Metastatic non-small cell lung cancer (NSCLC) is a highly fatal and immunogenic malignancy. Although the immune system is known to recognize these tumor cells, one mechanism by which NSCLC can evade the immune system is via overexpression of programmed cell death ligand 1 (PD-L1). Recent clinical trials of PD-1 and PD-L1 inhibitors have returned promising clinical responses. Important for personalizing therapy, patients with higher intensity staining for PD-L1 on tumor biopsies responded better. Thus, there has been interest in using PD-L1 tumor expression as a criterion for patient selection. Currently available methods of screening involve invasive tumor biopsy, followed by histological grading of PD-L1 levels. Biopsies have a high risk of complications, and only allow sampling from limited tumor sections, which may not reflect overall tumor heterogeneity. Circulating tumor cell (CTC) PD-L1 levels could aid in screening patients, and could supplement tissue PD-L1 biopsy results by testing PD-L1 expression from disseminated tumor sites. Towards establishing CTCs as a screening tool, we developed a protocol to isolate CTCs at high purity and immunostain for PD-L1. Monitoring of PD-L1 expression on CTCs could be an additional biomarker for precision medicine that may help in determining response to immunotherapies. These APCs help activate cytotoxic T-cells, helper T-cells and natural killer cells. All of these components work in concert against tumor cells. However, many metastatic tumors such as in non-small cell lung cancer (NSCLC) have adopted methods to evade immune detection and/or clearance3. One of the recently discovered pathways that tumors use is the overexpression of programmed cell death ligand 1 (PD-L1). PD-L1 binds to PD-1 on T-cells and suppresses its activity5. Immunotherapy based on inhibition of PD-1 or PD-L1 represents a breakthrough in the treatment of advanced cancers, particularly lung, renal, and melanoma cancers. Although only a minority of patients have clinical response, those that do often have a durable and lasting response12.The immune response to cancer involves a complex network of cellular interactions. Antigen presenting cells (APCs) can recognize neoantigens from some immunogenic tumors13. Past studies of renal cancer show tumors with high PD-L1 levels respond, while those with low levels do not respond10. Based on these findings, a clinical trial for anti-PD1 (pembrolizumab) was conducted where NSCLC patients were screened based on expression levels of PD-L1 on the primary tumor14. Response rate, progression free survival and overall survival with PD-1 inhibitors were greater in tumors with high tumor PD-L1 expression11. Although current guidelines call for anti-PD1 therapies after failure of standard chemotherapy, these are rapidly changing and recent phase I trial data suggests that anti-PD1 therapies may be effective as first line therapy15. First line therapy is especially promising for patients with higher PD-L1 expression on their biopsies and is associated with improved responses7.Finding biomarker predictors of response to PD-L1 blockade has proven challenging. Some studies have suggested that the level of PD-L1 expression on the initial tumor tissue can predict positive response, while other studies suggest that the level of infiltrating CD8+ lymphocytes are the most significant predictors of response7. Biopsy of multiple sites or serial biopsies during treatment could address some of these issues, however it may not be feasible due to the invasiveness of the procedure and the potential risks to the patient. In this regard, PD-L1 expression on circulating tumor cells (CTCs) could aid in screening and monitoring patients16. CTCs are tumor cells that are shed from various locations of the primary and/or metastatic tumors19. As such, they may represent a greater portion of the spectrum of genetic and epigenetic variability within a patient’s tumors 22. Most of these previous studies utilized specific surface markers for CTC capture, either with CellSearch or with similar magnetic bead technology, and did not isolate cells in a manner that is unbiased to surface expression. There are still other knowledge gaps, particularly in how PD-L1 expression on CTCs correlates with expression on tumor biopsies, what method to use for quantifying PD-L1 expression, on both CTCs and tumor biopsies, and how PD-L1 expression on CTCs varies, both at time of initial treatment and as therapy continues.There have been few studies to explore PD-L1 expression on CTCs, either in breast cancer (15) or in bladder cancerHere, we evaluate the PD-L1 expression on 31 CTC-containing samples, obtained from 22 patients with metastatic NSCLC who were scheduled to receive or were receiving PD-1 or PD-L1 inhibitors, including 11 metastatic NSCLC patients scheduled to receive the anti-PD-1 treatment pembrolizumab (one patient ended up receiving erlotinib) Fig. , Table 1This study included 32 volunteers for blood donation, 22 metastatic NSCLC patients and 10 healthy donors . All patients provided informed consent for participation in this study. After obtaining informed consent, 6–10 cc of blood were collected from each patient in EDTA tubes. Collected samples were processed to isolate CTCs within four hours of collection. Blood samples from healthy volunteers (n = 10) of various ages were similarly processed. All methods were performed in accordance with the relevant UCLA IRB guidelines and regulations.23 , H1703 (squamous carcinoma) and H3255 (adenocarcinoma) were cultured in RPMI media supplemented with 10% FBS and 1% pen/strep. Hela cells were cultured with DMEM media supplemented with 10% FBS and 1% pen/strep. At 70% confluence, cells were harvested using 0.25% trypsin and fixed with 2% paraformaldehyde. White blood cells (WBC) were isolated from healthy blood using RBC lysis buffer (eBioscience). WBCs were similarly fixed with 2% paraformaldehyde. During each staining experiment, an aliquot of each of these fixed cell solutions was stained in the same well-plate alongside with the CTC samples for normalization.23. All CTC and WBC counts were checked by two independent reviewers  for 10 min, permeabilized with 0.4% v/v Triton X-100  for 7 min, blocked with 5% goat serum (Invitrogen) for 30 min. To identify CTCs, cells were labelled for 40 minutes at 37 °C with 4,6-diamidino-2-phenylindole (DAPI) (Life Technologies), anti CD45-phycoerythrin , and a cocktail of primary antibodies to identify cytokeratin (CK) positive cells . To identify PD-L1 levels, cells were also stained with anti-PDL1 antibody (ProSci Inc), A secondary antibody labeled with Alexa Fluor 647 was used as the fluorescent reporter for the PD-L1 antibody. One set of cells consisting of A549, H1703 and healthy white blood cells (WBCs) were stained along with each patient sample. These additional cells were necessary to normalize for the staining process, antibody batch, and microscope conditions over the length of the study and report fluorescent intensities that could be compared between CTCs from many samples. The staining protocol for PDL1 was optimized to positively stain lung cancer cell lines  or PD-L1 positive. The quantity of PD-L1 expression was further categorized as either low , medium (0.4–0.7), or high (>0.7) as defined in the cutoff values in Fig. In order to quantify PD-L1 expression on CTCs, we developed a semi-automated imaging algorithm using a custom MATLAB script. The script was used to quantify the cell sizes and normalized fluorescence levels for each cell from each patient sample . The intensity signal from each cell was categorized as negative for PD-L1 expression (intensity between 0–0.04) or as positive (0.04–1). Positive cells were further categorized into low (intensity between 0.04–0.1), medium (0.1–0.2) and high levels (0.2–1), as indicated by the cutoff values shown in Fig. 26. Patients were categorized by the overall best response to treatment . The association between CTC and PD-L1+ counts with progression free survival was performed using Cox proportional-hazards models. In order to quantify the effects of interest, hazard ratios (HR) along with 95% confidence intervals (CI) were estimated by the model. All statistical analyses were performed using R V3.1.2  and IBM SPSS V23 . P values less than 0.05 were considered significant.We categorized patients by number of CTCs (<1.32 CTCs/mL or ≥1.32 CTCs/mL) and obtained clinical information  and immune related response criteria (irRC) based on imaging scan data when availableWe first confirmed that CTCs could be enriched from metastatic NSCLC patients with Vortex technology. A total of 22 patients were enrolled in this study, with 31 samples collected, processed with Vortex technology and enumerated for CTCs. 30/31 (96.8%) samples had at least 1 CTC, 15/31 (48.4%) samples at least 10 CTCs, with CTC total counts ranging from 0.1 to 9.67 CTCs/mL of blood Table . BesidesWe then developed a method for quantifying PD-L1 expression on lung cancer cells using immunofluorescence staining. To identify the optimal primary and secondary antibody concentrations, we utilized HeLa cells as a positive control and RBCs as a negative staining control for PD-L1 (Lee) We tested three commercially available PD-L1 antibodies  and determined that the ProSci antibody had the most intense specific staining while maintaining the least non-specific staining , and H3255 (squamous cell carcinoma) to develop and validate the PD-L1 fluorescence immunostaining protocol and quantification algorithms. The H1703 line was found to have the highest overall expression of PD-L1 while H3255 had minimal to no PD-L1 expression . Tumor biopsies prior to treatment were only available for 4 patients from the 22 patients in this study. For these 4, thin sections of tumor were cut from the paraffin block, stained and the resulting PD-L1 levels quantified as described above. Sample images for negative, low, medium, and high PD-L1 staining are shown in Fig. As PD-L1 expression is detectable on CTCs, we wanted to determine whether its detection can be an adjunct predictor for response to PD-1/PD-L1 inhibition. We determined progression using the irRC criteria. To assess the impact of overall CTC count and PD-L1 expression, we counted all CTCs and the subset that were PD-L1 positive Table . We limiAlthough for this initial study, we did not serially collect blood in all patients, we did so for two patients  are correlated with patient response, there is still a need for other ancillary and non-invasive biomarkers that can be used to monitor response to ultimately help guide the treatment course. CTCs have recently gained momentum as a non-invasive liquid biopsy for monitoring of treatment response and as a source of genetic material to understand treatment failures. Although the analysis of circulating cell-free tumor DNA (ctDNA) is also being explored, especially in identifying the presence of known druggable mutations, such an approach is not suited for identifying phenotypic changes such as levels of PD-L1 on tumor cells. There are no known consistent genetic lesions associated with up-regulated expression of PD-L1. Also, a variety of other cell types express PD-L1 (for example macrophages13) and could release protein, extracellular vesicles, or mRNA into the blood stream. CTCs are ideally suited to characterize PD-L1 expression through a non-invasive liquid biopsy in that they are short-lived markers of the active invasive tumors, with holistic phenotypic and proteomic information.Although immunotherapy represents a breakthrough in the treatment of selected cancers, only a fraction of patients respond. In metastatic NSCLC, the overall response rate is approximately 20%This study represents one of the first to examine PD-L1 expression on NSCLC CTCs in the context of anti-PD1/PD-L1 treatment. Some of the newly approved checkpoint inhibitors require a companion diagnostic indicative of tumor positivity for PD-L1 prior to administration . So, in this pilot study, we wished to develop algorithms to quantify PD-L1 expression on CTCs and tumor biopsies respectively and secondly, explore whether CTC PD-L1 expression was correlated with tumor expression. Although PD-L1 expression alone was