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Atomic Force Microscopy
Introduction
The atomic force microscope (AFM) or scanning force microscope (SFM) was invented in 1986 by Binnig, Quate and Gerber. Like all other scanning probe microscopes, the AFM utilizes a sharp probe moving over the surface of a sample in a raster scan. In the case of the AFM, the probe is a tip on the end of a cantilever which bends in response to the force between the tip and the sample.
The first AFM used a scanning tunneling microscope at the end of the cantilever to detect the bending of the lever, but now most AFMs employ an optical lever technique.
The diagram illustrates how this works; as the cantilever flexes, the light from the laser is reflected onto the split photo-diode. By measuring the difference signal (A-B), changes in the bending of the cantilever can be measured.
Since the Cantilever obeys Hooke's Law for small displacements, the interaction force between the tip and the sample can be found. The movement of the tip or sample is performed by an extremely precise positioning device made from piezo-electric ceramics, most often in the form of a tube scanner. The scanner is capable of sub-angstrom resolution in x-, y- and z-directions. The z-axis is conventionally perpendicular to the sample.
Feedback operation
The AFM can be operated in two principal modes
- with feedback control
- without feedback control
If the electronic feedback is switched on, then the positioning piezo which is moving the sample (or tip) up and down can respond to any changes in force which are detected, and alter the tip-sample separation to restore the force to a pre-determined value. This mode of operation is known as constant force , and usually enables a fairly faithful topographical image to be obtained (hence the alternative name, height mode ).
If the feedback electronics are switched off, then the microscope is said to be operating in constant height or deflection mode. This is particularly useful for imaging very flat samples at high resolution. Often it is best to have a small amount of feedback-loop gain,
to avoid problems with thermal drift or the possibility of a rough sample damaging the tip and/or cantilever. Strictly, this should then be called error signal mode. The error signal mode may also be displayed whilst feedback is switched on; this image will remove slow variations in topography but highlight the edges of features.
In principle, AFM resembles the record player as well as the stylus profilometer. However, AFM incorporates a number of refinements that enable it to achieve atomic- scale resolution:
- Sensitive detection
- Flexible cantilevers
- Sharp tips
- High-resolution tip-sample positioning
- Force feedback
Basic Set-Up of an AFM
In principle the AFM resembles a record player and a stylus profilometer. The ability of an AFM to achieve near atomic scale resolution depends on the three essential components: (1) a cantilever with a sharp tip, (2) a scanner that controls the x-y-z position, and (3) the feedback control and loop.
- Cantilever with sharp tip: The stiffness of the cantilever needs to be less effective spring constant holding atoms together, which is on the order of 1-10 nN/m. The tip should have a radius of curvature less than 20-50 nm (smaller is better) a cone angle between 10-20 degrees.
- Scanner: The movement of the tip or sample in the x,y, and z-directions is controlled by a piezo-tube scanner, similar to those used in Scanning tunneling microscope (STM). For typical AFM scanners, the maximum ranges are 90 x 90 Ī¼m in the x-y plane and 5 Ī¼m for the z ādirection.
- Feedback control: The forces that are exerted between the tip and sample are measured by the amount of bending (or deflection) of the cantilever. By calculating the difference signal in the photodiode quadrants, the amount of deflection can be correlated with a height. Because the cantilever obeys Hookās law for small displacements, the interaction force between the tip and the sample can be determined. The AFM can be operated with or without feedback control. If the electronic feedback is on, as the tip is raster-scanned across the surface, the piezo will adjust the tip-sample separation so that a constant deflection is maintained- or so the force is the same as its fairly faithful topographical (hence the alternative name, height mode). If the feedback is switched off, then the microscope is said to be operating in constant height or deflection mode. This is particularly useful for imaging very flat samples at high resolution.
Tip-sample interaction
Tapping mode is the next most common mode used in AFM. When operated in air or other gases, the cantilever is oscillated at its resonant frequency (often hundreds of kilohertz) and positioned above the surface so that it only taps the surface for a very small fraction of its oscillation period. This is still contact with the sample in the sense defined earlier, but the very short time over which this contact occurs means that lateral forces are dramatically reduced as the tip scans over the surface. When imaging poorly immobilised or soft samples, tapping mode may be a far better choice than contact mode for imaging.
Other (more interesting) methods of obtaining image contrast are also possible with tapping mode. In constant force mode, the feedback loop adjusts so that the amplitude of the cantilever oscillation remains (nearly) constant. An image can be formed from this amplitude signal, as there will be small variations in this oscillation amplitude due to the control electronics not responding instantaneously to changes on the specimen surface. More recently, there has been much interest in phase imaging. This works by measuring the phase difference between the oscillations of the cantilever driving piezo and the detected oscillations. It is thought that image contrast is derived from image properties such as stiffness and viscoelasticiy. Thus tapping mode measures:
- Phase lag measures composition, adhesion, friction and viscoelastic properties
- Identify two-phase structure of polymer blends
- Less damaging to soft samples than lateral force microscopy
- Identify surface contaminants that are not seen in height images
Non-contact operation is another method which may be employed when imaging by AFM. The cantilever must be oscillated above the surface of the sample at such a distance that we are no longer in the repulsive regime of the inter-molecular force curve. This is a very difficult mode to operate in ambient conditions with the AFM. The thin layer of water contamination which exists on the surface on the sample will invariably form a small capillary bridge between the tip and the sample and cause the tip to "jump- to-contact". Even under liquids and in vacuum, jump-to-contact is extremely likely, and imaging is most probably occurring using tapping mode.
