SPM Notes

 


Footnotes
 References
 Links

Footnotes

Inventors.  The first scanning probe microscope was the scanning tunneling microscope (STM) of Binnig and Rohrer (Binnig, G., Rohrer, H., et al., (1982) Phys. Rev. Lett., 49:57.).  Gerd Binnig and Heinrich Rohrer were awarded half of the 1986 Nobel Laureate in Physics   for their design of the scanning tunneling microscope.  Ivan Amato's 1997 article, "Atomic Imaging: Candid Cameras for the Nanoworld " (Science276(5321):1982-1985), entertainingly recounts the history of STM and AFM development.   The article is available on-line to Science subscribers.  A history of microscopy (Microscopy for Nanotechnologists by C. David  Eagle) provides perspective on the explosive development of this field.

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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.

The piezotube moves the specimen in a raster pattern at constant distance or force.
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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.

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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.  [ More detail ]

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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 .

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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 at the ThermoMicroscopes (also available in book form) and Digital Instruments websites are good examples.  The magnetically-driven cantilever system (MAC Mode TM ) is specific to Molecular Imaging .

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Interaction force. The z-axis (vertical) component of the force of interaction is calculated from the z-displacement of the cantilever and the spring constantof the cantilever. From Hooke's Law, F = - kz,where kis the spring constant. The spring constantfor a cantilever is provided by the cantilever supplier or can be determined by the investigator . A constant forceon the probe tip is maintained by feedback from measurement of the interaction force. The probe is moved up and down to maintain the measured constant force.

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Tapping modeTM, a trademark of Digital Instruments .  Tapping Mode imaging is implemented in ambient air by oscillating the cantilever assembly at or near the cantilever’s resonant frequency using a piezoelectric crystal..   To image in fluids, the entire fluid cell is oscillated to drive the cantilever into oscillation.

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SEM image of uncoated ULTRASHARP silicon cantileverTip 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 .     Tips are available from many suppliers.

Some very high aspect-ratio (ultrasharp) tips are available from ThermoMicroscopes and from MikroMasch (see image at right).

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The Effect of Instrumental Uncertainties on AFM Indentation Measurements.( 1998) Mark VanLandingham,  Materials Science Program and Center for Composite Materials,
University of Delaware.

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Home-brew Scanning Probe Microscopes

Several web sites offer information and/or references of use to those desiring to construct these instruments.  See for example, the site of the J.C. Davis group, Cornell University.  Construction of the tip is, perhaps, the most crucial aspect of the experiment.

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References

A Practical Guide to Scanning Probe Microscopy.  Howland, R. and Benatar, L. TM Microscopes, Sunnyvale , 2000, URL http://www.thermomicro.com/spmguide/contents.htm

Mervyn Miles published a useful overview of AFM technology and applications, Scanning Probe Microscopy:  Probing the Future (1997, Science277(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 recent scientific inquiries have included membrane assembled baterial toxin complexes, molecular motors (functional and structural), structure of chromatin, lamin B recceptor, analysis of model channels and nuclear structure.

For a bibliography of biological SPM research, Biological Bibliography from Digital Instruments. • Reviews • Nucleic Acids  • Chromatin / Chromosome Structure • Proteins • Crystals • Protein / DNA Interactions • Lipids • Membranes • Protein / Lipid Complexes • Cell and Virus Structure • Tissues • Carbohydrates  • Biomaterials and Polymers • Pharmaceutical Research • Nanoscale Forces  • Instrumentation and Techniques • Cantilevers and Tips • Relevant Books.

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. Another variation uses N-hydroxysuccinimide ester functionalized monolayers on a gold surface.  Digital Instruments has a large collection of application notes online.

Digital Instruments provides a variety of technical and scientific resources in their Library .  They also sponsor an  e-mail discussion group, The SPM Forum/Mailing List, "an open and unedited, but non-commercial, forum for discussing and exchanging technical information, views, issues, and applications of SPM." "Anyone actively involved in SPM usage is invited to join the Digest.[ follow link to subscribe ].

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Links

AFM Related Sitesfrom Dr. Peter Markiewicz, Surface Analysis Lab, Weizmann Institute of Science, Rehovot, Israel.  This site includes a page with comparative information on the unique file image formats and conversion formulae for equipment from many manufacturers.

Course Syllabus , atomic force microscopy to undergraduates by Nancy Burnham, Worcester Polytechnic Institute, Worcester, MA.

SPM-Related Software

SPM Research Group at the Department of Physics, University of Leuven, Belgium

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Revised:  October 31, 2005
Copyright © John W. Cross