Graphene is considered a promising substance in applications related to the capture and reduction of the environmental impact of fluorinated gases. However, further research is still required to explore all related possibilities. In this work, the potential use in this context of nanofluids (NFs), obtained by dispersing graphene nanosheets in fluorinated ionic liquids (FILs) is investigated. As a starting step, a thermal and structural characterization for this type of IoNanofluids (IoNFs) is presented. The highly nanostructured nature of FILs has been recently demonstrated. The presence of fluorinated moieties is responsible for enhancing the accommodation of solutes such as small gases. The strong tendency to self-assemble forming continuous and supramolecular structures, and the versatility to rearrange in several conformational features allows the stabilization of nano colloidal systems. It is essential to perform a comprehensive study of their structural features to understand the behavior of this type of heterogeneous systems. Therefore, we present screening on the phase and structural behavior of these novel IoNFs to discover and develop optimized systems where FILs turn out to be advantageous. Thermogravimetric analysis (TGA) was employed to evaluate IoNFs mass losses with temperature, and their solid–fluid phase transitions were located using a differential scanning calorimeter (DSC). Their rheological properties were also determined through oscillatory experiments, obtaining the viscous and loss moduli. In addition, the structural percolation transition was also identified. During the last decades, intensive research has been performed concerning ionic liquids (ILs) because they have been identified as environmentally friendly solvents in a wide range of applications , includiOn the other hand, graphene nanosheets have been recently employed in the formulation of nanofluids (NFs). A survey of the literature reveals that this application of graphene in NF formulation is finding application, for instance, in heat transfer processes and energy production optimization ,10,11, bIn particular, fluorinated ILs (FILs) are characterized by a higher molecular rigid structure and lower polarity as a result of the nature of carbon−fluorine bonds that promotes the formation of a nano segregated structure . Their u2, and their emissions have increased by 60% since 1990.In this work, we propose progress in the design of new absorbent media for the separation and capture of fluorinated gases (F-Gas). The atmospheric concentration of greenhouse gases (GHG) increases every year, contributing to the increases observed in the global temperature. In fact, most F gases have huge global warming potentials, some of them reaching 23,000 times the value of COF-Gas are used as an alternative to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) to prevent ozone depletion. In the literature, F-Gas have been shown to have high solubility in FILs, and thus, these innovative solvents have been proposed as separation and capture agents for these environmentally harmful gases. The combination of FILs with graphene nanosheets is proposed as a new step forward in this application. This tailoring of these innovative IoNFs is intended to enhance gas solubility through the selective adsorption of these gases on the surface of the nanosheets, which can be modified through chemical functionalization to increase affinity with F-Gas molecules. Nevertheless, the thermophysical and phase equilibrium transitions of these IoNFs are still poorly understood, and demand a rigorous characterization before considering them for the proposed application.The use of IoNFs in this application presents some potential advantages if compared with traditional NFs. We can cite, for instance, the improved nanofluid stability due to the FIL viscoelastic nature, which reduces agglomeration and settling. In addition, and due to the IL negligible vapor pressure, no concentration drifts are produced due to the solvent evaporation during sonication, and this feature also improves the traditional NF performance.With this objective in mind, the characterization of graphene-derived NFs will be performed using a highly surfactant FIL as a base fluid . Prelimi2 g−1 and individual flake thickness in the range of 1 to 5 nm. Nano powder was weighed with a Mettler AE-240 balance, whose accuracy is estimated to be 5 × 10–5 g, and then dispersed into a calculated volume of the base fluid, obtaining stable and homogeneous xGnP/FIL IoNFs, with percent weight concentrations of 1, 5, and 10 wt%. An ultrasonic bath  was used to obtain a correct dispersion. The FIL used in this case was 1-ethyl-3-methylpyridinium perfluorobutanesulfonate ([C2C1py][C4F9SO3]) (>99% mass fraction purity) supplied by Ionic Liquids Technologies. The thermophysical profile of this FIL has been determined in previous works [C4F9SO3] IoNF. Cooling was carried out using a refrigeration and thermostatic system able to reach 183.15 K. Dry nitrogen gas was used to purge the sample, with a 50 mL·min−1 flow rate. An amount of 12 to 15 mg of each studied sample was sealed in a standard aluminum sample tray, resistant to pressures up to 0.4 MPa. Eventual mass losses during the measuring process were avoided by weighing the sample holder before and after the experimental runs. This verification is essential for experimental data validation. IoNF samples were kept under sonication for one hour to ensure correct dispersion before each experimental measurement. DSC calibration was performed using indium  as standard. Samples temperature was then cooled to 183.15 K and maintained for 5 min. After that, they were heated to different temperatures, always respecting a minimum separation of 40 K between the last recorded transition and the cycle termination. Three repetitions of the cooling and heating cycles were performed for each sample at a constant 5 K·min−1 rate. This value of scanning rate ensures the best resolution and characterization of the various solid–fluid phase transitions. The DSC technique determines the thermal difference between two trays within a cell placed at the same conditions [C4F9SO3] were studied. A sample of 51 mg was heated from laboratory temperature to 1073 K at a constant rate of 5 K·min−1, maintaining an inert N2 atmosphere. Tonset = 686 K. Onset temperatures were obtained as the intersection between the baseline and the tangent to the derivative of the weight loss curve at the inflection point.Thermal stabilities of the FIL base fluid and the xGnP/[C2C1py][C4F9SO3], used as the base fluid, and of the three IoNF concentrations  were analyzed by temperature scans with cooling and heating rates of 5 K·min−1. In addition, repeated cycling tests were performed with five repetitions for each sample at the same conditions, certifying in all tests the same transitions.The solid–fluid transitions of the ionic liquid [C2C1py][C4F9SO3] and the xGnP IoNFs. The thermogram of the pure IL evidences that the material is polymorphic, presenting different solid phases in all cases. This behavior might be expected a priori because it has been shown [C4F9SO3] was also calculated due to its importance in different practical applications. This degree of supercooling can be computed from DSC results Strain SweepDuring the linear viscoelastic measurements, the linear viscoelastic regime (LVR) was determined as the first step by measuring both store (G′) and loss (G″) moduli of IoNFs in the strain range between 0.01% to 1000% at 10 rad·sIn these tests, the influence of the deformation of the fluids at a constant frequency on the viscosity was recorded, determining the region of linear viscoelasticity (LVR).−1 constant frequency, with a deformation range from 0.1% to 1000% and twenty points per decade.The values of storage (G′) and loss (G″) moduli, plotted against strain variation, are shown in As can be seen, the elastic component reached a point where torque was so low that the equipment found its measuring limit.It can be observed that the addition of nanoparticles produced an increase in both the elastic and viscous moduli because the dispersion had a higher resistance to flow. There was also a greater coherence in the data due to the arrangement in the structure of the ionic liquid with the aggregation of the nanoparticles.The linear regime in the viscoelastic behavior is clearly observed, where G″ was constant and strain independent. Afterward, for higher deformations, both moduli decreased for all samples. The structure resisted up to a certain tension when the internal structure was lost by the disaggregation of the suspended nanosheets.(2)Frequency SweepFor the pure FIL and 1 wt% IoNF, the viscous modulus was larger than the elastic one. However, at concentrations above the percolation concentration (described below), the elastic modulus G′ was higher than the viscous modulus G″. Then, the sample presented an interesting transition in its viscoelastic nature, evolving from a liquid-like a behavior at low concentrations, to a solid-like trend at higher graphene loads. Once the region of linear viscoelasticity was determined, the frequency tests were carried out with a selected deformation value of 0.1%.−1 at 293.15 K.