|Above: A probe used for atomic
Below: How a probe tip scans over a sample (not to scale).
Scanning probe microscopy covers several related technologies for imaging and measuring surfaces on a fine scale, down to the level of molecules and groups of atoms.
At the other end of the scale, a scan may cover a distance of over 100 micrometers in the x and y directions and 4 micrometers in the z direction. This is an enormous range. It can truly be said that the development of this technology is a major achievement, for it is having profound effects on many areas of science and engineering.
SPM technologies share the concept of scanning an extremely sharp tip (3-50 nm radius of curvature) across the object surface. The tip is mounted on a flexible cantilever, allowing the tip to follow the surface profile (see Figure).When the tip moves in proximity to the investigated object, forces of interaction between the tip and the surface influence the movement of the cantilever. These movements are detected by selective sensors. Various interactions can be studied depending on the mechanics of the probe
The three most common scanning probe techniques are:
Atomic Force Microscopy (AFM) measures the interaction force between the tip and surface. The tip may be dragged across the surface, or may vibrate as it moves. The interaction force will depend on the nature of the sample, the probe tip and the distance between them.
Scanning Tunneling Microscopy (STM) measures a weak electrical current flowing between tip and sample as they are held a very distance apart.
Near-Field Scanning Optical Microscopy (NSOM) scans a very small light source very close to the sample. Detection of this light energy forms the image. NSOM can provide resolution below that of the conventional light microscope.
There are numerous variations on these techniques.
AFM may operate in several modes which differ
according to the force between the tip and surface:
|Mode of Operation||Force of Interaction|
|contact mode||strong (repulsive) - constant force or constant distance|
|non-contact mode||weak (attractive) - vibrating probe|
|intermittent contact mode||strong (repulsive) - vibrating probe|
|lateral force mode||frictional forces exert a torque on the scanning cantilever|
|magnetic force||the magnetic field of the surface is imaged|
|thermal scanning||the distribution of thermal conductivity is imaged|
In contact mode, the tip is usually maintained at a constant force by moving the cantilever up and down as it scans. In non-contact mode or intermittent contact mode (tapping modeTM) the tip is driven up and down by an oscillator. Especially soft materials may be imaged by a magnetically-driven cantilever (MAC ModeTM). In non-contact mode, the bottom-most point of each probe cycle is in the attractive region of the force-distance curve. In intermittent contact mode the bottom-most point is in the repulsive region. Variations in the measured oscillation amplitude and phase in relation to the driver frequency are indicators of the surface-probe interaction.
To image frictional force, the probe is dragged along the surface, resulting in a torque on the cantilever. To image the magnetic field of the surface, a magnetically-susceptible probe is used. In other variations, the electric charge distribution on the surface or the surface capacitance is imaged. For thermal scanning microscopy (TSM) the thermal conductivity of the surface with is probed with a resistive tip that acts as a tiny resistance thermometer.
In addition to these modes, many instruments are also designed to plot the phase difference between the measured modes, for example frictional force versus contact profile. This plot is called phase mode.
Several types of probes with different tips are used in scanning probe
microscopy. Tip selection
depends on the mode of operation and on the type of sample.
|Inorganic and Synthetic Materials|
|Natural surface topography||Buckyballs and Nanotubes|
|Surface Chemistry||Surfaces of Polymers|
|Silicon wafers||Diffraction gratings|
|Data storage media||Integrated circuits|
|Polymers and Polymer Matrix||Biological Structures|
|Natural resins and gums||Bacterial flagellae|
|Plant cell walls||Cell and membrane surfaces|
Vibrating mode or intermittent contact modes are particularly suited for
imaging soft biological specimen. However, biological samples are successfully
imaged in the "harder" contact mode. Unfixed soft specimens are deformed
in the z-dimension to a degree dependent on the imposed probe force, although
spreading in the x-y plane may not be significant. Biological samples may
be hardened to reduce probe-induced deformation by aldehyde fixation or
frozen in a cryo-AFM. See the References for further information on technique
and sample preparation.
Accurately nanofabricated gratings are the basis for two and three-dimensional calibrations. Such calibration gratings and calibration software are commercially available.
Probe-Related Image Distortions. At very high magnifications and high-relief sample surfaces, the mode of imaging and the geometry of the probe tip can influence the scanned image. Knowledge of the probe geometry then becomes important for interpretation of the image.
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Revised: June 13, 2003
Copyright © John W. Cross