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Aleksandra Plačkić, Tilmann J. Neubert, Kishan Patel, Michel Kuhl, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Amaia Zurutuza, Roman Sordan, Kannan Balasubramanian

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Electrochemistry at the Edge of a van der Waals HeterostructureRESEARCH ARTICLEwww.small-journal.comElectrochemistry at the Edge of a van der WaalsHeterostructureAleksandra Plačkíc, Tilmann J. Neubert, Kishan Patel, Michel Kuhl, Kenji Watanabe,Takashi Taniguchi, Amaia Zurutuza, Roman Sordan,* and Kannan Balasubramanian*Artificial van der Waals heterostructures, obtained by stackingtwo-dimensional (2D) materials, represent a novel platform for investigatingphysicochemical phenomena and applications. Here, the electrochemistry atthe one-dimensional (1D) edge of a graphene sheet, sandwiched between twohexagonal boron nitride (hBN) flakes, is reported. When such anhBN/graphene/hBN heterostructure is immersed in a solution, the basalplane of graphene is encapsulated by hBN, and the graphene edge isexclusively available in the solution. This forms an electrochemicalnanoelectrode, enabling the investigation of electron transfer using severalredox probes, e.g., ferrocene(di)methanol, hexaammineruthenium, methyleneblue, dopamine and ferrocyanide. The low capacitance of the van der Waalsedge electrode facilitates cyclic voltammetry at very high scan rates (up to1000 V s−1), allowing voltammetric detection of redox species down tomicromolar concentrations with sub-second time resolution. The nanobandnature of the edge electrode allows operation in water without addedelectrolyte. Finally, two adjacent edge electrodes are realized in a redox-cyclingformat. All the above-mentioned phenomena can be investigated at the edge,demonstrating that nanoscale electrochemistry is a new application avenuefor van der Waals heterostructures. Such an edge electrode will be useful forstudying electron transfer mechanisms and the detection of analyte species inultralow sample volumes.1. IntroductionHeterostructures assembled using single sheets of 2D materi-als are highly promising due to the novel properties they exhibitA. Plačkíc, K. Patel, R. SordanL-NESS, Department of PhysicsPolitecnico di MilanoVia Anzani 42, Como 22100, ItalyE-mail: roman.sordan@polimi.itThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/smll.202306361© 2023 The Authors. Small published by Wiley-VCH GmbH. This is anopen access article under the terms of the Creative Commons AttributionLicense, which permits use, distribution and reproduction in anymedium, provided the original work is properly cited.DOI: 10.1002/smll.202306361and several advantages they offer in com-parison to their isolated counterparts.[1–3]Graphene has been widely explored as abuilding block in such heterostructures. Asa one-atom thick material, the propertiesof graphene are highly sensitive to its envi-ronment. Especially the substrate, on whichgraphene is placed, dictates not only thephysical properties but also the chemistry,such as reactivity and electron transfer (ET)properties.[4–6] By assembling a graphenesheet on an insulating 2D material such ashexagonal boron nitride (hBN), it is pos-sible to decouple graphene from the un-derlying substrate, which renders graphenewith properties close to that of free-standinggraphene.[7] The lattice constant of hBN issimilar to that of graphene, limiting thestress that can develop when graphene isplaced or grown on such a crystal.[8] As thedefect density in an hBN crystallite is or-ders of magnitude lower than in typical sub-strates such as silicon dioxide or glass, theintrinsic electronic properties of graphenecan be directly exploited to realize devicesshowing superior electronic performance,such as high charge carrier mobility.[7] Inan analogous manner, the environment towhich graphene is exposed from the top has a significant ef-fect on its physical properties. By using a van der Waals (vdW)heterostructure, wherein a graphene sheet is encapsulated be-tween two hBN layers, the sensitivity to the environment can beA. PlačkícBioSense InstituteUniversity of Novi SadDr Zorana Ðind̄íca 1, Novi Sad 21000, SerbiaT. J. Neubert, M. Kuhl, K. BalasubramanianSchool of Analytical Sciences Adlershof (SALSA), IRIS Adlershof &Department of ChemistryHumboldt-Universität zu BerlinUnter den Linden 6, 10099 Berlin, GermanyE-mail: nano.anchem@hu-berlin.deK. Watanabe, T. TaniguchiNational Institute for Materials Science1-1 Namiki, Tsukuba 305-0044, JapanA. ZurutuzaGraphenea Semiconductor SLUMikeletegi Pasealekua 83, San Sebastián 20009, SpainSmall 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (1 of 9)http://www.small-journal.commailto:roman.sordan@polimi.ithttps://doi.org/10.1002/smll.202306361http://creativecommons.org/licenses/by/4.0/mailto:nano.anchem@hu-berlin.dehttp://crossmark.crossref.org/dialog/?doi=10.1002%2Fsmll.202306361&domain=pdf&date_stamp=2023-12-18www.advancedsciencenews.com www.small-journal.comminimized thereby providing devices with high stability and im-proved electronic performance. This strategy has been exploitedto realize high mobility electronic devices, high frequency andoptical devices, memristors etc.