When localized proton transport occurs, it canbe a highly dynamic process, with additional transmission sites “opening”and a small number of sites “closing” under an appliedelectric field on the seconds time scale. Applying a simple equivalentcircuit model of ion transport through a cylindrical nanopore, thelocal transmission sites are estimated to possess dimensions (radii)on the (sub)nanometer scale, implying that rare atomic defects areresponsible for proton conductance. Overall, this work reinforcesSECCM as a premier tool for the structure–property mappingof microscopically complex (electro)materials, with the local ion-fluxmapping configuration introduced herein being widely applicable forfunctional membrane characterization and beyond, for example in diagnosingthe failure mechanisms of protective surface coatings.In 2014, it was reportedthat protons can traverse between aqueousphases separated by nominally pristine monolayer graphene and hexagonalboron nitride (h-BN) films (membranes) under ambient conditions. Thisintrinsic proton conductivity of the one-atom-thick crystals, withproposed through-plane conduction, challenged the notion that grapheneis impermeable to atoms, ions, and molecules. More recent evidencepoints to a defect-facilitated transport mechanism, analogous to transportthrough conventional ion-selective membranes based on graphene andh-BN. Herein, local ion-flux imaging is performed on chemical vapordeposition (CVD) graphene|Nafion membranes using an “electrochemicalion (proton) pump cell” mode of scanning electrochemical cellmicroscopy (SECCM). Targeting regions that are free from visible macroscopicdefects ( These G/A values representedthe intrinsic proton conductivity of the studied 2D crystals ,12 challengingthe widely accepted notion that pristine graphene is impermeable toall atoms, ions, and molecules under ambient conditions.14 Subsequent studies by several research groups18 have suggested that selectiveproton transport may be facilitated at defect sites  that are likely separate fromthe sites that facilitate the transport of other ions .19In 2014, anomalously highproton transport through nominally pristinemonolayer graphene and hexagonal boron nitride (h-BN) membranes  was reported,21 For example,a scalable “electrochemical proton pump cell” configurationin which macroscopic  graphene sheets producedby chemical vapor deposition (CVD) were deposited onto a commerciallyavailable perfluorosulfonic acid polymer (Nafion) film, was reported.12 CVD graphene-on-Nafion membranes  are able to achieve much higher protontransport rates ,11 while maintaining relatively high selectivity .19 Althoughit is well-known that CVD graphene possesses a distribution of intrinsicdefects , as yet there is no direct evidence for heterogeneoustransmission at particular locations on graphene|Nafion membranes.Conventional Raman spectroscopy lacks the spatial resolution and sensitivityto detect defects in high-quality graphene , and although high-resolution microscopy  has sufficient resolution to locally image defects, itis only able to provide a limited view in a macroscopic sense.15There has been interestin scaling up proton-selective membranesbased on graphene and related 2D materials.24 Scanningelectrochemical cell microscopy (SECCM)27 stands out as the idealtechnique for this application, as it uses a fluidic micropipet/nanopipetprobe to carry out local electrochemistry  within a confined region of an electrode surface,with a spatial resolution (down to tens of nanometer29) defined by the area of meniscus contact. In recent years, SECCMhas predominantly been used in conjunction with colocated microscopy/spectroscopyto reveal structure–activity in a diverse range of (electro)materials,30 including 2D materials such as graphene31 and transition metal dichalcogenides, such as MoS2,35 WS2,34 WSe2,36 MoS2/WS2 heterostructures,37etc. However, SECCM with a dual-channelprobe38 can also make real-time, localion conductance measurements on any type of surface, regardless ofelectrical conductivity.39 As we show herein,this configuration is crucial to land the meniscus cell on any partof the surface, irrespective of the local proton transmissibility.The extraction of large-scale statistics on local proton transmissionthrough graphene|Nafion requires a high-throughput technique thatcan directly probe/map ion flux with high spatial resolution overlarger areas of the membrane.11 graphene|Nafion membrane prepared by a hot-press method.The micropipet probe is deployed as an electrochemical ion (proton)pump cell40 to target regions of the graphene|Nafionmembrane that are free from macroscopic defects , revealing that, in these devices, most of the graphenesurface is impermeable to protons, with transmission typically occurringonly at ∼20–60 localized sites across a ≈0.003mm2 area. This localized proton transport process can alsobe highly dynamic, with a few additional transmission sites “openingup” on the seconds time scale when exposed to a proton-drivingvoltage (from the applied electric field) across the graphene|Nafionmembrane. By analogizing localized proton transmission to ion transportthrough a cylindrical nanopore,41 we can predict a simpleequivalent circuit model in which each site/pore possesses radii onthe (sub)nanometer scale and thus may be attributable to the presenceof one or more atomic defects in the graphene overlayer. All in all,this work further reinforces the status of SECCM as a premier toolfor local ion-flux mapping of microscopically complex (electro)materials.In this work, the synchronous electrochemical activity and ionconductance mapping capabilities of SECCM are exploited to probe localproton transmission through a previously reported–1 under the conditions explored).42 Themonolayer graphene film (situated on top of the Nafion 211 support)is investigated as a proton-selective membrane. Herein, the graphene|Nafionmembrane assembly was fabricated by a hot-press method similar tothat used previously,19 which, as established for themembrane electrode assemblies used in fuel cells,43 should ensure intimate interfacial contact between grapheneand the protogenic groups in Nafion, allowing for efficient protontransmission through the hydrated sandwich structure. Note that afterfabrication of the graphene|Nafion membranes, the quality of the grapheneoverlayer was assessed via SECCM measurements ofthe FcDM0/+ process (FcDM = ferrocenedimethanol). Afterthe tip was positioned, as for the proton conduction measurements(vide infra), this redox process was found to bekinetically facile in randomly selected spots, confirming the graphene preparation yieldeda surface of sufficient quality for electron tunneling measurements .To investigate local proton transport through graphene, spatiallyresolved electrochemical measurements were performed on graphene|Nafionmembranes using SECCM in the dual-channel configuration. The Nafion211 membrane (≈25 μm thickness) behaves as both a solidsupport and a highly conductive proton source/sink  on the scale of the SECCM probes used herein, assuming that intrinsic protontransfer occurs uniformly across the graphene surface via a through-plane conduction mechanism . Indeed, Nafion has previously been used as a graphenesupport for proton transmission measurements with similarly sized5 and larger macroscopic19 devices.Nafion is characterized by a complex, humidity-dependentnanostructure, with distinct domains of high and low ionic conductivity,corresponding to the hydrophilic sulfonate groups and hydrophobicfluorocarbon backbone, respectively.Ebias in idc in idc is highly sensitive to deformationof the meniscus cell,47 meaning that it can be used todetect meniscus–surface contact, enabling accurate positioningof the SECCM probe in three-dimensional (3D) space.48Herein, an “electrochemicalion (proton) pump cell”configuration of SECCM is introduced to measure local proton transmissionthrough graphene|Nafion membranes, as detailed in the Esurf = −(Eapp + Ebias/2) .47 Depending on the value of Esurf (vide infra), two proton-consuming reactions can take placeat the Pt WE, the hydrogen evolution reaction (HER) and/or the oxygenreduction reaction (ORR):Esurf, two scenarios are possible. Case (i):If the graphenefilm is ion-impermeable, the ionic pathway between the Pt WE and Ag/AgClQRCEs is blocked and no electrochemistry can place at the Pt WE, asshown in i.e., there is a continuous ionicpathway between the Pt WE and Ag/AgCl QRCEs) and protons flow fromthe meniscus cell into the Nafion film as the HER and/or ORR takeplace at the Pt WE (while the Ag/AgCl counter reaction takes placeat the QRCE), as shown in 40 whereby protons are “pumped” across the graphene filmin one direction (from meniscus to Nafion) in response to the proton-consumingreactions , through the graphene|Nafion membrane, the effective potentialat the WE surface is eactions 1 and 2 az-position, surface current (isurf), and idc fromrepresentative case (i) (red trace) and case (ii) (blue trace) measurementsare shown in i.e., t <0 s), (2) application of the electrochemical waveform , and (3) retract . From the plot of z-position in –1 in idc set point is triggered ; (2) after establishing meniscus–surfacecontact, zero or nonzero isurf is measuredat the Pt WE at 0 ≤ t ≤ 10 s, indicatingproton-impermeable [case (i)] and proton-permeable [case (ii)] regionsof the graphene|Nafion membrane, respectively; (3) in both cases, isurf returns to zero during retract, as meniscus–surfacecontact is broken. Note that the magnitude of isurf is dependent on Esurf andmay be limited by a combination of the charge-transfer resistance(Rct) associated with the electrode reaction, the micropipettip resistance (Rtip), and the resistanceof the proton transmission site(s) in the graphene film, exploredin greater detail below.In reference to the plotof idc in idc initially adopts a constant value of ≈7 nA, which decreasesduring approach, until reaching the idc set point , idc increases dramatically at 0 ≤ t ≤ 10 s, reflecting a percentage of the countercurrent flowing between the Pt WE and QRCEs ; (3) in both cases, idc tends back toward a stable value during retract , as the meniscus comes away from thesurface. Note that the idc–distancecharacteristic during the approach and retract of the SECCM tip dependson the tip size, initial meniscus size in air (here relatively large),and nature of the meniscus–substrate interaction . The gradual decrease in idc onapproach prior to triggering the feedback threshold at t = 0 s indicates substantial “squashing” of the meniscuscell47 over a distance of ≈500 nmin i.e., on the order of the micropipet probe radius).This idc–distance behavior is similarto that observed previously for SECCM on suspended graphene, wheremeniscus contact to the graphene surface was also evident from directsimultaneous measurements of isurf.49 Here, meniscus contact was additionally confirmedin separate measurements of isurf forthe direct electrochemistry of FcDM0/+ at the graphene|Nafionsubstrate (vide supra). It should also be noted that,herein, the idc set point (±500 pA)is taken relative to the value measured at the beginning of the approach,which means that it is both insensitive to drift in idc  and can be triggered by either a decrease  or increase  in the magnitude of