The Experts below are selected from a list of 26940 Experts worldwide ranked by ideXlab platform
A P Alivisatos - One of the best experts on this subject based on the ideXlab platform.
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Molecular Weight, Osmotic Second Virial Coefficient, and Extinction Coefficient of Colloidal CdSe Nanocrystals
2014Co-Authors: Alberto Striolo, J Ward, John M Prausnitz, Wolfgang J Parak, Daniela Zanchet, D. Gerion, D. Milliron, A P AlivisatosAbstract:Membrane osmometry is used to measure osmotic pressures of dilute solutions containing quasispherical CdSe Nanocrystals covered with polymer brushes in toluene in the range 31-45 °C. Osmotic-pressure data, as a function of Nanocrystal concentration, yield the molecular weight and the osmotic second virial coefficient of the Nanocrystals; the latter is related to the potential of mean force between two Nanocrystal particles in dilute solution. Coupled with molecular-weight data, extinction coefficients and oscillator strengths are also obtained for Nanocrystals of various sizes in toluene. CdSe Nanocrystal sizes were obtained either from transmission electron microscopy or from correlations between the wavelength of the absorbing peak and Nanocrystal size. Osmotic-pressure data are reduced with a simple perturbed-hard-sphere equation of state; the perturbation is due to long-range (London dispersion) attraction and a short-range interaction potential. The only adjustable parameter, the strength of this short-range potential, shows two-body repulsion or attraction, depending on the sample and on solution conditions
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improved efficiencies in light emitting diodes made with cdse cds core shell type Nanocrystals and a semiconducting polymer
Journal of Applied Physics, 1997Co-Authors: M C Schlamp, Xiaogang Peng, A P AlivisatosAbstract:We report experiments on bilayer light emitting diodes made with organically capped CdSe(CdS) core/shell type semiconductor Nanocrystals and an electroluminescent (EL) semiconducting polymer [poly(p-phenylenevinylene) or PPV]. The devices emit from red to green with external quantum efficiencies of up to 0.22% at brightnesses of 600 cd/m2 and current densities of 1 A/cm2. They have operating voltages as low as 4 V and lifetimes under constant current flow of hundreds of hours. Most of these numbers are significant improvements over similar devices made with CdSe Nanocrystals. The devices show either Nanocrystal-only EL or a combination of Nanocrystal and PPV EL, depending on Nanocrystal layer thickness. The Nanocrystal EL is dependent on Nanocrystal size. Some devices show a voltage dependent spectral output. The spectral output is consistent with a field dependent electron range in the Nanocrystal layer limited by carrier trapping.
Daniela Zanchet - One of the best experts on this subject based on the ideXlab platform.
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Molecular Weight, Osmotic Second Virial Coefficient, and Extinction Coefficient of Colloidal CdSe Nanocrystals
2014Co-Authors: Alberto Striolo, J Ward, John M Prausnitz, Wolfgang J Parak, Daniela Zanchet, D. Gerion, D. Milliron, A P AlivisatosAbstract:Membrane osmometry is used to measure osmotic pressures of dilute solutions containing quasispherical CdSe Nanocrystals covered with polymer brushes in toluene in the range 31-45 °C. Osmotic-pressure data, as a function of Nanocrystal concentration, yield the molecular weight and the osmotic second virial coefficient of the Nanocrystals; the latter is related to the potential of mean force between two Nanocrystal particles in dilute solution. Coupled with molecular-weight data, extinction coefficients and oscillator strengths are also obtained for Nanocrystals of various sizes in toluene. CdSe Nanocrystal sizes were obtained either from transmission electron microscopy or from correlations between the wavelength of the absorbing peak and Nanocrystal size. Osmotic-pressure data are reduced with a simple perturbed-hard-sphere equation of state; the perturbation is due to long-range (London dispersion) attraction and a short-range interaction potential. The only adjustable parameter, the strength of this short-range potential, shows two-body repulsion or attraction, depending on the sample and on solution conditions
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molecular weight osmotic second virial coefficient and extinction coefficient of colloidal cdse Nanocrystals
Journal of Physical Chemistry B, 2002Co-Authors: Alberto Striolo, J Ward, John M Prausnitz, Wolfgang J Parak, Daniela ZanchetAbstract:Membrane osmometry is used to measure osmotic pressures of dilute solutions containing quasispherical CdSe Nanocrystals covered with polymer brushes in toluene in the range 31−45 °C. Osmotic-pressure data, as a function of Nanocrystal concentration, yield the molecular weight and the osmotic second virial coefficient of the Nanocrystals; the latter is related to the potential of mean force between two Nanocrystal particles in dilute solution. Coupled with molecular-weight data, extinction coefficients and oscillator strengths are also obtained for Nanocrystals of various sizes in toluene. CdSe Nanocrystal sizes were obtained either from transmission electron microscopy or from correlations between the wavelength of the absorbing peak and Nanocrystal size. Osmotic-pressure data are reduced with a simple perturbed-hard-sphere equation of state; the perturbation is due to long-range (London dispersion) attraction and a short-range interaction potential. The only adjustable parameter, the strength of this sho...