[9–15] In such an architecture, thegraphene sheet is physically well-isolated from the environmentand the substrate.For studying chemistry and electrochemistry, anhBN/graphene/hBN vdW heterostructure provides newprospects, whose exploration is still in its infancy.[16,17] It isa suitable platform to study electrochemical intercalation instacked 2D materials.[15] In place of hBN, oxide layers could beused to realize graphene edges. By realizing pores in a graphenelayer sandwiched between two Al2O3 layers, ionic transportthrough the pore could be measured.[18] However, ET withredox active species at the edge has not been investigated inthat study. By contrast, the presence of hBN on both sides ofgraphene provides, in addition to an electronic isolation, alsoa clear chemical isolation of the basal plane of graphene fromthe environment,[4] opening up the possibility to study reactionsexclusively at the edge, virtually free of substrate and basalplane effects. Electrochemistry at the graphene edge has beeninvestigated using other fabrication strategies earlier.[19–23] Withthese edge electrodes, exclusive electrochemical modificationof the edge has also been demonstrated.[20,22] Furthermore, asis typical for nanoelectrodes, large current densities and highET rates have been estimated.[18,21,23] In all these cases, thegraphene sheet was directly placed on a silicon dioxide surface.The use of a vdW heterostructure effectively decouples the basalplane of graphene from the underlying surface and the environ-ment, preserves the favorable properties of its basal plane, andallows electrochemical investigation exclusively of its edge in adecoupled environment.There are specific advantages of studying electrochemistryat clean single graphene edges. Geometrically, the nanoscalesize of the electrode opens the possibility of enhanced masstransport.[23,24] This is fundamentally important for studying fastelectron transfer kinetics (typically > 1 cm s−1),[25] as has beenshown for single carbon nanotube electrodes.[] On the otherhand, the edge poses a different electronic structure in com-parison to the basal plane. Hence, improved electrochemical ac-tivity and catalytic effects are expected.[27–29] A graphene edgeis a one-dimensional system and functions like a nanobandelectrode.[19,30] Often such micro-/nano-band electrodes are re-alized using metals, e.g., Au or Pt.[31] The use of carbon as a na-noelectrode opens an avenue for the study of selected redox activespecies, e.g., nicotinamide adenine dinucleotide (NADH), whoseET rates are known to be higher on carbon than on metals.[23,32]A single carbon nanotube is a suitable candidate for a carbon-based linear nanoelectrode (two lateral dimensions < 10 nm).[26]The noise level in graphene has been reported to be lower thanthat in carbon nanotubes.[33] The basal part of graphene can beseen as a large current collector for measuring the response ofthe edge. In the ideal case, the redox current at every atom in theedge can be collected directly by the basal plane in parallel, i.e., in-dependent of the rest of the edge. Hence, it can be expected thatthe current at the edge can be acquired with low noise, whichis further minimized as the sheet is embedded in the vdW het-erostructure. On the other hand, single nanotubes have not beenreported for electrochemistry at high scan rates or in the contextof redox cycling.With this motivation, we present here the realization of 1Dgraphene edge electrodes with a length of few microns sand-wiched between two hBN multilayer sheets, where only the edgeof graphene is in contact with the solution. Exploiting the uniqueproperties of such an electrode, e.g., the small electrode areaand high interfacial resistance, we demonstrate that we can per-form electrochemistry at fast scan rates and work in water, freeof added supporting electrolyte. Finally, we also show that theseelectrodes can be realized in a redox-cycling format, wherein re-dox active species reduced or oxidized at one electrode can beshuttled for ET to a second edge electrode in its close vicinity.2. Results and DiscussionFigure 1 shows the fabrication steps and the layout of a typical vander Waals edge (vdWEdge) electrode, which comprises a mono-layer graphene sheet sandwiched between two hBN multilayerflakes and contacted on two ends by Au electrodes. Complete de-tails of the fabrication are given in the Methods section based onour previous work.[14,35] In short, the fabrication of such a devicestarted with the assembly of an hBN/graphene/hBN stack onto aSiO2/Si substrate using a hot pick-up technique (Figure 1(a)).