Friction Modulation via Photoexcitation in Two-Dimensional Materials.

We demonstrate a photoexcitation-friction coupling in bilayered black phosphorus, a two-dimensional semiconductor crystallized via van der Waals interaction, using density functional theory and the Prandtl-Tomlinson model. Under an experimentally accessible electron-hole density of 5 × 1013 cm-2, the energy barrier for interlayer sliding can be reduced by 13% and the resultant reduction of critical force for stick-slip transition can be up to 4.7%. With the carrier density being doubled, the frictional anisotropy can even be eliminated. Analysis based on Born-Oppenheimer approximation shows that photoexcitation-friction coupling can be universal for van der Waals crystals with interlayer electronic states responsive to both photoexcitation and interlayer sliding.