64 research outputs found
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.
Tight-binding g-Factor Calculations of CdSe Nanostructures
The Lande g-factors for CdSe quantum dots and rods are investigated within
the framework of the semiempirical tight-binding method. We describe methods
for treating both the n-doped and neutral nanostructures, and then apply these
to a selection of nanocrystals of variable size and shape, focusing on
approximately spherical dots and rods of differing aspect ratio. For the
negatively charged n-doped systems, we observe that the g-factors for
near-spherical CdSe dots are approximately independent of size, but show strong
shape dependence as one axis of the quantum dot is extended to form rod-like
structures. In particular, there is a discontinuity in the magnitude of
g-factor and a transition from anisotropic to isotropic g-factor tensor at
aspect ratio ~1.3. For the neutral systems, we analyze the electron g-factor of
both the conduction and valence band electrons. We find that the behavior of
the electron g-factor in the neutral nanocrystals is generally similar to that
in the n-doped case, showing the same strong shape dependence and discontinuity
in magnitude and anisotropy. In smaller systems the g-factor value is dependent
on the details of the surface model. Comparison with recent measurements of
g-factors for CdSe nanocrystals suggests that the shape dependent transition
may be responsible for the observations of anomalous numbers of g-factors at
certain nanocrystal sizes.Comment: 15 pages, 6 figures. Fixed typos to match published versio
Electron-Hole Correlations and Optical Excitonic Gaps in Quantum-Dot Quantum Wells: Tight-Binding Approach
Electron-hole correlation in quantum-dot quantum wells (QDQW's) is
investigated by incorporating Coulomb and exchange interactions into an
empirical tight-binding model. Sufficient electron and hole single-particle
states close to the band edge are included in the configuration to achieve
convergence of the first spin-singlet and triplet excitonic energies within a
few meV. Coulomb shifts of about 100 meV and exchange splittings of about 1 meV
are found for CdS/HgS/CdS QDQW's (4.7 nm CdS core diameter, 0.3 nm HgS well
width and 0.3 nm to 1.5 nm CdS clad thickness) which have been characterized
experimentally by Weller and co-workers [ D. Schooss, A. Mews, A. Eychmuller,
H. Weller, Phys. Rev. B, 49, 17072 (1994)]. The optical excitonic gaps
calculated for those QDQW's are in good agreement with the experiment.Comment: 3 figures, to appear in Phys.Rev.
Excitonic Transitions and Off-resonant Optical Limiting in CdS Quantum Dots Stabilized in a Synthetic Glue Matrix
Stable films containing CdS quantum dots of mean size 3.4 nm embedded in a solid host matrix are prepared using a room temperature chemical route of synthesis. CdS/synthetic glue nanocomposites are characterized using high resolution transmission electron microscopy, infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. Significant blue shift from the bulk absorption edge is observed in optical absorption as well as photoacoustic spectra indicating strong quantum confinement. The exciton transitions are better resolved in photoacoustic spectroscopy compared to optical absorption spectroscopy. We assign the first four bands observed in photoacoustic spectroscopy to 1se–1sh, 1pe–1ph, 1de–1dhand 2pe–2phtransitions using a non interacting particle model. Nonlinear absorption studies are done using z-scan technique with nanosecond pulses in the off resonant regime. The origin of optical limiting is predominantly two photon absorption mechanism
119Sn Mössbauer study of nickel-tin anodes for rechargeable lithium-ion batteries
International audienc
- …