ATOMIC FORCE MICROSCOPY

1). Background.

The Atomic Force Microscope is a scanning probe microscope where an image is collected based on the interaction between the sample and a point source, in this case the tip.

In AFM, a tip (most commonly made of silicon or silicon nitride (Si₃N₄) approaches the sample to within interatomic distances (approximately 10 Å). The tip (3-15 microns in length) is mounted at the end of a spring cantilever (approximately 100-500 microns in length).

The spring constant (between 0.10-0.34 N/m for Contact-Mode Silicon tips) of such a tip is less than the equivalent spring between atoms of the sample’s. As the tip is rastered across the sample, it fluctuates with respect to the surface features of the sample.

These fluctuations are caused by interactions (electrostatic, magnetic, capillary, Van der Waals) between the tip and the sample. By measuring the displacement of the tip, a topographical image can be generated.

2). Image Creation.

The AFM probe actually never moves; rather, it is the sample that is moved in the X, Y and Z, direction by a PZT (piezoelectric) actuator that defines the scan area. A laser beam is bounced off a prism and focused onto the end of the cantilever, slightly behind the tip, where it is reflected back to a 4-quadrant photo diode detector. The cantilever deflects as it responds to atomic force variations between itself and the sample and the detector measures the deflection.
 

The reflected beam is focused such that difference signal between the four sections of the diode is zero. When the cantilever deflects up, more light illuminates the upper half, creating a more positive signal; the converse is true when the cantilever deflects downward.

Because of the detector design, the instrument is sensitive to ambient light and electrical signals, which cause noise in the measurement. Thus, always be sure to cover the instrument with the protective magnetic shield. Additionally, extraneous vibrations from walking and talking may disrupt the integrity of the data collection process. The AFM is stabilized on a vibration isolation table to reduce the problem of vibrations due to movement. Acoustical vibrations are a common problem with no real systematic cure.

3). The Force-Distance Curve

As the atoms of the tip and sample are brought together they initially weakly attract each other. This attractive force increases until the interacting atoms are so close that their electron clouds begin to repel one another electrostatically. This electrostatic repulsion continues to weaken the attractive force as the separation distance decreases. The attractive force goes to zero (i.e., approaches the limit of zero) when the distance comes within the length of a chemical bond (a few Angstroms). Once the total Van der Waals forces (repulsive forces) are positive, the atoms are in “contact”.

Notice that the slope of the Van der Waals forces curve is quite steep in the contact region. As a result, the van der Waals forces dominate almost any other force that attempts to push the atoms closer together. Thus, in AFM, the cantilever bends when it pushes the tip against the sample’s surface, rather than probing it further into the sample

4). Scan Types.

Contact Mode – tip makes soft physical contact with the sample’s surface; heavily influenced by frictional and adhesive forces that can cause damage and distort image data; soft samples are often damaged by this mode

 Non-contact Mode –low resolution (more susceptible to noise), contaminants may interfere with oscillation

 Lateral Force – records measurements of friction; the degree of torsion of the cantilever is proportional to the surface friction caused by the lateral force exerted on the probe; measured by the left and right deflection of the laser

 Force Modulation – records measurements of elasticity or viscosity, particularly useful for distinguishing otherwise smooth surfaces

 Tapping Mode – eliminates frictional forces by intermittently contacting the surface and oscillating with sufficient amplitude to prevent it from being trapped in by adhesive forces; less destructive than contact mode; good compromise between contact and non-contact modes

Lift Mode – two-pass technique that measures magnetic and electric forces above the sample surface. On the first pass of the scan the sample’s surface topography is recorded; on the second pass, the tip scans some set distance above the surface and records the force measurements.

1). Open the Advanced True Image Software on your computer’s desktop.

2). Insert the probe mount into the probe holder.

•  Probes are extremely fragile – use extreme caution when handling the probe and the probe mount!
•  Before inserting the probe, use the COARSE RETRACT button on the approach controls to lower the scanning module so the probe mount may be inserted without contacting the sample stage.
•  Insert the probe mount into the probe mount holder using the exchange tool until it snaps. Make sure the probe mount is pushed all the way until it stops and snaps into place properly!
• Unscrew and remove the exchange tool.

