Recent novel research has been focussed on developing a green approach for scalable production of graphene. However, one important parameter, which is often neglected is the bulk temperature of the processing liquid. This paper follows our earlier work where optimal sono-exfoliation parameters of graphite in aqueous solutions were determined based on the measured acoustic pressure fields at various temperatures and input powers. Here, we take the next step forward and demonstrate using systematic characterisation techniques and acoustic pressure measurements that sonication-assisted liquid phase exfoliation (LPE) of graphite powder can indeed produce high quality few layer graphene flakes in pure water at a specific temperature, i.e. 40 °C, and at an optimised input generator power of 50%, within 2-h of processing. UV–vis analysis also revealed that the exfoliation, stability and uniformity of dispersions were improved with increasing temperature. We further confirmed the successful exfoliation of graphene sheets with minimal level of defects in the optimized sample with the help of Raman microscopy and transmission electron microscopy. This study demonstrated that understanding and controlling processing temperature is one of the key parameters for graphene exfoliation in water which offers a potential pathway for its large-scale production. Another study featured the effect of temperature on the graphene exfoliation in the presence of sodium cholate with a shear mixing technique et al.To the best of our knowledge, the effects of temperature on the graphene exfoliation in deionized water (DIW) have not been reported yet, despite a few earlier attempts in the literature. For example, Kim For this reason, this article is specifically aimed at finding the one-step facile ULPE route for producing graphene in DIW at an optimum solution temperature based on the role of cavitation dynamics and following our previous work in 2g RCF  for 30 min using SciSpin One Benchtop centrifuge to sediment un-exfoliated graphite particles/thick flakes to obtain supernatants. Subsequently, UV–vis absorption spectra of as-obtained fresh supernatants were recorded in the wavelength range of 200–800 nm with a Cary-60 spectrophotometer (Agilent Technologies) using quartz cuvettes . Dual-beam mode and baseline correction were used throughout the measurements to scan the samples. It is to be noted that experiments were repeated three times for each combination of parameters to validate the consistency in results. After getting reasonable findings from the preliminary UV–vis measurements, the examined supernatants were drop-cast onto a cleaned silicon substrate  and subsequently dried in a vaccum oven prior to Raman investigations. Further, micro-Raman analyses of the drop-cast samples were performed using a Horiba LabRAM HR Evolution confocal Raman spectrometer with 532 nm excitation. Data collection was performed in the range from 1200 to 3100 cm−1 using a 100 × objective with an average of 10 s acquisitions and used automated cosmic ray removal. Simultaneouly, 2–3 drops were put onto holey carbon coated copper grid  placed on a filter paper to wick away excess solvent and was dried completely for transmission electron microscopy (TEM) investigations. TEM analyses were performed to interrogate the individual flakes using a JEOL 2100F Field Emission Gun operating at 200 kV.A double walled borosilicate glass beaker  filled with 150 ml ultra-pure deionized water  was integrated to a recirculating cooler (Cole Parmer Stuart SRC5) through hose pipes allowing for temperature control. The cavitation intensity in the solution, under the experimental parameters was monitored using two advanced calibrated acoustic sensors. A 10 μm diameter fibre-optic hydrophone  calibrated between 300 kHz and 30 MHz and a 4 mm diameter needle hydrophone  calibrated between 8 and 400 kHz were positioned ∼2.5 cm underneath the sonotrode as shown in 33.1−1.m−1et al.et al.et al.2 carbon atoms and hydrophilic edges with dangling bonds. Therefore, oxygen atoms from water molecules preferentially direct toward the hydrophilic edge sites of the flake, which facilitate the stability of flakes in water. Even though, there are several other reported factors such as matching of surface energies between graphene and solvent Based on the UV–vis results, we further investigated the stability of the slurries (the amount of retained graphene (A/ℓ (at 660 nm)) after a certain period of time, estimated from Lambert-Beer’s law 3.2−1, respectively had been registered in each case. Both D and D’ bands are, attributed to the existence of defects such as edges, presence of functional groups and structural disorders 2 bonded carbon atoms D/IGI) (indicated by black squares) values increased up to ULPE at 40 °C followed by its downfall. We understand this observation is typical for solvent-exfoliated graphene flakes, the D band actuates due to the presence of edge sites with more active dangling bonds, produced with the cutting down of lateral size of flakes Raman spectroscopy is an effective tool for analyzing the defects and thickness of graphene sheets D/IGI ratio is inversally proportional to length of graphene flakes in accordance with equation D/IGI is the defect ratio and <L> is the mean lateral flake size.To put it in another way, the D’/IGI ratios (indicated by red circles) with temperature validates that edge defects are growing with temperature while the decline at 60 °C indicates closing of edge sites. Therefore, this disparate behaviour at 60 °C can be explained on the basis of re-stacking of scissored graphene flakes through reactive edge sites with the neighbouring flakes D/ID’I ratios (indicated by green triangles) for the registered flakes and they were found to be less than 3.5 for each temperature, which indicates the formation of edge defects in accordance to Eckmann et al. studies 2D/IGI ratios (indicated by blue triangles) up to 40 °C corroborates the formation of thinner graphene flakes 2D/IGI ratio for the flakes observed in 60 °C sample confirms the agglomeration/re-stacking of small sized sheets (if produced in large quantity), which is consistent with the trend of defect ratios as discussed earlier. This is also linked with FWHM of the G band  and area of the exfoliated flakes. From TEM observations, <L>/<W> ≠ 1 indicates the formation of asymmetric elongated flakes 2 determined with Image J software.In light of the results discussed above, the sample processed with 50 % input power at 40 °C was further assessed by TEM for morphological investigations. 3.4max), max obtained from FOH showed a similar trend with increasing temperature. Since the FOH is calibrated between 300 kHz and 30 MHz  pressure measurements with the FOH  can also be explained using the schematic presented in Therefore, combining the data from both calibrated sensors we can produce a wide frequency range from 8 kHz up to 30 MHz of registered pressures to analyse our cavitation activity occuring during ULPE under various temperature and input power conditions as shown Acoustic spectra from the NH . AnotherThus, it can be deduced that we can regulate the temperature and input power to such an extent that the intensity of the SWs is diminished  that are42. The contribution of the SW induced pressure was shown to be a useful factor in monitoring and optimising ULPE. In addition, an upward frequency shift along with reduction in high frequency (∼3 MHz) pressure peak intensity of SWs was observed with the increase in solution temperature. Interestingly, the corresponding wavelength of SWs is independent of the liquid temperature and kept in the range of 430–460 µm for all the solution temperature regimes. Even though the selection of low temperature environments may be desirable to produce a high intensity shock pressure field necessary for promoting ULPE, there is always a trade off that exists between the amount of emitted SWs and the extent of cavitation zone (bubbly cloud) formation required to achieve an optimum balance between the two that maximises the exfoliation output. The ability to use pure water to exfoliate graphene with good structural characteristics and reasonable size will stimulate its exploitation in medical applications such as cell imaging, bio-sensing, tissue engineering, cellular interactions in neuroscience and ecotoxicological studies.A systematic study of temperature controlled low frequency (24 kHz) ULPE configurations was performed to gauge the degree of exfoliation as a function of temperature with both characterization studies and acoustic pressure measurements. Based on the results, we conclude that ULPE process at 40 °C at the studied input powers  in pure DIW for 2-hours reduces the thickness of graphite crystallites to FLG, with some induced edge defects which are unavoidable in ULPE processes. Results are also in a very good agreement with previous estimations for the best sono-exfoliation conditions based on acoustic pressure measurements The data that support the findings of this study are available upon request from the corresponding author.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


