Multilayered graphene films prepared at moderate temperatures using energetic physical vapour deposition

Carbon films were energetically deposited onto copper and nickel foil using a filtered cathodic vacuum arc deposition system. Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and UV–visible spectroscopy showed that graphene films of uniform thickness with up to 10 layers can be deposited onto copper foil at moderate temperatures of 750 °C. The resulting films, which can be prepared at high deposition rates, were comparable to graphene films grown at 1050 °C using chemical vapour deposition (CVD). This difference in growth temperature is attributed to dynamic annealing which occurs as the film grows from the energetic carbon flux. In the case of nickel substrates, it was found that graphene films can also be prepared at moderate substrate temperatures. However much higher carbon doses were required, indicating that the growth mode differs between substrates as observed in CVD grown graphene. The films deposited onto nickel were also highly non uniform in thickness, indicating that the grain structure of the nickel substrate influenced the growth of graphene layers.     

Do CVD grown graphene films have antibacterial activity on metallic substrates?

Accurate assessment of the antibacterial activity of graphene requires consideration of both the graphene fabrication method and, for supported films, the properties of the substrate. Large-area graphene films produced by chemical vapor deposition were grown directly on copper substrates or transferred on a gold substrate and their effect on the viability and proliferation of the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli were assessed. The viability and the proliferation of both bacterial species were not affected when they were grown on a graphene film entirely covering the gold substrate, indicating that conductivity plays no role on bacterial viability and graphene has no antibacterial activity against S. aureus and E. coli. On the other hand, antibacterial activity was observed when graphene coated the copper substrates, resulting from the release of bactericidal cupric ions in inverse proportion to the graphene surface coverage.     

Tuning the electronic properties of graphene by hydrogenation in a plasma enhanced chemical vapor deposition reactor

Graphene films grown by chemical vapor deposition on copper foils were hydrogenated using commercially viable methods. Parameters such as plasma power, plasma frequency, and sample temperature were varied to determine the maximum possible hydrogenation without etching the film. The kinetic energy of the ions inside the plasma is critical, in that higher kinetic energy ions tend to etch the film while lower kinetic energy ions participate in the hydrogenation process. The film sheet resistance was shown to increase, while the hole mobility was shown to decrease with increasing hydrogenation. Variable temperature measurements demonstrate a transition from semi-metallic behavior for graphene to semiconducting behavior for hydrogenated graphene. Sheet resistance measurements as a function of temperature also suggest the emergence of a bandgap in the hydrogenated graphene films.     

Formation of self-assembled platinum particles on diamond and their embedding in diamond by microwave plasma chemical vapor depositions

Single-crystalline and polycrystalline diamond films containing platinum particles with sizes of ≤ ≈ 100 nm have been formed through a self-assembling process. Pt thin films pre-deposited on diamond were found to completely change in shape to grains during a subsequent diamond overgrowth process using a microwave-plasma chemical-vapor-deposition (MPCVD) technique. The self-assembled Pt grains on flat diamond surfaces had approximately spherical particles with rather uniform sizes when the pre-deposited Pt films were sufficiently thin or less than ≈ 1 μm in thickness. The average diameter of such Pt particles, D, was well controlled simply by changing the thickness of the pre-deposited Pt film, t_pt, since D was proportional to t_pt. Such spherical Pt particles were completely embedded after sufficient diamond overgrowth. Transmission electron microscope observations revealed that most of the spherical Pt particles were well crystallized and that the interfacial structures between the diamond overlayer and the buried Pt particles were sufficiently sharp without any appreciable mixing regions.     

