Nanoscale Order and Structure in Organic Materials: Diindenoperylene on Gold as a Model System

Abstract

Device performances in organic electronics are strongly influenced by the structure and morphology of the active layer. We investigated the thin film of diindenoperylene, a promising candidate for electronics, deposited on polycrystalline gold, and Au(100) and Au(111) single crystals using X-ray absorption and photoemission together with atomic force microscopy. The results show the influence of the substrate morphology, in this case via the roughness, on the structure of the obtained films. This affects the intermolecular interactions as evidenced by different details in X-ray absorption comparing the three cases. A further source of influence is the surface lattice geometry of the substrate that affects the shape of the islands. This behavior mimics very closely the behavior of atoms on metal growth, showing a general pattern in matter organization in the nanoscale regime.

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

“…The islands show a clear branched shape. XPEEM investigations 15 and thickness dependent measurements 16 using X-ray photoemission spectroscopy further confirm this Stranski-Krastanov growth mode. DIP thin films on various substrates have been investigated, revealing the tendency to Stranski-Krastanov growth.…”

“…The island distance is very small and the islands present similarities with DIP thin films grown on a variety of other substrates. 8,9,14,17 In particular, the shape of the DIP islands on Au(100) has an evident similarity with those of DIP on polycrystalline gold, where the latter is rough and with no preferential surface lattice geometry, 16 supporting the fact that the anisotropy in Au(100) is not sufficient to mark the island shape. The (110) faces show atomic troughs with strong anisotropy in the atomic distances giving rise to different degrees of possible interactions across or along the troughs.…”

Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface

“…The simultaneous use of this variety of techniques, in combination with controlled in-situ deposition, gives the opportunity not only to explore the complete electronic structure of the systems (occupied and unoccupied states, or orbitals) but to avoid possible artefacts and discrepancies due to slightly different preparations that could impact the morphology and the structure of the assemblies and consequently their electronic structure. [23][24][25] We demonstrate that a fractional charge transfer from the metal substrate to the physisorbed molecules occurs and that it exhibits a very specific and clearly recognizable fingerprint: This is a non-rigid shift of the XPS main lines and at the interface a strong alteration of the NEXAFS signal together with almost unperturbed occupied states. The experimental results are corroborated by calculations based on density-functional theory (DFT), as implemented in the exciting code.…”

Island shape and electronic structure in diindenoperylene thin films deposited on Au(110) single crystals

“…Details on NEXAFS normalization are given elsewhere. 28,29 No degradation of the samples was observed on the time scale of all presented experiments. AFM was performed ex situ using a Digital Instruments Nanoscope III Multimode microscope in tapping mode.…”

“…NEXAFS spectra are useful not only to gain information on the unoccupied states, stemming from transitions from the 1s states to excited bound states, 48 in this case, transitions from the C 1s core levels to the p* and s* molecular orbitals, 48 but also to investigate the structural properties of the films. 29,34,49,50 The NEXAFS features and their intensities are sensitive to the intermolecular interactions, 29,34,49,50 and NEXAFS dichroism allows determining the orientation of the molecules with respect to the substrate. 48,51 DIP NEXAFS spectra are characterized by two main groups of resonances that dominate the spectra (Fig.…”