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Excitation Intensities (excitation + intensity)
Selected AbstractsThe Effect of Thermal Treatment on the Morphology and Charge Carrier Dynamics in a Polythiophene,Fullerene Bulk Heterojunction,ADVANCED FUNCTIONAL MATERIALS, Issue 8 2005J. Savenije Abstract The influence of various thermal treatment steps on the morphology and the photoconductive properties of a non-contacted, 50,nm thick blend (50:50,wt.-%) of [6,6]-phenyl C61 -butyric acid methyl ester (PCBM) and poly(3-hexyl thiophene) (P3HT) spin-coated from chloroform has been studied using transmission electron microscopy (TEM) and the electrodeless time-resolved microwave conductivity technique. After annealing the film for 5,min at 80,°C, TEM images show the formation of crystalline fibrils of P3HT due to a more ordered packing of the polymer chains. The thermal treatment results in a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056,cm2,V,1,s,,1 for the non-annealed sample to 0.044,cm2,V,1,s,,1 for the sample annealed at 80,°C. In contrast, the temporal shape of the photoconductivity, with typical decay half-times, ,1/2, of 1,,s for the lowest excitation intensities, is unaffected by the temperature treatment. Further annealing of the sample at 130,°C results in the formation of three different substructures within the heterojunction: a PCBM:P3HT blend with PCBM-rich clusters, a region depleted of PCBM, and large PCBM single crystals. Only a minor increase in the amplitude, but a tenfold rise of the decay time of the photoconductivity, is observed. This is explained by the formation of PCBM-rich clusters and large PCBM single crystals, resulting in an increased diffusional escape probability for mobile charge carriers and hence reduced recombination. [source] Excitation relaxation in copper selenide nanowiresPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2009Gediminas Ju Abstract Ultrafast nonequilibrium charge carrier relaxation in highly ordered Cu2,xSe nanowires of 8 nm, 13 nm and 25 nm diameter was investigated by means of femtosecond pump,probe absorption spectroscopy. Transient absorption bleaching was observed in the region of the near infrared absorption band, whereas an induced absorption dominated at higher energies. The transient absorption kinetics is almost independent of the excitation and probe wavelength and shows a biexpoenetial charge carrier recombination with the excitation intensity dependent decay rates. The initial ultrafast relaxation, which gets slower at higher excitation intensities, is followed by the slower decay component emerging at high intensities. These relaxation peculiarities are discussed in terms of recombination enhancement by intragap states, and relaxation kinetics is described by a theoretical model of two concurrent relaxation channels involving deep and shallow impurity levels. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The evolution of the electric field in an optically excited semiconductor superlatticePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 8 2005Alvydas Lisauskas Abstract We report on time-resolved photocurrent spectroscopy of an intrinsic GaAs/Al0.3Ga0.7As superlattice subsequent to femtosecond optical excitation. Information on the spatio-temporal evolution of the densities of electrons and holes and on the internal electric field is obtained by tracing Wannier-Stark photocurrent spectra as a function of delay time for various bias fields and pump excitation intensities. The experimental results are supplemented by simulations. We employ the combined information to define the conditions to be met for succesful pump-probe Bloch gain experiments. In particular, we find that field screening sets on upper limit for the carrier density of 1016 cm,3, and that the time window during which gain should be found is defined by the duration of the sweep-out of the optically injected electrons from the superlattice which occurs within about 10 ps after excitation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Singlet Energy Dissipation in the Photosystem II Light-Harvesting Complex Does Not Involve Energy Transfer to CarotenoidsCHEMPHYSCHEM, Issue 6 2010Marc G. Müller Dr. Abstract The energy dissipation mechanism in oligomers of the major light-harvesting complex II (LHC II) from Arabidopsis thaliana mutants npq1 and npq2, zeaxanthin-deficient and zeaxanthin-enriched, respectively, has been studied by femtosecond transient absorption. The kinetics obtained at different excitation intensities are compared and the implications of singlet,singlet annihilation are discussed. Under conditions where annihilation is absent, the two types of LHC II oligomers show distributive biexponential (bimodal) kinetics with lifetimes of ,5,20 