Oscillatory AFM techniques were developed soon after the introduction of contact mode. Imaging of soft, delicate, and weakly adhering samples benefited from this new technique, as contact mode can deform, damage or move these types of samples. This technique is now used for a variety of applications, including the imaging of weakly-adsorbed single molecules like DNA and soft samples like some polymer materials and colloidal particles.

In Fig. 1, you can see an image of microdrops of a liquid n-alkane on a silanized silicon substrate. The use of non-contact mode allowed scanning without influencing the shape of the microdrops by tip-sample forces.

Noncontact mode

In noncontact mode, the long-distance van der Waals forces are sensed by an oscillating probe, which is excited at its resonance frequency by a piezoelectric transducer and brought in close proximity to a sample. The probe oscillation amplitudes are typically in the range of 1 nm or less. The resolution in this mode is defined by the tip-sample separation of a few nanometers. For the scanning feedback one can use ether the vibration amplitude (amplitude modulation mode, AM) or the frequency shift (frequency modulation, FM) of the probe.

To improve resolution in noncontact mode, one needs to enhance sensitivity to short-range tip-sample forces. To do this, small amplitudes of oscillation and small tip-sample distances are commonly used. Advances in the development of the noncontact mode allow for the discovery of single defects in atomic lattices. An image of Si 7x7 surface obtained in noncontact mode is presented in Fig.2.

Operation at distances near 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 one with a high spring constant between 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.

Not surprisingly, imaging in noncontact mode is performed with Si probes usually applied for tapping mode operation. It is likely that in the near future, commercial AFM instruments will operate in all oscillatory modes and the development of fast, broadband controllers will lead to new multi frequency techniques. These techniques might require the use of NanoTwist probes.

Tapping mode

Tapping mode is a form of the AM technique, in which large vibrations of the probe are applied for imaging. It allows for measurements to be made at ambient conditions. When the oscillating probe hits the sample, its short-time interactions with minimal shear are less destructive than the tip-sample forces in contact mode.

For identifying a type of tip-sample force, e.g. attractive or repulsive, recording of the phase behavior of the AFM probe (for example, as phase shift dependence on tip-sample separation) can be used. Actually, at small amplitudes operation in tapping mode is characterized by attractive tip-sample forces and overlaps with operation in the earlier introduced noncontact mode. The latter mode is characterized by small probe oscillations and net attractive interactions which not necessary mean the true noncontact regime.

Stable oscillations of the tapping probe are possible when the probe has enough energy to overcome adhesive and capillary forces of the samples in air. Traditionally, Si probes with stiffness in the 30-50 N/m, such as DP15/GP, are used for tapping mode experiments. Depending on the type of the sample and problem to solve, probes with stiffness ranging from 0.3 N/m to 400 N/m may be required. Probes with low stiffness should be applied to soft materials or particles with low adhesion to surface. Stiff probes are used for imaging at the elevated forces needed for the visualization of mechanically-different components of multicomponent samples.

The stiffness of the probe is only one of the parameters that influence tip-sample forces in tapping mode. The other parameters are two amplitudes: the amplitude of the free oscillating probe and the set-point amplitude used for the feedback. The difference between the set-point and free amplitude determines the tip-sample force level, which controls the image contrast.

Between two cantilevers with a given spring constant, the one with the higher resonant frequency is preferable because it allows for a faster scanning rate. Cantilevers with a high resonant frequency of 300 kHz and above provide the fastest possible scanning rates. It should also be taken into account that polymer surfaces become stiffer at higher frequencies. This property further reduces the possibility of sample damage when using a high resonant frequency cantilever.

High Q factor is desirable in tapping mode to optimize the sensitivity. For a given resonant frequency and spring constant, a rectangular cantilever generally has the higher Q factor in comparison to its triangular analog.

Important: If your AFM study involves heating the probe, Si cantilevers without any coating are preferred. Coated cantilevers can bend and/or twist at elevated temperatures.

HI'RES tips provide extremely high resolution when used with light tapping on smooth surfaces. See the application note on high resolution imaging for details.

Tapping mode in liquids

If your particular sample seems to be having problems with adhesive capillary tip-sample forces, you can substantially reduce these forces by immersing the tip and the sample in liquid, which generally requires the use of a liquid cell accessory. Further reduction of the attractive forces is possible by operating in appropriate solvents, especially ethanol. The liquid cell is also used for in situ electrochemical and biological experiments.

Conventional liquid cells have a drive frequency of about 10 kHz. This will match the resonant frequency of cantilevers that vibrate in air at about 50-70 kHz. Typical values of the Q factor in liquids are 1-3 due to increased hydrodynamic damping. Rectangular cantilevers suffer less damping in liquids than triangular.

Important: Al reflective coating is stable in water and ethanol. However, certain experimental conditions may cause corrosion of the coating. For example, Al dissolves readily in caustic alkali solutions to give Al hydroxide. It's generally recommended to use uncoated cantilevers or cantilevers with Cr-Au backside coating in liquids.

Further reading

Lift mode