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Faculty of Physics, University of Vienna, Boltzmanngasse 5, A-1090 Vienna, Austria.*These authors contributed equally to this work.†Corresponding author. Email: kimmo.mustonen@univie.ac.at (K.M.); jannik.meyer@univie.ac.at (J.C.M.)

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Buckyball sandwichesRasim Mirzayev,* Kimmo Mustonen,*† Mohammad R. A. Monazam, Andreas Mittelberger,Timothy J. Pennycook, Clemens Mangler, Toma Susi, Jani Kotakoski, Jannik C. Meyer†

Two-dimensional (2D) materials have considerably expanded the field of materials science in the past decade.Even more recently, various 2D materials have been assembled into vertical van der Waals heterostacks, andit has been proposed to combine them with other low-dimensional structures to create new materials withhybridized properties. We demonstrate the first direct images of a suspended 0D/2D heterostructure thatincorporates C60 molecules between two graphene layers in a buckyball sandwich structure. We find cleanand ordered C60 islands with thicknesses down to one molecule, shielded by the graphene layers from themicroscope vacuum and partially protected from radiation damage during scanning transmission electronmicroscopy imaging. The sandwich structure serves as a 2D nanoscale reaction chamber, allowing the analysisof the structure of the molecules and their dynamics at atomic resolution.

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INTRODUCTIONGraphene (1), a single layer of carbon in a hexagonal lattice, is thethinnest imaginable membrane, with unique intrinsic properties. Ithas no bulk, and it interfaces with other materials through van derWaals (vdW) forces. This has made graphene appealing as one ofthe main components for so-called vdW heterostructures consistingof either only two-dimensional (2D) materials (2) or mixtures of 2Dmaterials with structures of other dimensionalities (2, 3). One of theearliest examples ofmixed-dimensional hybrid structures was the car-bon peapod (4), in which a linear array of (quasi-)0D fullerenes (5) isconfined inside a (quasi-)1D carbon nanotube. Recently, collapseddouble-walled carbon nanotubes have been intercalated with C60 toform a nanoribbon-type peapod (6, 7).

Fullerenes have many interesting properties: They have a band gapunder ambient conditions (8), can become metallic when doped, andeven exhibit superconductivity under certain conditions (9). Fuller-enes belong to the rare group of saturable absorbers and can exhibitpeculiar ferromagnetic behavior at low temperatures and when poly-merized (10–12). They are commonly synthesized by ablating graphite,a process in which defects are thought to cause the carbon lattice tocurve into minimum-energy spheroids (13). In the carbon peapods,fullerenes can be studied in a confined and well-defined space withoutthe need of a suspension. This hasmade it possible to directly image theirdynamics and bonding under atomic-resolution electron microscopy(14, 15). However, fullerenes have also been reported to self-assembleinto hexagonally close-packed islands on surfaces and exhibit epitaxialordering on graphene and even on some transition metals (16, 17).

Here, we present the first example of suspended buckyball sand-wiches, where C60 molecules are confined between two graphene mono-layers. The resulting structures contain large atomically clean areas,allowing the direct study of C60 islands through scanning transmissionelectron microscopy (STEM). Similar to carbon peapods, the buckyballsandwiches are expected to have hybridized properties that could be suit-ed for applications ranging fromnanoscale lasers to spin cubit arrays andnanoscale mechanical elements (18, 19). Furthermore, alkali metal co-intercalation may turn it into a high-temperature superconductor (20, 21),similar to crystalline C60 (22, 23).We concentrate on understanding the

atomic-scale structure of the sandwiches and the dynamics of the con-tained fullerenes. We observe the diffusion of individual C60 and findthat they remain rotationally active during room temperature experi-ments. However, during continuous electron irradiation, some C60 firstbond to form dimers and then fused peanut-like structures, which con-tinue to rotate around the joint axis. This movement is hindered onlyfor structures involving three or more molecules.

