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.


The digital twin concept lays the foundation of the virtual vibrational analytics suggested in the current paper. The latter presents extended virtual experiments aimed at determining the specific features of the optical spectra of the studied molecules that provide reliable express analysis of the body spatial structure and chemical content. Reduced graphene oxide was selected as the virtual experiment goal. A set of nanosize necklaced graphene molecules, based on the same graphene domain but differing by the necklace contents, were selected as the relevant DTs. As shown, the Raman spectra signatures contained information concerning the spatial structure of the graphene domains, while the molecule necklaces were responsible for the IR spectra. Suggested sets of general frequency kits facilitate the detailed chemical analysis. Express analysis of a shungite carbon, composed of rGO basic structural units, revealed the high ability of the approach. This term refers to a very wide class of new molecular formations called necklaced graphene molecules (NGMs), presenting nanometer-sized graphene domains surrounded with heteroatom necklaces of various compositions. The number of corresponding rGO compositions is countless, differing both in the shape and size of the graphene domain and in the content of the heteroatomic necklace. The name ‘necklaced graphene molecules’ is common to all of this richness and better reflects its principle personality than rGO, which was only attributed to a chemically synthesized product.It is unlikely that there is at least one person today working in the field of materials science who would not know the widely advertised high-tech material called reduced graphene oxide (rGO). Its “discovery” and a base surge of popularity over the past 10–15 years have been facilitated by the special euphoria of a new direction in materials science-graphenics. Nevertheless, humanity, without realizing it, is familiar with the body from the first bonfire, from the first shovel of coal thrown into the furnace, from the first filler of wheel tires, from the first … and this listing can be continued indefinitely. The ash from a burnt fire, the deposits of natural coal, synthetic black carbon, and, finally, a laboratory product of the chemical reduction of oxidized nanoscale graphite—this is a short list of these unique materials, which are known as The graphene era revealed the wide demand for rGO for various applications, which immediately required the formulation and implementation of a reliable analysis of this material, which exists only in solid form. The structure and chemical composition, as a fundamental principle, and the chemical and physical properties as its consequences, became the basis for the development of appropriate analytics. The complexity of the analyzed object and the lack of standard structural and compositional features have inevitably led to the multi-method nature of the required analysis. The diffraction of X-rays and thermal neutron flux, as a source of information about the components of the structure of a solid; thermal gravimetric analysis, as the first cut of the atomic composition in gross as well as in the detection of the presence of hydrogen in the samples; chemical elemental analysis for the presence of carbon, hydrogen, nitrogen, and sulfur atoms; X-ray photoelectron spectroscopic (XPS) analysis for the presence of carbon, oxygen, and atoms of other heavy elements; and energy dispersive spectroscopic analysis of the distribution of these atoms in the mass of a solid all together allowed us to proceed to the stage of creating models of the atomic structure of the sample. Next comes the final analytic stage—verification and approval or rejection of the proposed models. At this stage, the main attention is given to spectral methods of investigation, the primary place among which is occupied by the vibrational spectroscopy of IR absorption and the combinational scattering of light. The latter was discovered independently by Mandelstam and Landsberg in the USSR  and Ramasp2 amorphous carbons [virtual vibrational analytics (VVA) applied to complicated carbon materials such as rGO. The VVA fundamentals and facilities are considered in the current paper. Digital twins (DTs) and a proper computational code constitute the VVA core. The former, as in the previous cases [sp2 amorphous carbon with experimental analytics [The recent development of virtual vibrational spectrometry (VVS) ,11,12,13nalytics ,5,6, onlgeneral frequency (GF) concept. The latter is a particular spectral signature of a selected individual covalent bond observed in the corresponding IR and Raman spectra [general frequency kits (GFKs), attributed to sets of similar covalent bonds. A particular feature of the study concerns a simultaneous consideration of both the density of vibrations and the IR spectra of the relevant DTs, which forms the grounds of the wished spectral marks. A successive result of such an approach is illustrated by the example of a VVA consideration of a shungite carbon.The analytic ability of the vibrational spectroscopy of multiatomic molecules, once presented as sets of covalent bonds, is traditionally based on the  spectra ,15. SuccThe rest of this paper is organized as follows. A general description of the VVS technique as well as the formation of DTs are given in VVS is a chain of successive steps leading to digitalized vibrational analytics according to a general scheme.Here, DTs are the molecular models under study, a virtual device is a carrier of the selected