Intercalated samarium as an agent enabling the intercalation of oxygen under a monolayer graphene film on iridium

Using thermal desorption time-of-flight mass spectrometry and thermionic methods, it is shown that oxygen does not intercalate under a graphene monolayer grown correctly on iridium, at least at temperatures of T = 300–400 K and exposures below 12000 L. However, if the graphene film on iridium is preliminary intercalated with samarium atoms (up to coverage of θ_Sm = 0.2–0.45), the penetration of oxygen atoms under the graphene film is observed. The oxygen atoms in the intercalated state are chemically bonded to samarium atoms and remain under graphene up to high temperatures (~2150 K).     

In-plane conductance measurement of graphene nanoislands using an integrated nanogap probe

The in-plane conductance of individual graphene nanoislands thermally grown on SiC substrate was successfully measured using an integrated nanogap probe without lithographic patterning. A Pt nanogap electrode with a 30?nm gap integrated on the cantilever tip of a scanning probe microscope enables us to image a conductance map of graphene nanoislands with nanometer resolution. Single-?and double-layer graphene islands are clearly distinguished in the conductance image. The size dependence of the conductance of the nanoislands suggests that the band gap opening is due to the lateral confinement effect.     

Ultrasonic wave characteristics of a monolayer graphene on silicon substrate

The present work deals with an ultrasonic type of wave propagation characteristics of monolayer graphene on silicon (Si) substrate. An atomistic model of a hybrid lattice involving a hexagonal lattice of graphene and surface atoms of diamond lattice of Si is developed to identify the carbon-silicon bond stiffness. Properties of this hybrid lattice model is then mapped into a nonlocal continuum framework. Equivalent force constant due to Si substrate is obtained by minimizing the total potential energy of the system. For this equilibrium configuration, the nonlocal governing equations are derived to analyze the ultrasonic wave dispersion based on spectral analysis. From the present analysis we show that the silicon substrate affects only the flexural wave mode. The frequency band gap of flexural mode is also significantly affected by this substrate. The results also show that, the silicon substrate adds cushioning effect to the graphene and it makes the graphene more stable. The analysis also show that the frequency bang gap relations of in-plane (longitudinal and lateral) and out-of-plane (flexural) wave modes depends not only on the y-direction wavenumber but also on nonlocal scaling parameter. In the nonlocal analysis, at higher values of the y-directional wavenumber, a decrease in the frequency band gap is observed for all the three fundamental wave modes in the graphene–silicon system. The atoms movement in the graphene due to the wave propagation are also captured for all the tree fundamental wave modes. The results presented in this work are qualitatively different from those obtained based on the local analysis and thus, are important for the development of graphene based nanodevices such as strain sensor, mass and pressure sensors, atomic dust detectors and enhancer of surface image resolution that make use of the ultrasonic wave dispersion properties of graphene.     

Bandgap engineering of zigzag graphene nanoribbons by manipulating edge states via defective boundaries

One of the most severe limits of graphene nanoribbons (GNRs) in future applications is that zigzag GNRs (ZGNRs) are gapless, so cannot be used in field effect transistors (FETs), and armchair GNR (AGNR) based FETs require atomically precise control of edges and width. Using the tight-binding approach and first principles method, we derived and proved a general boundary condition for the opening of a significant bandgap in ZGNRs with defective edge structures. The proposed semiconducting ZGNRs have some interesting properties one of which is that they can be embedded and integrated in a large piece of graphene without the need to completely cut them out. We also demonstrated a new type of high-performance all-ZGNR FET. Previous proposals of graphene FETs are all based on AGNRs.     

Formation of graphene by the thermal annealing of a graphite layer on silicon substrate in vacuum

In this letter we present our preliminary results about the preparation of graphene and carbon nanosheets by thermal annealing of graphite layer on silicon substrate in vacuum and at temperature 900 °C. The surface area of about several tens of micrometers and curved at the ends. The carbon nanosheets are very clean and look like a piece of paper. Investigations with Raman showed three main peaks D, G, G′ and small peak at 2420 cm⁻¹. This method is considered to be a simple and cheap method for preparing the carbon nanosheets directly on the silicon substrate for the application in the electronics.     

Formation of graphene by the thermal annealing of a graphite layer on silicon substrate in vacuum

In this letter we present our preliminary results about the preparation of graphene and carbon nanosheets by thermal annealing of graphite layer on silicon substrate in vacuum and at temperature 900 °C. The surface area of about several tens of micrometers and curved at the ends. The carbon nanosheets are very clean and look like a piece of paper. Investigations with Raman showed three main peaks D, G, G′ and small peak at 2420 cm⁻¹. This method is considered to be a simple and cheap method for preparing the carbon nanosheets directly on the silicon substrate for the application in the electronics.     

