Graphene Nanocrystallite Research Articles

A comparison between ion irradiation assisted- and electron irradiation assisted-oxygen plasma treatment on modification of nanostructured carbon films

Oxygen plasma treatment has been used for surface modification of carbon materials. However, the working mechanism is not well clarified. Here, the ion irradiation assisted- and electron irradiation assisted-oxygen plasma etching were proposed and realized in an electron cyclotron resonance plasma processing system. The effects of the ion assisted- and electron assisted-argon plasma and oxygen plasma etching on the surface morphology and nanostructure of graphene nanocrystallites embedded carbon film were compared. In the ion assisted plasma etching, compared with argon plasma, the oxygen introduction made the sp2/sp3 and ID/IG of the film upper layer further decrease. While in the electron assisted plasma etching, after O2 introduction, the sp2/sp3 and ID/IG in the film upper layer increased. Then the oxygen plasma treatment mechanism was proposed. In the ion assisted oxygen plasma treatment, the attracted oxygen positive ions took major effect, which would reduce the threshold energy for atom displacement and then implant more carbon or oxygen atoms into the film subsurface to make the film denser. In the electron assisted oxygen plasma treatment, the neutral oxygen took major effect, which would preferentially react with the more unstable amorphous carbon and then make the film equipped with more sp2 bonds.

The quality studies of vertical graphene nanosheets catalyst-free microwave plasma-enhanced chemical vapor deposited on glass and fused silica

Vertical graphene nanosheets (VGN) were prepared by a microwave plasma-enhanced chemical vapor deposition technique on glass and fused silica. Their quality was explored by Raman scattering spectroscopy and transient absorption spectroscopy (TAS). We have observed a correlation between Raman scattering spectroscopy data and TAS parameters (charge carrier relaxation times and relative transient absorption signal amplitude). The substrate-related effects were explained by competition between the changes in the size of the effects induced by the graphene nanocrystallite, the substrate-induced graphene self-doping, and the presence of on-site and vacancy defects along with the dominant boundary defects. The optical and electrical properties of VGN on glass deposited at low substrate temperature (600 °C) were similar or even better compared to VGN on fused silica. The ability to prepare high-quality VGN on glass could be very attractive for practical application due to the low substrate price.

Correlating Disorder Microstructure and Magnetotransport of Carbon Nanowalls

The carbon nanowalls (CNWs) grown by Plasma-Enhanced CVD reveal differences in the magnetotransport properties depending on the synthesis parameters. In this paper, we report the influence of the deposition temperature, which produces variations of the disorder microstructure of the CNWs. Relative low disorder leads to the weak localization with the transition to weak antilocalization. Higher disorder generates positive Hopping mechanism in low field with a crossover to a diffusion transport by graphene nanocrystallites. The samples reveal a similitude of the isoline density of the MR at a low temperature (<50 K), explained in the context of the magnetization. This effect is independent of the number of defects. We can achieve a desirable amount of control over the MT properties changing the CNWs’ microstructure.

Effect of Substrate Roughness on the Friction and Wear Behaviors of Laser-Induced Graphene Film

A rough substrate usually induces severe detriments limiting the performance of anti-friction materials that would lead to an increase in both the friction coefficient and wear rate. In this work, we found that a laser-induced graphene (LIG) film had a good friction adaptability on both mirror-polished and rough Si substrates. The friction coefficient of the LIG increased from 0.11 to 0.24 and the substrate roughness increased from 1.4 nm to 54.8 nm, while the wear life of the LIG was more than 20,000 cycles for both the mirror-polished and rough Si substrates. Optical microscope, Raman spectroscopy and scanning electron microscope analyses revealed a friction mechanism evolution of the LIG films on Si substrates with a different roughness. For the mirror-polished Si substrate, thick and dense graphene nanocrystallite transfer films could form on the counterpart balls, which guaranteed a long and stable wear. For the rough Si substrate, although the asperities on the rough surface would plough the counterpart balls and destabilize the transfer film formation, grooves could effectively store a compressed LIG, benefiting a stable anti-wear performance and reducing the abrasive wear at the friction interface. This work showed that a LIG film had outstanding friction adaptability on Si substrates with a different roughness and that it can be fabricated in a single-step economic process, indicating bright practical prospects in the solid lubrication fields.

