Browsing by Author "Shen, Ji-Lin"

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Coherent and Polarized Random Laser Emissions from Colloidal CdSe/ZnS Quantum Dots Plasmonically Coupled to Ellipsoidal Ag Nanoparticles
(2017-02)
Yao, Yung-Chi
;
Yang, Zu-Po
;
Hwang, Jung-Min
;
Su, Hai-Ching
;
Haung, Jing-Yu
This study demonstrates the capability of controlling optical anisotropy of lasing emissions by manipulating the coupling strength spatially and spectrally between the oscillated electric field of emitted light and the localized surface plasmon (LSP) resonance for a random lasing medium composed of colloidal CdSe/ZnS quantum dots (QDs) and ellipsoidal silver nanoparticles (Ag NPs). Distinctive from the amplified spontaneous emission generally observed on colloidal CdSe/ZnS QDs, it has been found that lasing emissions of the revealed system exhibit clear interference features (coherent optical feedbacks) with low-threshold characteristics, mainly attributed to enhanced light scatterings and optical gains arisen in peripheral surfaces of ellipsoidal Ag NPs. Importantly, the relative orientation, between the oscillated electric field of emitted light from colloidal CdSe/ZnS QDs and the ellipse axis of Ag NPs, plays a critical role in selective excitation of LSP resonances leading to laser emissions with preferential optical polarizations. The achievement of spatial and spectral coupling of LSP resonance with colloidal QDs, respectively, by spin coating thin QDs film on Ag NPs and control of the Ag NP geometry makes the present system one of the promising candidates of colloidal QDs-based random lasers to achieve coherent and polarized lasing emissions.
Effects of isomeric transformation on characteristics of Alq3 amorphous layers prepared by vacuum deposition at various substrate temperatures
(2007-06-15)
Jian, Zhi-An
;
Luo, Ying-Zi
;
Chung, Jia-Ming
;
Tang, Shiow-Jing
;
Kuo, Ming-Chin
Organic tris(8-hydroxyquinoline)aluminum (Alq3) amorphous layers are prepared by vacuum deposition at various substrate temperatures T-sub from 30 to 180 degrees C. The surface morphology and electrical characteristics of these as-deposited layers are studied by atomic force microscopy and current-density versus electric-field (J-E) curves. The temperature dependence of the dark electrical conductivity sigma(T) determined from J-E curves is also examined. These experimental results reveal that the surface and electrical properties of Alq3 amorphous layers deposited at T-sub between 90 and 120 degrees C exhibit an anomalous T-sub dependence. However, this anomalous T-sub dependence is not observed from infrared absorption measurements, and therefore is not the result of chemical degradation. The observed behavior is explained in terms of the property that the vacuum deposition of Alq3 with T-sub between 90 and 120 degrees C involves a thermal interconversion between meridional and facial Alq3 isomers. (c) x 2007 American Institute of Physics.
Enhanced Color-Conversion Efficiency of Hybrid Nanostructured-Cavities InGaN/GaN Light-Emitting Diodes Consisting of Nontoxic InP Quantum Dots
(2017-09-01)
Liu, Che-Yu
;
Chen, Tzu-Pei
;
Huang, Jhih-Kai
;
Lin, Tzu-Neng
;
Huang, Chia-Yen
Color-conversion efficiency enhancement of hybrid light-emitting diodes (LEDs) by cadmium-free colloidal quantum dots (QDs) and a novel selective area nanocavities structure has been demonstrated. Combining nanoimprinting and photolithography techniques, nanocavities array can be fabricated at designated locations on the LEDs. The color-conversion efficiency of selective area nanocavities LED can be enhanced by up to 13%. The significant color-conversion efficiency enhancement is attributed to resonance of InP QDs emission in nanocavities and nonradiative energy transfer from LED active layers to InPQDs, which has been investigated and characterized by finite domain time-domain simulation, electroluminescence, and time-resolved photoluminescence measurements. This hybrid nanostructured device, therefore, exhibits a great potential for the applications of multicolor lighting sources and micro-LED.
Enhanced Conversion Efficiency of GaAs Solar Cells Using Ag Nanoparticles
(2010-12-01)
Shu, Gia-Wei
;
Liao, Wei-Chuan
;
Hsu, Ching-Ling
;
Lee, Ja-Yuan
;
Hsu, I-Jen
We have studied the enhancement of conversion efficiency in GaAs solar cells using silver nanoparticles with particle average sizes between 11.4 to 17.2 nm. The maximum enhancement of 20.2% in conversion efficiency is achieved for the GaAs solar cells where the Ag nanoparticles have been incorporated. The simulations according to Mie theory suggest that the increased radiative scattering and conversion efficiency is due to a combined effect of the surface plasmon resonance and the interband transition.
