Physical Principles of Electron Microscopy: An Introduction to TEM ...

An Introduction to Microscopy 25
electron-beam instrument. Also, there is no way of implementing elemental
analysis in the AFM. An STM can be used in a spectroscopy mode but the
information obtained relates to the outer-shell electron distribution and is
less directly linked to chemical composition. And except in special cases, a
scanning-probe image represents only the surface properties of a specimen
and not the internal structure that is visible using a TEM.
Figure 1-20a shows a typical AFM image, presented in a conventional
way in which local changes in image brightness represent variations in
surface height (motion of the tip in the z-direction). However, scanningprobe
images are often presented as so-called y-modulation images, in which
z-motion of the tip is used to deflect the electron beam of the CRT display in
the y-direction, perpendicular to its scan direction. This procedure gives a
three-dimensional effect, equivalent to viewing the specimen surface at an
oblique angle rather than in the perpendicular to the surface. The distance
scale of this y-modulation is often magnified relative to the scale along the xand
y-scan directions, exaggerating height differences but making them more
easily visible; see Fig. 1-20b.
Figure 1-20. AFM images of a vacuum-deposited thin film of the organic semiconductor
pentacene: (a) brightness-modulation image, in which abrupt changes in image brightness
represent steps (terraces) on the surface, and (b) y-modulation image of the same area.
Courtesy of Hui Qian, University of Alberta.