Epioptics-11

The following sections are included:

• PREFACE

• CONTENTS

Lead phthalocyanine (PbPc) layers were deposited on GaAs(001)-c(4 × 4) and -(2 × 4) reconstructed surfaces and studied with the goal to determine a possible orientation with respect to the substrate in the layers by Raman spectroscopy. Moreover, the dependence of the molecular arrangement on the GaAs surface reconstruction was tested. For this purpose the intensity of the Raman peaks was measured as a function of a rotation of the sample around its surface normal. Since every molecule has a specific Raman tensor attached to its internal coordinates any order of the molecules in the layer should be indicated by specific variations in intensity with the rotation, except in case of a completely statistical arrangement (amorphous structure). Indeed periodic changes with rotation of the measured intensities of the molecular vibrational modes were observed. They are compared to the results of calculations with the corresponding Raman tensors and with the GaAs(OOl) phonon modes. The periodicity of the measured changes and their fixed relation to the substrate coordinates suggest well-ordered layers with preferential molecular orientations. Together with first observations of different structural layer properties on the -c(4 × 4) and -(2 × 4) surfaces this implies that the molecular orientation is induced by the atomic arrangement on the GaAs surfaces. Thus, we conclude that an epitaxial-like growth mode of PbPc molecules on reconstructed GaAs(OOl) surfaces takes place. This is an important result for organic devices like OLEDS or OFETS.

We present a very efficient recursive method to calculate the effective optical response of nano-structured metamaterials made up of particles with arbitrarily shaped cross sections arranged in periodic two or three-dimensional arrays. We consider dielectric particles embedded in a metal matrix with a lattice constant much smaller than the wavelength of the incident field. Neglecting retardation our formalism allows factoring the geometrical properties from the properties of the materials. If the conducting phase is continuous the low frequency behavior is metallic. If the conducting paths are nearly blocked by the dielectric particles, the high frequency behavior is dielectric. Thus, extraordinary-reflectance and transparent bands may develop at intermediate frequencies, whose properties may be tuned by adjusting the geometry.

In this work we describe a method, based on the linearization of the band dispersion, to compute the intraband contributions to the optical spectra of metals. To investigate its performance we test it on cubic iron and hexagonal magnesium. We compute the dielectric function and optical conductivity of these metals based on ab initio DFT-LDA band structure, in the RPA with a full inclusion of local-field effects. In particular we show that our method is capable to describe the anisotropic response of noncubic metals, without the need to resort to phenomenological parameterizations of the plasmon-pole type. We also introduce a method to recover the correct asymptotic trend in the ω → 0 limit and hence, the experimental value of the static conductivity.

Magneto-optic techniques provide non-contact and non-destructive characterization of magnetic materials. This includes embedded magnetic nanostructures, which are accessible due to the large penetration depth of optical radiation. Nonlinear magnetic second-harmonic generation (MSHG) can measure the surface and interlace magnetism of centrosymmetric magnetic films and nanostructures with sub-monolayer sensitivity. MSHG is briefly reviewed and examples from high symmetry interlaces and nanostructures described. Low symmetry systems, such as aligned magnetic nanostructures grown by self-organization on vicinal substrates, are more difficult to characterize, however, because of the large number of tensor components that may contribute to the signal. Normal incidence geometry simplifies the problem and this new approach is shown to allow the determination of hysteresis loops and the temperature dependent magnetic response of Au-capped Fe nanostructures grown on vicinal W(110).

We report on the characterization of organic layers adsorbed on semiconductor surfaces by means of reflectance anisotropy spectroscopy (RAS). At different organic adsorbate layers on GaAs(OOl) surfaces we show that RAS measurements are suitable to distinguish between physisorption and chemisorption of organic molecules on semiconductur surfaces. We also show that molecular layers can contribute to RAS signatures with new anisotropies due to intramolecular optical absorption. These molecular anisotropies can be utilzed to determine molecular orientation or to monitor the growth of organic layers directly for sub-monolayers as well as for thicker layers.

Recent work on the application of reflection anisotropy spectroscopy (RAS) to thiolate/metal interfaces is reviewed. The thiolate-metal surface bond is a key interaction in the attachment of alkanethiols and thiol-derivatised molecules to metal surfaces under ultra-high vacuum conditions and at the solid/liquid interface. Characteristic RAS signatures are obtained from thiolate/Cu(llO) and thiolate/Au(llO) surfaces, and these signatures can be simulated using three- and four-phase models.

Presenting the physics of X-FELs to a non-specialized audience is a challenge: not only the mathematical treatment is complex but the main properties are entangled together. We present a response to this challenge, dealing with properties such as optical gain, saturation, spectral bandwidth, electron energy spread and pulse time structure – and with the corresponding parameters. Our approach is an expansion and refinement of a previous simple treatment, improved in several ways and extended to formerly untreated aspects.

The theory of large radius polaron is derived with consideration for the interaction of charge particles both with bulk and with interface optical phonons in the quantum nanostructures. The polaron binding energies are obtained for quantum wells, quantum wires and quantum dots. For the quantum dot case, the contributions of bulk and interface phonons have commensurable quantities. Thus the inclusion of the interface phonons reduces to some changes of polaron binding energy value. The interface phonon role is much more important in the quantum wells and wires. The essential strengthening as well as the attenuation of polaron interaction is possible with the different interrelations of the structure and barrier dielectric properties.

The following sections are included:

• Introduction

• Preparation and characterization of ZrO₂ thin films with silver nanoparticles

• Analysis on absorption spectra using effective medium approximation (EMA)

• References

In this work we plan to use ab initio spectroscopy calculations to compute circular dichroism (CD) spectra of peptides. CD provides information on protein secondary structure content; peptides, instead, remain difficult to address, due to their tendency to adopt multiple conformations in equilibrium. Therefore peptides are an interesting test-case for ab initio calculation of CD spectra. As a first application, we focus on the (83-92) fragment of HIV-1 protease, which is known to be involved in the folding and dimerization of this protein. As a preliminary step, we performed classical molecular dynamics (MD) simulations, in order to obtain a set of representative conformers of the peptide. Then, on some of the obtained conformations, we calculated absorption spectra at the independent particle, RPA and TDLDA levels, showing the presence of charge transfer excitations, and their influence on spectral features.

The growing interest for the physical properties of graphene and its chemical functionalization is triggering a renewed attention to other more complex forms of sp² nanostructured carbon, e.g., carbon schwarzites. The structural complexity of these forms prevents in many cases the possibility of ab-initio calculations of their electronic and vibrational properties. On the other hand in systems with a certain level of conjugation many of these properties depend primarily on the topological features of the structure – notably on the graph describing the bonding network and the associated adjacency matrix. In this work, specifically devoted to planar schwarzites, it is shown that their vibrational spectrum as obtained from the diagonalization of the adjacency matrix (topological phonons) reflects the main features of ab-initio calculations. Thus the method is suitable to the study of thermodynamic and spectral phonon properties of defective schwarzites which involve integrals over the unperturbed phonon density of states. Calculations of the phonon density of states smallest planar schwarzites and the effects of various type of local perturbation are presented and thoroughly discussed.