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.     

Layer-by-layer synthesis of large-area graphene films by thermal cracker enhanced gas source molecular beam epitaxy

A thermal cracker enhanced gas source molecular beam epitaxy system was used to synthesize large-area graphene. Hydrocarbon gas molecules were broken by thermal cracker at very high temperature of 1200 °C and then impinged on a nickel substrate. High-quality, large-area graphene films were achieved at 800 °C, and this was confirmed by both Raman spectroscopy and transmission electron microscopy. A rapid cooling rate was not required for few-layer graphene growth in this method, and a high-percentage of single layer and bilayer graphene films was grown by controlling the growth time. The results suggest that in this method, carbon atoms migrate on the nickel surface and bond with each other to form graphene. Few-layer graphene is formed by subsequent growth of carbon layers on top of existing graphene layers. This is completely different from graphene formation through carbon dissolving in nickel and then precipitating from the nickel during rapid substrate cooling in the chemical vapor deposition method.     

Structural and optical properties of diamond and nano-diamond films grown by microwave plasma chemical vapor deposition

We compare structural and optical properties of microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films grown on mirror polished Si(100) substrates by microwave plasma chemical vapor deposition. The films were characterized by SEM, Raman spectroscopy, XRD, and AFM. Optical properties were obtained from transmittance and reflectance measurements of the samples in the wavelength range of 200–2000 nm. Raman spectrum of the MCD film exhibits a strong and sharp peak near 1335 cm⁻¹, an unambiguous signature of cubic crystalline diamond with weak non-diamond carbon bands. Along with broad non-diamond carbon bands, Raman spectra of NCD films show features near 1140 cm⁻¹, the intensity of which is significantly higher in the film grown at 600°C compared to the NCD film grown at higher temperature. The Raman feature near 1140 cm⁻¹ is related to the calculated phonon density of states of diamond and has been assigned to nanocrystalline or amorphous phase of diamond. XRD patterns of the MCD film show sharp peaks and NCD films show broad features, corresponding to cubic diamond. The rms surface roughness of the films was observed to be approximately 60 nm for MCD film that reduced substantially to 17 and 34 nm in the NCD films grown at 600 and 700°C, respectively. Tauc's optical gap for the diamond film is found to be approximately 5.5 eV. NCD grown at 700°C has a high optical absorption coefficient in the whole spectral region and the NCD film grown at 600°C shows very high transmittance (∼78%) in the near IR region, which is close to that of diamond. This indicates that the NCD film grown at 600°C has the potential for applications as optical windows since its surface roughness is significantly low as compared to the MCD film.     

Synthesis of large area carbon nanosheets for energy storage applications

We report the large area growth of highly conductive carbon nanosheets (CNS) composed of few layer graphene on 200 mm diameter Si substrates using conventional radio frequency plasma-enhanced chemical vapour deposition. Raman spectroscopy is used to characterise the evolution of the CNS nucleation and growth with time in conjunction with TEM revealing the nano-sized graphene-like nature of these films and the intimate contact to the substrate. An individual sheet can have edges as thin as 3 graphene layers. The influence of the growth support layer is also discussed as film growth is compared on titanium nitride (TiN) and directly on Si. Electrochemical cyclic voltammogram (CV) measurements reveal these layers to form an excellent electrical contact to the underlying substrate with excellent stability towards oxidation whilst having a large electrochemical surface area. The resistance of a 150 nm film was measured to be as low as 20 μohm cm. The high percentage of narrow few layer graphene edge sites exposed allows for faster electrochemical reaction rates compared to carbon nanotubes (CNTs) and other electrode materials (glassy carbon and Pt).     

Ex-situ spectroscopic ellipsometry studies of micron thick CVD diamond films

Micron thick diamond films have been studied by spectroscopic ellipsometry (SE). The films were grown, on previously prepared Si(100) substrates, by the plasma enhanced chemical vapor deposition (PECVD) technique. Ex situ SE measurements were carried out on samples grown under different conditions, such as substrate temperature and methane fraction in the gas mixture. An optical model consisting of five layers was constructed in order to explain the SE spectra and to provide the optical and structural parameters of the films. This model was deduced from results of various measurements performed by other characterization techniques (Raman spectroscopy, scanning electron microscopy, atomic force microscopy and positron annihilation spectroscopy) which have revealed the optical and structural parameters of the samples. Its sensitivity to the surface and interface roughness as well as to the absorption of the nondiamond phase of the film is demonstrated. Several values of the percentage of the nondiamond phase can be obtained, with the same fit quality, however, depending on the amorphous carbon reference used in the model. These references were obtained by performing SE measurements on various amorphous carbon films. Finally, our SE analysis has allowed us to monitor the lateral homogeneity of the thickness, surface and interface roughness and nondiamond phase concentration over the diamond film.     

