Research ArticleMATERIALS SCIENCE

Dynamic deformability of individual PbSe nanocrystals during superlattice phase transitions

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Science Advances  07 Jun 2019:
Vol. 5, no. 6, eaaw5623
DOI: 10.1126/sciadv.aaw5623
  • Fig. 1 Experimental setup and superlattice phase transition of PbSe nanocrystals.

    (A) Schematic procedures for preparing a carbon grid liquid cell for in situ TEM. (B) Sequence of in situ TEM images showing the oriented attachment of six PbSe nanocrystals during the phase transition from 2D hexagonal monolayer to 1D nanocrystal chain. (C) High-resolution TEM image of the nanocrystal chains in vacuum. Orange rectangles indicate nanocrystals elongated along the internanocrystal axis. Scale bars, 5 nm.

  • Fig. 2 Atomistic structure of the nanocrystals before and after oriented attachment.

    (A) HAADF-STEM images of three different nanocrystals before oriented attachment. Zone axes are labeled. (B) Corresponding atomic models of the three nanocrystals. Labels indicate the number of Pb layers along the <100> (orange), <110> (blue), and <111> (green) directions. (C) Histograms of the number of Pb layers along these three directions as obtained from analysis of 32 nanocrystals. (D) Reconstructed 3D model of the average nanocrystal before oriented attachment. Blue spheres, Pb; pink spheres, Se. Facets are labeled. (E) Representative HAADF-STEM image of a nanocrystal chain formed by oriented attachment. Vertical lines denote the nanocrystal boundaries, and labels indicate the number of Pb layers along the nanocrystal length (orange) and width (blue). (F) Histogram of nanocrystal length (orange) and width (blue) for 34 nanocrystals in several chains. Scale bars, 2 nm.

  • Fig. 3 Dynamic deformability of individual PbSe nanocrystals.

    (A) Sequence of in situ TEM images showing the elongation of nanocrystal 8 during the oriented attachment with nanocrystals 7 and 9. White lines in the third image indicate the matched {200} lattices of two attaching nanocrystals with a d-spacing of 3.1 Å. (B) Sequence of TEM images and reconstructed models showing reversible deformation of nanocrystal 11 during moving apart from the neighbor nanocrystal 12. (C) Superimposed FFT images of 11 in each movie frame during the reversible deformation. The zoomed-in region shows the overlap of the diffraction spot in each frame with a consistent {200} d-spacing of 3.1 Å. (D) Time-dependent changes of the length of 8 along the internanocrystal direction and the center-to-center distances between 7, 8, and 9. Green arrow marks the time that three nanocrystals attached. (E) Reversible changes of the long and short axial length of 11. (F) Time-dependent changes of center-to-center distance and gap distance between 11 and 12. (G) Two deformation modes corresponding to two different motions, i.e., the reversible deformation when nanocrystals move apart and the retained deformation when they move toward and attach. Scale bars, 5 nm.

  • Fig. 4 Reduced deformation during the superlattice transformation in EG.

    (A) Sequence of in situ TEM images showing reduced deformation of nanocrystals during the phase transition from oleate-capped hexagonal to epitaxially fused square superlattice. (B) Large-area TEM image observed in situ showing the uniformity of nanocrystals in the square superlattice. (C) STEM image of a dried square superlattice showing the necks between nanocrystals. (D) Ratio of the long and short axes of four representative nanocrystals (14 to 17) changes much smaller in EG than that of nanocrystal 11 in EDA solution. Scale bar, 5 nm (A) and 20 nm (B and C).

  • Fig. 5 Molecular dynamics simulation on the deformability of polar nanocrystals.

    Atom color code: blue, Pb; pink, Se. (A) Snapshots of the trajectory of single-dipolar nanocrystal at 300 K, starting with a dipole moment caused by fast ligand removal. Polar nanocrystal monomer exhibits the tendency to become symmetrized through surface atom diffusion; nevertheless, this does not lead to formation of extra lattice fringes along <100> directions. (B) Snapshots of the trajectory of two dipolar nanocrystals initially aligned along the dipole direction with a gap distance of 2 nm at 300 K. Translation and rotation of the two nanocrystals centers are not restricted, allowing them to move toward each other and get attached, thus to simulate the moving-toward motion in Fig. 3A. Although the two nanocrystals move toward each other, their deformation happens before they touch each other, and the resulting edge-to-edge distance is 12.3 nm (41 layers) larger than the distance shown by two nanocrystals rigidly touch with each other (12.0 nm, 40 layers). (C) Snapshots of the trajectory of three dipolar nanocrystals aligned with the same dipole direction with gap distances of 2 nm at 300 K. Translation and rotation of the two outside nanocrystal centers are eliminated, whereas the middle one is free to translate and rotate, imitating the situation of the boundary of two superlattice matrices. The reversible elongation phenomena for the center nanocrystal resembles that in Fig. 3B.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/6/eaaw5623/DC1

    Electron beam effects during in situ TEM characterization

    Internanocrystal interaction responsible for nanocrystal deformation

    Kinetics of superlattice phase transitions in EG and EDA/EG

    Perspective and open questions

    Fig. S1. Schematic illustration of the liquid cell fabrication and real-time in situ TEM imaging of nanocrystal superlattice phase transitions.

