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Lujun Wang, Simon Zihlmann, Ming-Hao Liu, Péter Makk, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Andreas Baumgartner, Christian Schönenberger

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[New Generation of Moiré Superlattices in Doubly Aligned hBN/Graphene/hBN Heterostructures](https://mdr.nims.go.jp/datasets/ae988b00-ddc9-4ddb-bd47-50b5b62d2b89)

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New Generation of Moiré Superlattices in Doubly Aligned hBN/Graphene/hBN HeterostructuresNew Generation of Moire ́ Superlattices in Doubly Aligned hBN/Graphene/hBN HeterostructuresLujun Wang,*,†,⊥ Simon Zihlmann,† Ming-Hao Liu,‡ Pet́er Makk,†,§ Kenji Watanabe,∥Takashi Taniguchi,∥ Andreas Baumgartner,*,†,⊥ and Christian Schönenberger†,⊥†Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland⊥Swiss Nanoscience Institute, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland‡Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan§Department of Physics, Budapest University of Technology and Economics and Nanoelectronics Momentum Research Group ofthe Hungarian Academy of Sciences, Budafoki ut 8, 1111 Budapest, Hungary∥National Institute for Material Science, 1-1 Namiki, Tsukuba, 305-0044, Japan*S Supporting InformationABSTRACT: The specific rotational alignment of two-dimensional lattices results in a moire ́ superlattice with alarger period than the original lattices and allows one toengineer the electronic band structure of such materials. Sofar, transport signatures of such superlattices have beenreported for graphene/hBN and graphene/graphene systems.Here we report moire ́ superlattices in fully hBN encapsulatedgraphene with both the top and the bottom hBN aligned tothe graphene. In the graphene, two different moire ́ super-lattices form with the top and the bottom hBN, respectively.The overlay of the two superlattices can result in a thirdsuperlattice with a period larger than the maximum period (14nm) in the graphene/hBN system, which we explain in a simple model. This new type of band structure engineering allows oneto artificially create an even wider spectrum of electronic properties in two-dimensional materials.KEYWORDS: hBN encapsulated graphene, moire ́ superlattice, three-layer moire ́ pattern, superlattice Dirac point, twistronicsSuperlattice (SL) structures have been used to engineerelectronic properties of two-dimensional electron systemsfor decades.1−8 Because of the peculiar electronic properties ofgraphene,9 SLs in graphene are of particular interest10−16 andhave been investigated extensively utilizing different ap-proaches, such as electrostatic gating,17−19 chemical doping,20etching,21−23 lattice deformation,24 and surface dielectricpatterning.25 Since the introduction of hexagonal boron nitride(hBN) as a substrate for graphene electronics,26 moire ́superlattices (MSLs) originating from the rotational alignmentof the two lattices have been first observed and studied byscanning tunneling microscopy (STM).27−29 It then triggeredmany theoretical30−33 and experimental studies, wheresecondary Dirac points,34−36 the Hofstadter Butterfly,34−38Brown-Zak oscillations,34−39 the formation of valley polarizedcurrents,40 and many other novel electronic device character-istics41−46 have been observed.Recently, another interesting graphene MSL system hasdrawn considerable attention, the twisted bilayer graphene,where two monolayer graphene sheets are stacked on top ofeach other with a controlled twist angle. For small twist angles,insulating states,47 strong correlations,48 and a network oftopological channels49 have been reported experimentally.More strikingly, superconductivity50,51 and Mott-like insulatorstates51,52 have been achieved, when the twist angle is tuned tothe so-called “magic angle”, where the electronic bandstructure near zero Fermi energy becomes flat, due