【讨论】关于非接触模式和敲击模式AFM的比较讨论

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Noncontact AFM mode
Surfaces of fragile samples can be easily destroyed when scanning in contact mode due to strong tip-sample interactions. Non-contact mode is free from this drawback. In this mode, the probe operates in attractive force region and tip-sample interaction is minimized. In Fig. 1, you can see image of microdrops of a liquid n-alkane on the silinized silicon substrate. The use of non-contact mode allowed scanning without influencing the shape of the microdrops by the tip-sample forces. In addition, this mode is the only AFM-mode that enables true atomic resolution (see fig. 2).

The tip-sample interaction in non-contact mode is much weaker, than one in contact mode.To enhance the sensitivity to attractive forces, modulation technique is used in this mode. In this technique, scanning tip is oscillated close to the sample at a frequency near to the resonant frequency of the cantilever. The tip-sample interaction forces change in both the effective resonant frequency and the amplitude of oscillations. During the scanning, the feedback loop keeps constant tip-sample distance by maintaining the resonant frequency of oscillation (frequency-modulation mode) or the amplitude of oscillation (amplitude-modulation mode).

To improve the resolution in Non-contact mode, one needs to enhance the sensitivity to short-range tip-sample forces. For the reason, small amplitudes of oscillation and small tip-sample distance are commonly used. But operation at distances near to the sample demands fine tip-force control and the absence of surface contamination. That is why this technique is mostly used in ultrahigh vacuum.

In most cases, the cantilever of choice for this mode is the one having high spring constant of 20-100 N/m so that it does not stick to the sample surface at small amplitudes. Between two cantilevers having the same spring constant, it is better to use the one with the higher resonant frequency. This lever is generally faster and less noisy. Low-frequency cantilevers are used with AFM systems that do not support probes with short lever arms.