Wenyan Xu

Intercalation of metal islands and films at the interface of epitaxially grown graphene and Ru(0001) surfaces

We report on intercalation of seven kinds of metals—Pt, Pd, Ni, Co, Au, In, and Ce—at the interface between an epitaxially grown graphene layer and a Ru(0001) substrate. Atomic resolution scanning tunneling microscopy images of perfect graphene lattice are obtained on top of these intercalated metals, showing that the high quality of the original graphene is, in the end, undisturbed by the intercalation. A model based theoretical calculation is proposed for the intercalation mechanism: metal atom-aided defect formation and self-healing of C–C bonds at high temperature. These intercalated materials include noble metals, magnetic metals, a IIIA group metal, and a rare earth metal, which indicates that intercalation through epitaxial graphene on Ru(0001) is a universal approach for metals.

Role of Cooperative Interactions in the Intercalation of Heteroatoms between Graphene and a Metal Substrate.

The intercalation of heteroatoms between graphene and a metal substrate has been studied intensively over the past few years, due to its effect on the graphene properties, and as a method to create vertical heterostructures. Various intercalation processes have been reported with different combinations of heteroatoms and substrates. Here we study Si intercalation between graphene and Ru(0001). We elucidate the role of cooperative interactions between hetero-atoms, graphene, and substrate. By combining scanning tunneling microscopy with density functional theory, the intercalation process is confirmed to consist of four key steps, involving creation of defects, migration of heteroatoms, self-repairing of graphene, and growth of an intercalated monolayer. Both theory and experiments indicate that this mechanism applies also to other combinations of hetero-atoms and substrates.

Sequence of Silicon Monolayer Structures Grown on a Ru Surface: from a Herringbone Structure to Silicene

Silicon-based two-dimensional (2D) materials are uniquely suited for integration in Si-based electronics. Silicene, an analogue of graphene, was recently fabricated on several substrates and was used to make a field-effect transistor. Here, we report that when Ru(0001) is used as a substrate, a range of distinct monolayer silicon structures forms, evolving toward silicene with increasing Si coverage. Low Si coverage produces a herringbone structure, a hitherto undiscovered 2D phase of silicon. With increasing Si coverage, herringbone elbows evolve into silicene-like honeycomb stripes under tension, resulting in a herringbone-honeycomb 2D superlattice. At even higher coverage, the honeycomb stripes widen and merge coherently to form silicene in registry with the substrate. Scanning tunneling microscopy (STM) was used to image the structures. The structural stability and electronic properties of the Si 2D structures, the interaction between the Si 2D structures and the Ru substrate, and the evolution of the distinct monolayer Si structures were elucidated by density functional theory (DFT) calculations. This work paves the way for further investigations of monolayer Si structures, the corresponding growth mechanisms, and possible functionalization by impurities.

The influence of annealing temperature on the morphology of graphene islands

We report on temperature-programmed growth of graphene islands on Ru (0001) at annealing temperatures of 700 °C, 800 °C, and 900 °C. The sizes of the islands each show a nonlinear increase with the annealing temperature. In 700 °C and 800 °C annealings, the islands have nearly the same sizes and their ascending edges are embedded in the upper steps of the ruthenium substrate, which is in accordance with the etching growth mode. In 900 °C annealing, the islands are much larger and of lower quality, which represents the early stage of Smoluchowski ripening. A longer time annealing at 900 °C brings the islands to final equilibrium with an ordered moire pattern. Our work provides new details about graphene early growth stages that could facilitate the better control of such a growth to obtain graphene with ideal size and high quality.

Constructing molecular structures on periodic superstructure of graphene/Ru(0001)

We review the way to fabricate large-scale, high-quality and single crystalline graphene epitaxially grown on Ru(0001) substrate. A moiré pattern of the graphene/Ru(0001) is formed due to the lattice mismatch between graphene and Ru(0001). This superstructure gives rise to surface charge redistribution and could behave as an ordered quantum dot array, which results in a perfect template to guide the assembly of organic molecular structures. Molecules, for example iron phthalocyanine and C60, on this template show how the molecule–substrate interaction makes different superstructures. These results show the possibility of constructing ordered molecular structures on graphene/Ru(0001), which is helpful for practical applications in the future.

High quality sub-monolayer, monolayer, and bilayer graphene on Ru(0001)

High quality sub-monolayer, monolayer, and bilayer graphene were grown on Ru(0001). For the sub-monolayer graphene, the size of graphene islands with zigzag edges can be controlled by the dose of ethylene exposure. By increasing the dose of ethylene to 100 Langmuir at a high substrate temperature (800 °C), high quality single-crystalline monolayer graphene was synthesized on Ru(0001). High quality bilayer graphene was formed by further increasing the dose of ethylene while reducing the cooling rate to 5 °C/min. Raman spectroscopy revealed the vibrational states of graphene, G and 2D peaks appeared only in the bilayer graphene, which demonstrates that it behaves as the intrinsic graphene. Our present work affords methods to produce high quality sub-monolayer, monolayer, and bilayer graphene, both for basic research and applications.

Intercalation of metals and silicon at the interface of epitaxial graphene and its substrates

Intercalations of metals and silicon between epitaxial graphene and its substrates are reviewed. For metal intercalation, seven different metals have been successfully intercalated at the interface of graphene/Ru(0001) and form different intercalated structures. Meanwhile, graphene maintains its original high quality after the intercalation and shows features of weakened interaction with the substrate. For silicon intercalation, two systems, graphene on Ru(0001) and on Ir(111), have been investigated. In both cases, graphene preserves its high quality and regains its original superlative properties after the silicon intercalation. More importantly, we demonstrate that thicker silicon layers can be intercalated at the interface, which allows the atomic control of the distance between graphene and the metal substrates. These results show the great potential of the intercalation method as a non-damaging approach to decouple epitaxial graphene from its substrates and even form a dielectric layer for future electronic applications.

Effects of graphene defects on Co cluster nucleation and intercalation

Four kinds of defects are observed in graphene grown on Ru (0001) surfaces. After cobalt deposition at room temperature, the cobalt nanoclusters are preferentially located at the defect position. By annealing at 530°C, cobalt atoms intercalate at the interface of Graphene/Ru (0001) through the defects. Further deposition and annealing increase the sizes of intercalated Co islands. This provides a method of controlling the arrangement of cobalt nanoclusters and also the density and the sizes of intercalated cobalt islands, which would find potential applications in catalysis industries, magnetism storage, and magnetism control in future information technology.

Stable Silicene in Graphene/Silicene Van der Waals Heterostructures

Silicene-based van der Waals heterostructures are theoretically predicted to have interesting physical properties, but their experimental fabrication has remained a challenge because of the easy oxidation of silicene in air. Here, the fabrication of graphene/silicene van der Waals heterostructures by silicon intercalation is reported. Density functional theory calculations show weak interactions between graphene and silicene layers, confirming the formation of van der Waals heterostructures. The heterostructures show no observable damage after air exposure for extended periods, indicating good air stability. The I-V characteristics of the vertical graphene/silicene/Ru heterostructures show rectification behavior.

Correction to Sequence of Silicon Monolayer Structures Grown on a Ru Surface: from a Herringbone Structure to Silicene

T binding energy for a single Si atom at an FCC site was inadvertently quoted as that of a Si atom at an HCP site in the original paper. The related sentence is on page 1162: “Whereas an isolated Si atom binds at a hexagonal-close-packed (HCP) hollow site with a binding energy of 4.95 eV, forming Si chains by occupying neighboring HCP hollow sites increases the binding to 5.41 eV/Si atom”. The revised sentence with the correct number is as follows: Whereas an isolated Si atom binds at a hexagonal-closepacked (HCP) hollow site with a binding energy of 5.38 eV, forming Si chains by occupying neighboring HCP hollow sites increases the binding to 5.41 eV/Si atom. The analysis and conclusions of the paper remain unaffected. We thank Professor Robert Kucharczyk and Dr. Rafal Topolnicki (University of Wroclaw) for bringing this to our attention.