<P> In contact mode, the tip is "dragged" across the surface of the sample and the contours of the surface are measured either using the deflection of the cantilever directly or, more commonly, using the feedback signal required to keep the cantilever at a constant position . Because the measurement of a static signal is prone to noise and drift, low stiffness cantilevers (i.e. cantilevers with a low spring constant, k) are used to achieve a large enough deflection signal while keeping the interaction force low . Close to the surface of the sample, attractive forces can be quite strong, causing the tip to "snap - in" to the surface . Thus, contact mode AFM is almost always done at a depth where the overall force is repulsive, that is, in firm "contact" with the solid surface . </P> <P> In ambient conditions, most samples develop a liquid meniscus layer . Because of this, keeping the probe tip close enough to the sample for short - range forces to become detectable while preventing the tip from sticking to the surface presents a major problem for contact mode in ambient conditions . Dynamic contact mode (also called intermittent contact, AC mode or tapping mode) was developed to bypass this problem . Nowadays, tapping mode is the most frequently used AFM mode when operating in ambient conditions or in liquids . </P> <P> In tapping mode, the cantilever is driven to oscillate up and down at or near its resonance frequency . This oscillation is commonly achieved with a small piezo element in the cantilever holder, but other possibilities include an AC magnetic field (with magnetic cantilevers), piezoelectric cantilevers, or periodic heating with a modulated laser beam . The amplitude of this oscillation usually varies from several nm to 200 nm . In tapping mode, the frequency and amplitude of the driving signal are kept constant, leading to a constant amplitude of the cantilever oscillation as long as there is no drift or interaction with the surface . The interaction of forces acting on the cantilever when the tip comes close to the surface, Van der Waals forces, dipole - dipole interactions, electrostatic forces, etc. cause the amplitude of the cantilever's oscillation to change (usually decrease) as the tip gets closer to the sample . This amplitude is used as the parameter that goes into the electronic servo that controls the height of the cantilever above the sample . The servo adjusts the height to maintain a set cantilever oscillation amplitude as the cantilever is scanned over the sample . A tapping AFM image is therefore produced by imaging the force of the intermittent contacts of the tip with the sample surface . </P> <P> Although the peak forces applied during the contacting part of the oscillation can be much higher than typically used in contact mode, tapping mode generally lessens the damage done to the surface and the tip compared to the amount done in contact mode . This can be explained by the short duration of the applied force, and because the lateral forces between tip and sample are significantly lower in tapping mode over contact mode . Tapping mode imaging is gentle enough even for the visualization of supported lipid bilayers or adsorbed single polymer molecules (for instance, 0.4 nm thick chains of synthetic polyelectrolytes) under liquid medium . With proper scanning parameters, the conformation of single molecules can remain unchanged for hours, and even single molecular motors can be imaged while moving . </P>

Difference between tapping mode and non contact mode