The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Paul A Alivisatos - One of the best experts on this subject based on the ideXlab platform.
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mechanistic study of precursor evolution in colloidal group ii vi Semiconductor nanocrystal synthesis
Journal of the American Chemical Society, 2007Co-Authors: Haitao Liu, And Jonathan S Owen, Paul A AlivisatosAbstract:The molecular mechanism of precursor evolution in the synthesis of colloidal group II−VI Semiconductor nanocrystals was studied using 1H, 13C, and 31P NMR spectroscopy and mass spectrometry. Tri-n-butylphosphine chalcogenides (TBPE; E = S, Se, Te) react with an oleic acid complex of cadmium or zinc (M−OA; M = Zn, Cd) in a noncoordinating solvent (octadecene (ODE), n-nonane-d20, or n-decane-d22), affording ME nanocrystals, tri-n-butylphosphine oxide (TBPO), and oleic acid anhydride ((OA)2O). Likewise, the reaction between trialkylphosphine selenide and cadmium n-octadecylphosphonic acid complex (Cd−ODPA) in tri-n-octylphosphine oxide (TOPO) produces CdSe nanocrystals, trialkylphosphine oxide, and anhydrides of n-octadecylphosphonic acid. The disappearance of tri-n-octylphosphine selenide in the presence of Cd−OA and Cd−ODPA can be fit to a single-exponential decay (kobs = (1.30 ± 0.08) × 10-3 s-1, Cd−ODPA, 260 °C, and kobs = (1.51 ± 0.04) × 10-3 s-1, Cd−OA, 117 °C). The reaction approaches completion at 70...
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mechanistic study of precursor evolution in colloidal group ii vi Semiconductor nanocrystal synthesis
Journal of the American Chemical Society, 2007Co-Authors: Haitao Liu, And Jonathan S Owen, Paul A AlivisatosAbstract:The molecular mechanism of precursor evolution in the synthesis of colloidal group II−VI Semiconductor nanocrystals was studied using 1H, 13C, and 31P NMR spectroscopy and mass spectrometry. Tri-n-butylphosphine chalcogenides (TBPE; E = S, Se, Te) react with an oleic acid complex of cadmium or zinc (M−OA; M = Zn, Cd) in a noncoordinating solvent (octadecene (ODE), n-nonane-d20, or n-decane-d22), affording ME nanocrystals, tri-n-butylphosphine oxide (TBPO), and oleic acid anhydride ((OA)2O). Likewise, the reaction between trialkylphosphine selenide and cadmium n-octadecylphosphonic acid complex (Cd−ODPA) in tri-n-octylphosphine oxide (TOPO) produces CdSe nanocrystals, trialkylphosphine oxide, and anhydrides of n-octadecylphosphonic acid. The disappearance of tri-n-octylphosphine selenide in the presence of Cd−OA and Cd−ODPA can be fit to a single-exponential decay (kobs = (1.30 ± 0.08) × 10-3 s-1, Cd−ODPA, 260 °C, and kobs = (1.51 ± 0.04) × 10-3 s-1, Cd−OA, 117 °C). The reaction approaches completion at 70...
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process for forming shaped group ii vi Semiconductor nanocrystals and product formed using process
2000Co-Authors: Paul A Alivisatos, Liberato MannaAbstract:A process for the formation of shaped Group II-VI Semiconductor nanocrystals comprises contacting the Semiconductor nanocrystal precursors with a liquid media comprising a binary mixture of phosphorus-containing organic surfactants capable of promoting the growth of either spherical Semiconductor nanocrystals or rod-like Semiconductor nanocrystals, whereby the shape of the Semiconductor nanocrystals formed in said binary mixture of surfactants is controlled by adjusting the ratio of the surfactants in the binary mixture.
Jin Z Zhang - One of the best experts on this subject based on the ideXlab platform.
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nature of the power dependent ultrafast relaxation process of photoexcited charge carriers in ii vi Semiconductor quantum dots effects of particle size surface and electronic structure
Journal of Chemical Physics, 1998Co-Authors: Trevor W Roberti, Nerine J Cherepy, Jin Z ZhangAbstract:The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI Semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the p...
