funCOS 4 focuses on structural aspects of organic adlayers and nanostructures including their correlations with the oxide substrate. We will take advantage of the strong expertise at the FAU in x ray and neutron diffraction in surface sensitive modes to characterize adlayer structures and adsorption dynamics. Reflectometry and GID focus on geometrical aspects including the full three-dimensional arrangement of the entire complex consisting of the substrate, the linker, and the functional molecule with accuracies in the sub-Ångstrom regime. The diffraction methods will be complemented by direct imaging, spectroscopic, microscopic (LEEM), and microspectroscopic methods x ray (PEEM, micro-NEXAFS). The latter will reveal geometric and electronic features from the atomic and molecular to the nano- and mesoscale. Spatially resolved NEXAFS probes unoccupied orbitals and monitors molecule-substrate interactions with superior sensitivity. Resonant XPS and Resonant AES applied to electronic relaxation processes in weakly bound molecules will elucidate their intermolecular and molecule–substrate interactions.
funCOS 4 aims at a deeper understanding of the basic processes in the formation of differently prepared organic thin films on various oxide surfaces (single crystal substrates, nanostructured oxides, amorphous substrates).
- This includes aspects of interactions, in particular the interplay of intermolecular interactions and molecule–substrate interactions, which are governing the growth of ultrathin organic films (perpendicular and lateral structures, molecular orientation and tilt).
- Utilizing probes, which offer information ranging from the atomic to the mesoscopic length scale, the structural properties of adsorbed organic films from the submonolayer to the multilayer regime can be accessed.
- Diffraction techniques (XRD/LEED) deliver structural information with accuracies better than 0.01 nm, and in-situ studies can make the film growth kinetics visible.
- Various microscopy techniques (PEEM, STXM, LEEM) shall be employed to provide the growth mode (layers, islands, or random growth) and film formation kinetics in real space with resolutions on the mesoscopic length scale, i.e., from lateral dimensions ranging from 20 to 10.000 nm.
The project will bridge the gap from model systems prepared under ultrahigh vacuum conditions to molecular depositions from solvents thus offering potential pathways to prepare highly ordered molecular systems without costly hardware.
Systems and strategy
The systems under investigation will focus on the funCOS toolbox with particular emphasis on metalloporphyrin systems (preferably Mg, Co and Pt metal centers) with functional ligands. For the study of self-assembled monolayers using tetraphenyl porphyrin frameworks (POR-SAMs) in particular -OH and -COOH linker groups are considered promising candidates to form a covalent bond with the surface hydroxyl groups via separation of water resulting in a self-assembled monolayer . As substrates we will use amorphous and single crystalline TiO2 and MgO (amorphous substrates will be prepared in an in-project cooperation with R. Fink via sputtering techniques, single crystals will be acquired commercially. Further, silicon-supported MgO nanocubes in the uncompressed phase and as textured coatings will be utilized as substrates. These will be prepared in funCOS 5. The unique nanoscale structure which can be tuned by the nanocube size and the compression is expected to influence the POR-SAM arrangement. The boundaries between the nanocubes will provide a highly reactive surface compared to the nanocube planes, and the POR-SAMs are expected to dock on in a highly correlated manner with these substrates. The orientational distribution will be assessed by us via powder-, single crystal- and thin film x-ray techniques in close cooperation with funCOS 5.
To monitor the film growth under UHV conditions, ultrathin MgO films grown on Ag(100) substrates shall be used. Again metalloporphyrins with planar and axial ligands will be employed. To improve the spectroscopic contrast in electron spectroscopy (contrast in binding energy with respect to the MgO substrate) we favor metalloporphyrins with Co and Pt centers. Deposition will be made using homemade Knudsen cells while the deposition is monitored by mass spectrometry and quartz microbalance or in-situ film growth (in PEEM).
The structure of the SAMs will be investigated first on our laboratory reflectometer, a new custom-modified D8-diffractometer with highly flexible x-ray optics including the options for Cu- and Mo-radiation. The reflectivity mode shows the structure along the surface normal of the monolayer , and the GID mode mirrors the lateral structure which represents the in-plane correlations . These data will yield comprehensive information on the thickness, the electron density and the roughness along the surface normal as well as the lateral structure of the monolayer. The resulting electron density profile will be chemically and geometrically interpreted to provide a model for the structure of the SAM. Particular interest lies in structural differences of the monolayer on the different oxide substrates. Referring to other systems like alkanoles on sapphire epitaxial behavior is expected for the single crystalline substrates as the lateral structure of the monolayer is expected to correlate with well-defined bonding sites . This is also expected to be the case for the MgO nanocube substrates as the boundary regions between the nanocubes are highly reactive compared to the nanocube planes.
Interruption of the SAM growth by taking substrates out off solution will yield information on the growth kinetics . The evolution of the layer thickness in these ex-situ measurements of interrupted growth will further show the growth mode. This information will be validated via complementary real space investigations such as AFM (funCOS 1) and chemical spectroscopy (funCOS 3) providing insight in the chemical bonding mechanism. In addition, our model from scattering data will be compared with theoretical considerations (funCOS 6).
A further strategic aim of this project is to perform in-situ measurements of the formation of the surface coating in the reaction bath. We have ample experience in building suitable sample environments. The measurements will be done in part on our laboratory instrument already mentioned with Mo radiation where we can penetrate up 1 cm through the reaction liquid while still having a dynamic range of more than 7 orders of magnitude! In addition to providing already valuable information, such data are highly desirable to prepare for and optimize measurements at synchrotron sources where high intensity instruments operating at shorter wavelengths are available. The synchrotron data to be taken in reflectivity, GID and GISAS are expected to make the growth of POR-SAMs under wet conditions visible in great detail .
In-situ neutron reflectivity and grazing incident diffraction measurements of the growth of POR-SAMs will complement whenever indicated the in-situ synchrotron data. Neutrons have the virtue of a high sensitivity for organic molecules in particular when H/D isotopic marking is being applied .
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