an excess of [S2-] during the synthesis, the particle diameter increases from 3.7 to 5.1 nm, which is reflected by a change in the luminescence properties. Photoluminescence measurements also showed the absence of the ZnS defect luminescence around 450 nm when an excess [S2-] is used during the synthesis. This effect is explained by the filling of sulfur vacancies. The ZnS luminescence is quenched with an activation energy of 62 meV, which is assigned to the detrapping of a bound hole from such a vacancy.

Synthesis and characterization of a low bandgap conjugated polymer for bulk heterojunction photovoltaic cells

Abstract: Low optical bandgap conjugated polymers may improve the efficiency of organic photovoltaic devices by increasing the absorption in the visible and near infrared region of the solar spectrum. Here we demonstrate that condensation polymerization of 2,5-bis(5-trimethylstannyl-2-thienyl)-N-dodecylpyrrole and 4,7-dibromo-2,1,3-benzothiadiazole in the presence of Pd(PPh3)2Cl2 as a catalyst affords a novel conjugated oligomeric material (PTPTB), which exhibits a low optical bandgap as a result of the alternation of electron-rich and electron-deficient units along the chain. By varying the molar ratio of the monomers in the reaction and fractionation of the reaction product, two different molecular weight fractions (PTPTB-I and PTPTB-II, see Experimental section) were isolated, containing 5–17 and 13–33 aromatic units respectively, as inferred from matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Thin films of PTPTB-I and PTPTB-II exhibit an optical bandgap of 1.60 and 1.46 eV, respectively. Photoinduced absorption (PIA) and photoluminescence spectroscopy of blends of PTPTB-I and a methanofullerene (1-(3-methoxycarbonyl)-propyl-1-phenyl-[6,6]C61, PCBM) gave direct spectral evidence of the photoinduced electron-transfer reaction from PTPTB-I as a donor to the fullerene derivative as an acceptor. Thin PTPTB-I:PCBM composite films were sandwiched between indium tin oxide/poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (ITO/PEDOT:PSS) and Al electrodes to prepare working photovoltaic devices, which show an open circuit voltage of 0.67 V under white-light illumination. The spectral dependence of the device shows an onset of the photocurrent at 1.65 eV (750 nm).

Photoluminescence Characteristics of $Y_2O_3 : Eu^{3+}$ Nanophosphors Prepared Using Sol-Gel Thermolysis

Abstract: Red emitting cubic $Y_2O_3:Eu^{3+}$ nanophosphor with an average particle size in the range of 10-20 nm was synthesized using a more facile gel-polymer pyrolysis process. The maximum relative luminescence yield obtained for the nanophosphor prepared with a urea and PVA combination is about 30% in relation to the bulk $Y_2O_3:Eu^{3+}$ industrial red phosphor. The photoluminescence excitation spectrum monitoring the dominant hypersensitive $^5D_0$ \rightarrow $^7F_2$ red emission of $Eu^{3+}$ comprises two parts, viz., the dominant $Eu^{3+}-O^2$ chargetransfer band and a weak excitonic band (or its tail) corresponding to the $Y^{3+}-O^{2-}$ host matrix absorption. The relative strengths of these two bands have a strong dependence on the particle size. Furthermore, in this nanocrystalline insulator system having a band gap of about 6 eV, it is possible to observe a size dependent blue shift $(\sim 600 cm^{-1})$ in the photoluminescence excitation band corresponding to the Urbach tail region of the yttria host matrix. Both the bulk and nanocrystalline $Y_2O_3:Eu^{3+}$ show storage luminescence, a phenomenon previously unknown in this system. The mechanisms responsible for this appear to be different in these systems. The storage luminescence in the bulk system can be attributed to lattice defects, whereas that in the nanocrystalline counterpart is from a meta-stable, photoinduced surface-states arising from chemisorbed species.

Quantum dot infrared photodetectors

Abstract: Self-assembled strained semiconductor nanostructures have been grown on GaAs substrates to fabricate quantum dot infrared photodetectors. State-filling photoluminescence experiments have been used to probe the zero-dimensional states and revealed four atomic-like shells (s,p,d,f) with an excitonic intersublevel energy spacing which was adjusted to ∼60 meV. The lower electronic shells were populated with carriers by n doping the heterostructure, and transitions from the occupied quantum dot states to the wetting layer or to the continuum states resulted in infrared photodetection. We demonstrate broadband normal-incidence detection with a responsivity of a few hundred mA/W at a detection wavelength of ∼5 μm.

