Hard X-ray helical dichroism of disordered molecular media

Chirality is a structural property of molecules lacking mirror symmetry that has strong implications in diverse fields, ranging from life sciences to materials science. Chirality-sensitive spectroscopic methods, such as circular dichroism, exhibit weak signal contributions on an achiral background. Helical dichroism, which is based on the orbital angular momentum (OAM) of light, offers a new approach to probe molecular chirality, but it has never been demonstrated on disordered samples. Furthermore, in the optical domain the challenge lies in the need to transfer the OAM of the photon to an electron that is localized on an angstrom-size orbital. Here we overcome this challenge using hard X-rays with spiral Fresnel zone plates, which can induce an OAM. We present the helical dichroism spectra of a disordered powder sample of enantiopure salts of the molecular complex of Fe(4,4 '-diMebpy)(3) at the iron K edge (7.1 keV) with OAM-carrying beams. The asymmetry ratios for the helical dichroism spectra are within one to five percent for OAM beams with topological charges of one and three. These results open a new window into the studies of molecular chirality and its interaction with the OAM of light. Linearly polarized orbital angular momentum-carrying hard X-ray beams are induced using spiral Fresnel zone plates. By sending the hard X-ray beams to disordered enantiopure molecular complexes, the helicity-dependent and chiral-sensitive signal is obtained.

Supramolecular assembly, chirality, and electronic properties of rubrene studied by STM and STS

This thesis presents the first experimental results of a scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) investigation of rubrene at the supramolecular, molecular and submolecular level. Based on its semiconducting and fluorescent properties, this molecule is of particular interest in view of the emerging fields of molecular electronics and optoelectronics which could one day replace the conventional technology relying on semiconductors such as silicon and gallium arsenide. The goal is the substitution of these inorganic materials by cheap and flexible layers of semiconducting organic molecules for a new class of diodes and transistors, as well as the realization of electronic switches based on individual molecules. One fundamental approach is to take advantage of the molecular self-assembly behavior which results in the creation of well-ordered supramolecular structures. The investigations of the self-assembly of rubrene adsorbed on metal surfaces (Au(111), Au(100), Ag(111), and Ag(100)), which were carried out within the framework of this thesis, show a surprising diversity of supramolecular structures. Amongst other shapes, the molecules organize themselves into geometries of perfect hexagonal and pentagonal symmetry and create multifaceted patterns on the surface. A fascinating peculiarity consists in the spontaneous construction of nested structures which are built up by a hierarchical self-assembly of individual molecules into pentagonal supermolecules which form in a second step perfect supramolecular decagons. The geometric shape of rubrene is characterized by a structural asymmetry leading to the existence of two mirror imaged versions of the molecule which are not superimposable to each other, such as for instance our left and right hand or the helical DNA. The aspect of chirality is crucial for basic processes in living systems and calls for a fundamental understanding of the interaction mechanisms occurring between chiral molecules. The experiments on rubrene reveal that the intermolecular bonding differentiates between the two chiral types of the molecule (chiral recognition), yielding the self-organization into homochiral structures. These assemblies exhibit a geometry which is again chiral, demonstrating a propagation of chirality throughout the three stages of the supramolecular hierarchy. The semiconducting behavior of rubrene is furthermore probed by STS measurements detecting the energetic positions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The experimental data uncover that different adsorption conformations exhibit characteristic HOMO energies and reveal adsorption conformations of rubrene which preserve the intrinsic electronic structure of the free molecule. Furthermore, a switching of the molecular conformation and the electronic structure of one rubrene conformer is induced with the STM.

EPFL2006

First-principles investigation of the structural and vibrational properties of vitreous GeSe2

Luigi Giacomazzi, Alfredo Pasquarello

Using a density-functional scheme, we study the structural and vibrational properties of vitreous germanium diselenide (v-GeSe2). Through the use of classical and first-principles molecular-dynamics methods, we generate a set of structural models showing a varying degree of chemical disorder. In particular, two types of structural concepts are represented: one in which the tetrahedral order is preserved to a very large extent, and one which reproduces the high degree of disorder in first-neighbor shells found in first-principles molecular-dynamics simulations of the liquid. The investigated structural properties include the angular distributions, the atomic arrangements in the first-neighbor shells, and the pair-correlation functions. In reciprocal space, we have calculated the x-ray and neutron total structure factors and the partial structure factors. Comparison with experiment gives overall good agreement for the models of either structural conception. We then investigate the vibrational properties via the vibrational density of states and the inelastic neutron spectrum. The considered models yield similar spectra and agree with experimental data. We also obtain infrared and Raman spectra through a density-functional scheme based on the application of finite electric fields. For these spectra, significant differences appear among the models. The comparison with experiment favors a model showing a high degree of chemical order. The Raman intensity is analyzed in terms of the underlying atomic vibrations. The assignment of the Raman companion line to Se motions in edge-sharing tetrahedra is supported.

2007

Supramolecular assembly, chirality, and electronic properties of rubrene studied by STM and STS

EPFL2006