    Fig. S2. STEM images of randomly oriented nanocrystals in initial unconnected hexagonal superlattice.

    Fig. S3. Moving-apart motion of neighboring nanocrystals during superlattice phase transitions.

    Fig. S4. Reversible deformation of nanocrystals marked by 7 and 10.

    Fig. S5. Retained deformation of nanocrystals during the oriented attachment in EDA solution.

    Fig. S6. Reduced deformation of nanocrystals during the superlattice transformation in EG.

    Fig. S7. Different pathways of superlattice transformations in EG and EDA/EG.

    Fig. S8. EDS analyses of superlattices before and after phase transitions.

    Fig. S9. Schematic of EDA-induced ligand removal.

    Fig. S10. Molecular dynamics simulation on the deformability of nondipolar PbSe nanocrystals.

    Movie S1. In situ liquid-phase TEM movie showing the phase transition of PbSe nanocrystals in EDA solution.

    Movie S2. Reconstructed 3D atomistic model of PbSe nanocrystals in unconnected hexagonal superlattices.

    Movie S3. In situ liquid-phase TEM movie showing the dynamic deformability of nanocrystals in EDA solution.

    Movie S4. In situ liquid-phase TEM movie showing the reversible deformation of nanocrystals in EDA solution.

    Movie S5. In situ liquid-phase TEM movie showing the transformation from a hexagonal superlattice to two nanocrystal chains in EDA solution.

    Movie S6. In situ liquid-phase TEM movie showing retained deformation of nanocrystals during the oriented attachment in EDA solution.

    Movie S7. In situ liquid-phase TEM movie showing the lattice alignment of neighboring nanocrystals during the oriented attachment in EDA solution.

    Movie S8. In situ liquid-phase TEM movie showing the phase transition from hexagonal to square superlattices in pure EG.

    Movie S9. In situ liquid-phase TEM movie showing reduced deformation of nanocrystals during the superlattice transformation in EG.

    Movie S10. In situ heating TEM movie showing no perceptible deformation of individual nanocrystals without introducing any EDA or EG solution.

  • Supplementary Materials

    The PDF file includes:

    • Electron beam effects during in situ TEM characterization
    • Internanocrystal interaction responsible for nanocrystal deformation
    • Kinetics of superlattice phase transitions in EG and EDA/EG
    • Perspective and open questions
    • Fig. S1. Schematic illustration of the liquid cell fabrication and real-time in situ TEM imaging of nanocrystal superlattice phase transitions.
    • Fig. S2. STEM images of randomly oriented nanocrystals in initial unconnected hexagonal superlattice.
    • Fig. S3. Moving-apart motion of neighboring nanocrystals during superlattice phase transitions.
    • Fig. S4. Reversible deformation of nanocrystals marked by 7 and 10.
    • Fig. S5. Retained deformation of nanocrystals during the oriented attachment in EDA solution.
    • Fig. S6. Reduced deformation of nanocrystals during the superlattice transformation in EG.
    • Fig. S7. Different pathways of superlattice transformations in EG and EDA/EG.
    • Fig. S8. EDS analyses of superlattices before and after phase transitions.
    • Fig. S9. Schematic of EDA-induced ligand removal.
    • Fig. S10. Molecular dynamics simulation on the deformability of nondipolar PbSe nanocrystals.
    • Legends for movies S1 to S10

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mov format). In situ liquid-phase TEM movie showing the phase transition of PbSe nanocrystals in EDA solution.
    • Movie S2 (.mov format). Reconstructed 3D atomistic model of PbSe nanocrystals in unconnected hexagonal superlattices.
    • Movie S3 (.mov format). In situ liquid-phase TEM movie showing the dynamic deformability of nanocrystals in EDA solution.
    • Movie S4 (.mov format). In situ liquid-phase TEM movie showing the reversible deformation of nanocrystals in EDA solution.
    • Movie S5 (.mov format). In situ liquid-phase TEM movie showing the transformation from a hexagonal superlattice to two nanocrystal chains in EDA solution.
    • Movie S6 (.mov format). In situ liquid-phase TEM movie showing retained deformation of nanocrystals during the oriented attachment in EDA solution.
    • Movie S7 (.mov format). In situ liquid-phase TEM movie showing the lattice alignment of neighboring nanocrystals during the oriented attachment in EDA solution. .
    • Movie S8 (.mov format). In situ liquid-phase TEM movie showing the phase transition from hexagonal to square superlattices in pure EG.
    • Movie S9 (.mov format). In situ liquid-phase TEM movie showing reduced deformation of nanocrystals during the superlattice transformation in EG.
    • Movie S10 (.mov format). In situ heating TEM movie showing no perceptible deformation of individual nanocrystals without introducing any EDA or EG solution.

    Files in this Data Supplement:

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