software, and the IT product covers a large set of computational results related to the DTs under different actions in light of the software explored. In the current study, the virtual device HF Specrodyn, implementing the software CLUSTER-Z1 , provide−1 if not pointed otherwise. The intensities are reported in arbitrary units, normalized per maximum values within each spectrum. Since the number of vibrational modes composing the spectra under consideration is too large, the excessive fine structure, statistically suppressed in practice, is covered by trendlines averaged over either 50 or 100 points of linear dispersion of 0.003 ÷ 0.010 cm−1 each. The procedure was applied to the plots of the density of vibrations and the Raman scattering spectra.Throughout the paper, the virtual spectra are presented by stick-bars convoluted with a Gaussian bandwidth of 10 cm66 (DT I) but surrounded with different necklaces. The first group involved compositions with necklaces outside the graphene domain, namely C66H22 (DT II), C66O22 (DT III), C66(COOH)20 (DT IV), and C66(OH)22 (DT V). The former were composed of densely packed monoatomic hydrogen, forming a complete framing of the domain with sp2C–H methines. The second concerned monoatomic oxygen, providing the domain framing with carbonyls sp2C=O. The next two DTs were related to necklaces consisting of individual carbonyls sp2C–COOH and hydroxyls sp2C–OH. All of the models corresponded to the maximal dense packing. These DTs were supplemented with a similar set of compositions whose necklaces consisted of only four units each, C66O4 (DT VI), C66(COOH)4 (DT VII), and C66(OH)4 (DT VIII), respectively. The second group of DTs resulted from the incorporation of oxygen atoms among the carbon edges of the domain, thus substituting the latter and forming necklaces as groups of esters, lactones, and other compositions, namely, C60O6 (DT IX), C56O10 (DT X), C70O6 (DT XI), C56O14 (DT XII), C62O8 (DT XIII), and C60O12 (DT XIV). Equilibrium structures of the species are presented in Unlike the usual attitude toward models for computational spectroscopy, the design of DTs was not subject to the requirement of achieving the best agreement with the experiment. The task of the DT concept is to identify the main including the so far hidden patterns that are characteristic of a given object. The achievement of this result is ensured by the wide variability of the model modifications. Following this trend, two groups of DTs simulating the rGO of different spatial structure and chemical contents were selected for the study. These were NGMs based on the only  graphene domain Csp2C–C bond pool of the graphene domains only. The second concerns the dependence of the spectra on structural factors such as the size and shape of the NGMs, on one hand, and the molecule packing, on the other. As it turned out, the IR spectra of the NGMs were practically insensitive to the geometrical factors, but readily responded to changes in the chemical compositions of the necklaces. Conversely, the Raman spectra were highly dependent on the former factor due to the extreme lability of the graphene domain bond structure [I) on the background of its density of vibrations (DOV). The domain was a flat sheet that was 1.2 × 1.1 nm2 in size. As shown by the DOV trendline, the vibration spectrum was clearly divided into two parts from 20 to 1100 cm−1 and from 1100 to 1800 cm−1, respectively. This two-part character of the vibration spectrum of NGMs also remained in the crystalline state [A comprehensive VVS analysis of IR absorption and the Raman scattering spectra of bare graphene domains has only recently been conducted. . In lightructure ,25, but ne state ,26,27.sp2C–C–C bending, benzenoid ring breathing, and sp2C–C–C puckering (1100–700 cm−1), in-plane and out-of-plane sp2C–C–C bending (700–400 cm−1) accompanied with low-frequency breathings of the graphene domain as a whole. In contrast, the region of 1100–1800 cm−1 was related to a large set of sp2C–C stretching. As seen in the figure, Raman scattering mainly involved the latter with a particular favor toward high-frequency ones. These modes also formed in the IR absorption spectrum of the domain [According to the fundamental consideration of the vibrational spectrum of the benzene molecule , the fire domain . Both spe domain . The sime domain .I with not only the presence of a complicated necklace, but with a larger size of its graphene domain , that is, 2.2 × 2.0 nm2. As seen in the figure, the DT Raman spectrum drastically changed, presenting the transformation of a broad multiband structure toward a single-band one. Such a transformation of the Raman spectra was also repeated for the DTs involving bare and hydrogenated  graphene domains and became more pronounced in the case of the  graphene ones [sp2C–H stretching.The Raman spectrum in ene ones . The matene ones ,31. In f crystal ,27, the sp2 amorphous carbon structures, the linear dimensions of the domains were smaller than Aside from the domain size, two more factors come into play. The first is due to the high tendency of graphene domains to stacking. Indeed, the single-layer graphene sheets observed in a number of experiments were only of crystalline structures, the micron linear dimensions of which far exceeded all of the known characteristic dimensional parameters of the graphene crystal quasiparticles, so that their behavior obeys the solid-state laws. Natural packing of the sheets unambiguously led to the sample graphitization. In contrast, in almost all of the 3spC–C bond of 1.53 Å, so that the unpaired electrons of carbon atoms in the domain basal