not predictive of progression free survival, we did note several potential trends in CTC count and PD-L1 expression. Even given a limited sample size, we did note that for patients with >1.32 CTCs/mL, those with >50% PD-L1 + had improved overall response (3 of 4 patients), though most of these patients were also PD-L1+ in their tumor biopsies. Heterogeneity of PD-L1 levels could indicate intra-tumoral or intra-patient heterogeneity, as each of these patients had multiple metastatic sites and the CTCs could break off from any or multiple of these sites.Nevertheless, the trends we find here are suggestive that PD-L1 status on CTCs may track that of tumor tissue and that this may be a useful correlate in helping to assess potential for response to immunotherapy. Our findings indicate that PD-L1 expression on CTCs can be quantified as an adjunct biomarker. This work also suggests that serial monitoring of patient overall CTC count and PD-L1 + CTC count may help to identify changes in response to treatment, and these cells may be a readily obtainable source to understand tumor evolution, work that needs to now be confirmed with larger prospective studies. Future work will examine the time course of CTCs and their PD-L1 levels during immunotherapy treatment as well as how changes in CTC count and PD-L1 expression may correlate with overall response.One limitation of this study or any CTC study in general is that it can be difficult to draw associations with few cells. In many cases, CTCs are present prior to treatment but their numbers can be limited, due in part to previous chemotherapeutic treatment or the fundamental patient-to-patient heterogeneity of tumor behavior. This fundamental issue is exacerbated by practical challenges with the limited number of patient samples that are available from patients on clinical trials for new therapies.27 can be tuned based on the cancer type known to be present.CTC PD-L1 quantified levels, when combined with the tumor biopsy results, could aid in identifying patients more likely to respond to therapy or likely to have become resistant to treatment when tracking levels over time. This study indicates that continued work is warranted to analyze and compare more CTC samples from patients on anti-PD-1 pathway treatment to more robustly determine how PD-L1 expression correlates to tumor levels, fluctuates in response to treatment, and is predictive of response. The methods we describe here are potentially applicable to any tumor type and to potentially any treatment course, as the size-based approach does not exclude cells on the basis of presence or absence of surface markers and the size selectivity criteriaSupplementary Figures


Charge density fluctuations can be measured with orders of magnitude higher precision than previously accomplished using the G-band shift with charge. The two 2D intrinsic peaks can be associated with the “inner” and “outer” Raman scattering processes, with the counterintuitive assignment of the phonon closer to the K point in the KM direction (outer process) as the higher energy peak. Even low charge screening lifts the phonon Kohn anomaly near the K point for graphene encapsulated in hBN, and shifts the dominant intensity from the lower to the higher energy peak.Pristine graphene encapsulated in hexagonal boron nitride has transport properties rivalling suspended graphene, while being protected from contamination and mechanical damage. For high quality devices, it is important to avoid and monitor accidental doping and charge fluctuations. The 2D Raman double peak in intrinsic graphene can be used to optically determine charge density, with decreasing peak split corresponding to increasing charge density. We find strong correlations between the  The 2D Raman mode is of particular interest, as it involves two D band phonons with opposite, non-zero momenta. Despite the higher order process and non-zero momentum phonons, the 2D mode is the strongest Raman mode in single layer graphene. The double resonance (DR) mechanism2 explains why this higher order phonon mode is so strong; resonant virtual electron and hole scattering picks phonon q vectors that satisfy momentum and energy conservation. The DR process together with the strong K-point Kohn anomaly is why the 2D line provides so much information about the electronic and phonon dispersions around the K and K’ points. The phonon Kohn anomaly near K softens the D band phonon energies conically near K, resulting in a discontinuity of the derivative of the dispersion at K3. The slope of the phonon dispersion is a direct measure of the electron-phonon coupling (EPC) strength3. The DR and phonon dispersion are responsible for the shift in the 2D band energy with changing laser excitation energy4. Similarly, the strong dependence of the 2D mode on dielectric screening is readily observable by the shift in the 2D position for different environments, e.g., suspended graphene, graphene on SiO2, graphene on top of hexagonal boron nitride (hBN) or encapsulated in hBN, etc5. Larger dielectric screening6 will reduce the Fermi velocity, vF, so that the DR conditions will select larger phonon energy. The screening affects vF since electron energies are much higher than expected from a single particle picture, with a significant self-energy correction in a 2D material, in particular for free standing graphene near the charge neutrality point (CNP)9. The self-energy is decreasing as ε−1, where ε is the effective dielectric constant of the environment6. Charging the graphene layer will also renormalize and reduce the self-energy and therefore vF. For example, a study of gated graphene on hexagonal boron nitride (hBN) examined with scanning tunnelling spectroscopy demonstrated a change of vF from 1.3 to 1.05 × 106 m/s as the doping increased one order of magnitude from 2 × 1011 cm−2 to 2 × 1012 cm−2 9. Ultraclean suspended graphene with mobility reaching 106 cm2/Vs have shown vF reaching 3 × 106 m/s, measured by Shubnikov-de Haas oscillations7. Screening will also have an effect on the phonon dispersion, as it may decrease the strength of the EPC, and thereby weakening the Kohn anomaly at K, resulting in higher phonon energies with increased screening11. The reduced vF and reduced Kohn anomaly with screening are both expected to lead to a higher energy 2D peak. Hence, changes to the dispersion of the Dirac cone and the phonon branch can be experimentally deduced from the 2D peaks.Raman spectroscopy is a versatile experimental technique for exploring fundamental properties of graphene and characterizing properties such as layer thickness, defect density, charge and strain13. The vectors are found by translating the iso-electronic energy contour determined by half the laser energy at K, with a vector that puts the K iso-electronic energy contour tangentially in contact with the slightly smaller iso-energy contour (EL/2 − hcωD) at K’, shown in Fig. q vectors associated with the 2D Raman peak. While the DR Raman scattering is a two-dimensional process, the strongest contributions come from the high symmetry direction ΓK and KM, so that we can consider the one-dimensional case where electron (hole) scattering from ΓK to K’Γ followed by hole (electron) scattering back from K’Γ to ΓK are referred to as the outer process and the similar process between KM and MK’ as the inner process13, as shown in Fig. K was cylindrically symmetric, the 2D peak would yield two very different phonon frequencies from the inner and outer process since qi > qo where q is measured from the K point with cylindrical coordinates, shown in Fig. 12. Indeed, early in the study of graphene, a symmetric 2D peak for graphene on SiO2/Si was identified as a signature of a single layer14. Later studies showed that the intrinsic 2D line shape of single layer suspended graphene is asymmetric17, but reverts to a symmetric line shape when charge is added by field doping to 2 × 1011 cm−2, indistinguishable in shape from graphene on SiO218. Hence, we can deduce that there is not a perfect compensation of the phonon trigonal warping at low doping condition, and that the two peaks can reveal the behaviour of the inner and outer scattering processes, respectively. In addition, the energy upshift of the 2D peak is indicative of screening due to changes in the Fermi velocity, which has been pointed out previously20, but also due to changes in the phonon velocities12, addressed here.The DR method can be used for mapping out a contour of participating phonon wave-vectors21. The accidental doping in these samples is approximately ~1011 cm−2 or less as deduced from transport measurements22, more than an order of magnitude lower than for graphene on SiO2. We find strong correlations between the 2D peak splitting and the relative peak intensities and linewidths. The decrease of the 2D2 peak energy with charge screening is opposite to expectations. It follows from a crossover, shown in Fig. 11. This is a consequence of screening the Kohn anomaly at K. One has to carefully consider the ratio of the inner and outer Fermi velocities with the ratio of the inner and outer phonon velocities before determining if 2D1 is an inner or outer process. Here we present evidence to suggest that the lower energy 2D1 peak originates from the inner process, based on the calculated stronger intensity from the inner process in the low doping regime12, and linewidth behaviour. The analysis below is based on the assumption that increasing the charge also increases the screening. This assumption is supported by the reduction in Fermi velocity with increasing charge level, especially close to the charge neutrality point, attributed to screening the self-energy correction of the electronic energy dispersion9.Here we are considering the effect of very low doping levels in ultra-clean graphene, dielectrically screened by encapsulation in hBN. Encapsulated graphene possesses many of the superior transport properties of suspended graphene with the added advantage of being mechanically more robust and easier to fabricate2D1 and 2D2 can be used as a measure of charge doping two orders of magnitude lower than is possible to measure using the G band response to doping24.On a practical level, the peak split between 2D1 (blue), and the higher energy peak is denoted 2D2 (black). The 2D peaks measured at different locations have different 2D1 and 2D2 peak separations, linewidths, energies and intensities as shown in Fig. Raman data is collected from a graphene sample encapsulated in hBN. Examples of 2D spectra (λ = 532 nm) are shown in Fig. 25. Figure 2D1 (blue symbols) and 2D2 (black symbols) versus the corresponding G peak position where the different data sets are boxed with their respective laser colour. The red laser selects shorter q vectors through the DR mechanism and therefore has a significantly lower 2D energy than the green laser, as expected. Both data sets exhibit strain lines, indicating some compression26. We first consider Fig. 2D1 peak has a well-defined strain-induced slope of 1.7 ± 0.13, very close to the calculated value for uniaxial strain/compression in a random lattice direction26. The strain variation across the sample is on the order of ±0.1% which is small enough not to cause strain-induced peak-splitting27. The presence of the small strain variation will increase the linewidth only marginally26. The higher energy peak 2D2 does not fall along a single, well defined line. Instead, the data falls mainly in two groups: a lower 2D2− vs G, and a higher 2D2+ vs G. Despite there being two 2D2 groups, there is only one 2D1 line. (Supplementary Information (SI) Fig. 2D2− vs G and 2D2+ vs G are 2.2 ± 0.3 and 3.0 ± 0.4 respectively. The variation and range of the strain slopes matches earlier studies of graphene/hBN sandwiches28. For reference, 2D1 and 2D2 values for λ = 532 nm for freely suspended graphene has been included in Fig. 18.In order to evaluate the range of 2D spectra across the sample, we plot all the data measured on the same sample for two laser lines, λ = 532 nm and λ = 633 nm, in a 2D versus G diagram which reveals the contribution to the peak position due to strain or charge densityper se, but rather the effect of low charge doping on the graphene Raman