Molecular Electronics (2)

Lunchlecture VVTP by Jan van Ruitenbeek

Technical University Delft 15-1-2015

A smartphone or tablet that you can bend without breaking it? Large scale production of cheap solar cells? Electronic circuits of which the components are single molecules? These are the topics that concern Professor Jan van Ruitenbeek, who works at the Leiden Institute of Physics. Invited by the VVTP, he came to share some knowledge on Molecular Electronics, which is his main research area. Molecular Electronics is the field of research that deals with electronics made of organic molecules. It promises to be very useful in the future, mainly because organic components are cheaper and easier to manipulate than metal components. Before you can use organic molecules as a substitute of metal in electronics, you must first find a way to transform the generally insulating molecules into conducting molecules. The reason molecules (read: organic molecules, from now on) do not conduct electricity whereas metals do, is that they have a very large band gap. This means that there is a large difference of energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The reason metals conduct electricity so well, is that they have small band gaps and metal atoms are closely together, so that the electrons orbiting the metal nuclei can reach the conducting band quite easy and when this conducting band is reached, the higher energy electron can already interfere with neighboring molecules. Organic molecules however, do not have such an organized and closely packed structure as metals do, and together with the large bandgap, conduction of electricity is much more harder to induce. There are however two ways to overcome this problem, resulting in ways you can use organic molecules in electric circuits:

  • Use the large band gap
  • Reduce the large band gap

A beautiful example of using the large band gap is the realization of photovoltaic cells: solar cells containing organic molecules with high band gaps. In these solar cells, organic molecules absorb sunlight. This absorption of sunlight causes an electron to jump from the HOMO to the LUMO, and this allows the electron to transfer to an atom nearby that has a lower LUMO than the electron, and this means that a current is running.

It is also possible to reduce the large band gap. This can be done using a fairly simple technique, for which Alan Heeger, Alan MacDiarmind and Hideki Shirakawa received the 1977 Nobel Prize in Chemistry. It basically comes down to oxidising a chain of polyacetylene  with halogens resulting in a molecule that can conduct electricity as good as copper does! The principle is shown below.Conducting PolyacetyleneIn conclusion, Professor Ruitenbeek told us that our economy as well as our technology is going to be more and more based on light and light-based technologies. It is therefore that 2015 is declared international year of light. For those who are interested in what this means, please visit:

http://www.light2015.org/

I want to encourage you all to visit this website and see what the year of light can mean to you, and see what you can do.

Kasper Spoelstra – wkspoelstra@hotmail.nl

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