Enhanced Conversion Efficiency of III-V Triple-junction Solar Cells with Graphene Quantum Dots
(2016-12-16)
Lin, Tzu-Neng
;
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 external quantum efficiency in GaN-based vertical-type light-emitting diodes by localized surface plasmons
(2016-03-03)
Yao, Yung-Chi
;
Hwang, Jung-Min
;
Yang, Zu-Po
;
Haung, Jing-Yu
;
Lin, Chia-Ching
Enhancement of the external quantum efficiency of a GaN-based vertical-type light emitting diode (VLED) through the coupling of localized surface plasmon (LSP) resonance with the wave-guided mode light is studied. To achieve this experimentally, Ag nanoparticles (NPs), as the LSP resonant source, are drop-casted on the most top layer of waveguide channel, which is composed of hydrothermally synthesized ZnO nanorods capped on the top of GaN-based VLED. Enhanced light-output power and external quantum efficiency are observed, and the amount of enhancement remains steady with the increase of the injected currents. To understand the observations theoretically, the absorption spectra and the electric field distributions of the VLED with and without Ag NPs decorated on ZnO NRs are determined using the finite-difference time-domain (FDTD) method. The results prove that the observation of enhancement of the external quantum efficiency can be attributed to the creation of an extra escape channel for trapped light due to the coupling of the LSP with wave-guided mode light, by which the energy of wave-guided mode light can be transferred to the efficient light scattering center of the LSP.
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.
Graphene Quantum Dot Vertical Cavity Surface-Emitting Lasers
(2019-11-01)
Lee, Ya-Ju
;
Yeh, Ting-Wei
;
Zou, Chen
;
Yang, Zu-Po
;
Chen, Jia-Wen
Nonzero-bandgap graphene quantum dots (GQDs) are novel optical gain materials promising for solution-processed light sources with high cost efficiency and device performance. Currently, there have only been a few reports on the realization of GQDs-based lasers. Herein, we demonstrate for the first time room-temperature lasing emission with green gamut from GQDs in a vertical optical cavity composed of Ta2O5/SiO2 dielectric distributed Bragg reflectors (DBRs). The lasing is enabled by the unique design of the DBR, which not only provides a wide stopband spectrally overlapping with the emission of the GQDs but also allows high transmittance of optical excitation in the UV-light region. This demonstration is a clear evidence of the use of GQDs as optical gain materials and represents an important step forward toward their potential applications in wide-gamut laser displays and projectors.
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
;
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.
Growth of gamma-In2Se3 films on Si substrates by metal-organic chemical vapor deposition with different temperatures
(2008-04-01)
Huang, Yen-Chin
;
Li, Zhen-Yu
;
Uen, Wu-Yih
;
Lan, Shan-Ming
;
Chang, K. J.
In2Se3 films were deposited on the p(+)-Si(1 1 1) substrates by metal-organic chemical vapor deposition (MOCVD) for the first time at temperatures higher than 500 degrees C. Trimethyl indium (TMI) and H2Se were used as the source reactants with the flow ratio of [H2Se]/[TMI] being maintained at 6.7. The growth temperature was varied from 350 to 650 degrees C. Powder X-ray diffraction analyses revealed that the In2Se3 film grown is polycrystalline and has a wurtzite structure when the growth temperature is below 450 degrees C. Instead, a rhombohedral structure began to appear when the growth temperature is above 450 degrees C. Besides, SEM observations indicated a surface morphology of columnar structure for all films grown. Optical properties were examined by photoluminescence (PL) measurements. It was found that by conducting the growth at 550 degrees C a strong PL emission at around 2.14 eV was observed at 20 K. The luminescence mechanism was found to be dominated by the free excitons based on the temperature-dependent PL spectra from 20 to 120 K. However, bound exciton emissions were also observed or even became the dominant band when the films were grown at temperatures lower than 500 degrees C. Otherwise, the intensity of the 2.14 eV peak reduced greatly when the growth temperature was higher than 5 50 degrees C. Power-dependent PL measurements with excitation power varying from 0.1 to 100 mW confirm the luminescent mechanism and the material quality of the film grown at 550 degrees C. It is concluded that single-phase gamma-In2Se3 films can be achieved at temperatures lower than 450 degrees C; however, good-quality In2Se3 films with the simultaneous appearance of gamma- and beta-phase structures can be grown at around 550 degrees C. (C) 2007 Elsevier B.V. All rights reserved.
Highly Efficient Flexible Hybrid Nanocrystal-Cu(In,Ga)Se-2 (CIGS) Solar Cells
(2015-01-21)
Liao, Yu-Kuang
;
Brossard, Mael
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Hsieh, Dan-Hua
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Lin, Tzu-Neng
;
Charlton, Martin D. B.
A novel scheme for hybridizing inkjet-printed thin film Cu(In,Ga)Se-2 (CIGS) solar cells with self-assembled clusters of nanocrystal quantum dots (NQDs), which provides a 10.9% relative enhancement of the photon conversion efficiency (PCE), is demonstrated. A non-uniform layer of NQD aggregates is deposited between the transparent conductive oxide and a CdS/CIGS p-n junction using low cost pulsed-spray deposition. Hybridization significantly improves the external quantum efficiency of the hybrid devices in the absorption range of the NQDs and in the red to near-IR parts of the spectrum. The low wavelength response enhancement is found to be induced by luminescent down-shifting (LDS) from the NQD layer, while the increase at longer wavelengths is attributed to internal scattering from NQD aggregates. LDS is demonstrated using time-resolved spectroscopy, and the morphology of the NQD layer is investigated in fluorescence microscopy and cross-sectional transmission electron microscopy. The influence of the NQD dose on the PCE of the hybrid devices is investigated and an optimum value is obtained. The low costs and limited material consumptions associated with pulsed-spray deposition make these flexible hybrid devices promising candidates to help push thin-film photovoltaic technology towards grid parity.
