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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 studies have shown that the selectivity for cation exchange to take place at different facets of the nanocrystal has played a major role in determining the resulting morphology of the binary heterostructure. The epitaxial attachments of copper sulfide ([Cu.sub.2]S) to the end facets of CdS nanorods have minimized the formation energy, thus making the interfaces stable throughout the exchange reaction.

The solution-liquid-solid (SLS) method is employed for the synthesis of the high-quality colloidal cadmium telluride (CdTe) quantum wires. The band gaps of these wires are shown to be extremely similar to the equidiameter cadmium telluride quantum dots.

The various spectroscopic properties of the colloidal indium phosphide quantum wires are discussed. These wires are found to have extremely low photoluminescence efficiencies.

The size dependence of the band gaps in CdSe quantum wires, and its comparison to the corresponding size dependence in CdSe quantum dots and rods is reported.

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.

While a large variety of organic and molecular materials have been found to exhibit charge memory effects, the underlying mechanism is not well-understood, which hinders rational device design. Here, we study the charge retention mechanism of a nanoscale memory system, an organic monolayer on a silicon substrate, with Au nanoparticles on top serving as the electrical contact. Combining scanning probe imaging/manipulation and density functional simulations, we observe stable charge retention effects in the system and attributed it to polaron effects at the amine functional groups. Our findings can pave the way for applications in nonvolatile memory, neuromorphic computing, and high dielectric breakdown devices. [ABSTRACT FROM AUTHOR]

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.

CH3NH3PbI3 is one of the most promising candidates for cheap and high-efficiency solar cells. One of its unique features is the long carrier diffusion length (>100 μm), but its carrier mobility is rather modest. The nature of the mobility is unclear. Here, using nonadiabatic wave function dynamics simulations, we show that the random rotations of the CH3NH3 cations play an important role in the carrier mobility. Our previous work showed that the electron and hole wave functions were localized and spatially separated due to the random orientations of the CH3NH3 cations in the tetragonal phase. We find that the localized carriers are able to conduct random walks due to the electrostatic potential fluctuation caused by the CH3NH3 random rotations. The calculated electron mobilities are in the experimentally measured range. We thus conclude that the carrier mobility of CH3NH3PbI3 is likely driven by the dynamic disorder that causes the fluctuation of the electrostatic potential. [ABSTRACT FROM AUTHOR]

An original ab initio procedure isdeveloped and applied to a ferroelectric nanocomposite, in order toreveal the effect of electrical vortices on electronic properties.Such procedure involves the combination of two large-scale numericalschemes, namely, the effective Hamiltonian (to incorporate ionic degreesof freedom) and the linear-scaling three-dimensional fragment method(to treat electronic degrees of freedom). The use of such proceduresheds some light into the origin of the recently observed currentthat is activated at rather low voltages in systems possessing electricalvortices. It also reveals a novel electronic phenomena that is a systematiccontrol of the type of the band-alignment (i.e., type I versus typeII) within the same material via the temperature-driven annihilation/formationof electrical topological defects. [ABSTRACT FROM AUTHOR]

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]

We developed an abinitio multiscale method for simulation of carrier transport in large disordered systems, based on direct calculation of electronic states and electron−phonon coupling constants. It enabled us to obtain the never seen before rich microscopic details of carrier motion in conjugated polymers, which led us to question several assumptions of phenomenological models, widely used in such systems. The macroscopic mobility of disordered poly(3-hexylthiophene) (P3HT) polymer, extracted from our simulation, is in agreement with experimental results from the literature. [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]

The electronic structure of nanowires in contact with metallic electrodes of experimentally relevant sizes is calculated by incorporating the electrostatic image potential into the atomistic single particle Schrödinger equation. We show that the presence of an electrode produces localized electron/hole states near the electrode. We found a strong nanowire size dependence of this localization effect. We calculate several electrode/nanowire geometries, with varying contact depths and nanowire radii. We demonstrate the change in the band gap of up to 0.5 eV in 3 nm diameter CdSe nanowires and calculate the magnitude of the applied electric field necessary to overcome the localization. [ABSTRACT FROM AUTHOR]

Type II core−shell nanowires based on III−V and II−VI semiconductors are designed to provide the highly desirable but not readily available featureefficient charge separationand concurrently address the different material challenges specific for a few key renewable energy applications:  including hydrogen generation via photoelectrochemical water splitting, dye-sensitized solar cells, and conventional solar cells. They also open up new avenues for studying novel physics and material sciences in reduced dimensionality of very unusual quasi-one-dimensional systems. A first-principles density function theory within the local density approximation (LDA) is used for the electronic structure calculation and a valence-force-field method for the strutural relaxation, and emprical corrections to the LDA errors are applied. [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]

The partial transformation of ionic nanocrystals through cation exchange has been used to synthesize nanocrystal heterostructures. We demonstrate that the selectivity for cation exchange to take place at different facets of the nanocrystal plays an important role in determining the resulting morphology of the binary heterostructure. In the case of copper(l) (Cut) cation exchange in cadmium sulfide (CdS) nanorods, the reaction starts preferentially at the ends of the nanorods such that copper sulfide (Cu2S) grows inward from either end. The resulting morphology is very different from the striped pattern obtained in our previous studies of silver(I) (Ag+) exchange in CdS nanorods where nonselective nucleation of silver sulfide (Ag2S) occurs (Robinson, R. D.; Sadtler, B.; Demchenko, D. O.; Erdonmez, C. K.; Wang, L.-W.; Alivisatos, A. P. Science 2007, 317, 355-358). From interface formation energies calculated for several models of epitaxial connections between CdS and Cu2S or Ag2S, we infer the relative stability of each interface during the nucleation and growth of Cu2S or Ag2S within the CdS nanorods. The epitaxial attachments of Cu2S to the end facets of CdS nanorods minimize the formation energy, making these interfaces stable throughout the exchange reaction. Additionally, as the two end facets of wurtzite CdS nanorods are crystallographically nonequivalent, asymmetric heterostructures can be produced. [ABSTRACT FROM AUTHOR]

The recent availability of semiconductor nanocrystals having anisotropic rod, wire, arrow, teardrop and tetrapod morphologies has renewed interest in how shape affects the electronic and optical properties of quantum nanostructures. The article discusses the size dependence of the band gaps in cadmiumselenide quantum wires and compares it to the corresponding size dependences in cadmiumselenide quantum dots and rods. It shows that the experimentally measured quantum wire band gaps are consistent with theoretical predictions, confirming that geometric dimensionality influences quantum confinement as expected. Our results quantify true 2-dimensional quantum confinement in the well-studied cadmiumselenide system.