Research ArticlePHYSICS

Evidence for a quantum spin Hall phase in graphene decorated with Bi2Te3 nanoparticles

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Science Advances  09 Nov 2018:
Vol. 4, no. 11, eaau6915
DOI: 10.1126/sciadv.aau6915
  • Fig. 1 Sample fabrication and characterization.

    (A to C) Atomic force microscopy (AFM) images of graphene Hall bar devices used for resistance measurements of Fig. 2. (B) Expansion of the dashed rectangle in (A). For (C), the channel and branch widths are ~1 μm. (D) Optical microscopy image of devices in (A) and (C), which are formed at neighboring position on the same segment of CVD-grown graphene (section S1). (E) AFM image of graphene decorated with Bi2Te3 nanoparticles at D ~ 4/1002 nm2 (~3% coverage ratio); the image is taken at the center of the six branches of (A). (F to H) XPS spectra of the samples.

  • Fig. 2 Four-probe resistances versus back-gate voltage (Vbg) measured on the samples shown in Fig. 1.

    (A to E) Current flows between contacts indicated by squares, and voltage is measured across circled contacts; no contact resistances are subtracted. In (A), the green line corresponds to undecorated graphene. All other data correspond to graphene with nanoparticles at a coverage ratio of ~3%. The green dashed line in each panel represents quantized resistances predicted for helical edge transport (RQ is the resistance quantum).

  • Fig. 3 Reconfirmation of QSH phase.

    (A) STS spectra for a sample around Vbg for R peak, recorded at bulk locations near two different nanoparticles (~50 to 80 nm from particles), showing the maximum (~20 meV; blue line) and minimum (~5 meV; black line) gaps, and at an edge point (~50 nm from edges and ~200 nm away from nanoparticles), revealing the gap disappearance (red line). a.u., arbitrary units. (B) Perpendicular magnetic field (B) dependence of conductance (G) corresponding to the inverse of the R peak values of Fig. 2 (A, blue symbol, and D, red symbol) and two off–R peak values (pink and green symbols in Fig. 2, A and D, respectively). (C) Magnetization curve for graphene decorated with Bi2Te3 nanoparticles at a coverage as large as ~10%, on application of B. (D) Zero-B temperature dependence of conductance (in Alenius plot format) corresponding to the inverse of the R peak values shown in Fig. 2 (A, blue symbol, and D, red symbol); the dashed lines serve as a guide to the eye.

  • Fig. 4 Theoretical calculations.

    (A and B) Side and top views of the atomic structure and charge-density difference of Bi2Te3/graphene. Red and blue colors indicate charge depletion and accumulation, respectively. (C and D) Band structure of Bi2Te3/graphene. (E) n-field configuration with red solid, blue hollow circles, and blank boxes denoting n = −1, 1, and 0, respectively. Summing the n-fields over half of the torus yields a nontrivial Z2 invariant.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/11/eaau6915/DC1

    Section S1. Formation of graphene to small Hall bar patterns with branches

    Section S2. Nanoneedle decoration of Bi2Te3 nanoparticles

    Section S3. DFT calculation methods

    Fig. S1. Designs of graphene Hall bars.

    Fig. S2. DFT calculation results for Bi10Te15/graphene using an 8 × 8 supercell.

    Fig. S3. Same as fig. S2, but for Bi11Te15/graphene in a 7 × 7 supercell.

    Fig. S4. Spatial distributions of states at the tips of the Dirac cones for Bi10Te15/graphene in a 7 × 7 supercell.

    Movie S1. How to decorate Bi2Te3 nanoparticles with very low amount by a nanoneedle

    References (3238)

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Formation of graphene to small Hall bar patterns with branches
    • Section S2. Nanoneedle decoration of Bi2Te3 nanoparticles
    • Section S3. DFT calculation methods
    • Fig. S1. Designs of graphene Hall bars.
    • Fig. S2. DFT calculation results for Bi10Te15/graphene using an 8 × 8 supercell.
    • Fig. S3. Same as fig. S2, but for Bi11Te15/graphene in a 7 × 7 supercell.
    • Fig. S4. Spatial distributions of states at the tips of the Dirac cones for Bi10Te15/graphene in a 7 × 7 supercell.
    • Legend for movie S1
    • References (3238)

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.MOV format). How to decorate Bi2Te3 nanoparticles with very low amount by a nanoneedle.

    Files in this Data Supplement:

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