Browsing by Author "Yuan, Chi-Tsu"

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Blinking Suppression and Anti-Bunching of Quantum Dots as Single-Photon Sources
(2010-01-01)
Yu, Pyng
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Yuan, Chi-Tsu
;
Ko, Hsien-Chien
;
Huang, Jane
;
Tang, Jau
We report here our study on suppression of fluorescence blinking of colloidal CdSe/ZnS quantum dots (QDs) for potential applications as high performance single-photon sources. Blinking is an interesting property for single QDs, but it is undesirable for their applications. We have demonstrated that by coupling these QDs to adjacent silver nanoprisms, we could not only completely suppress the blinking but also enhance the fluorescence quantum yield, and also increase the fluorescence decay rates. These single QDs also exhibit anti-bunching behavior which is a signature for a single-photon emitter. In addition, we have also achieved blinking suppression by embedded the QDs in agarose gel. The electrostatic environment around QDs due to negatively charged fibers of gel might strongly affect the extent of blinking suppression. The mechanisms of blinking suppression will be discussed in the frame work of diffusion-controlled electron transfer model.
Coordination-induced emission enhancement in gold-nanoclusters with solid-state quantum yields up to 40% for eco-friendly, low-reabsorption nano-phosphors
(2019-03-11)
Huang, Hsiu-Ying
;
Cai, Kun-Bin
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Talite, Maria Jessabel
;
Chou, Wu-Ching
;
Chen, Po-Wen
Colloidal quantum dots (CQDs) have gained much attention as light-emitting materials for light-conversion nano-phosphors and luminescent solar concentrators. Unfortunately, those CQDs involve toxic heavy metals and frequently need to be synthesized in the hazardous organic solvent. In addition, they suffer from severe solid-state aggregation-induced self-quenching and reabsorption losses. To address these issues, here we prepare Zn-coordinated glutathione-stabilized gold-nanocluster (Zn-GSH-AuNCs) assemblies without involving heavy metals and organic solvent. Unlike GSH-AuNCs dispersed in an aqueous solution with poor photoluminescence quantum yields (PL-QYs, typically similar to 1%), those Zn-GSH-AuNCs powders hold high solid-state PL-QYs up to 40 +/- 5% in the aggregated state. Such Zn-induced coordination-enhanced emission (CEE) is attributed to the combined effects of suppressed non-radiative relaxation and enhanced charge-transfer interaction. In addition, they also exhibit a large Stokes shift, thus mitigating both aggregation-induced self-quenching and reabsorption losses. Motivated by these photophysical properties, we demonstrated white-light emission from all non-toxic, aqueous-synthesis nano-materials.
Efficient synthesis and optical properties of highly luminescent copper nanoclusters
(2018-01-01)
Talite, Maria Jessabel
;
Chou, Wu-Ching
;
Yuan, Chi-Tsu
;
電子物理學系
;
Department of Electrophysics
Fluorescent nanomaterials such as metal nanoclusters (NCs) have become one of the most essential nanomaterial and attracted abundant attention in research due to their excellent photophysical properties and wide range of applications. Furthermore, metal nanoclusters have been predominantly investigated owing to its excellent optical properties, simple synthetic routes, low toxicity and excellent photostability. However, limited advancements and progress have been executed in fabricating hydrophilic and highly luminescent metal NCs. Copper is an eco-friendly, low-cost metal which is progressively advancing into focus for metal NCs research. In comparison to the extensively studied gold nanoclusters (Au NCs) and silver nanoclusters (Ag NCs), systematic and analytical applications of the copper nanoclusters (Cu NCs) are relatively limited and still at an early stage. In this review, we fixate on contemporary advances in the analytical applications of Cu NCs based on their behavior and properties of light. This work specifically addresses optical properties and some emerging applications of Cu NCs. The study seeks to unravel some unique photophysical properties of Cu NCs in its solid state, namely concentration/aggregation induced emission enhancement, thermally activated delayed fluorescence (TADF), by which both singlet and triplet excitons can be harvested. Furthermore, the as-synthesized Cu NCs used in this study was remarkably excellent, garnering similar to 20% quantum yield in colloidal and solid-state form, respectively. CuNC/PVA nano-composites can exhibit unique solid-state-induced dual-mode emissions of thermally activated delayed fluorescence (TADF) and phosphorescence at ambient environment. The outstanding performance of Cu NCs in solid-state makes it an excellent biocompatible nanoemitters light-emitting devices.
