Interpretation of Phase Imaging

I’ve been using Atomic Force Spectroscopy for over half a year now. And among the past 6 months I spent most of my time with contact mode. But now I prefer tapping mode because contrast in phase images almost always amazes me.

Using tapping mode still allows me to get the image of “height”. Furthermore, I am able to process the images of “amplitude” and “phase”. While the cantilever oscillates, phase lags relative to the signal that drives the cantilever take place during the process of scanning (James et al). But there are a lot of factors that could engender the differentiation in phase imaging, which makes the interpretation of phase images very complicated. Properties of materials, including adhesion, friction and viscoelasticity play roles in it. Even the shape of the tip could have an effect on the contact area with the sample surface, which results in different phase lag in nanoscopic scale (Wozniak et al). And even if the tip does not come into contact with the surface, but instead stay at the distance where only attractive force is primary, complicated intermolecular forces could twist the amplitude and phase anytime.

During my scanning, I’ve noticed that the contrast in phase imaging could reverse when I scan the same area. This should be due to the setpoint setting (Chen et al). Setpoint is defined as the ratio between cantilever oscillation amplitude during scanning and the cantilever free oscillation amplitude. The setpoint being too low may result in the reversal contrast.

Apparently when the cantilever first starts approaching the surface, it gets into the noncontact regime where the damping of the oscillation is the consequence of attractive force. In this regime, the phase shift is lower than 90 degrees. When the cantilever continues to approach, it enters the intermittent- contact regime, where the repulsive force comes into appearance. In this regime, the phase shift is between 90 and 180 degrees (James et al).

Even though most of the time I have enough understanding what to expect in my samples and intuitive interpretation could work for the phase imaging, I will need to build up my experience and starts quantitatively analysis.

 

Reference

Chen X., Davies M.C., Roberts C.J., Tendlers S.J.B., Williams P.M., Davies J., Dawekes A.C., Edwards J.C. (1998). Ultramicroscopy,75 , 171-181.

James P. J., Antognozzi M., Tamayo J., McMaster T. J., Newton J. M.,and

Miles M. J. (2001). Interpretation of Contrast in Tapping Mode AFM and

Shear Force Microscopy. A Study of Nafion,Langmuir, 17, 349-360

Wozniak M. J., Ryszkowska J. , Szymborski T., Chen G., Tteishi T., Kurzydlowski K. J. (2008) Application of phase imaging and force modulation mode for description of dispersion of carbon nanotubes in polyol matrix. Materials Science-Poland, Vol. 26, No. 1