The Experts below are selected from a list of 16143 Experts worldwide ranked by ideXlab platform
Younan Xia - One of the best experts on this subject based on the ideXlab platform.
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Colloidal Metal Nanocrystals with Metastable Crystal Structures.
Angewandte Chemie (International ed. in English), 2021Co-Authors: Annemieke Janssen, Quynh N Nguyen, Younan XiaAbstract:In addition to the conventional knobs such as composition, size, shape, and defect structure, the crystal structure (or phase) of Metal nanocrystals offers a new avenue for engineering their properties. Various strategies have recently been developed for the fabrication of Colloidal Metal nanocrystals in metastable phases different from their bulk counterparts. With a focus on noble Metals, we begin with a brief introduction to their atomic packing, followed by a discussion about five major synthetic approaches to their Colloidal nanocrystals in unconventional phases. We then highlight the success of synthesis in terms of mechanistic insights and experimental controls, as well as the enhanced catalytic properties. We end this Minireview with perspectives on the remaining issues and future opportunities.
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Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants.
Chemistry (Weinheim an der Bergstrasse Germany), 2018Co-Authors: Thenner S. Rodrigues, Kyle D. Gilroy, Tung-han Yang, Ming Zhao, Anderson G. M. Da Silva, Pedro H. C. Camargo, Younan XiaAbstract:There is a growing interest in controlling the synthesis of Colloidal Metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of Metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of Metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the Colloidal synthesis of Metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of Metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of Metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.
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Autocatalytic surface reduction and its role in controlling seed-mediated growth of Colloidal Metal nanocrystals
Proceedings of the National Academy of Sciences of the United States of America, 2017Co-Authors: Tung-han Yang, Kyle D. Gilroy, Shan Zhou, Legna Figueroa-cosme, Yi-hsien Lee, Younan XiaAbstract:The growth of Colloidal Metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of Colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of Colloidal Metal nanocrystals.
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Reduction rate as a quantitative knob for achieving deterministic synthesis of Colloidal Metal nanocrystals.
Chemical science, 2017Co-Authors: Tung-han Yang, Kyle D. Gilroy, Younan XiaAbstract:Despite the incredible developments made to the synthesis of Colloidal Metal nanocrystals, it is still challenging to produce them in a reproducible and predictable manner. This drawback can be attributed to the fact that the protocols continue to be built upon qualitative observations and empirical laws. Because of the vast number of intricately entangled experimental parameters in a synthesis, it is almost impossible to predict and control the outcome by knowingly alternating these parameters. In this Perspective article, we discuss the recent efforts in pushing nanocrystal synthesis towards a deterministic process based upon quantitative measurements. In particular, we focus on how the reduction rate of a salt precursor can be used as a quantitative knob for predicting and controlling the outcomes of both nucleation and growth. We begin with a brief introduction to the techniques that have been used to extract the kinetic information of a synthesis and then discuss how the reduction rate is correlated with the defect structure, shape/morphology, and elemental distribution of the resultant nanocrystals. We conclude by highlighting some of the recent advances related to in situ probing of nanocrystal synthesis, with an emphasis on the real-time, quantitative aspects with regard to both nucleation and growth.
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Seed-Mediated Growth of Colloidal Metal Nanocrystals.
Angewandte Chemie (International ed. in English), 2016Co-Authors: Younan Xia, Hsinchieh Peng, Kyle D. Gilroy, Xiaohu XiaAbstract:Seed-mediated growth is a powerful and versatile approach for the synthesis of Colloidal Metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of Metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.
Therese M. Cotton - One of the best experts on this subject based on the ideXlab platform.
