Electron beam-induced nanopores in Bernal-stacked hexagonal boron nitride

Abstract

Single-layer h-BN is known to have edges with unique magnetism, however, in the commonly fabricated multilayer AA-h-BN, edge relaxations occur that create interlayer bonds and eliminate the unpaired electrons at the edge. Recently, a robust method of growing the unconventional Bernalstacked h-BN (AB-h-BN) has been reported. Here, we use theoretical approaches to investigate the nitrogen-terminated zigzag edges in AB-h-BN that can be formed in a controlled fashion using a high-energy electron beam. We find that these "open" edges remain intact in bilayer and multilayer AB-h-BN, enabling researchers potentially to investigate these edge states experimentally. We also investigate the thermodynamics of the spin configurations at the edge by constructing a lattice model that is based on parameters extracted from a set of first-principles calculations. We find that the edge spins in neighboring layers interact very weakly, resulting in a sequence of independent spin chains in multilayer samples. By solving this model using Monte Carlo simulations, we can determine nm-scale correlation lengths at liquid-N2 temperatures and lower. At low temperatures, these edges may be utilized in magnetoresistance and spintronics applications.

Magnetism and Interlayer Bonding in Pores of Bernal-Stacked Hexagonal Boron Nitride

“…At regular temperatures (20 K < T < 500 K), we expect both the NC(30,150,270) and NC(90,90,270) pores to be present in single layer h-BN with pores with only trace amounts of NC(90,90,90). To the best of our knowledge, previous studies have only identified NC(90,90,90) [25,[44][45][46]. Although no probing of the local magnetism of edges or pores in h-BN has been reported, ferromagnetism has been shown to occur in h-BN at room temperature [47], which can be attributed to the existence of edges and pores in regular h-BN samples.…”

“…In the common AA -h-BN, consecutive layers are rotated by 60 • with respect to each other, and triangular pores do not neatly align. In contrast, multilayer nested pores are observed in AB-h-BN with perfectly parallel edges [15,25]. We report the geometries and electronic structures of all basic bilayer triangular pore combinations.…”

“…Until a robust and reproducible method for growing AB-h-BN was discovered, it was only observed in rare cases [17][18][19]. In AB-h-BN, three types of columns repeat to form the bulk material: (1) An infinite sheet of single-layer h-BN is a wide-gap insulator, yet, computational studies have shown that its edges and pores exhibit a rich collection of electronic and magnetic properties [11,[20][21][22][23][24][25]. The unpaired electrons that occupy dangling sp 2 hybrid orbitals at edges and pores result in magnetism which is absent in the pristine sheet.…”

“…These vacancies can be created by a high-energy electron beam in a transmission electron microscopy (TEM) chamber. As the beam is applied for longer periods of time, the vacancies grow into larger pores in predictable ways [15,[25][26][27][28]. The pores have three nitrogen-terminated zigzag edges that form an equilateral triangle.…”

QED theory of electron beam-induced electronic excitation and its effect on sputtering cross sections in 2D crystals

“…Therefore, the atoms lining the defect are more likely to sputter than their surface counterparts for a given beam energy. As the edge atoms continue to sputter away at a high rate, the nanopore grows, eventually extending up to a few nm in diameter [9,34,35]. Cretu et al measured the radial growth of these nanopores under electron irradiation for several temperatures ranging from 673 to 1473 K [9].…”