Our results show that individual molecules in a graphene sandwichcan be imaged at atomic resolution. Previously, graphene encapsulationhas been used to study small metal particles (24), colloidal nanocrystals(25), or other 2D materials systems (26, 27) through high-resolutionelectron microscopy. Encapsulation is known to reduce the rate of ra-diation damage (26, 27) and provide a sample support with a periodicstructure that can be easily subtracted from the image. As an additionalimportant feature, we find that the graphene sandwich provides a“clean” view of themolecules; the sandwich has only the contaminationthat is typical for graphene in transmission electronmicroscopy (TEM)imaging, whereas without the sandwich, no clear images of the fuller-enes could be obtained. The sandwich further provides a nanoscalereaction chamber that is less constrained than the 0D fullerene cage(28) or the 1D “test tube” inside a carbon nanotube (15) for studyingthe structure and properties of individual molecules trapped betweenthe layers.

RESULTS AND DISCUSSIONThe buckyball sandwiches were fabricated by thermally evaporatingC60 onto commercially available graphene-coated TEM grids, followedby the placement of a second graphene-coated grid on top and bringingthe two films into contact by evaporating a droplet of solvent (seeMaterials and Methods; figs. S1 and S2 show the general sample mor-phology, Raman spectrum of the sandwich, and the structure of themultilayers). The sandwiched C60 show remarkable stability underthe electron beam, similar to fullerenes encapsulated in carbon nano-tubes (14). On the basis of high-resolution images, it is easy to distin-guish between monolayer and multilayer regions, with an exampleshown in Fig. 1A. Figure 1B shows an atomically resolved close-up,and Fig. 1C shows its Fourier transform, from which the two graphenelayers (with a misorientation angle qGr ≈ 11°) and the fullerene latticecan be identified. Unlike on a single layer of graphene, the sandwichesalso contain fullerenemonolayers, whose edges exhibit nonlinear imagecontrast of themoiré pattern of the suspended graphene layers (fig. S3).

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Using the graphene lattice as a calibration standard, we measuredthe C60 lattice spacing (the distance between the centers of theneighboringmolecules) for crystallinemonolayer regions. Surprisingly,this spacing (9.6 ± 0.1 Å) is 4 to 5% smaller than that of 3D crystallites(29). This distance is also smaller than in 1D carbon peapods, whosereported values vary from 9.7 to 10.0 Å (possibly influenced by theendohedral metal atoms present in some studies) (14, 30, 31).

To understand this change in spacing, we used atomistic simula-tions to calculate the equilibrium lattice constants of the moleculesin bulk 3D and 2D C60 crystals and in a 2D sandwich (see Materialsand Methods). The simulated sandwich consisted of two graphenenanoribbons with an orientation mismatch of qGr ≈ 11°, with a 7 ×20 array of C60 in between (Fig. 1D). For 3D C60, we found an equilib-rium distance of 9.65 Å, whereas for an infinite 2D sheet, this distancewas 9.62 Å. Hence, the mere dimensionality of the crystallites is unableto explain the observed monolayer spacing. However, in the simulatedsandwich, the average equilibrium distance was 9.22 Å (with an SD of0.15 Å due to directional anisotropy), which represents a contractionof 4.7% compared with the bulk value. Thus, despite the difference inthe absolute distances, the interaction with the graphene sheets in thesimulated sandwich compresses the C60 lattice, in agreement withthe experiment.

Individual fullerenes are found to be highly mobile at the un-constrained edges of the C60 monolayers: Some C60 in the partiallyfilled outermost row of Fig. 2A have occupied and vacated edge sitesmultiple times during the acquisition of subsequent lines of thescanned image, resulting in the “interlaced” appearance of the fuller-enes. For example, the fullerene marked by an arrow has disappearedand reappeared at least 17 times during the acquisition of 100 scanlines (~1 s) in this region of the image. However, it typically remained

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stationary during the ~1ms the probe took to traverse the molecule ineach line of the scan. Evidently, it is possible to quantify even single-molecule diffusion in the sandwich structure using direct STEMimaging. Regardless of their mobility inside this area, it appears thatthemolecules remain confined in the graphenewrap and cannot com-pletely escape.