Synthesis of transfer-free graphene films on dielectric substrates with controllable thickness via an in-situ co-deposition method for electrochromic devices

The solutions and polymer supported materials in graphene transfer process would introduce lots of containments, defects and wrinkles, which weakens the performance of graphene. Herein, an in-situ co-deposition method is carried out to obtain transfer-free graphene films with controllable thickness on several dielectric substrates. The amorphous carbon (carbon source) and copper (catalyst) are co-deposited on dielectric substrates. Followed by an in-situ annealing process, the amorphous carbon is transformed to few-layer graphene. High co-deposition temperature could promote the decomposition of Cu(acac)₂ precursors, leading to the controllable thickness of amorphous carbon layer in Cu@C films. Finally, 3-, 5-, 8- and 10- layers graphene films with transmittance of up to 93.5% and square resistance of 0.8 kΩ·sq^?1 are obtained and a high-performance electrochromic device is fabricated using 3 layers graphene films as electrodes. The “color” and “bleach” time of the electrochromic device is 16.6 s and 6.8 s with the transmittance of 26.8% and 79.7% separately. This method paves an alternative way for the batch production of transfer-free graphene film as electrode materials.     

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.     

Photocatalytic activities and specific surface area of TiO<Subscript>2</Subscript> films prepared by CVD and sol-gel method

The photocatalytic activity of CVD grown films shows significant, non-linear (sigmoid-like) dependency on the film thickness. However, the photocatalytic activity of sol-gel grown film is almost independent of the film thickness. The specific surface area of sol-gel grown films is very small, regardless of the film thickness. Conversely, the specific surface area of CVD grown films indicates significant thickness dependency. The specific area and photocatalytic activity were found to show very similar dependencies on the film thickness.     

Multilayer graphene grown by precipitation upon cooling of nickel on diamond

Multilayer graphene is grown by precipitation upon cooling of a thin nickel film deposited by e-beam evaporation on single crystal diamond (0 0 1) oriented substrates. Nickel acts as a strong catalyst inducing the dissolution of carbon from diamond into the metal. Carbon segregation produces multilayers of graphene on the top surface. Characterization by Raman spectroscopy reveals that these thin layers display relatively narrow Raman phonon peaks that are typically associated with graphene. Atomic force microscope measurements reveal a multigrain structure that reproduces small domains in the nickel film. The multilayer graphene is transferred onto a optical microscope glass slide for further analysis. The thickness of the layers estimated from optical transmission measurements is 12 nm. The catalytic reaction found for nickel on diamond is not observed when glassy carbon is used as substrate. This method provides a venue for the fabrication of large area graphene films.     

Low temperature growth of complete monolayer graphene films on Ni-doped copper and gold catalysts by a self-limiting surface reaction

We report on the fabrication of completely uniform monolayer graphene on a metal thin film over a 150 mm Si substrate at a low temperature of 600 °C by inductively coupled plasma-enhanced chemical vapor deposition (ICPCVD). Through novel use of bimetallic catalyst such as CuNi and AuNi alloys we were able to control catalytic reaction at the metal surface and grow complete monolayer graphene with a Ni content less than 20 at.%. We also found that the 2D/G intensity ratio in the Raman spectra was almost invariant with growth time and the C 1s peak in the XPS spectra was observed only at the metal surface. This implies that monolayer graphene was possibly grown on these Ni-doped copper and gold catalysts by a self-limiting surface reaction under our CVD condition. From DFT calculations, it was shown that the catalytic activity of normally inactive Cu and Au could be enhanced through the addition of Ni atoms at surface sites, providing graphene growth at lower temperatures than pure Cu or Au. The carrier mobility of graphene films grown on these CuNi and AuNi alloy catalyst was measured to be over 9000 cm² V⁻¹ s⁻¹ at room temperature, which is comparable to that of CVD graphene film grown on Cu foil. Therefore, we suggest an efficient way in growing a complete monolayer graphene on thin films at low temperatures, which could be a key issue in the application of graphene devices.     

Controlled growth of carbon nanotube-graphene hybrid materials for flexible and transparent conductors and electron field emitters

We report a versatile synthetic process based on rapid heating and cooling chemical vapor deposition for the growth of carbon nanotube (CNT)-graphene hybrid materials where the thickness of graphene and density of CNTs are properly controlled. Graphene films are demonstrated as an efficient barrier layer for preventing poisoning of iron nanoparticles, which catalyze the growth of CNTs on copper substrates. Based on this method, the opto-electronic and field emission properties of graphene integrated with CNTs can be remarkably tailored. A graphene film exhibits a sheet resistance of 2.15 kΩ sq(-1) with a transmittance of 85.6% (at 550 nm), while a CNT-graphene hybrid film shows an improved sheet resistance of 420 Ω sq(-1) with an optical transmittance of 72.9%. Moreover, CNT-graphene films are demonstrated as effective electron field emitters with low turn-on and threshold electric fields of 2.9 and 3.3 V μm(-1), respectively. The development of CNT-graphene films with a wide range of tunable properties presented in this study shows promising applications in flexible opto-electronic, energy, and sensor devices.     