Rapid fabrication of ultra-wear-resistant graphene nanocrystallite film by direct laser writing

Developing protective films with high wear resistance under heavy load is an urgent challenge. Meanwhile, the conventional fabrication methods for anti-wear film usually require time-consuming and energy consumption processes with vacuum equipment. Here, we report an ultra-wear-resistant graphene nanocrystallite film that can be fabricated by direct laser writing on polyimide in a single step process. The graphene nanocrystallite film had a loose exploded structure but could surprisingly sustain more than 100,000 cycles of friction under 20 N with a stable friction coefficient around 0.17. The ultra-wear-resistant mechanism of the film was revealed that the loose exploded graphene nanocrystallites were restructured into a tough cross-linked nanocrystalline transfer film on the counterpart surface by friction. The cross-linked nano-structure could effectively alleviate the slip consumption between graphene sheets, which resulted in slow wear processes including lattice damage and amorphization of graphene sheets in transfer film. With a good practical significance, the graphene nanocrystallite film is expected to have broad application prospects with the merits of ultra-wear-resistance and easy rapid fabrication feature.

Graphitization vs tribo-oxidation governing friction behaviors of doped graphene nanocrystalline carbon films

Graphitization and tribo-oxidation are two common phenomena found at the friction interfaces of carbon-based films in atmospheric environment. However, the specific contributions of them to the low friction behavior are not well clarified. Here, we show two kinds of low friction behaviors of graphene nanocrystalline carbon (GNC) films doped with different elements (Ti, Al, Si). The promotion effects of graphene nanocrystallites on the graphitization and different doping elements on the tribo-oxidation were detailed analyzed for uncovering the low friction mechanisms. The results suggest that the relationship between graphitization and tribo-oxidation could be competitive or coexisting. For competitive type, the friction coefficient of Ti-doped GNC film first decreased to a low-friction stage due to graphitization, and then gradually increased into a steady-friction stage by tribo-oxidation. For coexisting type, the graphitization and tribo-oxidation co-induced the steady-low-friction behaviors of Al-doped and Si-doped GNC films. This work provides new insights into the friction mechanism, and the guidance for the design of doped GNC films with desired friction behaviors.

Photon excitation effect on formation of graphene nanocrystallites during carbon film growth process

In this article, we propose a method to deposit nanocrystallite embedded carbon films by electron cyclotron resonance plasma sputtering with photon irradiation cooperated with electron or Ar+ ion irradiation. We found photon irradiation can enhance the growth of graphene nanocrystallites during carbon film deposition. The energy transfer from the photon to the metastable carbon structure excites the growth of sp2 hybridized graphene nanocrystallites, and photon-excited electrons can be accelerated by the bias and further promote the graphene nanocrystallite growth. Photons are the second quantum medium we found that can be used to deposit nanocrystallite embedded carbon films, and their quantum properties with electric neutrality can help us to further understand the formation of the carbon nanocrystallite structure and may shed light on the quantum fabrication of desired materials.

Rapid Fabrication of Ultra-Wear-Resistant Graphene Nanocrystallite Film by Direct Laser Writing

Rapid Fabrication of Ultra-Wear-Resistant Graphene Nanocrystallite Film by Direct Laser Writing

Graphene nanocrystallites induced short run-in period with low electric power at current-carrying sliding interface

We reported the current-carrying friction behavior of graphene nanocrystallites carbon (GNC) film, which had hybrid structure with graphene nanocrystallites embedded in amorphous carbon (a-C) matrix. The GNC film exhibited high electrical conductance of 7.7 ~ 30.9 S/cm for effective current transmission, and short run-in period of 10 ~ 25 frictional cycles for achieving low friction (μ = 0.03 ~ 0.05) with low electric power of 0.25 ~ 0.60 W under the electric current of 0.5 A. As a comparison, a-C film showed low electrical conductance of 3.2 S/cm and long run-in period of 50 frictional cycles with high electric power of 1.25 W. The high electrical conductance and the short run-in period effectively reduce the electrical and mechanical energy consumption for the GNC film, thus presenting wider promising practical applications. Based on the nanostructure characterization, the role of graphene nanocrystallites was summarized as the electron transport channels for transmitting the electric current and the source of graphene nanocrystallite structure for promoting the low friction interface formation. These findings present potentials of the GNC film in protecting the current-carrying friction interface.