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Enhancement of Molecular Fluorescence near the Surface of Colloidal Metal Films
Analytical chemistry, 1998Co-Authors: Konstantin V Sokolov, George Chumanov, Therese M. CottonAbstract:Fluorescence enhancement was studied on silver Colloidal Metal films (CMFs) using two systems: (1) Langmuir−Blodgett monolayers of fluorescein-labeled phospholipids separated from the surface of the films by spacer layers of octadecanoic acid and (2) biotin−fluorescein conjugates captured by avidin molecules adsorbed on top of a multilayer structure formed by alternating layers of bovine serum albumin−biotin conjugate (BSA−biotin) and avidin. The dependence of fluorescence intensity on the number of lipid or protein spacer layers deposited on the surface of the CMF was investigated. The results demonstrate the requirement for adsorbate location within the region between Ag particles for maximal enhancement. The density of avidin molecules on the surface of the BSA−biotin/avidin multilayers adsorbed on the CMF was also determined. A procedure for forming a rigid, uniform silica layer around the Ag particles on the CMF is described. The layer protects the particles from undesirable chemical reactions such ...
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Ultrastructural characterization of Colloidal Metal films for bioanalytical applications by scanning force microscopy
Journal of Vacuum Science & Technology A: Vacuum Surfaces and Films, 1996Co-Authors: Wolfgang Fritzsche, Konstantin V Sokolov, George Chumanov, Therese M. Cotton, Eric HendersonAbstract:Colloidal Metal films (CMFs) are prepared by the attachment of silver, gold, or platinum (or other Metal) particles to a glass slide modified by silanization with 3‐mercaptopropyl silane. The covalent attachment of the Metal particles occurs through the Metal–sulfur bond. In these samples the local electromagnetic field is enhanced near the surface of the CMF due to excitation of plasmon resonances. This phenomenon can be used for a variety of analytical applications. Because the optical properties are strongly dependent on the morphology of the film, its structural characterization becomes of great importance. To further characterize CMFs we have used scanning force microscopy (SFM). Initial studies revealed lateral dimensions of the particles as well as the particle density. Height measurements were made using the three‐dimensional topographic image of the surface yielded by SFM, and were used to evaluate the selective deposition of a silica spacer layer onto the Metal particles. Comparative SFM measure...
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Reduction of Cytochrome c by Halide-Modified, Laser-Ablated Silver Colloids
The Journal of Physical Chemistry, 1996Co-Authors: Morgan S. Sibbald, And George Chumanov, Therese M. CottonAbstract:Silver colloids of 20 nm mean particle diameter were prepared by laser ablation and modified by adsorption of iodide and bromide ions. Addition of cytochrome c to this colloid resulted in the reduction of the protein, which was monitored by surface-enhanced resonance Raman scattering and absorption spectroscopies. Colloidal Metal films, prepared from the same Ag colloid, were employed to minimize contributions from aggregation. Effects of surface modification on the Ag plasmon resonance were studied in both Colloidal suspensions and Colloidal Metal films. The conclusion was made that adsorption of I- and Br- results in charging of the Ag particle as a whole and a shift of its potential to more negative values. The donated charge is delocalized in a thin surface layer and does not significantly affect the plasmon resonance frequency of the particle.
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Colloidal Metal Films as a New Substrate for Enhanced Spectroscopy of Biomolecules
Spectroscopy of Biological Molecules, 1995Co-Authors: George Chumanov, Konstantin Sokolov, Therese M. CottonAbstract:Recently we introduced a new substrate which provides an enhancement of both Raman scattering and fluorescence from adsorbed molecules [1]. Enhancement was tested with several nucleotides, as well as with an avidin/biotin complex. This system consists of silver or gold particles covalently attached to a glass substrate. Other substrates, such as silicon oxide, aluminum oxide, tin oxide and germanium, have been shown to form Colloidal Metal films. These films are formed by self-assembly of Metal particles from a Colloidal suspension onto substrates derivatized with mercaptosilanes. The films were characterized by UV-Vis spectroscopy, electron microscopy and atomic force microscopy. The advantages of these films relative to other substrates commonly used for enhanced spectroscopies (roughened Metal surfaces, Colloidal suspensions, island films, etc.) are their stability, reproducibility and greater enhancement. For example, the enhancement of the Raman spectrum of adenine on a silver Colloidal film was almost an order of magnitude larger than in an aggregated citrate silver colloid. Moreover, the aggregation state of these films can be controlled prior to their preparation. Because these films are very stable in different environments, they provide a unique possibility for studying fundamental properties of plasmon resonances in small Metal particles as a function of the dielectric medium. From a practical point of view, these highly reproducible and stable films offer the potential for development of quantitative analytical methods based on surface-enhanced spectroscopy.
