Search

We develop a novel approach to treat excitation energy transfer in hybrid nanosystems composed by an organic molecule attached to a semiconductor nanoparticle. Our approach extends the customary Förster theory by considering interaction between transition multipole moments of the nanoparticle at all orders and a point-like transition dipole moment representing the molecule. Optical excitations of the nanoparticle are described through an envelope-function configuration interaction method for a single electron-hole pair. We applied the method to the prototypical case of a core/shell CdSe/ZnS semiconductor quantum dot which shows a complete suppression of the energy transfer for specific transitions which could not be captured by Förster theory. [ABSTRACT FROM AUTHOR]

Due to the growing interest in the ferromagnetic properties of Fe-octaethylporphyrins (Fe-OEP) for applications in spintronics, methods to produce stable Fe-porphyrins with no Cl atoms are highly demanded. Here, we demonstrate the formation of Fe-OEP layers on Ag(111) single crystal by the ultra high vacuum in situ metalation of the free-base 2H-2,3,7,8,12,13,17,18-octaethylporphyrin (2H-OEP) molecules. The metalation proceeds exactly as in the case of 2H-5,10,15,20-tetraphenylporphyrin (2H-TPP) on the same substrate. An extensive surface characterization by means of X-ray photoemission spectroscopy, valence band photoemission, and NEXAFS with synchrotron radiation light provides information on molecular conformation and electronic structure in the monolayer and multilayer cases. We demonstrate that the presence of the ethyl groups affects the tilt of the adsorbed molecules, the conformation of the macrocycle, and the polarization screening in multilayers, but has only a minor effect in the metalation process with respect to 2H-TPP. [ABSTRACT FROM AUTHOR]

We demonstrate the possibility to achieve the doping of eumelanin thin films through K+ incorporation during the electrodeposition of the film. K-doping changes the optical properties of the eumelanin thin films, reducing the energy gap from 1.0 to 0.6 eV, with possible implications for the photophysical properties. We have identified the doping-related occupied and unoccupied electronic states and their spectral weight using resonant photoemission spectroscopy (ResPES) and x-ray absorption at the C and N K-edges (near edge x-ray absorption fine spectroscopy, NEXAFS). All data are consistently interpreted by ab initio calculations of the electronic structure within the frame of the macrocycle model developed for the eumelanin protomolecule. Our analysis puts in evidence the intercalation of K with one specific oligomer (a tetramer composed of one indolequinone and 3 hydroquinone monomers) in correspondence of the nitrogen macrocycle. The predicted variation of the tetramer spacing is also in agreement with the recent x-ray diffraction experiments. The charge donation from K to N and C atoms gives rise to new electronic states at the top of the valence band and in NEXAFS resonances of the unoccupied orbitals. The saturation of the tetramer macrocycles leaves an excess of K that bind to N and C atoms in alternative configurations, as witnessed by the occurrence of additional spectral features in the carbon-related ResPES measurements. [ABSTRACT FROM AUTHOR]

The electronic structure of single and multiple layers of C60 molecules deposited on a Rh(100) surface is investigated by means of valence photoemission spectroscopy and density functional theory calculations. The binding of the fullerene monolayer to the metal surface yields the appearance of a new state in the valence band spectrum crossing the Fermi level. Insight into the metallization of the metal/fullerene interface is provided by the calculated electronic structure that allows us to correlate the measured interface state with a strong hybridization between the Rh metal states and the highest and lowest molecular orbitals. This results in a net charge transfer of ≈0.5e–0.6e from the metal to the p states of the interfacial C atoms. The charge transfer is shown to be very short range, involving only the C atoms bound to the metal. The electronic structure of the second C60 layer is already insulating and resembles the one measured for C60 multilayers supported by the same substrate or calculated for fullerenes isolated in vacuum. The discussion of the results in the context of other C60/metal systems highlights the distinctive electronic properties of the molecule/metal interface determined by the Rh support. [ABSTRACT FROM AUTHOR]

