Field-induced Dissociation Research Articles

Understanding fundamental processes in poly(9,9-dioctylfluorene) light-emitting diodes via ultrafast electric-field-assisted pump-probe spectroscopy.

Femtosecond electric-field-assisted pump-probe measurements are a new approach to the study of fundamental processes within organic optoelectronic devices. Here we report a detailed study of organic light-emitting diodes based on poly(9,9-dioctylfluorene), using both temporal and spectral information to investigate polaron generation, due to field-induced singlet dissociation, and their subsequent recombination. The fundamental event in electroluminescence is time resolved: we find that initially free polarons coalesce into intermediate pairs of both singlets and triplets multiplicity which subsequently decay into the neutral state. Our results indicate that the efficiency of singlet formation, beta approximately 0.7, is much higher than expected from simple state degeneracy arguments (beta approximately 0.25).

Fast field-induced dissociation and recombination of optical excitations in a π-conjugated polymer

We present time resolved photoluminescence measurements on thin films of a phenyl-substituted poly(phenylene-vinylene) incorporated in a diode structure. Under reverse bias conditions rapid exciton dissociation is observed leading to luminescence quenching by up to 30%. In contrast, under forward bias conditions the initial quenching is substantially reduced due to shielding by space charges. At longer times thermally activated exciton quenching by injected polarons dominates the quenching process. At 3 ns after excitation, the external field is found to enhance the delayed luminescence. We attribute this to increased recombination of spatially correlated charge carrier pairs in the presence of the electric field, which are generated by exciton dissociation by bimolecular annihilation or on defect states.

On-chain defect emission in conjugated polymers – Comment on ‘Exciton dissociation dynamics in a conjugated polymer containing aggregate states’ [A. Haugeneder, U. Lemmer, U. Scherf, Chem. Phys. Lett. 351 (2002) 354

On-chain chemical defects have recently been identified as the source of broad, low-energy emission features in polyfluorenes. By considering the evolution of the time-resolved luminescence spectrum of a dilute solution of a ladder-type poly( p-phenylene) it is shown that on-chain defects also play an important role in the emission from these materials. A broad band centred at 560 nm, which has previously been attributed to aggregate emission, is identified in both solution and film, suggesting an incompatibility with the aggregate picture. This provides an explanation for the recent observation of a decrease of field-induced exciton dissociation with increasing emission wavelength as excitons are spatially more localised on chemical defects than on the polymer backbone.

Exciton dissociation dynamics in a conjugated polymer containing aggregate states

We study the exciton dynamics in ladder-type poly( p-phenylene) (LPPP) containing aggregate states under high electric field conditions. Time-resolved emission spectroscopy reveals a different dissociation behaviour of S 1-intrachain excitons as compared to excitons located in aggregate states. No acceleration of the decay of aggregate excitons due to field-induced dissociation is found for electric fields up to 2×10 6 V/cm. The time-integrated photoluminescence quenching does not exhibit a strong spectral dependence and is completely due to the dissociation of initially excited S 1-intrachain excitons.

Capillary electrophoresis detection of a complex sensitive to electric field

This study describes the capillary electrophoresis detection of the weak interaction occurring between the negative form of 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid (HEPES-) and nicotinamide-adenine dinucleotide (NAD + ). The ion-paired complex is formed probably by 1 molecule of each reactant and it is sensitive to the applied electric field, since it was found that the value of the apparent association constant decreases as a function of the field applied. The association constant at zero field was obtained by extrapolation of the values determined at different applied field strengths and was found to be 2.0 x 10 3 M -1 . The peak convolution observed at the detection window was representative of a dissociation process that started at the beginning of the separation. The rate of the field-induced dissociation corresponded to 6.4 × 10 -10 mol.cm .L -1 . V -1 . S -1 . Due to the complete absence of detectable signal shifts in 1 H-NMR experiments, the interaction should be ascribed to the negative sulpho-moiety of HEPES- and to the positive charge of the NAD + pyrimidine ring. To our knowledge, this is the first example of a secondary Wien effect observed at low field.

Direct interband transitions in tris-(8-hydroxyquinoline) aluminum thin films

The electroluminescent properties of three different device structures (A:ITO/SiO2/Alq3/SiO2/Al, B:ITO/Alq3/SiO2/Al, and C:ITO/SiO2/Alq3/Al) based on the tris-(8-hydroxyquinoline) aluminum (Alq3) were investigated. A blue electroluminescence at 457 nm was obtained from device (A) and (B), and the green emission at 518 nm was obtained from device (C). It is generally agreed that the green emission originates from the recombination of the singlet excitons. The blue emission, here, is attributed to the direct transitions between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital of Alq3. This is due to the electric field-induced excitons dissociation and the space charge accumulation at the interface. The high internal electric field enhances the dissociation of neutral singlet excitations into LUMO states and inhibits the formation of the singlet excitons, therefore enhances the probability for direct interband transitions of the relaxed carriers. The intensity of the blue emission is dependent on the operating frequency. This indicates that space charge accumulation time and effective internal electric field are responsible for the blue emission intensity.

