Grain Boundary Motion in Two-Dimensional Hexagonal Boron Nitride

Abstract

An in-depth understanding and precise controlling of grain boundary (GB) motion at the atomic scale are crucial for grain growth and recrystallization in polycrystalline materials. So far, the reported studies mainly focus on the GB motion in the ideal bicrystal system, while the atomic mechanisms of GB motion in polycrystals remain poorly understood. Herein, taking two-dimensional (2D) hexagonal boron nitride (h-BN) as a model system, we experimentally investigated the atomic-scale mechanisms of the GB motion in 2D polycrystals. Since GB motion is directly related to the GB structures, this article is organized following the configurations of GBs, which can be divided into straight (including symmetric and asymmetric GBs) and curved GBs. We revealed that (I) for symmetric GBs, the shear-coupled motion alone is insufficient to drive the continuous GB motion in polycrystalline materials, and GB sliding is also needed. (II) For asymmetric GBs, GB motion follows a defaceting–faceting process, in which dislocation reactions are crucial. (III) For curved GBs, shear-coupled GB motion (during grain shrinking) leads to grain rotation, and the rotation direction highly depends on the misorientation angles. (IV) Finally, we will discuss the characteristics of binary lattice h-BN and find that partial dislocations participate in the GB motion at high misorientation angles (>38°). Our results build up the framework of the atomic-scale mechanisms of the GB motion in 2D polycrystalline materials and will be instructive for technological applications such as grain growth and GB engineering.

Grain rotation and coupled grain boundary motion in two-dimensional binary hexagonal materials

“…The above analysis is consistent with recent experimental observations of 2D h-BN monolayers [6], where high-resolution TEM imaging of some time-evolving h-BN samples did show different types of atomic mechanisms of grain boundary coupled motion involving dislocation glide and climb at small vs large misorientations θ, associated with different dominated grain boundary defect configurations and dynamics. These included the motion of 5|7 dislocations for low-angle misorientations, similar to that of graphene, leading to a single positive mode of shear-coupled motion as also shown above, and more complicated processes at larger θ involving 5|8|4|7 and 5|7 dislocations, yielding the coupled motion of another negative mode or dual behavior of mode switching [6]. Richer types of defect transformation have been found in our PFC simulations of binary grain rotation.…”

“…to note that these PFC simulation results are consistent with those of a very recent experiment on the shrinking and rotation of h-BN grains [6] which observed the phenomenon of dual mode switching for initial θ 0 ∼ 35.6 • and the single-coupling-mode behavior for θ 0 < 22 • . A discrepancy occurs for 38 • < θ 0 < 60 • , where only the unidirectional rotation towards smaller θ (related to the negative branch of β coupling) was observed experimentally, while the simulation data shown in Figs.…”

“…This leads to the prevalence of 5|7 dislocation cores in single-component 2D hexagonal materials like graphene [1,3,[10][11][12], with other types of defect motifs being rare. In 2D binary hexagonal materials like h-BN and TMDs the situation is more complex, with a much richer variety of dislocation core structures such as 4|8, 5|7, 4|6, 4|10, 6|8, 4|4, 8|8, and others depending on the specific conditions of grain boundary [4][5][6][7][8][9]. This is related to the distinction between heteroelemental and homoelemental neighborings, and thus the formation of defect core structures containing more energetically favorable A-B heteroele-mental bonds.…”

“…These large-scale samples are often grown by techniques of heteroepitaxy, such as chemical vapor deposition (CVD). While large areas can be covered with atomically thin layers of the desired material via these techniques, the resulting monolayer film is typically polycrystalline, as a result of spontaneous nucleation at various locations on the substrate and the formation and evolution of topological defects including dislocations and grain boundaries as observed in experiments [1,[3][4][5][6][7][8].…”

“…The dynamics of these defects, particularly dislocation glide and climb, result in the motion of grain boundaries, with some grains growing at the expense of others. A large amount of research efforts, both experimentally [1,[3][4][5][6][7][8] and theoretically/computationally [9][10][11][12][13], have been put on the studies of defect dynamics and grain growth as well as the understanding and control of the corresponding mechanisms in various 2D material systems including graphene [1,3,[10][11][12], hexagonal boron nitride (h-BN) [4][5][6]9], and transition metal dichalcogenides (TMDs) [1,7,8].…”