17 research outputs found
Calculations on the Size Effects of Raman Intensities of Silicon Quantum Dots
Raman intensities of Si quantum dots (QDs) with up to 11,489 atoms (about 7.6
nm in diameter) for different scattering configurations are calculated. First,
phonon modes in these QDs, including all vibration frequencies and vibration
amplitudes, are calculated directly from the lattice dynamic matrix by using a
microscopic valence force field model combined with the group theory. Then the
Raman intensities of these quantum dots are calculated by using a
bond-polarizability approximation. The size effects of the Raman intensity in
these QDs are discussed in detail based on these calculations. The calculations
are compared with the available experimental observation. We are expecting that
our calculations can further stimulate more experimental measurements.Comment: 21 pages, 7 figure
Electric field and exciton structure in CdSe nanocrystals
Quantum Stark effect in semiconductor nanocrystals is theoretically
investigated, using the effective mass formalism within a
Baldereschi-Lipari Hamiltonian model for the hole states. General expressions
are reported for the hole eigenfunctions at zero electric field. Electron and
hole single particle energies as functions of the electric field
() are reported. Stark shift and binding energy of the
excitonic levels are obtained by full diagonalization of the correlated
electron-hole Hamiltonian in presence of the external field. Particularly, the
structure of the lower excitonic states and their symmetry properties in CdSe
nanocrystals are studied. It is found that the dependence of the exciton
binding energy upon the applied field is strongly reduced for small quantum dot
radius. Optical selection rules for absorption and luminescence are obtained.
The electric-field induced quenching of the optical spectra as a function of
is studied in terms of the exciton dipole matrix element. It
is predicted that photoluminescence spectra present anomalous field dependence
of the emission lines. These results agree in magnitude with experimental
observation and with the main features of photoluminescence experiments in
nanostructures.Comment: 9 pages, 7 figures, 1 tabl
Evolution of the electronic structure with size in II-VI semiconductor nanocrystals
In order to provide a quantitatively accurate description of the band gap
variation with sizes in various II-VI semiconductor nanocrystals, we make use
of the recently reported tight-binding parametrization of the corresponding
bulk systems. Using the same tight-binding scheme and parameters, we calculate
the electronic structure of II-VI nanocrystals in real space with sizes ranging
between 5 and 80 {\AA} in diameter. A comparison with available experimental
results from the literature shows an excellent agreement over the entire range
of sizes.Comment: 17 pages, 4 figures, accepted in Phys. Rev.
STABLE AMORPHOUS GERMANIUM FILMS PREPARED IN ULTRA HIGH VACUUM AND MEASURED IN-SITU : STRUCTURE AND ELECTRONIC PROPERTIES
Stable, homogeneous germanium films have been prepared by slow evaporation onto heated sapphire substrates in ultra high vacuum. Their properties do not change on annealing until they crystallise at 250°C (1,2). The structure of these films is characterised by low density, by a shift of the first diffraction peak to higher scattering angle, with respect to the crystal and by the existance of a diffraction pre-peak. The optical absorption edge is relatively sharp, has a shoulder at 0.85 eV, and the low photon energy refractive index has a lower value than for the crystal. The temperature T dependence of the d.c. electrical conductivity σ is characterised by an "S" shaped curve, when plotted as log σ versus T-¼. The photoconductivity shows marked long time ( t > 103 sec) relaxation effects at 77K and exhibits photoconductivity fatigue at 5K. These results are compared with amorphous germanium (a-Ge) films prepared under different conditions and with other amorphous and glassy materials. Some common trends in the structure and electronic properties are pointed out
Hydrostatic pressure, impurity position and electric and magnetic field effects on the binding energy and photo-ionization cross section of a hydrogenic donor impurity in an InAs Pöschl-Teller quantum ring
Using the variational method and the effective mass and parabolic band approximations,
the behaviour of the binding energy and photo-ionization cross section of a
hydrogenic-like donor impurity in an InAs quantum ring, with Pöschl-Teller confinement
potential along the axial direction, has been studied. In the investigation, the combined
effects of hydrostatic pressure and electric and magnetic fields applied in the direction
of growth have been taken into account. Parallel polarization of the incident radiation
and several values of the applied electric and magnetic fields, hydrostatic pressure, and
parameters of the Pöschl-Teller confinement potential were considered. The results
obtained can be summarised as follows: (1) the influence of the applied electric and
magnetic fields and the asymmetry degree of the Pöschl-Teller confinement potential on the
donor binding energy is strongly dependent on the impurity position along the growth and
radial directions of the quantum ring, (2) the binding energy is an increasing function of
hydrostatic pressure and (3) the decrease (increase) in the binding energy with the
electric and magnetic fields and parameters of the confinement potential (hydrostatic
pressure) leads to a red shift (blue shift) of the maximum of the photo-ionization cross
section spectrum of the on-centre impurity