(3)Graphene PercolationAs in strain sweep tests, the viscous component predominated over the elastic one in the entire region of linear viscoelasticity for the pure FIL, and it was the same case for low concentration of graphene. On the contrary, for concentrations above the percolation threshold, the opposite trend was observed. Then, the elastic modulus was predominant, being a feature of complex fluids approaching a solid-like trend. Both moduli increased with angular frequency, and also with the concentration of nanoparticles. This may be due to the structure of the pure liquid being unstable.The determination of the concentration of graphene percolation was obtained by extrapolation. Thus, five concentrations of graphene IoNF  were analyzed. Then, it was necessary to obtain the plateau of elastic modulus, from a frequency sweep within the (LVR), at the same conditions than frequency sweep, and 293.15 K, to ensure that all material was melted.0 is the threshold value for m, and the equation applies only in the vicinity of the percolation transition. This correlation is shown in The IoNFs experimental low frequency equilibrium moduli were modelled as a function of concentration following a percolation equation ,29:(2)loA value of the percolation concentration of 1.5% by weight was obtained, which demonstrates the solid internal structure of the IoNF due to the graphene addition.2C1py][C4F9SO3] at the same heating rates, with a 10 s−1 shear rate, obtaining the sample viscosity in these conditions, as displayed in Finally, temperature ramps were performed for the xGnP/[CSummarizing, samples of different concentrations of IoNFs obtained by dispersing graphene nanosheets in FIL have been analyzed in this work. The characterization of these IoNFs was carried out from the evaluation of the solid–fluid transitions using a differential scanning calorimetry technique, and in addition, a viscoelastic characterization was achieved using rheology. This allowed obtaining information about the internal structure of the IoNF, determining the influence of concentration of the suspended graphene.Previous research showed that 1-ethyl-3-ethylpyridinium perfluorobutanesulfonate is an optimum candidate to be used as a solvent in gas separation processes with a significantly reduced environmental impact. The formulation of IoNFs through the addition of graphene, or derivatives with a chemically functionalized surface, represents an option to enhance gas solubility, improving the performance in gas capture and separation.The obtained DSC profiles indicated a very complex thermal behavior of the solid–fluid transitions for the fluorinated ionic liquid, which are typical of polymorphic systems. For the pristine base fluid, a cooling ramp showed different exothermic peaks, revealing a succession of several polymorphic crystalline phases. This trend was remarkably modified on the addition of graphene nanosheets, resulting in an extremely rich and complex phase transition behavior. The final phase transition scenario combined features of enantiotropic and polymorphic solid–solid transitions for the pure IL, and the contributions of nucleation enhancement, solid–solid transition frustration and ultimate percolated structure due to the addition of the dispersed graphene nanosheets.The rheological profile found for these samples was also very complex. As a first and distinctive feature, all samples show non-Newtonian behavior, where modules G′ and G″ were practically constant in the low strain range, and both increased with graphene concentration, showing a clear transition from liquid-like to solid-like behavior with increasing nanoparticle load. A percolation concentration value of 1.5 wt% was obtained for this system, evidencing a solid internal structure of the IoNF.In conclusion, the addition of graphene allowed the phase transition and viscoelastic nature of the original FIL to be tuned. Graphene concentration controlled these two patterns, and small concentration variations produced a remarkable change in the system properties. Thus, the IoNF can be precisely tailored to match the required optimal features considering the use of this system in gas separation and capture applications.


New techniques to manipulate the electronic properties of few layer 2D materials, unveiling new physical phenomena as well as possibilities for new device applications have brought renewed interest to these systems. Therefore, the quest for reproducible methods for the large scale synthesis, as well as the manipulation, characterization and deeper understanding of these structures is a very active field of research. We here report the production of nitrogen doped bilayer graphene in a fast single step (2.5 minutes), at reduced temperatures (760 °C) using microwave plasma-enhanced chemical vapor deposition (MW-PECVD). Raman spectroscopy confirmed that nitrogen-doped bilayer structures were produced by this method. XPS analysis showed that we achieved control of the concentration of nitrogen dopants incorporated into the final samples. We have performed state of the art parameter-free simulations to investigate the cause of an unexpected splitting of the XPS signal as the concentration of nitrogen defects increased. We show that this splitting is due to the formation of interlayer bonds mediated by nitrogen defects on the layers of the material. The occurrence of these bonds may result in very specific electronic and mechanical properties of the bilayer structures. The absence of a band-gap, however, has prevented its implementation as a possible replacement in semiconductor industry10.Since graphene was effectively isolated in 200414 studies have demonstrated that an effective option towards its electronic modulation is nitrogen doping, which generates n-type extrinsic electrical conductivity15. The substitution of carbon by nitrogen disrupts the ideal sp2 hybridization of graphene’s lattice, locally inducing significant changes to its electronic properties, separating the conduction band from the valence band even at modest concentrations of 2%16. This possibility of tunning graphene’s electronic properities could lead to potential applications in electronic devices, from electrochemical biosensors to spintronics18.For monolayer graphene, both theoretical and experimental10. Last year, Y. Cao et. al. have shown that by twisting the relative angle between two graphene sheets the resulting electronic properties of the system can vary from a Mott-insulator19 to a superconductor20. Other groups have since then shown that twisting the angle between the layers of other 2D heterostructures - in what has been called twistronics - has also been achieved for materials such as MoSe2-WS221 and even heterostructures composed of three layers of atomic sheets alternating 2D Boron Nitride sheets with graphene monolayer22. In all of these cases, the appearance of a new periodicity in the rotated crystalline systems - called a moiré superlattice - results in new electronic properties of the systems. These properties can be so drastically different and so sensitive to the twist angle that changes in the relative rotation of the layers by no more than 0.2 degrees can cause all the remarkable effects observed to disappear20. The effective combination of 2D materials into heterostructures already appeared as a technique that would unveil an enormous new set physical properties that could be employed for a vast set of possible technological applications. The realization that the properties of these systems can be so dramatically altered by twisting the angle of stacking these monolayers indicates that these new possibilities stretch even further23.Another possibility for implementing a band-gap in graphene is working with few layer graphene films. It has long been known that the application of an electric field can induce the appearance of a band-gap in bilayer graphene25. Another relevant feature observed was the shifting of 2D peaks to lower energies, related to the introduction of electron donors (such as nitrogen) into the samples32. Using XPS we confirmed the presence of nitrogen defects in the synthesized material, identifying that pyridine defects prevail in our samples. However, as the concentration of nitrogen in the bilayer graphene is increased, we observed in the high resolution N1s spectrum an unexpected splitting in the peak associated to these defects.These exciting new findings have brought renewed interest to the study of bilayer grahene and methods to synthesize, dope and characterize these systems. Moreover, the comprehension of how atomic defects specific to bilayer systems affect their electronic and mechanical properties is of paramount importance. In this work we demonstrate a plasma-enhanced chemical vapor deposition (PECVD) process for the synthesis and doping of graphene bilayer structures in a single step. By carefully adjusting the variables of the plasma , and flow ratio of gases  we were able to obtain nitrogen-doped bilayer sheets in one single fast process (2.5 minutes). Moreover, we were able to carry out this process at low temperature (760 °C). Through Raman spectroscopy we could verify the effectiveness of the synthesis, determining that we obtained large area graphene bilayer films. We also confirmed doping thought the appearance of D′ band, attributed to the defect-induced intravalley double resonance scattering process33. In this conformation, an interlayer covalent bond forms, linking defects on the two layers of the material. We used quantum-mechanical parameter-free simulations, based on the density functional theory, to test this hypothesis. We simulated several defect complexes involving nitrogen and carbon vacancies, including pyridine and pyrrolic in different orientations. We also investigated how these defects could interact with a second layer of graphene, both pristine or containing other defects, employing different levels of simulations, with and without the explicit inclusion of van der Waals interaction. The ideal relative proportions of these defects in the synthesized material is readily given by their formation energies. For some defect complexes, we observed that the formation of covalent bonds between