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

An Introduction to Microscopy 23
A remarkable feature of the STM is the high spatial resolution that can be
achieved: better than 0.01 nm in the z-direction, which follows directly from
the fact that the tunneling current is a strong (exponential) function of the
tunneling gap. It is also possible to achieve high resolution (< 0.1 nm) in the
x- and y-directions, even when the tip is not atomically sharp (STM tips are
made by electrolytic sharpening of a wire or even by using a mechanical
wire cutter). The explanation is again in terms of the strong dependence of
tunneling current on gap length: most of the electrons tunnel from a single
tip atom that is nearest to the specimen, even when other atoms are only
slightly further away. As a result, the STM can be used to study the structure
of surfaces with single-atom resolution, as illustrated in Fig. 1-19.
Nevertheless, problems can occur due to the existence of “multiple tips”,
resulting in false features (artifacts) in an image. Also, the scan times
become long if the scanning is done slowly enough for the tip to move in the
z-direction. As a result, atomic-resolution images are sometimes recorded in
variable-current mode, where (once the tip is close to the sample) the
feedback mechanism is turned off and the tip is scanned over a short
distance parallel to the sample surface; changes in tunneling current are then
displayed in the image. In this mode, the field of view is limited; even for a
very smooth surface, the tip would eventually crash into the specimen,
damaging the tip and/or specimen.
Figure 1-19. 5 nm � 5 nm area of a hydrogen-passivated Si (111) surface, imaged in an STM
with a tip potential of –1.5 V. The subtle hexagonal structure represents H-covered Si atoms,
while the two prominent white patches arise from dangling bonds where the H atoms have
been removed (these appear non-circular because of the somewhat irregularly shaped tip).
Courtesy of Jason Pitters and Bob Wolkow, National Institute of Nanotechnology, Canada.