The reason for this is that the stronger thermal vibration reduces the load stress difference near the crack tip and, therefore, the crack tip can pass through the sp link. A crack propagates more easily along the zigzag direction than along the armchair direction. The out-of-plane tearing is more suitable than the in-plane tearing for graphene patterning. The path of crack propagation can be adjusted by changing the loading direction, e.g., a rectangular graphene ribbon can be produced by oblique tearing. This new understanding will benefit the application of graphene patterning via the tearing approach.The path of crack propagation in a graphene sheet is significant for graphene patterning via the tearing approach. In this study, we evaluate the fracture properties of pre-cracked graphene during the tearing process, with consideration of the effects of the aspect ratio, loading speed, loading direction, and ambient temperatures on the crack propagation in the monolayer sheet. Some remarkable conclusions are drawn based on the molecular dynamic simulation results, i.e., a higher loading speed may result in a complicated path of crack propagation, and the propagation of an armchair crack may be accompanied by  Cai et al. [Graphene  is a typectronic , thermalectronic , and mecectronic ,6,7 propectronic ,15,16,17ectronic  fabricatectronic  preparedectronic ,14 invesectronic ,19 develi et al.  introduci et al. ,22. Songi et al. ,24 modeli et al. ,26 builti et al.  also sug1/2. Xu et al. [1/2 for zigzag and 3.71 MPa·m1/2 for armchair cracks. Moura and Marder [2, which is about twice that of pristine graphene. Budarapu et al. [In preparing the graphene-based components in a nanodevice or nanostructure, several types of patterning techniques have been developed, e.g., lithographic techniques ,28,29,30u et al.  reportedd Marder  studied d Marder  estimated Marder  designedu et al.  studied u et al.  investigu et al.  also shoIn the above studies, which have provided a deep understanding of graphene fractures, much attention has been paid to functionalization , types oL1, has three options. The length of the moving area, i.e., L2, has four options, and determines the value of the aspect ratio (AR = L1/L2). Correspondingly, the three pairs of models are referred to as AC1, ZZ1; AC2, ZZ2; and AC3, ZZ3, with the values of AR listed in v, is constant in the tearing process. The moving direction of the moving area determines the tearing mode, e.g., in-plane , P(−30°), and P(30°)—marked in The tearing displacement is defined as the product of v) on the crack propagation in the graphene sheets with initial cracks as shown in sp2 carbon sheet, its bending deformation also leads to increasing potential energy, while an overlap of two parts of the sheet via van der Waals interaction reduces the potential energy of the system. For monitoring the state of the sheet during a tearing process, here we adopted a concept named variation of potential energy (VPE), which can be calculated by subtracting the initial potential energy (PE(0)) of the system from the current potential energy (PE(t)), i.e.,First, we studied the effect of loading speed , the cuL1 and L2 , e.g., In-AC1  at first, and then changes to P(−30°)  . Hence, L1 = 30.8 Å  before the sheet breaks in half. When observing the cracking processes, we found that the crack always propagated along with the path P(0°) perpendicular to the loading direction , the sheet still does not split off. For the sheet with an armchair crack, its path of crack propagation is mainly in the “SC” style. Conversely, the propagation path of a zigzag crack is mainly split off, with smooth edges along the initial direction of the crack, which benefits the graphene patterning by tearing.In the above discussion, the dynamic response of the crack in torn graphene is collected at an extremely low temperature, i.e., 1 K, which is difficult, but can be achieved in an experiment. It is known that the ambient temperature of the graphene sheet indicates the intensity of the thermal vibration of atoms. In general, a stronger thermal vibration of atoms leads to a lower strength of the sheet . At 1 K,sp carbon link appeared at the crack edge , which was obviously different from that at 1 K. The reason for this is that the stress distributes randomly in the sheet at higher temperatures . The top five rows of atoms at the zigzag edge were confined as the fixed area or the moving area for applying the tearing load. Five cases of oblique tearing together with the in-plane and out-of-plane tearing results were collected for comparison. For example, in According to the tearing direction shown in θ =75° or 90°, the crack always propagates along with the path P(0°)  for about 300 ps, and then changes to be aligned with P(−90°)  b. When θIn this study, the tearing process of graphene sheets is simulated using the molecular dynamics approach, and conducted on the open-source code LAMMPS . Each siStep 1: Build the graphene sheet with an initial crack;Step 2: Reshape the sheet by minimizing its potential energy;Step 3: Before tearing, relax the system in an NVT ensemble for 100 ps, using the Nose–Hoover thermostat ,56 to coStep 4: Fix the atoms in the fixed area, and apply a constant speed on the atoms in the moving area along a direction for 200 ps;Step 5: Collect the essential data for post-processing.In a simulation, the interactions between the carbon atoms in the graphene sheet are determined by the adaptive intermolecular reactive bond-order (AIREBO) potential , with a In this study, we estimated the fracture properties of pre-cracked graphene during the tearing process with constant velocity. The effects of the aspect ratio, loading speed, loading direction, and ambient temperatures on the crack propagation in the monolayer graphene sheet were investigated using the molecular dynamics simulation approach. Some remarkable conclusions can be drawn based on the numerical results for preparing a graphene ribbon by tearing; for example:Firstly, in an in-plane tearing process at an extremely low temperature, a high loading speed on the graphene sheet with an initial armchair crack may produce a complicated path of crack propagation. The influence can be reduced when the two loading areas  in the sheet contain more atoms. If the sheet has an initial zigzag crack, the crack propagates straight forward, i.e., along P(0°).sp carbon link may appear soon after the initial crack propagates through the whole graphene sheet. These appear more frequently in the graphene sheets with armchair cracks than in the sheets with zigzag cracks.Secondly, when the two loading areas contain fewer atoms, a new crack may occur at the edge of the shorter area—especially at a higher loading speed, e.g., >0.4 Å/ps. An sp carbon link may appear on the path of crack propagation, i.e., the crack tip passes through the carbon link due to a slight stress difference near the crack tip when the atoms have strong thermal vibration. Fortunately, the sp carbon link seldom appears in the graphenes sheet with initial zigzag cracks.Thirdly, in an in-plane tearing of a graphene sheet with an initial armchair crack, temperature influences the crack propagation. For example, at a temperature higher than 200 K, an Fourthly, in an out-of-plane tearing process, if the loading speed is less than 1 Å/ps, the armchair crack propagates along P(30°), while the zigzag crack propagates along P(0°), despite the length difference between the two loading areas and the difference in ambient temperature. Hence, out-of-plane tearing is more suitable for graphene patterning than in-plane tearing.Finally, when an oblique tearing process is applied to a larger sheet with an initial zigzag crack, the path of propagation depends on the loading direction.sp carbon links on the path of the armchair crack propagation should be avoided, and the two loading areas should have sufficient length to counteract the local stress concentration.From above, for preparing a graphene ribbon from a sheet by tearing, the


Conventional fabrication methods to produce graphene are cumbersome, expensive, and not ecologically friendly. This is due to the fact that the processing of a large volume of raw materials requires large amounts of acids and alkalis which, in turn, require special disposal. Therefore, it is necessary to develop new technologies or to refine existing ones for the production of graphene—and to create new, ecologically-safe and effective methods. Here, we utilized physical sonication to extract graphene films from natural mineral shungite rock. From our study of the structure of shungite by Raman spectrometry and X-ray phase analysis, we found that shungite is characterized by graphite-like mineral structures. Transmission electron microscopy images obtained from the processed material revealed graphene films—with surfaces as small as 200 nanometers long and several layers wide. Our green method of fabicating graphene can be widely used in a variety of fields, from electro-optics to ecology, to list a few. At the moment, graphene is a material that is being actively researched as it finds wide application in various industries ,2,3,4,5.Graphene films are produced by the following methods ,26,27: tEach of the presented methods has its pros and cons; during chemical processing, there is a need to wash the resulting graphene of the substances with which the material reacted, which increases the time to obtain graphene. During the mechanical processing (exfoliation) of graphite, large sheets of graphene are obtained; however, in this case, a large amount of tape is needed, which can also increase the amount of side debris, and the loss of some graphene on the tape. When grown on a substrate, graphene layers have a smaller film size than when machined. High energy consumption is required during plasma treatment. We chose ultrasound treatment because when using mechanical methods, films with large surfaces are formed. This method includes only two stages and is easy-to-use. In addition, it is environmentally friendly since it uses an aqueous medium without the addition of surfactants.Here, we used shungite in the sonication which consists of hybrid structures of sp1+sp2+sp3 forms of hybridized carbon atoms . Mostly, sp2 + sp3 forms structures are present in a small fraction of sp1 . It is kHere, we report the production of graphene from shungite rock, as illustrated in Shungite sample from the Zazhoginsky field were retrieved during geological processes. We investigated the surface of the shungite, the presence of mineral inclusions, and the composition of the elements. The studies were carried out using a scanning electron microscope (SEM) with a low vacuum and a Quanta 200 SEM tungsten electron source. The Quanta SEM electron microscope was also equipped with an energy-dispersive X-ray spectroscopy (EDS) system for elemental analysis. For the analysis of shungite, we also used the X-ray method. The studies were carried out using a PANalytical Empyrean multipurpose diffractometer for the analysis of powder and solid substances, nanomaterials, thin films, and suspensions.To test the technique, we used shungite from the Zazhoginsky deposit of group I (group I—more than 96% carbon). Before producing graphene, we broke up the shungite samples. A FRITSCH grinding mill 6 was used to grind shungite samples. Grinding time is 10 min at a room temperature of 25 We checked the shungite surface using a scanning electron microscope (SEM) to determine the type of shungite. We observed the heterogeneous and stepped surface of shungite; the surface-shell spalls characteristic of shungite, as can be seen from After detecting inclusions of various shapes on the surface, we investigated the chemical composition of shungite using energy-dispersive X-ray spectroscopy (EDS). We selected random regions in the shungite and selected several areas in each region with the following dimensions: shown in a,d. The We investigated two areas in each chosen region, as shown in Shungite is also characterized by graphite and graphene shapes, respectively, which have specific absorption peaks at specific wavelengths.Using the method of X-ray spectrometry and Raman spectroscopy, we studied how carbon binds to other elements. The data obtained are shown in graphite . The ban G bands a, it can G bands a, represTo determine the numbers of sp2 and sp3 carbon bonds, we studied the samples using X-ray photoelectron spectrometry. Data are presented in The XPS spectra of shungite after deconvolution is shown in After examining the shungite samples, we processed the samples via a mechanical method—sonication. Since large volumes of samples can be used with this method, there is no secondary contamination and there is no need to carry out subsequent sample processing.The XPS spectra of sonicated shungite after deconvolution is shown in ure data ,65, to aWe considered the production of graphene films from natural mineral shungite using a mechanical method—low-temperature  and optics to biotechnology , energy , and ecology .