[34]The 1D edges of graphene were exposed on all four sides of thestack, after the stack was shaped in rectangular form by reactive-ion etching (RIE) using a hard mask (Figure 1(b)). Two sidesof the stack were metallized to contact graphene along its 1Dedges, while the other two sides were left uncovered to expose the1D edges of graphene to the environment (Figure 1(c)). Finally,the electrodes were passivated using poly(methylmethacrylate)(PMMA). Electrical measurements were performed to confirmthe formation of ohmic contacts to graphene. The mean re-sistance was 2.44 kΩ (19 devices) with a standard deviation of0.75 kΩ for a graphene channel with a width of 10 μm and lengthof 4 μm. More details about the resistance of the devices can befound in Figure S1 (Supporting Information).An optical image of the final device showing the top-view, iden-tifying the different parts, is shown in Figure 2(a). Figure 2(b)shows a close-up scanning electron microscope (SEM) image ofthe edge of the assembled stack, where the top and bottom hBNlayers are clearly distinguishable, confirming that the basal planeof graphene is well protected. Further optical and atomic forcemicroscopy (AFM) images of a device with another layout areshown in Figure S2 (Supporting Information).We first evaluate ET with classical redox-active species atthe vdWEdge as shown in the cyclic voltammograms (CVs)in Figure 3. Figure 3(a) shows the oxidation of ferrocyanide(FeCN), dopamine (DA) and ferrocenemethanol (FcMeOH),while Figure 3(b) shows the reduction of hexaammineruthe-nium and methylene blue at a vdWEdge electrode at low scanrates. Except for DA and FeCN, the CVs show nearly a sig-moidal shape, signifying a quasi-steady-state response, typical fornanoelectrodes.[23,31,36] Due to the absence of diffusion limitation,we do not see characteristic peaks typically observed in diffusion-limited CVs at bulk electrodes.[37] Figure 3(c) presents CVs ofFcMeOH with varying scan rate (rates < 100 mV s−1), where itis apparent that the Faradaic response is independent of the scanSmall 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (2 of 9) 16136829, 2024, 21, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306361 by Cochrane Japan, Wiley Online Library on [24/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 1. Simplified workflow showing the fabrication of a vdWEdge device. a) An Al hard mask was fabricated on top of the hBN/graphene/hBN stack,which was assembled on top of a SiO2/Si substrate by a hot pick-up technique.[34] b) After RIE, the part of the stack unprotected by the mask was etchedaway. c) After removal of the mask, the Au electrodes were fabricated on the opposite sides of the stack forming contacts to the exposed 1D edges ofgraphene. d) The contacts were passivated by a PMMA layer such that the remaining 1D edges of graphene were exposed to the environment.Figure 2. Different views of a vdWEdge device. a) Optical image of a typ-ical device in top-view with the hBN/graphene/hBN stack in between theAu contacts as indicated. b) False-color high-resolution SEM image of theperipheral region of a vdWEdge device, where the two hBN crystals arediscernible. The graphene edge lies open in between the edges of thesetwo layers highlighted in yellow.rate. Only the capacitive current increases. These observationsconfirm that we indeed measured the response at the edge work-ing as a nanoelectrode.[23,31]Due to the miniscule size (typically 2 μm length at an esti-mated height of ≈1 nm) of the vdWEdge electrode, the masstransport rate is extremely high. Hence using such electrodes,we have the capability to measure a broad range of ET rates withredox active species.[25,31] One example of extracting heteroge-neous rate constant from our data is shown in Supporting Infor-mation. The non-sigmoidal nature of the CV with DA and FeCNsuggests that ET for these two cases is kinetically limited and isrelatively slow in comparison to FcMeOH at the vdWEdge. BothFeCN[6] and DA[38] are known to act as inner sphere probes oncarbon surfaces and are hence sensitive to the surface morphol-ogy, defects, the proportion of edges, and the chemical composi-tion of the terminal functional groups. The comparatively lowerET rate with FeCN is consistent with previous observations onmm-long graphene edge electrodes.[23] A relatively slow ET withDA at the vdWEdge indicates that the chemical composition ofour edge is most likely different from other graphitic or carbonelectrodes.[27,39–42] In previous work on ET with DA at carbon elec-trodes, often a quasi-reversible CV has been reported.[41] In con-trast, we were only able to observe an oxidation wave with DA.Even on the basal plane of our graphene electrodes, ET with DAappears to be quite inefficient (see Figure S3 in Supporting In-formation). We attribute the sluggish ET kinetics with DA to theabsence of deliberately introduced functional groups either at theedge or at the basal plane of the graphene sheet.[43] Moreover, theplasma conditions we use (e.g., SF6 plasma for shaping the edge)may render the edge with a chemistry that does not allow for afast electron transfer with DA. In the future, we plan to mitigatethis by evaluating other etching techniques such as Ar ion-beametching or wet etching to remove hBN using a sacrificial oxide orphotoresist layer.