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Markus Haase - One of the best experts on this subject based on the ideXlab platform.
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liyf4 yb liyf4 and liyf4 yb er liyf4 core shell nanocrystals with Luminescence decay times similar to ylf laser crystals and the upconversion Quantum Yield of the yb er doped nanocrystals
Nano Research, 2021Co-Authors: Frederike Carl, Ute Reschgenger, Bettina Grauel, Christian Wurth, Leonie Birk, Monica Pons, Markus HaaseAbstract:We developed a procedure to prepare luminescent LiYF4:Yb/LiYF4 and LiYF4:Yb,Er/LiYF4 core/shell nanocrystals with a size of approximately 40 nm revealing Luminescence decay times of the dopant ions that approach those of high-quality laser crystals of LiYF4:Yb (Yb:YLF) and LiYF4:Yb,Er (Yb,Er:YLF) with identical doping concentrations. As the Luminescence decay times of Yb3+ and Er3+ are known to be very sensitive to the presence of quenchers, the long decay times of the core/shell nanocrystals indicate a very low number of defects in the core particles and at the core/shell interfaces. This improvement in the performance was achieved by introducing two important modifications in the commonly used oleic acid based synthesis. First, the shell was prepared via a newly developed method characterized by a very low nucleation rate for particles of pure LiYF4 shell material. Second, anhydrous acetates were used as precursors and additional drying steps were applied to reduce the incorporation of OH− in the crystal lattice, known to quench the emission of Yb3+ ions. Excitation power density (P)-dependent absolute measurements of the upconversion Luminescence Quantum Yield (ΦUC) of LiYF4:Yb,Er/LiYF4 core/shell particles reveal a maximum value of 1.25% at P of 180 Wcm−2. Although lower than the values reported for NaYF4:18%Yb,2%Er core/shell nanocrystals with comparable sizes, these ΦUC values are the highest reported so far for LiYF4:18%Yb,2%Er/LiYF4 nanocrystals without additional dopants. Further improvements may nevertheless be possible by optimizing the dopant concentrations in the LiYF4 nanocrystals.
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nayf4 yb er nayf4 core shell nanocrystals with high upconversion Luminescence Quantum Yield
Angewandte Chemie, 2018Co-Authors: Christian Homann, Ute Reschgenger, Lisa Krukewitt, Florian Frenzel, Bettina Grauel, Christian Wurth, Markus HaaseAbstract:Upconversion core/shell nanocrystals with different mean sizes ranging from 15 to 45 nm were prepared via a modified synthesis procedure based on anhydrous rare-earth acetates. All particles consist of a core of NaYF4 :Yb,Er, doped with 18 % Yb3+ and 2 % Er3+ , and an inert shell of NaYF4 , with the shell thickness being equal to the radius of the core particle. Absolute measurements of the photoLuminescence Quantum Yield at a series of different excitation power densities show that the Quantum Yield of 45 nm core/shell particles is already very close to the Quantum Yield of microcrystalline upconversion phosphor powder. Smaller core/shell particles prepared by the same method show only a moderate decrease in Quantum Yield. The Quantum Yield of 15 nm core/shell particles, for instance, is reduced by a factor of three compared to the bulk upconversion phosphor at high power densities (100 W cm-2 ) and by approximately a factor of 10 at low power densities (1 W cm-2 ).
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liquid phase synthesis of doped nanoparticles colloids of luminescing lapo4 eu and cepo4 tb particles with a narrow particle size distribution
Journal of Physical Chemistry B, 2000Co-Authors: Karsten Riwotzki, Heike Meyssamy, And Andreas Kornowski, Markus HaaseAbstract:Nanocrystals of LaPO4:Eu and CePO4:Tb with a mean particle size of 5 nm and a narrow size distribution have been prepared by reacting the corresponding metal chlorides, phosphoric acid, and a base at 200 °C in tris(ethylhexyl) phosphate. Highly crystalline material was obtained as confirmed by X-ray powder diffraction measurements and high-resolution transmission electron microscopy. Successful doping with europium was evident from the splitting and the intensity pattern of the Luminescence lines. Luminescence lifetime measurements were used to confirm doping and energy transfer in both materials. Colloidal solutions of CePO4:Tb exhibit an overall Luminescence Quantum Yield of 16%.
