Tommaso Prosperi

Valence photoionization dynamics in circular dichroism of chiral free molecules: The methyl-oxirane

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients D(i)(omega) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

Carboxylic acid terminated monolayer formation on crystalline silicon and silicon nitride surfaces. A surface coverage determination with a fluorescent probe in solutionElectronic Supplementary Information (ESI) available: analytical data of the new compounds and general information on the instruments used for their characterization. See http://www.rsc.org/suppdata/jm/b3/b312273e/

Carboxylic acid terminated (–COOH) monolayers (ML) have been covalently anchored on the surface (S) of crystalline silicon (Si) and silicon nitride (Si3N4) by using wet-chemistry methods. Their concentration has been determined adopting a stepwise procedure with a fluorescent probe in solution. First, 7-amino-4-methylcoumarin (H2N–C10H7O2, AMC) is covalently bonded to the monolayer on the surface through an amidation reaction forming an amide-terminated surface monolayer, S–(CH2)n–CO–NH–C10H7O2, then the surface is intensely washed in order to remove any physisorbed species, finally AMC is detached from the surface via hydrolysis of the –CO–NH– bond and quantified by a fluorescence analysis in water solution. The yields of both the amidation and hydrolysis reactions have been evaluated. By this method, submonolayers ≤0.1 ± 0.03 ML can be determined. It has been possible to evaluate the different extent of surface functionalization changing the substrate between Si and Si3N4 and for Si changing the monolayer formation reaction: heat- (Δ) or light- (hν) promoted or via cathodic electrografting (CEG). It came out that submonolayers are formed in any case, Si3N4 is functionalized to a larger extent than Si on heating (0.52 vs. 0.15 ML) and Si is best functionalized via cathodic electrografting: Δ, 0.15; hν 0.20; CEG, 0.44 ML. Surface topography of some monolayers on Si and Si3N4 substrates was investigated by Atomic Force Microscopy (AFM). On clean Si, AFM showed topographical variations of 0.3–0.4 nm while for the clean Si3N4 the corrugation was around 3–4 nm. After functionalization the two surfaces were uniformly covered and the corrugation increased in both samples with a corrugation in topography of 1–2 nm for Si and 5–6 nm for Si3N4.

Controlled loading of oligodeoxyribonucleotide monolayers onto unoxidized crystalline silicon; fluorescence-based determination of the surface coverage and of the hybridization efficiency; parallel imaging of the process by Atomic Force Microscopy

Unoxidized crystalline silicon, characterized by high purity, high homogeneity, sturdiness and an atomically flat surface, offers many advantages for the construction of electronic miniaturized biosensor arrays upon attachment of biomolecules (DNA, proteins or small organic compounds). This allows to study the incidence of molecular interactions through the simultaneous analysis, within a single experiment, of a number of samples containing small quantities of potential targets, in the presence of thousands of variables. A simple, accurate and robust methodology was established and is here presented, for the assembling of DNA sensors on the unoxidized, crystalline Si(100) surface, by loading controlled amounts of a monolayer DNA-probe through a two-step procedure. At first a monolayer of a spacer molecule, such as 10-undecynoic acid, was deposited, under optimized conditions, via controlled cathodic electrografting, then a synthetic DNA-probe was anchored to it, through amidation in aqueous solution. The surface coverage of several DNA-probes and the control of their efficiency in recognizing a complementary target-DNA upon hybridization were evaluated by fluorescence measurements. The whole process was also monitored in parallel by Atomic Force Microscopy (AFM).

Chirality Transfer from a Single Chiral Molecule to 2D Superstructures in Alaninol on the Cu(100) Surface

The formation of 2D chiral monolayers obtained by self-assembly of chiral molecules on surfaces has been widely reported in the literature. Control of chirality transfer from a single molecule to surface superstructures is a challenging and important aspect for tailoring the properties of 2D nanostructures. However, despite the wealth of investigations performed in recent years, how chiral transfer takes place on a large scale still remains an open question. In this paper we report a coupling of scanning tunneling microscopy and low energy electron diffraction measurements with an original theoretical approach, combining molecular dynamics and essential dynamics with density functional theory, to investigate self-assembled chiral structures formed when alaninol adsorbs on Cu(100). The peculiarity of this system is related to the formation of tetrameric molecular structures which constitute the building blocks of the self-assembled chiral monolayer. Such characteristics make alaninol/Cu(100) a good candidate to reveal chiral expression changes. We find that the deposition of alaninol enantiomers results in the formation of isolated tetramers that are aligned along the directions of the substrate at low coverage or when geometrical confinement prevents long-range order. Conversely, a rotation of 14° with respect to the Cu(100) unit vectors is observed when small clusters of tetramers are formed. An insight to the process leading to a 2D globally chiral surface has been obtained by monitoring molecular assemblies as they grow from the early stages of adsorption, suggesting that the distinctive orientation of the self-assembled monolayer originates from a balance of cooperating forces which start acting only when tetramers pack together to form small clusters.

