Browsing by Author "Hsu, Chia-Hung"

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Anisotropic exciton relaxation in nanostructured metal (Zn and F16Zn)-phthalocyanine
(2015-02-09)
Ahn, Hyeyoung
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Liou, Wei-Hyun
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Chen, Huang-Ming Philip
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Hsu, Chia-Hung
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光電工程學系
We report ultrafast excited state dynamics of zinc phthalocyanine and zinc hexadecafluoro phthalocyanine thin films which have nanorod-like structures. Excitons in the singlet states undergo multi-exponential relaxation processes to the ground state and the singlet lifetime within a few tens of picoseconds is attributed to the diffusion-limited exciton annihilation process. Diffusive migration of the singlet excitons shows the anisotropic lifetimes depending on the polarization of probe beam. Similar anisotropy is observed in the X-ray diffraction data which exhibits long-range alignment of molecular columns along the long axis of nanorod, whereas disordered arrangement in lateral direction to the axis of nanorod. (C) 2015 Optical Society of America
Coherent acoustic phonon oscillation accompanied with backward acoustic pulse below exciton resonance in a ZnO epifilm on oxide-buffered Si(111)
(2016-08-17)
Lin, Ja-Hon
;
Shen, Yu-Kai
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Liu, Wei-Rein
;
Lu, Chia-Hui
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Chen, Yao-Hui
Unlike coherent acoustic phonons (CAPs) generated from heat induced thermal stress by the coated Au film, we demonstrated the oscillation from c-ZnO epitaxial film on oxide buffered Si through a degenerate pump-probe technique. As the excited photon energy was set below the exciton resonance, the electronic stress that resulted from defect resonance was used to induce acoustic wave. The damped oscillation revealed a superposition of a high frequency and long decay CAP signal with a backward propagating acoustic pulse which was generated by the absorption of the penetrated pump beam at the Si surface and selected by the ZnO layer as the acoustic resonator.
Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0001)
(2019-12-04)
Liu, Wei-Rein
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Pao, Chih-Wen
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Yu, Doris Weijyun
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Chin, Yi-Ying
;
Wu, Wen-Bin
We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0001) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L-2(,3) edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0005) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L-3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (V-Zn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous V-Zn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density V-Zn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.
Dielectric confinement effect in ZnO quantum dots embedded in amorphous SiO2 matrix
(2007-10-07)
Peng, Yu-Yun
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Hsieh, Tsung-Eong
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Hsu, Chia-Hung
;
材料科學與工程學系
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Department of Materials Science and Engineering
The dielectric confinement effect on the blue shift Delta E-g(a) of the ZnO quantum dots (QDs) embedded in the SiO2 matrix is evaluated by applying a multi-shell two-electron system model. The experimental measurement and the calculations of various dielectric structures indicate that the composite matrix structure provides a better estimation of the blue shift of the ZnO QDs-SiO2 system than the multi-shell structure. The proportionality factor x defined in this work exhibits a dependence of the dielectric confinement energy on the specific dimension ratio (the b/a ratio) and the dielectric constant epsilon(matrix) of the outer matrix. The result of the calculation also shows the limit of the two-electron system in estimating the ground-state energy of samples with high dot density. However, the correlation shows the existence of the strong dielectric confinement effect in ZnO QDs-SiO2 thin films and allows a better understanding of the semiconductor QDs-dielectric systems.
