(, 7/10/03).
The exponential (in vacuum) decay of the electron wavefunctions means that the tunnelling current is extremely sensitive to the tip-sample separation. This provides a very fine resolution of the surface.
Quantum Mechanical Tunnelling
The infinite potential walled particle in a box theory does not allow any of the wave function to escape the box as it would have to have more than infinity energy to cross the barrier. Allowing the potential energy well to be a finite number has the effect of making it possible for the wave function of a particle that is trapped in this potential well, to partially escape and thus have a presence outside the confines of the box.
The wave function can transverse the potential barrier, although it will decay exponentially through the barrier. Assuming that the wave function does not totally decay away before the end of the barrier, the particle can have a physical presence on the other side of the potential barrier. If the potential barrier is long range, then the wave function will decay away exponentially and tend towards zero. Upon reaching the end of the potential barrier, the particle will have an infinite small wave function and zero presence on the other side of the barrier. This property is known as quantum mechanical tunnelling.
(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. The current as a function of lateral position represents the surface image. This mode is only appropriate for atomically flat surfaces. If the surface were not flat, the STM tip would crash. An advantage of constant height mode is that it can be used at high scanning frequencies (up to 10 kHz).
(http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2002/sm242/stmdesign.htm, 12/10/03).
Constant current mode - In the constant-current mode, the current is used as the input to a feedback circuit that moves the scanner together with the tip up and down in the height direction. With an applied potential, the tip is brought close to the sample surface until the tunnelling current set point is detected, at which point the constant-current feedback loop is locked. When the tip moves laterally to a new position, any subtle sample-tip distance variation will lead to the fluctuation of the tunnelling current. Consequently, the feedback circuit will move the tip up and down until the current keeps the set-point value. As a result, the moving tip keeps the constant sample-tip distance, tracing the surface topography.
The main advantage of constant current mode is that the tip will not crash into a large cluster of atoms at the surface. Constant current mode can measure irregular surfaces with high precision, but the measurement takes more time, (http://std2.fic.uni.lodz. pl/stm.html, 13/10/03).
(http://instruct1.cit.cornell.edu/courses/ee476/FinalProjects/s2002/sm242/stmdesign.htm, 12/10/03).
Tip etching
The construction of the tip is one of the most crucial aspects of STM. The tip must be approximately one atom thick in order for the STM to be carried out effectively. “Surface pictures can appear to be distorted due to the presence of more than one sharp protrusion”, (Ouseph, P. J. & Gossman, M., 1998, 701-704). Some important characteristics of a tip are
- Sharp tips which allow high resolution STM observations
- Small resonance area
- Thick taper to reduce tip oscillation during STM scans
(http://www.mme.wsu.edu/~reu/poster2000/Ronald2000/ronald/ppframe.htm, 14/10/03)
Multiple tips can be formed when suspended particles are picked up from the etching solution, (NaOH, KCN). Multiple tips often lead to the occurrence of shadow effects and ghost images, (, 20/10/03).
Conditions
The STM can operate in many different kinds of environments. A simple STM can be run in ambient air. However, in order to achieve effective results in Surface Science the STM is run in ultra-high vacuum.
Vibration Isolation
As the tip is oscillating across a very small area, it is important to isolate vibration. Vibration is kept to a minimum by several methods. Placing the STM on a spring/damping table is one way to cut out vibrations travelling through the floor. The first typical isolation system is ‘the coiled spring suspension with magnetic damping’ and the second is ‘a stack of metal plates with viton dampers between each pair of steel plates’, (Kaiser, W. J., 1993, pp58). The basement level of a building is preferred because there are lower level of vibrations. To stop wave propagating through the air, a foam cover can be placed over the instrument, (, (19/10/03).
References
H. Strecker and G. Persch, Scanning tunnelling microscopy and technical applications
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
, 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
, 7/10/03
, 7/10/03
, (19/10/03)
http://www.mme.wsu.edu/~reu/poster2000/Ronald2000/ronald/ppframe.htm, 14/10/03
, 20/10/03
Bibliography
Bai, Chunli., (1995), Scanning tunnelling microscopy and its application, New York : Springer
Bonnell, D. A. (1993). Scanning tunnelling microscopy 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 scanning tunnelling microscopy investigation 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