idc, thereforeserving as a sensitive indicator of meniscus–surface contact,irrespective of proton transmission.From the plot of 50 A spatially resolved electrochemicalmovie, comprising 2601 independent cyclic voltammograms (CVs) across an 100 × 100 μm2 area (hopping distance = 2 μm) in the potential range−0.225 to +0.175 V vs Ag/AgClQRCE is shown in the SI Movie S1 (associatedmovie caption presented in the SI). A colocated“quasi-topographical” map, which reflects the (dynamic)topology of the underlying Nafion membrane  collected synchronously with the electrochemical data,is presented in the SI Figure S3. Notethat the ORR , obtained by integrating |isurf| from SI Movie S1 to calculate the charge (|Qsurf|) passedover the entire potential range to be equal to the tip area ]. Notethat the active pixels do not necessarily correspond to obvious featuresin the quasi-topographical map of the graphene|Nafion membrane , and the i–E response is different from that of the Nafion film itself(vide infra). Thus, the data in e.g., defects,18vide infra) across the macroscopic graphene|Nafionmembrane investigated herein. Indeed, due to the high sensitivityand low electronic noise of the SECCM setup, through-plane protonconduction at the “inactive” pixels (N = 2582) can be effectively ruled out in these devices, as the associatedlow isurf values . Considering the weight of statistics presentedin vide infra), and in these locations, proton transmissionrates (currents) are very high. However, as Nafion does not possessa uniform structure on the scale of (atomic) defects , the structure-dependent local proton conductivityof the acceptor Nafion membrane needs to be acknowledged. From theclassical cluster-network (inverted micelle) model for the morphologyof hydrated Nafion,51 proton conductionthrough the graphene|Nafion membrane could only occur if a proton-conductingdefect site  of graphenealigns with a proton-accepting water channel  of Nafion . Given the high hydration state of the Nafionmembrane , there would be an abundance of proton receptor sites on the 1–2μm2 scale , evidenced by the high functionality of graphene|Nafion membranesin previous macroscopic studies.19 Thus, while we are confident that we are not simplymeasuring a sparsity of graphene|Nafion wetting in the proton receptorphase (sink) in our measurements in e.g., 0.007 sites/μm2, vide supra) as a lower limit for these CVD graphene|Nafionmembranes.As discussed above, hydrated Nafion is effectively an isotropic(homogeneous) proton source/sink (electrolyte) on the scale of theSECCM probe  may also vary. In othercases, isurf is nonzero from the beginningof the potential sweep 52 relative to those employedherein. It should be noted, however, that during ultrashort/high-intensityvoltage pulses, the actual magnitude of the potential/electric fieldover the graphene membrane is expected to be dramatically reduceddue to double layer charging and uncompensated resistance, which,as discussed below, can be avoided entirely through the applicationof low-intensity voltage pulses for long times .As shown in the isurf spikesare always followed by a relativelyslow exponential decay with potential/time, taking place on the millisecondto second time scale  of themacroscopic Pt WE], through the uncompensated resistance of the cell,the time scale of which is characterized by the RC time constant (τ).54 As discussedin the next section (and outlined in detail in the SI), over an active proton transport site, R and C are estimated to be on the order of ≈100–1000MΩ and ≈2 nF, respectively, giving rise to τ valuesof 0.2–2 s, consistent with the time scale of the decay in isurf. In addition, in isurf is negative  and shows an exponential dependence on (over)potential,starting at ≈0 V vs Ag/AgClQRCE. This indicates that in the potential range of 0 to −0.225V vs Ag/AgClQRCE, both the relativelysluggish ORR kinetics at the Pt WE surface and the geometry of theactive transmission site may contribute some limitation to the magnitudeof isurf . These pointsare further discussed below.In each of the CVs extractedfrom active pixels 2c–i, indcale see 2g. As alvs Ag/AgClQRCE). A spatially resolved electrochemicalmovie, comprising 2601 independent CVs  across an 100 × 100 μm2 area (hopping distance = 2 μm) is shown in the SI Movie S2. The corresponding static imagesof electrochemical activity obtained for cycles 1 and 2 are shown in SI Figure S4). By consulting Movie S2 and 2. While a large proportion of the proton transmission sites are fixed,there are some sites that “open” and others that (partly)“close” on the time scale of the measurement  or dynamics of the acceptor Nafion phase57 . These are further reasons to consider the density of transmissionsites that we report as a lower limit. In any case, most of the graphene|Nafionmembrane is impermeable to protons, clearly demonstrated in the averageCVs (N = 2544) shown in To explore the dynamics of protontransport, particularly the potentialdependence, a voltammetric hopping mode SECCM experiment was performedon another area of the graphene|Nafion membrane, performing two cycleswithin the same potential window .Individual CVs extracted from representative active pixels areplotted in  landing 3c or aft landing 3d–gand st e.g., 3c–f, alt41 Applying the equivalent circuitmodel59 derived and discussed in the SI, the local electrochemical response  is rationalized and furtherused to estimate the geometry of the active transmission sites. Toachieve the latter, potential-step experiments were performed in the SECCM configuration, targeting bothdamaged and more intact areas of the graphene|Nafion membrane. Spatiallyresolved electrochemical movies, comprising 1421 independent chronoamperograms(CAs) across 120 × 70 μm2 areas  at Esurf = −0.5 V vs Ag/AgClQRCE (pulsetime = 10 s) are shown in the SI Movies S3 and S4. The corresponding static imagesof electrochemical activity ,obtained from damaged and more intact areas of the graphene|Nafionmembrane, are shown in Proton conductionthrough local transmission sites on the graphene|Nafionmembranes is likened to ion transport through an atomically thin,solid-state nanopore.i–t curve shown in e.g., cracks and holes) where the underlying Nafion extrudes throughthe graphene overlayer, relatively large isurf values of ≈−23 nA are measured throughout the entire i–t pulse, also shown in N = 24/1421), surrounded by contiguousareas of intact, proton-impermeable graphene. While many of the activepixels exhibit large isurf values  orafter an onset time , before decaying onthe millisecond to second time scale  tosteady values in the 270–1100 pA range. Again, the pixel-dependentdelayed onset of the isurf spike in Movies S3 and S4).Contrary to the SECCM scans shownabove 2 and 3, vs Ag/AgClQRCE  and long pulsetime of 10 s applied duringthese potential-step experiments permits quantitative treatment ofthe data, allowing the pore resistance (Rpore) to be calculated and the pore radii (rp) to be estimated. As seen from SI Figure S6, assuming Cdl and Rct can be neglected, and under conditions where Rpore is negligible , the series resistance(Rseries) ≈ Rtip, meaning that isurf is limitedby the resistance of the micropipet probe. Applying Ohm’s law(SI eq S13), Rseries is estimated to be ≈9 MΩ from isurf = −23 nA . Thus, landing on the Nafion film provides the “tip-limited” i–t response, and pixels with isurf values approaching approximately −23nA .The large driving potential of −0.5V Rtip is known (≈9MΩ), Rpore can be calculated (SI eq S14) for each of the individual activepixels highlightedin Rpore values of 730, 460, 320, and 170 MΩare calculated for isurf values of 270,430, 600, and 1100 pA, for the green-, red-, blue-, and pink-labeledpixels (–1 for 0.1 M HCl60) and taking Lp = 0.6 nm (theapparent thickness of graphene in water7), we estimated rp values of 0.4, 0.5,0.6, and 1.0 nm (SI eq S12) for the green-,red-, blue-, and pink-labeled pixels , both the access resistance(Rg ∝ 1/rp) and geometric resistance (Ra ∝ Lp/rp2) contribute significantly to Rpore,and the calculated rp values are sensitiveto Lp (SI eq S12). For instance, taking Rpore = 460 MΩ, rp is estimated to be 0.4 and 0.6 nm for Lp values of 0.34 nm  and 1 nm , respectively. In anycase, the estimated (sub)nanometer pore geometry from this simplemodel indicates that the local proton transmission sites through themacroscopic graphene|Nafion membrane likely coincide with relativelyrare atomic-scale defects  in the graphene overlayer film, consistent withsome previous reports.18Since dpixels 4a-ii,red pixels 4a-ii, reG/A valuesreported in previous studies, alongside the graphene preparation methodand size of the measured membrane, is reported in the SI Table S1. The G/A valuespreviously reported for macroscopic graphene|Nafion membranes variesover several orders of magnitude (≈0.09 to 30 S cm–2),19 which is perhapsunsurprising given that macroscopic defects such as pinholes, cracks,and other imperfections are known to be present. Indeed, comparing A summary of example –2) required to achieve the reported G/A values is also calculated, assumingan individual defect resistance of 170 MΩ·defect. As shownin Table S1, the lower end of defect densities(0.005 defects μm–2), obtained from high-quality,small-area graphene membranes produced by exfoliation  or CVD ,is in good agreement with the number of defects detected on the morepristine areas of the graphene|Nafion membrane, with values of 0.007,0.02, and 0.008 defects μm–2 calculated for G/A values of up to ≈1000mS cm–2),17 the densityof defects would need to increase by >2 orders of magnitude (assuminga constant defect resistance of 170 MΩ·defect) up to ∼2μm–2, such that on average each meniscus cellof size ≈2 μm2 , previousreports have shown that such defects can be introduced readily duringthe growth17 or postgrowth treatment  of CVDgraphene, producing highly conductive, proton-selective membranes.To contextualizethe results reported herein, the density of defects was employed to target a relativelydefective area of the membrane. A static map of electrochemical activitymade up of 4760 pixels across an 13.4 × 13.8 μm2 area  is shown in rt ≈ 0.6–1μm), with a small number of isolated (single-pixel) defectspossessing low activity  and alarge number of continuous (multipixel) defects possessing high activity. The synchronously collected,colocated quasi-topography map shown in e.g., vide supra), whereas the larger multipixel sitesrepresent areas where the underlying Nafion film has extruded throughthe graphene overlayer, most likely at pre-existing cracks or grainboundaries. Indeed, macroscopic defects of this type can also be observedby scanning electron microscopy (SEM) imaging, carried out on a nearbyarea of the graphene|Nafion membrane, as shown in To provide a closer inspectionof graphene|Nafion membranes, a much smaller nanopipet probe , relatively large areas of membrane can be coveredin a single SECCM scan. In addition, to accurately estimate Rpore from Rseries (SI eq S13), Rtip ≪ Rpore, which puts a lower limiton the probe size since Rtip ∝1/rtip (eq S6).It should be pointed out thatwhile a small SECCM probe provideshigh-resolution images, a relatively large probe (2) of the membrane, it canbe stated with confidence that the majority of the graphene overlayerdoes not