Control of rotation angles of multilayer graphene on SiC (0001‾) by substrate off-direction and angle.

Graphene on SiC (0001‾) tends to grow in multiple layers and does not have a single orientation relation with the SiC substrate. It has been considered impossible to control the rotation angle of multilayer graphene on SiC (0001‾). In this study, we grew graphene on off-axis SiC substrates with various off angles from 0° to 8° and investigated their in-plane rotation and electronic structures systematically. As the off angle toward the [112‾0]SiCdirection increased, graphene rotated by 30° with respect to SiC became less dominant and instead, graphene rotated by 30 ± 2.5° appeared. We also found that the uniformity of the graphene rotation angle was relatively high on SiC substrates with a small off angle toward the [11‾00]SiCdirection. Our results suggest that the step-terrace structure defined by the substrate off-direction and angle plays an important role in the controllability of the rotation angle of graphene.

Surface etching and edge control of hexagonal boron nitride assisted by triangular Sn nanoplates.

Hexagonal boron nitride (hBN) is an ideal insulating substrate and template for other two-dimensional (2D) materials. The combination of hBN and 2D materials of group IV atoms, such as graphene, is interesting, because it can offer attractive physical properties and promising applications. Here, we demonstrate the unique behavior of tin (Sn), one of the group IV elements, on multilayer hBN which was grown by chemical vapor deposition (CVD). At high temperatures, triangular nanoplates formed after thermal deposition of Sn on the hBN surface, with their orientations determined by the hBN lattice. The triangular Sn nanoplates moved on the hBN surface, leaving monolayer-deep nanotrenches. Low-energy electron microscopy (LEEM) revealed that the nanotrenches are aligned in the armchair directions of the hBN. Furthermore, an additional Ar annealing without supplying Sn vapor induced the structural change of the linear trenches to triangular pits, indicating the preferential formation of zigzag edges in the absence of Sn. Our work highlights the unique behavior of Sn on hBN and offers a novel route to engineer the hBN surface.

Epitaxial Intercalation Growth of Scalable Hexagonal Boron Nitride/Graphene Bilayer Moiré Materials with Highly Convergent Interlayer Angles.

Vertically stacked two-dimensional van der Waals (vdW) heterostructures with specific interlayer angles exhibit peculiar physical properties. Nowadays, most of the stacked layers are fabricated by mechanical exfoliation followed by precise transfer and alignment with micrometer spatial accuracy. This stringent ingredient of sample preparation limits the productivity of device fabrication and the reproducibility of device performance. Here, we demonstrate the one-pot chemical vapor deposition growth of hexagonal boron nitride (hBN)/graphene bilayers with a high-purity moiré phase. The epitaxial intercalation of graphene under a hydrogen-terminated hBN template leads to convergent interlayer angles of less than 0.5°. The near 0° stacking angle shows almost 2 orders of magnitude higher likelihood of occurrence compared with angles larger than 0.5°. The bilayers show a substantial enhancement of carrier mobility compared with monolayer graphene owing to protection from the top hBN layer. Our work proposes a large-scale fabrication method of hBN/graphene bilayers with a high uniformity and controlled interlayer rotation and will promote the production development for high-quality vdW heterostructures.

Isothermal Growth and Stacking Evolution in Highly Uniform Bernal-Stacked Bilayer Graphene.

Controlling the stacking order in bilayer graphene (BLG) allows realizing interesting physical properties. In particular, the possibility of tuning the band gap in Bernal-stacked (AB) BLG (AB-BLG) has a great technological importance for electronic and optoelectronic applications. Most of the current methods to produce AB-BLG suffer from inhomogeneous layer thickness and/or coexistence with twisted BLG. Here, we demonstrate a method to synthesize highly pure large-area AB-BLG by chemical vapor deposition using Cu-Ni films. Increasing the reaction time resulted in a gradual increase of the AB stacking, with the BLG eventually free from twist regions for the longer growth times (99.4% of BLG has AB stacking), due to catalyst-assisted continuous BLG reconstruction driven by carbon dissolution-segregation processes. The band gap opening was confirmed by the electrical measurements on field-effect transistors using two different device configurations. The concept of the continuous reconstruction to achieve highly pure AB-BLG offers a way to control the stacking order of catalytically grown two-dimensional materials.

