Graphene Films Research Articles

Intercross-linked aramid nanofibers/graphene hybrid films toward high mechanical strength and electrical conductivity

Although macro graphene films exhibit excellent electrical conductivity, the low tensile strength severely hampers the application of graphene films. Combined with polymers can improve the mechanical properties to a degree, but it usually dramatically decreases the electrical conductivities of graphene films. It is thus a great challenge to make a trade-off between high electrical conductivity and excellent mechanical properties of graphene films. Herein, we designed a homogeneous layer-by-layer microstructure combining reduced graphene oxide (rGO) nanosheets with aramid nanofiber (ANF) to obtain composite films referred to as rGO/ANF. By tuning the concentration of ANF, a balanced electrical and mechanical performance can be achieved. It is revealed that the tensile strength of the optimized composite film with 10 wt% ANF was dramatically improved by 194% comparing to the pure rGO film, and the electrical conductivity still maintained at a relatively high level (8495 S m−1). The remarkable overall properties can be attributed to the unique structure with abundant hydrogen bonds and π-π interactions formed between rGO and ANF. The results demonstrate that this is indeed an efficient strategy to balance the electrical conductivity and tensile strength of the graphene-polymer composites, making it promising in the application of multi-functional electronic devices.

Annealing Effects of ZnO Thin Film on Photocatalytic Performances of Graphene Composites

The hybrid structure of Graphene and ZnO (Graphene/ZnO) is emerging as a novel material used to achieve the high performance of photocatalysis. In this study, we examined the ZnO characteristics that affect the photocatalytic activity of graphene/ZnO using a lamellar structure of graphene and ZnO thin films. Graphene samples were synthesized via chemical vapor deposition, and a typical wet process was applied to transfer them on sputter-deposited ZnO thin films with and without annealing. We confirmed that graphene-deposited ZnO demonstrated more efficient photocatalytic behavior toward the decomposition of methylene blue (as a model organic compound) with ordinary sputtered ZnO thin films. Again, ZnO thin films annealed at 1000 °C in an N2 gas atmosphere with graphene performed better than unannealed films. XRD analysis confirmed that pre-thermal treatment of a ZnO thin film promoted re-crystallization, which had less impact on the photocatalytic improvement. The attachment of graphene to the film is considered to contribute to the enhancement. Raman analysis revealed that the graphene coverage areas on the post-annealed ZnO increased by two times compared to that of an unannealed film where the unannealed film had a higher graphene layer. The results presented in this study demonstrate that an annealed ZnO thin film forms a better attachment with graphene, resulting in a larger graphene coverage area with fewer multilayers, which effectively improves the photocatalytic activity in composite structures.

LiPON/Multilayer-Graphene Interface Enables High-Rate Charging and Discharging

Graphite is the anode-active material commonly used in LIBs. In LIB, solid electrolyte interphase (SEI) is formed by the reduction of the liquid electrolyte, and the SEI plays a role as a passive film. In the charging process, Li ions transport in the liquid electrolyte and SEI, desolvation reactions of Li ions occur, and then Li ions insert into the graphite anode. It has been reported that the desolvation reaction of Li ions is the rate-limiting process[1]. Lithium phosphorous oxynitride glass (LiPON) is a well-known material as a solid electrolyte. However, it has been reported that the electrochemical window of LiPON ranges from 0.68 V to 2.63 V[2]. Therefore, it is possible that the decomposition reaction of LiPON occurs when the electrochemical cell is charged. Since the decomposition products can be a resistance, LiPON that is not easily reduced is required. In this study, we focused on the lithium phosphorus oxynitride glass (LiPON) electrolyte/multilayered graphene (MGr) film interface as an example without the desolvation process. In addition, we have investigated the relationship between LiPON composition ratio, electronic properties, and interfacial resistance by changing the sputtering conditions for LiPON film. Consequently, we found that the charge-transfer resistance at the MGr/LiPON interface was significantly small, although the MGr/LiPON interface was supposed to have inorganic solid electrolyte interphase resulting from the LiPON reduction decomposition. The figure shows a schematic illustration of the Liquid Electrolyte/MGr interface and the LiPON/MGr interface. Figure 1

(Invited) Near-Field Optics and Its Applications in Nanophotonic Devices: A Review