Growth of Strontium Titanate Thin Films of Controlled Thickness by the Hydrothermal–Electrochemical Method

Strontium titanate thin films were grown on titanium electrodes up to ca. 2 μm in thickness by a hydrothermal–electrochemical method using electrolysis performed potentiostatically by the three-electrode cell technique. The film thickness increased monotonically with an increase in the quantity of electricity passed through the Ti electrode, and could be controlled accurately by this factor. Grown films were composed of a surface layer having an invariant thickness of ca. 0.2 μm and an inner layer grown by electrolysis. The lattice parameter of the cubic SrTiO₃ film was analyzed to be 3.919 A.     

Monolayer graphene as ultimate chemical passivation layer for arbitrarily shaped metal surfaces

Monolayer graphene was grown on polycrystalline Ru thin films on patterned fused silica. The Ru films grow with columnar structure with strongly aligned grains exposing flat (0 0 0 1) surface facets within the 3D geometric patterns and on the adjacent planar silica surface. The monolayer graphene was found to completely and uniformly cover the Ru films on the complex engineered substrates. In addition, we demonstrate that the single atomic layer graphene protects the underlying metal surface against reaction with ambient gases of particular importance for applications such as concave focusing mirrors, non-planar microelectrode arrays, etc.     

Homogeneous bilayer graphene film based flexible transparent conductor

Graphene is considered as a promising candidate to replace conventional transparent conductors due to its low opacity, high carrier mobility and flexible structure. Multi-layer graphene or stacked single layer graphenes have been investigated in the past but both have their drawbacks. The uniformity of multi-layer graphene is still questionable, and single layer graphene stacks require many transfer processes to achieve sufficiently low sheet resistance. In this work, bilayer graphene film grown with low pressure chemical vapor deposition was used as a transparent conductor for the first time. The technique was demonstrated to be highly efficient in fabricating a conductive and uniform transparent conductor compared to multi-layer or single layer graphene. Four transfers of bilayer graphene yielded a transparent conducting film with a sheet resistance of 180 Ω(□) at a transmittance of 83%. In addition, bilayer graphene films transferred onto the plastic substrate showed remarkable robustness against bending, with sheet resistance change less than 15% at 2.14% strain, a 20-fold improvement over commercial indium oxide films.     

Controllable growth of 1–7 layers of graphene by chemical vapour deposition

We report that graphene films with thickness ranging from 1 to 7 layers can be controllably synthesized on the surface of polycrystalline copper by a chemical vapour deposition method. The number of layers of graphene is controlled precisely by regulating the flow ratio of CH₄ and H₂, the reaction pressure, the temperature and the reaction time. The synthesized graphene films were characterized by scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. In addition, the graphene films transferred from copper to other substrates are found to have a good optical transmittance that makes them suitable for transparent conductive materials.     

Direct growth of vertically aligned carbon nanotubes on a planar carbon substrate by thermal chemical vapour deposition

Uniform, vertically aligned multiwalled carbon nanotube arrays (VACNTs) were grown on glassy carbon-like thin films by thermal chemical vapour deposition (CVD). Thin (5 nm) aluminum and iron catalyst layers were pre-deposited by evaporation on the carbon substrates and VACNTs were grown at 750 °C by water-assisted CVD using ethylene as the carbon source. The aluminum layer was shown to be essential for aligned nanotube growth. VACNT arrays adhered strongly to the carbon film with low contact resistance between the VACNTs and the substrate. The VACNT arrays grown directly on the planar conducting carbon substrate have attractive properties for use as electrodes. Excellent voltammetric characteristics are demonstrated after insulating the arrays with a dielectric material.     

