The effect of long-range order on intermolecular interactions in organic semiconductors: zinc octaethyl porphyrin molecular thin film model systems.

In order to optimize the performance of devices based on porphyrin thin films it is of great importance to gain a physical understanding of the various factors which affect their charge transport and light-harvesting properties. In this work, we have employed a multi-technique approach to study vacuum deposited zinc octaethyl porphyrin (ZnOEP) thin films with different degrees of long-range order as model systems. An asymmetrical stretching of the skeletal carbon atoms of the porphyrin conformer has been observed and attributed to ordered molecular stacking and intermolecular interactions. For ordered films, a detailed fitting analysis of the X-ray absorption near edge structure (XANES) using the MXAN code establishes a symmetry reduction in the molecular conformer involving the skeletal carbon atoms of the porphyrin ring; this highlights the consequences of increased π-π stacking of ZnOEP molecules adopting the triclinic structure. The observed asymmetrical stretching of the π conjugation network of the porphyrin structure can have significant implications for charge transport and light harvesting, significantly influencing the performance of porphyrin based devices.

Exploring the multiparameter nature of EUV-visible wave mixing at the FERMI FEL.

The rapid development of extreme ultraviolet (EUV) and x-ray ultrafast coherent light sources such as free electron lasers (FELs) has triggered the extension of wave-mixing techniques to short wavelengths. This class of experiments, based on the interaction of matter with multiple light pulses through the Nth order susceptibility, holds the promise of combining intrinsic ultrafast time resolution and background-free signal detection with nanometer spatial resolution and chemical specificity. A successful approach in this direction has been the combination of the unique characteristics of the seeded FEL FERMI with dedicated four-wave-mixing (FWM) setups, which leads to the demonstration of EUV-based transient grating (TG) spectroscopy. In this perspective paper, we discuss how the TG approach can be extended toward more general FWM spectroscopies by exploring the intrinsic multiparameter nature of nonlinear processes, which derives from the ability of controlling the properties of each field independently.

Nanoscale transient gratings excited and probed by extreme ultraviolet femtosecond pulses.

Advances in developing ultrafast coherent sources operating at extreme ultraviolet (EUV) and x-ray wavelengths allow the extension of nonlinear optical techniques to shorter wavelengths. Here, we describe EUV transient grating spectroscopy, in which two crossed femtosecond EUV pulses produce spatially periodic nanoscale excitations in the sample and their dynamics is probed via diffraction of a third time-delayed EUV pulse. The use of radiation with wavelengths down to 13.3 nm allowed us to produce transient gratings with periods as short as 28 nm and observe thermal and coherent phonon dynamics in crystalline silicon and amorphous silicon nitride. This approach allows measurements of thermal transport on the ~10-nm scale, where the two samples show different heat transport regimes, and can be applied to study other phenomena showing nontrivial behaviors at the nanoscale, such as structural relaxations in complex liquids and ultrafast magnetic dynamics.

First Evidence of Purely Extreme-Ultraviolet Four-Wave Mixing.

The extension of nonlinear optical techniques to the extreme-ultraviolet (EUV), soft and hard x-ray regime represents one of the open challenges of modern science since it would combine chemical specificity with background-free detection and ultrafast time resolution. We report on the first observation of a four-wave-mixing (FWM) response from solid-state samples stimulated exclusively by EUV pulses. The all-EUV FWM signal was generated by the diffraction of high-order harmonics of the FERMI free-electron laser (FEL) from the standing wave resulting from the interference of two crossed FEL pulses at the fundamental wavelength. From the intensity of the FWM signal, we are able to extract the first-ever estimate of an effective value of ∼6×10^{-24} m^{2} V^{-2} for the third-order nonlinear susceptibility in the EUV regime. This proof of principle experiment represents a significant advance in the field of nonlinear optics and sets the starting point for a manifold of techniques, including frequency and phase-resolved FWM methods, that are unprecedented in this photon-energy regime.

Optical constants modelling in silicon nitride membrane transiently excited by EUV radiation.

