Role of the Cu substrate in the growth of ultra-flat crack-free highly-crystalline single-layer graphene

Role of Bilayer Graphene Microstructure on the Nucleation of WSe2 Overlayers

Over the past few years, graphene grown by chemical vapor deposition (CVD) has gained prominence as a template to grow transition metal dichalcogenide (TMD) overlayers. The resulting two-dimensional (2D) TMD/graphene vertical heterostructures are attractive for optoelectronic and energy applications. However, the effects of the microstructural heterogeneities of graphene grown by CVD on the growth of the TMD overlayers are relatively unknown. Here, we present a detailed investigation of how the stacking order and twist angle of CVD graphene influence the nucleation of WSe₂ triangular crystals. Through the combination of experiments and theory, we correlate the presence of interlayer dislocations in bilayer graphene with how WSe₂ nucleates, in agreement with the observation of a higher nucleation density of WSe₂ on top of Bernal-stacked bilayer graphene versus twisted bilayer graphene. Scanning/transmission electron microscopy (S/TEM) data show that interlayer dislocations are present only in Bernal-stacked bilayer graphene but not in twisted bilayer graphene. Atomistic ReaxFF reactive force field molecular dynamics simulations reveal that strain relaxation promotes the formation of these interlayer dislocations with localized buckling in Bernal-stacked bilayer graphene, whereas the strain becomes distributed in twisted bilayer graphene. Furthermore, these localized buckles in graphene are predicted to serve as thermodynamically favorable sites for binding WSe_x molecules, leading to the higher nucleation density of WSe₂ on Bernal-stacked graphene. Overall, this study explores synthesis-structure correlations in the WSe₂/graphene vertical heterostructure system toward the site-selective synthesis of TMDs by controlling the structural attributes of the graphene substrate.

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.

Growth of Ultraflat Graphene with Greatly Enhanced Mechanical Properties

Graphene grown on Cu by chemical vapor deposition is rough due to the surface roughening of Cu for releasing interfacial thermal stress and/or graphene bending energy. The roughness degrades the electrical conductance and mechanical strength of graphene. Here, by using vicinal Cu(111) and flat Cu(111) as model substrates, we investigated the critical role of original surface topography on the surface deformation of Cu covered by graphene. We demonstrated that terrace steps on vicinal Cu(111) dominate the formation of step bunches (SBs). Atomically flat graphene with roughness down to 0.2 nm was grown on flat Cu(111) films. When SB-induced ripples were avoided, as-grown ultraflat graphene maintained its flat feature after transfer. The ultraflat graphene exhibited extraordinary mechanical properties with Young's modulus ≈ 940 GPa and strength ≈ 117 GPa, comparable to mechanical exfoliated ones. Molecular dynamics simulation revealed the mechanism of softened elastic response and weakened strength of graphene with rippled structures.

Structural Phase Transitions of Molecular Self-Assembly Driven by Nonbonded Metal Adatoms

The involvement of metal atoms in molecular assemblies has enriched the structural and functional diversity of two-dimensional supramolecular networks, where metal atoms are incorporated into the architecture via coordination or ionic bonding. Here we present a temperature-variable study of the self-assembly of the 1,3,5-tribromobenzene (TriBB) molecule on Cu(111) that reveals the involvement of nonbonded adatoms in the molecular matrix. By means of scanning tunneling microscopy and noncontact atomic force microscopy, we demonstrate the molecular-level details of a phase transition of TriBB assembly from the close-packed to porous honeycomb structures at 78 K. This is an unexpected transformation because the close-packed phase is thermodynamically favored in view of its higher molecular density and more intermolecular bonds as compared to the honeycomb lattice. A comprehensive density functional theory calculation suggests that Cu adatoms should be involved in the formation of the honeycomb network, where the Cu adatoms help stabilize the molecular assembly via enhanced van der Waals interactions between TriBB molecules and the underlying substrate. Both calculation and experimental results suggest no chemical bonding or direct charge transfer between the adatoms and the molecules, thus the electronic characteristics of the Cu adatoms trapped in the molecular confinement are close to the intrinsic ones on a clean metal surface and different from those in the traditional coordination-bonded framework. The nonbonded metal adatoms embedded self-assemblies may complement the metal-organic coordination system and can be used to tailor the chemical reactivity and electronic properties of supramolecular structures.