Dopamine detection on activated reaction field consisting of graphene-integrated silicon photonic cavity.

Graphene is widely recognized as an outstanding and multi-functional material in various application fields such as electronics, photonics, mechanics, and life sciences. We propose a neurotransmitter sensor with ultra-small volume for detecting the photonic light-matter response. Such detection can be achieved using surface-activated monolayer graphene sheets and CMOS-compatible silicon-photonic circuits. Patterned pieces of CVD-grown graphene are integrated on the top of a silicon micro-ring resonator, which induce the adsorption of catecholamine molecules originated from the π-stacking effect. We used dopamine to demonstrate such detection and examine the sensitivity of graphene-dopamine coupling. To avoid high optical insertion loss and degradation of resonance characteristics caused by a graphene's extremely high optical absorption coefficient in the near infrared region, a ring resonator with adjusted coupling design is used to compensate for the drawbacks. Owing to the advanced nano-sensing platform and measurement system, an activated graphene-sensing surface of only ∼30 µm2/ch enables π coupling to dopamine with enough sensitivity to detect less than 10-µM solution concentration. The detection mechanism through the surface reaction is also verified by optical simulation and atomic force microscopy measurement, revealing that the flowing dopamine molecules can only occupy the outermost surface of graphene. We expect this sensor to contribute to the development of an innovative label-free and disposable bio-sensing platform with accurate, sensitive, and fast response.

Synthesis of sub-millimeter single-crystal grains of aligned hexagonal boron nitride on an epitaxial Ni film.

Hexagonal boron nitride (h-BN), an insulating two-dimensional (2D) layered material, has attracted increasing interest due to its electrical screening effect, high-temperature-resistant gas barrier properties, and other unique applications. However, the presence of grain boundaries (GBs) in h-BN is a hindrance to obtain these properties. Here, we demonstrate the epitaxial growth of monolayer h-BN by chemical vapor deposition (CVD) on Ni(111) thin films deposited on c-plane sapphire. The Ni(111) films showed higher thermal stability than Cu(111) and Cu-Ni(111) alloy films, allowing us to perform CVD growth at a high temperature of 1100 °C. This resulted in an increase of the h-BN grain sizes to up to 0.5 millimeter, among the highest reported so far, and in a well-defined triangular grain shape. Low-energy electron microscopy (LEEM) revealed the epitaxial relationship between h-BN and the underlying Ni(111) lattice, leading to a preferential alignment of the h-BN grains. Both the large grain size and the alignment are expected to facilitate the synthesis of h-BN with a low density of GBs. We also found that the addition of N2 gas during the CVD improves the crystalline shape of the h-BN grains, changing from an irregular, truncated to a sharp triangle. The growth behavior of monolayer h-BN is further discussed in terms of the dependences on growth temperature and pressure, as well as on the structural evolution of the Ni metal catalyst. Our findings not only help understand the h-BN growth mechanism but also offer a new route to grow high-quality, monolayer h-BN films.

Visualization of three different phases in a multiphase steel by scanning electron microscopy at 1 eV landing energy.

In this study, we investigated an observation technique by super low energy scanning electron microscopy (SLESEM) at below 5 eV and its contrast mechanism for analyzing complex microstructures of a multiphase steel consisting of ferrite, martensite and austenite. With SLESEM at 1 eV, the three phases were observed as different brightness levels, ferrite as the darkest contrast, martensite as the second brightest and austenite as the brightest. These contrasts disappeared at 2 eV or higher. Similar contrasts and phenomena were also observed in the results of low energy electron microscopy (LEEM). According to the energy dependences of the LEEM intensities of the three phases, the threshold energies of the transition from electron reflection to surface impact were determined to be 0.00 eV, 0.15 eV and 0.39 eV for ferrite, martensite and austenite, respectively. These differences in thresholds indicate that the potentials on the surfaces of each phase are different, which is considered to result in the different brightness of each phase. This potential differences are probably due to the contact potentials generated when phases with different work functions contact each other. Although the sample is covered by a thin native oxide film (several nm thickness), the potentials can affect the incident electrons through the oxide film.