To understand these dynamics, we also calculated the relevant dif-fusion barriers using the nudged elastic band method (Fig. 2B; seeMaterials andMethods andmovies S1 and S2): one for a C60 diffusingalong the edge of the monolayer (234 meV) and another for a C60 dif-fusing from one edge to another across the gap (371 meV). Thesevalues are in reasonable agreement with those reported for fullerenediffusion onC60 crystallites (32, 33). These energies are easily availablefrom the electron beam, but these transitions might also be thermallyactivated at room temperature. From the second path, we could alsoestimate the barrier for moving the C60 inside the gap in the sandwichto be less than 5 meV. This explains why isolated fullerenes are neverobserved in the experimental images.

Different dynamics are observed in disordered C60 regions, asshown in Fig. 2 (C and D): In this case, voids in the C60 arrangementcan propagate inside the lattice. For example, between the two shownframes, a void propagated from location 1 to 2, and a single moleculeescaped outside the field of view from position 3. However, overall,this edge appears to be more tightly constrained by the two graphenelayers, displaying no oscillations like those seen in Fig. 2A.

Next, we consider the rotation and bonding of sandwiched C60 mol-ecules during imaging with 60-keV electrons. At least three types ofmodifications are possible: knock-on damage of the C60 [initially result-ing in the formation of C59 (34), with a reactive dangling bond next tothe vacancy], bonding between fullerenes, or the bonding of C60 to gra-phene; graphene knock-on damage is completely suppressed at thisenergy (35). Although the graphene lattice is clearly resolved in the orig-inal images of the time/electron dose sequence in Fig. 3 (A to C) (see the

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Fig. 1. Monolayer area of a buckyball sandwich. (A) Sandwich with bothmonolayered and multilayered C60 regions and (B) a higher magnification ofthe region labeled b. (C) Fourier transform of (B), with graphene reflectionsmarked by red dashed lines, and some of the fullerene peaks marked in yellow.(D) Model of a monolayer C60 sandwich (top and side views).

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Fig. 2. Dynamics of sandwiched fullerenes. (A) An oscillating C60 at the edge ofa gap in the fullerene monolayer, over which the graphene layers are suspended.At the location indicated by the arrow, a molecule disappears and appears at least17 times during 100 scan lines. (B) Nudged elastic band paths and energy barriersfor C60 diffusion along an edge of fullerene monolayer (vertical) and from oneedge of the gap to another (horizontal). (C and D) A disordered monolayershowing a void propagating from location 1 to 2, and the C60 at location 3escaping outside the field of view between the two consecutive frames.

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SupplementaryMaterials), no structure beyond spherical symmetry canbe discerned for isolated fullerenes at the beginning of the sequence. Be-cause the absorption energy of C60 on graphene is around 0.9 eV (36)and different absorption geometries are separated by <0.1 eV (16), eventhe graphene sandwich should not prevent C60 rotation under our ex-perimental conditions.We therefore attribute the lack of internal struc-ture of the C60 in our images to rotation, either thermally activated ordriven by the electron irradiation. Throughout the sequence in Fig. 3 (Ato C), all isolated fullerenes (unmarked objects) display only sphericalsymmetry, in contrast to clustered objects (marked by the yellowdashedlines) where internal structure is evident. Upon increasing the electrondose, some C60 clearly form bonds with their neighbors, becoming“locked” into a certain position and allowing their structure to be revealed.

Even more remarkable are the few fullerenes that appear to form abond with a neighbor without fusing together (especially the blue and

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red dashed lines in Fig. 3, A and B). Consider, for example, the trimermarked with I, II, and III in Fig. 3A: Here, the fullerene I that is boundto two neighbors is presumably locked in its rotation and shows clearinternal structure; however, those having been bonded only on oneside (II and III) apparently still rotate (or oscillate around a groundstate) because their structure is smeared out. The appearances of ro-tating and lockedmolecules are quite distinct, as revealed by the STEMimage simulation shown in Fig. 3E; the molecule on the left is an av-erage of 100 random orientations, and the one on the right is locked.On the basis of their appearance, we also conclude that the fullerenedimers or “peanuts” (37), highlighted by purple dashed lines in Fig. 3(A to C), exhibit rotation around their longitudinal axis that smearsout any internal structure (note the clear difference between the purplemarked dimers and polymers with additional cross-linking markedwith yellow).

