Synthesis and Properties of Stable Sub-2-nm-Thick Aluminum Nanosheets: Oxygen Passivation and Two-Photon Luminescence

The high reductivity of aluminum (Al) 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 to 1.5 nm. Within the (111) surface passivation, Al nanosheets have achieved satisfactory stability, ensuring the possibility to study thickness-dependent localized surface plasmon resonance from visible to near-infrared (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 applications of Al as a sustainable plasmonic material, and shows great potential in the synthesis of other active metal-based nanomaterials. [Display omitted] •Sub-2-nm Al nanosheets can be made in the presence of an oxygen-rich atmosphere•Unexpected stability is due to the selective oxygen passivation on (111) facets•Localized surface plasmon resonance can be tuned from visible to NIR region•Two-photon luminescence of the ultrathin Al nanosheets is efficiently enhanced The high reactivity of nano-sized aluminum requires an extremely strong reductant and inert gas protection for its synthesis; nevertheless, we have demonstrated here that Al nanosheets can be made in the presence of an oxygen-rich atmosphere. Unlike previously reported Al nanoparticles with more than 2-nm-thick surface oxide coating, the metallic Al nanosheets have been obtained with sub-2-nm-thickness and even with unexpected stability, thanks to the selective passivation on (111) facets. Two-photon luminescence of the ultrathin Al nanosheets was efficiently enhanced by tailoring the surface plasmon resonance in the near-infrared region matching with excitation. The ability to prepare the stable sub-2-nm Al nanosheets demonstrates the important role of selective oxygen adsorption in structural tailoring of the active nanometals and also enriches the synthetic toolbox for other inorganic nanostructures. The high reductivity of Al implies the utmost difficulty in achieving oxygen-resistant, ultrathin Al nanostructures. While sub-2-nm-thick Al nanosheets with unexpected ambient stabili