Enhanced Conversion Efficiency of III-V Triple-junction Solar Cells with Graphene Quantum Dots
(2016-12-16)
Lin, Tzu-Neng
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Santiago, Svette Reina Merden S.
;
Zheng, Jie-An
;
Chao, Yu-Chiang
;
Yuan, Chi-Tsu
Graphene has been used to synthesize graphene quantum dots (GQDs) via pulsed laser ablation. By depositing the synthesized GQDs on the surface of InGaP/InGaAs/Ge triple-junction solar cells, the short-circuit current, fill factor, and conversion efficiency were enhanced remarkably. As the GQD concentration is increased, the conversion efficiency in the solar cell increases accordingly. A conversion efficiency of 33.2% for InGaP/InGaAs/Ge triple-junction solar cells has been achieved at the GQD concentration of 1.2 mg/ml, corresponding to a 35% enhancement compared to the cell without GQDs. On the basis of time-resolved photoluminescence, external quantum efficiency, and work-function measurements, we suggest that the efficiency enhancement in the InGaP/InGaAs/Ge triple-junction solar cells is primarily caused by the carrier injection from GQDs to the
Enhanced Performance of GaN-based Ultraviolet Light Emitting Diodes by Photon Recycling Using Graphene Quantum Dots
(2017-08-02)
Lin, Tzu-Neng
;
Santiago, Svette Reina Merden
;
Yuan, Chi-Tsu
;
Chiu, Kuo-Pin
;
Shen, Ji-Lin
ablation. The synthesized GQDs can improve the optical and electrical properties of InGaN/InAlGaN UV light emitting diodes (LEDs) remarkably. An enhancement of electroluminescence and a decrease of series resistance of LEDs were observed after incorporation of GQDs on the LED surface. As the GQD concentration is increased, the emitted light (series resistance) in the LED increases (decreases) accordingly. The light output power achieved a maximum increase as high as 71% after introducing GQDs with the concentration of 0.9 mg/ml. The improved performance of LEDs after the introduction of GQDs is explained by the photon recycling through the light extraction from the waveguide mode and the carrier transfer from GQDs to the active layer.
Greener Luminescent Solar Concentrators with High Loading Contents Based on in Situ Cross-Linked Carbon Nanodots for Enhancing Solar Energy Harvesting and Resisting Concentration Induced Quenching
(2018-10-10)
Talite, Maria Jessabel
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Huang, Hsiu-Ying
;
Wu, Yao-Hsuan
;
Sena, Princess Genevieve
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Cai, Kun-Bin
A luminescent solar concentrator (LSC) is composed of loaded luminophores and a waveguide that can be employed to harvest and concentrate both direct and diffused sunlight for promising applications in solar windows. Thus far, most of efficient LSCs still relied on the heavy-metal-containing colloidal quantum dots (CQDs) dispersed into a polymer matrix with a very low loading (typically <1 wt %). Such low-loading constraint is required to mitigate the concentration induced quenching (CIQ) and maintain high optical quality and film uniformity, but this would strongly reduce the light absorbing efficiency. To address all issues, greener LSCs with high loading concentration were prepared by in situ cross-linking organosilane-functionalized carbon nanodots (Si-CNDs), and their photophysical properties relevant to LSC operation were studied. The PL emission is stable and does not suffer from the severe CIQ effect for cross-linked Si-CNDs even with 25 wt % loadings, thus exhibiting high solid-state quantum yields (QYs) up to 45 +/- 5% after the calibration of the reabsorption losses. Furthermore, such LSCs can still hold high optical quality and film uniformity, leading to low scattering losses and high internal quantum efficiency of similar to 22%. However, the reabsorption losses need to be further addressed to realize large-area LSCs based on earth-abundant, cost-effective CNDs.
Observation of localized surface plasmons in spatially controlled array structures
(2009-07-29)
Chou, J. W.
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Lin, Kao-Chin
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Lee, Yao-Jen
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Yuan, Chi-Tsu
;
Hsueh, Fu-Kuo
We observed the reflectance spectra of three different nano-scale array structures of Au-coated silicon nanorods. The trends of the reflectance spectra indicate that the localized surface plasmon modes can be spatially controlled by manipulating geometric parameters, namely the lattice constants of the array. In addition, the experimental results were compared with 2D numerical simulations based on the finite element method. Satisfactory agreement between the experimental observations and numerical results was obtained.
P-Type Doping of WS2 Quantum Dots via Pulsed Laser Ablation
(2018-12-01)
Caigas, Septem P.
