Epitaxial Growth of Diamond-Shaped Au1/2Ag1/2CN Nanocrystals on Graphene

Abstract

Epitaxial synthesis of inorganic nanomaterials on pristine 2D materials is of interest in the development of nanostructured devices and nanocomposite materials, but is quite difficult because pristine surfaces of 2D materials are chemically inert. Previous studies found a few exceptions including AuCN, AgCN, CuCN, and Cu0.5Au0.5CN, which can be preferentially synthesized and epitaxially aligned onto various 2D materials. Here, we discover that Au1/2Ag1/2CN forms diamond-shaped nanocrystals epitaxially grown on pristine graphene surfaces. The nanocrystals synthesized by a simple drop-casting method are crystallographically aligned to lattice structures of the underlying graphene. Our experimental investigations on 3D structures and the synthesis conditions of the nanocrystals imply that the rhombic 2D geometries originate from different growth rates depending on orientations along and perpendicular to 1D molecular chains of Au1/2Ag1/2CN. We also perform in situ TEM observations showing that Au1/2Ag1/2CN nanocrystals are decomposed to Au and Ag alloy nanocrystals under electron beam irradiation. Our experimental results provide an additional example of 1D cyanide chain families that form ordered nanocrystals epitaxially aligned on 2D materials, and reveal basic physical characteristics of this rarely investigated nanomaterial.

Spin Coating Promotes the Epitaxial Growth of AgCN Microwires on 2D Materials

“…The mechanism of this epitaxial alignment needs to be discussed further, because lattice matching, unlike conventional epitaxy, is generally not a main cause of van der Waals epitaxy . Previous studies have shown that various metal cyanides including AuCN, AgCN, CuCN, Cu 0.5 Ag 0.5 CN, and Au 1/2 Ag 1/2 CN present the alignment of 1D cyanide chains along the zigzag lattice directions of various 2D materials including graphene, hBN, MoS 2 , WS 2 , MoTe 2 , and WTe 2 . ,− The previous studies explained that the epitaxial alignment originates from orientation-dependent interactions of the 1D cyanide chains to hexagonal lattices. , Energetically favorable locations for metal cyanide adsorption onto 2D materials are hexagonally arranged, and because of this hexagonal arrangement, energetically favorable orientations of a straight molecular chain are along zigzag lattice directions of 2D materials (see Figure S12 for details) . Therefore, microwire axial directions, which are 1D chain directions of AgCN, are preferentially oriented along zigzag lattice directions of the underlying 2D materials.…”

“…The first one is gold nanowire epitaxy on MoS 2 , in which large deviations of crystallographic orientations are observed. The other type is the epitaxial growth of metal cyanide complexes on 2D materials. − After the discovery of solution-phase epitaxy of gold­(I) cyanide (AuCN) on graphene, five kinds of metal cyanides (AuCN, − AgCN, − CuCN, Cu 0.5 Au 0.5 CN, and Au 1/2 Ag 1/2 CN) have been confirmed to grow epitaxially on various 2D materials (graphene, MoS 2 , hBN, and so on) using drop casting of aqueous solutions. However, all the growth results of this metal cyanide epitaxy are highly unstable and nonuniform due to intrinsic limitations of drop casting: Most of the substrate areas either contain a few epitaxial crystals with extremely low areal densities or are covered by highly concentrated aggregates of nonepitaxial crystals formed from the bulk solution.…”