3). Align the laser beam on the probe.

•  Focus the optical microscope on the end of the probe. Use lamp to illuminate the sample compartment.
•  Use the X and Y probe alignment knobs, located on the right-side panel of the AFM unit and the far right side of the back panel, respectively, to align laser  slightly back from the very end of the probe (the tip is located here)

4). Prepare a sample.

• See lab experimental for this procedure.

5). Insert the sample.

•  Use the COARSE RETRACT button on the electronic controller to lower the scanning module so the sample can be inserted without damaging the probe.
•  Use the tweezers with curved ends to place it in the sample mount holder.
•  Push sample into place until the spring loaded ball plunger in the middle of the mount clicks.
•  If you are inserting a calibration sample, be sure to insert the sample mount such that the metal lines parallel to the Y-axis.
•  Select REFERENCE FORCE on the SPM MONITOR. Using the REFERENCE FORCE AMPLITUDE/CURRENT knob, adjust the REFERENCE FORCE to about 5.0 V (± 0.1 V).

6). Sample Positioning.

 CAUTION: The tips can crash very easily! Always watch how far you are moving the sample to prevent a broken or damaged tip.
• Use the COARSE APPROACH button on the rear panel of the instrument to bring the sample close to the tip.
•  Try to bring the tip to within one millimeter of the sample surface. The tip will be close to, but NOT touching, the sample.
•  Place the cover over the front opening.

7). Detector Alignment.

•  Begin by checking the probe alignment to ensure that the laser is still properly situated on the probe.
•  Be sure that the FEEDBACK ACTIVE light is off! If it is on, press the COARSE RETRACT button momentarily. If the light remains on, repeat this process.
•  Select the ACTUAL FORCE AMPLITUDE/CURRENT setting on the SPM MONITOR.
•  Select the Detector Alignment option from the Calibration menu in the Advanced True Imaging software window.
•  A target (the detector) and a cross hair (the laser) will appear on the screen. Using the fine X and Y detector alignment knobs, located on the left side panel and rear panel of the instrument, respectively, bring the cross hair to as close to the center of the target as possible.

•  Monitor the ACTUAL FORCE AMPLITUDE/CURRENT display while moving the cross hair. It must be adjusted as close to 0.0 V as possible (± 1.0 V).
•  Once properly aligned, click the Exit button in the Detector Alignment window.
•  If the FEEDBACK ACTIVE light is still on, press the COARSE RETRACT button monetarily to reset it.

8). Auto Approach.

•  Press the AUTO APPROACH momentarily; the button should illuminate. The scanning module begins to approach the sample until the preset reference force/amplitude is achieved. The auto approach stops and the feedback activates. When the FEEDBACK ACTIVE light is on the approach is completed.
•  Monitor the ACTUAL FORCE AMPLITUDE/CURRENT reading. It should be close to the reference force (5.0 ± 0.5 V).
•  If the ACTUAL FORCE AMPLITUDE/CURRENT is not within this range, retract the probe and re-verify the alignment of the laser and the detector.

9). Image Acquisition.

• Select Single from the Scan menu in the Advanced True Image Software window.
•  The image collected is user defined between X  greater than 291,432 Å and Y greater than 331,174 Å.

Atomic Force Microscopy is considered a wonderful technological advance because it has opened up the window to imaging non-conducting materials. The Atomic Force Microscope has a wide range of applications, from contact lens production to semiconductors to polymers, mostly due to its ability to image at the sub micron level (<10^-6m). A collection of images ranging from biochemical substances to actual atoms can be found at: http://www.di.com/theater/ut_surf.html. Similarly, http://www.thermomicro.com/apps/index.html categorizes a myriad of excellent AFM images by field.
 
 
 

Pre-Lab

- Experiment 2

- Experiment 3
 

Lab Report