1. Architecture and electronic properties of Self-assembled monolayers (SAMs)
SAMs are thin organic films that form spontaneously on solid surfaces. The high systematic and technological relevance of SAMs resides in the fact that they are expedient tools for customizing surfaces and for controlling the physical and chemical nature of the interface between a surface and its environment. Thus, the electric conductivity, the optical properties, corrosion resistance of the surface, among many other features, may be sensitively modified by SAM engineering. My involvement in SAM related research is chiefly associated with their usefulness for the purpose of optimizing sensors that detect chemical and biological species at the molecular level. Sensors are defined as instruments that convert input stimuli into a readily observable mechanical response. They may be realized by microscopic cantilevers coated by SAMs that selectively bind and immobilize the targeted agent. For instance, explosives attaching to SAMs were found to be detectible with sensitivities in the parts-per-trillion (ppt) range. This was achieved by measuring the mechanical deformation as well as the change in the resonance frequency of small SAM coated cantilevers upon adsorption of minute traces of the respective agent. To improve upon the selectivity of this and related methods, however, an in-depth understanding of the interaction of individual species with SAMs is needed, which is among the main goals of my research in this field.
Figure 1: Scanning Tunneling Microscopy (STM) image of a self-assembled monolayer of methanthiol molecules on a reconstructed gold surface (from: Phys. Rev. B - Rapid Comm. 80, 081401(R)(2009))
2. Carbon nanostructures
My work on carbon nanostructures focuses mostly on two prototypes, namely (a) fullerenes: cage-like species of which the spherical molecule C60 is the most famous representative, and cylinder-shaped units, so-called (b) carbon nanotubes. In the following, I characterize briefly my involvement in the computational research on these two types of materials.
Fullerenes may be used to encapsulate metal atoms into firm carbon enclosures. These metallofullerenes have turned out to be of major biomedical interest, most importantly in the area of magnetic resonance imaging (MRI). MRI employs magnetic heavy metal species (contrast agents) that are introduced into the examined tissue, as they improve the resolution of the MR image. A serious drawback of this practice, however, is the high toxicity of heavy metals which makes it necessary to administer them in a biocompatible form. A nanotechnological solution of this problem consists in enclosing the metal atoms into fullerenes. The high stability of the fullerene cage prevents them from intruding into the surrounding tissue and thus could make contrast agents based on metallofullerenes suitable for diagnostic use. My computational work on these complexes aims at obtaining basic information about their structure, stability, and magnetism from first principles. The practical goal of this research lies in designing and optimizing multifunctional contrast agents, to be employed in X-ray and MR imaging, as well as radioscopic therapy and diagnosis.
Figure 2: Representation of the prototypical cluster Gd3N@C80 (from: J.Phys.Chem.C 112, 5770 (2008)).
2b. Carbon nanotubes
A broad variety of applications have been identified for carbon nanotubes. Metallic nanotubes are very notable for their electric conductivity as they can be loaded with larger currents than any other known electric material. The mechanic properties of nanotubes are equally exceptional, as they have about one hundred times the strength of steel but only one-sixth of its weight. My work on carbon nanotubes is mainly concerned with their environmental use as extremely small and extremely efficient sensors of chemical species. The strong response of semiconducting nanotubes to external chemical agents can be exploited to design nanotube molecular sensors which react faster and with substantially higher sensitivity at room temperature than existing solid-state sensors. Correspondingly, my current work on SWNTs is motivated by the need to understand the changes of their materials properties, chiefly conductivity and magnetism, as a function of the nature and the number of external species interacting with the nanotube. Within this effort, the impact of these species on the magnetic properties of the nanotubes of finite length is of particular interest. This effect, in turn, utilizes the pronounced magnetism exhibited by ultra-short tubes, extending in length to some tens of nanometers. One of my recent projects is dedicated to the detailed exploration of the latter phenomenon.
Figure 3: The three types of finite SWNTs types considered, differing with respect to their termination mode: hydrogenation (a), truncation (b), and capping with fullerene hemispheres (c) (from: Phys. Rev. B 79, 115436 (2009))
Further computational projects of mine deal with silicon based clusters as potential media of hydrogen storage, and with the optical and photovoltaic properties of selected molecular systems.