鈴木 哲

Abstract We would like to improve detection sensitivity by making photoelectron transmission window (SiNx membrane) of liquid cell ultra-thin for liquid measurement using XPS or X-ray PEEM at UHV. In this study, thinning of the membrane using gas cluster ion beams (GCIB) was demonstrated and the burst pressure was compared with those thinned with atomic 400 eV Ar+ ions. It was shown that SiNx membranes thinned by GCIB was 2.5 times higher burst pressure than the Ar+ ions. In addition, improvement of sensitivity of characteristic X-ray from liquid-water induced by low-energy electrons was investigated. By using 4.5 nm thick SiNx membrane etched by GCIB, the X-ray intensity became 1.6 times higher than those from 11 nm thick pristine membrane at electron beam energy of 1.5 keV. This result showed good agreement with Monte Carlo simulation results of the electron-beam-induced X-ray emission from liquid-water beneath SiNx membrane.

We investigated the chemical composition of polytetrafluoroethylene (PTFE) under bending stress using hard X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy. Our measurements revealed the breaking of C–F bonds in the side chains and conspicuous observation of C–C bonds in the main chain only on the surface under bending stress (carbon-rich). Moreover, we found that the breaking of C–F bonds was dependent on the tensile strain caused by bending. Investigating the effects of tensile and compressive stresses induced by bending, the tensile stress was found to significantly contribute to the breaking of C–F bonds. However, the C–F bonds were hardly broken under uniaxial tensile stress. These findings suggest that tensile stress due to bending, rather than uniaxial tensile stress, causes significant C–F bond scission in the PTFE. This result is attributed to the force acting toward the center of curvature owing to bending, which does not occur under uniaxial tensile stress. Our results provide a better understanding of microscopic PTFE surfaces subjected to flexural tensile stress for nanofluidics and medical engineering applications. Additionally, our findings suggest that carbon-rich structures can be easily fabricated, which may lead to the development of processes for fabrication of two-dimensional materials.

We have completed a system that can achieve both deep x-ray lithography and submicron x-ray lithography with a single beamline by introducing the combination of x-ray plane and cylindrical mirrors. This x-ray lithography system can provide a large-scale microfabrication processing with 210 × 300 mm2 (A4 size). To exploit multiscale lithography, the beamline has a beam transport vacuum duct with a two-stage stacked structure and a 5-axis stage. This two-stage stacked structure allows us to fabricate both micron scale structures with high aspect ratios and submicron scale structures using the same beamline. In addition, x-ray imaging and computer tomography (CT) system are connected to the x-ray lithography system for nondestructive inspection and evaluation of the fabricated microstructures. The x-ray imaging system constructed this study has a relatively low energy range of x-ray energy in the beamline, which is in the range of 2–15 keV or less. Therefore, relatively good absorption contrast can be obtained for plastic materials, biomaterials, and the like. Since nondestructive imaging of the processed shape by x-ray lithography is possible, it is a very useful system in processing and evaluation can be performed simultaneously. This system also enables us to obtain the live images with keeping the creature alive in liquid using an indirect x-ray imaging system which converts x-ray images to visible light images through the fluorescent plate.

In near-ambient-pressure photoelectron spectroscopy, the photoelectron intensity is assumed to follow the Beer-Lambert law, that is, the intensity decreases exponentially with distance d between the sample and the aperture cone. In this study, the gas pressure dependence of photoelectron intensity is systematically studied in a wide range of d values from 0.3 up to 5 mm. The experimental results were reproduced by replacing d with d + do (do is a constant) in the Beer-Lambert law. The do value was evaluated as ∼1 mm, which is considerably larger than the normal d value of 0.3 mm. Fluid dynamics simulation results obtained using a structural model with a size close to that of the actual differential pumping system suggested that the residual gas in the long pumping path caused a large do value.

