Browsing by Author "Yuan, C. T."

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A facile method to prepare "green" nano-phosphors with a large Stokes-shift and solid-state enhanced photophysical properties based on surface-modified gold nanoclusters
(2017-12-15)
Cheng, C. H.
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Huang, H. Y.
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Talite, M. J.
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Chou, W. C.
;
Yeh, J. M.
Colloidal nano-materials, such as quantum dots (QDs) have been applied to light-conversion nano phosphors due to their unique tunable emission. However, most of the QDs involve toxic elements and are synthesized in a hazardous solvent. In addition, conventional QD nano-phosphors with a small Stokes shift suffered from reabsorption losses and aggregation-induced quenching in the solid state. Here, we demonstrate a facile, matrix-free method to prepare eco-friendly nano-phosphors with a large Stokes shift based on aqueous thiolate-stabilized gold nanoclusters (GSH-AuNCs) with simple surface modifications. Our method is just to drop GSH-AuNCs solution on the aluminum foil and then surface modified AuNCs (Al-GSH-AuNCs) can be spontaneously precipitated out of the aqueous solution. Compared with pristine GSH-AuNCs in solution, the Al-GSH-AuNCs exhibit enhanced solid-state PL quantum yields, lengthened PL lifetime, and spectral blue shift, which can be attributed to the aggregation induced emission enhancement facilitated by surface modifications. Such surface-treatment induced aggregation of AuNCs can restrict the surface-ligand motion, leading to the enhancement of PL properties in the solid state. In addition, the Al-GSH-AuNCs nano-phosphors with a large Stokes shift can mitigate the aggregation-induced PL quenching and reabsorption losses, which would be potential candidates for "green" nano-phosphors. (C) 2017 Elsevier Inc. All rights reserved.
Antibunching Single-Photon Emission and Blinking Suppression of CdSe/ZnS Quantum Dots
(2009-10-01)
Yuan, C. T.
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Yu, Pyng
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Ko, H. C.
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Huang, J.
;
Tang, Jau
We demonstrated that by property coupling to silver nanoprisms, single CdSe/ZnS semiconductor quantum dots (QDs) exhibited suppressed blinking behavior, an enhanced fluorescence intensity (similar to 2.5 fold), increased radiative decay rates (similar to 12.5,fold), and antibunching single-photon emission. All these modifications significantly promote the overall performance of the proposed single-photon sources based on colloidal semiconductor QDs.
Blinking suppression of colloidal CdSe/ZnS quantum dots by coupling to silver nanoprisms
(2009-06-15)
Yuan, C. T.
;
Yu, Pyng
;
Tang, Jau
;
光電工程學系
;
Department of Photonics
Fluorescence blinking of quantum dots (QDs) is a common but annoying behavior for practical applications as fluorescent markers or single photon sources. We demonstrate here, by coupling colloidal CdSe/ZnS QDs to silver nanoprisms, fluorescence blinking behavior can be suppressed. For most of coupled QDs, two emissive states with different fluorescence intensities and lifetime were observed. We suggest that energy transfer could play an important role in suppressing blinking behavior.
Blinking suppression of single quantum dots in agarose gel
(2010-01-04)
Ko, H. C.
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Yuan, C. T.
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Lin, S. H.
;
Tang, Jau
;
應用化學系
Fluorescence blinking is commonly observed in single molecule/particle spectroscopy, but it is an undesirable feature in many applications. We demonstrated that single CdSe/ZnS quantum dots in agarose gel exhibited suppressed blinking behavior. In addition, the long-time exponential bending tail of the power-law blinking statistics was found to be influenced by agarose gel concentration. We suggest that electron transfer from the light state to the dark state might be blocked due to electrostatic surrounding of gel with inherent negatively charged fibers.
Blocked Electron Transfer and Suppressed Blinking of Single CdSe/ZnS Quantum Dots in Agarose Gel
(2010)
Ko, H. C.
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Yuan, C. T.
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Lin, S. H.
