The Experts below are selected from a list of 58341 Experts worldwide ranked by ideXlab platform
M.a. Van Hove - One of the best experts on this subject based on the ideXlab platform.
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Green Emission in zno nanostructures examination of the roles of oxygen and zinc vacancies
Applied Surface Science, 2013Co-Authors: Yu Hang Leung, X.y. Chen, Mu Yao Guo, Fangzhou Liu, Wai Kin Chan, Xingqiang Shi, Aleksandra B Djurisic, M.a. Van HoveAbstract:Abstract Green defect Emission is commonly observed in ZnO nanostructures. It is frequently attributed to oxygen vacancies and used to evaluate performance and study physical mechanisms in a variety of applications, such as gas sensing and photocatalysis. However, competing hypotheses have been proposed to explain Green Emission, which raises questions about the role of oxygen vacancies in sensing and photocatalytic processes. The major problem in correct experimental identification of defects in ZnO is the abundance of defects present, while theoretically there are problems with accurate calculation of a defect energy level in the gap. Thus, here we adopted a different approach and studied experimentally and theoretically the interaction of ZnO with different chemical substances (hydrogen and a silane-based molecule). Based on theoretical predictions and experimental results, we can conclude that Green Emission can likely be assigned to defect complexes, which may contain zinc vacancies.
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Green Emission in ZnO nanostructures—Examination of the roles of oxygen and zinc vacancies
Applied Surface Science, 2013Co-Authors: Yu Hang Leung, X.y. Chen, Mu Yao Guo, Fangzhou Liu, Aleksandra B. Djurišić, Wai Kin Chan, Xingqiang Shi, M.a. Van HoveAbstract:Abstract Green defect Emission is commonly observed in ZnO nanostructures. It is frequently attributed to oxygen vacancies and used to evaluate performance and study physical mechanisms in a variety of applications, such as gas sensing and photocatalysis. However, competing hypotheses have been proposed to explain Green Emission, which raises questions about the role of oxygen vacancies in sensing and photocatalytic processes. The major problem in correct experimental identification of defects in ZnO is the abundance of defects present, while theoretically there are problems with accurate calculation of a defect energy level in the gap. Thus, here we adopted a different approach and studied experimentally and theoretically the interaction of ZnO with different chemical substances (hydrogen and a silane-based molecule). Based on theoretical predictions and experimental results, we can conclude that Green Emission can likely be assigned to defect complexes, which may contain zinc vacancies.
Jun Ding - One of the best experts on this subject based on the ideXlab platform.
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Green Emission in carbon doped ZnO films
AIP Advances, 2014Co-Authors: L. T. Tseng, T. S. Herng, X. Y. Zhang, G. Z. Xing, Haibo Fan, X. Luo, Mihail Ionescu, Jun DingAbstract:The Emission behavior of C-doped ZnO films, which were prepared by implantation of carbon into ZnO films, is investigated. Orange/red Emission is observed for the films with the thickness of 60–100 nm. However, the film with thickness of 200 nm shows strong Green Emission. Further investigations by annealing bulk ZnO single crystals under different environments, i.e. Ar, Zn or C vapor, indicated that the complex defects based on Zn interstitials are responsible for the strong Green Emission. The existence of complex defects was confirmed by electron spin resonance (ESR) and low temperature photoluminescence (PL) measurement.
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Strong Green Emission in ZnO films after H2 surface treatment
Journal of Physics D: Applied Physics, 2012Co-Authors: T. S. Herng, Houkun Liang, Nina Bao, Tupei Chen, Jen It Wong, Junmin Xue, Jun DingAbstract:Using a two-step fabrication technique (pulsed laser deposition (PLD) and H2 surface treatment), we fabricated ZnO thin films that could emit ultra-strong Green Emission with coexistence of random lasing phenomenon. After PLD deposition, the as-prepared undoped ZnO thin films (200–500 nm) were annealed in Ar 95%–H25% ambient at 500 °C. The H2 treatment led to the formation of a porous structure that creates substantial optical cavities (diameter ∼1.3 µm). Surprisingly, these optical cavities tremendously amplified the Green Emission rather than ultraviolet (UV) Emission. There was insignificant change in Emission intensity after high-temperature annealing (700 °C) in O2 and acetone dipping, indicating the samples are thermally and chemically stable. The samples exhibited a high quantum yield of 32%. We studied the origin of this ultra-strong Green Emission using low-temperature photoluminescence, extensive structural study and cyclic annealing. The results suggested that neither hydrogen nor VO plays a role in Green Emission. The Green Emission was attributed mainly to the complex defects and the presence of structural defects in the porous structure. In addition, we demonstrated the feasibility of large-scale Green Emission ZnO fabrication via micro-size patterning, paving a way to practical optoelectronic applications.