peaks. The charge present is much lower than can be evaluated with help of a 2D versus G plot25, or by the G band linewidth or peak position24  to the right. Figure 2D1 and 2D2 band versus peak splitting, and Fig. 2D1 and 2D2 evolve as the splitting decreases. Figure Here we are not interested in the strain 11 cm−2 based on the back-gate voltage at the CNP. The charge variation is 0.4 × 1011 cm−2 given the residual conductivity at the CNP (see supporting information section 2). Comparing with our Raman data, which has a mean and standard deviation of −8.1 and 3.6 cm−1 for the 2D peak splitting, we find that for each cm−1 of 2D peak splitting, the doping changes by 0.11 × 1011 cm−2 making the 2D peak splitting a very sensitive probe of local charge density. Extrapolating linearly, we also find from this relationship that the CNP corresponds to a 2D peak splitting of 20.4 cm−1 (see supporting information section 3). While we know the mean and standard deviations for the doping in the sample used for transport measurements with high precision, and we know the mean and standard deviations for the 2D peak splitting for the sample used for Raman measurements, there is sample-to-sample variation which introduces uncertainty in our values for the slope of the linear correspondence and 2D peak splitting at CNP. We estimate a roughly 5% sample-to-sample variation, which translates into uncertainty in charge per wavenumber of 0.11 × 1011 ± 0.02 × 1011 cm−2 per cm−1, and 20.4 ± 1.1 cm−1 for the 2D peak splitting at the CNP.We establish a quantitative relationship between 2D peak splitting and doping by assuming a linear relationship between split and charge, and comparing means and standard deviations between the two. Transport measurements on a similarly prepared sample show that the mean doping level is 1.4 × 102/Si produces optical excitations of defect transitions in hBN and charge transfer to the graphene. In fact, light exposure during gating quickly returns the graphene to the charge neutrality point, undoing the gate doping29. The sign of the charge is also undetermined, but recent measurements30 on ultraclean samples have shown a symmetric electron and hole blue-shift of the 2D peak (single peak fitting) for charge density up to 2 × 1012 cm−2. Hence, the 2D peak response for holes and electron doping is indistinguishable on the Raman spectra at such low charge density. Early gated Raman measurements31 on graphene on SiO2 showed a weak difference between electron vs hole doping below 1012 cm−2 in contrast to the newer easurements30. This illustrates the need for excellent charge homogeneity to observe the low charge density effects achieved on suspended or hBN encapsulated graphene.While it would be advantageous to use electrostatic field gating to confirm, attempts to control the charge level via back-gating samples for optical measurements are complicated due to light-induced charge screening of the gate voltage for graphene on hBN. This is because light illumination on a gated graphene/hBN sample on SiO15, see Fig. q-dependent scattering matrix elements of the phonon intensity have shown that the inner process dominates despite the stronger vector nesting for the outer process13. Our results show the 2D1 peak with the strongest intensity at the lowest doping level. A higher 2D1 intensity has also been observed experimentally previously on both suspended18 and hBN encapsulated17 graphene. Based on the observed and predicted higher intensity for the inner process for pristine graphene, we tentatively assign the 2D1 peak as originating from the inner process, and show below how this assignment is supported by the results. Figure qi is larger than the outer vector qo, but the inner KΓ phonon dispersion is flatter than the steeper outer KM phonon branch32, as illustrated schematically in Fig. 2D1 as the inner process.Generally, a high density of vector nesting creates a stronger Raman peak. This would indicate that the outer process, with flatter parts of the trigonal warping of the iso-energy contour matched by the phonon contour, has a stronger contribution to the Raman spectra than the inner process2D1 to 2D2 with increased charge screening may be an effect of changing wave vectors and phonon energies that will affect both the scattering matrix terms in the numerator, as well as the denominator of the Raman intensity scattering expression12, which will change the relative strengths of the 2D1 and 2D2 peak due to quantum interference. Regardless of the cause for the intensity behaviour of the two 2D peaks, it is clear that added charge to graphene in hBN changes the dominant intensity from the 2D1 peak (inner) to the 2D2 peak (outer) with increasing charge.Intensity behaviour as a function of charge has not yet been explored theoretically. We suggest that the observed switch in intensity from 2D1 and 2D2 components and their linewidths is the ratio of the phonon velocity vph to the Fermi velocity vF for the inner and outer process. Using the DR mechanism and the simplifying assumption of perfectly linear electronic and phonon dispersions around the K and K’ points (but with different slopes for the inner and outer directions)13, and using vph ≪ vF, we have−1) at the K point (q = 0), γ is the imaginary part of the electronic self energy (eV), h is Planck’s constant, and c is the speed of light. The subscripts i and o refer to the inner and outer phonons, respectively. Equations 33, ∂ω/∂EL, of the 2D1 and 2D2 band for “pristine” and charged graphene and use Eqn.  of the inner branch, as expected11.It is counterintuitive that the rge Fig. , since iqi Fig. . The red2D1 and 2D2 are not indicative of the phonon lifetime, but rather determined by the electronic lifetime (imaginary part of the self-energy) and ratio of the phonon to electron velocity. However equation δqi,o. Phonons that fall within the blurred contour make a significant contribution to the 2D peak, and we will regard them as participatory phonons, and the blurred line as the participatory contour. The middle of the participatory contour is mapped almost perfectly to the triangularly warped iso-energy phonon contour with the “corners” pointing towards Γ rather than M. The width of the participatory contour in the KΓ direction (inner), δqi, has been calculated12 to be broader than the width in KM direction (outer), δqo, which provides a method for assigning 2D peaks to inner and outer processes. Using equations  The electronic self-energy is momentum dependent, and the momenta that contribute to the 2D1 and 2D2 peaks are different. Our observations are then consistent with an electronic self-energy that increases more greatly with charge for momenta that contribute to the 2D2 peak than momenta that contribute to the 2D1 peak. 2) The Fermi velocity decreases with charge less dramatically along the direction of the 2D2 process than the 2D1 process. If this is the case, then the 2D2 phonon velocity reduces by smaller amount than the 2D1 phonon velocity, and will result in a 2D2 linewidth that increases more rapidly with charge.Charge affects the 2D linewidth through two primary mechanisms: scattering and phonon velocity renormalization. Eqn.  shows thses Fig. . It is g2D1, has the highest intensity. The screening of the K-point Kohn anomaly with increasing charge has not yet been addressed experimentally on suspended graphene. A significant difference is the substantial decrease in the peak split at the CNP for λ = 633 nm, not observed for suspended graphene where the 2D peak-split stays nearly constant (~13 ± 3 cm−1) over a laser energy range from 1.5–2.7 eV18. This indicates that the dielectrically screened graphene is less well fit with a linear model than suspended graphene: The peak split EL, Berciaud et al.18 dismissed the theory of the peak split originating from the difference in the inner and outer 2D phonons. The phonon linewidth is expected to decrease with lower laser energy, since γeh ∝ EL, and Berciaud et al.18 observed an increasing linewidth for laser line energies below ~1.7 eV for suspended graphene. We note that it is possible that the predicted increase in phonon dispersion slope closer to the K point11, probed with lower laser energies, dominates over the decrease in electronic linewidth, γeh.The results presented here on hBN encapsulated graphene have both similarities and differences to suspended graphene. Both types of samples exhibit a decrease in the 2D double peak split with increased charge, and the lower energy peak, io, Eqn. , which iK-point Kohn anomaly with screening observed here is a sign of the weakened EPC3. Excitation of D band phonons by electrons is a fundamental bottleneck for ballistic transport at high fields35. Hence, it is possible that encapsulated graphene with low and uniform charge screening could have better transport properties at high fields than suspended graphene if the screening sufficiently reduces the strength of the EPC of the D phonons, without introducing other scattering mechanisms.The lifting of the 2D1 and 2D2 peak separation removes influence of strain, and reveals strong correlations between charge, peak intensities, peak positions and linewidths. The peak split can be used to estimate the charge density with sensitivity ~ 1010 cm−2 per cm−1 2D peak split, an improvement of two orders of magnitude compared to use of the G band Kohn anomaly at the Γ point. Hence, the 2D peak split could be very informative for detecting low amounts of charge, with the caveat that the peak split with charge behaviour is known for the particular substrate screening and laser line used. The method also reveals information about the origin of the double peak and the effect of increased charge, for example the intensity shifts from the inner to the outer phonon with increased charge. We associate the lower energy 2D1 peak with the so called inner process, and the higher energy 2D2 peak with the outer process. Even the low amount of charge puddling found in these samples is enough to significantly alter the strength of the D band Kohn anomaly at the K point.In conclusion, we have used Raman spectroscopy of graphene encapsulated in hBN to explore the intrinsic double peaked 2D phonon behaviour under low charge screening. The analysis of the 2D data as a function of 21. This particular sample did not have functioning edge contacts, but similar samples have a room temperature mobility of ~3.2 × 104 cm2/Vs.The encapsulated graphene sample was fabricated at Columbia University by Carlos Forsythe with their pick-and-place method that avoids direct contact between the pick-up polymer and graphene layer or the accompanying hBN surfaces that contact the grapheneEL = 2.33 eV) and red laser with wavelength of 633 nm (EL = 1.96 eV), are applied at 1 µm by 1 µm spacing with a laser spot of approximately 1µm2 with typical spectra shown in Fig. The collected 2D Raman spectral map is fitted with two Voigt peak functions at all different locations on the hBN encapsulated graphene. Different incident laser photon energies, green laser with wavelength of 532 nm (The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.Supplementary Info