L E Brus - One of the best experts on this subject based on the ideXlab platform.
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surface states in the photoionization of high quality cdse core shell Nanocrystals
ACS Nano, 2009Co-Authors: Michael L Steigerwald, L E BrusAbstract:We use electric force microscopy (EFM) to study single Nanocrystal photoionization in two classes of high-quality Nanocrystals whose exciton luminescence quantum yields approach unity in solution. The CdSe/CdS/ZnS core/shell Nanocrystals do not photoionize, while the CdSe/CdS Nanocrystals do show substantial photoionization. This verifies the theoretical prediction that the ZnS shell confines the excited electron within the Nanocrystal. Despite the high luminescence quantum yield, photoionization varies substantially among the CdSe/CdS Nanocrystals. We have studied the Nanocrystal photoionization with both UV (396 nm) and green (532 nm) light, and we have found that the magnitude of the charge due to photoionization per absorbed photon is greater for UV excitation than for green excitation. A fraction of the photoionization occurs directly via a “hot electron” process, using trap states that are either on the particle surface, within the ligand sphere, or within the silicon oxide layer. This must occur wi...
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imaging the photoionization of individual cdse cds core shell Nanocrystals on n and p type silicon substrates with thin oxides
Journal of Physical Chemistry B, 2004Co-Authors: Oksana Cherniavskaya, Liwei Chen, L E BrusAbstract:The low-intensity photoionization of individual semiconductor Nanocrystals, at 23 °C in dry nitrogen, is time-resolved over many hours for both S (532-nm excitation) and P (395-nm excitation) Nanocrystal excited states using electrostatic force microscopy. Over 7000 calibrated charge measurements have been made on 14- and 21-A-thick oxide layers. Photoexcited electrons tunnel across the oxide into the silicon, and multiple charges can build up on individual Nanocrystals at intensities of only 0.1−0.01 W/cm2. The silicon dopant type influences the net Nanocrystal charging via the interfacial band bending; P-type subtrates show a faster Nanocrystal reneutralization rate due to their higher interfacial electron concentration. There is a huge range of photoionzation behavior for individual Nanocrystals. This behavior is different for 395- and 532-nm excitation in the same Nanocrystal. This individuality seems in part to reflect tunneling through spatially localized defect states in the oxide. The line widths ...
Haoran Yang - One of the best experts on this subject based on the ideXlab platform.
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the effects of the size and the doping concentration on the power factor of n type lead telluride Nanocrystals for thermoelectric energy conversion
Nano Letters, 2014Co-Authors: Haiyu Fang, Zhiqiang Luo, Haoran YangAbstract:For the first time, we demonstrate a successful synthesis of colloidal n-type lead telluride Nanocrystals doped with iodine. By tuning the reaction time and iodine concentration in the precursor solution, Nanocrystals with different sizes and doping concentrations are synthesized. The Seebeck coefficient and electrical conductivity of the Nanocrystals are measured on Nanocrystal thin films fabricated by dip-coating glass substrates in the Nanocrystals solution. Investigations on the influence of size and doping concentration on the electrical properties have been performed. The results show that the size of the Nanocrystals significantly influences the electrical conductivity but not the Seebeck coefficient of Nanocrystal films, while higher doping concentration leads to lower Seebeck coefficient but higher electrical conductivity in the Nanocrystal films. Proof-of-concept thin-film thermoelectric modules are also fabricated using both p-type and n-type PbTe Nanocrystals for the conversion of thermal ener...