We investigated the mechanism of polytetrafluoroethylene (PTFE) chain scission through in situ hard X-ray photoelectron spectroscopy at room temperature, 200 °C, and 230 °C. The C–C bonds in the main chain and C–F bonds in the side chains were broken, and F desorption from the PTFE surface was observed at room temperature. The formation of CF3 was also observed from the recombination of broken C–C bonds in the main chain and detached F, which were not induced by soft X-rays. In contrast, when the PTFE substrate was irradiated with hard X-rays at 200 °C, the CF3 intensity initially produced by recombination reactions decreased with irradiation time, and the photoelectron spectrum retained the original PTFE spectrum. Under these conditions, the F1s/C1s intensity ratio did not change with the irradiation time; hence, the fragment containing only CF2, the chemical composition of the original PTFE, was desorbed. When the substrate temperature was 230 °C, the CF3 intensity increased in relation to that at 200 °C. This result indicated that the formation of CF3 via recombination reactions of broken molecular chains is enhanced by thermal assistance. These phenomena were considered to be based on the balance between recombination and desorption by photochemical and pyrochemical reactions. These results will lead to a better understanding of the use of X-ray-irradiated fluorine resins and PTFE in potential space-based environments. This study will also promote the improvement of PTFE microfabrication methods and thin-film formation using synchrotron radiation.

The competition between magnetic shape anisotropy and the induced uniaxial magnetic anisotropy in the heterojunction between a ferromagnetic layer and a ferroelectric substrate serves to control magnetic domain structures as well as magnetization reversal characteristics. The uniaxial magnetic anisotropy, originating from the symmetry breaking effect in the heterojunction, plays a significant role in modifying the characteristics of magnetization dynamics. Magnetoelastic phenomena are known to generate uniaxial magnetic anisotropy; however, the interfacial electronic states that may contribute to the uniaxial magnetic anisotropy have not yet been adequately investigated. Here, we report experimental evidence concerning the binding energy change in the ferromagnetic layer/ferroelectric substrate heterojunction using X-ray photoemission spectroscopy. The binding energy shifts, corresponding to the chemical shifts, reveal the binding states near the interface. Our results shed light on the origin of the uniaxial magnetic anisotropy induced from the heterojunction. This knowledge can provide a means for the simultaneous control of magnetism, mechanics, and electronics in a nano/microsystem consisting of ferromagnetic/ferroelectric materials.

The charging effect often complicates photoemission spectroscopy and x-ray absorption spectroscopy of an insulating material. Here, monolayer graphene was used as a conductive layer to prevent the charging effect of insulating substrates such as glass and LiNbO3. Charging-free spectra were obtained with various photon energies ranging from vacuum ultraviolet light to hard x-rays. This method could also be applied to photoemission spectroscopy of epoxy adhesives and to photoemission electron microscopy of an insulating film. Photoelectron transmissivities for the transferred graphene film were evaluated over a wide kinetic energy range from 29 to 7910eV. A minimum transmissivity of similar to 0.1 was found at a kinetic energy of similar to 60eV, which rose to 0.86 at 7910eV. In terms of the kinetic energy dependence of the transmissivity, this method is especially suitable for conventional and hard x-ray photoelectron spectroscopy.

X-ray-radiolysis-induced photochemical reaction of a liquid solution enables the direct synthesis and immobilization of nano/micro-scale particles and their aggregates onto a desired area. As is well known, the synthesis, growth and aggregation are dependent on the pH, additives and X-ray irradiation conditions. In this study, it was found that the topography and composition of synthesized particles are also dependent on the types of substrate dipped in an aqueous solution of Cu(COOCH3)(2) in the X-ray-radiolysis-induced photochemical reaction. These results are attributed to the fact that a secondary electron induced by the X-ray irradiation, surface or interface on which the particles are nucleated and grown influences the particle shape and composition. This study will shed light on understanding a novel photochemical reaction route induced under X-ray irradiation. The development of this process using the X-ray-radiolysis-induced photochemical reaction in aqueous liquids enables us to achieve the rapid and easy operation of the synthesis, growth and immobilization of special nano/micro-scale complex materials or multifunctional composites.

