Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) are two types of scanning probe microscopy techniques used to visualize surfaces at the nanoscale.
The major difference between AFM and STM is that AFM uses a mechanical probe to interact with the surface, whereas STM uses a conductive probe and tunneling current.
AFM and STM have different working principles that allow imaging of various sample properties. While both techniques can achieve atomic resolution, they are suited to measuring different characteristics of a surface. Understanding the differences in instrumentation and operational modes can help select the appropriate technique for nanoscale analysis.
Instrumentation and Operational Modes
AFM
AFM utilizes a sharp tip on the end of a flexible cantilever that physically touches and scans across the sample surface. As the tip raster scans the surface, forces between the tip and sample cause the cantilever to deflect. This deflection is measured by a laser reflected off the back of the cantilever into a photodetector. Feedback mechanisms keep the applied force constant during imaging.
Various operational modes are used in AFM:
- Contact mode – Maintains a constant deflection as the tip is dragged across the surface. Provides high resolution but can damage soft samples.
- Tapping mode – Oscillates the cantilever near its resonance frequency and lightly “taps” the surface. Reduces sample damage but lower resolution than contact mode.
- Non-contact mode – Oscillates the cantilever at a frequency higher than the cantilever’s resonance and does not contact the surface. Used on delicate or liquid samples. Resolution limited by weak tip-sample forces.
STM
In STM, an atomically sharp conductive tip is brought in close proximity to a conductive sample. As the bias voltage between the tip and sample is changed, electrons tunnel across the gap between them, generating a tunneling current. The tip raster scans the surface at a constant tunneling current, providing information about the electron density of states at the surface.
Operational modes of STM include:
- Constant current mode – Varies the height of the tip to maintain a set tunneling current as the tip scans. Provides topographic images.
- Constant height mode – Rasters the tip at a fixed height and measures changes in current. Faster but less precise.
- Scanning tunneling spectroscopy – Measures current while varying voltage at a specific location. Probes electronic structure and properties.
Table of Differences Between Afm and Stm
| Atomic Force Microscopy | Scanning tunneling microscopy |
|---|---|
| Mechanical probe interacts with sample surfaces through interatomic forces | Conductive probe interacts with surface electron states via quantum tunneling |
| Can image insulators and conductors | Limited to conductive or semiconductive surfaces |
| Force sensitivity allows high resolution of surface structure and properties | Current sensitivity provides electronic structure and properties |
| The tip is maintained nanometers from sample surface | The force applied can temporarily deform soft samples |
| The tip is maintained nanometers from the sample surface | Ambient and liquid imaging are possible |
| The non-contact technique does not modify the sample | Requires high vacuum conditions |
| Slower scan rates | Faster scan rates |
When to Choose AFM and STM?
The choice between AFM and STM depends on the nature of the sample and the type of information desired.
- AFM provides high-resolution topographic images and information on sample mechanics, making it ideal for imaging delicate biological samples, polymers, and insulators.
- STM excels at atomic-scale electronic characterization of flat conductive surfaces like metals, semiconductors, or graphene. STM can map variations in surface conductance.
- STM requires ultra-high vacuum while AFM can operate at ambient pressure or in liquid. Liquid AFM is used for imaging live biological samples.
- AFM has a lower resolution than STM on flat conductive samples. STM may be preferred when characterizing structures like surface reconstructions or electron scattering patterns.
- AFM tip deformation can provide mechanical property measurements through force-distance curves. STM has higher precision for spectroscopy measurements.