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nature of the power dependent ultrafast relaxation process of photoexcited charge carriers in ii vi Semiconductor quantum dots effects of particle size surface and electronic structure
Journal of Chemical Physics, 1998Co-Authors: Trevor W Roberti, Nerine J Cherepy, Jin Z ZhangAbstract:The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI Semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the power-dependent decay is only weakly dependent on surface, size, and electronic structure. With the assistance of a power-dependent nanosecond fluorescence study, we have assigned the power-dependent decay primarily to exciton–exciton annihilation. This decay mechanism becomes dominant under high intensity excitation where multiple photoexcited charge carriers are created in each quantum dot, leading to trap state saturation and an accumulation of band edge excitons. Auger recombination may also play an important role at very high excitation intensities.The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI Semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the p...
A K Geim - One of the best experts on this subject based on the ideXlab platform.
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indirect excitons in van der waals heterostructures at room temperature
Nature Communications, 2018Co-Authors: E V Calman, M M Fogler, L V Butov, Artem Mishchenko, A K GeimAbstract:Indirect excitons (IXs) are explored both for studying quantum Bose gases in Semiconductor materials and for the development of excitonic devices. IXs were extensively studied in III-V and II-VI Semiconductor heterostructures where IX range of existence has been limited to low temperatures. Here, we present the observation of IXs at room temperature in van der Waals transition metal dichalcogenide (TMD) heterostructures. This is achieved in TMD heterostructures based on monolayers of MoS2 separated by atomically thin hexagonal boron nitride. The IXs we realize in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD and their energy is gate controlled. The realization of IXs at room temperature establishes the TMD heterostructures as a material platform both for a field of high-temperature quantum Bose gases of IXs and for a field of high-temperature excitonic devices.
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indirect excitons in van der waals heterostructures at room temperature
Nature Communications, 2018Co-Authors: E V Calman, M M Fogler, L V Butov, Artem Mishchenko, S Hu, A K GeimAbstract:Indirect excitons (IXs) are explored both for studying quantum Bose gases in Semiconductor materials and for the development of excitonic devices. IXs were extensively studied in III–V and II–VI Semiconductor heterostructures where IX range of existence has been limited to low temperatures. Here, we present the observation of IXs at room temperature in van der Waals transition metal dichalcogenide (TMD) heterostructures. This is achieved in TMD heterostructures based on monolayers of MoS2 separated by atomically thin hexagonal boron nitride. The IXs we realize in the TMD heterostructure have lifetimes orders of magnitude longer than lifetimes of direct excitons in single-layer TMD and their energy is gate controlled. The realization of IXs at room temperature establishes the TMD heterostructures as a material platform both for a field of high-temperature quantum Bose gases of IXs and for a field of high-temperature excitonic devices. Indirect excitons, composed of a spatially separated electron and hole, could find applications in excitonic devices for signal processing and communication, however they are normally detected at low temperatures. Here, the authors observe room-temperature indirect excitons in van der Waals transition metal dichalcogenide heterostructures.
Hilmi Volkan Demir - One of the best experts on this subject based on the ideXlab platform.
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sub single exciton optical gain threshold in colloidal Semiconductor quantum wells with gradient alloy shelling
Nature Communications, 2020Co-Authors: Nima Taghipour, Tze Chien Sum, Savas Delikanli, Sushant Shendre, Mustafa Sak, Furkan Isik, Ibrahim Tanriover, Burak Guzelturk, Hilmi Volkan DemirAbstract:Colloidal Semiconductor quantum wells have emerged as a promising material platform for use in solution-processable lasers. However, applications relying on their optical gain suffer from nonradiative Auger decay due to multi-excitonic nature of light amplification in II-VI Semiconductor nanocrystals. Here, we show sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient-alloyed shell quantum wells. This sub-single exciton ensemble-averaged gain threshold of (Ng)≈ 0.84 (per particle) resulting from impeded Auger recombination, along with a large absorption cross-section of quantum wells, enables us to observe the amplified spontaneous emission starting at an ultralow pump fluence of ~ 800 nJ cm−2, at least three-folds better than previously reported values among all colloidal nanocrystals. Finally, using these gradient shelled quantum wells, we demonstrate a vertical cavity surface-emitting laser operating at a low lasing threshold of 7.5 μJ cm−2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers. Colloidal quantum wells are highly promising for applications of solution-processed lasers, but their performance is limited by multi-excitonic nature of the materials. Here, the authors demonstrate optical gain in graded alloy core/shell CdSe/CdS@CdZnS quantum wells at less than one exciton per particle resulting in ultralow thresholds.
