Pressure-Induced Formation and Mechanical Properties of 2D Diamond Boron Nitride

Understanding phase transformations in 2D materials can unlock unprecedented developments in nanotechnology, since their unique properties can be dramatically modified by external fields that control the phase change. Here, experiments and simulations are used to investigate the mechanical properties of a 2D diamond boron nitride (BN) phase induced by applying local pressure on atomically thin h-BN on a SiO₂ substrate, at room temperature, and without chemical functionalization. Molecular dynamics (MD) simulations show a metastable local rearrangement of the h-BN atoms into diamond crystal clusters when increasing the indentation pressure. Raman spectroscopy experiments confirm the presence of a pressure-induced cubic BN phase, and its metastability upon release of pressure. Å-indentation experiments and simulations show that at pressures of 2-4 GPa, the indentation stiffness of monolayer h-BN on SiO₂ is the same of bare SiO₂, whereas for two- and three-layer-thick h-BN on SiO₂ the stiffness increases of up to 50% compared to bare SiO₂, and then it decreases when increasing the number of layers. Up to 4 GPa, the reduced strain in the layers closer to the substrate decreases the probability of the sp²-to-sp³ phase transition, explaining the lower stiffness observed in thicker h-BN.