Multilayer Graphene Films Research Articles

Picosecond laser-direct fabrication of graphene-based electrodes for a gas sensor module with wireless circuits

This paper proposes an ultrafast-laser process for ablating multilayer graphene film to form the conductive electrodes for a gas sensor module with wireless circuits. In this study, a picosecond laser source was directly used to perform double scanning for precisely fabricating the spiral electrode pattern as the sensing test region. The ablated surface of multilayer graphene structures fabricated using sufficiently strong pulses with a pulse width of 7 ps and a wavelength of 532 nm was investigated through scanning electron microscopy. The graphene-based electrode was integrated with wireless circuits to form a gas sensor module. The relationship between the spiral electrode design and sensitivity to gas detection was analyzed. Electrical detection of gas was studied by employing different electrode designs, and change in the detection limit at room temperature with change in resistance was examined. Furthermore, the role of graphene-based devices comprising spiral electrodes fabricated using the laser process and integrated with wireless circuits to form a module in gas detection was analyzed.

Direct formation of continuous multilayer graphene films with controllable thickness on dielectric substrates

Direct formation of graphene films on dielectric substrates is investigated by the “etching-precipitation” method which converts metal-carbon mixed films to graphene films by etching metal away by Cl2 at 600–650 °C. Here we report a new approach for improved control of the layer number and continuity of the graphene films. Reactive sputtering of Fe in C2H4/Ar enabled fine control of the carbon concentrations and thicknesses of the initial Fe-C films, which yielded continuous multilayer graphene films of controllable average layer numbers of ~10–40, low resistivity down to ~240 μΩ cm, and high Raman G-band to D-band intensity ratio up to 16 directly on SiO2 substrates. We also show that the carbon concentration of the initial Fe-C films determines the film continuity and crystallinity of the graphene.

Graphene Oxide and Derivatives: The Place in Graphene Family

Graphene oxide (GO) is useful and promising material for graphene based applications in electronic, optics, chemistry, energy storage and biology. At the beginning of graphene history GO was only a simple and cheap step for preparation of single and multilayer graphene films and bulk structures by reduction. The further studies revealed the substantial structure imperfection of graphene oxide derived materials due to the defects in initial graphite and incompletion of reducing process. However, the results of recent research demonstrated a great amount of unique chemical, optical and electronic properties of graphene oxide that allow regarding it as independent nanomaterial possessing a large area of applications. In general, it represents the ultra-large organic molecule containing 2D carbon mesh. Unlike conventional graphene it provides wide range of chemical methods for attachment of various functional groups to its surface for control optical transparency, electrical and thermal conductance. Recently developed methods for preparation of graphene oxide derivatives saturated by carboxyl groups open the new attractive application areas in green technologies including energy storage and utilizing nuclear wastes. The goal of the review is to summarize the results of recent studies of graphene oxide, derivatives and reveal the most promising directions to focus the efforts of researchers.

Ultrathin nitrogen-doping graphene films for flexible and stretchable EMI shielding materials

Ultrathin, flexible and highly conductive materials that possess excellent electromagnetic interference (EMI) shielding performance are greatly needed, especially for the fabrication of stretchable shielding materials in practical applications such as wearable and foldable electronics. Graphene oxide (GO) sheets are modified with ethylenediamine to prepare cross-linked graphene films using a pressure-assisted self-assembly technique. FTIR and XPS results demonstrate that amine monomers are chemically bonded to GO sheets, with simultaneous reduction of GO sheets. After thermal annealing and followed with compression, the 6.6-μm-thick nitrogen-doping graphene film (rGO-EDA-2) is obtained with ultrahigh electrical conductivity of 8796 S cm−1. The excellent electrical conductivity is mainly attributed to nitrogen-doping effect, defects repair during chemical functionalization and removal of oxygenated groups. Ultrahigh electrical conductivity, multilayer structure and modified electronic structure with nitrogen doping lead to outstanding shielding performance for the rGO-EDA-2 film, with excellent shielding effectiveness (SE) of 58.5 dB and the specific SE/thickness of 43902 dB cm2 g−1, respectively. By fixing the rGO-EDA-2 film on the pre-stretched wavy substrate, the stretchable shielding composite is obtained, with constant EMI SE of 56.3 dB after repeated stretching. The pre-stretched wavy substrate allows the multilayer graphene film to achieve wavy structure after strain release, which is capable of bearing tensile strain up to 32.6%. This study could be significant in the applications of stretchable and wearable electronic devices.