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Colloidal Metal films as a substrate for surface-enhanced spectroscopy
The Journal of Physical Chemistry, 1995Co-Authors: George Chumanov, Konstantin V Sokolov, Brian W. Gregory, Therese M. CottonAbstract:Colloidal films of gold and silver were prepared on glass or quartz slides. The slides were derivatized with (3-mercaptopropyl)trimethoxysilane and subsequently reacted with aqueous Metal colloids for variable time periods. The formation of the sulfur-Metal bond provides a stable Colloidal film on the surface. Because of the electrostatic interaction between individual particles, a semiregular structure is produced, as can be seen from electron micrographs. The unique property of the Colloidal film is that they possess the optical properties of Colloidal Metals and the convenience of solid substrates. The effect of the dielectric constant of solvents on the optical frequencies, as well as the specific interaction of the solvent molecules with the Metal on the plasmon resonances, was examined in detail. The Colloidal films exhibit strong enhancement of Raman scattering and fluorescence emission from molecules adsorbed on the surface. Enhancement of fluorescence was observed for fluorescein-labeled molecules spaced 0-200 A away from the surface. These substrates can be used in a number of analytical applications, such as surface-enhanced spectroscopies as well as for fundamental studies of plasmon resonances in small Metal particles. 40 refs., 6 figs., 1 tab.
George Chumanov - One of the best experts on this subject based on the ideXlab platform.
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Enhancement of Molecular Fluorescence near the Surface of Colloidal Metal Films
Analytical chemistry, 1998Co-Authors: Konstantin V Sokolov, George Chumanov, Therese M. CottonAbstract:Fluorescence enhancement was studied on silver Colloidal Metal films (CMFs) using two systems: (1) Langmuir−Blodgett monolayers of fluorescein-labeled phospholipids separated from the surface of the films by spacer layers of octadecanoic acid and (2) biotin−fluorescein conjugates captured by avidin molecules adsorbed on top of a multilayer structure formed by alternating layers of bovine serum albumin−biotin conjugate (BSA−biotin) and avidin. The dependence of fluorescence intensity on the number of lipid or protein spacer layers deposited on the surface of the CMF was investigated. The results demonstrate the requirement for adsorbate location within the region between Ag particles for maximal enhancement. The density of avidin molecules on the surface of the BSA−biotin/avidin multilayers adsorbed on the CMF was also determined. A procedure for forming a rigid, uniform silica layer around the Ag particles on the CMF is described. The layer protects the particles from undesirable chemical reactions such ...
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Ultrastructural characterization of Colloidal Metal films for bioanalytical applications by scanning force microscopy
Journal of Vacuum Science & Technology A: Vacuum Surfaces and Films, 1996Co-Authors: Wolfgang Fritzsche, Konstantin V Sokolov, George Chumanov, Therese M. Cotton, Eric HendersonAbstract:Colloidal Metal films (CMFs) are prepared by the attachment of silver, gold, or platinum (or other Metal) particles to a glass slide modified by silanization with 3‐mercaptopropyl silane. The covalent attachment of the Metal particles occurs through the Metal–sulfur bond. In these samples the local electromagnetic field is enhanced near the surface of the CMF due to excitation of plasmon resonances. This phenomenon can be used for a variety of analytical applications. Because the optical properties are strongly dependent on the morphology of the film, its structural characterization becomes of great importance. To further characterize CMFs we have used scanning force microscopy (SFM). Initial studies revealed lateral dimensions of the particles as well as the particle density. Height measurements were made using the three‐dimensional topographic image of the surface yielded by SFM, and were used to evaluate the selective deposition of a silica spacer layer onto the Metal particles. Comparative SFM measure...