Martin Vacha - One of the best experts on this subject based on the ideXlab platform.
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Intrachain Aggregates as the Origin of Green Emission in Polyfluorene Studied on Ensemble and Single-Chain Level C
The Journal of Physical Chemistry, 2018Co-Authors: Tomonori Nakamura, Shuzo Hirata, Dharmendar Kumar Sharma, Martin VachaAbstract:Polyfluorenes are conjugated polymers that show strong blue Emission and as such have been explored for potential applications in light-emitting devices. However, heat treatment, prolonged exposure to air, or extended operation in electroluminescent devices can lead to an appearance of parasitic Green Emission that degrades the material performance. This phenomenon has been extensively studied over the past two decades, and two main and conflicting explanations, i.e., oxidation and formation of fluorenone species on the one hand and inter- or intrachain aggregation on the other, have been put forward. There is abundant experimental evidence to support either of these theories, and the question is far from settled. Here, we aim at getting deeper insight into the problem of the Green Emission origin using single-molecule spectroscopy performed on individual chains of poly(9,9-di-n-octylfluorene) (PFO) to resolve the Green Emission band and reveal its spectral and temporal heterogeneity. We disperse single PFO chains in solid thin-film matrices of polystyrene (PS) and poly(methyl methacrylate), as well as in solutions of cyclohexane, toluene, or PS/toluene, to simulate good and poor-solvent environments and environments with different permeabilities and diffusions of oxygen, to systematically study the effects of intrachain aggregation as well as oxidation on the appearance and characteristics of the Green band. The studies are complemented by direct measurement of individual chain conformation by atomic force microscopy and by bulk measurements of photoluminescence (PL) lifetimes and quantum yield. The single-molecule results reveal two PL spectral forms in the region of the Green Emission, a vibrationally resolved type located around 500 nm and broad structureless type located toward lower energies, none of them sensitive to the presence of oxygen. These two types are characterized by different lifetimes of 1.4 and 5.1 ns, respectively, and their oscillator strengths are 2 orders of magnitude smaller compared to those of the blue Emission band. These results point to two different optical transitions comprising the Green band, and these have been assigned to the Emission of H-aggregates and charge transfer or indirectly excited excimer states, respectively.
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Single-molecule electroluminescence and photoluminescence of polyfluorene unveils the photophysics behind the Green Emission band
Nature communications, 2014Co-Authors: Yoshihiro Honmou, Shuzo Hirata, Hideaki Komiyama, Junya Hiyoshi, Susumu Kawauchi, Tomokazu Iyoda, Martin VachaAbstract:Optoelectronic properties of polyfluorene, a blue light-emitting organic semiconductor, are often degraded by the presence of Green Emission that originates mainly from oxidation of the polymer. Here, we use single-molecule electroluminescence (EL) and photoluminescence (PL) spectroscopy on polyfluorene chains confined in vertical cylinders of a phase-separated block copolymer to spectrally resolve the Green band and investigate in detail the photophysical processes responsible for its appearance. In both EL and PL, a substantial fraction of polyfluorene chains shows spectrally stable Green Emission which is ascribed to a keto defect. In addition, in EL, we observe a new type of vibrationally resolved spectra distributed over a wide range of frequencies and showing strong spectral dynamics. Based on quantum chemical calculations, this type is proposed to originate from charge-assisted formation and stabilization of ground-state aggregates. The results are expected to have broad implications in the fields of photophysics and material design of polyfluorene materials.
Yu Hang Leung - One of the best experts on this subject based on the ideXlab platform.
