Mechanisms of Postsynthesis Doping of Boron Nitride Nanostructures with Carbon from First-Principles Simulations

Electron-beam-mediated postsynthesis doping of boron-nitride nanostructures with carbon atoms [Nature (London) 464, 571 (2010); J. Am. Chem. Soc. 132, 13 692 (2010)] was recently demonstrated, thus opening a new way to control the electronic properties of these systems. Using density-functional theory static and dynamic calculations, we show that the substitution process is governed not only by the response of such systems to irradiation, but also by the energetics of the atomic configurations, especially when the system is electrically charged. We suggest using spatially localized electron irradiation for making carbon islands and ribbons embedded into BN sheets. We further study the magnetic and electronic properties of such hybrid nanostructures and show that triangular carbon islands embedded into BN sheets possess magnetic moments, which can be switched on and off by electrically charging the structure.

Figure 1

Examples of the structures studied in our calculations, including single carbon atom impurities in the $h$-BN sheet, four carbon atom islands with boron ($4{\mathrm{C}}_{1\mathrm{B}3\mathrm{N}}$ structure) and nitrogen ($4{\mathrm{C}}_{3\mathrm{B}1\mathrm{N}}$) termination, as well as 16-atom ($16{\mathrm{C}}_{6\mathrm{B}10\mathrm{N}}$ structure) and 25-atom ($25{\mathrm{C}}_{15\mathrm{B}10\mathrm{N}}$ structure) islands.

Figure 2

Formation energies $E_f$ of substitutional defects in different charge states as functions of electron chemical potential ${\mu }_e$ (a)–(d). Single carbon atom substitutional impurity in B-atom (a) and N-atom (b) positions. Four C atoms substituting 3 B and 1 N-atom (c) and 1 B and 3 N atoms (d). All the results are for the 200-atom supercell with different defects (PBE calculation). The numbers stand for the charge states. $E_f$ as a function of $K_{\mathrm{C}}$ for various charge states and for preferential substitution of N and B atoms (e). $E_f$ vs $K_{\mathrm{C}}$ for positive charge states and for preferential B substitution giving the lowest formation energies for each $K_{\mathrm{C}}$ (f). Note that the values of $E_f$ are negative.

Figure 3

Lowest formation energies of the substitutional defects presented in Fig. 2 in different charge states as functions of ${\mu }_e$ calculated using the PBE and HSE functionals. Defects with nitrogen (a) and boron (b) termination. ${\mu }_e$ is normalized to the corresponding value of the gap. It is evident that the PBE and HSE results agree well.

Figure 4

Magnetic moments of triangular carbon islands embedded into BN matrix as a function of the difference of the number of substituted nitrogen ($K_{\mathrm{N}}$) and boron ($K_{\mathrm{B}}$) atoms. The insets show the magnetic moments of individual C atoms and four-atom defects as a function of ${\mu }_e$. Green and red lines correspond to preferential N/B substitution. Depending on the defect type, the magnetic moment can be switched on and off.