However, the most important advantage of the introduced metallic alloy-decorated graphene is its distinct stability in acid medium; the retention in the electrocatalytic performance after 1,000 successive cycles is approximately 98%. This finding is attributed to the high corrosion resistance of the NiFe alloy. The kinetic study indicates that the number of the transferred electrons is 3.46 and 3.89 for the introduced and Pt/C (20 wt%) electrodes, respectively which concludes a high activity for the proposed nanocomposite. The suggested decorated graphene can be synthesized using a multi-thermal method. Typically, nickel acetate, iron acetate, graphene oxide and urea are subjected to MW heating. Then, sintering with melamine in an Argon atmosphere at 750 °C is required to produce the final electrocatalyst. Overall, the introduced NiFe@ N-doped Gr nanocomposite shows remarkable electrochemical activity in the acid medium with long-term stability.NiFe nanoparticles-decorated & N-doped graphene is introduced as an effective and stable non-precious electrocatalyst for ORR in the acid medium. Compared to conventional Pt/C electrodes under the same conditions, the proposed nanocatalyst shows closer onset potential and current density. Typically, the observed onset potentials and current densities for the synthesized and Pt/C electrodes are 825 and 910 mV (vs. NHE) and −3.65 and −4.31 mA.cm Undoubtedly, the confirmed near depletion of fossil oils is forcing the researchers to find effective, low cost, and environmentally safe energy devices. In addition to this dilemma, the serious negative impact to the climate promotes the elimination of the fossil fuels utilization. Flexibility, ease of management, and applicability make fuel cells the most promising energy devices to overcome the aforementioned problems. For example, wide utilization of the proton exchange fuel cells (PEMFCs) was one of the closing remarks of the 2015 Paris Climate Conference (COP21).1. Although there are numerous efforts to develop new membranes for the alkaline media fuel cells2, commercially, the acidic solutions-based PEMFCs are prevailing due to the excellent performance of the Nafion membrane. However, working in an acid medium limits the selection of the electrode materials as most of the metals show low corrosion resistance to acids. Accordingly, precious metals  became inevitable choices for manufacturing the commercial electrode materials3. In acid media, platinum (Pt) is the most effective catalyst for both hydrogen oxidation and oxygen reduction5. In addition to Pt, palladium-based compounds also have considerable activity7. Though, the high cost and rarity of the precious metal electrodes are constraining the wide application of the PEMFCs8.The main constituents of any PEMFC are anode, cathode, and membrane. Typically, oxygen is reduced at the surface of an active cathode while the fuel  is oxidized at the anode surface9. Recent research has indicated that bimetallic nanoparticles containing a first-row transition metal possess magnetic, optical, composition-dependent and catalytic properties that are different from mono metallic nanoparticle components10.Disregarding the stability, many non-precious materials have shown good performance as anodes11 and CNT/graphene mixture12), supporting of effective bimetallic nanoparticles on a proper N-doped carbonaceous material can distinctly enhance the ORR electrocatalytic activity13. Besides enhancing the electrocatalytic activity, N-doped carbon nanostructural supports showed more stability than nitrogen-free ones due to the high number of surface nucleation sites, which allows for anchorage and high dispersion of the catalyst nanoparticles on surface of the support material16. Moreover, due to the strong electron donor behavior of nitrogen, the doping process improves the durability of the produced carbon-support catalysts because of enhancement of π bonding18 and their basic properties19.Considering that the most effective non-precious ORR catalysts are primarily nitrogen-doped nanocarbons (e.g. N-doped carbon nanotubes (CNT)2 g−1), strong mechanical strength, and excellent electrical conductivity. Consequently, it was exploited to enhance the performance of several promising electrode materials in the electrochemical devices22. Moreover, compared to other carbon nanostructures, the chemical route for graphene synthesis provides a good chance for functionalization by active groups which aids in decoration surface by metallic nanoparticles9.Graphene is a charming carbon material with excellent characteristics including a large theoretical surface area  Exploiting the alloy structure of the transition metals to enhance the stability in the acid media. (2) Maximizing the nitrogen content in the carbonaceous support to strength the catalyst activity toward the ORR. (3) Synthesis of the graphene sheets supports by the chemical route to develop chemical anchors for the transition metals nanoparticles which strongly improves the attachment and consequently ameliorates the activity of the final catalyst. Overall, the obtained results indicated that the proposed nanocomposite have very good electroactivity toward ORR with distinct stability in the acid media.25. However, after the violent oxidation, the graphite peak disappears and a new diffraction peak appears at 2θ of 10.5° 25. On the other hand, due to the reduction process, the reduced graphene oxide shows a broad peak that can be fitted using a Lorentzian function into three peaks, which are centered at 2θ = 20.17°, 23,78° and 25.88°, which correspond to the interlayer distances of 4.47, 3.82 and 3.53 Å, respectively. These XRD results are related to the exfoliation and reduction processes of GO and the processes of removing intercalated water molecules and the oxide groups27. The observed broaden peak indicates the smaller crystalline size of graphene in the single-layer or few-layer structure. Accordingly, from left inset , iron and nickel are identified by the standard peaks at 2θ of 44.67°, 65.02° and 82.33°, and 44.05°, 54.85° and 76.37° corresponding to (110), (200) and (211), and (111), (200) and (220), respectively which indicates that some nanoparticles have a physical Fe/Ni mixture. Additionally, no peaks attributed to oxides or carbides were detected. According to the utilized characterizations which confirmed formation of pure Fe and Ni and considering the difficulty of evaporation of these metals due to the high melting points , the weight of metals in the final produce can be estimated. Moreover, it was reported that calcination of the graphene oxide prepared by a similar chemical route to 750 °C leads to lose around 60 wt%29. Consequently, the Ni:Fe:C ratio in the produced composite can be determined to be 41:40:19 wt%, respectively.Accordingly, the prepared GO was used to prepare the proposed FeNi-decorated & N-doped graphene as it was explained in the experimental section. The XRD pattern of the synthesized composite is displayed in the left inset Fig. . The obs30.Although XRD is highly trustable analytical technique, its utilization is limited to the crystalline materials. Therefore, to investigate nitrogen doping, X-ray photoelectron spectroscopy (XPS) was exploited. The obtained XPS spectra Fig.  indicateFigure −1 scan rate and room temperature. The synthesized electrode reveals comparable performance with the precious metal electrode in the form of current density and onset potential. Based on previous reports, the onset potential is defined as the potential at which the background subtracted current density is equal to 0.1 mA.cm−2 32. The onset potential for the introduced decorated graphene was close to that of Pt/C ), Fig. 19, while the metallic nanoparticles have more adsorption capacity for the molecular oxygen. Delivery of electrons, from the anode to the cathode, leads to complete the oxygen-hydrogen combination to form water molecules; ORR reaction. From the chemistry point of view, the pyridinic nitrogen has more basic characteristic than pyrrolic because the nitrogen lone pair electrons do not share in the pyridine cycle resonance. However, in case in pyrrole, the lone pair electrons contribute in the cycle resonance which negatively affects the basic characteristic. Accordingly, it is expected that pyridinic nitrogen has more attraction capacity for the hydrogen ions that reflects more contribution in the ORR reaction. It is noteworthy mentioning that the expected adsorption influence of the utilized N-doped graphene support can partially enhance the electrocatalytic activity of the proposed composite. However, the main impact can be assigned to avoiding the agglomeration of unsupported bimetallic nanoparticles which strongly improves a very important parameter in the heterogeneous catalytic reactions; contact area between the reactants and the catalyst surface. Furthermore, the excellent electrical conductivity of the utilized support has also a regarded effect. The proposed mechanism of O2 reduction on the surface of the introduced catalyst is visualized in Fig. Doping the carbon nanostructures with heteroatoms such as N can distinctly change the properties of carbon. For instance, doping of carbon by nitrogen strongly enhances the oxidation resistance capability and its ORR catalytic activity. For example, doping of carbon nanofibers by nitrogen led to increase the onset potential of the oxygen reduction reaction by 70 mV with a corresponding electron transfer number of approximately 437. The stability of the introduced and the precious (Pt/C) electrodes was first investigated by cyclic voltammetry analysis for 1,000 successive cycles. Figure th and 850th cycles of the introduced FeNi-N-Gr  of 1.23 V is difficult to be obtained. Instead, the steady-state rest potential of the platinum electrode in the oxygen-saturated solutions is around 1.06 V, a mixed value of the thermodynamic potentials of Pt/PtO and On) in the ORR is another important factor. Rotating ring-disk electrode (RRDE) analysis is typically invoked to measure the number of the utilized electrons. This technique can give indication about the relative importance of the H2O2 routes in the overall oxygen reduction reaction process. However, in the literature, for the same material, several n values can be found. These different values of the number of transferred electrons can be attributed to the history of the electrocatalytic material which can distinctly affect the ORR rates. For instance, at the ring, in addition to the oxidation of hydrogen peroxides that generates an anodic current, decomposition of H2O2 without current flow may take place. Moreover, the Pt ring potential can influence the number of electrons, e.g., by varying the relative amount of the formed PtO, and also the synthesis conditions of the investigated material can play a strong role. Therefore, other techniques have been introduced to estimate the number of electrons, such as scanning electrochemical microscopy41 and cyclic voltammetry42. In the cyclic voltammetry-based procedure, the analysis is performed at different scan rates and the peak currents (Ip) increase linearly with the scan rate that is a typical characteristic of the reaction occurs on the surface of the electrode. The number of the electron can be estimated from the slope according to this equation:n is the number of the electron transfer, A is the electrode active area (0.073 × 10−4 m2), v is the scan rate (mV/s), and S is the concentration of the adsorbed oxygen on the electrode surface . From the Ip vs. v linear relation slope, the number of electrons n can be calculated. The voltammograms for the FeNi-N-Gr and Pt/C electrodes are introduced in the Fig. R2 = 0.998) for the FeNi-N-Gr electrode data. Accordingly, the number of electrons was determined to be 3.89 and 3.465 for Pt/C (linear regression is not shown) and the introduced electrode, respectively.In addition to the current density and onset potential, the number of sharing electrons , −4 M) at 20 °C, −9 m2.s−1), υ is the kinematic viscosity of the solution at 25 °C (1 × 10−6 m2.s−1). As can be distinguished from the value of the linear regression parameter (R2 = 0.994), the experimental results strongly validate the model, moreover the high linearity obtained suggests first-order of the reaction kinetics toward the concentration of dissolved oxygen43. The estimated number of electrons from eq. ; Fig. In summary, graphene sheets were decorated with FeNi nanoparticles by hydrothermal treatment of graphene oxide in the presence of nickel acetate and iron acetate. Moreover, the presence of urea in the reaction medium followed by calcination in an argon atmosphere leads to the incorporation of nitrogen atoms in the graphene skeleton. The produced FeNi-decorated and N-doped graphene can be exploited as a stable and effective electrocatalyst for the oxygen reduction reaction process in acid medium. The high performance was attributed to the good affinity of the metallic nanoparticles and nitrogen atoms to oxygen molecules and hydrogen ions, respectively. Moreover, the good stability in acidic media can be assigned to the alloy structure of the metallic nanoparticles.2SO4 (assay 95–97%); these chemicals were purchased from Sigma–Aldrich. All the used chemicals were utilized without further modification. DI