planes of neighboring layers cannot be paired. However, as has been shown theoretically [3spC–C bond can be considered broken when the interatomic distance is greater than or equal to 2.5 Å. Later on, the existence of “long” single 3spC–C bonds was forced to be admitted experimentally [sp2 hybridization. At the same time, the vibrations of carbon atoms are not limited in space, which is supported by the occurrence of special out-of-plane phonons in a multilayer graphene crystal, the existence of which significantly changes the form of its Raman spectrum [3dynamic spC–C bonds between the domain layers, supported with the displacement of carbon atoms between the layers toward each other [sp2C–C stretching caused by the presence of oxygen atoms in the molecule carbon core [sp2 amorphous carbons into a sign marking the stack structure of the substance. In fact, the doublet mark of the sp2 amorphous carbons disappeared when their BSU stacks were completely destroyed [Indeed, the distance between adjacent layers in graphite-like structures is close to the van der Waals diameter of a carbon atom and constitutes 3.35–3.50 Å [etically , a singlmentally . Howeverspectrum ,35,36,37ch other . Furtherbon core . Additiobon core ,35,36,37estroyed . On the sp2 amorphics [sp2C–C bonds of their graphene domains. In the case of DTs related to the considered NGMs [sp2C–C stretching −1 because of the tight interconnection between sp2C–C stretching in the DOV shown in sp2C–C bond structure, the less As shown by a detailed analysis of the Raman spectra of an extended set of morphics , the forred NGMs ,13, the brations . This feonfirmed .sp2 amorphics composed of BSUs with predetermined size [As for  domains ,43. A st domains ,45. Howened size  did not Concluding the description of the main characteristics of the vibrational spectra of graphene domains, I–V against the background of their DOVs. The set was opened by the reference spectra related to the bare  graphene domain DT I. As seen in the figure, the DOV spectra of all of the derivatives strongly retained the common two-part structure, definitely separating the high-frequency stretching from compositions of the deformational modes. In all the spectra, this division occurred at 1100 cm−1. The main changes in the DOVs concern the stretching part. A detailed comparison of the DT DOVs with the reference one can be traced in We started from the general overlook of the virtual spectra of the NGMs forming the DTs of the first group. Presented in sp2C–C bonds of the pattern graphene domains, caused by the corresponding necklaces, on one hand, as well as the appearance of additional heteroatom covalent bonds, on the other. As for the first fact, it is important to note that even the necklace, consisting of only one hydrogen atom, disturbed the distribution of the sp2C–C covalent bonds of the domains quite significantly [sp2 amorphous carbons in practice [As seen in ficantly . Despitepractice .sp2C–H modes provided the spectrum shape. When the necklace included only oxygen atoms  graphene domain caused by the derivatization. As seen in the figure, the hydrogenation concerned all of the vibrational modes. In turn, the plot in II makes it possible to use the reference data in the table belonging to the benzene molecule. As seen in the table, the suggested GFK was well-coherent with the reference data and correlated well with experimental data related to the NGMs, for which the methine-unit content of the necklaces was firmly established [The plot in ablished ,24,46.3spC–H2, 3spC–CH2, and 3spC–CH3. The HF Spectrodyn allows for the reliable consideration of all of the cases, based on the  graphene domain DTs, as was, say, in the case of 3spC–CH3 [Obviously, in practice, hydrogeneous necklaces of NGMs are much richer and may involve a variety of hydrogen-containing groups such as sp3C–CH3 . In its sp3C–CH3 ,20 offerOxygen is the second most frequent element present in carbon materials. Its atoms combine willingly to carbon ones, forming a wide spectrum of organic functionalities from carboxylic acids to ketones and ethers. sp2 amorphous carbons, which were thoroughly spectroscopically studied [I–V is given in III–V were substantially emptied to four units in each case. The corresponding DTs VI, VII, and VIII are shown in −1, in general. At the same time, the observed similarity of the spectra allows us to suggest quite reliable GFKs with the above-mentioned accuracy.As described in  studied ,8,24. A To achieve this goal, we needed reliable reference data similar to those of the benzene molecule discussed in the connection of the GFKs listed in III–V were treated in the manner described previously for the necklaced graphene hydrides in the previous section. The The calculated data related to DTs III is composed of monoatomic oxygens, the addition of which fully inhibits the dangling bonds of the edge carbon atoms. The circumference composition is extremely similar to that of polycarbonyls , sp2C–C (1500–1600 cm−1), and sp2C=O stretching (1800–1900 cm−1). Accordingly, stretching dominated in the IR spectrum, among which the sp2C–O–C ones covered the largest frequency area, sp2C–C ones presented carbon atom vibrations stimulated by the presence of chemically added oxygens [sp2C=O ones were the characteristic signature of carbonyls.The necklace of graphene oxide DT yls  graphene domain, form the basis of the second group of DTs. Their equilibrium structures are presented in XI provided the simplest IR spectrum, revealing mainly two types of vibrational modes related to the sp2C–O–C and sp2C=O stretching. It should be noted that the latter