Synthesis of well-aligned millimeter-sized tetragon-shaped graphene domains by tuning the copper substrate orientation

We present a simple processing method for synthesizing well-aligned millimeter-sized tetragon-shaped graphene domains on a polycrystalline copper substrate via low-pressure chemical vapor deposition. The tetragonal shape is achieved simply by wet loading the copper substrate with processing conditions previously used for the growth of millimeter-sized hexagon-shaped graphene domains. Electron backscatter diffraction (EBSD) shows that the wet loaded copper substrate is uniformly textured with a surface plane between Cu (1 0 0) and Cu (1 1 0). The in-plane rotation of the crystalline orientation across the Cu grains is very small. However, the EBSD showed that the surface orientation of the dry loaded substrate is close to the (1 1 1) crystal plane. The different surface orientation of the wet and dry loaded samples is attributed to the different surface oxygen concentration, which changes the relative stability of the (0 0 1), (1 1 0), and (1 1 1) plane during copper sublimation and recrystallization. These results provide an approach to tune the surface crystal orientation and thus the shape and orientation of the graphene domains.     

Discontinuity and misorientation of graphene grown on nickel foil: Effect of the substrate crystallographic orientation

Graphene growth by chemical vapor deposition on low cost metal foils is a promising approach for the production of large-scale graphene. However, the precise control of the uniformity of synthesized mono- and multilayer graphene requires elucidation of the factors affecting deposition and growth. In this study, we investigate the influence of the crystallographic orientation of nickel on multilayer graphene growth using electron-backscatter diffraction, Raman and energy dispersive X-ray spectroscopies, as well as scanning electron and atomic force microscopies. We correlated the discontinuities of the graphene sheets on polycrystalline nickel foils with crystallographic orientations of nickel grains. In addition, we observed indications of misoriented (twisted) multilayer graphene on particular grain orientations. We demonstrate that the Raman signature from these misoriented multilayer graphene areas is highly similar to that previously reported for twisted bilayer graphene. Using microscopy methods, we demonstrated dramatic morphological changes in the nickel substrate induced by graphene growth.     

Transformation of chemical vapor deposited individual graphene crystal with oxidation of copper substrate

Here, we reveal the structural transformation process of as-synthesized individual graphene crystals with oxidation of a copper (Cu) foil. We found that the transformation of a graphene crystal with Cu oxidation is significantly different for the thermal annealing and room temperature long-term atmospheric oxidation. Annealing creates large cracks in an individual graphene crystal due to the thermal stress and strain created by rapid oxidation of Cu surface. The cracks are further enhanced with longer annealing duration enabling oxygen diffusion through cracks, thereby accelerating oxidization of Cu. Eventually, the graphene crystals are completely damaged, leaving behind the highly oxidized Cu surface. On the other hand, in case of room temperature long-term atmospheric oxidation, oxygen diffusion occurs underneath of a graphene crystal through the reactive edge without any large cracks formation. The graphene crystals decouple from Cu surface during the oxygen diffusion and oxidation process, however no structural deformation is observed. This finding shows the significant contrast of structural change of graphene crystal and oxidation behaviors of Cu surface with thermal annealing and room temperature atmospheric oxidation.     

Room temperature reduction of multilayer graphene oxide film on a copper substrate: Penetration and participation of copper phase in redox reactions

A self-reduction of graphene oxide (GO) at room temperature after prolonged storage on a copper substrate is evidenced by decrease of oxygen content and a dramatic, 6 orders in magnitude, increase in dc conductivity. Experiments revealed that the stored GO film contains copper hydroxide phase embedded in the reduced GO structure.     