We find the adsorption is dominated by different mechanisms in sulphur and lithium polysulfides, i.e. van der Waals attraction and formation of coordinate covalent Li–O bonds. The adsorption strength is dependent on the inter-layer distance and electron rich functional groups. Through these mechanisms, sulphur and lithium polysulfides can be successfully retained in porous graphene, leading to improved conductivity and charge transfer in the cathode of Li–S batteries.Lithium–sulfur (Li–S) batteries are emerging as one of the promising candidates for next generation rechargeable batteries. However, dissolution of lithium polysulfides in the liquid electrolyte, low electrical conductivity of sulfur and large volume change during electrochemical cycling are the main technical challenges for practical applications. In this study, a systematic first-principles density functional theory calculation is adopted to understand the interactions between graphene and graphene with oxygen containing functional groups  and sulphur (S via moderate interactions, leading to improved conductivity and charge transfer in the cathode of Li–S batteries.Functionalized graphene can successfully anchor sulfur compounds  Among the various types of carbon materials, graphene has attracted much attention due to its good electrical conductivity and high surface area. Another alternative is reduced graphene oxide (rGO), a layered material with graphene domains and residual functional groups such as hydroxyl, epoxy and carboxyl groups.18,19 Consequently, attempts were made to design cathodes with porous graphene and rGO.20–23 Chen's group24 employed a three-dimensional rGO sponge to produce a sulfur nanogranular film-coated composite cathode with a reversible capacity of 1080 mA h g−1 at 0.1C rate and 86.2% capacity retention after 500 cycles at 1.0C rate.Recently, there has been an increasing demand for higher energy density in energy storage systems such as batteries and supercapacitors for portable devices and electric vehicles. With an energy capacity of 1673 mA h g25–27 However it was realized that anchoring materials with non-polar surfaces, alone cannot mitigate the polysulfide shuttle as they fail to make sufficient interaction with polar lithium polysulfides, and further surface modification is generally needed to chemically bind polysulfides onto the carbonaceous matrix. Recently, density functional theory (DFT) studies were carried out to investigate the discharge mechanisms in Li–S batteries,28,29 and several investigators modelled the interactions between defective graphene, heteroatom doped graphene, and lithium polysulfides.30–32 However, the anchoring mechanisms of porous graphene functionalized with different oxygen groups during lithiation process have not been well understood. In addition, it is unclear if polar groups can facilitate the adsorption of non-polar S8 as they interact with lithium polysulfides and if the functional groups can build up a barrier for electron transfer at the interface between S8 and the substrate.In General, microporous anchoring materials (pore size < 2 nm) could successfully confine polysulfides to achieve good electro activity and long-term cyclability.In this paper, we report a systematic first-principle investigation on the anchoring effects of microporous epoxy-, hydroxyl- and carboxyl-functionalized graphene and elucidate the mechanisms responsive for interfacial interaction and electron transfer.8 (∼0.7 nm).33,34 To model oxygen functionalized graphene (OFG) which consists of hydroxyl, epoxy and carboxyl functional groups, three distinct microporous structures are built using the basic model shown in 35,36 hydroxyl and epoxy groups are introduced on the basal plane and a carboxyl group is introduced at the edge of the plane corresponding to oxygen atomic concentration of 1.4–2.6%.As shown in 37,38 of Materials Studio 2016. Electron–electron exchange correlations are described by generalized gradient approximations (GGA) of the Perdew–Burke–Ernzerhof (PBE) functional.39 To consider the van der Waals interaction, a semi-empirical dispersion potential of DFT-D2 method of Grimme is used.40 Energy, maximum force and maximum displacement are set to 2.0 × 10−5 ha, 4 × 10−3 ha A−1 and 5 × 10−3 A, respectively. Self-consistent-field density is set to 1 × 10−5 eV. Double numerical plus polarization (DNP) is selected as the basis set and effective core potentials (ECP) are employed to describe the core electrons. The corresponding k point grid is generated by the Monkhrost–Pack technique41 for the Brillouin zone sampling and simulations are carried out using a 5 × 5 × 1 grid.Molecule configurations of S compounds, adsorption energies of S compounds to microporous structures (graphene and OFG), charge transfer from S compounds at different lithiation stages, and density of states near Fermi energy region are examined using DFT calculations with DMol3 packageEbind) of each Li2Sx molecules can be calculated as, Ebind = [E2SxLi − (2ELi + ExS)]/2, where E2SxLi, ELi and ExS are the energies of Li2Sx, Li atom and Sx molecule respectively. The adsorption energy (Eads) of each S species to various graphene structures are calculated according to, Eads = ETotal − (Egraphene + ES), where ETotal, Egraphene and ES are the total energies of the system, graphene structure and S containing cluster , respectively. According to the equation, more negative adsorption energy indicates stronger interaction between graphene structure and S cluster. Initially, sulfur species were placed with different orientations inside the graphene structure. Eight different potential configurations of each S compound were considered and after relaxation, corresponding adsorption values were compared to obtain the most stable configuration  and decomposed structure (Li + LiS8/Li + LiS4) is calculated according to, ΔE = E(Li2Sx + graphene) − E(Li + LiSx + graphene), where x = 4 and 8.The binding energy  and the binding energies of lithium polysulfides are shown in To find the structures with the minimum energy in S8 is reduced to S82−, S62−, S42− forming high-order lithium polysulfides and further gets reduced to S22− and S2− forming low-order lithium polysulfides at the latter stages.2 According to our calculations, as x in Li2Sx decreases from 8 to 4, the average Li–S distance decreases from 2.450 Å to 2.405 Å, implying that the interaction between Li and S atom strengthens as the x decreases. The average S–S bond lengths of Li2S8, Li2S6 and Li2S4 are 2.103, 2.138 and 2.118 Å, respectively. Our results revealed that S–S bond lengths of polysulfides are larger than that of S8 molecule which is 2.092 Å due to the less covalent nature of the S cluster within the lithium polysulfide. Due to the weak interaction between Li and S, high order polysulfides tend to ionize easily into Li and polysulfide ions in the electrolyte than low-order Li2S2 and Li2S. MPA is a qualitative method to understand how the charge is distributed among each atom. According to MPA, the average net charge of Li is 0.44, 0.44 and 0.46 in Li2S8, Li2S6 and Li2S4 respectively. Therefore, the interaction between Li and S in Li2S8 is the weakest due to the largest Li–S distance and the lowest average net charge of Li. Moreover, Li2S8 exhibits the lowest binding energy of −3.39 eV, suggesting that it is the most soluble compound among these lithium polysulfides during a charge/discharge cycle, consistent with previous experiments.44 As an intermediate which presents in both high and low voltage plateau regimes of charge/discharge cycle, Li2S4 plays a vital role in the Li–S redox reaction. Elemental S is the starting point of the multi-electron-transfer cathode reaction, and hence the retention of S8 is important for the long lasting performance of a Li–S cell. For these reasons, the interaction between S8, Li2S8 and Li2S4 and the porous graphene deserves further investigation.The most stable configuration of sulfur is the cycloocta-S which consists of covalently bonded 8 sulfur atoms in a crown formation with a