The Oxygen Vacancy as the Origin of a Green Emission in Undoped ZnO

Abstract: We have investigated undoped ZnO single crystals, which are commercially available from Eagle-Picher, by photoluminescence (PL) and optically detected magnetic resonance (ODMR) spectroscopy. The electrical properties of this material are very similar to the samples investigated. The total residual shallow donor concentration is about 1 x 10{sup 17} cm{sup -3}. The low temperature emission is dominated by the donor bound exciton (D{sup 0}X) at 3.366 eV. At 2.45 eV the broad, unstructured ''green'' emission is located, its full width at half maximum is 320 meV. The temperature dependence of the PL reveals that this green band maintains its peak energy up to 450 K, which is a feature typical of a recombination within a localised defect, while the D{sup 0}X emission follows the shrinkage of the bandgap with increasing temperature. (orig.)

Role of the energy transfer in the optical properties of undoped and Er-doped interacting Si nanocrystals

Abstract: In this article the luminescence properties of Si nanocrystals (nc) formed by plasma enhanced chemical vapor deposition and their interaction with Er ions introduced by ion implantation are investigated in detail. Si nc with different size distributions and densities were produced and all show quite intense room temperature luminescence (PL) in the range 700–1100 nm. It is shown that the time-decay of the luminescence follows a stretched exponential function whose shape tends towards a single exponential for almost isolated nc. This suggests that stretched exponential decays are related to the energy transfer from smaller towards larger nc. Indeed, by comparing samples with similar nc size distributions, but with very different nc densities, it is demonstrated that the PL has a quite strong redshift in the high density case, demonstrating a clear energy redistribution within the sample. Excitation cross sections have been measured in all samples yielding a value of ∼1.8×10−16 cm2 for isolated nc excited w...

Fine structure on the green band in ZnO

Abstract: An emission band at 2.4 eV, called the green band, is observed in most ZnO samples, no matter what growth technique is used. Sometimes this band includes fine structure, which consists mainly of doublets, repeated with a longitudinal-optical-phonon-energy spacing (72 meV). We have developed a vibronic model for the green band, based on transitions from two separate shallow donors to a deep acceptor. The donors, at energies 30 and 60 meV from the conduction-band edge, respectively, are also found from Hall-effect measurements.

Size-Dependent Optical Spectroscopy of a Homologous Series of CdSe Cluster Molecules

Abstract: The optical properties and electronic structure of a homologous series of CdSe cluster molecules covering a size range between 0.7 and 2 nm are investigated. CdSe cluster molecules with 4, 8 10, 17, and 32 Cd atoms, capped by selenophenol ligands, were crystallized from solution and their structures determined by single-crystal X-ray diffraction. The cluster molecules are composed of a combination of adamanthane and barylene-like cages, the building blocks of the zinc blende and the wurtzite structures of the bulk CdSe. The onset of the room temperature absorption and low-temperature photoluminescence excitation spectra exhibit a systematic blue shift with reduced cluster size manifesting the quantum confinement effect down to the molecular limit of the bulk semiconductor. Blue-green emission, shifted substantially to lower energy from the absorption onset, is observed only at low temperature and its position is nearly independent of cluster size. The wavelength dependence of both photoluminescence and ph...

Catalytic synthesis and photoluminescence of β-Ga2O3 nanowires

Abstract: Monoclinic gallium oxide (β-Ga2O3) nanowires were synthesized by heat treating a composite material of GaAs and pre-evaporated Au at 1240 °C in dry oxygen atmosphere. The catalytic Au metal generated liquid nanoclusters that serve as reactive sites confining and directing the growth of β-Ga2O3 nanowires during the vapor-liquid-solid growth process. The β-Ga2O3 nanowires have diameters ranging from 20 to 50 nm and lengths of several micrometers. Photoluminescence measurement under excitation at 250 nm shows that the bulk β-Ga2O3 nanowires have a stable blue emission at 475 nm and an ultraviolet emission at 330 nm, which may be related to the defects such as the oxygen vacancy and the gallium–oxygen vacancy pair.

Decreased Aggregation Phenomena in Polyfluorenes by Introducing Carbazole Copolymer Units

Abstract: To decrease aggregation phenomena and improve the luminescence of the blue-light-emitting polymer polyfluorene, carbazole units were copolymerized to introduce disorder in the polymer backbone. A “kink” linkage was introduced into the polyfluorene chain by forming a 9:1 and 7:3 copolymer with the carbazole group at the 3,6 positions. The disordered polyfluorene was made though a Ni-catalyzed reaction, which resulted in high yield and high molecular weight polymers. The structure and physical properties were confirmed by NMR, SEC, UV absorption, photoluminescence, elemental analysis, and quantum yield measurements. The copolymers exhibited better spectral properties both in solution and in film compared to previously synthesized polyfluorene homopolymers. Annealing studies showed both improved thermal and UV stability of the copolymer over previously reported homopolymers. Cyclic voltammetry studies showed a slightly lower highest occupied molecular orbital (HOMO) than the homopolymer.

