| 1. |
- Han, Tiantian, et al.
(författare)
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Room-temperature photoluminescence of doped 4H-SiC film grown on AlN/Si(100)
- 2007
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Ingår i: Applied Physics A. - : Springer Science and Business Media LLC. - 0947-8396 .- 1432-0630. ; 86:1, s. 145-149
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Tidskriftsartikel (refereegranskat)abstract
- Well-defined room-temperature photoluminescence (PL) was observed from 4H-SiC films on AlN/Si(100) complex substrates grown at temperatures below 1150 degrees C by the chemical vapor deposition method. The PL spectrum consists of three major emission peaks in the vicinities of 3.03, 3.17 and 3.37 eV. By the combination of experimental measurements and theoretical analysis, the origins of the PL emission peaks have been identified and associated with N donors, Al acceptors in the 4H-SiC films and the band-to-band transition between the second minimum of the conduction band and the top of valance band of the 4H-SiC. The room-temperature SiC PL can be much utilized for optoelectronic high-power, high-frequency and high-temperature applications in the ultraviolet spectral regime.
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| 2. |
- Fu, Ying, et al.
(författare)
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Design of semiconductor CdSe core ZnS/CdS multishell quantum dots for multiphoton applications
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:17, s. 173102-1-173102-3
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Tidskriftsartikel (refereegranskat)abstract
- Optical properties of colloidal II-VI semiconductor CdSe cores with ZnS and CdS multishell quantum dots (QDs) have been studied by experimental characterization and theoretical analysis. Due to the large number of energy states densely compacted in both conduction and valence bands of the quantum dots, strong interband and intraband optical couplings are induced by the multiphoton excitation, implicating an efficient fluorescence of such II-VI-based core-multishell CdSe QDs. This fact in combination with the advantage of the size tolerance of II-VI QDs with respect to the narrow fluorescence bandwidth make these systems excellent candidates for applications using fluorescence induced by multiphoton excitation.
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| 3. |
- Fu, Ying, et al.
(författare)
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Dynamic analysis of multiple-photon optical processes in semiconductor quantum dots
- 2006
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 18:39, s. 9071-9082
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Tidskriftsartikel (refereegranskat)abstract
- Semiconductor quantum dots (QDs) have been gaining much attention because of their outstanding properties for multiple-photon microscopy applications. By solving nonperturbatively the time-dependent Schrodinger equation, it has been shown that the large number of energy states densely compacted in both the conduction and valence bands of the QD greatly enhance the inter-band and intra-band optical couplings between two energy states induced by multiple photons from ultra-fast and ultra-intense lasers. The multiphoton absorption processes are further enhanced by many energy relaxation processes in commonly used semiconductors, which are generally represented by the relaxation energy in the order of tens of meV. Numerical calculation of multiphoton processes in QDs agrees with experimental demonstration. After proper designing, QDs can be activated by infrared radiation to emit radiation in the visible optical regime (up-conversion) for bioimaging applications.
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| 4. |
- Fu, Ying, et al.
(författare)
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Multiphoton excitation of quantum dots by ultrashort and ultraintense laser pulses
- 2006
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:22, s. 221114-1-221114-3
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Tidskriftsartikel (refereegranskat)abstract
- Multiphoton optical processes in semiconductor quantum dots (QDs) excited by ultrafast (femtosecond) and ultraintense (GW/cm(2)) lasers are considered as the ultimate tags for cellular bioimaging. By solving the time-dependent Schrodinger equation unperturbatively, experimentally observed strong multiphoton excitation is reproduced when optical transitions among all confined states and a few hundred more extended states are taken into account. Model calculations indicate a significant excitation of a CdS QD of 3.7 nm in radius by a 100 fs laser pulse with a 10 GW/cm(2) peak optical power. The excitation rate is almost constant between 1300 and 560 nm, a useful region for bioimaging.
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| 5. |
- Han, Tiantian, et al.
(författare)
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Dynamic photon emission from multiphoton-excited semiconductor quantum dots
- 2008
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 103:9, s. 093703-1-093703-6
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Tidskriftsartikel (refereegranskat)abstract
- The dynamic process of multiphoton optical transitions in semiconductor quantum dots (QDs) has been studied by a Monte Carlo scheme. The scheme includes optical transitions of all electrons, initially occupying the valence-band confined states in the QD, among the confined states in valence and conduction bands. The optical transition probabilities are calculated by the time-dependent Schrodinger equation, and nonradiative phonon scattering processes have been included. Assisted by a two-photon excitation by a continuous-wave laser (one photon energy equals half of the QD energy band gap), an assembly of the QDs shows an emission peak around the band gap in the optical emission spectrum, while an ultrafast pulsed laser, whose photon energy is below the QD band gap, also induces a similar narrow but weaker emission peak, which results in a nonstrict multiphoton excitation condition for many potential applications including biophotonics. Extension of the theoretical study to the spherical CdS/Cd0.5Zn0.5S/ZnS-multicoated CdSe QD has reproduced the experimental absorption and multiphoton emission spectra.