water was used as a solvent.In the introduced decorated graphene, the used precursors for the metal nanoparticles were iron (II) acetate , and nickel acetate tetrahydrate . The reduced graphene oxide was prepared by chemical route using graphite powder (particle size <20 μm), hydrazine monohydrate, hydrogen peroxide, and H44. Briefly: treated twice by 5% HCl five grams of graphite was placed in ice bath concentrated H2SO4 (130 mL). Later on, 15 g of KMnO4 was added gradually to the mixture under zero Celsius temperature condition with stirring for 2 h. Then, distilled water was added to the mixture which results in increasing the temperature to 98 °C. After that the mixture was cold to room temperature, and H2O2  was added, the mixture was kept under stirring for 24 h. Later on, the synthesized GO was separated by filtration under vacuum and washed by10% aqueous HCl several time and then dried at 50 °C. NiFe alloy nanoparticle-decorated and N-doped graphene was synthesized from mixing 0.5 mM iron (II) acetate and 0.5 mM nickel (II) acetate tetrahydrate aqueous solutions with 250 mg of urea (as a source of nitrogen) and stirring for 2 h, followed by ultrasonication for 30 min. As maximizing the nitrogen content in the final product was an important target during the synthesis process, using of urea during preparation of the FeNi-decorated graphene was done (in the reflux step) to incorporate nitrogen atoms within the graphene cycles which effectively increases the nitrogen content during the thermal treatment with melamine45. It is worth mentioning that, based on our studies and others, acetate salt was chosen due to the complete reduction during calcination at relatively high temperature under inert atmosphere to produce the corresponding metals rather than the expected metal oxides47. In another beaker, two hundred mg of the prepared graphene oxide were treated in a microwave oven for two minutes at around 600 W to achieve the thermal exfoliation. The two solutions were mixed and the obtained slurry was then refluxed for twelve hours at 150 °C. The produced slurry was filtered, and the solid material was dried for one day at 80 °C under vacuum. The dried powder was ground with twice as much melamine and calcined under argon atmosphere at 1 atm for four hours at 750 °C. High calcination temperature was chosen to ensure complete reduction of graphene oxide to graphene and avoid formation of metal oxides46. Both urea and melamine were used as nitrogen precursors to enhance the nitrogen content in the final product.The graphene was prepared chemically from reduction of exfoliated graphene oxide (GO). Typically, GO was synthesized from natural graphite powder by a modified Hummer’s method2) of the glassy carbon electrode which was then subjected to drying process at 80 °C for 20 min. Cyclic voltammetry measurements were carried out in 0.5 M H2SO4 solution and the sweep potential range was adjusted from −0.2 to 1.0 V [vs. Ag/AgCl].Information about the phase and crystallinity was obtained by using Rigaku X-ray diffractometer  with Cu Kα (λ = 1.5406 Å) radiation over Bragg angle ranging from 10 to 80°. Normal and high resolution images were obtained with transmission electron microscope  operated at 200 kV equipped with EDX analysis. The electrochemical measurements were performed on a VersaSTAT 4 (USA) electrochemical analyzer and a conventional three-electrode electrochemical cell. A Pt wire and an Ag/AgCl electrode were used as the auxiliary and reference electrodes, respectively. All potentials were quoted regarding to the Ag/AgCl electrode. Glassy carbon electrode was used as working electrode. Preparation of the working electrode was carried out by mixing 2 mg of the functional material, 20 µL Nafion solution (5 wt%) and 400 µL isopropanol. The slurry was sonicated for 30 min at room temperature. 15 µL from the prepared slurry was poured on the active area (0.073 cm


We present Raman studies of graphene films grown on copper foil by atmospheric pressure CVD with n-decane as a precursor, a mixture of nitrogen and hydrogen as the carrier gas, under different hydrogen flow rates. A novel approach for the processing of the Raman spectroscopy data was employed. It was found that in particular cases, the various parameters of the Raman spectra can be assigned to fractions of the films with different thicknesses. In particular, such quantities as the full width at half maximum of the 2D peak and the position of the 2D graphene band were successfully applied for the elaborated approach. Both the G- and 2D-band positions of single layer fractions were blue-shifted, which could be associated with the nitrogen doping of studied films. The XPS study revealed the characteristics of incorporated nitrogen, which was found to have a binding energy around 402 eV. Moreover, based on the statistical analysis of spectral parameters and the observation of a G-resonance, the twisted nature of the double-layer fraction of graphene grown with a lower hydrogen feeding rate was demonstrated. The impact of the varied hydrogen flow rate on the structural properties of graphene and the nitrogen concentration is also discussed. Due to its high charge carrier mobility , it has e planes –6. For SThe controlled injection of defects, which cause strain, is an extra degree of freedom in addition to the number of layers. This may account for the unique properties of TG. In particular, it was shown that the strained TG bilayer could be an ideal platform for the realization of the high-temperature zero-field quantum valley Hall effect . From a 4) is the most common hydrocarbon used in the CVD process to grow graphene. However, the use of hydrocarbons other than CH4 compounds is a challenging task. The successful implementation may offer the possibility to tune the growth process and to pave the way to graphene synthesis with desirable parameters, such as type of defects and their concentration.TG can be obtained by different methods, e.g., by means of graphene folding, graphene layer stacking, thermal decomposition of SiC  or chemin-decane as a precursor in the presence of nitrogen flow.In this work, we investigate the experimental conditions at which the APCVD growth of large area, nitrogen-doped TG can be realized utilizing 2 sample was placed in the middle of the reactor. One side of the copper foil covered the inner wall of the reactor. The copper foil was annealed for one hour at 1050 °C in the presence of N2 and H2 gas flow at a rate of 100 and 150 cm3/min, respectively. The purity of the nitrogen gas was 99.95%. We utilized a commercial hydrogen generator (GVCh-12D) as a H2 source. The resulting purity of the hydrogen gas was 99.99%. The temperature was controlled by a thermocouple placed inside the heating block, next to the reactor wall.A custom-made APCVD set-up with a 14 mm diameter tubular quartz reactor was employed for the experiment. Polycrystalline copper foil  with a thickness of 60 μm was used as the catalyst. Prior to the APCVD, the foil was electrochemically polished for 5 min in 1 M phosphoric acid at a bias voltage of a 2.3 V. The 35 × 45 mmn-decane. In addition, in previous publications [The above experimental procedure has been previously reported in –14. The ications –14, the 2 and H2 gas flow with the rates of 100 and 60 cm3/min, respectively. Sample B was prepared under similar conditions, except the H2 gas flow rate was reduced to 6 cm3/min.In this article, we present the results related to two samples: sample A and sample B. Sample A was prepared at 1050 °C in the presence of Nn-decane (C10H22), a member of the homologous series of alkane hydrocarbons. n-Decane has a molecular mass approximately one order of magnitude greater than that of methane, which influences the growth kinetics of graphene. As a representative n-alkane, n-decane forms chains of radicals with a high reactive nature during thermal decomposition. This could stimulate numerous chemical reaction pathways and promote doping. In fact, CVD makes it possible to dope graphene by nitrogen in situ, which not only tolerates the ground state of graphene via additional electrons but also introduces a strain to graphene because of the difference in ionic radii [n-decane could lead to the decomposition of the nitrogen molecule, which in fact has one of the strongest binding energies. The resulting atomic nitrogen can be embedded into the graphene lattice.The APCVD depends essentially on the hydrocarbon precursor . To thisic radii . The radn-decane was introduced into the tubular quartz reactor via barbotage system for 30 min. The feeding rate of n-decane was estimated to be 4 μL/min (for both samples). Afterwards, the tubular quartz reactor was cooled at a rate of 50 °C/min in the presence of N2 gas flow. The obtained properties of samples A and B are typical for samples synthesized under similar conditions.The 2 forming gas annealing [2-based annealing [2(598 nm)/Si substrates without the use of a polymer support. This was performed in two steps. First, one side  of the copper foil was treated for 3 min in a solution of H2NO3 and H2O mixed in a volume ratio of 1:3, and then the copper foil was totally dissolved in a water solution of FeCl3. The graphene film was gently washed several times in a bath with distilled water prior to the transfer onto the substrate.The transfer of graphene from the original to the arbitrary substrate without deteriorating the crystallinity of the graphene is still a challenging task . Currentnnealing , O2-basennealing , and in nnealing ), deterinnealing . In this−1 for both spectrometers. A 3D scanning laser confocal Raman microscope (Confotec NR500) allowed for the acquisition of two kinds of images within a single scan: a Rayleigh image, using laser light reflected from a sample, and a spectral image by Raman scattering. More details about the laser beam size and spectra accumulation time are presented further in the text.The graphene samples were analyzed by Raman spectroscopy using the Nanofinder HE with 532 nm and 473 nm excitation wavelengths and a Confotec NR500 confocal micro-Raman spectrometer with 473 nm excitation wavelength. The spectral resolution was about 3 cmhν = 1486.6 eV) was used for X-ray photoelectron spectroscopy (XPS) measurements. The base pressure in the analytical chamber was lower than 2 × 10−7 Pa. The 20 eV and 40 eV pass energy values of a hemispherical electron energy analyzer were used for the survey and high resolution spectra acquisition, respectively. The energy scale of the system was calibrated with respect to Au 4f7/2, Ag 3d5/2 and Cu 2p3/2 peak positions. ESCALAB 250Xi Avantage software was used for the peak deconvolution and fitting procedure using a sum of Lorentzian–Gaussian (70:30) functions. The samples were analyzed as received and no surface cleaning procedure was applied. Finally, the transmittance was measured using the PROSCAN MC-121 spectrometer.A Thermo Scientific ESCALAB 250Xi spectrometer with monochromatic Al Kα radiation  was performed for the same sample area as in I2D/IG  utilizing the same laser beam conditions as for 473 nm excitation. By analogy with the previous Raman maps, the non-monotonic distribution of mapped values was observed. As an example, in the inset of ID/IG ratio is becoming a common method for the point defect concentration evaluation [In many aspects, the theory of Raman scattering in graphene is very well understood and the quantitative analysis of Raman spectra parameters provides sufficient information about structural and electrical properties of graphene. For example, the analysis of the line shape of the 2D band along with its spectral position can provide important information about the number of layers in graphene and the interlayer interaction . From thaluation –29.