;
Tang, Jau
;
應用化學系
Fluorescence blinking is an interesting phenomenon commonly observed in single molecule/particle spectroscopy and has attracted wide interest recently. Such behavior in quantum dots is undesirable for applications as fluorescence markers. In this report, we observed suppressed blinking for single CdSe/ZnS quantum dots mixed in agarose gel We attributed the suppression to an increase in activation energy so that the charge transfer is blocked from the neutral light state to the charged dark state as the gel concentration increases
Distance dependence of energy transfer from InGaN quantum wells to graphene oxide
(2013-08-01)
Lin, T. N.
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Huang, L. T.
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Shu, G. W.
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Yuan, C. T.
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Shen, J. L.
We report the distance-dependent energy transfer from an InGaN quantum well to graphene oxide (GO) by time-resolved photoluminescence (PL). A pronounced shortening of the PL decay time in the InGaN quantum well was observed when interacting with GO. The nature of the energy-transfer process has been analyzed, and we find the energy-transfer efficiency depends on the 1/d(2) separation distance, which is dominated by the layer-to-layer dipole coupling. (C) 2013 Optical Society of America
Eco-friendly luminescent solar concentrators with low reabsorption losses and resistance to concentration quenching based on aqueous-solution-processed thiolate-gold nanoclusters
(2017-09-15)
Huang, H. Y.
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Cai, K. B.
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Chang, L. Y.
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Chen, P. W.
;
Lin, T. N.
Heavy-metal-containing quantum dots (QDs) with engineered electronic states have been served as luminophores in luminescent solar concentrators (LSCs) with impressive optical efficiency. Unfortunately, those QDs involve toxic elements and need to be synthesized in a hazardous solvent. Recently, biocompatible, eco-friendly gold nanoclusters (AuNCs), which can be directly synthesized in an aqueous solution, have gained much attention for promising applications in 'green photonics'. Here, we explored the solid-state photophysical properties of aqueoussolution- processed, glutathione-stabilized gold nanoclusters (GSH-AuNCs) with a ligand-tometal charge-transfer (LMCT) state for developing 'green' LSCs. We found that such GSH-AuNCs exhibit a large Stokes shift with almost no spectral overlap between the optical absorption and PL emission due to the LMCT states, thus, suppressing reabsorption losses. Compared with GSH-AuNCs in solution, the photoluminescence quantum yields (PL-QYs) of the LSCs can be enhanced, accompanied with a lengthened PL lifetime owing to the suppression of non-radiative recombination rates. In addition, the LSCs do not suffer from severe concentration-induced PL quenching, which is a common weakness for conventional luminophores. As a result, a common trade-off between light-harvesting efficiency and solid-state PL-QYs can be bypassed due to nearly-zero spectral overlap integral between the optical absorption and PL emission. We expect that GSH-AuNCs hold great promise for serving as luminophores for 'green' LSCs by further enhancing solid-state PL-QYs.
Efficient energy transfer from InGaN quantum wells to Ag nanoparticles
(2013)
Shu, G. W.
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Chiu, C. H.
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Huang, L. T.
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Lin, T. N.
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Yang, C. C.
Nonradiative energy transfer from an InGaN quantum well to Ag nanoparticles is unambiguously demonstrated by the time-resolved photoluminescence. The distance dependence of the energy transfer rate is found to be proportional to 1/d(3), in good agreement with the prediction of the dipole interaction calculated from the Joule losses in acceptors. The maximum energy-transfer efficiency of this energy transfer system can be as high as 83%.
Enhancement of light emission in GaAs epilayers with graphene quantum dots
(2015-01-01)
Lin, T. N.
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Chih, K. H.
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Cheng, M. C.
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Yuan, C. T.
;
Hsu, C. L.