Jan Valenta - One of the best experts on this subject based on the ideXlab platform.
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boron incorporating silicon nanocrystals embedded in sio 2 absence of free carriers vs b induced defects
Scientific Reports, 2017Co-Authors: Daniel Hiller, S Gutsch, Margit Zacharias, J Lopezvidrier, Michael Wahl, Wolfgang Bock, Alexander Brodyanski, Michael Kopnarski, Keita Nomoto, Jan ValentaAbstract:Boron (B) doping of silicon nanocrystals requires the incorporation of a B-atom on a lattice site of the Quantum dot and its ionization at room temperature. In case of successful B-doping the majority carriers (holes) should quench the photoLuminescence of Si nanocrystals via non-radiative Auger recombination. In addition, the holes should allow for a non-transient electrical current. However, on the bottom end of the nanoscale, both substitutional incorporation and ionization are subject to significant increase in their respective energies due to confinement and size effects. Nevertheless, successful B-doping of Si nanocrystals was reported for certain structural conditions. Here, we investigate B-doping for small, well-dispersed Si nanocrystals with low and moderate B-concentrations. While small amounts of B-atoms are incorporated into these nanocrystals, they hardly affect their optical or electrical properties. If the B-concentration exceeds ~1 at%, the Luminescence Quantum Yield is significantly quenched, whereas electrical measurements do not reveal free carriers. This observation suggests a photoLuminescence quenching mechanism based on B-induced defect states. By means of density functional theory calculations, we prove that B creates multiple states in the bandgap of Si and SiO2. We conclude that non-percolated ultra-small Si nanocrystals cannot be efficiently B-doped.
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effects of inter nanocrystal distance on Luminescence Quantum Yield in ensembles of si nanocrystals
Applied Physics Letters, 2014Co-Authors: Jan Valenta, Michael Greben, S Gutsch, Daniel Hiller, Margit ZachariasAbstract:The absolute photoLuminescence (PL) Quantum Yield (QY) of multilayers of Silicon nanocrystals (SiNCs) separated by SiO2 barriers were thoroughly studied as function of the barrier thickness, excitation wavelength, and temperature. By mastering the plasma-enhanced chemical vapor deposition growth, we produce a series of samples with the same size-distribution of SiNCs but variable interlayer barrier distance. These samples enable us to clearly demonstrate that the increase of barrier thickness from ∼1 to larger than 2 nm induces doubling of the PL QY value, which corresponds to the change of number of close neighbors in the hcp structure. The temperature dependence of PL QY suggests that the PL QY changes are due to a thermally activated transport of excitation into non-radiative centers in dark NCs or in the matrix. We estimate that dark NCs represent about 68% of the ensemble of NCs. The PL QY excitation spectra show no significant changes upon changing the barrier thickness and no clear carrier multipli...
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step like enhancement of Luminescence Quantum Yield of silicon nanocrystals
Nature Nanotechnology, 2011Co-Authors: Dolf Timmerman, Jan Valenta, K Dohnalova, W A De Boer, T GregorkiewiczAbstract:Carrier multiplication by generation of two or more electron-hole pairs following the absorption of a single photon may lead to improved photovoltaic efficiencies and has been observed in nanocrystals made from a variety of semiconductors, including silicon. However, with few exceptions, these reports have been based on indirect ultrafast techniques. Here, we present evidence of carrier multiplication in closely spaced silicon nanocrystals contained in a silicon dioxide matrix by measuring enhanced photoLuminescence Quantum Yield. As the photon energy increases, the Quantum Yield is expected to remain constant, or to decrease as a result of new trapping and recombination channels being activated. Instead, we observe a step-like increase in Quantum Yield for larger photon energies that is characteristic of carrier multiplication. Modelling suggests that carrier multiplication is occurring with high efficiency and close to the energy conservation limit.
Miki Hasegawa - One of the best experts on this subject based on the ideXlab platform.