calculated average S–S bond length of 2.092 Å. During the initial phase of the discharge process, SEads) increases as the pore size decreases. The adsorption energy of S compounds to a typical anchoring material of the cathode depends on (1) the chemical interaction between the lithium polysulfide and the anchoring material in which a covalent bond can be formed between the Li atom in Li2Sx and the functional group of the anchoring material and/or between the S atom of Li2Sx and the functional group of the anchoring material and (2) the physical van der Waals attraction. Unlithiated S8 is a non-polar molecule, and does not form any chemical interaction with graphene domains. Consequently, the interfacial interaction is mainly governed by the physical interaction. However, as the lithiation begins, apart from the physical interaction, a chemical interaction between Li atoms and the anchoring material is formed. In graphene, regardless of different lithiation stages the physical interaction overpowers the chemical interaction and there is a significant increase of adsorption strength as the pore size decreases from 10 Å to 7.5 Å. When the pore size is 7.5 Å, the adsorption energies are −1.55 eV, −1.50 eV and −1.22 eV for Li2S8, Li2S4 and S8, respectively.As shown in ρ = ρTotal − (ρgraphene + ρS), where ρTotal, ρgraphene and ρS are the electron densities of the system, graphene structure and the S-containing cluster respectively. It can be seen that charge is transferred inside the S species and inside the graphene surfaces, but no apparent charge transfer occurs between them, suggesting no strong chemical interaction. However, by thoroughly comparing the deformation charge density of Li2S8 and Li2S4 with their adsorption sites  for the adsorption system of S8 in graphene and OFG, in which the Ef is set to 0 eV and represented by a vertical dotted line. For S8 adsorbed graphene system, the DOS value at Ef is found to be 4.33 electrons per eV and it reduces to 4.00 and 3.92 electrons per eV when S8 gets adsorbed to hG and cG, respectively . These Lewis-base sites are attracted by the terminal Li atoms of Li2S8 and 2.41 Å to 2.46 Å in Li2S4  on the lithium anode.49Though there is a relatively strong interaction between lithium polysulfides and the OFG, the polysulfide molecule itself remains intact without being dissociated into short chain lithium polysulfides upon adsorption. We observed that after adsorption, the internal Li–S bond length increased slightly from 2.45 Å to 2.48 Å in Li Table S4. Various2Sx by the weakening of the Li–S bond due to the attraction of OFG, we considered the adsorption of the decomposed LiS8 and LiS4 structures along with an isolated Li atom and calculated the energy difference (ΔE) between the decomposed structure and the intact structure (Li2S8 and Li2S4). A negative value for ΔE indicates that the intact structure has lower energy than the decomposed structure and the intact structure is energetically stable. 8 and Li + LiS4 clusters on graphene, hG, eG and cG. According to ΔE values in all the cases, the intact structure proved to be energetically preferable over the decomposed structure. Therefore microporous graphene decorated with oxygen functional groups could be considered as an effective choice as a cathode material for Li–S batteries which strikes a balance between adsorption strength and intactness of high order lithium polysulfides.To further investigate the possibility of the decomposition of Li2S8/Li2S4, we analysed the atomic partial density of states (PDOS) near Ef for the adsorption systems  which closely approaches that of pristine graphene, even at an atomic C/O ratio of ∼10.The DOS near Fermi energy for hG, eG and cG before and after the anchoring of S2S8/Li2S4 by the OFG is clearly observed from the DOS values of hG, eG and cG after anchoring of Li2S8 and Li2S4. When Li2S8 and Li2S4 are adsorbed, due to the newly formed Li–O covalent bond between Li atom of Li2S8/Li2S4 and O atom of the functional groups, more electrons are transferred to the adsorption sites and the DOS curve exhibits a higher value at Ef when compared with the S8 adsorbed system. Therefore polar groups maintain a strong interaction with the lithium polysulfides in the discharge/charge cycle of the Li–S battery and in the meantime improve the electrical conductivity of the cathode by facilitating the charge transfer at the interface.The strong affinity towards Li8, Li2S8 and Li2S4 sulfonimide lithium salt (LiTFSI) dissolved in a mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME). To investigate the influence of the electrolyte towards the discharge/charge cycle of the battery, we calculated the adsorption energy values of DOL and DME towards Snd Li2S4  and comp8 is controlled by the van der Waals interaction, and it is highly unlikely for S8 being dissolved in the electrolyte due to the poor attraction of the solvents towards S8 molecules. A microporous substrate is more suitable to confine S8 as it can be confined inside a narrow pore which induces a strong physical interaction. Surface functionalization of graphene has no additional influence on attracting S8 to its surface since almost no chemical interaction can be formed between functional groups and non-polar S8. Furthermore functional groups adversely affect the electron transfer and increase the ohmic resistance at the substrate/S8 interface. However functional groups have a major impact on mitigating the shuttle effect and improving the capacity retention of Li–S cell by ensuring a strong interaction with Li2S8/Li2S4. On the other hand, the strong interaction with lithium polysulfides facilitates the interfacial electron transfer through the Li–O bond which can be explained by the Lewis-acid base theory.Based on the insights gained from our theoretical study, we can conclude that adsorption strength and electron transfer at the interface are two critical aspects which have to be considered when selecting an anchoring material for the cathode in Li–S batteries. At the initial stage of the discharge cycle, confinement of S8) and long chain lithium polysulfides (Li2S8 and Li2S4), via density functional theory computations. During the initial unlithiated stage, we find the interaction between sulphur and pristine graphene and OFG are dominated by van der Waals attraction. Although graphene and OFG exhibits almost the same adsorption capability towards non-polar S8, the functional groups develop a slight barrier for electron transfer at the interface and increase the ohmic resistance. The highest adsorption energy is observed when the distance between two graphene layers approaches to about 7.5 Å. During the lithiation stage, surface functionalization of graphene significantly enhances the interaction with Li2S8/Li2S4 by forming a coordinate covalent Li–O bond. Due to the covalent nature of the Li–O bond, polysulfides are well retained inside the cathode and it also improves the conductivity of the electrode upon the deposition of Li2S8 and Li2S4 by facilitating the interfacial charge transfer. Furthermore, our work explains the reason why porous graphene with oxygen functional groups is more effective as a cathode material compared to materials with too strong interactions which could cause destruction effects on the adsorbed lithium polysulfides. Due to the moderate binding affinity, OFG strikes a balance between adsorption strength and intactness of high order lithium polysulfides. Therefore based on our simulations, we suggest that microporous graphene decorated with hydroxyl, epoxy and carboxyl functional groups can successfully anchor sulfur and lithium polysulfides via moderate interactions, leading to improved conductivity and charge transfer in the cathode of Li–S batteries.In this work, we have investigated the interactions between graphene with oxygen containing functional groups  and sulphur (SThe authors declare no conflict of interest.RA-008-C7RA11628D-s001