Effects of annealing on copper substrate surface morphology and graphene growth by chemical vapor deposition

Understanding the mechanism of graphene synthesis by chemical vapor deposition and the effect of process parameters is critical for production of high-quality graphene. In the present work, we investigated the effect of H₂ concentration during annealing on evolution of Cu surface morphology, and on deposited graphene characteristics. Our results revealed that H₂ had a smoothening effect on Cu surface as its surface roughness was reduced significantly at high H₂ concentration along with the formation of surface facets, dents and nanometer-sized particles. Furthermore, H₂ content influenced the graphene morphology and its quality. A low H₂ concentration (0% and 2.5%) during annealing promoted uniform and good quality bilayer graphene. In contrast, a high concentration of H₂ (20% and 50%) resulted in multilayer, non-uniform and defective graphene. Interestingly, the annealed Cu surface morphology differed considerably from that obtained after deposition of graphene, indicating that graphene deposition has its own impact on Cu surface.     

Microstructure in electrodeposited copper layers; the role of the substrate

The microstructures of Cu layers, ranging in thickness from 3 to 12 μm, were investigated. The layers were electrodeposited from an acidic copper electrolyte onto two distinct substrate materials important for the micro-components industry: an Au layer with a pronounced 〈111〉-texture, and a nano-crystalline NiP layer. The evolutions of surface topography, morphology and crystallographic texture in the layers were investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction analysis, respectively. Distinct surface topographies were observed for Cu layers deposited on the Au and NiP substrates. Deposition onto the Au substrate resulted in a very smooth surface of all Cu layers, whereas the NiP substrate caused an irregular surface for 3-μm-thick layers of Cu. The crystallographic texture in the Cu layers in the first few micrometres depended strongly on the crystallographic texture in the substrate. The Cu crystallites inherited the 〈111〉-orientation of the Au substrate, whilst no preferred crystallographic orientation was observed in the Cu crystallites on the nano-crystalline NiP substrate. For Cu layers thicker than 3 μm a 〈110〉-fibre texture developed on both the substrates.     

The orientation of silver nuclei on a platinum substrate

The initial stages of silver electrocrystallization on a platinum electrode have been investigated. A pulse method was used, in which an electron pulse-generator was combined with a pulse potentiostat. A single, square potentiostatic pulse, with height and duration chosen in a way that single crystal nuclei were formed, was superposed on an initial potential at the beginning of the experiment. The crystals grew further on account of the potential initially set, and their growth was continuously followed microscopically.

The crystal orientations obtained in the various overvoltage regions, are: for the orientation axis [111], 20–50 mV; for [100], 60–100 mV; for [110], 120–170 mV; for [113], 170–200 mV; for [210], over 200 mV. The experimentally obtained orientation axes are in full agreement with those theoretically calculated for metals with a f.c.c. lattice.     

Relationship between thickness of polymer films and their oxidation on copper substrate

This research was undertaken to understand how the thickness of polyethylene films oxidized on a copper substrate influences the accumulation of carbonyl groups (measured by an IR‐spectroscopy technique) and of metal from the substrate (determined by polarography analysis). It was found that the whole polymer became inhibited by the time the copper stopped transferring into the specimen. Plots of copper concentration versus film thickness have two thickness sections: section I is found between 0 and 70 μm and section II between 80 and 170 μm. Between these two sections the metal concentration varies drastically. This situation can be explained by two schemes by which PE changes to inhibited condition. According to Scheme I (for section I, short oxidation time) this change has only one step: the inhibited layer gradually becomes thicker beginning from the interface and moving toward the outer surface. The second scheme (for section II) shows that the polymer becomes inhibited in two steps. It is typical of thicker films. In this case the oxidation process shifts and localizes in the outer surface because of longer treatment. As a result, transfer of metal and formation of an inhibited layer are interrupted for some time. The metal accumulation in the film only resumes when low‐molecular‐weight products of thermooxidative degradation—formed in the specimen outer surface—enter the region of adhesional contact. A so‐called second transfer stage for metal is realized during which the whole polymer becomes inhibited. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 671–675, 2003     

Nanopatterning of graphene with crystallographic orientation control

The recent papers on the nanopatterning of graphene and cutting of graphene nanoribbons were reviewed. It was found that until now the simultaneous control of crystallographic orientation and of the ribbon width in the range of nanometers was possible only by scanning tunneling lithography. The cutting process by local anodic oxidation under the AFM tip is a similar process, but due to the different physical interaction mechanisms of the STM and AFM tip with the substrate, and due to the larger radius of the AFM tip, the resolution of AFM lithography is poorer. The various cutting processes based on mobile, catalytic nanoparticles yield trenches with well defined crystallographic orientation, but have a major drawback: the location of the nanoparticles and the control of the direction in which the cutting will start are currently not predictable. The first promising results of a solid phase reduction reaction of the SiO₂ substrate at the graphene edge indicate the possibility of developing a new type of lithography that will allow the realization of complex nanopatterns. Recent results pointing to the possibility of the engineered modification of graphene edges may prove useful to all lithographic processes.     