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computational study on the Luminescence Quantum Yields of terbium complexes with 2 2 bipyridine derivative ligands
Physical Chemistry Chemical Physics, 2018Co-Authors: Miho Hatanaka, Ayato Osawa, Tomonari Wakabayashi, Keiji Morokuma, Miki HasegawaAbstract:Terbium complexes are widely used as luminescent materials because of their bright green emission and sharp emission spectra and the independence of their emission wavelengths from the surrounding environment. The Luminescence Quantum Yield (LQY), however, heavily depends on the surroundings, and an appropriate ligand design is indispensable. In this study, we focus on a Tb3+ complex coordinated by a 2,2′-bipyridine derivative ligand (L1), whose LQY is almost zero at room temperature [M. Hasegawa et al., New. J. Chem. 2014, 38, 1225] and compare it with a Tb3+ complex with a bipyridine ligand, which is widely used as a photo-antenna ligand. To discuss the LQYs of the complexes, we computed their energy profiles, i.e. the energetic and structural changes during the emission and quenching processes. The low LQY of the TbL1(NO3)2 complex was explained by the stability of the minimum energy crossing point between the potential energy surfaces of the ligand-centered lowest triplet state and the ground state, which was induced by the out-of-plane bending of the azomethine moiety. The most efficient way to improve the LQY by modification of the ligand is to replace the azomethine moieties by other functional groups, such as ether or reduced azomethine groups, whose minimum energy crossing points are unstable enough to reduce the rate of the quenching processes.
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Luminescence behaviour in acetonitrile and in the solid state of a series of lanthanide complexes with a single helical ligand
New Journal of Chemistry, 2014Co-Authors: Miki Hasegawa, Hideki Ohtsu, Daisuke Kodama, Takeshi Kasai, Shoya Sakurai, Ayumi Ishii, Kengo SuzukiAbstract:Luminescence mechanisms of EuIII, TbIII, GdIII and NdIII complexes with a hexadentate ligand (abbreviated to EuL, TbL, GdL, and NdL, respectively), which have two bipyridine moieties bridged by an ethylenediamine unit, have been examined. Our molecular design is that each complex forms a single helical polar structure based on the chelate ring to retain solubility in solutions. EuL and NdL show comparably bright emission from ff transitions both in acetonitrile solution and in the solid state. To understand the mechanism of the emission in detail, the energy level of the triplet (T) state of the ligand L has been estimated based on the phosphorescence measurements of GdL, because GdIII shows no ff emission. The donor level of the T state of L and the acceptor level of EuIII or NdIII can overlap, indicating that the excited photon localized on L has been used for the efficient ff emission, while not for ππ* emission. For TbL, the Luminescence Quantum Yield is significantly dependent on temperature and the state: in the solid state of TbL, the Quantum Yield of ff emission is over 90% at 77 K, while no Luminescence is observed at room temperature, and in solution TbL shows no emission. This observation suggests that the emissive f-level of TbIII and the energy donor level of the excited T state of L are in thermal equilibrium. The described lanthanide complexes are stable and retain their molecular structure even in solutions and show characteristic Luminescence behaviour based on the energy relaxation process of each lanthanide ion. Furthermore the HoIII complex with L (HoL) has been prepared and its structure has been analyzed. HoL has a twisted arrangement of the bipyridine moiety surrounding HoIII due to the small ionic radius of HoIII.
T Gregorkiewicz - One of the best experts on this subject based on the ideXlab platform.
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enhancement of Luminescence Quantum Yield of 1 5 µm emission from er doped sio 2 sensitized with si nanocrystals
MRS Proceedings, 2014Co-Authors: S Saeed, T GregorkiewiczAbstract:Excitation of multiple Er3+ ions upon absorption of a single high-energy photon increases Er-related emission at 1.5 μm, and therefore enhances UV/visible-to-IR photon conversion efficiency. Here we investigate this effect for layers of Er-doped SiO2 sensitized with silicon nanocrystals by measuring the Quantum Yield of 1.5 µm Er-related emission. We demonstrate dramatic increase of the emission commencing for excitation energies above a certain threshold value, as the number of Er3+ ions excited upon absorption of a single incoming photon increases. By comparing differently prepared materials, we show that the actual value of this threshold energy and the rate of the observed increase of the Quantum Yield depend on sample characteristics – the size of Si nanocrystals and the ratio of Er3+ ions and nanocrystals concentrations.