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The Effects of the Size and the Doping Concentration on the Power Factor of n‑type Lead Telluride Nanocrystals for Thermoelectric Energy Conversion
2014Co-Authors: Haiyu Fang, Zhiqiang Luo, Haoran YangAbstract:For the first time, we demonstrate a successful synthesis of colloidal n-type lead telluride Nanocrystals doped with iodine. By tuning the reaction time and iodine concentration in the precursor solution, Nanocrystals with different sizes and doping concentrations are synthesized. The Seebeck coefficient and electrical conductivity of the Nanocrystals are measured on Nanocrystal thin films fabricated by dip-coating glass substrates in the Nanocrystals solution. Investigations on the influence of size and doping concentration on the electrical properties have been performed. The results show that the size of the Nanocrystals significantly influences the electrical conductivity but not the Seebeck coefficient of Nanocrystal films, while higher doping concentration leads to lower Seebeck coefficient but higher electrical conductivity in the Nanocrystal films. Proof-of-concept thin-film thermoelectric modules are also fabricated using both p-type and n-type PbTe Nanocrystals for the conversion of thermal energy into electrical energy
Xiaogang Peng - One of the best experts on this subject based on the ideXlab platform.
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size dependent dissociation ph of thiolate ligands from cadmium chalcogenide Nanocrystals
Journal of the American Chemical Society, 2005Co-Authors: Jose Aldana, Natasha Lavelle, Yunjun Wang, Xiaogang PengAbstract:A method, pseudo steady-state titration, is introduced for determining the precipitation pH of Nanocrystals coated by electron-donating ligands. CdSe Nanocrystals coated with hydrophilic deprotonated thiol (thiolate) ligands were studied systematically. For comparison, CdTe and CdS Nanocrystals coated with the same types of ligands were also examined. The results show that the precipitation of the Nanocrystals is caused by the dissociation of the Nanocrystal−ligand coordinating bonds from the Nanocrystal surface. The ligands are removed from the surface due to protonation in a relatively low pH range, between 2 and 7 depending on the size, approximately within the quantum confinement size regime, and chemical composition (band gap) of the Nanocrystals. In contrast, the redispersion of the Nanocrystals was found to be solely determined by the deprotonation of the ligands. The size-dependent dissociation pH of the ligands was tentatively used as a means for determining the size-dependent free energy associa...
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photochemical instability of cdse Nanocrystals coated by hydrophilic thiols
Journal of the American Chemical Society, 2001Co-Authors: Jose Aldana, And Andrew Y Wang, Xiaogang PengAbstract:The photochemical instability of CdSe Nanocrystals coated by hydrophilic thiols was studied nondestructively and systematically in water. The results revealed that the photochemical instability of the Nanocrystals actually included three distinguishable processes, namely the photocatalytic oxidation of the thiol ligands on the surface of Nanocrystals, the photooxidation of the Nanocrystals, and the precipitation of the Nanocrystals. At first, the thiol ligands on the surface of a Nanocrystal were gradually photocatalytically oxidized using the CdSe Nanocrystal core as the photocatalyst. This photocatalytic oxidation process was observed as a zero-order reaction in terms of the concentration of the free thiols in the solution. The photogenerated holes in a Nanocrystal were trapped onto the thiol ligands bound on the surface of the Nanocrystal, which initiated the photooxidation of the ligands and protected the Nanocrystal from any photooxidation. After nearly all of the thiol ligands on the surface of the ...
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improved efficiencies in light emitting diodes made with cdse cds core shell type Nanocrystals and a semiconducting polymer
Journal of Applied Physics, 1997Co-Authors: M C Schlamp, Xiaogang Peng, A P AlivisatosAbstract:We report experiments on bilayer light emitting diodes made with organically capped CdSe(CdS) core/shell type semiconductor Nanocrystals and an electroluminescent (EL) semiconducting polymer [poly(p-phenylenevinylene) or PPV]. The devices emit from red to green with external quantum efficiencies of up to 0.22% at brightnesses of 600 cd/m2 and current densities of 1 A/cm2. They have operating voltages as low as 4 V and lifetimes under constant current flow of hundreds of hours. Most of these numbers are significant improvements over similar devices made with CdSe Nanocrystals. The devices show either Nanocrystal-only EL or a combination of Nanocrystal and PPV EL, depending on Nanocrystal layer thickness. The Nanocrystal EL is dependent on Nanocrystal size. Some devices show a voltage dependent spectral output. The spectral output is consistent with a field dependent electron range in the Nanocrystal layer limited by carrier trapping.
