attoMICROSCOPY
PAGE 102
Atomic Force & Scanning Tunneling Microscopy (AFM/STM)
fundamentals
The attoAFM/STM is a very versatile instrument, in that it combines two different scanning probe techniques, namely
non-contact mode atomic force and scanning tunneling microscopy. In contrast to
the attoAFM III, the attoAFM/STM uses a
horizontally aligned tuning fork, with an
ultra-sharp etched tungsten tip as a probe
on one prong. This configuration allows
for scanning tunneling spectroscopy measurements even on nominally non-conductive samples with just a few conductive
patches.
One of the advantages of this instrument is
its large scan range compared to conventional STM designs, since it employs an attocube ANS scanner instead of a piezo scan
tube. This feature substantially simplifies
finding and identifying a particular region
of interest on a nanostructured sample.
This can sometimes prove difficult with
conventional STMs, since they are usually
based on piezo tubes with very limited
scan range, especially at low temperatures. Often, conventional STMs are even
completely lacking lateral positioning
capabilities over macroscopic ranges.
In AFM mode, the tuning fork is excited
mechanically by a small dither piezo,
usually at its resonance frequency. This
allows for AFM imaging of non-conductive
parts of the sample structure in non-contact mode operation, and enables both
safe and relatively quick mapping of the
surface topography. Alternatively, the tip
can be electrically connected to a current/
voltage amplifier for measuring the tunneling current in STM mode. A bias voltage
is applied between tip and sample, which
yields a tunneling current that varies exponentially with their mutual distance. In
constant height mode, the tip is scanned
in a plane parallel to the surface. In the
case of a constant density of states (DOS)
of the sample, the tunneling current between tip and sample reflects the sample
topography.
In this mode, adjustment of the tip-sample
separation is not requ