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

Mechanism of Corrugated Graphene Moiré Superstructures on Transition-Metal Surfaces

A graphene layer on a transition-metal (TM) surface can be either corrugated or flat, depending on the type of the substrate and its rotation angle with respect to the substrate. It was broadly observed that the degree of corrugation generally decreases with the increase of rotation angle or the decrease of Moiré pattern size. In contrast to a flat graphene on a TM surface, a corrugated graphene layer has an increased binding energy to the substrate and a concomitant elastic energy. Here, we developed a theoretical model about the competition between the binding energy increase and the elastic energy of corrugated graphene layers on TM surfaces in which all the parameters can be calculated by density functional theory (DFT) calculations. The agreement between the theoretical model and the experimental observations of graphene on various TM surfaces, for example, Ru(0001), Rh(111), Pt(111), and Ir(111), substantiated the applicability of this model for graphene on other TM surfaces. Moreover, the morphology of a graphene layer on an arbitrary TM surface can be theoretically predicted through simple DFT calculations based on the model. Our work thus provides a theoretical framework for the intelligent design of graphene/TM superstructures with the desired structure.

Influence of the Underlying Substrate on the Physical Vapor Deposition of Zn-Phthalocyanine on Graphene

Graphene shows great promise not only as a highly conductive flexible and transparent electrode for fabricating novel device architectures but also as an ideal synthesis platform for studying fundamental growth mechanisms of various materials. In particular, directly depositing metal phthalocyanines (MPc's) on graphene is viewed as a compelling approach to improve the performance of organic photovoltaics and light-emitting diodes. In this work, we systematically investigate the ZnPc physical vapor deposition (PVD) on graphene either as-grown on Cu or as-transferred on various substrates including Si(100), C-plane sapphire, SiO2/Si, and h-BN. To better understand the effect of the substrate on the ZnPc structure and morphology, we also compare the ZnPc growth on highly crystalline single- and multilayer graphene. The experiments show that, for identical deposition conditions, ZnPc exhibits various morphologies such as high-aspect-ratio nanowires or a continuous film when changing the substrate supporting graphene. ZnPc morphology is also found to transition from a thin film to a nanowire structure when increasing the number of graphene layers. Our observations suggest that substrate-induced changes in graphene affect the adsorption, surface diffusion, and arrangement of ZnPc molecules. This study provides clear guidelines to control MPc crystallinity, morphology, and molecular orientations which drastically influence the (opto)electronic properties.

Interaction of CO with Pt nanoclusters on a graphene-covered Ru(0001) surface

The adsorption of CO on Pt nanoclusters on a single layer of graphene epitaxially grown on the Ru(0001) surface [Gr/Ru(0001)] was studied with reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). The graphene layer was grown through exposure to ethylene using a method that has previously been shown to completely cover the surface. As CO adsorbs on Ru(0001) but not on graphene, the complete coverage of the Ru(0001) surface by graphene was verified with TPD as no CO adsorption was detectable. Previous work has demonstrated that Pt nanoclusters nucleate in the moiré unit cells of the Gr/Ru(0001) surface. Exposure of the Pt/Gr/Ru(0001) surface to CO gives rise to strong RAIRS peaks at 2065-2085 cm^-1 assigned to CO at Pt atop sites and at 1848 cm^-1 due to CO at Pt bridge sites. The CO TPD peak areas were used to quantify the CO coverage, which allowed for the determination of the RAIRS peak areas per CO molecule. It was found that the RAIRS intensity for CO on Pt/Gr/Ru(0001) is as much as nine times the intensity of CO on Ru(0001) on a per molecule basis. A more modest intensity enhancement was observed compared to CO on Pt islands on the Ru(0001) surface.

The alignment-dependent properties and applications of graphene moiré superstructures on the Ru(0001) surface

The moiré superstructure of graphene on a lattice-mismatched metal substrate has profound effects on the electronic properties of graphene and can be used for many applications. Here, we propose to systematically tune the moiré superstructure of graphene on the Ru(0001) surface by rotating the graphene layer. Our study reveals two kinds of graphene moiré superstructures: (i) the ultra-flat graphene layers with height variations of less than 0.1 Å for rotation angles greater than 20° that have the same structural and electronic properties everywhere, and (ii) the highly corrugated graphene moiré superstructures with height variations from 0.4 to 1.6 Å for rotation angles less than 20°, whose electronic properties are highly modulated by the interaction with the substrate. Moreover, these rotated graphene moiré superstructures can serve as templates to produce matrices of size-tunable metal clusters from a few to ∼100 atoms. This study reveals the causes of the structural fluctuation of moiré superstructures of graphene on the transition metal surface and suggests a pathway to tune graphene's electronic properties for various applications.

Epitaxial growth and physical properties of 2D materials beyond graphene: from monatomic materials to binary compounds

This review highlights the recent advances of epitaxial growth of 2D materials beyond graphene.