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Fig. 3. Fullerene reactions. (A to C) A dose series of a C60 sandwich, smoothed to reduce pixel noise, Fourier-filtered to remove the graphene contribution (fig. S4), andcontrast-enhanced tomaximize the visibility of the C60 structures (unfiltered images and images with different contrast settings are given as figs. S5 and S6). The blue dashedlines highlight the C60 that appear to be locked in position by two-directional bonding, showing a visible internal structure and those stationary due to polymerization withyellow color. The dimers (purple) and molecules with a one-directional bond (red) still appear to be moving, obscuring their internal structure. (D) STEM image of a pair ofrotating C60 [IV and V in (A)] and a simulation of a rotating (left) and a stationary (right) C60 in (E). (F) A loosely bound pair of fullerenes and a simulated C59–C59 dimer with onebond in (G) and a tightly bound pair and a simulatedC59–C59with three bonds in (H) and (I). A typical peanut dimer in (J) and an image simulation of C120 in (K). The red dashesin (D) indicate the integration width of the line profiles plotted in (D) to (K).

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Already one bond between two fullerenes should make the struc-ture rotationally stiff because of the overlap of the p orbitals on theconnecting C atoms. However, the energy input from the electronbeam can overcome this barrier. Movie S3 shows a density functionaltheory (DFT) molecular dynamics simulation (see Materials andMethods) of a dimer where one molecule is prevented from rotatingby fixing atoms on its back wall, and one atom on the side of the othermolecule is given a high kinetic energy, emulating an electron impact.This mimics the situation where a fullerene is bonded by a single C–Cbond to an already rotationally locked neighbor. As can be seen fromthe video, the simulated impact results in a noticeable rotation of thesecond molecule even while the bond is preserved.

We further analyzed the dimer bonding bymeasuring the apparentcenter-to-center distances (d) of paired molecules from Fig. 3A. Forcomparison, Fig. 3D shows a pair of free molecules (molecules IVand V), whose center-to-center distance d′ is set to 9.6 Å based onthe average monolayer spacing. To extract the d of bound molecules,we fitted Gaussian line shapes to the outer edges of the intensity pro-files andused thepeak-to-peak separation (D) to calculated=d′+D−D′,where D′ is the reference distance measured for nonbonded mol-ecules. Figure 3F shows what is possibly a singly bonded C59–C59 di-mer with d = 9.6 Å (molecules I and III), closely resembling thesimulated equivalent in Fig. 3G. (We also simulated the spacing of asingly bonded C60–C59 and a C60–C60 dimer with two bonds, yieldingslightly larger values of 9.9 and 10.0 Å, respectively; see fig. S7.) Anapparently more tightly bound pair (molecules I and II) is comparedin Fig. 3 (H and I): This might be a pair of C59 molecules connectedby three bonds, based on the measured d = 8.6 Å, matching well to asimulated value of 8.7 Å. The peanut structures are compared in Fig. 3(J and K), where we see that the experimental structure (molecule VI)is slightly smaller than the simulated one, 8.5 versus 8.9 Å. However,stable fullerene peanuts will likely have fewer than 120 atoms and thusmany possible structures.

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CONCLUSIONSIn conclusion, we have created and characterized amixed-dimensionalsandwich heterostructure of C60 fullerenes encapsulated by twolayers of graphene. The fullerene monolayers inside the sandwichexhibit a lattice spacing of 9.6 Å, ~5% smaller than the bulk spacing.Dynamics of entire molecules can be observed, with weakly boundfullerenes oscillating between different positions at the edges of 2DC60 molecular crystals where the graphene layers are suspended overnanometer areas, along with mobile vacancies in disordered 2D full-erene layers. Last, we observed the transition from rotating individualfullerenes through dimers with suppressed rotation to molecularclusters locked into position, allowing their internal structure to be re-vealed. The graphene sandwich thus provides a nanoscale reactionchamber, a clean interface to the microscope vacuum, some suppres-sion of radiation damage, and a low-contrast background that can besubtracted from the image.

