(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 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 was 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”, (………). 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)
It appears that multiple tips are formed when suspended particles are picked up from the etching solution, (……same as above).
