A Derivative of the Blatter Radical as a Potential Metal-Free Magnet for Stable Thin Films and Interfaces.

Organic radicals are fascinating materials because of their unique properties, which make them suitable for a variety of applications. Their synthesis may be challenging, and big efforts have focused on chemical stability. However, introducing a new material in electronics not only requires chemically stable molecules but also stable monolayers and thin films in view of their use in devices. In this work, we have investigated the thin films of a derivative of the Blatter radical that was synthesized bearing in mind the thermodynamic factors that govern thin film stability. We have proved our concept by investigating the electronic structure, the paramagnetic character, and stability of the obtained films under UHV and ambient conditions by in situ X-ray photoelectron spectroscopy, ex situ atomic force microscopy, and electron paramagnetic resonance spectroscopy.

Electronic structure and stability of fluorophore-nitroxide radicals from ultrahigh vacuum to air exposure.

Thin film processes of organic radicals remain widely unknown, although these materials may have a significant technological potential. In aiming at their use in applications, we explore the electronic structure of thin films of a nitronyl nitroxide radical attached to a fluorophore core. According to our findings, this molecule maintains its radical function and, consequently, its sensing capabilities in the thin films. The films are characterized by a high structural degree of the molecular arrangement, coupled to strong vacuum and air stability that make this fluorophore-nitroxide radical an extremely promising candidate for application in electronics. Our work also identifies a quantitative correlation between the results obtained by the simultaneous use of X-ray photoemission and electron paramagnetic resonance spectroscopy. This result can be used as a standard diagnostic tool in order to link the (in situ-measured) electronic structure with classical ex situ paramagnetic investigations.

Substrate-induced effects in thin films of a potential magnet composed of metal-free organic radicals deposited on Si(111).

We deposit a paramagnetic pyrene derivative of the nitronyl nitroxide radical on Si(111). The molecules experience a strong chemical interaction with the substrate that influences the film growth. We also study the time evolution of the nitronyl nitroxide radical under a micro-focused soft X-ray beam, observing a stable radical as a product. This result hints at the possibility of using this class of materials in dosimeters and sensors.

Island shapes and aggregation steered by the geometry of the substrate lattice.

We find that island shapes and aggregation in diindenoperylene deposited on Au(100), Au(110), and Au(111) single crystals are steered by the anisotropy due to the lattice geometry of the substrate. This phenomenon may be exploited as a tool for molecular patterning of surfaces.

Resonant Raman spectra of diindenoperylene thin films.

Resonant and preresonant Raman spectra obtained on diindenoperylene (DIP) thin films are interpreted with calculations of the deformation of a relaxed excited molecule with density functional theory (DFT). The comparison of excited state geometries based on time-dependent DFT or on a constrained DFT scheme with observed absorption spectra of dissolved DIP reveals that the deformation pattern deduced from constrained DFT is more reliable. Most observed Raman peaks can be assigned to calculated A(g)-symmetric breathing modes of DIP or their combinations. As the position of one of the laser lines used falls into a highly structured absorption band, we have carefully analyzed the Raman excitation profile arising from the frequency dependence of the dielectric tensor. This procedure gives Raman cross sections in good agreement with the observed relative intensities, both in the fully resonant and in the preresonant case.

Unusual energy shifts in resonant photoemission spectra of organic model molecules.

We study the electronic structure of zinc phthalocyanine (ZnPc) and 1,4-octa-decyl substituted zinc phthalocyanine [(Dec)(8)PcZn] thin films (approximately 6-15 nm) using resonant photoemission spectroscopy and X-ray absorption spectroscopy (XAS) at room temperature and at liquid He temperature. From XAS we conclude that the probability amplitude of the lowest unoccupied molecular orbital is located predominantly at the inner C and N atoms of the molecules. Nonlinear energy shifts in resonant photoemission were observed; large shifts are explained by reduced electrical conductivity of inhomogeneously oriented molecules.

From interfaces to surfaces: soft x-ray spectromicroscopy investigations of diindenoperylene thin films on gold.