In our presentation, we will offer an overview of aperture-type scanning near field optical microscopy (SNOM) – a family of nano-optical imaging techniques derived from scanning probe microscopy which are capable of subwavelength resolution, and the development of three dimensional (3D) SNOM methods undertaken by our group to locally image the distribution of the electromagnetic radiation in the proximity of nanoparticles and nano-objects.[1] We will discuss a few applications in which we took advantage of 3D-SNOM to design specific optical nanosystems for light harvesting device applications. Specific case studies that will be presented include the design of plasmonic thin-film solar cells enhanced by random arrays of copper nanoparticles,[2] and the use of 3D-SNOM for characterizing evanescent waveguides self-assembled from of copper nanoparticles assembled on thin films of graphene.[3] In the final part of our talk, we will we present near-field scanning thermoreflectance imaging (NeSTRI), a new pump-probe technique invented in our group (see Figure 1) in which an aperture-type SNOM is used to contactlessly determine the thermal conductivity of inhomogeneous thin films and low-dimensional systems at the nanoscale for heat-spreading and thermoelectric applications.[4,5] These examples well represent the versatility of SNOM imaging and its potential for designing an even wider family of nano-optical devices.[1] P Bazylewski, S Ezugwu, G Fanchini, Applied Sciences 7 (2017) 973[2] S Ezugwu, H Ye, G Fanchini, Nanoscale 7 (2016) 252-260[3] T Ouyang, A Akbari-Sharbaf, J Park, R Bauld, MG Cottam, G Fanchini, RSC Advances 5 (2015) 98814-98821[4] S Ezugwu, S Kazemian, DYW Choi, G Fanchini, Nanoscale 9 (2017) 4097-4106[5] S Kazemian, S Ezugwu, G Fanchini, Proc. SPIE 10926, Quantum Sensing and Nano Electronics and Photonics XVI, 109260L (1 February 2019); doi: 10.1117/12.2509828 Figure 1

Cryo-electron microscopy at the Faculty of Biology of Lomonosov Moscow State University

This paper demonstrates an example of a successful upgrade of a JEOL JEM-2100 analytical transmission electron microscope to a low-resolution cryo-electron microscope designed for routine tasks of sample preparation and quality evaluation. As a result of the upgrade, the instrument allows the subnanometer resolution of protein molecule reconstructions (within 8 Å). The influence of graphene and amorphous carbon support films to prevent the effect of preferred orientation of protein particles in the frozen sample is discussed.

Free-Standing Carbon Nanotube Embroidered Graphene Film Electrode Array for Stable Neural Interfacing.

Implantable neural probes that are mechanically flexible yet robust are attractive candidates for achieving stable neural interfacing in the brain. Current flexible neural probes consist mainly of metal thin-film electrodes integrated on micrometer-thick polymer substrates, making it challenging to achieve electrode-tissue interfacing on the cellular scale. Here, we describe implantable neural probes that consist of robust carbon nanotube network embroidered graphene (CeG) films as free-standing recording microelectrodes. Our CeG film microelectrode arrays (CeG_MEAs) are ultraflexible yet mechanically robust, thus enabling cellular-scale electrode-tissue interfacing. Chronically implanted CeG_MEAs can stably track the activities of the same population of neurons over two months. Our results highlight the potential of ultraflexible and free-standing carbon nanofilms for stable neural interfacing in the brain.

Nonepitaxial Wafer-Scale Single-Crystal 2D Materials on Insulators.

Next-generation nanodevices require 2D material synthesis on insulating substrates. However, growing high-quality 2D-layered materials, such as hexagonal boron nitride (hBN) and graphene, on insulators is challenging owing to the lack of suitable metal catalysts, imperfect lattice matching with substrates, and other factors. Therefore, developing a generally applicable approach for realizing high-quality 2D layers on insulators remains crucial, despite numerous strategies being explored. Herein, a universal strategy is introduced for the nonepitaxial synthesis of wafer-scale single-crystal 2D materials on arbitrary insulating substrates. The metal foil in a nonadhered metal-insulator substrate system is almost melted by a brief high-temperature treatment, thereby pressing the as-grown 2D layers to well attach onto the insulators. High-quality, large-area, single-crystal, monolayer hBN and graphene films are synthesized on various insulating substrates. This strategy provides new pathways for synthesizing various 2D materials on arbitrary insulators and offers a universal epitaxial platform for future single-crystal film production.