conduct protons in the investigated CVD graphene|Nafion membranedevices. Proton transmission was shown to be a site-specific process,occurring at ∼0.007–0.02 sites μm–2, giving rise to very high local conductance values . In addition, protontransmission was strongly potential- and time-dependent, with additionaltransmission sites dynamically “opening” and a smallnumber shutting off during the measurements. Reasons for this behaviorhave been suggested. A simple equivalent circuit model was proposed,analogizing these transmission sites to electrolyte-filled circularnanopores in the graphene film, which were estimated to possess dimensions(radii) on the (sub)nanometer scale, implying that atomic defectsare responsible for local proton transport, in agreement with recentmodeling16 and experimental18 works. The potentiality of SECCM for rapidly assessing the qualityof ion-selective membranes was further demonstrated by deploying afine nanopipet probe, producing high-throughput, high-resolution electrochemicaland (quasi-)topographical images that gave a more detailed pictureof the local proton transmission sites.In this study, an “electrochemicalion (proton) pump cell”configuration of SECCM has been used to probe the spatially dependentproton permeability of CVD graphene|Nafion membranes. Due to the sheerweight of statistics  over severallarge areas  and conversion ; separation technologies; and biological systems.Beyond membranes, SECCM may also have application in any areas whereion transport and/or reactive flux is a highly localized phenomenon,for example, in the characterization of corrosion-resistant coatings.Graphene has been proposed as a corrosion-resistant coating,61 and based on the results presented herein, itis clear that high-resolution, dual-channel SECCM could be deployedto rapidly assess local protection efficiency, in particular, by identifyingactivity “hot-spots” where the protective barrier maybe compromised.Overall, the resultspresented herein demonstrate the strong potentialof SECCM as a multifunctional membrane characterization tool, producinghigh-fidelity images that provide a wealth of information on spatiallyresolved ion-selective transport/transmission. Although CVD graphene|Nafionmembranes have been exclusively considered herein, ion-selective transportthrough membranes plays an important role in many applications, toname a few: electrochemical energy storage , 1,1′-ferrocenedimethanol , and potassium chloride  were used as supplied by the manufacturer. All aqueous solutionswere prepared with ultrapure deionized water .vide infra) are high-quality, with nodetectable D-peak near 1350 cm–1, indicating a relativelack of graphene defects with edge-plane character.19 Further characterization with X-ray photoelectron spectroscopy (XPS)revealed successful graphene transfer onto the Nafion membrane, withno detectable impurities or surface contamination from copper . The XPS survey reveals the expectedelemental composition  for both unmodified Nafion 211 and graphene|Nafion membranes(see SI Table S2). The C 1s spectra ofan unmodified Nafion 211 membrane show only one major main carbonpeak at 291 eV, attributed to the CF2 group of the fluorocarbonbackbone (the other low intensity peak at 284.8 eV is assigned toadventitious carbon impurities). In contrast, C 1s spectra from thegraphene|Nafion membranes show two main peaks, corresponding to theCF2 groups of Nafion and sp2 carbon atoms ofgraphene at 291 and 284.1 eV, respectively.The Nafion 211 membraneswere purchased from the Fuel Cell Store . Themonolayer CVD graphene (supported on copper foil) was purchased fromACS Material . In previous studies, Raman spectroscopyindicated that these graphene-on-copper substrates , prior to use as a WE. Ag/AgClQRCEs were prepared by anodizing 125 μm diameter Ag wire  in an aqueous saturated KCl solution. The Ag/AgClQRCEs possessed a stable reference potential  on the hours timescale in 0.1 M HCl, consistent with a previous report.62The nanocrystallinePt WE was prepared by evaporating a 2 nm Cradhesion layer followed by a 75 nm Pt layer on a borosilicate glassmicroscope slide. The glassy carbon plate was purchased from HTW Germanyand was polished with a suspension of 0.05 μm Al19 In brief,a ≈2 × 2 cm2 square of copper-supported graphenewas placed on top of a Nafion 211 disk with a diameter of ≈1.9cm and a thickness of ≈25 μm. Furthermore, two piecesof Teflon-reinforced fiberglass (of diameter ≈1.9 cm) wereplaced below and atop the Nafion|graphene on copper to serve as protectivelayers. This assembly was then placed into a hot press  and pressed at 140 °C for 2 min. Next, the Nafion|graphene|copperassembly was placed into a 0.3 M ammonium persulfate solution andallowed to react until the copper layer was fully etched away by visualinspection. Note that in previous studies,19 monolayer graphene sheets were shown to survive the Nafion hot-pressingand copper-etching processes intact, without any significant creationof additional defects or contamination from the copper substrate,as revealed by Raman spectroscopy and XPS.Nafion|graphenesandwich structures were fabricated at Clemson University, U.S.A.,using a previously reported procedure.2 nanocrystalline Pt WE withadhesive tape, ensuring intimate contact between the Nafion and Pt.The constructed Pt|Nafion|graphene electrode assembly was then fittedinto a custom sample holder with a surrounding moat of deionized water,47 effectively fixing the local relative humidity at >70%. Due tothereportedly long equilibration times associated with Nafion hydration,63 the electrode assembly was stored under these conditions overnight.Following this procedure ensured that, during SECCM experiments onthe several hours time scale, the prehydrated Nafion 211 membrane(1) possessed high bulk proton conductivity (on the order of 0.02–0.06S cm–1)42 and (2) didnot undergo significant changes in volume . An electrical connection was made by fixinga copper wire to the Pt WE surface with conductive silver epoxy resin, taking care to avoid making a connection  with the graphene overlayerfilm. Scanning electron microscopy imaging was carried out on thePt|Nafion|graphene electrode assembly, after SECCM, with a GeminiSEM500 system .Prior to scanningwith SECCM, the graphene|Nafion assembly was rinsed in deionized waterand fixed to a 2 × 2 cmi.e., calculated from the overalldiameter of the dual barrel) in the ≈10–11 to ≈10–9 cm2 (nanopipets) and≈10–8 cm2 (micropipets) ranges,were fabricated from filamented quartz and borosilicate  theta capillaries, respectively, using a CO2 laser puller . After fabrication,both barrels of the probes were backfilled with analyte solution  using a MicroFil syringe , before adding a thin layerof silicone oil  on top to minimize evaporationfrom the back of the pipet during prolonged scanning, as previouslyreported.33 Ag/AgCl QRCEs were then insertedinto each barrel, through the silicone oil layer, into the analytesolution, to finalize the SECCM probe, rendering it ready for use.After being scanned, the SECCM probes were carefully emptied and rinsedwith deionized water (using a clean MicroFil syringe) before imagingthe tip on a GeminiSEM 500 system.Double-barreledpipet probes, withtotal tip areas  was mounted on a z-piezoelectric positioner , and the Pt|Nafion|grapheneelectrode assembly  was mounted atop an xy-piezoelectric positioner.As schematized in Ebias) of0.05 V was applied between the QRCEs to induce a dc ion current (idc) between the barrels to enable meniscus positioningon the substrate.48 The SECCM probe wasinitially positioned above the WE using coarse xy-micropositioners  and subsequentlylowered into the near-surface position using a stepper motor in tandemwith an optical camera .Local electrochemicalmeasurements were carried out in the SECCM format on a home-builtscanning electrochemical probe microscopy (SEPM) workstation at theUniversity of Warwick, U.K., as previously reported.–8 cm2) was approached to the graphene overlayer film  surface using an idc thresholdof ∼500 pA to detect when the meniscus–surface contacthad been made and to stop further translation. Note that the glassportion of the probe never contacted the graphene surface. Electrochemicalmeasurements  wereperformed in the confined area defined by the meniscus cell createdbetween the SECCM probe tip and graphene surface  at voltammetricscan rates (υ) of 0.1 or 0.2 V s–1 for 1 or2 cycles, respectively. During chronoamperometry, the potential atthe Pt WE was held at −0.5 V vs Ag/AgClQRCE (0.1 M Cl–) for 10 s. Mapping was carriedout using a standard hopping mode protocol, as previously reported.50 In brief, the SECCM probe was approached to the graphene surfaceat a series of locations in a predefined grid pattern, and upon eachlanding, an independent electrochemical measurement was made, buildingup spatially resolved chronoamperometric (i–t) or voltammetric (i–E) “images” of the substrate surface. In addition, thefinal position of the z-piezoelectric positionerat approach was used to synchronously construct a “quasi-topographical”map of the Pt|Nafion|graphene electrode assembly surface. Note that,in context, “quasi” refers to the fact that the underlyingNafion membrane possesses a dynamic physical structure (topology)due to small changes in volume  on the time scale of SECCM scanning(vide supra).The SECCMprobe , held at a common ground, was measuredwith a home-built electrometer. Note that during the SECCM measurements,unless otherwise stated , the graphene membrane itself was floating .The isurf and idc were measured every 4 μs and averaged in 256 blocks to givean effective data acquisition rate of 4 × (256 + 1) = 1028 μs,where one extra iteration was used to transfer the data to the hostcomputer. A home-built eighth-order (low-pass) brick-wall filter unit(time constant = 1–10 ms) was utilized during data (current)collection. Instrumental control and data acquisition were carriedout using an FPGA card (PCIe-7852R) controlled by a LabVIEW 2016  interface running the Warwick ElectrochemicalScanning Probe Microscopy  software.The SEPM setup was located ona vibration isolation platform  located within an aluminum faraday cage equippedwith heat sinks and acoustic foam to minimize mechanical vibration,electrical noise, and thermal drift (<10 nm per minute) duringprolonged scanning.isurf data, log10|isurf|, was plotted vs xy position to create a seriesof time-resolved (chronoamperometry) or potential-resolved (cyclicvoltammetry) images, which were combined and presented as dynamicelectrochemical movies.30 The static images ofelectrochemical activity ,presented in the main text, were constructed by integrating |isurf| with respect to time to calculate surfacecharge, |Qsurf|, which was plotted aslog10|Qsurf| vs xy position. In all electrochemical images and movies, pixels withlog10|isurf| or log10|Qsurf| values less than 3× theelectrical noise level are assigned a gray color and represent proton-impermeable regionsof the graphene|Nafion membrane. The proton transmission site densitywas estimated as ≈active pixels/,taking the area wetted by the meniscus cell during contact to be equalto the tip area of the employed pipet probe. Data plotting was carriedout using the Matlab R2020 and OriginPro 2019b  software packages. Note that all electrochemical maps and moviesare presented without any data interpolation.Afteracquisition, theraw SECCM data were processed using the Matlab R2020a  software package. The logarithm of


