Twisted Epitaxial Growth of Gold Nanodiscs Confined in Twisted Bilayer Molybdenum Disulfide

Epitaxy, which describes crystal growth on a substrate with crystallographic registry in an ordered manner [1], has been utilized in synthesizing planar or core-shell heterostructures with applications in transistors, light-emitting diodes, lasers, quantum devices, etc. The scope has been expanded to van der Waals (vdW) epitaxy [2], remote epitaxy [3], and confined epitaxy [4], owing to the recent development of two-dimensional (2D) vdW materials. Thus far, research has mainly been based on the framework of growing one crystal epitaxially onto 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 (⁠{11¯1}⁠) and the hexagonal MoS2 planes ({001}) [5]. In this work, we expand the concept of epitaxy to a new regime of “twisted epitaxy” with the epilayer crystal growth between two twisted single crystal layers. Both layers interact with an intermediate gold layer in an appreciable way and influence its crystallographic registry by the relative orientation of the two layers, opening up opportunities of using the relative orientation of a bilayer as a control parameter. We demonstrate this twisted epitaxy concept by depositing epitaxial Au nanoparticles onto exfoliated MoS2, encapsulating them with a second layer of MoS2 with varying misorientation (θm) with respect to the first layer from 0o to 60o and then annealing at 500 oC for 2 hours in an inert atmosphere. Fig. 1 shows the schematic synthetic process of the MoS2-Au-MoS2 sandwich structure. Transmission electron microscopy (TEM) shows the as-deposited Au to be nanoparticles ∼ 3 nm in diameter epitaxial with the first substrate. Selected area electron diffraction (SAED) patterns show that Au sandwiched in the bilayer MoS2 aligns with {220} planes parallel to {110} planes of MoS2. After annealing in argon at 500 oC for 2 hours, Au nanodiscs with average diameter about 50 nm are observed as larger, less dark contrast regions within ∼ 600 nm of the sandwiched region, by plan-view bright field TEM examination (Fig. 2). We found that the original epitaxial orientation of Au (θa) was modified by the orientation of the upper MoS2 layer, as determined by electron diffraction and Moiré pattern image analysis. The orientation of Au {220} planes still match closely with the bottom MoS2 {110} planes. However, small deviations from the ideal epitaxy are observed with θa determined to be 2.0o in the same direction as θm, in Fig. 2. In addition, the Moiré fringe spacings from overlap of the Au and MoS2 layers were measured to be 1.47 nm, smaller than the theoretical spacing (1.63 nm) of epitaxial Au on MoS2, indicating the 2o misorientation of Au. These observations are consistent with the upper and lower interfacial energies of Au-MoS2 calculated using Density Functional Theory. This concept of twisted epitaxy implies the possibility of using the van der Waals spacing of a stacked bilayer 2D material as a nanoreactor to confine material growth as well as tuning the lattice orientation, motivating a new class of experiments to synthesize other 2D materials and heterostructures using such a structure [6]. Schematic synthesis procedure of Au nanodiscs between twisted single crystal MoS2 layers, showing the physical twisting and annealing steps. a) Bright field TEM image of MoS2-Au-MoS2 with a bilayer twist angle θm = 20o. A white dashed line denotes the edge of the top MoS2. b) Bright field TEM image of a typical Au nanodisc marked in a white dashed box in (a). Green lines note the Moiré fringes originating from the top and bottom MoS2. Orange lines note the Moiré fringes originating from Au and the bottom MoS2. c) SAED pattern of MoS2-Au-MoS2 in (b). Yellow lines show the ideal alignment of Au with the bottom MoS2 and the small epitaxial twist is noted by Δga. Scale bar: 2 nm-1.