Over 1.5 μm light emission from InAs quantum dots embedded in InGaAs strain-reducing layer grown by metalorganic chemical vapor deposition

Abstract: We demonstrated the 1.52 μm light emission at room temperature from self-assembled InAs quantum dots embedded in the In0.45Ga0.55As strain-reducing layer. By capping InAs quantum dots with an InGaAs strain-reducing layer instead of GaAs, the photoluminescence peak of InAs quantum dots can be controlled by changing the indium composition of the InGaAs strain-reducing layer. The full width at half maximum is as narrow as 22 meV. The wavelength of 1.52 μm is the longest wavelength so far achieved in self-assembled InAs quantum dots, which would be promising to quantum-dot lasers on GaAs substrate for application to light sources in long-wavelength optical communication systems.

Incorporation of nitrogen in nitride-arsenides: Origin of improved luminescence efficiency after anneal

Abstract: A key to the utilization of nitride-arsenides for long wavelength optoelectronic devices is obtaining low defect materials with long nonradiative lifetimes. Currently, these materials must be annealed to obtain device quality material. The likely defect responsible for the low luminescence efficiency is associated with excess nitrogen. Photoluminescence and capacitance–voltage measurements indicate the presence of a trap associated with excess nitrogen which decreases in concentration upon anneal. Our films are grown by elemental source molecular beam epitaxy and the background impurity concentration is low, thus we have investigated the role of crystalline defects. High resolution x-ray diffraction showed improved crystal quality after anneal. We observed that the lattice parameter does not decrease linearly with nitrogen concentration for levels of nitrogen above 2.9 mol % GaN. The fact that Vegard’s law is not observed, despite theoretical calculations that it should, indicates that nitrogen incorporat...

Luminescence properties of nanocrystalline Y2O3:Eu3+ in different host materials

Abstract: In this study, the optical properties of nanocrystalline europium doped yttria, Y2O3:Eu3+ were investigated in dependence on different caging hosts such as porous MCM-41, porous silica, and porous alumina with pore sizes ranging between 2.7 to 80 nm. These results were compared to nanopowders measured in air and aqueous solution whose particle sizes were 5 nm and 8 nm, respectively. All these results were compared to a commercial lamp phosphor powder with a grain size of about 5 μm. The structural properties of the samples were determined by x-ray diffraction and transmission electron microscopy. Investigated optical properties are the photoluminescence emission spectra, the excitation spectra, the lifetimes, and the quantum efficiencies. A heavy dependence of the charge transfer process on the surrounding will be reported and discussed.

Ordered indium-oxide nanowire arrays and their photoluminescence properties

Abstract: Ordered semiconductor In2O3 nanowire arrays are uniformly assembled into hexagonally ordered nanochannels of anodic alumina membranes (AAMs) by electrodeposition and oxidizing methods. Their microstructures were characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. A blue-green photoluminescence (PL) band in the wavelength range of 300–650 nm was observed in the In2O3/AAM assembly system. The PL intensity and peak position depend on the annealing temperature, which is mainly attributed to the singly ionized oxygen vacancy in the In2O3 nanowire array system.

Organic-Capped ZnO Nanocrystals: Synthesis and n-Type Character

Abstract: Wurtzite ZnO nanocrystals capped with trioctylphosphine oxide or alkylamines are synthesized and characterized. These ZnO nanocrystals can be made n-type either by electron transfer doping from reducing species in solution or by above band gap photoexcitation with a UV lamp. The n-type nanocrystals exhibit a strong intraband infrared absorption, an extensive bleach of the interband band-edge absorption, and a complete quenching of the photoluminescence.

Conformational Effects on the Photophysics of Conjugated Polymers: A Two Species Model for MEH−PPV Spectroscopy and Dynamics

Abstract: We report transient and steady state photoluminescence results along with absorption and NMR data to support the existence of two distinct morphological species in MEH−PPV solutions. NMR data provide evidence for the close packing of polymer chains, a consequence of solvent quality reduction. These data are correlated with optical properties of the aggregated species in poor solvents and the isolated chains in good solvents. We infer that steric hindrance of backbone motions increases effective conjugation length and leads to a spectral red shift in absorption and emission. At the same time, interchain excitations with negligible luminescence can be formed, leading to a dramatic reduction in photoluminescence quantum yield. While spectral changes are observed as packing is induced, we show that interchain state formation and its subsequent back-transfer to excitons are particularly sensitive to the interchain registry of the highly packed chains.