Preparation of Electrodes with β-Nickel Hydroxide/CVD-Graphene/3D-Nickel Foam Composite Structures to Enhance the Capacitance Characteristics of Supercapacitors.

Supercapacitors have the characteristics of high power density, long cycle life, and fast charge and discharge rates, making them promising alternatives to traditional capacitors and batteries. The use of transition-metal compounds as electrode materials for supercapacitors has been a compelling research topic in recent years because their use can effectively enhance the electrical performance of supercapacitors. The current research on capacitor electrode materials can mainly be divided into the following three categories: carbon-based materials, metal oxides, and conductive polymers. Nickel hydroxide (Ni(OH)2) is a potential electrode material for use in supercapacitors. Depending on the preparation conditions, two crystal phases of nickel hydroxide, α and β, can be produced. When compared to α-NiOH, the structure of β-Ni(OH)2 does not experience ion intercalation. As a result, the carrier transmission rate of α-Ni(OH)2 is slower, and its specific capacitance value is smaller. Its carrier transport rate can be improved by adding conductive materials, such as graphene. β-Ni(OH)2 was chosen as an electrode material for a supercapacitor in this study. Homemade low-pressure chemical vapor deposition graphene (LPCVD-Graphene) conductive material was introduced to modify β-Ni(OH)2 in order to increase its carrier transport rate. The LPCVD method was used to grow high-quality graphene films on three-dimensional (3D) nickel foam substrates. Then, a hydrothermal synthesis method was used to grow β-Ni(OH)2 nanostructures on the 3D graphene/nickel foam substrate. In order to improve the electrical properties of the composite structure, a high-quality graphene layer was incorporated between the nickel hydroxide and the 3D nickel foam substrate. The effect of the conductive graphene layer on the growth of β-Ni(OH)2, as well as its electrical properties and electrochemical performance, was studied. When this β-Ni(OH)2/CVD-Graphene/3D-NF (nickel foam) material was used as the working electrodes of the supercapacitor under a current density of 1 A/g and 3 A/g, they exhibited a specific capacitance of 2015 F/g and 1218.9 F/g, respectively. This capacitance value is 2.62 times higher than that of the structure without modification with a graphene layer. The capacitance value remains at 99.2% even after 1000 consecutive charge and discharge cycles at a current density of 20 A/g. This value also improved compared to the structure without graphene layer modification (94.7%).

Solution-processed graphene films for electrochemical monitoring of extracellular nitric oxide released by breast cancer cells

Nitric oxide (NO) plays an important role in cardiovascular function, immune response, and intercellular signaling. However, due to its short lifetime, real-time detection of NO is challenging. Herein, an electrochemical sensor based on fibronectin-modified, solution-processed graphene ink for NO detection is developed using a facile fabrication method involving spin-coating and hot-plate annealing. The sensor is first electrochemically characterized with a NO donor, spermine NONOate, exhibiting a dynamic range of 10–1000 μM. The fibronectin-functionalized graphene supports the attachment and growth of MDA-MB-231 breast cancer cells, as confirmed by optical microscopy. Extracellular NO production is stimulated using the amino acid L-arginine. NO production results in morphological changes to the adhered cells, which are reversible upon the addition of the NO synthase antagonist Nω-nitro-L-arginine methyl ester. The production of NO is also confirmed using real-time amperometric measurements with the fibronectin-functionalized graphene sensors. While this work focuses on NO detection, this potentially scalable platform could be extended to other cell types with envisioned applications including the high-throughput evaluation of therapeutics and biocompatible coatings.