We hereby report on a set of transient optical reflectivity and transmissivity measurements performed on silicon nitride thin membranes excited by extreme ultraviolet (EUV) radiation from a free electron laser (FEL). Experimental data were acquired as a function of the membrane thickness, FEL fluence and probe polarization. The time dependence of the refractive index, retrieved using Jones matrix formalism, encodes the dynamics of electron and lattice excitation following the FEL interaction. The observed dynamics are interpreted in the framework of a two temperature model, which permits to extract the relevant time scales and magnitudes of the processes. We also found that in order to explain the experimental data thermo-optical effects and inter-band filling must be phenomenologically added to the model.

Characterization of ultrafast free-electron laser pulses using extreme-ultraviolet transient gratings.

The characterization of the time structure of ultrafast photon pulses in the extreme-ultraviolet (EUV) and soft X-ray spectral ranges is of high relevance for a number of scientific applications and photon diagnostics. Such measurements can be performed following different strategies and often require large setups and rather high pulse energies. Here, high-quality measurements carried out by exploiting the transient grating process, i.e. a third-order non-linear process sensitive to the time-overlap between two crossed EUV pulses, is reported. From such measurements it is possible to obtain information on both the second-order intensity autocorrelation function and on the coherence length of the pulses. It was found that the pulse energy density needed to carry out such measurements on solid state samples can be as low as a few mJ cm-2. Furthermore, the possibility to control the arrival time of the crossed pulses independently might permit the development of a number of coherent spectroscopies in the EUV and soft X-ray regime, such as, for example, photon echo and two-dimensional spectroscopy.

Graphene nanobubbles on TiO2 for in-operando electron spectroscopy of liquid-phase chemistry.

X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Absorption Spectroscopy (XAS) provide unique knowledge on the electronic structure and chemical properties of materials. Unfortunately this information is scarce when investigating solid/liquid interfaces and chemical or photochemical reactions under ambient conditions because of the short electron inelastic mean free path (IMFP) that requires a vacuum environment, which poses serious limitation on the application of XPS and XAS to samples present in the atmosphere or in the presence of a solvent. One promising approach is the use of graphene (Gr) windows transparent to both photons and electrons. This paper proposes an innovative system based on sealed Gr nanobubbles (GNBs) on a titanium dioxide TiO2 (100) rutile single crystal filled with the solution of interest during the fabrication stage. The GNBs were successfully employed to follow in-operando the thermal-induced reduction of FeCl3 to FeCl2 in aqueous solution. The electronic states of chlorine, iron and oxygen were obtained through a combination of electron spectroscopy methods (XPS and XAS) in different phases of the process. The interaction of various components in solution with solid surfaces constituting the cell was obtained, also highlighting the formation of a covalent C-Cl bond in the Gr structure. For the easiness of GNB fabrication and straightforward extension to a large variety of solutions, we envisage a broad application of the proposed approach to investigate in detail electronic mechanisms that regulate liquid/solid electron transfer in catalytic and energy conversion related applications.

Fullerene C60-silver nanoparticles hybrid structures: optical and photoelectric characterization.

A detailed optical and photoelectric characterization of pristine fullerene C60 films deposited onto n-silicon substrates (C60/Si), C60 films crosslinked by means of the solvent-free chemical functionalization with 1,8-diaminooctane (C60-DA/Si), and the pristine and crosslinked films decorated with silver nanoparticles (C60-Ag/Si and C60-DA-Ag/Si), was carried out. The reflectance spectra obtained allowed to calculate the absorption coefficient (alpha = 4pik/lamda) spectral dependencies and the spectra of light transmittance in layered barrier structures metal(Au)/fullerene/Si. Photoelectric properties of the films were investigated as well. The experimentally measured values of band gap were in a good agreement with mobility gap values (2.3 +/- 0.1 eV). The decoration of fullerene films (both pristine and chemically crosslinked) with silver nanoparticles did not change the photocurrent spectra as compared to those for undecorated fullerene films, but lowered the values of internal quantum efficiency Qint. The photocurrent generated in fullerene/Si heterostructure, showed a maximum value at lamda to appromimately 450 nm (Qint max approximately = 0.25 for decorated and undecorated C60-DA/Si films), and it was higher for the samples based on pristine C60 films, in accordance with their higher absorption coefficient. Diminishing of Qint for C60-DA/Si and C60-DA-Ag/Si films was observed for the spectral range of photocurrent generated in Si layer. The analysis of dark current-voltage characteristics showed that the barrier properties differ insignificantly, though a certain increase of series resistance was observed for the C60-DA/Si samples.