An inexpensive, simple, and high-activity catalyst preparation method has been introduced in this work. Pt and RuOx catalysts were fabricated by soaking inexpensive graphite electrodes (pencil-lead graphite rod: PGR) in catalyst precursor solutions and using a simple flame-annealing method, which results in lower amount of Pt and RuOx catalyst layers. From X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure analysis, it has been found that platinum and ruthenium were deposited as zero-valence metal (Pt) and oxide (RuOx), respectively. Catalytic activities of Pt/PGR and RuOx/PGR for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were evaluated using neutral 1 M Na2SO4 aqueous electrolyte, respectively. Although HER and OER currents using PGR without catalysts were -16 mA cm(-2) (at -1.5 V vs Ag/AgCl) and +20 mA cm(-2) (at +2.0 V vs Ag/AgCl), they were improved to -110 and +80 mA cm(-2) with catalysts (Pt and RuOx), respectively. Such an inexpensive and rapid catalyst electrode preparation method on PGR using flame-annealing is a very significant method in the initial catalyst activity evaluation requiring a large amount of trial and error.

Synthesis and immobilization of caltrop cupric particles onto a Si substrate using X-ray radiolysis directly from a liquid solution of Cu(COOCH3)(2) is demonstrated. Caltrop cupric oxide particles are formed in the X-ray radiolysis of aqueous solutions of Cu(COOCH3)(2), which also contain methanol, ethanol, 2-propanol or 1-propanol as OH scavenger. The blade lengths of the caltrop particles are dependent on the alcohol chain length. In particular, it was found that an alkyl alcohol whose chain length is longer than four is unable to synthesize any particles in aqueous solutions of Cu(COOCH3)(2) in X-ray radiolysis. These results are attributed to the alkyl alcohol chain length influencing the rate of reaction of radicals and determines the solvable ratio of its alcohol into water. In addition, it was found that the synthesized particle geometric structure and composition can also be controlled by the pH of the aqueous solution in the X-ray radiolysis. This study may open a door to understanding and investigating a novel photochemical reaction route induced under X-ray irradiation. The development of the X-ray radiolysis process enables us to achieve the rapid and easy process of synthesis and immobilization of higher-order nano/microstructure consisting of various materials.

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 pi-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 similar to 30 mu m(2)/ch enables pi coupling to dopamine with enough sensitivity to detect less than 10-mu M 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. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Submonolayer h-BN was grown on Ni foil in ultra-high vacuum by the diffusion and precipitation method and the growth process was studied by X-ray photoelectron spectroscopy. Formation of h-BN started to be observed at 600 degrees C. All through the process, the surface was always slightly B-rich, which is consistent with the fact that B which is soluble in Ni at a high temperature can diffuse in Ni by the conventional bulk diffusion and insoluble N cannot. Moreover, both formation and decomposition of h-BN were found to occur at elevated temperatures possibly depending on provision of N atoms to the surface. On the Ni surface, decomposition of h-BN was observed at a relatively low temperature of 800 degrees C. (C) 2019 The Japan Society of Applied Physics

To achieve the three dimensional additive manufacturing process, we investigated X-ray radiolysis-induced chemical reaction of Cu(CH3COO)2 solution. Here, we demonstrated synthesis and immobilization of cupric oxide particles onto a silicon or aluminium substrate using X-ray radiolysis directly from a liquid solution. The X-ray radiolysis of Cu(CH3COO)2 solutions was observed to produce curious shaped microstructures consisting of cupric oxide (CuO, Cu2O, Cu4O3) particles and Cu particles. The sizes of the particles depended on the additive type of alcohol. The results indicate that there are several routes and reaction processes for these particles and aggregation to be synthesized. In addition, we demonstrated the synthesis of these particles using X-ray radiolysis cell combined with a solution flow system. The developed technique of X-ray radiolysis enables us to achieve the rapid and easy synthesis of higher-order structures consisting of cupric oxide and copper particles onto the desired area.

Inexpensive and sensitive graphite electrodes were fabricated by applying flame annealing to pencil-graphite rods (PGRs) as electrodes for water electrolysis cells. The resin (polymer, binder) on the surface of PGR was removed by flame annealing to make it porous, and the graphite electrodes with high activity and low cost were obtained. By flame annealing the PGR, although the PGR electrode became active upon water electrolysis, the PGR electrode became instable for long-time operation. The effects of flame annealing on PGR for water electrolysis were analyzed by SEM, FT-IR spectroscopy, Raman spectroscopy, NEXAFS, and electrochemical impedance spectroscopy (EIS).

