Research ArticleMATERIALS SCIENCE

Cryo-mediated exfoliation and fracturing of layered materials into 2D quantum dots

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Science Advances  15 Dec 2017:
Vol. 3, no. 12, e1701500
DOI: 10.1126/sciadv.1701500
  • Fig. 1 Procedure of cryo-exfoliation for preparing pristine 2D QDs (represented by MoS2).

    (A) Schematic illustration of the steps for preparing 2D QDs. The outlined step in a red rectangle is the cryo-pretreatment. (B to D) AFM results showing the evolution of nanosheets to QDs, (B) ultrathin MoS2 nanosheets at the delamination stage, (C) small MoS2 nanosheets with tiny holes (the red arrows point to the holes.), and (D) pure MoS2 2D QDs after vacuum filtration.

  • Fig. 2 Morphological and crystalline structure of the 2D QDs.

    (A to C) AFM images and their statistic thickness distribution (insets). (A) GQDs, (B) MoS2 QDs, and (C) WS2 QDs. (D to F) Low-resolution TEM images of (D) GQDs, (E) MoS2 QDs, and (F) WS2 QDs, respectively (insets showing the lateral size distribution). (G to I) High-resolution TEM images of (G) GQDs, (H) MoS2 QDs, and (I) WS2 QDs, respectively.

  • Fig. 3 Cryo-exfoliation applied to fabricate 2D QDs from different bulk-layered material powders in various solvents.

    AFM images of QDs for (A) h-BN, (B) TiS2, (C) NbS2, (D) Bi2Se3, (E) MoTe2, and (F) Sb2Te3. (G) Statistic average thickness of MoS2 QDs as a function of cryo-pretreatment duration (the blue curve, with fixed 1-hour ultrasonication time) and sonication time (the red curve, with fixed 1-hour cryo-pretreatment time). The average thickness of MoS2 QDs as-exfoliated in different solvents with varied polar/dispersive ratio (H) and surface tension (I).

  • Fig. 4 Optical properties of the pristine 2D QDs.

    (A) PL spectra excited at different wavelengths of GQDs. Insets show digital images of the corresponding QDs under day and UV light. a.u., arbitrary units. (B) Pump-probe transient absorption spectra of GQDs and monolayer graphene. (C) The QY and (D) collective optical bandgap for selected 2D QDs. The inset in (C) shows epoxy filled with MoS2 QDs exhibiting blue fluorescence under UV irradiation.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/12/e1701500/DC1

    fig. S1. The AFM images of the as-exfoliated MoS2 nanosheets with only sonication-assisted LPE and without cryo-pretreatment.

    fig. S2. XRD patterns of the as-obtained QDs and raw powder of layered materials.

    fig. S3. XPS spectra of the as-obtained QDs.

    fig. S4. XPS spectra of the corresponding raw bulk powders.

    fig. S5. UV-Vis absorbance spectra of MoS2 samples with a series of cycles of cryo-pretreatment and liquid exfoliation treatment.

    fig. S6. Digital image of MoS2 supernatants without vacuum filtration with series cycles of cryo-pretreatment and liquid exfoliation.

    fig. S7. AFM images of MoS2 QDs produced in different solvents.

    fig. S8. The thickness statistics of the as-exfoliated GQDs with different cryo-pretreatment duration and solvents.

    fig. S9. Photoluminescence spectra excited at different wavelengths for MoS2 QDs and WS2 QDs.

    fig. S10. Tauc plots used to determine the optical bandgaps of various 2D QDs derived from UV-vis spectra.

    fig. S11. XRD pattern of the as-synthesized MoTe2 powders.

    fig. S12. The AFM image of the MoS2 sample obtained in a process with 1-hour gap between the cryo-pretreatment and the ultrasonication.

    fig. S13. ICP-MS results of the WS2 QDs concentration with different sonication time at a fixed cryo-pretreatment duration of 1 hour.

    fig. S14. Thickness statistics of the as-exfoliated MoS2 QDs and the background thickness signal of the new mica substrate.

    table S1. Surface tension and polar/dispersive ratio of the solvents adopted for the cryo-exfoliation of MoS2.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. The AFM images of the as-exfoliated MoS2 nanosheets with only sonication-assisted LPE and without cryo-pretreatment.
    • fig. S2. XRD patterns of the as-obtained QDs and raw powder of layered materials.
    • fig. S3. XPS spectra of the as-obtained QDs.
    • fig. S4. XPS spectra of the corresponding raw bulk powders.
    • fig. S5. UV-Vis absorbance spectra of MoS2 samples with a series of cycles of cryo-pretreatment and liquid exfoliation treatment.
    • fig. S6. Digital image of MoS2 supernatants without vacuum filtration with series cycles of cryo-pretreatment and liquid exfoliation.
    • fig. S7. AFM images of MoS2 QDs produced in different solvents.
    • fig. S8. The thickness statistics of the as-exfoliated GQDs with different cryp-pretreatment duration and solvents.
    • fig. S9. Photoluminescence spectra excited at different wavelengths for MoS2 QDs and WS2 QDs.
    • fig. S10. Tauc plots used to determine the optical bandgaps of various 2D QDs derived from UV-vis spectra.
    • fig. S11. XRD pattern of the as-synthesized MoTe2 powders.
    • fig. S12. The AFM image of the MoS2 sample obtained in a process with 1-hour gap between the cryo-pretreatment and the ultrasonication.
    • fig. S13. ICP-MS results of the WS2 QDs concentration with different sonication time at a fixed cryo-pretreatment duration of 1 hour.
    • fig. S14. Thickness statistics of the as-exfoliated MoS2 QDs and the background thickness signal of the new mica substrate.
    • table S1. Surface tension and polar/dispersive ratio of the solvents adopted for the cryo-exfoliation of MoS2.

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