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RESEARCH PRODUCT

Self-assembly of Organic Molecules on Insulating Surfaces

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description

Molecular self-assembly is known to provide a powerful tool for creating functional structures, with the ultimate structure and functionality encoded in the molecular building blocks. Upon molecule deposition onto surfaces, functional structures have been created ranging from defect-free, highly symmetric two-dimensional layers to complex assemblies with dedicated functionality. Especially organic molecules play a key role for molecular self-assembly due to their impressive structural flexibility and the high degree of control by chemical synthesis. Furthermore, the surface itself provides another exciting dimension: adjusting the subtle balance between intermolecular and molecule-surface interactions allows creating a broad variety of structures and explains the great success when confining molecular self-assembly to surfaces. While most of the structures realized so far have been fabricated on metallic substrates, comparatively little is known about molecular self-assembly on insulating surfaces. However, extending the materials basis to insulating substrates is of increasing interest to benefit from self-assembly strategies for emerging technologies such as molecular (opto)electronics. For the latter and further related applications, decoupling of the electronic structure from the underlying substrate is mandatory for the device functionality. Moreover, future applications will require the molecular structure to be stable at room temperature rather than at low temperatures. On insulating support surfaces, however, most attempts to create self-assembled molecular structures at room temperature have been hampered by the weak molecule-surface interactions. Thus, considerable effort has been made to establish means for increasing the molecule-surface interaction and to provide strategies for tailoring the anchoring of the molecules towards the surface. Non-contact atomic force microscopy has been proven to provide an indispensable tool for direct imaging of molecular structures on electronically insulating surfaces with molecular resolution. Besides imaging, this technique also offers the potential to obtain complementary information, for example when performing Kelvin probe force microscopy or three-dimensional force mapping. In this chapter, we report the current status of molecular self-assembly on insulating surfaces in ultra-high vacuum with a special emphasis on structures stable at room temperature. Basic molecular self-assembly principles are reviewed and applied to the specific situation of prototypical insulating surfaces. Strategies for anchoring organic molecules towards insulating surfaces are discussed in the view of macroscopic as well as microscopic parameters such as the surface energy and detailed molecule-surface binding motifs. Based on structure optimization at the molecular level, the molecule-surface interaction can be tuned to adjust the final assembly, ranging from largely unperturbed molecular bulk crystals to strongly substrate-templated overlayers. Finally, current research topics are outlined, focusing on efforts directed towards steadily increasing the control of the resulting structures.