Atom probe analysis of the dissociation of CO and N2 gases on a W(110)-oriented tip

We studied the dissociation of CO and N2 gases on a W(110)-oriented tip by an atom probe (AP) technique. We paid attention to the effect of electric field induced dissociation as well as the temperature dependence of dissociation probabilities. We first show that the AP can discriminate between physisorbed and trapped states based on the difference in desorption field strength. Then we show that field-induced dissociation can be properly measured. Finally, we conclude the activation energy required for the conversion from the trapped state to the dissociated state to be 30 meV for CO gas and 22 meV for N2 gas.

Atom probe analysis of the dissociation of CO and N 2 gases on W(110)-oriented tips

In order to extend the application range of an atom probe (AP) to the study of the gas-surface catalytic phenomena, the proper interpretation of the electric field induced dissociation of molecular gases is greatly needed. We first show that the AP can discriminate between physically absorbed and trapped states based on the difference in desorption field strength. Then we show that the field-induced dissociation of CO and N 2 gases on a W(110)-oriented tip can be estimated. Finally, the activation energy required for conversion from the trapped state to the dissociated state is deduced from the temperature dependence of the dissociation probability.

Electrical impedance tomography study of biological processes in a single cell.

An in vivo electrical impedance tomography (EIT) study of single plant cells of Chara corallina is reported. When these aquatic cells grow in alkaline conditions, proton-translocating ATP synthases in the plasma membrane operate in reverse, utilizing ATP to translocate protons against an electrochemical gradient to the periplasm and creating localized acidic regions along the cell's cylindrical surface. These acidic regions, which appear as radial bands, approximately 5 mm long, between narrower alkaline bands, facilitate the uptake of bicarbonate, the plant's source of inorganic carbon for photosynthesis in the carbon dioxide-depleted alkaline conditions. Our EIT study of cell ultrastructure in the acidic and alkaline regions provides evidence that the plasma membrane is folded in localized regions (e.g., charasomes) in the acidic bands. The very low frequency capacitance dispersions were very similar to those of double fixed-charge structures. Such charge distributions are known to be present in the membrane-bound F0 portion of the ATP synthase. The theoretical dependence of the fixed-charge concentrations on pH in the proteins is shown to broadly account for the observed correlations between pH, membrane potential, conductance, and capacitance in these regions. In synthetically formed double fixed-charge membranes, electric field-induced dissociation of water into H+ and OH- occurs. This leads to the speculation that H+/OH- fluxes in ATP synthases located in the alkaline regions of Chara cells might also involve the electric field-induced dissociation of water.

Field-Induced Deposition of Hard C-H Coatings on the Steel Surface in a Rich-Propane Flame Flow

The process of a chemical deposition of hard wear-resistant C-H coatings on a steel surface has been developed in a premixed propane-rich flame flow. The experimental investigations are performed under conditions of a field-induced dissociation and ionization of propane. The influence of a process of the sheath-convection of radical ions from a flame flow to the exposed steel surface on the hard C-H film growth is studied in a range of the applied field voltages from 0.6 to 2.4 kV. Infrared spectroscopy and electron microscopy are used as the tools for the characterization and identification of C-H coatings.

Dynamical quenching of field-induced dissociation of H2+ in intense infrared lasers

The dynamics of dissociation of the hydrogen molecular ion H2+ in an intense infrared (IR) field is studied by a series of wave packet simulations. In these simulations, the molecular ion is assumed to be instantly prepared at the initial time by a sudden ionization of the ground-state H2 parent molecule, and a variety of frequency and intensity conditions of the laser field are considered. A new stabilization mechanism, called dynamical dissociation quenching, is found operative in the IR spectral range. In a time-resolved picture, this effect is shown to arise when a proper synchronization between the molecular motions and the laser field oscillations is ensured. In the Floquet, dressed molecule picture, the effect is related to interferences between the Floquet resonances that are excited initially by the nonadiabatic, sudden preparation of the ion. The Floquet analysis of the wave packets in this low frequency regime reveals important intersystem couplings between Floquet blocks, reflecting the highly multiphoton character of the dynamics.

