Scanning Probe Microscopy

Scanning probe microscopy includes the techniques of both Atomic Force Microscopy (AFM) and Scanning Tunnelling Microscopy (STM).

In STM, electrons tunnel through the potential barrier at the surface when a small voltage is applied to a tip held just above the surface. If the tip is held very close to the surface, approximately 1 nm, then atomic resolution is possible because the probability of tunneling falls exponentially with distance from the surface and so only the last few atomic layers of the tip participate. The surfaces however must be able to conduct electrons.

In AFM, sharpened tips are scanned across a surface to measure topography. The technique became possible with the advent of semiconductor fabrication techniques which enable cantilevers to be made with low spring constants so that the surface and probe suffer only minimal damage.

The Centre possesses two scanning probe microscopes. A Nanoscope 3 multimode SPM and a Dimension 3100, both from Veeco Instruments. Processing the data is performed on a separate computer using the Scanning Probe Image Processor (SPIP), from Image Metrology.

Nanoscope3

Nanoscope3 Multi-mode Scanning Probe Microscope

The Nanoscope3 is a versatile scanning probe microscope capable of both STM and AFM. The microscope is capable of operating in air, liquids and under electrochemical control.

Operating as an AFM in contact mode it is also capable of lateral force, ie friction, and force modulation mode. In lateral force mode the surface is scanned perpendicular to the cantilever so that changes in frictional forces cause a twisting of the cantilever. In force modulation mode a small oscillation is applied to the tip so that the mechanical properties of the surface can be measured. This is best performed in liquid to prevent the surface tension of adsorbed water layers masking changes due to the mechanical proerties of the surface.

The AFM may also be operated in tapping mode, where a large amplitude oscillation is applied to the tip as it moves above the surface, and changes in topography alter the amplitude of oscillation. This mode results in less damage to tip and surface than contact mode, and also has the advantage that surface debris is less likely to foul the tip. Measuring changes in the frequency of oscillation, known as phase imaging, may reveal mechanical properties of the surface. The microscope may also be operated in electric force and magnetic modes.

The microscope is mounted on an active isolation unit form Halcyonics, and image processing carried out using the Scanning Probe Image Processor (SPIP) from Image Metrology.

Dimension 3100

Dimension 3100 Scanning Probe Microscope

The Dimension 3100 is an atomic force microscope capable of operating in contact mode, including lateral force mode, tapping mode, icluding phase imaging and electric force modes. The microscope can accomodate large samples and has a vertical descent scanner which facilitates accurate placement of the tip on the sample surface, where the surface is scanned perpendicular to the cantilever so that changes in frictional forces cause a twisting of the cantilever.

The microscope may also be operated in tapping mode, where a large amplitude oscillation is applied to the tip as it moves above the surface, and changes in topography alter the amplitude of oscillation. This mode results in less damage to tip and surface than contact mode, and also has the advantage that surface debris is less likely to foul the tip. Measuring changes in the frequency of oscillation, known as phase imaging, may reveal mechanical properties of the surface.The microscope may also be operated in electric force mode to measure the potential at the surface reletive to the tip.

The microscope is mounted on an air table from TMC for vibration isolation. Image processing is carried out using the Scanning Probe Image Processor (SPIP) from Image Metrology.

Equipment booking

Experimental Officer Dr Ben Spencer
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