to thestrong interlayer coupling.So far, MSL engineering in graphene has concentratedmostly on MSLs based on two relevant layers (2L-MSLs).However, fully encapsulated graphene necessarily forms twointerfaces, namely at the top and at the bottom, which canresult in a much richer and more flexible tailoring of thegraphene band structure. Because of the 1.8% larger latticeconstant of hBN, the largest possible moire ́ period that can beachieved in graphene/hBN systems is limited to about 14nm,29 which occurs when the two layers are fully aligned. Thissituation changes when both hBN layers are aligned to thegraphene layer. Here, we report the observation of a new MSLwhich can be understood by the overlay of two 2L-MSLs thatform between the graphene monolayer and the top and bottomhBN layers of the encapsulation stack, respectively. Figure 1Received: December 19, 2018Revised: February 21, 2019Published: February 25, 2019Letterpubs.acs.org/NanoLettCite This: Nano Lett. 2019, 19, 2371−2376© 2019 American Chemical Society 2371 DOI: 10.1021/acs.nanolett.8b05061Nano Lett. 2019, 19, 2371−2376This is an open access article published under a Creative Commons Non-Commercial NoDerivative Works (CC-BY-NC-ND) Attribution License, which permits copying andredistribution of the article, and creation of adaptations, all for non-commercial purposes.Downloaded via NATL INST FOR MATLS SCIENCE (NIMS) on February 20, 2020 at 04:40:30 (UTC).See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.pubs.acs.org/NanoLetthttp://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.nanolett.8b05061http://dx.doi.org/10.1021/acs.nanolett.8b05061http://pubs.acs.org/page/policy/authorchoice/index.htmlhttp://pubs.acs.org/page/policy/authorchoice_ccbyncnd_termsofuse.htmlillustrates the formation of the MSLs when both hBN layersare considered. On the right side of the illustration, only thetop hBN (blue) and the graphene (black) are present, whichform the top 2L-MSL with period λ1. The bottom hBN (red)forms the bottom 2L-MSL with graphene, shown on the leftwith period λ2. In the middle of the illustration, all three layersare present and a new MSL (3L-MSL) forms with a longerperiod, indicated with λ3. The influence of the MSL can bemodeled as an effective periodic potential with the samesymmetry. The periodic potentials for the top 2L-MSL and thebottom 2L-MSL are calculated following the model introducedin ref 29, shown as insets in Figure 1. To calculate thepotentials for the 3L-MSL, we sum over the periodic potentialsof the top 2L-MSL and the bottom 2L-MSL. The period of the3L-MSL from the potential calculation matches very well theone of the lattice structure in the illustration. In the transportmeasurements, we demonstrate that MSL with a period longerthan 14 nm can indeed be obtained in doubly aligned hBN/graphene/hBN heterostructures, coexisting with the graphene/hBN 2L-MSLs. These experiments are in good agreement witha simple model for the moire ́ periods for doubly aligned hBN/graphene/hBN devices.We fabricated fully encapsulated graphene devices with boththe top and the bottom hBN layers aligned to the grapheneusing a dry-transfer method.53 We estimate an alignmentprecision of ∼1°. A global metallic bottom gate is used to tunethe charge carrier density n, and one-dimensional Cr/Au edgecontacts are used to contact the graphene53 (see inset of Figure2a). Transport measurements were performed at 4.2 K usingstandard low-frequency lock-in techniques.The two-terminal differential conductance, G, of one device,shown as inset of Figure 2c, is plotted as a function of n inFigure 2a (data from other devices with similar characteristics,including bilayer graphene devices, are presented in theSupporting Information). The charge carrier density n iscalculated from the gate voltage using a parallel plate capacitormodel. The average conductance is lower on the hole side (n <0) than on the