Ultrasmooth nanocrystalline carbon film induced by low concentration doping: Carbide disorienting graphene nanocrystallite

The fabrication of nanocrystalline films is usually accompanied by undesired surface roughening, which limits their applications in tribology, optics and electronics. Here, we report a technique to reduce the average roughness of graphene nanocrystalline carbon film from 18.3 nm to 0.66 nm by ∼5 at.% Ti-doping. The surface of the Ti-doped graphene nanocrystalline carbon film stay ultrasmooth from few nanometer thick up to 1 μm thick. Based on the nanostructure characterization, the ultrasmooth mechanism is interpreted that Ti-doping leads to the formation of Ti–C bond, and changes the growth of graphene nanocrystallite from preferred vertical-aligned orientation to random orientation. The ultrasmooth mechanism is general, as shown by similar effects with low concentration doping of Si, Al, Cr, Zr and W. This approach paves a way for fabricating ultrasmooth nanocrystalline carbon films without a requirement of ion-impact-induced downhill current, and significantly improving the hardness of the graphene nanocrystalline carbon film.

Room-temperature anisotropic magnetoresistance of magnetic carbon films with vertically-grown graphene nanocrystallites

In this study, graphene nanocrystallited carbon films were deposited with electron irradiation induced deposition in electron cyclotron resonance plasma. The orientation of nanocrstallites was revealed with transmission electron microscope (TEM) images. The TEM results showed that the graphene nanocrystallites aligned perpendicular to substrate surface. The anisotropic-magneto-resistive performances of the films were investigated under diowed that from 400 to 2 K, the film resistance varied periodically as the film rotated along its surface axial, exhibiting a sinusoidal feature. In room temperature, the average angular resolution of the film was better than 0.5%/degree, and the largest relative changing rate of the film MR ratio reached over 400% as magnetic field increased from 3 to 9 T. The mechanism of this performance was investigated through electrical transport measurement and magnetic hysteresis loops, which revealed the spin-enhanced magnetism of the nanocrystallites were the main origin of this anisotropic magnetic response behaviors.

Structural and tribological behaviors of graphene nanocrystallited carbon nitride films

Structural and tribological performances of graphene nanocrystallited carbon nitride (GNCN) films were investigated. The GNCN films were prepared in a multifunctional electron cyclotron resonance plasma system with the variation of the microwave power from 300 to 700 W. An increase in the growth rate and a decrease in the nitrogen atomic concentration were clearly observed with increasing microwave power. Whereas, the residual stress, surface roughness (Ra), and nano-scratch depth of the GNCN films were independent of the microwave power. Raman and XPS characterizations of the GNCN films indicated a gradual increase in sp2 carbon bonding structures as well as the size of the graphene nanocrystallite. When rubbing against the Si3N4 balls, low friction coefficients of approximately 0.05 were achieved in nitrogen gas atmosphere. Low friction was attributed to the formation of a uniform tribofilm on the wear scar of the worn Si3N4 ball surface. TEM-EELS analysis of the transfer film evidenced the formation of graphene nanocrystallites and the loss of nitrogen atoms in the topmost layer. It is strongly argued that the evolution of carbon sp2 structure on the contact interface is favorable for obtaining low friction coefficient of the GNCN films in nitrogen gas environment.