Because these models are carefully designed to avoid a serious deformation of graphene in the tension of sandwich-like composite models, the strengthening mechanism close to the experimental theory where graphene just serves to transfer a load can be studied dynamically. The boundary comprising of two phases is confirmed to hinder the motion of dislocations, while the crack grows along the interface beside graphene, forming a fracture surface of orderly arranged Al atoms. The results indicate that single-layer graphene (SLG) gives rise to an increase of 1.2 or 0.4 GPa in tensile strength when stretched in in-plane or normal direction, while bilayer graphene (BLG) brings a clear rise of 1.2–1.3 GPa in both directions. In both in-plane and normal stretching directions, the mechanical properties of the composite can be improved clearly by graphene giving rise to a strong boundary, new crack path, and more dense structure.The powder metallurgy process of the Al–graphene system is conducted by molecular dynamics (MD) simulations to investigate the role of graphene. During the sintering process, graphene is considered to reduce the pore size and metal grain size based on the volume change and atomic configuration of the Al parts in the composite. Compared with the pure Al system, the space occupied by the same number of Al atoms in the sintered composite is 15–20 nm Owing to the superior mechanical properties, graphene is naturally expected to work as an effective reinforcement phase to improve metal matrix composite materials. Existing studies have provided quite a number of methods to fabricate metal–graphene composites such as powder metallurgy sintering, composite electroplating, differential speed rolling, etc. In those techniques, powder metallurgy sintering is supposed to be an efficient way to prepare metal–graphene composite in bulk because of the simple process and precise control, drawing considerable attention of researchers recently ,2,3.Generally, most of the investigations on sintering metal–graphene composites are performed by experiments. Some focus on problems of graphene dispersion hindering the preparation of a homogeneous structure. Mu et al. applied an electroless plating method to prepare Ni-decorated graphene nanoflakes as a reinforcement to be uniformly dispersed in a Ti matrix, and then spark plasma sintering (SPS) and hot-rolling are followed to gain a titanium matrix composite with an enormous strength increase . Ju et aThe experimental studies listed in Nevertheless, most of the MD papers only consider the system of pure metal particles ,13 or meIn the sintering systems of Al–graphene composites, MD models shown in During the sintering process, the composite structure will be heated from 300 K to 773 K in 300 ps, with the external pressure increasing from 1 atm to 500 atm (50 MPa). Then, the system maintains such temperature and pressure for 300 ps. At last, the external conditions of sintered composite will return to 300 K and 1 atm after a cooling stage of 300 ps. The NPT ensemble is also applied in the entire sintering process. It should be noted that parameter settings above such as the heating and cooling rate are selected according to the similar sintering process of the Al–graphene system in reference .Then, tensile simulation is conducted on sintered composite to examine the mechanical behaviors. Another relaxation process will be carried out at 300 K for 100 ps to eliminate thermal stress before tensile testing. The NVE ensemble is applied to operate quasi-static tension by changing the shape of the simulation box in both in-plane and normal directions, and the temperature is reset by explicitly rescaling their velocities. Verlet algorithm is used to solve Newton’s equations of motions and the time step is set to 1.0 femtosecond. The eam potential is applied to the interaction between Al atoms [airebo potential is used to link carbon atoms in graphene [morse potential [All the MD simulations are carried out by LAMMPS and the generated data are visualized by VMD software. The velocity Al atoms , the filgraphene , as commgraphene ,21,22 tootential , which iotential . 3 to 816.9 nm3 in the heating process, while the configures in 3 to 801.0 nm3; the surface morphology of sintered bulk has been basically established and the final drop of volume can be simply attributed to cold shrink in the cooling stage. This change is mainly in line with expectations, which have been reported in many experimental studies [As is known to all, the powder metallurgy process can promote densification of metal particles by virtue of plastic deformation and material transfer during the sintering, leading to a decline in porosity, a shrinkage in volume, and an increase in density. To validate the MD model in this paper, a sintering process of pure Al particles is operated, firstly, after relaxation on the initial model, followed by monitoring of the changes in system volume and surface morphology. As expected, the system temperature and pressure are successfully controlled during the sintering process a. The bl studies ,9. 3 to 806.3 nm3 during the heating stage, which keeps a lower than pure Al curve after about 150 ps, and then falls to 777. 4 nm3 at constant temperature and pressure. The curves also indicate a possibility of further decline in volume, as reported in [3, which is a little less than 759.6 nm3 of the pure Al model, even though there is an extra SLG. Therefore, the same number of Al atoms in the Al-SLG composites occupies a smaller space, as presented in Based on the pure Al model, Al–graphene composite bulks are sintered to investigate the role of graphene on the powder metallurgy process. The initial structures for sintering are prepared by inserting the graphene nanoplatelet into the gap between Al particles. Firstly, a single-layer graphene (SLG) is placed in the center gap of the pure Al model. It means both sides of the SLG are adjacent to four Al particles, forming a pore containing graphene, as marked in orted in  with impTo gain insights into the inner structure of sintered bulk, centro-symmetry parameter (CSP) analysis is performed, and the local lattice disorder is tinted in CSP values. Blue balls stand for Al atoms with bulk lattice, red balls represent surface atoms, and other colorful balls are classified as defects such as atoms in dislocation or grain boundary. Therefore, the size of Al grains can be directly determined by color configurations of atomic structures. Since red atoms imply surface atoms appearing inside, the pores can be easily found in the sintered structure. As marked with yellow circles in In order to study the effect of multilayer graphene on the powder metallurgy process, SLG in the models is substituted by bilayer graphene (BLG) to gain the Al–BLG composite. Then, the sintering simulation and structure analysis are performed as before. In comparison with the composite structure of SLG, BLG is likely to make little difference in the volume and structure of the Al–graphene composite. Based on The strengthening mechanism of graphene has been illumined experimentally by some static micro images of composite structures, where high density of dislocations accumulates around the boundary comprising two phases and this boundary is expected to hinder the motion of dislocations effectively ,28. It sy or z axis where the tension is roughly parallel or normal to graphene. Then, the stress–strain curve is drawn to compare strength and deformation, and CSP analysis of Al atoms is used to trace the change of inner structure including dislocation motion, propagation of the cavities, and morphology of fracture surface. Uniaxial tensile simulations are performed here to investigate the enhancement mechanism of graphene in metal matrix composites, and the stretching direction is along the It is remarkable that quite a few studies simply attribute the wonderful mechanical performance to graphene sharing most of the stress, where the sizes of graphene are the same as the plane size in models. However, the distribution of atom stress in The stress–strain curves of models stretched in in-plane and normal directions are drawn in 3 smaller than the pure Al structure, and the amount of arranged Al atoms in the sintered composite is about 5000 fewer too. The effect of graphene in the mechanical property is studied minutely through the tensile process on the sintered composite. The composite with the SLG can exhibit an increase of 1.2 or 0.4 GPa in tensile strength when stretched in in-plane or normal direction, while the composite with the BLG can achieve a clear rise of 1.2–1.3 GPa in both directions. This suggests that graphene is of great importance for composites to improve the mechanical performance whether in in-plane or normal tensile direction. This improvement can be attributed to the effect of graphene, contributing to the formation of two-phase boundary, new crack path, and more compact structure, which can be easily found in view of the atomic configures. MD simulations on the powder metallurgy process are performed to study the sintering process and the strengthening mechanism of the Al–graphene system. Details on volume and local crystal structure of Al atoms in a composite, which cannot be measured through the experimental method, indicate that graphene is helpful for promoting the sintering progress, improving the density of sintered structures, and refining metal grain size. The space occupied by the same number of Al atoms in the sintered composite is 15–20 nm