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Cheng, Min-Chiang
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Lin, Tzu-Neng
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Santiago, Svette Reina Merden S.
;
Yuan, Chi-Tsu
Doping provides an advantage to engineering the optical and electrical characteristics of transition-metal dichalcogenides (TMDs). Here, we report the doping of WS2 quantum dots (QDs) with diethylenetriamine (DETA) using pulsed laser ablation. The synthesized DETA-doped 2H-WS2 QDs with an average size of similar to 6 nm have been demonstrated by transmission electron microscopy. With the introduction of DETA during pulsed laser ablation, current modulation, carrier concentration, and field-effect mobility are greatly enhanced, demonstrating a successful doping in WS2 QDs. The positive shift of the threshold voltage in gate dependent conductance measurements reveals p-type doping for DETA-doped WS2 QDs. A remarkable improvement in photoluminescence in WS2 QDs by 74-fold has been achieved after DETA doping. An anomalous dopant-dependent negative photoconductivity was observed for WS2 QDs, originating from light-induced desorbing of water (oxygen) molecules on the surface. The proposed doping approach can provide a vehicle to modulate the optical and electrical properties in WS2 QDs and could be important in the performance improvement of WS2-QD-based devices.
Short-time power-law blinking statistics of single quantum dots and a test of the diffusion-controlled electron transfer model
(2009-08-14)
Tang, Jau
;
Lee, Der-Hau
;
Yeh, Yi-Cheun
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Yuan, Chi-Tsu
;
光電工程學系
In this work we analyzed the blinking statistics data of single CdSe/ZnS quantum dots at vey short times to test some predictions of the diffusion-controlled electron transfer (DCET) model. Using autocorrelation function (ACF) approach we could extract the exponent of the inverse power-law blinking statistics down to 1 mu s. Such an approach also minimizes human subjectivity in choosing a bin time and an on-off threshold. We showed that the observed stretched exponential decay in the ACF and its relationship to the blinking statistics are consistent with the DCET model, and we set an upper bound for the characteristic time constant t(c).
Simple and Fast Method To Fabricate Single-Nanoparticle-Terminated Atomic Force Microscope Tips
(2013-06-27)
Cheng, Hui-Wen
;
Chang, Yuan-Chih
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Yuan, Chi-Tsu
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Tang, Song-Nien
;
Chang, Chia-Seng
This paper introduces a simple, yet controllable scheme to pick up a single 13 nm Au nanoparticle (Au-NP) using the tip of an atomic force microscope (AFM) probe through the application of electrical biases between the tip and the Au-NP. Transmission electron microscope (TEM) images were acquired to verify that a single Au-NP was attached to the AFM probe. We postulate that the mechanism underlying the ability to manipulate individual Au-NPs at the apex of the AFM probe tip is Coulomb interaction induced by tip bias. The AFM tip with the attached Au-NP was then used to study the interaction between a single quantum dot (QD) and the Au-NP. The blinking behavior of single colloidal CdSe/ZnS core/shell QD was significantly suppressed with the approach of the 13 nm Au-NP attached to the AFM tip.
Study of fluorescence enhancement of colloidal CdSe/ZnS quantum dots bound to hexadecylamine by single-molecule measurements
(2007-10-25)
Yuan, Chi-Tsu
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Chou, Wu-Ching
;
Chen, Yueh-Nan
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Chou, Jui-Wen
;
Chuuji, Der-San
Fluorescence enhancement of colloidal CdSe/ZnS quantum dots (QDs) by the binding of hexadecylamine (HDA) ligands was investigated at the single-QD level. The blinking behavior of individual CdSe/ZnS QDs can be dramatically modified by additional HDA ligands. Both the ratio of the mean on/off time lengths and the total number of emitted photons within the on-time duration can be enhanced, by factors of up to 2.8 and 13, respectively. Consequently, the blinking rate can be suppressed down to 18%. The fluorescence decay curve is closer to a single-exponential decay profile and has a longer measured lifetime. These results suggest that HDA ligands can further improve surface passivation.