Direct deposition of multilayer graphene on dielectrics via solid-phase precipitation from carbon-doped cobalt with a copper capping layer

A method for producing a uniform multilayer graphene (MLG) film directly on SiO2 via solid-phase precipitation from carbon-doped cobalt (Co–C) with a Cu capping layer has been developed for a large scale integration (LSI) interconnect application. One advantage is that no transfer process is required. A 20 nm thick MLG film was grown uniformly from a 100 nm thick Co–C (20 at%) catalyst layer with a Cu capping layer. Cross-sectional TEM/EDX images revealed that the optimized Cu capping layer prevents Co agglomeration during annealing and promotes C precipitation on the SiO2 while eliminating it on the top surface of the catalytic metals.

Optimization of a nanoscale field emission cell with a planar blade-shaped multilayer graphene/SiC emitter

The paper reports an investigation of planar field emission cathodes with a nanoscale interelectrode gap in high electric fields. The multilayer graphene film is used as a rectangular blade-shaped cathode. The authors carry out simulations of the distribution of the electric field strength along the emitting surface. Six types of distribution of the field enhancement factor are revealed by the simulations. The localized enhancement of the electric field strength at the blade corners is observed. The influence of the anode parameters on the emission homogeneity along the emitting surface of the cathode in strong electric fields is determined. The analysis of the simulation results suggests that an increase in the rounding-off radius of the emitter corner does not contribute to the complete elimination of the field enhancement factor heterogeneity. Additional anode modification makes it possible to obtain a homogeneous field enhancement factor along the emitting surface of the cathode and to improve the stability of the emission.

Получение высокопроводящих и оптически прозрачных пленок со структурой мультиграфена путем карбонизации полиимидных пленок Ленгмюра-Блоджетт

For the first time, multilayer graphene films on quartz glass were obtained by carbonization of the Langmuir-Blodgett films of polyimide. On the Raman spectra of the carbonized polyimide films, the G, D bands and a wide band in the region of 2300–3200 cm-1 were observed. Using the method of transmission microscopy, it was shown that films consist of “stacks” of layers with a interlayers distance of 0.36 nm. The surface resistance of films 5 nm thick was 1.2±0.2 kOmega, and the transmittance was 87% at lambda=550 nm. In the infrared region of the spectrum, the transmittance was 97%, and in the UV region 70-80%.

Multilayer-graphene-stabilized lithium deposition for anode-Free lithium-metal batteries.

The will to circumvent capacity fading, Li dendrite formation, and low coulombic efficiency in anode-free Li-metal batteries (AFLMBs) requires a radical change in the science underpinning new materials discovery, battery design, and understanding electrode interfaces. Herein, a Cu current collector formed with ultrathin multilayer graphene grown via chemical vapor deposition (CVD) was used as an artificial layer to stabilize the electrode interface and sandwich-deposited Li with Cu. A multilayer graphene film's superior strength, chemical stability, and flexibility make it an excellent choice to modify a Cu electrode. Fabricating an anode bigger than the cathode improved the alignment of the electrodes during assembly, minimizing interfacial stress. Here, 19 mm electrodes when paired with a commercial LiFePO4 cathode (mass loading: ∼12 mg cm-2) delivered the first-cycle discharge capacities of 147 and 151 mA h g-1 for bare and multilayer-graphene-protected electrodes, respectively, which could alleviate the big hurdle (initial capacity loss) in anode-free batteries. After 100 round-trip cycles, bare Cu and multilayer-graphene-protected electrodes retained ∼46 and ∼61% of their initial capacities, respectively, in an ether-based electrolyte at the rate of 0.1 C.