In-plane lattice thermal conductivities of multilayer graphene films

The in-plane lattice thermal conductivities of a single layer and multilayer graphene films are investigated using nonequilibrium molecular dynamics simulations. It is found the thermal conductivity of a single layer graphene is higher than that of multilayer graphene. Increasing the bonding strength between neighboring layers will reduce the in-plane thermal conductivity for multilayer graphene films. The constraints from the neighboring layer play the role of impeding phonon transport along the in-plane direction in multilayer graphene films. This observation implies the thermal conductivity of a single layer graphene will be reduced in practical applications once it is bonded on a substrate.     

Interaction between carbon nanotubes and substrate and its implication on field emission mechanism

The interaction between carbon nanotubes (CNTs) and substrate plays an important role in the process of field emission. A double-barrier model is adopted to analyze the difference of field emission for various CNT films. Result shows that the width of interface barrier determines the emission performance. For CNTs on titanium, the best emission performance is attributed to the removal of interlayer barrier by the formation of conductive titanium carbide. These facts might shed new light on the field emission mechanism for CNTs.     

三种铁基体上碱性镀铜工艺的比较

为了考察EDTA体系无氰碱性镀铜工艺的性能,在铁基体上获得结合力优良的镀铜层,作为后续镀铅工艺的中间层,从镀液性能(如阴极极化、电流效率、均镀能力、深镀能力、整平能力、沉积速率等)和镀层性能(如孔隙率、结合力、表面形貌等)两方面对氰化体系、柠檬酸-酒石酸体系和EDTA体系等3种镀铜工艺进行了比较.结果表明,传统氰化镀铜...     

Interaction between single gold atom and the graphene edge: a study via aberration-corrected transmission electron microscopy

Interaction between single noble metal atoms and graphene edges has been investigated via aberration-corrected and monochromated transmission electron microscopy. A collective motion of the Au atom and the nearby carbon atoms is observed in transition between energy-favorable configurations. Most trapping and detrapping processes are assisted by the dangling carbon atoms, which are more susceptible to knock-on displacements by electron irradiation. Thermal energy is lower than the activation barriers in transition among different energy-favorable configurations, which suggests electron-beam irradiation can be an efficient way of engineering the graphene edge with metal atoms.     

Interaction between Ca,Zn, and porous cordierite substrate at elevated temperatures

The interactions of both Ca and Zn with cordierite substrates were studied to better understand how Ca and Zn individually contribute to physio chemical changes in the cordierite diesel particulate filters (DPF) during high temperature engine exhaust conditions. The Ca and Zn doped cordierite samples were heated to temperatures of 300, 500, 900, and 1100?°C. Thermal treatment consisted of 1, 3, and 10 h exposure, separately, to simulate the duration of heat experienced by DPFs during use. Among the temperature profiles studied, SEM, XRD and EPMA mapping results indicate that the zinc–cordierite interaction is more severe in contributions to cordierite degradation than the calcium–cordierite interaction. The corrosion pathway generally followed the cordierite honeycomb porous structures. Appraised by the EPMA mapping, the element concentration across a given cordierite cross-section tends to increase for Zn doped samples, where calcium did not. XRD analysis indicate both the Ca and Zn can chemically interact with cordierite and alter the crystalline phases present at temperatures above 300?°C.     