MATERIALS AND METHODSSandwich fabricationC60 was thermally evaporated at a pressure of ~10−5 mbar onto com-mercial graphene-coated TEM grids from Graphenea Inc. Before dep-osition, the grids were annealed for 30 min at 400°C in air, and the C60

powder was annealed for 120 min at 200°C under vacuum. The evap-

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oration took place in a vacuum chamber (Mantis HEX deposition sys-tem) with the target at 100°C at a rate of ~1 Å/s, measured with aquartz-crystal monitor. A second TEM grid used for stacking wasplaced on top and adhered via the tensile forces resulting from evapo-rating a droplet of isopropyl alcohol (semiconductor-grade PURANAL,Sigma-Aldrich), followed by drying and then separating the gridswith tweezers.

Scanning transmission electron microscopyAtomic-resolution imaging was conducted using an aberration-corrected Nion UltraSTEM 100 STEM operated with a 60-keVprimary beam energy, with a near-ultrahigh vacuum of 2 × 10−7

Pa at the sample. The angular range for the medium-angle annulardark-field (MAADF) detector was 60 to 200 mrad.

Raman spectroscopyRaman spectra (fig. S1E) were acquired using an ND-MDT NTEGRASpectra AFM/Raman instrument with a 473-nm excitation wavelength(2.62 eV) under ambient conditions. The output laser powerwas~4mWover a focused spot with a diameter of ~0.5 mm.

STEM simulationsSTEM simulations used QSTEM version 2.31 with a chromatic aber-ration coefficient of 1 mm, a spherical aberration coefficient of 1 mm,energy spread of 0.48 eV, and MAADF detector angle range set to theexperimental range of 60 to 200 mrad. The illumination semianglewas 35 mrad. In Fig. 3E, to mimic the experimental contrast of therotating fullerene, graphenewas included in the simulations and, later,Fourier-filtered.

Atomistic simulationsMost simulations were carried out with the LAMMPS (Large-scaleAtomic/Molecular Massively Parallel Simulator) code (38), using anAIREBO (Adaptive Intermolecular Reactive Empirical Bond Order)potential augmented by aMorse potential for the intermolecular inter-action (A-M) to describe the covalent and vdW interactions, respec-tively (39, 40). To study the intermolecular spacing and diffusionbarriers in sandwichedC60 layers, we used amodel supercell consistingof seven rows of C60 with periodic boundary conditions along theoverlying and underlying graphene sheet edges. The misorientationangle between the graphene lattices was set to 11.6°, resulting in amoiré pattern periodicity of ~12 Å, which matched that in theexperiments. This unit cell was repeated 5 and 20 times, resulting ina supercell consisting of 33,820 carbon atoms, including the C60 mol-ecules. The total energy was minimized by relaxing the layers withoutconstraints until the forces were <10−4 eV Å−1.

For bulk C60 crystals, we additionally compared these results withDFT, calculated using two different vdW functionals (C09 and DF2)(41, 42).We relaxed theminimal single-molecule unit cells of both 3Dface-centered cubic and 2D crystallites using the plane wave mode intheGPAWsimulation package (43) (600-eV cutoff energy, and 5 × 5 ×1 and 5 × 5 × 5 Monkhorst-Pack k-point meshes for the 2D and 3Dcases, respectively). The results agreed with the A-M calculation inthat there is no significant difference between the 2D and 3D spacingof the fullerenes in the absence of the graphene sandwich. For 3D bulkC60, we found 0-K equilibrium distances of 9.65 Å (A-M), 9.86 Å(C09), and 10.03 Å (DF2), whereas for an infinite 2D sheet, thedistances were 9.62 Å [A-M; consistent with that from the study ofReddy et al. (44)], 9.91 Å (C09), and 10.02 Å (DF2).

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For the diffusion barrier simulations, we cut out part of the fullerene-containing area of the model, created a gap inside the fullerene mono-layer, and fixed the outermost rows of fullerenes and the edges of thegraphene layers. The barriers were calculated using the nudged elasticband method (45) with a spring constant of 10 eV/Å.

