Carbon Nanoflakes Research Articles

Catalyst-free synthesis of carbon and boron nitride nanoflakes using RF-magnetron sputtering

Catalyst-free boron nitride (BN) and carbon (C) nanoflakes have been produced by direct radio frequency (RF)-magnetron sputtering on molybdenum and tungsten substrates at or above temperatures of 1000 °C and 800 °C, respectively. Selected-area electron diffraction (SAED) shows that the films are polycrystalline and contain disordered graphite and hexagonal BN. Transmission electron microscopy (TEM) reveals curved or twisted flakes up to several hundred nanometres in length. High resolution transmission electron microscopy (HRTEM) confirms the nanoflake structure to be turbostratic, which is intermediate between an amorphous phase and the ordered layered phases of hexagonal BN or graphite.

Electron field emission properties of carbon nanoflakes prepared by RF sputtering

Carbon nanoflakes (CNFs) with corrugated geometry were synthesized using RF sputtering process with Ar/CH4 gas mixture. Transmission electron microscopic examination reveals that the introduction of H2 in sputtering chamber leads to the preferential etching of amorphous carbons, while maintaining integrity for the nano-crystalline phases. The proportion of nano-sized crystalline clusters is thus increased, which improved the electron field emission (EFE) properties of the materials, viz. with turn-on field of E 0 = 6.22 V/μm and FEE current density of J e = 90.1 μA/cm2 at 11.0 V/μm. The cathodes made of screen printing of CNFs-Ag paste exhibit even better EFE properties than the as-deposited CNFs. The EFE of the CNFs cathodes can be turned on at E 0 = 5.71 V/μm, achieving J 0 = 340.1 μA/cm2 at 11.0 V/μm applied field. These results showed that the CNFs are inheritantly more robust in device fabrication process than the other carbon materials and thus possess better potential for electron field emitter applications.

Enhanced Electron Field Emission Characteristics of Carbon Nanoflakes Prepared by Modified RF Sputtering

Carbon films with flakelike geometry, exhibiting excellent electron field emission (EFE) properties were synthesized by a modified RF sputtering process, which used an Ar:CH4:H2 gas mixture, instead of a conventional Ar:H2 mixture. These films contain nanosized diamonds and allotropic phases (i-carbons and graphites) embedded in amorphous carbon films. The EFE process can be turned on at E0 = 4.83 V/µm, achieving an EFE current density of Je = 280 µA/cm2 at an 8 V/µm applied field. The EFE properties of these flakelike carbon films essentially do not change with processing parameters, indicating that these carbon films are more process-reliable and hence have better potential for application as electron field emitters.

Arrays of carbon nanoflake spherules realised on copper substrate

Arrays of carbon nanoflake spherules (CNSs) have been grown on the copper substrate by the microwave plasma assisted chemical vapour deposition method. Uniform sized CNSs of diameters of 700–950 nm have been produced. The CNSs are free from any other carbon nano-structures. The growth of CNSs has been found to be strongly affected by the undulating surface (crest–trough morphology) of the substrate with preferential growth along the direction of crests. Micro-Raman spectroscopic analysis reveals highly active surface of the graphitic edges of as-grown CNSs. A possible growth mechanism has been proposed which could be used for a large yield of arrays of the CNSs. The field-emission measurements show potential applications of the CNSs in the vacuum electronics devices with a low turn-on field of 7.4 V/μm.

Magnetic Properties and Large-Scale Synthesis of Novel Carbon Nanocomposites via Benzene Decomposition over Ni Nanoparticles

Nanorods composed of carbon nanoflakes have been synthesized via catalytic decomposition of benzene at temperatures as low as 350 °C over Ni nanoparticles derived from sol−gel synthesis followed by hydrogen reduction. Field-emission scanning electron microscopic and high-resolution transmission electron microscopic images reveal that the carbon nanoflakes are tightly stacked. We investigated the effects of reaction temperature and catalyst amount on the yield, morphology, and magnetic properties of the products. Above 500 °C, the main product was carbon nanotubes. After optimization of reaction parameters, the maximum purity and yield of carbon nanoflakes at 460 °C were 97.9 wt % and ca. 4579% whereas those of nanotubes at 550 °C were 96.7 wt % and ca. 2892%, respectively. Compared to the methods reported in the literature, the approach described herein has the advantages of being simple, low-cost, and environmentally friendly and is suitable for the mass production of one-dimensional carbon nanocomposite...