Direct Backward Third Harmonic Generation in Nanostructures
(2010-01-01)
Chang, Chieh-Feng
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Chen, Hsing-Chao
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Chen, Miin-Jang
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Liu, Wei-Rein
;
Hsieh, Wen-Feng
We theoretically and experimentally demonstrated that direct backward third harmonic generation (THG) waves can be comparable in magnitude with forward THG waves in nanostructures, such as ZnO thin films and nanoparticles of CdSe and Fe3O4. (C) 2010 Optical Society of America
Direct backward third-harmonic generation in nanostructures
(2010-03-29)
Chang, Chieh-Feng
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Chen, Hsing-Chao
;
Chen, Miin-Jang
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Liu, Wei-Rein
;
Hsieh, Wen-Feng
Direct-backward third harmonic generation (DBTHG) has been regarded as negligible or even inexistent due to the large value of wave-vector mismatch. In the past, BTHG signals were often interpreted as back-reflected or back-scattered forward-THG (FTHG). In this paper, we theoretically and experimentally demonstrate that backward third harmonic waves can be directly generated, and that their magnitude can be comparable with FTHG in nanostructures. Experimental data of DBTHG from ZnO thin films, CdSe quantum dots and Fe(3)O(4) nanoparticles agree well with simulation results based on the Green's function. An integral equation was also derived for fast computation of DBTHG in nano films. Our investigation suggests that DBTHG can be a potentially powerful tool in nano-science research, especially when combined with FTHG measurements. (C) 2010 Optical Society of America
Effects of interface bonding configuration on photoluminescence of ZnO quantum dots-SiOxNy nanocomposite films
(2008-04-01)
Peng, Yu-Yun
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Hsieh, Tsung-Eong
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Hsu, Chia-Hung
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材料科學與工程學系
;
Department of Materials Science and Engineering
Nanocomposite films containing ZnO quantum dots (QDs) and SiOxNy matrix were prepared by target-attached radio frequency sputtering. Photoluminescence (PL) dominated by violet and blue emissions was observed from all ZnO QD-SiOxNy nanocomposite films with dot diameters ranging from 2.77 to 6.65 nm. X-ray photoemission spectroscopy (XPS) revealed the formation of nitrogen-correlated bonding configurations in both the SiOxNy matrix and the dot/matrix interfaces. The nitrogen-correlated configuration at the interface produced a substantial polarization effect at dot surface. The suppression of green-yellow emission observed in photoluminescence spectra of all samples was ascribed to the hole-trapping process promoted by the enhancement of the surface polarization.
Exciton Localization of High-Quality ZnO/MgxZn1-x Multiple Quantum Wells on Si (111) with a Y2O3 Buffer Layer
(2018-08-01)
Liu, Wei-Rein
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Huang, Wei-Lun
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Wu, Yung-Chi
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Lai, Liang-Hsun
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Hsu, Chia-Hung
We report the structural and optical properties of ten-period ZnO/MgxZn1-x multiple quantum wells (MQWs) prepared on the most widely used semiconductor material, Si. The introduction of a nanometer thick high-k Y2O3 transition layer between Si (111) substrate and a ZnO buffer layer significantly improves the structural perfection of the MQWs grown on top of it. The high structural quality of the ZnO/MgxZn1-xO MQWs is evidenced by the appearance of pronounced high order satellite peaks in X-ray crystal truncation rods; high resolution cross-sectional transmission electron microscopy images also confirmed the regularly arranged well and barrier layers. When the well width is less than similar to 2.7 nm, the quantum-confined Stark effect in MQWs can be negligible. Not only the increasing exciton-binding energy but also reducing exciton-phonon coupling determined in temperature-dependent photoluminescence spectra indicate quantum-size effect. Our results demonstrate that ZnO/MgxZn1-xO MQWs integrated on Si have great potential in UV optoelectronic device applications.
Flexible Organometal-Halide Perovskite Lasers for Speckle Reduction in Imaging Projection
(2019-05-01)
Wang, Yu-Chi
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Li, Heng
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Hong, Yu-Heng
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Hong, Kuo-Bin
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Chen, Fang-Chung
Disorder is emerging as a strategy for fabricating random laser sources with very promising materials, such as perovskites, for which standard laser cavities are not effective or too expensive. We need, however, different fabrication protocols and technologies for reducing the laser threshold and controlling its emission. Here, we demonstrate an effectively solvent-engineered method for high-quality perovskite thin films on a flexible polyimide substrate. The fractal perovskite thin films exhibit excellent optical properties at room temperature and easily achieve lasing action without any laser cavity above room temperature with a low pumping threshold. The lasing action is also observed in curved perovskite thin films on flexible substrates. The lasing threshold can be further reduced by increasing the local curvature, which modifies the scattering strengths of the bent thin film. We also show that the curved perovskite lasers are extremely robust with respect to repeated deformations. Because of the low spatial coherence, these curved random laser devices are efficient and durable speckle-free light sources for applications in spectroscopy, bioimaging, and illumination.
