Electron Cyclotron Resonance Plasma Sputtering Research Articles

Silver ion beam formation and implantation on nano-pyramidal template for isolated nano-dot formation

We report the development of silver (Ag) ion-beam by Electron Cyclotron Resonance (ECR) plasma sputtering and synthesis of Ag nano-dot arrays by implantation of the Ag+ ions on a pre-fabricated silicon nano-template. Ag atoms are mixed with the argon plasma by sputtering a biased silver plate placed inside the ECR chamber. The extracted and mass-separated Ag+ ions are transported to a UHV target chamber for implantation. The variation of the flux of Ag+ ion beam is studied by changing the various parameters, such as applied bias voltage of Ag plate, microwave power, argon gas pressure and specifically, the position of silver plate from the ECR plasma center. It is observed that the sputtering rate of Ag and thereby the final Ag beam current strongly depends on the applied bias and position of the plate. The dependence of microwave power and gas pressure on beam intensity is also investigated and explained in terms of ionization and mean free path of the ionized atoms. Finally, 6 keV Ag+ ions are implanted on pre-patterned nano-pyramidal templates to form an array of Ag nano dots on a large area, by exploiting the geometrical shape of the templates and the directionality of the ion beam.

Effect of Graphene Sheets Embedded Carbon Films on the Fretting Wear Behaviors of Orthodontic Archwire-Bracket Contacts.

Carbon films were fabricated on the orthodontic stainless steel archwires by using a custom-designed electron cyclotron resonance (ECR) plasma sputtering deposition system under electron irradiation with the variation of substrate bias voltages from +5 V to +50 V. Graphene sheets embedded carbon (GSEC) films were fabricated at a higher substrate bias voltage. The fretting friction and wear behaviors of the carbon film-coated archwires running against stainless steel brackets were evaluated by a home-built reciprocating sliding tribometer in artificial saliva environment. Stable and low friction coefficients of less than 0.10 were obtained with the increase of the GSEC film thickness and the introduction of the parallel micro-groove texture on the bracket slot surfaces. Particularly, the GSEC film did not wear out on the archwire after sliding against three-row micro-groove textured bracket for 10,000 times fretting tests; not only low friction coefficient (0.05) but also low wear rate (0.11 × 10−6 mm3/Nm) of the GSEC film were achieved. The synergistic effects of the GSEC films deposited on the archwires and the micro-groove textures fabricated on the brackets contribute to the exceptional friction and wear behaviors of the archwire-bracket sliding contacts, suggesting great potential for the clinical orthodontic treatment applications.

The O-doped layer under electron irradiation induced high sensitivity simultaneous electrochemical determination of colorants in the graphene intercalated carbon film

In this work, we studied the high electrochemical activity induced by the O-doped layer on the graphene intercalated carbon (GIC) film under low energy electron irradiation. The GIC film was prepared by using the electron cyclotron resonance (ECR) plasma sputtering system and the top layer was deposited with 5 % oxygen doped under electron irradiation with different energy. Based on the nanostructure characterization, the electron irradiation played the key role to activate the bonding with oxygen and carbon atoms. In our experiments, doping 5 % oxygen under electron irradiation with energy of 50 V promoted more graphene edges and functional groups formed. The GIC film with O-doped top layer under electron irradiation of 50 V exhibits high electrochemical activity in Fe(CN)64−/3− and Fe3+/2+ redox systems, and realized the simultaneous detecting of sunset yellow (SY) and tartrazine (TT). The mechanism could be that the graphene edge and functional group formed by the activated oxygen atoms provide more active sites to accelerate the electron transfer.

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.