Fasciola gigantica excretory-secretory products (FgESP) shows better diagnostic value than the FgESP, but its immunomodulatory function and potential as a molecular vaccine candidate are unknown. Thus, the effect of F22 on the mitogen-induced proliferation of buffalo peripheral blood mononuclear cells (PBMCs) in the innate immune response was preliminarily studied, and its components were also explored. The results indicated that F22 significantly decreased the proliferation of PBMCs stimulated with mitogen. Two hundred and sixteen were identified as components of the F22, and these included eighty-six proteins present in more than one pathway and some with robust immunomodulatory ability. Further studies should be performed to investigate the immunomodulatory function of F22 in the adaptive immune response, and the components of F22 can be further studied as potential vaccine candidate molecules.The 22nd chromatography component (F22) of the Fasciola gigantica excretory-secretory products (FgESP) shows better diagnostic value than the FgESP, and diagnostic methods based on F22 have also been established. Thus, exploring its immunomodulatory function and potential as a molecular vaccine candidate is attractive. In the present study, the effect of F22 on the mitogen-induced proliferation of buffalo peripheral blood mononuclear cells (PBMCs) in the innate immune response was preliminarily studied using the FgESP as a control. PBMCs were incubated with concanavalin A (ConA) and phytohemagglutinin (PHA) at optimal (1 µg/well) or suboptimal (0.25 µg/well) doses coupled with FgESP and F22 at different doses (1–16 µg/well). Cell proliferation was then assessed by microenzyme reaction colorimetry -2,5-diphenyl tetrazolium (MTT) assay). In addition, the components of F22 were also explored by mass spectrometry and then subjected to Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis to infer their functions. The results indicated that FgESP decreased the proliferation of PBMCs stimulated with ConA and PHA at specific doses, whereas F22 significantly decreased the proliferation of PBMCs stimulated with ConA and PHA at both optimal and suboptimal doses (p < 0.05). Two hundred and sixteen proteins were identified in F22, and these included 86 proteins that could be assigned to more than one pathway and some with robust immunomodulatory ability. Further studies should be performed to investigate the immunomodulatory function of F22 in the adaptive immune response, and the components of F22 can be further studied as potential vaccine candidate molecules.The 22nd chromatography component (F22) of the  Fasciola hepatica, Fasciola gigantica, and their intermediate form that causes great global losses annually _15305.fasta) was selected for library comparison of F22. Proteins for which the number of unique peptides ≥ 2 were selected for subsequent analysis to detect the number of proteins related to Fasciola spp. Proteome Discoverer 1.3 (Thermo Scientific) software was used to match and score the original map files (raw files) from the peptide identification with the Q Exactive chromatographic mass spectrometer through SEQUEST software. According to the criterion FDR < 0.01, the peptides were screened based on the confidence, and the presence of at least two unique peptides was used for identification to obtain highly reliable qualitative results.FgESP and F22 were collected as previously described [FgESP obtained by gel filtration chromatography showed four higher peaks of UV280 , and F22 was located under the P1 peak (not shown). F22 obtained after elution was collected for concentration determination of the concentration, and F22 was then adjusted to specific concentration for further use.escribed . The chrFgESP at multiple concentrations (1~8 µg/well) had no effect on the proliferation of PBMCs stimulated with ConA at either dose, whereas FgESP at 16 µg/well decreased the proliferation of PBMCs stimulated with ConA at a suboptimal dose (p < 0.05) ( < 0.05) a.FgESP had no effect on the proliferation of PBMCs stimulated with PHA at the optimal dose. With the suboptimal dose of PHA, FgESP at 2 and 4 µg/well significantly decreased proliferation (p < 0.05) (FgESP is shown in  < 0.05) b. The pep < 0.01). With a suboptimal dose of ConA, F22 at 1~16 µg/well decreased PBMC proliferation (p < 0.01).The ConA-induced proliferation of PBMCs was decreased by F22 (2~16 µg/well) a. With ap < 0.01). With a suboptimal dose of PHA, F22 at 4~16 µg/well clearly decreased PBMC proliferation (p < 0.01), whereas at lower doses (1 and 2 µg/well), F22 had no regulatory effect on PBMC proliferation. The percent inhibition induced by F22 is shown in The PHA-induced proliferation of PBMCs was also decreased by F22 (4~16 µg/well) b. With aFgESP and F22 were subjected to mass spectrometry analysis (with a Q Exactive combined mass spectrometer), and proteins with ≥2 unique peptides were selected for analysis. FgESP contains 548 proteins, whereas F22 contained 216 proteins (FgESP score: 323.31), calcium-binding protein (CaBP) (FgESP score: 323.31), and heat-shock protein (HSP) (FgESP score: 323.31), were present in FgESP. Cathepsin L (CatL) (F22 score: 143.76), thioredoxin peroxidase (TPx) (F22 Score 103.37), and fibronectin type II domain (F22 Score 100.63) were present in F22 with high scores  of proteins), whereas “Digestive system” had the highest number of proteins within “Organismal Systems” (21.60% (27 of proteins) . MoreoveFgESP functions poly-directionally in F. gigantica—buffalo interaction and attracts a great deal of attention. However, its complex composition hindered relevant research to some extent. Encouragingly, the chromatography component of FgESP, F22, shows substantial diagnostic value and immunosuppressive ability, which indicates that chromatography may act as a tool in complex, mixed-component identification and can be applied to immunomodulatory molecular screening.As is well known, F. gigantica Ras-related protein Rab10 (rFgRab10) enhances the apoptosis and migration of PBMCs, promotes the phagocytic ability of monocytes, and significantly inhibits cell proliferation [Filarial nematodes (Onchocerca volvulus and Acanthocheilonema viteae) cystatins suppress the proliferation of human PBMCs and murine spleen cells [Haemonchus contortus (Hc-AK) decreased the proliferation and increases the apoptosis of goat PBMCs [H. contortus excretory-secretory 15-kDa protein induced decreased migration ability of PBMCs and stimulated PBMCs to increase NO production [ConA and PHA are two mitogenic substances that can stimulate the proliferation and activation of T lymphocytes. These substances can activate different T-cell subpopulations, and activated T lymphocytes then function in the cellular immune response. In a previous study , ConA stferation . Filariaen cells . Argininat PBMCs . Recombioduction .FgESP decreased mitogen-stimulated PBMC proliferation at specific doses, which is consistent with the findings reported by Zhang [FgESP, it can be inferred that FgESP may consist of inhibitory and activator components with immunomodulatory functions. Specific molecules in FgESP, such as Fg14-3-3e and FgTPx, have been identified, and relevant research is underway [FgESP components in the immunomodulatory activity of FgESP. F22 decreased mitogen-induced PBMC proliferation stimulated by both ConA and PHA. As the results indicate, it can be inferred that the components of F22 may affect different T-cell subpopulations, and identifying the correlation between specific molecules in F22 and their target subpopulations will deepen our understanding of specific immunomodulatory processes.In the present study, by Zhang . Howeverunderway ,21. FurtFgESP Score: 323.31) and CaM (FgESP Score: 263.37) have been preliminarily explored. For CaBP2 and CaM2, a previous study showed that calmodulin-targeted drugs can impede the growth of Schistosoma mansoni, Plasmodium falciparum, and malarial parasites [Fg-CaBP4 protein can actively bind to the surface of goat PBMCs and regulate the immune response of PBMCs [FgCaBP2 and FgCaM2 may need to be explored further.As components of FgESP, the immunomodulatory roles of CaBP  proliferation in vitro; then, FhESP was fractionated, and a proliferation assay showed that the suppressive activity of FhESP is shown to correlate with the presence of Cat L proteases [FhCat L1 and FhCat L2 show a protective range of 33% to 79% against F. hepatica infection [TPx and cathepsin L, which exhibited high scores in the mass spectrometry analysis of both us study  showed troteases . Recombinfection .FgESP, which is helpful for the screening of potential vaccine candidate molecules. FND (F22 Score: 100.63), β-TUB (F22 Score: 26.29), and HDM-1 (F22 Score: 20.95) were identified in F22. Fibronectin type III domain-containing 5 (FNDC5) is a transmembrane glycoprotein that can be hydrolyzed and lysed to form irisin, which has many benefits in preventing human diseases [F22 exhibits stronger immunosuppressive ability than the diseases . β-TUB idiseases . HDM-1 cSchistosoma mansoni Hsp70 may be associated with regulated cecal invasion as well as cecal transformation [Fg14-3-3e protein, which inhibits the proliferation and migration of goat PBMCs, inhibits phagocytosis of monocytes and may play an important role in the interaction of F. gigantica with goat PBMCs [KEGG analyses indicated that 86 of the proteins in F22 are active in more than one pathway. Heat-shock protein 70 (Hsp70) is a very conserved and prevalent protein , and theormation ,33. The ormation ,36,37,38at PBMCs . Glutathat PBMCs , and subIn the present study, F22 decreased mitogen-induced naïve buffalo PBMC proliferation, indicating its strong immunosuppressive effect on the initial buffalo immune response. Further studies on F22 and its specific component-mediated immunomodulation under the adaptive immune state may facilitate the screening of potential vaccine candidate molecules for the prevention of fascioliasis.