Y We present the synthesis of nano/micro-scale metallic and oxidized particles onto various substrates using the X-ray radiolysis with a manufactured flow system. As a typical example, the cupric particles are deposited onto the substrates. Scanning electron microscopy with energy dispersive X-ray analysis, X-ray diffraction, micro-Raman spectroscopy have been demonstrated to investigate and understand the physical and chemical mechanisms to synthesize the particles. These analyses enable us to provide the understanding that the radical ions nucleated by the X-ray irradiation in the solution play a significant role in synthesizing and ripening of particles and clusters. The process using X-ray radiolysis sheds light on the novel three dimensional additive manufacturing process for three dimensional structure formation and additive process.

The electronic structure of monolayer hexagonal boron nitride grown on Ni by the diffusion and precipitation method was studied by x-ray absorption spectroscopy, emission spectroscopy, x-ray photoelectron spectroscopy and micro-ultraviolet photoemission spectroscopy. No indication of hybridization between h-BN pi and Ni 3d orbitals was observed. That is, the monolayer h-BN was found to be in the quasi-free-standing state. These results are in striking contrast to those of previous studies in which h-BN was strongly bound to the Ni surface by the orbital hybridization. The absence of hybridization is attributed to absence of a Ni(111) surface in this study. The lattice-matched Ni(111) surface is considered to be essential to orbital hybridization between h-BN and Ni.

Polarized transmission and reflection spectra in the terahertz region were obtained from a graphene complementary split ring resonator device. The complementary structure combined with an ion gel gate electrode rendered the optical properties of the device tuneable. The oscillator strength at the intraband plasmon resonance was largely enhanced with the gate-voltage-induced doping, and absorption exceeded 2.3%/layer of the interband transition. The resonance frequency could also be largely increased with the gate voltage. These results suggest the possibility of graphene-based metamaterials with tuneable permeability or permittivity and tuneable resonance frequencies. (C) 2017 The Japan Society of Applied Physics

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 (similar to 100 mu m) was formed along the [112] direction on (111) surfaces. Our results show that boron stripes are first formed and the following nitridation converts them into h-BN stripes. (C) 2017 The Japan Society of Applied Physics

We show millimeter-scale graphene single crystals synthesized on commercial Cu foils by the atmospheric pressure chemical vapor deposition (CVD) method, which does not involve the routine use of a specially designed CVD reactor or long-term processes. Upon the designed annealing step in the Ar environment, the natural oxide layer covering on Cu catalysts is to a large extent maintained and is further used to protect the surface passivation and restrict the graphene nucleation. Moreover, for Cu foils placing on certain solid supports, we found that the graphene deposition is highly related to the environments proximate to each surface (referred to as open or confined space, respectively). For instance, the domain size of as-grown graphene is larger (smaller), while the nucleation density is higher (lower) on the back (top) surface. The possible mechanism to interpret the discrepancy on either side is discussed in the frame of the graphene nucleation and growth kinetics. At the nucleation stage, the thermal decomposition of the oxide layer leads to oxygen (O) desorption at high temperature on the open side and dominates the temperature dependence of nucleation density. On the confined side, the 0 desorption is suppressed due to the collision rebound effect, but highly concentrated active carbon species will be trapped in the vicinity of the back surface, which may promote the threshold of nucleation on the O-containing Cu surface. The following growth of graphene islands is edge-attachment limited on both sides of the Cu foil but with different enlargement rates. The roughness of support substrates also affects the deposition of graphene. With an optimized annealing condition and a polished quartz support, similar to 3 mm isolated graphene islands with an average growth rate of similar to 25 mu m/min were obtained. The as-grown hexagonal domains were further confirmed to be uniform, monolayer, single-crystalline graphene with a field-effect mobility of similar to 4900 cm(2) s(-1) at room temperature.

Both transmission and reflection spectra were obtained from graphene microribbons on a SiC substrate, which resonantly coupled with terahertz light through plasmon excitation. An absorption spectrum was also derived from the transmission and reflection spectra. Absorption by the confined intraband plasmons reached 5-10%/layer, which is considerably larger than the wavelength-independent interband absorption of 2.3%/layer. The absorption was found to be larger when the microribbons/substrate sample was illuminated from the back surface than from the ribbon surface because of an interference effect at the surface. (C) 2016 The Japan Society of Applied Physics

We propose a new neurotransmitter sensor with a high-Q silicon ring resonator surrounded by surface activated mono-layer graphene. A minimum sensitivity of <10 mu M is demonstrated in a sophisticated mu-fluidic bio-sensing system.