nitrogen in one layer with one of the atoms in the layer adjacent to it is stable. We then performed calculations of level shifts associated to core electrons of nitrogen in different sets of defects, simulating the XPS signature of each of these defects complexes, comparing to our experimental data. We conclude that the splitting of the XPS peak in our bilayer graphene samples as the concentration of nitrogen increased is likely related to the formation of the interlayer bonds mediated by nitrogen defect complexes. The presence of these interlayer covalent bonds can affect the mechanical as well the electronic properties of bilayer graphene in significant contrast to the way these defects modulate the properties of monolayer graphene. Therefore, the induction or suppression of such defects can represent another parameter to be taken into consideration when tailoring graphene and other bilayer or few layer heterostructures for specific applications.If the observed splitting in the XPS peak was associated to new types of defects becoming more frequent in the material as the concentration of nitrogen increases, we would expect similar behavior to occur in the monolayer graphene, but we have found no reports of a similar splitting in the literature for this material. Therefore, we hypothesized that this splitting could be caused by new conformations of the usual pyridinic defects exclusive to bilayer graphene, called clipping2/Si substrate for pristine (PG) and nitrogen-doped samples . The intensity, the shape and the peak position are strong indicatives of the number of layers, graphene structure and type of dopants36.We performed Raman analyses of our graphene samples to study their structural properties as well as dopant content. Figure D2/IG) and by the full width at half-maximum (FWHM) of the 2D band37. A monolayer graphene usually presents ID2/IG values of 2 or bigger with a FWHM ~30 cm−1, while bilayer structures present this intensity ratio ranging from around 1 to 1.5 and a FWHM ~50 cm−1 40. Therefore, the structures synthesized in our experiments show typical intensity relation values of bilayer graphene. Raman mapping studies also confirm the high-yield growth of bilayer graphene as can be seen in Supplementary Information Fig. 2 substrate, as well as careful removal of carbon and other impurities left on the sample during the transfer to the substrate. Therefore, we believe that the Raman analyses described above is more indicated in our case.In the spectra of all samples, Fig. −1 (D band), and an additional peak located at ~1620 cm−1 (D′ peak) are also observed. The D and D′ peaks are activated by inter and intra-valley double resonant Raman process25, in which the defects are responsible for the missing momentum that completes the resonant process. Therefore, the increase in the concentration of defects leads to an increase in both intensities42. Pristine sample shows a moderate amount of defects (D band) even without nitrogen doping. This is expected as the growth occurs in the presence of hydrogen plasma, which would contain energetic ions and radicals that induce defect formation44.Peaks at 1350 cm45. With the increase of nitrogen incorporated in the sample, the D band increase together with the 2D band decrease and broadening then indicates the formation of more disordered structures32, which is further confirmed by the increase on the ID/IG ratio39. Moreover, it has been shown that the in-plane vibrational G band of graphene and bilayer graphene is blue-shifted for both electron and hole doping, due to the non-adiabatic removal of the Kohn anomaly at the Γ point. On the other hand, the 2D band is red-shifted for electron doping and blue-shifted for hole doping, due to the charge transfer induced modification of the equilibrium lattice parameter46.The 2D peak, on the other hand, originates from two-phonon double resonant process, which does not need defects to satisfy the resonant condition. Its intensity is affected by the electron/hole scattering rate. The presence of nitrogen dopants in graphene would both create defects and introduce electron doping, which increases the electron scattering rate and, consequently, decreases the intensity of the 2D peak−1, 2684 cm−1 and 2679 cm−1, while the blue-shifted G peaks in 1593 cm−1, 1594 cm−1 and 1595 cm−1 for NG1, NG2 and NG3 samples, respectively. Therefore, the value and evolution on peak intensities combined with position shift consolidate strong evidences of bilayer graphene formation, with nitrogen dopants concentration increase proportional to nitrogen flow during the PECVD process.In our analysis, we found that the 2D band red-shifts while the G band blue-shifts as the nitrogen content increase during the synthesis process. In Fig. 2 hybridized carbon and confirms the formation of graphene during the synthesis. A second and third bands, centered at 286 eV and 288.5 eV obtained by deconvolution of previously described peak, reflects the binding of both oxygen or nitrogen to sp2 C and sp3 C respectively36. Previous studies47 show that N-doped graphene is more susceptible for oxygen functionalization. This fact might be ascribed to the increased reactivity of atoms at the edge of defects and pyridinic N sites. These sites react with oxygen-containing species when exposed to the atmosphere, creating a sample with higher oxygen content. As our samples were exposed to laboratory air before the XPS analysis, the oxygen signature might be a consequence of the described phenomenon. The higher intensity at α-C sp3 position and of α-C sp2 band with the increase in nitrogen corroborates the observed Raman results, as these changes are caused by the increase of ligands concentration, which further confirms graphene doping.XPS was used to investigate chemical state changes on graphene structure as a result from nitrogen flow increasing during the synthesis, i.e., doping process. Fig. 48. Figure Nevertheless, a proper comprehension of chemical state changes required the analysis of high resolution N spectra, which are presented in Fig. 3N, in which the compound was fabricated by reactive radio frequency sputtering of copper targets in nitrogen and argon environment49. Nevertheless, it is known that copper nitrate is not stable at temperatures higher than 400 °C49. Therefore, the formation of such compound is inconsistent with our experimental conditions (760 °C temperature synthesis), which indicates that the observed peak might arise from other binding structures present in doped bilayer graphene. To further investigate the chemical state changes detected by our XPS measurements, we performed fully quantum atomistic simulations based on the density functional theory (DFT) method.Previous studies have assigned this second peak in the XPS analysis to N–Cu bonding in CuIn Fig. 54 which aims to accurately describe van der Waals forces in DFT simulations. The interlayer distances of AB and AA stackings considering vdW forces remained nearly the same as those obtained with LDA 3.34 Å and 3.54 Å. The binding changed markedly to −33 meV per atom, or three times more than predicted by LDA. We also simulated defects using optB86b + vdW-DF functional. In the Supplemental Information Fig. We expect that coulomb interaction will favor the formation of complexes composed by defects of opposing charges, specially in bilayer graphene, in which defects of opposite charge on different layers can approach each other more easily than in the monolayer. In the Supplemental Information we discuss how we estimate the electronic state of the defects indicated in Fig. 33, and is illustrated in Fig. Ef) calculated in the same supercell, and the difference of this formation energy relative to the sum of the formation energies of the defective monolayers that make up the defect complex (ΔEf). This difference in energy gives an idea of the interaction energy between the defective monolayers when combined. We also indicate whether the interlayer covalent bond remained stable during the optimization of the atomic positions in our simulations of these complexes and what are the formation energies when interlayer bonds do remain stable. As discussed previously for monolayer graphene and the bilayer with one isolated defect, we tested the effect of optB86b + vdW-DF functional in these results, as shown in Tables Having established that isolated defects in monolayer and bilayer graphene are not markedly different and thus are not expected to produce different XPS signatures, we investigate complexes of defects in the bilayer. Specifically, complexes in which defects are in adjacent layers. The proximity of two defects will not only induce structural distortions through electrostatic interaction, but also due to the presence of frustrated bonds, that can lead to significant reconstructions. In some cases, the interaction of defects in opposite layers of the bilayer can lead to the formation of covalent bonds between the layers. This phenomenon is called clippingi.e. different bonding arrangements).As shown in Table 12.We simulated the XPS experiment in our defective graphene bilayers computing the electron core level shift for different configurations of the defects. The core level shifts are not only related to the orbital of the electron within the nucleus of an atom but also to how this atom bonds with its neighbors. The difference in the bonding structure of an atom results in different binding energies for core electrons in the same atom. This energy difference is usually enough to distinguish between a nitrogen in the substitutional defect, Fig. 55. In this method, we compute the energy difference of a system in its ground state with the same system with one electron removed from the core of