Catalyst-Selective Growth of Single-Orientation Hexagonal Boron Nitride toward High-Performance Atomically Thin Electric Barriers.

The ability to control the crystal orientation of 2D van der Waals (vdW) layered materials grown on large-scale substrates is crucial for tailoring their electrical properties, as well as for integration of functional 2D devices. In general, multiple orientations, i.e., two or four orientations, appear through the crystal rotational symmetry matching between the material and its substrate. Here, it is reported that hexagonal boron nitride (h-BN), an ideal electric barrier in the family of 2D materials, has a single orientation on inclined Cu (1 0 1) surfaces, where the Cu planes are tilted from the (1 0 1) facet around specific in-plane axes. Density functional theory (DFT) calculation indicates that this is a manifestation of only one favored h-BN orientation with the minimum vdW energy on the inclined Cu (1 0 1) surface. Moreover, thanks to the high interfacial strength with the underlying Cu, the single-orientation h-BN is free of thermal wrinkles, and exhibits a spatially homogeneous morphology and tunnel conductance. The findings point to a feasible approach to direct growth of single-orientation, wrinkle-free h-BN thin film for high-performance 2D electrical devices, and will be of benefit for controllable synthesis of other vdW materials.

Surface-Mediated Aligned Growth of Monolayer MoS2 and In-Plane Heterostructures with Graphene on Sapphire.

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS2) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS2 grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS2-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS2, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS2 growth on sapphire was further developed to the rational synthesis of an in-plane MoS2-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS2 parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS2 was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures.

Ultrafast Terahertz Nonlinear Optics of Landau Level Transitions in a Monolayer Graphene.

We investigated the ultrafast terahertz (THz) nonlinearity in a monolayer graphene under the strong magnetic field using THz pump-THz probe spectroscopy. An ultrafast suppression of the Faraday rotation associated with inter-Landau level (LL) transitions is observed, reflecting the Dirac electron character of nonequidistant LLs with large transition dipole moments. A drastic modulation of electron distribution in LLs is induced by far off-resonant THz pulse excitation in the transparent region. Numerical simulation based on the density matrix formalism without rotating-wave approximation reproduces the experimental results. Our results indicate that the strong light-matter coupling regime is realized in graphene, with the Rabi frequency exceeding the carrier wave frequency and even the relevant energy scale of the inter-LL transition.

Spatially Controlled Nucleation of Single-Crystal Graphene on Cu Assisted by Stacked Ni.

In spite of recent progress of graphene growth using chemical vapor deposition, it is still a challenge to precisely control the nucleation site of graphene for the development of wafer-scale single-crystal graphene. In addition, the postgrowth patterning used for device fabrication deteriorates the quality of graphene. Herein we demonstrate the site-selective nucleation of single-crystal graphene on Cu foil based on spatial control of the local CH4 concentration by a perforated Ni foil. The catalytically active Ni foil acts as a CH4 modulator, resulting in millimeter-scale single-crystal grains at desired positions. The perforated Ni foil also allows to synthesize patterned graphene without any postgrowth processing. Furthermore, the uniformity of monolayer graphene is significantly improved when a plain Ni foil is placed below the Cu. Our findings offer a facile and effective way to control the nucleation of high-quality graphene, meeting the requirements of industrial processing.

Etchant-free graphene transfer using facile intercalation of alkanethiol self-assembled molecules at graphene/metal interfaces.

