Zettl Research Group
SCANNING TUNNNELING MICROSCOPE
A Scanning Tunneling Microscope is a non-optical apparatus which directly measures electronic states. With precision control, the microscope can obtain atomic resolution by imaging the states of individual atoms. On a surface, the microscope is also sensitive to localized defect states and quantum confined states.
Placing the microscope into an Ultra High Vacuum environment allows for controlled surface preparation. Operation in air or other "dirty" environments complicates the interpretation of atom-sized features. In a clean environment, however, both the surface and the atoms placed upon it can be carefully controlled.
Operation at Cryogenic Temperatures reduces electronic noise and eliminates some smearing between separate energy levels. At small enough temperatures, these different levels can be independently probed.
In addition, low temperatures freeze adsobed atoms and molecules in place on a surface. Using the microscope tip, Atomic Manipulation can move the otherwise frozen adsorbates. If stable over long periods of time, unique structures may be built up on an atom-by-atom basis.
By applying a Magnetic Field and rotating the samples within the field, additional effects may be studied. Some energy levels will be split into multiple levels in a strong field, in which case the STM may observe spin polarized states. Other systems, such as superconductors and ferromagnets, have more complicated behaviors in magnetic fields.
The Low-Temperature UHV STM has been designed for the purpose of assembling and characterizing nanoscale materials with interesting electronic behaviors. The following is a brief list of the types of research possible with such an instrument.
Clusters with a relatively small number of atoms enable us to study how metals and ferromagnets arise from the properties of individual atoms. By studying the electronic states of clusters as a function of cluster size, much can be learned about the onset of bulk properties.
As the semiconductor industry pushes the size of electrical elements smaller and smaller, quantum effects will gradually come to dominate the classical behaviors. A metal wire, if only two atoms wide, won't behave like any wire available on a spool. Using a combination of high resolution lithography and atomic manipulation capabilities, such wires and dots can be studied in both metallic and semiconducting systems. Thus we can observe the limits of classical "wiring" and find new elements dominated by quantum effects.
Many new materials are being discovered with nanoscopic dimensions. Currently, the most famous example of this type of materials is carbon nanotubes. Measuring transport properties of these materials is critical to understanding whether some will have practical applications. Many calculations suggest various types of nanotubes and nanowires may have unique properties leading directly to practical applications. Materials such as these can be studied in our system.
Any of the systems described above may be modified in situ. Modifications are a powerful tool for understanding a new material and probing its properties. Doping, or intercalation, of a material with donor atoms is one method of changing a material. For example, some superconductors are quite sensitive to doping levels. Another modification method is the tip-induced production of local defects in a material. Such defects in a mesoscopic conductor, for example, may dominate the transport properties. A "quantum corral," as built by researchers at IBM Almaden, is another example of a special local potential which can be formed by this instrument.
Low temperature ultra high vacuum scanning tunneling microscope
We have built a low temperature ultra high vacuum scanning tunneling microscope (STM) designed specifically to study nanoscale structures on surfaces. The microscope operates at 4.9 Kelvin. The entire UHV system has a base pressure of 1.8 x 10-10 Torr, and has a variety of in-situ sample preparation capabilities including substrate temperature control, thermal and e-beam evaporation, ion beam sputtering, and low-coverage surface dosing. The system also includes LEED and Auger analysis for further surface characterization. STM can be fitted with a split-coil magnet to apply a 5.5 Tesla rotatable field to samples.This microscope will aid in the characterization of new materials and provide insights into nanoscale electronic elements. Using this instrument, it is possible to immediately characterize novel purified nanomaterials synthesized in our laboratory.
We have variable temperature STM/AFM capable of imaging samples from 20 to 1000 K purchased from RHK. The microscope has optical access to the sample, quick sample turn around time, and tip-exchange capability. The microscope system includes standard surface analysis chamber with Auger and LEED and preparation chamber with electron beam evaporators. We use the system to investigate properties nanoscale structures using novel scanning probe techniques such as spin-polarized tunneling and STM-induced photoluminescence.
Bundle of carbon nanotubes
[Image by Low temperature UHVSTM]
[Image by Low temperature UHVSTM]