Optical functions of tris (8-hydroxyquinoline) aluminum (Alq3) by spectroscopic ellipsometry

Optical functions of tris (8-hydroxyquinoline) aluminum (Alq₃) have been studied in the spectral range from 1.55 eV to 5 eV using spectroscopic ellipsometry. The samples have been deposited by thermal evaporation on glass substrates. Optical functions of Alq₃ deposited on unheated substrates and on substrates kept at 100 °C have been determined. The optical functions have been modeled using point-to-point fitting, with the conventional oscillator model and modified oscillator model. It has been found that point-to-point fitting gives the best agreement with the experimental data, and that the modified oscillator model yields better agreement with the experimental data than the conventional oscillator model. Received: 3 September 2001 / Accepted: 22 March 2002 / Published online: 5 July 2002 RID="*" ID="*"Corresponding author. Fax: +852-2559/8738, E-mail: dalek@eee.hku.hk     

Proton-irradiation effect on the luminescence of the MEH-PPV conjugated polymer

We report proton-irradiation effect on luminescence in the MEH-PPV conjugated polymer. While the luminescence was severely weakened by the low-energy irradiation with a high dosage, it was affected only weakly by the high-energy irradiation with a low dosage. The electroluminescence was affected by the irradiation more severely than the photoluminescence was, preserving the external quantum efficiency. While the luminescence spectrum of the severely damaged polymer was broadened with a blue-shift, a weak damage resulted only in a decrease in the luminescence intensity apparently preserving the spectral shape. The change in the luminescence spectra may be explained by chain conformational disorders as well as chain scission induced by the irradiation.     

Analysis by using X-ray photoelectron spectroscopy for polymethyl methacrylate and polytetrafluoroethylene etched by KrF excimer laser

The C 1s, F 1s, and O 1s electron spectra for polymethyl methacrylate (PMMA) and polytetrafluoroethylene (PTFE) irradiated by KrF excimer laser with 248 nm wavelength were analyzed by the X-ray photoelectron spectroscopy (XPS) method. The results show that, after irradiation by the laser, the percentage of the carbon atoms of C-C bond decreases and the percentage of CO bond increases for PMMA, while for PTFE percentages of both C-C bond and CF₂ group decrease drastically, respectively. Moreover, it was found that C-O bond and other complex carbon-oxygen groups appeared for PTFE. The photon-chemical processes associated with the energy level transitions, energy diversion, and dissociation of bonds in the interaction were theoretically analyzed based on the chemical structures of PTFE and PMMA. Our analyses can successfully explain that PMMA can be effectively etched by KrF excimer laser with 248 nm wavelength can efficiently etch the PMMA, but the surface of PTFE can only be modified by it.     

Optical properties of Eu(fod)3-doped polymers using supercritical carbon dioxide

X-ray diffraction pattern of solvated Eu(fod)₃(H₂O)_1,5(i-PrOH)_0,5 form of Eu(fod)₃ complex (fod=6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate) is presented. The photoluminescence (PL) spectra of Eu³⁺ ions in polycrystalline powder Eu(fod)₃ and also in Eu(fod)₃-doped polymethylmethacrylate (PMMA), polypropylene (PP) and polydimethylsiloxane (PDMS) were investigated. It is revealed that the matrix influences the temperature quenching of PL intensity in the range 20-100 °C. A polycrystalline powder Eu(fod)₃ and Eu(fod)₃-doped PP are the most effective materials for PL quenching. It is shown that water molecules in the first coordinating sphere of Eu³⁺ ions increase PL intensity temperature quenching.     

Quasi-persistent photocurrent in the UV-irradiated MEH-PPV conjugated polymer

We report the quasi-persistent photocurrent in the MEH-PPV conjugated polymer, induced by UV-irradiation in air. It is attributed to the irradiation-induced defects, which also act to accelerate its decay.     