Scanning near-field optical microscopy was used to image hybrid polaritons in graphene/α-MoO3. We demonstrate the topological transition and characterize hybrid modes, which can be tuned from surface waves to bulk waveguide modes, traversing an exceptional point arising from the anisotropic plasmon-phonon coupling. Graphene/α-MoO3 heterostructures offer the possibility to explore dynamical topological transitions and directional coupling that could inspire new nanophotonic and quantum devices.Polaritons in hyperbolic van der Waals materials—where principal axes have permittivities of opposite signs—are light-matter modes with unique properties and promising applications. Isofrequency contours of hyperbolic polaritons may undergo topological transitions from open hyperbolas to closed ellipse-like curves, prompting an abrupt change in physical properties. Electronically-tunable topological transitions are especially desirable for future integrated technologies but have yet to be demonstrated. In this work, we present a doping-induced topological transition effected by plasmon-phonon hybridization in graphene/α-MoO Hyperbolic phonon polaritons – mixed states of photons and anisotropic lattice vibrations – offer appealing properties for nanophotonic applications. Here, the authors show that the plasmon-phonon hybridization upon electronic doping in graphene/α-MoO3 heterostructures can induce topological transitions of the polariton wavefront. The term “hyperbolic” refers to the shape of the isofrequency surface, which is hyperboloidal rather than ellipsoidal like in common anisotropic media, because of the opposite signs of the real dielectric permittivities of metallic and dielectric media. Hyperbolic isofrequency surfaces give rise to an increased photonic density of states, which can help engineer spontaneous2 and thermal3 emission. Furthermore, the unique properties of hyperbolic media have allowed for the development of subwavelength imaging4, polarization converters5, and negative refraction6, among many other technologies.Hyperbolic optics first rose to prominence with the development of hyperbolic metamaterials constructed by periodically alternating metallic and dielectric media9 and α-phase molybdenum trioxide (α-MoO3)11 have been found to exhibit hyperbolicity around anisotropic phonon resonances that drive permittivity to negative values along one crystal axis or plane14. Van der Waals crystals display relatively weak phonon damping, enabling propagating phonon polaritons that behave like waveguide modes with deeply subdiffractional wavelengths15. Phonon polaritons in the two reststrahlen bands of uniaxial h-BN are out-of-plane hyperbolic, meaning that the out-of-plane optic axis and its normal plane have opposite-signed permittivities. h-BN polaritons can propagate along any in-plane direction9. On the other hand, α-MoO3 is biaxial with three infrared reststrahlen bands. The upper band hosts out-of-plane hyperbolic modes 3) is rooted in crystal structure. Heterostructuring and hybridizing hyperbolic modes with other modes can modify their properties22, but adjusting tuning parameters typically requires altering device design. With α-MoO3, for example, in-plane hyperbolic phonon polaritons can be modified by fabricating double-layer structures with a predefined twist angle as the tuning parameter19. The isofrequency contour (IFC) characterizing the propagation-direction-dependent momenta of in-plane hyperbolic modes can be tuned through a topological transition23 from an open hyperbola to a closed curve by sweeping twist angle. On the other hand, dynamical tuning of h-BN phonon polaritons has been demonstrated by means of hybridization with graphene surface plasmons20. Surface plasmon-phonon polariton (SP3) and hyperbolic plasmon-phonon polariton (HP3) hybrid modes have gate-tunable dispersions enabled by the carrier-density-dependent Fermi energy 24. However, graphene/h-BN admits only modest electronic tunability without topological transitions.Integrated nanophotonics requires dynamical tuning of hyperbolic polaritons, but this is challenging since hyperbolicity in the two most widely studied systems  where the topological transition occurs. Finite difference time domain simulations (Methods) show that, without doping, graphene is devoid of plasmonic response and the heterostructure is optically equivalent to bare α-MoO3 . In the cleavage plane of the [100] (x) and [001] (y) crystal axes  phonon frequency 25 has recognized that the transition occurs at an exceptional point (EP), above which plasmons experience Rabi splitting from strong coupling to phonons. Below the EP, there is no mode splitting: plasmons and phonons are weakly coupled in a manner akin to electromagnetically-induced transparency.Along the [001] direction in the middle reststrahlen band, the α-MoO3 with high-frequency permittivities set to unity. The total dielectric function of Drude-like graphene with electron coupling to in-plane hyperbolic phonons modulated by the quantity 3 to SP3 in the finite slab.We now show quantitatively that tuning mentclass2pt{minimely Fig. . \docume}Γ− Fig. . Splittix) to dope graphene26 by a charge transfer process (Methods). Doping graphene using thin high-work-function materials like WOx or α-RuCl3 has proven to be nondetrimental to s-SNOM imaging28. Furthermore, by stacking either monolayer or bilayer tungsten diselenide (WSe2) prior to oxidation, we can tune the 3 between ~0.60 eV and 0.45 eV, respectively. Doping levels were determined by fitting  was used to image polaritons and evaluate their energy-momentum  onto SiO2/Si substrates. A monolayer or bilayer WSe2 on graphene heterostructure is then transferred onto a predetermined α-MoO3 flake. Finally, we treat samples in a UV-ozone generator (Jelight UVO-cleaner) at 300 K to oxidize the topmost WSe2 monolayer into WOx37.High-quality WO38.A Neaspec neaSNOM near-field microscope was used with a DRS Daylight Solutions Hedgehog tunable quantum cascade laser TLS-SK-41112-HHG (λ=10.86–11.67 µm). Standard PtIr-coated Arrow tips with 75 kHz resonance frequencies were used with tapping amplitudes of about 40 nm in contact. The signal localized under the apex of the tip is isolated in the backscattered signal by demodulation at the 1st−5th tip tapping harmonics in a pseudoheterodyne detection scheme3 heterostructures were performed with the Ansys Lumerical finite difference time domain (FDTD) solver for Maxwell’s equations . We used a point dipole source positioned 20 nm above the top surface of the graphene/MoO3 structure and polarized along the z direction. Electric field monitors were placed on the top surface. The boundary conditions were set to the perfectly matched layer option. The NumPy fast Fourier transform library was used to compute isofrequency contours from FDTD wavefronts. In particular, we used the fft2 function followed by the fftshift function to set the zero-frequency component to the center.Simulations of the electric field distributions on graphene/MoOTo extract polariton momenta −1.Raman measurements were conducted on a commercial Bruker Senterra system using an excitation wavelength of 532 nm at 2 mW with a 60 second integration time. A grating of 1200 line/mm was used to attain an energy resolution of 3–5 cmSupplementary Information


The tunneling current–bias voltage (It − bV) properties of GWMHs can be tuned by 5 × 106 times in magnitude for current increasing from 0.2 nA to 1 mA with applied bias voltage increasing from 10 mV to 2 V. Moreover, the transfer properties of GWMHs exhibit n-type conduction at bV = 0.1 V and bipolar conduction at bV = 2 V; these findings are explained well by direct tunneling (DT) and Fowler–Nordheim tunneling (FNT), respectively. The results show the great potential of GWMHs for high-power field-effect transistors (FETs) and next-generation logic electronic devices.Van der Waals heterojunctions, formed by stacking two-dimensional materials with various structural and electronic properties, opens a new way to design new functional devices for future applications and provides an ideal research platform for exploring novel physical phenomena. In this work, bilayer graphene/WS Graphene FETs have the advantages of high carrier mobility and large current density ,3,4,5,6,Among the device applications of Van der Waals heterojunctions, tunneling transistors based on the carrier tunneling effect have particular significance due to their advantage of a large on/off ratio and the associated promising prospect of low power consumption. To achieve a sufficiently high on-state current density and a very low off-state current density in graphene-based tunneling transistors, a suitable tunneling layer should be delicately designed. Among 2D materials, hexagonal boron nitride (h-BN) is widely used as the tunneling layer ,30 becau2) is an n-type semiconductor [2 is 4.2 eV [2, taking advantage of graphene’s gate-voltage controllability. Hence, WS2 is an ideal material for investigating the tunneling mechanism and improving the performance of graphene-based tunneling devices [Tungsten disulfide  were fabricated via a controlled dry transfer method (see details in Methods) [2 substrate on the monolayer graphene. Moreover, Cr/Au was selected as the top metal because Cr’s work function (~4.6 eV) [2 barrier and the Cr/Au electrode with a relatively low barrier, resulting in a high on-state current density (4 × 107 A m−2) and a high on/off ratio (5 × 106) by sweeping the bias voltages. More interestingly, the GWMHs device exhibits n-type conduction at 0.1 V bias voltage and bipolar conduction at 2 V bias voltage, which is unusual in semiconducting devices but can be explained well by direct tunneling (DT) and Fowler–Nordheim tunneling (FNT), respectively. The results of this study not only pave the way to polarity-controllable high-performance 2D transistors but also provide an ideal paradigm for designing next-generation logic electronic devices.In this study, bilayer graphene–WSMethods) . Bilayer~4.6 eV)  is closeDevice fabrication. Few-layer bottom graphene and a WS2 tunneling barrier layer were mechanically exfoliated from bulk graphite and WS2 single crystals from the HQ Graphene company  [2/Si substrate, and the exfoliated WS2 layer was transferred onto a polydimethylsiloxane (PDMS) membrane. Then, the vertical graphene–WS2 Van der Waals heterostructures were carefully assembled with the dry transfer method [2 layers were deposited with Cr/Au electrodes using standard e-beam lithography and an e-beam evaporation process. The deposited film thicknesses of Cr and Au electrodes were 5 nm and 50 nm, respectively.erlands) . After er method  using a Structure characterization. The Raman spectra of graphene and WS2 layers were measured by a confocal Raman spectrometer  at room temperature in ambient conditions. The excitation wavelength was 532 nm. The laser spot was about 1 μm in diameter and had a power of 1 mW. The thickness of the WS2 layer was determined by atomic force microscopy using the tapping mode .Electrical transport measurements. The tunneling I–V characteristics of our devices were measured by a low-temperature probe station equipped with source meters and lock-in amplifiers. The samples were placed in a vacuum chamber (<10−6 Torr) with a variable temperature from 300 to 5 K.2 layers were mechanically exfoliated from high-quality bulk crystals and then vertically stacked layer-by-layer via a dry transfer method to form the heterostructures. The bottom graphene layer was electrically attached to Cr/Au metal electrodes, and the whole heterostructure device was placed on an Si substrate with a 290 nm SiO2 layer on top . 