[44]In contrast to previously reported mm-long edgeelectrodes,[19,20,23] the electrode area in a vdWEdge electrodeis smaller by at least two orders of magnitude and the currentswere < 1 nA. Hence, a simpler two-electrode setup can beused avoiding the need for a third current-collecting counterelectrode.[31,45] This configuration is customary when workingSmall 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (3 of 9) 16136829, 2024, 21, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306361 by Cochrane Japan, Wiley Online Library on [24/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 3. CVs of selected redox-active species at vdWEdge electrodes. a) CVs of species undergoing oxidation: ferrocyanide (0.1 mM), dopamine (1 mM),ferrocenemethanol – FcMeOH (1 mM). b) CVs of molecules undergoing reduction: methylene blue – MB (0.1 mM), hexaammineruthenium – HARu(0.1 mM). c,d) CVs at varying scan rates for the oxidation of FcMeOH (1 mM slow scan, 5 mM fast scan) at a vdWEdge electrode – low scan rates(c) and high scan rates (d). The potentials are measured with reference to a commercial Ag/AgCl (3 M KCl) reference electrode. Scan rate was 100 mV s−1in (a) and (b). For dopamine and FcMeOH, a phosphate buffer (pH 6.7, 100 mM ionic strength) was used. The other measurements were performed in0.1 M KCl.with ultramicroelectrodes and nanoelectrodes. Using our elec-trodes, we were able to detect redox active species down to micromolar concentrations (see Figure S4 in Supporting Information).As is typical for nanoelectrodes, the small area of the vdWEdgenanoelectrode corresponds to a drastically low double layer capac-itance, which is lower than our system capacitance of 57 pF (seeFigure S5, Supporting Information for details). The system ca-pacitance is also much larger than the capacitance of the basalplane of the graphene channel (see page S4 in Supporting Infor-mation). Hence, using our current setup we cannot estimate theactual capacitance of the vdWEdge. The low capacitance can how-ever be exploited to investigate ET at high scan rates. Figure 3(d)shows the obtained response for FcMeOH at a vdWEdge at highscan rates (20–1000 V s−1). Although the capacitive current in-creases with scan rate, we are still able to observe the redoxwaves of FcMeOH, superimposed over the current due to capac-itive contribution. Since the area of vdWEdge electrode is muchsmaller than mm-long edge electrodes,[23] there is no diffusionlimitation and hence the sigmoidal response is observable evenat high scan rates.We do not expect to see any tunneling through the hBN lay-ers, since we have an insulating hBN crystallite of several lay-ers assembled tightly on and below the graphene sheet.[46] Thisis further justified by considering that multilayer hBN with avery high resistance constitutes a large energy barrier for elec-tron tunneling[47] and hence blocks ET completely to redox ac-tive species in solution. What we cannot however completely ex-clude is the possibility that not just the atomic edge, but a certainwidth (< 5 nm) of the graphene sheet in the edge region encoun-ters the solution. This could be thought of as a nanoscale flat-band electrode. However, we cannot confirm this only from theCVs. Nevertheless, the diffusion profile is expected to be conver-gent also in this case.[48,49] This could also be one of the reasonswhy we sometimes observe higher currents than the theoret-ically estimated mass-transport-limited current (see discussionon Estimation of mass-transport-limited current in SupportingInformation).We have exploited the ability to work at high scan ratesto explore the use of fast-scan cyclic voltammetry (FSCV)[50,51]at our vdWEdge electrodes. FSCV has been widely used forthe detection of redox-active species such as neurotransmit-ters, nucleobases and antioxidants.[45,51–54] Due to the high timeresolution achievable, it has found application for the detec-tion of release of DA and other molecules in vivo in differ-ent biological species.[55–57] Furthermore, by an appropriate de-sign of the voltage waveform, multiplex detection, ET mecha-nisms as well as improved detection of antioxidants have beendemonstrated.[58–59] Performing FSCV in a two-electrode setupprovides a further advantage of a simplified cell setup and com-pact instrumentation.Small 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (4 of 9) 16136829, 2024, 21, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306361 by Cochrane Japan, Wiley Online Library on [24/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 4. FSCV of FDM at a vdWEdge electrode. a) A color map showingthe voltammetric response during the entire measurement cycle. The mea-surement starts at t = 0 s with only 0.1 M KCl. At t = 4 s, FDM is injectedto achieve a final concentration of 0.5 mM. The voltage is continuouslyscanned in the range of 0 to 0.5 V at a rate of 100 V s−1 during the entiremeasurement. Only the forward cycle is shown here. b) A cross-sectionof the measured voltammetric response in (a), showing the evolution ofcurrent at a voltage of +0.45 V. The data are background subtracted usingthe