Study of delocalized and localized states in ZnSeO layers with photoluminescence, micro-photoluminescence, and time-resolved photoluminescence
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In this work, we present a detailed investigation of the low temperature emission properties of ZnSeO alloys by means of photoluminescence, micro-photoluminescence, and time-resolved photoluminescence. We show that the low temperature broad photoluminescence spectrum of ZnSeO attributed to the recombination of localized excitons is composed of sharp lines related to individual trapping states. Based on studies of photoluminescence thermal quenching from individual trapping states and photoluminescence dynamics, the mechanism of nonradiative recombination in ZnSeO alloys is discussed. Moreover, an unexpected decrease of the low temperature Stokes shift with increasing oxygen content is observed in contrast to what has been reported for GaAs based highly mismatched alloys. The possible origin of this effect is proposed.Keywords:
Photoluminescence excitation
We present a photoluminescence and photoluminescence excitation study of CdTe quantum dots, prepared via a novel organometallic approach. The global photoluminescence (excited at the energy above the absorption edge) showed a red shift of 75 meV with respect to the first absorption peak. This band edge emission was found to be strongly dependent on the excitation photon. Resonant emission spectra showed a pronounced spectral shift and line narrowing with decreasing excitation energy. The resonant Stokes shifts were extracted from photoluminescence and photoluminescence excitation data. The minimum magnitude of the resonant Stokes shift of 14 meV was obtained at room temperature.
Photoluminescence excitation
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Photoluminescence excitation
Porous Silicon
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We resolve the characteristic emission features of excitons in a single GaAs quantum wire using near-field photoluminescence spectroscopy. We report emission from both localized excitons and quasi-one-dimensional excitons that are delocalized over a length of up to several /spl mu/m.
Semiconductor nanostructures
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Low temperature (2 K) photoluminescence (PL), PL excitation (PLE), and resonant excitation (RE) measurements on a very high quality, 150-Å-wide GaAs/Al0.3Ga0.7As multiquantum well structure are reported, with a focus on exciton localization effects. The PL spectra show evidence of effective submonolayer well width fluctuations. Excitons observed via RE are found to be predominantly localized; i.e., they decay from the same effective well-width region in which they are formed, while both localized and delocalized excitons are observed in PLE measurements. Delocalized excitons are free to diffuse to differing effective well-width regions which are energetically accessible. These results are supported by time-resolved PL measurements, which show essentially identical time-responses for differing effective well-width regions under resonant excitation conditions, thereby demonstrating the localized nature of resonantly created excitons.
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Abstract Designing molecular materials with very large exciton diffusion lengths would remove some of the intrinsic limitations of present-day organic optoelectronic devices. Yet, the nature of excitons in these materials is still not sufficiently well understood. Here we present Frenkel exciton surface hopping, an efficient method to propagate excitons through truly nano-scale materials by solving the time-dependent Schrödinger equation coupled to nuclear motion. We find a clear correlation between diffusion constant and quantum delocalization of the exciton. In materials featuring some of the highest diffusion lengths to date, e.g. the non-fullerene acceptor Y6, the exciton propagates via a transient delocalization mechanism, reminiscent to what was recently proposed for charge transport. Yet, the extent of delocalization is rather modest, even in Y6, and found to be limited by the relatively large exciton reorganization energy. On this basis we chart out a path for rationally improving exciton transport in organic optoelectronic materials.
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A delocalized exciton state is revealed by resonant Raman scattering in GaAs-AlGaAs multiple-quantum-well heterostructures where only lower energy, confined quasi-two-dimensional excitons had previously been observed. In spite of its extension across the abrupt GaAs-AlGaAs interface, the delocalized exciton remains a well-defined state inhomogeneously broadened to widths between 6 and 14 meV. We estimate an exciton binding energy greater than 2 meV. Scattering of the delocalized state by the exciton-optical phonon interaction results in transitions to both delocalized and quasi-two-dimensional localized excitons.
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We show that optical two-dimensional (2D) spectroscopy can recover ultrafast heterogeneous dynamics of closely spaced delocalized exciton states from a molecular exciton manifold characterized by a single absorption band. The complete experimental third-order nonlinear optical response from room-temperature J-aggregates in liquid phase is reproduced for the first time with self-consistent Frenkel exciton theory combined with modified Redfield theory. We show that exciton relaxation between the exciton states and nuclear-motion-induced exchange-narrowed energy fluctuations of individual delocalized exciton states can be distinguished because these two processes lead to a distinctively different evolution of the absolute 2D spectrum. Our technique also allows recovery of the variation of the exciton relaxation rates as well as the degree of exciton delocalization across the absorption band.
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Using photoluminescence, photoluminescence excitation, and secondary ion mass spectroscopy (SIMS) investigations we show in this paper that two luminescence objects with different PL excitation spectra exist in PS. Two stages of photoluminescence aging were observed which are connected with desorption and oxidation processes.
Photoluminescence excitation
Porous Silicon
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Several ultrafast coherence spectroscopies are employed to study initially delocalized excitons and their subsequent sub-100 fs localization dynamics in LH2. Rigorous simulations of the exciton relaxation process allow determination of the time-dependent exciton size.
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