were characterized by 200 cm−1 less frequency in comparison with that of the DTs of the first group. This mode feature was repeatedly observed when acid anhydride and lactone OCCUs were present in the molecules [XII or pure lactones, located at the zigzag (DT XIII) and armchair (DT XIV) edges. All of the discussed peculiarities are presented in Acid anhydrides of DT olecules ,22,47,48In the present work, we abandoned the desire for a virtual substantiation of the experimental spectrum of certain molecules and focused on revealing the characteristic spectral characteristics inherent in the chosen class of molecules. The digital twin concept assists in achieving this goal. The concept lays the foundation of the virtual representations of physical objects or processes along with the connections of data and information that tie the two together ,51,52. ITo illustrate the possibilities of VVA, in this work, practical graphenics was chosen as an area that occupies one of the central places in modern materials science. Methodical approaches and the levels of analysis of the obtained results were presented for an important high-tech material such as reduced graphene oxide, thus demonstrating that VVA can already today be attributed to modern analytical methods of a new type.sp2 amorphous carbons, the DTs used in this work were molecular models constructed by taking into account the entire body of knowledge acquired to date concerning these solids. The central place in the information obtained is occupied by a clear idea of the multilevel structure of these solids, which is based on nanosized quasi-molecular formations, or BSUs, which are graphene domains, necklaced with heteroatoms that terminate dangling bonds of the domains edge atoms. The choice of these BSUs as DTs and the possibility of a wide variation in the size and chemical composition of their necklaces is the first fact, which ensures the successful implementation of the VVA. The second element of success is the correct choice of a virtual device that allows for a large number of calculations to be performed at the modern level of the molecular theory of the object under study. In this case, BSUs are stable polyatomic radicals, or molecules with open shells, which determine the use of the UHF version of the HF Spectrodyn as an effective device. A comparative analysis of a large set of calculated data obtained was the completion of the digital cycle. As a result of extensive virtual experiments carried out using two dozen DTs, started in a previous work [Since rGO belongs to the class of real ous work  and comp1.The carbon atoms of the graphene domains of the BSUs and heteroatoms of their necklaces participate in the optical vibrational spectra in different ways: the vibrations of the former form the Raman spectra of the BSUs, while the IR spectra are assigned to the vibrations of the heteroatoms of the necklaces.2.sp2 amorphics of various origins. Being multiband in the case of individual BSUs, the Raman spectrum accepts a standard D–G doublet view when packing the BSUs into stacks. In this case, the width of the bands depends on the linear size of the BSUs themselves and the thickness of the stacks. It sharply decreases with an increase in these parameters as their size approaches or exceeds the critical value determined by the mean free path of the optical phonons The carbon-domain nature of the Raman spectra underlies their unique uniform appearance for a wide range of 3.sp2 amorphous carbon, accommodating numerous types of rGOs. For example, the typical Raman spectrum of shungite carbon shown in These conclusions make it possible to carry out an express analysis of the structure of any 4.sp2 carbons strongly depend on the chemical content of the necklaces of their BSUs [The IR spectra of amorphous eir BSUs . It has 5.−1 blue shift of the former, on average.The DTs considered in the work, which are close in size to empirical BSUs, consist of a hundred atoms, due to which eigenvectors of harmonic vibrations of the molecules involve many hundreds of components. This makes the attribution of the considered vibrational mode to individual atomic pairs, or particular chemical bonds, practically impossible. However, the empirical IR spectra of amorphics retain a certain similarity to the spectra of small molecules, which allows us to speak about the selection of atomic compositions in the realization process of the absorption spectrum. However, if in the case of small molecules such selection leads to the formation of an extensive pool of GFs corresponding to individual covalent bonds, in the present case, we can speak about GFKs, implying the excitation of a set of bonds. In the present work, by changing the composition of the necklaces of the considered DTs in a given way, we succeeded in revealing the GFKs that are related to the composition of the BSU necklaces of different rGOs. A comparison of calculated data with the reference ones revealed a ~250 cm6.sp2 amorphous carbon based on the shape of its IR absorption spectrum. Thus, the IR spectrum of a shungite carbon in sp2C–H groups and sp2C=O carbonyls. There were no hydroxyls in the necklace, while a final conclusion concerning the presence of carboxyl groups, although extremely improbable [As listed in probable , should The given express analysis of the Raman and IR absorption spectra of an rGO sample is only the beginning of the in-depth analysis and is given as an example. Nevertheless, they provide a reliable entry level of analysis, requiring further confirmation using all of the advanced methods of analytical chemistry and spectroscopy .


