Yui Ogawa

Catalytic Growth of Graphene: Toward Large-Area Single-Crystalline Graphene.

For electronic applications, synthesis of large-area, single-layer graphene with high crystallinity is required. One of the most promising and widely employed methods is chemical vapor deposition (CVD) using Cu foil/film as the catalyst. However, the CVD graphene is generally polycrystalline and contains a significant amount of domain boundaries that limit intrinsic physical properties of graphene. In this Perspective, we discuss the growth mechanism of graphene on a Cu catalyst and review recent development in the observation and control of the domain structure of graphene. We emphasize the importance of the growth condition and crystallinity of the Cu catalyst for the realization of large-area, single-crystalline graphene.

Polycrystalline Graphene with Single Crystalline Electronic Structure

We report the scalable growth of aligned graphene and hexagonal boron nitride on commercial copper foils, where each film originates from multiple nucleations yet exhibits a single orientation. Thorough characterization of our graphene reveals uniform crystallographic and electronic structures on length scales ranging from nanometers to tens of centimeters. As we demonstrate with artificial twisted graphene bilayers, these inexpensive and versatile films are ideal building blocks for large-scale layered heterostructures with angle-tunable optoelectronic properties.

Epitaxial growth and electronic properties of large hexagonal graphene domains on Cu(111) thin film

Large hexagonal single-crystalline domains of single-layer graphene are epitaxially grown by ambient-pressure chemical vapor deposition over a thin Cu(111) film deposited on c-plane sapphire. The hexagonal graphene domains with a maximum size of 100 µm are oriented in the same direction due to the epitaxial growth. Reflecting high crystallinity, a clear band structure with the Dirac cone is observed by angle-resolved photoelectron spectroscopy (ARPES), and a high carrier mobility exceeding 4,000 cm2 V-1 s-1 is obtained on SiO2/Si at room temperature. Our epitaxial approach combined with large domain growth is expected to contribute to future electronic applications.

Chiral atomically thin films

Chiral materials possess left- and right-handed counterparts linked by mirror symmetry. These materials are useful for advanced applications in polarization optics, stereochemistry and spintronics. In particular, the realization of spatially uniform chiral films with atomic-scale control of their handedness could provide a powerful means for developing nanodevices with novel chiral properties. However, previous approaches based on natural or grown films, or arrays of fabricated building blocks, could not offer a direct means to program intrinsic chiral properties of the film on the atomic scale. Here, we report a chiral stacking approach, where two-dimensional materials are positioned layer-by-layer with precise control of the interlayer rotation (θ) and polarity, resulting in tunable chiral properties of the final stack. Using this method, we produce left- and right-handed bilayer graphene, that is, a two-atom-thick chiral film. The film displays one of the highest intrinsic ellipticity values (6.5 deg μm(-1)) ever reported, and a remarkably strong circular dichroism (CD) with the peak energy and sign tuned by θ and polarity. We show that these chiral properties originate from the large in-plane magnetic moment associated with the interlayer optical transition. Furthermore, we show that we can program the chiral properties of atomically thin films layer-by-layer by producing three-layer graphene films with structurally controlled CD spectra.

Structure and transport properties of the interface between CVD-grown graphene domains.

During the chemical vapor deposition (CVD) growth of graphene, graphene domains grown on a Cu surface merge together and form a uniform graphene sheet. For high-performance electronics and other applications, it is important to understand the interfacial structure of the merged domains, as well as their influence on the physical properties of graphene. We synthesized large hexagonal graphene domains with controlled orientations on a heteroepitaxial Cu film and studied the structure and properties of the interfaces between the domains mainly merged with the same angle. Although the merged domains have various interfaces with/without wrinkles and/or increased defect-related Raman D-band intensity, the intra-domain transport showed higher carrier mobility reaching 20,000 cm(2) V(-1) s(-1) on SiO2 at 280 K (the mean value was 7200 cm(2) V(-1) s(-1)) than that measured for inter-domain areas, 6400 cm(2) V(-1) s(-1) (mean value 2000 cm(2) V(-1) s(-1)). The temperature dependence of the mobility suggests that impurity scattering dominates at the interface even for the merged domains with the same orientation. This study highlights the importance of domain interfaces, especially on the carrier transport properties, in CVD-grown graphene.