Single molecule spectroscopy of oligofluorenes: how molecular length influences polymorphism

Polyfluorene represents a unique model to study the influence of intramolecular conformation on the electronic properties of chromophores with an extended π-conjugation. According to the degree of planarity between the adjacent repeat units the electronic and optical properties can change substantially. This peculiar spectroscopic behavior has been described by identifying different phases, namely the glassy, the γ- and the β-phase. Here, we present low-temperature single-molecule spectroscopy of a series of oligofluorenes differing in the number of monomeric units, in order to gain information on the influence of chain length on the polymorphism. By monitoring the energy of the 0-0 transition we have classified single molecules belonging to the different phases. We demonstrate that a large number of molecules start to form the β-phase only when more than 9 repeat units constitute the molecular chain. The implications for the control of morphology in polyfluorene thin films are discussed.     

Time-resolved spectroscopy of soantaneous luminescence of CdSSe1−x quantum dots

Picosecond time-resolved spectroscopy of the edge luminescence band of CdS _x Se_1–x quantum dots with crystallite diameters as small as a few nanometers under band-to-band excitation reveals strong enhancement of the radiative recombination rate compared to bulk CdS owing to quantum confinement. The splitting of the luminescence band into two lines originates from near-band-gap absorption. Analysis of the temperature as well as the spectral dependence of the decay time (leading to a red shift of the luminescence with increasing time) and of the total-light-decay law result in a new model for the dominant radiative recombination channel: donor-acceptor pair recombination instead of an excitonic mechanism as claimed in previous publications.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Visible photoluminescence from hydrogenated amorphous carbon films prepared by pulsed laser ablation of polymethyl methacrylate (PMMA)

Visible photoluminescence (PL) has been observed from the hydrogenated amorphous carbon (a-C:H) films prepared by ArF pulsed laser ablation of polymethyl methacrylate (PMMA) in the presence of hydrogen gas (H₂). With increasing hydrogen concentration the PL intensity increases and the intensity maximum blue-shifted. The PL intensity also increases after hours of light illumination, exhibiting a light soaking enhancement. Increasing the deposition temperature decreases the PL and increases the ratio of sp³/sp² bonds.Z.F. Li is on leave from Department of Physics, Nanjing University, Nanjing 210093, P.R. China.     

Concentration dependence of photoluminescence properties in poly[(2-methoxy,5-octoxy)1,4-phenylenevinylene] thin films

Photoluminescence and selective excitation photoluminescence measurements at room temperature have been performed on poly[(2-methoxy,5-octoxy) 1,4-phenylenevinylene] (MO-PPV) thin films, which are prepared from MO-PPV chloroform solutions of different concentrations. The position of the S0→S1 absorption peak shows red-shift and broadened relative to that in an MO-PPV solution form due to the solidification effect, while no relative shifts between the absorption spectra of these film samples are observed. A long wavelength emission component near 630 nm has been identified as S2→S0 vibronic transition through the Gaussian decomposition method and confirmed by below-gap PL and selective-excitation PL experiments. This second vibronic component cannot be observed in the spectra of thick films. The PL efficiency of MO-PPV thin film is also investigated through comparison with that of an MEH-PPV thin film and explained by the side substituent effect.     

Photoluminescence in amorphous (PbLa)TiO3 thin films deposited on different substrates

Pb_1−XLa_XTiO₃ thin films, (X=0.0; 13 and 0.27 mol%) were prepared by the polymeric precursor method. Thin films were deposited on Pt/Ti/SiO₂/Si (1 1 1), Si (1 0 0) and glass substrates by spin coating, and annealed in the 200-300°C range in an O₂ atmosphere. X-ray diffraction, scanning electron microscopy and atomic force microscopy were used for the microstructural characterization of the thin films. Photoluminescence (PL) at room temperature has been observed in thin films of (PbLa)TiO₃. The films deposited on Pt/Ti/SiO₂/Si substrates present PL intensity greater than those deposited on glass and silicon substrates. The intensity of PL in these thin films was found to be dependent on the thermal treatment and lanthanum molar concentration.     