The synthesis of graphene nanowalls on a diamond film on a silicon substrate by direct-current plasma chemical vapor deposition

Graphene nanowalls have been synthesized on diamond by direct-current plasma enhanced chemical vapor deposition (CVD) on silicon substrates pre-seeded with diamond nanoparticles in gas mixtures of methane and hydrogen. Switching from diamond CVD to graphene CVD is done by increasing the methane concentration and decreasing the plasma power without breaking the vacuum. Graphene nanowalls stand on the CVD diamond film to form a 3-dimensional network. Scanning electron microscopy, high-resolution transmission electron microscopy, UV and visible Raman scattering and electrochemical cyclic voltammetry measurements are used to characterize the multi-layer turbostratic graphitic carbon nanostructure and demonstrate its electrochemical durability with a low background current in a wide electrochemical potential window.     

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.     

Comparison of GaN and AlN nucleation layers for the oriented growth of GaN on diamond substrates

In this study, {0001} oriented GaN crystals have been grown on freestanding, polycrystalline diamond substrates using AlN and GaN nucleation layers (NLs). XRD measurements and SEM analysis showed that the application of a thin AlN NL gives the best structural results, because AlN has a thermal expansion coefficient in between GaN and diamond and thus delocalizes the stress to two interfaces. The optical quality of the layers, investigated with Raman microscopy and photoluminescence spectroscopy, is similar. Although no lateral epitaxy is obtained, new insight is gained on the nucleation of GaN on diamond substrates facilitating the growth of GaN epilayers on polycrystalline diamond substrates.     

Novel high frequency pulsed MW-linear antenna plasma-chemistry: Routes towards large area, low pressure nanodiamond growth

Current experimental microwave plasma enhanced chemical vapour deposition (MW PECVD) concepts for diamond thin films do not allow scaling up towards large areas, which is essential for microelectronic industries. Also, current growth temperatures are rather high and not compatible with processing technologies. In the current work we demonstrate a breakthrough concept using a high frequency (HF) pulsed MW-linear antenna plasma configuration, allowing a scalable concept. By using HF pulses non-linear MW absorption conditions are reached, allowing a reduction of input power to 4 W/cm² compared with typically 100-200 W/cm² for resonance cavity applicators. Despite the factor of 50 power reduction, the growth rate obtained at 450 °C is comparable to or higher than that of resonance cavity systems. Our concept is a significant improvement as compared to [1,3] previous methods of nanodiamond growth. The resulting diamond films show columnar growth, i.e. resembling classical nano-crystalline diamond (NCD) films [3], with high crystallinity compatible with silicon on diamond chip technology. We present data from plasma diagnostics, showing HF pulsed data from optical emission spectroscopy (OES) for the CH₄-CO₂-H₂ gas chemistry and discuss the basic properties of the layers prepared. In comparison to the work [1] we have succeeded in suppression of re-nucleation during the growth and prepared high quality NCD films with 3-7% sp² carbon, depending on the growth conditions used, based on Raman measurements for layers as thin as 40 nm.     

Raman scattering characterization of CVD graphite films

Raman spectroscopic study has been performed for thin graphite films grown on nickel substrates by chemical vapor deposition from a mixture of hydrogen and methane activated by a direct current discharge. Depending on the growth conditions, the CVD films are composed of graphene layers parallel to the substrate surface or of plate-like crystallites with the predominant orientation of their graphene layers perpendicular to the substrate surface. A comparison of the Raman spectra for the CVD films and for the highly oriented pyrolytic graphite has been performed. The mechanisms governing the Raman scattering process in the films are discussed. An important role of a double resonance mechanism in the Raman spectra of these graphite-based materials has been revealed. The Raman band positions and intensities and their dependence on excitation wavelength confirm a high degree of the structural order in the CVD graphite films.     