Enhanced graphene-edge-induced magnetism by O2 introduction during electron irradiation assisted carbon film growth

We report the effects of the O2 introduction during electron irradiation assisted film growth on the structure and magnetic behavior of the graphene nanocrystallites embedded carbon (GNEC) films. It was found that although O2 was introduced into the chamber during film deposition, there were nearly no oxygen atoms incorporated into the film. The introduced O2 mainly had an effect on selectively removing the amorphous carbon (a-C) phase and gradually promoting the graphene nanocrystallite growth. Such effect brought about more graphene edges in the GNEC film, and then the saturation magnetization of the film Ms at 300 K increased from 0.10 emu/g to 0.28 emu/g. This research proposes an effective approach to enhance graphene-edge-induced carbon magnetism, providing fundamental basis for potential applications of graphene-based materials in magnetic devices.

Ultra-sensitive flexible strain sensor based on graphene nanocrystallite carbon film with wrinkle structures

Flexible strain sensor is under high demand for detecting the various movements of human, but it is still a challenge to integrate high sensitivity and high stretchability simultaneously in a single flexible strain sensor. Here, based on a multi-scale structure design idea, an ultra-sensitive and highly stretchable strain sensor is fabricated using graphene nanocrystallite carbon film with wrinkle structure. In nano-scale, the intrinsic piezoresistive property of graphene nanocrystallite and the tunneling effect between graphene nanocrystallites bring an ultrahigh gauge factor of 1071. In micro-scale, the strain induced ordered parallel wrinkles and perpendicular microcracks to the strain direction can guarantee the flexible strain sensor stable under a maximum strain of 15%. Application tests of detecting the movement of muscles around eye, throat and wrist are conducted. These application tests prove that this strain sensor has high sensitivity, wide measurement range and rapid responsibility, suggesting a great potential in monitoring human activities.

Bias-Modulated High Photoelectric Response of Graphene-Nanocrystallite Embedded Carbon Film Coated on n-Silicon.

We propose that bias-modulated graphene-nanocrystallites (GNs) grown vertically can enhance the photoelectric property of carbon film coated on n-Si substrate. In this work, GN-embedded carbon (GNEC) films were deposited by the electron cyclotron resonance (ECR) sputtering technique. Under a reverse diode bias which lifts the Dirac point of GNs to a higher value, the GNEC film/n-Si device achieved a high photocurrent responsivity of 0.35 A/W. The bias-modulated position of the Dirac point resulted in a tunable ON/OFF ratio and a variable spectral response peak. Moreover, due to the standing structured GNs keeping the transport channels, a response time of 2.2 μs was achieved. This work sheds light on the bias-control wavelength-sensitive photodetector applications.

Edge Effect on the Photodetection Ability of the Graphene Nanocrystallites Embedded Carbon Film Coated on p‐Silicon

A sensitive photodetector of graphene nanocrystallites embedded carbon (GNEC) film coated on p‐silicon has been proposed. Different from the conventional growth mode of graphene, GNEC film contains a large amount of vertically grown graphene nanocrystallites (GNs). Edges of GNs act as electron trapping centers, increasing the ability to capture electrons. Different types of films are prepared under various deposition biases (20, 40, 60, and 80 V), which have different density of edges (Nedge). Edge entrapment improves the photocurrent responsivity of 40 V film (high Nedge) to 0.401 A W−1, compared with 0.126 A W−1 of 20 V film (amorphous, no Nedge) and 0.194 A W−1 of 80 V film (low Nedge). A high specific detectivity of 1.34 × 1012 cm Hz1/2 W−1 is exhibited at zero bias. GNs maintain a charge transport channel, which makes it have a fast response time τrise = 260 ns.

Friction-induced rapid restructuring of graphene nanocrystallite cap layer at sliding surfaces: Short run-in period

Amorphous carbon film is vastly applied for low-friction protective coatings at contact sliding surfaces. However before reaching steady low friction status, the film always endures a high friction period known as the “run-in” period, sometimes taking thousands of sliding cycles, causing remarkable energy dissipation. Here, we report that the run-in period of amorphous carbon film could be drastically shortened to 22 ± 5 cycles by fabricating a 5-nm graphene nanocrystallite cap layer. The cap layer gave rise to rapid formation of graphene nanocrystallized transfer film, which responds to the short run-in period. We found two key factors for the rapid formation of transfer film. Firstly, the cap layer had lower wear resistance than the amorphous carbon, severing as a quick-wearing sacrificial layer. Secondly, the nanocrystallization of transfer film was mainly due to friction-induced restructuring of graphene nanocrystallite but not friction-induced heat. In addition, the friction test of amorphous carbon film covered with multilayer graphene micro-flakes also verified that friction-induced rapid restructuring of graphene sheets at sliding surfaces resulted in short run-in period.

