Multi-channel scanning probe microscopy
Correlation of different physical properties at microscopic to nanometer scales is crucial to understanding the basic physics in nano-matters. It should be realized that most existing SPM probes are single-channel and therefore can only image one physical property at a time. This motivated us to develop multi-channel SPM. The first microwave/optical dual-channel microprobe was demonstrated recently in our laboratory for simultaneous mapping of microwave and optical properties. The key element of this probe is the open-ended coaxial resonator terminated by a tapered and metal-coated fiber optic tip. Microwave is emitted/collected at the tip via the metal coating and light is emitted/collected via the core of the fiber. We extended the application of this metal-coated fiber tip to scanning tunneling microscopy (STM). Since the metal-coating (Gold) terminates at the tip apex but not covering the optical aperture, it facilitates the injection of tunneling electron. With this probe, STM and near-field scanning optical microscopy (NSOM) experiments can be carried out. There is no doubt that these multi-channel SPM probes will become versatile tools for material research and nanoscience and we are currently carrying out some exciting applications.
Imaging of different physical properties of a sample can also be realized by combining SPM techniques with macroscopic measurements. For example, we have integrated the microwave microprobe with electrical current-voltage (I-V) measurement. This allows us to excite the sample locally using the focused microwave from the probe and measure the sample voltage response. By recording this voltage response as a function of probe position, we can visualize the current flow pattern in a conductor. We can use this technique to map the current distribution in HTS coated conductors and locate defects that impede the current flow. Moreover, other information about the sample area under investigation can be acquired. A microwave map reflecting the nonuniformity of structure and chemical composition through the HTS layer will reveal location and form of defects. This method may provide a unique room-temperature technique for electrical current mapping in long-length HTS coated conductors.