In this study synchrotron radiation was used to investigate the electronic properties of a thin film of zinc-phthalocyanine (ZnPc) deposited on Si(001)-2×1 and progressively doped with K atoms. The molecular orientation was probed by angular-dependent x-ray absorption spectroscopy and the molecules were found to lie with the macrocycle plane roughly perpendicular to the surface. The evolution of the electronic properties of the film was then followed by measuring the photoemission spectra upon in situ evaporation of K atoms on the pristine ZnPc film. The results show that doping proceeds through charge donation from the K atoms to the molecular units whose lowest unoccupied molecular orbital (LUMO) becomes progressively filled. Despite the fact that the LUMO spectral weight increases as the stoichiometry x in the KxZnPc compound varies from about 1 to 4 (as determined by core level photoemission), no detectable density of states was observed at the Fermi level, showing that the film remains insulating for all the investigated stoichiometries. On the other hand the C 1s spectra, which appear merely broadened at the earliest stages of doping (x≈1), clearly develop two distinct components when x exceeds 2, suggesting that the charge state is not the same for all the molecules. At the same time, the modification of the valence band points towards the coexistence of two distinct phases with x=2 and x=4. [ABSTRACT FROM AUTHOR]

We present a new high performance configuration interaction code optimally designed for the calculation of the lowest-energy eigenstates of a few electrons in semiconductor quantum dots (also called artificial atoms) in the strong interaction regime. The implementation relies on a single-particle representation, but it is independent of the choice of the single-particle basis and, therefore, of the details of the device and configuration of external fields. Assuming no truncation of the Fock space of Slater determinants generated from the chosen single-particle basis, the code may tackle regimes where Coulomb interaction very effectively mixes many determinants. Typical strongly correlated systems lead to very large diagonalization problems; in our implementation, the secular equation is reduced to its minimal rank by exploiting the symmetry of the effective-mass interacting Hamiltonian, including square total spin. The resulting Hamiltonian is diagonalized via parallel implementation of the Lanczos algorithm. The code gives access to both wave functions and energies of first excited states. Excellent code scalability in a parallel environment is demonstrated; accuracy is tested for the case of up to eight electrons confined in a two-dimensional harmonic trap as the density is progressively diluted up to the Wigner regime, where correlations become dominant. Comparison with previous quantum Monte Carlo simulations in the Wigner regime demonstrates power and flexibility of the method. [ABSTRACT FROM AUTHOR]

The reorganization of the morphologically complex amorphous carbon nanoparticle layers deposited on a Si substrate into ordered sp2 entities was investigated by means of two complementary techniques, synchrotron radiation x-ray photoelectron microscopy and scanning electron microscopy, which provided the necessary chemical and structural information at submicrometer length scales. The studies were focused on the effect of the local thickness of the carbon nanoparticle layer on the extent of the restructuring process. The lateral distribution of the phases formed on the surface and their local chemical configuration were revealed by measuring C 1s and the Si 2p core level spectra with photoelectron spectromicroscopy. The results showed that an effective graphitization, which on the basis of complementary characterization techniques was associated to the formation of single-wall nanotubes, occurs only in areas with a high density of nanoparticles. These findings indicate that tube self-assembling is promoted by the interaction between neighboring nanoparticles. [ABSTRACT FROM AUTHOR]

Core level and valence band photoemission measurements combined with near edge x-ray absorption fine structure measurements were performed on a single C70 layer adsorbed on polycrystalline Al (1 ML-C70/Al) (ML—monolayer), pure and doped with sodium atoms. The data obtained from the pure ML chemisorbed on Al surface show a semiconducting behavior of the system, which is characterized by a covalent bond between the adsorbate and the substrate. The same data show also that the C70 molecules tend to orient themselves with the C5v axis perpendicular to the surface in analogy to what observed for 1 ML-C70/Cu(111). By doping the sample with sodium atoms a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the C70 molecules takes place, as underlined by the gradual increasing intensity of the C70 LUMO peak as a function of doping. Nevertheless, no metallic phases are observed for any doping step. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The composition, stability, and thermal evolution of nitric oxide on Rh(100) have been characterized by means of synchrotron radiation fast x-ray photoelectron spectroscopy and low-energy-electron diffraction. At 140 K, there is a single NO chemisorbed species (α[sub 2]–NO) at saturation, forming a disordered phase which develops in an ordered p(4[square_root_of_2]×[square_root_of_2])R45° structure after heating the layer to 373 K. A further annealing induces desorption and a progressive dissociation of the NO molecules, which is completed above 425 K, where a c(2×2) phase consisting of coadsorbed N and O atoms is observed. At low NO coverage, a different NO molecular species (α[sub 1]–NO), characterized by ≈1.7 eV lower N1s binding energy, is detected on Rh(100). α[sub 1]–NO is observed to undergo dissociation under the photon beam. The N1s line shape modifications due to irradiation by 500 eV photons were monitored in real time by using fast photoemission. The different stability of the α[sub 1]–NO and α[sub 2]–NO species is discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A detailed experimental and theoretical investigation of the structure of nitric oxide adsorption layers formed at 140 K on Rh(100) has been carried out by means of x-ray photoelectron spectroscopy, x-ray photoelectron diffraction, near-edge x-ray absorption fine structure, and ab initio calculations. At saturation, a single NO species is present. Both theory and experimental results indicate a bridge adsorption site for NO in this phase, with the molecules standing upright on the surface. At low NO coverage, the presence of a different molecular species is experimentally revealed and it is fully characterized by ab initio calculations. This species has been identified with a NO molecule with the molecular axis almost parallel to the surface, lying above a Rh(100) hollow site with the two atoms in asymmetric bridge sites. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The nature of bonding in amorphous carbon nitride was studied with core level spectroscopies such as x-ray absorption, photoemission, and Auger electron spectroscopy, using synchrotron radiation as the excitation source. The changes in local structure were systematically studied as a function of sample preparation conditions. The correspondence between nitrogen local environment and spectral features is discussed. It is found that thermal annealing increases the average coordination number and decreases bond multiplicity at nitrogen sites. [ABSTRACT FROM AUTHOR]