Optical characteristics and microstructure of ZnO quantum dots-SiO2 nanocomposite films prepared by sputtering methods
(2006-11-20)
Peng, Yu-Yun
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Hsieh, Tsung-Eong
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Hsu, Chia-Hung
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材料科學與工程學系
;
Department of Materials Science and Engineering
ZnO quantum dots (QDs)-SiO2 nanocomposite films were prepared using the target-attached radio-frequency sputtering. The transmission electron microscopy revealed the uniform dispersion of ZnO QDs with diameters about 2-7 nm in amorphous SiO2 matrix. The photoluminescence showed that small ZnO QDs are able to emit white light with luminescence spectra similar to those of the present GaN-based light emitting diode (LED). The calculated chromaticity coordinates of emitting light evidenced the feasibility of ZnO QDs-SiO2 nanocomposite films as the fluorescence material in optoelectronic devices. (c) 2006 American Institute of Physics.
Pressure-induced structural phase transition in bulk Zn0.98Mn0.02O by angular dispersive X-ray diffraction
(2014-08-15)
Lin, Chih-Ming
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Lin, Kung-Liang
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Chern, Yu-Ker
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Hsu, Chia-Hung
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Sheu, Hwo-Shuenn
The high pressure induced phase transition in bulk Zn0.98Mn0.02O at ambient temperature have been investigated using angular-dispersive X-ray diffraction (ADXRD) under high pressure up to around 13.80 GPa. For loading run, in situ ADXRD measurements found that a wurtzite-to-rocksalt structural phase transition pressure of bulk Zn0.98Mn0.02O began at 7.35 GPa. The fitting of volume compression data to the third-order Birch-Murnaghan equation of state yielded that the zero-pressure isothermal bulk moduli and the first-pressure derivatives were 157(8) GPa and 8(3) for the B4 phase, respectively. When decompress bulk Zn0.98Mn0.02O to ambient pressure a large part of the bulk Zn0.98Mn0.02O reverted to the B4 phase and only a small amount of the metastable B1 phase remained. We have exhibits the pressure dependence of lattice constant a and c axis, and the normalized ratio c/a of the bulk Zn0.98Mn0.02O. Possible pressure-induced phase transition mechanisms were explored by examining the cell parameters and the internal structural parameter (u) with pressures. The effect of the 3d electrons of manganese to increase the nearest-neighbor distance of O to Zn (Mn) parallel to the c axis may be the main reason for the phase transition of bulk Zn0.98Mn0.02O. An increase in the u value with pressure indicates that the B4-to-B1 phase transformation in bulk Zn0.98Mn0.02O is likely via the hexagonal path. (C) 2014 Elsevier B.V. All rights reserved.
Saturation and beating of acoustic phonon oscillations excited near the exciton resonance of strained polar ZnO/Zn0.8Mg0.2O multiple quantum wells
(2018-01-01)
Liu, Wei-Rein
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Lin, Ja-Hon
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Chen, Jyun-Sian
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Cheng, Hsin-Ming
;
Li, Sheng-Jie
Saturation and beating of coherent acoustic phonon (CAP) oscillations were observed and attributed to the screening of a built-in electric field with increasing pump power using degenerate pump-probe measurements near the exciton resonance of polar ZnO/Zn0.8Mg0.2O multiple quantum wells (MQWs). After purifying the CAP signals by using an empirical mode decomposition, we found not only that the CAP amplitude follows the trend of the band gap renormalization (BGR) and shows saturation at high pump power, but also that the CAP oscillation period coincides with that of the MQWs, consistent with the XRD and TEM results. An additional low-frequency oscillation modifying the CAP signal is revealed due to the negative change in refractive index caused by BGR as the pump power increases.
