Investigation of B2C Hole Defect*

The structural, thermal, and mechanical stability of B2C monolayers has been explored in depth by several teams. The edges of a material are the most active, and to increase the number of active sites on a surface, we need to introduce hole defects. However, no systematic study of hole defects in B2C has been conducted, which is crucial for its catalytic properties. This research aims to understand the hole defects in B2C monolayers, along with bonding mechanisms around the hole defects in the more favorable and less favorable scenarios.

Our primary approach to the question is to account for all permutations and then relax using ab initio calculation, specifically DFT calculation with finite-difference mode and PBE functional, done by GPAW as parameters for preliminary calculations. We manually removed all surrounding atoms individually within a 4-atom radius from a single atom to be removed as 2-atom removed cases. We did a convergence test of raw material and formation energies on the most favorable structure for Plane-wave mode with different cutoff energies (from 350eV to 1000eV) and k-point sampling (from 1x1x1 to 6x6x1). The convergence test on the formation energy suggests that the energies start to converge at 500eV with a 4x4x1 k-point mesh. To avoid bias, we set an energy threshold to be the largest energy gap (2.042eV) in each case and re-calculated the formation energies of all structures with the converged parameters. To understand the bonding mechanism, we manually assigned +1 magnetic moments on all atoms around the hole defects of all top and bottom cases and relaxed the spins to see the existence of unpaired electrons. 

Our study found the most favorable hole defects, with up to 4 atoms being removed. The energies to take out one Boron and one Carbon atom are -9.005eV and -12.143eV. The lowest formation energies of 2, 3, and 4-atom-removed are -3.136eV, -5.037eV and -8.256eV respectively. For now, the most favorable hole defect consists of multiple small symmetrical rings. Unsurprisingly, there are no magnetic moments on all atoms around the hole defects, suggesting that no unpaired electrons on the atoms forming the defects are present. The dangling boron atoms in the hole defect in less favorable cases show magnetic moments after relaxation, suggesting sp2 hybridization with unpaired electrons.