Search

Atomistic semiempirical quantum mechanical calculations were performed on the interaction between CdSe and CdSe/CdS core/shell nanocrystals and MPOH. Results suggest that the experimentally observed changes in binding energy are due to the distribution of surface facets on the nanocrystals and not related to band gap.

Charging effects in a CdSe nanotetrapod are theoretically investigated by using an atomistic pseudopotential method. It is found that surface polarization can significantly change the electron wave functions and depending on the situation the tetrapod can behave like a single coherent dot to weakly coupled multiple dots.

Equilibrium geometries, surface energies, and surfactant binding energies are calculated for selected bulk facets of wurtzite CdSe with a fist-principle approach. Passivation of the surface Cd atoms with alkyl phosphonic acids or amines lowers the surface energy of all facets, expect for the polar 0001 facet.

The electronic spectrum of CdS colloidal quantum dots (QDs) with a radius of 1.0-2.3 nm was studied by low-temperature photoluminescence excitation spectroscopy. CdS QDs were found to exhibit a resonant Stokes shift of ~20-70 meV, which was ~4 times larger than similarly sized CdSe QDs.

Atomistic empirical pseudopotential calculations are used for determining the influence of external charges on the radiative recombination rate of an electron-hole pair and the photoluminescence of a CdSe quantum dot. It is seen that when the negative external charge is near the surface of the CdSe quantum dot, then its Coulomb potential could be very strong and capable of pulling the hole away from the electron, thus resulting in the reduction of the radioactive recombination rate.

We present large-scale calculations of electronic structure of strongly disordered conjugated polymers. The calculations have been performed using the density functional theory based charge patching method for the construction of single-particle Hamiltonian and the overlapping fragments method for the efficient diagonalization of that Hamiltonian. We find that the hole states are localized due to the fluctuations of the electrostatic potential and not by the breaks in the conjugation of the polymer chain. The tail of the density of hole states exhibits an exponentially decaying behavior. The main features of the electronic structure of the system can be described by an one-dimensional nearest neighbor tight-binding model with a correlated Gaussian distribution of on-site energies and constant off-site coupling elements. [ABSTRACT FROM AUTHOR]

Electronic structure of disordered semiconducting conjugated polymers was studied. Atomic structure was found from a classical molecular dynamics simulation, and the charge patching method was used to calculate the electronic structure with the accuracy similar to the one of density functional theory in local density approximation. The total density of states, the local density of states at different points in the system, and the wave functions of several states around the gap were calculated in the case of poly(3-hexylthiophene) (P3HT) and polythiophene (PT) systems to gain insight into the origin of disorder in the system, the degree of carrier localization, and the role of chain interactions. The results indicated that disorder in the electronic structure of alkyl-substituted polythiophenes comes from disorder in the conformation of individual chains, while in the case of polythiophene there is an additional contribution due to disorder in the electronic coupling between the chains. Each of the first several wave functions in the conduction and valence band of P3HT is localized over several rings of a single chain. It was shown that the localization can be caused in principle both by ring torsions and chain bending; however, the effect of ring torsions is much stronger. PT wave functions are more complicated due to larger interchain electronic coupling and are not necessarily localized on a single chain. [ABSTRACT FROM AUTHOR]

Recent experiments have indicated that 3-mercapto-1-propanol ligands display a size-dependent binding energy of attachment to the surface of II−VI semiconductor nanocrystals. Using semiempirical calculations, we exhaustively calculate the energy of this bond at each surface site, for CdSe and CdSe/CdS core/shell nanocrystals ranging from 1.8 to 4.1 nm in diameter. Our results suggest that the experimentally observed changes in binding energy are due to the distribution of surface facets on the nanocrystals, and not related to the band gap, as proposed in the experimental paper. [ABSTRACT FROM AUTHOR]