HOW TO CHOOSE

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Soft or Fragile

Since its very discovery, AFM has held the great promise as a technique of choice for non-destructive surface measurements. To achieve this goal, one need to decrease to a minimum the tip-sample interaction force. The strong interaction force can bring about deformation and/or destruction of the surface of soft, fragile, liquid and weakly adhering samples.

Fig. 1. Images of liquid crystalline nanotexture of polypeptide with mini-dendritic groups. Images are obtained in tapping mode on Veeco Dimension 5000 AFM. Images are courtesy of Dr. S. Magonov (Veeco). Sample courtesy of Prof. V. Percec, (University of Pennsylvania, Phyladelphia, USA).

Soft and fragile samples can be imaged in Noncontact and Tapping modes only. Contact mode AFM is characterized by the presence of uncontrollable shear forces able to modify the morphological features.

To avoid sample deformation due to mechanical contact with the AFM tip in Tapping mode, the tip-sample interaction force must be decreased to a minimum by using sharp tips, soft cantilevers, smaller amplitudes, and higher set-point ratios. High-resolution images of soft samples can be obtained by regular General Purpose probes; however, a better performance is achieved using High Resolution probes. 

Tapping mode

Hi'Res-C probes (Rtip < 1nm) having soft cantilevers with a spring constant of 5 N/m or lower are recommended for scanning delicate samples in Tapping mode. "Light" tapping conditions including small free cantilever amplitudes (0.1 - 0.2V) and high setpoint ratios (0.8 - 0.95) provide stable imaging. Under light tapping conditions, Hi'Res-C probes do not break and remain clean during long-term measurements.

However, extra attention should be paid to engagement of Hi'Res-C probes. Scan rate should start at below 1 Hz. Scan size should start out at 50 nm x 50 nm.

Noncontact mode

Non-destructive surface imaging is also attainable in non-contact mode under ultra high vacuum conditions using regular silicon or Hi'Res-C probes. Cantilevers of the 15 Series having high spring constant of  20 - 100 N/m and high resonance frequency over 300 kHz are usually used in this mode.

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