Nanoelectronics group of the University of Basel

The nanoelectronics group of the University of Basel does experiments with nanodevices to explore fundamental electrical properties in charge transport through electrically confined geometries.

The devices encompass a size regime from a few 100 of nanometers down to the molecular electronics scale of 1 nanometer. The devices are fabricated, on the one hand, along traditional lines employing state-of-the-art electron-beam lithography and conventional material systems (e.g. semiconductors and metals). On the other hand, alternative approaches, such as the bottom-up assembly, trapping of molecules and clusters in nanojunctions, and the use of nanowires and nanotubes grown by a self-organization process are used as well.

In the field of metallic and semiconducting nanostructures the focus has been on fluctuation phenomena (shot noise) and correlation spectroscopy of quantum-coherent systems. In molecular electronics, key results have been electrical measurements of single molecules trapped in break junctions. There is ample of experience in electrical studies of carbon nanotubes (CNTs), semiconducting nanowires (NWs) and graphene. CNTs and NWs are used as quantum wires and to define quantum dots with major results in the area of spintronics, and the superconducting proximity effect in reduced dimension and in the regime of strong interaction. Finally, we conduct applied research by exploiting CNTs and NWs as new materials for on-chip biochemical sensing.

List of selected publications

1. Aharonov-Bohm oscillations in carbon nanotubes
   A. Bachtold et al., Nature, vol 397, 673-675 (1999)
   When electrons pass through a cylindrical electrical conductor aligned in a magnetic field, their wave-like nature manifests itself as a periodic oscillation in the electrical resistance as a function of the enclosed magnetic flux. This phenomenon reflects the dependence of the phase of the electron wave on the magnetic field, known as the Aharonov-Bohm effect, which causes a phase difference, and hence interference, between partial waves encircling the conductor in opposite directions. Such oscillations have been observed in micrometre-sized thin-walled metallic cylinders and lithographically fabricated rings. Carbon nanotubes are composed of individual graphene sheets rolled into seamless hollow cylinders with diameters ranging from 1 nm to about 20 nm. They are able to act as conducting molecular wires, making them ideally suited for the investigation of quantum interference at the single-molecule level caused by the Aharonov-Bohm effect. Here we report magnetoresistance measurements on individual multi-walled nanotubes, which display pronounced resistance oscillations as a function of magnetic flux. We end that the oscillations are in good agreement with theoretical predictions for the Aharonov-Bohm effect in a hollow conductor with a diameter equal to that of the outermost shell of the nanotubes. In some nanotubes we also observe shorter-period oscillations, which might result from anisotropic electron currents caused by defects in the nanotube lattice.
2. Electrical conduction through DNA molecules
   H.-W. Fink et al., Nature, vol 398, 407-410 (1999)
   The question of whether DNA is able to transport electrons has attracted much interest, particularly as this ability may play a role as a repair mechanism after radiation damage to the DNA helix. Experiments addressing DNA conductivity have involved a large number of DNA strands doped with intercalated donor and acceptor molecules, and the conductivity has been assessed from electron transfer rates as a function of the distance between the donor and acceptor sites. But the experimental results remain contradictory, as do theoretical predictions. Here we report direct measurements of electrical current as a function of the potential applied across a few DNA molecules associated into single ropes at least 600 nm long, which indicate efficient conduction through the ropes. We end that the resistivity values derived from these measurements are comparable to those of conducting polymers, and indicate that DNA transports electrical current as efficiently as a good semiconductor. This property, and the fact that DNA molecules of specific composition ranging in length from just a few nucleotides to chains several tens of micrometres long can be routinely prepared, makes DNA ideally suited for the construction of mesoscopic electronic devices.
3. The Hanbury Brown and Twiss Experiment with Fermions
   S. Oberholzer et al., Physica E 6, 314-317 (2000)
   We realized an equivalent Hanbury Brown and Twiss experiment for a beam of electrons in a two dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter which is used to partition the incident beam into transmitted and reflected partial beams. The current fluctuations in the reflected and transmitted beam are fully anticorrelated demonstrating that fermions tend to exclude each other (anti- bunching). If the occupation probability of the incident beam is lowered by an additional gate, the anticorrelation is reduced and disappears in the classical limit of a highly diluted beam.
4. The Electrochemical Carbon Nanotube Field-Effect Transistor
   M. Krüger et al., Appl. Phys. Lett. 78, 1291-1293 (2001)
   We explore the electric-field effect of carbon nanotubes (NTs) in electrolytes. Due to the large gate capacitance, Fermi energy (E_F) shifts of order ±1 V can be induced, enabling to tune NTs from p to n-type. Consequently, large resistance changes are measured. At zero gate voltage the NTs are hole doped in air with |E_F|»0.3...0.5 eV, corresponding to a doping level of »10¹³ cm^-2. Hole-doping increases in the electrolyte. This hole doping (oxidation) is most likely caused by the adsorption of oxygen in air and cations in the electrolyte.
5. Quantum dot in the Kondo regime coupled to superconductors
   M.R. Buitelaar et al., Phys. Rev. Lett. 89, 256801 (2002)