Effect of titanium metal in the prenucleation of ultrananocrystalline diamond film growth at low substrate temperature

Ultrananocrystalline diamond (UNCD) film is usually grown in methane–argon plasma unlike methane–hydrogen plasma conventionally used to deposit microcrystalline diamond film. The prenucleation and growth mechanism of these two types of diamond films are different as well. The present study introduces titanium metal powder during ultrasonication of silicon substrate to enhance the nucleation density of UNCD. A titanium thin film was also used at the interface to find the effect of metal on the growth of diamond film. The nucleation density of as-grown film was estimated from the FE-SEM images. After 20 min of growth, nucleation density reaches to 10¹¹/cm² on a surface pretreated by titanium mixed nanodiamond powder. Raman study was carried out for qualitative analysis of different carbon phase present in the UNCD films. X-ray photoelectron spectroscopy (XPS) was used to understand the growth mechanism by detecting the formation of carbon phase and metal carbide formation at the surface after stopping the growth at different time intervals.     

Incorporation of hydrogen in diamond thin films

In this investigation, diamond thin films with grain size ranging from 50 nm to 1 µm deposited using hot filament chemical vapor deposition (HFCVD) have been analyzed by elastic recoil detection analysis (ERDA) for determining hydrogen concentration. Hydrogen concentration in diamond thin films increases with decreasing grain size. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) results showed that part of this hydrogen is bonded to carbon forming C–H bonding. Raman spectra also indicated the increase of non diamond phase with the decrease in crystallite size. Incorporation of hydrogen in the samples and increase of hydrogen content in nanocrystalline sample are discussed. Large separation between filament and substrate used for the synthesis of nanocrystalline film helped to understand the large incorporation of hydrogen in nanocrystalline diamond films during growth. The study addresses the hydrogen trapping in different samples and higher hydrogen concentration in nanocrystallites by considering the synthesis conditions, growth mechanisms for different grain sized diamond films and from the quality of CVD diamond films.     

Ultra-fast synthesis of graphene by melt spinning

Large-scale few-layer graphene (FLG) films were prepared by an industrial single-roller melt spinning technique based on molten alloy quenched carbon self-segregation using nickel and carbon as precursors. A formation mechanism of FLG based on rapid diffusion and non-equilibrium segregation of carbon is discussed. This ultra-fast thin film preparation technique can be extended and used to produce ultrathin sheets of two-dimensional materials other than graphene.     

Improvements in the Formation of Boron-Doped Diamond Coatings on Platinum Wires Using the Novel Nucleation Process (NNP)

In order to increase the initial nucleation density for the growth of boron-doped diamond on platinum wires, we employed the novel nucleation process (NNP) originally developed by Rotter et al. [1]. This pretreatment method involves (i) the initial formation of a thin carbon layer over the substrate followed by (ii) ultrasonic seeding of this “soft” carbon layer with nanoscale particles of diamond. This two-step pretreatment is followed by the deposition of boron-doped diamond by microwave plasma-assisted CVD. Both the diamond seed particles and sites on the carbon layer itself function as the initial nucleation zones for diamond growth from an H₂-rich source gas mixture. We report herein on the characterization of the pre-growth carbon layer formed on Pt as well as boron-doped films grown for 2, 4 and 6 h post NNP pretreatment. Results from scanning electron microscopy, Raman spectroscopy and electrochemical studies are reported. The NNP method increases the initial nucleation density on Pt and leads to the formation of a continuous diamond film in a shorter deposition time than is typical for wires pretreated by conventional ultrasonic seeding. The results indicate that the pre-growth layer itself consists of nanoscopic domains of diamond and functions well to enhance the initial nucleation of diamond without any diamond powder seeding.     

Raman and photoluminescence spectra of indented cubic boron nitride and polycrystalline cubic boron nitride

The use of Raman spectroscopy, and in particular Raman line shifts, to measure stress in diamond and nitrides such as gallium nitride (GaN), is well known. In both diamond and GaN the application is principally to study stresses in thin films and at the substrate–thin film interface. Stresses in polycrystalline diamond composites have also been measured by this method. Typically stresses of the order of GPa can be determined with a spatial resolution of a few micrometers. In this paper, Raman spectra of indentations on cubic boron nitride (cBN) crystals and polycrystalline cubic boron nitride (PcBN) composites are presented. Shifts of the cBN Raman lines from their unstressed positions quantify the residual stresses in the boron nitride due to the deformation brought about by the indentation. Making use of the measured coefficient of shift of 3.39 cm⁻¹/GPa for the transverse optical Raman peak, these are of the order of 1 GPa. These measurements illustrate, for the first time, the use of Raman spectroscopy to study residual stresses in boron nitride. Plastic deformation is usually associated with the creation of vacancies. To investigate the possible presence of vacancy defects and vacancy-related defects, the indented boron nitride samples were also studied with photoluminescence spectroscopy.     