PEDOT:PSS-patched magnetic graphene films with tunable dielectric genes for electromagnetic interference shielding and infrared stealth

The intelligent era brings electronics closer to humans, but also produces a large scale of electromagnetic (EM) radiation simultaneously, which causes serious harm to health and high sophisticated equipment. Exploring the underlying response logic of EM materials is urgently needed to face the challenge of EM interference (EMI) and secondary EM pollution better. Herein, PEDOT:PSS-patched magnetic graphene films are fabricated by vacuum-assisted molecular patching engineering, with tunable EM wave response. Based on the observation of micro-nano structure, the dielectric genes are visually revealed, which offers a bran-new horizon for the optimization of EM properties. Impressively, the constructed films achieve double band shielding toward gigahertz wave and infrared radiation. The optimal EMI shielding efficiency exceeds 99 %, and covers the entire X-band. Meanwhile, the green shielding index rises from 0.3 to 0.6, indicating that it is a potential green EMI shielding materials. Furthermore, the periodic macroscopic interfaces and the inherent thermal anisotropy endow the films with thermal insulation and flexible infrared stealth functions in simulated thermal environments. This work refreshes the insight into multi-band shielding, providing a new idea to EM energy governance.

Room‐Temperature Ferromagnetism and Fermi Level Pinning in Multilayer Graphene Films Derived from Ferrocene

AbstractSpin polarization is a very important condition for spintronics applications which can easily be achieved by using ferromagnetic materials. Graphene‐based magnetic materials, which have long spin lifetime and diffusion length, are very promising as spin channel materials. In this work, Ferrocene molecules are chosen as the precursor carbon source using the chemical vapor deposition method, facilitating the synthesis of multilayer graphene films decorated with magnetic nanoclusters. This resultant material manifested planar ferromagnetism with a Curie temperature exceeding room temperature (≥300 K) and a saturation magnetization as high as 4.1 × 10−7 emu mm−2. The robust ferromagnetic coupling can plausibly be attributed to the presence of magnetic nanoclusters interspersed within the synthesized graphene, enhancing the localized magnetic moment and promoting ferromagnetic long‐range ordering. Furthermore, the Fermi level pinning at the Dirac point is observed and predominantly ascribed to the pervasive defects in the graphene derived from ferrocene. The oxygen functional groups within these defects act as charge traps, effectively anchoring the Fermi level to the Dirac point, intrinsically suppressing the ambipolar behavior of graphene. This work sheds light on potential avenues for the exploration of novel, high‐performance carbonaceous spintronics materials with economic feasibility.

Ultrasensitive Biochemical Sensing Platform Enabled by Directly Grown Graphene on Insulator.

To fabricate label-free and rapid-resulting semiconducting biosensor devices incorporating graphene, it is pertinent to directly grow uniform graphene films on technologically important dielectric and semiconducting substrates. However, it has long been intuitively believed that the nonideal disordered structures formed during direct growth, and the resulted inferior electrical properties will inevitably lead to deteriorated sensing performance. Here, graphene biosensor chips are constructed based on direct plasma-enhanced chemical vapor deposition (PECVD) grown graphene on a 4-inch silicon wafer with excellent film uniformity and high yield. To surprise, optimal operations of graphene biosensors permit ultrasensitive detection of SARS-CoV-2 virus nucleocapsid protein with dilutions down to sub-femtomolar concentrations. Such impressive limit of detection (LOD) is comparable to or even outperforms that of the state-of-the-art biosensor devices based on high-quality graphene. Further noise spectral characterizations and analysis confirms that the LOD is limited by molecular diffusion and/or known interference signals such as drift and instability of the sensors, rather than the electrical merits of the graphene devices along. Hence, result sheds light on processing directly grown PECVD graphene into high-performance sensor devices with important economic benefits and social significance.

Free-standing graphene film growth synaptic network-like nickel-vanadium double hydroxide for supercapacitors

Herein, a novel graphene/synaptic network-like nickel-vanadium double hydroxide (rGO/SNNiVDH) film was fabricated by a one-step hydrothermal method. The exquisite structure design is not merely conducive to the rapid transmission of electrons, but also provides a large number of active site for capacitive storage. Correspondingly to innovative structures is excellent electrochemical performance, the rGO/SNNiVDH obtains an excellent specific capacitance of 1726.7 F g−1 at a current density of 1 A g−1 and good rate capability with the capacitance value can still maintain 53.5 % even at the current density of 10 A g−1. Furthermore, the assembled asymmetric supercapacitor comprising rGO/SNNiVDH and active carbon (AC) as positive and negative electrodes respectively achieves a high energy density of 77.3 Wh kg−1 even under a power density of 7250 W kg−1. The rGO/SNNiVDH//AC exhibits excellent cycle stability with high capacitance retention of 96.3 % after 5000 cycles at a high current density of 10 A g−1. The study provides a new research perspective for the growth of other transition metal double hydroxides on the surface of rGO to obtain more transition metal double hydroxides with special morphology for high performance supercapacitors.

