In our continuing series on ways to use an AFM - let’s look at force measurements.  These measurements are important in life science, polymer structures, semiconductors, and composite materials.  Force measurements can be made in air or liquid and under controlled conditions like temperature and humidity. 

Force Modulation mode is a fast, very sensitive imaging method that is especially useful to measure and detect variations in a surface’s mechanical properties, including stiffness and elasticity. In this technique, a modulated driving signal at a constant frequency is applied to the AFM cantilever while the AFM tip is in contact with the sample, and the amplitude variation and phase lag during the scan are measured. Force modulation provides the user with simultaneous surface topography measurements, material elasticity or stiffness (the amplitude of the modulated signal), and energy dissipation characteristics of the sample (from the phase of the cantilever response). When an AFM cantilever is modulated with the driving signal, elastic materials will result in relatively larger modulated amplitude compared to stiffer materials because the AFM tip can indent an elastic material.

If you want more information on Agilent AFM’s go to our website at www.agilent.com/find/afm

Those unfamiliar with the flexibility of an AFM (atomic force microscope) don’t realize that one can oscillate the tip up and down.  Contact mode AFM often has a disadvantage for samples that are either weakly bound or soft because the tip can simply move or damage the surface feature and the resulting images are generally not high resolution. The advent of AC mode AFM, which operates in the intermittent contact regime or in the non-contact regime, provides a solution to this problem. The Agilent AFM’s have two oscillating modes - magnetic and acoustic.   

Magnetic AC Mode (MAC)

To achieve MAC Mode imaging, a cantilever coated with a magnetic material is driven into oscillation by an AC magnetic field generated by a solenoid positioned close to the cantilever housing. The result of MAC Mode™ is a gentle, clean cantilever response that has no spurious background signals (“forest of peaks”) like other AC modes can have.  Because the cantilever (and only the cantilever) is driven directly by the magnetic field, the need to shake the cantilever holder at large amplitudes is eliminated. Background resonance is absent, signal to noise is improved and setup becomes straightforward. Better signal-to-noise means that much smaller amplitudes can be used, This decreases damage to the sample and preserves asperities on the probe, contributing to greatly improved resolution. MAC Mode has even greater advantages when the cantilever is vibrated in liquid. 
 

Acoustic AC Mode (AAC)

In Acoustic AC Mode (AAC) the cantilever is excited by high frequency acoustic vibration from a piezoelectric transducer attached to the cantilever holder.  AAC mode can be classified into two categories, intermittent contact mode and non-contact mode, depending on the force regime and the tip-sample separation distance. The interaction between the tip and the sample is predominately vertical, thus negligible lateral forces are encountered. Consequently, AC mode AFM does not suffer from the tip or sample degradation effects that are sometimes observed after many scans in contact mode AFM, and it is a technique for imaging soft samples. In AC mode, tip-sample force interactions cause changes in amplitude, phase and the resonance frequency of the oscillating cantilever. The spatial variation of the change can be presented in height (topography) or interaction (amplitude or phase) images that can be collected simultaneously. The system monitors the resonant frequency or amplitude of the cantilever and keeps it constant by a feedback circuit that moves the scanner up and down. The motion of the scanner at each probe location is used to generate a topographic data set. The amplitude change at each probe location forms the amplitude image. The phase data is the result of the phase lag between the AC drive input and the cantilever oscillation output at each probe location. Consequently, contrast in phase images, which are due to differences in material properties, can provide very useful information. In addition, fine morphological features are easily observed in amplitude and phase images.

If you would like to learn more about the Agilent AFM’s - go to www.agilent.com/find/afm.

Atomic Force Microscopes (AFM) do more than just provide a topographical image of the surface.  So for the next few blogs I’m going to describe some of the “modes” of the AFM.  Let’s start with contact mode.

In contact mode AFM, interatomic van der Waals forces become repulsive as the AFM tip comes in close contact with the sample surface. The force exerted between the tip and the sample in contact mode is on the order of about 0.1-1000nN. Under ambient conditions two other forces besides van der Waals interactions, are also generally present. These forces include the capillary force from a thin layer of water in the atmosphere, and the mechanical force from the cantilever itself. The capillary force is due to the fact that water can wick its way around the tip, causing the AFM tip to stick to the sample surface. The magnitude of the capillary force should vary with the tip-sample distance. The mechanical force resulting from the cantilever is similar to the force of a compressed spring and its magnitude and sign (repulsive or attractive) is dependent on the cantilever deflection and the cantilever’s spring constant. Consequently, in contact mode AFM, the repulsive van der Waals forces arising for the AFM tip to sample interaction must balance the sum of the forces arising from the capillary force plus the mechanical force from the cantilever.

The thin layer of water present on many surfaces in air exerts an attractive capillary force and holds the tip in contact with the surface. Thus, when scanner pulls the tip away from the surface, the cantilever bends strongly towards the surface and the scanner has to retract further so that the tip can snap off of the surface. The cantilever returns to its original unbent status as the scanner moves the tip away from the surface beyond the snap-out point. In liquid, since the large capillary force is isotropic and the total force that the tip exerts on the sample can be reduced to some extent.

In order to improve imaging in air and liquid environments, Agilent offers Magnetic AC mode (MAC Mode). In MAC Mode, a cantilever that is coated with a magnetic material is driven into oscillation by an AC magnetic field that is generated by a solenoid positioned close by the cantilever. The result is a clean cantilever response that has no artifacts or background signals when the cantilever is vibrated in air or in liquid.

For more information on Agilent AFM’s - visit the www.agilent.com/find/afm