a specific atom. In the final state approximation, the electrons are allowed to relax, screening the localized core hole. This energy difference gives the binding energy of the respective core electron. Absolute energy values are not meaningful, but the core level shift, that is, the difference of the binding energy of the same core electron in different chemical environments  is accurate to within 50 meV55. Our referential is the core eigenvalue of one 1 s electron from substitutional nitrogen on monolayer graphene, see Fig. ECLS is the energy of the core level shift, c in the configuration of interest and s electron from substitutional nitrogen on monolayer graphene, this energy is −406.4 eV.We computed the core eigenvalues using the final state approximation within VASPWhen considering the combination of a monolayer containing a nitrogen defect and a pristine monolayer, we observed an almost rigid translation in the core level shift, as shown in Table When defects in adjacent layers interact, a significant difference between clipped and not-clipped structures can be seen Table . ConsideWhen a pyrrole defect occurs in the geometry shown in Fig. As stated in the Experimental Results and Discussion, the analyses of the XPS survey . To further verify this conclusion, we analyzed cooper defects on monolayer and bilayer graphene. In Fig. Finally, we also investigated defect complexes involving copper impurities. Since we used Cu substrate to grow and analyze our samples, there is a possibility that this impurity could be responsible for some of the features observed in our experimental XPS analyses. As a matter of fact, other authors have proposed this interaction of Cu with N defects as the cause of the splitting in the XPS peak that we have also observed56.The results of formation energies and CLS regarding the copper-related defect complexes described above can be found in Table Considering the CLS of nitrogen atoms in the copper-related defect complexes, from monolayer to bilayer graphene, we verified a rigid shift of at most 2 eV among the results of all the analyzed defects. Moreover, the nitrogen CLS energies of the copper-related defect complexes are about 20 eV distant from those values obtained for the nitrogen defects  and high crystallinity. The plasma/metal coupling causes localized, rapid heating of the foil, leading to a fast synthesis (2.5 min). This localization also reduces the post-growth cooling time (10 min). The combination of hydrogen, nitrogen and methane plasma serves to remove the copper native oxide layer as the synthesis occur, minimizing the number of steps taken to obtain a stable graphene sheet. It was also shown that the change in N2 during synthesis. These nitrogen related defects must be further studied, but are expected to alter mechanical and electronic properties of bilayer graphene in a markedly different way from monolayer graphene. These defects are likely to be present in other types of few layer 2D films and heterostructures and we indicate a route to identify their occurrence through XPS characterization and computer simulations. Moreover, their are likely to play a role in the new field of twistronics that needs to be further investigated. The suppression or incorporation of these defects in 2D few layer structures can be another parameter to tune in order to adjust their properties for specific applications.We performed state-of-the-art, parameter-free simulations based on the density functional theory (DFT) to verify the cause of this splitting of the XPS signal. Using the local density approximation as well as implementations of the DFT that explicitly include van der waals interactions (optB86b + vdW-DF) we obtained the same trend for the formation energy of several monolayer graphene defects, bilayer graphene defects and defect complexes. When defects are close enough in adjacent layers, they can induce the formation of interlayer covalent bonds, a phenomenon known as “clipping”. Calculations of the shifts in core level energy of these interacting defect complexes suggest that the features observed in the high resolution N1s XPS spectrum of doped samples (NG2 and NG3) are related to the clipping of defect complexes in bilayer graphene. The formation of interlayer bonds changes the chemical environment around the impurity atom, affecting the core electron binding energy detected by the XPS measurement. Our simulations show that interaction between defect complexes are highly dependent on experimental conditions and can be tuned by controlling the ratio of N57. In this system, the sample is mounted onto the top of a quartz tube cap inside the plasma chamber, and the bottom surface of the sample can be viewed by an optical pyrometer positioned outside the chamber. Copper films with dimensions of 5 × 5 × 0.025 mm3 were used as substrates. First the MW-PECVD chamber was pumped down to 15 Torr, followed by an argon purge, in which the system reached a pressure of 30 Torr. The synthesis was then started by plasma activation. To start the plasma, the microwave power was adjusted to a value of 500 W, resulting in a voltage of 85 V, and generating a locally confined plasma of ~5 cm diameter around the sample. The plasma parameters resulted in the heating of the substrate to 760 °C, which remained constant throughout the synthesis. After the plasma activation, the argon flow was interrupted and the 10 standard-cubic centimeters-per-minute (s.c.c.m.) of room temperature hydrogen gas was added. Keeping the hydrogen flow constant, several flow ratios of methane and nitrogen gases were added into the chamber, as described in Table Graphene films were grown in an in-house built microwave plasma-enhanced chemical vapor deposition (MW-PECVD) system, which configuration is described in details elsewhere2 layer on Si by PMMA transfer method in order to obtain a spectra without copper background reflectance interference58. Raman spectroscopy was performed on a confocal micro-Raman spectrometer (Witec Alpha 300R), equipped with a 532 nm laser source (power at sample below 0.1), 100× objective and charge coupled device (CCD) detector. The system generated a laser spot size of 300 mm (diameter) and spectral resolution of 3 cm−1. The wavenumber calibration was based on standard values for crystalline silicon band checked at 532 cm−1. Measurements were carried out at room temperature and peaks were fitted using a Lorentzian function.For Raman spectroscopy, graphene films were transferred onto a 300 nm thick SiOKα radiation (hν = 1486.6 eV) and pass energy of 10 eV, operating at base pressure of 3e-9 Torr. Background was subtracted according to Shirley model, and peak fitting was performed with a product of Gaussian and Lorentzian shapes. Initial quantification was based on Scofield sensitivity factors59 and a model of uniform elemental distribution. The chemical composition of the samples  were estimated from survey XPS spectra as shown in Fig. Film composition was investigated by X-ray photoelectron spectroscopy (XPS) using a UNI-SPECS UHV spectrometer with monochromatic Al ab initio total energy calculations based on the density functional theory (DFT)61 with the Vienna Ab Initio Simulation Package (VASP)63 using the local density approximation (LDA)64. Results obtained by this approximation were compared with results using van der Waals corrections through optB86b + vdW-DF exchange and correlation functional54. For the Brillouin zone sampling, an optimized Γ-centered 1 × 1 × 1 mesh was used for geometry relaxations and Γ-centered 2 × 2 × 1 for electronic properties. The atomic positions in all structures were relaxed using the CG algorithm until residual forces were below 0.025 eVÅ−1. Pristine monolayer and bilayer graphene supercells considered have 160 and 320 atoms, respectively. We used 20 Å of vacuum in the z direction to prevent spurious interaction between our simulated supercells and their images given that periodic boundary conditions (pbc) are used. The projector augmented-wave method (PAW)65 was used and the kinetic energy cutoff for the plane wave expansion was optimized to 560 eV.All simulations were performed with Characterization of nitrogen doped graphene bilayers synthesized by fast, low temperature microwave plasma-enhanced chemical vapor deposition