However, the specific capacitance of graphene reduced at 800 °C can be increased to 137 F g−1 after nitric acid oxidation treatment, and is only 39 F g−1 after pore forming on graphene surface, demonstrating that the oxygen functional groups can improve the capacitive performances of graphene electrochemical supercapacitors.Graphene is a promising electrode material for supercapacitors due to its superior physical and chemical properties, but the influence of its oxygen functional groups on capacitive performance still remains somewhat uncertain. In this work, graphene sheets with different oxygen content have been prepared through thermal reduction in argon. Furthermore, oxidation and pore-forming treatment of graphene annealed at 800 °C are also performed to explore the important effect of oxygen functional groups. The effects of disorder degree, surface area and oxygen functional groups on the specific capacitance were explored systematically. The content and species of oxygen functional groups are found to be significant factors influencing the electrochemical supercapacitor performance of graphene electrodes. The specific capacitances of graphene annealed at 200, 400 and 800 °C are 201, 153 and 34 F g The content and species of oxygen functional groups are significant factors influencing the electrochemical supercapacitor performance of graphene electrodes. Commonly, porous carbon  is used as the electrode material for supercapacitors.4–8 Graphene, a monolayer of sp2 hybridized carbon atoms in honeycomb structure, has recently attracted extensive interests as a promising supercapacitor electrode material owing to its large theoretical specific surface area, high intrinsic electrical conductivity, outstanding mechanical flexibility and high chemical stability.9–11Supercapacitors, one of the most promising energy-storage devices with high power density and long cycle life, have drawn considerable attention due to their wide potential in hybrid power sources, such as electrical vehicles and uninterruptible power supplies.et al.12 and Vivekchand et al.13 pioneered supercapacitors based on graphene by chemical and thermal method, respectively, there have been numerous reports on the application of graphene-based electrode materials as supercapacitors.11,14–18 Generally, chemical attribute is also a critical factor for the capacitive performance of porous carbon besides specific surface area, electrical conductivity and pore geometry.19 Furthermore, surface functional groups including nitrogen, oxygen and phosphorous can considerably increase the capacitance through pseudo-capacitance effects, as well as improving the wettability of porous carbon with electrolytes.20–25 It have been widely demonstrated that the doping heteroatoms , which contribute to the pseudo-capacitance, is an effective approach to improve the capacitive performance of graphene.26–29 Nevertheless, the oxygen functional groups also play significant role in affecting the capacitive property of graphene as supercapacitors electrode material, to which little attention have been payed.30–32 In most cases for graphene with high specific capacitance value, C/O ratios are relatively low depending on the synthetic rout.11 In addition, Xu et al. reported that graphene oxide (GO) shows a higher capacitance than graphene because its abundant oxygen-containing functional groups provide additional pseudo-capacitance.19 However, Banks and his coworkers demonstrated the inverse results, in which the specific capacitance increases when the oxygenated species reduces.30 While only the electrochemical performance of graphene and GO were investigated in the two works mentioned above. Moreover, in most researches, graphene electrode materials prepared by reducing GO inevitably possess a certain amount of oxygen functional groups.33–37 Thus, it should never be neglected that the effect of oxygen functional groups to capacitive behavior of graphene electrochemical supercapacitor. Consequently, the systematic investigation is necessary and urgent to clarify the contradictory understanding.Since Ruoff −1 after oxidation treatment. This work probably provides a new insight for designing and synthesizing graphene-based materials of supercapacitors.In this work, we present a systematic investigation about influence of oxygen functional groups on capacitive properties of graphene as the electrode material for supercapacitors. Graphene with different oxygen content were synthesized by changing thermal reduction temperature. The effects of structural parameters such as disorder degree, surface area and oxygen content on specific capacitance were studied in detail. Our results indicate that oxygen functional groups in graphene are the critical factor improving their electrochemical capacitor performance. The specific capacitance of graphene annealed at 800 °C can be raised from 34 to 137 F g38 The final suspension of GO was frozen in a fridge (−18 °C), and then transferred into a freeze drying equipment and evaporated in vacuum at the temperatures below 0 °C for 2 days to obtain GO powders. Graphene was prepared by thermally reducing GO powders in a simple horizontal tube furnace in argon atmosphere with heating rate of 10 °C min−1. The annealing temperature was varied from 200 to 800 °C for 1 h to control the deoxygenation degrees of graphene. The obtained samples were denoted as G-200, G-400 and G-800, respectively.GO was synthesized from natural graphite flakes using a modified Hummers method.G-800 was oxidized by nitric acid to study the importance of oxygen functional groups. Typically, 0.2 g G-800 and 60 mL nitric acid were mixed at room temperature and subsequently stirred for 12 h and then washed and filtered. After drying under vacuum, oxidized G-800 (OG-800) were obtained.39 and the detailed synthetic procedure is as following. GO suspension mixed homogeneously with cobalt acetate solution through severe stirring. The mass ratio of GO/Co was 10/1. After sonication, the final solution was frozen, transferred into freeze drying equipment. The dry mixture powder was heated at 800 °C for 1 hour in a tube furnace under a flowing Ar gas. Finally, the black powder was treated by diluted hydrochloric acid, followed by washing with water. The resulting powder was dried under vacuum to obtain PG.Porous graphene (PG) was prepared by a metal etching method as reported in our previous work,SEM images were acquired using a scanning electron microscopy . X-ray photoelectron spectra (XPS) were carried out by an AXIS ULTARDLD spectroscopy from Kratos. Nitrogen adsorption/desorption measurements were recorded at 77 K in Micromeritics ASAP 2020M instrument. Raman spectra analysis was conducted by a Renishaw inVia Reflex Raman Spectrometer, with a 532 nm-wavelength laser. FT-IR spectra were collected using a Bruker Tencor II instrument. Contact angle meter SL200B  was employed to measure the contact angle of the samples.N-methylpyrrolidone solution. Then the slurry was casted on nickel foam and dried at 100 °C for 12 h in vacuum. Finally, the coin cell, which was a symmetrical two-electrode unit cell, was assembled using a glass fiber filter paper as the separator. Several drops of 5 M KOH as the electrolytes were added before the cell was pressed to seal for measurement. The cyclic voltammogram (CV) and galvanostatic charge/discharge tests were carried out using CHI 760E. The cycling performance was tested through galvanostatic charge/discharge using a LAND CT2001A battery test system. The electrochemical impedance spectroscopy (EIS) was characterized by using a Princeton Verstat3 electrochemical workstation. The frequency range for impedance spectra was from 100 kHz to 0.01 Hz with 10 mV voltage amplitude.The electrochemical properties of as-prepared samples for supercapacitors were investigated using CR2032 coin cells at room temperature. A slurry was prepared first by mixing the active materials, polyvinylidene fluoride and Super P with a weight ratio of 8 : 1 : 1 in Cs were calculated according to the following equations:Ccell is the specific capacitance of the coin cell, m is the total mass of two electrodes, v is the scan rate, (Vc − Va) represents the sweep potential range, and I(V) denotes the response current.To quantitatively compare the capacitance performance of these electrode materials, the values of specific capacitance 27 Moreover, the graphene nanosheets of the three samples are transparent due to their very thin thickness. In addition, Raman spectra of these samples are depicted in −1 (D band) and 1595 cm−1 (G band) are resolved for all samples, which assigned to the E2g phonon of C sp2 atoms and breathing mode of κ-point phonon of A1g symmetry, respectively.31,40,41 The intensity ratios of the D to G bands (ID/IG) are listed in Table S1,ID/IG ratios of G-200, G-400 and G-800 are 0.96, 0.94 and 0.93, respectively. The decreasing ID/IG ratio with increasing the pyrolytic temperature indicates the reduction of disorder degree and the increase in size of sp2 clusters.32 Furthermore, the BET surface area of G-200, G-400 and G-800 are tested to be 248, 302 and 251 m2 g−1, respectively. The much lower value than the theoretical limit of pristine graphene is probably caused by partial overlap and coalescence