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Biology LibreTexts

3.3.7: Scanned-Probe Microscopy

Scanned-probe microscopy uses a fine probe rather than a light-beam or electrons to scan the surface of a specimen and produce a 3D image.

LEARNING OBJECTIVES

Describe the different types of scanning probe techniques and their advantages over other types of microscopy

KEY TAKEAWAYS

Key Points

  • Scanned-probe microscopy has enabled researchers to create images of surfaces at the nanometer scale with a probe.
  • The probe has an extremely sharp tip that interacts with the surface of the specimen.
  • There are several variations of scanned-probe microscopy of which atomic force microscopy, scanning tunneling microscopy, and near-field scanning optical microscopy are most commonly used.

Key Terms

  • micrometer: An SI/MKS unit of measure, the length of one one-millionth of a meter. Symbols: µm, um, rm

3-D Images

Scanned-probe microscopy (SPM) produces highly magnified and three-dimensional-shaped images of specimens in real time. SPM employs a delicate probe to scan the surface of the specimen, eliminating the limitations that are found in electron and light microscopy. SPM covers several related technologies for imaging and measuring surfaces on a fine scale, down to the level of molecules and groups of atoms.

image

Scanning tunneling microscopy: Schematic diagram of a scanning tunneling microscope.

A scan may cover a distance of over 100 micrometers in the x and y directions and 4 micrometers in the z direction. SPM technologies share the concept of scanning a sharp probe tip with a small radius of curvature across the object surface. The tip is mounted on a flexible cantilever, allowing the tip to follow the surface profile. 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. Selective sensors detect these movements. Various interactions can be studied depending on the mechanics of the probe.

There are three common scanning probe techniques: 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 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.