MATS6008 Chap.7 Scanning Probe Microscopy
Scanning Probe Microscopy
This chapter treats scanning probe microscopy as a feedback-controlled interaction rather than a direct photograph. It compares STM and AFM, constant-current and constant-height operation, and contact, tapping and non-contact modes. Tip shape, scan direction, setpoint and feedback speed are used to diagnose convolution, double-tip images, deformation and tracking artefacts.
What this chapter covers
- 01
Probe, detector, setpoint and feedback loop
- 02
STM tunnelling current and conductivity boundary
- 03
AFM force detection for insulating surfaces
- 04
Contact, tapping and non-contact choices
- 05
Tip convolution and double-tip artefacts
- 06
Trace, retrace and scan-direction controls
Worked example · free
Mode choice for a soft insulating film
- +1Exclude STM because the specimen lacks the required conductive path.
- +1Recognise that contact AFM can drag or deform a soft film.
- +1Choose tapping-mode AFM to reduce sustained lateral contact.
- +1Report tip radius, setpoint and environment because they affect apparent roughness.
Key terms
- Setpoint
- The target interaction signal maintained by a scanning-probe feedback controller.
- Constant-current STM
- An STM mode that adjusts tip height to keep tunnelling current fixed.
- Tapping mode
- An AFM mode using an oscillating cantilever to reduce sustained lateral contact.
- Tip convolution
- Broadening or distortion caused by the finite geometry of the imaging probe.
- Double-tip artefact
- Repeated duplication caused by two active asperities on one probe.
Scanning Probe Microscopy FAQ
Why can STM not image an ordinary insulator?
STM requires a tunnelling current between the tip and specimen, so both sides need a conductive electronic pathway under the measurement conditions. Standard STM therefore cannot map an ordinary insulating surface, while AFM can use force-related interactions and does not impose the same conductivity requirement.
When is tapping AFM preferred over contact AFM?
Tapping mode is useful for soft, adhesive or weakly attached structures because the oscillating probe reduces continuous lateral drag. It still needs a carefully chosen amplitude or force setpoint, and an overly aggressive setting can deform the sample or change apparent dimensions.
How can a double-tip artefact be recognised?
Objects repeat with a consistent separation and orientation fixed to the probe rather than the specimen. Cleaning or changing the tip, rotating scan direction and imaging a calibration surface can test the diagnosis. Real particle pairs should not preserve a tip-fixed duplicate pattern across those controls.
Why does AFM broaden narrow objects?
The recorded surface is a geometric interaction between the finite tip and specimen. When tip radius is comparable with the object, the sides are contacted before the apex reaches the true edge, so lateral width is exaggerated. Height may be more reliable for isolated rigid objects. Use a calibration standard or sharper tip before interpreting width quantitatively.
How should trace and retrace disagreement be interpreted?
A mismatch can indicate feedback lag, drift, contamination, tip change or forces that depend on scan direction. Compare scan rates and directions, inspect line profiles and repeat on a stable calibration surface. If an apparent feature moves or reverses with the scan, it is less likely to be fixed specimen topography and should not be quantified as structure.
What must accompany a quantitative roughness value?
State scan size, pixel sampling, filtering, probe condition, setpoint and environment. Roughness depends on the spatial window and processing, so values from unmatched scans are not directly comparable. Preserve raw data and verify lateral and vertical calibration when the conclusion depends on a small change between specimens.
Exam move
Redraw the feedback loop and label measured signal, setpoint, error and piezo correction. Make a mode-selection tree using conductivity, softness and roughness. Study artefacts through diagnostic reversals: change scan direction, compare trace and retrace, vary setpoint, and replace the tip. Always distinguish apparent lateral width from height.
Choose a method by excluding impossible pathways first: an ordinary insulating sample rules out standard STM, while a delicate surface makes sustained lateral contact risky. Then state the measured interaction and feedback variable because an image channel is not automatically pure topography.
Compare constant-current STM, which adjusts height to hold tunnelling current, with constant-height operation, which scans faster on a very flat surface but increases crash risk. For AFM, connect contact, tapping and non-contact modes to lateral force, adhesion and signal stability. Draw how a finite-radius or double tip transforms a narrow object; lateral width is commonly broadened even when height is more reliable.
Build an artefact checklist around dependence. Real structure remains fixed to specimen coordinates, whereas feedback lag, drift and tip artefacts change with scan rate, direction, setpoint or probe replacement. Trace and retrace disagreement should trigger rate and direction tests before quantification. Repeated objects with a fixed separation suggest a double tip and call for cleaning, replacement or a calibration surface.
For each roughness value, record scan size, filtering, force or amplitude setting and environment so comparisons use the same definition. End an interpretation with one independent control rather than trusting a visually convincing image.