Observation of Surface 2D Electron Gas in Highly Bulk-Insulating Bi4I4

Topological insulators are usually good thermoelectric materials, and it is intriguing to investigate if the topological surface states enhance the thermoelectric performance. For the observation of exotic surface-state-dominated transport properties, it is crucial to tune the Fermi energy into the bulk bandgap. However, this is of great challenge, and a controllable method to tune the Fermi energy for the as-grown bulk crystals is highly desirable. In this work, realize the post-synthesis tuning of the Fermi energy in the topological insulator Bi₄I₄ is successfully realized by annealing. The annealing process excites the escape of iodine and changes the carrier type from hole to electron. The annealed crystals can be highly insulating by showing an extremely low carrier concentration in the order of 10¹⁵ cm⁻³, one of the lowest values among the reported topological insulators. Benefiting from the highly insulating bulk state, a 2D surface state induced by band bending is identified by quantum oscillations. These results shed light on the post-synthesis way of Fermi energy engineering for bulk crystals, and on the preparation of 2D electron gas on the surface of bulk crystals, which can be instructive to resolve the surface-states contribution to various physical properties including thermoelectric transport properties.