Breakthrough on the electron transport of molecules and macroscopic systems

Thursday 25 Apr 19


Jens Ulstrup
Professor, emeritus, dr. scient.
DTU Chemistry
+45 45 25 23 59
DTU Chemistry is part of research team who has discovered a system that improves our understanding of a central element in nanoscience, namely how electron transport proceeds in the transition range from single molecules to macroscopic systems. The discovery can be important for the development of new nanoscale electronics.

In nanoelectronics, you work on making components as small as possible. As the components get increasingly smaller, they also change their molecular properties, affecting their electron transport. Scientists have now reached a step closer in the understanding of the behavior of the molecules, as the team of researchers has succeeded in mapping an electron transport system the transition range from single molecules to large molecular systems.

"Electron transport can be followed right down to the level of the individual molecule, but the transition between the behaviour of single molecules and molecules in large assemblies - what is called macroscopic behaviour - is not nearly as well understood. But now a system that describes this transition has actually been found, ”says Professor Emeritus Jens Ulstrup, DTU Chemistry, who emphasizes that exactly the transition between macroscopic and single-molecule behaviour is the core of nanotechnology and central when new nanoelectronics is to be developed.

The researchers reached the discovery by transmitting electric current through a "smart" molecule, ferrocene (Fc) linked to two gold electrodes. Using a quantum mechanical technology called scanning tunnel ling microscopy, they recorded the conductivity by tracing the electron transport in the molecules one at a time.

“Thousands of electrical single-molecule measurements were carried out. The data showed that the electron transport for the individual molecule displayed large variations, but when the average was calculated, the macroscopic behaviour emerged. Based on the number of single molecule measurements, the transition from the individual molecule's "random behaviour" to macroscopic thermodynamic behaviour, could be precisely determined,"Jens Ulstrup explains.

The scientific team was led by Professor Nongjian Tao from Arizona State University and his associates. In addition to Jens Ulstrup from DTU Chemistry’s Nanochemistry group, other research team members came from Nanjing, Fudan and Shanghai Universities. The results have just been published in the recognized journal Proceedings of the National Academy of Sciences of the USA, PNAS.

The behaviour of the molecules at several scales is crucial
Nanotechnology is not just about very small systems (1 nanometer = one billionth of a meter), but also about the way they they are small.

"When we go from "large", macroscopic systems down to the individual molecule, the properties of the systems begin to depend on their size from approx. 100 nm and further down. For example, when a chunk of gold is cut into smaller and smaller pieces, and we reach approx. 100 nm, the gold chunks change color and other properties on their further way down to the individual gold atom, “says Jens Ulstrup.

In these size ranges, each 1-100 nm unit also has characteristics slightly different from its neighbours. Molecules on a surface are bonded slightly differently and conduct electrical current differently. It is said that the molecular properties fluctuate. Only when the average properties of a very large number of molecules or measurements have been determined the well-known macroscopic properties will be obtained. Several thousand measurements were therefore carried out to map the transition to macroscopic behaviour.

Electron transport is the most important elementary step in chemical and biological processes and also controls nanoelectronics such as transistors and diodes. It is notable that electron transport can now be followed down right down to the level of the individual molecule, but the transition between the single molecules and the macroscopic behaviour of large assemblies of single molecules and molecules is far from understood.

The aim in nanoelectronics is that the components become as small as possible, but as the molecular size ("molecular electronics") is approached the problem of properties that become size-dependent and fluctuate, also called stochastic phenomena is encountered. The transition between macroscopic and single-molecule behaviour is the core of nanotechnology and central when new nanoelectronics is to be developed.

Important knowledge for the development of future electronics
The fact that the researchers have now found a new system for mapping the transition between single-molecule behaviour and the molecular behaviour in large molecular assemblies can be of importance for future development of ultra-small transistors and other electronic components, which are still in need of becoming even smaller.

"The chip in our PC’s, for example, consists of transistors that control the electronic signals, and the need for more computer power requires more transistors on the same area. Today, transistors can be manufactured down to approx. 40 nm or smaller, and there is an ongoing need for development towards even smaller units. It is therefore important to know when the molecular units become so small that the molecules begin to behave randomly and their properties fluctuate. Our results are therefore important for nanotechnology,” explains Jens.

The method

The new study is based on "ferrocene", Fc, where an iron atom is bound between two flat ligand molecules. The molecules are "smart" by appearing both in reduced, Fc and oxidized form Fc +, where Fc+ conducts electrical current best. "Smart" molecules have far more molecular electronics perspective than simple, not so "smart" molecules that only occur in a single form.

Fc / Fc+ can be equipped with linker groups that attach Fc / Fc+ to two gold electrodes in a scanning tunnelling microscope. Very well controlled electrical current can be mapped on a molecular scale in this way. The combination of a "smart" molecule with electron transport between the electrodes is a key perspective in understanding when a molecular system goes from being a single molecule to macroscopic.



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