Search

In this paper, we present results concerning ab initio calculations of the optical properties of group IV two-dimensional sheets and compare them with the prediction of an analytical model for excitons in two dimensions, which we explicitly derive. This comparison helps to understand the physics at the basis of the strong many-body effects present in the optical spectra of this class of materials. Although it demonstrates the need of full ab initio calculations for an accurate description of the two-dimensional excitons, it validates, at the same time, this simple approach to estimate, through a density functional theory calculation of the two-dimensional electronic and optical properties, the binding energy and the radius of the excitons in this class of materials.

Colloidal nanocrystals electronic energy levels are determined by strong size-dependent quantum confinement. Understanding the configuration of the energy levels of nanocrystal superlattices is vital in order to use them in heterostructures with other materials. A powerful method is reported to determine the energy levels of PbS nanocrystal assemblies by combining the utilization of electric-double-layer-gated transistors and advanced ab-initio theory.

Paper is an important material for many applications. During the centuries, it has been the most widely used writing support and therefore paper degradation is a major issue for cultural heritage. The main component of paper is cellulose, one of the most abundant biomaterials on Earth. Cellulose oxidation is mainly responsible for the yellowing of the ancient samples, through the formation of chromophores. In order to investigate this issue and the chromophores’ role, we exploit the optical properties combining non-destructive experiments and theoretical calculations based on ab-initio techniques. In this paper, we illustrate the method and show its application to three ancient paper samples. The procedure we describe is a precious tool for cultural heritage preservation: indeed, the approach we present is based on non-invasive and non-destructive measurements that allows a microscopic understanding of cellulose-based artifacts degradation.

Spectral properties of chromophores are used to probe complex biological processes in vitro and in vivo, yet how the environment tunes their optical properties is far from being fully understood. Here we present a method to calculate such properties on large scale systems, like biologically relevant molecules in aqueous solution. Our approach is based on many body perturbation theory combined with quantum-mechanics/molecular-mechanics (QM/MM) approach. We show here how to include quasi-particle and excitonic effects for the calculation of optical absorption spectra in a QM/MM scheme. We apply this scheme, together with the well established TDDFT approach, to indole in water solution. Our calculations show that the solvent induces a redshift in the main spectral peak of indole, in quantitative agreement with the experiments and point to the importance of performing averages over molecular dynamics configurations for calculating optical properties.

The microscopic study of complex systems has reached a high level of accuracy that allows for a deep understanding of their structure, electronic properties, and optical spectra. The theoretical investigation of surfaces is nowadays routinely done within density functional theory, for ground state properties, and, with a larger computational load, within many-body perturbation theory, for excited states properties. In this paper we present and discuss examples of calculations for group IV two-dimensional systems such as a clean silicon surface, a tin–germanium interface, graphene, and graphane, pointing out the importance of a pertinent treatment of many-body effects.

We review the theoretical framework of ab-initio excited state properties calculations showing the application of these methods to systems of different dimensionality. Many body perturbation theory within the GW approximation of the self energy for the calculation of electronic band structures is presented and applied to liquid water. The Bethe Salpeter equation for the computation of optical properties is illustrated and used for the case of surfaces and liquid water. An alternativemethod for the calculation of optical properties, the Time Dependent Density Functional Theory, is also briefly illustrated. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We show an approximate method to calculate the polarizability of a many-electron system within Green's function theory in a similar way as within time-dependent density functional theory (TDDFT). The basic idea is to join the computational simplicity of the latter with the accuracy of the former. We apply this approach to a prototype system, LiF. For comparison, we also show results obtained within other approximated TDDFT-based methods. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the results of a first principles study of the adsorption of acetylene on Si(001). Several adsorption sites are investigated, and the electronic structures and reflectance anisotropy spectra (RAS) are calculated for the energetically most favorable geometries. We find that the end-bridge adsorption sites, both at 0.5 ML and at 1ML C2H2 coverage do not give metallic surfaces, at odd with other calculations. First principles study of optical properties for different configurations of acetylene adsorbed on Si(001) show a strong change of the spectra, thus suggesting that RAS experiments may give a definitive answer on the adsorption sites and saturation coverage. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The microscopic structure of the GaP(001) surface is not well understood, in marked contrast to other III–V compounds, in particular GaAs(001) (see Ref. [1] for a recent review). It has been suggested that ion bombardment and annealing of GaP(001) results in a (4 2) reconstructed Ga-rich surface [2, 3] in analogy to GaAs. On the other hand, based on ion scattering spectroscopy and low-energy electron diffraction (LEED) experiments, a (2 4) translational symmetry has been proposed [4, 5]. In solid-source molecular beam epitaxy a (2 4) diffraction pattern shows up under both Ga and P supply [6]. In addition, (4 4) spots have been observed. A slightly different behaviour is found for samples grown by metal-organic vapour phase epitaxy or chemical beam epitaxy as well as for GaP(001) surfaces prepared by thermal desorption of a protective arsenic/phosphorous double layer cap under ultrahigh vacuum conditions [7–9]. After annealing to 690 K the de-capped GaP(001) surfaces show a (2 1)=(2 2)-like LEED pattern. Annealing to higher temperatures (785 K) leads to a (2 4) translational symmetry. The many-step annealing procedure is accompanied by a transition from a more P-rich to a more Ga-rich GaP(001) surface. Previous total-energy (TE) calculations [8, 9] show the stability of several (2 4) reconstructions. For Ga-rich surfaces they are characterized by single Ga–Ga or mixed Ga–P dimers on top of a complete Ga layer. For lower Ga contents one Ga pair is missing in this layer and one (d structure) or two P dimers (b2 structure) terminate the surface. (4 2) reconstructions with Ga or P dimers in the uppermost atomic layer are unstable. The comparison of measured and calculated reflectance anisotropy spectra support the mixed-dimer reconstruction of the Ga-rich GaP(001) surface [8, 9]. This has recently been confirmed by a comparison of calculated and measured scanning-tunneling microscopy (STM) images [10]. The (2 4) mixed-dimer structure agrees with recent measurements of the surface core-level shifts [5]. The structure of the (2 1)=(2 2) reconstruction of the GaP(001) surface remains unknown. Auger electron spectroscopy indicates a P-rich surface. The similarities with

We present a calculation of the electronic and optical properties of the GaAs(110) : H surface performed within the first-principles density functional theory (DFT) in the local density approximation (LDA). The geometry and electronic structure are analyzed and compared with those of the clean surface. The reflectance anisotropy spectrum and the differential reflectivity are then computed. Taking into account the usual underestimation of the LDA gaps with respect to the experimental values, the comparison between our theoretical spectra with available experimental reflectance data shows a satisfactory agreement, allowing for the interpretation of the main structures in terms of electronic transitions.

We discuss the progress in realistic calculations of the optical response of semiconductor surfaces. It concerns numerical developments as well as the better inclusion of many-body effects. We show that the starting point of such calculations, the atomic structure and the surface equilibrium phase, can be now derived with high accuracy. The different steps are described in detail for the computation of the reflectance anisotropy and the exploration of the In-rich InP(001)2 4 surfaces.