Preparation and characterization of boron doped diamond electrodes on diamond anvil for in situ electrical measurements under high pressure

A layer of boron doped diamond (BDD) film was deposited selectively on a diamond anvil and employed as electrodes for measuring the electrical resistivity of matter under high pressure. Both heavily doped and lightly doped electrodes were characterized by Raman spectroscopy and scanning electron microscopy. Though the BDD film electrodes contain sp² carbon, it is still suitable for in situ high pressure electrical measurements. The dependability of diamond film electrodes was tested at high pressure up to 36 GPa, by measuring the electric resistance of C60 fullerene powder, and no damage of the electrodes was observed.     

The influence of dark feature on optical and thermal property of DC Arc Plasma Jet CVD diamond films

Different grades of CVD diamond films were prepared by 100 kW DC Arc Plasma Jet system. The films were characterized using optical microscope (OM), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and Raman spectroscopy. The results show that dark feature mainly is inclusions in CVD diamond films, the concentration are amorphous carbon and nitrogen. As for transparent optical grade diamond film, it has very high IR transparency and high thermal conductivity. The appearance of dark feature degraded the quality of CVD diamond film, apparently influencing IR transparency and thermal conductivity. But even in optical grade diamond film, there are very strong absorption features in the 7–9 μm region, this will limit the practical applications of diamond films grown by Plasma Jet as IR windows for CO₂ lasers.     

A composite material made of carbon nanotubes partially embedded in a nanocrystalline diamond film

A composite material, made of carbon nanotubes (CNTs) partially embedded in a nanocrystalline diamond film was produced. The diamond film was first decorated with palladium or nickel nanoparticles. An array of nanopores was drilled in the film in a hot filament CVD (HFCVD) reactor thanks to the anisotropic etching that takes place under the nanoparticles. During this etching process, the metallic particles penetrate the diamond film to a controlled depth, thus remaining at the bottom of the nanopores. The buried nanoparticles remain catalytically active and are used to grow a multiwall carbon nanotube forest using HFCVD in the same reactor without breaking the vacuum. The quality of the CNTs was assessed by scanning electron microscopy and Raman spectroscopy. The interface between the carbon nanotubes and the diamond was characterized by ultrasonication, lateral force microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. As a result of these characterizations, we demonstrate that the buried carbon nanotubes exhibit higher mechanical stability and improved electrical behavior compared to CNTs directly grown on the diamond surface.     

Bulk growth of mono- to few-layer graphene on nickel particles by chemical vapor deposition from methane

A method for the bulk growth of mono- to few-layer graphene on nickel particles by chemical vapor deposition from methane at atmospheric pressure is described. A graphene yield of about 2.5% of the weight of nickel particles used was achieved in a growth time of 5 min. Scanning and transmission electron microscopy, Raman spectroscopy, thermogravimetry, and electrical conductivity measurements reveal the high quality of the graphene obtained. Suspended graphene can be prepared during this process, bridging the gaps between nearby nickel grains. After the growth of graphene the nickel particles can be effectively removed by a modest FeCl₃/HCl etching treatment without degradation of the quality of the graphene sheets.     

Multi-layer structure for chemical vapor deposition diamond on electroplated diamond tools

In this work we established a process to overcome the deposition difficulty on electroplated diamond tools by a multi-layer structure. The process consists of the following steps: (1) diamond powder aggregation with nickel (this step is the conventional method for the production of electroplated tools); (2) electrochemical deposition of a chromium layer, but leaving the diamond grains partially uncovered; (3) nitridation of the chromium layer; and (4) deposition of the chemical vapor deposition (CVD) diamond layer. This method uses the advantages and overcome the disadvantages of each step. Electroplating with nickel is conventionally used due to its relatively good wettability to diamond. The direct aggregation of the diamond powder with a chromium layer results in looser grains and is not usable. The nickel layer is inadequate for diamond deposition; even after treatment in hydrogen atmosphere, diamond does not grow on it. The chromium nitride layer is well known to be very suitable for diamond growth, however, the thermal stress between these layers is very high limiting film thickness and its applicability. With the multi-layer structure obtained the CVD diamond film is deeply anchored by the diamond grains into the metal matrix and helps considerably to decrease the stress. The process has been developed on flat surfaces and tested on small conventional diamond burrs. The diamond films have been characterized by scanning electron microscopy (SEM), energy dispersive X-rays (EDS) and Raman spectroscopy (RS).