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Dynamic Vertical Mode Force Microscopy


In Dynamic Vertical Mode Force Microscopy, the AFM cantilever oscillates, typically at frequencies in the kHz up to 100s of kHz. The oscillations are such that the free end of the cantilever and the tip move along a gently curved trajectory on a plane perpendicular to the XY plane. The tip motion is therefore not strictly perpendicular to the XY plane, but the departure from perpendicularity is often negligible.

The cantilever is usually mounted at an angle relative to the X,Y plane. This angle, which is typically around 10-15 degrees, is necessary to ensure the cantilever’s free end and the tip are closer to the sample surface than the cantilever’s fixed end, and so that the tip apex is the first point of contact between the sample and the cantilever when the two approach. For those applications that require tip oscillations substantially perpendicular to the X,Y plane, some tips are manufactured in a way that the apex of the tip is angled to compensates for the cantilever‘s tilt.

Generally, the amplitude of the oscillations at the free end of the cantilever are in the nanometers or tens of nanometers, and occasionally a few hundred nanometers.

There are at least three methods of operating an AFM in a Dynamic Vertical Mode. These are usually called Noncontact Mode AFM, Frequency Modulation AFM (FM-AFM), and Amplitude Modulation AFM (AMAFM). The latter is also referred to as Slope Detection Mode AFM. Amplitude Modulation is an unfortunate choice of name, because unlike in FM-AFM, where the instrument modulates the frequency of cantilever oscillations, in AM-AFM, the instrument does not modulate the amplitude. Similarly, Slope Detection is poor nomenclature because the quantity that is detected (and controlled) in this mode is simply the amplitude of the cantilever oscillations, not the cantilever oscillations, not the rate of change of one thing with another as the word slope would suggest. But this nomenclature is now widely in use. Here, we use “Intermittent Contact Mode AFM” instead, which is also widely used and adopted in the literature, but which is mode accurately descriptive. Frequency Modulation and Noncontact Mode AFM are described elsewhere in detail, and we do not cover in this tutorial.4


4 For an excellent review, see “Dynamic atomic force microscopy“ by R. Garcia and R. Perez, in Surface Science Reports, 2002, 47.

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