We investigated the interaction of C[sub 70] with the Cu(111) surface using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The data point to a net charge transfer from the Cu substrate to the C[sub 70] molecules directly bonded to the Cu atoms, providing a metallic character for a single layer of C[sub 70] (monolayer) adsorbed on this surface. A strong dependence of the x-ray absorption spectra from the light polarization is also observed. By comparing these experimental data with density-functional calculations of the contribution to the x-ray absorption spectral line shape of the five inequivalent carbon atoms in the C[sub 70] cage, we observed that the molecules are mainly oriented with the C[sub 5]V axis perpendicular to the Cu surface. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The optimal design for infrared second harmonic generation (SHG) is determined for a GaAs-based quantum device using a recently developed genetic approach. Both the compositional parameters and electric field are simultaneously optimized, and the quantum limit for SHG, set by the trade-off between large dipole moments (favoring electron delocalization) and large overlaps (favoring electron localization), is determined. Optimal devices are generally obtained that have an asymmetric double quantum well shape with narrow barriers and a graded region sideways to the largest well. An electric field is not found to lead to improved SHG if the compositional parameters are optimized. [ABSTRACT FROM AUTHOR]

We report on k → · p → calculations of Rashba spin-orbit coupling controlled by external gates in InAs/InAsP core-shell nanowires. We show that charge spilling in the barrier material allows for a stronger symmetry breaking than in homogenous nano-materials, inducing a specific interface-related contribution to spin-orbit coupling. Our results suggest additional wavefunction engineering strategies to enhance and control spin-orbit coupling. [ABSTRACT FROM AUTHOR]

The probability is estimated that at a given frequency to the action of an external, monochromatic radiation field acting on a hydrogen atom yields a fixed positive convergence radius of the Rayleigh-Schrödinger perturbation expansion for the quasienergy resonances. [ABSTRACT FROM AUTHOR]

A general methodology is presented to model the opto-electronic properties of a dye molecule in the presence of a semiconductor nanoparticle (NP), a model system for the architecture of dye-sensitized solar cells. The method is applied to the L0 organic dye solvated with acetonitrile in the neighborhood of a TiO2 NP. The total reaction potential due to the polarization of the solvent and the metal oxide is calculated by extending the polarizable continuum model integral equation formalism. The ground state energy is computed by using density functional theory (DFT) while the vertical electronic excitations are obtained by time-dependent DFT in a state-specific corrected linear response scheme. We calculate the excited state oxidation potential (ESOP) for the protonated and deprotonated forms of the L0 dye at different distances and configurations with respect to the NP surface. The stronger renormalizations of the ESOP values due to the presence of the TiO2 nanostructure are found for the protonated dye, reaching a maximum of about -0.15 eV. The role of protonation effect is discussed in terms of the atomic Löwdin charges of the oxidized and reduced species. On the other hand, we observed a weak effect on the L0 optical excitation gap due to the polarization response of the NP. [ABSTRACT FROM AUTHOR]

