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Probing the electron states and metal-insulator transition mechanisms in molybdenum disulphide vertical heterostructures. Nat Commun 2015; 6:6088. [PMID: 25586302 DOI: 10.1038/ncomms7088] [Citation(s) in RCA: 154] [Impact Index Per Article: 17.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/06/2014] [Accepted: 12/11/2014] [Indexed: 12/23/2022] Open
Abstract
The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron-electron interactions has been used in modelling the metal-insulator transition in molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer molybdenum disulphide. In addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties are accessed.
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52
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Schmidt H, Rode JC, Smirnov D, Haug RJ. Superlattice structures in twisted bilayers of folded graphene. Nat Commun 2014; 5:5742. [PMID: 25475084 PMCID: PMC4268689 DOI: 10.1038/ncomms6742] [Citation(s) in RCA: 59] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/14/2014] [Accepted: 11/03/2014] [Indexed: 11/16/2022] Open
Abstract
The electronic properties of bilayer graphene strongly depend on relative orientation of the two atomic lattices. Whereas Bernal-stacked graphene is most commonly studied, a rotational mismatch between layers opens up a whole new field of rich physics, especially at small interlayer twist. Here we report on magnetotransport measurements on twisted graphene bilayers, prepared by folding of single layers. These reveal a strong dependence on the twist angle, which can be estimated by means of sample geometry. At small rotation, superlattices with a wavelength in the order of 10 nm arise and are observed by friction atomic force microscopy. Magnetotransport measurements in this small-angle regime show the formation of satellite Landau fans. These are attributed to additional Dirac singularities in the band structure and discussed with respect to the wide range of interlayer coupling models. The properties of bilayer graphene can be tuned by twisting the layers relative to one another. Schmidt et al. now demonstrate the twist angle dependence of magnetotransport in this material system and uncover the formation of satellite Landau fans in the small-angle regime because of superlattice formation
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Affiliation(s)
- Hennrik Schmidt
- 1] Institut für Festkörperphysik, Leibniz Universität Hannover, Hannover 30167, Germany [2] Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, Singapore 117546, Singapore
| | - Johannes C Rode
- Institut für Festkörperphysik, Leibniz Universität Hannover, Hannover 30167, Germany
| | - Dmitri Smirnov
- Institut für Festkörperphysik, Leibniz Universität Hannover, Hannover 30167, Germany
| | - Rolf J Haug
- Institut für Festkörperphysik, Leibniz Universität Hannover, Hannover 30167, Germany
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53
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Yankowitz M, Xue J, LeRoy BJ. Graphene on hexagonal boron nitride. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2014; 26:303201. [PMID: 24994551 DOI: 10.1088/0953-8984/26/30/303201] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.
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54
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Yeh CH, Lin YC, Chen YC, Lu CC, Liu Z, Suenaga K, Chiu PW. Gating electron-hole asymmetry in twisted bilayer graphene. ACS NANO 2014; 8:6962-6969. [PMID: 24999754 DOI: 10.1021/nn501775h] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/03/2023]
Abstract
Electron-hole symmetry is one of the unique properties of graphene that is generally absent in most semiconductors because of the different conduction and valence band structures. Here we report on the manipulation of electron-hole symmetry in the low-energy band structure of twisted bilayer graphene, where symmetric saddle points form in the conduction and valence bands as a result of interlayer coupling. By applying a gate voltage to a twisted bilayer with a critical rotation angle, enhanced electron resonance between the two saddle points can be turned on or off, depending on the electron-hole symmetry near the saddle points. The appearance of a 2D(+) peak, a gate-tunable Raman feature found near the critical angle, indicates a reduction of Fermi velocity in the vicinity of the saddle point to/from which electrons are inelastically scattered by phonons in the round trip of the double-resonance process.