Nanofabrication research pursues the miniaturization of patterned feature size. In the current state of the art, micron scale areas can be patterned with features down to ~30 nm pitch using electron beam lithography. Here, we demonstrate a nanofabrication technique which allows patterning periodic structures with a pitch down to 16 nm. It is based on focused ion beam milling of suspended membranes, with minimal proximity effects typical to standard electron beam lithography. The membranes are then transferred and used as hard etching masks. We benchmark our technique by electrostatically inducing a superlattice potential in graphene and observe bandstructure modification in electronic transport. Our technique opens the path towards the realization of very short period superlattices in 2D materials, but with the ability to control lattice symmetries and strength. This can pave the way for a versatile solid-state quantum simulator platform and the study of correlated electron phases. Focused-ion beam (FIB) lithography enables high-resolution nanopatterning of 2D materials, but usually introduces significant damage. Here, the authors report a FIB-based fabrication technique to obtain high quality graphene superlattices with 18-nm pitch, which exhibit electronic transport properties similar to those of natural moiré systems. Accordingly, the push to further improve nanofabrication techniques is an ongoing effort to fit more electronic components per unit area or to develop quantum devices and quantum bits, relying on quantum coherent control in the nanoscale regime3. Another important application is in quantum materials, where materials’ properties can be tuned in situ. Arrays of quantum dots with sufficiently large Coulomb interactions could lead to the observation of metal-Mott insulator quantum phase transitions and potentially even the d-wave superconducting phase4. Recent discoveries of correlated phases in twisted bilayer graphene due to the superlattice (SL) potential6 motivate further exploration of graphene SLs with more versatility in the lattice design and in situ control.Nanoscale fabrication is at the heart of the technological revolution of semiconductor technology and scientific breakthroughs in nanotechnology, progressing rapidly for more than five decades8, but is impractically slow for micron-scale lithography. Other techniques, such using self-assembled polymers, can achieve high resolution and yield9, at the expense of limiting design flexibility in what patterns can be implemented. A higher throughput alternative is ultraviolet optical lithography, the main lithography technique in the semiconductor industry, which is faster but is not suited for academic research and development.Nanofabrication techniques always involve a trade-off between patterning resolution, throughput and flexibility. For example, scanning tunneling microscopy, can be utilized to rearrange materials on the atomic scale12.In academic research, the most common nanopatterning technique is based on electron beam lithography (EBL), which can be thought of as a compromise offering reasonable throughput and nanoscale resolution. Forward scattered electrons limit the minimal patternable feature size, while secondary electrons due to (inelastic scattering) interactions with the resist limit the pitch resolution. As such, using high acceleration voltages and thin organic resists, patterns with 35 nm pitch can be obtained, but significant reduction of the feature size has not been demonstrated18. However, two major limiting factors affect He-FIB-based nanofabrication: (1) Milling of materials directly on a substrate is accompanied by deposition of He bubbles inside the substrate, contaminating it and affecting surface topography19. (2) Secondary collisions damage the quality and integrity of the patterned material20. It is possible to reduce the damage by milling suspended materials (where substrate scattering is absent) or using resist lithography21. However, suspending the material is complex and the amount of FIB induced unintentional damage is still expected to modify the material properties in the form of ion implantation and/or amorphization24. Here, we propose and demonstrate a nanofabrication technique that combines the high resolution of He-FIB milling with minimal damage to the patterned material.Another common nanofabrication approach uses focused-ion beam (FIB) milling to pattern the target material directly. In particular, helium (He) FIB routinely achieves single digit single feature resolution and pitch on the order of 10 nm25 or graphene/hexagonal boron nitride (hBN) heterostructures27), it is possible to engineer SLs by applying a periodic electrostatic potential28. When applied to low-dimensional materials, such as graphene, this modifies its electronic band structure when the SL period is much smaller than the electronic coherence length29. Several approaches have been explored to introduce such potentials: from patterning the graphene directly12, to inducing a periodic potential modulation by patterning the dielectric material31 or the gate electrode34. In all cases, it is essential to reduce damage to the patterned material, keeping it flat and free of external contamination. However, to realize the full potential of artificial SLs, a new lithographic technique is required, one which can create high quality periodic patterning on the sub-20-nm scale, approaching the moiré length in graphene aligned on hexagonal boron nitride or magic angle twisted bilayer grapheme . The resolution of our method is demonstrated directly by atomic force microscopy (AFM), whereas the quality is demonstrated by applying our technique to engineer SL potentials in graphene via patterned graphite electrodes. Graphene could be very sensitive to the presence of charge traps, impurities and patterning errors in the patterned gate electrode. However, transport data from our samples indicates minor unintentional damage to the patterned electrodes, even for highly dense patterns with an 18 nm pitch. Similar to moiré systems , which is commercially available on a large scale and high quality. Importantly, the suspended membrane can be removed from its supporting frame and transferred with a polymeric stamp Fig. . The memask Fig.  with a s nm Fig. .Fig. 2FaTo quantify the disorder of our lattices, we perform Fourier analysis of the AFM images Fig. , finding32. By tuning the silicon backgate voltage (Si BG) and the patterned gate voltage (PBG), one can modulate the strength of the SL and the carrier type in the graphene layer to observe the cloned Dirac cones, see Fig. Figure The second device studied in this work, Dev 2, has an 18 nm period square SL. To the best of our knowledge, there is no experimental work about electrostatically induced SLs with such a short periodicity. In this case, there is a top gate electrode (TG) apart from the PBG. This combination of gates allows us to control the SL strength and the total carrier density of the system independently. Therefore, we keep PBG fixed at certain voltages and sweep TG to change the carrier density. In longitudinal resistivity, we observe clear signatures of the emergence of satellite peaks at sed Fig. . Furthersed Fig. , i.e., nThe prominence of the SL signature is different in the two devices. To understand this, we must examine the specific characteristics of both devices. As a rough approximation for the electrostatic potential modulation along the graphene layer for the formation of the SL, we consider a periodic one-dimensional system with a patterned bottom gate, global top gate . Prior to milling in FIB, the membranes are annealed in a HThe membranes are mounted on a hole in the sample holder, to improve imaging contrast and loaded into the He focused-ion beam microscope (HIM). The HIM is then aligned to high precision on metallic features in the sample, with typical parameters being 5 After initial alignment, the membrane area is located in the HIM . The sample is usually allowed to rest for 30–90 min , to minimize the effects of piezo drift in the sample stage. Following this resting period and additional finer alignment, the membrane is milled using a preprogrammed point-by-point deflection list. For large features, a milling dose comparable to 0.9 In addition to the majority of the samples produced during our research, using He-FIB, we were also able to produce similar results (but with larger milling periods) using a Gallium focused-ion beam microscope (Ga FIB). These samples were processed similarly to the above described samples milled by HIM, with the exception being that the membrane was mounted with the window facing down (situated above a hole in the sample holder), in order to facilitate locating the intended milling area with the FIB. The resolution of the specific Ga FIB microscope used in our experiments was restricted by various technical issues, limiting the size of the spot effected by the Ga ions to about 20 nm and the lattice period to the order of 50 nm. With a more narrowely focused FIB, we estimate, based on our experience, that a ~10 nm single feature and ~30 nm lattice period should be achievable.2 substrate at a temperature of 158 °C to clean its interfaces, and subsequently temperature was increased to 180 °C to melt the poly (bisphenol A carbonate) (PC) film42. After dissolving the PC film in chloroform, the heterostructure quality is checked through AFM imaging and Raman spectroscopy. Finally, the heterostructure is again picked up and released following the same procedure on the patterned graphite gate. It is important to note that the hBN flakes between the graphene layer and the patterned gate electrode must be thin to allow for efficient doping modulation.The fabrication process of the patterned graphite gate is explained in the main text. After the cleaning process, we prepared an hBN/graphene/hBN (hBN/graphene/hBN/graphite/hBN in the case of Dev 2) heterostructure. We first dropped it on a clean Si/SiO43 to our heterostructure. In the case of the contacts, since they are right on top of the patterned graphite flake, we perform first an SF6 etching process44 to remove only the top hBN, followed by an O2/Ar etching to remove the graphene, but not the thin bottom hBN that will prevent leakage current between the bottom graphite gate and the electrodes. We deposit Cr(3 nm)/Au(40 nm) followed by a lift-off in acetone.Standard EBL is used to pattern the Hall bar geometry and make one-dimensional edge contactsElectrical measurements were performed in a He flow cryostat from ICE Oxford operating at To remove substrate height variations, we apply a high pass filter with a cut off frequency Supplementary informationPeer Review File


In this letter, we propose a structure based on double-layer graphene-based planar chiral metasurface with a J-shaped pattern to generate asymmetric transmission for circularly polarized waves in the mid-infrared region. Asymmetric transmission of the double-layer structure can reach to 16.64%, which is much larger than that of the monolayer. The mechanism of asymmetric transmission is attributed to enantiomerically sensitive graphene's surface plasmons. Besides, asymmetric transmission can be dynamically tuned by changing the Fermi energy and is affected by intrinsic relaxation time. All simulations are conducted by the finite element method. Our findings provide a feasibility of realizing photonic devices in tunable polarization-dependent operation, such as asymmetric wave splitters and circulators. In this paper, we propose a structure based on double-layer graphene-based planar chiral metasurface with J-shaped pattern to generate asymmetric transmission, which can reach to 16.64%. Asymmetric transmission of chiral metasurfaces for circularly polarized waves can be defined as the total transmission difference between the same handedness waves propagating in the opposite directions or the opposite handedness waves propagating in the same direction.12 With the development of research on chirality with asymmetric transmission, electromagnetic devices applied for both linearly and circularly polarized waves have been designed, such as some polarization transformers, polarization analysers and polarization-controlled devices.13–16 They are widely used to promote the evolution of optical communication and photonics. The excitation of enantiomerically sensitive surface plasmons is a collective mode of electron oscillation at the interface of conductor and medium, which is in charge of asymmetric transmission for circularly polarized waves in planar chiral metasurfaces.17–19In recent years, chiral metasurfaces have become more and more popular in many fields such as optics. Chirality means that an object cannot be coincident with its mirror image. Existing studies of chiral metamaterials mainly focus on circular20–22 And for graphene