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Colloidal Metal Films as a New Substrate for Enhanced Spectroscopy of Biomolecules
Spectroscopy of Biological Molecules, 1995Co-Authors: George Chumanov, Konstantin Sokolov, Therese M. CottonAbstract:Recently we introduced a new substrate which provides an enhancement of both Raman scattering and fluorescence from adsorbed molecules [1]. Enhancement was tested with several nucleotides, as well as with an avidin/biotin complex. This system consists of silver or gold particles covalently attached to a glass substrate. Other substrates, such as silicon oxide, aluminum oxide, tin oxide and germanium, have been shown to form Colloidal Metal films. These films are formed by self-assembly of Metal particles from a Colloidal suspension onto substrates derivatized with mercaptosilanes. The films were characterized by UV-Vis spectroscopy, electron microscopy and atomic force microscopy. The advantages of these films relative to other substrates commonly used for enhanced spectroscopies (roughened Metal surfaces, Colloidal suspensions, island films, etc.) are their stability, reproducibility and greater enhancement. For example, the enhancement of the Raman spectrum of adenine on a silver Colloidal film was almost an order of magnitude larger than in an aggregated citrate silver colloid. Moreover, the aggregation state of these films can be controlled prior to their preparation. Because these films are very stable in different environments, they provide a unique possibility for studying fundamental properties of plasmon resonances in small Metal particles as a function of the dielectric medium. From a practical point of view, these highly reproducible and stable films offer the potential for development of quantitative analytical methods based on surface-enhanced spectroscopy.
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Colloidal Metal films as a substrate for surface-enhanced spectroscopy
The Journal of Physical Chemistry, 1995Co-Authors: George Chumanov, Konstantin V Sokolov, Brian W. Gregory, Therese M. CottonAbstract:Colloidal films of gold and silver were prepared on glass or quartz slides. The slides were derivatized with (3-mercaptopropyl)trimethoxysilane and subsequently reacted with aqueous Metal colloids for variable time periods. The formation of the sulfur-Metal bond provides a stable Colloidal film on the surface. Because of the electrostatic interaction between individual particles, a semiregular structure is produced, as can be seen from electron micrographs. The unique property of the Colloidal film is that they possess the optical properties of Colloidal Metals and the convenience of solid substrates. The effect of the dielectric constant of solvents on the optical frequencies, as well as the specific interaction of the solvent molecules with the Metal on the plasmon resonances, was examined in detail. The Colloidal films exhibit strong enhancement of Raman scattering and fluorescence emission from molecules adsorbed on the surface. Enhancement of fluorescence was observed for fluorescein-labeled molecules spaced 0-200 A away from the surface. These substrates can be used in a number of analytical applications, such as surface-enhanced spectroscopies as well as for fundamental studies of plasmon resonances in small Metal particles. 40 refs., 6 figs., 1 tab.
Tung-han Yang - One of the best experts on this subject based on the ideXlab platform.
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Synthesis of Colloidal Metal Nanocrystals: A Comprehensive Review on the Reductants.
Chemistry (Weinheim an der Bergstrasse Germany), 2018Co-Authors: Thenner S. Rodrigues, Kyle D. Gilroy, Tung-han Yang, Ming Zhao, Anderson G. M. Da Silva, Pedro H. C. Camargo, Younan XiaAbstract:There is a growing interest in controlling the synthesis of Colloidal Metal nanocrystals and thus tailoring their properties toward various applications. In this context, choosing an appropriate combination of reagents (e.g., salt precursor, reductant, capping agent, and stabilizer) plays a pivotal role in enabling the synthesis of Metal nanocrystals with diversified sizes, shapes, and structures. Here we present a comprehensive review that highlights one of the key reagents for the synthesis of Metal nanocrystals via chemical reduction: the reductants. We start with a brief introduction to the compounds commonly employed as reductants in the Colloidal synthesis of Metal nanocrystals by showing their oxidation half-reactions and the corresponding oxidation potentials. Then we offer specific examples pertaining to the controlled synthesis of Metal nanocrystals, followed by some fundamental aspects covering the general mechanisms of Metal ion reduction based on the Marcus Theory. Afterwards, we present a case-by-case discussion on a wide variety of reductants, including their major properties, reduction mechanisms, and additional effects on the final products. We illustrate these aspects by selecting key examples from the literature and paying close attention to the underlying mechanism in each case. At the end, we conclude by summarizing the highlights of the review and providing some perspectives on future directions.