We report a novel etchant-free transfer method of graphene using the intercalation of alkanethiol self-assembled monolayers (SAMs) at the graphene/Cu interfaces. The early stage of intercalation proceeds through graphene grain boundaries or defects within a few seconds at room temperature until stable SAMs are formed after a few hours. The formation of SAMs releases the compressive strain of graphene induced by Cu substrates and make graphene slightly n-doped due to the formation of interface dipoles of the SAMs on metal surfaces. After SAM formation, the graphene is easily delaminated off from the metal substrates and transferred onto insulating substrates. The etchant-free process enables us to decrease the density of charged impurities and the magnitude of potential fluctuation in the transferred graphene, which suppress scattering of carriers. We also demonstrate the removal of alkanethiol SAMs and reuse the substrate. This method will dramatically reduce the cost of graphene transfer, which will benefit industrial applications such as of graphene transparent electrodes.

Direct growth of graphene on SiC(0001) by KrF-excimer-laser irradiation.

In this report, we propose a direct patterning method of graphene on the SiC(0001) surface by KrF-excimer-laser irradiation. In this method, Si atoms are locally sublimated from the SiC surface in the laser-irradiated area, and direct graphene growth is induced by the rearrangement of surplus carbon on the SiC surface. Using Raman microscopy, we demonstrated the formation of graphene by laser irradiation and observed the growth process by transmission electron microscopy and conductive atomic force microscopy. When SiC was irradiated by 5000 shots of the laser beam with a fluence of 1.2 J/cm2, two layers of graphene were synthesized on the SiC(0001) surface. The number of graphene layers increased from 2 to 5-7 with an increase in the number of laser shots. Based on the results of conductive-atomic force microscopy measurements, we conclude that graphene formation was initiated from the step area, after which the graphene grew towards the terrace area by further Si evaporation and C recombination with increasing laser irradiation.

Energy Dissipation in Graphene Mechanical Resonators with and without Free Edges.

Graphene-based nanoelectromechanical systems (NEMS) have high future potential to realize sensitive mass and force sensors owing to graphene's low mass density and exceptional mechanical properties. One of the important remaining issues in this field is how to achieve mechanical resonators with a high quality factor (Q). Energy dissipation in resonators decreases Q, and suppressing it is the key to realizing sensitive sensors. In this article, we review our recent work on energy dissipation in doubly-clamped and circular drumhead graphene resonators. We examined the temperature (T) dependence of the inverse of a quality factor ( Q - 1 ) to reveal what the dominant dissipation mechanism is. Our doubly-clamped trilayer resonators show a characteristic Q - 1 -T curve similar to that observed in monolayer resonators: Q - 1 ∝ T 2 above ∼100 K and ∝ T 0.3 below ∼100 K. By comparing our results with previous experimental and theoretical results, we determine that the T 2 and T 0.3 dependences can be attributed to tensile strain induced by clamping metals and vibrations at the free edges in doubly-clamped resonators, respectively. The Q - 1 -T curve in our circular drumhead resonators indicates that removing free edges and clamping metal suppresses energy dissipation in the resonators, resulting in a linear T dependence of Q - 1 in a wide temperature range.

On-chip FRET Graphene Oxide Aptasensor: Quantitative Evaluation of Enhanced Sensitivity by Aptamer with a Double-stranded DNA Spacer.

We propose a molecular design for a biomolecular probe to realize an on-chip graphene oxide (GO) aptasensor with enhanced sensitivity. Here, GO works as an excellent acceptor for fluorescence resonance energy transfer. We inserted a rigid double-stranded DNA as a spacer between the GO surface and the aptamer sequence to extend the distance between a fluorescence dye and the GO surface during molecular recognition. We examined the dependence of the sensitivity on the length of the spacer quantitatively by using a 2×2 linear-array aptasensor. We used the modified aptamer with 10 and 30 base pair (bp) double-stranded DNA spacers. The signal with a 30bp-spacer was about twice as strong that with a 10bp-spacer as regards both thrombin and prostate specific antigen detections. The improvement in the sensitivity was supported by a model calculation that estimated the effect of spacer length on fluorescence recovery efficiency.

