Welcome to the workgroup of Prof. Möller
Surface Science Let's have a look at atoms and molecules!
Welcome to the site of the Möller Group!
Surfaces are of major importance in daily life. On the one hand friction allows us to run or to drive a car; on the other hand it causes that costy energy is degraded to heat which is dissipated. Chemical processes at surfaces lead as well to corrosion as to catalytic reactions. Furthermore surfaces and interfaces are becoming more and more important for the development of devices in microelectronics if the size is reduced to the nanometer scale.
In our group surfaces and interfaces are studied by various microscopic and spectroscopic techniques.
News
Contact Electrification
Contact electrification and triboelectricity are ubiquitous phenomena which are well known in daily life, e.g. encountering an electric shock when touching a door handle. However, the detailed processes are still not yet fully understood.
In our group, we have developed a new experimental technique which enables us to study in great detail the charge transfer occurring when two objects come into contact. It relies on new electronic charge amplifiers that measure very small charges on a time scale of microseconds. To study the contact electrification a ball with a diameter of 1 millimeter is dropped onto a plate from a height of a few centimeters, so that it bounces several times. By measuring the charge induced in the plate, it can be analyzed how the charge changes each time that it comes into contact with the surface.
The photo shows the inner part of the experimental setup. The wheel at the left side is used to transport the sphere to the upper level. A ball track, which is not visible, here guides the spheres to the entrance hole of the upper plate of the parallel plate capacitor. The lower plate is connected to a charge sensitive amplifier measuring the induced charge.
Please find details here.
Dynamics of contact electrification Mirco Kaponig, Andre Mölleken, Hermann Nienhaus und Rolf Möller
The video displays in slow motion how the charge (red line) measured at the lower plate evolves in time. The blue line shows the corresponding height which can be deduced from the data. Between successive contacts the ratio between the two signals is given by the charge on the sphere (and a constant factor given by the geometry).
Terahertz STM
Ultrafast dynamics of adsorbed nanoobjects
Scanning tunneling microscopy (STM), owing to its high spatial resolution (sub nanometer), is a tool of choice to investigate, address and manipulate single nanoobjects adsorbed on surfaces. Many of the investigated nanoobjects, be it atoms, molecules or nanostructures, exhibit rich dynamics such as charge transfer, spin/vibration excitation and change of conformation, which are usually too fast to be resolved using typical STM instrumentation. This is because the main quantity being measured, the tunneling current, is relatively small (sub nanoampere), and its detection is typically slow (time resolution on the order of millisecond for conventional instruments).
To overcome the time resolution of the detector, measurements can be performed in a stroboscopic manner as done in the field of ultrafast optics. Pump and probe voltage pulses, with an adjustable delay between them, are repeatedly applied to the sample. The dynamics of the investigated systems are inferred from the evolution of the corresponding quasi-static tunneling current as a function of the delay between the pulses. Loth and coworkers have implemented this concept in STM using an all-electronic scheme [LOT10]. They reached a time resolution on the order of 10 ns, which is essentially dictated by the ability of the wires to transmit fast pulses with limited distortion. To overcome limitations due to the wiring, Cocker and coworkers used electromagnetic THz pulses to effectively induce ultrafast voltage transients in the STM junction [COC13], which was for instance employed to track the ultrafast motion of a single molecule with a sub-picosecond time resolution [COC16].
In a close collaboration with the group of Manuel Gruber, we are implementing THz STM on a custom-made STM and using commercial THz emitters. We aim at investigating various ultrafast dynamics of individual adsorbates on surfaces at variable temperatures (10 to 300 K) under ultra-high vacuum.
[LOT10] Loth et al., Science 329, 1628 (2010).
Steering structural phase transitions of CO monolayers
Our reserach on CO monolayers on Cu(111) got published in the journal NanoLetters. In particular we find, that the electric field between the STM tip and the sample surface can selectively tune the ordering of a CO monolayer on Cu(111). Thereby, a phase transition between two ordered structural CO phases is induced. Moreover, the control by the STM tip also allows to write arbitrary patterns of the phases on the Cu(111) surface to create artificial potential landscapes with it. Together with our cooperation partners in San Sebastian and Barcelona, our results were recently published in the journal Nanoletters.
Teaching-Award 2014 for Christian Bobisch
On the occasion of this years "Dies Academicus", Christian Bobisch was awarded a second award within short time. There he was awarded with the "Teaching Award of the University of Duisburg-Essen". This award is endowed with a price money of 5000 €, which is dedicated to further improve the teaching. For Christian Bobisch this award is especially important since the students themself, i.e. the student representatives, are nominating the awardees. At the ceremony, the development and the integration of new learing concepts and methods within the physics couse was honorably mentioned.
Adresse
Arbeitsgruppe Möller
Fakultät für Physik
Universität Duisburg-Essen
Lotharstr.1-21
Gebäude MG/MF, 2. Etage
47048 Duisburg
