Amplitude-modulation (intermittent-contact, semicontact)
mode is widely spreaded oscillating mode and generally speaking can be
interpreted and described by the amplitude, phase, frequency and
dissipation on one another or on the cantilever-sample distance
dependences.
The study of such dependences is necessary according
to following circumstances. First of all it relates to obtaining
high-grade images (without noise and with high resolution). Then the
study of suitable dependences can help in determining the nature of
tip-sample interaction, defining forces included in this interaction
and formation SPM images. At last the study of suitable curves can help
in obtaining more contrastive images and quantitative parameters of
sample under investigation.
Obtained in Amplitude-modulation mode
images are determined by the row of factors related to the sample as
well as to conditions of measurement and values of scanning parameters.
For
interpreting results of amplitude-modulation mode usage one can to
study dependence of oscillation amplitude the tip-sample distance.
Suitable amplitude-distance (a-d) curves (their typical view one can
see on the animated picture) can be monotonic or can to have areas of
bistability and hysteresis. The presence of the bistability (as is
shown on the same picture) leads to arising of the artifacts on the
images obtained in Amplitude-modulation mode.
Origin of
bistability lies in possibility of simultaneously co-existence
oscillations predominantly in attractive or predominantly in repulsive
potentials.
The bistability also can arise in complicated shape of
tip-sample potential when in initial area cantilever stiffness is
greater than potential derivative and the potential derivative becames
greater tha cantilever stiffness.
With suitable choise of
set-point amplitude of cantilever oscillation, its stiffness, sharpness
of the tip one can reach conditions when over all sample surface under
investigation areas with bistability are absent.