current response in the first cycle. The potentials are with reference toan Ag/AgCl wire reference electrode.Figure 4 presents FSCV data obtained at a vdWEdge electrodefor the detection of 0.5 mM ferrocenedimethanol (FDM) in a two-electrode setup. Figure 4(a) shows a map of the measured current(after background subtraction) starting in 0.1 M KCl and uponaddition of FDM at time t = 4 s. A clear response for oxidation isseen at anodic potentials > 0.15 V. A time profile (cross-sectionacross the map in Figure 4(a)) is shown in Figure 4(b), where itis apparent that we can detect the voltammetric response at thevdWEdge with sub-second resolution.Moreover, there is an increase in current after the addition ofFDM, which shows an initial slow relaxation followed by a fasterrelaxation to approach a final steady state with a constant cur-rent. This indicates that a stable diffusion layer is set up veryfast (in less than a second) and there is a constant rate of oxi-dation afterward.[31] This is analogous to observations on metal-lic nanoelectrodes and ultramicroelectrodes.[37,60] Since we needjust a two-electrode setup, such measurements can be carriedout even in a very small droplet, enabling detection in ultra-lowsample amounts. For the data in Figure 4, the analyte volumewas 5 μL and an analyte amount of just 2.5 nanomoles. By ap-propriate miniaturization of the reference electrode, we believethat it would be possible to reduce this amount further by atleast an order of magnitude. Figure S6 in Supporting Informa-tion shows another example of FSCV at a smaller concentrationof 10 μM. For concentrations much lower than this value, the sig-nals were buried in noise. This can be understood by consideringthat the miniscule electrode area sets a lower limit of current thatcan be measured free of noise. Hence the detection limit is inthe micromolar range and is somewhat higher than substrate-supported graphene edge.[23] Nevertheless, the low capacitanceand the small footprint give a clear advantage of very fast mea-surement as well as rapid equilibration of the diffusion layer.There are some differences to previously reported FSCVwork.[51,52] The most widely used analyte is DA,[53] the detectionof which often requires adsorption on the electrode, which is typ-ically enabled by using a hold potential between two consecutiveCV cycles in an FSCV experiment.[39] Other analytes could bedetected either via adsorption or by diffusion.[61,62] At our elec-trodes, we were not able to observe an adsorption-based responsesystematically. In cases where there was an indication of adsorp-tion, the electrode was increasingly blocked irreversibly with ev-ery measurement cycle. This could most likely be mitigated inthe future by performing appropriate chemical modification[56]of the edge.Another important aspect of a vdWEdge is that the interfa-cial resistance, when operating in liquids, is quite high,[45] sinceonly a small electrode region encounters the solution. Hence, itis interesting to look at the possibility of working in electrolyte-free media.[63] Already several decades ago, it was proposed andshown that nanoelectrodes with sub-micron dimensions are suit-able for the study of redox species in solutions free of support-ing electrolyte.[64] A two-electrode configuration is more advanta-geous, since it reduces noise and allows for a simple setup.[64–66]Figure 5(a) compares typical background-subtracted two-electrode CVs of FDM in water measured at the same vdWEdgeelectrode at 0.1 V s−1 in 0.1 M KCl (red curve) and without anyadded supporting electrolyte (blue curve). The CV in water showsa slightly higher current and an apparent cathodic shift in theelectrochemical response. Otherwise, the two curves are similar.The higher quasi-steady state current can be explained by a re-duction in resistance after oxidation of the neutral FDM speciesto a positively charged ion, which accumulates/diffuses at thevdWEdge-electrolyte interface and decreases the resistance.[67–70]Nearly no hysteresis was observed in the background-subtracted CVs during the forward and backward scans, suggest-ing that we are able to obtain a stable quasi-steady-state responseeven in the absence of added electrolyte. Although we do not de-liberately add any supporting electrolyte, trace amounts of ionicspecies cannot be avoided,[71] which enabled the observation of aquasi-sigmoidal ET response.[72] We attribute the shift in the CValong the potential scale to a shift in the electrode potential of thereference electrode. A chloride coated silver wire served as com-bined reference and counter electrode for both measurements.For the measurement in water, there was no added chloride.Hence, the chloride concentration is determined by the equilib-rium between the AgCl coating on the silver wire and residualchloride ions in solution, as determined by the low solubilityproduct of AgCl. The electrode potential of the Ag/AgCl wire insuch a situation is lower (cathodic offset) than the potential when0.1 M KCl is present in the solution.In previous studies at ultramicroelectrodes, a shift in thehalf-wave potential has been predicted and observed for theoxidation of ferrocene when the supporting electrolyte concen-tration is reduced.