Growth and Optical Properties of High-Quality Monolayer WS2 on Graphite.

Atomic-layer transition metal dichalcogenides (TMDCs) have attracted appreciable interest due to their tunable band gap, spin-valley physics, and potential device applications. However, the quality of TMDC samples available still poses serious problems, such as inhomogeneous lattice strain, charge doping, and structural defects. Here, we report on the growth of high-quality, monolayer WS2 onto exfoliated graphite by high-temperature chemical vapor deposition (CVD). Monolayer-grown WS2 single crystals present a uniform, single excitonic photoluminescence peak with a Lorentzian profile and a very small full-width at half-maximum of 21 meV at room temperature and 8 meV at 79 K. Furthermore, in these samples, no additional peaks are observed for charged and/or bound excitons, even at low temperature. These optical responses are completely different from the results of previously reported TMDCs obtained by mechanical exfoliation and CVD. Our findings indicate that the combination of high-temperature CVD with a cleaved graphite surface is an ideal condition for the growth of high-quality TMDCs, and such samples will be essential for revealing intrinsic physical properties and for future applications.

On-chip graphene oxide aptasensor for multiple protein detection.

The versatility of an on-chip graphene oxide (GO) aptasensor was successfully confirmed by the detection of three different proteins, namely, thrombin (TB), prostate specific antigen (PSA), and hemagglutinin (HA), simply by changing the aptamers but with the sensor composition remaining the same. The results indicate that both DNA and RNA aptamers immobilized on the GO surface are sufficiently active to realize an on-chip aptasensor. Molecular selectivity and concentration dependence were investigated in relation to TB and PSA detection by using a dual, triple, and quintuple microchannel configuration. The multiple target detection of TB and PSA on a single chip was also demonstrated by using a 2×3 linear-array GO aptasensor. This work enables us to apply this sensor to the development of a multicomponent analysis system for a wide variety of targets by choosing appropriate aptamers.

Raman spectroscopic investigation of polycrystalline structures of CVD-grown graphene by isotope labeling.

Topological defects, such as point defects, dislocations and grain boundaries, have a dramatic influence on the chemical and physical properties of large-scale graphene grown by chemical vapor deposition (CVD) method. Here we demonstrate the Raman visualization of polycrystalline structures in an isotopically modified CVD graphene. By means of the reversible reaction of methane on a copper catalyst, the etching of (12)C-lattice and surface deposition of (13)C-atoms occur in CVD graphene by sequentially introducing hydrogen and isotopic methane after standard growth of graphene with full monolayer coverage. Spatial Raman spectroscopic mapping on labeled graphene reveals pronounced network-like (13)C-rich regions, which are further identified to exist along the grain boundaries of graphene by low-energy electron microscopy. The structural defects inside the graphene grains are also targeted in the isotope labeling process. Our work opens a new way to investigate multiple grain structures in CVD graphene with a simple spectroscopic technique.

Direct chemical vapor deposition growth of WS2 atomic layers on hexagonal boron nitride.

Atomically thin transition metal dichalcogenides (TMDCs) have attracted considerable interest owing to the spin-valley coupled electronic structure and possibility in next-generation devices. Substrates are one of the most important factors to limit physical properties of atomic-layer materials, and among various substrates so far investigated, hexagonal boron nitride (hBN) is the best substrate to explore the intrinsic properties of atomic layers. Here we report direct chemical vapor deposition (CVD) growth of WS2 onto high-quality hBN using a 3-furnace CVD setup. Triangular-shaped WS2 grown on hBN have shown limited crystallographic orientation that is related to that of the underlying hBN. Photoluminescence spectra of the WS2 show an intense emission peak at 2.01 eV with a quite small fwhm of 26 meV. The sharp emission peak indicates the high quality of the present WS2 atomic layers with high crystallinity and clean interface.

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.