We present the results of photoemission electron microscopy investigations on diindenoperylene (DIP) thin films deposited on polycrystalline gold, prepared in order to have a roughness much larger than the molecular size. Our investigations revealed the ability of the DIP molecule to form well-organized films, exhibiting a different molecular orientation with respect to the already known λ and σ phases. In locally thicker film regions, the energy of the films is minimized by a molecular arrangement that has an asymptotic tendency to the σ phase.

A high-resolution near-edge x-ray absorption fine structure investigation of the molecular orientation in the pentacene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) pentacene/system.

We present x-ray photoemission spectroscopy and highly resolved near-edge x-ray absorption fine structure spectroscopy measurements taken on pentacene thin films of different thicknesses deposited on a spin coated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrate. Thin films of pentacene were prepared by using organic molecular beam deposition in situ using strictly controlled evaporation conditions. Our investigations show that pentacene thin films on PEDOT:PSS are characterized by upright standing molecules. Due to the strong dichroic behavior, the calculated values of the molecular orientation give a clear indication not only of the real molecular arrangement in the films but also of a high orientational order. This high degree of molecular orientation order is a characteristic already of the first layer. The films show the tendency to grow on the PEDOT:PSS substrate following an island-fashion mode, with a relatively narrow intermixing zone at the interface between the pentacene and the polymer blend. The peculiarity of the growth of pentacene on PEDOT:PSS is due to the fact that the substrate does not offer any template for the nucleated films and thus exerts a lateral order toward the crystal structure arrangement. Under these conditions, the upright orientation of the molecules in the films minimizes the energy required for the system stability.

Influence of the preparation conditions on the morphology of perylene thin films on Si(111) and Si(100).

Thin films of perylene on Si(111) and Si(100) substrates have been investigated using a variety of experimental techniques. We find that the structural and morphological properties as well as the growth modes strongly depend on the preparation parameters. In general, we observe the existence of a relatively weak coupling between perylene and the two single crystal substrates. However, under special preparation conditions, it is possible to obtain a multilayer phase on the Si(111) substrate that is characterized by flat-lying, parallel-oriented molecules, and strong coupling with the substrate in the first layer. This phase has different structural, electronic, and intermolecular bonding properties as compared to the known crystalline phases. On Si(100), by varying the deposition rate between 0.1 and 10 nm/min, it is possible to observe a transition from island growth mode, with large and isolated crystallites, to homogeneous film growth. These findings contribute to the basic knowledge for film engineering. Thus, the film morphology could be designed ranging from the growth of very large single grains suitable for a complete nanodevice to homogenous films for application in large displays.

High-resolution inner-shell excitation spectroscopy of H2-phthalocyanine.

We report on a combined experimental and theoretical carbon and nitrogen K-edge near-edge x-ray absorption fine structure investigation on condensed metal-free phthalocyanine (H2Pc). Based on the results from improved virtual orbital calculations, all resonances in the experimental high-resolution data can be assigned to various electronic transitions. The comparison between experiments and calculations further shows that a significant influence of the core hole, which affects both the transition energies and the cross sections, is present and must be considered in theoretical approaches. Moreover, additional fine structure is clearly resolved for the first N 1s-->pi* transition, which can be interpreted as vibronic coupling to the electronic core excitation.

Bonding and structure of glycine on ordered Al2O3 film surfaces.

The interaction between glycine (NH2CH2COOH) layers and an ultrathin Al2O3 film grown epitaxially onto NiAl(110) was studied by temperature-programmed desorption, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, work function measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. At monolayer coverages at 110 K, there are two coexisting molecular forms: the anionic (NH2CH2COO-) and the zwitterionic form (NH3+CH2COO-) of glycine. As deduced from the photoemission data, the buildup of multilayers at 110 K leads to a condensed phase predominantly in the zwitterionic state. In contrast to the monolayer at 110 K, the monolayer formed at 300 K consists primarily of glycine molecules in the anionic state. The latter species is adsorbed with the oxygen atoms of the carboxylic group pointing toward the substrate. The polarization-dependent C K- and O K-edge NEXAFS spectra indicate that the glycinate species in the monolayer at 300 K is oriented nearly perpendicular to the surface, with the amino group pointing away from the surface.