High-Speed Variable-Temperature STM
The Scanning Probe experiments in our group rely heavily on the use of home-built, special-purpose Scanning Probe Microscopes. The first special SPM instrument developed by the Interface Physics Group is a High-Speed, Variable-Temperature Scanning Tunneling Microscope. The technical aspects of this microscope are described in [1,2,3,4].
The high speed of the STM is obtained by a combination of fast analog electronics and a fast digital control system, which is interfaced to an ensemble of three T800-transputers and a Silicon Graphics workstation. This system acquires up to 100.000 pixels per second, which can be configured into e.g. 10 images per second of 100 x 100 pixels per image. A clever scheduling of tasks between the transputers ensures that the images are recorded without any time delay between the last pixel of an image and the first pixel of the next image. Currently, we are working on the development of a fully digitally controlled analog feedback and control system that will go up to video rates, i.e. 25 images/s, at a more mature image size of 256 x 256 pixels. This system should be operational in the course of 2002.
The temperature range available to our present STM is 30 K - 800 K. Anywhere in this temperature range, the temperature can be swept over 300 K, while a single area on the surface can be kept "in view" of the tip, without the need for mechanical position adjustments of the tip, either parallel or perpendicular to the surface. This superb thermal stability has been reached as the result of a computer optimization of the design. This was performed in the form of a collection extensive finite-element computer calculations of the thermal behavior of the microscope+sample combination, during temperature ramps of the sample.
The figure shows the result of the computer optimization. A central tube scanner is used to scan the sample surface (pink). The coarse approach is performed via a pivoting mechanism on the sample holder. A wobble stick can be used to exchange the entire scanner unit, and the sample holder can be exchanged by use of the same wobble stick. the photograph below shows the microscope in reality.
Below is a photograph of the symmetrically expanding and contracting sample holder, with the pivoting mechanism for the coarse approach. The holder contains a heating element and a thermocouple or Pt-resistance thermometer.
The microscope is built into an ultrahigh vacuum system, with LEED, Auger, and other tools for surface preparation and diagnostics.
Here's a picture of Raoul van Gastel, pretending to do useful things with it, and to have fun with it at the same time...
We're still working on the variable-temperature STM, and hope to improve its thermal stability even further in the near future...
|||"Surfaces in motion: a variable-temperature scanning tunneling microscopy study"
Ph.D. thesis: Leiden University, June, 1998
|||"Design and performance of a programmable-temperature scanning tunneling microscope"
M.S. Hoogeman, D. Glastra van Loon, R.W.M. Loos, H.G. Ficke, E. de Haas, J.J. van der Linden, H. Zeijlemaker, L. Kuipers, M.F. Chang, M.A.J. Klik, and J.W.M. Frenken
Rev. Sci. Instrum. 69 (1998) 2072
|||"Surface Dynamics Studied with a High-Temperature High-Speed Scanning Tunneling Microscope"
Ph.D. thesis: Amsterdam University, June, 1994
|||"Design and Performance of a High-Temperature High-Speed STM"
L. Kuipers, R.W.M. Loos, H. Neerings, J. ter Horst, G.J. Ruwiel, A.P. de Jongh and J.W.M. Frenken
Rev. Sci. Instr. 66 (1995) 4557
|||More publications in the group's publication list.|