Conformational Effects in Photoelectron Circular Dichroism of Alaninol

A photoelectron circular dichroism (CD) study of the valence states of 2-amino-1-propanol (alaninol) in the gas phase is presented. The aim of the investigation is to reveal conformer population effects in the valence-state photoelectron spectrum. The experimental dispersion of the dichroic D parameter of valence states as a function of the photon excitation energy is compared with its theoretical value calculated by employing a multicentric basis set of B-spline functions and a Kohn-Sham Hamiltonian. The theoretical values are in very good agreement with the experimental data when the conformer population distribution is taken into account. Moreover, thanks to a comparison between experiment and theory, a clear assignment of the molecular orbital character and conformer geometry is given to the features of the photoelectron spectrum. This work indicates in a detailed experimental analysis that CD in photoelectron spectroscopy is an effective technique to disentangle the conformer assignment in photoelectron spectra.

Vibrational state dependence of β and D asymmetry parameters: The case of the highest occupied molecular orbital photoelectron spectrum of methyl-oxirane

The beta angular asymmetry and D dichroic asymmetry parameters of the methyl-oxirane highest occupied molecular orbital (HOMO) band have been experimentally investigated with vibrational resolution using synchrotron radiation. A theoretical calculation of the Franck-Condon factors between vibrational ground state and different ionic vibrational states, in the Born-Oppenheimer harmonic approximation, has been performed in order to gain information on the vibrational states mainly involved in the HOMO photoelectron band. The general good agreement between theoretical and experimental results allows a reliable assignment of the major features. The experimental determination of beta and D shows their dependence on the different final vibrational states. This paper reports, for the first time, experimental evidence of the dependence of the dichroic D parameter on the vibrational excitation of the ion.

Conformational Sensitivity in Photoelectron Circular Dichroism of 3-Methylcyclopentanone

A study of (R)-3-methylcyclopentanone [(R)-3-MCP] by photoelectron spectroscopy and photoelectron circular dichroism (PECD) is presented. The synchrotron radiation gas-phase photoelectron spectra of (R)-3-MCP were measured and are discussed on the basis of different theoretical methodologies. The experimental dichroism of (R)-3-MCP for selected deconvoluted valence states and for the carbonyl carbon 1s core state are reported and reproduced well by calculated dispersions generated by considering the contributions of two different conformers. The theoretical dichroic parameters are calculated by employing a multicentre basis set of B-spline functions and a Kohn-Sham Hamiltonian. Temperature-dependent PECD studies of the HOMO state and the carbonyl carbon 1s core level allowed the separation of the contributions of each conformer by photoelectron dichroism. This new approach clearly shows how the PECD methodology is sensitive to conformational and structural changes of unoriented (R)-3-MCP in the gas phase, opening up new perspectives in the characterisation of chiral molecular systems.

D-Alaninol Adsorption on Cu(100) : Photoelectron Spectroscopy and First-Principles Calculations

The adsorption of a single molecule of the D-enantiomer of alaninol (2-amino-1-propanol) on the surface of Cu(100) is investigated through density functional theory calculations. Different possible adsorption sites for D-alaninol are tested, and it is found that the most stable configuration presents both amino and hydroxyl group covalently interacting with on top copper atoms. The electronic structure is analyzed in detail and compared with experimental photoelectron spectra. Another adsorption structure in which a dehydrogenation process is assumed to occur on the amino group is analyzed and provides a possible explanation of the valence band electronic structure and of the experimentally observed N 1s core-level shift at full coverage, where a self-assembled ordered chiral monolayer is formed on the copper surface.

Microscopic investigation of the strain distribution in InGaAs/GaAs quantum well structures grown by molecular beam epitaxy

Extended X-ray absorption fine structure in the glancing angle geometry has been used to study the strain accommodation in quantum well structures of In x Ga 1-x As/GaAs (x<0.25). The results show that a number of Ga-As bond lengths are stretched. Indeed, two Ga-As bonds distances coexist: 2.45±0.01 A and 2.64±0.02 A, which correspond to the Ga-As and In-As bond distances in the binary compounds GaAs and InAs, respectively. This result is independent of the In molar fraction in the strained alloy layers

Glancing‐angle extended x‐ray absorption fine structure study of strained InGaAs/GaAs heterostructures

The structural properties of strained InGaAs grown by molecular beam epitaxy on GaAs(100) substrates, have been studied by glancing‐angle extended x‐ray absorption fine structure (EXAFS). The very low incidence angle of the x‐ray beam on the sample makes it possible to collect the signal coming from a thin quasi‐surface layer allowing the study of a single strained sample built up by only 6 ML of InGaAs. The EXAFS results show that a slight deformation of the first shell Ga–As distance occurs and that the strain is accommodated also by bond‐bending mechanism as deduced by the second and third coordination shells analysis. The lattice expands in the growth direction in agreement within the limits predicted by the elastic theory.