Gas sensing applications of multilayer graphene grown on Co-Ni/Al<SUB align="right">2O<SUB align="right">3 substrate by chemical vapour deposition

The gas sensing applications of multilayer graphene (MLG) films have been investigated in this study. The MLG was synthesised by chemical vapour deposition (CVD) over Co-Ni/Al2O3 substrate. The gas sensing experiment was performed by measuring resistance across the MLG film at different times while passing ammonia, hydrogen and, methane through. The MLG-based sensor was exposed to different concentrations of the gases for a period of 15 min at a fixed voltage of 1.5 V. When different gases were introduced to the test chamber at a steady flowrate, the resistance increased and reached a saturation level. The findings showed that the MLG-based sensor device was most sensitive to NH3 and H2 gases whereas it displayed least sensitivity to CH4 gas. This study has demonstrated the suitability of the MLG as a material that can be employed as a sensor device for gas sensing applications selective to NH3 and H2.

Construction of Schottky junction solar cell using silicon nanowires and multi-layered graphene

Schottky junction solar cells based on n-type silicon nanowires (SiNWs) and multi-layered graphene were assembled. The textured substrates enhanced the light-harvesting compared to the planar substrates. Multi-layered graphene films produced by chemical vapor deposition and layer-by-layer stacking mitigated damages and increased the work function of graphene. Thus, both the power conversion efficiency and fill factor of Schottky junction solar cells could be improved with multi-layered graphene. It was found in the present study that the optimum photovoltaics performance of solar cells was achieved when double-layered graphene was used. The performance degraded with the number of graphene layers when it was more than 2 layers. While damages could be mitigated and the work function could be improved using multi-layered graphene in SiNWs Schottky junction solar cells, the optical transmittance of graphene would be reduced with excessive number of graphene layers.

The piezoconductive effect of suspended ultrathin graphene film

We study a vertical pressure-modulated conductance of suspended graphene in which the CC coupling is adjusted by external pressure. Through the tight-binding model and the non-equilibrium Green's function method, the relative piezoconductance is calculated. It is found that the perfect graphene monolayer and multi-layer films have similar piezoconductive properties. When dephasing processes are considered, the multi-layer graphene is fragile, its piezoconductance is dramatically reduced. Concerning the nonideal ohmic contact, the relative piezoconductance is highly layer number dependent. The more layers, the greater the impact. It means the piezoconductive effect is highly affected by the dephasing processes and quality of ohmic contact. Because the dephasing processes and ohmic contact are both the most common factors in experiments, the piezoconductive effect in pressure modulated ultrathin graphene, and its dependance on the dephasing precesses and device fabrication, would help us to understand the electromechanical properties of graphene for future applications.

Transfer-free synthesis of multilayer graphene on silicon nitride using reusable gallium catalyst

Direct growth of graphene film on dielectric substrates is significant for the applications in electronic devices, which can avoid additional complicated and damaging treatments. Despite the advances, a facile way is urgently required to prepare graphene directly on insulators at a low temperature. We presented a gallium (Ga)-assisted atmosphere chemical vapor deposition to synthesize graphene films on silicon nitride (Si3N4). Liquid Ga was dropped on the substrate as a catalyst and flowed away after growth, achieving graphene on Si3N4 (100 nm)/SiO2 (100 nm)/Si and recovering the most of Ga. Multilayer graphene film was formed at the interface between Ga and the substrate. The growth of graphene was investigated by regulating the growth conditions, and an interfacial precipitation mechanism was proposed. Graphene synthesized with recovered Ga droplet is comparable to that prepared using pure Ga, revealing the reusability of Ga catalysts.

