| 1. |
- Chen, Junsheng, et al.
(author)
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Surface plasmon inhibited photo-luminescence activation in CdSe/ZnS core-shell quantum dots
- 2016
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In: Journal of Physics: Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 28:25
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Journal article (peer-reviewed)abstract
- In a composite film of Cdx Sey Zn1-x S1-y gradient core-shell quantum dots (QDs) and gold nanorods (NRs), the optical properties of the QDs are drastically affected by the plasmonic nanoparticles. We provide a careful study of the two-step formation of the film and its morphology. Subsequently we focus on QD luminescence photoactivation - a process induced by photochemical changes on the QD surface. We observe that even a sparse coverage of AuNRs can completely inhibit the photoactivation of the QDs' emission in the film. We demonstrate that the inhibition can be accounted for by a rapid energy transfer between QDs and AuNRs. Finally, we propose that the behavior of emission photoactivation can be used as a signature to distinguish between energy and electron transfer in the QD-based materials.
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| 2. |
- Lenngren, Nils, et al.
(author)
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Hot electron and hole dynamics in thiol-capped CdSe quantum dots revealed by 2D electronic spectroscopy
- 2016
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 18:37, s. 26199-26204
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Journal article (peer-reviewed)abstract
- Colloidal quantum dots (QDs) have attracted interest as materials for opto-electronic applications, wherein their efficient energy use requires the understanding of carrier relaxation. In QDs capped by bifunctional thiols, used to attach the QDs to a surface, the relaxation is complicated by carrier traps. Using 2D spectroscopy at 77 K, we follow excitations in thiol-capped CdSe QDs with state specificity and high time resolution. We unambiguously identify the lowest state as an optically allowed hole trap, and identify an electron trap with excited-state absorption. The presence of traps changes the initial dynamics entirely by offering a different relaxation channel. 2D electronic spectroscopy enables us to pinpoint correlations between states and to easily separate relaxation from different starting states. We observe the direct rapid trapping of 1S3/2, 2S3/2, and 1S1/2 holes, and several competing electron relaxation processes from the 1Pe state.
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| 3. |
- Osipov, Vladimir Al, et al.
(author)
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Nature of relaxation processes revealed by the action signals of intensity-modulated light fields
- 2016
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In: Physical Review A (Atomic, Molecular and Optical Physics). - 1050-2947. ; 94:5
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Journal article (peer-reviewed)abstract
- We present a generalized theory and experimental results of the action signals induced by the absorption of two photons from two phase-modulated laser beams. In our experiment, the phases of the laser beams are modulated at the frequencies φ1 and φ2, respectively. Their collinear combination leads to the modulation of the total intensity at the frequency φ=φ2-φ1. The action signals, such as photoluminescence and photocurrent, which result from the absorption of two photons, are isolated at frequencies mφ, m{0,1,2,»}. We demonstrate that the ratio of the amplitudes of the secondary (m=2) and the primary (m=1) signals A2φ:Aφ is sensitive to the type of relaxation processes in the media. Such sensitivity originates from the cumulative effects of the nonequilibrated state of the matter between the pulses. When the cumulative effects are small, i.e., the relaxation time is much shorter than the laser repetition rate or the laser intensity is high enough to dominate the system behavior, the ratio achieves the reference value of 1:4. This ratio decreases monotonically as the relaxation time increases. Our experimental results from fluorescent molecules rhodamine 6G and rubrene support these theoretical findings. In the case of a second-order relaxation process, the ratio changes rapidly with the excitation intensity. When the recombination rate in the second-order process is significantly slower than the repetition rate of the laser, we observe nonmonotonic behavior of the ratio as a function of excitation population at low excitation intensity, and when the recombination rate and the excitation intensity are high, the ratio approaches the value of 1:4. We also use the model to determine the value of the recombination rate of charge carriers in a GaP photodiode.
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| 4. |
- Zheng, Kaibo, et al.
(author)
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Trap States and Their Dynamics in Organometal Halide Perovskite Nanoparticles and Bulk Crystals
- 2016
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:5, s. 3077-3084
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Journal article (peer-reviewed)abstract
- Organometal halide perovskites have attracted tremendous attention for optoelectronic applications. Charge carrier trapping is one of the dominant processes often deteriorating the performance of devices. Here, we investigate the details of trap behavior in colloidal nanoparticles (NPs) of CH3NH3PbBr3 perovskites with mean size of 8 nm and the corresponding bulk crystals (BCs). We use excitation intensity dependence of photoluminescence (PL) dynamics together with comprehensive simulation of charge carrier trapping and the trap-state dynamics. In the bulk at very low excitation intensities the PL is quenched by trapping. A considerable fraction of the traps become filled if excitation fluence is increased. We identified two different traps, one exhibiting ultralong lifetime (∼70 μs) which leads to efficient accumulation of trap filling even at relatively low excitation intensities. In colloidal NPs, the average number of surface traps is estimated to be 0.7 per NP. It means about 30% excitation would undergo trap-free radiative recombination. The trapping time constant of 7 ns is orders of magnitude longer than the usual trapping times in typical colloidal quantum dots indicating semipassivation of the trap states by a large barrier which slows down the process in the perovskite NPs. We also note that due to the localized character of photogenerated electron-hole pairs in NPs the trapping efficiency is reduced compared to the freely moving charges in BCs. Our results offer insight into the details of photophysics of colloidal perovskite nanoparticles which show promise for light-emitting diode and laser applications.
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