The root system architecture (RSA) of plants is highly dependent on the surrounding nutrient environment. The uptake of essential nutrients triggers various signaling cascades and fluctuations in plant hormones to elicit physical changes in RSA. These pathways may involve signaling components known as small signaling peptides (SSPs), which have been implicated in a variety of plant developmental processes. This review discusses known nutrient-responsive SSPs with a focus on several subclasses that have been shown to play roles in root development. Most functionally well-characterized cases of SSP-mediated changes in RSA are found in responses to nitrogen (N) and phosphorus (P) availability, but other nutrients have also been known to affect the expression of SSP-encoding genes. These nutrient-responsive SSPs may interact downstream with leucine-rich repeat receptor kinases (LRR-RKs) to modulate hormone signaling and cellular processes impacting plant root development. SSPs responsive to multiple nutrient cues potentially act as mediators of crosstalk between the signaling pathways. Study of SSP pathways is complicated because of functional redundancy within peptide and receptor families and due to their functionality partly associated with post-translational modifications; however, as genomic research and techniques progress, novel SSP-encoding genes have been identified in many plant species. Understanding and characterizing the roles of SSPs influencing the root phenotypes will help elucidate the processes that plants use to optimize nutrient acquisition in the environment. Plants require macronutrients and micronutrients from the soil to grow and develop. Due to various geological processes and soil chemistry, these nutrients are heterogeneously distributed in patches and gradients . Thus, pOne way in which plants sense changes in nutrient availability in the environment and convey the information to downstream physiological processes is through the expression of nutrient-responsive small signaling peptides (SSPs). SSPs are a class of proteins ranging from 5–75 amino acids in length. Many SSPs are derived from nonfunctional precursor proteins, which are cleaved and post-translationally-modified to mature forms which may then carry out their function as short- or long-distance mobile signals. However, other SSP families may be derived from functional precursors, the 5’ region of mRNA, primary transcripts of miRNA, or small open reading frames (sORFs) ,7. TheseRegardless of the current lack of receptor identification, SSPs have been implicated in a broad range of downstream processes throughout the plant, such as meristem maintenance, cell proliferation and expansion, reproduction, and response to pathogens ,11,12,13This review examines how nutrient signals are interpreted by SSP pathways to alter root morphology for the improvement of nutrient uptake. Most current evidence suggests that a few SSP families play important roles in regulating macronutrient-responsive changes in RSA, particularly in regard to N- or P-availability or maintaining nutrient uptake and homeostasis . However3−) is highly mobile and easily leaches due to the flow of water; in certain plant species, this may encourage the root system that is “steep, cheap, and deep” to acquire NO3− that has stratified in the lower soil levels [4+) can elicit different changes in RSA. When N-depleted plants grow into a NO3−-rich patch, lateral root elongation is stimulated in order to acquire more N, but in NH4+-rich patches, more lateral roots are initiated and existing roots become more branched [3− conditions, the RSA profile exhibits long primary roots with short lateral roots, while excess NH4+ inhibits primary root growth [3− supply after a period of N-starvation also induces root hair development [3− modulates RSA has been studied in depth and involves complex interactions between the NITRATE TRANSPORTER 1.1 (NRT1.1) and auxin signaling pathways [Variation in availability of the essential macronutrient nitrogen (N) is known to elicit significant changes in plant RSA. Within the soil profile, nitrate  gene expression is induced, and mature CLE3 peptides are produced and secreted from the root pericycle, which bind to the CLV1 LRR-RLK expressed in the phloem companion cells. This interaction signals to yet uncharacterized downstream components to repress the lateral root development. This model was suggested from an experiment validating the ligand-receptor relationship in transgenic Arabidopsis lines that overexpress CLE3, which have a decreased lateral root density correlating with the increased CLE3 transcript accumulation when CLV1 is present [CLE3 peptide-encoding gene is induced by NO3−-starvation, it is induced by NH4+ supply after N-starvation [N-availability alters the expression of various SSPs linked to these observed changes in RSA. One peptide family in which select members have exhibited N-responsiveness is the CLAVATA3 EMBRYO SURROUNDING REGION RELATED (CLE) family. CLEs are 12–13 amino acid peptides that control plant development at various stages. While there are thirty-two CLE family members in Arabidopsis, more have been identified in leguminous species where CLE peptides are known to be involved in the autoregulation of nodulation (AON) in addition to RSA response ,23. The  present . Additioarvation , indicatCEP genes leads to the repression of primary root growth and lateral root initiation, while the knockout lines of CEP3 generate larger root systems with a higher lateral root density under N-limited conditions [CEP gene expression increases in roots undergoing local N-starvation, and mature CEP peptides translocate to the shoot as long-distance signals to interact with the CEPR1 and CEPR2 LRR-RKs, which induces a shoot-derived signal mediated by the CEP DOWNSTREAM1 (CEPD1) and CEPD2 polypeptides [Medicago truncatula, low nitrogen upregulates MtCEP1, which interacts with the CEPR1 homolog COMPACT ROOT ARCHITECTURE 2 (MtCRA2) to inhibit the lateral root growth [Another SSP family with members exhibiting responsiveness to N is the C-TERMINALLY ENCODED PEPTIDE (CEP) family. The fifteen members of this family of Arabidopsis are processed to mature 15-amino-acid peptides hydroxylated at the proline residues . Overexpnditions . The CEPpeptides . This sipeptides . Despitepeptides . In Medit growth ,33. Addit growth .Phosphorus (P) is another essential macronutrient known to impact RSA. P is relatively immobile in the soil and is typically localized in higher concentrations in the topsoil layer . P-deficRGF1, RGF2, and RGF3 are induced in the meristematic cortex and epidermis of the root tip under inorganic phosphate (Pi) deprivation [RGF2 exhibit short primary roots and a hypersensitivity to low P-environments, while RGF1 was shown to control circumferential root growth through the repression of radial cellular divisions in response to a Pi deficiency. These changes are proposed to occur through the RGF-mediated signaling pathways, acting on manipulating gradients of PLETHORA (PLT) transcription factors along the root [One of the best characterized SSP families responsive to P-availability is the ROOT GROWTH FACTOR/GOLVEN/CLE-LIKE (RGF/GLV/CLEL) family. rivation . These trivation . A more the root . Throughthe root . RGF-pepthe root ,37.M. truncatula. Members of the INFLORESCENCE DEFICIENT IN ABCISSION (MtIDA), PLANT PEPTIDE CONTAINING SULFATED TYROSINE (MtPSY), and PAMP-INDUCED SECRETED PEPTIDE (MtPIP) families are upregulated upon P-deficiency, but exogenous application of synthetic MtIDA18, MtPSY2, and 9 MtPIP peptides enhances the total root length, especially of the primary root [M. truncatula.While the RGF/GLV/CLEL family regulates RSA in response to P-availability in Arabidopsis, other families have been suggested to play a role in the response based on experiments in ary root . It is, Nicotiana attenuata, NaRALF1 additionally promotes root hair formation, which is proposed to improve P-uptake [SSP pathways governing root developmental processes may still influence the plant’s ability to acquire various nutrients, even if the expression of the SSP-encoding gene is not responsive itself to nutrient availability. The function of the CASPARIAN STRIP INTEGRITY FACTOR (CIF) sulfated peptides represents an example of such constitutive actions of SSPs ,45. CIF1P-uptake ,48. ThesOther non-nutrient responsive SSP pathways are known to impact and modulate RSA, which may indirectly affect nutrient uptake. Many of these established pathways have been shown to have downstream effects on cell structure and maintenance. Among the members of the CLE family, CLE40-ACR signaling is integral for the maintenance of the root apical meristem . IDA pep3− supply, but this response is ablated in the tga1/tga4 double knockout; however, the direct effect of CAPE expression on root hair phenotype, once again, remains to be observed or functionally characterized.Only a small percentage of SSPs have been functionally characterized for their involvement in nutrient response and RSA. Potential candidates for further research are largely identified after their nutrient responsiveness are exhibited at the transcriptional level. Relevance of SSPs to regulatory pathways responding to nutritional availability and their potential influence on root development can be mined from the transcriptome datasets examining the effect of nutrient supply on gene expression or RSA. Microarray data of Arabidopsis grown on split-root media, with half of the root system exposed to N-replete environments and the other half in N-deficient conditions, shows a differential expression of thirty-one SSPs (including various CLEs and CEPs) from multiple SSP families ,51. ThesM. truncatula transcriptome after S- and K-deprivation showed that seventy-two SSP-encoding genes were differentially regulated by S, and one (MtLegin13) was downregulated by a K-deficiency [N and P are indeed the well-characterized macronutrients affecting the SSP-mediated changes in RSA, likely because they induce the most dramatic visible changes in root morphology. However, other macronutrients, such as sulfur (S) and K, as well as various micronutrients may also act through SSP pathways to influence growth and metabolism in the root. Analysis of ficiency . AdditioOne further challenge to interpret the impact of nutrients on the RSA is that most of the experiments reported in the literature only examine the effect of a single nutrient. Plants growing in soil, however, experience combinatorial nutrient cues that could independently cause contradictory changes in RSA. The moderate effects of certain nutrients, such as S and K, on RSA may also be magnified when combined with fluctuations in other nutrient availabilities . CrosstaCLE3 expression is repressed by nitrate, the more desirable source of N used by the plant, but highly induced by ammonium [3− but also to sulfate and phosphate, while other CLE family members exhibit responsiveness to glucose and iron availabilities [LjCLEs) in the Lotus japonicus have been shown to be upregulated in response to P-supply, although these have not been linked to any RSA changes [M. truncatula to N, P, K, and S starvation and resupply. Two hundred forty nutrient-responsive SSPs were purportedly involved in the receptor-mediated signaling; while 61% of these genes were responsive to a single macronutrient deprivation in the roots, the remainder, including the MtCLE family members MtCLE05 and MtCLE34, were responsive to two or three conditions [These cues may be derived from different functional forms of the same nutrient. For example, ammonium ,55. Thisbilities . Corrobo changes . These tnditions . ResearcM. truncatula, recent genome re-annotations have identified 1970 homologs of known SSP gene families and 2455 potentially novel SSP-encoding genes [http://bioinformatics.psb.ugent.be/webtools/PlantSSP/, accessed on 8 November 2018), which predicts close to 40,000 SSPs and over 4000 SSP families based on the annotations of 32 plant species, many of which have agricultural relevance [The field of SSP research is greatly expanding. In Arabidopsis, over a thousand peptide encoding genes have been identified . Likewisng genes . Increaselevance . DespiteMany experiments performed to study SSPs have relied on forward and reverse genetics using transgenic overexpression lines of peptide-encoding genes or knockout mutant lines. One study has also suggested the use of antagonistic peptides to artificially generate a dominant-negative effect to examine the loss of peptide activity . HoweverAnother specific consideration is that many experiments rely on the use of exogenously applied synthetic peptides. While these may indeed phenocopy the transgenic overexpression lines and provide a relatively quicker means of analysis, many synthetically-generated peptides lack post-translational modifications that are necessary for proper biological functioning of the SSPs . Conservclv1 bam double mutant results in a much more severe phenotype than the single mutants in clv1 or bam [Lastly, challenges are present not only in studying SSPs themselves, but also in determining associated LRR-RKs. Many studies on the downstream developmental effects of SSP pathways rely on using the mutant lines of proposed receptors. However, SSPs may bind to multiple receptors and individual receptors may also interact with multiple SSPs ,44,45. F1 or bam . This re1 or bam ,68,69,701 or bam . Aside f1 or bam ,72, or w1 or bam ,74.M. truncatula may be further translated in crops or other leguminous species. More directly, application of validated synthetic peptides to seeds or plants growing in nutrient-limited environments is proposed as a non-transgenic means to improve plant growth and yield. Current research in the field has identified the key roles of SSPs in N- and P-responsive root development, as well as other SSPs either responsive to other nutrients or influential in nutrient uptake and homeostasis. Large-scale genomic datasets have also introduced a better understanding of the number and variation in the putative SSP families, which could potentially impact the RSA or be involved in the crosstalk of various nutrient cues. Further research advancement in this area to elucidate the roles of potentially hundreds of nutrient-responsive SSPs is imperative to fully understand the integration of nutrient cues in RSA phenotypes.While there remain significant technical challenges, SSP signaling modules identified recently have shown a great promise as regulators of root development, controlled in response to nutrient environmental cues. SSP signaling and biology can be applied to various agricultural practices. Improving the basic understanding of these pathways and how they mediate nutrient-responsive changes in the RSA in model organisms such as Arabidopsis and


Lateral graphene p–n junctions are important since they constitute the core components in a variety of electronic/photonic systems. However, formation of lateral graphene p–n junctions with a controllable doping levels is still a great challenge due to the monolayer feature of graphene. Herein, by performing selective ion implantation and in situ growth by dynamic chemical vapor deposition, direct formation of seamless lateral graphene p–n junctions with spatial control and tunable doping is demonstrated. Uniform lattice substitution with heteroatoms is achieved in both the boron-doped and nitrogen-doped regions and photoelectrical assessment reveals that the seamless lateral p–n junctions exhibit a distinct photocurrent response under ambient conditions. As ion implantation is a standard technique in microelectronics, our study suggests a simple and effective strategy for mass production of graphene p–n junctions with batch capability and spatial controllability, which can be readily integrated into the production of graphene-based electronics and photonics. Fabricating lateral graphene p–n junctions with controlled doping levels is instrumental to realize ultrafast and efficient optoelectronic devices. Here, the authors report a seamless graphene based photodetector doped by selective ion implantation and in-situ chemical vapour deposition. To realize the full potential of graphene, fabrication of graphene p–n junctions is vital as they are the basic building blocks of modern semiconductor devices such as diodes, bipolar transistors, photodetectors, light-emitting diodes, and photovoltaic cells6. However, current techniques to produce graphene p–n junctions mainly based on multiple electrostatic gates11, charge transfer from adsorbed chemical dopants15, and electrostatic substrate engineering16, all are difficult to implement in practical applications. In contrast, substitutional doping by introducing dopants into the graphene lattice during chemical vapor deposition (CVD) is an effective means to mass produce doped graphene with the desirable carrier types 20 and tunable Fermi energy levels 23. Using CVD doping, vertical graphene p–n junctions can be created by transfer and stacking of graphene with the desired doping type. However, the fabrication process has severe limitations including contamination and stacking orientation control which must be addressed25. Alternatively, a modulation-doped CVD strategy has been proposed to create lateral graphene heterostructures including i–n junctions26 and p–n junctions6. As the lateral graphene p–n junctions are formed by grafted growth of N-doped graphene from randomly nucleated B-doped graphene, it is challenging to obtain p–n junctions with good spatial control. In this respect, ion implantation which is routinely used in silicon processing to precisely dope bulk semiconductors offers excellent spatial and dose control. Recently, ion implantation has been extended to two-dimensional (2-D) materials including graphene for chemical doping and incorporation of N and B has been demonstrated29. However, the ion energy  much lower than the standard condition for IC industry is required to suppress the formation of implantation-induced defects. Owing to the monolayer or bilayer nature of graphene, the carbon atoms displaced during ion implantation with moderate energy cannot remain as interstitials within graphene basal plane and its immediate vicinities to participate in subsequent damage recovery through recombination of Frenkel pairs30. Therefore, the damage-curing annealing process routinely used in microelectronics processing cannot be applied directly to two-dimensional graphene.Graphene with unique properties has large potential in atomic-level electronic/photonic systems12 cmHz1/2 W−1) and responsivity (1.4~4.7 AW−1). As the ion implantation technology has been widely adopted in the current IC manufacturing, the fabrication of seamless lateral graphene p–n junctions with good spatial controllability, using the combination of ion implantation and in situ CVD, may pave the way for the industrial application of graphene in electronic and photonic devices.Herein, we describe a well-controlled approach to fabricate seamless lateral graphene p–n junctions by selected-area pre-implantation of heteroatoms into the catalytic substrate followed by in situ growth of doped graphene by dynamic CVD. The doping type and dopant concentration are strictly determined by the pre-implanted heteroatom species and pre-designed fluence, respectively. Since the doping process is analogous to in situ doping of epitaxial semiconductor layers in the integrated circuits (IC) industry, both the p-type and n-type regions in the graphene p–n junction are expected to possess an excellent crystalline quality comparable to that of pristine graphene grown under identical conditions. Our strategy combines the substitutional doping advantage rendered by CVD and the spatial and dose control provided by ion implantation. The obtained lateral graphene p–n junctions exhibit a typical rectification behavior together with a superior photo-response rate. The photodetector fabricated on the lateral graphene p–n junction shows excellent performance in the range of 532–1550 nm with high detectivity  to the B-doped graphene (orange). The micro-Raman spectra acquired along the p–n junction confirm that N shifts the 2D band to a smaller wavenumber, whereas B shifts it to a larger wavenumber relative to the pristine undoped graphene. At the junction between the N-doped and B-doped graphene, two characteristic bands corresponding to N doping and B doping emerge simultaneously, as shown in Fig. s of the B-doped graphene and the slightly asymmetric B-1s peak suggests that the predominant bonding for B is B-C33. The deconvoluted high-resolution N-1s spectrum , the B-K edge at 190 eV36 and N-K edge at 403 eV37 are observed from the B-doped and N-doped graphene, respectively. The EELS elemental maps show that either B or N is uniformly distributed throughout graphene as described in Supplementary