Tribological properties of Ti/Pb-MoS2-Graphene composite films under high environment temperature

For purpose of enhancing the tribological properties of molybdenum disulfide (MoS2) films, Ti/Pb doping and graphene dripping have been synchronously introduced to restrain the oxidation and wear under the environment temperature of 25, 100, 200, 300 and 400 °C. The MoS2, Ti-MoS2, Pb-MoS2 and Ti/Pb-MoS2 films have been prepared on the GCr15 steel substance by magnetron sputtering; and then graphene has been doped on the surface of the MoS2-based films to investigate the synergistic effects of graphene and MoS2-based films on the tribological performances tested by the UMT multifunctional friction and wear tester. Experimental results show the friction coefficient and wear rate of the MoS2-based films increase with the rising of the environment temperature. Wear mechanisms of tested under various temperatures are the co-action of oxidation wear and abrasive wear. Comparing the MoS2-graphene composite film, the Ti, Pb or Ti/Pb doped MoS2-graphene composite films tested under the environment temperature of below 400 ℃ show a low friction coefficient (less than 0.1) and a low wear rate, which is owing to the redox reaction of graphene and metal (Ti and Pb).

Synthesis of N-Doped Graphene Photo-Catalyst for Photo-Assisted Charging of Li-Ion Oxygen Battery.

In this work, nitrogen (N)-doped graphene film is synthesized, as a photo-catalyst, on one side of the copper foam by chemical vapor deposition and the copper foam is directly used as an electrode after porous Pd@rGO cathode loading to the other side of the foam for the photo-assisted charging of the Li-ion oxygen battery. The amount of urea (CO(NH2)2), which is used as N atom source, is optimized to get maximum photo-anodic currents from the n-type graphene films. The optical band gap and the valance band edge potential of the optimized N-doped graphene film are determined as 2.00eV and 3.71VLi+/Li, respectively. X-ray photoelectron spectra provided that the atomic percent of N atoms in the graphene film is 1.34% and the graphitic, pyrrolic and pyridinic N atom percentages are 54.01%, 42.20% and 3.79%, respectively. The photo-assisted charging tests indicated that the N-doped graphene film photo-catalyst reduced the charging potential significantly even at 1000mAg-1 (0.1mAcm-2) current density and improved the cyclic discharge-charge performance of the Li-ion oxygen battery considerably.

Probing the Intrinsic Strain in Suspended Graphene Films Using Electron and Optical Microscopy.

Quantifying the intrinsic properties of 2D materials is of paramount importance for advancing their applications. Large-scale production of 2D materials merits the need for approaches that provide direct information about the role of growth substrate on 2D material properties. Transferring the 2D material from its growth substrates can modify the intrinsic properties of the asgrown 2D material. In this study, suspended chemical vapor deposition (CVD) graphene films are prepared directly on their growth substrates in a high-density grid array. The approach facilitates the quantification of intrinsic strain and doping in suspended CVD graphene films. To achieve this, transmission electron microscopy and large-area Raman mapping are employed. Remarkably, the analysis reveals consistent patterns of compressive strain (≈-0.2%) both in the diffraction patterns and Raman maps obtained from these suspended graphene films. By conducting investigations directly on the growth substrates, the potential influences introduced during the transfer process are circumvented effectively. Consequently, the methodology offers a robust and reliable means of studying the intrinsic properties of 2D materials in their authentic form, uninfluenced by the transfer-induced alterations that may skew the interpretation of their properties.

Transparent Patternable Large-Area Graphene p-n Junctions by Photoinduced Electron Doping.