electron side (n > 0), which we attribute to n-type contact doping resulting in a p−n junction near thecontacts. The sharp dip in conductance at n = 0 is the mainDirac point (MDP) of the pristine graphene. Our device showsa large field-effect mobility of ∼90 000 cm2 V−1 s−1, extractedfrom a linear fit around the MDP. The residual doping is of theorder δn ≈ 1 × 1010 cm−2, extracted from the width of theMDP. In addition to the MDP, we find two pairs ofconductance minima symmetrically around the MDP at higherdoping, labeled A and C, which we attribute to two MSLs. Theminima on the hole side are more pronounced than theircounterparts on the electron side, similar to previouslyreported MSLs.29,34−36On the basis of the simple model of periodic potentialmodulation,11,29,31 superlattice Dirac points (SDPs) areexpected to form at the superlattice Brillouin zone boundariesat k = G/2, where G 4 /( 3 )π λ| | = is the length of thesuperlattice wavevector and λ is the moire ́ period. Forgraphene, k is related to n by k nπ= . The position of theSDPs in charge carrier density for a given period λ is then ns =Figure 1. Illustration of three different MSLs formed in a hBN/graphene/hBN heterostructure. Blue, black, and red hexagonal latticesrepresent top hBN, graphene, and bottom hBN lattices, respectively.ϕ1 (ϕ2) is the twist angle between top (bottom) hBN and graphene.θ1 (θ2) indicates the orientation of the corresponding MSL withrespect to graphene. The resulting moire ́ periods are indicated withλ1,2,3. The 3L-MSL (middle part) has a larger period than both 2L-MSLs (left and right parts). Insets: moire ́ potential calculations.Figure 2. Electronic transport at 4.2 K. (a) Two-terminal differentialconductance G as a function of charge carrier density n. In addition tothe MDP, there are four other conductance minima at nsA ≈ ±2.4 ×1012 cm−2 (green dashed lines) and nsC ≈ ±1.4 × 1012 cm−2 (bluedashed lines), respectively. The top axis shows the moire ́ periodsn4 /3 sλ π= . The red dashed lines indicate the longest period(lowest density) for a graphene/hBN MSL. Inset: schematic of thecross section of our device. (b) dG/dn as a function of n and B of thesame device. Filling factors fan out from all DPs, except for the blueone on the electron side and are indicated on top of the diagram,calculated as ν ≡ nh/(eB), where n is counted from each DP. (c)Zoom-in on the left side of (b). There are additional lines fanning outfrom an even higher density nsB ≈ 5.2 × 1012 cm−2, labeled B. Thefilling factors of these lines are 34, 38, 42, 46 and 50, respectively.Inset: micrograph and experimental setup of the presented device. “S”and “D” are the source and drain contacts, respectively.Nano Letters LetterDOI: 10.1021/acs.nanolett.8b05061Nano Lett. 2019, 19, 2371−23762372http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.8b05061/suppl_file/nl8b05061_si_002.pdfhttp://dx.doi.org/10.1021/acs.nanolett.8b050614π/(3λ2). The pair of conductance minima at nsA ≈ ± 2.4 ×1012 cm−2 can be explained by a graphene/hBN 2L-MSL witha period of about 13.2 nm. However, the pair of conductanceminima at nsC ≈ ± 1.4 × 1012 cm−2 cannot be explained by asingle graphene/hBN 2L-MSL, because it corresponds to asuperlattice period of about 17.3 nm, clearly larger than themaximum period of ∼14 nm in a graphene/hBN moire ́ system.We attribute the presence of the conductance dips at nsC to anew MSL that is formed by the three layers together: top hBN,graphene and bottom hBN. This 3L-MSL can have a periodconsiderably larger than 14 nm.To substantiate this claim, we now analyze the data obtainedin the quantum Hall regime. Figure 2b shows the Landau fanof the same device, where the numerical derivative of theconductance with respect to n is plotted as a function of n andthe out-of-plane magnetic field B. Near the MDP, we observethe standard quantum Hall effect for graphene with plateaus atfilling factors ν ≡ nh/(eB) = ±2, ±6, ±10, ... with h as thePlanck constant and e as the electron charge. This spectrumshows the basic Dirac nature of the charge carriers in graphene.The broken symmetry states occur for B ⩾ 2T, suggesting ahigh device quality. Around the SDPs at nsA ≈ ±2.4 ×1012cm−2, the plot also shows filling factors ν ≡ (n − nsA)h/(eB) = ±2, ±6, ... consistent with previous graphene/hBNMSL studies.34 Around the SDPs at nsC ≈ ± 1.4 × 1012 cm−2,there are also clear filling factors fanning out on the hole sidewith ν ≡ (n − nsC)h/(eB) = ±2, which is consistent with aDirac spectrum at nsC, while on the electron side thecorresponding features are too weak to be observed. Inaddition, lines fanning out from a SDP located at density n <− 3 × 1012 cm−2 are observed. A zoom-in is plotted in Figure2c. The lines extrapolate to a density of about −5.2 × 1012cm−2, denoted nsB with filling factors ν = 34, 38, 42, 46, ... Thisdensity cannot be explained by the “tertiary” Dirac pointoccuring at the density of about 1.65nsA, which comes from aKekule ́ superstructure on top of the graphene/hBN MSL.54However, nsB matches the SDP from a MSL with a period ofabout 9 nm. We therefore attribute it to a 2L-MSL originatingfrom the alignment of the second hBN layer to the graphenelayer.As derived in refs 29 and 33, the period λ for a graphene/hBN MSL is given bya(1 )2(1 )(1 cos ) 2λ δδ ϕ δ= ++ − + (1)where a (2.46 Å) is the graphene lattice constant, δ (1.8%) isthe lattice mismatch between hBN and graphene and ϕ(defined for −30° to 30°) is the twist angle of hBN withrespect to graphene. The moire ́ period is maximum at ϕ = 0with a value of λ ≈ 14 nm. This corresponds to the lowestcarrier density of nmin ≈ ±2.2 × 1012 cm−2 for the position ofthe SDPs (red dashed lines in Figure 2a). The orientation ofthe MSL is described by the angle θ relative to the graphenelatticetansin(1 ) cosθ ϕδ ϕ= −+ − (2)For the graphene/hBN system, one finds |θ | ≲ 80°.29 Thesetwo equations describe the top 2L-MSL and the bottom 2L-MSL, as shown schematically in Figure 1. The functionaldependence of λ and θ on ϕ is plotted in SupportingInformation Figure S1.In a fully encapsulated graphene device, not only one, butboth hBN layers can be aligned to the graphene layer so thattwo graphene/hBN 2L-MSLs can form. In this case, thepotential modulations of the two 2L-MSLs are superimposedand form a MSL with a third periodicity. The values of theresulting periods can be understood based on Figure 3a. Thevectors g⃗, b⃗1, and b⃗2 denote one of the reciprocal lattice vectorsfor the graphene, the top hBN, and the bottom hBN layers,respectively. The twist angle between the top (bottom) hBNand graphene is denoted ϕ1 (ϕ2). Following the derivations inrefs 29 and 33, one of the top 2L-MSL (bottom 2L-MSL)reciprocal lattice vectors k1⃗ (k2⃗) is given by the vectorconnecting g⃗ to b⃗1 (b⃗2). The moire ́ period λ1,2 is then given byk4 /( 3 )1,2 1,2λ π= | ⃗ | , which is explicitly described by eq 1 as afunction of the twist angle ϕ1,2. Since the reciprocal lattices ofthe top 2L-MSL and the bottom 2L-MSL are triangular, thesame as those for graphene and hBN, we can use the sameapproach to derive the 3L-MSL, which is described by theFigure 3. (a) Schematics in reciprocal space for the formation ofdifferent MSLs, where g⃗, b⃗1, b⃗2, k1⃗, k2⃗, and k3⃗ are one of the reciprocallattice vectors for graphene, top hBN, bottom hBN, top 2L-MSL,bottom 2L-MSL, and 3L-MSL, respectively. N is an integer, which canbe 1, 2, or 3. (b) λ3 plotted as a function of ϕ1 and ϕ2 for all possibletwist angles. (c) Zoom-in of (b) for small twist angles. Numbers onthe contour lines indicate the values of λ3 in nm.Nano Letters LetterDOI: 10.1021/acs.nanolett.8b05061Nano Lett. 2019, 19, 2371−23762373http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.8b05061/suppl_file/nl8b05061_si_002.