Universal Oriented van der Waals Epitaxy of 1D Cyanide Chains on Hexagonal 2D Crystals

Deformation twin traces on gold surfaces: A pathway to tailored epitaxial growth of 1D semiconductors

The field of one-dimensional semiconducting materials holds a wide variety of captivating applications, such as photovoltaic cells, electronic devices, catalysis cells, lasers, and more. The tunability of electrical, mechanical, or optical attributes of a semiconductor crystal relies on the ability to control and pattern the crystal's growth direction, orientation, and dimensions. In this study, we harvest the unique properties of crystallographic defects in Au substrates, specifically twin boundaries, to fabricate selective epitaxial growth of semiconducting nanocrystals. Different crystallographic defects were previously shown to enhance materials properties, such as, screw dislocations providing spiral crystal growth, dislocation outcrops, and vacancies increasing their catalytic activity, dislocation strengthening, and atomic doping changing the crystal's electrical properties. Here, we present a unique phenomenon of directed growth of semiconductor crystals of gold(I)-cyanide (AuCN) on the surface of thin Au layers, using traces of deformation twins on the surface. We show that emergence of deformation twins to the {111} Au surface leads to the formation of ledges, exposing new {001} and {111} facets on the surface. We propose that this phenomenon leads to epitaxial growth of AuCN on the freshly exposed {111} facets of the twin boundary trace ledges. Specific orientations of the twin boundaries with respect to the Au surface allow for patterned growth of AuCN in the <110> orientations. Nano-scale patterning of AuCN semiconductors may provide an avenue for property tuning, particularly the band gap acquired.

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

Epitaxial growth of inorganic crystals on 2D materials is expected to greatly advance nanodevices and nanocomposites. However, because pristine surfaces of 2D materials are chemically inert, it is difficult to grow inorganic crystals epitaxially on 2D materials. Previously, successful results were achieved only by vapor-phase deposition at high temperature, and solution-based deposition including spin coating made the epitaxial growth unaligned, sparse, or nonuniform on 2D materials. Here, we show that solvent-controlled spin coating can uniformly deposit a dense layer of epitaxial AgCN microwires onto various 2D materials. Adding ethanol to an aqueous AgCN solution facilitates uniform formation of the thin supersaturated solution layer during spin coating, which promotes heterogeneous crystal nucleation on 2D material surfaces over homogeneous nucleation in the bulk solution. Microscopic analysis confirms highly aligned, uniform, and dense growth of epitaxial AgCN microwires on graphene, MoS₂, hBN, WS₂, and WSe₂. The epitaxial microwires, which are optically observable and chemically removable, enable crystallographic mapping of grains in millimeter-sized polycrystalline graphene as well as precise control of twist angles (<∼1°) in van der Waals heterostructures. In addition to these practical applications, our study demonstrates the potential of 2D materials as epitaxial templates even in spin coating of inorganic crystals.

Commensurate Assembly of C60 on Black Phosphorus for Mixed-Dimensional van der Waals Transistors

2D crystals can serve as templates for the realization of new van der Waals (vdW) heterostructures via controlled assembly of low-dimensional functional components. Among available 2D crystals, black phosphorus (BP) is unique due to its puckered atomic surface topography, which may lead to strong epitaxial phenomena through guided vdW assembly. Here, it is demonstrated that a BP template can induce highly oriented assembly of C₆₀ molecular crystals. Transmission electron microscopy and theoretical analysis of the C₆₀ /BP vdW heterostructure clearly confirm that the BP template results in oriented C₆₀ assembly with higher-order commensurism. Lateral and vertical devices with C₆₀ /BP junctions are fabricated via a lithography-free clean process, which allows one to investigate the ideal electrical properties of pristine C₆₀ /BP junctions. Effective tuning of the C₆₀ /BP junction barrier from 0.2 to 0.5 eV and maximum on-current density higher than 10⁴  mA cm^-2 are achieved with graphite/C₆₀ /BP vertical vdW transistors. Due to the formation of high-quality C₆₀ film and the semitransparent graphite top-electrode, the vertical transistors show high photoresponsivities up to ≈100 A W^-1 as well as a fast response time under visible light illumination.

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

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.

Self-catalytic growth of one-dimensional materials within dislocations in gold

Dislocations in metals affect their properties on the macro- and the microscales. For example, they increase a metal's hardness and strength. Dislocation outcrops exist on the surfaces of such metals, and atoms in the proximity of these outcrops are more loosely bonded, facilitating local chemical corrosion and reactivity. In this study, we present a unique autocatalytic mechanism by which a system of inorganic semiconducting gold(I) cyanide nanowires forms within preexisting dislocation lines in a plastically deformed Au-Ag alloy. The formation occurs during the classical selective dealloying process that forms nanoporous Au. Nucleation of the nanowire originates at the surfaces of the catalytic dislocation outcrops. The nanowires are single crystals that spontaneously undergo layer-by-layer one-dimensional growth. The continuous growth of nanowires is achieved when the dislocation density exceeds a critical value evaluated on the basis of a kinetic model that we developed.

Describing chain-like assembly of ethoxygroup-functionalized organic molecules on Au(111) using high-throughput simulations

Due to the low corrugation of the Au(111) surface, 1,4-bis(phenylethynyl)-2,5-bis(ethoxy)benzene (PEEB) molecules can form quasi interlocked lateral patterns, which are observed in scanning tunneling microscopy experiments at low temperatures. We demonstrate a multi-dimensional clustering approach to quantify the anisotropic pair-wise interaction of molecules and explain these patterns. We perform high-throughput calculations to evaluate an energy function, which incorporates the adsorption energy of single PEEB molecules on the metal surface and the intermolecular interaction energy of a pair of PEEB molecules. The analysis of the energy function reveals, that, depending on coverage density, specific types of pattern are preferred which can potentially be exploited to form one-dimensional molecular wires on Au(111).

Unidirectional Alignment of AgCN Microwires on Distorted Transition Metal Dichalcogenide Crystals

Van der Waals epitaxy on the surface of two-dimensional (2D) layered crystals has gained significant research interest for the assembly of well-ordered nanostructures and fabrication of vertical heterostructures based on 2D crystals. Although van der Waals epitaxial assembly on the hexagonal phase of transition metal dichalcogenides (TMDCs) has been relatively well characterized, a comparable study on the distorted octahedral phase (1T' or T_d) of TMDCs is largely lacking. Here, we investigate the assembly behavior of one-dimensional (1D) AgCN microwires on various distorted TMDC crystals, namely 1T'-MoTe₂, T_d-WTe₂, and 1T'-ReS₂. The unidirectional alignment of AgCN chains is observed on these crystals, reflecting the symmetry of underlying distorted TMDCs. Polarized Raman spectroscopy and transmission electron microscopy directly confirm that AgCN chains display the remarkable alignment behavior along the distorted chain directions of underlying TMDCs. The observed unidirectional assembly behavior can be attributed to the favorable adsorption configurations of 1D chains along the substrate distortion, which is supported by our theoretical calculations and observation of similar assembly behavior from different cyanide chains. The aligned AgCN microwires can be harnessed as facile markers to identify polymorphs and crystal orientations of TMDCs.