2. The thicknesses of the graphene bottom layer and the WS2 middle layer were measured by atomic force microscopy (AFM) and were about 0.89 and 6.4 nm  at one edge of the device, as shown in the −1, integration width: 20 cm−1). The shapes of the graphene and WS2 layers are clearly visible, correlating well with the optical image. Raman spectroscopy was used to characterize the number of layers of WS2 and graphene, as shown in 2 exhibits two characteristic peaks—the E1g2 peak at 350 cm−1 and the A1g peak at 420 cm−1—which are consistent with the previous Raman studies of few-layer WS2 [Both the graphene and WSayer WS2 . The numayer WS2 ,38,39.jt − bV) characteristics were performed in the GWMHs devices. By applying a bias voltage (bV) between the bottom graphene layer and the top Cr/Au electrode, the tunneling current density (jt) through the multilayer WS2 could be detected. jt − bV curves under various gate voltages (Vg) at 300 K, and jt as a function of the bias voltage bV and the gate voltage Vg at 300 K. jt − bV curves and the jt map at 5 K, respectively. As illustrated in jt) of GWMHs can be tuned by a factor of 5 × 106, e.g., from 8 A m−2 to 4 × 107 A m−2, with the bias voltage changing from 10 mV to 2 V at 5 K. Under a very small bias at 10 mV, current leakage still exists with magnitude 8 A m−2, resulting from host point defects in the WS2 layer [6 to 6 × 104 because of the increased current leakage at higher temperatures [6 A m−2 at 25 V bias) [7 A m−2 at 2 V bias).After structural characterization, electrical measurements of the tunneling current density–bias voltage  , our GWMjt−Vg) exhibit typical n-type conduction at a low bias (bV = 0.1 V) but bipolar conduction at a high bias (bV = 2 V). By changing the gate voltage, the Fermi level of bottom graphene can be tuned. 2, but the Fermi level of the top metal electrodes cannot be tuned because of their high DOS, as shown in 2, lowering the tunneling barrier and increasing the tunneling current, as shown in As depicted in 2–metal heterojunction device, it is possible to tune the Fermi level of the bottom graphene and tailor the shape of the tunneling barrier, providing a platform for exploring the tunneling mechanism under different bias voltages. At low bias voltages, direct tunneling occurs, and the tunneling barrier width is equal to the WS2 barrier thickness, which is unaffected by the bias and gate voltages. The direct tunneling current density DTj can be approximated by the following equation [In the graphene–WSequation ,42:(1)jD2 barrier can be further shaped by higher bias voltages, converting it from a trapezoid to a triangle. In this circumstance, the tunneling barrier width is no longer equal to the WS2 barrier thickness, which is reduced as bV is increased. Therefore, the tunneling probability is markedly increased, and the resistance of bottom graphene dominates the total resistance. Fowler–Nordheim tunneling (FN tunneling) occurs, which can be described by the following equation [The WSequation ,42:(2)jFm and m* are the masses of free and effective electrons, respectively. d are the effective barrier height and width, respectively, determined by the difference between graphene’s Fermi level and the bottom of WS2’s conduction band at the graphene–WS2 interface, respectively. h is Planck’s constant, and e is the unit charge of the electron.In Equations (1) and (2), jt − bV characteristics under different back-gate voltages at a temperature of 5 K. The Equation (2) can be transformed into [Because of the high electric field generated during the FN tunneling process, the Joule heating effect frequently causes a large thermionic emission current over the triangle-shaped barrier. To suppress the thermionic current and determine the effective barrier height med into ,42:(3)lnjt − bV curves and establish a reference point for defining the tunneling barrier height. tj|/bV2) as a function of 1/bV in the GWMHs tunneling device at different Vg values at 5 K. At a higher bV, the larger negative slope indicates that the FN tunneling dominated the carrier transport and current flow in this region, while at lower bV, direct tunneling of charge carriers through the ultrathin WS2 layer dominated. If the effective mass value of the electron (m*) is set at 0.3 m [Equation (3) helps investigate the at 0.3 m , then thtion (4) ,42:(4)k=Vg changes from zero to negative values (hole doping). The gate-tunability of the carrier tunneling regions and the barrier height in our GWMHs devices opens a new road to control the characteristics of vertical field-effect tunneling transistors. In addition, with the continuous progress of lithography technology, lateral tunneling heterojunctions have also attracted much attention [ttention ,45,46. H2–metal heterojunctions (GWMHs) was studied in this work. The tunneling current density (jt) of GWMHs changes by a factor of 5 × 106, with the applied bias voltages changing from 10 mV to 2 V, and a high on-state current density (4 × 107 A m−2) was achieved. Moreover, the transfer properties of GWMHs exhibit n-type conduction at a 0.1 V bias and bipolar conduction at 2 V, which are well-explained by direct tunneling (DT) and Fowler–Nordheim tunneling (FNT), respectively. Our results show great potential for GWMHs in high-power FET devices and next-generation logic electronic devices.In summary, a tunable field-effect tunneling transistor based on graphene–WS


Multimodal platforms combining electrical neural recording and stimulation, optogenetics, optical imaging, and magnetic resonance (MRI) imaging are emerging as a promising platform to enhance the depth of characterization in neuroscientific research. Electrically conductive, optically transparent, and MRI-compatible electrodes can optimally combine all modalities. Graphene as a suitable electrode candidate material can be grown via chemical vapor deposition (CVD) processes and sandwiched between transparent biocompatible polymers. However, due to the high graphene growth temperature (≥ 900 °C) and the presence of polymers, fabrication is commonly based on a manual transfer process of pre-grown graphene sheets, which causes reliability issues. In this paper, we present CVD-based multilayer graphene electrodes fabricated using a wafer-scale transfer-free process for use in optically transparent and MRI-compatible neural interfaces. Our fabricated electrodes feature very low impedances which are comparable to those of noble metal electrodes of the same size and geometry. They also exhibit the highest charge storage capacity (CSC) reported to date among all previously fabricated CVD graphene electrodes. Our graphene electrodes did not reveal any photo-induced artifact during 10-Hz light pulse illumination. Additionally, we show here, for the first time, that CVD graphene electrodes do not cause any image artifact in a 3T MRI scanner. These results demonstrate that multilayer graphene electrodes are excellent candidates for the next generation of neural interfaces and can substitute the standard conventional metal electrodes. Our fabricated graphene electrodes enable multimodal neural recording, electrical and optogenetic stimulation, while allowing for optical imaging, as well as, artifact-free MRI studies. In addition, it is of paramount importance to monitor neural activity systematically to uncover the interconnections between the neurons and neural clusters. In recent years, several methods such as optical imaging 3, optogenetics4, and magnetic resonance imaging (MRI)6 have emerged to assist neuroscientists to decipher the neural structure and function. These, combined with electrical neural recording and stimulation in a multimodal fashion, can pave the way towards a much deeper understanding and mapping of the nervous system7.Neural interfaces are tools that enable bidirectional interactions with the human nervous system. To allow for personalized therapies, which is the ultimate goal of bioelectronic medicine, the functional neural behavior has to be well understood. Conventional neural recording and stimulation methods provide insufficient spatio-temporal resolution for neuroscientific research9. Platinum-iridium (Pt-Ir) alloy electrodes, on the other hand, cause image artifacts in MRI due to the magnetic susceptibility of Pt being different from that of the surrounding tissue11.Conventional noble metal electrodes, such as gold (Au) or platinum (Pt), are the most common tools for recording neural activity and stimulating neurons due to their good electrical performance, high biocompatibility, and chemical stability. However, due to their opaque nature, they prevent any in vivo optical imaging at the site of stimulation (underneath the electrodes). In addition, due to photoelectrochemical effects, Au electrodes might produce photo-induced artifacts when used for electrophysiology in optogenetic devices14. Among all transparent carbon-based electrodes, graphene is the most attractive material due to its high thermal/electrical conductivity, broad-spectrum transparency, and flexibility15. In addition, graphene-coated copper wires16 and graphene-fiber electrodes made of graphite oxide17 have been proven to be MRI compatible due to their magnetic susceptibilities being close to that of tissue. Therefore, graphene has the potential to be the ideal electrode material candidate for the next generation of optically transparent, and MRI-compatible multimodal neural interfaces.Therefore, there is a need for optically transparent and MRI-compatible electrodes. Indium-Tin-Oxide (ITO) and carbon-based electrodes are the most commonly used transparent conductive electrodes. However, ITO cannot be used in flexible devices due to its brittleness that might cause crack formation18. However, its electrical conductivity does not reach that of pristine graphene19. More importantly, a cytotoxicity concern towards different types of cells using GO and rGO has been raised recently20.The majority of research on graphene-related materials concerns graphene-oxide (GO) and reduced-graphene-oxide (rGO) materials. However, due to the electrically insulating properties of GO, its combination with other conducting materials, such as conductive polymers and metals, is necessary to fabricate electrodes. rGO’s large effective surface area leads to low impedance and high charge-injection capacity (CIC) that are both important for neural recording and stimulation21. However, the required high graphene growth temperature  prevents direct graphene growth on wafers with already present polymers, a fundamental component of flexible implants. Therefore, current state-of-the-art graphene electrode fabrication has been mostly focusing on graphene transfer processes, where