Supplementary information files for 'Synthesis and properties of stable sub-2-nm-thick aluminum nanosheets: Oxygen passivation and two-photon luminescence'

The high reductivity of aluminum implies the utmost difficulty in achieving oxygen resistant ultrathin Al nanostructures. Herein, we demonstrate that sub-2 nm thick Al nanosheets with ambient stability can be synthesized through a facile wet chemical approach. Selective oxygen adsorption on the (111) facets of the face-centered cubic (fcc) Al has been revealed as the reason of controlling the morphology and stability of Al nanosheets, tailoring the thickness from 18 nm down to 1.5 nm. Within the (111) surface passivation, Al nanosheets have achieved satisfactory stability which ensures the possibility to study thickness-dependent localized surface plasmon resonance from visible to the Near-IR region, and significantly enhanced two-photon luminescence. This work demonstrates, for the first time, the feasibility in obtaining stable ultrathin nanostructures of Al metal, which paves the way toward optical application of Al as a sustainable plasmonic material; it also shows the great potential of the controllable synthesis for investigation of other active metal- based nanomaterials.