of graphene nanosheets during reduction process.42Graphene nanosheets with different content of oxygen functional groups were prepared by reducing GO at different pyrolytic temperatures from 200 to 800 °C, which were denoted as G-200, G-400 and G-800. Their morphology and nanostructure were characterized by SEM observation. Typically crumpled structures are observed for all samples with a lightly trend toward higher level of wrinkling when annealing temperature increased .27 Moreo43,44 The spectrum of GO indicates the presence of C PBM data was replaced with SVG by xgml2pxml:<glyph-data id="z.dbd" format="PBM" resolution="300" x-size="8" y-size="10" xml:space="preserve">00000000000000000000000000000000111111110000000011111111000000000000000000000000</glyph-data><svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata>Created by potrace 1.16, written by Peter Selinger 2001-2019</metadata><g transform="translate scale" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg>O (1732 cm−1), aromatic CC (1623 cm−1), carboxy C–O (1390 cm−1), and epoxy or alkoxyl C–O (1050 cm−1). While the band at ca. 3423 cm−1 could be due to the O–H stretching mode of intercalated water. After thermal treatment, the peaks for oxygen functional groups were reduced significantly, and the peak at 1390 cm−1 for carboxy C–O was nearly entirely removed. After annealed at 800 °C, the carboxyl groups are considerably decreased. The surface chemistry of GO before and after thermal reduction were further characterized using X-ray photoelectron spectroscopy (XPS). The XPS spectra of GO, G-200, G-400 and G-800 over a wide range of binding energy (0–1000 eV) are shown in Fig. S2.2 C–C, C–OH, CO and OC–OH groups, respectively.40,45,46 It should be mentioned that these graphene samples also possess an epoxy group (C–O–C), which have a C 1s binding energy similar to C–OH.40,46 The area percentages of each peak, which corresponds to the content of these four C species are summarized in Table S1.2 C–C increases, which matches well with the Raman results. In a word, there are obvious differences among the content and species of oxygen functional groups of the three grapheme samples. Thus the electrochemical properties of these graphene electrode materials may be different.The as-prepared samples were characterized by FTIR spectra. Fig. S147−1 scan rate in 5 M KOH electrolyte between 0 and 0.8 V. The CV curves of G-200 and G-400 show a box-like shape with a well-broadened peak at 0–0.6 V, inferring the presence of pseudo-capacitance. However, the CV curves of G-800 shows a nearly rectangular shape, which is typical of a capacitance behavior. Moreover, the capacitive behavior difference among the three samples is confirmed by galvanostatic charge–discharge curves at a current density of 1 A g−1, as shown in 19,20 In addition, the CV curves for these three samples at different scan rates ranging from 1 to 200 mV s−1 were measured . The specific capacitance decreases when the oxygenated species reduces, demonstrating a remarkable contribution of oxygen-containing functional groups in raising the capacitance of graphene. The rate performances of these samples were also investigated by galvanostatic charge/discharge at different current densities . In contrast, the specific capacitance of OG-800 is as high as 107 F g−1, approximate five times larger than the value of G-800, demonstrating that the oxygen functional groups can greatly improve the capacitive performance of graphene electrode materials. However, not all these groups are beneficial for reversible electrochemical redox reaction. Previous researches reveal that redox reaction between quinone and phenol groups contribute additional pseudocapacitance, while surface acidic functional groups (carboxyl-type) play negative role in supercapacitors.31,48,49 G-400 and OG-800 were chose for comparison because the oxygen contents of the two samples are relatively close. As shown in Table S1,−1, which is higher than that of OG-800. Moreover, the CO and COOH group contents of G-400 are 9.40% and 12.91%, respectively, which are lower than that of OG-800. However, the C–OH group content of G-400 is 17.89%, which is larger than that of OG-800 (13.17%). Therefore, we infer that the C–OH group is more contributive to the improvement of capacitive performance. The pseudo-capacitance of graphene probably comes from the electrochemical reactions, e.g. PBM data was replaced with SVG by xgml2pxml:<glyph-data id="z.dbdsl" format="PBM" resolution="300" x-size="9" y-size="15" xml:space="preserve">011000000001100000000110000000011000000001100000000110000000010000000000000000010000000110000001100000011000000110000001100000011000000</glyph-data><svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="10.400000pt" height="16.000000pt" viewBox="0 0 10.400000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata>Created by potrace 1.16, written by Peter Selinger 2001-2019</metadata><g transform="translate scale" fill="currentColor" stroke="none"><path d="M80 1160 l0 -40 40 0 40 0 0 -40 0 -40 40 0 40 0 0 -40 0 -40 40 0 40 0 0 -40 0 -40 40 0 40 0 0 -40 0 -40 40 0 40 0 0 -40 0 -40 40 0 40 0 0 -40 0 -40 40 0 40 0 0 80 0 80 -40 0 -40 0 0 40 0 40 -40 0 -40 0 0 40 0 40 -40 0 -40 0 0 40 0 40 -40 0 -40 0 0 40 0 40 -40 0 -40 0 0 40 0 40 -80 0 -80 0 0 -40z M560 520 l0 -40 -40 0 -40 0 0 -40 0 -40 -40 0 -40 0 0 -40 0 -40 -40 0 -40 0 0 -40 0 -40 -40 0 -40 0 0 -40 0 -40 -40 0 -40 0 0 -40 0 -40 -40 0 -40 0 0 -40 0 -40 80 0 80 0 0 40 0 40 40 0 40 0 0 40 0 40 40 0 40 0 0 40 0 40 40 0 40 0 0 40 0 40 40 0 40 0 0 40 0 40 40 0 40 0 0 80 0 80 -40 0 -40 0 0 -40z"/></g></svg>C–OH ⇔ CO + H+ + e−, etc., at the electrode interfaces.50,51 Certainly, the detailed electrochemical reaction mechanism still need be further studied.The electrochemical performances of PG and OG-800 were also measured, as shown in e curves  also demd Fig. S9. The sha−1. 52 Moreover, the semicircles are mainly related to the summation of the contact resistance and charge transfer resistance. The equivalent series resistance (ESR) value determined from the first intersection of the semicircle with the real axis.31,52 The ESR of PG, G-800 and OG-800 are 0.89, 1.29 and 1.34 Ω, respectively. The small semicircle diameter and ESR observed for all three electrodes, which match well with their outstanding rate performance.The cycling stability was tested by galvanostatic charge/discharge at a current density of 2 A g−1, the specific capacitance of G-800 can be increased from 23 to 107 F g−1 after nitric acid oxidation treatment. However, the specific capacitance of PG is only 27 F g−1. These results have special references to design and prepare graphene-based supercapacitor electrode materials.In summary, graphene sheets with different content of oxygen functional groups have been prepared through thermal reduction of GO, and their electrochemical performances were investigated as electrode materials for supercapacitors. Moreover, the electrochemical performance of G-800, OG-800 and PG were compared. The structure and surface properties, especially the effect of oxygen functional groups on the specific capacitance of graphene electrodes were explored systematically. The content and species of oxygen functional groups were proved to be critical factors influencing the capacitive performance of graphene electrochemical supercapacitors. The specific capacitance decreases when the oxygenated species reduces. In addition, at 20 mV sThere are no conflicts to declare.RA-008-C7RA12425B-s001


