Graphene Films Research Articles

Impact of material and tunnel barrier quality on spin transport in a CVD graphene non-local spin valve device array

Wafer-scale graphene films produced via chemical vapor deposition (CVD) are now commercially available, however these films inherently contain randomly distributed defects such as adlayers and grain boundaries. This report discusses the impact of these defects on the signal integrity of an array of graphene-based non-local spin valves (NLSVs). It was found that critical spin parameters fluctuate drastically between adjacent identical devices. Investigation of the channel quality indicated that adlayers do not affect spin signal significantly even when located directly in the spin transport region of the device. In contrast, grain boundary defects within the spin transport region have significant impact on spin signal. Poor tunnel barrier integrity due to residue from the fabrication process also remains a dominant factor driving device variability.

Biodegradable Chitosan-Based Films as an Alternative to Plastic Packaging.

The impact of synthetic packaging on environmental pollution has been observed for years. One of the recent trends of green technology is the development of biomaterials made from food processing waste as an alternative to plastic packaging. Polymers obtained from some polysaccharides, such as chitosan, could be an excellent solution. This study investigated the biodegradability of chitosan-metal oxide films (ZnO, TiO2, Fe2O3) and chitosan-modified graphene films (CS-GO-Ag) in a soil environment. We have previously demonstrated that these films have excellent mechanical properties and exhibit antibacterial activity. This study aimed to examine these films' biodegradability and the possibility of their potential use in the packaging industry. The obtained results show that soil microorganisms were able to utilize chitosan films as the source of carbon and nitrogen, thus providing essential evidence about the biodegradability of CS, CS:Zn (20:1; 10:1), and CS:Fe2O3 (20:1) films. After 6 weeks of incubation, the complete degradation of the CS-Fe2O3 20:1 sample was noted, while after 8 weeks, CS-ZnO 20:1 and CS-ZnO 10:1 were degraded. This is a very positive result that points to the practical aspect of the biodegradability of such films in soil, where garbage is casually dumped and buried. Once selected, biodegradable films can be used as an alternative to plastic packaging, which contributes to the reduction in pollution in the environment.

Asymmetric Pt nanoparticles with high lattice strain grown onto 3D graphene film as efficient integrated catalyst layer for proton exchange membrane fuel cells

Reducing Pt loading in proton exchange membrane fuel cells (PEMFCs) with improved performance is highly demanded for their wide adoption.Herein, asymmetric Pt nanoparticles (NPs) onto 3D graphene porous film (3D-GF) as integrated electrode was developed for ultra-low Pt loading PEMFCs. The 3D-GF featuring highly graphitic degree and rich macro-porosity as the framework of constructing catalyst layer was prepared by arc-discharge method and Pt NPs were grown onto 3D-GF via atomic layer deposition. The Pt NPs prefer to grow along the surface of graphene, presenting unique characteristics of high lattice strain with extended contact interface with 3D-GF. The specific structure is completely distinguished from Pt based NPs with symmetric structures synthesized by wet chemical methods. The cell with Pt/3D-GF at 100 μgPt cm−2 as the cathode exhibits outstanding activity and durability, in which its maximum power density (MPD) is 2.07 W cm−2 and 0.88 W cm−2 under H2/O2 and H2/Air at 150.0 kPa respectively, much higher than that of the cell with Pt/C at 200 μgPt cm−2, as the mass activity (MA) of Pt/3D-GF is 3.0 times higher than that of commercial Pt/C catalysts, and its performance suffers from the MPD loss of 25 % and 7.6 % after accelerated durability tests (ADTs) with 30 K cycles at 0.6–0.95 V and 5 K cycles at 1.0–1.5 V respectively, which is also significantly lower than that of commercial Pt/C catalyst. The asymmetric Pt NPs on 3D-GF provides a new concept for achieving ultra-low Pt loading PEMFCs.