The optical properties of stacked graphene microribbons in the terahertz region were simulated by the finite element method. The microribbons, which couple with terahertz light through the excitation of plasmons, were stacked with micrometer-scale vertical spacing (similar to 0.1 lambda or larger). Reflection and absorption spectra were found to strongly depend on the direction of incident light (forward or backward incidence), when the stacking structure was made slightly asymmetric by changing the ribbon width or the chemical potentials in each layer. At a certain frequency, light reflection is almost completely suppressed only for one incidence direction. The high directivity is considered to be due to the phasing effects of electromagnetic waves emitted from each layer like in a Yagi-Uda antenna. (C) 2015 AIP Publishing LLC.

We are conducting a wide range of research on graphene and related two-dimensional materials from basic physics to device applications towards their electronics/photonics applications. In this paper, we review our recent research activities on the growth and functionalization of graphene, hexagonal boron nitride, and transition metal dichalcogenides with the emphasis on the following four topics; Raman spectroscopic visualization of grain boundaries in chemical-vapor-deposition (CVD) grown graphene by isotope labeling, CVD of h-BN on heteroepitaxial Co films, synthesis of layer-controlled MoS2 and WS2 sheets by sulfurization of thin metal films, and photodetection with graphene p-n junction fabricated using interface modification.

A back-gate graphene p-n junction was achieved by selective interfacial modification of a chemical vapor deposition (CVD)-grown graphene field effect transistor (FET). Silane self-assembled monolayer (SAM) patterns were used to fabricate uniform p-and n-doped regions and a sharp p-n junction in the graphene FET channel. A gate-dependent photocurrent response was observed at the graphene p-n junction, and exhibited a maximum signal between two Dirac point voltages of SAM-doped graphene regions. A spatial photocurrent map shows that the photocurrent generated at the junction region was much larger than that from graphene/electrode junctions under the same incident laser power. This single-peak characteristic photocurrent in CVD graphene is dominated by the photothermoelectric contribution, and is highly sensitive to the power of incident laser. The SAM interfacial modification method provides a feasible route for the fabrication of efficient graphene-based photodetectors.

We investigate the tunneling properties of large-area monolayer hexagonal boron nitride (BN) grown via chemical vapor deposition (CVD) by fabricating metal/BN/metal devices on rigid and flexible substrates and compare the properties to metal/exfoliated BN/graphite devices. The measured current of the tunneling devices sandwiched by metal electrodes is linear around zero bias and increases exponentially at higher biases, a behavior consistent with direct tunneling. We also investigate the effect of PMMA contamination on the tunneling current by comparing the zero-bias resistances of the BN devices that have undergone PMMA cleaning by acetone and by heat treatment. Annealing under Ar/O-2 at 500 degrees C proves to be the best heat treatment for removing the PM.MA contaminants introduced during BN transfer, though extra care must be given because this condition can also roughen the bottom electrodes. Further, from tunneling theory, we estimate the barrier height for tunneling to be similar to 2.5 eV, and the dielectric strength to be 3.78 +/- 0.83 GV m(-1) which are comparable to those of exfoliated monolayer BN. Our results demonstrate that CVD-grown BN can be a perfect alternative to exfoliated BN for tunneling applications, such as vertical transistors and spintronics, with an advantage of being available in a large area.

Transition metal dichalcogenides (TMDs) have emerged as exciting 2D materials beyond graphene due to their promising applications in the field of electronics and optoelectronics. Hence, the ability to produce controllable and uniformly thick TMD sheets over a large area is of utmost important for large-scale applications. Here, a facile method of synthesizing large-area, layer-controlled WS2, and MoS2 sheets by sulfurization of their corresponding thin metal films is reported. A metal film, which is deposited by magnetron sputtering method, can be adjusted to produce, with great control, the desired sheet thickness down to a monolayer. Various characterization techniques, such as Raman, photoluminescence, and transmission electron microscopy, were used to evaluate the grown films. The results confirmed some of the exotic properties of TMDs such as the thickness dependent band-gap transition (indirect to direct band gap) and Raman shift. Devices made directly on the as-grown film showed modest mobility, ranging from 0.005 to 0.01 cm(2) V-1 s(-1). Our synthesis method is simple and could also be used to synthesize other TMDs. (C) 2014 AIP Publishing LLC.