graphene is grown on a copper (Cu) catalyst, and subsequently transferred to the required polymer used for the implant25. Sacrificial polymer supporting layers, such as polymethylmethacrylate (PMMA), facilitate the transfer process. This method, despite its popularity, has reliability and scaling issues26 regarding preserving the quality of the material after transfer, polymer residues from the supporting layer, or an additional cleaning process to remove any polymer residues27. Finally, metallic particles from the, typically, non-biocompatible Cu catalyst layer can impact the implant’s biocompatibility. Apart from that, in such processes the first polymer layer is present before the graphene transfer. This limits the electrode post-processing options that have the potential to e.g., improve the conductivity28.The most common fabrication method for growing graphene is chemical vapor deposition (CVD) which has the advantage of simplicity and the possibility to create high-quality graphene on a metal catalyst that can span a large surface area29, or laser carbonization of parylene-C to create graphitic carbon as a coating on metal electrodes30, have also been reported. However, to date, laser pyrolysis fabrication has been successfully used only for devices with relatively large electrodes (200–700 μm diameter). The main limitation of this technology is the laser resolution, in comparison with the resolution achieved by photolithographic methods, crucial for miniaturization and the formation of high-density arrays. In addition, these low-quality carbon-based electrodes are not highly optically transparent and suffer from reproducibility issues.Other techniques to fabricate graphene electrodes, such as direct laser pyrolysis of porous graphene on a polyimide substrate31. Although monolayer graphene has shown compatibility with neuro-imaging and optogenetics27, previously reported works suggest that monolayer graphene in an undoped state suffers from low sheet conductivity32. This prevents the use of graphene instead of long metal tracks, reducing the total implant transparency. Moreover, graphene made of fewer layers is more prone to damage during the fabrication and implantation processes.Therefore, using CVD graphene is, so far, the best approach for developing neural electrodes. CVD graphene itself can be created as a monolayer or multilayer, depending on the metal catalyst and the process parameters used34. Recent research in the field of supercapacitors showed that multilayering of graphene (up to a threshold of 4 to 6 layers) could result in higher electrochemical capacitance35, and previously reported stacked four monolayer graphene neural electrodes have demonstrated good electrochemical characteristics36. However, literature suggests that adding graphene layers in a transfer-based process requires more transfer steps, which, in turn, leads to more polymer residues between layers and therefore lower optical transparency34.On the other hand, increasing the number of graphene layers reduces the sheet resistance but also reduces the optical transparency37, which uses a transfer-free method to grow graphene on a Molybdenum (Mo) catalyst38, to create the neural electrodes. This method enables the fabrication of a multilayer graphene electrode without any transfer involved. The electrodes’ impedance, charge storage capacity (CSC), and CIC are assessed and compared to Pt and Au electrodes with the same size and geometry. In addition, the developed electrodes were assessed for compatibility with optogenetic stimulation and MRI, versus Au and Pt electrodes, respectively.The aim of the current study is to use CVD multilayer graphene to create fully-transparent and MRI-compatible neural electrodes with better electrochemical performance. To prevent the presence of polymer residues caused by the transfer process, but also, to make the process more compatible with conventional wafer-scale fabrication and post-processing technologies, we have adapted the process reported inMultilayer graphene neural electrodes were fabricated as illustrated in Fig. 2 and O2 gas flows, respectively. Graphene is selectively grown on Mo as shown in Fig. 2, and CH4 gas flows, respectively, at 25 mbar pressure for 20 min and cooled to room temperature under an Ar atmosphere.First, 2 and 4 μm plasma-enhanced chemical vapor deposition (PECVD) oxide was deposited on the front-and backside of a double-sided polished (DSP) 100 mm silicon (Si) wafer /Ti stack  is sputtered at 50 °C on top of the existing graphene layer and photolithographically patterned (wet etching performed using a 0.55% hydrofluoric acid (HF) solution) to cover the graphene features  is needed to prevent damaging the graphene layer while etching the polymer over the electrodes and contact pads. However, since the adhesion of Al to graphene is poor, an additional titanium (Ti) layer, due to a better microstructure of the film, the Al %Si/Ti st2 and HBr gas flows, respectively, with a long over-etching time with 15 and 30 sccm Cl2 and HBr gas flows, respectively, to remove potential Al particles from the polymer layer)  . Mo is removed at this stage by wet etching in hydrogen peroxide (H2O2). Graphene will not be accidentally removed in this step as it has already adhered well to the top polymer. Subsequently, the second parylene layer is deposited on both sides of the wafer encapsulating the implant.Finally, a DRIE process on the backside of the wafer lands on the frontside oxide Fig. g, which 6 and 185 sccm of O2), landing on the Al protective layer, which is then, together with the hard mask, removed in 0.55% HF  with 40 W LF power, 15 sccm of SFDetails of the mask design used for the electrode arrays can be found in Fig. SFor rapid prototyping and to investigate and compare the electrode properties of graphene with those of Pt and Au of similar size and geometry, non-suspended devices were fabricated. For this version, parylene insulation is substituted by photoresist to simplify the processing. The fabrication process for these devices is shown in Figs. S2O2 bath to remove Mo only on the electrodes. The Mo is kept on the contact pads to make a better contact with the attached metal wire.For the graphene variant, the devices are at this point placed inside a HDifferent growth times  were used to create graphene with various thicknesses. Longer growth times result in a larger number of layers. To compare these, both their sheet resistance and optical transmittance are measured. For the sheet resistance, Van der Pauw structures were made, and four-point probe measurements were performed with a Cascade Microtech probe station  and can be found in the To evaluate the quality of a transparent conductive film, a figure of merit (FOM) is used; it is calculated for all graphene thicknesses based on the optical transmittance (T) at 550 nm wavelength and the sheet resistance  was used to assess the electrochemical properties of the electrodes. The measurements were performed in phosphate-buffered saline (PBS) in a three-electrode setup with a Pt electrode (3 mm diameter (BASI Inc.)) as a counter electrode (CE), a leakless miniature silver/silver chloride (Ag/AgCl) (eDAQ) as a reference electrode (RE), and the graphene, Au and Pt electrodes fabricated in this work as the working electrodes (WE). The setup was kept inside a Faraday cage during the measurements. All the electrodes were connected to a potentiostat (Autolab PG-STAT302N) that applied a 10 mV RMS sinusoidal voltage between the WE and the RE and measured the current between the WE and the CE43. This measurement was also performed using the same three-electrode setup. The water window for graphene was chosen from −0.8 to 0.6 V and used as the CV potential range. As the CSC highly depends on the scan rate, the measurements were performed with various scan rates . Both the total and cathodic CSC were calculated.Cyclic voltammetry (CV) is frequently used to calculate the amount of charge that an electrode can inject into the tissue44. The voltage transient was recorded in the same three-electrode configuration by applying a cathodic-first biphasic symmetric current pulse between the WE and CE  in the PBS solution. In the voltage transients between the WE and the RE, an immediate resistive potential drop ) is observed at the onset of the cathodic pulse followed by a gradual potential decrease due to the capacitive charging of the electrode-tissue interface. The potential reaches its minimum value at the end of this pulse. The interface polarization (Ep) is evaluated by eliminating the resistive potential drop from this minimum potential (Ep = Emin − Va). Next, the applied current amplitude is increased until the interface polarization reaches the cathodic water window extracted from the CV measurement. It should be noted that the anodic interface polarization must also not exceed the anodic water window. Finally, the maximum cathodic CIC of the electrode is calculated based on the maximum current amplitude multiplied by the pulse width and divided by the electrode surface area44.Voltage-transient measurements are used to estimate the maximum charge that can be injected by means of a constant current stimulation pulse9.When shining light on the metal electrode, electrons from the metal surface might be ejected and a small transient potential is created that could interfere with the recorded signal from the neurons. This signal is created due to the photoelectrochemical effect and is called a photo-induced artifact2 for short pulses from 0.5 to 50 ms45. In this experiment, rhythmic rectangular pulse stimulation with 10 ms pulse duration at 10 Hz and 50 mW/mm2 light intensity was applied to both graphene and the Au electrodes while immersed in a PBS solution. The power spectrum of the recorded signal was investigated for light-induced artifacts. In addition, this test was performed for three different graphene thicknesses to compare the effect of thickness on the produced artifact.Here, we tested our multilayer graphene in comparison with gold electrodes using an optical fiber coupled with an 470 nm LED. The setup used for this test is shown in Fig. STo investigate the MRI compatibility of multilayer graphene and Pt electrodes, samples were prepared as follows. To simulate a brain-tissue environment, a phantom was prepared by dissolving 1 g agarose in 100 ml PBS in a Petri dish, where the suspended graphene  sequence; (2) Multi-slice GRE sequence with single-shot EPI (echo-planar imaging) readout; (3) Low resolution T2* mapping performed with a multi-echo GRE sequence; (4) Ultra high resolution B0 mapping based on multi-echo GRE phase imaging.An image artifact is usually detected as a specific signal dropout that clearly obstructs a portion of the image around the electrodes and prevents visualization of brain structures where neural signals are recorded, or electrical stimulation is applied. The MR images of the phantom were acquired with a clinical 3 T scanner . The following sequences were used to acquire MRI images: (1) High resolution 3D T20 maps are analyzed to quantitatively assess B0 field distortion introduced by the electrodes. A region-of-interest (ROI) is defined to detect the field shifts induced by the electrodes. Then, a background field removal (BFR) method is performed using a high-pass or Gaussian filter with a standard deviation of 23 to remove the field distortions originating from outside of the ROI. The sequences and their corresponding parameters to acquire the MRI images are provided in detail in Table SBThe final suspended graphene electrode with parylene substrate is shown in Fig. a. The po2 as explained in Fig. SOptical images of the 340 μm diameter electrodes before and after Mo removal are shown in Fig. −1 with a full-width at half maximum (FWHM) of 61.02 cm−1, a G peak (green) at 1586 cm−1 related to the sp2 C–C bonds forming the graphene lattice and having a FWHM of 33.32 cm−1, and a 2D peak (yellow) around 2670 cm−1 with a FWHM of 62.20 cm−1. The ratio between the intensities of the D and the G peaks (ID/IG = 0.38) indicates the defects in the graphene layer, which in this case indicates a low number of defects after Mo removal. This ratio matches with the reported values for graphene on Mo for gas sensing applications46. The ratio between the intensities of the 2D and the G peaks (ID2/IG = 0.74) confirms the presence of multilayer graphene as the ratio is less than 140. In addition, from the shape of the single-peaked 2D band, it can be postulated that the graphene is turbostratic48.Raman spectroscopy using a laser with a 633 nm wavelength on the electrode surface was performed after Mo removal to confirm the presence of graphene on the electrode surface. As shown in Fig. sh) was measured on 27 Van der Pauw structures for different graphene growth times. The average (plus sign) values for Rsh are depicted in the box plot in Fig. sh and the location on the wafer for all conditions. The structures in the center of the wafer showed the lowest Rsh, and the structures towards the edge showed higher Rsh. This is possibly due to the single zone heating element in the chamber causing a higher temperature close to the center of the wafer, which results in thicker graphene with a lower defect density.The sheet resistance  catalyst that the rate of isothermal graphite growth slows down with increasing exposure time, which might be due to the increased coverage of the catalyst surface with graphite that blocks the precursor supply from the Ni catalyst49. Another explanation is the low solubility of Mo (0.0026 weight % at 1000 °C) for carbon atoms. Mo will be saturated faster in the middle, and thus the thickness will not increase further. Therefore, we postulate that with a longer growth time, the thickness of the graphene on the edges of the wafer becomes more similar to the thickness in the center.Furthermore, the average R41. According to these measurements, 20, 40, and 60 min graphene growth times lead to ~7, 10, and 17 graphene layers, respectively. These confirm that increasing the growth time increases the thicknesses of graphene and reduces the optical transmittance.Optical transmittance measurements performed on graphene grown for 20, 40, and 60 min after removing the contribution of the glass slide are shown in Fig. 50 and also higher than the theoretical value of (2.55) calculated from the same equation for an undoped monolayer graphene in51. Finally, a 20 min graphene growth time was chosen for the final electrode to achieve a higher optical transparency.The calculated FOM is reported in Table EIS measurements were performed on 15 graphene electrodes with 20 min growth time, and the obtained graphs can be found in Fig. In the Bode plots, the deviation from the average impedance and phase is shown in the shaded gray area. This could be related to slight variations of graphene growth over the Si wafer. EIS measurements were performed for 3 graphene electrodes with 40 min growth time and 3 electrodes with 60 min growth time as shown in Fig. Ss is the resistance of the solution, ZCPE is the constant phase element representing the Helmholtz double layer capacitance. Rct is the charge-transfer resistance used to simulate Faradaic reactions and ZWB is the bounded Warburg impedance used to simulate the diffusion process.The proposed equivalent circuit model for multilayer graphene electrodes is shown in Fig. q) caused by the limited electronic density of states (DOS)52. Cq is relatively small for monolayer graphene and therefore dominant at low frequencies. Recent research shows that, by increasing the number of graphene layers, Cq is increased and its effect on total capacitance becomes less dominant35. It has also been shown that for multilayer graphene another capacitance is added in series with Cq, which is called the dielectric capacitance (Cdiel). This capacitance is caused by a shielding effect inside the electrode due to a generated electric field. By increasing the number of graphene layers, this shielding region expands leading to a reduction in Cdiel53.It was found that the double layer capacitance for graphene is in series with the quantum capacitance  using the equations presented in the EIS measurements were performed on Au and Pt electrodes with the same dimensions Fig. d, e. The54.Cyclic voltammetry was performed on the same 15 graphene electrodes. The CV curves for graphene (20 min growth time), Au, and Pt are shown in Fig. The comparison between the CSC calculated for graphene electrodes with different thicknesses was inconclusive as the variation between the CSC of the electrodes is insignificant. Therefore, a larger number of samples is needed for 40 and 60 min graphene growth to be able to study the impact of thickness on the CSC.25. Moreover, the graphene electrodes reported in24 show a high CSC at 1.0 V/s scan rate but still lower than the one reported in this work. Such a high CSC for the graphene electrode reported in24 was related to the larger potential window used for the CV measurement.Furthermore, as shown in Table 2 for graphene, Au, and Pt electrodes, respectively. It should be emphasized that by reducing the current pulse width, the current amplitude could be increased to ensure that the current is high enough to elicit neural activation, as pulse widths shorter than 0.6 ms are generally employed in neural stimulation43. However, this result still can be used as an indication of the CIC for neural stimulation.The results of voltage-transient measurements for graphene, Au, and Pt are shown in Fig. The power spectra of the recorded signals for Au and graphene electrodes while shining 10 Hz light pulses on their surface are shown in Fig. *-weighted images were acquired because they accentuate local susceptibility effects. However, no electrode-related image artifact was detected in these images , respectively. This shows that the Pt electrode causes about 18.6 times higher magnetic field distortion than the graphene electrode due to its higher magnetic susceptibility than the surrounding tissue.However, the Bdes Fig. e. The fi55. Moreover, the thermal expansion coefficient (CTE) of Mo in comparison with Cu and Ni is much closer to the one of Si, hence, Mo is less prone to wrinkle creation during high temperature graphene growth57. Additionally, catalyst residues are an important concern in an implantable device. Cu has shown toxicity after histopathological evaluation in the cerebral cortex and categorized as a toxic material for the human body59. Mo has shown biocompatibility60and biodegradability62 and therefore, is a great substitute for Cu as a catalyst material for biomedical applications. In addition, energy dispersive X-ray (EDX) analysis performed on our graphene electrode after Mo removal revealed only 0.03% weight percentage of Mo residue on the electrode surface as shown in Fig. SMultilayer graphene electrodes were fabricated using a wafer-scale transfer-free process. The use of CVD processes for graphene synthesis gives the opportunity of developing graphene layers only over desired areas, since the catalyst used can be patterned before graphene growth. Mo is chosen as a catalyst layer due to the possibility of growing thin and uniform layers of graphene because of its extremely low carbon solubility, thus creating a self-limiting growth process26. Therefore, the resulting graphene implant performance might have a variation from device-to-device and wafer-to-wafer.The use of a transfer-free process adds significant advantages to the fabrication process. The graphene transfer method is a complicated process and the graphene layer is prone to crack formation, polymer contamination, catalyst residues, wrinkling, and folding63. This could provide the possibility of monolithic integration of active circuitry to the device prior to graphene growth. The proposed fabrication process can be also an advantageous method for the fabrication of optoelectronic devices.However, due to the transfer-free process used in this work, less defects and misalignment are expected in a graphene layer compared to transferred graphene. Subsequently, the absence of any polymer residues results in high optical transparency. More importantly, the transfer-free process is more compatible with conventional wafer-scale fabrication processes and results in a higher yield, as shown by the authors in ref. 64 and65 also shows the use of a multilayer stack for the encapsulation. In these cases, the mechanical properties of the device can be tuned by changing the thickness of each layer based on the application.The process also allows for the addition of arbitrary polymers at the end of the fabrication process based on their mechanical characteristics and the application requirements. Reference33.Multilayer graphene could cause a lower sheet resistance for graphene tracks compared to monolayer graphene as the sheet resistance is inversely proportional to the thickness of the film. Moreover, having multiple graphene layers provides additional transport paths for the charge carriers, which increases the conductivity of graphene. Recent research shows that increasing the number of layers to reduce the sheet resistance in a transfer process leads to optical transmittance reduction not only due to the added layers but also due to the polymer residues on each layer from the transfer process. Furthermore, since the transfer process can induce defects in the graphene lattice, for the same number of layers, fewer transfers show lower sheet resistance66. Therefore, 20 min graphene growth was chosen to make graphene-based devices that are optically transparent enough to be used for modern neuroscientific research such as optogenetics and in vivo optical imaging. It should be noted that doping could decrease the sheet resistance even further but this was not the focus of this work.The results obtained by the sheet resistance and optical transmittance measurements in this work show sheet resistance and optical transparency reductions by increasing the graphene growth time. Besides, the additional layers are expected to enhance the mechanical and electrical reliabilityA thorough characterization of the properties of the graphene electrodes presented here was conducted and results are summarized in Table A comparison between our multilayer graphene with Au and Pt electrodes showed only 