Nanophotonics - Nanoelectronics

Research

Nanophotonics, the use of light to explore and exploit nanostructured materials, uses near-field effects to push the limits of optics for application and spectroscopy below the diffraction limit of light. The wide range of energies and interactions with matter made optical spectroscopy the most powerful characterization technique in physics and material science. Taking these techniques to the nanoworld thus opens new dimensions of understanding in physics and material research.

Many materials only exist at the nanoscale such as biological macromolecules while many others that are believed to be well characterized completely change their properties below a critical size. The potentially new properties of these nano-structured materials that are often governed by their surface properties are only about to become visible. As a counterpart, the disappearance of some bulk properties at the nanoscale, imposes the final frontiers for integration in semiconductor industries.

Our first focus is on ferroelectrics. These materials exhibit a macroscopic polarization that can be used e.g. as a non-volatile binary state, almost like for ferromagnets. To date, the critical size for this property is subject of intense research and different from ferromagnets, the electromechanical boundary conditions e.g. from electrodes play a crucial role. Scanning probe can reconstruct the polarization state and helps exploring related properties such as piezo- and pyroelectricity and even the coexistence of ferroelectricity and magnetic ordering in multiferroics. Some limitations of piezoresponse force microscopy (PFM) as a contact mode method have motivated us to investigate non-contact mode techniques for domain imaging.

In addition to capacitive hysteresis effects in ferroelectrics, we investigate resistance hysteresis in binary and ternary oxides. Resistive switching is an almost universal material property known since decades but the understanding of the nanostructural origin, indispensible in order to optimize materials for R-RAMs, is still in its infancy. We investigate these materials by conductive AFM and macroscopic schemes.

We combine scanning probe and optical microscopy in two dedicated tip-enhanced Raman spectroscopy (TERS) microscopes. A surface plasmon resonance at the tip-apex of the SPM tip provides enhanced sensitivity with an optical resolution as small as 15 nm FWHM. Depending on whether the sample is conductive or not, we use scanning tunnelling (STM) or atomic force microscopy (AFM) to control the sample-tip distance. Especially on insulators, this method is unrivalled since charging effects blur the lateral resolution of electron microscopes. Just as conventional Raman spectroscopy, TERS is a non-destructive, chemically sensitive method that additionally provides information on the structure, the orientation and the strain in a material. Whenever possible, we use complementary electron microscopy techniques including the facilities at the Canadian Light Source.

The transfer of these techniques to piezoelectric biominerals (e.g. hydroxylapatite in bones and teeth) and isolated macromolecules (e.g. in membranes) is the next challenge in scanning probe. We play an active role in the development of novel techniques and in the interpretation of scanning probe data.