The Lightest 2D Nanomaterial: Freestanding Ultrathin Li Nanosheets by In Situ Nanoscale Electrochemistry.

As the lightest solid element and also the simplest metal, lithium (Li) is one of the best representations of quasi-free electron model in both bulk form and the reduced dimensions. Herein, the controlled growth of 2D ultrathin Li nanosheets is demonstrated by utilizing an in situ electrochemical platform built inside transmission electron microscope (TEM). The as-grown freestanding 2D Li nanosheets have strong structure-anisotropy with large lateral dimensions up to several hundreds of nanometers and thickness limited to just a few nanometers. The nanoscale dynamics of nanosheets growth are unraveled by in situ TEM imaging in real-time. Further density-functional theory calculations indicate that oxygen molecules play an important role in directing the anisotropic 2D growth of Li nanosheets through controlling the growth kinetics by their facet-specific capping. The plasmonic optical properties of the as-grown Li nanosheets are probed by cathodoluminescence spectroscopy equipped within TEM, and a broadband visible emission is observed that contains contributions of both in-plane and out-of-plane plasmon resonance modes.