SPM is the acronym of scanning probe microscopy.  An AFM (atomic force microscope) is a subset of SPM. 

STM is the acronym of scanning tunneling microscopy.  STM is a mode of AFM.  So what is it, you ask? 

STM (Scanning Tunneling Microscopy) was invented in 1981 by G. Binnig and H Rohrer who shared the 1986 Nobel Price in Physics for their invention. STM uses a sharp conducting tip and it applies a bias voltage between the tip and the sample. When the tip is brought close to the sample electrons can “tunnel” through the narrow gap either from the sample to the tip or from the tip to the sample, depending on the sign of the bias voltage. This tunneling current changes with tip-to-sample distance, it decays exponentially with the distance, which gives STM its remarkably high precision in positioning the tip (sub-angstrom vertically and atomic resolution laterally). For the electron tunneling to take place, both the sample and the tip must be conductive. Therefore STM cannot be used on insulating materials.

STM can image a sample surface in either constant-current mode or constant-height mode, as shown in the image above. In constant-current mode, in order to keep the tunneling current constant STM uses feedback to adjust the height of the scanner at each measurement point, e.g. when the system senses a tunneling current increase, it adjusts the voltage applied to the piezoelectric scanner so that the scanner lifts the tip and give an increase in the tip-sample distance. The scanner height measured at each location on the sample surface constitutes the topographic image. The constant-current mode is thus generally used to acquire surface height data, its scan speed is limited by the feedback response and thus it takes longer to image an irregular surface at a larger scan size. In constant-height mode, the tip scans at a constant height above the sample and the tunneling current changes due to the topography and the local surface electronic properties of the sample. The current image is a result of measured tunneling current at each location on the sample surface. The constant-height mode can acquire data faster because the system doesn’t have to move the scanner in the vertical direction, so it is most often used for imaging relatively smooth surfaces.

Strictly speaking STM tunneling current is correlated to the surface electronic density of states, i.e., the number of filled or unfilled electron states near the Fermi level, within an energy range determined by the bias voltage. So STM measures constant tunneling probability instead of the physical topography at the surface. STM tunneling spectroscopy, looking at the current-voltage relationship at a constant tip-sample distance or the current-distance relationship at a constant bias voltage, is a useful tool to study the electronic structure and property of a sample surface at the atomic level.

This entry was posted on Wednesday, August 1, 2007 at August 1, 2007 and is filed under General, Microscopy. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.