Ardie D. Walser

Bimolecular reactions of singlet excitons in tris(8‐hydroxyquinoline) aluminum

We have studied singlet excited state dynamics in thin films of tris(8‐hydroxyquinoline) aluminum (Alq3), a well known emitter for organic electroluminescent devices. Our experimental results show that bimolecular recombination dominates the singlet exciton decay in pristine films at high intensities, thus decreasing the photoluminescence quantum efficiency and the singlet lifetime. Because of the relatively low carrier concentration at which light emitting diodes operate the electroluminescence efficiency is not affected. The measured rate constant of singlet–singlet annihilation in Alq3 films is γss=(3.5±2.5)×10−11 cm3 s−1. The diffusion coefficient and the diffusion length of singlet excitons estimated from γss are Ds=(1.2±0.8)×10−5 cm2 s−1 and L=98±40 A, respectively.

Photooxidation effects on picosecond photoluminescence and photoconductivity in tris-(8-hydroxyquinoline) aluminum (Alq3).

Abstract The photogeneration of radiative excited states and charge carriers in pristine and photooxidized tris-(8-hydroxyquinoline) aluminum (Alq 3 ) was studied by measuring both its photoluminescence and photoconductive response simultaneously, with subnanosecond resolution. 25 psec laser pulses at 355nm were used for excitation of vacuum evaporated 2–4 μm thick films of Alq 3 . We observed that photooxidation of Alq 3 with oxygen and UV light substantially decreases the fluorescence lifetime and quantum yield as well as the photoconductive response. This decrease is shown to be a result of quenching of singlet molecular excitations by products of photooxidation of Alq 3 .

Third-order nonlinear optical properties of a cadmiun sulfide-dendrimer nanocomposite

We have investigated the nonlinear optical response of thin films consisting of a poly(propyleneimine) dendrimer matrix having diaminobutane core with and without small 2.2nm diameter CdS quantum dots. Large nonlinear coefficients and low nonlinear absorption losses were observed at 532 and 1064nm. The high nonlinearity and low two-photon absorption yielded promising figures of merit for nonlinear optical switching applications.

Enhanced optical nonlinearity of surfactant-capped CdS quantum dots embedded in an optically transparent polystyrene thin film

Nanocomposites of surfactant-capped CdS quantum dots embedded in polystyrene thin film have been prepared and characterized by z-scan measurements. Large values of the two- and three-photon absorption coefficients were observed at 532nm (252cm∕GW) and 1064nm (160cm3∕GW2). The cross sections of absorption were ∼1×10−44cm4s/photon (532nm) and 8×10−73cm6s2∕photon2 (1064nm). These results are among the highest values of nonlinear absorption reported and orders of magnitude larger than CdS quantum dots capped by inorganic materials or in other matrix material. The high multiphoton absorption suggests that this class of materials is extremely promising for optical applications.

Temperature dependence of the singlet excited state lifetime in alq3

Abstract The temperature dependence of the lifetime of the singlet excited state in thin films of tris(8-hydroxyquinoline) aluminum (Alq 3 ) is investigated. An increase in the excited state lifetime and fluorescence quantum yield with decreasing temperature from 300K to 77K is observed. A diffusion based singlet exciton trapping model is used to explain these results.

Temperature dependence of transient photoluminescence in tris(8-hydroxyquinoline) aluminum (Alq3)

Abstract We have studied the temperature dependence of the dynamics of singlet excited states in pristine films of tris(8-hydroxyquinoline) aluminum (Alq 3 ). Measurements of the transient photoluminescence (PL) response revealed an increase in the radiative excited state lifetime and fluorescence quantum yield with decreasing temperature from 300 to 77 K. Bimolecular singlet exciton annihilation measurements show a decrease in the singlet exciton diffusion coefficient and diffusion length at 77 K. A singlet exciton trapping model is used to explain these results.

Fluorescence lifetime of terbium(III) as a probe for the ion‐binding properties of tactic poly(methacrylic acids)

The binding properties of trivalent metal ions to polyelectrolytes were investigated through the use of terbium [Tb(III)] in fluorescence studies. The fluorescence intensity and lifetimes of the lanthanide ions are directly dependent upon the number of water molecules bound to their inner coordination sphere. The more efficiently a ligand coordinates to a lanthanide ion, the more water molecules are expelled and consequently, the greater the fluorescence intensity and lifetime. This effect was used to probe for differences in the complexation behavior of tactic polymers. Aqueous solutions of isotactic and syndiotactic poly(methacrylic acid) (PMA) were neutralized and complexed with Tb(III) ions. The fluorescence intensity of the 286 nm hypersensitive excitation band was monitored and the lifetimes were measured using several excitation wavelengths. It was found that the isotactic PMA/Tb(III) complex exhibited a six times greater fluorescence intensity than the syndiotactic PMA complex. Lifetime measurements gave the number of water molecules coordinated by Tb(III) in the isotactic complex to be 2.4 while 3.4 waters remained bound to the Tb(III) ion in the syndiotactic PMA complex. These results indicate that isotactic PMA has the greater binding affinity towards Tb(III) ions.

Dynamics of photoexcited states and charge carriers in organic thin films: Alq3

We demonstrate a method for observing and analyzing the dynamics of photoexcited molecular states and charge carriers in organic thin films, by measuring simultaneously the transient photoluminescence and photoconductivity. We show experimentally in tris‐(8‐hydroxyquinoline) aluminum (Alq3) that exciton–exciton annihilation dictates the behavior of the photoluminescence and photoconductivity at high excitation intensities. The experimental results are corroborated by calculations based on an exciton‐exciton annihilation model.

Nonlinear optical responses in two-dimensional transition metal dichalcogenide multilayer: WS 2 , WSe 2 , MoS 2 and Mo 0.5 W 0.5 S 2

Synthetic two-dimensional transition metal dichalcogenides such as, tungsten disulphide (WS₂), tungsten diselenide (WSe₂), molybdenum disulphide (MoS₂) as well as mixed molybdenum tungsten disulphide (Mo_0.5W_0.5S₂) single crystals were grown by the chemical vapor transport method using halogens (bromine or chlorine) as transport agents. Multi- layer samples were cleaved from the single crystals, and their nonlinear optical (NLO) properties were obtained from both open aperture and closed aperture Z-scan measurements using a picosecond mode-locked Nd: YAG laser operating at a wavelength of 1064 nm, with pulse duration of 25 ps, and 20 Hz repetition rate. Both WS₂ and MoS₂ exhibited nonlinear saturable absorption (SA), whereas WSe₂ and Mo_0.5W_0.5S₂ showed nonlinear two-photon absorption (2PA). A large 2PA coefficient β as high as + 1.91x10^-8 cm/W was obtained for the Mo_0.5W_0.5S₂, and an index of refraction coefficient γ = -2.47x10^-9 cm²/W was obtained for the WSe₂ sample.

Polymerization of Diphenylbutadiyne by Gamma Rays Irradiation in the Molten State

Diphenylbutadiyne was irradiated with Gamma ray in the molten state (90°C), and its polymerization process was studied. The amorphous product has a number average molecular weight of around 1300. The yield reached around 20% with a dose of 3000 kGy, which is far greater than that in the case of irradiation in the solid state at room temperature. From the number of spins and molecular weights the product is thought to be mainly composed of cyclic oligomers. The third order nonlinear optical susceptibility of the products was found to be 3–6 × 10−10 esu determined by the Z-scan technique.