Molecular self-assembly on surfaces
Dr. Tobias Weidner
Self organization of functional molecules at interfaces plays an important role for surface modification, molecular electronics and biomaterial surface design. To improve the quality of existing films and to develop new molecular designs it is important to understand microscopic details of molecular structure, binding and film assembly. We use a complementary suite of surface analytical tools to provide that information about polymer surfaces and novel self-assembled monolayers (SAMs). Sum frequency generation (SFG) spectroscopy is used to probe molecular alignment and orientation. Figure 1 shows an example how interfacial structure affects electronic device performance. SFG spectra allow a quantitative analysis of the phenyl ring orientations at the surface of a polystyrene film spin-coated onto hafnium oxide with and without annealing. The differences in ring orientation caused by the annealing step influences the structure of pentacene layers grown on these films, which changes field-effect transistor characteristics.
Fig. 1: 1: Left) SFG spectra of untreated and annealed spin coated polystyrene films on HfOx surfaces. SFG spectra show different orientations of the phenyl rings depending on the surface treatment. Right) AFM micrographs of pentacene films deposited into the different polystyrene surfaces. The polystyrene structure affects the pentacene film morphology and device performance in transistor applications.
Molecular designs involving oligodentate binding units are particularly promising for
immobilizing large, sterically demanding functional groups on surfaces. Architectures with
two, three and more anchors have been used in this collaboration with Prof. Ulrich
Siemeling at the University of Kassel. Figure 2 displays a disk-like phthalocyanine-based
precursor molecule bearing eight thioether binding units. This molecule is intended to
provide a basis for subsequent vertical ligand attachment, which could, e.g., be uses as
a basis for metal-organic frameworks. For this study, SFG was combined with near-edge
X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy
(XPS) to probe the binding chemistry and molecular orientation
. Information about SAM structure and quality provides essential feedback for synthetic chemists in the field and our aim is to develop general design guidelines for the architecture of complex precursor molecules.
Fig. 2: Disk-like adsorbate molecules for vertical anchoring of ligands on gold. These manganese(III) phthalocyanines have eight thioether linker for robust binding to gold surfaces.
U. Siemeling, C. Schirrmacher, U. Glebe, C. Bruhn, J. E. Baio, L. Árnadóttir, D. G. Castner, T. Weidner. Phthalocyaninato complexes with peripheral alkylthio chains: disk-like adsorbate species for the vertical anchoring of ligands on gold surfaces, Inorganica Chimica Acta, in press.
O. Acton, D. Hutchins, L. Arnadottir, T. Weidner, G. G. Ting, T.-W. Kim, N. Cernetic, D. G. Castner, H. Ma, A. K.-Y. Jen. Spin-Cast and Patterned Organo-Phosphonate Self-Assembled Monolayer Hybrid Dielectrics on Metal Oxide Activated Si, Advanced Materials, 23, 1899 (2011).
O. Acton, M. Dubey, T. Weidner, K. O’Malley, T.-W. Kim, G. G. Ting, D. Hutchins, J. Baio, T. C. Lovejoy, A. H. Gage, D. G. Castner, H. Ma, A. K.-Y. Jen. Simultaneous Modification of Bottom-Contact Electrode and Dielectric Surfaces for Organic Thin Film Transistors Through Single-Component Spin-Cast Monolayer, Advanced Functional Materials, 21, 1476 (2011).
T. Weidner, M. Zharnikov, J. Hoßbach, D. G. Castner U. Siemeling. Adamantane-Based Tripodal Thioether Ligands Functionalised with a Terminal Ferrocenyl Moiety for Redox-Active Self-Assembled Monolayers, Journal of Physical Chemistry C, 114, 14975 (2010).
M. Dubey, T. Weidner, L. J. Gamble, D. G. Castner. Structure and Alignment of Organophosphonate Self-Assembled Monolayers on Si(100), Langmuir, 26, 14747 (2010).
Y. Wang, O. Acton, G. Ting, T. Weidner, H. Ma, D. G. Castner, A. K.-Y. Jen. Low-Voltage High-Performance Organic Thin Film Transistors with Thermally Annealed Polystyrene/Hafnium Oxide Dielectric, Applied Physics Letters 95, 243302 (2009).
T. Weidner, N. Ballav, U. Siemeling, D. Troegel, T. Walter, R. Tacke*, D. G. Castner, M. Zharnikov Tripodal Binding Units for Self-Assembled Monolayers on Gold: A Comparison of Thiol and Thioether Headgroups Journal of Physical Chemistry C 113, 19609 (2009).