Substrate-induced strain in 2D layered GaSe materials grown by molecular beam epitaxy
(2020-07-31)
Liu, Cheng-Wei
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Dai, Jin-Ji
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Wu, Ssu-Kuan
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Nhu-Quynh Diep
;
Sa-Hoang Huynh
Two-dimensional (2D) layered GaSe films were grown on GaAs (001), GaN/Sapphire, and Mica substrates by molecular beam epitaxy (MBE). The in situ reflective high-energy electron diffraction monitoring reveals randomly in-plane orientations of nucleated GaSe layers grown on hexagonal GaN/Sapphire and Mica substrates, whereas single-orientation GaSe domain is predominant in the GaSe/GaAs (001) sample. Strong red-shifts in the frequency of in-plane E2g2 vibration modes and bound exciton emissions observed from Raman scattering and photoluminescence spectra in all samples are attributed to the unintentionally biaxial in-plane tensile strains, induced by the dissimilarity of symmetrical surface structure between the 2D-GaSe layers and the substrates during the epitaxial growth. The results in this study provide an important understanding of the MBE-growth process of 2D-GaSe on 2D/3D hybrid-heterostructures and pave the way in strain engineering and optical manipulation of 2D layered GaSe materials for novel optoelectronic integrated technologies.
Synthesis of Silver Nanoprisms and Nanodiscs an Applications in Fluorescence Blinking Suppression
(2010)
Yu, Pyng
;
Huang, Jane
;
Yuan, Chi-Tsu
;
Tang, Jan
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光電工程學系
In this work we investigated the influence of reagents in the synthesis of triangular silver nanoprisms through direct chemical reduction. We identified the chemical role of each reagent, i e H(2)O(2), poly(vinylpyrrolidone) (PVP) and NaBH(4), by systematically varying one reagent concentration while keeping the concentration of other reagents constant We also discuss the applications of nanoprisms in blinking suppression of single semiconductor quantum dots. In addition, we also investigated the synthesis of silver nanodiscs through the reconstruction of triangular nanoprisms using halide ions. The results from different halide ions presented the different face-selective etching rates on the well-identified facet in the silver nanoprism.
Tunnel injection from WS2 quantum dots to InGaN/GaN quantum wells
(2018-01-01)
Santiago, Svette Reina Merden
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Caigas, Septem P.
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Lin, Tzu-Neng
;
Yuan, Chi-Tsu
;
Shen, Ji-Lin
We propose a tunnel-injection structure, in which WS2 quantum dots (QDs) act as the injector and InGaN/GaN quantum wells (QWs) act as the light emitters. Such a structure with different barrier thicknesses has been characterized using steady-state and time-resolved photoluminescence (PL). A simultaneous enhancement of the PL intensity and PL decay time for the InGaN QW were observed after transfer of charge carriers from the WS2-QD injector to the InGaN-QW emitter. The tunneling time has been extracted from the time-resolved PL, which increases as the barrier thickness is increased. The dependence of the tunneling time on the barrier thickness is in good agreement with the prediction of the semiclassical Wentzel-Kramers-Brillouin model, confirming the mechanism of the tunnel injection between WS2 QDs and InGaN QWs.
Tunnel injection from WS2 quantum dots to InGaN/GaN quantum wells (vol 8, pg 15399, 2018)
(2018-01-01)
Santiago, Svette Reina Merden
;
Caigas, Septem P.
;
Lin, Tzu-Neng
;
Yuan, Chi-Tsu
;
Shen, Ji-Lin
Visible-Transparent Luminescent Solar Concentrators Based on Carbon Nanodots in the Siloxane Matrix with Ultrahigh Quantum Yields and Optical Transparency at High-Loading Contents
(2020-01-16)
Talite, Maria Jessabel
;
Huang, Hsiu-Ying
;
Cai, Kun-Bin
;
Co, King Chester Capinig
;
Santoso, Phoa Adelina Cynthia
Visible-transparent luminescent solar concentrators (VT-LSCs) can be integrated with solar cells for designing solar glasses. Recently, rare-earth complexes, semiconductor nanocrystals, and carbon nanodots (CNDs) have been applied in developing VT-LSCs. However, several challenges still existed, such as quantum yields (QYs) at high-loading contents, scattering/reabsorption losses, and stability. Here, highly luminescent and visible-transparent composites based on organosilane-functionalized CNDs (Si-CNDs) cross-linked in the siloxane matrix were prepared. The composites with a high-loading content (similar to 10 wt %) possess ultrahigh QYs of similar to 94% due to surface passivation, cross-linking-enhanced emission, and negligible inter-CND energy transfer. Moreover, they still appear exceptionally transparent and, thus, are suitable for VT-LSCs. Eco-friendly VT-LSCs without colored tinting were fabricated, yielding high internal and external quantum efficiencies of similar to 66% and similar to 3.9%. Our demonstration would pave a bright way for the utilization of eco-friendly VT-LSCs in solar glasses.