At Advanced Surface Microscopy, we are very enthusiastic about AFM and we like to say that any surface you can touch is a potential candidate for study. The following examples emphasize common and/or relatively inexpensive materials.
Contact or Tapping Mode AFM – Height Imaging
Here are some ideas that emphasize nm- to um-scale topography
- Aluminum foil – compare shiny and dull sides. What does this tell you about the process for making the foil?
- Salt and sugar crystals – look at facetting, defects, motion of atomic steps over time. Does humidity affect the rate of surface diffusion?
- Paper and paper coatings – comparing cheap photocopier paper with expensive “photo quality” paper for ink jet printers
- Polymer fibers: compare cotton and rayon. Both are made of cellulose, but the surface structure is very different. Reading – see Encyclopedia of Polymer Science and Engineering to learn what cellulose and rayon are. Which one can crystallize? Which one can only form amorphous structures?
- Hair: see the overlapping segments (like shingles on a roof). Compare the proximal (near scalp) and distal (far end) surfaces. Does hair wear out? Look for shampoo and/or dirt deposits.
- Magnetic hard drive. take apart a dead drive, removing the recording head and media:
- identify and scan the pole tip region of the recording head
- scan different regions of the disk surface. Why is the surface not perfectly smooth?
- CD-R (compact disc-recordable). Remove the coating from the label side. Image the dye-coated surface. Using alcohol or other suitable solvent, wash off the dye from a spot and scan the grooved surface. What is the function of this surface? Reading: Ken C. Pohlmann, The Compact Disc Handbook.
- “water spots”. Apply a droplet of a dilute solution, such as tap water, to a smooth surface, such as glass. Let dry. AFM images will often show rings of residual material. Why do
rings form when a droplet evaporates? Reading – research papers by Sidney Nagel and co-workers (Physics, U. Chicago.):
PHYSICS AT THE BREAKFAST TABLE
–Coffee Rings: Solute Deposits at the Contact Line of a Drying Sessile Drop
Tapping Mode Height and Phase Imaging
Tapping Mode height images often show higher resolution than contact mode, because the tip is sharper and lateral forces, which deform the surface, are practically absent. Phase imaging adds the new capability of mapping material domains.
An intriguing specimen for demonstrating phase contrast of material domains at high resolution (10 nm) is our Model PT-1.
Most of the materials listed for Contact Mode imaging also show phase contrast:
- Aluminum foil – apply a viscous oil and then wipe the surface, leaving only microdroplets behind. Scans near the microdroplets should contain some nanodroplets, which normally appear dark, indicating strong adhesion and low stiffness.
- “Water spots”
- Hair
- Hard disk drive: Different regions of the magnetic recording head have different phase contrast.
Additional suggestions:
- The Platinum-coated 10-um pitch 2-dimensional grid specimen commonly supplied with Digital Instruments AFMs up to about 2000 often becomes contaminated with nm-scale bumps. These show good phase contrast.
Magnetic Force Imaging
- Image the data and servo marks on magnetic recording tape or hard disks.
- Advanced: connect the appropriate leads of a magnetic recording head to a dc power supply. Image the magnetic field at the pole tips with and without applied current. Caution: use a resistor to limit the current to about 1-10 mA. Hint: the appropriate leads are the ones connected to the inductive write head. You may need a microscope to identify these. Older recording heads are easier to work with because everything is larger.
Electric Force Imaging
Metallic specimens with sharp edges will illustrate geometric variations in electric force gradients. Aluminum foil or our Model 750-HD can be used.
References
- “Atomic Force Microscopy”, Donald A. Chernoff and Sergei Magonov, chapter 19 in “Comprehensive Desk Reference of Polymer Characterization and Analysis”, R. Brady, ed., Oxford University Press, New York. 2003 This chapter describes AFM starting with the basics and continuing with various applications so that the reader can understand each technique, the information it provides, how to prepare the samples and capture the images, and how to interpret the data. Although the examples emphasize polymers, anyone examining materials with AFM can benefit. The discussion uses very little mathematics.
- ASTM E 2382–04 “Guide to Scanner and Tip Related Artifacts in Scanning Tunneling Microscopy and Atomic Force Microscopy”, under jurisdiction of ASTM Committee E42 on Surface Analysis and direct responsibility of Subcommittee E42.14 on STM/AFM. Published by ASTM International. September 2004. www.astm.org. This document helps users understand how to recognize many types image errors. Remember: all scientific instruments have artifacts. Recognizing the artifacts is a required skill for an advanced user.