ps and ,200,400 ps having transient spectra typical for chlorophyll excited states. The data can be described kinetically by a two-state compartment model involving only chlorophyll excited states. Evidence is provided that neither carotenoid excited nor carotenoid radical states are involved in the quenching mechanism at variance with earlier proposals. We propose instead that a chlorophyll,chlorophyll charge-transfer state is formed in LHC II oligomers which is an intermediate in the quenching process. The relevance to non-photochemical quenching in vivo is discussed. [source] Optical characterization of Zn0.97Mn0.03Se/ZnSe0.92Te0.08 type II multiple-quantum-well structuresPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2007D. Y. Lin Abstract The optical characterization of type II Zn0.97Mn0.03Se/ZnSe0.92Te0.08 multiple-quantum-well structures have been studied using photoluminescence (PL), temperature-dependent PL, polarized PL, power-dependent PL, and photoreflectance (PR) in this study. The PL data reveal that the band alignment of the ZnMnSe/ZnSeTe system is type II. Comparing with the theoretical calculation based on the Schrodinger equation, the valence band offset is estimated to be 0.6 eV. From the power-dependent PL spectra, it is observed that the peak position of PL spectra shows a blueshift under different excitation power. The blueshift can be interpreted in terms of the band-bending effect due to spatially photoexcited carriers in a type II alignment. The thermal activation energy (EA) for quenching the PL intensity was determined from tem- perature-dependent PL spectra. The thermal activation energy was found to decrease as the thickness of ZnMnSe and ZnSeTe layers decreased. The polarized PL spectra exhibit a large in-plane polarization with the polarization degree up to 50%. The polarization does not depend on the excitation intensity as well as temperature. The large polarization is an inherent orientation of the interface chemical bonds. The higher transition features observed in PR spectra show a blueshift with the similar trend observed in the PL spectra as decreasing the thickness of ZnSeTe layer. This result provides a consistent evidence for the assumption that square-like well shapes were built in the ZnSeTe layers. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Impurity levels in the layered semiconductor p-GaSe doped with group V elements As, Bi and SbPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 15 2005S. Shigetomi Abstract The radiative and non radiative recombination mechanisms in the As, Sb and Bi-doped GaSe have been investigated on the basis of photoluminescence (PL) and Hall effect measurements. The PL features (at 77 K) related to the impurity levels coming from the As, Sb and Bi atoms are dominated by a broad emission band at about 1.7 eV. From the temperature dependences of the peak energy and PL intensity and the dependence of excitation intensity of peak energy, it was found that the 1.7 eV emission band is due to the transition from the shallow donor level at about 0.08 eV below the conduction band to the deep acceptor. In addition it was found, from the temperature dependence of hole concentration, that a deep acceptor level at about 0.6 eV above the valence band is formed by the doping atoms. It is associated with defects or defect complexes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The photo-induced phase transition in Et2Me2Sb[Pd(dmit)2]2PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 1 2009Tadahiko Ishikawa Abstract The large photo-induced change of reflectivity has been observed in the unique charge separated phase of the Et2Me2Sb[Pd(dmit)2]2 crystal. We observe a clear spectral change due to the disappearance of the charge separated phase by femtosecond photo-excitation in the [Pd(dmit)2] dimer. The dynamics of the photo-excited state depends on the excitation intensity. Obtained results strongly suggest that cooperative effect plays a key role in the photo-excited state of this system. We tentatively classify the observed phenomena as a photo-induced phase transition due to the melting of the charge separation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Anti-Stokes photoluminescence in CdSe self-assembled quantum dotsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 4 2003Aishi Yamamoto Abstract Anti-Stokes photoluminescence (ASPL) as well as Stokes photoluminescence (SPL) were studied in CdSe self-assembled quantum dots using a micro-photoluminescence technique. A few ASPL peaks were observed, and they also appeared in the SPL spectra. Photoluminescence intensities of both ASPL and SPL increase linearly with the excitation intensity. The emission mechanism of the observed ASPL is discussed. [source] | ||||||||||