117 research outputs found
Time-Resolved Intraband Relaxation of Strongly-Confined Electrons and Holes in Colloidal PbSe Nanocrystals
The relaxation of strongly-confined electrons and holes between 1P and 1S
levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond
transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor
nanocrystals, both electrons and holes are strongly confined in PbSe
nanocrystals. Despite the large electron and hole energy level spacings (at
least 12 times the optical phonon energy), we consistently observe picosecond
time-scale relaxation. Existing theories of carrier relaxation cannot account
for these experimental results. Mechanisms that could possibly circumvent the
phonon bottleneck in IV-VI quantum dots are discussed
Control of surface plasmon resonance in out-diffused silver nanoislands for surface-enhanced Raman scattering
Abstract We present the studies of self-assembled silver nanoislands on the surface of silver ion-exchanged glasses. The nanoislands were formed by out-diffusion of reduced silver atoms from the bulk of the glass to its surface. Control of silver ions distribution in the glass by thermal poling after the ion exchange allowed formation of relatively big, up to 250 nm, isolated silver nanoislands while without the poling an ensemble of silver nanoislands with average size from several to tens of nanometers with random size distribution was formed. The nanoislands were characterized using atomic force microscopy and spectral measurements. We used optical absorption spectroscopy for ârandomâ nanoislands and dark field scattering spectroscopy for isolated ones, corresponding spectra showed peaks in the vicinity of 450 nm and 600 nm, respectively. The ârandomâ nanoislands significantly enhanced Raman scattering from Rhodamine 6G, also the modification of Raman signal from deposited on the surface of the samples bacteriorhodopsin in purple membranes was registered
Dark-field spectroscopy of plasmon resonance in metal nanoislands: effect of shape and light polarization
Abstract We present the experimental dark-field scattering studies and the simulation of plasmonic properties of isolated silver nanoislands. The nanoislands were fabricated on a soda- lime glass substrate using silver-sodium ion exchange, subsequent thermal poling and annealing of the processed glass substrate in hydrogen. The morphology of the nanoislands was characterized with atomic force microscopy and scanning electron microscopy; the dimensions were 100-180 nm in base and 80-160 nm in height. We measured and modeled dark-field scattering spectra of the silver hemiellipsoidal nanoparticles differing in size and shape. The SPR position varied from 450 nm to 730 nm depending on the particle shape and dimensions. Both experiments and simulation showed a red shift of the SPR for bigger nanoislands of the same shape. Losing the axial symmetry in nanoislands resulted in the resonance splitting, while their elongation led to an increase in the scattering of p-polarized light
Complementary detection of confined acoustic phonons in quantum dots by coherent phonon measurement and Raman scattering
Coherent acoustic phonon oscillation is observed in PbSe quantum dots embedded in phosphate glass by femtosecond pump-and-probe. The size dependence of the oscillation is investigated. Distinct low-frequency peaks are observed in Raman spectrum for the same samples. The size-dependence of the frequencies is well explained by elastic sphere model, but the observed modes are different to each other for coherent phonon and Raman scattering. Coherent phonon measurement and Raman scattering are found to give complementary information on confined acoustic phonons in this system
Stationary structures in two-dimensional continuous Heisenberg ferromagnetic spin system
Stationary structures in a classical isotropic two-dimensional continuous
Heisenberg ferromagnetic spin system are studied in the framework of the
(2+1)-dimensional Landau-Lifshitz model. It is established that in the case of
\vec S (\vec r, t)= \vec S (\vec r - \vec v t) the Landau-Lifshitz equation is
closely related to the Ablowitz-Ladik hierarchy. This relation is used to
obtain soliton structures, which are shown to be caused by joint action of
nonlinearity and spatial dispersion, contrary to the well-known one-dimensional
solitons which exist due to competition of nonlinearity and temporal
dispersion. We also present elliptical quasiperiodic stationary solutions of
the stationary (2+1)-dimensional Landau-Lifshitz equation.Comment: Archive version is already official Published by JNMP at
http://www.sm.luth.se/math/JNMP
Photoluminescence Stokes shift and exciton fine structure in CdTe nanocrystals
The photoluminescence spectra of spherical CdTe nanocrystals with zincblende
structure are studied by size-selective spectroscopic techniques. We observe a
resonant Stokes shift of 15 meV when the excitation laser energy is tuned to
the red side of the absorption band at 2.236 eV. The experimental data are
analyzed within a symmetry-based tight-binding theory of the exciton spectrum,
which is first shown to account for the size dependence of the fundamental gap
reported previously in the literature. The theoretical Stokes shift presented
as a function of the gap shows a good agreement with the experimental data,
indicating that the measured Stokes shift indeed arises from the electron-hole
exchange interaction.Comment: 8 pages, 4 figures, LaTe
Strong confinement of PbSe and PbS quantum dots
We synthesized PbSe and PbS quantum dots in strong-confinement regime, and measured energy relaxation time by using pump-and-probe experiments. Energy relaxation time of PbSe dots in phosphate glasses showed clear correlation with the average radius. Smaller dots were shown to have shorter decay times. This dependence is ascribed to the relaxation to the surface of the dots
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