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.8b05061/suppl_file/nl8b05061_si_002.pdfhttp://dx.doi.org/10.1021/acs.nanolett.8b05061vector connecting k2⃗ to k1⃗, denoted k3⃗. The 3L-MSL period isthen given by k4 /( 3 )3 3λ π= | ⃗ | .In order to calculate λ3 using eq 1, we first need to find thenew a, δ, and ϕ. Because of symmetry, we only consider ϕ1 <ϕ2, so λ2, the smaller period of the two graphene/hBN 2L-MSLs, becomes the new a and the new δ will then be given by(λ1 − λ2)/λ2. The new ϕ, denoted ϕ3, is determined by |θ1 −θ2|, where θ1 (θ2) is the relative orientation of the top 2L-MSL(bottom 2L-MSL) with respect to the graphene lattice,described by eq 2. Different cases occur for ϕ3 due to the60° rotational symmetry of the lattices. Since ϕ in eq 1 isdefined for −30° to 30°, we subtract multiples of 60° to bringϕ3 to this range if it is larger than 30°, given asFor the first case, the 3L-MSL is effectively the MSL formedby the two hBN layers, as illustrated in the left panel of Figure3a. Another case is shown in the right panel, where multiples of60° are subtracted, which is equivalent to choosing anotherreciprocal lattice vector for k2⃗ so that it makes an angle within±30° with k1⃗.Figure 3b plots all possible values for λ3, as a function of ϕ1and ϕ2, by using eq 1 with the new parameters. Theoreticallyλ3 varies from below 1 nm to infinity, but one finds valueslarger than 14 nm only for small twist angles (see Figure 3c).For most angles λ3 is very small, which explains why MSLswith periods larger than 14 nm have not been reported inprevious studies, where only one hBN layer was alignedintentionally to the graphene layer.Most of Figure 3c can be understood intuitively. On the lineof the right diagonal with ϕ1 ≡ ϕ2, we have λ1 = λ2 and θ1 = θ2,therefore ϕ3 = 0, which results in λ3 =∞. This case is similar tothe twisted bilayer graphene with a twist angle of 0, which doesnot form a MSL (or a MSL with infinitely large period). Onthe diagonal line in the left part with ϕ1 ≡ −ϕ2, one has λ1 =λ2, but θ1 = −θ2. As |ϕ1 | = | ϕ2| increases, θ1 = −θ2 evolves (seeSupporting Information Figure S1). Therefore, ϕ3 can havenonzero values, resulting in different λ3 values. This case isagain similar to the twisted bilayer graphene, but with a tunabletwist angle. Whenever the difference of the orientation of thetop 2L-MSL and the bottom 2L-MSL becomes multiples of60° (i.e., θ1 = −θ2 = 30° or 60°), the arrangement is equivalentto the full alignment of the two 2L-MSL due to the 60°rotational symmetry of the MSLs. In this case, ϕ3 is reset to 0,therefore λ3 diverges, giving rise to the two maxima, which isequivalent to the diagonal on the right part. The kinks on thecontour lines come from the 60° rotational symmetry of thelattices, where |ϕ3| = 30°.We now compare this simple model to our experiments.From the SDPs at nsA ≈ ±2.4 × 1012 cm−2, we calculate thecorresponding moire ́ period λ1 ≈ 13.2 nm and the twist angle|ϕ1| ≈ 0.34. Similarly, for the extrapolated SDP at nsB ≈ − 5.2 ×1012 cm−2, we obtain λ2 ≈ 9 nm and |ϕ2| ≈ 1.2°. The two twistangles give us two points in the map in Figure 3c: ∼17.2 nmfor (0.34°, 1.2°) and ∼27.1 nm for (−0.34°, 1.2°). The ∼17.2nm matches very well the value ∼17.3 nm extracted from thenew-generation SDPs at nsC ≈ ±1.4 × 1012 cm−2 in thetransport measurement, which confirms that the new-generation SDPs come from the 3L-MSL.We fabricated five hBN/graphene/hBN heterostructures intotal, two of which exhibit 3L-MSL features. Data from devicesof the second heterostructure are presented in the SupportingInformation, which has a 3L-MSL with λ3 ≈ 29.6 nm.In conclusion, we have demonstrated the emergence of anew generation of MSLs in fully encapsulated graphene deviceswith aligned top and bottom hBN layers. In these devices, wefind three different superlattice periods, one of which is largerthan the maximum graphene/hBN moire ́ period, which weattribute to the combined top and bottom hBN