One-step fabrication and field emission properties of petal-like carbon nanotubes

We have used bias-assisted microwave plasma chemical vapor deposition to synthesize (i) carbon nanotubes (CNTs) possessing a graphitic carbon nanoflake (CNF) morphology, (ii) CNF spheres, and (iii) a pure carbon nanoflake film (CNFF) on stainless-steel substrates. We employed CH 4 and CO 2 as the precursors to these carbon nanomaterials (CNMs); the use of CO 2 aided the destruction of the CNT surfaces and reduced the growth temperature range to 380–450 °C at 300 W, as a result of the presence of oxygen-based species in the plasma. Applying an external bias of − 150 V led to the growth of CNTs possessing specifically oriented graphitic CNFs. We used scanning electron microscopy and high-resolution transmission electron microscopy to observe the morphologies and graphitic structures of these CNMs. Field emission measurements of a CNT/CNF-coated electrode revealed that the turn-on field was ca. 2.73 V/μm.

Catalyst-free, low-temperature growth of high-surface area carbon nanoflakes on carbon cloth

We have used a microwave plasma-enhanced chemical vapor deposition system to synthesize two-dimensional (2D) and three-dimensional (3D) structures of carbon nanoflakes (CNFs). This catalyst-free, low-temperature synthesis involved introducing CO2 and CH4 as reactants at a specified ratio of 2:3. We obtained uniform 2D arrays of CNFs at lower microwave powers (200 or 300W) and substrate temperatures (up to 420°C); their thickness was close to 1μm. At a microwave power of 400W, we obtained a 3D architecture comprising the smallest nanoflakes reported to date. The specific surface area of the 3D structure was double that of the corresponding 2D arrays. We suspect that such structured carbon materials might have great potential for energy storage applications.

Growth of carbon with vertically aligned nanoscale flake structure in capacitively coupled rf glow discharge

Carbon nanoflakes (CNFs) have been deposited on Si (1 0 0) wafer substrates at a substrate temperature of 670 °C from a glassy carbon target by capacitively coupled rf (13.56 MHz) glow discharge using a mixture discharge gas of Ar and CH 4 with a total pressure of 14.5 Pa. Microstructures of deposited carbon were investigated by a field emission scanning electron microscope (FESEM) and a high-resolution transmission electron microscope (HRTEM). Under the given conditions, vertically aligned CNFs with a flake length of about 1 μm and thickness of about 20 nm were grown. High intensity and symmetry of electron diffraction pattern indicate that the CNFs deposited by capacitively coupled rf glow discharge have three-dimensionally perfect crystallinity with a graphene interlayer spacing of 335 pm. In particular, there is little disorder in stacking of the layer structure. It was further found that the thickness of the flakes was less dependent of deposition time while the length of the flakes increases to about 1 μm with increasing deposition time to 3 h. The growth rate of a graphite sheet parallel to (0 0 1) stacking layers was much higher than that perpendicular to (0 0 1) stacking layer, resulting in anisotropic growth of a flake-like structure. The formation mechanisms of CNFs are discussed from the viewpoint of the difference in residence time of carbon atoms on CNF surfaces parallel to and perpendicular to (0 0 1) direction and anisotropic heat conductivity of graphite.

Functional Carbon Nanoflakes with High Aspect Ratio by Pyrolysis of Cured Templates of Block Copolymer and Phenolic Resin

We describe a procedure to design functional high aspect ratio nano-objects based on the self-assembly of block copolymer templates and selective swelling of the lamellar domain by phenolic resins. This procedure yields either flat “hairy” nano-objects or after the controlled pyrolysis high surface area microporous carbon based nanoflakes.