To study the bonding of the fullerenes, we placed two C60 in thecenter of a 32 × 20 × 20 Å unit cell and relaxed their atomic structureusing the C09 functional in the GPAW finite difference mode (0.19 Ågrid spacing, Gamma point only) (46). To study the effect of irradiationon apparent bond length, we first removed one C atom from the secondC60 and then relaxed the structure and then further removed another Catom from the first C60 and relaxed that structure, resulting in dimerswith either one or three bonds. For the C60–C59 system, we further ranmolecular dynamics simulations in the linear combination of atomicorbitals DFT mode (15-eV kinetic energy; movie S3) to simulate the ro-tation of the C59 caused by electron impact on one of the carbon atoms.

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SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/6/e1700176/DC1Sample morphologyC60 MultilayersStem contrast of the sandwichFourier filteringUnfiltered stem images of the C60 sandwichAdditional filtered views of the C60 sandwichStem simulations of a C60-C60 with two bonds and singly bonded C60-C59fig. S1. Overview STEM micrographs, electron diffraction patterns, and Raman spectra.fig. S2. Structure of C60 multilayers.fig. S3. Moiré contrast at the edges of the C60 monolayer.fig. S4. Fourier filtering.fig. S5. Unfiltered version of the STEM micrographs of Fig. 3 (A to C).fig. S6. Fourier-filtered views of the rotating and fusing fullerenes.fig. S7. STEM simulations of merged fullerenes.movie S1. C60 diffusing away from a row of molecules enveloped by graphene.movie S2. C60 diffusing along a row of molecules enveloped by graphene.movie S3. Bond rotation in fullerene dimer after a 15-eV electron impact.

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Mirzayev et al., Sci. Adv. 2017;3 : e1700176 9 June 2017

AcknowledgmentsFunding: This work was supported by the European Research Council Starting Grant no.336453-PICOMAT and the Austrian Science Fund (FWF) under project nos. P 25721-N20,I1283-N20, and P 28322-N36. K.M. acknowledges support from the Finnish Foundations’Post Doc Pool, and T.J.P. acknowledges funding from the European Union’s Horizon 2020Research and Innovation Programme under the Marie Skłodowska-Curie grant agreementno. 655760—DIGIPHASE. J.K. acknowledges funding from Wiener Wissenschafts- Forschungs-und Technologiefonds through project MA14-009. Computational resources from theVienna Scientific Cluster are gratefully acknowledged. Author contributions: R.M. andA.M. prepared samples. R.M., A.M., T.J.P., and C.M. performed STEM experiments. R.M.performed Raman spectroscopy. K.M., R.M., and J.C.M. analyzed experimental data. M.R.A.M.,A.M., T.S., J.K., and K.M. carried out simulations and analyzed results. K.M., T.S., J.K., R.M.,and J.C.M. wrote the paper with input and comments from all authors. J.C.M. conceivedand supervised the study with cosupervision from J.K. and T.S. Competing interests: Theauthors declare that they have no competing interests. Data and materials availability: Alldata needed to evaluate the conclusions in the paper are present in the paper and/or theSupplementary Materials. Additional data related to this paper may be requested from the authors.

Submitted 17 January 2017Accepted 13 April 2017Published 9 June 201710.1126/sciadv.1700176

Citation: R. Mirzayev, K. Mustonen, M. R. A. Monazam, A. Mittelberger, T. J. Pennycook,C. Mangler, T. Susi, J. Kotakoski, J. C. Meyer, Buckyball sandwiches. Sci. Adv. 3, e1700176 (2017).

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Buckyball sandwiches

Mangler, Toma Susi, Jani Kotakoski and Jannik C. MeyerRasim Mirzayev, Kimmo Mustonen, Mohammad R. A. Monazam, Andreas Mittelberger, Timothy J. Pennycook, Clemens

DOI: 10.1126/sciadv.1700176 (6), e1700176.3Sci Adv 

ARTICLE TOOLS http://advances.sciencemag.org/content/3/6/e1700176

MATERIALSSUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2017/06/05/3.6.e1700176.DC1

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