Scanning Probe Microscopy in Materials Science
The quest toward understanding the behavior of condensed matter has relied on measuring structure, bonding, and properties at increasingly local levels. This has driven advances in techniques that probe both soft and hard materials directly as well as indirectly. Many of these advances are described in other chapters of this volume. While structure and bonding-based probes have accessed molecular and atomic scales for decades, local determination of properties has been elusive. The emergence of scanning probes filled this gap to some extent. There are three major classes of scanning probe techniques that access electronic, magnetic, optical, and mechanical properties. Scanning tunneling microscopy (STM) was the first and is based on electrons tunneling between a metal tip and a sample. The distance sensitivity of tunneling imparts intrinsically high spatial resolution and the voltage dependence yields local density of states. It is, however, applicable only to conducting materials. Another class of techniques is based on local optical responses induced and/or detected with a very fine optical fiber. These techniques, e.g., near field optical microscopy, find extensive application in organic and biological systems. The focus of this chapter will be those probes that exploit the interactions of a small tip with a surface that are detected via the properties of a cantilever to which the tip is attached. The original cantilever probe, atomic force microscopy (AFM), is based on van der Waals interactions at the tip/surface junction. As a cantilever is mechanically oscillated near its resonant frequency (usually 10–500 kHz) and, at relatively small sample-tip separations (usually 0.5–100 nm), the van der Waals interaction causes a force that alters the oscillation. Cantilever motion is detected with laser reflection into a photodiode. If this measurement is made at every point as the tip is scanned across a surface, and a signal is used to maintain a profile at constant force, the topographic structure of the surface is mapped. It was almost immediately understood that other interactions can be detected with this scheme and, furthermore, that these interactions might be distinguished by their distance dependence. For example, at a 10-nm sample/tip separation the van der Waals interactions can be used to determine the topographic structure, while at 200 nm an electrostatic force (EFM) or magnetic force (MFM) would dominate the measurement.
KeywordsScan Probe Microscopy Atomic Resolution Physical Review Letter Apply Physic Letter Magnetic Force Microscopy
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