“Molecular chains could be useful for the electronics of the future”14.09.2023
Christian Wäckerlin already worked at PSI in the past, while completing his Master’s thesis and doctorate. After three other career stages, an Eccellenza research grant from the Swiss National Science Foundation (SNSF) allowed him to return to PSI at the start of 2023 to investigate novel types of molecular chains. One of the questions his fundamental research is attempting to answer is whether these metal-organic nanowires could possibly be used in the electronics of the future.
Mr Wäckerlin, what exactly is the device we have here?
It’s a scanning probe microscope, which provides high-resolution images of a sample. It does so using an extremely fine tip to scan the surface of the sample line by line, detecting miniscule height differences in the process. For about 10 years now, this type of microscope has made it possible to visualise molecules lying on an otherwise smooth surface with such precision that the position of the individual atoms can be identified. In other words, we can see the actual chemistry, which in itself is fascinating. So we are now using our scanning probe microscope to examine molecular chains and measure their electronic properties.
What are these molecular chains?
They are basically very long molecules where a specific arrangement of atoms is consistently repeated, like pearls of a necklace. Our molecular chains are metal-organic, which means the repeating entity is composed of metal and carbon atoms. Our research group is one of the global specialists in the production of these molecular chains.
And you’ve received an Eccellenza grant for five years to investigate these molecular chains. What are you hoping to achieve?
The grant gives us the opportunity to engage in completely open-ended research. We want to characterise these molecular chains, or nanowires, ideally down to the very last atom. On the one hand we can visualise the chains on the surface. But we can also pick up one end with the scanning tip. Then a short chain, perhaps 50 nanometres in length, hangs there at the tip and touches the surface with its lower end. If we then apply an electrical voltage, we see whether our molecular chain is electrically conductive or electrically insulating. Such fundamental investigations have long been carried out for large objects, but not yet on the nano scale. In the case of copper wires and magnets in common use, for example, the electrical and magnetic properties can still be easily examined separately. But if you go down to the level of the individual atoms, they are interwoven and influence each other.
So everything is pure fundamental research?
To some extent, but we are also trying to determine whether these molecular chains could potentially play a role in a novel type of electronics for the future. The great thing is that we are already very skilled at controlling the production of these chains. Just like a Lego set, we can fit the building blocks together and predict how the molecular chain will ultimately look. We can use different metallic atoms: cobalt, nickel, iron, etc. We can control how many metal atoms sit in the pearls of the chain and then measure the effects on the chain’s properties.
How do you get from nano chains to electronics?
The key question at the moment is whether the electronics of the future still need to be based on silicon. There are two main trends here: one is quantum computers – also a major research topic at PSI – and the other is the search for more energy-efficient forms of computing. For the latter, we are looking for innovative materials or systems for information processing. Our molecular chains could be interesting in this respect.
I recently spent a year in Prague, where I learned new experimental techniques for scanning probe microscopy. We also conducted our first investigations with molecular chains there, and we could see that they are electrically conductive initially. But when exposed to a light source, they repeatedly revert to an insulating state. If light can be used to switch electrical transport on and off, this is a basis for optoelectronic elements. So we are keen to discover, among other things, whether different light colours could have different influences. And also how quickly this switching happens. We didn’t have the right resources in Prague to do this. PSI, on the other hand, has excellent facilities for time-resolved measurements, both at the Swiss Light Source SLS and at the SwissFEL X-ray free-electron laser. So we are very excited to be looking for answers to this question here at PSI.
Who exactly is we?
The Eccellenza grant does not just fund my position – which, by the way, is split between EPFL, the Swiss Federal Institute of Technology Lausanne, and PSI – but has also allowed me to recruit three doctoral students. I’ve been able to assemble a strong team: the first doctoral student is a chemist who now wants to conduct surface research. The second is a physicist with an affinity for computers. And the third was also originally a chemist, but specialising in magnetism and therefore at home in the space between physics and chemistry. As for myself, I am a bit of a nothing (laughs).
I don't quite buy that.
Originally, I studied nano sciences in Basel, which includes physics, chemistry and biology. Especially in the nano world, it often helps not to treat these fields as separate disciplines.
I went on to complete my Master’s and PhD theses at PSI, where I came across molecules on surfaces. Back then I already began, as a little side project, to manufacture metal-organic nano systems by chemical means. We were one of the first research groups to do so, back in the “Dark Ages” of this particular field.
After that, I worked at EPFL, where we studied the magnetism of metal-organic systems and looked into the question of whether a single atom could be a stable magnetic storage bit. We showed that it is possible in principle – however, only at extremely low temperatures.
Next, I worked for the research institute Empa which, like EPFL, is a fellow institute of PSI within the ETH Domain. At Empa we investigated the chemical synthesis of organic and inorganic nano materials on surfaces. And after that came my year in Prague.
So you have quite a varied skill set.
Yes, you could say I now have all the pieces of the jigsaw puzzle needed to tackle this five-year project. What I really meant was that the three doctoral students I’ve found each have a different background from my own and can therefore approach things from a different angle. That’s exactly what I wanted and is why I am so excited about this collaboration.
Text: Paul Scherrer Institut/Laura Hennemann