−1 and 67 cm−1, respectively. This suggests that within the same sample there are at least two graphene ”systems” with quite different numbers of layers. Indeed, the presence of two maxima in the 2D-position distribution  could be caused by a double layer structure of graphene [−1 . The obtained results are in reasonable agreement with each other. Therefore, in order to demonstrate the universality of the elaborated method, for sample B on a SiO2/Si substrate, the Raman data set splitting was based on another criterion, namely, the 2D-band position with a splitting value of 2712 cm−1 . The maxima of the G-band position distribution for the SLG fraction of sample A and B are centered at ≈1590 cm−1  and ≈158−1  [2C peak position is usually explained by the charge transfer phenomena that take place in the substrate–graphene system [Eb can be related to the difference in concentration of sp2C–H bonds which cause the down shift of the Eb[3C bonds [2C–N bonds [In the inset of  Pt(111) –33 to 28 Pt(111) –36. Thise system . Since bof the Eb. The ori3C bonds –40. Fina–N bonds –44. The –N bonds . A numbe–N bonds –38. TherEb = 399.7 eV and 402.1 eV (sample B) (Eb = 401.8 eV (sample A)). The peak at Eb = 399.7 eV observed for both samples is related to the adsorbed nitrogen. Its position was quantified by the measurement on the bare substrate area, see inset of Eb ≈ 402 eV is greater for sample A. We would like to emphasize that these observations are consistent with the Raman spectroscopy results, from which it follows that the G-band blue shift is greater for sample A.Finally, the quantified surface atomic concentrations determined by XPS for samples A and B are summarized in 15 cm−2, the average surface concentration of the nitrogen atoms in sample B can be evaluated as nN ≈ 1.5 × 1013 cm−2.The XPS measurements provide the atomic concentration of nitrogen in the studied samples, as shown in ID/IG provides information about the concentration of point defects n [On the other hand, the Raman integral intensity ratio efects n ,ID/IG ≈ 1, and the energy of the laser Elaser ≈ 2.33 eV, we get n ≈ 2.2 × 1011 cm−2.From the experimental results, nN value may be the following. The nitrogen atoms could be located in such a configuration that almost does not affect the ID value. In particular, the pyrrolic nitrogen due to the symmetry breaking of hexagon rings should have a strong impact on the intensity of the D band and cannot be considered as the main type of defect in the studied films.The reason for lower defect density as compared to the Now we focus on the Raman data associated with the double layer graphene. In For the domains corresponding to the spectra presented in a is the lattice parameter of graphene (2.46 Å),  is the reduced Planck’s constant, vF is the Fermi velocity in monolayer graphene (106 m/s) and Elaser is the laser energy. Within this approach we obtain θ = 13.4° and 11.9° for the laser excitation wavelengths of 473 and 532 nm, respectively. The estimated θ values mean that in the investigated domains, the layers of graphene could be electronically decoupled [where aphene 2.6 Å,  is The higher G band intensities were observed at different locations within the same probed area for different excitation wavelengths. This fact confirms that the presence of the G-resonance is a consequence of the twisted nature of the graphene. However, the ratio of integrated G-band intensities for the double layer TG and SGL is ≈60 and ≈30 for excitation wavelengths of 532 and 473 nm, respectively . In our It is worth highlighting that the above presented Raman mapping has been performed on different sections of samples (not shown here). One of the main results of this study is the uniformity of the samples, confirming their macroscopic homogeneity. For sample B we have provided sets of data where SLG fraction was significant (≈50%), which helps to evaluate and compare the structural properties of SLG and TG. However, for other sets of data, the fraction of double layer TG dominates.We also proved homogeneity directly by the light transmittance measurement of samples transferred on a glass substrate. The diameter of probed area was ≈0.5 cm. In Now we would like to discuss the two most prominent phenomena observed in our experiments: (i) the morphology change of the films with hydrogen flow rate and (ii) nitrogen atom incorporation into graphene, which we believe to be correlated phenomena. Nevertheless, as a first approach, we will discuss them separately.Let us underline the main features that distinguish the morphology of studied samples. Sample A is characterized by the presence of hexagonally shaped domains with a few and single layer graphene domains. In turn, sample B contains the mixture of single and double layer domains where the double layer fraction dominates. Our studies do not indicate the presence of any regularity in shape and distribution of SLG and double layer domains at the measured scale. Since the hydrogen flow rate value was the only technological parameter we have varied during the synthesis, we will focus on the impact of hydrogen on the graphene. First of all, the hydrogen partial pressure strongly affects the shape of the graphene edges. It was demonstrated that higher hydrogen pressure favors the hexagonal shape whereas low hydrogen pressure makes dendritic-like growth preferable –52. Our 10H22 decomposition there is enough hydrogen to terminate the graphene edges [The formation of few layer graphene requires formation of the additional graphene layer (layers) on the top or underneath of a SLG. These processes are usually described by wedding cake (WC) –54 and ine edges .2 gas was used as the nitrogen source during the CVD growth [2 gas as a nitrogen source arises from the fact that nitrogen molecule possesses one of the strongest bonds with an energy of 226 kcal/mol, which means that the temperature of the CVD process is not enough to decompose a significant amount of nitrogen molecules into atoms. The authors of the work [•C) or hydrocarbon (•CxHy) radicals may attack on nitrogen molecules [2 dissociation process. For example, for the •CH + N2 = N + •CHN reaction this value was determined as 21.2 ± 0.7 kcal/mol [n-decane just by thermal decomposition [Finally, we turn to the problem of nitrogen incorporation into graphene. Nitrogen can be incorporated into graphene sheet (i) in situ, using ammonia as a component of the gas carrier mixture  or with D growth . In our the work  did not olecules . The endkcal/mol . Moreoveposition n-decane decomposition at elevated temperature over copper catalyst is beyond the scope of our study. We can only reasonably assume that the different partial pressure of H2 in a gas mixture could favor some decomposition path. As a consequence, it enhances the concentration of radicals with specific configurations. Obviously, the enthalpy of the hydrocarbon reaction with N2 depends on the hydrocarbon radical configuration [However, the issue of the exact reaction route of guration . Therefo•CxHy + N2 reaction has been proposed.In summary, the graphene films have been studied by using the micro-Raman technique. The samples have been grown on copper foil by APCVD using n-decane as a precursor and a mixture of nitrogen and hydrogen as a carrier gas with the use of different hydrogen flow rates. A special analysis approach to the Raman data was employed based on the statistical analysis of spectral line parameters. This approach allowed the association of Raman spectra to fractions of the films with different thicknesses. Based on the values of the 2D peak FWHM and 2D band position, the double layer fraction of graphene grown with the lower hydrogen feeding rate has been established. Moreover, the analysis of the Raman spectra revealed the presence of graphene spots with the G-resonance for both excitation wavelengths used in our experiments (473 nm and 532 nm). The observation of the G-resonance directly confirms the twisted nature of graphene. The obtained blue shift of the G and 2D band positions of the SLG fractions is caused by nitrogen doping, which has been proved by the XPS study. The binding energy of incorporated nitrogen has been evaluated to be around 402 eV. The amount of the G-band shift for each sample is consistent with the XPS data. It has been found that at the wavelength of 550 nm the transmittance for the film grown with the lower hydrogen feeding rate is equal to 94%, which corresponds to 2–3 graphene layers. This is in good agreement with the micro-Raman findings. We suppose that the variation in the morphology is presumably related to the variation of the hydrogen flow in our experiments, as it has been also demonstrated by other authors. Finally, the possible mechanism of the nitrogen concentration incorporated in graphene based on a variation of the endothermicity of a Further work related to the mechanisms of the nitrogen doping and relation between transport properties of graphene and its microstructure is in progress now.


The as-synthesized composite exhibits excellent electrochemical properties by using the advantage of NCS nanontube core as superhighway for electron and ion transport, and CNS nanosheets shell as high active area pseudocapacitive material. The 3D graphene layer serves as excellent surface area to support 3D NCS/CNS; moreover, it provides excellent electrical conductivity between nickel foam current collector and the 3D NCS/NCS composite. Using these hybrid advantages the as-synthesized graphene/NCS/CNS composite electrode exhibits high areal capacitance of 15.6 F/cm2 at current density of 10 mA/cm2; excellent cycling stability of 93% after 5000 of cycles and excellent rate capability of 74.36% as current increase from 10 to 100 mA/cm2. Moreover, a prototype of asymmetric device fabricated using graphene/NCS/CNS as positive electrode and RGO as negative electrode exhibits high energy density of 23.9 Wh/kg and power density of 2460.6 W/kg at high operating current of 100 mA. Such high performance electrode material may get great application in future energy storage device.Three-dimensional (3D) core/shell structure of nickel cobalt sulfide is nano-engineered by using series of hydrothermal steps on a CVD grown graphene for supercapacitor application. This core/shell is composited of NiCo Based on energy storage mechanism supercapacitors are of two types, namely, electrical double-layer capacitors (EDLCs) in which energy storage is dominated by electrostatic charge diffusion and accumulation at the interface of the electrode and electrolyte; and pseudocapacitors in which energy storage is by reversible Faradaic reactions that occur at the electrode surface6.The advancement in use of portable electronic devices and high standards of living imposed the world to consume large amount of energy. So, to overcome this demand designing highly efficient energy storage device is an important issue. Among the energy storage devices, supercapacitors (SCs) are one of the promising devices due to their ultra-high power density, long cycling stability, wide range of operating temperature, and improved safety8. Different morphologies of ternary metal sulfides were investigated as high performance psudocapcitive materials 13. Even though ternary metal sulfides showed promising performance, practical application of ternary metal sulfides as electrode material is still limited because of two main challenges, namely, low rate performance at high current densities and short cycling stability.It is well known as the performance of SCs devices mainly depends on the properties of the materials used; and the key to achieve outstanding performance is to seek appropriate materials and designing suitable electrode architecture that support maximum efficiency of the proposed material. Recently, nanostructured nickel cobalt sulfides’ composites have attracted much research attention as pseudocapacitor electrode materials15. Recently, considerable effort has been devoted in synthesizing core/shell nanostructures for energy storage applications to increase the active area, cycling stability and rate capability of the device. For example, Kong et al.16 reported Core-shell NiCo2S4 nanostructures supported on nickel foam with capacitance value of 1.948 F/cm2 at 1 mA/cm2 and 79.36% of rate retention at 20 mA/cm2; Fu et al.17 reported Cobalt sulfide nanosheets coated on NiCo2S4 nanotube arrays as core/shell structure with highest capacitance of 4.74 F/cm2 at 5 mA/cm2 and rate capability of 47.67% at current density of 50 mA/cm2; similarly, Zhang et al.18 has reported nanoforest of hierarchical Co3O4@NiCo2O4 nanowire arrays synthesized via a facile strategy for electrochemical supercapacitors. Unfortunately, the