Low friction of graphene nanocrystallite embedded carbon nitride coatings prepared with MCECR plasma sputtering

Graphene nanocrystallite embedded carbon nitride (GNECN) coatings were fabricated with the mirror confinement electron cyclotron resonance (MCECR) plasma sputtering system under low energy electron irradiation at various N2/Ar ratios. N2/Ar ratio was clarified to be an effective deposition parameter for tailoring the composition and structure of the GNECN coatings. It was observed that the deposition rate, N/C atomic ratio, internal stress, surface roughness, and scratch depth of the prepared GNECN coatings change greatly with the increasing N2/Ar ratio from 1/11 to 1/3. Graphene nanocrystallite was clearly identified in the amorphous carbon nitride structure from the analyses of TEM, XPS, and Raman spectroscopy. Moreover, the friction behavior of the GNECN coatings sliding against Si3N4 balls in both ambient air and N2 gas stream showed less dependency on the N2/Ar ratio. Specifically, high friction coefficients ranging from 0.10 to 0.15 were obtained in ambient air, whereas, stable and low friction coefficients of <0.05 were achieved in N2 gas stream. Optical images and Raman spectra of the worn surfaces on the Si3N4 balls and GNECN coatings suggested that a homogeneous tribofilm on the mating ball surface together with the generation and evolution of the nano-size graphene structure in the GNECN coating are the key points in achieving stable and low friction coefficients of <0.05 of the GNECN coatings in N2 gas stream. Finally, it was argued that the combination of low energy electron irradiation and nitrogen atom incorporation in the MCECR plasma sputtering system results in the embedding of soft graphene nanocrystallites into hard carbon nitride matrix, providing a beneficial architecture for achieving stable and low friction coefficients of GNECN coatings in N2 gas stream.

Low-energy electron excitation effect on formation of graphene nanocrystallites during carbon film growth process

We report a low-energy electron excitation effect on the formation of graphene nanocrystallites embedded in carbon films in an electron cyclotron resonance plasma. In this work, carbon films were deposited at different irradiation electron energies and fluxes monitored using a retarding field energy analyzer, while the deposition temperature was measured using an infrared thermometer. We found that the size and concentration of graphene nanocrystallites strongly depend on the irradiation electron energy and the electron flux but not the temperature, which indicates that the electron excitation effect of the covalent bonds dominates the formation process of these nanocrystallites. This finding shed light on the new fabrication method of 2D materials.

Nanostructured ferrite/graphene/polyaniline using for supercapacitor to enhance the capacitive behavior

Graphene nanosheets, polyaniline (PANI), and nanocrystallites of transition metal ferrite {Fe3O4 (Mag), NiFe2O4 (NiF), and CoFe2O4 (CoF)} have been prepared and characterized via XRD, FTIR, SEM, TEM, UV–vis spectroscopy, cyclic voltammetry, galvanostatic charge discharges, and impedance spectroscopy. Electrochemical measurements showed that supercapacitances of hybrid electrodes made of the ternary materials are higher than that of hybrid electrode made of binary or single material. The ternary hybrid CoF/graphene (G)/PANI electrode exhibits a highest specific capacitance reaching 1123 Fg−1, an energy density of 240 Wh kg−1 at 1 A g−1, and a power density of 2680 Wkg−1 at 1 A g−1 and outstanding cycling performance, with 98.2% capacitance retained over 2000 cycles. The extraordinary electrochemical performance of the ternary CoF/G/PANI hybrid can be attributed to the synergistic effects of the individual components. The PANI conducting polymer enhances an electron transport. The Ferrite nanoparticles prevent the restocking of the carbon sheets and provide Faradaic processes to increase the total capacitance.