Permanent Dipole Moment in a Quantum-Confined Two-Dimensional Metal Revealed by Electric Double Layer Gating

The tunable optical properties of metals through size-dependent quantum effects have attracted attention due to synthesis of chemically stable, ultrathin, and two-dimensional metals. Gate tunability, from the reduced screening of low-dimensional metals, adds an additional route for control over optical properties. Here, two-dimensional (2D) Ga is synthesized via confinement heteroepitaxy and patterned into electric-double-layer (EDL) gated transistors. 2D Ga is predicted to have an out-of-plane permanent dipole moment resulting from a non-centrosymmetric interface. Alternating current EDL gating induces a measurable change in 2D Ga reflectivity of ΔR/R ∼ 8 × 10^-4. The optical response is dominated by a linear Stark shift of 1.8 meV, corresponding to a 0.4 D change in the permanent dipole moment between the ground and excited states of 2D Ga. These results are the first demonstration of 2D metal gating and the first direct evidence of a permanent dipole moment in a 2D metal.