This page is a personal description of my experience as a postdoc in single-molecule electronics. It aims at describing what is single-molecule electronics, trace back its history and explain my own research activity in this context. In brief, single molecule electronics consists in the study of electronic transport trough a single molecule. In practice, it boils down to contact one molecule to two metal or semiconducting contacts and study the flow of current while the difference between the electron potential of the contacts is changed (measurement of current- voltage traces). The difficulty does not lie in the measurement itself (though tedious and boring). They are made using standard electronic and cryogenic techniques. In fact it is the fabrication that is challenging. How is it possible to position one single molecule of a few atoms between two contacts ?
A few years ago this topic was boiling, but the hype has since faded out. The reason is that irreproduced results have made their way to the top (nature, science) in the field of molecular electronics. In particular, the Hendrik Schön affair has cooled down the trend, deceiving a lot of scientists. Probably there is now the feeling that it is not so simple after all, that it might take time and too much effort to make things really reproducible or get published. And today a result has to be obtained fast to survive financially. However, if pysiscists want to understand properties of small scale objetcs, there is no other way than putting efforts into getting a reproducible technique, no matter how painfull and off the track it may be. This page explains a potential road to reach this goal. It is entirely biased and personal, so do not take anything said here for granted (as you should always do).
Electrochemistry : a possible route
Looking for a place to work !
I will probably will be unemployed starting from 1st of November and I am looking to work in europe with people who have an interest in developong single molecular contacts for electronic transport. Ideally, I would like to continue my work (see articles). You can see my CV here. For contact my present email adress is click here.
Electronics is mostly known as a technology. To most people, it is as such, a complete bore. Their every day life is invaded by the electronic products but they surely do not want to waste their time understanding how work the components hidden behind the nice plastic cover. But electronics is more than that. It is above all the part of physics studying the transport of electron in materials. Everyone knows (or should know for its own safety) that materials are divided into three classes according to their electronic properties (insulator, semiconductor and conductor). The conduction properties of the materials can be unserstood theoritically by looking at how electrons are influenced by the supposed perfectly periodic array of atomic cores in a macroscopic material. If we take free electrons, those behaves as waves. When placed in an empty box matching the dimension of a piece of material (with atomic length resolution), then there density oscillate in space like waves in a closed pool. The periodicity can take only certain values in accordance with the boundary separation (at the boundary there is a dip in the density). However in a macroscopic material, the boundary condition are rather loose and there are a huge number of possible oscillation frequency. People say that there is a (quasi continuum) of oscillation or states.
However in a material there are some atoms placed in a periodic array. These atoms exert an attractive force on the electrons. If the electrons density oscillates at a frequency very different from the atomic spacing (or a multiple frequency of the atomic spacing) then the effect is next to none due to cancellation. But if the frequency is very close , then each atoms will contribute constructively.
Why molecular electronics is of any (scientifc) interest.
Basically it is the part of physics which aims at understanding electronic transport through molecules.
What is transport.
Molecules are small atomic aggregates (two atoms forms a molecule) where atoms are linked by covalent bonds. Up to now the electronic structure of molecules and atoms have been sorted out using the interaction between incoming photons and the electronic cortege of a large numbers of these isolated objects (for instance in a gaz phase).
Mechanically adjusted contacts
Problems: single molecules ? clean contacts ?
Links:
How to connect a single molecule
There has been many theoritical works on how a single molecules should conduct electrons. In someof these works striking features in the conductance characteristics (ie conductance versus potential difference between the electrodes) have been predicted, depending on the molcular structures and the way it is contacted to the electrodes. However, in the current status, there is much more theorictical predictions than actual clear reproducible measurements. Why, well because of the simple question that everyone should ask himself before reading any paper in the field. How can one connect a single molecule to two electrodes, in a reproducible way and be sure it is actually there, well connected, and alone. Off course you can make a nice drawing. Up to now, no actual image (using available microscopy technique) has ever been produced. So you have to believe the authors, but in this troubled time where everyoe runs for grants, jobs and thus publication, faith is dangerous. The best way is thus to understand the different available method and judge them by yourself (you can find the detailed presentation in the second chapter of my theseis, see downlad adress). Off course, I will in the end give my own opinion of the matter and try to explain why the method I used in my thesis is an interesting candidate.Scanning tunneling microscope
Break junctions
Three terminal devices obtained by lithography
Electrochemistry : a possible route
- Electrochemically fabricated nanometre-sized molecular junctions
This is my Ph'd thesis made in Delft. You can download it here (~20 MB). The first chapter is an easy introduction to electron transport in small structure, where quantization of electron energy due to confinment strongly influence transport. The second chapter is a general overview of the status of single molecular electronics from the beginnings until october 2004.