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Green Emission in zno nanostructures examination of the roles of oxygen and zinc vacancies
Applied Surface Science, 2013Co-Authors: Yu Hang Leung, X.y. Chen, Mu Yao Guo, Fangzhou Liu, Wai Kin Chan, Xingqiang Shi, Aleksandra B Djurisic, M.a. Van HoveAbstract:Abstract Green defect Emission is commonly observed in ZnO nanostructures. It is frequently attributed to oxygen vacancies and used to evaluate performance and study physical mechanisms in a variety of applications, such as gas sensing and photocatalysis. However, competing hypotheses have been proposed to explain Green Emission, which raises questions about the role of oxygen vacancies in sensing and photocatalytic processes. The major problem in correct experimental identification of defects in ZnO is the abundance of defects present, while theoretically there are problems with accurate calculation of a defect energy level in the gap. Thus, here we adopted a different approach and studied experimentally and theoretically the interaction of ZnO with different chemical substances (hydrogen and a silane-based molecule). Based on theoretical predictions and experimental results, we can conclude that Green Emission can likely be assigned to defect complexes, which may contain zinc vacancies.
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Green Emission in ZnO nanostructures—Examination of the roles of oxygen and zinc vacancies
Applied Surface Science, 2013Co-Authors: Yu Hang Leung, X.y. Chen, Mu Yao Guo, Fangzhou Liu, Aleksandra B. Djurišić, Wai Kin Chan, Xingqiang Shi, M.a. Van HoveAbstract:Abstract Green defect Emission is commonly observed in ZnO nanostructures. It is frequently attributed to oxygen vacancies and used to evaluate performance and study physical mechanisms in a variety of applications, such as gas sensing and photocatalysis. However, competing hypotheses have been proposed to explain Green Emission, which raises questions about the role of oxygen vacancies in sensing and photocatalytic processes. The major problem in correct experimental identification of defects in ZnO is the abundance of defects present, while theoretically there are problems with accurate calculation of a defect energy level in the gap. Thus, here we adopted a different approach and studied experimentally and theoretically the interaction of ZnO with different chemical substances (hydrogen and a silane-based molecule). Based on theoretical predictions and experimental results, we can conclude that Green Emission can likely be assigned to defect complexes, which may contain zinc vacancies.
T. S. Herng - One of the best experts on this subject based on the ideXlab platform.
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Green Emission in carbon doped ZnO films
AIP Advances, 2014Co-Authors: L. T. Tseng, T. S. Herng, X. Y. Zhang, G. Z. Xing, Haibo Fan, X. Luo, Mihail Ionescu, Jun DingAbstract:The Emission behavior of C-doped ZnO films, which were prepared by implantation of carbon into ZnO films, is investigated. Orange/red Emission is observed for the films with the thickness of 60–100 nm. However, the film with thickness of 200 nm shows strong Green Emission. Further investigations by annealing bulk ZnO single crystals under different environments, i.e. Ar, Zn or C vapor, indicated that the complex defects based on Zn interstitials are responsible for the strong Green Emission. The existence of complex defects was confirmed by electron spin resonance (ESR) and low temperature photoluminescence (PL) measurement.
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Strong Green Emission in ZnO films after H2 surface treatment
Journal of Physics D: Applied Physics, 2012Co-Authors: T. S. Herng, Houkun Liang, Nina Bao, Tupei Chen, Jen It Wong, Junmin Xue, Jun DingAbstract:Using a two-step fabrication technique (pulsed laser deposition (PLD) and H2 surface treatment), we fabricated ZnO thin films that could emit ultra-strong Green Emission with coexistence of random lasing phenomenon. After PLD deposition, the as-prepared undoped ZnO thin films (200–500 nm) were annealed in Ar 95%–H25% ambient at 500 °C. The H2 treatment led to the formation of a porous structure that creates substantial optical cavities (diameter ∼1.3 µm). Surprisingly, these optical cavities tremendously amplified the Green Emission rather than ultraviolet (UV) Emission. There was insignificant change in Emission intensity after high-temperature annealing (700 °C) in O2 and acetone dipping, indicating the samples are thermally and chemically stable. The samples exhibited a high quantum yield of 32%. We studied the origin of this ultra-strong Green Emission using low-temperature photoluminescence, extensive structural study and cyclic annealing. The results suggested that neither hydrogen nor VO plays a role in Green Emission. The Green Emission was attributed mainly to the complex defects and the presence of structural defects in the porous structure. In addition, we demonstrated the feasibility of large-scale Green Emission ZnO fabrication via micro-size patterning, paving a way to practical optoelectronic applications.
