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Antonio Rossi, Cameron Johnson, Jesse Balgley, John C. Thomas, Luca Francaviglia, Riccardo Dettori, Andreas K. Schmid, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Matthew Cothrine, David G. Mandrus, Chris Jozwiak, Aaron Bostwick, Erik A. Henriksen, Alexander Weber-Bargioni, Eli Rotenberg

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[Direct Visualization of the Charge Transfer in a Graphene/α-RuCl<sub>3</sub> Heterostructure via Angle-Resolved Photoemission Spectroscopy](https://mdr.nims.go.jp/datasets/ed036742-4510-4bbd-be3e-188774d0a8d2)

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Direct Visualization of the Charge Transfer in a Graphene/α-RuCl3 Heterostructure via Angle-Resolved Photoemission SpectroscopyDirect Visualization of the Charge Transfer in a Graphene/α-RuCl3Heterostructure via Angle-Resolved Photoemission SpectroscopyAntonio Rossi,* Cameron Johnson, Jesse Balgley, John C. Thomas, Luca Francaviglia, Riccardo Dettori,Andreas K. Schmid, Kenji Watanabe, Takashi Taniguchi, Matthew Cothrine, David G. Mandrus,Chris Jozwiak, Aaron Bostwick, Erik A. Henriksen,* Alexander Weber-Bargioni, and Eli RotenbergCite This: Nano Lett. 2023, 23, 8000−8005 Read OnlineACCESS Metrics & More Article Recommendations *sı Supporting InformationABSTRACT: We investigate the electronic properties of agraphene and α-ruthenium trichloride (α-RuCl3) heterostructureusing a combination of experimental techniques. α-RuCl3 is a Mottinsulator and a Kitaev material. Its combination with graphene hasgained increasing attention due to its potential applicability innovel optoelectronic devices. By using a combination of spatiallyresolved photoemission spectroscopy and low-energy electronmicroscopy, we are able to provide a direct visualization of themassive charge transfer from graphene to α-RuCl3, which canmodify the electronic properties of both materials, leading to novelelectronic phenomena at their interface. A measurement of thespatially resolved work function allows for a direct estimate of theinterface dipole between graphene and α-RuCl3. Their strong coupling could lead to new ways of manipulating electronic propertiesof a two-dimensional heterojunction. Understanding the electronic properties of this structure is pivotal for designing nextgeneration low-power optoelectronics devices.KEYWORDS: Graphene, a-RuCl3, p−n junction, electronic structure, angle-resolved photoemission spectroscopy,low energy electron microscopyIn recent years, there has been a surge in interest inheterostructures composed of different two-dimensional(2D) materials.1−3 These systems offer unique electronicproperties that arise from their interfacial interactions, makingthem promising candidates for novel electronic and optoelec-tronic devices.4,5 The absence of a covalent chemical bondbetween the layers opens the route toward designing 2Dquantum systems that hold the promise to unlock the post-Moore era.6,7 One particularly exciting development is therecent discovery of permanent charge transfer induced ingraphene by proximity to α-RuCl3 (RuCl3 hereafter). In turn,this offers a route to explore the physics of charge-doped Mottinsulators.8−12RuCl3 is a layered transition metal compound with ahoneycomb lattice structure similar to graphene. However,unlike graphene, it is a Mott material with insulating behaviorarising from strong electronic correlations.13 At low temper-atures, the complex competition of magnetic interactionsultimately stabilizes a zigzag antiferromagnet in RuCl3.14However, the influence of doping, for instance by photo-induced charged carriers, is predicted to stabilize ferromagneticorder.15 Additionally, RuCl3 is classified as a Kitaev materialdue to its strong spin−orbit coupling, crystal field, andelectronic correlations, which lead to anisotropic exchangeinteractions that favor the formation of a quantum spin liquidexpected to host Majorana fermions.16−20 These quasiparticleshave non-Abelian statistics and are essential for topologicalquantum computation.21 Nonetheless, the evidence for suchbehavior in RuCl3 is still under debate22 and seems to bestrongly affected by the presence of crystal defects, whichpromote impurity scattering and non-Kitaev interactions.23When graphene is brought in contact with RuCl3, a chargetransfer occurs between the two materials due to their differentwork functions and electronic structures.8 This charge transfercan modify and hybridize the electronic properties of bothmaterials10 as well as influence the magnetism in RuCl3.15,24The coupling between graphene and RuCl3 can modify theelectronic band structure of RuCl3 and enhance its spin−orbitcoupling, potentially impacting the Kitaev physics in thematerial.9,24 Anomalous quantum oscillations have beenReceived: May 26, 2023Revised: August 21, 2023Published: August 28, 2023Letterpubs.acs.org/NanoLett© 2023 The Authors. Published byAmerican Chemical Society8000https://doi.org/10.1021/acs.nanolett.3c01974Nano Lett. 2023, 23, 8000−8005This article is licensed under CC-BY 4.0Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on October 22, 2023 at 02:17:33 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.https://pubs.acs.org/action/doSearch?field1=Contrib&text1="Antonio+Rossi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Cameron+Johnson"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Jesse+Balgley"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="John+C.