Moreover, the graphene–TPP photodetector has wavelength and angle selectivity, which can be applied in LiDAR detecting, sun sensors, laser beacon tracking, and navigational instruments in the future.Recently, two-dimensional materials have attracted attention owing to their special optical characteristics and miniaturization, with low thickness as well as extremely high responsivity. Additionally, Tamm plasmon polariton (TPP) resonance can be observed by combining a metal film and a one-dimensional (1D) photonic crystal (PC), where an electric field confinement is located at the metal–1D PC interface. In this study, a graphene layer combined with a TPP is proposed as a wavelength- and angle-selective photodetector. The graphene layer is located where the strong field confinement occurs, and the photocurrent response is significantly enhanced with increasing absorption by over four times (from 62.5 μA⋅W Light detection and ranging (LiDAR) is a widely used technology that emerged after the invention of the laser in the 1960s , artificTwo-dimensional (2D) material with atomic-scale thicknesses, such as transition-metal dichalcogenides (TMDCs), black phosphorus, and graphene, have been introduced in a variety of applications ,13,14, iThe PV , photothQ-factor) and strong field confinement located at the DBR-metal interface. Compared with SPPs, TPPs can be directly excited without any coupler or prism; likewise, the structure of TPPs is more easily fabricated by an evaporation system than that of SPPs because of its layer-by-layer structure. Moreover, TPP resonance can be adjusted by changing either the thickness of the dielectric layers of the DBR or the incident angle of the light source, which is a promising utilization for narrow band filters [Plasmonic resonance is a well-known plasmon state that includes SPPs. It creates strong localization of electric fields and has been extensively studied for various applications, including nanoantennas , metasur filters , thermal filters ,48, and  filters ,50.In this study, an idea was developed for a graphene photodetecting device employing the TP structure. As graphene was transferred onto the DBR, a metal film was fabricated on the graphene layer. The proposed graphene–TPP photodetector was devised and fashioned using gold electrodes to collect electrical signals. The graphene layer does not affect the TPP resonance thanks to the atomic layer thickness of graphene; however, the graphene locates the localized electric field formed by the TP structure. The structure converts incident light into TPPs that are collected and transferred into photocurrents by the graphene layer. The photocurrent generation mechanism of our device is related to the PV effect. In our proposed graphene–TPP photodetector, the photoresponse of graphene can be enhanced by the localized electric field, and the device facilitates angle and wavelength selectivity by means of the TP structure. Our device can be utilized as a miniaturized LiDAR detector for suitable applications, lowering the processing cost, and optimizing the processing yield.3)3 solution (33 wt.%) at room temperature (23–26 °C) for approximately 12 h. Subsequently, the stack was cleaned for 1 h with deionized (DI) water to wash off the residual etchant. After the floating stack became etchant-free, it was transferred to the target substrate, which was treated with UV-Ozone prior to the transfer for achieving better quality graphene. The UV-Ozone treatment enhanced the surface energy, which decreased the contact angle, resulting in a free adjustment of the stack on the substrate. Following the transfer process, the sample was left to dry in an oven at room temperature until complete dehydration was achieved. The sample was immersed in acetone to remove the photoresist and subsequently rinsed with isopropanol (IPA).Single-layer graphene grown on a Cu foil using the chemical vapor deposition technique was transferred using the wet chemical method. Initially, the graphene was covered with a thin layer of photoresist (PMMA-A4) by spin coating. The graphene/photoresist stack was removed after the Cu foil was etched away in a Fe(NOTwo graphene–TPP photodetecting devices were designed to prove that our device could work in both the near-infrared (NIR) and visible light regions. In 2, refractive index n = 1.475) layers is approximately 144 nm, and that of all the titanium dioxide  layers is about 95 nm, giving rise to the central wavelength of the DBR at 850 nm, which could be considered as an NIR-region sample.  