Spin-dependent electron scattering at graphene edges on Ni(111)

Stripe-Like hBN Monolayer Template for Self-Assembly and Alignment of Pentacene Molecules

Metallic surfaces with unidirectional anisotropy are often used to guide the self-assembly of organic molecules along a particular direction. Such supports thus offer an avenue for the fabrication of hybrid organic-metal interfaces with tailored morphology and precise elemental composition. Nonetheless, such control often comes at the expense of detrimental interfacial interactions that might quench the pristine properties of molecules. Here, hexagonal boron nitride grown on Ir(100) is introduced as a robust platform with several coexisting 1D stripe-like moiré superstructures that effectively guide unidirectional self-assemblies of pentacene molecules, concomitantly preserving their pristine electronic properties. In particular, highly-aligned longitudinal arrays of equally-oriented molecules are formed along two perpendicular directions, as demonstrated by comprehensive scanning tunneling microscopy and photoemission characterization performed at the local and non-local scale, respectively. The functionality of the template is demonstrated by photoemission tomography, a surface-averaging technique requiring a high degree of orientational order of the probed molecules. The successful identification of pentacene's pristine frontier orbitals underlines that the template induces excellent long-range molecular ordering via weak interactions, preventing charge transfer.

Spin filtering by proximity effects at hybridized interfaces in spin-valves with 2D graphene barriers

We report on spin transport in state-of-the-art epitaxial monolayer graphene based 2D-magnetic tunnel junctions (2D-MTJs). In our measurements, supported by ab-initio calculations, the strength of interaction between ferromagnetic electrodes and graphene monolayers is shown to fundamentally control the resulting spin signal. In particular, by switching the graphene/ferromagnet interaction, spin transport reveals magneto-resistance signal MR > 80% in junctions with low resistance × area products. Descriptions based only on a simple K-point filtering picture (i.e. MR increase with the number of layers) are not sufficient to predict the behavior of our devices. We emphasize that hybridization effects need to be taken into account to fully grasp the spin properties (such as spin dependent density of states) when 2D materials are used as ultimately thin interfaces. While this is only a first demonstration, we thus introduce the fruitful potential of spin manipulation by proximity effect at the hybridized 2D material / ferromagnet interface for 2D-MTJs.

Direct CVD Growth of Graphene on Traditional Glass: Methods and Mechanisms

Chemical vapor deposition (CVD) on catalytic metal surfaces is considered to be the most effective way to obtain large-area, high-quality graphene films. For practical applications, a transfer process from metal catalysts to target substrates (e.g., poly(ethylene terephthalate) (PET), glass, and SiO₂ /Si) is unavoidable and severely degrades the quality of graphene. In particular, the direct growth of graphene on glass can avoid the tedious transfer process and endow traditional glass with prominent electrical and thermal conductivities. Such a combination of graphene and glass creates a new type of glass, the so-called "super graphene glass," which has attracted great interest from the viewpoints of both fundamental research and daily-life applications. In the last few years, great progress has been achieved in pursuit of this goal. Here, these growth methods as well as the specific growth mechanisms of graphene on glass surfaces are summarized. The typical techniques developed include direct thermal CVD growth, molten-bed CVD growth, metal-catalyst-assisted growth, and plasma-enhanced growth. Emphasis is placed on the strategy of growth corresponding to the different natures of glass substrates. A comprehensive understanding of graphene growth on nonmetal glass substrates and the latest status of "super graphene glass" production are provided.