We present a novel approach to achieve n-type doping in graphene and create graphene p-n junctions through a photoinduced electron doping method using photobase generators (PBGs). The unique properties of PBGs allow us to spatially and temporally control the doping process via light activation. The selective irradiation of specific regions on the graphene film enables switching their doping from p- to n-type, as confirmed by changes in the electromotive force and Seebeck and Hall coefficients. We demonstrate a stable (over 2 months) high electron mobility exceeding 1000 cm2 V-1s-1 using Hall effect measurements. The precise control of doping and the creation of p-n junctions in graphene offer exciting possibilities for various electronic, optoelectronic, and thermoelectric applications. Furthermore, we fabricate transparent graphene thermocouples with a high electromotive force of approximately ca. 80 μV/K, which validates the reliability and effectiveness of our approach for temperature sensing applications. This work paves the way for high-performance graphene-based electronic devices via controlled doping and patterning techniques. These findings provide valuable insights for the practical implementation of graphene in various fields.

Stable vortex waves generation by a flexible antenna made of highly conductive graphene film

Electromagnetic waves carrying orbital angular momentum (OAM and vortex waves) attract much attention due to their specific characteristics and prospects for use in wireless communication, biomedical engineering, and imaging. Vortex waves are complex spatial entities; therefore, their stable generation is a quite complicated task, especially in the radio frequency and terahertz wave domains, considering that the corresponding antennas must be precisely adjusted. Such adjusting is very difficult to achieve for flexible antennas, which are now being actively introduced into practice. Here, we propose a design of a flexible antenna that is able to stably generate waves carrying OAM even when being subjected to some bending. The antenna is composed of a ring-shaped resonator made of a highly conductive graphene film deposited on a thin polydimethylsiloxane substrate. The antenna is operated on two mutually orthogonal degenerate TMm1 modes excited by two supply strip lines to generate vortex waves with a desired topological charge. We verify this ability numerically and in a microwave experiment on two antenna prototypes deriving the OAM mode purity by changing the bending radius of the antenna bearing surface, giving it either positive or negative curvature. The obtained experimental results confirm that the antenna has good performance and can be used for flexible electronic devices and communication systems.

Positronium transmission through graphene thin films supported with lacy carbon in the energy range of a few keV

We studied the transmission of positronium through thin graphene films supported with lacy carbon using an energy-tunable positronium beam recently developed based on photodetachment of positronium negative ions. By analysing the results of the measurements conducted with different number of graphene layers, the transmittance per single-layer graphene without lacy carbon was estimated to be 70% and 74% at 2.3 keV and 3.3 keV, respectively.Graphical

Enhancing supercapacitor performance through design optimization of laser-induced graphene and MWCNT coatings for flexible and portable energy storage

The field of supercapacitors consistently focuses on research and challenges to improve energy efficiency, capacitance, flexibility, and stability. Low-cost laser-induced graphene (LIG) offers a promising alternative to commercially available graphene for next-generation wearable and portable devices, thanks to its remarkable specific surface area, excellent mechanical flexibility, and exceptional electrical properties. We report on the development of LIG-based flexible supercapacitors with optimized geometries, which demonstrate high capacitance and energy density while maintaining flexibility and stability. Three-dimensional porous graphene films were synthesized, and devices with optimized parameters were fabricated and tested. One type of device utilized LIG, while two other types were fabricated on LIG by coating multi-walled carbon nanotubes (MWCNT) at varying concentrations. Characterization techniques, including scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and voltammetry, were employed to analyze the fabricated devices. AFM analysis revealed a surface roughness of 2.03 µm for LIG due to laser treatment. SEM images displayed compact, dense, and porous surface morphology. XRD analysis confirmed the presence of graphene and graphene oxide, which was further supported by energy-dispersive X-ray spectroscopy (EDX) data. Raman spectroscopy indicated that the fabricated samples exhibited distinct D and G bands at 1362 cm–1 and 1579 cm–1, respectively. Cyclic voltammetry (CV) results showed that LIG's capacitance, power density, and energy density were 6.09 mF cm–2, 0.199 mW cm–2, and 3.38 µWh cm–2, respectively, at a current density of 0.2 mA cm–2. The LIG-MWCNT coated electrode exhibited a higher energy density of 6.05 µWh cm–2 and an areal-specific capacitance of 51.975 mF cm–2 compared to the LIG-based devices. The fabricated device has potential applications in smart electronics, nanorobotics, microelectromechanical systems (MEMS), and wearable and portable electronics.