Figure 38.Figure ray Fig. . The corrum Fig.  shows th15 to 4 × 1016 atoms/cm2, the 2D band of the B-doped graphene shifts gradually from ~2708 to ~2720 cm−1, whereas a monotonic decrease from ~2680 to ~2666 cm−1 is observed from the N-doped graphene , both N-1s and B-1s signals begin to rise at ~ 400 eV and ~ 190 eV, respectively. As the ion implantation fluence is increased from 8 × 1015 to 4 × 1016 atoms/cm2, N-1s, and B−1s peaks become resolved and their intensity increase accordingly. Based on the XPS intensity, Fig. In ion implantation, the dopant concentration can be controlled accurately. Figure ene Fig. . Electro41 as shown in the inset in Fig. * and 1s to σ* transitions, respectively. The sharp excitonic features at 291.5 eV indicate the long-range order in the electronic structure in the B-doped and N-doped graphene. The angular dependence in the C K-edge NEXAFS spectra in Fig. 43. Moreover, the B and N K-edge NEXAFS spectra are similar to the C K-edge NEXAFS spectra showing the 1s → π* and 1s → σ* transitions 44. The two representative STM topographic images obtained from B-doped and N-doped graphene show that the clear honeycomb lattice is interspersed with bright protrusions, which correspond to the substitutional dopants or the intrinsic defects are present as shown in Fig. 45, appears. As the STM tip moves towards the B dopant, the second minimum, i.e., a depression, emerges at the positive sample bias, which is ascribed to the Dirac point (ED = eVD) of graphene. The Dirac point is located at the positive sample bias, which is above the Fermi level, suggesting that the graphene film is doped with holes46. In addition, a noticeable shift in the Dirac point towards a more positive sample bias with enhanced depletion is observed as the STM tip gets closer to the B dopant due to electronic coupling between the B dopant and its neighboring C atom and the maximum shift is observed at exactly the center of the B atom. The recurrence of the Dirac point shift as the STM tip moves away from the B dopant verifies that the Dirac point is associated with B and the shift in the Dirac point is induced by hole charge doping due to B. As for the N-doped graphene, besides the Fermi level (0 eV), the depression associated with the Dirac point is also observed but at a negative sample bias, suggesting that the graphene film is electron doped. Furthermore, due to the distinct STS spectra obtained from B-doped and N-doped graphene, it is concluded that the observed protrusions originate from the dopants, not the suspected defects. Because of the similar electronic coupling effect, the circuitous shift of the Dirac point location is also observed when the STM tip moves across the N-doped region47. The same dependence has been previously reported for graphene supported by individual water clusters on Au (111), where nanoscale water clusters introduce the strong doping of electrons in graphene and the doping level is controlled by the cluster size48.To gain insights into the incorporation of dopants as well as the local electronic structure of the doped graphene, scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) are performed at 77 K on the doped graphene grown on Cu-like alloyIDS−VG curve reveals a typical symmetry in the electron and hole conductance with the charge neutrality at nearly zero volt  and compared with pristine graphene as shown in Fig. 53, as shown in Figure 19 and Supplementary Note 55 ) and near-infrared light  with a fixed power density of 15 mW/cm2 are chosen to excite the p–n junction region. Figure Ids−Vds characteristics of the lateral graphene p–n junction in the dark and under light illumination with different wavelengths. Importantly, a clear current rectification behavior, which is a typical feature for semiconductor p–n junction, is observed for the dark current, as shown in the inset of Fig. 58. However, PV effect should not be the dominant effect, since it usually leads to photocurrents in the same direction. While, as observed in Fig. + or nn−, as described previously62. Although the doping seems homogeneous across large area of doped graphene, the non-uniform doping still exists locally, especially near the p–n junction interface  and responsivity (1.4~4.7 AW−1). Recently, high detectivity (~1012 cmHz1/2 W−1) is also achieved in the broad spectral range (300–950 nm) from the graphene p–n vertical-type tunnelling diodes, but the responsivity (0.4~1.0 AW−1) is inferior to our lateral graphene p–n junction63. The performance of photodetector built on graphene p–n junction in further evaluated by the pulsed light illumination (1550 nm) with variable intensity, as summarized in Fig. 2, and decreases subsequently as the intensity attenuates back to 15 mW/cm2. It should be noted that the photocurrent barely varies under the light illuminations at a fixed intensity, which suggests the photodetector built on graphene p–n junction exhibit good repeatability. In addition, the good reproducibility of the photodetector device is also confirmed by the measurements from multiple devices, as summarized in Supplementary Figure tr) and fall time (tf) are evaluated to be 1.2 and 0.8 μs, respectively. The response speed is superior to that reported previously from graphene-based photodetectors65. We believe that such an ultrafast response should be attributed to the lateral structure of our graphene p–n junctions at which fast transport of photo-generated carriers is maintained along the lateral direction due to the high in-plane mobility, which agrees with TCAD simulation results  mix atmosphere. Finally, CH4 was shutoff and the sample was cooled down to room temperature in a mixture of Ar and H2 (200:10 sccm flow rate). The optimized thermal procedures are summarized in Supplementary Figures The Ni/Cu bilayer substrate was formed by the deposition of a 300 nm thick Ni layer on a high-purity Cu foil  by an electron beam evaporator. Then, 60 keV B and N ions with different fluences were implanted into the selected areas specified by photolithography. The Ni/Cu bilayer substrates were firstly cleaned by acetone, isopropyl alcohol, and deionized water several times and then annealed at 950 °C for 30 min in Ar, HThe doped-graphene films were transferred onto an oxidized silicon substrate or quartz slides for analysis and characterization by a PMMA assisted wet transfer method. Detailed experimental procedures are provided in and Supplementary Note Raman scattering (HORIBA Jobin Yvon HR800) was performed to evaluate the thickness, quality, and uniformity of the doped graphene films. Crystallographic information and the number of doped-graphene layers were determined by transmission electron microscopy (TEM) (FET-Tecnai G2F20 S-7WIN). The STM measurements were performed in the constant current mode by applying a bias voltage to the sample. The polarization dependence of the π- and σ-resonances of the graphene- and graphite-functionalized systems were investigated by NEXAFS at beamline 20A1 of National Synchrotron Radiation Research Center at Taiwan. XPS  and EELS (FEI Tecnai G2 F20) were employed to determine the chemical composition of the doped graphene. The photoelectric measurements were conducted under ambient conditions using an Agilent (B1500A) semiconductor parameter analyzer together with the Keithley 4200 semiconductor characterization system.Supplementary Information


In this paper, we propose a broadband absorption-controllable absorber based on nested nanostructure graphene and a narrowband frequency-tunable absorber utilizing gold-graphene hybrid structure in the terahertz regime. The numerical simulation results showed that the absorption of the broadband absorber can be changed from 27% to more than 90% over 0.75 to 1.7 THz by regulating the chemical potential of graphene. With the same regulation mechanism, the absorbing peak of the narrowband absorber can be moved from 2.29 to 2.48 THz continuously with absorption of 90%. Furthermore, via the cascade of the two types of absorbers, an independently tunable dual-band absorber is constituted. Its absorption spectrum is the superposition of absorption-controllable absorber and frequency-tunable absorber. The absorptivity and operating frequency of the two absorbing bands can be tuned independently without mutual inference. Moreover, it is insensitive to the polarization and it maintains high absorption over a wide range of incident angle. For the flexibility, tunability as well as the independence of polarization and angle, this design has wide prospects in various applications. In recent years, terahertz (THz) wave, lying between the microwaves and infrared light in the electromagnetic spectrum, has attracted great attention of researchers for its potential applications in communication, imaging, sensing, spectroscopy, etc. ,2,3,4. WHence, the research on development of tunable THz absorbers based on graphene has been expanding fast recently. To enhance the absorption and tunability, THz absorbers usually take advantage of patterned graphene, graphene-gold hybrid structure, and gold patches along with monolayer graphene ,15,16,17In this paper, we propose a dual-band THz graphene absorber with a broad absorption-controllable band and a narrow frequency-tunable band using the cascade method. Firstly, a broadband absorption-controllable absorber is designed. This absorber consists of periodic nanostructure graphene layer placed over dielectric layers backed on a reflector. The nested square ring and patch of graphene sheet achieves a high switching intensity of absorption over a wide band when adjusting the chemical potential via external gate voltage. Secondly, a narrowband frequency-tunable absorber is proposed. It is composed of a