The electronic properties of heterostructures containing (Cd,Mn)Te quantum wells are probed by soft x-ray spectroscopies. We provide experimental evidence that Mn ions are in the Mn2+ (3d5) electronic configuration, and rule out the possibility that charge transfer and crystal field effects can reduce the magnetic moment of each Mn ion to the value (0.61) extracted from the fitting of the magnetization curve with a Brillouin function. These results confirm that the observed magnetic behavior can be correctly explained by assuming an antiferromagnetic coupling between nearest-neighbor S=5/2 Mn2+ ions, rather than a paramagnetic response from an ensemble of low-spin Mn ions. [ABSTRACT FROM AUTHOR]

We experimentally and theoretically investigate the photoluminescence broadening of different excitonic complexes in single self-assembled GaAs/AlGaAs quantum dots. We demonstrate that the excitonic fine-structure splitting leads to a sizable line broadening whenever the detection is not resolved in polarization. The residual broadening in polarized measurements is systematically larger for the exciton with respect to both the trion and the biexciton recombination. The experimental data agree with calculations of the quantum confined Stark effect induced by charge defects in the quantum dot (QD) environment, denoting the role of the QD spectator carrier rearrangement in reducing the perturbation of the fluctuating environment. [ABSTRACT FROM AUTHOR]

High-resolution photoemission spectroscopy was used to study Rh decorated single-walled C nanotubes before and after the exposure to NO2. We found that the metal adatoms form ordered nanocrystallites exhibiting a surface termination close to the (111) oriented crystal. After NO2 uptake at 200 K, the thermal evolution of the NOx adspecies indicates for the Rh nanocrystallites a catalytic behavior similar to the low index Rh surfaces. The Rh 3d5/2 line shape discloses the bonding of Rh atoms to NO2 and its fragments, whereas the oxidation of the metal/nanotubes contact is monitored by the energy shift of the C 1s peak. [ABSTRACT FROM AUTHOR]

We suggest that order-ot-magnitude reduction of the longitudinal-acoustic phonon scattering rate, the dominant decoherence mechanism in quantum dots, can he achieved in coupled structures by the application of an external electric or magnetic field. Modulation of the scattering rate is traced to the relation between the wavelength of the emitted phonon and the length scale of delocalized electron wave functions. Explicit calculations for realistic devices, performed with a Fermi golden rule approach and a fully three-dimensional description of the electronic quantum states, show that the lifetime of specific states can achieve tens of microseconds. Our findings extend the feasibility basis of many proposals for quantum gates based on coupled quantum dots. [ABSTRACT FROM AUTHOR]

The magnetic and structural properties of InAs:Mn self-organized diluted magnetic quantum dots grown by low-temperature (∼270 °C), solid-source molecular-beam epitaxy using a very low InAs growth rate (<0.1 ML/s) are investigated. A Curie temperature (TC) of ∼350 K is measured in a sample grown with a Mn/In flux ratio of 0.15. Electron energy-loss spectroscopy confirms that most of the Mn remains within the InAs quantum dots. We propose as a possible explanation for this high TC the effects of magnetic and structural disorder introduced by a random incorporation and inhomogeneous distribution of Mn atoms amongst the InAs quantum dots. [ABSTRACT FROM AUTHOR]

We calculate the near-field optical spectra of excitons and biexcitons in semiconductor quantum dots naturally occurring at interface fluctuations in GaAs-based quantum wells, using a nonlocal description of the response function to a spatially modulated electromagnetic field. The relative intensity of the lowest, far-field forbidden excitonic states is predicted; the spatial extension of the ground biexciton state is found, in agreement with recently published experiments. [ABSTRACT FROM AUTHOR]

Core level photoemission spectra from a free-standing bundle of single-wall carbon nanotubes have been measured using a high-flux soft x-ray spectromicroscope. The good signal-to-noise ratio for the C1s emission provides information on fundamental quantities such as the core-hole lifetime and binding energy, free from uncontrolled interactions between the nanotubes and the substrate or between the nanotubes and contaminants. We show that it is possible to distinguish chemically different nanotubes from the binding energy and line shape of the C1s core level. This finding opens unique opportunities to probe in situ the response of the nanotube electronic properties and chemical activity to mechanical actions, doping, and functionalization. [ABSTRACT FROM AUTHOR]