Large-area, Freestanding Single-Crystal Gold of Single Nanometer Thickness

Two-dimensional single-crystal metals are highly sought after fornext-generation technologies. Here, we report large-area (>10^4 μm2),single-crystal two-dimensional gold with thicknesses down to a single-nanometerlevel, employing an atomic-level-precision chemical etching approach. Theultrathin thickness and single-crystal quality endow two-dimensional gold withunique properties including significantly quantum-confinement-augmented opticalnonlinearity, low sheet resistance, high transparency and excellent mechanicalflexibility. By patterning the two-dimensional gold into nanoribbon arrays,extremely-confined near-infrared plasmonic resonances are further demonstratedwith quality factors up to 5. The freestanding nature of two-dimensional goldallows its straightforward manipulation and transfer-printing for integrationwith other structures. The developed two-dimensional gold provides an emergingplatform for fundamental studies in various disciplines and opens up newopportunities for applications in high-performance ultrathin optoelectronic,photonic and quantum devices.