Facile Identification of Graphene's Crystal Orientations by Optical Microscopy of Self-Aligned Microwires

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

“…From the epitaxial AgCN microwires observable with conventional optical microscopy, one can come up with an idea to use the microwire directions as visualization markers of crystal structures (crystallographic orientations and grain boundaries) of underlying 2D materials. For metal cyanide nanomaterials, this idea has been suggested and experimentally verified with scanning electron microscopy (SEM) of AuCN nanowires , and optical microscopy of AgCN microwires. − While these previous works were limited to provide a proof-of-concept in small areas due to nonuniform and sparse growth characteristics, the spin-coating method in this study further improves reliability and practicality of the crystallographic identification. First, we quantitatively evaluate how precisely crystal orientations of 2D materials can be measured from optical images of the spin-coated AgCN microwires.…”

“…Although all three methods applied onto the five 2D materials can form AgCN microwires preferentially oriented with 3-fold rotational symmetry (Figure B and Figure S3 describe the epitaxial relationship), their uniformities and areal densities are largely different. Drop casting of an aqueous AgCN solution, the conventional method, − leads to sparse and nonuniform growth of microwires on all five 2D materials (left images in Figure C; Figures S1A and S2A). In addition, there exist defocused or irregularly shaped particles on the 2D materials, and these particles are nonepitaxial crystals formed by homogeneous nucleation in the bulk solution.…”

“…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.…”

“…Three different methods to synthesize AgCN microwires on 2D materials are described here: (i) Drop casting of an aqueous solution is a conventional approach used in the previous reports. − A AgCN aqueous solution is prepared by dissolving AgCN (13.3 mg, 2 mmol, 99% purity, Sigma-Aldrich) in an ammonia solution (50 mL, 14.8 M, Samchun). A 5 mL amount of an aqueous solution is dropped on the surface of 2D materials and dried at 150 °C on a hot plate for 10 min in air.…”

“…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.…”