Broadband optical antireflection enhancement by integrating antireflective nanoislands with silicon nanoconical-frustum arrays

Based on conventional colloidal nanosphere lithography, we experimentally demonstrate novel graded-index nanostructures for broadband optical antireflection enhancement including the near-ultraviolet (NUV) region by integrating residual polystyrene antireflective (AR) nanoislands coating arrays with silicon nano-conical-frustum arrays. This is a feasible optimized integration method of two major approaches for antireflective surfaces: quarter-wavelength AR coating and biomimetic moth's eye structure.     

Yield and shape selection of graphene nanoislands grown on ni(111)

The catalytic decomposition of hydrocarbons on transition-metal surfaces has attracted increasing interest as a method to prepare high quality graphene layers. Here, we study the optimal reaction path for the preparation of graphene nanoislands of selected shape using controlled decomposition of propene on Ni(111). Scanning tunneling microscopy performed at different stages of the reaction provides insight into the temperature and dose-dependent growth of graphene islands, which precedes the formation of monolayer graphene. The effect of postreaction annealing on the morphology of the islands is studied. By adjusting the initial propene dose, reaction temperature, and postannealing procedure, islands with a triangular or hexagonal shape can be selectively obtained.     

Modification of the shape of large germanium nanoislands on silicon surface by low-energy ion bombardment

Germanium nanoislands on silicon substrates were irradiated by hydrogen and argon ions with energies below 350 eV. In the initial stage, ion bombardment leads to the division of large islands into several small islands irrespective of the ion type. The resulting surface is more homogeneous than the initial and is stable with respect to further ion irradiation.     

Topographic and spectroscopic characterization of electronic edge states in CVD grown graphene nanoribbons

We used scanning tunneling microscopy and spectroscopy (STM/S) techniques to analyze the relationships between the edge shapes and the electronic structures in as-grown chemical vapor deposition (CVD) graphene nanoribbons (GNRs). A rich variety of single-layered graphene nanoribbons exhibiting a width of several to 100 nm and up to 1 μm long were studied. High-resolution STM images highlight highly crystalline nanoribbon structures with well-defined and clean edges. Theoretical calculations indicate clear spin-split edge states induced by electron-electron Coulomb repulsion. The edge defects can significantly modify these edge states, and different edge structures for both sides of a single ribbon produce asymmetric electronic edge states, which reflect the more realistic features of CVD grown GNRs. Three structural models are proposed and analyzed to explain the observations. By comparing the models with an atomic resolution image at the edge, a pristine (2,1) structure was ruled out in favor of a reconstructed edge structure composed of 5-7 member rings, showing a better match with experimental results, and thereby suggesting the possibility of a defective morphology at the edge of CVD grown nanoribbons.     

Inducing electronic changes in graphene through silicon (100) substrate modification

We have performed scanning tunneling microscopy and spectroscopy (STM/STS) measurements as well as ab initio calculations for graphene monolayers on clean and hydrogen(H)-passivated silicon (100) (Si(100)/H) surfaces. In order to experimentally study the same graphene piece on both substrates, we develop a method to depassivate hydrogen from under graphene monolayers on the Si(100)/H surface. Our work represents the first demonstration of successful and reproducible depassivation of hydrogen from beneath monolayer graphene flakes on Si(100)/H by electron-stimulated desorption. Ab initio simulations combined with STS taken before and after hydrogen desorption demonstrate that graphene interacts differently with the clean and H-passivated Si(100) surfaces. The Si(100)/H surface does not perturb the electronic properties of graphene, whereas the interaction between the clean Si(100) surface and graphene changes the electronic states of graphene significantly. This effect results from the covalent bonding between C and surface Si atoms, modifying the π-orbital network of the graphene layer. The local density of states shows that the bonded C and Si surface states are highly disturbed near the Fermi energy.     

Field emission microscopy of graphene on iridium point emitter

The field electron emission from the surface of an iridium point emitter covered by a monolayer graphene film has been studied. An analysis of the field emission images showed that electron emission takes place at the boundaries between graphene islands with dimensions up to several dozen nanometers. Intercalation of alkali metal (cesium) atoms under the graphene film decreases the work function of the emitter but does not change the image. Field ion desorption images obtained in the fields where the surface diffusion of Cs atoms is impossible reveal the presence of a submonolayer concentration of cesium at the defects representing graphene island contacts.     

Li absorption and intercalation in single layer graphene and few layer graphene by first principles

We present an exhaustive first-principles investigation of Li absorption and intercalation in single layer graphene and few layer graphene, as compared to bulk graphite. For single layer graphene, the cluster expansion method is used to systemically search for the lowest energy ionic configuration as a function of absorbed Li content. It is predicted that there exists no Li arrangement that stabilizes Li absorption on the surface of single layer graphene unless that surface includes defects. From this result follows that defect-poor single layer graphene exhibits significantly inferior capacity compared to bulk graphite. For few layer graphene, we calibrate a semiempirical potential to include the effect of van der Waals interactions, which is essential to account for the contribution of empty (no Li) gallery to the total energy. We identify and analyze the Li intercalation mechanisms in few layer graphene and map out the sequence in stable phases as we move from single layer graphene, through few layer, to bulk graphite.     