Radial textured carbon nanoflake spherules

A unique type of carbon structure, radial textured carbon nanoflake spherules, has been synthesized by a microwave plasma assisted chemical vapor deposition method. The spherules with a diameter of 1.2–35μm consist of a number of radially distributed carbon nanoflakes growing from a common core. The constituent nanoflakes are interlaced and perpendicular to the surface of spherules, forming a large amount of open edge planes. Thus, the carbon nanoflake spherules are isotropic graphite with a larger surface area and higher surface activity, which can be demonstrated by Raman scattering spectroscopy with two characteristic peaks of 860 and 1140cm−1.

Synthesis and field-emission testing of carbon nanoflake edge emitters

A nanometer-edged carbon structure [carbon nanoflake (CNF)] has been synthesized on 50–150-nm-diameter nickel arrays. The Ni dot arrays are patterned using a polystyrene nanosphere lithography technique capable of creating arrays of regularly spaced nanometer-diameter structures of Ni. The flake-like morphology was formed by an inductively coupled rf plasma-enhanced chemical vapor deposition system, with H2 as the carrier gas and CH4 as the carbon source. Typical deposition conditions are: substrate temperature of 680 °C, chamber pressure of 70–90 mTorr, overall gas flow rate of 10 sccm, and CH4 concentration in the range of 10%–40%. Scanning electron microscopy (SEM) shows CNF preferentially growing on the Ni dots, with the irregular carbon flakes standing vertically on the substrate. The flake edge widths are ∼10 nm and the interflake spacing on a given Ni dot is on the order of 15–100 nm. Experiments show that the density, height, and interspacing of the flakes are controllable by varying patterning and deposition parameters. The structures show no degradation or vibration under small-spot SEM imaging, indicating good thermal stability. Raman spectra show a typical carbon feature with D and G peaks at 1350 and 1580 cm−1, respectively. Intensity ratio of these two peaks, I(D)/I(G), increases with CH4 concentration. The work function of this structure determined by Kelvin probe measurement is about 4.3 eV, which is near that of graphitic carbon. Preliminary results of I–V curve testing indicate that this structure could act as a conductive, robust, edge emitter.

スパッタリング法によるカーボンナノフレークの形成

Carbon nano-flakes were formed on Si (001) surface by rf-magnetron sputtering. A graphite plate was used as the sputtering target. Ar-methane mixture gas was introduced into the sputtering chamber as the discharge gas. The structure of the sputter deposited samples was investigated by field emission type scanning electron microscope, high resolution transmission electron microscope and electron diffraction method. Carbon nano-flakes were observed on the Si substrate after a few minute sputter deposition. No other allotropes such as nano-tubes, soots, nano-fibers and nano-horns were observed. The flakes were found to be graphite sheets and to grow to the plane direction with increasing time. The length of the flake seemed to increase approximately proportionally to the sputter deposition time, while the thickness increase was very small. The formation process of the flake was discussed considering the adsorption and desorption process of carbon atoms on graphite surface.

Uniform carbon nanoflake films and their field emissions

Films of uniformly distributed carbon nanoflakes have been prepared by using hot filament chemical vapor deposition. A large amount of carbon flakes with a thickness of less than 20 nm interlaced together to form a layer of carbon nest-like film with their sharp edges almost perpendicular to the Si substrate. Raman spectroscopy showed that the films were characteristic of pyrolytic graphite and became more disordered with the increase of substrate temperature and acetylene concentration. The growth mechanism of the carbon nanoflake films was discussed. The field emission performance has been done, showing a turn-on field of about 17 V/μm. Considering the ease of large-area preparation, the carbon nanoflake films might have a potential application in vacuum electronic devices.

Prediction of electronic properties of carbon-based nanostructures

We present first-principles total-energy calculations that allow us to predict two new properties of carbon-based nanostructures. First, it is shown that carbon nanotubes (CNTs) encapsulating C 60, now commonly called carbon peapods, are one-dimensional conductors with different types of carriers. Second, CNTs and carbon nanoflakes with zigzag edges are predicted to have ferromagnetic ordering. It is also shown that hetero-nanotubes and hetero-sheets consisting of boron and nitrogen in addition to C are also nano-scale ferromagnets.