The role of ion dose on C implanted multilayered graphene films in Ni as host substrate

In this paper, we report formation of single and multilayer graphene synthesized by varied doses of implanted carbon ions in Ni as a host medium. 1 MeV energetic C ions with a dose in the range from 1012 ions/cm2 to 1015 ions/cm2 were implanted in thermally evaporated Ni films on glass and Si substrates. XRD, SEM and AFM were employed to study the structure and morphology of the synthesized graphene. A sharp Raman G band was observed, which overlapped with a very broad Raman D band due to presence of amorphous C in the synthesized structures. The lineshape analysis of Raman 2D bands confirmed the formation of monolayer, bilayers and four layers of graphene layers. The splitting observed in the Raman 2D mode was attributed to the interlayer disorders caused by Ni traces. In the end it was concluded that the dose rate and quality of Ni film were critical for the synthesis of graphene or graphene like multilayered films. The synthesis of graphene was also found to depend on the substrate as adhesion and growth of Ni on various substrates was different. In present experiments, dose of 1015 ions/cm2 of 1 MeV C ions was suitable in case of Si substrates where as 1014 ions/cm2 dose rate was acceptable lower limit for the synthesis of single layer grapheme on glass substrate.

Unraveling the Effect of Material Properties and Geometrical Factors on Ballistic Penetration Energy of Nanoscale Thin Films

It is crucial to investigate the dynamic mechanical behavior of materials at the nanoscale to create nanostructured protective systems that have superior ballistic impact resistance. Inspired from recent experimental advances that enable ballistic materials testing at small scales, here we report a comparative analysis of the dynamic behavior of nanoscale thin films made from multilayer graphene (MLG), polymer, gold, and aluminum under high-speed projectile impact. We employ atomistic and coarse-grained (CG) molecular dynamics (MD) simulations to measure the ballistic limit velocity (V50) and penetration energy (Ep) of these nanoscale films and investigate their distinctive failure mechanisms over a wide range of impact velocities (Vi). For the local penetration failure mechanism observed in polymer and metal films, we find that the intrinsic mechanical properties influence Ep at low Vi, while material density tends to govern Ep at high Vi. MLG films uniquely show a large impact propagation zone (IPZ), which transfers the highly localized impact energy into elastic deformation energy in a much larger area through cone wave propagation. We present theoretical analyses that corroborate that the size of IPZ should depend not only on material properties but also on a geometrical factor, specifically, the ratio between the projectile radius and film thickness. This study clearly illustrates how material properties and geometrical factors relate to the ballistic penetration energy, thereby allowing a quantitative comparison of the nanoscale ballistic response of different materials.

Photocurrent generation of biohybrid systems based on bacterial reaction centers and graphene electrodes

The direct conversion of sunlight into chemical energy via photosynthesis is a unique capability of plants and some bacterial species. Aimed at mimicking this energy conversion process, the combination of inorganic substrates and organic photoactive proteins into an artificial biohybrid system is of a great interest for artificial bio-photovoltaic applications. It also allows to better understand charge transfer processes involved in the photosynthetic chain. In this work, single layer graphene (SLG) and multilayer graphene (MLG) electrodes are used as a platform for the immobilization of reaction centers (RCs) from purple bacteria Rhodobacter sphaeroides, a protein complex responsible for the generation of photo-excited charges. Electrochemical experiments with graphene electrodes and redox molecules reveal fundamental differences in the charge transfer processes for SLG and MLG films. We demonstrate that both graphene-based materials enable the immobilization of RCs without loss of functionality, attested by a photocurrent generation under illumination with IR-light at a wavelength of 870 nm. Furthermore, we report on the dependence of the generated photocurrent on the applied bias voltage, as well as on the presence of charge mediators in the surrounding electrolyte. This work demonstrates that SLG and MLG are a suitable platform for RC immobilization and subsequent photocurrent generation, suggesting a promising potential for graphene-based materials in bio-photovoltaics.

Fast and controllable reduction of graphene oxide by low-cost CO2 laser for supercapacitor application

Direct reduction of graphene oxide has been regarded as the economically viable route for large-scale synthesis of graphene. However, the currently known methods suffer from either poor reduction efficiency or involve multi-step and energy-intensive reduction processes. Here, we demonstrate a remarkably fast, single step as well as highly efficient reduction technique to produce high-quality multilayer graphene film using a compact and low-cost CO2 laser pyrolysis. Thanks to the intrinsically high absorptivity of graphene oxide in the near- and mid-infrared regions, the irradiation of CO2 laser generates instantaneous and strong localized heating on it and thus burst apart the oxygen functional groups from the graphene oxide layers. The extent of reduction in the synthesized multilayer graphene films can be fruitfully controlled by variation of laser processing parameters such as laser intensity, scanning speed and shifting pitch. To prove the worth of this method, the graphene films were used as the binder-free and self-standing electrode for symmetric supercapacitor cell. The electrochemical performance data shows that specific capacitance and cyclic stability has a contrasting relation with the reduction efficiency. We believe that this CO2 laser-based reduction method could guarantee a high outturn of multilayer graphene and its composites for innumerable applications.