of the TPP resonance. The TP structure and excited TPP resonance were designed using the finite-element method in COMSOL Multiphysics . In this model, a plane-wave light source of 700–1000 nm obliquely illuminates the designed TP structure, and it is transverse magnetic (TM) polarization. The different simulated TPP resonance peaks are as shown in Tamm Plasmonic structures have been extensively investigated; we can finely tune the wavelength of the target resonance by changing the incident angle or the thickness of the top dielectric layer of the DBR, and we can also change the different metal films  to achieve a higher quality factor (R = photocurrent intensity (IPC)/Light source power (Plight)), the red solid curve in −1 when the incident light power is 0.24 mW. Although both the peaks from the simulation and experiments happen at the same angle of 50, it can be noticed that there is still little deviation between the red and green curves because the simulated TPP (green curve) owns a better-quality factor in resonance than the measured TPP (red curve). Next, we removed the DBR layer to create a W/O TPP sample as a control group (for a detailed schematic of the control group, please see As we have already discussed the angle selectivity in −1 when the power of the 517 nm incident light is 0.32 mW. We also prepared a W/O TPP sample as a control group (for a detailed schematic of the control group, please see The contribution of this work is the simplification of the structure of the photodetector device by stacking layers to form the plasmonic structure. The results confirm that our Graphene–TPP photodetector can be universally applied to various optoelectronic materials. With the field confinement located at the metal–DBR interface, the TPP signal can be detected and converted into electrical signals.−1 at an incident angle of 50°, as the absorption increased from 0% to 60%; hence, it can be noted as an angle-selective device. On the other hand, the responsivity in Device B was enhanced from 60 mA/W to 271 μA⋅W−1 at the wavelength of the light source of 517 nm, as the absorption increased from 15% to 65%; therefore, it can be considered as a wavelength-selective device. Additionally, two devices working in the visible and NIR regions were demonstrated, indicating that the graphene–TPP device can work in both regions.In conclusion, a Graphene–TPP photodetector device was developed. By illuminating the TP structure, a strong localized field was generated by the structure and detected by graphene, which converted to the photocurrent response. Different photocurrent responsivities were observed at different incident angles and light source wavelengths. The responsivity of the photodetector can be affected by the absorption of the structure. In Device A, the responsivity could be enhanced from approximately 0 mA/W to 330 μA⋅W2 and MoS2, and the wavelength and angle selectivity can be applied to next-generation miniature LiDAR detecting, sun sensors, and laser beacon tracking and navigational instruments.To enhance responsivity, the graphene layer in the developed device can be replaced by materials such as WS


N-methyl-2-pyrrolidone (NMP) is recognized as the most efficientmethod for both the exfoliation and dispersion of graphene. Unfortunately,NMP is neither sustainable nor suitable for up-scaling productiondue to its adverse impact on the environment. Here, we show the realpotential of green solvents by revealing the independent contributionsof their exfoliation efficiency and graphene dispersibility to thegraphene yield. By experimentally separating these two factors, wedemonstrate that the exfoliation efficiency of a given solvent isindependent of its dispersibility. Our studies revealed that isopropanolcan be used to exfoliate graphite as efficiently as NMP. Our findingis corroborated by the matching ratio between the polar and dispersiveenergies of graphite and that of the solvent surface tension. Thisdirect evidence of exfoliation efficiency and dispersibility of solventspaves the way to developing a deeper understanding of the real potentialof sustainable graphene manufacturing at a large scale.Achieving a sustainableproduction of pristine high-quality grapheneand other layered materials at a low cost is one of the bottlenecksthat needs to be overcome for reaching 2D material applications ata large scale. Liquid phase exfoliation in conjunction with  Deconvolution of the overall grapheneyield of green solventsinto exfoliation efficiency and dispersibility. This deconvolutionreveals the high exfoliation efficiency of green solvents. Forsuch applications, graphene is generally produced by simple, scalable,and inexpensive liquid phase exfoliation (LPE) techniques wherebygraphite crystals are either shear-mixed (SM) or ultrasonically exfoliatedin a solvent. In LPE, there are three key factors that influence andgovern the graphene yield: the (i) graphite quality, the (ii) exfoliationefficiency, and the (iii) dispersibility of the solvent. The qualityof graphite, defined by the graphite size, crystallinity, and impurities,greatly affects its wettability.7 The higher the graphite quality,the lower its wettability, and if a solvent cannot wet the graphite,it cannot penetrate between the graphite layers to peel them off.Intuitively, this means that large, highly crystalline graphite flakesare more difficult to exfoliate than smaller, less crystalline, anddefect-containing graphite powders. The exfoliation efficiency isdefined by the ability of the solvent to “peel off”individual layers of graphene from the graphite crystal. In contrastto this, the graphene dispersibility is the