Enhanced resonance frequency in Co2FeAl thin film with different thicknesses grown on flexible graphene substrate

The flexible materials exhibit more favorable properties than most rigid substrates in flexibility, weight saving, mechanical reliability, and excellent environmental toughness. Particularly, flexible graphene film with unique mechanical properties was extensively explored in high frequency devices. Herein, we report the characteristics of structure and magnetic properties at high frequency of Co2FeAl thin film with different thicknesses grown on flexible graphene substrate at room temperature. The exciting finding for the columnar structure of Co2FeAl thin film lays the foundation for excellent high frequency property of Co2FeAl/flexible graphene structure. In-plane magnetic anisotropy field varying with increasing thickness of Co2FeAl thin film can be obtained by measurement of ferromagnetic resonance, which can be ascribed to the enhancement of crystallinity and the increase of grain size. Meanwhile, the resonance frequency which can be achieved by the measurement of vector network analyzer with the microstrip method increases with increasing thickness of Co2FeAl thin film. Moreover, in our case with graphene film, the resonance magnetic field is quite stable though folded for twenty cycles, which demonstrates that good flexibility of graphene film and the stability of high frequency magnetic property of Co2FeAl thin film grown on flexible graphene substrate. These results are promising for the design of microwave devices and wireless communication equipment.

Characterization of graphene films formed using radical plasma species

Previously the authors of this paper have reported the synthesis of highly tethered, planarized graphene films on 6H-SiC(0001) using a two-step CF4-based plasma assisted synthesis (PAS) process. In the first step, silicon was selectively etched from the SiC surface to form a carbon-rich overlayer, and in the second step, a rapid thermal anneal was used to form the graphene film from this carbon-rich layer. The etch process was performed in an inductively coupled plasma-reactive ion etching system and utilized both physical etching due to ions and chemical etching due to radicals. This paper reports the plasma assisted synthesis of few-layer graphene films using only the radical species produced in a CF4-based plasma. That is, only chemical etching was involved in the selective etching process. This was achieved by placing a physical mask over the substrate to block the ion flux and allowing the microplasma formed from the ambient radical species to etch the SiC surface. This process followed by the rapid thermal anneal is referred to as microplasma assisted synthesis (MPAS) of graphene. The resulting graphene films were characterized using x-ray photoelectron spectroscopy, Raman spectroscopy and atomic force microscopy. These analyses reveal that the MPAS films are less tethered than those produced by the PAS process, and they reflect the step and terrace structure of the initial SiC substrate. Consequently, the few layer graphene films formed using MPAS show marked progress toward the quasi-freestanding graphene films formed by high temperature sublimation. Overall, these results suggest that a radical based etching process may provide a chemical route to the synthesis of quasi-freestanding graphene on SiC. Additionally, these results provide insight into the roles of radicals and ions in the etching process.

Study on a differential pressure microflow sensor on microfluidic chip

Based on the principle of differential pressure flow measurement, graphene film was used to measure the differential pressure in the micro channel of microfluidic chip and finally to obtain the micro flow rates. The relationship between differential pressure and flow rate in micro channel was studied and validated by CFD simulation. The fiber F-P cavity using graphene film was fabricated as the pressure sensitive structure, and the relationship between the measured pressure and the optical power output from the pressure sensitive structures was studied. The mechanism model used to measure the micro flow rates in the micro channel of microfluidic chip was established, and a sample of micro flow sensor was fabricated. The relationship between various quantities in the process of converting flow rate to optical power was verified through experiments, and the mechanism model established was verified. The experimental results demonstrate that fabricated sample could achieve accurate measurement of micro flow rate in the micro channel of microfluidic chip.

Cooking oil waste from AYAMAS as a carbon source in forming multilayer graphene films

ABSTRACT Industrial oil waste has become a novel carbon source for graphene due to its carbon-rich and renewable nature. A new approach was proposed to produce multilayer graphene from waste industrial cooking oil (WICO) sourced from project collaborators, the AYAMAS Food Corporation Sdn Bhd. Graphene was synthesized from WICO through a double thermal chemical vapor deposition (DT-CVD) method. Nickel foil was used as a substrate to initiate the process. The amount of WICO added was varied by 10 µL increments, ranging from 10 to 50 µL. The precursor and deposition temperatures were fixed at 350°C and 1000°C, respectively. Eventually, precipitation and segregation resulted in the formation of graphene. Raman spectroscopy analysis of the graphene obtained revealed an I2D/IG ratio of less than 0.5, indicating its multilayered nature. Besides, it has 4 graphene layers by calculating the number of atomic layers. This finding was further supported by outcomes shown through Ultraviolet – visible (UV-Vis) spectroscopy, where sharp peaks of 30 µL and 40 µL were observed at 250 nm. Finally, Atomic Force Microscopy (AFM) representative images demonstrated an inverse relationship between layer numbers and surface roughness in high-WICO graphene, where a gain in layers smoothened rather than roughened its appearance. Multilayer graphene has high conductivity and a wide bandgap. These properties are applicable in electrical and thermal applications. Through a method known as DT-CVD, it was achievable to successfully produce multilayer graphene by using the usage of AYAMAS, which is usage from the frying oil industry, as a carbon source.

