Strain-Induced Growth of Twisted Bilayers during the Coalescence of Monolayer MoS2 Crystals

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 MoS2 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.

调控晶界（grain boundaries, GBs）与二维（2D）晶体间的扭转角，是确定性实现莫尔激子、涌现磁特性或单光子发射等预期应用的两大关键合成挑战。本研究揭示了在化学气相沉积过程中，两个生长中的单层二硫化钼（MoS₂）晶体发生碰撞与融合，进而合成扭转二维双层结构的机制。研究发现，扭转双层（TB）的莫尔角与碰撞晶体的取向差角（θ）完全一致。可通过扭折传播模型对扭转双层区域的形貌进行合理解释，该模型可预测融合晶体所形成的晶界。光学光谱表征结果显示，带有缝合晶界的晶体中存在与取向差角θ相关的长程应变，且扭转双层结构的出现存在明确阈值（θ>20°），该结构可有效缓解上述应变。反应分子动力学模拟揭示了该应变的起源：融合过程中晶体持续生长，原子被嵌入晶界处的未饱和缺陷中，当缺陷完全饱和后，该原子嵌入过程会自发终止。模拟结果还复现了原子分辨电子显微镜观测到的晶界刻面现象，该现象源于生长过程诱导产生的长程应变。这些刻面与碰撞晶体的晶轴对齐，为扭转双层结构的第二层生长提供了有利形核位点，且该扭转双层的扭转角与取向差角θ保持一致。应变产生与定向形核之间的这种协同作用，为合成具有确定扭转角的扭转双层结构提供了可控合成路径。