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Autocatalytic surface reduction and its role in controlling seed-mediated growth of Colloidal Metal nanocrystals
Proceedings of the National Academy of Sciences of the United States of America, 2017Co-Authors: Tung-han Yang, Kyle D. Gilroy, Shan Zhou, Legna Figueroa-cosme, Yi-hsien Lee, Younan XiaAbstract:The growth of Colloidal Metal nanocrystals typically involves an autocatalytic process, in which the salt precursor adsorbs onto the surface of a growing nanocrystal, followed by chemical reduction to atoms for their incorporation into the nanocrystal. Despite its universal role in the synthesis of Colloidal nanocrystals, it is still poorly understood and controlled in terms of kinetics. Through the use of well-defined nanocrystals as seeds, including those with different types of facets, sizes, and internal twin structure, here we quantitatively analyze the kinetics of autocatalytic surface reduction in an effort to control the evolution of nanocrystals into predictable shapes. Our kinetic measurements demonstrate that the activation energy barrier to autocatalytic surface reduction is highly dependent on both the type of facet and the presence of twin boundary, corresponding to distinctive growth patterns and products. Interestingly, the autocatalytic process is effective not only in eliminating homogeneous nucleation but also in activating and sustaining the growth of octahedral nanocrystals. This work represents a major step forward toward achieving a quantitative understanding and control of the autocatalytic process involved in the synthesis of Colloidal Metal nanocrystals.
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Reduction rate as a quantitative knob for achieving deterministic synthesis of Colloidal Metal nanocrystals.
Chemical science, 2017Co-Authors: Tung-han Yang, Kyle D. Gilroy, Younan XiaAbstract:Despite the incredible developments made to the synthesis of Colloidal Metal nanocrystals, it is still challenging to produce them in a reproducible and predictable manner. This drawback can be attributed to the fact that the protocols continue to be built upon qualitative observations and empirical laws. Because of the vast number of intricately entangled experimental parameters in a synthesis, it is almost impossible to predict and control the outcome by knowingly alternating these parameters. In this Perspective article, we discuss the recent efforts in pushing nanocrystal synthesis towards a deterministic process based upon quantitative measurements. In particular, we focus on how the reduction rate of a salt precursor can be used as a quantitative knob for predicting and controlling the outcomes of both nucleation and growth. We begin with a brief introduction to the techniques that have been used to extract the kinetic information of a synthesis and then discuss how the reduction rate is correlated with the defect structure, shape/morphology, and elemental distribution of the resultant nanocrystals. We conclude by highlighting some of the recent advances related to in situ probing of nanocrystal synthesis, with an emphasis on the real-time, quantitative aspects with regard to both nucleation and growth.
Younghoon Kim - One of the best experts on this subject based on the ideXlab platform.
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Strategies to Achieve High Circularly Polarized Luminescence from Colloidal Organic–Inorganic Hybrid Perovskite Nanocrystals
ACS nano, 2020Co-Authors: Younghoon Kim, Yaxin Zhai, E. Ashley Gaulding, Severin N. Habisreutinger, Taylor Moot, Bryan A. Rosales, Abhijit Hazarika, Roman Brunecky, Lance M. WheelerAbstract:Colloidal Metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high pho...
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Highly Efficient Light-Emitting Diodes of Colloidal Metal–Halide Perovskite Nanocrystals beyond Quantum Size
ACS nano, 2017Co-Authors: Younghoon Kim, Christoph Wolf, Young-tae Kim, Himchan Cho, Woosung Kwon, Aditya Sadhanala, Chan Gyung Park, Shi-woo RheeAbstract:Colloidal Metal–halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter DB (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the Colloidal perovskite nanocrystals (NCs) in a dimension > DB (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than DB show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete ...
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highly efficient light emitting diodes of Colloidal Metal halide perovskite nanocrystals beyond quantum size
ACS Nano, 2017Co-Authors: Younghoon Kim, Christoph Wolf, Young-tae Kim, Himchan Cho, Woosung Kwon, Aditya Sadhanala, Chan Gyung Park, Shi-woo Rhee, Richard H Friend, Taewoo LeeAbstract:Colloidal Metal–halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter DB (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the Colloidal perovskite nanocrystals (NCs) in a dimension > DB (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than DB show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete ...