Browsing by Author "Chuu, D. S."

Now showing 1 - 16 of 16

Coulomb scattering rates of excited carriers in moderate-gap carbon nanotubes
(2006-06-01)
Chiu, C. W.
;
Shyu, F. L.
;
Chang, C. P.
;
Chuu, D. S.
;
Lin, M. F.
Temperature induces some free carriers in moderate-gap carbon nanotubes, which, thus, creates the low-frequency intraband e-h excitations. Such excitations are the effective deexcitation channels for the excited electrons (holes) in conduction (valence) bands, while they are very weak. The Coulomb decay rates of the lowest two conduction bands are sensitive to the changes in wave vector, radius, and temperature. However, those of the band-edge states depend on chirality weakly. These results directly reflect the strong energy dispersions and the temperature-dependent electron distributions. The calculated results are roughly consistent with the experimental measurements from the ultrafast optical spectroscopies.
Decoherence of a charge qubit embedded inside a suspended phonon cavity
(2008-01-01)
Liao, Y. Y.
;
Chen, Y. N.
;
Chou, W. C.
;
Chuu, D. S.
;
電子物理學系
This study elucidates the theory of phonon-induced decoherence of a double dot charge qubit that is embedded inside a suspended semiconductor slab. The influences of the lattice temperature, width of the slab, and positions of the dots on the decoherence are analyzed. Numerical results indicate that the decoherence in the slab system is weaker than that in a bulk environment. In particular, the decoherence is markedly suppressed by the inhibition of the electron-phonon coupling. Such a system with low decoherence may be useful for manipulating the qubits.
Electric-field-controlled electron relaxation in lateral double quantum dots embedded in a suspended slab
(2008-11-15)
Liao, Y. Y.
;
Chuu, D. S.
;
Jian, S. R.
;
電子物理學系
;
Department of Electrophysics
This study investigates phonon-induced electron relaxation in a lateral double quantum dot that is embedded in a suspended slab. Exact calculations are made in electric fields. The dependence of the relaxation rate on the parameters of the dots and the slabs is analyzed. Numerical results indicate that the relaxation rate depends strongly on the phonon character of the slab. Unlike in the bulk environment, phonon-subband quantization clearly influences the behavior. In particular, the relaxation rate can be greatly suppressed or enhanced by tuning the electric fields. This fact may be useful in manipulating the relaxation rate in lateral double quantum dots.
Electron transport of a driven three-level system in an asymmetric double quantum dot irradiated by an external field
(2007-03-01)
Liao, Y. Y.
;
Chuu, D. S.
;
Chen, Y. N.
;
電子物理學系
;
Department of Electrophysics
Electron tunneling through a three-level system in an asymmetric double quantum dot irradiated by an external field is investigated. For a resonant external field, two symmetric peaks occur in the current spectrum. If the field frequency is detuned, unequal contributions from two channels lead to two asymmetric peaks with population inversion, which can be observed with increasing Rabi frequency. On the other hand, as the ground states in two dots are equal, a suppression of current occurs around the resonant frequency. In contrast, an enhanced behavior is found for the case of unequal ground states.
Electronic structure of a two-dimensional graphene monolayer in a spatially modulated magnetic field: Peierls tight-binding model
(2008-01-01)
Chiu, Y. H.
;
Lai, Y. H.
;
Ho, J. H.
;
Chuu, D. S.
;
Lin, M. F.
Magnetoelectronic properties of a two-dimensional (2D) monolayer graphene are investigated by the Peierls tight-binding model. They are dominated by the period, strength, and direction of a spatially modulated magnetic field. Such a field could induce the reduction in dimensionality, change of energy dispersions, anisotropy at low energy, composite behavior in state degeneracy, extra band-edge states, and asymmetry of energy bands. There are partial flatbands at the Fermi level and one-dimensional parabolic bands at others. These make density of states exhibit delta-function-like structure and asymmetric prominent peaks, respectively. Energies of the extra band-edge states strongly depend on the period, while those of the original band-edge states exhibit little dependence. Both of them grow as the strength increases. The modulated and uniform magnetic fields differ from each other in energy dispersion, state degeneracy, and dimensionality. Important differences between a monolayer graphene and a 2D electron gas also exist.
Fluorescence signals of core-shell quantum dots enhanced by single crystalline gold caps on silicon nanowires
(2009-04-22)
Christiansen, S. H.
;
Chou, J. W.
;
Becker, M.
;
Sivakov, V.
;
Ehrhold, K.
We use nanoscale (20-300 nm in diameter) single crystalline gold (Au)-caps on silicon nanowires (NWs) grown by the vapor-liquid-solid (VLS) growth mechanism to enhance the fluorescence photoluminescence (PL) signals of highly dilute core/shell CdSeTe/ZnS quantum dots (QDs) in aqueous solution (10(-5) M). For NWs without Au-caps, as they appear, for example, after Au etching in aqua regia or buffered KI/I(2)-solution, essentially no fluorescence signal of the same diluted QDs could be observed. Fluorescence PL signals were measured using excitation with a laser wavelength of 633 nm. The signal enhancement by single crystalline, nanoscale Au-caps is discussed and interpreted based on finite element modeling (FEM).
Fluorescence signals of quantum dots influenced by spatially controlled array structures
(2009-10-14)
Chou, J. W.
;
Lin, K. C.
;
Tang, Y. T.
;
Hsueh, F. K.
;
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.
Local Strained Channel (LSC) nMOSFETs by different poly-Si gate and SiN capping layer thicknesses: Mobility enhancement, size dependence, and hot carrier stress
(2006)
Lee, Yao-Jen
;
Fan, Chia-Hao
;
Yang, Wen-Luh
;
Lin, Wen-Yan
;
Huang, Bohr-Ran
Magnetic energy bands of a 2D graphite layer in the spatially modulated magnetic field
(2008-04-01)
Chiu, Y. H.
;
Lai, Y. H.
;
Ho, J. H.
;
Chuu, D. S.
;
Lin, M. F.
A 2D graphite layer exists in a spatially modulated magnetic field. The low magnetic energy bands, which are calculated from the Peierls tight-binding model, strongly depend on the period and strength of a modulated magnetic field. There are a lot of I D parabolic bands. Such bands exhibit the double degeneracy, strong energy dispersions, and extra band-edge states. Density of states owns many asymmetric square-root divergent structures, mainly owing to the band-edge states in 1D parabolic bands. The band-edge state energies decrease with the increasing field period, while the opposite is true for the dependence on the field strength. That a modulated magnetic field reduces dimensionality and thus drastically changes electronic properties could be examined by the experimental measurements. (c) 2007 Elsevier B.V. All rights reserved.
Photoinduced fluorescence enhancement in colloidal CdSeTe/ZnS core/shell quantum dots
(2008-05-05)
Yuan, C. T.
;
Chou, W. C.
;
Chuu, D. S.
;
Chen, Y. N.
;
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.
Pressure-dependent Raman scattering and photoluminescence of Zn1-xCdxSe epilayers
(2007-04-01)
Lin, Y. C.
;
Chiu, C. H.
;
Fan, W. C.
;
Yang, S. L.
;
Chuu, D. S.
Raman and photoluminescence spectra of cubic Zn1-xCdxSe (0 <= x <= 0.32) epilayers were obtained at high pressure. The impurity mode I observed in the phonon Raman spectra at low temperature confirms the intermediate phonon mode behavior. A split transverse optical phonon mode was found in the down-stroke high-pressure Raman scattering. Additionally, the pressure-dependent longitudinal optical (LO) phonon frequencies and the Gruneisen parameter (gamma(LO)) were obtained by quadratic polynomial fitting. Pressure-driven resonant Raman scattering effect was observed in samples with a high Cd concentration (x >= 0.18). The critical pressure of semiconductor-to-metal phase transition (Pt) decreases as the Cd content increases. As the Cd concentration increases from 0 to 0.32, Pt falls from 13.6 to 9.4 GPa, according to Pt (GPa)=13.6-6.8x-20.3x(2). (c) 2007 American Institute of Physics.
Pressure-induced metallization and resonant Raman scattering in Zn(1-x)Mn(x)Te
(2008-07-01)
Lin, Y. C.
;
Fan, W. C.
;
Chiu, C. H.
;
Ke, F. K.
;
Yang, S. L.
Pressure-induced resonant Raman scattering is adopted to analyze the zone-center optical phonon modes and crystal characteristics of Zn(1-x)Mn(x)Te (0 <= x <= 0.26) thin films. The pressure (Pt) at which the semiconducting undergoes a transition to the metallic phase declines as a function of Mn concentration (x) according to the formula Pt(x)=15.7-25.4x+19.0x(2) (GPa). Pressure-dependent longitudinal and transverse optical phonon frequencies and the calculated mode Gruneisen parameters were adopted to investigate the influence of Mn(2+) ions on the iconicity. The experimental results indicate that the manganese ions tend to increase the iconicity of ZnTe under ambient conditions, whereas an external hydrostatic pressure tends to reduce the iconicity and the bond length of Zn(1-x)Mn(x)Te. (c) 2008 American Institute of Physics.
Quantum-dot exciton dynamics with a surface plasmon: Band-edge quantum optics
(2009-03-01)
Chen, Y. N.
;
Chen, G. Y.
;
Chuu, D. S.
;
Brandes, T.
;
電子物理學系
We examine the spontaneous emission of a two-level emitter, quantum-dot exciton, into surface plasmons propagating on the surface of a cylindrical nanowire. The numerically obtained dispersion relations are found to influence the spontaneous emission rate strongly. At certain values of the exciton band gap, the emission rates can go to infinity due to the band-edge feature of the dispersion relations. Borrowing the idea from the photonic crystals, we model the quantum-dot exciton dynamics with a non-Markovian way and demonstrate that the decay can undergo an oscillatory behavior. In addition, we also show that the remote entangled states can be generated via super-radiance by using the one-dimensional propagating feature of the nanowire surface plasmons.
Raman scattering of longitudinal-optical-phonon-plasmon coupling in Cl-doped ZnSe under high pressure
(2007-12-15)
Lin, Y. C.
;
Chiu, C. H.
;
Fan, W. C.
;
Chia, C. H.
;
Yang, S. L.
The vibrational, electronic, and crystalline properties of n-type chlorine-doped ZnSe (ZnSe:Cl) layers with a carrier concentration from 8.2x10(15) to 1.8x10(18) cm(-3) are studied by Raman spectroscopy. The spectral line shapes of the longitudinal-optical-phonon and plasmon coupling mode are analyzed using the Raman scattering efficiency and the dielectric function to obtain the electron densities and mobility. The splitting of the transverse-optical (TO) phonon and the redshift of the chlorine-related impurity vibration mode are clearly observed when pressure is applied. The semiconductor-to-metal phase transition pressure of ZnSe:Cl layers declines as the carrier concentration increases, indicating that n-type doping reduces crystal stability. Additionally, the pressure-induced weakening of the longitudinal-optical-phonon-plasmon coupling efficiency suggests that pressure tends to degrade the n-type characteristic of ZnSe:Cl because of the emergence of the new deep donorlike state. (c) 2007 American Institute of Physics.
Single fluorescent gold nanoclusters
(2009-08-31)
Yuan, C. T.
;
Chou, W. C.
;
Tang, J.
;
Lin, C. A.
;
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.
;
Lin, Y. C.
;
Chen, Y. N.
;
Chiu, Q. L.
;
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.