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Affiliation(s)
- Chao-Hui Yeh
- Department of Electrical Engineering, National Tsing Hua University , Hsinchu 30013, Taiwan
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55
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Chen JJ, Meng J, Yu DP, Liao ZM. Fabrication and electrical properties of stacked graphene monolayers. Sci Rep 2014; 4:5065. [PMID: 24861035 PMCID: PMC4033920 DOI: 10.1038/srep05065] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/07/2014] [Accepted: 05/08/2014] [Indexed: 11/21/2022] Open
Abstract
We develop a simple method to fabricate the two-stacked graphene monolayers and investigate the electronic transport in such a system. The independence of the two graphene monolayers gives rise to the asymmetric resistance-gate voltage curves and an eight-fold degeneracy of Landau level. The position of the maximum resistance of the transfer curves shifts towards higher gate voltage with increasing magnetic field, which is attributed to the magnetic field induced interlayer decoupling of the stacked graphene monolayers.
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Affiliation(s)
- Jing-Jing Chen
- State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, P.R. China
| | - Jie Meng
- State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, P.R. China
| | - Da-Peng Yu
- 1] State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, P.R. China [2] Collaborative Innovation Center of Quantum Matter, Beijing, China
| | - Zhi-Min Liao
- 1] State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, P.R. China [2] Collaborative Innovation Center of Quantum Matter, Beijing, China
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56
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Zhang J, Triola C, Rossi E. Proximity effect in graphene-topological-insulator heterostructures. PHYSICAL REVIEW LETTERS 2014; 112:096802. [PMID: 24655268 DOI: 10.1103/physrevlett.112.096802] [Citation(s) in RCA: 31] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/03/2013] [Indexed: 06/03/2023]
Abstract
We formulate a continuum model to study the low-energy electronic structure of heterostructures formed by graphene on a strong three-dimensional topological insulator (TI) for the cases of both commensurate and incommensurate stacking. The incommensurability can be due to a twist angle between graphene and the TI surface or a lattice mismatch between the two systems. We find that the proximity of the TI induces in graphene a strong enhancement of the spin-orbit coupling that can be tuned via the twist angle.
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Affiliation(s)
- Junhua Zhang
- Department of Physics, College of William and Mary, Williamsburg, Virginia 23187, USA
| | - C Triola
- Department of Physics, College of William and Mary, Williamsburg, Virginia 23187, USA
| | - E Rossi
- Department of Physics, College of William and Mary, Williamsburg, Virginia 23187, USA
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57
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Luican-Mayer A, Kharitonov M, Li G, Lu CP, Skachko I, Gonçalves AMB, Watanabe K, Taniguchi T, Andrei EY. Screening charged impurities and lifting the orbital degeneracy in graphene by populating Landau levels. PHYSICAL REVIEW LETTERS 2014; 112:036804. [PMID: 24484160 DOI: 10.1103/physrevlett.112.036804] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/19/2013] [Indexed: 06/03/2023]
Abstract
We report the observation of an isolated charged impurity in graphene and present direct evidence of the close connection between the screening properties of a 2D electron system and the influence of the impurity on its electronic environment. Using scanning tunneling microscopy and Landau level spectroscopy, we demonstrate that in the presence of a magnetic field the strength of the impurity can be tuned by controlling the occupation of Landau-level states with a gate voltage. At low occupation the impurity is screened, becoming essentially invisible. Screening diminishes as states are filled until, for fully occupied Landau levels, the unscreened impurity significantly perturbs the spectrum in its vicinity. In this regime we report the first observation of Landau-level splitting into discrete states due to lifting the orbital degeneracy.
