Researchers in Germany studied how a multitude of electronic interactions govern the encounter between a molecule called porphine and copper and silver surfaces
As we continue to shrink electronic components, top-down manufacturing methods begin to approach a physical limit at the nanoscale. Rather than continue to chip away at this limit, one solution of interest involves using the 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, researchers at the Technical University of Munich have characterized the contributions of all interaction components, such as covalent bonding and van der Waals interactions between molecules and between molecules and a surface.
"In an ideal case, the smallest possible device has the size of a single atom or molecule," said Katharina Diller, who worked as a postdoctoral researcher in the group of Karsten Reuter at the Technical University of Munich. Reuter and his colleagues present their work this week in The Journal of Chemical Physics, from AIP Publishing.
One such example is a single-porphyrin switch, which occupies a surface area of only one square nanometer. The porphine molecule, which was the object of this study, 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 from TU Munich assessed the 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. Their simulations were performed at the high-performance supercomputer SuperMUC at Leibniz-Rechenzentrum in Garching.
The metallic substrates the researchers chose for the 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, which allow the molecules to exhibit a smooth adsorption environment. Additionally, copper and silver each react differently with porhyrins - 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, the researchers 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, in order to observe the interplay of the individual interactions.
Diller and Reuter, along with colleagues Reinhard Maurer and Moritz Müller, who is first author on the paper, found that 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."
The article, "Interfacial charge rearrangement and intermolecular interactions: Density-functional theory study of free-base porphine adsorbed on Ag(111) and Cu(111)," is authored by Moritz Müller, Katharina Diller, Reinhard J. Maurer, and Karsten Reuter. It will appear in the Journal of Chemical Physics on January 11, 2016 (DOI: 10.1063/1.4938259). After that date, it can be accessed at: http://scitation.
ABOUT THE JOURNAL
The Journal of Chemical Physics publishes concise and definitive reports of significant research in the methods and applications of chemical physics.
Jason Socrates Bardi | EurekAlert!
What happens when we heat the atomic lattice of a magnet all of a sudden?
18.07.2018 | Forschungsverbund Berlin
Subaru Telescope helps pinpoint origin of ultra-high energy neutrino
16.07.2018 | National Institutes of Natural Sciences
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.
Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
19.07.2018 | Materials Sciences
19.07.2018 | Earth Sciences
19.07.2018 | Life Sciences