Twisted Epitaxy of Gold Nanodiscs Grown in Twisted Bilayer Molybdenum Disulfide

The concept of epitaxy is important in materials science for many technological applications, and it describes crystal growth on a single crystal substrate with crystallographic registry [1]. The recent development of two-dimensional (2D) van der Waals (vdW) materials has expanded the scope of epitaxy to vdW epitaxy [2], remote epitaxy [3], and confined epitaxy [4]. Thus far, all epitaxy research has been based on the framework of growing one crystal epitaxially onto or into one substrate. For example, previous studies of gold (Au) through vacuum deposition onto single crystal molybdenum disulfide (MoS₂) show that there is an epitaxial relationship between the FCC close packed Au planes ({111}) and the hexagonal MoS₂planes ({001}) [5,6]. Here we expand the concept of epitaxy to a new regime of “twisted epitaxy” with the epilayer crystal growth between two MoS₂substrates with varying mutual crystal orientations. Both substrates interact with intermediate Au nanoparticles in an appreciable way and influence its crystallographic orientation and registry, opening up opportunities of using the relative orientation of two substrates as a control parameter.<br/><br/>To demonstrate the concept of twisted epitaxy, we propose a method to synthesize 2D twisted epitaxial Au nanodiscs by depositing epitaxial Au nanoparticles onto exfoliated MoS₂, encapsulating them with a second layer of MoS₂with varying orientation from 0^o to 60^o and then annealing at 500 ^oC for 2 hours in argon atmosphere. The orientation of the crystalline Au nanodiscs with respect to the MoS₂lattice was studied by selected area electron diffraction (SAED) and Moire fringe image analysis using transmission electron microscopy (TEM) and it can be altered in a small but controllable fashion via the bilayer twist angle. Specifically, Au aligns mid-way between the orientation of the top and bottom MoS₂when the twist angle of the bilayer is small (&lt; ~7^o). For larger twist angles, Au aligns close to the bottom MoS₂, with a small misorientation varying approximately sinusoidally with the twist angle of the bilayer MoS₂ [[7]. These observations are consistent with the combined upper and lower interfacial energies of Au- MoS₂calculated using Density Functional Theory [8]. In addition, 4D STEM analysis is used to study the strain variations (&lt; |±1%|) in the Au nanodiscs associated with the twisted epitaxy. The discovery of twisted epitaxy therefore provides opportunities for tri-lattice heterostructure Moiré engineering and for structure-property investigation of 2D materials with advanced scanning transmission electron microscopy (STEM) [9].<br/><br/><b>References:</b><br/>[1] Cho, A. Y. & Arthur, J. R. Prog. Solid State Chem. 10, 157-192 (1975).<br/>[2] Chen, Z., et al. Sci. Adv. 7, eabk0115 (2021).<br/>[3] Kim, Y., et al. Nature 544, 340-343 (2017).<br/>[4] Briggs, N., et al. Nat. Mater. 19, 637-643 (2020).<br/>[5] Takayanagi, K., et al. Surf. Sci. 205, 637-651 (1988).<br/>[6] Reidy, K., et al. Nat. Commun. 12, 1-9 (2021).<br/>[7] Cui, Y., et al. Microsc. Microanal. in press (2023).<br/>[8] Cui, Y., et al. Science 383, 212-219 (2024).<br/>[9] This work was partly supported by Office of Basic Energy Sciences of the U.S. Department of Energy (DOE), Division of Materials Sciences and Engineering. The authors also acknowledge the use and support of the Stanford Nano Shared Facilities (SNSF).