Changing a material by putting a voltage on a nearby electrode – a gate – separated from it by an . The voltage pulls electrons into the material or pushes them out, as in every . Because a is so thin, the gate reaches all of it, so the number of electrons can be dialled up and down continuously: a single device can be turned from insulator to metal, its magnetism strengthened, or switched on. Two gates, above and below, can also apply an electric field across the layer.
Going deeper
A voltage on a gate pulls electrons into a 2D layer, and because the layer is so thin the whole of it responds. Two gates set the electron density and an electric field across the layer independently; ions from a liquid, piled up a nanometre from the surface, induce ten times more charge – enough to make MoS2 superconduct.
A voltage that adds electrons
A gate, its insulator and the 2D layer form a capacitor. A positive voltage on the gate draws electrons into the layer, a negative one draws them out and leaves holes, and the number induced follows the gate capacitance: a silicon back gate under 300 nanometres of silicon dioxide adds about 7 × 1010 electrons per square centimetre for every volt. In a bulk crystal this charge would sit in a thin skin at the surface; in a 2D layer it fills the whole material, so its electronic properties change throughout. That is the field effect with which graphene was first characterised in 2004, and the basis of every 2D transistor.
Two gates and a field
With a gate above and another below, two voltages can be set independently: their sum fixes the electron density, their difference the electric field across the layer, called the displacement field. In graphene that field breaks the symmetry between the two layers and opens a of up to about 0.25 eV, which can be tuned continuously. In twisted and materials, dual gating moves the electrons through their one filling at a time, which is how with and superconductivity are mapped out in a single device.
Ions for higher densities
Solid break down before they reach the densities of a doped metal. An ionic liquid or a solid electrolyte goes further: under a voltage its ions pile up within about a nanometre of the surface, forming an electric double layer whose capacitance is far larger, and the induced density reaches 1014 electrons per square centimetre or more. That is enough to make semiconducting MoS2 superconduct and to raise the of thin Fe3GeTe2 to room temperature.
Ions have side effects. They can slip between layers or react with the surface, so what looks like electrostatic may be electrochemistry; they move only above a freezing temperature, so the gate is set warm and the sample then cooled; and changes may not reverse completely. Distinguishing the two needs checks such as reversibility and structural measurements before and after.
For specialists
Electrostatic control of carrier density and electric field through a gate coupled capacitively across a dielectric: the induced density equals the gate capacitance times the voltage beyond threshold, divided by the electron charge, reaching a few 1013 cm−2 with oxide or hBN gates before breakdown. Dual gating sets density and perpendicular displacement field independently, opening the gap of bilayer graphene and tuning moiré flat bands. Ionic-liquid and solid-electrolyte gating form an electric double layer about a nanometre thick and reach 1014–1015 cm−2, enough to induce superconductivity in MoS2 or raise the Curie temperature of Fe3GeTe2, but can or react electrochemically and work only while the ions are mobile. from traps, contact effects and quantum capacitance are the usual caveats.