Exploring graphene

Delta, 21 April 2010

PhD student Jeroen Oostinga has published leading scientific papers on the bizarre behaviour of electrons in single-atom layered carbon sheets, known as graphene. But his group has now left for Geneva. “A loss for Delft”, says his PhD supervisor.

Carbon exists in many forms. Despite their obvious differences, both coal and diamond have a three-dimensional crystal structure. But in the early years of the 21st century other forms were revealed as well, such as the one-dimensional carbon tubes and ‘buckyballs’ (football-like molecules containing sixty carbon atoms).  It was therefore rather logical to assume that two-dimensional carbon would also exist, although others had predicted that a freestanding atomic sheet could never be stable.
In 2004, dr. Andre Geim and physicists from the University of Manchester isolated the first flat atomic sheet of carbon, known as graphene. The way these researchers managed to produce the world’s first graphene became known as the Scotch tape method. Knowing that drawing with a pencil produces tiny fragments of graphene, the researchers repeatedly split graphite fragments into ever-thinner pieces using Scotch tape, until the carbon specks became transparent. They then deposited the Scotch taped flakes onto a silicon wafer, thus proving the existence of graphene, which has the structure of chicken-wire with regular hexagonal holes. Each node consists of a carbon-atom connected to three others in the same plane (at a distance of 0.142 nanometres), and each atom has one free electron that can move across the remarkably regular atomic plane.
In 2006, Jeroen Oostinga (MSc) started his research of the electronic properties of graphene at the Kavli Institute of Nanoscience (faculty of Applied Sciences), under the supervision of professor Alberto Morpurgo. Two years later Oostinga published an article in Nature Materials on the world’s first graphene-based transistor with a tunable bandgap.
It turned out that electrons in an atomic plane behave much differently compared to three-dimensional conductors or semi-conductors.  Mathematically, electrons in graphene behave very much like mass-less neutrinos; however, the much lower speed of the electrons offers physicists unique possibilities to verify certain quantum mechanical predictions, which cannot be easily measured for neutrinos. Graphene became the physicists’ new playground, filled with exciting new phenomena.
“The flat crystal structure is very strong and stable”, Oostinga says over the phone from Geneva. “It means that there are very few defects in the crystal, which in turns results in a large mobility of electrons. This is very promising in terms of low electrical resistance and fast electronics, which is why IBM is interested.” Another consequence of the strong and regular lattice is the relative low disturbance of crystal vibrations onto the electrons, which is an interesting property for quantum transport applications, a subject of research in the group led by professor Lieven Vandersypen, also based at the Kavli Institute of Nanoscience.
Oostinga discovered a new phenomenon when measuring the electrical conductivity over two-layered graphene (and later three-layered as well). It turned out that double-layered graphene can be persuaded to display transistor-like properties. Oostinga explains: “Normally graphene acts like a conductor, also when it consists of two layers. But when you apply an electric field perpendicular to the graphene, you disturb the symmetry and create a bandgap, just as a semiconductor has.” Macroscopically this results in a lower conductivity, which can be externally controlled by varying an electric field, just like in a transistor. The range over which the conductivity changes is still limited (factor 30-100) compared to typical silicon characteristics.
All was going well with Delft graphene research until 2008, when prof. Morpurgo accepted an offer from the University of Geneva to set up a new quantum electronics group. To add insult to injury, three TU Delft PhD students (jocularly known the as the ‘Three Musketeers’: Jeroen Oostinga, Ignacio Gutiérrez Lezama and Hangxing Xie) followed him there, along with the professor’s research equipment. “This gave rise to huge conflicts with TU Delft, which made it an extremely unpleasant period for everyone involved”, Oostinga recalls.
Professor Huub Salemink, who at the time was vice-president of the Nanoned research programme and chairman of the Kavli Institute, then stepped in as the TU Delft PhD supervisor to limit the damage. “PhD students shouldn’t suffer the consequences”, he explains.
Next week Oostinga, the second of these displaced PhD students, will defend his thesis. TU Delft has suffered a serious loss, Salemink admits, but at least graphene research on quantum transport continues under professor Vandersypen.
Jeroen Oostinga, ‘Quantum transport in graphene’, thesis defense on 27 April 2010. PhD supervisors professor Alberto Morpurgo and professor Huub Salemink.

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