1,720,987 research outputs found
Electronic and Optical Properties of Si, Ge and Sige Low Dimensional Systems: Ab-Initio Results
No full textIn recent years,considerable efforts have been done to study silicon, germanium, and silicon/germanium slabs, nanocrystals, and nanowires for optoelectronics and photovoltaic solar energy applications. These zero- one- and two-dimensional systems, with sizes ranging from few to some tenths of nanometers, show unique electronic, optical, and transport properties that are intrinsically associated with their low dimensionality and to the quantum confinement effect. The possibility of understanding the microscopic properties of these systems and modulating their characteristics by doping and passivation can open new perspectives in the development of new, advanced, photovoltaics and optoelectronics devices. In this talk, we will discuss ab-initio theoretical results obtained by our group in the study of electronic, optical, and transport properties of silicon, germanium, and silicon/germanium low dimensional systems. The role played by size, passivation, and doping will be discussed. Moreover, we will show how the interaction between different nanostructures is a promising route to foster the establishment of third-generation photovoltaics
Scattering operators for E1-E2 x-ray resonant diffraction
No full textResonant x-ray diffraction in noncentrosymmetric crystals is studied by considering E1-E2 processes in the fast-collision approximation. The scattering amplitude is expressed in terms of polar and magnetoelectric operators of the valence states, which are involved in the resonance. Near-edge Bragg peaks from ferroelectric, antiferroelectric, and magnetoelectric structures are predicted
Oxygen vacancy effects on the Schottky barrier height at the Au/TiO2(100) interface: a first principle study
No full textMotivated by the pioneering work of McFarland and Tang on multilayer photovoltaic devices, we discuss here structural and electronic properties of the Au/TiO2(110) interface for a coverage of 1 monolayer (1 ML) of gold, both for a stoichiometric and a reduced (Ti-rich) rutile surface. A detailed analysis of theSchottky barrier height for such systems is presented and the effects generated on this barrier by the presence of an oxygen vacancy (localized on the rutile support) are discussed
X-ray dichroism in noncentrosymmetric crystals
No full textIn this paper the authors analyze near-edge absorption of x rays in noncentrosymmetric crystals. The work is motivated by recent observations of x-ray dichroic effects which stem from parity-nonconserving electron interactions. We provide a theoretical description of these experiments and show that they are sensitive to microscopic polar and magnetoelectric properties of the sample. Our derivation extends previous theoretical work on centrosymmetric systems and identifies interesting directions in the microscopic analysis of crystalline materials using x-ray-absorption spectroscopy
Optical dichroism: E1-M1 integral relations
No full textThe present paper discusses optical dichroism in noncentrosymmetric systems. The cases of circular and linear polarizations are considered. Integrated spectra are interpreted using effective two-electron operators, which are derived within a localized (atomic) model. As a consequence, our theory is not suitable for quantitative predictions; nevertheless, it identifies microscopic origins of natural, nonreciprocal and Jones' dichroisms
First-principles calculations of electronic coupling effects in silicon nanocrystals: Influence on near band-edge states and on carrier multiplication processes
No full textArrays of closely packed nanocrystals show interesting properties that can be exploited to induce new features in nanostructured optoelectronic devices. In this work we study, by first principles calculations, effects induced on near band-edge states and on carrier multiplication by nanocrystals interplay. By considering both hydrogenated and oxygenated structures, we prove that interaction between silicon nanocrystals can alter both the energy gap of the system and dynamics of excited states with a relevance that depends on the nanocrystal-nanocrystal separation, on nanocrystals orientation and on nanocrystals surface properties
Red-shifted carrier multiplication energy threshold and exciton recycling mechanisms in strongly interacting silicon nanocrystals.
No full textWe present density functional theory calculations of carrier multiplication properties in a system of strongly coupled silicon nanocrystals. Our results suggest that nanocrystal-nanocrystal interaction can lead to a reduction of the carrier multiplication energy threshold without altering the carrier multiplication efficiency at high energies, in agreement with experiments. The time evolution of the number of electron-hole pairs generated in a system of strongly interacting nanocrystals upon absorption of high-energy photons is analyzed by solving a system of coupled rate equations, where exciton recycling mechanisms are implemented. We reconsider the role played by Auger recombination which is here accounted also as an active, nondetrimental process
Silicon Nanocrystals for Photonics and Photovoltaics: Ab-initio Results
No full textReview Article on electronic and optical properties of silcon nanocrystals for photonic and photovoltaic application
Angular and polarization dependence of x-ray resonant elastic scattering in transition metals
No full textWe present a comparison of the x-ray elastic scattering at the 2p threshold of Ni calculated both in a single-ion picture with a full description of the multiplet manifolds of the electron configurations and in a one-electron model using a multiple scattering approach for the excited intermediate state. We study the case of perpendicular geometry of the circularly polarized incoming light and analyze the variation of the intensity of the emitted light along a conical scan. We discuss the results obtained in the two approaches for the total and dichroic signals, that confirm the presence of a significant anisotropy as previously predicted in the inelastic case.We present a comparison of the x-ray elastic scattering at the 2p threshold of Ni calculated both in a single-ion picture with a full description of the multiplet manifolds of the electron configurations and in a one-electron model using a multiple scattering approach for the excited intermediate state. We study the case of perpendicular geometry of the circularly polarized incoming light and analyze the variation of the intensity of the emitted light along a conical scan. We discuss the results obtained in the two approaches for the total and dichroic signals, that confirm the presence of a significant anisotropy as previously predicted in the inelastic case. © 2006 The American Physical Society
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