Research ArticlePHYSICS

Distinct conducting layer edge states in two-dimensional (2D) halide perovskite

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Science Advances  12 Jul 2019:
Vol. 5, no. 7, eaau3241
DOI: 10.1126/sciadv.aau3241
  • Fig. 1 Direct observations of the layer edge current between adjacent insulating terraces.

    (A) Schematic of the 2D (C4H9NH3)2PbI4 pero-MQW structure with a QW layer thickness of 6.3 Å and a barrier layer thickness of 7.6 Å. (B) Schematic layout for c-AFM measurement in contact mode, where the QW plane is parallel to the metal substrate. (C and D) 3D image of (C) topographical and (D) current map of the 2D pero-MQW sample. (E and F) Corresponding (E) height profile (z-profile) and (F) comparison of first derivative of z-profile and current profile, along a longitudinal line in the x-axis direction (designated in fig. S1, C and D).

  • Fig. 2 Layer edge current along the contour direction.

    Local area analysis near the layer edge region along the edge contour (dashed arrow) in the X-Y plane: (A) topographical and (B) current images showing the starting position. (C) 3D topographical and current models of the analytical local unit marked in (A) and (B), and the schematic showing the ECIR that is exclusively responsible for conduction during each contact between the probe and the sample. (D) Corresponding current, z-profile, and current integration (along the in-plane x direction) curves at the starting position. (E) Edge height, peak current, and integrated current across the layer edges located at different positions along the edge contour direction [dashed arrow in (A)], and (F) charge carrier density distribution along the edge contour direction, summarized from 26 analytical units in the contour direction.

  • Fig. 3 Scan rate effects on the layer edge current.

    (A) Topographical and current images under different scan rates. (B) Average z-profile and (C) the corresponding average current profile extracted from the analytical unit, along the arrow direction in (A), under different scan rates. (D and E) Dependence of the (D) current integration and (E) charge carrier density on the scan rate at layer edge positions (i.e., positions B and C), and at the terrace region (position A).

  • Fig. 4 Layer edge current polarity study.

    (A) Topographical and current images under the trace (from left to right) and retrace (from right to left) scan directions. (B) Average z-profile and the corresponding trace and retrace current profile, extracted from the analytical unit, along the arrow direction in (A). (C) Comparison of charge carrier density obtained from both trace and retrace scanning, at two different layer edge positions of positions A and C (various sample biases were applied to the sample with the probe being grounded, which shows negligible influence on the carrier density due to the insulating feature of the 2D pero-MQWs in the out-of-plane direction, as detailed in note S3).

  • Fig. 5 Illumination influence on the layer edge current.

    (A) Topographical and current images under different light intensities. (B) Average z-profile and (C) the corresponding average current profile extracted from the analytical unit, along the arrow direction in (A), under different light intensities. (D and E) Dependence of (D) the current and (E) charge carrier density on the illumination intensities at layer edge position (i.e., positions A and B), and at the terrace region (position C).

  • Fig. 6 PL study showing the lower energetic states at the layer edges.

    Confocal PL mapping measurements on (C4H9NH3)2(CH3NH3)3Pb4I13 (n = 4) pero-MQWs: the PL landscape probed at (A) 1.89 eV and (B) 1.66 eV, and the (C) PL spectra of different topographical positions. Confocal PL mapping measurements on (C4H9NH3)2(CH3NH3)2Pb3I10 (n = 3) pero-MQWs: the PL landscape probed at (D) 1.95 eV and (E) 1.66 eV, and the (F) PL spectra of different topographical positions. Proposed layer ES mechanism: (G) schematics of MQW structure and (H) correlated electronic band structure. The electron-hole pairs in the bulk terrace region are tightly bonded, while at the layer edges, additional ES electrons reside, which are responsible for the detected current by the c-AFM measurement. The ES PL peak at 1.66 eV in n = 3 and 4 samples suggests that the ES has a lower optical bandgap than that in the bulk terrace region. Eg, energy bandgap. a.u., arbitrary units.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/7/eaau3241/DC1

    Fig. S1. 2D pero-MQW structural and optical properties and direction observations of the layer edge current.

    Fig. S2. Layer edge current development along the contour direction.

    Fig. S3. Layer edge charge distribution along the contour direction.

    Fig. S4. Scan rate effects on layer edge current.

    Fig. S5. Layer edge current polarity study.

    Fig. S6. Repeatability of the layer edge current.

    Fig. S7. SEM and confocal Raman mapping study of layer ES.

    Fig. S8. Optical image of pero-MQWs.

    Fig. S9. Hertz method analysis on the tip contact with samples.

    Fig. S10. Possible mechanisms for the current polarity conversion at the layer edges.

    Note S1. Estimation of ECIR and charge carrier density.

    Note S2. Conjecture of the current polarity conversion at the layer edges.

    Note S3. Sample bias influence on layer edge currents.

    Note S4. Large exciton binding energy in 2D pero-MQWs.

    Note S5. Surface charge effect exclusion study by SEM and confocal Raman mapping measurements.

    Appendix S1. The c-AFM edge current on samples of (C4H9NH3)2(CH3NH3)n−1PbnI3n+1 (n = 2, 3, 4, and ∞).

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. 2D pero-MQW structural and optical properties and direction observations of the layer edge current.
    • Fig. S2. Layer edge current development along the contour direction.
    • Fig. S3. Layer edge charge distribution along the contour direction.
    • Fig. S4. Scan rate effects on layer edge current.
    • Fig. S5. Layer edge current polarity study.
    • Fig. S6. Repeatability of the layer edge current.
    • Fig. S7. SEM and confocal Raman mapping study of layer ES.
    • Fig. S8. Optical image of pero-MQWs.
    • Fig. S9. Hertz method analysis on the tip contact with samples.
    • Fig. S10. Possible mechanisms for the current polarity conversion at the layer edges.
    • Note S1. Estimation of ECIR and charge carrier density.
    • Note S2. Conjecture of the current polarity conversion at the layer edges.
    • Note S3. Sample bias influence on layer edge currents.
    • Note S4. Large exciton binding energy in 2D pero-MQWs.
    • Note S5. Surface charge effect exclusion study by SEM and confocal Raman mapping measurements.
    • Appendix S1. The c-AFM edge current on samples of (C4H9NH3)2(CH3NH3)n−1PbnI3n+1 (n = 2, 3, 4, and ∞).

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