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| 6. |
- Han, Tiantian, 1980-
(författare)
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Optical Properties of Low Dimensional Semiconductor Materials
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation presents a serial study on optical properties of different semiconductor materials. Three main types of studies are addressed: The role of doping levels of N and Al atoms in the room-temperature photoluminescence (PL) of 4H-SiC films for optoelectronic applications; the use of a basic Monte Carlo method combined with probability calculations of the time-dependent Schroedinger equation to manifest multi-photon absorption and emission of II-VI compound quantum dots (QDs) for bioimaging; a theoretical quantum chemistry approach to study of structure and optical properties of InGaAsN and GaAs clusters for laser technology applications. 4H-SiC films were grown on AlN/SiC(100) substrates by a chemical vapour deposition (CVD) system. Three well-defined room-temperature PL peaks close to the band-gap energy were observed. By a detailed theoretical analysis of optical transitions in the samples, it was found that the PL peaks are most probably due to the optical transitions between impurity levels and band edges, and the transition between the second minimum of the conduction band and the top of the valance band. Special attention has been paid to effects of doping levels of N and Al impurities. Optical transitions in several II-VI semiconductor QDs have been studied by a quantum Monte Carlo method. We model the QD energy band structure by a spherical square quantum well and the electrons in the conduction band and holes in the valence band by the effective mass approximation. The optical probabilities of optical transitions induced by ultrafast and ultraintense laser pulses are calculated from the time-dependent Schroedinger equation. With the inclusion of the nonradiative electron-phonon processes, the calculated absorption and emission spectra are in agreement with experimental work. The dynamic processes and up-conversion luminescence of the QDs, required for many applications including bio-imaging, are demonstrated. Quantum chemistry is used to study InGaAsN and GaAs nano systems. The molecular structures of a series of dilute-nitride zinc blende InGaNAs clusters are examined from the energy point of view with a semi-empirical method. The optimum cluster configurations are identified by which we can identify the detailed bonding structures and the effects of In mole fraction. After proper geometry construction, an effective central insertion scheme has been implemented to study the electronic band structures of GaAs at the first-principles level. The formation of energy bands and quantum confinement effects have been revealed, thus providing theoretical support for laser design.
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| 7. |
- Han, Tiantian, et al.
(författare)
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Optical properties of multi-coated CdSe/CdS/ZnS quantum dots for multiphoton applications
- 2008
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 41:11, s. 115104-
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Tidskriftsartikel (refereegranskat)abstract
- CdSe-core CdS/Cd(0.5)Zn(0.5)S/ZnS multishell quantum dots (QDs) were synthesized by using the successive ion layer adsorption and reaction method, and their optical properties were characterized experimentally. Based on probability calculations of the time-dependent Schrodinger equation, a kinetic Monte Carlo method was used to simulate and analyse the absorption and spontaneous emission spectra of multiphoton-excited QDs. It has been shown that the blue shift of the photoluminescence peak of the core CdSe QD after coating is most probably due to the Zn diffusion during the synthesis of the ZnS coating layer.
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| 8. |
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| 9. |
- Han, Tiantian, et al.
(författare)
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Radiative emission from multiphoton-excited semiconductor quantum dots
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:6, s. 063712-1-063712-6
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Tidskriftsartikel (refereegranskat)abstract
- Optical transitions in CdS semiconductor quantum dots (QDs) have been studied by the Monte Carlo method based on probability calculations of the time-dependent Schrodinger equation. It has been demonstrated that excited by a continuous-wave laser, an assembly of CdS QDs, whose radii range from 2 to 5 nm centered at 3.7 nm, shows an emission peak around 2.65 eV in the optical emission spectrum, which corresponds to optical transitions among degenerate sublevels close to the ground sublevels in the conduction and valence bands of a CdS QD having a radius of 3.7 nm. For resonant one-photon excitation, the emission peak is very sharp, while for resonant two-photon excitation, the emission peak becomes blueshifted and broadened. The inclusion of the nonradiative electron-phonon processes makes the two-photon excitation peak significantly sharper and shows a better agreement with experimental work, thus demonstrating the upconversion luminescence of the QDs required for many applications including bioimaging.
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| 10. |
- Han, Tiantian, et al.
(författare)
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Structural analysis of dilute-nitride zinc blende InxGa1−xNyAs1−y cluster by a semiempirical quantum chemistry study
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 101:12, s. 123707-1-123707-6
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the total energy of a series of dilute-nitride zinc blende Inx Ga1-x Ny As1-y cluster configurations using a semiempirical quantum chemistry method. It was found that In-N bonding is favorable from an energy point of view when the N atom is substitutional (replacing an As atom at the regular zinc blende lattice site) and the In mole fraction is smaller than 25%. In-N bonding is always favorable when the N atom is interstitial. Furthermore, an analysis of the incorporation of N-N pairs showed that substitutional incorporation is favored over interstitial. In addition, the dissociation of a N-N pair was found to depend on the local environment, being either In rich or In-free, along the dissociation trajectory when the average In mole fraction is high. The theoretical results are in agreement with experimental results.
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