+Thomas"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Luca+Francaviglia"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Riccardo+Dettori"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Andreas+K.+Schmid"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Andreas+K.+Schmid"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Kenji+Watanabe"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Takashi+Taniguchi"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Matthew+Cothrine"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="David+G.+Mandrus"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Chris+Jozwiak"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Chris+Jozwiak"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Aaron+Bostwick"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Erik+A.+Henriksen"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Alexander+Weber-Bargioni"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/doSearch?field1=Contrib&text1="Eli+Rotenberg"&field2=AllField&text2=&publication=&accessType=allContent&Earliest=&ref=pdfhttps://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.3c01974&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?goto=articleMetrics&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?goto=recommendations&?ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?goto=supporting-info&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=tgr1&ref=pdfhttps://pubs.acs.org/toc/nalefd/23/17?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/17?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/17?ref=pdfhttps://pubs.acs.org/toc/nalefd/23/17?ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://pubs.acs.org?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01974?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-ashttps://pubs.acs.org/NanoLett?ref=pdfhttps://pubs.acs.org/NanoLett?ref=pdfhttps://acsopenscience.org/researchers/open-access/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/reported in the Gr/RuCl3 heterostructure and explained withinthe Kitaev-Kondo lattice model.25−27 The strong chargetransfer has also been used to create modulation-dopedgraphene where a lateral thickness variation of a tunnel barrierchanges the magnitude of the charge transfer betweengraphene and RuCl3,11 enabling ultrasharp (less than 5 nmwide) p−n junctions,12 which were also observed in nano-bubbles of graphene on RuCl3.28 The interaction betweengraphene and RuCl3 can also lead to plasmon polaritons at theinterface.29 The coupling between plasmon polaritons and theMott physics in RuCl3 could unlock new ways of manipulatinglight and electronic properties, with potential applications insensing, imaging, and communication. Moreover, by leveragingthe unique passive doping control (no gating needed) of RuCl3over graphene, we envision the creation of low-power devicesthat exhibit enhanced light-harvesting capabilities and precisecontrol over optical signals.30Here, we employ a combination of experimental techniquesto better investigate the electronic properties of the interfacebetween RuCl3 and graphene. Nanometer-scale spatiallyresolved photoemission spectroscopy (nanoXPS) and low-energy electron microscopy (LEEM) are used to explore theelectronic properties of the heterostructure, allowing for adirect visualization of its charge transfer.It is possible to effectively map the core levels of 2D systemsand the dispersive electronic band structure of the hetero-structure with submicron spatial resolution via nanoXPS andangle-resolved photoemission spectroscopy (nanoARPES).31LEEM allows imaging of the morphology and electronicproperties of heterostructures with high spatial resolution. Byusing low-energy electrons to probe the surface of the material,we can investigate local variations in electronic properties tostudy their evolution over time. By comparing ourexperimental results with the calculations present in theliterature, we validate our findings and provide a morecomplete understanding of the electronic properties of theheterostructure.The experimental data show a massive charge transfer fromgraphene to RuCl3, clearly visible in the nanoARPES data andreflected in the core levels measured via nanoXPS. LEEManalysis provides a value of the shift in work function that ismuch lower than the band shift measured via nanoARPES,consequent to the charge transfer between the layers. Thisdiscrepancy can be attributed to the presence of a dipole at theinterface that greatly affects the work function value.32Moreover, the appearance of a band below the Fermi level,not observed in other ARPES experiments, can be attributed tothe effect of charge transfer on RuCl3. The most straightfor-ward interpretation suggests that electrons from graphene havepartially occupied the typically unoccupied upper Hubbardband, causing it to shift below the Fermi level. Alternatively, ina study of adatom-doped RuCl3, Zhou et al. identified theappearance of such a band between the lower Hubbard bandand Fermi level as an unconventional Mott transition driven bythe charge transfer.33 Either way, this new spectral weightshows that charge transfer from graphene to RuCl3 inducessignificant changes in the electronic structure of the system.The presence of this band provides further evidence of thecomplex electronic interactions occurring at the interface andhighlights the role of charge transfer in driving unconventionalelectronic phenomena in this system.We fabricated a heterostructure composed of a thickhexagonal boron nitride (h-BN) substrate, with three othermaterials exfoliated on top: graphene, thin h-BN (2 nm), andRuCl3 (Figure 1(a)). The fabrication and experimental detailsare reported in the Supporting Information. The device hasthree distinct regions: one with graphene on thick h-BN as areference (Gr/h-BN), one with graphene directly on RuCl3(Gr/RuCl3), and one with a thin h-BN flake sandwichedbetween graphene and RuCl3 (Gr/h-BN/RuCl3). The thin h-BN acts as a buffer to decrease the Gr-RuCl3 interactions.11,28The thick h-BN substrate provides a stable and flat surface forother materials and minimizes the effect of the underlyingsubstrate on the electronic properties of the heterostructure. Asketch of the three regions is reported in Figure 1(b) with acoherent color scheme. Figure 1(c) displays the hetero-structure contour, where the contrast is given by the counts ofthe photoelectrons coming from the valence band of RuCl3 at abinding energy of −1.3 eV. The contrast allows for identifyingthe three regions described above. The color scheme for thethree colored squares on the map is consistent with the sketchin panel (b) and confirmed by the core level analysis via XPS.We focus on the peaks originating from Cl, Ru, and C corelevels, reported in Figure 1(d,e). The most informative peakregarding the location of the RuCl3 region is the Cl 2p corelevel. Its signal decreases when the h-BN buffer is present andFigure 1. (a) Optical image of the analyzed device. False-colorcontours are used to highlight the different layers. (b) Sketch of theside view of three regions of interest. RuCl3 must be considered asmultiple layers, even though one layer is depicted for neatness. (c)Photoelectron intensity map collected at E − EF = −1.3 eV. Thecolored profiles match the flakes highlighted in panel (a). (d) The Cl2p core level collected in the regions highlighted in panel (c) with thecorresponding color scheme. (e) Ru 3d and C 1s core levels collectedfrom the same points of panel (d). (f) Fit of the Ru 3d and C 1s levelfor the data reported in panel (e).Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01974Nano Lett. 2023, 23, 8000−80058001https://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.3c01974/suppl_file/nl3c01974_si_001.pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig1&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig1&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01974?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asdisappears entirely outside the RuCl3 flake. The Ru 3d3/2 corelevel partially overlaps with that of C 1s. It is possible to fit andtrack the evolution of the C 1s peak for the three differentregions (Figure 1(f)). The fitting is performed considering oneDoniach-Sunjic (DS) asymmetric line shape34 for grapheneand one DS for Ru 3d3/2, plus a Gaussian peak to take intoaccount the broad and weak contribution from the 3d5/2 peak.While the Ru level remains roughly at fixed position, the Cpeak progressively shifts toward lower binding energy whenincreasing the coupling strength between graphene and RuCl3.An overall shift of about −750 meV is observed for C 1s fromthe Gr/h-BN region. RuCl3 is expected to induce a significantelectron depletion in graphene8,9 that is reflected on anelectrostatic shift of the C core levels and the whole grapheneband structure.To directly visualize the electronic properties of the system,we conducted a nanoARPES study. This study enabled us toobserve the electronic band structure in three specific regionshighlighted in Figure 1(c). In Figure 2(a), we present thebands of the Gr/h-BN region, which are close to the neutralitypoint. In Figure 2(b,c), we show the band structures of the Gr/h-BN/RuCl3 and Gr/RuCl3 regions, respectively. Notably, thegraphene bands in these regions are shifted upward byapproximately 500 meV where the h-BN layers separate thegraphene and RuCl3 crystals and by 750 meV where RuCl3 isin direct contact with graphene, indicating a progressive p-doping when reducing the distance between graphene andRuCl3. The charge transfer from graphene to RuCl3 isresponsible for the shift of the Dirac cone and is similarlymanifested in the position of the C 1s core level, where theobserved chemical shift, as illustrated in Figure 1, can be alsoascribed to the presence of an electrostatic dipole effect.35 Theinfluence of charge transfer and the resulting interface dipole isevident in the upward shift of the h-BN bands (indicated bywhite arrows) by approximately 1 eV, relative to the regionwithout RuCl3 as shown in Figure 2(a).Because of the short mean free path of the photoelectrons,the RuCl3 bands are visible only in the region where grapheneis in direct contact, with no intervening h-BN buffer. TheRuCl3 bands are identified by studying the energy distributioncurves (EDCs) taken along the dashed line in panels (a−c) ofFigure 2 (Figure 2(d)). The RuCl3 electronic structure,highlighted by three red arrows, displays two dispersionlessbands centered at binding energies of ∼0.5 and ∼1.3 eV and athird more dispersive band at a deeper binding energy (∼3.8eV).The band centered at −1.3 eV is very likely to correspond tothe lower Hubbard band, as indicated by Biswas et al.9However, it is worth noting that the observed energy positionof the lower Hubbard band may appear to be lower than thatpredicted by computational models. This discrepancy could beattributed to the challenges in accurately estimating the energygap using, e.g., density functional theory (DFT) calculations.Factors such as electron−electron interactions and correlationeffects, which are not fully captured in DFT calculations, caninfluence the energy position of the lower Hubbard band.Regarding the band at −3.8 eV, it is identified as an in-planeorbital and labeled as Cl p bands, originating from the Clorbitals within the RuCl3 structure.33The presence of the band with spectral weight centered atabout 0.5 eV below the Fermi level, which is typically notobserved in ARPES experiments conducted on bulk RuCl3,33,36suggests that its emergence is a result of the interactionbetween RuCl3 and graphene. This band can be understood asthe upper Hubbard band, which is typically unoccupied, beingpartially filled by electrons transferred from graphene andconsequently shifted below the Fermi level. Anotherexplanation put forth by Zhou et al. proposes that theintroduction of dopants on the surface of RuCl3 leads to theFigure 2. (a−c) Band structure collected around graphene K (point as depicted by panel (a) inset) from the three regions described above with aconsistent color scheme. The horizontal dashed line is the Fermi level. The white arrows highlight the h-BN bands, the red arrows the RuCl3 bands.(d) EDCs collected from panels (a−c) along the vertical dashed line. The red arrows highlight the corresponding states in panel (c). (e−g) Fermisurface of the band structure from the sample regions with the corresponding color scheme. The dashed red circle in panel (f) approximates thegraphene Fermi surface. (h) MDCs collected across the dashed lines in panels (e−g).Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01974Nano Lett. 2023, 23, 8000−80058002https://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig2&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig2&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01974?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-aspopulation of new bands near the Fermi level.33 These bandsare attributed to an unconventional Mott transition, asdescribed by the authors. It is possible that RuCl3 undergoesa similar Mott transition when in contact with graphene, asobserved in the case of Rb and K doping in ref 33.A more quantitative estimate of the total amount of chargetransferred between the layers, with and without the h-BNbuffer layer, is given by considering the Fermi surface for eachof the three regions, as reported in Figure 2(e−g). The Fermisurface of graphene exhibits a characteristic area of reducedintensity known as the “dark corridor”. This phenomenonarises due to the interference of photoelectrons that areemitted from the two identical carbon atoms within each unitcell of graphene’s honeycomb lattice.37 The momentumdistribution curves (MDCs) collected along the dashed lineson the Fermi surface are displayed in Figure 2(h). By fittingwith two Lorentzian curves, the position of their maxima isused to evaluate the Fermi surface area, approximated as acircle. By means of the Luttinger theorem,38−40 we can extractthe amount of charge tunneling from graphene to RuCl3, about4.1 × 1013 cm−2, consistent with previous, if indirect,experimental and computational observations.8−10,24,29 Whenthe spacing between layers is increased with a few h-BN layers,the tunnel barrier thickens, resulting in a decreased chargetransfer and therefore a lower p-doping level in graphene (∼1.7× 1013 cm−2).Finally, we can quantify the electric dipole generated at theinterface by the charge transfer to the RuCl3. It is possible tocompute the magnitude of the dipole by measuring thevariation of the work function across the different regions ofthe system. By applying a positive voltage to the sample, theincident LEEM electrons transition from mirror mode, withthe electrons reflecting before touching the sample surface, toLEEM mode, where the electrons are scattered from samplesurface with a landing energy proportional to the appliedsample bias. In LEEM mode, the incident electrons can beaccepted into unoccupied bands of the sample surface causinga lower reflected intensity than in the mirror mode. Theinflection point of this drop in intensity from mirror mode toLEEM mode can be interpreted as the work function of thesample surface when accounting for the work function of theLEEM cathode.In Figure 3(a), selected LEEM images collected just belowthe mirror mode transition display the boundary of the threeregions discussed above. The line profiles in Figure 3(b−d)show the average work function across each boundary in thedirections indicated by the arrows in the corresponding LEEMimages. The profile analysis highlights a difference in the workfunction of about 230 meV across the interface between Gr/RuCl3 and Gr/h-BN. When the h-BN buffer layer is alsopresent, the shift in the work function is reduced by 160 meV.This difference with respect to the Gr/h-BN region agrees withthe 70 meV difference between the Gr/h-BN/RuCl3 and Gr/RuCl3 regions. Previously, Yu and co-workers demonstratedthat the work function of graphene can be substantially affectedby the dipole formed by surface adsorbates.32 Analogously,here we estimate the magnitude of the electric field at theinterface knowing the value of the chemical potential and thevalue of the work function, with respect to pristine graphene.In the presence of a dipole at the interface, the work functionof graphene can be written as= +W W W Esample D gr F0(1)where ΔWD is the offset of work function due to the dipole atthe interface, Wgr0 is the intrinsic work function of the undopedgraphene, and EF is the position of the Fermi level.32 We cantherefore evaluate the magnitude of the electric dipole withrespect to the pristine sample simply considering the measuredwork function difference across the different regions andadding this to the corresponding difference in the EF positionwith respect to the Dirac point. This results in an electricdipole energy of ∼1 eV and ∼660 meV for Gr/RuCl3 and Gr/h-BN/RuCl3 regions, respectively.In conclusion, we used a combination of experimentaltechniques, including nanoXPS and nanoARPES and LEEM,to investigate the electronic properties of the Gr/RuCl3heterostructure. The results showed direct evidence ofsignificant charge transfer from graphene to RuCl3, leadingto a doping-induced Mott transition and potential enhance-ment of the Kitaev physics. LEEM measurement also allowedus to provide an estimate of the dipole moment formed at theinterface between RuCl3 and graphene, instrumental forcomprehensive device modeling. This work lays out valuableinsights into the electronic properties of Gr/RuCl3 hetero-structures and its potential for future applications, where thepassive control of the doping level in graphene is at thefoundation of low-power electronics and light-harvestingdevices.