metasurface, novel optical properties are dynamically tunable by changing the Fermi energy through voltage control or the changing intrinsic relaxation time through chemical doping.23–26 Compared with metallic surface plasmons, graphene surface plasmons have the advantages of low loss, and high localization.27 Thus, graphene will revolutionize metamaterials and metadevices, and promote the development of nano-optics.28 There are plenty of researches on metallic chiral metamaterials,29 but they are generally not tunable, which can be easily realized in graphene metasurface. Recently, people propose kinds of monolayer graphene chiral metasurface,30,31 and study the asymmetric transmission for circularly polarized waves. The researches of graphene planar chiral structure will deepen the understanding of the interaction between light and matter in planar chiral structure, which can be applied to tunable polarization sensitive devices, and circular polarizers. But the asymmetric transmission of monolayer graphene chiral metasurface is only up to 5%.As a kind of promising electrically tunable plasmonic material, periodical graphene metasurfaces can excite graphene surface plasmons in the mid-infrared and terahertz (THZ) region.T can reach a small value of 2.05%. When another J-shaped pattern, which is designed to be rotated by 90 degrees after being mirror symmetric operation with respect to the previous layer, is added, the asymmetric transmission ΔT can achieve 16.64%. The mechanism of asymmetric transmission can be explained with the enantiomerically sensitive graphene surface plasmons, which induce the charge-field excitations. Also, the peaks of asymmetric transmission blue shift with increasing Fermi level, and the values of peaks increase with increasing electron scattering time. Finite Element Method (FEM) simulations are conducted to verify our findings.In this letter, we propose a double-layer graphene-based planar chiral metasurface structure to achieve giant enhancement of asymmetric transmission for circularly polarized waves incidence in mid-infrared region. In the monolayer metasurface which is composed of J-shaped hollow graphene patterns, the asymmetric transmission Δp of the structure is set to be 270 nm, and the other parameters of the pattern are displayed in the ax = 230 nm and m = 125 nm, respectively. The length of J-shaped pattern is ay = 210 nm, and the width of hollow area is w = 20 nm. Periodic boundary conditions are set along x and y directions for our periodical structure and perfectly matching layers are set in z direction to achieve the minimal reflection for the outgoing electromagnetic wave.Firstly, we discuss the monolayer graphene metasurface, and one unit is shown in 32σintra represents intraband conductivity, it can be calculated as:σinter represents interband conductivity, it is characterized by:  Here,  can be expressed as:According to the Kubo formula, the conductivity of graphene consists of two parts: intraband and interband elements: can be expressed as:e is the electron charge, kb is Boltzmann constant, T (= 300 K) is temperature, ℏ is Planck constant, ω is the angular frequency, and Ef is Fermi energy. Here, the Fermi energy of the graphene is fixed to be 0.80 eV, and it can be changed by altering the gate voltage applied on the graphene planar metasurface. The relaxation time τ is defined as μEf/evF2. Here, the Fermi velocity and DC mobility are vF ≈ 106 m s−1 and μ = 10 000 cm2 V–1 S−1, respectively,22,33 thus τ is 0.8 ps.Ei = tijE0j, the indices i, j represents right-handed circularly polarized (RCP) or left-handed circularly polarized (LCP) components. The transmission matrix elements are defined as Tij = |tij|2. The circular transmission T can be applied to any dispersive optical system under the condition of coherent light incidence. The transmission matrix elements are the same for circularly polarized waves of opposite handedness propagating in opposite directions. It means that the LCP(RCP) waves incident forward equal to the RCP(LCP) waves incident backward. Thus, we can define the transmittances of the LCP waves with opposite propagation directions asTf and Tb represent the transmittances in the forward and backward incident directions for LCP. TLL and TRR are the direct transmission, and TRL and TLR are the circular polarization conversion transmission. For our chiral structure with non-C4 symmetry, the direct transmissions of planar chiral metasurface are insensitive to polarization or propagation directions (TLL = TRR). The asymmetric transmission ΔT is defined as the difference between Tf and Tb.LCPcirc is the asymmetric transmission for LCP incidence, and ΔRCPcirc is the asymmetric transmission for RCP incidence.The pattern of the J-shaped graphene metasurface has a character of chirality, which can reveal asymmetric transmission when the incident circularly polarized waves have different handedness or propagation directions. Thus, the asymmetric transmission is only decided by circular polarization conversion transmission.Ef is fixed to be 0.80 eV, for the monolayer J-shaped graphene metasurface. Black and red lines represent LCP incidence along the forward and backward directions, respectively. It is found that the direct transmissions (TLL and TRR) always keeps the same with each other with the wavelength changing, and reaches minimum when the incident wavelength is 14.83 μm, shown as TRL is always a little larger than TLR with the wavelength ranging from 13 to 17 μm, and the maxima of TRL and TLR are 15.07% and 13.02% at 14.83 μm, respectively. Thus, the asymmetric transmission for LCP waves reaches the maximum value of 2.05% at the wavelength of 14.83 μm, as shown in T is small, compared with that of metallic planar chiral metamaterial structure.34,35 To realize larger asymmetric transmission, a double-layer structure is proposed based on the monolayer graphene-based J-shaped metasurface structure. y-axis. The medium between the two graphene layers is quartz, and the thickness of the quartz is 125 nm. The LCP waves are still coming perpendicularly from the front and the back of graphene-based planar chiral metasurfaces, respectively.Although the asymmetric transmission has been obtained, the ΔEf is fixed to be 0.80 eV for the double-layer structure shown as TLL and TRR are still identical, which proves that asymmetric transmission is independent of direct transmission. In TRL is slightly higher than that of the monolayer chiral metasurface shown in TLR goes down sharply to almost zero at the wavelength of 14.90 μm. Compared with the monolayer J-shaped metasurface, the resonant wavelength of graphene surface plasmon redshifts a little. Due to the existence of the near to zero value of TLR, the asymmetric transmission ΔT can be increased a lot. As shown in the T can reach 16.64% at the wavelength of 14.90 μm, which is much larger than 2.05% of the monolayer graphene-based planar chiral metasurface. Similarly, the asymmetric transmission peak of RCP incident waves reaches −16.64%, just the opposite of LCP. The giant enhancement of circular asymmetric transmission suggests that the structure of double-layer J-shaped graphene metasurface can greatly improve chirality.36 The excitation level is decided by the direction and polarization state of the incident waves. Here, graphene localized surface plasmon is excited along the J-shaped edge because the electric field is enhanced around the J-shaped edge shown in Ey of the induced electric field for LCP(RCP) incidences can reflect the excitation level of localized plasmon, and explain the difference between TLR and TRL. Here, the yellow and green arrows was painted to demonstrate the components Ey along y-directions as shown in TLR is smaller than TRL, and an asymmetric transmission (ΔT = TRL − TLR) of 2.05% is achieved for the monolayer metasurface structure. Ey for the LCP incidence indicated by yellow arrows in TRL of double-layer increases to 17.13%, compared with 15.07% of the monolayer. As shown in the 17,36 Thus, TLR is near to zero, and the asymmetric transmission of the double-layer metasurface can be greatly increased and ΔT can reach 16.64%.In order to understand the physical mechanisms behind the asymmetric transmission, the induced electric field in the hollow area of the bottom layer of graphene metasurface is studied in Kspp = ħω2/2α0Efc ∝ 1/Lg37 where α0 = e2/ħc is the fine structure constant. The resonant wavelength λ can be written as:Lg represents the resonant characteristic length of the graphene hollow pattern. Consequently, it is clearly demonstrated that increasing Fermi energy Ef can cause blue shift of these resonant wavelengths. In addition, when Fermi energy increases, the peak amplitude of asymmetric transmission increases from 15.82% to 19.19%. τ and asymmetric transmission, when the Ef is 0.80 eV. Here, μ is usually in the range of 3000 cm2 V–1 S−1 to 10 000 cm2 V–1 S−1, so intrinsic relaxation time τ can vary from 0.24 to 0.80 ps. With the increase of intrinsic relaxation time τ, the peak value of the asymmetric transmission increases from 5.95% to 16.64%, and the peak position remains unchanged, always at the wavelength of 14.90 μm. This increase of the asymmetric transmission peak value with increasing electron scattering time τ is caused by the reduced free-carrier loss in graphene, which can lead to a increase in the circular polarization conversion transmission TRL but no change of TLR. Therefore, the frequency peak position of asymmetric transmission can be tuned because the Fermi energy can be changed by tuning the gate voltage, compared with the metallic chiral metamaterials. For tuning the Fermi energy of graphene in the potential experiments, we can cover the structure with a layer of ion gel following the procedure described in 39To study the dynamical tunability of asymmetric transmission, we changed the Fermi energy and intrinsic relaxation time of graphene layers. When increasing the Fermi energy from 0.76 to 1.00 eV, the peak of asymmetric transmission has a blue shift changing from 15.30 to 13.30 μm, as shown in Ef is fixed to be 0.80 eV. y-axis, respectively. TLR and TRL varies with wavelength. For the structures of λ locates at 15.1 and 14.8 μm, shown as Other double-layer structures of J-shaped pattern have been also studied, and circular polarization conversion are calculated, when In summary, we demonstrated and studied the double-layer graphene-based planar chiral metasurfaces structure. Through FEM numerical simulation, we chose the most suitable orientation of double-layer graphene pattern, and the asymmetric transmission can reach to 16.64%, which is much larger than that of the monolayer. In addition, the induced electric field below the graphene chiral metasurfaces are analyzed for understanding the physical mechanism of asymmetric transmission. We attribute the asymmetric transmission for circularly polarized waves to the enantiomerically sensitive plasmonic excitations. Besides, the peak of asymmetric transmission could have frequency shift and amplitude changes when the Fermi energy and intrinsic relaxation times of graphene are changed. We believe our findings can be applied to tunable polarization sensitive devices, circular polarizers and other fields.There are no conflicts to declare.