3–4 times higher impedance (1 kHz) for graphene electrodes. The multilayer graphene electrodes fabricated in this work showed a lower area-normalized impedance compared to other undoped CVD-based graphene electrodes.CV measurements showed that our graphene electrodes are comparable to Pt electrodes in terms of CSC. Graphene electrodes outperform Pt electrodes when using slower scan rates for CV measurements. This could be related to the high graphene surface roughness that could be more accessible for ion fluxes at lower frequencies. The CSC at different scan rates was measured to be able to compare the result with state-of-the-art graphene electrodes. It was shown that our multilayer graphene has the highest CSC reported so far for CVD graphene electrodes.The significant improvement in CSC for the multilayer graphene compared to monolayer graphene could be explained by the effect of the quantum capacitance in series with the double layer capacitance. By increasing the number of graphene layers, the quantum capacitance is increased. Therefore, this capacitance is no longer dominant for multilayer graphene and the total capacitance will be increased.2, respectively) and low impedance due to their high surface area are other electrode candidates43. These have been added as coating materials on graphene to improve its characteristics68.On the other hand, voltage-transient measurements showed comparable CIC for both graphene and Pt. However, to substitute conventional metal electrodes, the CIC could be further improved using chemical dopants or surface functionalization methods to give graphene the possibility to compete with Pt electrodes. In fact, other transparent materials such as poly  polystyrene sulfonate (PEDOT:PSS) and carbon nanotube (CNT) with great CIC  light source did not show any artifact9.The photo-induced artifact test performed in this work using an LED light source, showed no artifact on the power spectrum of the recorded signal picked up from the graphene electrode. However, visible peaks were observed using the Au electrode. The same measurement with different thicknesses of graphene still did not show any artifact. This could prove the lack of dependence of photoelectrochemical effect on the graphene thickness. However, to be able to conclusively argue about such independence, additional characterization would be needed. More importantly, LEDs were used as the light source in this test. It is possible that when a coherent light source, i.e., a laser diode, is used instead, photo-induced artifacts will be generatedMoreover, it should be noted that for a thorough investigation of the photo-induced artifact, this test must be performed in an in vivo condition as the light scattering and absorption in tissue differs from that in a simple PBS environment. However, this PBS test is a good first indicator and can additionally provide information about the effect of increased thickness on any generated artifact.16 and graphene fibers17 has been recently confirmed. The MRI test performed in this work shows that CVD graphene electrodes encapsulated with parylene-C can be considered MRI compatible. This could be due to the small difference between the magnetic susceptibility of graphene and the human body. The exact value of magnetic susceptibility of graphene is unknown. However, carbon (C) in graphite form is reported to have a highly anisotropic diamagnetic susceptibility (−8.5 ppm)69, which is very close to that of brain tissue (−9.2 to −8.8 ppm)70.The MRI compatibility of graphene encapsulated Cu wiresOn the other hand, Pt electrodes were expected to show image artifact in MRI. However no artifact was detected. Therefore, using Pt electrodes with a larger thickness or in an MRI scanner with a higher magnetic field strength (7 T or more) might generate even higher magnetic field distortion leading to more image artifacts.No substantial heating was detected with a room temperature IR thermometer. However, the use of a phantom instead of real tissue might lead to a different temperature distribution and thus a different degree of image artifacts. Therefore, an in vivo MRI test with graphene electrodes implanted would be advantageous.Apart from a magnetic susceptibility difference, the material conductivity and the eddy currents induced in the material by gradient switching and the RF field might cause MRI artifacts. However, the eddy current induced artifact was assumed to be negligible.We presented the development and characterization of fully-transparent CVD-based multilayer graphene electrodes using a wafer-scale transfer-free process for the next generation of optically transparent and MRI-compatible neural interfaces. The electrodes were fabricated directly on a patterned Mo catalyst resulting in a multilayer graphene electrode.2 CSC was achieved based on CV measurements for graphene at a 100 mV/s scan rate that is the highest value reported for CVD graphene electrodes to date. The CIC was also calculated for graphene electrodes (44 μC/cm2) using voltage-transient measurements. Our graphene electrodes illuminated with light pulses with a repetition rate of 10 Hz did not reveal any photo-inducted artifacts for all thicknesses measured. Moreover, the fully-transparent electrodes did not show any image artifact in a 3 T MRI scanner. These results show that graphene multilayer electrodes with a high CSC and a low impedance could be used for the next generation of neural interfaces, enable multimodal electrical and optical recording and stimulation, and substitute the current standard metal electrodes, to additionally allow for MRI studies of the nervous system.The electrode showed low impedance (27.4 kΩ) at 1 kHz that is quite comparable to those of Au and Pt electrodes with the same size and geometry. A 3.5 mC/cmSupplementary material


Cellulose nanocrystals (CNCs) have received a significant amount of attention due to their excellent physiochemical properties. Herein, based on bioinspired layered materials with excellent mechanical properties, a CNCs-graphene layered structure with covalent linkages (C-C bond) is constructed. The mechanical properties are systematically studied by molecular dynamics (MD) simulations in terms of the effects of temperature, strain rate and the covalent bond content. Compared to pristine CNCs, the mechanical performance of the CNCs-graphene layered structure has significantly improved. The elastic modulus of the layered structure decreases with the increase of temperature and increases with the increase of strain rate and covalent bond coverage. The results show that the covalent bonding and van der Waals force interactions at the interfaces play an important role in the interfacial adhesion and load transfer capacity of composite materials. These findings can be useful in further modeling of other graphene-based polymers at the atomic scale, which will be critical for their potential applications as functional materials. Cellulose nanocrystals (CNCs) show exceptional mechanical properties, biodegradability and environmental friendliness ,2,3. TheCNCs with a high degree of crystallinity can easily be formed into smooth films with uniform thickness by appropriate treatment ,22,23. TIn this study, the mechanical properties of CNCs-graphene layered nanocomposites with covalent bonding are investigated utilizing non-equilibrium molecular dynamics (NEMD) under typical temperature and strain rate. The effects of covalent bonding on the interfacial interactions are also predicted by comparing the elastic modulus of CNCs-graphene layered structure with that of pristine CNCs. In addition, the mechanism behind the mechanical property is further explained with the help of the atomic configuration. The simulation results could provide some important connections between mechanical behavior and covalent bonding in layered nanocomposites, and it could provide some insights for optimizing the mechanical properties of other polymers.The NEMD simulations were carried out using the LAMMPS  program package . All intThe NEMD simulations were conducted by first minimizing the energy of the system using the conjugate gradient method. The time step was set to 0.5 fs for all the simulation processes. The simulation temperature was controlled by using a Nosé-Hoover thermostat. Before starting the tensile simulation, the nanostructures reached the global energy minimum using the steepest descent minimization algorithm. After equilibration, the system was stretched for 200 ps under the NVT ensemble. Energy minimization was performed after each deformation increment to maintain the lowest energy state of the system. For each initial configuration, the axial tensile loading modeling was repeated three times with different velocity initialization seeds. The elastic modulus calculated according to Hooke’s law was obtained by averaging the results of three independent simulation trials.−4 fs−1 at temperatures of 300 K, 400 K, and 500 K. As shown in The calculated stress–strain behaviors of the pristine CNCs and CNCs-graphene layered structure subject to x-direction strain are shown in In order to further verify the above results, the atomic configurations of two nanocomposites under tensile stresses with the increasing temperature are shown in The stress-strain behavior of the two nanocomposites with different strain rates at the temperature of 300 K is shown in The elastic modulus of pristine CNCs and CNCs-graphene layered composites are given in –4/fs, an obvious local stress concentration appears in the pristine CNCs. Some local fractures of graphene layer in layered composite also appear. This fact proves that the covalent bonds at the interface can strengthen the stress transfer capacity. As the displacement in the tensile direction increases, an attractive restoring force arising from the covalent bonds in CNCs-graphene layered composite attempts to hold the layers together. Moreover, van der Waals forces also provide some interface strengths due to the large specific surface area between CNCs and graphene layers [To understand the dependence of strain rate on the mechanical properties of the two nanocomposites, the configuration of atoms with the increasing strain rates are shown in e layers .The mechanical properties of the CNCs-graphene layered structure at each content level of covalent bonds are shown in In this work, the mechanical properties of the CNCs-graphene layered structure with covalent bonding are investigated by molecular dynamics simulation. Compared to the pristine CNCs, the mechanical properties of the CNCs-graphene layered structure can be prominently improved due to the covalent bond interactions and van der Waals forces at the interfaces. The elastic modulus negatively correlates with temperature but has a positive relationship with strain rates and content of the covalent bonds. The atomic structures of pristine CNCs and CNCs-graphene layered structure during the tensile process are analyzed to reveal the relationship between structure and mechanical behavior. These results have a particular incentive effect on theoretical research into polymers with layered structures and their practical applications.