[72,73] The shift was however anodic, which isnot what we observed here. Another alternative explanation forthe potential shift could be an iR drop (parasitic voltage dropdue to uncompensated solution resistance), due to a differencein resistance with and without electrolyte, caused by changesSmall 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (5 of 9) 16136829, 2024, 21, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306361 by Cochrane Japan, Wiley Online Library on [24/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 5. Electrochemistry of 100 μM FDM in water free of supportingelectrolyte. a) Comparison of background-subtracted CV measured at ascan rate of 0.1 V s−1 without added supporting electrolyte (blue curve)to that measured with added 0.1 M KCl (red curve). b) CVs measured atvarying scan rates without added supporting electrolyte. The potentials arewith reference to an Ag/AgCl wire reference electrode.in ionic composition in the diffusion layer as a result of theredox reaction.[63,74] However, in such a situation, a significantdistortion of the entire CV would be expected, which we did notobserve in our CVs (see Figure S7 in Supporting Information).This kind of distortion has, however, mainly been reported insolvents of low dielectric strength.[75] Most likely, it appears thatthe iR drop is minimized due to the ultra-small interfacial areaand pA currents. With a solution resistance in the MΩ range,the ohmic drop is significantly below 1 mV. Figure 5(b) presentsbackground-subtracted CVs at three different scan rates mea-sured in water, where it is apparent that the quasi steady stateresponse is preserved without any shift in potential or distortionin the CV or increase in hysteresis even up to a scan rate of10 V s−1. From these data, it is clear that our vdWEdge electrodescan reliably be used in solutions without added electrolyte.The vdWEdge electrode constitutes a nanoelectrode, two ofwhich can be assembled facing each other in a straightfor-ward manner using our fabrication methodology. The avail-ability of two such nanoelectrodes in close vicinity opens anavenue for specialized electrochemical experiments such asredox cycling.[76,77] Here one electrode serves as a genera-tor where a redox species is oxidized, while the generatedoxidized species is reduced at the other. This allows for apositive feedback using which the redox species shuttles be-tween the two electrodes leading to an amplification of thecurrent.[78] Moreover, such an assembly can be used for studyingmechanisms of redox kinetics, electrochemiluminescence andtitrations.[79–81]We have evaluated the capability of redox cycling, by realiz-ing two vdWEdge electrodes facing each other, with an opennanochannel in between, as illustrated by the schematic in Figure6(a), an optical image in Figure 6(b) and an SEM image inFigure 6(c). We have also realized other layouts in interdigitatedformat (see another example in Figure S8 in Supporting Informa-tion). However, the current responses were qualitatively similarin all electrodes. Figure 6(d) shows typical background-subtractedcurrent response for 0.6 mM FDM (in 0.1 M KCl) in generator-collector mode at a redox cycling vdWEdge gap. Such a measure-ment was carried out by biasing the two vdWEdge electrodeswith respect to a chloridified Ag wire as the counter/referenceelectrode (see Methods section for details). While the collectorelectrode was maintained at a constant potential of −0.05 V, thegenerator potential was scanned in the displayed potential rangeand the currents at both electrodes were measured. It is clear inFigure 6(d) that, at potentials > 0.15 V, the FDM molecules ox-idized (red curve) by the generator electrode are reduced at thecollector, which results in a cathodic current (blue curve). How-ever, the current at the collector electrode is only 20% of the cur-rent observed at the generator electrode. We attribute this to therather large electrode spacing of 200 nm in comparison to theedge electrode dimension of a few nm.[76] Moreover, the conver-gent diffusion profile[23] leads to a rapid diffusion of the gener-ated reduced species, explaining the low collection efficiency of≈20%. Nevertheless, the data confirm that the two electrodes areable to work in unison, through exchange of species from oneelectrode to the other. The collection efficiency could be improvedby realizing closed nanochannels or having a smaller electrodespacing.We also explored the possibility of redox cycling in supporting-electrolyte-free water as shown in Figure 6(e). The red curveshows the response obtained at the collector electrode withoutconnecting the generator electrode, while the blue curve showsthe response when the generator electrode was kept at a constantpotential of +0.4 V. In the latter case, the generator produced re-duced species, whose concentration increased locally, resulting ina higher current (indicated by the blue arrow) at the collector elec-trode. With this, we have demonstrated that using the vdWEdgeelectrode it is also possible to investigate redox cycling in mediafree of added electrolyte. In such electrodes, not only the diffu-sion layer but also the double layer of the two electrodes mustoverlap each other.