Schematic depiction of different energy terms contributing to the adsorption energy, and charge density difference of 2H-P after adsorption onto Cu(111) at 12.8 Angstrom separation. Credit: M. Müller/TU Munich
As we continue to shrink electronic components, top-down manufacturing methods begin to approach a physical limit at the nanoscale. A solution involves bottom-up self-assembly of molecular building blocks to build nanoscale devices.
Successful self-assembly is an elaborately choreographed dance, in which the attractive and repulsive forces within molecules, between each molecule and its neighbors, and between molecules and the surface that supports them, have to all be taken into account. To better understand the self-assembly process, Technical University of Munich researchers have characterized all interaction components, eg covalent bonding and van der Waals interactions. “In an ideal case, the smallest possible device has the size of a single atom or molecule,” said Katharina Diller.
As example is a single-porphyrin switch, 1 square nanometer. The porphine molecule is even smaller than this. Porphyrins are a group of ringed chemical compounds which notably include heme – responsible for transporting oxygen and carbon dioxide in the bloodstream – and chlorophyll. In synthetically-derived applications, porphyrins are studied for their potential uses as sensors, light-sensitive dyes in organic solar cells, and molecular magnets.
The researchers assessed interactions of the porphyrin molecule 2H-porphine by using density functional theory, a quantum mechanical computational modelling method used to describe the electronic properties of molecules and materials. Simulations were performed at the high-performance supercomputer SuperMUC at Leibniz-Rechenzentrum in Garching.
The metallic substrates chosen for porphyrin molecules to assemble on, the close packed single crystal surfaces of copper and silver, are widely used as substrates in surface science. This is due to the densely packed nature of the surfaces, so molecules exhibit smooth adsorption. The molecule adsorbs more strongly on copper, whereas silver does a better job of keeping the electronic structure of the molecule intact – allowing the researchers to monitor a variety of competing effects for future applications.
In their simulation, porphyrin molecules were placed on a copper or silver slab, which was repeated periodically to simulate an extended surface. After finding the optimal geometry in which the molecules would adsorb on the surface, they altered the size of the metal slab to increase or decrease the distance between molecules, thus simulating different molecular coverages. The computational setup gave them a switch to turn the energy contributions of neighboring molecules on and off, to observe individual interactions.
The weak long-range van der Waals interactions yielded the largest contribution to the molecule-surface interaction, and showed that the often employed methods to quantify the electronic charges in the system have to be used with caution. Surprisingly, while interactions directly between molecules are negligible, the researcher found indications for surface-mediated molecule-molecule interactions at higher molecular coverages.
“The analysis of the electronic structure and the individual interaction components allows us to better understand the self-assembly of porphine adsorbed on copper and silver, and additionally enables predictions for more complex porphyrine analogues,” Diller said. “These conclusions, however, come without yet considering the effects of atomic motion at finite temperature, which we did not study in this work.” http://www.alphagalileo.org/ViewItem.aspx?ItemId=160014&CultureCode=en
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