   The Kondo effect and superconductivity are both prime examples of many-body phenomena. Here we report transport measurements on a carbon nanotube quantum dot coupled to superconducting leads that show a delicate interplay between both effects. We demonstrate that the superconductivity of the leads does not destroy the Kondo correlations on the quantum dot when the Kondo temperature, which varies for different single-electron states, exceeds the superconducting gap energy.
6. Quantum Shot Noise
   Carlo Beenakker and C.S., Physics Today, 56-5,37-42 (2003)
   Physics Today
   Copyright (2003) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.
7. Electrical conductance of atomic contacts in liquid environments
   L. Grüter et al., Small 1, 1067-1070 (2005)
   We present measurements of the electrical conductance G at room temperature of mechanically controllable break junctions (MCBJ) fabricated from Au in different solvents (octane, DCM, DMSO, and toluene) and compare with measurements in air and vacuum. In the high conductance regime (G <≈ G₀ = 2e²/h), the environment plays a minor role, as proven by the measured conductance histograms, which do not depend on the environment. In contrast, the environment significantly affects the electrical properties in the low conductance (tunneling) regime G << G0. Here, we observe a systematic and reproducible lowering of the tunneling barrier height f. At shorter distances, a transition to a strongly suppressed apparent barrier height is observed in octane, providing evidence for the layering of solvent molecules at small inter-electrodes separations. The presented experimental configuration offers interesting perspectives for the electrical characterization of single molecules in a controlled environment.
8. Electric field control of spin transport
   S. Sahoo et al., Nature Physics 1, 99 (2005)
   Spintronics is an approach to electronics in which the spin of the electrons is exploited to control the electric resistance R of devices. One basic building block is the spin-valve, which is formed if two ferromagnetic electrodes are separated by a thin tunneling barrier. In such devices, R depends on the orientation of the magnetisation of the electrodes. It is usually larger in the antiparallel than in the parallel configuration. The relative difference of R, the so-called magneto-resistance (MR), is then positive. Common devices, such as the giant magneto-resistance sensor used in reading heads of hard disks, are based on this phenomenon. The MR may become anomalous (negative), if the transmission probability of electrons through the device is spin or energy dependent. This offers a route to the realisation of gate-tunable MR devices, because transmission probabilities can readily be tuned in many devices with an electrical gate signal. Such devices have, however, been elusive so far. We report here on a pronounced gate-field controlled MR in devices made from carbon nanotubes with ferromagnetic contacts. Both the amplitude and the sign of the MR are tunable with the gate voltage in a predictable manner. We emphasise that this spin-field effect is not restricted to carbon nanotubes but constitutes a generic effect which can in principle be exploited in all resonant tunneling devices.
9. Reversible Formation of Molecular Junctions in Two-Dimensional Nanoparticle Arrays
   J. Liao et al., Adv. Mat. 18, 2444 (2006)
   Molecular electronics is attracting an increasing research attention, primarily supported by the possibilities offered by synthetic chemistry for the tailoring of single molecules to achieve specific electronic functions. Whereas various experimental approaches have been devised to form and electrically study molecular junctions, the integration of individual junctions into functional electronic circuits remains a demanding task, requiring innovative approaches in fabrication philosophy and circuit structure.We show here that an approach combining the self-assembly and micro-contact printing of ligand-protected metallic nanoparticles, followed by an in-situ ligand exchange reaction, allows the preparation of stable two-dimensional networks of molecular junctions. A significant decrease in resistance (up to three order of magnitude) after the exchange of alkanethiols ligands with conjugated, double-ended organic wires (thiolated oligo(phenylene ethynylene), OPE) confirms a proper interlinking of neighbouring nanoparticles. We also demonstrate that the formation of the molecular junctions is reversible, making nanoparticle networks a valuable platform for the development of molecular electronic circuits. The flexibility of this approach lets envision the realisation of more complex networks, for instance by intermixing ensembles of bimodal nanoparticles of different materials.
10. Molecular Junctions based on Aromatic Coupling
    Songmei Wu et al., Nature Nanotech. 3, 569 (2008)
    If individual molecules are to be used as building blocks for electronic devices, it will be essential to understand charge transport at the level of single molecules. Most existing experiments rely on the synthesis of functional rod-like molecules with chemical linker groups at both ends to provide strong, covalent anchoring to the source and drain contacts. This approach has proved very successful, providing quantitative measures of single-molecule conductance, and demonstrating rectification and switching at the single- molecule level. However, the influence of intermolecular interactions on the formation and operation of molecular junctions has been overlooked. Here we report the use of oligo-phenylene ethynylene molecules as a model system, and establish that molecular junctions can still form when one of the chemical linker groups is displaced or even fully removed. Our results demonstrate that aromatic π-π coupling between adjacent molecules is efficient enough to allow for the controlled formation of molecular bridges between nearby electrodes.