A green and one-step synthesis of graphene quantum dots (GQDs) has been implemented using pulsed laser ablation from aqueous graphene. The synthesized GQDs are able to enhance the photoluminescence (PL) of GaAs epilayers after depositing them on the GaAs surface. An enhancement of PL intensity of a factor of 2.8 has been reached at a GQD concentration of 1.12 mg ml(-1). On the basis of the PL dynamics, the PL enhancement in GaAs is interpreted by the carrier transfer from GQDs to GaAs due to the work function difference between them.
Environmental effects on photoinduced electron transfer and fluorescence blinking of single semiconducting nanocrystals in various matrices
(2010-01-01)
Ko, H. C.
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Yuan, C. T.
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Yu, Ping
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Huang, Jane
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Tang, Jau
In this report we explore how the surrounding environments around a single semiconducting nancrystal affect the photoinduced electron transfer, charge trapping fluorescence lifetime and fluorescence blinking. Using the time-correlated single photon counting techniques combined with confocal microscopy, we investigated photoluminescence of single CdSe/ZnS quantum dots embedded in agarose gel and on conductive substrates as examples. Understanding of the underlying mechanisms would allow us to better control of the photoluminescence properties of nanoparticles and to improve their performance in biophotonics and optoelectronics applications such as fluorescence markers, single-photon sources, photovoltaics and quantum dot lasers.
Fluorescence signals of quantum dots influenced by spatially controlled array structures
(2009-10-14)
Chou, J. W.
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Lin, K. C.
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Tang, Y. T.
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Hsueh, F. K.
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Lee, Yao-Jen
Fluorescence signals of quantum dots (QDs) influenced by different array structures of gold-coated silicon nanorods (SiNRs) were investigated via experimental observations and two-dimensional (2D) finite element method (FEM) simulations. On the densest gold-coated SiNRs array structure, the highest QD fluorescence quenching rates were observed and on the sparsest array structure, the highest QD fluorescence enhancement rates were observed. By developing a new technique which obtains the optical image of the array structures without losing information about the QD locations, we were able to further investigate how the QD fluorescence is influenced by spatially controlled array structures.
Observation of Inverted Regime Electron Transfer in CdSe/ZnS QDs from pH-Sensitive Single-Particle and Ensemble Fluorescence Measurements
(2011-07-28)
Ko, H. C.
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Yuan, C. T.
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Lin, S. H.
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Tang, Jau
;
應用化學系
In this report, we investigated the pH dependence of the photoluminescence of CdSe/ZnS quantum dots (QDs). We present experimental results and theoretical analysis of both the blinking behavior of single-QDs as well as the fluorescence intensity time trace from an ensemble of QDs in agarose gel fibers at different pH environments. Such a combined approach of confined QDs by single-particle and ensemble measurements has not been used previously. This study allows us to elucidate the electron transfer processes from the light state to the dark state. The observed increase in both the activation energy for the charge transfer and the free energy gap between the light and the dark states at an increased concentration of H(+) ions supported the Marcus inverted-regime, electron transfer.
Photo-induced Doping in GaN Epilayers with Graphene Quantum Dots
(2016-03-18)
Lin, T. N.
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Inciong, M. R.
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Santiago, S. R. M. S.
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Yeh, T. W.
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Yang, W. Y.
We demonstrate a new doping scheme where photo-induced carriers from graphene quantum dots (GQDs) can be injected into GaN and greatly enhance photoluminescence (PL) in GaN epilayers. An 8.3-fold enhancement of PL in GaN is observed after the doping. On the basis of time-resolved PL studies, the PL enhancement is attributed to the carrier transfer from GQDs to GaN. Such a carrier transfer process is caused by the work function difference between GQDs and GaN, which is verified by Kelvin probe measurements. We have also observed that photocurrent in GaN can be enhanced by 23-fold due to photo-induced doping with GQDs. The improved optical and transport properties from photoinduced doping are promising for applications in GaN-based optoelectronic devices.
Photoinduced fluorescence enhancement in colloidal CdSeTe/ZnS core/shell quantum dots
(2008-05-05)
Yuan, C. T.
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Chou, W. C.
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Chuu, D. S.