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Affiliation(s)
- Adina Luican-Mayer
- Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
| | - Maxim Kharitonov
- Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
| | - Guohong Li
- Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
| | - Chih-Pin Lu
- Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
| | - Ivan Skachko
- Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
| | - Alem-Mar B Gonçalves
- Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
| | - K Watanabe
- Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - T Taniguchi
- Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan
| | - Eva Y Andrei
- Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
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58
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Santos EJG, Kaxiras E. Electric-field dependence of the effective dielectric constant in graphene. NANO LETTERS 2013; 13:898-902. [PMID: 23339637 DOI: 10.1021/nl303611v] [Citation(s) in RCA: 72] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
The dielectric constant of a material is one of the fundamental features used to characterize its electrostatic properties such as capacitance, charge screening, and energy storage capability. Graphene is a material with unique behavior due to its gapless electronic structure and linear dispersion near the Fermi level, which can lead to a tunable band gap in bilayer and trilayer graphene, a superconducting-insulating transition in hybrid systems driven by electric fields, and gate-controlled surface plasmons. All of these results suggest a strong interplay between graphene properties and external electric fields. Here we address the issue of the effective dielectric constant (ε) in N-layer graphene subjected to out-of-plane (E(ext)(⊥)) and in-plane (E(ext)(||)) external electric fields. The value of ε has attracted interest due to contradictory reports from theoretical and experimental studies. Through extensive first-principles electronic structure calculations, including van der Waals interactions, we show that both the out-of-plane (ε(⊥)) and the in-plane (ε(||)) dielectric constants depend on the value of applied field. For example, ε(⊥) and ε(||) are nearly constant (~3 and ~1.8, respectively) at low fields (E(ext) < 0.01 V/Å) but increase at higher fields to values that are dependent on the system size. The increase of the external field perpendicular to the graphene layers beyond a critical value can drive the system to a unstable state where the graphene layers are decoupled and can be easily separated. The observed dependence of ε(⊥) and ε(||) on the external field is due to charge polarization driven by the bias. Our results point to a promising way of understanding and controlling the screening properties of few-layer graphene through external electric fields.
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Affiliation(s)
- Elton J G Santos
- School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States.
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59
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Kim Y, Yun H, Nam SG, Son M, Lee DS, Kim DC, Seo S, Choi HC, Lee HJ, Lee SW, Kim JS. Breakdown of the interlayer coherence in twisted bilayer graphene. PHYSICAL REVIEW LETTERS 2013; 110:096602. [PMID: 23496735 DOI: 10.1103/physrevlett.110.096602] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/15/2012] [Indexed: 06/01/2023]
Abstract
Coherent motion of electrons in Bloch states is one of the fundamental concepts of charge conduction in solid-state physics. In layered materials, however, such a condition often breaks down for the interlayer conduction, when the interlayer coupling is significantly reduced by, e.g., a large interlayer separation. We report that complete suppression of coherent conduction is realized even in an atomic length scale of layer separation in twisted bilayer graphene. The interlayer resistivity of twisted bilayer graphene is much higher than the c-axis resistivity of Bernal-stacked graphite and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation and incoherent conduction is a main transport channel between the layers of twisted bilayer graphene.
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Affiliation(s)
- Youngwook Kim
- Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea
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60
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Brihuega I, Mallet P, González-Herrero H, Trambly de Laissardière G, Ugeda MM, Magaud L, Gómez-Rodríguez JM, Ynduráin F, Veuillen JY. Unraveling the intrinsic and robust nature of van Hove singularities in twisted bilayer graphene by scanning tunneling microscopy and theoretical analysis. PHYSICAL REVIEW LETTERS 2012; 109:196802. [PMID: 23215414 DOI: 10.1103/physrevlett.109.196802] [Citation(s) in RCA: 121] [Impact Index Per Article: 10.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/13/2012] [Indexed: 05/13/2023]
Abstract
Extensive scanning tunneling microscopy and spectroscopy experiments complemented by first-principles and parametrized tight binding calculations provide a clear answer to the existence, origin, and robustness of van Hove singularities (vHs) in twisted graphene layers. Our results are conclusive: vHs due to interlayer coupling are ubiquitously present in a broad range (from 1° to 10°) of rotation angles in our graphene on 6H-SiC(000-1) samples. From the variation of the energy separation of the vHs with the rotation angle we are able to recover the Fermi velocity of a graphene monolayer as well as the strength of the interlayer interaction. The robustness of the vHs is assessed both by experiments, which show that they survive in the presence of a third graphene layer, and by calculations, which test the role of the periodic modulation and absolute value of the interlayer distance. Finally, we clarify the role of the layer topographic corrugation and of electronic effects in the apparent moiré contrast measured on the STM images.