Structural and optical characteristics of non-polar ZNO epitaxial films grown by radio-frequency magnetron sputtering
(2018-08-01)
Hsu, Chia-Hung
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Lin, Bi-Hsuan
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Liu, Wei-Rein
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Hsieh, Wen-Fong
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光電工程學系
Structural Transitions in Nanosized Zn0.97Al0.030 Powders under High Pressure Analyzed by in Situ Angle-Dispersive X-ray Diffraction
(2016-07-01)
Lin, Chih-Ming
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Liu, Hsin-Tzu
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Zhong, Shi-Yao
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Hsu, Chia-Hung
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Chiu, Yi-Te
Nanosized aluminum-doped zinc oxide Zn1-xAlxO (AZO) powders (AZO-NPs) with x = 0.01, 0.03, 0.06, 0.09 and 0.11 were synthesized by chemical precipitation method. The thermogravimetric analysis (TGA) indicated that the precursors were converted to oxides from hydroxides near 250 degrees C, which were then heated to 500 degrees C for subsequent thermal processes to obtain preliminary powders. The obtained preliminary powders were then calcined at 500 degrees C for three hours. The structure and morphology of the products were measured and characterized by angle -dispersive X-ray diffraction (ADXRD) and scanning electron microscopy (SEM). ADXRD results showed that AZO-NPs with Al content less than 11% exhibited wurtzite zinc oxide structure and there was no other impurity phase in the AZO-NPs, suggesting substitutional doping of Al on Zn sites. The Zn0.97Al0.030 powders (A3ZO-NPs) with grain size of about 21.4 nm were used for high-pressure measurements. The in situ ADXRD measurements revealed that, for loading run, the pressure-induced wurtzite (B4)-to-rocksalt (B1) structural phase transition began at 9.0(1) GPa. Compared to the predicted phase-transition pressure of 12.7 GPa for pristine ZnO nanocrystals of similar grain size (similar to 21.4 nm), the transition pressure for the present A(3)ZO-NPs exhibited a reduction of 3.7 GPa. The significant reduction in phase -transition pressure is attributed to the effects of highly selective site occupation, namely Zn2+ and Al3+, were mainly found in tetrahedral and octahedral sites, respectively.
Structure-Dependent Photoluminescence in Low-Dimensional Ethylammonium, Propylammonium, and Butylammonium Lead Iodide Perovskites
(2020-01-29)
Lin, Chang-Wei
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Liu, Fangzhou
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Chen, Ting-Yang
;
Lee, Kuan-Hua
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Chang, Chung-Kai
Hybrid organic-inorganic perovskites have attracted great attention as the next generation materials for photovoltaic and light-emitting devices. However, their environment instability issue remains as the largest challenge for practical applications. Recently emerging two-dimensional (2D) perovskites with Ruddlesden-Popper structures are found to greatly improve the stability and aging problems. Furthermore, strong confinement of excitons in these natural quantum-well structures results in the distinct and narrow light emission in the visible spectral range, enabling the development of spectrally tunable light sources. Besides the strong quasi-monochromatic emission, some 2D perovskites composed of the specific organic cations and inorganic layer structures emit a pronounced broadband emission. Herein, we report the light-emitting properties and the degradation of low-dimensional perovskites consisting of the three shortest alkylammonium spacers, mono-ethylammonium (EA), n-propylammonium (PA), and n-butylammonium (BA). While (BA)(2)PbI4 is known to form well-oriented 2D thin films consisting of layers of corner-sharing PbI6 octahedra separated by a bilayer of BA cations, EA with shorter alkyl chains tends to form other types of lower-dimensional structures. Nevertheless, optical absorption edges of asprepared fresh EAPbI(3), (PA)(2)PbI4, and (BA)(2)PbI4 are obviously blue-shifted to 2.4-2.5 eV compared to their 3D counterpart, methylammonium lead iodide (MAPbI(3)) perovskite, and they all emit narrow excitonic photoluminescence. Furthermore, by carefully optimizing deposition conditions, we have achieved a predominantly 2D structure for (PA)(2)PbI4. However, unlike (BA)(2)PbI4, upon exposure to ambient environment, (PA)(2)PbI4 readily transforms to a different crystal structure, exhibiting a prominently broadband light from similar to 500 to 800 nm and a gradual increase in intensity as structural transformation proceeds.