Resolving the Chemically Discrete Structure of Synthetic Borophene Polymorphs

Atomically thin two-dimensional (2D) materials exhibit superlative properties dictated by their intralayer atomic structure, which is typically derived from a limited number of thermodynamically stable bulk layered crystals (e.g., graphene from graphite). The growth of entirely synthetic 2D crystals, those with no corresponding bulk allotrope, would circumvent this dependence upon bulk thermodynamics and substantially expand the phase space available for structure-property engineering of 2D materials. However, it remains unclear if synthetic 2D materials can exist as structurally and chemically distinct layers anchored by van der Waals (vdW) forces, as opposed to strongly bound adlayers. Here, we show that atomically thin sheets of boron (i.e., borophene) grown on the Ag(111) surface exhibit a vdW-like structure without a corresponding bulk allotrope. Using X-ray standing wave-excited X-ray photoelectron spectroscopy, the positions of boron in multiple chemical states are resolved with sub-angström spatial resolution, revealing that the borophene forms a single planar layer that is 2.4 Å above the unreconstructed Ag surface. Moreover, our results reveal that multiple borophene phases exhibit these characteristics, denoting a unique form of polymorphism consistent with recent predictions. This observation of synthetic borophene as chemically discrete from the growth substrate suggests that it is possible to engineer a much wider variety of 2D materials than those accessible through bulk layered crystal structures.

Tuning ultrafast electron injection dynamics at organic-graphene/metal interfaces

We compare the ultrafast charge transfer dynamics of molecules on epitaxial graphene and bilayer graphene grown on Ni(111) interfaces through first principles calculations and X-ray resonant photoemission spectroscopy. We use 4,4'-bipyridine as a prototypical molecule for these explorations as the energy level alignment of core-excited molecular orbitals allows ultrafast injection of electrons from a substrate to a molecule on a femtosecond timescale. We show that the ultrafast injection of electrons from the substrate to the molecule is ∼4 times slower on weakly coupled bilayer graphene than on epitaxial graphene. Through our experiments and calculations, we can attribute this to a difference in the density of states close to the Fermi level between graphene and bilayer graphene. We therefore show how graphene coupling with the substrate influences charge transfer dynamics between organic molecules and graphene interfaces.

Switching Vertical to Horizontal Graphene Growth Using Faraday Cage-Assisted PECVD Approach for High-Performance Transparent Heating Device

Plasma-enhanced chemical vapor deposition (PECVD) is an applicable route to achieve low-temperature growth of graphene, typically shaped like vertical nanowalls. However, for transparent electronic applications, the rich exposed edges and high specific surface area of vertical graphene (VG) nanowalls can enhance the carrier scattering and light absorption, resulting in high sheet resistance and low transmittance. Thus, the synthesis of laid-down graphene (LG) is imperative. Here, a Faraday cage is designed to switch graphene growth in PECVD from the vertical to the horizontal direction by weakening ion bombardment and shielding electric field. Consequently, laid-down graphene is synthesized on low-softening-point soda-lime glass (6 cm × 10 cm) at ≈580 °C. This is hardly realized through the conventional PECVD or the thermal chemical vapor deposition methods with the necessity of high growth temperature (1000 °C-1600 °C). Laid-down graphene glass has higher transparency, lower sheet resistance, and much improved macroscopic uniformity when compare to its vertical graphene counterpart and it performs better in transparent heating devices. This will inspire the next-generation applications in low-cost transparent electronics.

Complex Magnetic Exchange Coupling between Co Nanostructures and Ni(111) across Epitaxial Graphene

We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(111) mediated by an epitaxial graphene layer. X-ray magnetic circular dichroism and scanning tunneling microscopy combined with density functional theory calculations reveal that Co atoms occupy two distinct adsorption sites, with different magnetic coupling to the underlying Ni(111) surface. We further report a transition from an antiferromagnetic to a ferromagnetic coupling with increasing Co cluster size. Our results highlight the extreme sensitivity of the exchange interaction mediated by graphene to the adsorption site and to the in-plane coordination of the magnetic atoms.

Hexagonal Boron Nitride-Graphene Heterostructures: Synthesis and Interfacial Properties

Research on in-plane and vertically-stacked heterostructures of graphene and hexagonal boron nitride (h-BN) have attracted intense attentions for energy band engineering and device performance optimization of graphene. In this review article, recent advances in the controlled syntheses, interfacial structures, and electronic properties, as well as novel device constructions of h-BN and graphene heterostructures are highlighted. Firstly, diverse synthesis approaches for in-plane h-BN and graphene (h-BN-G) heterostructures are reviewed, and their applications in nanoelectronics are briefly introduced. Moreover, the interfacial structures and electronic properties of h-BN-G heterojunctions are discussed, and a zigzag type interface is found to preferentially evolve at the linking edge of the two structural analogues. Secondly, several synthetic routes for the vertically-stacked graphene/h-BN (G/h-BN) heterostructures are also reviewed. The role of h-BN as perfect dielectric layers in promoting the device performance of graphene is presented. Finally, future research directions in the synthesis and application of such heterostructures are discussed.