Investigation of omnidirectional reflection band in ZnTe/ZnSe distributed Bragg reflector
(2015-11-15)
Huang, Ying-Shin
;
Hu, Sheng-Yao
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Lee, Yueh-Chien
;
Chang, Chung-Cheng
;
Tiong, Kwong-Kau
We report the characteristics of reflectance spectra of the 15- and 20-period ZnTe/ZnSe distributed Bragg reflector grown on GaAs (001) substrates by molecular beam epitaxy. The reflectance spectra measured at various incident angles and polarizations were investigated by the theoretical curves simulated using transfer matrix method. The wavelength variation of the refractive indices described by Sellmeier equation and random thickness model were also considered for the interpretation of the experimentally observed curves. An omnidirectional reflection range defined from the edge of incident-angle-dependent reflection band with TE and TM polarizations is about 15 nm, and is consistent with the observed experimental curves. The results showed that the selected ZnTe and ZnSe materials are suitable for constructing multilayer structures having omnidirectional reflection band. (C) 2015 Elsevier B.V. All rights reserved.
Measuring the Junction Temperature of GaInP/GaInAs/Ge Multijunction Solar Cells Using Photoluminescence
(2011-09-01)
Shu, Gia-Wei
;
Tung, Chiun-Hsiang
;
Tung, Shr-Chang
;
Su, Po-Chen
;
Shen, Ji-Lin
The junction temperatures of the three individual subcells of InGaP/InGaAs/Ge solar cells were measured using photoluminescence (PL) with three different excitation lasers. With the illumination of an extra xenon-mercury lamp, the linear relationship between the PL energy and the illumination level is clearly observed and advantageously used for deriving the junction temperature. Using the Varshni relationship between the PL peak energy and the heat-sink temperature allows us to determine the junction temperature in each subcell. (C) 2011 The Japan Society of Applied Physics
Optical and Electrical Properties of GaN-Based Light Emitting Diodes Grown on Micro- and Nano-Scale Patterned Si Substrate
(2011-07-01)
Chiu, Ching-Hsueh
;
Lin, Chien-Chung
;
Deng, Dong-Mei
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Lin, Da-Wei
;
Li, Jin-Chai
We investigate the optical and electrical characteristics of the GaN-based light emitting diodes (LEDs) grown on micro-and nano-scale patterned silicon substrate (MPLEDs and NPLEDs). The transmission electron microscopy images reveal the suppression of threading dislocation density in InGaN/GaN structure on nano-pattern substrate due to nano-scale epitaxial lateral overgrowth. The plan-view and cross-section cathodo luminescence mappings show less defective and more homogeneous active quantum-well region growth on nano-porous substrates. From temperature-dependent photoluminescence (PL) and low temperature time-resolved PL measurement, NPLEDs have better carrier confinement and higher radiative recombination rate than MPLEDs. In terms of device performance, NPLEDs exhibit smaller electroluminescence peak wavelength blue shift, lower reverse leakage current and decrease in efficiency droop when compared with the MPLEDs. These results suggest the feasibility of using NPSi for the growth of high quality and power LEDs on Si substrates.
P-Type Doping of WS2 Quantum Dots via Pulsed Laser Ablation
(2018-12-01)
Caigas, Septem P.
;
Cheng, Min-Chiang
;
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.
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
;
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 and luminescent properties of strong blue light-emitting Al2O3/ZnO nanocables
(2008)
Hsiao, Chi-Sheng
;
Kuo, Wan-Lin
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Chen, San-Yuan
;
Shen, Ji-Lin
;
Lin, Chin-Ching
Photoelectronic characteristics are investigated in well-aligned aluminum-coated ZnO nanorods (Al2O3/ZnO nanocables) grown on Si substrates buffered with ZnO film at a low temperature. Photoluminescence measurement indicates that a strong blue emission peak at similar to 450 nm appears at 400 and 600 degrees C in O-2 and N-2 atmospheres, respectively. A 30x enhancement of the relative intensity ratio of blue emission (I-B) to ultraviolet emission (I-UV) has been observed for the Al2O3/ZnO nanocables. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy analyses reveal that the origin of the strong blue emission can be attributed to the structure transition and cleavage of the oxygen-hydrogen bond (OH) of the Al2O3/ZnO nanocable, which leads to the formation of singly ionized oxygen vacancies (Al-O-center dot). (C) 2008 The Electrochemical Society.
Tunnel injection from WS2 quantum dots to InGaN/GaN quantum wells
(2018-01-01)
Santiago, Svette Reina Merden
;
Caigas, Septem P.
;
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