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Scanning Tunnelling Microscopy.

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Introduction

Scanning Tunnelling Microscopy

Introduction

        The STM with its unmatched combination of high vertical and lateral resolution in a promising new tool that can be operated under ambient conditions, yielding three-dimensional detailed images, (H. Strecker and G. Persch, 1990, p441-445).  The scanning tunnelling microscope (STM) is a solid-state microscope based on the principle of quantum mechanical tunnelling of electrons between a sharp tip and a conducting sample. Surfaces can be studied by allowing the individual atoms to be imaged in real space. By scanning the tip across a sample surface it is possible to image directly the three dimensional structure of a surface down to atomic scale resolutions. Prior to the invention of STM, the only way that surface structures could be deduced was by more indirect methods such as low energy electron diffraction (LEED) or medium-energy ion scattering (MEIS).

Technique

        The tip used in STM is very sharp and ideally terminates into a single atom. The tip is mounted onto a system of piezo electric drives, which provide movement in three dimensions.

image00.png

        (http://nanowiz.tripod.com/stmbasic/stmbasic.htm, 14/10/03).

The movement is controllable with sub-atomic scale accuracy and can be brought within a few Amstroms of the conducting sample surface.

...read more.

Middle

image03.png

(http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmdet.htm,10/10/03).

        The quantum mechanical phenomenon creates the high degree of sensitivity necessary for atomic scale imaging of surfaces. The quantum mechanical tunnelling current is highly dependent on the tip-surface distance. The distance between tip and surface is usually of the order of 0.3 nm and the tunnelling voltage V ranges from a few mV up to a few V, depending on the conductivity of the surface. The tunnelling current typically varies between 10 pA and 1 nA, (http://www.fys.kuleuven.ac.be/vsm /spm/introduction.html12/10/03). ‘The tunnel current decreases to 1/10 of its initial value for every 0.1 nm increase in gap separation’, (Kaiser, W. J. & Stroscio, J. A., 1993, p78)

        The essential aspect of STM is the extreme sensitivity of the tunnelling current to the tip sample separation. It is therefore important to realise that the tunnelling current is a quantum phenomenon. In classical physics the current could not flow across a gap.

Modes of Operation

Constant height mode - In this mode the vertical position of the tip is not changed, equivalent to a slow or disabled feedback. The tunnelling current varies depending on topography and the local surface electronic properties of the sample.

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Conclusion

   Applied Surface Science, Volume 46, Issues 1-4, December 1990, Pages 441-445

Kaiser, W, J. &Stroscio, J. A. (1993) Scanning Tunnelling Microscopy, Academic

   Press Inc: California

Ouseph, P. J. & Gossman, M. Effects of self-modifying multiple tips on STM surface

pictures,  Meas. Sci. Technol. Volume 9, 1998, pages 701-704

Websites

http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2002/sm242/stmdesign.htm, 12/10/03

http://nanowiz.tripod.com/stmbasic/stmbasic.htm, 14/10/03

http://std2.fic.uni.lodz. pl/stm.html, 13/10/03

http://www.chembio.uoguelph.ca/educmat/chm729/STMpage/stmdet.htm,10/10/03

http://www. chem.qmw.ac.uk/surfaces/scc/, 7/10/03

http://www.chem.qmw.ac.uk/surfaces/scc/, 7/10/03

http://www.mme.wsu.edu/~reu/Matt/Matt.htm, (19/10/03)

http://www.mme.wsu.edu/~reu/poster2000/Ronald2000/ronald/ppframe.htm, 14/10/03

http://www.physik.tu-berlin.de/institute/IFFP/daehne/ index.htm?/institute/ IFFP/daehne /forsch/rs-tips.htm, 20/10/03

Bibliography

Bai, Chunli., (1995), Scanningtunnellingmicroscopy and its application, New York : Springer

Bonnell, D. A. (1993). Scanningtunnellingmicroscopy and spectroscopy: theory, techniques, and Applications, New York, N.Y: VCH

Coughlin, M., (2002) An XPS/STM study of the Chemistry of small molecules on

bulk and Nano-Particulate Copper. Unpublished PhD Thesis, University of Cardiff.

Dunstan, Peter R. (1997) A soft x-ray photoelectron spectroscopy and scanningtunnellingmicroscopyinvestigation of Si/GaAs(110) interface, Thesis (Ph.D.)

Scanning tunnelling microscopy and technical applications, Applied Surface Science, Volume 46, Issues 1-4, 2 December 1990, Pages 441-445

Practical applications of scanning tunnelling microscopy, Ultramicroscopy, Volume 33, Issue 2, August 1990, Pages 83-92

...read more.

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