Cutting a crystal to make a surface leaves the outermost atoms with fewer neighbours than atoms deep in the bulk — each missing bond is a dangling bond, and it carries extra energy. Real surfaces lower that energy by reconstructing: shifting atoms away from their ideal bulk-truncated positions into a new, often lower-symmetry, periodic pattern.
- (100) — the classic case (silicon): neighbouring surface atoms pair up into dimers, each dimer bond replacing two dangling bonds with one new one. This doubles the surface period along one axis — the real Si(100) 2×1 reconstruction.
- (110) — open, low-coordination face. Many metals (Pt, Au, Ir) remove every second surface row entirely — the real missing-row 1×2 reconstruction — trading atoms for a lower-energy, more close-packed remainder.
- (111) — already the most close-packed face, so most metals barely reconstruct; atoms mainly relax outward/inward a little instead. Toggle reconstruction to compare against the ideal truncated lattice.
The electron cloud does not stop exactly at the last atomic layer — it spills into vacuum, leaving the ion cores slightly positive behind it. That separation of charge is the surface dipole layer (visualised as the glowing band above the slab). It sits in series with the bulk electrostatic potential and directly sets the work function Φ — the minimum energy to pull an electron from the surface into vacuum. Because coordination, relaxation and dipole strength all depend on which face is cut, real crystals show a genuinely different Φ per facet (e.g. clean copper: Φ(100)≈4.59 eV, Φ(110)≈4.48 eV, Φ(111)≈4.94 eV) — reproduced schematically here.
Φ = ΔV(dipole) − E_F(bulk)
dangling bonds ↓ on reconstruction
→ surface energy ↓, dipole ↓, Φ shifts
Drag to orbit, scroll/pinch to zoom. Toggle reconstruction on any face to watch the surface layer snap between the ideal bulk-truncated lattice and its real reconstructed geometry.