potentialmodulation. Whereas our model describes qualitatively thedensities where these 3L-MSL features occur, the precisenature of the band structure distortions is unknown. Thealignment of both hBN layers to graphene opens newpossibilities for graphene band structure engineering, thereforeproviding motivation for further studies. Our new approach ofMSL engineering is not limited to graphene with hBN butapplies to two-dimensional materials in general, such as twistedtrilayer graphene, graphene with transition metal dichalcoge-nides, and so forth, which might open a new direction in“twistronics”.55,56■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.8b05061.Discussions about fabrication, functional dependence ofmoire ́ period λ and orientation θ on the twist angle ϕ for2L-MSL, data of other devices from the first hBN/graphene/hBN heterostructure and data of devices fromthe second heterostructure (PDF)■ AUTHOR INFORMATIONCorresponding Authors*E-mail: lujun.wang@unibas.ch.*E-mail: andreas.baumgartner@unibas.ch.ORCIDLujun Wang: 0000-0002-5447-0484Pet́er Makk: 0000-0001-7637-4672Christian Schönenberger: 0000-0002-5652-460XNotesThe authors declare no competing financial interest.■ ACKNOWLEDGMENTSThis work has received funding from the Swiss NanoscienceInstitute (SNI), the ERC project TopSupra (787414), theEuropean Union Horizon 2020 research and innovationprogramme under Grant Agreement 696656 (GrapheneFlagship), the Swiss National Science Foundation, the SwissNCCR QSIT, Topograph, ISpinText FlagERA network andfrom the OTKA FK-123894 grants. P.M. acknowledgessupport from the Bolyai Fellowship, the Marie Curie grantand the National Research, Development and Innovation Fundof Hungary within the Quantum Technology NationalExcellence Program (Project Nr. 2017-1.2.1-NKP-2017-00001). M.-H.L. acknowledges financial support from TaiwanMinister of Science and Technology (MOST) under GrantNano Letters LetterDOI: 10.1021/acs.nanolett.8b05061Nano Lett. 2019, 19, 2371−23762374http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.8b05061/suppl_file/nl8b05061_si_002.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.8b05061/suppl_file/nl8b05061_si_002.pdfhttp://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.8b05061/suppl_file/nl8b05061_si_002.pdfhttp://pubs.acs.orghttp://pubs.acs.org/doi/abs/10.1021/acs.nanolett.8b05061http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.8b05061http://pubs.acs.org/doi/suppl/10.1021/acs.nanolett.8b05061/suppl_file/nl8b05061_si_002.pdfmailto:lujun.wang@unibas.chmailto:andreas.baumgartner@unibas.chhttp://orcid.org/0000-0002-5447-0484http://orcid.org/0000-0001-7637-4672http://orcid.org/0000-0002-5652-460Xhttp://dx.doi.org/10.1021/acs.nanolett.8b05061107-2112-M-006-004-MY3. Growth of hexagonal boronnitride crystals was supported by the Elemental StrategyInitiative conducted by the MEXT, Japan and the CREST(JPMJCR15F3), JST. The authors thank David Indolese andPeter Rickhaus for fruitful discussions.■ REFERENCES(1) Weiss, D.; Klitzing, K. V.; Ploog, K.; Weimann, G. Magneto-resistance Oscillations in a Two-Dimensional Electron Gas Inducedby a Submicrometer Periodic Potential. Europhys. Lett. 1989, 8, 179.(2) Weiss, D.; Roukes, M. L.; Menschig, A.; Grambow, P.; vonKlitzing, K.; Weimann, G. Electron pinball and commensurate orbitsin a periodic array of scatterers. Phys. Rev. Lett. 1991, 66, 2790−2793.(3) Pfannkuche, D.; Gerhardts, R. R. Theory of magnetotransport intwo-dimensional electron systems subjected to weak two-dimensionalsuperlattice potentials. Phys. Rev. B: Condens. Matter Mater. Phys.1992, 46, 12606−12626.(4) Ferry, D. Quantum magnetotransport in lateral surfacesuperlattices. Prog. 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The revised paper was publishedon March 12, 2019.Nano Letters LetterDOI: 10.1021/acs.nanolett.8b05061Nano Lett. 2019, 19, 2371−23762376http://dx.doi.org/10.1021/acs.nanolett.8b05061