Photoexfoliation Synthesis of 2D Materials

Abstract

Quantum two-dimensional (2D) materials discovered in the early 21st century have outsmarted existing nanomaterials in various frontiers of applications. Among the preparation of 2D materials, molecular beam epitaxy, atomic layer deposition, and chemical vapor deposition are nonscalable costly methods, whereas sonochemical and Hummer’s exfoliation methods provide functionalized sheets. Conversely, ultrafast liquid-phase laser processing promises quick delivery of defect-free 2D quantum materials. We report photoexfoliation synthesis of atomic graphene layers, boron nitride (BN), and molybdenum disulfide (MoS2) by the intense KrF laser irradiation into aqueous dispersions of parent material powders in DMF taken in quartz beakers. The number of atomic layers and the lateral size of the sheets gradually decrease with an increase in the laser irradiation duration. Also, the laser fluence becomes the critical control parameter of the lateral size and the number of layers. The average lateral size shrinks from ∼400 nm at 1.5 J/cm2 to 20–30 nm at 4 J/cm2, which accompanies a surge in the ratio of sheets with fewer layers. We correlate the laser processing parameters with the sample size and analyze the molecule-atom-scale interactions. Simulation and DFT calculations suggest the mild out-of-plane thermal expansion of atomic layers followed by solvent intercalation stretches interlayer distance to ∼6.68 Å and thereby lowers the activation energy of exfoliation. The optimum photon fluence at the solvent-assisted condition reduces the activation barrier, enabling us to synthesize 2D crystals in the solution phase. Photoexfoliation synthesis of pure crystals of 2D materials can be promising for next-generation electronic devices.