Mechanistic Understanding of Improved Performance of Graphene Cathode Inverted Organic Light-Emitting Diodes by Photoemission and Impedance Spectroscopy.

Modification of multilayer graphene films was investigated for a cathode of organic light-emitting diodes (OLEDs). By doping the graphene/electron transport layer (ETL) interface with Li, the driving voltage of the OLED was reduced dramatically from 24.5 to 3.2 V at a luminance of 1000 cd/m2. The external quantum efficiency was also enhanced from 3.4 to 12.9%. Surface analyses showed that the Li doping significantly lowers the lowest unoccupied molecular orbital level of the ETL, thereby reducing the electron injection barrier and facilitating electron injection from the cathode. Impedance spectroscopy analyses performed on electron-only devices (EODs) revealed the existence of distributed trap states with a well-defined activation energy, which is successfully described by the Havriliak-Negami capacitance functions and the temperature-independent frequency dispersion parameters. In particular, the graphene EOD showed a unique high-frequency feature as compared to the indium tin oxide one, which could be explained by an additional parallel capacitance element.

Thermal-assisted direct transfer of graphene onto flexible substrates

We explored an easy technique to transfer directly large-area graphene grown by chemical vapor deposition (CVD) onto flexible substrates through substrate heating at appropriate temperatures. Polyethylene (PE) films were selected as the flexible substrate. The critical factor to ensure successful transfer was the good contact between the substrate and graphene. By heating the PE films merely approaching their glass-transition temperature, film morphology did not change. The stickiness of PE films further enabled the van der Waal forces to ensure close interaction between the viscoelastic substrate and graphene. This method did not require an intermediate transfer membrane such as polymethylmethacrylate (PMMA) that needs removal afterwards. Though this transfer method, monolayer and multilayer graphene films were transferred onto PE substrates with sheet resistance as low as 158 Ω/sq. All these results can be applied in existing and new graphene CVD applications on flexible substrates.

Novel Hollow Graphene Flowers Synthesized by Cu‐Assisted Chemical Vapor Deposition

AbstractNovel hollow graphene flowers (HGFs) are synthesized by a Cu‐assisted chemical vapor deposition. Multilayer graphene films and vertical‐oriented graphene nanosheets are grown on hollow silica spheres successively by controlling the CH4/H2 flow and growth time. After graphene growth, HSS templates are removed to achieve HGFs. The π‐conjugated systems and unique morphology of HGFs lead to apparent adsorption ability for dyes and phenol molecules. This work provides a facile way to produce unique hollow structure graphene with massive edges for different applications in the future.

In situ direct growth of graphene/hexagonal boron nitride heterostructure on SiO2 substrate without metal catalyst

Here, we report the in situ direct growth of a graphene/hexagonal boron nitride (hBN) heterostructure on a SiO2 substrate without a metal catalyst by chemical vapor deposition (CVD). The hBN could be grown easily on a SiO2 substrate, while graphene growth was difficult and time-consuming as graphite could be grown only partially on the dielectric substrate, even after 5 h. Graphene was grown directly on this hBN/SiO2 substrate sequentially, which demonstrated easy and quick growth of a fully covered and high-quality graphene multilayer film within 40 min. The effect of hydrogen on the direct growth of hBN on a SiO2 substrate was also studied, and it was found that when a higher flow rate of hydrogen was used, the domain size was larger and higher quality of hBN could be grown. The quality of the grown hBN and graphene/hBN samples were confirmed by UV–vis, Raman, and atomic force microscopy (AFM). This new method can be used for graphene multilayer coating on dielectric substrates, on which it is difficult to grow graphene directly, for industrial or scientific applications.