| 1. |
- Zheng, Kaibo, et al.
(författare)
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Direct Experimental Evidence for Photoinduced Strong-Coupling Polarons in Organolead Halide Perovskite Nanoparticles
- 2016
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Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7:22, s. 4535-4539
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Tidskriftsartikel (refereegranskat)abstract
- Echoing the roaring success of their bulk counterparts, nano-objects built from organolead halide perovskites (OLHP) present bright prospects for surpassing the performances of their conventional organic and inorganic analogues in photodriven technologies. Unraveling the photoinduced charge dynamics is essential for optimizing the optoelectronic functionalities. However, mapping the carrier-lattice interactions remains challenging, owing to their manifestations on multiple length scales and time scales. By correlating ultrafast time-resolved optical and X-ray absorption measurements, this work reveals the photoinduced formation of strong-coupling polarons in CH3NH3PbBr3 nanoparticles. Such polarons originate from the self-trapping of electrons in the Coulombic field caused by the displaced inorganic nuclei and the oriented organic cations. The transient structural change detected at the Pb L3 X-ray absorption edge is well-captured by a distortion with average bond elongation in the [PbBr6]2- motif. General implications for designing novel OLHP nanomaterials targeting the active utilization of these quasi-particles are outlined.
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| 2. |
- Bamini, Sesha, et al.
(författare)
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Time-resolved terahertz spectroscopy reveals the influence of charged sensitizing quantum dots on the electron dynamics in ZnO
- 2017
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 19:8, s. 6006-6012
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Tidskriftsartikel (refereegranskat)abstract
- Photoinitiated charge carrier dynamics in ZnO nanoparticles sensitized by CdSe quantum dots is studied using transient absorption spectroscopy and time-resolved terahertz spectroscopy. The evolution of the transient spectra shows that electron injection occurs in a two-step process, where the formation of a charge transfer state (occurring in several picoseconds) is followed by its dissociation within tens of picoseconds. The photoconductivity of electrons injected into the ZnO nanoparticles is lower than that of charges photogenerated directly in ZnO. We conclude that the motion of injected electrons in ZnO nanoparticles is strongly influenced by their interaction with positive charges left in the sensitizing quantum dots.
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| 3. |
- Bokareva, O. S., et al.
(författare)
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Optimized long-range corrected density functionals for electronic and optical properties of bare and ligated CdSe quantum dots
- 2017
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Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 13:1, s. 110-116
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Tidskriftsartikel (refereegranskat)abstract
- The reliable prediction of optical and fundamental gaps of finite size systems using density functional theory requires to account for the potential self-interaction error, which is notorious for degrading the description of charge transfer transitions. One solution is provided by parametrized long-range corrected functionals such as LC-BLYP, which can be tuned such as to describe certain properties of the particular system at hand. Here, bare and 3-mercaptoprotionic acid covered Cd33Se33 quantum dots are investigated using the optimally tuned LC-BLYP functional. The range separation parameter, which determines the switching on of the exact exchange contribution, is found to be 0.12 bohr-1 and 0.09 bohr-1 for the bare and covered quantum dot, respectively. It is shown that density functional optimization indeed yields optical and fundamental gaps and thus exciton binding energies, considerably different compared with standard functionals such as the popular PBE and B3LYP ones. This holds true, despite the well established fact that the leading transitions are localized on the quantum dot and do not show pronounced long-range charge transfer character.
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| 4. |
- Chen, Junsheng, et al.
(författare)
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Photo-stability of CsPbBr3 perovskite quantum dots for optoelectronic application
- 2016
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Ingår i: SCIENCE CHINA Materials. - : Springer Science and Business Media LLC. - 2095-8226 .- 2199-4501. ; 59:9, s. 719-727
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Tidskriftsartikel (refereegranskat)abstract
- Due to their superior photoluminescence (PL) quantum yield (QY) and tunable optical band gap, all-inorganic CsPbBr3 perovskite quantum dots (QDs) have attracted intensive attention for the application in solar cells, light emitting diodes (LED), photo detectors and laser devices. In this scenario, the stability of such materials becomes a critical factor. We hereby investigated the long-term stability of as-synthesized CsPbBr3 QDs suspended in toluene at various environmental conditions involving the light illumination, atmosphere, and temperature. We found light illumination would induce dramatic degradation of CsPbBr3 QDs reflecting as decreasing absorbance and PL intensity together with red-shifted emission band. Such light instability can be attributed to the photo-induced surface degradation and aggregation. The steady-state spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD) technics verified that CsPbBr3 QDs trend to aggregate to form larger particles under continuous light soaking. In addition, decreasing PL QY of the QDs during light soaking indicates the formation of trap sites. Such trap sites lead to the red-shifted emission with increasing PL lifetime. Our work reveals that the main origin of instability in CsPbBr3 QDs and provide reference to engineer such QDs towards optimal device application.