Graphene resonant sensors have shown strong competitiveness with respect to sensitivity and size. To advance the applications of graphene resonant sensors, the damage behaviors of graphene harmonic oscillators after thermal annealing and laser irradiation were investigated by morphology analysis and frequency domain vibration characteristics. The interface stress was proven to be the key factor that directly affected the yield of resonators. The resulting phenomenon could be improved by appropriately controlling the annealing temperature and size of resonators, thereby achieving membrane intactness of up to 96.4%. However, micro-cracks were found on the graphene sheets when continuous wave (CW) laser power was more than 4 mW. Moreover, the fluctuating light energy would also cause mechanical fatigue in addition to the photothermal effect, and the threshold damage power for the sinusoidally modulated laser was merely 2 mW. In this way, based on the amplitude-frequency surface morphology of the graphene resonator, the thermal time constant of the order of a few microseconds was confirmed to evaluate the damage of the graphene oscillator in situ and in real time, which could be further extended for those resonators using other 2D materials. Micro resonant sensors have been widely applied in aviation, aerospace engineering and automation due to their high sensitivity, stable performance and direct frequency signal output. The resonator is the key element of resonant sensors, which dominantly affects the performance of the whole system. To develop a high-performance resonant sensor, the material of the micro resonators should be stiff, robust and stable.9 cycles [Graphene, an atom-thick two-dimensional material with a single-layer thickness of 0.335 nm, has demonstrated excellent mechanical , optical9 cycles , which m9 cycles , actuate9 cycles . This pe9 cycles ,12,13. TThe damage of graphene resonators can be attributed to the manufacturing process and operating conditions. For the fabrication of resonators, suspended transfer of graphene is a common step that might cause damage to the thin suspended membrane. Generally, graphene transfer in micro-mechanical systems is performed with the help of the widely used polymethyl methacrylate (PMMA) , PolydimThe Raman spectroscopy technique has been used to evaluate the extent of damage by calculating the intensity of the D and G peaks . HoweverIn this section, the damage to the graphene harmonic oscillator after thermal annealing is presented. Graphene membranes annealed at a temperature of 300, 375 or 450 °C were first observed under an optical microscope and scanning electron microscope (SEM)  to analyse the breakage rate of the membrane, on micro- and nano-scales, respectively. Then, the resonant characteristics, including quality factor and resonant frequency, were investigated by the aforementioned all-fibre experimental system.After the annealing process at each temperature, the intactness of the graphene was observed by an optical microscope. It has been found that graphene with a more than 95% free-standing area has the potential to be fabricated as a well-behaved harmonic oscillator; thus, this kind of membrane was noted to be intact in this paper. To be specific, 300 graphene membranes after each annealing process were randomly selected, and the number of intact membranes was counted. The statistical results are listed in Besides the stress between the graphene and PMMA, there was also a thermal interfacial interaction between the MLG and substrate in the heating process, which would also cause damage to the graphene, especially at the edge of the entire membrane. A photo of a typical damaged graphene caused by thermal interaction between the MLG and substrate is shown in Furthermore, in order to graphically depict the damage to the graphene membrane on a nano scale, the SEM photographs of the suspended graphene are given in dark/numall, wherein numdark and numall represent the number of dark pixels and all pixels, respectively. It could be concluded that a higher annealing temperature T or a larger suspended radius R would result in a larger broken area. Taking the graphene , for example, the graphene exhibited perfect intactness (100%). However, once the suspended diameter was increased to 100 μm, the breakage rate increased to 2.5%. When the annealing temperature was increased to 450 °C, the breakage rate rose rapidly to 30.9%. In the aforementioned experimental set up, the graphene breakage rate showed higher sensitivity to the annealing temperature than the suspended diameter.In this way, these SEM images were further binarized to calculate the proportion of the damaged area. Thus, the microscopic broken rate of the membrane could be calculated as numω0/Δω, where ω0 is the natural frequency and Δω is the 3 dB bandwidth of the amplitude–frequency curve. Thus, the resonant characteristics of the graphene resonators were investigated at the aforementioned three annealing temperatures, as illustrated in Besides the breakage rate and the surface morphology of the membrane, which has been mentioned above, the resonant characteristics were also investigated, including the resonant frequency and quality factor. Among these, the resonant frequency indicates the fundamental frequency of the oscillator, and the quality factor represents the energy loss per oscillation cycle, which is calculated by f20 = 1254.1 kHz (D = 20 μm), f60 = 338.9 kHz (D = 60 μm) and f100 = 153.5 kHz (D = 100 μm), and the standard deviation (STD) of frequencies were confirmed as σ20 = 192.7 kHz (D = 20 μm), σ60 = 59.0 kHz (D = 60 μm) and σ100 = 46.2 kHz (D = 100 μm), respectively , σ60 = 2.50 (D = 60 μm) and σ100 = 3.34 (D = 100 μm), respectively  and f60 = 661.3 kHz (D = 60 μm), and the STD values were calculated to be 641.8 kHz and 348.4 kHz, accordingly. The Q factors were measured to be 8.93 (D = 20 μm) and 6.85 (D = 60 μm) with the corresponding STD values of 2.03 and 2.72, respectively , the ratio of the inner strain of 20 μm, 60 μm and 100 μm membranes were estimated to be S1:S2:S3 = 2.69:1.78:1. This strain possibly resulted from the Van der Waals force between the graphene and the copper sidewall. At different temperatures, different mechanical energy might be introduced via distortions of the graphene lattice [For a circular membrane under tension, the fundamental frequency can be expressed as (1)f=2.4 lattice . For theNote that when the annealing temperature increased to 450 °C, the fabricated resonators of all sizes were damaged with the damaged areas as shown in Damage not only occurs in the fabrication process, but also in the working process of the harmonic oscillator. In the previous study on the laser-induced damage, the graphene was often tested by a Raman spectrometer  or microAs the modulated laser was verified with a higher possibility of damaging the membrane, the effect of the modulated laser on the graphene resonant characteristics was further explored. Thus, the graphene membrane was excited with a modulated laser whose power was gradually increased from 1 mW to 5 mW. Meanwhile, the motion was recorded by a CW laser with an extremely small amount of power, so that this laser would barely damage the structure of the graphene. The experiment results were recorded in From the frequency domain, z in the frequency domain could be written as [τ is the thermal time constant, R is the thermal resistance, C is the thermal capacitance (RC = τ), α is an effective thermal-expansion coefficient and P is the heating power. After taking the derivative of Formula (2), the imaginary part of the response function reached the maximum amplitude when ωτ = 1. Thus, the thermal time constant τ of the graphene membrane was calculated. According to Metzger et al. , a thermitten as : (2)z(ω)τ started to deviate at about 3 mW  of the resonator showed no synchronous improvement compared to the counterparts annealed at 300 °C (Q20 = 9.8).From the perspective of the application of graphene resonant sensors, the effect of the temperature-dependent annealing treatment on the intactness of suspended multilayer graphene was investigated through surface morphology observation to further evaluate the resonant behaviours of graphene resonators after the thermal annealing process. The experimental results showed that an annealing temperature of 300 °C leaves a certain degree of PMMA residue, which can prevent the breakage of graphene while annealing. In contrast, when the annealing temperature rose to 375 °C, more cracks, or even a total collapse, occur in the suspended membrane. In this way, although resonant frequencies that are twice as high could be achieved, the atmosphere pressure quality factor (Besides the annealing treatment, the effect of the laser irradiation on the intactness of the suspended multilayer graphene was also investigated. The damage was mainly caused by the modulated laser, which would induce both a thermal effect and mechanical fatigue. The damage threshold power for the modulated laser was found to be about 2–3 mW, which is about half the CW laser. Thus, the modulated laser power should be controlled carefully in application. Moreover, it was found that the fluctuation of the thermal time constant could be applied to evaluate this damage in situ and in real time.


