From Aluminum Foil to Two-Dimensional Nanocrystals Using Ultrasonic Exfoliation

Al nanostructures have unique optical properties such as widely tunable surface plasmon resonances from deep UV to NIR that can be used for label-free fluorescence enhancement and surface-enhanced Raman scattering. Various Al nanostructures have been fabricated using sophisticated “top-down” lithographic and “bottom-up” colloidal methods. Here, we developed a simple and efficient method of synthesizing two-dimensional (2D) aluminum (Al) nanocrystals from commercially available Al foil using ultrasonic exfoliation under ambient environment. 2D Al nanocrystals with sizes from a few hundred nanometers to several micrometers and thickness in the tens of nanometers were isolated through centrifugation separation. The exfoliated 2D Al nanocrystals are covered with a passivated Al₂O₃ nanolayer. The determined exfoliation mechanism is a combination of the preferred cleavage along the (111) surface planes and layer-by-layer Al₂O₃ exfoliation from the surface of the 2D Al nanocrystals. We demonstrate that the 2D Al nanocrystals can be assembled at water/air interface and transferred to different substrates to form 2D Al nanocrystal films. These 2D Al nanocrystal films exhibit surface plasmon resonance in the visible spectral range and show enhanced Raman signals of adenine using a 532 nm excitation. These 2D Al nanocrystal films could be further developed for new optical and sensing applications.