Alkali metal intercalation and rapid exfoliation of two-dimensional fullerene frameworks

Abstract

Alkali-metal intercalation significantly shapes the electronic and structural properties of molecular C₆₀ crystals and two-dimensional (2D) layered materials, underpinning breakthroughs in electronic structure tuning and superconductivity. For the first time, we investigated the vapor-phase intercalation in a 2D framework consisting of molecular C₆₀ (Mg₄C₆₀), revealing distinct potassium and sodium intercalation mechanisms. Potassium insertion yields a stage-I intercalation compound (estimated as K₃Mg₄C₆₀) at 400 °C, characterized by pronounced A2 g Raman mode shifts (−16 cm⁻¹) and interlayer expansion from 8.34 Å to 8.75 Å. Sodium, however, initially induces cage compression (A2 g blue-shift of +4 cm⁻¹) at lower temperatures and, upon heating to 500 °C, approaches an estimated Na₃Mg₄C₆₀ composition as the A2 g band downshifts. Quenching the intercalation compounds with benzonitrile leads to rapid release of mono-/bilayer 2D-C₆₀ sheets. Note that the entire process is very facile and can be done in less than 10 minutes. The exfoliated sheets display four-fold higher photocurrent response than bulk Mg₄C₆₀. Our integrated intercalation–quenching strategy provides extensive opportunities for future chemical functionalization of 2D-C₆₀ by quenching the charge with a wide range of electrophiles, in addition to the fundamental intercalation mechanism and rapid exfoliation.