[82] Future work will shed more light on the ef-fect of this overlap. Furthermore, ion migration and electrostaticeffects are expected to play an important role when using chargedredox active species.[67]Small 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (6 of 9) 16136829, 2024, 21, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306361 by Cochrane Japan, Wiley Online Library on [24/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comFigure 6. Redox cycling in a vdWEdge nanogap device. a) Schematic of the dual vdWEdge electrode with a nanogap. The scheme shows one example ofredox cycling where the collector electrode on the right is maintained at a constant cathodic potential, while the generator electrode potential is scannedin a selected voltage range where oxidation occurs. b) Optical and c) false-color SEM images showing top view of such a redox cycling vdWEdge electrodewith a gap of 200 nm. d) CVs obtained in 0.1 M KCl for the redox cycling of FDM with the collector electrode maintained at a potential of −0.05 V asexemplified in (a). e) CVs obtained at the collector electrode in water (without added electrolyte) for the redox cycling of FDM with the generator electrodeeither disconnected or maintained at an anodic potential. The potentials are measured with reference to an Ag/AgCl wire reference electrode.3. ConclusionIn conclusion, we demonstrated the capability of performingelectrochemistry at the 1D edge of a single graphene sheet sand-wiched between two hBN crystallites. The unique geometry ofthe vdWEdge electrode and its small size opens up the possibil-ity to study interfacial ET with several advantages. We showedthat using such electrodes, FSCV could be deployed to study ETprocesses at very high scan rates with a very good time resolution.In this work, we have utilized FSCV in its simplest form. In thefuture, complex waveforms may be experimented for the detec-tion of species via adsorption or for the sensing of multianalytespecies.[58] We also described that it is possible to record CVs andperform redox cycling in water without added supporting elec-trolyte. Currently, the passivation layer on our devices is not wellsuited for experiments in organic solvents. For the future, morestable passivation layers can be utilized to enable this possibility.In such devices, the actual effect of iR drop can be more care-fully investigated when working in organic solvents of lower di-electric constant.[75,83] The ability to study ET in the absence ofelectrolyte may provide a platform for an unambiguous under-standing of ET mechanisms, free of diffusion and supportingelectrolyte effects.[31,84] For sensing applications, our electrodesare more expensive in comparison to commercially available sen-sors, such as screen-printed electrodes. However, there are otherbenefits. For example, measurements can be carried out in a sim-pler two-electrode configuration with just a few μL of sample so-lution. Another advantage of our lithography-based technology isthat, in the future, we can realize arrays of vdWEdge electrodes ona small chip. Finally, chemical functionalization can be utilizedto modulate ET at the vdWEdge, thereby expanding the spectrumof heterogeneous reactions that can be studied. Chemical mod-ification of the edge may also help improve the sensitivity, e.g.,by introduction of oxygen or nitrogen containing groups or bydecoration with metallic nanoparticles.4. Experimental SectionDevice Fabrication: A hot pick-up technique[34] was used for the as-sembly of hBN/graphene/hBN vdW stacks. A glass slide with a poly-dimethylsiloxane block covered by polypropylene carbonate (PPC) wasused to assemble the stacks. Due to the thermoplastic properties of PPC,hBN and graphene flakes were picked up from the initial substrate at 40 °Cand subsequently released on the target substrate at 80—110 °C. The tar-get substrate was highly resistive Si (resistivity of 5 kΩcm) with a 1-μm-thick SiO2 layer on top. After the deposition, the heterostructures werebaked for 5 minutes at 160 °C and cleaned in acetone for ≈30 minutes. Thethickness of the chosen hBN flakes was in the range of 25–40 nm to pro-vide sufficient chemical and electrical isolation (e.g., to prevent electrontunneling through the flakes). The hBN flakes were exfoliated from syn-thetic hBN crystals, while both exfoliated graphene and graphene grownSmall 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (7 of 9) 16136829, 2024, 21, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306361 by Cochrane Japan, Wiley Online Library on [24/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comwww.advancedsciencenews.com www.small-journal.comby chemical vapor deposition were used. Once a vdW stack was immersedin a solution, no percolation of the electrolyte was expected because therewas not much difference between the vdW gap between graphene and hBNand the interlayer space inside hBN.