A mechanism for self-pulsation in a proposed graphene-on-silicon microring device is studied. The relevant nonlinear effects of two photon absorption, Kerr effect, saturable absorption, free carrier absorption, and dispersion are included in a coupled mode theory framework. We look at the electrical tunability of absorption and the Kerr effect in graphene. We show that the microring can switch from a stable rest state to a self-pulsation state by electrically tuning the graphene under constant illumination. This switching is indicative of a supercritical Hopf bifurcation since the frequency of the pulses is approximately constant at 7 GHz and the amplitudes initial grow with increasing Fermi level. The CMOS compatibility of graphene and the opto-electronic mechanism allows this to device to be fairly easily integrated with other silicon photonic devices. Self-puGraphene comes with a few benefits: Its linear dispersion means it is wavelength independent. It can operate over a large bandwidth because it has no bandgap. It is CMOS compatible , which a22O3 gate layer is deposited. Graphene is placed over the gate oxide over top of the waveguide and covers 10 percent of the ring. Metal contacts are deposited over the graphene and doped silicon. The contacts are sufficiently far away from the waveguide to prevent optical losses. By applying a bias to the graphene across the aluminum oxide, the graphene can be charged and its Fermi level can be modulated. To find the distribution of the electric field in the waveguide a finite difference eigenmode solver was used. The horizontal electric field forms the majority of the energy in the mode. The interaction of the light with the graphene is strongest when the electric field is parallel to the graphene because it is treated as a surface conductivity. We operate the ring in quasi TE-mode and in this way the graphene on top of the waveguide has more influence than the graphene on the side of the waveguide. Applying a voltage bias to the graphene changes its optical properties; however the overall distribution of the light in the waveguide changes very little. We approximate the optical energy distribution in the waveguide to be constant with changing Fermi level of graphene. Voltage bias does change the effective refractive index and these effects are captured in the next section as perturbations. We obtain the unperturbed effective refractive index, n0, from the eigenmode solver when the Fermi level is 0.1 eV. This Fermi level was chosen because the optical behavior of graphene is relatively constant there.The structure of the studied device is given in 3u is normalized so that |u|2 is the energy of the mode in the ring. Light is coupled into the ring via a nearby bus waveguide with input power Ppump. The external quality factor Qe determines the strength of the coupling Γc = ω0/Qe. The imaginary terms in δ = ω0 − ωd, where ω0 is the resonant frequency of the ring and ωd is the frequency of the input mode. Using the detuning parameter, the ring is biased close to resonance. The cavity itself has an internal quality factor Q0 which encompasses the constant linear losses . Ql is the loaded quality factor, taken to be half of Q0 to approach critical coupling.The optical behavior of the ring is captured by treating it as a cavity , 36. We σFCA, σe, and σh terms in N, in the waveguide is roughly uniform due to diffusion effects and can be pulled out of the integral. We only account for the free carriers in the rib of the waveguide so that Vcar = 2πRwH instead of integrating over the whole microring cavity which includes the graphene, oxides and surrounding air.γFC encapsulates the weighted average over the mode energy.We can account for the free carrier and the nonlinear bulk effects on the dispersion and absorption in the cavity by using the first term in ll known  and are y common . This isωd = ω0 in low light intensity conditions. The first term will saturate at high intensities representing saturable absorption of the graphene.Only the intraband contribution can account for the thermal distribution of electrons around the Fermi level analytically, so numerical integration with I is above the saturation intensity Isat. It has been shown that the absorption of graphene decreases by I [Saturable absorption has been modelled nonperturbatively as it gives more accurate values when the intensity e by 1/I . Note the by 1/I , 36. Howe by 1/I , 36 we fσ3 as simply σ3. The expression of the third order conductivity of graphene is long and complicated, but is given in [Graphene as a material has a significant Kerr coefficient. The Kerr effect is strongly dependent on the Fermi level and comes from the third order surface conductivity. Since we are only interested in the Kerr effect, and not four-wave mixing we refer to given in  and is ggiven in  to obtaiτcar.We restate 4Due to the Pauli exclusion principle, it is Fermi level relative to half the photon energy that primarily determines conductivity. If the Fermi level is below half the photon energy, an electron can be excited from the valence band to the conduction band. However, if the Fermi level is above half the photon energy, an electron cannot be excited because the state in the conduction band is filled. In this way, the optical behavior of graphene is switched on and off by changing the Fermi level. The temperature and electron scattering rates effectively blur the states between on and off by introducing a probability whether the state is filled or not. They primarily only have an effect when the Fermi level is roughly equal to half the photon energy. The change in absorption is the dominant effect overall, but the dispersion effects also change. The change in the effective refractive index of the ring is minor due to the small portion of light in the graphene compared to the rest of the waveguide. Graphene as a material has a strong Kerr effect, however in this device its overall effect is negligible compared to the Kerr effect in silicon. This is primarily because there is far more silicon in the microring compared to graphene. Additionally, the electric field is stronger in the middle of the waveguide and weaker at the surface where graphene sits. The absorption of graphene is still the dominant effect, and dominates over the linear and two-photon absorption loss in the ring when it is on. This is important, because it controls the overall photon lifetime in the ring. In order for self-pulsation to occur, the photon lifetime and the free carrier lifetime must be roughly the same order of magnitude. When they are not the same order of magnitude, the ring decays to a constant steady state.The self-pulsation behavior comes from the oscillations of the free carrier concentration. The nonlinear behavior of the graphene has very little to do with the self-pulsation. In fact, the saturable absorption makes switching more difficult at higher light intensities because the change in absorption is smaller. The electrical tunability of the linear absorption of graphene merely controls the photon lifetime in the ring. When the Fermi level is low, the absorption is high, and light is absorbed before it has the chance to generate a significant number of free carriers. When the Fermi level is high, the absorption is low, and light is allowed to generate free carriers. The free carriers begin to dominate the absorption and dispersion effects. As free carriers increase, the absorption increases and the ring is pushed further away from resonance see . This ca5We have discussed the electrical tunability of the linear and nonlinear optical effects in graphene. This paper has described how graphene on a silicon microring can be modelled in coupled mode theory. The optical properties of graphene were shown to change the behavior of the microring and were electrically tunable. We have demonstrated that this tunability can be utilized to switch the microring from stable state to a self-pulsating state. This effect is fundamentally due to Pauli blocking and disappearance of the interband conductivity at high Fermi energies. The self-pulsation behavior is due to the oscillations of the free carrier concentration in the microring. The frequency of the pulses was roughly 7 GHz and could be slightly tuned by adjusting the pump power. This device used graphene primarily for its electrically tunable absorption to effectively switch from a low-intensity linear regime to a high intensity nonlinear regime. In principle, this could be used in other devices to electrically activate other nonlinear behaviors.