See the electron orbital signature
(Nanowerk News) No one will ever be able to see a purely mathematical construct like a perfect sphere. But now, scientists using supercomputer simulations and atomic-resolution microscopes have imaged the orbital signatures of electrons, which are determined by the mathematical equations of quantum mechanics and predict where an atomic electron is most likely to be.
Scientists at UT Austin, Princeton University, and ExxonMobil have directly observed the electron orbital signatures in two different transition metal atoms, iron (Fe) and cobalt (Co), which are present in the metal-phthalocyanine. The signature is clearly visible in forces as measured by atomic force microscopy, which often reflect the underlying orbitals and can be interpreted as such.
Their study was published as an Editor’s Spotlight in the journal Nature Communications (“Observation of the electron orbital signature of a single atom in a metal-phthalocyanine using atomic force microscopy”).
“Our collaborators at Princeton University found that although Fe and Co are adjacent atoms on the periodic table, implying similarities, the corresponding force spectra and their measured images show reproducible experimental differences,” said study co-author James R. Chelikowsky, WA “Tex” Moncrief, Jr. Chair of Computational Materials and professor in the Departments of Physics, Chemical Engineering, and Chemistry in the College of Natural Sciences at UT Austin. Chelikowsky also serves as director of the Center for Computational Materials at the Oden Institute for Computational Engineering and Sciences.
Without a theoretical analysis, the Princeton scientists were unable to pinpoint the source of the differences they found using high-resolution non-contact atomic force microscopy (HR-AFM) and spectroscopy which measures molecular-scale forces on the order of piconewtons (pN), one trillionth of a Newton.
“When we first observed experimental images, our initial reaction was to marvel at how experiments were able to capture such subtle differences. This is a very small force,” Chelikowsky added.
“By directly observing the electron orbital signature using techniques such as atomic force microscopy, we can gain a better understanding of the behavior of individual atoms and molecules, and potentially even how to design and engineer new materials with specific properties. This is very important in fields such as materials science, nanotechnology and catalysis,” said Chelikowsky.
The required electronic structure calculations are based on density functional theory (DFT), which starts from basic quantum mechanical equations and serves as a practical approach to predicting material behavior.
“Our main contribution is that we validated through our real-space DFT calculations that the observed experimental differences mainly stem from the different electronic configurations in the 3d electrons of Fe and Co near the Fermi level, the highest energy state an electron can occupy in an atom. ,” said co-author of the first study Dingxin Fan, a former graduate student who worked with Chelikowsky. Fan is now a postdoctoral research fellow at the Princeton Materials Institute.
The DFT calculations included copper substrates for Fe and Co atoms, adding several hundred atoms to the mix and requiring intense computation, for which they were awarded the Stampede2 supercomputer at the Texas Advanced Computing Center (TACC), funded by the National Science Foundation.
“In our model, at a certain height, we move the AFM carbon monoxide tip over the sample and calculate the quantum forces at each grid point in real space,” says Fan. “This requires hundreds of different calculations. The built-in software package on TACC’s Stampede2 helps us make data analysis much easier. For example, the Visual Molecular Dynamics software accelerates our analysis of computational results.”
“Stampede2 has provided excellent computing power and storage capacity to support our various research projects,” added Chelikowsky.
By demonstrating that the electron orbital signature can indeed be observed using AFM, the scientists suggest that this new knowledge can extend the application of AFM to different areas.
What’s more, their study used an inert molecule probe tip to get close to another molecule and accurately measure the interactions between the two molecules. This allows science teams to study specific surface chemical reactions.
For example, suppose a catalyst can speed up a certain chemical reaction, but it is unknown which molecular site is responsible for the catalysis. In this case, the AFM tip prepared with the reactant molecule can be used to measure interactions at different sites, ultimately determining the chemically active site or sites.
In addition, because orbit-level information can be obtained, scientists can gain a much deeper understanding of what will happen when a chemical reaction occurs. As a result, other scientists can design more efficient catalysts based on this information.
Says Chelikowsky: “Supercomputers, in many ways, allow us to control how atoms interact without having to go into a lab. Such work can guide the discovery of new materials without laborious ‘trial and error’ procedures.”