[10]The stacks were patterned in two RIE steps using an Al mask. The maskswere patterned on top of the stacks by electron-beam (e-beam) lithography(using Raith eLINE at 10–30 keV) and deposited by evaporating Al in ane-beam evaporator at a base pressure of ≈10−6 mbar. In the first RIE step,only the top hBN flake was etched by SF6 forming a rectangle (typically11 μm × 8 μm) protected by a 40-nm-thick Al mask. The final device layoutwas defined in the second RIE step when the entire hBN/graphene/hBNstack was etched, exposing graphene edges. The hBN flakes were etchedby SF6 and graphene by O2 plasma using an 80-nm-thick Al mask. The Almasks were removed after each RIE step by a mixture of tetramethylam-monium hydroxide and H2O.The edge contacts to graphene were defined at the opposite sides of thestacks by e-beam lithography and e-beam evaporation of 80 nm of pureAu. The deposition of Au was followed by a lift-off process. Finally, theAu electrodes were passivated by a 200-nm-thick layer of PMMA to avoidtheir contact with the solution used in electrochemical measurements. A2-μm-long opening in the passivation layer (between the contacts) waspatterned by e-beam lithography to expose the edges of graphene to theenvironment.Electrochemical Instrumentation: Electrochemical measurements in athree-electrode configuration were performed using an Ivium Compact-stat bipotentiostat with the vdwEdge as working electrode, a Pt wire as acounter electrode, and Ag/AgCl as reference electrode. The reference elec-trode was either a commercial DRIREF-450 (WPI Inc.) or a homemadechloridified Ag wire (50 μm diameter). The potential offset of the latter in0.1 M KCl with respect to the commercial electrode was less than 50 mV.For the two-electrode configuration, two homemade setups were utilized,where the vdWEdge acted as the working and the Ag/AgCl wire as thecounter electrode. In a first setup, Vortis Controller of a Bruker JPK Nan-oTracker 4 system was used. Specifically a voltage source channel wasused for applying the potential and an analog-to-digital converter (ADC)for measuring voltage input. The current passing through the electrochem-ical cell was amplified using an I/V-converter (Femto DLPCA-200) and thevoltage was sampled by the ADC at a suitable frequency between 500 and20 kHz depending on the scan rate. In a second setup, a two-channel Keith-ley source measure unit (SMU) 2636B was used to measure currents byapplying a desired voltage waveform. In this case, the devices were placedin a FormFactor Summit 11000 probe station. For redox-cycling measure-ments, the currents from both electrodes were measured independentlyusing the two channels. The Keithley SMU was also used to measure theresistance of the encapsulated graphene between the two Au electrodes.All measurements were carried out using an appropriate low-noise setupequipped with a low-noise amplifier and a shielded Faraday cage.Surface Characterization: Optical images were obtained on a LeicaDM6 M, AFM images using a Bruker JPK Nanowizard 4 or a Veeco Innova,and SEM images using a Raith eLINE e-beam system.Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsA.P. and T.J.N. equally contributed to this work. This project was partlyfunded by the Deutsche Forschungsgemeinschaft (DFG, German Re-search Foundation) – 425219379; INST 276/754-1. This work is supportedthrough the EU Horizon 2020 project Graphene Flagship Core 3 (grantagreement ID 881603) and ANTARES project that has received fundingfrom the European Union’s Horizon 2020 research and innovation pro-gramme under grant agreemnt SGA CSA. No. 739570 under FPA No.664387. Funding from the DFG as part of the excellence initiative via theGraduate School of Analytical Sciences Adlershof (GSC1013 SALSA) wasgratefully acknowledged.Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from thecorresponding author upon reasonable request.Keywords2D Materials, Cyclic voltammetry, Graphene, Nanoelectrochemistry, vander Waals heterostructure, EdgeReceived: July 26, 2023Revised: October 19, 2023Published online: December 18, 2023[1] A. K. Geim, I. V. Grigorieva, Nature 2013, 499, 419.[2] K. S. Novoselov, A. Mishchenko, A. Carvalho, A. H. 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Interf. Electrochem.1986, 213, 189.[84] S. R. Belding, J. G. Limon-Petersen, E. J. F. Dickinson, R. G. Compton,Angew. Chem., Int. Ed. 2010, 49, 9242.Small 2024, 20, 2306361 © 2023 The Authors. Small published by Wiley-VCH GmbH2306361 (9 of 9) 16136829, 2024, 21, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/smll.202306361 by Cochrane Japan, Wiley Online Library on [24/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.small-journal.comhttps://doi.org/10.1039/B413177K Electrochemistry at the Edge of a van der Waals Heterostructure 1. Introduction 2. Results and Discussion 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords