In microsystems technology, the role of surfaces, and hence the forces acting between surfaces is becoming more and more important in the efforts to reduce dimensions. The physicist Prof. Dr. Paul Leiderer and his team are investigating the adhesive forces between nanoparticles and a variety of different surfaces in the search for innovative methods to remove them. As the researcher reports in an interview with BIOPRO, his team particularly focuses on the optical, electrical and mechanical properties of these nanoparticles.
In principle, I find it very difficult to make concrete predictions on the fields in which nanotechnology can achieve decisive breakthroughs. Of course, there are a number of examples where nanomaterials are used. For example in terms of materials, the nanostructuring of surfaces, the well-known lotus effect and coating. Nanoparticles are also used for medical applications such as cancer therapy. Although these particles are very small, they still continue to be used in macroscopic quantities (in the gram to kilogram range). We are mainly interested in the characteristics of individual particles and structures that can be created using electron beam lithography or other nanostructuring methods.
Miniaturisation in the field of microsystems technology requires us to gain a better understanding of the importance of surfaces, and hence of the forces that act between surfaces. Let me give you two examples: the adhesive forces that occur between small particles and surfaces, which are mainly generated through van der Waals interactions, and the capillary forces that are generated when droplets of liquid are located on surfaces. These two types of forces also occur when macroscopic particles are used, but in this case they play a lesser role than frictional and accelerating forces. My group is investigating the adhesive forces between small particles and different surfaces, and we are looking for innovative methods to remove the particles. We use a method that is known as "laser cleaning" which involves pulsed laser beams. Another method we use is laser interference lithography which enables the simple generation of spot and line patterns in the submicrometre range on surfaces using pulsed, superordinated laser beams. Our research also relates to applications such as microcontact stamps that can be used to apply chemical patterns to surfaces.
The procedure is astonishingly simple and can be compared with stamps used by the postal service to make postmarks; however our stamps have much smaller structures and use special "inks", mostly organic molecules, which adhere well to the matrix to be structured. The stamps are made of a rubber-like polymer and are produced from a "master" using a mould. The chemical patterns that can thus be applied to glass or metal change the wetting behaviour locally and can be used as initial structures for simple electrical circuits, as sensors for biomolecules or for the study of cell growth. It is astonishing to see that this simple principle works on the micrometre scale and even below.
The optical characteristics of materials mainly refer to optical near fields, i.e. the "light intensity" in the direct vicinity of nanoparticles, which can be a thousand times more intensive locally than the light intensity applied. Using specifically formed nanostructures, so-called "optical antennas", we can efficiently capture the light energy on surfaces and guide it to the location where we want the light to have an effect. Such structures include, for example, active elements of solar cells. With regard to the materials' electrical characteristics, we investigate the transport of charge (electrons) through nanoscopic structures and the influence of light on this transport. We are mainly interested in studying whether and what kind of new effects occur on this length scale; however, direct applications of these investigations cannot be expected in the near future. With regard to the materials' mechanical characteristics, we are interested in the oscillations and resonances of nanostructures, which are similar to those of a tuning fork. However, while tuning forks vibrate at audible frequencies, the mechanical resonances of the nanostructures are so small that they occur at frequencies one million times higher than those of tuning forks.
It is known that small silver particles have an antibacterial effect. In addition, several years ago, researchers found that small gold particles had excellent catalytic characteristics. Nobody had actually expected this, because solid gold cannot be used as a catalyst. Therefore, I envisage that other nanoparticles will also be able to be used for life sciences applications.
We are working with a chemistry department at the University of Ulm in an investigation into the catalytic effect of small particles. Our contribution to this work is a process that enables the creation of the small particles. We are also working with a group from Karlsruhe to study nanocontacts that are opened and closed electrochemically in an electrolyte. In this project, we are investigating the effect of light on the electrical transport in these structures.
We work mainly with particles between some ten nanometres (millionth millimetres) and a few micrometres in size. The majority of particles are between 50 and 500 nanometres.
We use lasers to structure surfaces in the micro- and submicrometre range, to investigate optical near fields and characterise their spatial distribution in order to remove particles from surfaces. We also use lasers as "optical tweezers" to manipulate small particles. We use pulsed lasers with pulses of between a few nanoseconds to around one hundred femtoseconds, as well as continuous wave (CW) lasers. However, we do not actually focus on laser technology ourselves, as we only use commercially available lasers.
Prof. Dr. Paul Leiderer
Department of Physics
University of Konstanz
Tel.: +49 7531 883793
Fax: +49 7531 883091