Ultrafast field-induced dissociation of excitons in a conjugated porphyrin polymer

Ultrafast field-induced dissociation of excitons in a conjugated porphyrin polymer

Ultrafast field-induced dissociation of excitons in a conjugated porphyrin polymer

We present a study of field-induced modulation of the ultrafast transient photo-induced response in a conjugated zinc porphyrin polymer. This was in the form of aggregated strands in a host polymer matrix. The photobleaching of the polymer Q-band is significantly reduced by the application of an electric field. This field modulation occurs at very short times (< 0.4 ps). We attribute this to field-assisted dissociation of excitons to give charge separation, as supported by the measured photovoltaic response. By the measurement of the field-dependent photo-induced Q-band bleaching, we are able to extend the study of the photogeneration of charge in conjugated polymers to very much shorter times than hitherto. Our results indicate that charge generation is closely coupled with exciton formation.

Comment on “Theory of Exciton Migration and Field-Induced Dissociation in Conjugated Polymers”

A Comment on the Letter by M. C. J. M. Vissenberg and M. J. M. de Jong, Phys. Rev. Lett. 77, 4820 (1996). The authors of the Letter offer a Reply.

Theory of Exciton Migration and Field-Induced Dissociation in Conjugated Polymers.

The interplay of migration, recombination, and dissociation of excitons in disordered media is studied theoretically in the low temperature regime. An exact expression for the photoluminescence spectrum is obtained. The theory is applied to describe the electric field-induced photoluminescence-quenching experiments by Kersting et al. [Phys. Rev. Lett. 73, 1440 (1994)] and Deussen et al. [Synth. Met. 73, 123 (1995)] on conjugated polymer systems. Good agreement with experiment is obtained using an on-chain dissociation mechanism, which implies a separation of the electron-hole pair along the polymer chain.

Field-induced dissociation of optical excitations in conjugated polymers

A theory of the field induced quenching of photoluminescence in films of conjugated polymers is presented. Time-resolved photoluminescence measurements show that the quenching effect evolves on a time scale from 0.5 ps to ∼ 100 ps after excitation. This ultrafast temporal behavior of the quenching process is explained in terms of a field-assisted dissociation of localized neutral singlet excitations into geminate electron hole pairs while undergoing a random walk among polymer segments. The model considers photocarrier generation as a secondary process resulting from exciton dissociation.

Electric field-induced photoluminescence quenching in thin-film light-emitting diodes based on poly(phenyl- p-phenylene vinylene)

We report on electric field-induced fluorescence quenching in thin-film light-emitting diode (LED) structures made from blends of poly(phenyl- p-phenylene vinylene) and polycarbonate. The dependence of fluorescence quenching on excitation wavelength, electric field and concentration has been measured. Efficient fluorescence quenching is observed, which is attributed to field-induced dissociation of excitons. The same effect is expected to be operative in LED devices limiting electroluminescence quantum yield. An estimate of the exciton binding energy of E B ≈ 0.4 eV in this class of system is inferred from the field dependence by comparison with model calculations.

Field-induced oxygen layer formation from H 2O and its titration by hydrogen on a Pt-emitter

FEM and FIM studies are presented showing the field-induced formation of an O ads island on the apex of a platinum field emitter. This occurs during the well known bimolecular, field-induced water dissociation reaction 2H 2O ads → H 3O gas + + OH ads + e (Pt) − which is followed by a disproportionation reaction of the OH ads species to H 2O and O ads without any noticeable face specifity. The titration reaction of the O ads layer with H 2 displays face specific conversion rates. FIM reveals chemical waves propagating with sharp boundaries on the atomic scale. Implications for the oscillating H 2 O H 2 reaction system are discussed.

Formation and decay dynamics of excitonic photoluminescence in a GaAs/AlGaAs superlattice under an electric field

Photoluminescence (PL) dynamics of localized excitons in a strongly coupled 40 Å/40 Å GaAs/Al0.2Ga0.8/As superlattice under an electric field along the growth direction is reported. At zero field, a delayed PL formation due to the relaxation of dark excitons into radiative states is observed. With increasing field, both the PL decay time and the PL amplitude are strongly reduced. A rate equation analysis of the measured PL transients gives evidence that the field-induced dissociation of nonradiative excitons is important.

Ultrafast field-induced dissociation of excitons in conjugated polymers.

We present a time-resolved study of the field-induced luminescence quenching in poly(phenylphenylenevinylene) using femtosecond luminescence spectroscopy. The dynamic quenching of the luminescence reveals a delay between the generation of excitons and their breakup into geminate electron-hole pairs. We explain the data by the interplay of the dissociation and the spectral relaxation of the excitons within an inhomogeneously broadened density of states. Our experimental results suggest that the photocarriers in conjugated polymers are not formed directly but result from dissociation of excitons.