(Invited) Fabrication and Characterization of Ge-Based MIS Structures

Germanium (Ge) is a very attractive channel material for future high-performance metal-insulator-semiconductor (MIS) devices because it exhibits much higher low-field electron (two times higher) and hole (four times higher) mobilities than Si. In addition, the smaller band gap of Ge than that of Si should allow the further reduction of power consumption in advanced integrated circuits and the lower melting point of Ge than that of Si also enables Ge-based device integration at low processing temperatures. However, until the early years of the 2000s, the reported electrical properties of high-k/Ge interfaces had been so poor that one of the critical issues in establishing Ge-based MIS technology was the effective passivation of Ge surfaces by dielectrics with superior interface properties. Although the quality of the Ge-MIS structure has been remarkably improved in the past decade, the relationship between the processing methods and the quality has not been discussed precisely.In this paper, we report on the fabrication of Ge MIS structures with various high-k dielectric materials and interlayer materials through various methods. We also introduce the convenient method to characterize the interface trap densities (Dit's) of the MIS capacitors by spectroscopic measurement of the interface trap density in an insulator/Ge interface at room temperature [1]. The effects of the insertion of interlayers into the interface of high-k insulator/Ge and the application of heat treatment to the Ge MIS structures will be also presented.For instance, the Dit of Ta2O5/GeNx/Ge fabricated by electron-cyclotron-resonance (ECR) plasma nitridation and sputtering was estimated to be 4×1011 cm-2 · eV-1 [2], that of Si3N4/GeNx/Ge by ECR plasma nitridation to be 1×1011 cm-2 · eV-1 [3-6], and that of Al2O3/GeO2/Ge by ECR plasma oxidation and sputtering to be 4.5×1010 cm-2 · eV-1 [7-9]. These results have summarized that the adequate surface preparations of the interfaces and films by the ECR plasma process with low energy plasma irradiation have been effective to realize the low Dit of MIS structures with GeO2 and/or GeNx.These results have expanded to another MIS structures fabricated by more conventional film formation technique such as the atomic layer deposition (ALD) and the radio frequency (RF) magnetron sputtering. The Dit of Al2O3/Al germanate/Ge by ALD with trimethylaluminum (TMA) and microwave-generated atomic oxygen was estimated to be 2×1011cm-2 · eV-1 [10], and that of HfO2/Ge by RF magnetron sputtering to be 5.9×1011 cm-2 · eV-1. These results have summarized that the heat treatment after the fabrication of MIS strucures have affected interface properties of germanate/Ge and oxide/Ge.Although the mechanism by which the process conditions act on the interface state is now under estimation, it has been speculated that the fabrication method, the insertion of the interlayers, and the application of heat treatment have a great influence on the interface characteristics. The structures and the fabrication method of various MIS will be discussed.Part of this work was carried out under the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.

Robust low friction performance of graphene sheets embedded carbon films coated orthodontic stainless steel archwires

Reducing the friction force between the commercial archwire and bracket during the orthodontic treatment in general dental practice has attracted worldwide interest. An investigation on the friction and wear behaviors of the uncoated and carbon film coated stainless steel archwires running against stainless steel brackets was systematically conducted. The carbon films were prepared at substrate bias voltages from +5 to +50 V using an electron cyclotron resonance plasma sputtering system. With increasing substrate bias voltage, local microstructures of the carbon films evolved from amorphous carbon to graphene nanocrystallites. Both static and stable friction coefficients of the archwire-bracket contacts sliding in dry and wet (artificial saliva) conditions decreased with the deposition of carbon films on the archwires. Low friction coefficient of 0.12 was achieved in artificial saliva environment for the graphene sheets embedded carbon (GSEC) film coated archwire. Deterioration of the friction behavior of the GSEC film coated archwire occurred after immersion of the archwire in artificial saliva solution for different periods before friction test. However, moderate friction coefficient of less than 0.30 sustained after 30 days immersion periods. The low friction mechanism is clarified to be the formation of salivary adsorbed layer and graphene sheets containing tribofilm on the contact interfaces. The robust low friction and low wear performances of the GSEC film coated archwires make them good candidates for clinical orthodontic treatment applications.

Multiscale frictional behaviors of sp2 nanocrystallited carbon films with different ion irradiation densities

sp2 nanocrystallited carbon films with large nanocrystallite sizes, smooth surfaces, and relative high hardness were prepared with different ion irradiation densities regulated with the substrate magnetic coil current in an electron cyclotron resonance plasma sputtering system. Their multiscale frictional behaviors were investigated with macro pin-on-disk tribo-tests and micro nanoscratch tests. The results revealed that, at an ion irradiation density of 16 mA/cm2, sp2 nanocrystallited carbon film exhibits the lowest friction coefficient and good wear resistant properties at both the macroscale and microscale. The film sliding against a Si3N4 ball under a contact pressure of 0.57 GPa exhibited a low friction coefficient of 0.09 and a long wear life at the macroscale. Furthermore, the film sliding against a diamond tip under a contact pressure of 4.9 GPa exhibited a stable low friction coefficient of 0.08 with a shallow scratch depth at the microscale. It is suggested that sp2 nanocrystallites affect the frictional behaviors in the cases described differently. At the macroscale, the contact interface via the small real contact area and the sp2 nanocrystallited transfer layer dominated the frictional behavior, while the sp2 nanocrystallited structure in the film with low shear strength and high plastic resistivity, as well as the smooth surface morphology, decided the steady low nanoscratch properties at the microscale. These findings expand multiscale tribological applications of sp2 nanocrystallited carbon films.