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step like enhancement of Luminescence Quantum Yield of silicon nanocrystals
Nature Nanotechnology, 2011Co-Authors: Dolf Timmerman, Jan Valenta, K Dohnalova, W A De Boer, T GregorkiewiczAbstract:Carrier multiplication by generation of two or more electron-hole pairs following the absorption of a single photon may lead to improved photovoltaic efficiencies and has been observed in nanocrystals made from a variety of semiconductors, including silicon. However, with few exceptions, these reports have been based on indirect ultrafast techniques. Here, we present evidence of carrier multiplication in closely spaced silicon nanocrystals contained in a silicon dioxide matrix by measuring enhanced photoLuminescence Quantum Yield. As the photon energy increases, the Quantum Yield is expected to remain constant, or to decrease as a result of new trapping and recombination channels being activated. Instead, we observe a step-like increase in Quantum Yield for larger photon energies that is characteristic of carrier multiplication. Modelling suggests that carrier multiplication is occurring with high efficiency and close to the energy conservation limit.
Yang Zhou - One of the best experts on this subject based on the ideXlab platform.
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doping enhanced short range order of perovskite nanocrystals for near unity violet Luminescence Quantum Yield
Journal of the American Chemical Society, 2018Co-Authors: Zijun Yong, Shaoqiang Guo, Junying Zhang, Yameng Chen, Binbin Zhang, Yang Zhou, Jie Shu, Lirong Zheng, Osman M BakrAbstract:All-inorganic perovskite nanocrystals (NCs) have emerged as a new generation of low-cost semiconducting luminescent system for optoelectronic applications. The room-temperature photoLuminescence Quantum Yields (PLQYs) of these NCs in the green and red spectral range approach unity. However, their PLQYs in the violet are much lower, and an insightful understanding of such poor performance remains missing. We report a general strategy for the synthesis of all-inorganic violet-emitting perovskite NCs with near-unity PLQYs through engineering local order of the lattice by nickel ion doping. A broad range of experimental characterizations, including steady-state and time-resolved Luminescence spectroscopy, X-ray absorption spectra, and magic angle spinning nuclear magnetic resonance spectra, reveal that the low PLQY in undoped NCs is associated with short-range disorder of the lattice induced by intrinsic defects such as halide vacancies and that Ni doping can substantially eliminate these defects and result i...
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cs4pbbr6 cspbbr3 perovskite composites with near unity Luminescence Quantum Yield large scale synthesis Luminescence and formation mechanism and white light emitting diode application
ACS Applied Materials & Interfaces, 2018Co-Authors: Yameng Chen, Zijun Yong, Shaoqiang Guo, Junying Zhang, Yang Zhou, Qing Zhao, Tongtong Xuan, Jing Wang, Yoshihiro Kuroiwa, Chikako MoriyoshiAbstract:All-inorganic perovskites have emerged as a new class of phosphor materials owing to their outstanding optical properties. Zero-dimensional inorganic perovskites, in particular the Cs4PbBr6-related systems, are inspiring intensive research owing to the high photoLuminescence Quantum Yield (PLQY) and good stability. However, synthesizing such perovskites with high PLQYs through an environment-friendly, cost-effective, scalable, and high-Yield approach remains challenging, and their Luminescence mechanisms has been elusive. Here, we report a simple, scalable, room-temperature self-assembly strategy for the synthesis of Cs4PbBr6/CsPbBr3 perovskite composites with near-unity PLQY (95%), high product Yield (71%), and good stability using low-cost, low-toxicity chemicals as precursors. A broad range of experimental and theoretical characterizations suggest that the high-efficiency PL originates from CsPbBr3 nanocrystals well passivated by the zero-dimensional Cs4PbBr6 matrix that forms based on a dissolution–cr...