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sub single exciton optical gain threshold in colloidal Semiconductor quantum wells with gradient alloy shelling
arXiv: Applied Physics, 2019Co-Authors: Nima Taghipour, Tze Chien Sum, Savas Delikanli, Sushant Shendre, Mustafa Sak, Furkan Isik, Ibrahim Tanriover, Burak Guzelturk, Hilmi Volkan DemirAbstract:Colloidal Semiconductor quantum wells have emerged as a promising material platform for use in solution-processable light-generation including colloidal lasers. However, application relying on their optical gain suffer from a fundamental complication due to multi-excitonic nature of light amplification in common II-VI Semiconductor nanocrystals. This undesirably increases the optical gain threshold and shortens the net gain lifetime because of fast nonradiative Auger decay. Here, we demonstrate sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient alloyed shell colloidal quantum wells. This sub-single exciton ensemble-averaged gain threshold of Ng = 0.80 (per particle) resulting from impeded Auger recombination along with a large absorption cross-section of quantum wells enables us to observe the amplified spontaneous emission starting at a low pump fluence of 800 nJ cm-2, at least three-folds better than the previously best reported values among all colloidal Semiconductor nanocrystals. Moreover, long optical gain lifetimes of 800 ps accompanied with modal gain coefficients of 2,000 cm-1 are achieved. Finally, using these gradient shelled quantum wells, we show a vertical cavity surface-emitting colloidal laser operating at an ultralow lasing threshold of 7.5 micro-joule cm-2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers.
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green stimulated emission boosted by nonradiative resonant energy transfer from blue quantum dots
Journal of Physical Chemistry Letters, 2016Co-Authors: Guannan Yu, Yue Wang, Cuong Dang, Hilmi Volkan DemirAbstract:Thanks to their tunability and versatility, the colloidal quantum dots (CQDs) made of II–VI Semiconductor compound offer the potential to bridge the “green gap” in conventional Semiconductors. However, when the CQDs are pumped to much higher initial excitonic states compared to their bandgap, multiexciton interaction is enhanced, leading to a much higher stimulated emission threshold. Here, to circumvent this drawback, for the first time, we show a fully colloidal gain in green enabled by a partially indirect pumping approach assisted by Forster resonance energy transfer process. By introducing the blue CQDs as exciton donors, the lasing threshold of the green CQDs, is reduced dramatically. The blue CQDs thus serve as an energy-transferring buffer medium to reduce excitation energy from pumping photons in a controlled way by injecting photoinduced excitons into green CQDs. Our newly developed colloidal pumping scheme could enable efficient CQD lasers of full visible colors by a single pump source and casc...
Tze Chien Sum - One of the best experts on this subject based on the ideXlab platform.
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sub single exciton optical gain threshold in colloidal Semiconductor quantum wells with gradient alloy shelling
Nature Communications, 2020Co-Authors: Nima Taghipour, Tze Chien Sum, Savas Delikanli, Sushant Shendre, Mustafa Sak, Furkan Isik, Ibrahim Tanriover, Burak Guzelturk, Hilmi Volkan DemirAbstract:Colloidal Semiconductor quantum wells have emerged as a promising material platform for use in solution-processable lasers. However, applications relying on their optical gain suffer from nonradiative Auger decay due to multi-excitonic nature of light amplification in II-VI Semiconductor nanocrystals. Here, we show sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient-alloyed shell quantum wells. This sub-single exciton ensemble-averaged gain threshold of (Ng)≈ 0.84 (per particle) resulting from impeded Auger recombination, along with a large absorption cross-section of quantum wells, enables us to observe the amplified spontaneous emission starting at an ultralow pump fluence of ~ 800 nJ cm−2, at least three-folds better than previously reported values among all colloidal nanocrystals. Finally, using these gradient shelled quantum wells, we demonstrate a vertical cavity surface-emitting laser operating at a low lasing threshold of 7.5 μJ cm−2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers. Colloidal quantum wells are highly promising for applications of solution-processed lasers, but their performance is limited by multi-excitonic nature of the materials. Here, the authors demonstrate optical gain in graded alloy core/shell CdSe/CdS@CdZnS quantum wells at less than one exciton per particle resulting in ultralow thresholds.