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Chen, Y. N.
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Lin, C. A.
Photoinduced fluorescence enhancement (PFE) in colloidal CdSeTe/ZnS core/shell quantum dots (QDs) was investigated by monitoring ensemble fluorescence and single-QD fluorescence blinking behavior upon illumination. Ensemble fluorescence was increased in air and in vacuum with different enhanced factors. At the single-QD levels, fluorescence was also enhanced for some individual QDs. Relatively long on times, high quantum yields within the on times, and multilevel on states were found in fluorescence time traces. We suggest that the PFE origin from single-QD viewpoint is attributed to the contributions of surface passivation by photoinduced charged carriers and the formation of neutral core/charged shell QD states. (C) 2008 American Institute of Physics.
Single fluorescent gold nanoclusters
(2009-08-31)
Yuan, C. T.
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Chou, W. C.
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Tang, J.
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Lin, C. A.
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Chang, W. H.
Both ensemble and single-molecule measurements were performed to explore the fluorescence properties of Au nanoclusters (NCs). Photoinduced fluorescence enhancement was observed for ensemble NCs in solution, but photobleaching was found at ambient environments. At the single-molecule level, fluorescence blinking and single-step photobleaching were observed. Furthermore, their time-resolved fluorescence shows a single exponential decay with a lifetime of similar to 7 ns and is insensitive to changes in fluorescence intensity. The lifetime distribution is more homogeneous within ensemble Au NCs as compared to CdSe QDs. Therefore, Au NCs have potential applications as nontoxic fluorescent labels for lifetime-based imaging microscopy. However, their low quantum yields and poor photostability are disadvantageous factors, which require further improvement. (C) 2009 Optical Society of America
Studies on the electronic and vibrational states of colloidal CdSe/ZnS quantum dots under high pressures
(2007-05-09)
Yuan, C. T.
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Lin, Y. C.
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Chen, Y. N.
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Chiu, Q. L.
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Chou, W. C.
The electronic and vibrational states of colloidal core/shell CdSe/ZnS quantum dots are studied at room temperature by using high pressure optical measurements. Pressure-induced quadratic variations of lattice constants can be observed clearly from both photoluminescence ( PL) and Raman spectra up to similar to 7 GPa. This quadratic relationship is consistent with the theoretical prediction. The pressure coefficients of linear and quadratic terms are 32 meV GPa(-1), -1 meV GPa(-2) for PL and 4.2 cm(-1) GPa(-1), -0.1 cm(-1) GPa(-2) for Raman measurements, respectively.
Temperature dependence of excitonic emission in cubic CdSe thin film
(2008-01-01)
Chia, C. H.
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Yuan, C. T.
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Ku, J. T.
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Yang, S. L.
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Chou, W. C.
A detailed photoluminescence investigation of the thermal redshift and broadening of the excitonic line of cubic CdSe film grown by molecular beam epitaxy is presented. Free excitonic emission from the cubic Use film was observed at low temperature. Temperature-dependent measurement was performed to obtain material parameters related to exciton-phonon interaction by fitting the experimental data to the phenomenological model. The relative contribution of both acoustic and optical phonon to the band gap shrinkage and exciton linewidth broadening are discussed. Exciton binding energy of 16 +/- 1.5 meV was determined from the Arrhenius analysis. (C) 2007 Elsevier B.V. All rights reserved.
Waveguide based energy transfer with gold nanoclusters for detection of hydrogen peroxide
(2014)
Lin, T. N.
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Chang, Y. L.
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Shu, G. W.
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Yuan, C. T.
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Shen, J. L.
H2O2 detection that uses fluorescence resonance energy transfer from InGaN quantum wells to Au nanoclusters via optical waveguiding has been developed. Steady and time-resolved photoluminescence studies have been used to demonstrate the waveguide-based energy transfer. H2O2 detection is achieved by the quenching of the red emission from Au nanoclusters. Advantages of the sensing technique include the capability of visual detection and large area analysis.