F. Meinardi

Photoluminescence of Sn-doped SiO2 excited by synchrotron radiation

Abstract Changes in the photoluminescence (PL) of amorphous SiO2 due to Sn-doping have been investigated by synchrotron radiation. Sn-doped SiO2 samples have been produced by a controlled sol–gel procedure as well as Ge-doped samples prepared for comparison. Detailed maps of the PL and PL excitation pattern have been obtained up to the band-to-band transition energy. The results confirm the analysis of Skuja as regards the emission at 3.1 and 4.2 eV excited at about 5 eV. At higher energies, our data show that the 3.1 and 4.2 eV PL bands have another excitation region with structures at 6.7, 7.2 and 8.0 eV. Lifetimes of about 10 ns for the 4.2 eV PL and 10 μs for the 3.1 eV PL are observed independently of the excitation energy. Data between 10 and 300 K are presented and compared with data from Ge-doped samples. The results show that high energy excitation of the 3.1 eV PL is not thermally activated, in contrast to the 4.9 eV excitation channel. Effects of the different spin–orbit coupling constants at Ge and Sn sites on PL intensity suggest that the high energy excitation channels arise from intra-center singlet-to-singlet transitions.

NIR emitting ytterbium chelates for colourless luminescent solar concentrators

A new oxyiminopyrazole-based ytterbium chelate enables NIR emission upon UV excitation in colorless single layer luminescent solar concentrators for building integrated photovoltaics.

Tin doped silica by sol-gel method : doping effects on the SiO2 Raman spectrum

Abstract Glasses of Sn-doped SiO2 were obtainted by the sol–gel method from two different Sn precursors, Sn(OtBu)4 and Sn(OOCCH3)2(C4H9)2. Structural modifications from xerogel to glass were monitored by Raman spectroscopy at different steps of the condensation process. The shifts of the SiO2 intrinsic vibrational modes show that Sn-doping can influence the structural properties of SiO2 even at relatively low tin concentrations. The changes depend on the thermal treatment and the preparation method. It is suggested that the formation of the SiO2 network is affected by the tin doping process during the early stages of the condensation.

Novel erbium complexes with low band-gap for infrared laser and optical amplifiers

Two new NIR luminescent erbium(III) crown-ethers have been synthesized and studied by optical spectroscopies. Both the complexes show absorption bands in the visible spectral region (absorption edge at about 500 nm and 600 nm) and exhibit the NIR lanthanide luminescence following excitation of the organic ligand. The absorption bands of these complexes, well in the visible region of the spectrum, make them ideal candidates as active molecules for efficient optically pumped laser and optical amplifier by excitation with commercially available lasers.

The origin of radiative emission of quaterthiophene ultra-thin films

The origin of the excited species giving rise to radiative emission in ultra-thin films of quaterthiophene grown on potassium acid phthalate single crystal by organic molecular beam deposition is investigated. In the steady-state photoluminescence (PL) spectrum collected at 80 K two emission peaks at 2.52 eV and 2.43 eV with their vibronic progression, are detected. The PL emission at 2.52 eV decays on the time scale of 500 ps, while the lifetime of the second one is only 4 ps. These results allow to relate the two emissions with localised and delocalised states, respectively.

Exciton quenching in emitter blends for organic light emitting devices probed by electric field–dependent time-resolved luminescence

We investigate quenching mechanisms of excited states in emitter layers for organic light emitting diodes (LEDs). An extensive study of a strong electric field-induced modulation (over 50%) of the time-resolved luminescence in a diamine derivative (TPD): polycarbonate blend films doped with an organic complex of europium are presented as a typical example of an important class of emitters for organic monochromatic LEDs. Using this method allowed us to identify the quenched species as the excited ligand precursors of the emissive europium ion states. Manipulating the electrode materials and their electrical bias, the electric field-enhanced dissociation, and interaction with injected charge could be separated and found as principal quenching mechanisms. We show the first one to follow the three-dimensional Onsager theory of geminate recombination, and the second one raised by their interaction with the TPD-transported holes. The interaction rate constant is found to be underlain by the three-dimensional diffusion of excited ligand singlets, combining the exciton diffusion coefficient and long-range (Forster type) energy transfer parameters. The dynamic parameters of the hole-precursor excitons interactions, extracted from the experimental data, allow us to establish the criteria for identifying useful ligands and matrices in the optimized design of electrophosphorescent, linelike emitting molecules, and device structure for organic LEDs.

Temperature activated de-trapping processes in vacuum deposited sexithiophene thin films

Photoluminescence (PL) emission spectra of sexithiophene (6T) thin films grown on different substrates by organic molecular beam deposition (OMBD) are studied in the range of temperature from 300 to 7 K. Besides the two vibronic progressions detectable between 300 and 50 K, related to the radiative recombination of the free exciton and to a structural defect state, a new emission is observed at temperatures lower than 40 K, whose origin is discussed in terms of exciton bound to a structural defects.

Growth of quaterthiophene thin films on potassium acid phthalate by organic molecular beam deposition

Absorption measurements and atomic force microscopy images of quaterthiophene (4T) thin films grown by molecular beam deposition on an organic single crystal substrate provide interesting information on the growth mode of these films, which is demonstrated to be strongly influenced by the interaction between the 4T molecules and the substrate. Evidence of a macroscopic film orientation and local crystal order is achieved.

Role of impurities in the 5.16 eV optical absorption band of Ge-doped silica

Abstract The optical absorption band at 5.16 eV and the related photoluminescence were investigated in Ge-doped silica samples as a function of the Ge content to study the relation between impurities and oxygen deficient defects. The data, collected in a wide doping range, showed that the dependence of the intensity of the 5.16 eV absorption band on the Ge content depends on the thermochemical parameters of the preparation process. Further, two distinct ranges of Ge-doping were individuated showing different dependence of the density of optically active center on the impurity concentration.

Absorption and emission properties of α, ω-dihexyl-quaterthiophene thin films grown by organic molecular beam deposition

The optical properties of thin films of α,ω-dihexyl-quaterthiophene grown by organic molecular beam deposition on different substrates are illustrated. Vibronic transitions in both the absorption and the emission spectra are observed, which originate from the coupling between vibronic states in the solid, similarly as in unsubstituted quaterthiophene films. Nevertheless, the alkyl chains in α,ω-dihexyl-quaterthiophene are responsible for a shift in both the absorption and emission peaks with respect to unsubstituted quaterthiophene and for an increased mesoscopic disorder.