Towards a graphene transparent conducting electrode for perovskite/silicon tandem solar cells

AbstractIndium‐based transparent conducting electrodes (TCEs) are a major limiting factor in perovskite/silicon tandem cell scalability, while also limiting maximum cell efficiencies. In this work, we propose a novel TCE based on electrostatically doped graphene monolayers to circumvent these challenges. The electrode is enabled by a thin film dielectric that is charged and interfaced to a graphene film, optimally exploiting electrostatic doping. The field effect mechanism allows the modulation of charge carriers in monolayer graphene as a function of charge concentration in the dielectric thin film. Electrostatic charge was deposited on SiO2 membranes, and graphene transferred onto them exhibited a reduction in sheet resistance because of the induced charge carriers. We show a reduction in sheet resistance of graphene by 60% in just 3 min of dielectric charging, without impacting the transmission of light through the film stack. Hall effect measurements indicated that the mobility of the films was not significantly degraded. The deposition of negative electrostatic charge reversed this effect, allowing for precise tunability of charge concentration from n‐ to p‐type. We develop a model to determine the required sheet resistance of a graphene TCE with 97% transmittance in a perovskite/silicon tandem cell. As the technique here reported does not impact transmittance, a graphene TCE with a sheet resistance below 50 Ω/□ could enable efficiencies up to 44%, presenting a promising alternative to indium‐based TCEs.

Advancements in Plasma-Enhanced Chemical Vapor Deposition for Producing Vertical Graphene Nanowalls.

In recent years, vertical graphene nanowalls (VGNWs) have gained significant attention due to their exceptional properties, including their high specific surface area, excellent electrical conductivity, scalability, and compatibility with transition metal compounds. These attributes position VGNWs as a compelling choice for various applications, such as energy storage, catalysis, and sensing, driving interest in their integration into next-generation commercial graphene-based devices. Among the diverse graphene synthesis methods, plasma-enhanced chemical vapor deposition (PECVD) stands out for its ability to create large-scale graphene films and VGNWs on diverse substrates. However, despite progress in optimizing the growth conditions to achieve micrometer-sized graphene nanowalls, a comprehensive understanding of the underlying physicochemical mechanisms that govern nanostructure formation remains elusive. Specifically, a deeper exploration of nanometric-level phenomena like nucleation, carbon precursor adsorption, and adatom surface diffusion is crucial for gaining precise control over the growth process. Hydrogen's dual role as a co-catalyst and etchant in VGNW growth requires further investigation. This review aims to fill the knowledge gaps by investigating VGNW nucleation and growth using PECVD, with a focus on the impact of the temperature on the growth ratio and nucleation density across a broad temperature range. By providing insights into the PECVD process, this review aims to optimize the growth conditions for tailoring VGNW properties, facilitating applications in the fields of energy storage, catalysis, and sensing.

Stacking transfer of wafer-scale graphene-based van der Waals superlattices

High-quality graphene-based van der Waals superlattices are crucial for investigating physical properties and developing functional devices. However, achieving homogeneous wafer-scale graphene-based superlattices with controlled twist angles is challenging. Here, we present a flat-to-flat transfer method for fabricating wafer-scale graphene and graphene-based superlattices. The aqueous solution between graphene and substrate is removed by a two-step spinning-assisted dehydration procedure with the optimal wetting angle. Proton-assisted treatment is further used to clean graphene surfaces and interfaces, which also decouples graphene and neutralizes the doping levels. Twist angles between different layers are accurately controlled by adjusting the macroscopic stacking angle through their wafer flats. Transferred films exhibit minimal defects, homogeneous morphology, and uniform electrical properties over wafer scale. Even at room temperature, robust quantum Hall effects are observed in graphene films with centimetre-scale linewidth. Our stacking transfer method can facilitate the fabrication of graphene-based van der Waals superlattices and accelerate functional device applications.

