Anomalous stability of non-van der Waals bonded B₄C nanosheets through surface reconstruction

Boron carbide (B 4C) has been well studied both theoretically and experimentally in its bulk form due to its exceptional hardness and use as a high-temperature thermoelectric. However, the properties of its two-dimensional nanosheets are not well established. In this paper, using van der Waals-corrected density-functional theory simulations, we show that bulk B 4C can be cleaved along different directions to form B 4C nanosheets with low formation energies. We find that there is minimal dependence of formation energies on cleavage planes and surface terminations, even though the bulk is not van der Waals layered. This anomalous stability of B 4C nanosheets is found to be a result of surface reconstructions that are unique to B-rich systems. While the density of states of the bulk B 4C indicate that it is a semiconductor, the B 4C nanosheets are found to be predominantly metallic. We attribute this metallic behavior to a redistribution of charges on the surface bonds of the films. The Seebeck coefficients of the B 4C films remain comparable to those of the bulk and are nearly constant as a function of temperature. Our results provide guidance for experimental synthesis efforts and future application of B 4C nanosheets in nanoelectronic and thermoelectric applications.