High Electrochemical Activity Induced by Doping Oxygen in Graphene Sheets Embedded Carbon Film

AbstractIn this work, the high electrochemical activity induced by doping different contents of oxygen on the surface of graphene sheets embedded carbon (GSEC) film is studied, which is prepared by electron cyclotron resonance (ECR) plasma sputtering system under electron irradiation. Transmission electron microscopy observation and Raman analysis confirm that doping oxygen induced more graphene edges and oxygen‐containing functional groups on the surface. The electrochemical activities of the films are measured in the redox system and the results show that a proper doping content of oxygen is beneficial to reduce the oxidation‐reduction peak separation. The surface O‐doped GSEC film with 5% content realizes the simultaneous detection of uric acid (UA), xanthine (XA) and hypoxanthine (HXA) with high sensitivity. The mechanism could be that the isolated electron at graphene edge and the polar fraction of oxygen‐containing functional groups provide more active sites to accelerate the electron transfer. These results indicate that surface O‐doped GSEC films are promising electrode materials to construct sensitive electrochemical biosensors.

Thermally induced atomic and electronic structure evolution in nanostructured carbon film by in situ TEM/EELS analysis

The atomic and electronic structures of nanostructured carbon film are of fundamental importance due to their dominant role in carbon films' physical, chemical and mechanical properties. Here, we reported thermally induced atomic and electronic structure evolution of nanostructured carbon film by in-situ transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Two nanostructured carbon films were synthesized by electron cyclotron resonance (ECR) plasma sputtering under ion and electron irradiation, respectively. We observed that even though the two carbon films had similar initial sp2/sp3 hybridization ratio, they exhibited very different performance during the same heating process. The electron irradiated film only had a relative slight clustering of sp2 carbon crystallites and almost maintained its sp2/sp3 ratio till 1000 °C; while the ion irradiated film had a significant sp2 crystallites clustering, started its sp3-to-sp2 conversion from 200 °C and almost completed at 400 °C. Argon signal was only found in the ion irradiated carbon film and its content reduced simultaneously with the sp3-to-sp2 conversion process. We suggest that the heated argon trapped inside the carbon film were responsible for triggering the sp3-to-sp2 conversion. The result will guide the future application of the nanostructured carbon film in the proper operation temperature range and help to choose appropriate irradiation method for thin film surface modification.

N-doped graphene sheets induced high electrochemical activity in carbon film

In this study, we prepared a carbon film with clearly shaped N-doped graphene sheets by electron cyclotron resonance (ECR) plasma sputtering under low-energy electron irradiation. We found the N-doped graphene sheets remarkably improved the electrochemical activity of carbon film. The charge-transfer resistance was decreased from 21.62 Ω cm2 to 1.37 Ω cm2, and the redox peak separation was reduced to a low value of 65.4 mV in Fe(CN)64−/3− redox system. The high electrochemical activity of N-doped graphene sheets embedded carbon (N-GSEC) films was ascribed to the formation of smaller sized N-doped graphene sheets. The smaller sized N-doped graphene sheets with high electronic density of states produced abundant edge defects, which served as active sites, facilitated the adsorption of Fe(CN)64−/3− on film surface and enhanced the electron transfer. In detecting DNA base of adenine, the N-GSEC film showed a low oxidation potential and high sensitivity. These results demonstrate the N-GSEC film is a promising candidate material for construction sensitivity electrochemical biosensor.

Ion excitation and etching effects on top-surface properties of sp2 nanocrystallited carbon films

We proposed the low energy ion irradiation effects (ion excitation and ion etching) on the formation and the top-surface properties of sp2 nanocrystallited carbon films in electron cyclotron resonance (ECR) plasma sputtering system. In this work, the ion etching rate during film deposition was measured and a threshold voltage for physical etching was found to be about 35 V. Below the threshold voltage, the film growth was induced by ion excitation effect (ion exciting electrons of top-most atoms), as a result, the sp2 dominant nanocrystallite size in the films was large. When the ion energy exceeded the threshold voltage, the film growth was induced by ion etching effect (ion breaking covalent bond), and the nanocrystallite size was small confirmed by the transmission electron microscopy (TEM) observation and Raman analysis. Furthermore, the top-surface properties of sp2 nanocrystallite carbon films prepared with different ion irradiation effects were examined by using the atomic force microscopy (AFM). Results showed that the ion etched carbon films had smaller surface roughness, scratch depth and adhesive force compared with the ion excited carbon films. These findings shed light on the fabrication and applications of carbon based nanostructured materials.