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Affiliation(s)
- I Brihuega
- Departamento Física de la Materia Condensada, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
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61
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Ohta T, Robinson JT, Feibelman PJ, Bostwick A, Rotenberg E, Beechem TE. Evidence for interlayer coupling and moiré periodic potentials in twisted bilayer graphene. PHYSICAL REVIEW LETTERS 2012; 109:186807. [PMID: 23215315 DOI: 10.1103/physrevlett.109.186807] [Citation(s) in RCA: 56] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/12/2012] [Indexed: 05/13/2023]
Abstract
We report a study of the valence band dispersion of twisted bilayer graphene using angle-resolved photoemission spectroscopy and ab initio calculations. We observe two noninteracting cones near the Dirac crossing energy and the emergence of van Hove singularities where the cones overlap for large twist angles (>5°). Besides the expected interaction between the Dirac cones, minigaps appeared at the Brillouin zone boundaries of the moiré superlattice formed by the misorientation of the two graphene layers. We attribute the emergence of these minigaps to a periodic potential induced by the moiré. These anticrossing features point to coupling between the two graphene sheets, mediated by moiré periodic potentials.
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Affiliation(s)
- Taisuke Ohta
- Sandia National Laboratories, Albuquerque, New Mexico 87185, USA
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62
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Garcia AGF, Neumann M, Amet F, Williams JR, Watanabe K, Taniguchi T, Goldhaber-Gordon D. Effective cleaning of hexagonal boron nitride for graphene devices. NANO LETTERS 2012; 12:4449-54. [PMID: 22866696 DOI: 10.1021/nl3011726] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
Hexagonal boron nitride (h-BN) films have attracted considerable interest as substrates for graphene. ( Dean, C. R. et al. Nat. Nanotechnol. 2010 , 5 , 722 - 6 ; Wang, H. et al. Electron Device Lett. 2011 , 32 , 1209 - 1211 ; Sanchez-Yamagishi, J. et al. Phys. Rev. Lett. 2012 , 108 , 1 - 5 .) We study the presence of organic contaminants introduced by standard lithography and substrate transfer processing on h-BN films exfoliated on silicon oxide substrates. Exposure to photoresist processing adds a large broad luminescence peak to the Raman spectrum of the h-BN flake. This signal persists through typical furnace annealing recipes (Ar/H(2)). A recipe that successfully removes organic contaminants and results in clean h-BN flakes involves treatment in Ar/O(2) at 500 °C.
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Affiliation(s)
- Andrei G F Garcia
- Department of Applied Physics, McCullough Building, Stanford University, Stanford, California 94305-4045, United States.
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63
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Graphene and boron nitride lateral heterostructures for atomically thin circuitry. Nature 2012; 488:627-32. [DOI: 10.1038/nature11408] [Citation(s) in RCA: 681] [Impact Index Per Article: 56.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/16/2012] [Accepted: 07/10/2012] [Indexed: 12/22/2022]
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64
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Chae DH, Zhang D, Huang X, von Klitzing K. Electronic transport in two stacked graphene monolayers. NANO LETTERS 2012; 12:3905-3908. [PMID: 22823447 DOI: 10.1021/nl300569m] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2023]
Abstract
We report on interlayer and lateral electronic transport measurements in two stacked graphene monolayers which have separate electrical contacts. The current-voltage characteristic across the two layers shows linear Ohmic behavior at zero magnetic field. At high magnetic fields, sequences of quantum Hall plateaus of the overlap region with filling factors 4, 8, and 12 are observed which can be explained by equilibration of the edge channel potentials of the individual graphene layers. An anomaly is observed at total filling factors ±2 in the overlap region. The I-V characteristic for interlayer transport turns nonlinear, and the Hall signal vanishes, indicating a magnetic field induced electrical decoupling of the two graphene layers.
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Affiliation(s)
- Dong-Hun Chae
- Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany.
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