Self-doping in boron sheets from first principles: A route to structural design of metal boride nanostructures

Based on first-principles methods, we present a self-doping picture in atomically thin boron sheets: this shows that for two-dimensional boron nanostructures, adding or removing boron atoms is essentially equivalent to simply adding or removing electrons from a fixed electronic structure. This picture allows us to propose a general design rule for pure boron nanostructures and explains the occurrence of known stable nanostructures. In addition, self-doping provides a powerful tool for finding stable metal boride nanostructures. We illustrate this last point by showing an unexpectedly stable MgB₂ sheet structure which is likely the precursor of MgB₂ nanotubes. Our results are easily generalized to other stoichiometries and other choices of metals.