Controllable Friction on Graphene via Adjustable Interfacial Contact Quality.

Despite the numerous unique properties revealed through tribology research on graphene, the development of applications that utilize its rich tribological properties remains a long-sought goal. In this article, a novel approach for reversible patterning of graphene's frictional properties using out-of-plane mechanical tapping is presented. The friction force between the atomic force microscopy (AFM) tip and the graphene film is increased by up to a factor of two, which can be attributed to variations in the interfacial binding strength between the graphene and substrate through the tapping process. The reversible and repeatable frictional properties of graphene make it a promising material for information storage applications with a high storage capacity of ≈1600 GB inch-2 , allowing for direct writing and erasing of information, akin to a blackboard. These findings highlight the potential for friction tuning in lamellar materials and emphasize the significance of understanding nanoscale friction on graphene surfaces.

Magnetically controlled graphene field-effect transistor biosensor for highly sensitive detection of cardiac troponin I

Herein, we have constructed a magnetic graphene field-effect transistor biosensor (MGFETs) for highly sensitive detection of cardiac troponin I (CTNI). Graphene films transferred to ITO conductive glass as conductive channels. CTNI aptamer was immobilized onto the graphene film via 1-pyrene-butanoic acid succinimidyl ester (PBASE) to capture CTNI. Magnetic nanobeads (MBs) modified with CTNI antibody were added to the reaction chamber to form an aptamer/CTNI/antibody/magnetic nanobeads sandwich-type complex. We found that the magnetic force exerted on the complex leads to an impedance change of the graphene film. The reason for this result is that the magnetic field exerts an influence on the MBs, causing CTNI aptamer strand to bend, resulting in a change in the distance between the double conductive layers of the graphene film surface and the test solution. With periodic sampling integration, different concentrations of CTNI can be detected with high sensitivity. Due to the stringent recognition capability and high affinity between the CTNI aptamer and CTNI, MGFETs have the potential to detect various types of proteins. Furthermore, MGFETs also have the potential to be utilized for the detection of DNA or specific cells in the future.

(Invited) Non-Enzymatic Glucose Sensor Fabricated By Ni-Nanowires Decorated Graphene Gated FETs

In this study, an innovative non-invasive glucose sensor made of nickel nanowire-decorated graphene-gated field-effect transistors is demonstrated. Due to the redox reaction between nickel nanowires and glucose molecules in weakly alkaline solution, electron exchange occurs when Ni(III) reacts with glucose to form Ni(II) and gluconolactone. In order to improve the sensitivity of the glucose sensor and reduce the detection limit, we compared the characteristics of the glucose sensor fabricated by nickel nanowires on gold surface and graphene/gold surface, respectively. Since the CVD-grown graphene film has excellent electrical conductivity and no defects, the electrons generated by the redox reaction can be quickly and evenly dispersed and are not easily neutralized, resulting in a change on the gate potential, thereby affecting the current between the source and the drain metal, and then the glucose concentration in the solution can be deduced. It can be seen from the experiments that the glucose sensor with graphene film has better current stability, lower detection limit and better linear relationship between current and concentration. The demonstrated nickel wire-decorated graphene-gated FET biosensor can be used for the quantification of glucose with a linear range of 10-50 mM and a detection limit of 51nM. Figure 1

(Invited) Rapid, Large Area Preparation of Few-Layer Graphene Films Using Xenon Flash Lamp Pyrolysis for High-Performance Structural Micro-Supercapacitors