reported structures still suffer from low capacitance, inferior rate performance and low utilization rate of active materials and low cycling performance at high currents. Even though electrode materials possessing high capacity, long cycling stability, and excellent rate capability are highly desirable, it is difficult to achieve all these objectives simultaneously from a single materials. So, hybridizing of different materials with different properties is becoming interesting research area19. Recently, hybridizing ternary metal sulfides with nanocarbon based material is becoming important research area. From nanocarbons, graphene is one of hot research area for energy storage application due to its unique properties, including high surface area, excellent conductivity and excellent mechanical and chemical stability which are desirable properties for SCs application21. In ternary metal sulfide/graphene hybrid composite, graphene serve as flexible high surface area to physically support pseudocapacitive ternary metal sulfides and excellent electron transport channels and resulted in improved capacitance, cycling stability and rate capability. Du et al.22 demonstrated facile synthesis and superior electrochemical performances of CoNi2S4/graphene nanocomposite suitable for supercapacitor electrodes with maximum specific capacitance of 2009.1 F/g at a discharge current density of 1 A/g; Wang et al.23 reported electrodeposited nickel cobalt sulfide nanosheet arrays on 3D graphene/nickel foam for high performance supercapacitors with good electrochemical performance; similarly, Patil et al.24 and Beka et al.25 reported the improvement in capacitance performance after hybridizing of ternary metal sulfides with graphene. Even though there are many works mentioned in the literature for ternary metal sulfide/graphene composite, the electrochemical performance is still in its infancy compared to the theoretical expectations. As a result, design of an integrated smart architecture ternary metal sulfide porous nanostructure that consists of 3D building blocks grown on 3D graphene is significantly desired, to use synergistic advantages of graphene and the ternary metal sulfides.In general to address these kinds of problems a wide range of nanomaterial design concepts have been developed such as decreasing the material features size to nanoscale with the structures such as core/shell, hallow particles and tubesHerein, we proposed a porous 3D nickel cobalt sulfide core/shell (NCS/CNS) nanostructure grown on 3D graphene decorated nickel foam for supercapacitor applications. Initially, the 3D graphene was grown on nickel foam using CVD process; then, using series of hydrothermal treatments 3D NCS/CNS core/shell is grown on the graphene layer. This hybrid composite uses the high surface area and excellent mechanical properties of graphene to support 3D porous NCS/CNS core/shell; moreover, the excellent conductivity of graphene leads to super electron transport channels between the collector and the active material. The 3D NCS/CNS core/shell nanocomposite contains numerous nanosheet shells which results in excellent porosity and high active area which are favorable for energy storage application. Using these synergistic effects of graphene and ternary metal sulfides the as-fabricated nanocomposite structure shows excellent specific capacitances with outstanding capacitance retention at high current densities and excellent cycling stability. To further evaluate our electrode material for practical application we assembled a prototype asymmetric device of 3D NCS/CNS core/shell/on graphene decorated nickel foam as positive electrode and RGO as negative electrode, and the assembled device receives an excellent electrochemical performance. The results obtained in this nanocomposite showed promising performance of hybrid structure of porous core/shell ternary metal sulfide and graphene for future energy storage applications.26 and labeled as GNF/NCS nanotube. Lastly, CNS porous nanosheets were grown on the NCS nanotube and the sample is labeled as GNF/NCS/CNS. Figure 27. However, the electronic and mechanical properties of graphene are strongly influenced by the thickness or number of layers of graphene present in the composite. As a result, the dimensional characterization of the as-synthesized graphene on nickel foam is crucial. So, to know the average thickness of our graphene we conducted Raman spectroscopy test on different places and almost the sample shows approximately any of the three graphs shown in Fig. 28.The fabrication process of our composite electrode is schematically represented in Fig. Figure 2S4 (JCPDS 43–1477). On the other hand, the crystal phase of the sample after the formation of shell is illustrated by using Fig. 3S2 (PDF # 24–0334). From the XRD pattern of the as-synthesized composite Co atoms cannot be directly seen, this is because the atomic radii of Co and Ni are quite similar and the partial substitution of Ni atoms by Co atoms does not change the crystal structure of Ni3S2 except change in lattice parameters29. So, substituting some of the Ni atoms by the Co atoms the composite can be indexed to a chemical composite of a form CoxNi3−xS2 where 0 < × < 3. Especially, the existence of additional peaks in the GNF/NCS/CNS core/shell composite compared to GNF/CNS sample proves the successful transformation of the nanotube structure into GNF/NCS/CNS core/shell structures.XRD diffraction pattern was used to investigate the crystal phase of as synthesized samples. In Fig. 3/2 and Co 2p1/2, respectively. The existence of 15.1 eV energy band difference between Co 2p3/2 and Co 2p1/2 spectrum confirms the existence of both Co2+ and Co3+ on the surface of the composite30. While in the core/shell composite the amount of Co is reduced and only one weak peak is observed at 781.4 eV for Co 2p3/2, suggesting the existence of only Co3+; the reduction in the cobalt amount is in agreement with the XRD analysis. Similarly, Fig. 3/2 and Ni 2p1/2, respectively. The energy band difference between Ni 2p3/2 and Ni 2p1/2 spectrum is 17.5 eV which indicates the existence of both Ni2+ and Ni3+ on the surface of the composite before and after the formation of core/shell31; while the other peaks are satellite peaks. The S 2p XPS spectrum is shown in Fig. 1/2 and S 2p3/2, respectively32. In the core/shell composite the peaks are reduced due to less number of S atoms in the shell (CoxNi3−xS2) compared to the number of S atoms in the core (Ni Co2S4). Finally, Fig. 2 hybridized carbon atoms forming honeycomb, implying existence of graphene in the composite34.The chemical states and electronic configuration of each element in the composite is studied in Fig. 35 the capacitance of pristine graphene on nickel foam is very low and we didn’t consider in this paper for comparison. The CV and GCD plots of GNF/NCS/CNS core/shell and the respective comparison with control samples are shown in Fig. 2 is shown in Fig. To measure the electrochemical performance of GNF/NCS/CNS core/shell structure we conducted comparative electrochemical tests with respect to control samples such as nickel foam (NF)/NCS/CNS core/shell and NF/NCS nanotube (without shell) structures which are grown on the pristine nickel foam (nickel foam without graphene). According to our previous report2 at current density of 10 mA/cm2. For more detail, Table 2) of GNF/NCS/CNS core/shell at different current densities. Even at high current density of 100 mA/cm2 our porous GNF/CNS core/shell shows high areal capacitance of 11.6 F/cm2. Thus, the as synthesized structure shows 74.36% capacitance retention as current increase from 10 to 100 mA/cm2. On the other hand, the NF/NCS/CNS core/shell and NF/CNS core show the highest specific capacitance of 10.9 and 8 F/cm2 at current density of 10 mA/cm2, respectively. Moreover, Fig. The specific capacitances of the as-synthesized composites are calculated using equation  and 2)..2). The 2 and the result is shown in Fig. 36.Cycling stability is one of key parameter to evaluate the performance of SCs. As a result, we conducted a comparative cycling stability test for GNF/NCS/CNS core/shell, NF/NCS/CNS core/shell and NF/NCS core electrodes by applying constant current density of 50 mA/cmESR), the high frequency semicircle diameter corresponds to the charge transfer resistance (Rct) and the more slope line in low frequency region is related to good capacitance behavior and low diffusion resistance37. The magnified scale high frequency region EIS plot is shown in Fig. ct values are 0.1, 0.155 and 0.32 Ω for GNF/NCS/CNS core/shell, NF/NCS/CNS core/shell and NF/NCS electrodes, respectively. Similarly, the approximate RESR values are 0.49, 0.5, and 0.52 Ω for GNF/NCS/CNS core/shell, NF/NCS/CNS core/shell and NF/NCS core electrodes, respectively. Moreover, the more slope line /(C−ΔV−)43, where, C+ and ΔV+ are correspondingly the mass specific capacitance and working voltage window of GNF/NCS/CNS core/shell electrode tested in the three electrode and similarly C− and ΔV− are correspondingly the mass specific capacitance and working voltage window of RGO electrode tested in three electrode. Hence, 44 mg/cm2 of RGO is required to balance 8 mg/cm2 of GNF/NCS/CNS core/shell positive electrode. Figure 43; thus, clearly shows as our asymmetric device can support a wide potential window of 1.6 V. Figure To investigate the practical application of GNF/NCS/CNS core/shell structure we assembled a prototype asymmetric electrode consisting of GNF/NCS/CNS core/shell as positive electrode and reduced graphene oxide (RGO) as the negative electrode. Asymmetric electrodes have been found to be an effective approach to meet the ever increasing demand of high energy density SCs, through providing a larger voltage window by combination of a carbon-based material as negative electrode with pseudocapacitive based materials as positive electrode. Cui 2 at current density of 15 mA/cm2. Figure 2 and illustrating excellent rate capability of 80%. The energy and power densities are calculated using a relation given in equation ,,2 at curin situ grown on graphene decorated nickel foam current collector by using stepwise hydrothermal process. The excellent capacitive performance of our GNF/NCS/CNS core/shell structure is due to the contribution of high surface area graphene which supported 3D porous NCS/CNS core/shell structure. Moreover, the 3D porous shell structure leads to increase of active area and resulted in high areal capacity. Additionally, the graphene layer sandwiched between the NCS/CNS core/shell and nickel foam current collector serves as a super high way for electron transport and leads to better rate performance. Using these synergistic advantages of graphene and NCS/CNS core/shell 3D porous structure we got superior areal capacitance of 15.6 F/cm2 at current density of 10 mA/cm2, excellent cycling stability of 93% after 5000 of cycles and excellent rate capability of 74.36% as current increase from 10 to 100 mA/cm2. This kind of hybrid structure may hold great potential for high performance energy storage devices in the future.In summary, Porous NCS/CNS core/shell structure was 2 was cleaned with deionized (DI) water and absolute ethanol (Et) and dried.All the reagents used were analytical grade without farther purification. Prior to the synthesis process, nickel foam of area 2 × 4 cm2 (100 Sccm) and CH4 (2 sccm) gas were flew into the quartz tube for 90 minutes to grow 3D graphene layer on nickel foam (GNF). Finally, the sample was cooled down rapidly to room temperature with the gas protection of H235.The pre cleaned nickel foam was placed in the quarz tube of CVD furnace and heated to 1023 °C in 80 minutes, maintained at 1023 °C for 20 minutes under argon (100 Sccm) gas atmosphere. Then, H2, 0.95 g of NiCl2, 0.72 g of urea and 60 ml of DI water were mixed by magnetic stirring until a transparent homogenous pink solution was formed and the solution was transferred to 80 ml Teflon-lined stainless steel autoclave. Then, the pre-prepared GNF was diagonal placed into the solution and the autoclave was sealed and kept in an oven at a temperature of 120 °C for 6 hour. After cooling down to room temperature, NCS nanoneedls precursor grown