Browsing by Author "Cheng, HM"

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Band gap engineering and spatial confinement of optical phonon in ZnO quantum dots
(2006-06-26)
Lin, KF
;
Cheng, HM
;
Hsu, HC
;
Hsieh, WF
;
光電工程學系
Both band gap engineering and spatial confinement of optical phonon were observed depending upon the size of ZnO quantum dots at room temperature. Size-dependent blueshifts of photoluminescence and absorption spectra reveal the quantum confinement effect. The measured Raman spectral shift and asymmetry for the E-2(high) mode caused by localization of optical phonons agree well with that calculated by using the modified spatial correlation model. (c) 2006 American Institute of Physics.
Band gap variation of size-controlled ZnO quantum dots synthesized by sol-gel method
(2005-06-30)
Lin, KF
;
Cheng, HM
;
Hsu, HC
;
Lin, LJ
;
Hsieh, WF
ZnO quantum dots were synthesized successfully via a simple sol-gel method. The average size of quantum dots can be tailored using well-controlled concentration of zinc precursor. Size-dependent blue shifts of photoluminescence and absorption spectra revealed the quantum confinement effect. The band gap enlargement is in agreement with the theoretical calculation based on the effective mass model. Furthermore, as the particle size decreases, we observed an increase in the size-dependent Stokes shift of the photoluminescence peak relative to the absorption onset. (c) 2005 Elsevier B.V. All rights reserved.
Efficient UV photoluminescence from monodispersed secondary ZnO colloidal spheres synthesized by sol-gel method
(2005-04-15)
Cheng, HM
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Hsu, HC
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Chen, SL
;
Wu, WT
;
Kao, CC
ZnO colloidal spheres were synthesized by sol-gel method with a narrow size distribution. We controlled the size of the spheres during the secondary reaction with various amounts of primary reaction supernatant. The optimized condition of ZnO powders with an average diameter of 185 nm was obtained for structure analysis and study of optical properties. Transmission electron microscopy and X-ray diffraction reveal that the spheres are polycrystalline in the pure wurtzite phase. Markedly enhanced near-band-edge ultraviolet pbotoluminescence and significantly reduced defect-related visible emission were also observed. We attribute this observation to the localized bound excitons with high density of defect states. In addition, broad yellow emission and green emission were observed when the powders were post-annealed at 500 degrees C in air ambience. The origins of the defect-induced visible emissions are attributed to oxygen interstitials and oxygen vacancies. (c) 2005 Elsevier B.V. All rights reserved.
Enhanced resonant Raman scattering and electron-phonon coupling from self-assembled secondary ZnO nanoparticles
(2005-10-06)
Cheng, HM
;
Lin, KF
;
Hsu, HC
;
Lin, CJ
;
Lin, LJ
Self-assembled secondary ZnO nanoparticles, recognized with the agglomeration of crystalline subcrystals, are successfully synthesized by a simple sol-gel method. TEM images display that one artificial cluster behaves in a single-crystal-like wurtzite structure because subcrystals coagulate as the same crystal orientation. Moreover, from the resonant Raman scattering, the as-grown sample exhibits phonon red shift; meanwhile, the coupling strength between electron and longitudinal optical phonon, determined by the ratio of second to first-order Raman scattering cross sections, diminishes compared with the samples after postannealing at 350 and 500 degrees C. The size dependence of electron-phonon coupling is principally as a result of the Frohlich interaction.
Luminescence of selective area growth of epitaxial ZnO nanowires and random-growth-oriented nanobelts
(2006-03-14)
Hsu, HC
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Cheng, HM
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Wu, CY
;
Huang, HS
;
Lee, YC
Epitaxial ZnO nanowires and random-growth-oriented nanobelts were grown on c-plane sapphire with and without a pre-coated ZnO epilayer film. On the pre-coated ZnO epilayer, ZnO nanowires are vertically aligned with good in-plane alignment as a result of homoepitaxy, whereas on the bare c-plane sapphire, besides a few nanowires vertically aligned with [0001]znO parallel to [0001](Al2O3), the nanowires were properly aligned with three-fold rotation symmetry. The ZnO nanowires are well-defined hexagonal crystals with diameters of 70-500 nm and lengths of up to several micrometres. In the junction regions between the pre-coated epilayer and the bare sapphire surface, however, ZnO nanobelts (nanoribbons) were found. Cathodoluminescence measurements revealed that the emission at 3.26 eV is correlated with free-exciton recombination and the broad green emission at 2.48 eV is attributed to surface defects. The stronger green emission implies that more surface defects exist on the side walls of nanowires and nanobelts. In Raman scattering, the E1 (LO) mode is sensitive to the orientation of nanostructure that is consistent with the cathodoluminescence results.
