Written and maintained by CASRAI Editorial Board
Last updated
Scanning tunneling microscopy (STM) images a conductive surface by holding a sharp metal tip a tiny distance above it and measuring the tunneling current that flows across the gap. Because that current changes sharply with distance, the tip can follow the surface at atomic scale. The result is a map of the surface built from current, not from light or an electron beam.
Short answer
An STM scans a sharp tip line by line over a biased, conductive sample. Feedback electronics react to the tunneling current and move the tip with piezoelectric elements so that the tip-sample distance stays constant; the tip’s motion is recorded as the image. The signal comes from the outermost atom of the tip, which is why atomic resolution is possible. The image also reflects electron density, so it is not a literal photograph of atom positions.
Definition and scope
STM is a scanning probe technique. It uses quantum tunneling to probe a conductive surface with atomic resolution (Tufts University, Sykes lab). The gap between tip and sample can be air, vacuum or liquid (Nanoscience Instruments). Piezoelectric materials, which STM uses to position the tip, are also used in most other scanning probe techniques, so STM shares hardware ideas with the atomic force microscopy imaging modes used on non-conductive samples.
Binnig and Rohrer received the 1986 Nobel Prize in Physics for the instrument, according to Nanoscience Instruments. For wider context on how scanning probes fit among research microscopes, see what a research microscope is and what nanotechnology is.
How STM works in practice
Tunneling current
A voltage (bias) is applied between tip and sample, positive or negative, so a tunneling current flows across the gap. The current decays exponentially as the gap grows, so very small changes in tip-sample separation produce large changes in current (University of Hamburg, Wiesendanger group). That sensitivity is the source of the technique’s vertical precision.
Single-atom tip
Tips are made by etching or tearing a thin metal wire. Because the current depends so steeply on distance, it is carried essentially by the outermost atom of the tip; the Hamburg group describes the sample surface as being scanned by a single atom, with the second-nearest tip atoms carrying only a negligible share of the current.
Piezo scanner and feedback loop
Piezoelectric elements move the tip in x, y and z. The z-piezo ties the tunneling current to the motion of the tip, and the tip is scanned line by line following the topography of the sample (Tufts; Hamburg).
Constant-current versus constant-height imaging
The two imaging modes differ in whether the feedback loop is running, and that determines what the recorded image actually is.
| Question | Constant current | Constant height |
|---|---|---|
| Feedback loop | On: the tip height is adjusted to hold the current fixed | Off: the tip is not adjusted |
| What is displayed | The tip’s motion as it follows the surface | Only the current |
| Surface condition named by the source | General case | Very flat surfaces |
Nanoscience Instruments names these the constant current image and the constant height image, and states that the feedback loop can be turned off for very flat surfaces so that only the current is displayed. A practical consequence follows directly: a constant-current image encodes how far the feedback had to move the tip, while a constant-height image encodes the raw current at a fixed position, so the two should not be compared as if they carried the same quantity.
Types and related concepts
Scanning tunneling electron microscopy
People sometimes write “scanning tunneling electron microscopy.” STM is a probe technique in which the measured quantity is the tunneling current between tip and sample; no electron beam illuminates the sample. For beam-based instruments, which work differently, see SEM versus TEM.
Related methods on CASRAI
Atomic force microscopy is the scanning probe method covered in AFM imaging modes compared. The diffraction limit that motivates scanning probe approaches for fine detail is covered in the optical microscopy resolution limit.
Where it matters for labs and instrument decisions
A short checklist for deciding whether STM fits a sample and a question:
- Is the sample conductive? STM probes conductive surfaces (Tufts). A sample that cannot carry a tunneling current falls outside the technique.
- What medium is the gap? Air, vacuum or liquid are all named as gap media (Nanoscience Instruments).
- Is the surface very flat? Only then is the feedback-off, constant-height mode described as an option (Nanoscience Instruments).
- What will the image be used to claim? The STM also measures electron density changes of the surface (Tufts), so a feature in the image should be described as a feature in the measured signal, not automatically as an atom position.
A well-known demonstration of atomic-scale control is the quantum corral: standing-wave patterns on a Cu(111) surface formed by iron atoms and point defects (Nanoscience Instruments).
Common misunderstandings
- “STM is an electron microscope.” The signal is a tunneling current across a gap, not a transmitted or scattered beam.
- “The image shows where the atoms are.” It reflects electron density changes of the surface (Tufts).
- “The whole tip contributes.” The current is carried by the outermost tip atom (Hamburg).
- “Constant height and constant current images are interchangeable.” One records the tip’s response to feedback; the other records the current directly.
FAQ
What is scanning tunneling microscopy used for?
Imaging conductive surfaces at atomic resolution by measuring a tunneling current between a sharp tip and the sample (Tufts; Hamburg).
What is the difference between constant-current and constant-height STM?
In constant-current mode the feedback loop adjusts the tip to hold the current fixed. In constant-height mode, used for very flat surfaces, feedback is off and only the current is displayed (Nanoscience Instruments).
Does STM need a conductive sample?
Yes. STM is described as a way to probe conductive surfaces at atomic resolution, because it relies on a tunneling current between tip and sample (Tufts).
Is scanning tunneling electron microscopy the same as STM?
The phrase usually refers to STM. STM uses no electron beam; the measured quantity is the tunneling current between tip and sample.
Sources
- University of Hamburg, Wiesendanger group, “Scanning Tunneling Microscopy”: physik.uni-hamburg.de (checked 2026-10-29)
- Tufts University, Sykes lab, “How a Scanning Tunneling Microscope Works”: chem.tufts.edu (checked 2026-10-29)
- Nanoscience Instruments, “Scanning Tunneling Microscopy”: nanoscience.com (checked 2026-10-29)
