A scanning tunneling microscope (STM) holds a sharp conducting tip a few Ångströms above a conductive surface. Electrons "tunnel" across that gap even though classically they shouldn't have enough energy to cross it — a purely quantum effect. The tunneling current falls off exponentially with distance, so the tip can sense the surface without ever touching it, and it can also be used to drag single loosely-bound adatoms sideways across the surface — the technique IBM researchers used in 1989 to spell out their logo with 35 xenon atoms.
I(d) ∝ I₀ · e⁻²ᵀᴸ
κ ≈ √(2mΦ) / ℏ (Φ = work function)
- Scan height — the nominal tip-to-surface distance; lower it and the current rises exponentially even for a fraction of an Ångström of extra closeness.
- Drag an atom — click and hold a bright adatom, drag it across the lattice, release to drop it at the new site (this is how single-atom positioning is actually done).
- Arrange: IBM logo — snaps every adatom into the dot-matrix pattern IBM used for its famous 1989 demonstration.
Real-world relevance: this atom-by-atom placement underlies atomic-scale data storage research, quantum-dot fabrication and fundamental studies of surface chemistry, one atom at a time.