Green and red up-conversion emissions of Er-Yb-codoped Al2O3 powders prepared by the nonaqueous sol-gel method

The 1 mol% Er³⁺- and 0-20 mol% Yb³⁺-codoped Al₂O₃ powders have been prepared by the nonaqueous sol-gel process using aluminum isopropoxide as precursor, acetylacetone as chelating agent, nitric acid as catalyzer, and hydrated erbium and ytterbium nitrate as dopant under isopropanol environment. The two crystalline types of doped Al₂O₃, γ and θ, and a stoichiometric compound, (Yb,Er)₃Al₅O₁₂, were obtained for all the Er³⁺-Yb³⁺-codoped Al₂O₃ powders at the sintering temperature of 1000 °C. The maximal intensity of both the green and red up-conversion emissions centered at about 523, 545, and 660 nm was observed for the 1 mol% Er³⁺- and 10 mol% Yb³⁺-codoped Al₂O₃ powders. The intensity ratio of the red to green up-conversion emission (I_red/I_green) increased with increasing the Yb³⁺ doping concentration for the Er³⁺-Yb³⁺-codoped Al₂O₃ powders. Furthermore, the intensity ratio of the green up-conversion emission at about 523 to 545 nm (I₅₂₃/I₅₄₅) was proportional to the Yb³⁺ doping concentration and pump electric current, which was associated with the elevated temperature of powders.     

Ultraviolet Luminescence Depending on Zn Interstitial in ZnO Polycrystalline Films

The ultraviolet emission line at 3.315eV is observed at 8K in ZnO polycrystalline films and investigated by temperature-dependent photolumineseence spectra and cathodoluminescence spatial image. The relative intensity of 3.315 eV emission line depends strongly on growth and annealing conditions. The cathodoluminescence image shows that the 3.315 eV emission localizes on the surface and ridge of ZnO grain. These results suggest that the 3.315 eV emission attributes to Zn interstitials at the grain surface and ridge. This emission is stable in the range from 8 K to 300 K and contributes to the room temperature ultraviolet band.     

Studies on influence of light on fluorescence of Tris-(8-hydroxyquinoline)aluminum thin films

Tris-(8-hydroxyquinoline)aluminum (Alq₃) thin films, the most widely used electron transport/emissive material in the organic electroluminescent (EL) devices, have been deposited on glass substrates by thermal evaporation process. Alq₃ thin films were exposed to light for various time periods under normal ambient. The fluorescence of as-prepared and light exposed Alq₃ thin films and formation of luminescent quencher have been studied using fluorescence, Mass, MALDI-ToF-MS, ¹H & ¹³C NMR, and FT-IR spectroscopy. It is observed that among the three 8-hydroxyquinoline (HQ) units in Alq₃ molecule, one HQ unit is affected during the light exposure in the normal ambient. It is found that the affected resultant Alq₃ molecule containing the carbonyl group acts as fluorescent quencher and the energy of excitons in the Alq₃ molecule in the light exposed Alq₃ thin films can be non-radiatively transferred to the neighboring fluorescent quencher, quenching the fluorescence of light exposed Alq₃ thin films in the normal ambient.     

Vacuum Ultraviolet Excited Photoluminescence Properties of Novel Na3Y9O3（BO3）8：Tb^3＋ Phosphor

The novel vacuum ultraviolet （VUV） excited Na3 Y9O3 （BO3）8：Tb^3＋ （NYOB：Tb^3＋） green phosphor is prepared. Strong VUV photoluminescence and high quenching concentration of Tb^3＋ （20 wt%） are observed in NYOB： Tb^3＋ and the strong emission are correlated with the unique layer-type structure of NYOB. All the characteristic 4 f - 5d transitions of Tb^3＋ and the host absorption band in VUV region are identified in the excitation spectrum. Based on the results, the energy levels scheme of Tb^3＋ in NYOB：Tb^3＋ is first established. This newly developed NYOB：Tb^3＋ phosphor shows excellent optical properties when compared with the commercial Zn2SiO4：Mn^2＋ and would be a potential VUV-excited green phosphor.     