■ ASSOCIATED CONTENT*sı Supporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974.Figure 3. (a−c) Mirror mode LEEM images of selected regions ofinterest: (a) RuCl3-hBN-Gr/Gr, (b) RuCl3-Gr/Gr, (c) RuCl3-Gr/RuCl3-hBN-Gr. (d−f) Surface work function measured across theboundary separating the different regions along the arrows drawn inthe microscopy panels.Nano Letters pubs.acs.org/NanoLett Letterhttps://doi.org/10.1021/acs.nanolett.3c01974Nano Lett. 2023, 23, 8000−80058003https://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?goto=supporting-infohttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig3&ref=pdfhttps://pubs.acs.org/doi/10.1021/acs.nanolett.3c01974?fig=fig3&ref=pdfpubs.acs.org/NanoLett?ref=pdfhttps://doi.org/10.1021/acs.nanolett.3c01974?urlappend=%3Fref%3DPDF&jav=VoR&rel=cite-asDevice fabrication methods including atomic forcemicroscopy characterization of the device. Experimentaldetails for nXPS and nARPES experiment includingphoton energy and light polarization used. LEEMmeasurements details (PDF)■ AUTHOR INFORMATIONCorresponding AuthorsAntonio Rossi − Advanced Light Source and The MolecularFoundry, Lawrence Berkeley National Laboratory, Berkeley,California 94720, United States; Center for NanotechnologyInnovation @ NEST, Istituto Italiano di Tecnologia, Pisa56127, Italy; orcid.org/0000-0003-4574-7215;Email: antonio.rossi@iit.itErik A. Henriksen − Department of Physics and Institute forMaterials Science and Engineering, Washington University inSt. Louis, St. Louis, Missouri 63130, United States;orcid.org/0000-0002-4978-2440; Email: henriksen@wustl.eduAuthorsCameron Johnson − The Molecular Foundry, LawrenceBerkeley National Laboratory, Berkeley, California 94720,United StatesJesse Balgley − Department of Physics and Institute forMaterials Science and Engineering, Washington University inSt. Louis, St. Louis, Missouri 63130, United StatesJohn C. Thomas − The Molecular Foundry, LawrenceBerkeley National Laboratory, Berkeley, California 94720,United States; orcid.org/0000-0002-2151-7725Luca Francaviglia − The Molecular Foundry, LawrenceBerkeley National Laboratory, Berkeley, California 94720,United StatesRiccardo Dettori − Physical and Life Sciences Directorate,Lawrence Livermore National Laboratory, Livermore,California 94550, United States; orcid.org/0000-0002-4678-1098Andreas K. Schmid − The Molecular Foundry, LawrenceBerkeley National Laboratory, Berkeley, California 94720,United StatesKenji Watanabe − Research Center for Functional Materials,National Institute for Materials Science, Tsukuba 305-0044,Japan; orcid.org/0000-0003-3701-8119Takashi Taniguchi − International Center for MaterialsNanoarchitectonics, National Institute for Materials Science,Tsukuba 305-0044, Japan; orcid.org/0000-0002-1467-3105Matthew Cothrine − Material Science & Technology Division,Oak Ridge National Laboratory, Oak Ridge, Tennessee37831, United StatesDavid G. Mandrus − Material Science & Technology Division,Oak Ridge National Laboratory, Oak Ridge, Tennessee37831, United States; orcid.org/0000-0003-3616-7104Chris Jozwiak − Advanced Light Source, Lawrence BerkeleyNational Laboratory, Berkeley, California 94720, UnitedStates; orcid.org/0000-0002-0980-3753Aaron Bostwick − Advanced Light Source, Lawrence BerkeleyNational Laboratory, Berkeley, California 94720, UnitedStatesAlexander Weber-Bargioni − The Molecular Foundry,Lawrence Berkeley National Laboratory, Berkeley, California94720, United StatesEli Rotenberg − Advanced Light Source, Lawrence BerkeleyNational Laboratory, Berkeley, California 94720, UnitedStates; orcid.org/0000-0002-3979-8844Complete contact information is available at:https://pubs.acs.org/10.1021/acs.nanolett.3c01974Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work was supported as part of the Center for NovelPathways to Quantum Coherence in Materials, an EnergyFrontier Research Center funded by the U.S. Department ofEnergy, Office of Science, Basic Energy Sciences. Work wasperformed at the Molecular Foundry and at the AdvancedLight Source, which was supported by the Office of Science,Office of Basic Energy Sciences, of the U.S. Department ofEnergy under contract no. DE-AC02-05CH11231. A.W.-B.and J.C.T. acknowledge support from the U.S. Department ofEnergy, Office of Science, National Quantum InformationScience Research Center, Quantum Systems Accelerator. A.R.received funding from the European Union’s Horizon 2020research and innovation programme under grant agreement881603. R.D. performed this work under the auspices of theU.S. Department of Energy by Lawrence Livermore NationalLaboratory under Contract DE-AC52-07NA27344. E.A.H.acknowledges support by the Office of the Under Secretaryof Defense for Research and Engineering under award numberFA9550-22-1-0340 and the Moore Foundation ExperimentalPhysics Investigators Initiative award no. 11560. We acknowl-edge support for device fabrication by the Institute of MaterialsScience and Engineering at Washington University in St. Louis.The authors thank Dr. Luca Moreschini from University ofCalifornia, Berkeley and Dr. Stiven Forti from Istituto Italianodi Tecnologia, Pisa for the fruitful discussion.■ REFERENCES(1) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Castro Neto, A.H. 2D Materials and van Der Waals Heterostructures. Science 2016,353 (6298), aac9439.(2) Liu, Y.; Weiss, N. O.; Duan, X.; Cheng, H.-C.; Huang, Y.; Duan,X. Van Der Waals Heterostructures and Devices. Nat. Rev. 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