Relaxing layers of silicon carbide grown on a silicon substrate by magnetron sputtering

Epitaxial layers of silicon carbide of the 3C-polytype are prepared by magnetron sputtering on Si(111) substrates of structural perfection with ω_θ = 1.4°. The crystalline structure and surface morphology of the 3C-SiC/Si(111) heterostructures depending on film thickness are studied by X-ray diffraction, Raman scattering, and atomic-force microscopy. It is found that the additional energy of ionized particles that is imparted to the Si(111) substrate during magnetron sputtering contributes the formation of strong C-C and β-SiC bonds, which hinders the crossing of the grain boundary by dislocations with increasing growth time.     

Clean transfer of graphene on Pt foils mediated by a carbon monoxide intercalation process

Noble metals such as Pt are a perfect substrate for the catalytic growth of monolayer graphene. However, the requirements of the subsequent transfer process are not compatible with the traditional etching method. In this work, we find that the interaction of graphene with Pt foil can be weakened through the intercalation of carbon monoxide （CO） under ambient pressure. This intercalation process occurs on both hexagonal-shape graphene islands and irregular graphene patches on changing the CO partial pressure from 0 to 0.6 MPa, as observed by scanning electron microscopy （SEM）, Raman spectroscopy and X-ray photoemission spectroscopy. We demonstrate that, on a practical timescale, the intercalation ratio is proportional to the partial pressure of CO. Furthermore, we develop a clean transfer method of CO-intercalated graphene with water as a peeling agent. We show that this method enables the transfer of tens of micrometer-scale graphene patches onto SiO2/Si, which are free from metal or oxide particle contamination. This transfer method should be a significant step towards the dean transfer of graphene, as well as the recydable use of noble metal substrates.     

石墨/聚苯乙烯插层复合材料的介电性能研究

采用浓酸处理和高温快速热冲击的工艺制备了单层石墨。用高分辨透射电镜观察了其有序的微观结构,并做了选区电子衍射分析。使用热压成型的工艺制备了单层石墨/聚苯乙烯(PS)复合材料。介电性能测试表明该复合材料的渗流阈值仅为0.1%(质量分数),这样低的渗流阈值有利于使用该类材料的电容器向微型、轻质发展。同时还发现,采用电场诱导和不施加电场的情况下制备的单层石墨/聚苯乙烯复合材料的介电性能有较大区别。     

Recovery of an arbitrary edge on an existing surface mesh using local mesh modifications

This study describes an algorithm for recovering an edge which is arbitrarily inserted onto a pre‐triangulated surface mesh. The recovery process does not rely on the parametric space of the surface mesh provided by the geometric modeller. The topological and geometrical validity of the surface mesh is preserved through the entire recovery process. The ability of inserting and recovering an arbitrary edge onto a surface mesh can be an invaluable tool for a number of meshing applications such as boundary layer mesh generation, solution adaptation, preserving the surface conformity, and possibly as a primary tool for mesh generation. The edge recovery algorithm utilizes local surface mesh modification operations of edge swapping, collapsing and splitting. The mesh modification operations are decided by the results of pure geometrical checks such as point and line projections onto faces and face‐line intersections. The accuracy of these checks on the recovery process are investigated and the substantiated precautions are devised and discussed in this study. Copyright © 2001 John Wiley & Sons, Ltd.     

Effect of the vicinal character of substrate on nucleation of nanoislands in lattice-mismatched systems

A model of the nucleation of three-dimensional nanoislands by the Stranski-Krastanov mechanism on a lattice-mismatched vicinal substrate is proposed. It is shown that the work of island formation at the step edge is a function of two variables: number of particles in the island and its aspect ratio. The activation barrier for the nucleation corresponds to a saddle point of the work of formation. The vicinal character of the surface leads to a change in the work of island formation and significantly decreases the nucleation activation barrier. Data on the nucleation activation barrier and energetically favorable aspect ratio, which are necessary to simulate particular systems, are obtained.     

Thick-layer growth of n+ silicon on n− silicon substrate by liquid-phase epitaxy

A solution technique is proposed for the growth of thick epitaxial layers with high doping of arsenic on undoped silicon substrates. The interface between the epitaxial layer grown and the substrate was investigated by spreading resistance measurements and a quadruple crystal X-ray diffractometer. The results showed that (a) a fairly abrupt change of the n-type dopant arsenic concentration was obtained at the interface between the epitaxial layer and the silicon substrate, and (b) no lattice mismatch between the epitaxial layer and the substrate was found from the results of the rocking curves by the quadruple crystal X-ray diffractometer, even if the arsenic-doping level in the epitaxial layer was high, 2×10¹⁹ atoms cm⁻³. This revised version was published online in July 2006 with corrections to the Cover Date.