Previous and Current Research

Neural network devices
Artificial neural networks are systems that permit computers to function in a manner analogous to that of the human brain. Instead of manipulation of “0”s and “1”s, they create weighted connections (synapses) between switching elements (neurons). This allows for data to be read and stored in ways that resemble human learning and memory.

We found a simultaneous occurrence of synaptic and neuron-like behavior in nanoscopic tunneling elements which consist of two ferromagnetic metallic electrodes separated by an ultrathin insulator (MgO). By applying current pulses with appropriate intensity and duration, the resistance of the elements shows a memristive behavior which enables mimicking synaptic and neuronic functionalities. These devices represent the simplest implementation of neuromorphic hardware available to date. Since in a neural network synapses outnumber neurons by orders of magnitude, synapses and neurons would be represented by different elements. However, since both could be etched from the same wafer, memristive MTJs may become very attractive for mass fabrication of artificial neural networks. A comparison of the Excitatory Postsynaptic Potential (EPSP) in Long Term Potentiation (LTP) recorded from a hippocampal synapse (reproduced from Bilss et al., 1993. Nature, 361: 31) and the typical change of the conductivity of a memristive tunnel junction induced by 16 train voltage pulses with 500mV demonstrates this similarity. The conductivity represents the strength of the artificial synapse.

Molecule manipulation
DNA-based single-molecule studies, nanoelectronics and nanocargos require a precise placement of DNA in an orientation-defined manner. Until now, there is a lack of orientation-defined alignment and immobilization of DNA over distances smaller than several micrometers. We tried to realize a defined immobilization by designing bifunctionalized DNA. The DNA is then put on a microchip with two different electrodes separated by a gap. An electric ac field then aligns the DNA and those strands oriented with the functionalized ends towards the specific binding partner are immobilized. In the example shown at the bottom right, DNA was functionalized on one end with thiol and the other with (3-aminopropyl) triethoxysilane. With this method, we succeeded in defined orientation of pUC19 DNA. Alignement was done with an electric ac field of 2V (p-p) at a frequency of 1 MHz.

Future Projects and Aims

In future projects, the group first will integrate memristive tunnel junctions in first working logic circuits. The aim of this work is to emulate learning with memristive devices.

We second aim for using nanoelectronic components such as miniaturized thermocouples, coils and sensors to tackle signal creation and transmission on a single cell level.