Inversion Symmetry Breaking in Lithium Intercalated Graphitic Materials.

Intercalation is a unique degree of freedom for tuning the physical and chemical properties of two-dimensional (2D) materials, providing an ideal system to study various electronic states (such as superconductivity, ferromagnetism, and charge density waves). Here, we demonstrate the inversion symmetry breaking in lithium (Li)-intercalated ultrathin graphite (about 20-100 graphene layers) by optical second-harmonic generation (SHG). This inversion symmetry breaking is attributed to nanoscale inhomogeneities (i.e., lattice distortion and dislocations) in lithiated graphite. In addition, the efficiency of the SHG signal in an ultrathin graphite flake is widely tunable by the electrochemical lithiation process, and the efficiency of fully lithiated graphite (LiC6) is comparable to that of other noncentrosymmetric 2D crystals. Our results reveal a novel intercalation-induced inversion symmetry breaking effect and open up possibilities for building 2D intercalated-compounds-based nonlinear optical devices.