Yu Yiling, Jung Gang Seob, Liu Chenze, Lin Yu-Chuan, Rouleau Christopher M, Yoon Mina, Eres Gyula, Duscher Gerd, Xiao Kai, Irle Stephan, Puretzky Alexander A, Geohegan David B
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
Computational Sciences and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
ACS Nano. 2021 Mar 23;15(3):4504-4517. doi: 10.1021/acsnano.0c08516. Epub 2021 Mar 2.
Tailoring the grain boundaries (GBs) and twist angles between two-dimensional (2D) crystals are two crucial synthetic challenges to deterministically enable envisioned applications such as moiré excitons, emerging magnetism, or single-photon emission. Here, we reveal how twisted 2D bilayers can be synthesized from the collision and coalescence of two growing monolayer MoS crystals during chemical vapor deposition. The twisted bilayer (TB) moiré angles are found to preserve the misorientation angle (θ) of the colliding crystals. The shapes of the TB regions are rationalized by a kink propagation model that predicts the GB formed by the coalescing crystals. Optical spectroscopy measurements reveal a θ-dependent long-range strain in crystals with stitched grain boundaries and a sharp (θ > 20°) threshold for the appearance of TBs, which relieves this strain. Reactive molecular dynamics simulations explain this strain from the continued growth of the crystals during coalescence due to the insertion of atoms at unsaturated defects along the GB, a process that self-terminates when the defects become saturated. The simulations also reproduce atomic-resolution electron microscopy observations of faceting along the GB, which is shown to arise from the growth-induced long-range strain. These facets align with the axes of the colliding crystals to provide favorable nucleation sites for second-layer growth of a TB with twist angles that preserve the misorientation angle θ. This interplay between strain generation and aligned nucleation provides a synthetic pathway for the growth of TBs with deterministic angles.
调整二维（2D）晶体之间的晶界（GBs）和扭转角是两项至关重要的合成挑战，其目的是确定性地实现诸如莫尔激子、新兴磁性或单光子发射等预期应用。在这里，我们揭示了在化学气相沉积过程中，如何通过两个生长中的单层MoS晶体的碰撞和合并来合成扭曲的二维双层结构。发现扭曲双层（TB）的莫尔角保留了碰撞晶体的取向差角（θ）。通过扭折传播模型对TB区域的形状进行了合理化解释，该模型预测了合并晶体形成的GB。光谱测量揭示了具有缝合晶界的晶体中存在与θ相关的长程应变，以及TB出现的尖锐（θ>20°）阈值，该阈值缓解了这种应变。反应分子动力学模拟解释了这种应变是由于在合并过程中晶体的持续生长，这是由于沿GB的不饱和缺陷处插入原子所致，当缺陷饱和时该过程会自我终止。模拟还重现了沿GB刻面的原子分辨率电子显微镜观察结果，结果表明这是由生长诱导的长程应变引起的。这些刻面与碰撞晶体的轴对齐，为具有保留取向差角θ的扭转角的TB的第二层生长提供了有利的成核位点。应变产生与对齐成核之间的这种相互作用为具有确定角度的TB的生长提供了一条合成途径。
