Profile: Leo Gross - Playing with atoms

Editorial

Rebecca Pool

Thursday, April 3, 2014 - 08:30

Top image: Leo Gross, SPM pioneer at IBM Research: "When it comes to research, you cannot do it entirely on your own."

In 2009, Leo Gross revealed the first ever images of individual bonds between the atoms of a molecule that he had taken using a modified atomic force microscope (AFM). At the same time, he used this technique to measure the charge state of single atoms, again, a world first.

And then, just over a year ago, the IBM Research-Zurich scientist imaged the different chemical bonds between carbon molecules. Yet again, this marked a first as scientists had never before seen the physical differences between these different bond types.

The delicate inner structure of a pentacene molecule imaged with an atomic force microscope. [IBM Research-Zurich]

Each of these breakthroughs has delivered a deeper understanding of chemical reactions, but ask Gross about his success and you'll receive a modest answer. "I think good teamwork is important, or at least for me it has always been very important," he says. "If you see things, you can discuss these with your colleagues so it's very good to have close collaborations with colleagues and friends. You discuss your results and this generates new ideas."

Today, and since 2005, Gross has been working alongside Gerard Meyer, who heads up IBM's scanning tunnelling microscopy-related research. Meyer himself pioneered low temperature scanning tunnelling microscope (STM), which is now used worldwide, and Gross describes the researcher as a 'great role model'.

"He's also responsible for most of the instrumental achievements that we have," explains Gross. "As well as having knowledge on so many different fields, in addition to Physics, he has all the skills you need to build such machines. He's constantly improving the low temperature AFM electronic software and hardware."

Different chemical bonds in nanographene molecules, imaged by non contact atomic force microscopy using a carbon monoxide functionalised tip. The carbon-carbon bonds in the imaged molecule appear with different contrast and apparent lengths. [IBM Research-Zurich]

It is this passion for instrumentation that has also driven Gross. As part of his degree in physics from the Free University of Berlin, Germany, the young scientist spent a year working in Tulane University, New Orleans, with Professor Ulrike Diebold. Diebold, now at the Vienna University of Technology, is renowned for her STM studies of atoms, notably titanium dioxide.

During his time with Diebold, Gross also studied titanium dioxide, encountering STM for this first time, an experience that stirred initial curiosity in these microscopes. "Titanium dioxide is now one of the most studied materials, especially for catalysis, but it was really the techniques we used that caught my interest," he says.

"I liked that you get these visual images directly and that you can really optimise images while you work," he explains. "So I enjoyed the whole process of measuring and analysing the data, and it is at this time that I decided I wanted to work with scanning probe microscopy"

With this in mind, and having started out with Diebold, Gross continued to gain experience with the great and the good from the world of scanning probe microscopy (SPM). From Berlin and New Orleans, he moved to the University of Muenster, Germany, to join an STM group led by Professor Harald Fuchs, a leading light in SPM and nanotechnology. And after completing his Master's Diploma here, he then returned to the Free University of Berlin in 2001, to join STM and nanofabrication pioneer, Professor Karl-Heinz Rieder. Here, using low temperature STM, he explored the behaviour of organic molecules on metallic surfaces, a fundamental step to building molecular electronics devices, and was hooked.

As he explains, low temperature STM - close to absolute zero - brings the possibility of manipulating atoms. "You pull and you push atoms with the apex of your tip to build up an experiment, and that's really nice," he says.

As part of his thesis, he took a so-called six-leg benzene-based molecule, designed by collaborating chemists from CEMES-CNRS, Toulouse, France, and found that by moving it along a copper surface with an STM tip, he could collect and carry up to six copper adatoms. These adatoms could then be further manipulated to construct molecular-metal atomic-scale structures.

The technique as since described as a "molecular hoover", and as Gross says: "You just took this molecule and used it to collect single atoms and then make a small cluster out of these atoms. This was really fun."