graphene-gold hybrid layer, a substrate spacer, and a gold grid, exhibiting a typical sandwich structure. The graphene strips deposited on the top layer dissipate the resonant power of gold structure, thus forming an absorption band. Besides, via controlling the external gate voltage, the resonant frequency can be regulated for the perturbation caused by the regulation of chemical potential in graphene strips. Finally, replacing the gold reflector of absorption-controllable absorber with the frequency-tunable absorber of the sandwiched structure, an independent tunable dual-band absorber is constructed.Bk is the Boltzmann’s constant, e is the charge of an electron, and h/2π is the reduced Planck’s constant, FE is the chemical potential (Fermi energy), τ is the relaxation time set as 0.1 ps, which is typical for experimentally studied graphene [H(Ω) is defined as:Graphene is considered as an infinitesimally thin material, which can be modeled as a two-dimensional sheet ,27. The graphene , T is rogZ = 1/σg. The real and imaginary parts of the surface impedance as a function of chemical potential is illustrated in FE, the surface impedance decreases gradually. Therefore, the surface impedance of graphene can be effectively regulated by changing the chemical potential.According to the Pauli exclusion principle, the inter-band contribution of graphene conductivity can be safely neglected in low THz region . Then, tFv is the Fermi velocity, 0ε and rε are the permittivity of free space and the substrate between electrodes, separately. gV is the gate voltage, e is the electron charge, and sd is the thickness of the spacer between electrodes. Therefore, by regulating the surface impedance of graphene through the gate voltage, the intensity of surface plasmon resonance can be tuned effectively, thus controlling the absorption of THz absorber based on graphene.To manipulate the chemical potential of graphene via electrostatic doping, an external gate voltage is applied between the graphene layer and underneath layer. The relation between chemical potential and gate voltage can be approximately expressed by the equation ,33:(5)EF2-doped Si-SiO2 sandwich structure, a gold ground. The nested square ring and patch of graphene can form resonance and excite localized surface plasmon resonances, which are hybridized with each other leading to a broadband absorption band [2 layers function as spacers to separate the graphene, doped Si, and gold. It is a normal substrate and doesn’t have effects on the absorption. Doped Si plays the role of another electrode gate to realize the dynamical control of chemical potential of graphene [εAu = 1−ωp2/ω(ω+iγ0), where bulk plasmon frequency of gold is ωAu = 1.37 × 1016 S−1 and the collision frequency is γ0 = 4.08 × 1013 S−1 [Firstly, a broadband absorption-controllable absorber is proposed. ion band . The diagraphene ,35,36. T1013 S−1 . The str2-gold sandwich structure. The front layer is a square gold patch array surrounded by the gold grid, and graphene strips are evenly deposited at the bottom of the slot. The patch-grid periodic metal structure generates a parallel capacitance and inductance (LC) resonance [2 acts as a spacer and also as a support layer. The back gold grid in staggered arrangement plays the role of electrode gate. As mentioned above, graphene sheet behaves like the series of inductance and resistance. By increasing the voltage between front and back layer, the inductance of graphene drops gradually, the total paralleled inductance of the resonance decreases as well, thus moving the absorbing frequency. The detailed structural parameters are shown in the caption of Secondly, a narrowband frequency-tunable absorber is designed. esonance ,40, whos1f is controlled by the voltage between nanostructure graphene and doped Si layer 1V and the frequency of the second operating band 2f is tuned via the voltage between the gold-graphene hybrid layer and back grid layer 2V. Since the nanostructure graphene is almost transparent to incident wave of 2f and the frequency-tunable absorber reflects all of the transmitted wave of 1f, two absorbing bands can be controlled individually without impacting each other. In theory, the doped Si can play electorate for both two different absorbers, but the distance between doped Si and gold-graphene hybrid structure is quite long, which may lead to an extremely high bias voltage. The introduction of cross feed network can solve this problem effectively. Besides, as shown in Finally, an independently tunable dual-band absorber is constituted by substituting the frequency-tunable absorber for the gold ground of the broadband absorption-controllable absorber. The absorption of the first operating band R = ǀS11ǀ2 and T = ǀS21ǀ2. Then, absorption can be obtained through A = 1−R−T. The influence of the chemical potential on the absorption spectrum of the broadband absorption-controllable absorber is given in FE = 0.7 eV. As FE decreases from 0.7 to 0 eV, the surface impedance of graphene drops rapidly, the absorption changes continuously from 90% to 27% corresponding the intense of resonances weakened. In the process of chemical potential decline, the rate of change in absorption is gradually accelerated, consistent with that of surface impedance shown in The full-wave simulation is conducted in the Computer Simulation Technology (CST) Microwave Studio (2015). It is a high-performance 3D EM analysis software package for designing, analyzing and optimizing electromagnetic (EM) components and systems . The refs1t) is shown in the s1t, the absorption curves appear a red shift. Besides, the absorptivity drops down gradually at the upper frequency while grows slowly at the lower frequency. In ground backed absorber, the thickness of substrate between patterned graphene and ground has a negative relation with the resonant frequency. Hence, the high-frequency resonance is weakened while the high-frequency one is enhanced. As a result, the operating band of the broadband absorber shifts to lower frequency with the increase of s1t.The absorption spectrum of the broadband absorption-controllable absorber with different thick spacer thickness  sustains a continuous growth from 30% to more than 90%. At the same time, the second absorbing peak located at 2f = 2.3 THz maintains the absorption over 90% without frequency shift due to the fixed F2E. However, as shown in F1E, which leads to the gradual expansion of effective operating bandwidth (over 90%) of the second absorbing peak. F2E with F1E fixed at 0.7 eV. The effective absorbing band from 0.72 to 1.6 THz remains unchanged while the operating frequency of the second peak 2f moves from 2.3 to 2.5 THz, as F2E grows from 0 to 0.3 eV. During the blue shift of 2f, the effective bandwidth of the absorbing peak becomes narrow slowly, which is attributed to the decreased absorption of nanostructure graphene from 2.3 to 2.5 THz. In brief, the dual-band absorber combines the characteristics of broadband absorption-controllable absorber and narrowband frequency-tunable absorber. The absorption at 1f and the position of 2f can be tuned independently and freely.F1E and F2E are fixed at 0.7 and 0 eV. ϕ of incident wave. As ϕ varies from 0° to 90°, the absorption spectrum has no change since the C4 symmetry of the structure ensures the robustness. For TE (Transverse electric) and TM (Transverse magnetic) polarized wave under oblique incident angle θ, the absorption spectrums are shown in 1f and 2f remains over 90% up to 50° for TE polarized wave. However, as θ continues to rise, there is a decline tendency for the absorption at narrow absorbing peak 2f and the low frequency portion of the broad absorbing band 1f. The absorptions at these regions is triggered by the magnetic resonance on the same layer. With the increase of incident angle for TE waves, the transverse magnetic field decreases gradually, so does the intensity of magnetic resonance, resulting in a drop in absorption. As for TM polarized wave, the magnetic resonance and localized surface plasmon resonance are not affected by the incident angle so that the absorption remains more than 90% over a wide range of incidence angles up to 60°. Besides, the broad absorbing band is expanded to higher frequency when θ = 50°, because some parasitic resonances occur and become stronger at large incident angle [The independence of absorption with respect to angle and polarization is essential in practical applications. Therefore, the absorption under different incident angles and polarized wave is investigated. Here, the chemical potentials nt angle ,18. The A dual-band tunable absorber has been designed and demonstrated theoretically, by cascading two different types of absorber. The first broad absorbing band is produced by the hybridization of different resonances in the nanostructure graphene. By changing the voltage between the graphene and doped Si to control the resonance intensity, the absorption can be continuously tuned from 30% to 90% over 0.72 to 1.6 THz. The second narrow absorbing band is formed by the magnetic resonance of gold-graphene hybrid structure. The absorbing peak with 90% absorptivity can be varied between 2.3 to 2.5 THz via controlling the voltage between hybrid layer and back grid layer. The two operating bands are controlled with different electrode gates and can be independently regulated without interference, thus greatly expanding the scope of its applications. This paper presents the general method of designing broadband absorption-controllable absorber and frequency-tunable absorber. In addition, the integration technique for combining the mentioned reconfiguration capabilities is elaborated, which is also available in other frequency for the scalability.