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Subatomic deformation driven by vertical piezoelectricity from CdS ultrathin films

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Science Advances  01 Jul 2016:
Vol. 2, no. 7, e1600209
DOI: 10.1126/sciadv.1600209
  • Fig. 1 Schematic illustration of local characterization of in-plane piezoelectricity and vertical piezoelectricity.

    In-plane piezoelectricity (piezo) (d11, d22) of ultrathin materials is the planar electromechanical couple behavior, where the applied stress and produced piezoelectric potential are located at the in-plane of exposed lattice plane. Vertical piezoelectricity (d33) focus on electromechanical interaction occurred in the vertical axis, which is perpendicular to the surface of materials. The high-precision deformation actuator can be implemented using accurate positioning of the materials surface by vertical inverse piezoelectricity.

  • Fig. 2 CdS thin films with three different shapes.

    (A to E, G, and H) Typical optical images (A, D, and G) and AFM images (B, C, E, and H) of CdS thin films with uniform disc-like structure, Janus structure, and the structure of rounded microparticle at its center. The AFM line scan in (C) indicates that the thickness of the CdS ultrathin film is ~3.2 nm, corresponding to approximately five lattices of CdS. The AFM line scans in (E) and (H) show that the Janus structure has two kinds of thickness (2.1 and 3.7 nm) at one thin film, and the structure of the microparticle is grown on the thin film with a height (h) of ~1.4 μm and radius of ~5 μm. (F and I) Corresponding structural diagram.

  • Fig. 3 Spectroscopic characterization of CdS thin film.

    (A) Energy (E) band structure in the vicinity of the Γ-point of the Brillouin zone, showing the photon emission process. (B and C) PL spectrum of CdS thin film from points i and ii marked in (C) with plus signs, showing strong band edge emission (506 nm) of CdS ultrathin film and defect-related emission (595 nm). (C) Optical image of CdS thin film with a rounded microparticle at its center. a.u., arbitrary units. (D and E) PL mapping at an emission of 514 nm with a different scale bar, demonstrating high uniformity and homogeneity of CdS thin films at the outside area. (F) PL mapping at the 595-nm emission, indicating that the defect-related emission only occurs at the thicker CdS microparticle.

  • Fig. 4 Noncontact SKFM and standard contact PFM investigation for CdS thin film.

    (A and B) Schematic illustration of SKFM (A) and PFM (B) measurements. (C) Band diagram of tip and sample when they are electrically separated (top graph) and electrically contacted (bottom graph). d, distance; VL, vacuum levels; q, electronic charge; Vc, contact potential difference. (D) Optical image of CdS thin films. (E and F) Topography (E) and phase (F) images observed by SKFM mode for the single CdS thin film marked in (D). (G to I) Corresponding potential mappings with tip voltages of 3, 6, and 9 V, respectively. Insets show histograms of the surface potential distributions. The CdS ultrathin film has a higher positive voltage (~0.9 V) than the substrate, demonstrating that a large amount of charges are accumulated at a CdS thin film after contact PFM scanning. (J) Amplitude images observed by contact PFM technology with tip voltages from 1 to 6 V, showing remarkable inverse piezoelectricity. (K) Average amplitude variations versus applied voltages calculated from (J). Error bars indicate 1 SD. Scale bars, 2 μm (E to J). The linearly fitted line shows that the measured piezoelectric coefficient deff is ~16.4 pm·V−1, whereas the vertical piezoelectric coefficient d33 is ~32.8 pm·V−1.

  • Fig. 5 DART-PFM technology for characterization of CdS thin film.

    (A to D) Topography (A), resonance frequency (B), phase (C), and amplitude (D) images for a single CdS thin film, showing remarkable resonance frequency variations. (E to H) Top-view 3D topography (E), resonance frequency (F), phase (G), and amplitude (H) images for marked area in (A). (I) Overlap image of topography and resonance frequency. Marked areas present the notable change of resonance frequency. (J and K) Height profiles measured by line scans along lines 1 and 2 in (E), and shadow areas (I, III, and V) corresponding to marked areas in (I).

  • Fig. 6 Simulation of vertical piezoelectricity and subatomic deformation actuator.

    (A) Three-dimensional image of potential drop on CdS film. (B) Scanning electron microscopy image of a conductive tip for PFM characterization. (C and D) Bottom and side views of stress distribution on CdS film. (E to G) Simulation for subatomic deformation actuator. Different potentials were applied to surface deformation curves (E), mappings (G), and vertical deformation (F) of CdS thin films.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/7/e1600209/DC1

    Supplementary Materials and Methods

    fig. S1. Growth of CdS thin films.

    fig. S2. Statistical analysis for the size of CdS thin films.

    fig. S3. Raman spectra of as-obtained CdS sample.

    fig. S4. PL spectra and PL mapping from the CdS thin films.

    fig. S5. SKFM characterization of CdS thin film.

    fig. S6. Topography and phase images of PFM characterization of CdS thin film when different tip voltages (1 to 6 V) were applied.

    fig. S7. The average amplitude change can be established from eight different areas of CdS sample.

    fig. S8. Illustration of weak indentation and strong indentation.

    fig. S9. Thickness versus piezoelectric coefficient distribution from previous literature results.

    fig. S10. Hall device of CdS thin film.

    fig. S11. Electrical characterization of CdS thin film.

    fig. S12. Schematic illustration of experimental setup for DART-PFM.

    fig. S13. DART-PFM characterization for the boundary of CdS thin film.

    table S1. Summary of piezoelectric coefficient from different materials.

    References (4050)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials and Methods
    • fig. S1. Growth of CdS thin films.
    • fig. S2. Statistical analysis for the size of CdS thin films.
    • fig. S3. Raman spectra of as-obtained CdS sample.
    • fig. S4. PL spectra and PL mapping from the CdS thin films.
    • fig. S5. SKFM characterization of CdS thin film.
    • fig. S6. Topography and phase images of PFM characterization of CdS thin film when different tip voltages (1 to 6 V) were applied.
    • fig. S7. The average amplitude change can be established from eight different areas of CdS sample.
    • fig. S8. Illustration of weak indentation and strong indentation.
    • fig. S9. Thickness versus piezoelectric coefficient distribution from previous literature results.
    • fig. S10. Hall device of CdS thin film.
    • fig. S11. Electrical characterization of CdS thin film.
    • fig. S12. Schematic illustration of experimental setup for DART-PFM.
    • fig. S13. DART-PFM characterization for the boundary of CdS thin film.
    • able S1. Summary of piezoelectric coefficient from different materials.
    • References (4050)

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