ability of the solventto form uniform and stable graphene dispersions over a given time.Typically, 1-methyl-2-pyrrolidone (NMP) or dimethylformamide (DMF)is used as a solvent in LPE since both are ideal for exfoliating graphiteand producing stable dispersions of graphene.8 However, their toxicity and high boiling points, for example, 153and 202 °C for DMF and NMP, respectively, are an issue in thecontext of sustainable graphene manufacturing, and therefore, alternativeLPE solutions must be sought out.14 High-boiling-point solvents are difficult to remove and result inmajor loss of the material, degradation of the material quality, andthe generation of toxic waste.15 Therefore, major significantefforts have gone into finding suitable and sustainable solvent alternatives,so-called green solvents.16 Green solvents areeco-friendly solvents with low boiling points, for example, <100°C, and low toxicity.17 Although previouswork has shown that green solvents can indeed be used to produce grapheneby means of LPE, it has not progressed much further. To enhance thegraphene yield in green solvent LPE (GS-LPE), which currently is ca.50–75% lower than in DMF or NMP,19additives suchas surfactants and/or dispersants  have been intensively explored.23 Unfortunately, additives remainon the graphene and are difficult to remove after exfoliation, andtherefore, the as-produced graphene is no longer pristine, which canaffect its properties and negatively influence its performance.24 In the pursuit of improving the graphene yieldof the GS-LPE process, solvent exchange methods have also been investigated,whereby graphite was first “pre-treated” and/or exfoliatedin NMP or DMF, followed by re-dispersion of the as-produced graphenein a green solvent.27 While the graphene yield could indeed be increasedthrough the solvent exchange, toxic NMP and DMF solvents are stillrequired for the exfoliation. This will contribute in developing anunderstanding of the actual role that the solvent plays in the LPEmechanism and how it specifically contributes toward the exfoliationprocess. The current evaluation method of the graphene yield in greensolvents is hindered by the limited dispersibility of the as-producedgraphene in green solvents. The low amount of collected (and detected)graphene gives the false impression that green solvents are unsuitablefor the efficient production of graphene. This study paves the wayfor the development of a set of post-exfoliation procedures for theenhancement of the graphene yield in green solvents through the optimizationof their dispersibility. This route opens up new opportunities toexplore the real potential of green solvents, thought of as inefficientin the production of graphene via LPE.Grapheneis widely investigated as the next generation materialfor application in flexible electronics,S1.1), we employed shear mixing LPE  in conjunction with distinctively differentgraphite materials , and their surface areas of 13.15(G50) and 1.88 m2/g (G150) were measured by the Brunauer–Emmet–Teller(BET) N2 absorption isotherms. The experimental methodsused for the exfoliation and re-dispersion of the exfoliated grapheneare detailed in S1.2. The characterizationtechniques used for the graphite and graphene materials are detailedin S2. Following the work by Coleman etal., it is generally accepted that a higher graphene yield can beachieved if the surface tension or solubility parameters of a solventare similar to that of graphene.28 This rule-of-thumbis typically employed to select the solvents for the LPE of layeredmaterials.29 The surface energy of idealgraphene, that is, atomically perfect graphene, is 68 mJ/m2.30 This value changes for defective grapheneand may vary for graphite due to, for example, graphite morphology,lateral size, defect density, edge defects, d-spacing,additional functional groups, and so forth.31 Wedeliberately selected graphite starting materials with different degreesof defect in our study (S3). G50 graphiteis more defective than G150, with a higher D peak at (1350 cm–1 Raman shift), as analyzed through Raman spectroscopy. Herein, we employed SM-LPEin a range of green solvents  while keeping shearmixing parameters constant (S1.2.1). Asthe shear mixing product usually consists of well-separated/-dispersedgraphene and graphite aggregates composed of stacked graphene andpartially exfoliated graphite . We compared theabsorptivity values for GR150 and GR50 in both NMP and D.I. water. Absorptivity values, ε,for GR150 and GR50 in NMP are used for the concentration calculationbased on the Lambert–Beer law due to the high graphene dispersibilityof NMP. ε for GR150 and GR50 are 1178 and 1589 mL mg–1 m–1, respectively. In most cases, the grapheneyield or exfoliation efficiency is correlated with the initial weightof graphite used for LPE, and it is generally accepted that the graphitesource is represented by its mass,33 whilethe graphite surface area is neglected or not considered to be important.For example, Lund et al. claimed that the concentration of grapheneproduced using LPE is independent on the size of the graphite source.34 