Chemical vapor deposition of thin graphite films of nanometer thickness

Well-ordered graphite films with a thickness of a few graphene layers have been grown on Ni substrates by chemical vapor deposition (CVD) from a mixture of hydrogen and methane activated by a DC discharge. According to Auger, Raman and scanning tunneling microscopy (STM) data the CVD graphite film thickness is about 1.5 ± 0.5 nm. The graphene layers were perfectly adhered to the substrate surface except for upthrusted ridges of a few tens of nanometers in height. STM has revealed an atomically smooth surface with the atomic arrangement typical of graphite between the ridges. A difference in the thermal expansion coefficients of nickel and graphite is considered as a reason for the ridge formation.     

Synthesis and characterization of electrically conductive polyethylene-supported graphene films

We describe a simple mechanical approach for low-density polyethylene film coating by multilayer graphene. The technique is based on the exfoliation of nanocrystalline graphite (few-layer graphene) by application of shear stress and allows to obtain thin graphene layers on the plastic substrate. We report on the temperature dependence of electrical resistance behaviors in films of different thickness. The experimental results suggest that the semiconducting behavior observed at low temperature can be described in the framework of the Efros-Shklovskii variable-range-hopping model. The obtained films exhibit good electrical conductivity and transparency in the visible spectral region.

PACS

72.80.Vp; 78.67.Wj; 78.66.Qn; 85.40.Hp     

Cross-sectional transmission electron microscopy of thin graphite films grown by chemical vapor deposition

Graphene has been the subject of an extraordinary upsurge of interest due to its intriguing properties and potential applications. Recent work has shown that excellent electronic properties are exhibited by large-scale ultrathin graphite films, grown by chemical vapor deposition on a polycrystalline metal and transferred to a device-compatible surface. The properties of such multilayered graphene films could depend strongly on film thickness and uniformity. Unlike the other common methods for analysis in the literature, cross-sectional transmission electron microscopy (TEM) would provide direct, straightforward analysis of film thickness and quality, as well as provide a means to investigate specific defect structures (e.g. wrinkles). However, this approach has not often been pursued due to the sensitivity of graphite to the electron and ion beam damage in such a procedure. Here, an approach to creating cross-sectional TEM samples using a focused ion beam lift-out method is presented, along with the resulting TEM images of thin graphite films and several wrinkle defects. Samples removed from as-grown films on polycrystalline nickel and films removed from measured devices are presented. The benefits and limitations to this approach are discussed.     

Oxide Thickness Measurement in the Electron Probe Microanalyzer

An X-ray method for the measurements of the thickness of supported thin films in the microprobe is modified for silica films on silicon nitride and silicon carbide. The intensities of the oxygen Kα line are measured on bulk SiO₂ and on the film. The derivation of the caliboration curve giving the thickness of the film from the ratio of these intensities is outlined. The method has been used for silica films thinner than 1 μm with a lateral resolution of a few micrometers.     

Copper and brass aged at open circuit potential in slightly alkaline solutions

Surface oxide films were grown on 99.99% copper and brass (copper–zinc alloy, Cu77Zn21Al2) in 0.1 mol L⁻¹ borax solution at open circuit potential and were characterized using various experimental techniques. The composition of the passive films formed in situ on the different materials was studied using differential reflectance spectroscopy. The thickness of the oxide layers on copper and brass was compared by chronopotentiometric curves and potentiodynamic reductions. The electrical properties of each oxide were analyzed by means of electrochemical impedance spectroscopy. Their influence on the oxygen reduction reaction was also investigated using voltammetry hydrodynamic tools such as the rotating disk electrode. The results show that the incorporation of Zn to Cu in brass changes the composition and the thickness of the surface film. The films grown on brass tend to be thicker but less resistive and Zn compounds incorporate to the film. This is supported by results from reflectance and impedance spectroscopy. The kinetics of oxygen reduction is strongly inhibited on oxidized electrodes, particularly in the case of brass. The global number of exchanged electrons remains close to four and seems to be independent of the presence of surface oxides.