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 C-12-lattice and surface deposition of C-13-atoms occur in CVD graphene by sequentially introducing hydrogen and isotopic methane after standard growth of graphene with full monotayer coverage. Spatial Raman spectroscopic mapping on labeled graphene reveals pronounced network-like C-13-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.

Graphene is attracting attention in electrical and optical research fields recently. We measured the optical absorption characteristics and polarization dependence of single-layer graphene (SLG) on sub-micrometer Si waveguide. The results for graphene lengths ranging from 2.5 to 200 pm reveal that the optical absorption by graphene is 0.09 dB/mu m with the TE mode and 0.05 dB/mu m with the TM mode. The absorption in the TE mode is 1.8 times higher than that in the TM mode. An optical spectrum, theoretical analysis and Raman spectrum indicate that surface-plasmon polaritons in graphene support TM mode light propagation.

Low-energy electron microscopy (LEEM) has been used to study the structure, initial growth orientation, growth progression, and the number of layers of atomically thin hexagonal boron nitride (h-BN) films. The h-BN films are grown on heteroepitaxial Co using chemical vapor deposition (CVD) at low pressure. Our findings from LEEM studies include the growth of monolayer film having two, oppositely oriented, triangular BN domains commensurate with the Co lattice. The growth of h-BN appears to be self-limiting at a monolayer, with thicker domains only appearing in patches, presumably initiated between domain boundaries. Reflectivity measurements of the thicker h-BN films show oscillations resulting from the resonant electron transmission through quantized electronic states of the h-BN films, with the number of minima scaling up with the number of h-BN layers. First principles density functional theory (DFT) calculations show that the positions of oscillations are related to the electronic band structure of h-BN. [Figure not available: see fulltext.] © 2013 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

A thin film consisting of boron, carbon, and nitrogen (BCN) was grown on a polycrystalline Ni substrate by thermal chemical vapor deposition. The local elemental composition of the BCN film was analyzed by scanning Auger electron spectroscopy. The film is elementally highly inhomogeneous and consists of domains with a typical size of 1-10 μm and irregular shapes. The domain structure is strongly related to the structure of the grains of the polycrystalline Ni film beneath the domain. A thick domain is often formed on a small Ni grain. On a large and flat Ni grain, the film thickness is relatively small, and both the boron and nitrogen contents are often below the detection limit, indicating that it is a graphene domain. Boron and nitrogen contents are highly correlated, which is consistent with formation of hexagonal boron nitride. However, unbalanced boron and nitrogen contents are observed from thick domains. © 2012 Materials Research Society.

A heating treatment is often used in graphene research to remove adsorbates and resist materials from graphene. Heating graphene followed by air exposure is also known to result in heavy hole doping in graphene, although the role of heating has been unclear. Here, we demonstrate that a practical graphene sample fabricated using the commonly used growth and transfer techniques is unstable against heating in a high vacuum. Structural disorder likely due to defect formation is induced by heating, and the disorder is accompanied by hole doping. Our analysis shows that the main cause of the defect formation is graphene reacting with O-2 and H2O molecules inserted between graphene and the substrate. The hole doping caused by air exposure after heating is explained by gas adsorption at the defect sites.

The doping and scattering effect of substrate on the electronic properties of chemical vapor deposition (CVD)-grown graphene are revealed. Wet etching the underlying SiO2 of graphene and depositing self-assembled monolayers (SAMs) of organosilane between graphene and SiO2 are used to modify various substrates for CVD graphene transistors. Comparing with the bare SiO2 substrate, the carrier mobility of CVD graphene on modified substrate is enhanced by almost 5-fold; consistently the residual carrier concentration is reduced down to 10(11) cm(-2). Moreover, scalable and reliable p-and n-type graphene and graphene p-n junction are achieved on various silane SAMs with different functional groups. (C) 2013 AIP Publishing LLC.