Graphene-based microwave devices have enabled reconfigurability, thus paving the way to the realization of flexible wireless terahertz systems with featured performances. Despite great progress in the development of graphene-based terahertz devices in the literature, high insertion loss and wide tunable range are still significant challenges at such high frequencies. In this work, we introduce the use of graphene to implement a reconfigurable printed ridge gap waveguide (RPRGW) structure over the terahertz frequency range for the first time. This guiding structure is suitable for both millimeter and terahertz wave applications due to its supporting quasi-TEM mode, which exhibits low dispersion compared to other traditional guiding structures. The presented solution is featured with low loss as the signal propagates in a lossless air gap, which is separated from the lossy graphene elements responsible for the reconfigurable behavior. In addition, this guiding structure is deployed to implement a tunable RPPGW power divider as an application example for the proposed structure. Reconfigurable systems can be constructed through deploying sub-systems having the ability to reversibly change their functionality under some controlling process. One of the major revolutions of reconfigurability in electronics is the field-programmable gate array (FPGA), which has a powerful impact on digital design methodologies alongside wireless systems3. In a related context, few trials are initiated to develop reconfigurable microwave components providing an additional level of functionality for wireless systems. These components can be implemented using a traditional guiding structure such as a microstrip line, rectangular waveguide, and substrate integrated waveguide6. However, these guiding structures have high material losses and fabrication limitations over higher frequency bands. Therefore, the research community exerts more effort to develop innovative guiding structures such as ridge gap waveguide (RGW) to overcome the high frequency limitations and satisfy the technical requirements for future wireless technology9. RGW is considered as a promising guiding structure for millimeter and terahertz wave applications due to the recently reported advantages such as supporting the propagation of quasi-TEM (Q-TEM) mode with lower loss compared with other guiding structures14.Reconfigurable systems are gaining considerable attention as they are expected to play an important role in the development of terahertz wireless technologies. Reconfigurability can result in flexible, cost-effective, and smart transceivers overcoming the design challenges at such high frequencies and achieving the technological future demands19. One of the promising techniques to realize these switches are using graphene. Graphene has incited great interest due to its attractive features that lead to potential usage in different applications22. Graphene’s transport characteristics and conductivity can be tuned by either electrostatic or magnetostatic gating, or via chemical doping, thereby leading to the possibility of developing various electronic devices25. Based on the possibility of controlling the graphene conductivity (impedance) by field effects, several studies have been conducted showing a possibility to reconfigure the performances of microwave devices and antennas33. However, few studies have addressed the effect of tuning the graphene on the frequency response of guiding structures such as coplanar waveguide, parallel plate waveguide, rectangular waveguide, microstrip line, and substrate integrated waveguide40.Reconfigurability of microwave components can be achieved through various mechanisms such as switches, variable reactive loading, and mechanical and material variation. The most common technique used is deploying a switch such as MEMS, PIN diode, and field-effect transistor (FET), which can alter the performance of components in a discrete manner36. A reconfigurable Electromagnetic Band Gap (EBG) unit cell is investigated to construct the PRGW guiding structure. Simulated results show that inserting graphene in the EBG unit cells influences the higher frequency of the band gap. This is achieved by altering the chemical potentials of the loaded graphene. Thus, the operating bandwidth of the PRGW line can be varied corresponding to changing the chemical potentials of graphene. As an application of the proposed RPRGW line, a wideband reconfigurable PRGW power divider is simulated. The results verify that the proposed RPRGW can provide an effective guiding structure for reconfigurable terahertz devices.To our best knowledge, the use of graphene to implement a reconfigurable printed ridge gap waveguide (RPRGW) at terahertz is proposed for the first time in this paper. The PRGW structure offers the separation between the graphene and the signal path, which imply by nature the low loss characteristics compared with SIW graphene-based structure as the graphene exists in the signal pathThe aforementioned applications and challenges in the terahertz motivate the scientific community to develop novel solutions that can overcome significant losses and provide reconfigurable features. We introduce for the first time the deployment of graphene in the printed ridge gap waveguide technology to enable the reconfigurable behavior of the entire structure. We propose the placement of the graphene in the cell structure away from the signal path to avoid the loss of contribution of the graphene, which will be discussed in detail in the following section. Before going through the proposed structure, we will give a brief discussion about the graphene material characteristics and modes of operation.T is temperature in kelvin37. The chemical potential is determined by the carrier density, where varying the carrier density will affect the mobility of the electron-hole, thus changing the material conductivity. The carrier density can be controlled by the application of a gate voltage, wherein in the ungated case both the carrier density and chemical potential can be considered zero24. The conductivity of graphene e = 1.60217662 × 10Many articles have discussed the electronic model of graphene, which can be modeled as an infinitesimally thin surface characterized by a surface conductivity 41. It can be noticed that the imaginary part of the conductivity is negative, meaning that the graphene can be modeled by a surface impedance 42:The effect of chemical potential variation of the graphene’s real and imaginary conductivity is studied and plotted in Fig. The real and imaginary part of the graphene impedance is plotted in Fig. 9. The signal propagates in the form of a Q-TEM mode between the ridge and the top ground, while the cells prevent any leakage within the operating bandwidth43. The cell stop band represents the operating bandwidth of the entire structure. This structure has been investigated intensively in the past decade, where various components have been proposed based on this promising technology45. This technology provides a wide band operation with minimal dispersion, which is suitable for high-frequency applications. In addition, a printed version of the same structure has been proposed to provide the same advantages in a low-cost guiding mechanism. Different unit cell structures have been examined in the literature for (RGW) Ridge Gap waveguides and PRGW—Printed Ridge Gap waveguides14. In this work, we aim to utilize a novel unit cell structure, where some metallic parts can be replaced by graphene. Afterward, applying a controlling voltage will vary the conductivity of cell parts replaced by graphene. As a result, the effective cell shape will depend on the applied controlling voltage. Hence, the operating bandwidth of the entire structure can be adjusted through the controlling voltage.Ridge gap waveguide is considered among the emerging guiding structures, introduced for the first time in 2009First of all, we propose an innovative cell structure to ensure the operation at the target frequency band, where some metallic parts will be replaced by graphene in a later design stage. The proposed cell construction, row of cells, and a ridge surrounded by three successive cells are illustrated in Fig. 7. The dispersion relation extraction setup is shown in Fig. 7:l, and m is the mode number, while The proposed cell is modified by adding the graphene material in four rectangular cuts within the cell perimeter to provide tuning elements. It is worth mentioning that the band gap of the conventional unit cell can be determined using the eigenmode solver, which is a numerical technique available in many 3D EM analysis software packages such as CST and HFSS. However, the eigenmode solver packages do not support lossy materials solution, accordingly, they fail to calculate the dispersion diagram of the proposed graphene doped unit cell. Other solvers can be used to solve the lossy materials unit cell dispersion diagram however, a mathematical method based on the scattering parameters is proposed to extract the dispersion relation and study the effect of the graphene material on the band gap of the unit cellTo obtain the dispersion relation, the simulated scattering parameters are obtained using the model shown in Fig. mentclass2pt{minimThe same outcomes are obtained through the simulated response of the model presented in Fig. 47.The geometrical configuration of the proposed T-junction power divider implemented using the graphene-based terahertz PRGW line that has been discussed in the previous sections is shown in Fig. }% 0.8–1. THz at a}% 0.8–1. THz at aThe investigation of the use of graphene in implementing a reconfigurable printed ridge gap waveguide (RPRGW) at terahertz has been proposed in this article. A printed ridge gap waveguide has well-known advantages compared with other guiding structures including low radiation and material loss, which is suitable for very high frequencies. An electronic model of graphene at terahertz has been studied in detail, where an infinitesimally thin surface characterized by surface conductivity has been deployed to model the graphene. A novel unit cell structure has been proposed, where some metallic parts have been replaced by graphene to achieve a reconfigurable electromagnetic band gap by changing the chemical potential. In addition, the graphene is not allocated in the signal path, which leads to a featured reconfigurable configuration with minimal losses. An innovative mathematical methodology based on the scattering parameters has been proposed to extract the dispersion relation and study the effect of the graphene material on the band gap of the unit cell. As an application example of the proposed technology, a dynamic PRGW line, and power divider has been proposed, where the effect of altering the chemical potential on operating bandwidth has been studied. For the future extension, this work can be applied to different guiding structures such as groove waveguides and substrate integrated waveguides, however, more losses will occur.