Surface plasmons in graphene provide a compelling strategy for advanced photonic technologies thanks to their tight confinement, fast response and tunability. Recent advances in the field of all-optical generation of graphene’s plasmons in planar waveguides offer a promising method for high-speed signal processing in nanoscale integrated optoelectronic devices. Here, we use two counter propagating frequency combs with temporally synchronized pulses to demonstrate deterministic all-optical generation and electrical control of multiple plasmon polaritons, excited via difference frequency generation (DFG). Electrical tuning of a hybrid graphene-fibre device offers a precise control over the DFG phase-matching, leading to tunable responses of the graphene’s plasmons at different frequencies across a broadband (0 ~ 50 THz) and provides a powerful tool for high-speed logic operations. Our results offer insights for plasmonics on hybrid photonic devices based on layered materials and pave the way to high-speed integrated optoelectronic computing circuits. Nano-photonic devices based on 2D materials offer a potential solution for the miniaturization of optical computing technologies. Here, the authors demonstrate the implementation of high-speed logic operations via the all-optical generation and electrical control of multiple plasmon polaritons in a hybrid graphene device. In recent years, intense research in the field of optical computing has led to novel and advanced solutions for high-speed modulation and demodulation5, metamaterial analog processing7, optoelectronic convolution9, machine learning13 and quantum computing14. Since logic gates are the most fundamental building-block of classical and quantum computing15, their development is nowadays of paramount importance. While classical electrical computing is based on transistors, a gold standard for optical logic gates is still elusive. For instance, by exploiting the linear interference in micro-rings and micro-couplers, optical logic operations have been successfully obtained on-chip20. However, the diffraction limit21 hinders further miniaturization of such devices and creates a bottleneck for industrial expansion.Nano-photonic technologies hold great promises for high-speed signal processing, as they circumvent light-electron conversion and offer the ideal solution to the problems of heat generation and limited bandwidth that are typical of electronic circuits23, graphene plasmons are promising candidates for the realization of miniaturized photonic integrated signal-processing elements beyond the diffraction limit of light25. In addition, in contrast to noble metals, graphene’s plasmons display flexible electrical tunability28, which in turn offers a powerful knob for dynamic information processing. Thanks to the strong nonlinear response of graphene32, such electrically tunable plasmons can be generated all-optically and subsequently manipulated and detected directly on-chip34, providing a viable strategy for planar integration in high-density optoelectronics. The main downside of the conventional methods that are used for the nonlinear generation of plasmons in graphene is the necessity of a careful laser scanning with a tunable range up to 10 THz, which is typically slow and has limited bandwidth and thus prevents the realization of multiple plasmonic generations for ultrafast logic operations. For instance, in ref. 33, only one graphene plasmon mode with a single terahertz frequency was generated.On the other hand, due to their high resonant frequencies (≈tens of THz) and unique field confinement (≈hundreds of nm)36 are counter-propagating inside a hybrid graphene/D-shaped fibre and are used as pump and probe pulses for the DFG in a broad frequency range up to 50 THz. In particular, we simultaneously detect multiple plasmon peaks thanks to the broadband frequency-momentum conservation. Finally, by electrically tuning the graphene’s Fermi level, we show individual modulation of each of the co-generated plasmons, thanks to the distinct phonon-plasmon interactions in different branches. This provides a powerful method to realize different logic operations  on a single integrated photonic device.Here, we exploit difference-frequency-generation (DFG) to demonstrate multiple plasmons co-generation in a hybrid graphene photonic device. In this scheme, two synchronized and stabilized mode-locked lasers (i.e. laser frequency combs)37. The fibre core is 6 μm in diameter and light-graphene interaction occurs via evanescent waves38; the total linear losses of the graphene-fibre heterostructure are <0.5 dB at 1560 nm when the |EF | of graphene is >0.4 eV. Clearly, by tuning the graphene Fermi level, the transmission loss of the device will change. Further details on the nanofabrication and characterization of the GDF are provided in Supplementary Note χ(2) of the GDF, the high-frequency pump photon (fpump) splits into a lower frequency probe photon  and a plasmon . The DFG requires both energy conservation fsp + fprobe = fpump and momentum conservation kpump = ksp + kprobe, where ħksp, ħkprobe and ħkpump are the momenta of the plasmon, probe and pump, respectively. Considering the scalar dispersion relation k = 2πfn/c, the momentum conservation can be re-written as fspnsp = fpumpnpump + fprobenprobe. In order to generate plasmons in graphene via the DFG process, all the optical modes must have transverse magnetic (TM) polarization that probes the non-centrosymmetric out-of-plane direction of the hybrid device, while graphene is centrosymmetric for in-plane excitation40. As an example, Fig. fpump = 192 THz and fsp = 10 THz.Figure 41, this on-fibre Au-graphene-Au transistor is driven by current rather than gate voltage42. Within the Drude’s model, the surface-plasmon frequency fsp is defined by the graphene’s dispersion relation which, in turn, depends on the Fermi level: a higher carrier density induces a higher fsp. In our GDF architecture, by changing the source-drain voltage (VD) in the range 0 to 1 V, we can tune the effective graphene Fermi level |EF | from ≈0 to 0.4 eV. Figure EF | as a function of VD. When VD = 0 V graphene is intrinsically positively charged (p-doped) with |EF | ≈ 0.2 eV. By increasing VD we drive a current in the graphene channel and shift EF towards the Dirac point and the upper Dirac cone (n-doping). The graphene’s resistance reaches the maximum (Dirac point) when VD = 0.16 V. We notice that it is experimentally impossible to achieve EF = 0 eV due to the presence of puddles and inhomogeneities43. However, for simplicity we define EF = 0 eV as the point where we measure the minimum conductivity in the GDF. Such behaviour is typical of a graphene bipolar junction42, as discussed in detail in Supplementary Note VD dependence of EF, we have also characterized the doping via in-situ Raman spectroscopy , in analogy with the widely used process of optical parametric amplification45.Figure fsp = 10 THz, we will observe a peak on the flat spectrum of Comb 2 (probe enhancement) at ≈1645.57 nm (182.308 THz). To further prove that this peak arises from DFG, we modulated the pump at 500 kHz and observed the same modulation in the counter-propagating probe comb  of the probe comb, this scheme enables us to find high-frequency plasmons beyond the limitation of any near-infrared tunable lasers.For example, for a pump wavelength of 1560 nm 192.308 THz) and a nonlinearly excited plasmon at 08 THz anksp∝ fsp2):25 The blue curves in Fig. |EF | = 0.1 eV and 0.4 eV. On the other hand, the phase-matching condition for DFG can be re-written as (c/2π)ksp = −fspnprobe + fpump(npump + nprobe). When tuning the pump comb from lower to a higher frequency  while the probe pulse is broadband. Figure 23 at |EF | = 0.1, 0.2 and 0.3 eV, respectively. Here, we considered two surface-optical phonon resonances47 of the silica substrate (fibre), located at f1 = 24 THz and f2 = 35 THz (white solid lines). The phase-matching condition for the DFG is marked by the white dashed line and we used the refractive inside the fibre’s core npump ≈ nprobe = 1.45. When |EF | = 0.1 eV . When |EF | = 0.2 eV . Finally, when |EF | = 0.3 eV . In addition, based on the dispersion relation nsp = (fpumpnpump + fprobenprobe)/(fpump − fprobe), we can estimate the effective refractive index of the plasmonic modes. For instance, when fsp ≈ 7.5, 27 THz and 36 THz we obtain nsp ≈ 68, 19 and 14, suggesting strong confinement of the plasmons. The simulated fibre mode indices are shown in Supplementary Note Figure  eV Fig.  the grapIDFG. In particular, the generation of plasmons and the consequent intensity enhancement at fprobe rely on the consumption of the pump. We thus fix the probe power to 10 mW while increasing the pump power, and we measure both the pump transmission and the total ΔIDFG at different values of |EF | . For low pump power values (<8 mW), the curves at |EF | = 0.1 eV and 0.2 eV are almost identical, while the curve at |EF | = 0.3 eV shows higher transmission, likely due to reduced absorption in the thermally broadened Fermi Dirac distribution (considering the pump photon energy of ≈0.8 eV). Initially, the transmission curves for all |EF | values increase with pump power and reach a plateau at ≈12 mW due to saturable absorption48. Subsequently, for pump powers >12 mW, graphene is fully saturated and a further increase of the power leads to a lower transmission due to pump consumption via the DFG process. For the same value of the pump power (>12 mW) we observe that the ΔIDFG raises from the noise floor and subsequently increases linearly  = [χ(2)]2IpumpIprobe/Lsp2, where Lsp is the EF dependent transmission loss of the plasmon  reaches 10−2 m/V, and [χ(2)]2/Lsp2 is on sub 10−4 W−1 level, in agreement with theoretical values30.For a D-shaped fibre without graphene, we neither observe any plasmon generation nor DFG. This indicates that the process originates from graphene instead of from the gold contacts , in analogy with an auto-correlation measurement , VD = 0.4 V ( | EF | = 0.2 eV) and VD = 0.6 V ( | EF | = 0.3 eV), respectively. At zero delay time we observe the largest nonlinear enhancement ΔIDFG for all EF values. In this measurement, the central pump wavelength is 1560 nm and the time delay is tunable from −5 to +5 ps. The soliton pulse width for the pump and probe combs are ≈ 430 fs and ≈ 110 fs, respectively  are used as input and each of them can provide either 0 V  or 0.5 V . By combining the different states of VA and VB , the graphene’s EF can be tuned to 0, 0.24 and 0.4 eV. The pump and probe beams are launched into the GDF in opposite directions and we detect the gate-tunable plasmon generation via the DFG as an increase in the probe intensity (ΔIDFG) at specific frequencies. For logic operations, three ΔIDFG peaks (defined by the three plasmonic dispersion branches) with fsp in the 0–50 THz band are filtered using three bandpass filters (BPF) based on-fibre Bragg gratings . On the other hand, as previously discussed ; the ΔIDFG from BPF2 is detectable when either VA or VB is ‘ON’ (OR gate) while the ΔIDFG from BPF1 is detectable only when both VA and VB are ‘OFF’ (NOR gate). Figure VA and VB . Accordingly, the different BPFs provide at the output the logic operation expected from the AND, OR and NOR gates. Specifically, the signal-to-noise ratios of the AND, OR and NOR gate outputs are higher than 89%, 82% and 77%, respectively, thus allowing precise discrimination of the logic operation. In future, the plasmonic co-generation and control scheme demonstrated here could be easily extended to on-chip integrated graphene optoelectronic devices suitable for large-scale fabrication, such as silicon nitride waveguides33 and micro-rings49. Recent advances in the field of heterogeneously integrated microcombs on silicon50 also provide a powerful tool to exploit pulsed dual-comb optical sources for the on-chip generation and control of the multiple graphene plasmons generated by DFG, further boosting the practicality of this scheme on versatile device platforms.Figure In conclusion, we used two synchronized counter-propagating laser combs to excite multiple electrically tunable plasmons via difference-frequency generation and parametric amplification in a graphene/D-shaped fibre device and demonstrate their potential for integrated logic operations. By electrically tuning the graphene’s Fermi level, the co-generated plasmons in the frequency range 7.5–37 THz can be independently controlled and, after proper filtering, they can be used to realize three optoelectronic logic gates  in one single device. The optical generation and electrical control of multiple graphene’s plasmons provide a platform for nanoscale integrated optoelectronic devices with a high potential impact in advanced applications such as ultrafast light-field manipulating, signal processing and optoelectronic computing.χ(2) of graphene is estimated using the density matrix equations. More details are discussed in Supplementary Note The electrical field distribution inside the GDF was modelled using the finite element method. The graphene plasmonic dispersion and phonon coupling were calculated within the random phase approximation. The The D-shaped fibre samples were prepared by side-polishing a commercial single-mode silica fibre. The length of the D-shaped region is 2 mm and the insertion loss of the D-shaped fibre at 1560 nm is <1 dB. Single-layer graphene (SLG) was grown via CVD on Cu foil (99.8% pure) and then transferred onto the D-shaped fibre by using the polymethyl methacrylate (PMMA)-based wet transfer method. After bathing the PMMA/SLG/D-shaped fibre in acetone and water, the PMMA was removed. Then 30 nm/10 nm Au/Ti electrodes were deposited via the masked sputtering method. The effective area of the on-fibre graphene interacting with light is 10 × 200 μm. Details are shown in Supplementary Note A stabilized mode-locked fibre laser with a central wavelength 1560 nm, fixed repetition 38 MHz and pulse width ≈ 300 fs is divided into two paths. The first one has 3 dB spectral bandwidth ≈ 6 nm and it is used as the pump, while the second one is supercontinuum-broadened to cover the spectral range 1560–2100 nm and it acts as the probe. To ensure the highest accuracy during measurement, the frequency combs are stabilized with total relative intensity noise < −120 dB and phase noise < −150 dBc/Hz at 10 kHz. For the DFG plasmon detection we carefully implemented the following points: (1) both the pump and the probe are TM polarized, in order to maximize the graphene-light interaction and to probe its out-of-plane direction; (2) a tunable free-space delay line is used to precisely control the counter-launched pulses, with motion accuracy <1 μm; (3) the maximum peak power of the pump reaches 10 kW, much higher than the graphene saturable absorption threshold. The different logic gates  are selected via three different bandpass filters based on-fibre Bragg gratings with a slightly tunable (≈ 10 nm) central wavelength for the optimization of the output intensity. Details of the experimental setup are shown in Supplementary Note Supplementary InformationPeer Review FileDescription of Additional Supplementary FilesSupplementary Movie 1


Here, we report a new strategy to chemically functionalize graphene for use in FETs without affecting the electrical performance. The key idea is to control the Fermi level of the graphene using the consecutive treatment of gold nanoparticles (AuNPs) and thiol-SAM (self-assembled monolayer) molecules, inducing positive and negative doping effects, respectively, by flipping the electric dipoles between AuNPs and SAMs. Based on this method, we demonstrate a Dirac voltage switcher on a graphene FET using heavy metal ions on functionalized graphene, where the carboxyl functional groups of the mediating SAMs efficiently form complexes with the metal ions and, as a result, the Dirac voltage can be positively shifted by different charge doping on graphene. We believe that the nanoparticle-mediated SAM functionalization of graphene can pave the way to developing high-performance chemical, environmental, and biological sensors that fully utilize the pristine properties of graphene.Since its discovery, graphene has attracted much attention due to its unique electrical transport properties that can be applied to high-performance field-effect transistors (FETs). However, mounting chemical functionalities onto graphene inevitably involves the breaking of sp Consecutive treatment of gold nanoparticles and thiol-SAM molecules induce the recovery of electronic state of pristine graphene. Chemical functionalization can allow graphene FETs to be realized as various chemical19–31 and biological sensing32–41 platforms. The most frequent way to implement such functions has been accomplished by the direct covalent bonding of molecules to graphene,19,20,42–46 such as through an azide group.47,48Graphene was the first of the recently realized ideal two dimensional materials.2 hybrid orbital, which is the intrinsic characteristic of graphene, to a broken sp3 character, and thus inevitably degrades the pristine electrical property of graphene.42 In addition, if not that severe, adopting the heterogeneous species including functional molecules, in general, cannot avoid a significant doping effect, which substantially shifts the charge neutrality point (VCNP) (Dirac point voltage) from the pristine value and limits the detection range in real applications. Thus, our interest is to determine how to introduce functional groups on graphene FETs without impairing the pristine sp2 orbitals.This method, however, converts the spIn this work, we have proposed a novel method to modulate the electrical properties of graphene FET devices by adopting gold nanoparticles and thiol-SAM molecules. Consecutive treatment of the nanoparticle and SAM molecules induces the p-type and n-type doping effects, respectively. By analyzing the doped characteristics both electrically and optically, we realized a functionalized graphene FET device that preserves the close-pristine electronic state of graphene.2/Si) wafers, and then the as-prepared CVD  graphene was transferred onto Au electrodes (step 1). Next, we incorporated the gold nanoparticles onto the graphene surface, and this is the key step (step 2) in our experiments. As shown by the step 3 process in 4−), induced by the redox potential difference  between them.49,50 This electroless method is cost-effective and straightforward since it does not require any extra linking molecules or reducing agents. Only the dipping time, along with precursor concentration (in stock solution), needed to be controlled  analysis confirmed that our deposited gold nanoparticles are uniformly distributed, at ∼4 nm in height  was used to verify that the desired treatment of each step was achieved through the dipping method. The spectra on the far left of each row in 4 exist in the Au3+ state, and when they are reduced to Au NPs, they exist in the Au0 state, or some in the Au+ state. Each binding energy corresponding to 4f7/2 of Au0, Au+, and Au3+ is about 84–84.5, 85–86, and 86–87 eV.55–59 In Δ, is separated in the Au 4f region due to orbital energy splitting as a result of spin–orbit coupling in the 4f orbital of the heavy metal elements. The value of Δ between Au 4f7/2 and 4f5/2 is 3.67 eV,55,57 and this can be found from XPS spectra for the Au NPs. In this process, Au NPs are reduced by the graphene film and the surface of graphene is doped as p-type  change depending on the character of the adsorbed molecule. It has been known that thiol functionalization is likely to induce a negative doping effect, which is electron donating, on the gold nanoparticle surface.65 Such SAM-induced electron donation on the gold nanoparticle then concurrently induces negatively charged doping (n-type doping) on the graphene. Therefore, such succeeding p-type (Au nanoparticles) and n-type (thiol-SAM) doping effects finally result in the restoration of the electronic state of graphene that is close to its pristine state, and this can be used as a metal ion captor in the next step through the functionalized pendent chemical group. In order to confirm its generality, we also compared the FET responses using other kinds of thiol-SAM molecules, showing that all of the thiol-SAM molecules induce a similar n-type doping effect on graphene, while exhibiting different doping degrees depending on the molecular type. Such differences might be associated with molecular dipole moments65 ] as a mercury ion captor. Step 4 in 0 peak in + peak increased significantly. As a control experiment, we confirmed that without 4-MBA SAM treatment, a mercury ion solely induces an opposite negative (electron) doping effect on gold nanoparticle-deposited graphene FETs  value at each step. Although some variation was observed (device to device variation) at each step, sequential treatments generally induced the change in doping state in graphene, from pristine to p-type doped to n-type doped to p-type doped (VCNP: 8.14 → 95.86 → 24.03 → 90.86 V). We calculated the hole mobility of the doped graphene, estimated using the standard model of the metal-oxide–semiconductor FET (MOSFET). The corresponding band structures at the K point in the Brillouin zone of the pristine monolayer graphene are shown in IDS, is calculated using the approximation of:where μ is the carrier mobility, Cox is the capacitance of the gate material, 300 nm of SiO2 (1.08 × 10−8 F cm−2), W is the gate width (100 μm), L is the gate length (40 μm), VGS is the applied gate voltage and VDS is the drain-source voltage difference (1 mV). The carrier mobility finally becomes:In the linear region of the transfer curve , the current from drain to source, μh, of graphene can be measured indirectly. VCNP values: 10 236 → 6563.1 → 6087.3 → 5401.2 cm2 V−1 s−1. Compared with a number of previous reports,5,72,73 our system shows relatively high mobility, which is advantageous in sensing capability. This can be attributed to the low charge puddles or surface defect concentration (see ID/IG ratio) induced by the unbroken sp2 bond character.Using this equation, the hole carrier mobility, 74−1, respectively,75 and such G and 2D peaks signify the frequencies of the phonon interactions at the Γ and K points in the Brillouin zone, respectively. In our case, as shown in −1 and 2682.75 ± 3.47 cm−1, respectively. After the gold nanoparticles were deposited, the peaks shifted to the higher positions at 1594.36 ± 2.98 cm−1 and 2692.59 ± 4.59 cm−1 and, after thiol-SAM (4-MBA) treatment, returned to 1585.95 ± 1.22 and 2684.96 ± 2.98 cm−1, respectively. Finally, mercury ion treatment induces a shift of the G and 2D peak positions to 1590.64 ± 1.50 and 2687.72 ± 0.34 cm−1, respectively.Raman spectroscopy is a valuable optical technique for analyzing the electrical properties of graphene.et al.,75 Raman spectroscopy can be used to optically differentiate between the strain effect and the charge doping effect in graphene samples. versus 2D peak position of our samples, indicating that the point of O  originally corresponds to the strain-free and charge-neutral state. If tensile strain or a p-type doping effect is applied to this graphene, the point  moves in the direction  or , respectively . Even though our pristine graphene state (black square) is slightly far away from the original point of O, it is still positioned along the blue dashed line, which means that only compressive strain has been applied to our pristine sample. Since graphene has an intrinsically negative thermal expansion coefficient in the range of 200–400 °C,76–79 annealing at 300 °C and cooling to room temperature unavoidably results in compressive strain on graphene, and this is the situation in our case. Next, our experiments showed that, in the consecutive treatment of the gold nanoparticles, thiol-SAM molecules and mercury ions, the graphene states in the plot (VCNP in graphene FETs.According to Lee the plot  change fI2D/IG) is also a good parameter to evaluate the doping strength in a graphene sample. On the other hand, the intensity ratio between the 2D and G peaks  of pristine graphene is compared with those with the 514 nm and 633 nm lasers. In the upper figure (514 nm), we can clearly observe the G and 2D peak characteristics of graphene, but in the lower figure (633 nm), the intensities of the two peaks are relatively low. Therefore, we used the 514 nm laser in this research. To observe the SERS effect by the Au NPs, the Raman spectra of each step  structure rather than a unidentate or bridging structure.83 Moreover, in the same process, the peak near 1630 cm−1 was removed because the stretching mode of C PBM data was replaced with SVG by xgml2pxml:<glyph-data id="z.dbd" format="PBM" resolution="300" x-size="8" y-size="10" xml:space="preserve">00000000000000000000000000000000111111110000000011111111000000000000000000000000</glyph-data><svg xmlns="http://www.w3.org/2000/svg" version="1.0" width="13.200000pt" height="16.000000pt" viewBox="0 0 13.200000 16.000000" preserveAspectRatio="xMidYMid meet"><metadata>Created by potrace 1.16, written by Peter Selinger 2001-2019</metadata><g transform="translate scale" fill="currentColor" stroke="none"><path d="M0 440 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z M0 280 l0 -40 320 0 320 0 0 40 0 40 -320 0 -320 0 0 -40z"/></g></svg>O in the carboxyl acid form disappeared as it formed the bidentate form.83,88In the Raman analyses of  step in  with a 6VCNP and G peak positions.92–94 In the standard carrier density model,94,95 the hole concentration of the graphene film is quantitatively calculated by the following equation:where ε0 is the vacuum permittivity (8.854 × 10−12 F m−1), κox is the relative permittivity of the gate material, 300 nm of SiO2 (3.9), e is the elementary charge (1.602 × 10−19 C) and Tox is the thickness of the gate material. In this experiment, the gate material was SiO2 and its thickness, Tox, was 300 nm. Generally, in Raman spectra of graphene, the G peak can be used to measure the charge concentration,93 indicating that 1 × 1012 cm−2 of the hole concentration corresponds to 1–3 cm−1 of the G peak positional measurements (1 cm−1). Therefore, we can demonstrate that the carrier concentrations, n, derived by Raman measurements, are comparable to the value for FET, showing the reliability of our FET measurements in each step.We performed Raman spectral mapping, using a 514 nm laser, on graphene films throughout each step in RS) of graphene films, as another form of mapping image. A square graphene film with one side of 5 cm was transferred onto the PET substrate, and after doping by each step process  on the basal plane of graphene, AuNP-mediated thiol-SAM functionalization maintains the mechanical and electrical properties of pristine graphene without affecting the spThere are no conflicts to declare.NA-003-D0NA00603C-s001