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sub single exciton optical gain threshold in colloidal Semiconductor quantum wells with gradient alloy shelling
arXiv: Applied Physics, 2019Co-Authors: Nima Taghipour, Tze Chien Sum, Savas Delikanli, Sushant Shendre, Mustafa Sak, Furkan Isik, Ibrahim Tanriover, Burak Guzelturk, Hilmi Volkan DemirAbstract:Colloidal Semiconductor quantum wells have emerged as a promising material platform for use in solution-processable light-generation including colloidal lasers. However, application relying on their optical gain suffer from a fundamental complication due to multi-excitonic nature of light amplification in common II-VI Semiconductor nanocrystals. This undesirably increases the optical gain threshold and shortens the net gain lifetime because of fast nonradiative Auger decay. Here, we demonstrate sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient alloyed shell colloidal quantum wells. This sub-single exciton ensemble-averaged gain threshold of Ng = 0.80 (per particle) resulting from impeded Auger recombination along with a large absorption cross-section of quantum wells enables us to observe the amplified spontaneous emission starting at a low pump fluence of 800 nJ cm-2, at least three-folds better than the previously best reported values among all colloidal Semiconductor nanocrystals. Moreover, long optical gain lifetimes of 800 ps accompanied with modal gain coefficients of 2,000 cm-1 are achieved. Finally, using these gradient shelled quantum wells, we show a vertical cavity surface-emitting colloidal laser operating at an ultralow lasing threshold of 7.5 micro-joule cm-2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers.
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ultralow threshold two photon pumped amplified spontaneous emission and lasing from seeded cdse cds nanorod heterostructures
ACS Nano, 2012Co-Authors: Guichuan Xing, Yile Liao, Sabyasachi Chakrabortty, Xinfeng Liu, Edwin K L Yeow, Yinthai Chan, Tze Chien SumAbstract:Ultralow-threshold two-photon pumped amplified spontaneous emission (2ASE) and lasing in seeded CdSe/CdS nanodot/nanorod heterostructures is demonstrated for the first time. Such heterostructures allow the independent tunability of the two-photon absorption (2PA) cross-section (σ2) through varying the CdS rod size, and that of the emission wavelength through varying the CdSe dot size. With an enhanced σ2, 2ASE in these heterostructures is achieved with an ultralow threshold fluence of ∼1.5 mJ/cm2, which is as much as one order less than that required for spherical Semiconductor NCs. Importantly, by exploiting this unique property of the seeded nanorods exhibiting strong quantum confinement even at relatively large rod sizes, a near reciprocal relation between the 2ASE threshold and the 2PA action cross-section (σ2η) (where η is the quantum yield) was found and validated over a wide volume range for II–VI Semiconductor nanostructures. Ultrafast optical spectroscopy verified that while the Auger processes i...
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dynamics of bound exciton complexes in cds nanobelts
ACS Nano, 2011Co-Authors: Yanyuan Zhao, Edbert J Sie, Bo Liu, Sandy Adhitia Ekahana, Qike Jiang, Jianbo Wang, Handong Sun, Tze Chien Sum, C H A Huan, Yuan Ping FengAbstract:Intrinsic defects such as vacancies, interstitials, and anti-sites often introduce rich luminescent properties in II−VI Semiconductor nanomaterials. A clear understanding of the dynamics of the defect-related excitons is particularly important for the design and optimization of nanoscale optoelectronic devices. In this paper, low-temperature steady-state and time-resolved photoluminescence (PL) spectroscopies have been carried out to investigate the emission of cadmium sulfide (CdS) nanobelts that originates from the radiative recombination of excitons bound to neutral donors (I2) and the spatially localized donor−acceptor pairs (DAP), in which the assignment is supported by first principle calculations. Our results verify that the shallow donors in CdS are contributed by sulfur vacancies while the acceptors are contributed by cadmium vacancies. At high excitation intensities, the DAP emission saturates and the PL is dominated by I2 emission. Beyond a threshold power of approximately 5 μW, amplified spont...