Graphene offers excellent electrical conductivity and very high surface area, which holds great promise for energy storage applications including supercapacitors. Current preparation methods for graphene require relatively long processing times, extremely high temperatures within controlled atmospheres, and/or involve multi-step reactions that present challenges for high throughput fabrication of graphene-based energy storage devices. We report a novel photothermal route to large-scale production of graphene within milliseconds using a commercially available polymers (polybenzoxazine, polyacrylonitrile and polyaniline) and a high intensity xenon flash lamp on various substrates, including carbon fibers, at ambient conditions. The xenon flash lamp provides large-area illumination and a wide emission band (300 nm –1100 nm) that was used to convert the polymeric material directly into few layer graphene upon millisecond exposures. The precursor material is heated to extremely high temperatures in a fraction of a second – a duration that is much shorter than the timescale for thermal equilibrium. This enabled the conversion of the polymeric material into few layer graphene in air and at room temperature, and without thermally damaging the substrate. Conversion to few-layer graphene was strongly dependent on pulse energy and the local temperature which was controlled via pulse power modulation. The obtained graphene composites at optimized conditions exhibited excellent conductivity (0.1 ohm-cm) with ID/IG ratio of 0.3 (Figure 1a). Particularly, graphene derived from polyaniline resulted in the formation of macroporous network (Figure 1b) with good adhesion to the carbon fiber, all of which facilitate the transport of ions through the material. The fabricated supercapacitor devices exhibited a very high areal capacitance of 200 mF/cm2 at 10 mV/s with retention of more than 70% of their capacitance, even at scan rates up to 100 mV/s. Moreover, mechanically, and structurally stable graphene derived from polyaniline on carbon fiber was utilized for the preparation of structural energy storage devices. We assembled symmetric structural supercapacitor comprised of carbonaceous material (carbon fiber and few-layer graphene) and polymer gel electrolyte. This resulted in the preparation of large area device (40 cm2) as shown in the Figure 1c with the capacitance exceeding 2000 mF and almost complete retention of capacitance after 5000 cycles.The preparation of high-quality graphene via photothermal pyrolysis of polymer is amenable to high throughput processing, enabling large-scale production of high performance structural electrochemical energy storage devices. Figure 1

Graphene nanospacer layer modulated multilayer composite structures of precious metals and their SERS performance.

Graphene(G)-noble metal-ZnO hybrid systems were developed as highly sensitive and recyclable surface enhanced Raman scattering (SERS) platforms, in which ultrathin graphene of varying thickness was embedded between two metallic layers on top of a ZnO layer. Due to the multi-dimensional plasmonic coupling effect, the Au/G/Ag@ZnO multilayer structure possessed ultrahigh sensitivity with the detection limit of Rhodamine 6 G (R6G) as low as 1.0×10-13 mol/L and a high enhancement factor of 5.68×107. Both experimental and simulation results showed that graphene films could significantly regulate the interlayer plasmon resonance coupling strength, and single-layer graphene had the best interlayer regulation effect. Additionally, the SERS substrate structure prepared through physical methods exhibited high uniformity, the graphene component of the substrate possessed excellent molecular enrichment ability and silver oxidation inhibition characteristics, resulting in a substrate with high stability and exceptional reproducibility. The signal change was less than 15%. Simultaneously, due to the excellent photocatalytic performance of the low-cost and wide-band-gap semiconductor material ZnO, the SERS substrate exhibited exceptional reusability. Even after five cycles of adsorption-desorption, the SERS performance remained stable and maintained a reliable detection limit. The study introduced a novel approach to creating multilayer composite SERS substrates that exhibited exceptional performance, offering a new analytical tool with high sensitivity, stability, and reusability.

Low T direct plasma assisted growth of graphene on sapphire and its integration in graphene/MoS2 heterostructure-based photodetectors

We report on outstanding photo-responsivity, R > 103 A/W, fast response (~0.1 s), and broadband sensitivity ranging from the UV to the NIR in two terminal graphene/MoS2 photodetectors. Our devices are based on the deterministic transfer of MoS2 on top of directly grown graphene on sapphire, and their performance outperforms previous similar photodetectors using large-scale grown graphene. Here we devise a protocol for the direct growth of transparent (transmittance, Tr > 90%), highly conductive (sheet resistance, R□ < 1 kΩ) uniform and continuous graphene films on sapphire at 700 °C by using plasma-assisted chemical vapor deposition (CVD) with C2H2/H2 gas mixtures. Our study demonstrates the successful use of plasma-assisted low-temperature CVD techniques to directly grow graphene on insulators for optoelectronic applications.

