Maria Benedetta Casu

Determination of transport levels of organic semiconductors by UPS and IPS

In this paper, we present a comprehensive discussion on the determination of transport levels of organic semiconductors using a combination of UV and inverse photoemission spectroscopies (UPS and IPS, respectively). We discuss the key question of how the spectra should be evaluated to obtain the correct values for the transport levels. Our evaluation results in much smaller exciton binding energies than generally assumed, ranging from 0.1 to 0.4 eV for CuPc, PTCDA and Alq3. Moreover, we find that polarization effects, vibrations, experimental resolution and inhomogeneous broadening contribute very little to the position and width of the broad photoelectron spectroscopy (PES) and IPS signals. Thus, we suggest a model that explains much of the broadening as a result of dynamic charge delocalization. We discuss our data in analogy to those obtained for inorganic semiconductors (IOSCs), which are presented in a separate paper.

Buried interfacial layer of highly oriented molecules in copper phthalocyanine thin films on polycrystalline gold.

The growth of copper phthalocyanine thin films evaporated on polycrystalline gold is examined in detail using near edge x-ray absorption fine structure spectroscopy and surface sensitive x-ray photoemission spectroscopy. The combination of both methods allows distinguishing between the uppermost layers and buried interface layers in films up to approximately 3 nm thickness. An interfacial layer of approximately 3 ML of molecules with an orientation parallel to the substrate surface was found, whereas the subsequent molecules are perpendicular to the metal surface. It was shown that even if the preferred molecular orientation in thin films is perpendicular, the buried interfacial layer can be oriented differently.

Locally resolved core-hole screening, molecular orientation and morphology in thin films of diindenoperylene deposited on Au(111) single crystals

Organic electronics has reached the market in a very short period of time, since the first organic light emitting device has been demonstrated.[1] Growth of organic thin films has been the focus of intensive investigations that in the recent years have gained knowledge about the involved mechanisms: substrate-molecule interaction versus molecule-molecule interaction, preparation parameters, substrate morphology (i.e., roughness, local defects, steps), and post growth treatment.[2–6] Here we investigate thin films of diindenoperylene deposited on Au(111) single crystals by using photoelectron emission microscopy (PEEM). We also discuss the effect of the growth on the film structure using the molecular orientation (i.e., the angle between the molecular axis and the substrate).

Molecular orientation in diindenoperylene thin films deposited on polycrystalline gold

We present x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine structure spectroscopy measurements taken on diindenoperylene (DIP) thin films of different thicknesses deposited on polycrystalline gold. Our investigations show that DIP films present flat lying molecules in the first layer, while the angle between the molecular axis and the surface increases with thickness. By using XPS, we find evidence for Stranski–Krastanov growth mode.

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.

Nanoscale Assembly of Paramagnetic Organic Radicals on Au(111) Single Crystals

The successful thin-film deposition of a pyrene-substituted nitronyl nitroxide radical under controlled conditions has been demonstrated. The electronic properties, chemical environment at the interface, and morphology of the thin films have been investigated by a multitechnique approach. Spectroscopic and morphological analyses indicate a Stranski-Krastanov growth mode and weak physisorption of molecules onto the metallic surface. Electron spin resonance (ESR) spectroscopy shows that evaporation processes and deposition do not affect the paramagnetic character of the molecules. Useful concepts for the engineering of new, purely organic-based magnets, which may open the way to fruitful exploitation of organic molecular-beam deposition for assembly on solid surfaces in view of future technological applications, are presented.

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.

Thin‐Film Properties of DNA and RNA Bases: A Combined Experimental and Theoretical Study

The structures of the DNA and RNA bases cytosine, uracil, and thymine in thin films with a nominal film thickness of about 20 nm are studied by using X-ray photoemission spectroscopy (XPS) and Fourier-transform infrared spectroscopy. The molecules are evaporated in situ from powder on a gold foil. The experimental results indicate that cytosine is composed of two energetically close tautomeric forms, whereas uracil and thymine exist in only one tautomeric form. Additionally, quantum chemical calculations are performed to complement the experimental results. The relative energies of the tautomeric forms of cytosine, uracil, and thymine are calculated using Hartree-Fock (HF), density functional theory (DFT), and post-HF methods. Furthermore, the assignment of the XPS spectra is supported by using simple model considerations employing Koopmans ionization energies and Mulliken net atomic charges.

Chemisorption, morphology, and structure of a n-type perylene diimide derivative at the interface with gold: influence on devices from thin films to single molecules.

We have investigated thin films of a perylene diimide derivative with a cyano-functionalized core (PDI-8CN2) deposited on Au(111) single crystals from the monolayer to the multilayer regime. We found that PDI-8CN2 is chemisorbed on gold. The molecules experience a thickness-dependent reorientation, and a 2D growth mode with molecular stepped terraces is achieved adopting low deposition rates. The obtained results are discussed in terms of their impact on field effect devices, also clarifying why the use of substrate/contact treatments, decoupling PDI-8CN2 molecules from the substrate/contacts, is beneficial for such devices. Our results also suggest that perylene diimide derivatives with CN bay-functionalization are very promising candidates for single-molecule electronic devices.

Nanoscale assembly, morphology and screening effects in nanorods of newly synthesized substituted pentacenes

We report our investigation on the nanorods of two newly synthesized substituted pentacenes, δ4-substituted (2,3-X2-9,10-Y2) pentacene with X = Y = methoxy group (MOP) and X = F, Y = methoxy (MOPF), by using X-ray photoemission spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS), and atomic force microscopy (AFM). The nanorods were deposited on Au(111) single crystals. Energy dependent photoemission spectra show complex features, including a rich satellite structure that we have analyzed in detail by using a best-fit procedure applying constraints based on stoichiometry, electronegativity, and bond strength. This analysis reveals the presence of surface core level shifts due to the high electronegativity of the fluorine atoms. The distinctive features of growth and morphology of the nanorods are subjected to a template effect by the substrate lattice geometry, leading to morphological well-organized assemblies. Fluorine atoms play an important role not only in the electronic structure but also in the morphology of the nanorod assemblies.