Scanning Probe Microscopy Studies of Functional Molecular Structures Prepared via Electrospray Deposition

Functional molecular materials built-up by self-assembly on surfaces in ultrahigh vacuum (UHV) require rationally designed complex and appropriately equipped elementary building blocks. Access to increasingly complex and large precursor molecules with functional groups can offer interesting features as a result of newly accessible properties. A well-established method to allow high-resolution scanning probe microscopy (SPM) studies of such large and complex molecular structures on surfaces with the necessary cleanliness and reasonable control over the coverage is electrospray deposition (ESD). Thereby, molecular solution droplets are electrically accelerated towards surfaces in UHV through a differential pumping setup.
In this thesis, SPM studies of large two-dimensional supramolecular islands on surfaces in UHV, formed by ESD of already large, complex and functionalized molecules, are first performed to reveal specific responses to external stimuli and their tuning. In a second step, solution synthesized nanoribbons are deposited by ESD to investigate their novel and interesting structures and properties on even previously not accessible surfaces in UHV.
First, the influence of the deposition method on adsorption properties and surface morphology is evaluated in comparison to thermal evaporation in a model study with non-contact atomic force microscopy (nc-AFM). Then, the thermal response of molecular networks due to alkyl chains is investigated for a large spoked wheel (SW) molecule with nc-AFM at 300 K, scanning tunneling microscopy (STM) and CO functionalized AFM at 5 K. In combination with molecular dynamics (MD) simulations this reveals a mechanism of thermal expansion based on alkyl chain mobility. This mechanism is applied to tune the thermal response of hexabenzocoronene (HBC) based molecules equipped with different alkyl chain lengths. STM and nc-AFM imaging at 5 K, 77 K and 300 K unveil an inverse correlation of chain length and thermal expansion coefficient, shorter chains result in a higher expansion.
In a second part of this thesis, two solution synthesized graphene nanoribbons (GNRs) containing fragile moieties, alkyl chains, tert -butyl and methoxy edges, are investigated. Nc-AFM studies at room temperature demonstrate a significant influence of edge functionalization and structure. A planar and flat rigid structure is observed for a methoxy functionalized ”cove”-type GNR, while a non-planar twisted and significantly more flexible structure is observed for the three-dimensionally designed fjord-GNR. Subsequent depositions of this GNR on non-metallic surfaces open high-resolution studies to fragile GNRs even on previously inaccessible surfaces and when annealing should be avoided.