Graphene growth and properties on metal substrates

Graphene-metal interface as one of the interesting graphene-based objects attracts much attention from both application and fundamental science points of view. This paper gives a timely review of the recent experimental works on the growth and the electronic properties of the graphene-metal interfaces. This work makes a link between huge amount of experimental and theoretical data allowing one to understand the influence of the metallic substrate on the electronic properties of a graphene overlayer and how its properties can be modified in a controllable way. The further directions of studies and applications of the graphene-metal interfaces are discussed.

Epitaxial B-Graphene: Large-Scale Growth and Atomic Structure

Embedding foreign atoms or molecules in graphene has become the key approach in its functionalization and is intensively used for tuning its structural and electronic properties. Here, we present an efficient method based on chemical vapor deposition for large scale growth of boron-doped graphene (B-graphene) on Ni(111) and Co(0001) substrates using carborane molecules as the precursor. It is shown that up to 19 at. % of boron can be embedded in the graphene matrix and that a planar C-B sp(2) network is formed. It is resistant to air exposure and widely retains the electronic structure of graphene on metals. The large-scale and local structure of this material has been explored depending on boron content and substrate. By resolving individual impurities with scanning tunneling microscopy we have demonstrated the possibility for preferential substitution of carbon with boron in one of the graphene sublattices (unbalanced sublattice doping) at low doping level on the Ni(111) substrate. At high boron content the honeycomb lattice of B-graphene is strongly distorted, and therefore, it demonstrates no unballanced sublattice doping.

Observation of single-spin Dirac fermions at the graphene/ferromagnet interface

With the discovery and first characterization of graphene, its potential for spintronic applications was recognized immediately. Since then, an active field of research has developed trying to overcome the practical hurdles. One of the most severe challenges is to find appropriate interfaces between graphene and ferromagnetic layers, which are granting efficient injection of spin-polarized electrons. Here, we show that graphene grown under appropriate conditions on Co(0001) demonstrates perfect structural properties and simultaneously exhibits highly spin-polarized charge carriers. The latter was conclusively proven by observation of a single-spin Dirac cone near the Fermi level. This was accomplished experimentally using spin- and angle-resolved photoelectron spectroscopy, and theoretically with density functional calculations. Our results demonstrate that the graphene/Co(0001) system represents an interesting candidate for applications in devices using the spin degree of freedom.

Breaking of symmetry in graphene growth on metal substrates

In graphene growth, island symmetry can become lower than the intrinsic symmetries of both graphene and the substrate. First-principles calculations and Monte Carlo modeling explain the shapes observed in our experiments and earlier studies for various metal surface symmetries. For equilibrium shape, edge energy variations δE manifest in distorted hexagons with different ground-state edge structures. In growth or nucleation, energy variation enters exponentially as ∼e(δE/k(B)T), strongly amplifying the symmetry breaking, up to completely changing the shapes to triangular, ribbonlike, or rhombic.

Temperature-driven changes of the graphene edge structure on Ni(111): substrate vs hydrogen passivation

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 °C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges.

Spin-crossover and massive anisotropy switching of 5d transition metal atoms on graphene nanoflakes

In spin crossover phenomena, the magnetic moment of a molecule is switched by external means. Here we theoretically predict that several 5d-transition metals (TMs) adsorbed on finite graphene flakes undergo a spin crossover, resulting from multiple adsorption minima, that are absent in the zero-dimensional limit of benzene and the two-dimensional limit of graphene. The different spin states are stable at finite temperature and can be reversibly switched with an electric field. The system undergoes a change in magnetic anisotropy upon spin crossover, which facilitates read-out of the spin state. The TM-decorated nanoflakes thus act as fully controlled single-ion magnetic switches.