Here, we demonstrate that the nature of thestarting graphite material matters because the exfoliation efficiencyand dispersibility are greatly affected by the crystallinity  and the size of the graphite crystal. Both theconventional GS-LPE .To study the green solvent exfoliationefficiency and dispersibilityof graphene independently, we performed SM-LPE in a range of greensolvents and then removed the solvent and re-dispersed the subsequentpowder in NMP (with high dispersibility) for analysis only, referredto as the NMP re-dispersion (NMP-R) from hereon. Here, NMP merelyserves as a standard dispersing medium, that is, tool, to investigatethe exfoliation efficiency and dispersibility, independently. Withthis approach, it is possible to compare the actual yield of graphenein green solvents 1d. For tatersee S2. Absoratersee S2. Absoratersee S2. AbsorFigure S5a). Interestingly, the exfoliation efficienciesof both, IPA and EtOH, exceed that of NMP for GR150, whereas for GR50,the exfoliation efficiencies of IPA and EA are comparable to thatof NMP , in which G150 and G50 were firstshear-mixed in NMP. The graphene-containing suspension was then filteredand washed with EtOH in order to remove any NMP residues, dried, andre-dispersed in selected green solvents. This evaluation explicitlyshows that IPA has the best graphene dispersibility for a green solventfor GR150, followed by the EtOH/D.I. water, whereas for GR50, EtOH/D.I.water possesses better dispersibility than IPA  are shown in Figure S5c,d.In conventional GS-LPE, the poor graphene dispersibilityof thegreen solvents (besides NMP) results in a considerably lower grapheneconcentration in the supernatant after centrifugation than the actualamount of graphene produced 2a,a1; thS7). This resulted in the enhancement of GR50yield by 16 times when re-dispersed in IPA (from 0.0005 to 0.0077mg/mL) and 60 times (from 0.0005 to 0.0298 mg/mL) when re-dispersedin EtOH/D.I. water (S8.1), without the need of compressing it intopellets—required for a conventional contact angle approach.The properties  of the testliquids used for the contact angle measurement are shown in Table S1. The contact angle obtained are usedfor the Owens–Wendt–Rabel and Kaelble (OWRK) plot . The interfacial contact angle between all tested solvents andG150 and G50 is lower than 90° (p/σd)36 The polar component, S8). The analysis showsthe difference in σp/σd betweenG150 and G50 and the solvent [Δσ(p/d)] . Δσ(p/d) forNMP is lower than that for green solvents while maintaining a highexfoliation efficiency, regardless of its large surface tension differencewith G150 and G50 graphite. Similarly, IPA shows low Δσ(p/d),which explains its high exfoliation efficiency, close to that of NMP.On the contrary, Ace and D.I. water show high Δσ(p/d),which causes the low exfoliation efficiency.IPA has the highest dispersibility and exfoliation efficiencyforGR150, that is, it produces the highest graphene yield 3a. For GI. water 3c, implyRK) plot 3d for ththan 90° 3e, confithan 90° 3d, whichFigure S8). The evaluation of ID/ID′37 for all exfoliated graphene samples showed that the type of defectsis similar to that of NMP-R graphene, except for that of the EA-exfoliatedGR150 (ID/IG) asit is confirmed to be a more accurate way to determine the latticedisorder within graphene.39 The G peak fwhm ishigher for GR50 exfoliated in all investigated solvents; hence, GR50has a higher lattice disorder than GR150  nor the number of layers (2D peak). Atomicforce microscopy (AFM) examination was used to determine the thicknessof GR150 and G50 exfoliated in IPA. The exfoliated GR150 shows a veryhomogeneous leaf-like structure with a lateral size close to the micronscale and with the thickness ranging from 5 to 9 layers  the solvent exfoliationefficiency and (ii) the solvent’s ability to disperse the exfoliatedgraphene. We systematically evaluated a wide range of green solventsand their mixtures together with the state-of-the-art NMP solvent,which is known as the best for LPE, as a control. Specifically, westudied the potential of NMP-R that we developed in assessing andquantifying the exfoliation efficiency of green solvents and the GS-Rin assessing the graphene dispersibility of green solvents. From thestudy, the IPA solvent has a high exfoliation efficiency toward theexfoliation of GR150 graphene, which is comparable to that of NMP(despite its poor graphene dispersibility), and EtOH/D.I. water providesthe highest GR50 yield mainly because of its good graphene dispersibility.We also verified the superior exfoliation efficiency of IPA by experimentallystudying the surface energy and surface tension between graphite andthe solvent using the Washburn method. By re-dispersing the grapheneexfoliated in the solvent with a high exfoliation efficiency but poordispersibility into a solvent with a relatively high dispersibility,the graphene concentration can be improved up to 16 times. Our NMP-Rand GS-R approaches can be applied to LPE on any type of layered materialin order to evaluate the potential of green solvents for obtainingthe highest possible yield of the material.