[ Applications ]
[ Choice of Technique ]
[ Accuracy and Calibration ]
[ Instrument Makers ]
[ Footnotes, References and Links ]
[ Books about SPM ]
[ Graphic equivalent ]
These 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.
As the tip passes over bumps in the surface, the vertical deflection of the cantilever (delta-z) is measured.
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 or are detected by selective sensors. Various interactions can be studied depending on the probe sensors used.
The three most common types of 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.
Scanning Tunneling Microscopy (STM) measures a weak electrical current flowing between tip and sample.
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:
Force of Interaction
To image frictional force, the probe is dragged along the surface,
in a torque on the cantilever. To image the magnetic field of the
surface, a magnetically-susceptible probe is used. In other
the electric charge distribution on the surface or the surface
For thermal scanning microscopy (TSM) the thermal conductivity of the surface with is probed with a bimetallic tip out of contact with the sample surface.
Several types of probes with different tips are used in scanning
microscopy. Tip selection depends
the mode of operation and on the type of sample.
Vibrating mode or intermittent contact modes are particularly suited
for imaging soft biological specimen. However, biological samples are
imaged in the "harder" contact mode. Unfixed soft specimens are
in the z-dimension to a degree dependent on the imposed probe force,
spreading in the x-y plane may not be significant. Biological samples
be hardened to reduce probe-induced deformation by aldehyde fixation or
frozen in a cryo-AFM. See the Footnotes
for further information on technique and sample
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.
Many of these firms offer general-purpose instruments that can handle a variety of imaging modes, while others are more specialized. A number of physical scientists have built their own SPM instruments. The Homebrew STM Page provides a resource for those who wish to construct their own STM.
Scanning. The probe (or the
sample under a stationary probe) generally is moved by a
piezoelectric tube. Such scanners are designed to be moved precisely in any of the three
perpendicular axes (x,y,z). By following a raster pattern, the sensor data forms an image of the
probe-surface interaction. Feedback from the sensor is used to maintain the probe at a constant
force or distance from the object surface. For atomic force microscopy the sensor is a
position-sensitive photodetector that records the angle of reflection from a laser bean focused on the
top of the cantilever.
AFM systems detect the z-displacement of the cantilever by the reflection of a laser beam focused on the top surface of the cantilever. The feedback from this sensor maintains the probe at a constant force.
STM systems measure the quantum tunnelling current between a wire or metal-coated silicon tip and the object surface. An electronic feedback system maintains a constant current by positioning the tip to exactly contact the surface.
NSOM systems scan an optical fiber probe over the sample. The probe has an opaque material covering its surface, except for a small aperture at the tip. The light (usually a laser source is used) is emitted through this aperture. Image data can be gathered in transmission, reflection or fluorescence mode. The transmission mode provides a higher signal throughput. It can be used with specimens that are transparent and have low or moderate light absorption, particularly biological subjects. Reflection mode is for highly scattering and opaque samples. The resolution of optical microscopes has been limited by the wavelength of light, in practice about 400 - 500 nm. By placing a point source of light less than that distance from the sample, NSOM improves this resolution by an order of magnitude. An NSOM is available from ThermoMicroscopes.
Modes of operation. The manufacturers' web sites have references and application notes that are useful in understanding the advantages and disadvantages of the various modes. The on-line guides on the Digital Instruments website is a good examples. The magnetically-driven cantilever system (MAC ModeTM) is specific to Molecular Imaging.
Tip Selection. AFM tips are generally made of silicon or silicon nitride. For most applications, pyramidal silicon nitride tips are used. They are relatively durable and present a hydrophobic surface to the sample. Conical silicon tips are often used for bio-molecular applications because they are very sharp and present a hydrophilic surface. However, they are relatively less durable. For the ultimate sharpness, tips of carbon nanotubes have been made. The Rice group also has a tutorial for mounting carbon nanotube tips on commercial cantilevers. In other cases selective modification of silicon nitride tips has been used to provide for measurement of specific molecular interactions. STM tips are made of mechanically-formed or electrochemically-etched wire, usually noble metals or tungsten. Digital Instruments has a useful Tip Selection Guide. Some very high aspect-ratio tips are available from ThermoMicroscopes. Tips are available from many suppliers.
Effect of Instrumental Uncertainties on AFM Indentation Measurements.(1998)
Mark VanLandingham, Materials Science Program and Center for
University of Delaware.
Mervyn Miles recently published a useful overview of AFM technology and applications, Scanning Probe Microscopy: Probing the Future (1997, Science 277(5333):1845). The article is available on-line to Science subscribers.
Sean Morgan's Scanning Probe Microscopy Page offers many links to galleries of micrographs and the home pages of SPM/STM users.
Zhifeng Shao (University of Virginia) has developed a cryo-AFM for applications in structural biology. His work on GroES, a protein chaperon, is particularly interesting.
Bibliography of Biological SPM Research (pdf file).
Syllabus, atomic force microscopy to undergraduates by
Sample preparation is of great importance in SPM as in other areas of microscopy. SPM analysis of biological macromolecules places particularly high demands on the quality of the substrate. Freshly-cleaved mica surfaces has been particularly useful. Mica presents a charged, hydrophilic surface to which proteins and other biomolecules readily bind. Moreover, mica surfaces are nearly flat on an atomic scale and are quite clean when fresh, conditions that are ideal for scanning at high resolution. For certain applications, covalent attachment to the surface is be required. A particularly useful approach has been the preparation of gold surfaces coated with protein-reactive monolayers. Several investigators have used monolayers composed of alkanethiols and dithioalkanes. Peter Wagner (Department of Biochemistry, Stanford University) has developed a variation using N-hydroxysuccinimide ester functionalized monolayers on a gold surface.
Digital Instruments sponsors an e-mail discussion group, "We sponsor an Internet SPM Mailing List, or "Digest," a forum for interchanging SPM-related technical information. Anyone actively involved in SPM usage is invited to join the Digest."
So.. ..who's interested in SPM? Scientists world-wide are investigating this new technology. Find out who's reading this page.
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Revised: December 5, 2003
Copyright © John W. Cross