Design Principles for Metastable Standing Molecules

Abstract

Molecular nanofabrication with a scanning probe microscope (SPM) is a promising route toward the prototyping of metastable functional molecular structures and devices which do not form spontaneously. The aspect of mechanical stability is crucial for such structures, especially if they extend into the third dimension vertical to the surface. A prominent example is freestanding molecules fabricated on a metal which can function as field emitters or electric field sensors. Improving the stability of such molecular configurations is an optimization task involving many degrees of freedom and therefore best tackled by computational nanostructure design. Here, we use density functional theory to study 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) standing on the Ag(111) surface as well as on the tip of a scanning probe microscope. We cast our results into a simple set of design principles for such metastable structures, the validity of which we subsequently demonstrate in two computational case studies. Our work proves the capabilities of computational nanostructure design in the field of metastable molecular structures and offers the intuition needed to fabricate new devices without tedious trial and error.

Concept for the Real-Time Monitoring of Molecular Configurations during Manipulation with a Scanning Probe Microscope

“…O carb is also expected to form bonds with the Au atoms in the close-packed Au(111) surface if the Au–O carb distance is not too large. While these bonds will be weaker due to the higher coordination of the Au atoms in the surface, they still have a considerable impact on the measured force gradients (Figure b). Such molecule–surface bonding is more prominently observed on the Ag(111) surface, where it leads to a distortion of PTCDA, with the O carb atoms bending toward the surface to a distance of 2.66 Å above the topmost atomic plane of the substrate .…”

“…20 Moreover, the rest of the tip apex beyond the metal−oxygen bond plays only a minor role in the behavior of the tip−molecule junction. 26 ■ METHODS Primary and Duplicate Configurations. The atomic lattice of the Au(111) surface constitutes the natural frame of reference relative to which all tip−molecule configurations are defined.…”

“…This is a reasonable simplification since the oxygen atom prefers to bind on top of the tip apex atom and not to a bridge or hollow site . Moreover, the rest of the tip apex beyond the metal–oxygen bond plays only a minor role in the behavior of the tip–molecule junction …”

“… 20 Moreover, the rest of the tip apex beyond the metal–oxygen bond plays only a minor role in the behavior of the tip–molecule junction. 26 …”