Reduction of graphene oxide at the interface between a Ni layer and a SiO2 substrate

Reduction of graphene oxide (GO) was carried out on SiO₂ using a thin Ni overlayer as a catalyst. A Ni/GO/SiO₂ structure was heated at 800 °C in high vacuum for 6 min. After removing the Ni overlayer, formation of graphene was confirmed by Raman spectroscopy. For the Ni overlayer thinner than 40 nm, GO was reduced to graphene on-site. For the thicker Ni overlayer, however, GO was completely decomposed and graphene was formed in a segregation and/or precipitation process. The use of GO with a thin Ni overlayer enabled on-site and transfer-free fabrication of graphene without use of such flammable gases as methane and hydrogen.     

Biaxial orientation of poly(vinyl chloride) compounds: Interaction between drawing,structure, and properties

Rigid poly(vinyl chloride) (PVC) sheets were stretched uniaxially (at constant width), equally biaxially, and unequally biaxially to various draw ratios. Tensile properties, density, and birefringence of the stretched sheets were measured, and their wide angle X-ray diffraction traces recorded. Comparison of results showed the highest crystallite and overall orientation and density in the uniaxial samples, and lowest values for the equal biaxial samples. Drawing resulted in the alignment of some existing crystallites in the plane of the film, together with the formation of new mesomorphous structures. Changes in tensile strength were attributed to overall orientation.     

The role of cotranslation in protein folding: a lattice model study

Computational studies of protein folding have implicitly assumed that folding occurs from a denatured state comprised of the entire protein. Cotranslational folding accounts for the linear production and release of a protein from the ribosome, allowing part of the protein to explore its conformation space before other parts have been synthesized. This gradual ‘extrusion’ from the ribosome can yield different folding kinetics than direct folding from the denatured state, for a lattice folding model. First, in model proteins containing chiefly short-ranged (local in sequence) contacts, cotranslational folding is shown to be significantly faster than direct folding from the denatured state. Secondly, for model proteins with two competing native states, cotranslational folding tilts the apparent equilibrium toward the state with a more local-contact dominant topology.     

Modulation of the band gap of graphene oxide: The role of AA-stacking

The unique electronic properties of graphene make it an advantageous material for use in many applications, except those that require a band gap. Much work has been done to introduce an appropriately tuned band gap into graphene, including uniaxial strain and oxidation, with varying levels of success. We report here that the stacking configuration of the sheets in multilayered graphene oxide can have a significant impact on the band gap. Through comparison of X-ray absorption near-edge spectra of multilayered pristine graphene sheets with spectra simulated using density functional theory, we have found that AA-stacking pushes unoccupied states closer to the Fermi level than AB-stacking by widening the π¹ resonance in both graphene oxide and graphene. If the near-Fermi states have been removed such that the nearest unoccupied state to the Fermi level is the π¹ band, then AA-stacked multilayered graphene oxide will have a smaller band gap than AB-stacked graphene oxide. We have confirmed this by measuring the band gap of graphene oxide and reduced graphene oxide indirectly using X-ray absorption near-edge spectroscopy and X-ray emission spectroscopy. Controlling the stacking configuration of multilayered graphene oxide may provide a novel method for tuning its band gap.     

Photo-thermal chemical vapor deposition of graphene on copper

We demonstrate the synthesis of single-layer graphene films on copper by photo-thermal chemical vapor deposition (PTCVD) realized using a rapid thermal processing system typically used in CMOS processing. Influence of the temperature on the low-pressure (10 mbar) graphene synthesis using methane precursor was characterized by analyzing the crystalline quality, thickness and electronic properties of the films. Using a growth time of only 60 s, for graphene fabricated at 950 °C the sheet resistance and mobility show equivalent quality compared to thermal CVD graphene. Moreover, μ-Raman mapping reveals very low defect density and high 2D to G band ratio similar to the fingerprint of exfoliated single-layer graphene. The synthesis process was found to exhibit a threshold at around 900 °C at which (and below) the single-layer graphene film does not contain adlayer flakes typically observed in high temperature CVD graphene on copper. Our study shows that PTCVD can be used for the high throughput fabrication of high-quality single-layer graphene on copper and is therefore a promising method while pursuing cost-effective graphene fabrication.