Come the end of his PhD, this research had hit the cover page of prestigious journal, Nature Materials, an achievement many fresh post-graduates only dream of. But by this time, Gross had moved to IBM Zurich - where STM has been invented in 1981 - and was already focusing on new research.

From STM to AFM

During his first years at IBM, Gross looked at nanostencil lithography, a tool for the fabrication and in-situ characterization of ultraclean nanostructures for fundamental surface science research. However, he hadn't forgotten his low temperature STM research, and come 2007, was investigating how to apply these techniques to AFM.

IBM-Zurich scientists including Leo Gross and Gerhard Meyer reviewing the first-principles density functional theory calculations that corroborated images of the Pentacene molecule. Front: STM/AFM used for the experiments. [IBM Research-Zurich]

While STM had been used to image atomic-scale features on surfaces, resolving single atoms within molecules remained a challenge as the tunnelling current was sensitive to the electron density of states near to the Fermi energy level. Using non-contact AFM would alleviate this problem but moving from low temperature STM to low temperature AFM also had issues.

"AFM is a little bit more challenging than STM. You have more feedback loops and technically [low temperature AFM] is more challenging," says Gross.

A first step was to implement the so-called qPlus sensor - a quartz tuning fork with a much higher stiffness than standard silicon cantilevers - with a metal tip into a low temperature STM/AFM to ensure stable operation at 5K and in a ultrahigh vacuum.

Sensor success in hand, Gross and colleagues then stumbled across something that would lead to the AFM images of molecules that stunned scientists worldwide. As Gross explains, during STM, carbon monoxide molecules are a well known surface contaminant that can be inadvertently picked up by a cantilever tip.

"In our first [non-contact AFM] images of molecules, we started seeing some strange contrasts that we just didn't understand," he says. "We tried to reproduce them and realised that these contrasts were actually due to a carbon monoxide molecule that was accidentally on the tip."

Gross and colleagues repeated imaging experiments with a single carbon monoxide attached to the tip, now described as a CO-modified tip. And as the researcher says, the images just got 'clearer and clearer', boosting AFM resolution to resolve the atomic positions and bonds inside a planar pentacene molecule, precisely revealing the atomic structure.

The actual scanning tunneling/atomic force microscope used to image the Pentacene molecule with atomic resolution. [IBM Research-Zurich]

Further studies at IBM Zurich confirmed Pauli repulsion forces were the source of the unprecedented atomic resolution, and Gross and colleagues went onto publish their results and stunning images in Science, in 2009.

The breakthroughs that followed have built on Gross's modified AFM and a growing understanding of the imaging mechanism. For example, further imaging of pentacene revealed differences in the brightness its bonds.

Later electron density calculations indicated these could be due to bond order, so the researchers repeated imaging on molecules, such as C60 fullerene, with more distinct differences in bond order. They were able to map subtle differences in charge density and bond length in molecular systems and correlate them with bond order. Results were published, again in Science, and in short, the researchers had captured the first images of different chemical bonds.

And at the same time, the team was also working on other SPM developments, including Kelvin probe force microscopy to measure the charge distribution within a single molecule. As Gross says: "For me, it's been very important that this work is fun, and at the moment I'm still having fun working with these machines."

Gross and colleagues are currently experimenting with different tip terminations, in a bid to find out how resolution changes with different molecules at the tip and also to gauge which termination best suits certain materials. "Xenon works very well and gives you a realistic topography of the electron density of a molecule," he says. "We've also tried different halogens such as iodine, bromine and chlorine, as well as krypton, nitrogen oxide, copper and gold."

Alongside his research, Gross has scooped award after award, including the first Gerhard Ertl Young Investigator Award, for outstanding research in surface science, and the Feynman Prize for experiment in Nanotechnology, which he won with collaborators, Meyer and Professor Jascha Repp.

And for Gross, collaboration remains key. "I have always had a good working atmosphere with my colleagues, and this, I think it very important to successful research," he says. "When it comes to research, you cannot do it entirely on your own."

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