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Atomically thin layers of B–N–C–O with tunable composition

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Science Advances  31 Jul 2015:
Vol. 1, no. 6, e1500094
DOI: 10.1126/sciadv.1500094
  • Fig. 1 Atomically thin 2D-BNCO sheets.

    (A) A bright-field TEM image of a typical BNCO sample transferred onto a lacey carbon-coated TEM grid. (B) Typical SAED pattern from various locations demonstrating honeycomb-like lattice structure in monolayer or rotated bilayer forms. (C and D) SEM images of a BNCO sample as grown on Cu substrate. The samples shown here were grown with different oxygen flow rates in the CVD chamber. See the Supplementary Materials, section S1, for details of sample synthesis. (E) Percent surface coverage of domains and graphitic areas as extracted from the analysis of SEM images, as a function of O2 flow rate into the chamber. (F) Typical XPS survey scan from BNCO-12 sample. The B1s and N1s peaks (regions marked by arrows) are only visible in high-resolution detailed scans (see the Supplementary Materials, section S4). (G) Relative occurrence of oxygen atoms as a function of the fractional coverage of the samples by domains, providing direct evidence of the presence of oxygen in the domain areas.

  • Fig. 2 Chemical composition and structure of the 2D-BNCO samples through XPS studies.

    (A) Typical high-resolution O1s XPS spectrum. The oxygen atoms were mostly found to attach with N-, B-, and C-based moieties that could include B and N (see text). Direct bond formation with Cu (the growth substrate) was almost negligible in all samples. a.u., arbitrary unit. (B) Typical high-resolution C1s spectrum, which could be resolved into a number of peaks associated with known bonds –sp2 (graphene-like) carbon, as well as bonds with B, N, and O entities. (C and D) Specific bonds whose relative occurrences were found to (C) grow or (D) decrease/remain negligible as the oxygen flow rate was increased during CVD growth of samples. Specifically, some of the flow rate dependences seen in (C) appear to have a reasonable similarity to the flow rate dependence of domain area coverage as shown in Fig. 1E, implying that these bonds were instrumental in the formation of these domains. Similarly, the bonds that decreased or remained negligibly low as seen in (D) could be less associated with the 2D-BNCO domains.

  • Fig. 3 Structural, morphological, and Raman analyses of the 2D-BNCO samples.

    (A and B) SEM (A) and AFM (B) images of the same location of a 2D-BNCO sample mechanically transferred onto a SiO2 (300 nm)/Si substrate. The 2D-BNCO domains could not be “seen” in the SEM images after they were transferred to the insulating silica substrate. AFM images confirmed that the “darker” contrast seen in (A) is due to the presence of a second layer. The thinner layer was found to have a height of 0.6 nm with respect to the SiO2 substrate, indicating a possible monolayer region. (C) Typical Raman spectra obtained from the monolayer and bilayer regions showing peaks that could be identified with the G, G′, and D peaks of graphene. Decomposed G′ peaks could be fitted with one or four peaks confirming their mono- and bilayer natures, respectively. (D) Typical composition of the Raman peaks about 1355 cm−1 showing the presence of the D peak, along with h-BN and B–C bonds only in the bilayer area. (E to G) Optical image (E), G peak (F), and h-BN peak (G) Raman maps of a 2D-BNCO domain and the surrounding 2D-BNC matrix from sample BNCO-3 on SiO2 (300 nm)/Si substrate. (H) Percentage occurrence of regions containing h-BN (as obtained from Raman spectra from 30 to 35 randomly selected regions) and h-BN–free graphene-rich regions, as a function of increasing oxygen content.

  • Fig. 4 DFT-computed electronic properties of representative atomic configurations of 2D-BNCO nano patches.

    (A) (a to c) Optimized structures of distinct stable configurations of model BNCO structures including the spatial distribution of the differential spin density Δρ = ρ − ρ. Regions of |Δρ| > 0 were found to be associated with the local BO2 motifs, which leads to non-zero magnetic moments. (B) Side views showing the out-of-plane displacement of B atoms closest to O atoms. (C) DOS of the BNCO units. The Fermi level is aligned with the top of the valence band. The lines in red and blue correspond to opposite spin polarizations. (D) A table of bond lengths, electronic band gap, and average magnetic moment on the B atoms closest to the O atoms.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/6/e1500094/DC1

    Text

    Fig. S1. CVD setup for BNCO growth.

    Fig. S2. Morphology of growth substrate (copper) with and without boron and nitrogen source.

    Fig. S3. Morphological characterizations of sample and substrate after growth.

    Fig. S4. XPS analysis of the chemical composition of the 2D-BNCO samples.

    Fig. S5. 2D-BNCO surface coverage analysis.

    Fig. S6. Detailed Raman spectroscopic analysis.

    Fig. S7. DFT-computed electronic properties of metastable structures of C38(BN)3B2O4 and C35(BN)3B3O6 nano patches.

    Fig. S8. Source-drain current as a function of back gate voltage measured from BNCO-6 sample.

    Fig. S9. Absorption spectroscopy studies.

    Fig. S10. Variation of resistance with temperature in a typical BNCO sample, demonstrating semiconducting behavior with a negative temperature coefficient of resistance (TCR).

    Fig. S11. Temperature dependence of resistance.

    Fig. S12. Phototransmittance characteristics of 2D-BNCO.

    Table S1. Calculated TCR values for BNCO samples.

    References (3638)

  • Supplementary Materials

    This PDF file includes:

    • Text
    • Fig. S1. CVD setup for BNCO growth.
    • Fig. S2. Morphology of growth substrate (copper) with and without boron and nitrogen source.
    • Fig. S3. Morphological characterizations of sample and substrate after growth.
    • Fig. S4. XPS analysis of the chemical composition of the 2D-BNCO samples.
    • Fig. S5. 2D-BNCO surface coverage analysis.
    • Fig. S6. Detailed Raman spectroscopic analysis.
    • Fig. S7. DFT-computed electronic properties of metastable structures of C38(BN)3B2O4 and C35(BN)3B3O6 nano patches.
    • Fig. S8. Source-drain current as a function of back gate voltage measured from BNCO-6 sample.
    • Fig. S9. Absorption spectroscopy studies.
    • Fig. S10. Variation of resistance with temperature in a typical BNCO sample, demonstrating semiconducting behavior with a negative temperature coefficient of resistance (TCR).
    • Fig. S11. Temperature dependence of resistance.
    • Fig. S12. Phototransmittance characteristics of 2D-BNCO.
    • Table S1. Calculated TCR values for BNCO samples.
    • References (36−38)

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