Jan van Ruitenbeek Lab (Atomic and Molecular Conductors)
Our interest in single-molecule junctions has evolved logically from the invention of the Mechanically Controllable Break Junction (MCBJ) technique in our group, and the research on single-atom contacts that was made possible by it. We have focused initially on very simple molecules (H2, H2O, C6H6, etc) connected between Pt leads. Experiments at low-temperature permit detailed characterization of the junctions, for which we exploited the measurement of shot noise, and vibration mode spectroscopy in the differential conductance. More recently the work is continued in three directions:
- Tree-terminal single-molecule junctions. In collaboration with the group of Herre van de Zant at Delft University of Technology we have developed microfabricated break junctions that include a third electrode as a gate. By these we can tackle much larger molecules, such as thiol-terminated zinc-pophyrin.
- Shot noise provides information on the quantum conductance channels. It is usually measured at low voltage bias only, but at higher bias we have discovered that inelastic scattering becomes visible. We are extending these experiments to higher bias and higher frequencies (op to 10MHz) in order to information on the statistics of occupation of the molecular vibration levels and the “lattice temperature” of the molecules.
- Low-temperature STM. The disadvantage of break junction experiments lies in the large variation in molecular bonding configuration, for which it remains difficult to obtain information from the experiments. By means of low-temperature scanning tunneling microscopy this problem can be largely remedied and the detail of the measurements will permit a much more direct comparison with theory and the investigation of the more subtle interesting effects in electron transport.
Scanning Tunneling Microscopy: into the third dimension
STM permits imaging and manipulation of atoms and molecules on a metal surface. For experiments of controlled contacting of individual molecules it is necessary to extend the instrument. The first problem we face is that we need to follow a complex trajectory in space in order to gently peal a molecule off a surface. For the 3D- control of the tip motion we have implemented a motion tracking system that follows a light held by the hand of the operator in space, and translates this into the nanoscale motion of the tip. The second problem is that we lose the imaging capability of STM as soon as we have made contact with the tip to the molecule. We have built a real-time molecular dynamics simulation, controlled by the same motion sensor, in order to guide the motion by the operator. This instrument opens new avenues for manipulation of atoms and molecules and for the investigation of nanoscale systems.
Filamentary conduction in resistive memory devices
Thin films of insulating materials (mostly metal oxides) when contacted with metallic films from both sides often show electrical break down above a given threshold voltage. In many cases it turns out to be possible to reversibly switch the conductance of the structure formed after break down between a low conductance and a high conductance state, simply by reversing the bias polarity. This phenomenon is widely investigated for memory applications. We searching for evidence for quantum conductance channels in the high-conductance state as a result of the formation of metallic filaments.
graphene-graphene edge tunnel junctions
The research aims at fabrication of a new device concept for electrically recognizing single molecules and for sequencing biomolecules, including DNA and proteins, challenging current sequencing strategies through a novel architecture able to resolve single nucleobases. Reliably distinguishing individual building blocks in biomolecules (individual amino acids, individual nucleotides, individual molecule conformations) has, to date, been hampered by the size of the sensing volume. Graphene – a one-atom thin sheet of graphite – has the potential to remedy this problem. In this project we closely collaborate with the group of Dr. Grégory Schneider.