The following projects can be conducted in our group:
We are looking for PhD candidates regarding the following themes:
Nanostructure Arrays on Graphite for Scanning Tunneling Microscopy
Nanosphere lithography (NSL) is an efficient and simple method to create large-scale arrays of metal nanostructures, The key idea is to let polymer nanospheres assemble themselves on a surface, and use this assembly as a mask for subsequent metal deposition. While this method is routinely applied for several years in our group, most experiments have been conducted using insulating substrates or on semiconductors with an insulating surface layer which are not well suited for scanning tunneling microscopy (STM). In this project a truly conductive, yet weakly interacting substrate shall be investigated: highly oriented pyrolytic graphite (HOPG). HOPG is a commonly used material for surface studies, mainly because it is chemically and thermally inert, most deposited materials only interact weakly, and it is well-suited for STM. The aim is to prepare nanostructure arrays, optimize preparation parameters, and investigate the samples using scanning force microscopy (AFM) and / or STM.
Stamping of Molecular Patterns with Amine Termination for Cell Adhesion Studies
Osteoblast adhesion is important for ingrowth of implants. Cell adhesion includes a number of steps such as seeding, migration, settling and spreading. To understand which cues are relevant in migration and anisotropic spreading molecular landscapes of amine-terminated species can be used. In aqueous solution the amine groups become positively charged, thus lateral electric fields will be present on the surface. Micro-contact printing is a method to prepare monolayer patterns of molecules on surfaces. Silicone stamps are produced as molds from glass masters, immersed in the molecule solution and contacted to the substrate in order to transfer molecules from the stamp to the glass. The task is to produce and characterize such molecule landscapes and participate in scanning ion conductance measurements of adhered cells thereon.
Electrochemical Etching of Pt-Ir Tips for Multiprobe Applications
Scanning probe microscopes are known for their extraordinary resolution, even enabling imaging of individual atoms. The most critical component is the tip which must be very sharp, ideally reaching an effective diameter of only a single atom. In most cases the basis for such tips is electrochemical etching yielding a highly defined shape. While this method is routinely used for tungsten tips many applications require inert, non-oxidizing materials. In this project wires made from a Pt-Ir alloy are used for tip etching. Resulting tips are characterized by electron microscopy and by the actual imaging quality "at work" in a multiprobe scanning probe microscope.
Conductive Organic Films on Insulators for Surface Charge Mitigation
Surface charges frequently lead to critical situations in space applications. In a project together with the DLR (German Aerospace Center) we explore mechanisms of charge deposition and charge control on the nanoscale, where the primary tool is a scanning electron microscope (SEM). A promising class of materials is conducting polymers, due to their optical transparence and cost-efficiency. This Bachelor project aims at controlling the geometric properties of ultrathin films and / or nanostructures prepared under ambient conditions. Charge deposition is accomplished in a scanning electron microscope, and charge transport properties can be measured by a sheet resistance meter. The question is if and how surface charges on insulating materials are affected by conductive organic films.
Immersion Process of Metal Nanoparticles Embedded into thin Polymer Films
Functional mapping in an atomic force microscope (AFM) often suffers from crosstalk between topographic and functional channels. For experiments using optical mapping embedded nanoparticles are a promising system because they exhibit local near fields with little geometric surface corrugation. In this project the embedding process into polymer films is investigated by a combination of AFM, optical microscopy, and/or reflectometry.
- Ion Current in Nanopipettes near Surfaces
In scanning ion conductance microscopy a nanopipette is brought close to a surface, and the ion current in a conductive liquid is used as a probe for nanoscale surface imaging. Due to surface charges inside the pipette, as well as on the surfaces to be investigated, the ion current may be affected in a characteristic way. For example, rectifying behavior is observed in analogy to semiconductor heterojunctions. In this project the ion current is investigated as a function of external parameters such as voltage, ion concentration, pipette dimensions and -material, and the surface material (determining the surface charge). Observations may be analyzed experimentally in view of biological samples (e.g. living cells) or in conjunction with numerical simulations taking into account the essential geometry.
- Nano-Tribology of Synovial Fluid Components on Artificial Joint Materials (Biophysics)
Human joints such as the knee or the hip show astonishing performance with regard to lubrication and wear. In collaboration with the department of orthopedics (UMR) we investigate friction and wear processes (also called tribology) on a nonoscale using a scanning force microscope (AFM). Although the exerted force is orders of magnitude smaller compared to the real joint, the pressure is of comparable order of magnitude due to the small interaction area. In this work the lateral friction of model fluids on realistic artificial joint materials shall be investigated by AFM. The aim is to better understand how the materials behave on the nanometric scale, and which mechanisms are dominating friction and wear.
More topics are available upon request.
- Nonlinear Oscillation Dynamics of Atomic Force Microscopy Cantilevers
The amplitude spectrum of a cantilever in an atomic force microscope (AFM) in vicinity to a sample surface is rich in structure if the tip-sample interaction is sufficiently nonlinear. In this project the eigenfrequencies and mixing modes due to external forces for different cantilevers, as well as nonlinear mapping modes are explored.
- Mapping Light-Induced Forces on Nanostructures Samples
Light-matter interaction can lead to various phenomena such as optical forces, photovoltages, and plasmon excitation. In this project the possibility to map such effects with dynamic atomic force microscopy (AFM) is explored on nanostructured surfaces. Possible approaches are Kelvin probe force microscopy (KPFM), amplitude modulated force detection, or tip-induced light scattering in the near field.
- Long-Range Order of Self-Assembled Organic Molecules Prepared in Ultra-High Vacuum
Photoemission electron microscopy (PEEM) enabled simultaneous spatially and energy resolved investigation of the electronic and optical properties of dye molecules adsorbed on a surface. In this project a dedicated ultra-high vacuum (UHV) evaporator for molecular species is being built and used for well-controlled preparation of ultrathin fluorophore layers. The geometric, optical and electronic properties are investigated using atomic force microscopy (AFM), fluorescence microscopy, and PEEM.
- Mapping the Work Function of Heterogeneous Metal-Semiconductor Nanostructures
The spatially dependent work function of metal-semiconductor nanostructures at surfaces contains vital information on the material distribution and the band topology. Using Kelvin probe force microscopy (KPFM) or photoemission electron microscopy (PEEM) the work function can be mapped with high spatial resolution. This capability is applied to nanostructures prepared by evaporation in vacuum or by ex-situ nanolithography.
- Morphology of Live Cells on Structured Surfaces Studied by Scanning Ion Conductance Microscopy (Biophysics)
Adhesion of cells to surfaces can include several steps such as initial physical or chemical binding, rearrangements in the cells surface region, spreading, and migration. The aim is to uncover surface structure dependent interaction mechanisms on the basis of time-dependent morphology studies.
For the State Examination ("Staatsexamen") a variety of projects are available which are closely related to running activities in our research group. Possible topics are:
- Dynamic atomic force microscopy (AFM)
- Growth of molecular nanostructures
- Luminescence properties of dye aggregates on surfaces
- Morphology of metallic nanostructures on surfaces
None at the moment.