993 research outputs found
Ab initio Simulation of Optical Limiting: The Case of Metal-Free Phthalocyanine
We present a fully ab initio, non-perturbative description of the optical
limiting properties of a metal-free phthalocyanine, by simulating the effects
of a broadband electric field of increasing intensity. The results confirm
reverse saturable absorption as leading mechanism for optical limiting
phenomena in this system and reveal that a number of dipole-forbidden
excitations are populated by excited-state absorption, at more intense external
fields. The excellent agreement with the experimental data supports our
approach as a powerful tool to predict optical limiting, in view of
applications
Excitons in carbon nanotubes: an ab initio symmetry-based approach
The optical absorption spectrum of the carbon (4,2) nanotube is computed
using an ab-initio many-body approach which takes into account excitonic
effects. We develop a new method involving a local basis set which is symmetric
with respect to the screw symmetry of the tube. Such a method has the
advantages of scaling faster than plane-wave methods and allowing for a precise
determination of the symmetry character of the single particle states,
two-particle excitations, and selection rules. The binding energy of the
lowest, optically active states is approximately 0.8 eV. The corresponding
exciton wavefunctions are delocalized along the circumference of the tube and
localized in the direction of the tube axis.Comment: 4 pages, 1 LaTex file + 4 eps figure
Quantum dot states and optical excitations of edge-modulated graphene nanoribbons
We investigate from first principles the electronic and optical properties of edge-modulated armchair graphene nanoribbons, including both quasiparticle corrections and excitonic effects. Exploiting the oscillating behavior of the ribbon energy gap, we show that minimal width-modulations are sufficient to obtain confinement of both electrons and holes, thus forming optically active quantum dots with unique properties, such as the coexistence of dotlike and extended excitations and the fine tunability of optical spectra, with great potential for optoelectronic applications
Electronic properties of polymer crystals: The effect of interchain interactions
We present a theoretical study of the transport parameters in a prototype conjugated-polymer, poly-para-phenylenevinylene, in two different possible crystalline packings. Our analysis is performed through density-functional electronic structure calculations, and allows one to obtain the fundamental parameters describing charge transport. The transfer integrals are found to be a crucial quantity to appreciate the effects of crystalline aggregation on conduction properties: our results indicate that interchain interactions can be viewed as a tunable parameter for the design of efficient electronic devices based on organic materials.The sensitivity of interchain interactions to the specific three-dimensional (3D) structure was demonstrated. It was confirmed that the interactions can be used to tailor the transport properties of conjugated-polymer films
Optical properties of graphene nanoribbons: The role of many-body effects
We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination.We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination. © 2008 The American Physical Society
Tailoring the electronic properties of silicon with cysteine: A first principle study
We discuss the electronic structure modifications induced on the dihydride-terminated Si(001) surface upon cysteine adsorption by means of ab initio calculations: several stable functionalization schemes are presented, providing different routes for biological recognition, surface nanostructuring, and biomolecular electronics applications. The resulting hybrid systems are discussed and compared in terms of stability, structural, and electronic properties. Based on our results, we propose STM and photoemission experiments to determine unambiguously the adsorption mechanism involved and the attached functional group
Ab initio complex band structure of conjugated polymers: Effects of hydrid DFT and GW schemes
The non-resonant tunneling regime for charge transfer across nanojunctions is
critically dependent on the so-called \beta{} parameter, governing the
exponential decay of the current as the length of the junction increases. For
periodic materials, this parameter can be theoretically evaluated by computing
the complex band structure (CBS) -- or evanescent states -- of the material
forming the tunneling junction. In this work we present the calculation of the
CBS for organic polymers using a variety of computational schemes, including
standard local, semilocal, and hybrid-exchange density functionals, and
many-body perturbation theory within the GW approximation. We compare the
description of localization and \beta{} parameters among the adopted methods
and with experimental data. We show that local and semilocal density
functionals systematically underestimate the \beta{} parameter, while
hybrid-exchange schemes partially correct for this discrepancy, resulting in a
much better agreement with GW calculations and experiments. Self-consistency
effects and self-energy representation issues of the GW corrections are
discussed together with the use of Wannier functions to interpolate the
electronic band-structure.Comment: Accepted for publication on Physical Review B v2: fixed some typo
Accurate ab initio tight-binding Hamiltonians: Effective tools for electronic transport and optical spectroscopy from first principles
The calculations of electronic transport coefficients and optical properties require a very dense interpolation of the electronic band structure in reciprocal space that is computationally expensive and may have issues with band crossing and degeneracies. Capitalizing on a recently developed pseudoatomic orbital projection technique, we exploit the exact tight-binding representation of the first-principles electronic structure for the purposes of (i) providing an efficient strategy to explore the full band structure E-n (k), (ii) computing the momentum operator differentiating directly the Hamiltonian, and (iii) calculating the imaginary part of the dielectric function. This enables us to determine the Boltzmann transport coefficients and the optical properties within the independent particle approximation. In addition, the local nature of the tight-binding representation facilitates the calculation of the ballistic transport within the Landauer theory for systems with hundreds of atoms. In order to validate our approach we study the multivalley band structure of CoSb3 and a large core-shell nanowire using the ACBN0 functional. In CoSb3 we point the many band minima contributing to the electronic transport that enhance the thermoelectric properties; for the core-shell nanowire we identify possible mechanisms for photo-current generation and justify the presence of protected transport channels in the wire
Dynamical-charge neutrality at a crystal surface
For both molecules and periodic solids, the ionic dynamical charge tensors
which govern the infrared activity are known to obey a dynamical neutrality
condition. This condition enforces their sum to vanish (over the whole finite
system, or over the crystal cell, respectively). We extend this sum rule to the
non trivial case of the surface of a semiinfinite solid and show that, in the
case of a polar surface of an insulator, the surface ions cannot have the same
dynamical charges as in the bulk. The sum rule is demonstrated through
calculations for the Si-terminated SiC(001) surface.Comment: 4 pages, latex file, 1 postscript figure automatically include
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