Scanning Tunneling Microscope: Constant-Current Imaging

Tunneling Parameters

Work function φ (eV) 4.5

Setpoint current I_set (pA) 50

Scan speed 1.0×

Live Readout

Tunneling current

50.0 pA

Tip-sample gap

5.00 Å

Decay constant κ

1.09 Å⁻¹

Scan line

0 / 48

How it works

A sharp conducting tip is held a few Ångströms above a conductive surface. Applying a small bias voltage lets electrons quantum-mechanically tunnel across the vacuum gap, producing a current that falls off exponentially with the gap width d:

I(d) ∝ V · exp(-2κd)
κ = √(2mφ) / ħ  ≈  0.51·√φ[eV]  (Å⁻¹)

Because κ sits in the exponent, the current drops by roughly an order of magnitude for every Ångström of extra gap — this is what gives STM its extreme, sub-atomic vertical sensitivity.

Constant-current mode (the standard STM imaging mode): a feedback loop continuously raises or lowers the tip so the measured current stays locked to I_set. The recorded tip-height trace is the topographic image — that's exactly what the small panel below the 3D view is drawing, pixel by pixel, as the tip rasters across the lattice.
Constant-height mode: the tip instead glides over the surface at a fixed height, and the current itself is recorded as the image. This is faster but risks a tip crash over tall features, so it's normally only used on very flat terraces.
Work function φ sets how sharply current decays with distance — a higher barrier (larger φ) makes the tip hover further out for the same setpoint current.
I_set is the target tunneling current the feedback loop tracks; raising it pulls the tip physically closer to the surface.

Real-world relevance: this constant-current feedback loop is the actual working principle behind every commercial STM, first built by Binnig & Rohrer in 1981 (1986 Nobel Prize), and is how single atoms on a metal surface are routinely imaged and even individually manipulated.

Tip & scan raster Surface height

Reconstructed STM image
(builds up as the tip scans)

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