The alkali halides are ionic compounds. Each alkali atom donates an electron to a halogen atom, leading to ions with full shells. The valence band is mainly located on halogen atoms, while, in a traditional picture, the conduction band is mainly located on alkali atoms. Scanning tunnelling microscopy of NaCl at 4 K actually shows that the conduction band is located on Cl- because the strong Madelung potential reverses the order of the Na+ 3s and Cl- 4s levels. We verify this reversal is true for both atomically thin and bulk NaCl, and discuss implications for II-VI and I-VII compounds.

The current work finds that ordinary table salt behaves unexpectedly. The chloride ions alone dominate the electronic states at both edges of its band gap although sodium ions are also present. This is important when NaCl is used as an insulator.