Nanosized graphene sheets induced high electrochemical activity in pure carbon film

We found that nanosized graphene sheets induced high electrochemical activity in pure carbon films, which prepared by electron cyclotron resonance (ECR) plasma sputtering under low-energy electron irradiation condition. The electrochemical properties were studied by electrochemical impedance spectroscopy and cyclic voltammetry. The graphene sheets embedded carbon (GSEC) films showed a wide potential window over 3.2 V. The charge transfer resistance and the oxidation-reduction peak separation (ΔEP) of the GSEC films are lower than amorphous carbon films in several redox systems (Fe(CN)64−/3-, Ru(NH3)62+/3+, dopamine and ascorbic acid), especially in the inner-sphere system, the ΔEP is only half of amorphous carbon films. The high electrochemical activity of GSEC films originated from the nanosized graphene sheets, which offered faster electron transfer path and more reaction active sites. Our results indicate the GSEC films have great potential to be an electrochemical biosensor in detecting biomolecules with high oxidation potential.

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.

Note: Easy-to-maintain electron cyclotron resonance (ECR) plasma sputtering apparatus featuring hybrid waveguide and coaxial cables for microwave delivery.

The branched-waveguide electron cyclotron resonance plasma sputtering apparatus places quartz windows for transmitting microwaves into the plasma source not in the line of sight of the target. However, the quartz windows must be replaced after some time of operation. For maintenance, the loop waveguide branching from the T-junction must be dismounted and re-assembled accurately, which is a time-consuming job. We investigated substituting the waveguide branches with two sets of coaxial cables and waveguide/coaxial cable converters to simplify assembly as far as connection and disconnection go. The resulting hybrid system worked well for the purposes of plasma generation and film deposition.

Variability Improvement by Si Surface Flattening of Electrical Characteristics in MOSFETs With High-k HfON Gate Insulator

Variability improvement of metal-oxide-semiconductor field-effect transistors (MOSFETs) characteristics with high-k HfON gate insulator by Si surface flattening was investigated. The Si surface flattening process was carried out by Ar/4.9%H 2 anneal utilizing rapid thermal annealing system. The HfON gate insulator was formed by the in-situ Ar/O 2 plasma oxidation of HfN utilizing electron cyclotron resonance plasma sputtering followed by the post deposition anneal at 600 °C for 1 min. The Si surface flattening was found to significantly improve the threshold voltage variability (ΔV TH ) of MOSFET with HfON gate insulator for the first time. Furthermore, the stability of the device characteristics was improved up to the measurement temperature of 100 °C by introducing the Si surface flattening process.

The Effects of Diamond-Like Carbon Films on Fretting Wear Behavior of Orthodontic Archwire-Bracket Contacts.

This study aims to assess the effects of diamond-like carbon (DLC) films on fretting wear behavior of orthodontic archwire-bracket contacts. 'Mirror-confinement-type electron cyclotron resonance (MCECR) plasma sputtering' was utilized to deposit carbon films on stainless steel archwires and brackets. Nanostructure of carbon films such as the bonding structure, cross-sectional thickness and surface roughness were studied. The fretting wear behavior of various archwire-bracket contacts were investigated by using a self-developed tester in ambient air and artificial saliva. The results indicated that DLC-coated wires showed significantly low friction coefficient than the uncoated wires independently of the applied environments. Nevertheless, the DLC-coated and uncoated brackets showed no significant differences in the friction coefficient. Microscopic analysis showed that low wear took place for the DLC-coated surfaces. It is proposed that the application of DLC coating on archwires can decrease the orthodontic fretting wear and coefficient of friction. Unfortunately it does not affect the frictional properties for brackets at present.

Epitaxial ZnO/LiNbO3/ZnO stacked layer waveguide for application to thin-film Pockels sensors

We produced slab waveguides consisting of a LiNbO3 (LN) core layer that was sandwiched with Al-doped ZnO cladding layers. The ZnO/LN/ZnO stacked layers were grown on sapphire C-planes by electron cyclotron resonance (ECR) plasma sputtering and were subjected to structural, electrical, and optical characterizations. X-ray diffraction confirmed that the ZnO and LN layers were epitaxial without containing misoriented crystallites. The presence of 60°-rotational variants of ZnO and LN crystalline domains were identified from X-ray pole figures. Cross-sectional transmission electron microscopy images revealed a c-axis orientated columnar texture for LN crystals, which ensured operation as electro-optic sensors based on optical anisotropy along longitudinal and transversal directions. The interfacial roughness between the LN core and ZnO bottom layers as well as that between the ZnO top and the LN core layers was less than 20 nm, which agreed with surface images observed with atomic force microscopy. Outgrowth of triangular LN crystalline domains produced large roughness at the LN film surface. The RMS roughness of the LN film surface was twice that of the same structure grown on sapphire A-planes. Vertical optical transmittance of the stacked films was higher than 85% within the visible and infrared wavelength range. Following the approach adopted by Teng and Man [Appl. Phys. Lett. 56, 1734 (1990)], ac Pockels coefficients of r33 = 24-28 pm/V were derived for c-axis oriented LN films grown on low-resistive Si substrates. Light propagation within a ZnO/LN/ZnO slab waveguide as well as within a ZnO single layer waveguide was confirmed. The birefringence of these waveguides was 0.11 for the former and 0.05 for the latter.