-Nanometer-spaced electrodes with calibrated separation
Y. -V. Kervennic, H. S. J. van der Zant, A. F. Morpurgo, L. Gurevich, and L. P. Kouwenhoven,
Applied Physics Letters 80, p. 321 (2002).
pdf, doi: 10.1063/1.1433914.
-Planar nanocontacts with atomically controlled separation
Y. -V. Kervenic, D. Vanmaekelbergh, L. P. Kouwenhoven and H. S. J. van der Zant,
Applied Physics Letters 83, p. 3782 (2003).
pdf, doi: 10.1063/1.1623317
-Molecular three-terminal devices: fabrication and measurements
H. S. J. van der Zant, Y. -V. Kervennic, M. Poot, Kevin O'Neill, Z. de Groot, J. M. Thijssen, H. B. Heersche, N. Stuhr-Hansen, T. Bjørnholm, D. Vanmaekelbergh, C. A. van Walree and L. W. Jenneskens,
Faraday Discussions 131, p.347 (2006).
pdf, doi: 10.1039/b506240n
-Charge Transport in Three-Terminal Molecular Junctions Incorporating
Sulfur End-Functionalized Tercyclohexylidene Spacers
Y. -V. Kervennic, H. S. J. van der Zant, D. Vanmaekelbergh, C. A. van Walree and L. W. Jenneskens,
accepted for publication in Angewandte Chemie. (back to index)
- The quantum transport group where I did my Phd under the direction of Leo Kouwenhoven. This group now focuses on semiconducting quantum dots (with a view to make qbits) and superconducting qbits (based on aluminum oxide used as a confining barrier and a plan to use few nanometer scale wires instead).
-The molecular electronics group in Delft, a fusion of the molecular electronics activities of three previous groups of Delft. My supervisor, Herre van der Zant, who directed the second half of my thesis is one of the leader together with Alberto Morpurgo.
Groups working on single-molecule contacts
Europe
The Netherlands
-The molecular electronics group of TU delft, presently works on electromigrated gold contacts
-The Leiden group of Prof. Jan van Ruitenbeek (mechanical break junctions)
-The group of Prof. Bart van Wees. This group worked on connecting light-sensitive molecules using mechanical break junctions
-The group of Prof. Cess Dekker in Delft. This group is now focused on biophysics on the nano scale. Previously, it also worked on protein based or DNA based transistor and did some pioneering work on nanotubes electronic spectroscopy.
Sweden
-The quantum device physics goup, in Chalmers, Göteborg. My present research group.
Denmark
-Chemistry group of Pr. Thomas Bjørnholm in Copenhagen . This group has worked on the synthesis of OPVs used in single-molecules transport experiments performed in Chalmers and Delft.
Germany
-Group of Prof. Heiko Weber in Erlangen . This group has worked on contacting single organic molecules using break junction technique in vacuum. They have studied rectification of current in molecules
France
-CEMES website. A CNRS group in Toulouse who has made noticed work on molecular interaction with metal contacts using STM.
-LEM website. The laboratoire d'electronique moleculaire is a CEA group in Saclay (near Paris) who was one of the first to work on contacting organic molecules using mechanical break junctions (after Mark Reed in the USA).
Switzerland
-The group of Prof. Christian Schonenberger in Basel. This group mainly worked on transport in nanotubes and DNA.
Italy
-The Lecce nanocenter. In this center a group has studied transport in contacted amino acids .
Israel
U.S.A
Cornell
Harvard
Arizona State University
-The group of Prof. N. J. Tao. Group of chemical physicisits at the Arizona State University. Prof N. J. Tao was a pioneer in using electrochemistry to make atomic contacts.