Raman scattering and efficient UV photoluminescence from well-aligned ZnO nanowires epitaxially grown on GaN buffer layer
(2005-05-12)
Cheng, HM
;
Hsu, HC
;
Tseng, YK
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Lin, LJ
;
Hsieh, WF
Optical phonon confinement and efficient UV emission of ZnO nanowires were investigated in use of resonant Raman scattering (RRS) and photoluminescence (PL). The high-quality ZnO nanowires with diameters of 80-100 nm and lengths of several micrometers were epitaxially grown through a simple low-pressure vapor-phase deposition method at temperature 550 &DEG; C on the precoated GaN(0001) buffer layer. The increasing intensity ratio of n-order longitudinal optical (LO) phonon (A(1) (nLO)/E-1 (nLO)) with increasing scattering order in RRS reveals the phonon quantum confinement as shrinking the diameter of ZnO nanowires. The exciton-related recombination near the band-edge transition dominate the UV emissions at room temperature as well as at low temperature that exhibits almost no other nonstoichiometric defects in the ZnO nanowires.
Selective growth of ZnO nanorods on pre-coated ZnO buffer layer
(2004-02-01)
Hsu, HC
;
Tseng, YK
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Cheng, HM
;
Kuo, JH
;
Hsieh, WF
Hexagonal ZnO nanorods have been selectively synthesized via vapor-solid process without gold catalysis on. a precoated ZnO buffer layer. The presence of nanometer-sized pits or hills on the surface of ZnO buffer layer provides nucleation sites to which the zinc vapor is transferred and condensed. Followed by immediate oxidation the ZnO nanorods were grown on the. buffer layer. Contrarily, the SEM images hardly show growth of irregular ZnO nanometer-sized products on the bare sapphire substrate. Besides a strong ultra-violet emission at 3.26 eV observed at room temperature, the coupling strength of the radiative transition to LO-phonon polarization field was deduced in use of the Huang-Rhys factor. from low temperature photoluminescence spectra to show that single crystalline ZnO nanorods. (C) 2003 Elsevier B.V. All rights reserved.
Size dependence of band gap variation and electron-phonon coupling in ZnO quantum dots
(2005)
Hsieh, WF
;
Cheng, HM
;
Lin, KF
;
Hsu, HC
;
光電工程學系
Photoluminescence and resonant Raman scattering (RRS) were investigated in various sizes of ZnO quantum dots. The size dependent shift of exciton emission reveals quantum confinement effect as changing the band gap. Besides, it was also found the enhancement of RRS intensity and electron-phonon coupling diminishes with decreasing particle's diameter.
Size dependence of photoluminescence and resonant Raman scattering from ZnO quantum dots
(2006-06-26)
Cheng, HM
;
Lin, KF
;
Hsu, HC
;
Hsieh, WF
;
光電工程學系
ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum confinement effect. From the resonant Raman scattering, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Frohlich interaction.
Structural and optical properties of ZnO nanosaws
(2006-01-18)
Wu, CY
;
Hsu, HC
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Cheng, HM
;
Yang, S
;
Hsieh, WF
The randomly oriented ZnO nanosaws were synthesized on Si substrates by a simple vapor transport method. For structure characterization of the products, the SEM micrograph clearly reveals that length of nanosaw exceeds several tens of mu m and the side of sharp teeth of a saw has a length of about 1-5 mu m and diameters of about 100nm. The results of TEM, XRD, and Raman measurements indicate that the ZnO nanosaw is the single-crystal structure with the wurtzite phase. Under low excitation density, this UV emission peak measured at room temperature is attributed to the recombination of free excitons. Several sharp peaks emerge at photon energy around 3.18 eV with FWHM less than 4 meV when the sample is excited above a certain threshold. We refer that the random lasing in ZnO nanosaws results from the multiple coherent scattering in highly random-oriented ZnO nanosaws. (c) 2005 Elsevier B.V. All rights reserved.
The substrate effect on the in-plane orientation of vertically well-aligned ZnO nanorods grown on ZnO buffer layers
(2005-12-01)
Cheng, HM
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Hsu, HC
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Yang, S
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Wu, CY
;
Lee, YC
Vertically well-aligned ZnO nanorods were synthesized without employing any metal catalysts on various substrates including glass, Si(111), 6H-SiC(0001) and sapphire (0001), which were pre-coated with c-oriented ZnO buffer layers, by simple physical vapour deposition. The alignments of the ZnO nanorods on the different substrates depend on the crystallographic alignments of the pre-coated ZnO buffer layers. The ZnO nanorods grown on glass and Si(111) are vertically aligned but randomly oriented in the in-plane direction. In contrast, the vertically aligned ZnO nanorods on 6H-SiC(0001) and sapphire (0001) show an in-plane alignment with azimuthally sixfold symmetry, which indicates the epitaxial relationship between ZnO and the substrate. Similarly, photoluminescence measurements show the distinct appearance of ZnO nanorods on different substrates. Besides the UV band, which was attributed to the recombination of free excitons near the band edge, defect-related visible emissions were also observed for the samples grown on both glass and Si(111) substrates. However, the ZnO nanorods exhibit only strong band edge emission peaks with no noticeable deep level emissions when grown on the 6H-SiC(0001) and sapphire (0001) substrates, which confirms the good crystalline and optical quality of the epitaxial ZnO nanorods.