Photothermal deflection spectroscopy of polymer thin films

Photothermal Deflection Spectroscopy (PDS) is known to be one of the most sensitive techniques for measuring the absorption of weakly absorbing materials. We have applied PDS for measuring the optical absorption of a few polymer thin-film samples over the wavelength region from 0.4–2.0 m. The results are useful for optical evaluation of these polymers.     

Excimer-laser-induced electric conductivity in thin-film C60

The electrical conductivity of thin-film C₆₀ has been changed by more than seven orders of magnitude with KrF (248nm) excimer-laser irradiation. Specific conductivities of 1 ^–1 · cm^–1 have been obtained. The onset of conductivity is consistent with a laser-induced metal-insulator phase transition. The threshold for KrF-laser ablation of C₆₀ has been determined to be 20±2 mJ/cm². This laser-induced process generates an all-carbon semiconductor-metallic junction which may have important technological applications.     

Photoluminescence properties of vacuum-deposited organic molecule-oxide (MePTCDI-SiO2) mixed layers

Thin films of the perylene derivative N,N′-dimethylperylene-3,4,9,10-bis-dicarboximide (MePTCDI) incorporated in SiO₂ matrix at various concentrations are obtained by condensation of host and dye in high vacuum. Photoluminescence spectroscopy is applied to study the spectral properties of the layers. Significant alterations in luminescence spectra in dependence on dye quantity are explained as a consequence of dye aggregation and resonant energy transfer. We demonstrate that the deposition geometry and preparation conditions offer an effective way to reduce the possibilities for non-radiative transitions, thus increasing the photoluminescence quantum efficiency.     

Photoluminescence in amorphous TiO2-PbO systems

Photoluminescence (PL) at room temperature has been achieved in amorphous thin films and powders of the TiO₂–PbO system. They were prepared by the polymeric precursor method with [PbO]/[TiO₂] molar ratios ranging from 0.0 to 1.0. The energy position of maximum PL emission and the PL intensity showed dependence on Pb concentration. The Pb addition suggests an increase in the number of non-bridging oxygens (NBO) in the amorphous TiO₂ network. These results support the relationship between photoluminescence and structure in TiO₂-based amorphous materials. Received: 7 February 2001 / Accepted: 8 February 2001 / Published online: 27 June 2001     

Structure and properties of ZnO films grown on Si substrates with low temperature buffer layers

Zinc oxide (ZnO) thin films were grown on Si (1 0 0) substrates by pulsed laser deposition (PLD) using two-step epitaxial growth method. Low temperature buffer layer (LTBL) was initially deposited in order to obtain high quality ZnO thin film; the as-deposited films were then annealed in air at 700 °C. The effects of LTBL and annealing treatment on the structural and luminescent properties of ZnO thin film were investigated. It was found that tensile strain was remarkably relaxed by employing LTBL and the band-gap redshifted, correspondingly. The shift value was larger than that calculated from band-gap theories. After annealing treatment, it was found that the annealing temperature with 700 °C has little influence on strains of ZnO films with LTBLs other than directly deposited film in our experiments. Interestingly, the different behaviors in terms of the shift of ultraviolet (UV) emission after annealing between films with and without buffer were observed, and a tentative explanation was given in this paper.     

Li掺杂8-羟基喹啉铝的密度泛函理论研究

采用密度泛函理论研究了Li原子掺杂8-羟基喹啉铝(Alq₃)分子的几何构型、 前线分子轨道及电子转移特性. 研究结果表明, Li原子掺杂Alq₃后, Li原子与Alq₃的O, N原子键合, 形成电子转移复合物. Li原子将部分电子转移到Alq₃的吡啶环上, 在Alq₃的带隙内形成施主能级, 这种n型掺杂结构有效地提高了电子的传输效率; 但过多的Li原子的掺杂会使Alq₃分解, 从而减弱其电子传输能力. 为使Alq₃的电子传输能力达到最高, Li原子的掺杂应保持在2:1左右的比例.