High-orientation, defect-rich and porous graphene films for excellent electromagnetic shielding and thermal management

With the rapid development of portable electronics, the demand for flexible shielding and thermal management materials is increasing steadily. Herein, we reported a scalable flexible graphene film preapred following hydrogen iodide chemical reducing and graphitization thermal reducing. Benifitting from the coordinated reduction, the fabricated graphene film is equipped with varous characteristics of high orientation, rich defect and three-dimensional porosity. Then the graphene film deliver outstanding mechanical property and electrical conductivity (1.2 × 105 S m−1). Moreover, superior electromagnetic interference (EMI) and thermal conductivity are realized compared with the simplex thermal reduction. The EMI effectiveness can reach a high value of 70 dB when the thickness is only 35 μm, and the specific EMI effectiveness is 380 dB/g cm−3. And the relationship between the effectiveness and structure parameters (thickness and density) is uncovered. The thermal conductivity could be up to 864 W m−1 K−1. This research may not only provide a new avenue to the development of flexible shielding and thermal management materials, but also promote the application of graphene.

Sub-second ultrafast yet programmable wet-chemical synthesis

Wet-chemical synthesis via heating bulk solution is powerful to obtain nanomaterials. However, it still suffers from limited reaction rate, controllability, and massive consumption of energy/reactants, particularly for the synthesis on specific substrates. Herein, we present an innovative wet-interfacial Joule heating (WIJH) approach to synthesize various nanomaterials in a sub-second ultrafast, programmable, and energy/reactant-saving manner. In the WIJH, Joule heat generated by the graphene film (GF) is confined at the substrate-solution interface. Accompanied by instantaneous evaporation of the solvent, the temperature is steeply improved and the precursors are concentrated, thereby synergistically accelerating and controlling the nucleation and growth of nanomaterials on the substrate. WIJH leads to a record high crystallization rate of HKUST-1 (~1.97 μm s−1), an ultralow energy cost (9.55 × 10−6 kWh cm−2) and low precursor concentrations, which are up to 5 orders of magnitude faster, −6 and −2 orders of magnitude lower than traditional methods, respectively. Moreover, WIJH could handily customize the products’ amount, size, and morphology via programming the electrified procedures. The as-prepared HKUST-1/GF enables the Joule-heating-controllable and low-energy-required capture and liberation towards CO2. This study opens up a new methodology towards the superefficient synthesis of nanomaterials and solvent-involved Joule heating.

Facile construction of flexible multilayered graphene-based composite films for excellent electromagnetic interference shielding with ultralow-reflection

Precisely tailored green electromagnetic interference shielding materials with absorption-dominated ultrahigh shielding performance are highly desired, but they have been constantly challenged by the conflict between superb shielding efficiency, ultralow reflection loss, and low thickness. In this work, the compatibility of excellent shielding capability, imperceptible reflection, and low thickness is successfully achieved in a flexible graphene-based film by simply integrating out a foam-like graphene network on fabric skeleton (RGO@F) as a surface wave trapping layer of graphene film (GF). The composite films exhibit exceptional shielding performance of > 80 dB, minuscule reflection as low as 0.82 dB, and a thickness of merely 0.8 mm, which are superior to most literature reports and first achieved for film-based shields. The power coefficient of reflection (R) of RGO@F/GF is only 0.172, much lower than that of original GF (R > 0.992), manifesting the shielding mechanism of GF has been readily converted from reflection-dominated to absorption-dominated. Several proof-of-concept studies are given to demonstrate the broad generality of RGO@F to virtually any other reflection-dominated shielding material. This work represents a critical breakthrough in tailoring advanced green shielding materials and paves the way for facilely fabricating thin, absorption-dominated and ultra-strong performance shielding films from accessible common materials.

Quality Assessment of Processed Graphene Chips for Biosensor Application.

The quality of graphene intended for use in biosensors was assessed on manufactured chips using a set of methods including atomic force microscopy (AFM), Raman spectroscopy, and low-frequency noise investigation. It is shown that local areas of residues on the graphene surface, formed as a result of the interaction of graphene with a photoresist at the initial stage of chip development, led to a spread of chip resistance (R) in the range of 1-10 kOhm and to an increase in the root mean square (RMS) roughness up to 10 times, which can significantly worsen the reproducibility of the parameters of graphene chips for biosensor applications. It was observed that the control of the photoresist residues after photolithography (PLG) using AFM and subsequent additional cleaning reduced the spread of R values in chips to 1-1.6 kOhm and obtained an RMS roughness similar to the roughness in the graphene film before PLG. Monitoring of the spectral density of low-frequency voltage fluctuation (SU), which provides integral information about the system of defects and quality of the material, makes it possible to identify chips with low graphene quality and with inhomogeneously distributed areas of compressive stresses by the type of frequency dependence SU(f).