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| 5. |
- Chen, Junsheng, et al.
(författare)
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Size-And Wavelength-Dependent Two-Photon Absorption Cross-Section of CsPbBr3 Perovskite Quantum Dots
- 2017
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 8:10, s. 2316-2321
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Tidskriftsartikel (refereegranskat)abstract
- All-inorganic colloidal perovskite quantum dots (QDs) based on cesium, lead, and halide have recently emerged as promising light emitting materials. CsPbBr3 QDs have also been demonstrated as stable two-photon-pumped lasing medium. However, the reported two photon absorption (TPA) cross sections for these QDs differ by an order of magnitude. Here we present an in-depth study of the TPA properties of CsPbBr3 QDs with mean size ranging from 4.6 to 11.4 nm. By using femtosecond transient absorption (TA) spectroscopy we found that TPA cross section is proportional to the linear one photon absorption. The TPA cross section follows a power law dependence on QDs size with exponent 3.3 ± 0.2. The empirically obtained power-law dependence suggests that the TPA process through a virtual state populates exciton band states. The revealed power-law dependence and the understanding of TPA process are important for developing high performance nonlinear optical devices based on CsPbBr3 nanocrystals.
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| 6. |
- Chen, Junsheng, et al.
(författare)
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Surface plasmon inhibited photo-luminescence activation in CdSe/ZnS core-shell quantum dots
- 2016
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Ingår i: Journal of Physics: Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 28:25
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Lenngren, Nils, et al.
(författare)
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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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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 18:37, s. 26199-26204
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Zheng, Kaibo, et al.
(författare)
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High Excitation Intensity Opens a New Trapping Channel in Organic–Inorganic Hybrid Perovskite Nanoparticles.
- 2016
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Ingår i: ACS Energy Letters. - : American Chemical Society (ACS). - 2380-8195. ; 1:6, s. 1154-1161
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the excited-state dynamics of CH3NH3PbBr3 perovskite nanoparticles (NPs) and bulk crystals under various excitation intensity regimes using transient absorption spectroscopy. We confirmed the sub-band gap hole trap states with optical transition to the conduction band in both samples. In bulk crystals, the excited-state dynamics is independent of pump intensity. However, in NPs, pronounced intensity dependence appears. At low intensities, the hole trap states do not affect the excited-state dynamics due to the potential barrier between the photogenerated holes and the surface trap states. When the excitation density is much higher than one per NP, charge accumulation makes hot holes overcome the barrier and get trapped with electrons long living in the conduction band (≫10 ns). This explains the high emissive properties of such NPs despite the existence of surface traps. However, in the application of emitting devices requiring high excitation intensity, the surface trapping becomes significant.
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| 9. |
- Zheng, Kaibo, et al.
(författare)
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Inter-phase charge and energy transfer in Ruddlesden-Popper 2D perovskites : Critical role of the spacing cations
- 2018
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Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7488 .- 2050-7496. ; 6:15, s. 6244-6250
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Tidskriftsartikel (refereegranskat)abstract
- Photo-generated charge carrier dynamics in Ruddlesden-Popper 2D perovskites with linear (n-BA) and branched (iso-BA) butylamine as spacing cations have been studied by using transient absorption and time-resolved photoluminescence spectroscopies. Both n-BA and iso-BA perovskites consist of mixed-phase 2D quantum wells with various layer thicknesses, where the photo-generated charges undergo inter-phase charge transfer from thinner quantum wells to thicker ones. By shortening the spacer from n-BA to iso-BA, the transfer rates are significantly increased, which can also diminish the charge accumulation in thin quantum wells induced by the unbalanced electron and hole charge transfer rates. Under high excitation intensity, the shorter spacing cation is found to further facilitate the energy transfer, which can compete with fast high-order carrier recombination and consequently improve the charge transfer efficiency. Intriguingly, we observe the existence of extra bulk 3D phases embedded within iso-BA perovskites, which can efficiently collect the confined charges within 2D phases and then transport them with faster carrier mobility and slower recombination rates.
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| 10. |
- Zheng, Kaibo, et al.
(författare)
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Trap States and Their Dynamics in Organometal Halide Perovskite Nanoparticles and Bulk Crystals
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:5, s. 3077-3084
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Tidskriftsartikel (refereegranskat)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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