Dense Arrays of Highly Aligned Graphene Nanoribbons Produced by Substrate‐Controlled Metal‐Assisted Etching of Graphene

Dense arrays of aligned graphene nanoribbons (GNRs) are fabricated by substrate-controlled etching of large-area single-layer graphene. An adequate choice of etching substrate and catalyst deposition method allows densities up to 25 nanoribbons μm(-1) to be obtained with average widths of 19 nm. The efficacy of the method is evidenced by the high on/off ratios of back-gated transistors made with these GNRs, which can go up to 5000.

Lattice-Oriented Catalytic Growth of Graphene Nanoribbons on Heteroepitaxial Nickel Films

Graphene nanoribbons (GNRs) are a promising material for electronic applications, because quantum confinement in a one-dimensional nanostructure can potentially open the band gap of graphene. However, it is still a challenge to synthesize high-quality GNRs by a bottom-up approach without relying on lithographic techniques. In this work, we demonstrate lattice-oriented catalytic growth of single-layer GNRs on the surface of a heteroepitaxial Ni film. Catalytic decomposition of a poly(methyl methacrylate) film on the Ni(100) film at 1000 °C gives narrow nanoribbons with widths of 20-30 nm, which are aligned along either [011] or [011] directions of the Ni lattice. Furthermore, low-energy electron microscope (LEEM) analysis reveals that orientation of carbon hexagons in these GNRs is highly controlled by the underlying Ni(100) lattice, leading to the formation of zigzag edges. This heteroepitaxial approach would pave a way to synthesize nanoribbons with controlled orientation for future development of electronic devices based on graphene nanostructures.

On the nucleation of graphene by chemical vapor deposition

We demonstrate that homogeneous single-layer graphene can be grown by simply annealing crystalline Cu(111)/c-plane sapphire (α-Al2O3) at 900 and 1000 °C without additional carbon supply. The resulting graphene film shows a high carrier mobility of 1210 cm2 V−1 s−1. However, the annealing at a lower temperature of 800 °C gives an amorphous carbon film. Further investigations indicate that graphitization of amorphous carbon and/or adsorbed carbon atoms during chemical vapour deposition (CVD) is not simply a consequence of carbon supersaturation, it is also affected by CVD temperature, and crystallographic plane of the underlying metal, which are essentially correlated to the energy barrier of nucleation. Our results provide the direct experimental evidence to elucidate the influencing factors of graphitization of amorphous carbon, and contribute fundamental insight into the nucleation and growth of graphene to improve its quality for applications.

Electron Microscopy in Air: Transparent Atomic Membranes and Imaging Modes

Environmental Scanning Electron Microscopy (ESEM) where differential pumping and a pressurelimiting aperture [1] has enabled electron imaging in partial atmosphere environments. The typical imaging gas path length (GPL) is 2~5 electron mean free paths (mfp), resulting in lower contrast than a pure vacuum SEM. More recently, the use of thick (100nm) SiNx membranes to separate the electron optics in vacuum and the specimens in atmosphere has led to Atmospheric Scanning Electron Microscopy (ASEM) [2] where imaging in liquid or air without a specimen chamber is made possible by placing specimens in contact with the membrane. However, the resolution and contrast in both ESEM and ASEM are compromised by the multiple electron scattering. By keeping the sample away from the electron transparent membrane, thinner membranes can be used (and reused). This has led to the airSEM [3,4], where a thin SiNx window is used to separate electron optic and air from an optically-aligned sample (Fig 1(a)). The airSEM is able to image specimens in air with high throughput – a few minutes per sample at low magnification [3].

Initial stage of hexagonal boron nitride growth in diffusion and precipitation method

This study investigated the initial stage of hexagonal boron nitride (h-BN) formation on a Ni plate by the diffusion and precipitation method. Regular triangle-shaped domains with two orientations were observed on several Ni faces, as commonly observed in the chemical vapor deposition method. On (213) surfaces, strained triangle-shaped domains with unique orientation were observed, suggesting the possible formation of large single-crystalline domains. Moreover, stripe-shaped h-BN with lengths comparable to Ni grain size (~100 µm) was formed along the direction on (111) surfaces. Our results show that boron stripes are first formed and the following nitridation converts them into h-BN stripes.