Low-temperature crystallization and high-temperature instability of hydroxyapatite thin films deposited on Ru, Ti, and Pt metal substrates

We investigated the crystallinity, orientation, and stability of hydroxyapatite (HAp) thin films on structural materials including SiO2 and metals (Ru, Ti, and Pt). The HAp films were deposited by electron cyclotron resonance (ECR) plasma sputtering with an Xe sputtering gas under simultaneous flow of H2O vapor. The HAp films as-crystallized during deposition and solid-phase crystallized by post annealing were analyzed with X-ray diffraction, Fourier-transform infrared spectroscopy, and Raman scattering. The lowest solid-phase crystallization temperatures were 350°C on Ru, 300°C on Ti, and 300°C on Pt. These crystallization temperatures are much lower than those on silicon and SiO2 substrates (550°C). For HAp films as-crystallized at temperatures above 400°C, the elements out diffused and reacted with substrates. Precipitation of CaO was observed for HAp films deposited on SiO2 and Ru substrates. Diffusion of PO43− into SiO2 and Ru-catalyzed decomposition of HAp crystals are suggested for the mechanism. When deposited on Pt substrates, CaPt2O4 alloy byproducts were created for solid-phase crystallized films and CaPt2 alloy byproducts were created for as-crystallized films, indicating reaction of Ca atoms with Pt substrates.

Relation between process parameters of ZnO host films and optical activation of doped Er3+ ions

Abstract We investigated how the crystallinity of ZnO host films deposited on Si(1 0 0) substrates by electron cyclotron resonance (ECR) plasma sputtering affects the optical activity of Er 3+ ions doped in them under resonant excitation with 532-nm laser light. Photoluminescence from ZnO:Er films deposited at room temperature (RT) exhibited a main emission peak at 1538 nm with a sub-peak at 1553 nm. The emission intensity reached a steady state at Er content between 0.6 and 3 at.%, and attenuated above 3 at.%. This concentration quenching correlates with derivation of X-ray diffraction angles from the Vegard's law. Deposition at RT produced non-equilibrium state of ZnO:Er films possibly stabilized with vacancies to maintain charge neutrality. After post annealing of ZnO:Er films with Er content less than 1 at.%, the emission intensities were markedly reduced as the crystalline lattice was rearranged. For films in the concentration quenching range, however, post annealing increased the emission peak width while maintaining emission intensity, indicating that Er 3+ ions migrated and populated various emission-active sites. In this case, the main peak wavelength blue shifted to 1536 nm and produced a new sub peak at 1546 nm. When the deposition temperature exceeded 500 °C, emissions from Er 3+ ions were greatly enhanced. This will result from improvement in crystallinity and fixing Er 3+ ions at six-fold coordinated C 4v symmetry sites.

Frictional Behavior of Carbon Film Embedded with Controlling-Sized Graphene Nanocrystallites

Graphene nanocrystallites embedded in amorphous carbon matrix can bring excellent tribological, electrical and magnetical properties to the carbon films. But too large size of graphene nanocrystallite would lead to degradation of the tribological performance. So it is necessary to clarify the dependence of frictional behavior of the carbon film on graphene nanocrystallite size. In order to control the size, different electron irradiation densities were introduced during film growth in the electron cyclotron resonance plasma sputtering process. Frictional tests on the films were carried out with a Pin-on-Disk tribometer. The evolution of graphene nanocrystallite size along with electron irradiation density was examined by transmission electron microscopy and Raman spectroscopy. The results showed that the graphene nanocrystallite size increased with increasing of the electron irradiation density. The film with a graphene nanocrystallite size of 1.09 nm exhibited a low friction coefficient of 0.03 and a long wear life. When nanocrystallite size increased, the friction coefficient increased and the wear life decreased. Observation on transfer film revealed that the nanocrystallite in transfer film grew larger when initial size was 1.09 nm, and changed smaller when initial size was 1.67 nm. The results suggested that embedded graphene nanocrystallite played an important role in the formation of transfer film, the initial size of graphene nanocrystallite strongly affected the frictional behavior of the film, and the graphene nanocrystallite needed to be controlled under a certain size in order to keep the good tribological performance.