In addition to its superior resolution, the AFM has these key advantages:
- Compared with Scanning Electron Microscopes (SEM), the AFM provides extraordinary topographic contrast, direct height measurements and unobscured views of surface features (no coating is necessary).
- Compared with Transmission Electron Microscopes, 3-dimensional AFM images are obtained without expensive sample preparation and yield far more complete information than the 2-dimensional profiles available from cross-sectioned samples.
ZnS Nanoparticles
- Compared with Optical Interferometric Microscopes (Optical Profilers), the AFM provides unambiguous measurement of step heights, independent of reflectivity differences between materials.
Image display
Height image data obtained by the AFM is three-dimensional. The usual method for displaying the data is to use a color mapping for height, for example black for low features and white for high features. A popular choice of color scheme is shown on the left.
Similar color mappings can be used for non-topographical information such as phase or potential.
Tip effects
One of the most important factors influencing the resolution which may be achieved with an AFM is the sharpness of the scanning tip. The first tips used by the inventors of the AFM were made by gluing diamond onto pieces of aluminum foil. Commercially fabricated probes are now universally used. The best tips may have a radius of curvature of only around 5nm. The need for sharp tips is normally explained in terms of tip convolution. This term is often used (slightly incorrectly) to group together any influence which the tip has on the image. The main influences are
- broadening
- compression
- interaction forces
- aspect ratio
Tip broadening arises when the radius of curvature of the tip is comparable with, or greater than, the size of the feature trying to be imaged. The diagram illustrates this problem; as the tip scans over the specimen, the sides of the tip make contact before the apex, and the microscope begins to respond to the feature. This is what we may call tip convolution.
Compression occurs when the tip is over the feature
Applications
These techniques have the ability to operate on a scale from microns down to nanometers and can image clusters of individual atoms and molecules. STM relies on the electrical conductivity of the sample, so at least some features on the sample surface must be electrically conductive to some degree. AFM is used for studies of non-conductors and is the technique more commonly used for studies of macromolecules and biological specimens. AFM has been used for measurements on a wide variety of sample types, including:
Inorganic and Synthetic Materials Surfaces Nanostructures
Natural surface topography Buckyballs and Nanotubes Surface Chemistry Surfaces of Polymers Silicon wafers Diffraction gratings Data storage media Integrated circuits
Ceramics Carbon Nanotubes Biological Materials
Polymers and Polymer Matrix
Biological Structures
Natural resins and gums Bacterial flagellae
Muscle proteins Amyloid-beta DNA Chromosomes Plant cell walls Cell and membrane surfaces
Choice of Technique
The analytic mode is chosen based on the surface characteristics of interest and on the hardness or stickiness of the sample. Contact mode is most useful for hard surfaces. However, a tip in contact with a surface is subject to contamination from removable material on the surface. Excessive force in contact mode can also damage the surface or erode the sharpness of the probe tip. Near-contact mode has fewer tendencies to deform a soft surface, but is more sensitive to environmental vibrations and to variations in the film of moisture that coats samples in a normal atmosphere. Moisture or other thin liquid films expert an attractive capillary force on the probe as it is withdrawn from the surface. In non-contact mode this force becomes significant. Vibrating mode or intermittent
contact modes are particularly suited for imaging soft biological specimen and carbon nanotubes.
Laboratory Procedures
You have the opportunity to use state-of-art AFM instrument from Digital Instruments to characterize your samples. You will use (tapping and contact mode to image small gratings. Samples can be mounted to the magnetic sample holder (puck) using double sided tape. The samples are loaded on the J-scanner of a Multimode AFM, whose maximum scan size is 90 x 90 Ī¼m 2 , and whose vertical range is 5 Ī¼m. You will practice mounting tips into the tip holder using tweezers. Once the sample is loaded and the tip is placed above the sample, you need to align the laser beam onto the cantilever. You can get a rough idea where the beam is using the optical microscope situated above the AFM. Once the signal is maximized, you can bring the tip close to the surface and start the engage process. Specific operational details can be found in the training manual designed by Dr. Gajendra Shekhawat at www.nuance.northwestern.edu/nifit/manuals/htm.
Questions for laboratory write-up
- What was the resolution of the probe tip you were using?
- What types of tips did you use for these experiments?
- What were the spring constants and resonant frequencies of the tips?
- How small (laterally and vertically) were the sizes of the gratings in tapping mode? In contact mode? Is any imaging mode preferred for these types of samples?
- Comment on the difference between the topographic and deflection/amplitude images of the patterned gratings. How useful was the deflection imaging mode?