The Conductive Property of ZnO QDs-SiO(2) and ZnO QDs-SiO(x)N(y) Nanocomposite Films
(2009-08-01)
Peng, Yu-Yun
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Hsieh, Tsung-Eong
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Hsu, Chia-Hung
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材料科學與工程學系
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Department of Materials Science and Engineering
The dc and ac conductive properties of ZnO QDs-SiO(2) and ZnO QDs-SiO(x)N(y) nancomposite films prepared by the target-attached sputtering method are investigated. Both two nanocomposite samples reveal a typical characteristic of a metal-oxide varistor (MOV) on the J-E plots with distinct threshold electric fields (E(th)) which are affected by the molecular bonding configurations in the matrix and at the dot/matrix interfaces. The two systems exhibit dissimilar dependences of dc conductivity (sigma(dc)) on the ZnO content, revealing the limitation of conventional percolation models in which the surface interaction term is usually ignored. The ac conduction behaviors of the two nanocomposite systems were also analyzed and their percolation concentrations V(c) (22.75% for ZnO QDS-SiO(2) and 20.78% for ZnO QDs-SiO(x)N(y)) are determined by the construction of master curves. Analytical results illustrated that the dielectric matrix type indeed affects the defect configuration and transport behaviors inside ZnO dots. Hence, manipulation of dielectric matrix type can possibly achieve various optical and electrical properties in ZnO QDs-dielectric nanocomposite systems.
The dominant effect of non-centrosymmetric displacement on the crystal-field energy splitting in the strained a-plane ZnO epi-films on r-plane sapphires
(2017-06-28)
Wu, Yung-Chi
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Liu, Wei-Rein
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Chen, Hou-Ren
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Hsu, Chia-Hung
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Hsieh, Wen-Feng
The polarization-dependent photoluminescence (PL) of the a-plane ZnO (a-ZnO) films grown on r-plane sapphire substrates shows that the crystal-field energy splitting (Delta(CF)) becomes larger with increasing film thickness from 24 nm to 291 nm. In the use of the nonlinear fitting of the stress-strain tensor to the X-ray diffraction data, we found crystal symmetry breaking from wurtzite to monoclinic in these films and determined the corresponding lattice constants and the relaxed lattice constants. With the available lattice variables, we further used the tight-binding method to calculate the band energies and compared with the PL spectra. The results confirm that the crystal deformation and the non-centrosymmetric displacement had opposite effects on the Delta(CF) at the Gamma point and the displacement induced by the strain is the dominant effect on Delta(CF).
The effect of thermal annealing on the optical and electrical properties of ZnO epitaxial films grown on n-GaAs (001)
(2015-01-01)
Liu, Wei-Rein
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Lin, Bi-Hsuan
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Lin, Chi-Yuan
;
Yang, Song
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Kuo, Chin-Chia
Wurtzite ZnO epitaxial layers grown on n-type GaAs (001) by pulsed laser deposition (PLD) exhibited n-type conductivity. Post-growth annealing leads the conversion of carrier type from electron to hole, as revealed by Hall effect measurements, although only moderate structural improvement was observed. The carrier type conversion is attributed to thermally activated arsenic diffusion from the substrate, confirmed by secondary ion mass spectrometry and photoluminescence. The surface electrical properties of both the as-deposited n-type and annealed p-type ZnO epitaxial layers were thoroughly characterized by Kelvin force microscopy (KFM) and electrostatic force microscopy (EFM). The results indicated the existence of a high density of surface states close to the ZnO midgap with a density of a few 10(14) cm(-2) eV(-1). The Fermi levels (E-F) of n- and p-type ZnO epitaxial layers were found to be 1.06 eV below the conduction-band minimum (CBM) and 1.612-1.769 eV above the valence-band maximum (VBM), respectively. The small EF difference between the n- and p-type ZnO epitaxial layers implies Fermi level pinning at the surface of both n-and p-type ZnO epitaxial layers.
White-light emitting ZnO-SiO2 nanocomposite thin films prepared by sputtering method
(2006)
Peng, Yu-Yun
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Hsieh, Tsung-Eong
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Hsu, Chia-Hung
;
材料科學與工程學系
;
Department of Materials Science and Engineering
ZnO-SiO2 nanocomposite thin films were prepared using RF sputtering method without substrate heating. The photoluminance (PL) measurements showed that the ZnO-SiO2 nanocomposite thin films are able to emit white light consisting of violet, blue, and green-yellow band emissions. The blue emission was attributed to the large amount of ZnO/SiO2 interfaces, which enlarges the depletion layer width and then enhances the related transition. Further analyses indicated that the defect structure of samples could be manipulated by the amount and distribution of ZnO nanoparticles in SiO2 matrix to yield distinct luminescence property.