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SPM Images

Although the word “map” describes them more accurately, SPM maps are most often called “images,“ and in keeping with the soft surfaces. common practice, we will usually use the word image.

The most common SPM images are topography images, which are most often created with Atomic Force Microscopy (AFM) or Scanning Tunneling Microscopy (STM). We will study both AFM and STM in some detail. The acronyms AFM and STM also stand for atomic force microscope, which was invented in 1986,1 and its predecessor scanning tunneling microscope, invented in 1982.2

In AFM topography images, the third dimension, Z, at a given X,Y coordinate pair, is the relative height of the sample surface at those coordinates. This interpretation implies the AFM’s sharp probe does not deform the sample surface, either reversibly or irreversibly. The harder the sample surface, the more accurate is this interpretation, because the AFM tip penetrates a harder sample surface less than it does a softer one. In other words, the AFM tip follows the height variations of hard surface with higher fidelity than it does soft surfaces.

AFM height measurements are in general calibrated against height standards. Those height standards themselves are often measured with methods other than SPM. In this sense, AFM topography (height) images may often readily be compared for quantitative information, even if they are collected on different instruments, so long as the calibrations are accurate; the instruments work well; and the AFM users operate the instrument properly, and know how to correctly extract information from the raw data.

In STM topography images, the interpretation of the third dimension, Z, as the relative height of the sample surface is sometimes less straight forward than it is in AFM. (See the section on Scanning Tunneling Microscopy).

In other types of SPM images, the third dimension of the image is a measure of the relative strength of a detectable interaction, between the probe and the sample, that may or may not have a correlation with the topography. The image is usually recorded simultaneously with, and displayed side-by-side, the topography image of the same in-plane coordinates (same areas on the sample). This helps reveal any correlation that may exist between topography and the interaction recorded in the second image.

In a few cases, the measurement of the interaction may be calibrated, so that in addition to mapping the relative strength of this interaction, the image may also yield quantitative information beyond topography about the sample. An example is Surface Potential image or Scanning Kelvin Probe Microscopy (SKPM) image. But generally, nontopographic SPM images serve to identify only relative variations of the interaction.

1 G. Binnig, C. Gerber, C. Quate, Physical Review Letters, 56, p 930, 1986.

2 G. Binning, H.Rohrer, C. Gerber, E. Weibel, “Surface studies by scanning tunneling microscopy,” Physical Review Letters, 49, pp. 57-61, 1982.

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