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Probing Buried and Exposed Interfaces with Submolecular Precision


The chemistry of nanoscale materials and self-assembled monolayers are largely determined by interfacial properties, structural dimensionality, bonding and lattice structures, and defect types and densities. Bottom-up design strategies differ from prototypical top-down nanofabrication approaches in that the aim is to control and to place single molecules and atoms precisely, which may provide routes to breaking the current lithography limits. Scanning tunneling microscopy (STM) is able to probe the molecular world and to extract information from single molecules, groups of molecules, and even submolecular features. Here, we use STM in both ultrastable and ambient conditions to test the nature of dipolar interactions between molecules, to probe novel

building blocks for assembly, to manipulate surface binding modes, to examine a holey graphene framework against chemical deposition as a means in patterning, and to explore the nature of buried hydrogen-bonding networks in self-assembled monolayer matrices. The simultaneous usage of STM in topographic imaging mode and local barrier height (LBH) mode, in an ultrahigh vacuum (≤10-12 torr) and low temperature (4 K) environment, enables the measurement of both molecular topography and the buried dipolar interface. Using a block-matching approach, the correlation between molecular apexes and dipolar extrema is computed, thus enabling the visualization of molecular orientations at the submolecular scale. We also use STM in ambient conditions to examine the assembly of p-mercaptobenzoic acid derivative of carboranethiol on a Au{111} surface, where the carboxyl-modified carboranethiol backbone provides a surface available for further chemical functionality. Adding a second thiol group attachment in 1,2-carboranedithiol and 9,12-carboranedithiol isomers enables the control of valency between neighboring thiol/thiolate attachment sites. The assembly and manipulation of the two different binding modes using simple acid-base chemistry is evaluated and measured at a local level. Two-dimensional graphene has garnered much interest recently due to extraordinary two-dimensional properties. The design and placement of holes within a protecting sheet is tested against the deposition of molecules, where the holey network acts as a mask that may find use in lithographic processing. Hydrogen bonds also exhibit strong dipoles, where STM in simultaneous topographic and LBH mode may be able to measure the topographic map along with buried amide bond dipoles. We use an amide-containing self-assembled network to evaluate the nature of these ‘hidden’ networks at the single-molecule scale. The embodiment of this work aims to provide a fundamental understanding of chemical properties at the nanoscale and a complementary tool with averaging and ensemble techniques.

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