on GNF was washed with DI water and Et several times and dried at a temperature of 50 °C for 4 hours. Then, the resulted, GNF/NCS-precursor was hydrothermally treated in 80 ml Teflon-lined stainless steel autoclave containing a solution of 1.8 g of Na2S and 60 ml of DI water at temperatures of 180 °C for 8 hours. Then, after cooling down to room temperature and washing we get GNF/NCS nanotube.In this step, 1.855 g of CoCl2, 0.25 g of NiCl2, 1.2 g of urea and 12 ml of DI water, 12 ml Et and 12 ml of ethyl glycol were mixed by magnetic stirring until a transparent homogenous pink solution was formed and the solution was transferred to 60 ml Teflon-lined stainless steel autoclave and finally GNF/NCS was placed in the solution and it was hydrothermally treated at a temperature of 250 °C for 1 hours and after normally cooling down and washing we dried at a temperature of 50 °C for 4 hours and we got GNF/NCS/CNS core/shell.In this step, 0.5 g of CoClIt was prepared by hydrothermal reduction of GO; in detail, 120 mg of GO was immersed in 48 ml of DI water, then ultrasonicated for 5 hour at a temperature of 60 °C. Then, the solution was transferred to a 80 ml Teflon-lined stainless steel autoclave and heated at a temperature of 180 °C for 6 hrs, after cooling down to normal temperature the resulting sample is washed by vacuum filtration using DI water several times and dried at a temperature of 60 °C for 6 hr.Kα radiation (λ = 0.1542 nm). X-ray photoelectron spectroscopy (XPS) was used to study chemical state of the elements in the composite. Raman spectroscopy was carried out by HR800 with an excitation wavelength of 633 nm to study graphene on the nickel foam.To study the morphology of as synthesized sample scanning electron microscopy (SEM) Hitachi S-4800 was used. To study how atoms are arranged in crystal structures, we conducted X-ray powder diffraction (XRD) patterns on a Rigaku Max-2200 X-ray diffractometer with monochromatized Cu2 area GNF/NCS/CNS core/shell sample was sandwiched between two pieces of nickel foams and pressed at pressure of 3 MPa and used as working electrode. Moreover, we also tested the electrochemical performance of assembled prototype working device. In this case, we used GNF/NCS/CNS core/shell as positive electrode and RGO as negative electrode. The RGO negative electrode was prepared by coating a mixture of RGO, acetylene black and poly (tetrafluoroethylene) in a weight ratio of 80: 10: 10 on nickel foam. Then, the electrode was dried for 12 hour at 60 °C in a vacuum. The average areal specific capacitance and mass specific capacitance values in three electrode measurement were calculated from GCD curves, using equations (1) and (2)45:CA(F/cm2), CS(F/g), I(A), t(S), m(g), S(cm2) and ΔV(V) are specific capacitance, areal capacitance, discharge current, discharge time, mass of active material, area of active electrode and potential window for charge discharge, respectively. On the other hand, the specific capacitance of two-electrode system is calculated using the relation given by refs cell is the specific capacitance of an asymmetric cell, I is discharge current of the cell, ΔV is the operating voltage window of the cell, m’ is sum mass of active materials on both electrode  after obtaining the balanced mass of the two electrodes. Energy density and power density are key parameters to measure the practical performance of electrodes. So, the energy and power densities are calculated using the following relations:44.The electrochemical properties of the as-synthesized sample were tested by conducting cyclic Voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). All electrochemical performance measurements were carried out using electrochemical workstation  using 3 M KOH aqueous electrolytes. In a three-electrode system measurement, the as-synthesized active material was used as a working electrode; Platinum plate and Ag/AgCl with saturated KCl are used as a counter and reference electrodes, respectively. A 1 × 1 cm


The use of electric fields for signalling and control in liquids is widespread, spanning bioelectric activity in cells to electrical manipulation of microstructures in lab-on-a-chip devices. However, an appropriate tool to resolve the spatio-temporal distribution of electric fields over a large dynamic range has yet to be developed. Here we present a label-free method to image local electric fields in real time and under ambient conditions. Our technique combines the unique gate-variable optical transitions of graphene with a critically coupled planar waveguide platform that enables highly sensitive detection of local electric fields with a voltage sensitivity of a few microvolts, a spatial resolution of tens of micrometres and a frequency response over tens of kilohertz. Our imaging platform enables parallel detection of electric fields over a large field of view and can be tailored to broad applications spanning lab-on-a-chip device engineering to analysis of bioelectric phenomena. Detection of electric fields, central to chemical and biological processes, has been limited to measurements of current  and secondary reporters . Here, the authors demonstrate an optical platform capable of imaging electric field dynamics with high spatio-temporal resolution. Signalling and manipulation through the control of electric field distributions is ubiquitous to both biological and physical systems. For example, intercellular electrical activity is central to the signalling and computation of excitable cells such as cardiac and neuronal cells12467810Over the past decade, there has been a concerted effort to develop new techniques to observe and analyse the dynamic field fluctuations in liquids, such as the development of multitransistorarrays1214151617101919πα∼2.3% at all frequenciesAtomically thin materials like graphene offer exceptional opportunities for electric field sensing due to their unique physical properties and intrinsic sensitivity to their environment. Over the past decade, graphene's optoelectronic properties have been studied extensively23−4. However, physical and biological applications require orders of magnitude improvement in voltage sensitivity. To increase the light-matter interaction in graphene, researchers have explored different approaches such as coupling graphene to silicon waveguides262728293132Realizing this goal requires new optical designs that optimize the detection sensitivity and parallel readout simultaneously. For example, simple transmission mode imaging of gated monolayer graphene yields a 1% change of transmitted light over a gate voltage change of 200 mV around the Pauli blocking region. This results in a voltage sensitivity of only 2 mV, accounting for a typical laser with noise levels of 10Here we demonstrate highly sensitive, high-speed optical imaging of local electric field dynamics in solutions using graphene and a critical-coupled planar waveguide. Using a custom numerical simulation, we designed the critically coupled waveguide amplified platform to obtain the so-called critical coupling condition2O5 (n=2.0856), forming a high refractive index waveguide. We use an equilateral SCHOTT SF-11 glass (SF-11) prism (n=1.743) with a 1,000 nm top layer of SiO2 (n=1.444) to evanescently couple a collimated 1.55 μm beam to and from the CAGE sensor. The reflected light is then collected by an infrared objective and imaged onto an InGaAs camera and photodiode.2 and Ta2O5/solution interfaces. Under this framework, we can describe the waveguide as a Fabry-Perot cavity with high reflection coefficients |r1| and |r2| at the two interfaces, where |r1| and |r2| deviate from unity due to the frustrated total internal reflection from the finite SiO2 thickness at the first interface and the absorption of graphene at the second interface. At resonance coupling, the total reflection R from the Fabry-Perot cavity is described by |r2| can be varied in situ by electrostatic gating of graphene. To attain the highest sensitivity to local electric fields, we gate the graphene to a specific Fermi energy that generates the largest relative change in optical reflection for a given local electric field. At the critical-coupling condition, |r1|=|r2| and the total reflection, R, equals zero, at which point all of the light is absorbed by graphene. Consequently, close to the critical coupling condition, the light-graphene interaction is strongly enhanced and the background reflection is very weak contributing to the high voltage sensitivity of CAGE imaging.The concept of waveguide critical coupling is illustrated in ibed by  . The val2 layer, the frustrated total internal reflection |r1|=98.2%. The reflection coefficient at the Ta2O5/solution |r2| is 97.5% at the charge neutral point (Vg=−0.14 V) due to graphene absorption, which leads to a total reflection of R ∼1.5%. Graphene absorption can be set subsequently by electrostatic gating through the solution. The critical coupling condition |r1|=|r2| is realized at Vg=+0.41 V (electron doped) and Vg=−0.69 V (hole doped), resulting the lowest total reflectivity, R . Further increase of carrier doping leads to a decrease of graphene absorption corresponding to |r1|<|r2| and one obtains an increase in total reflectivity. The grey dashed line in R/R caused by a change in voltage induced by the local electric field, ΔV. R/R)/ΔV as a function of the gate voltage from Vg=0.53 V. This sensitivity is over 200 times higher than that of the direct transmission configuration /ΔV=1.2% per mV reported in Vpp=100 μV with a signal to noise ratio (SNR) of 6.5. This measurement reflects optical detection capabilities of ∼15 μV or, equivalently, a change of carrier concentration of 2.3 × 108 electrons per cm2 in graphene. The noise level at 0.017% in CAGE optical detection of small electric field fluctuations is demonstrated in C is the capacitance of the graphene/electrolyte interface and R is governed by graphene conductance. For local electrical field fluctuations, the effective area and capacitance is small and the response speeds can be very high. Experimentally, we characterized the frequency response of the CAGE detector using a relatively large graphene area (200 by 400 μm), underscoring the applicability of our measurements to large fields of view. R/R value remains largely constant up to 10 kHz and decreases at higher frequencies. These measurements yield a 3 dB frequency of 13 kHz  and 150 nm Ta2O5 (waveguide layer) deposited on one face of a 1 cm equilateral SF-11 glass prism by ion-assisted deposition. The structure was designed using a custom Python simulation (2 area graphene was transferred onto the waveguide surface by poly(methyl methacrylate) (PMMA) transfer. We obtain high-quality large-area graphene with near-uniform optical absorption at the device/solution interface. The Pt (2 nm)/Au (60 nm) electrodes were deposited on the graphene to make electrical contacts. The metal electrodes were insulated with nitrocellulose lacquer to prevent water-Au chemical reactions during measurements. The device was mounted in a solution chamber printed from PR48 resin (Autodesk), which permits access to the device from both the top and bottom. All data were obtained in a saline solution  except for imaging data  was applied through a Ag/AgCl electrode in solution to set the Fermi energy of graphene and to test the optical response of CAGE detection under electrostatic gating. For the spatially resolved measurements, an external gate voltage Vg (1.1 V) was applied through the microelectrode, whose high impedance at the electrode/solution interface requires a larger applied voltage but yields the same voltage bias and critical coupling condition at the detector's graphene/solution interface. A 10 mV electrical pulse generates a 1.8 mV local field at the graphene/solution interface beneath the Pt/Ir microelectrode tip   . To thisThe data that support the findings of this study are available from the corresponding author upon request.How to cite this article: Horng, J. et al. Imaging electric field dynamics with graphene optoelectronics. Nat. Commun.7, 13704 doi: 10.1038/ncomms13704 (2016).Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.Supplementary Figures, Supplementary Notes and Supplementary References.