Research ArticleMATERIALS SCIENCE

Creation of two-dimensional layered Zintl phase by dimensional manipulation of crystal structure

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Science Advances  28 Jun 2019:
Vol. 5, no. 6, eaax0390
DOI: 10.1126/sciadv.aax0390
  • Fig. 1 Creation of 2D layered ZnSb.

    (A) Schematic illustration of the dimensional manipulation of a crystal structure from 3D-ZnSb to 2D-ZnSb via Li alloying and etching processes. The Li alloying into 3D-ZnSb was conducted by thermal and electrochemical reactions (ERs) (see Materials and Methods and fig. S1). The selective etching of Li ions was conducted by reacting with polar solvent solution reaction (SR). A reversible process of alloying and etching occurs in the mean of electrochemical reaction (ER). (B) XRD patterns of 3D-ZnSb and 2D-LiZnSb. The 2D-LiZnSb polycrystal and single crystal were synthesized by using the synthesized 3D-ZnSb as a precursor. All patterns are well matched with the simulated patterns of corresponding compounds. a.u., arbitrary units. (C) XRD patterns of 2D-ZnSb crystals obtained by solution reaction and electrochemical reaction processes. For the solution reaction process, water-based solutions [DI water and dimethyl sulfoxide (DMSO) with 1 volume % of DI water, and hexamethyl phosphoric triamide (HMPA) with 1 volume % of DI water] were used. For the electrochemical reaction process, 1 M LiPF6 dissolved in 1:1 mixture of ethylene carbonate and diethyl carbonate solution was used as an electrolyte. The interlayer distances were calculated from the angle of highest intensity. (D to I) Scanning electron microscopy (D to F) and optical images (G to I) of 2D-LiZnSb and 2D-ZnSb created by the solution reaction and electrochemical reaction processes (see also fig. S2 for Na- and K-etched samples). The flakes of 2D-ZnSb were exfoliated by mechanical cleaving using 3M tape. The exfoliated nanosheets obtained from the solution reaction and ultrasonication processes are shown in fig. S4. (J to L) X-ray photoelectron spectroscopy (XPS) spectra of Li 1s (J), Zn 2p (K), and Sb 3d (L) for 3D-ZnSb, 2D-LiZnSb, and 2D-ZnSb, respectively. The Li 1s peak (54.6 eV) of 2D-LiZnSb indicates the Li1+ state. While the binding energies of the Zn 2p3/2 (1019.8 eV) and Sb 3d5/2 (525.8 eV) are significantly lower than the Zn 2p3/2 (1021.5 eV) and Sb 3d5/2 (527.6 eV) in 3D-ZnSb, the binding energies of Zn 2p3/2 (1022.1 eV) and Sb 3d5/2 (528.2 eV) of 2D-ZnSb are slightly higher than those of 3D-ZnSb.

  • Fig. 2 Crystal structure of 2D layered ZnSb.

    (A and B) Atomic resolution STEM-HAADF (high-angle annular dark-field) images of 2D-LiZnSb along the [110] (A) and [001] (B) zone axes, respectively. (C) Atomic resolution STEM-EDS elemental mapping for 2D-LiZnSb along the [110] (top) and [001] (bottom) zone axes. (D and E) Atomic resolution STEM-HAADF images of 2D-ZnSb along the [110] (D) and [211] (E) zone axes. The determined crystal structure of 2D-ZnSb, together with the results in fig. S5, is visualized in fig. S6, where the detailed structural parameters are listed. The atomic distances of 2D-ZnSb are compared with those of 3D-ZnSb and 2D-LiZnSb, as shown in fig. S6. From the observation at [211] zone axis of 2D-ZnSb, the honeycomb lattice is slightly tilted, as explained in fig. S6. (F) Atomic resolution STEM-EDS elemental mapping for 2D-ZnSb along the [110] (top) and [211] (bottom) zone axes. For the detection of lithium, the STEM–EELS (electron energy-loss spectroscopy) technique was used, showing the clear existence and absence of lithium in 2D-LiZnSb and 2D-ZnSb, respectively, as shown in fig. S3. (G) Cohesive energy (ΔEcoh) calculation of predictable 2D-ZnSb structures. Structure I that is determined from the STEM observations exhibits the lowest energy compared with other candidates, showing excellent agreement between experiments and calculations.

  • Fig. 3 Electronic properties of 2D layered ZnSb.

    (A to C) Temperature dependence of electrical resistivity (A), Hall mobility (B), and carrier concentration (C) for 3D-ZnSb, 2D-LiZnSb, and 2D-ZnSb. The bidimensional polymorphs of 3D-ZnSb and 2D-ZnSb show the metal-insulator transition. (D to F) Electronic band structures of 3D-ZnSb (D), 2D-LiZnSb (E), and 2D-ZnSb (F). The density of states for 3D-ZnSb, 2D-LiZnSb, and 2D-ZnSb are shown in fig. S9. The band structures of 3D-ZnSb (D) and 2D-LiZnSb (E) indicate that both are semiconductors with a well-defined indirect bandgap of 0.05 and 0.29 eV, respectively. A low electrical resistivity and a high carrier concentration of 2D-LiZnSb indicate a heavily doped semiconducting behavior.

  • Fig. 4 Dimensional manipulation of a crystal structure for the bidimensional polymorphic ZnSb.

    (A and B) In situ synchrotron powder XRD patterns using 3D-ZnSb (A) and 2D-ZnSb (B) via the electrochemical reaction. The alloying and etching processes were controlled by reducing and increasing voltage potential, respectively. The inset (bottom left) of (A) shows the peak shift of (002) plane for 3D-ZnSb. The inset (top left) of (A) shows the disappearance of diffraction peaks’ corresponding (002) and (101) planes at 11.1° and 11.7° of 2D-LiZnSb with Li etching, indicating the transformation to 2D-ZnSb. The inset (middle) shows the appearance and disappearance of Li1+xZnSb by-product with discharging and charging reactions, respectively. The insets of (B) show the same changes observed in the insets (top left and middle) of (A). No diffraction peaks of 3D-ZnSb were observed during the reversible structural transformation by Li alloying and etching processes. The detailed evolution and degradation of diffraction peaks during the transformation are shown in fig. S10. (C) Schematic illustration of the dimensional manipulation of a crystal structure, along with the transition of hybridized bonding characters from sp3 of 3D-ZnSb to sp2 of 2D-LiZnSb and 2D-ZnSb. The displacement of the blue arrow in Sb fifth to Zn fourth orbital depicts the covalent bonding character between Zn and Sb in the honeycomb lattice. The electron transfer from Li to sp3-hybridized state of 3D-ZnSb enables the transition to sp2-hybridized state of honeycomb ZnSb lattice in 2D-LiZnSb and 2D-ZnSb.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/6/eaax0390/DC1

    Fig. S1. Crystal structure and synthetic pathway of 2D-AZnSb.

    Fig. S2. 2D layered ZnSb polymorphs by the selective etching of alkali metals.

    Fig. S3. Lithium detection for 2D-LiZnSb and 2D-ZnSb.

    Fig. S4. Exfoliated nanosheets of Li-etched 2D-ZnSb.

    Fig. S5. Structural confirmation of 2D-LiZnSb and determined structure of 2D-ZnSb.

    Fig. S6. Crystal structure of 2D-ZnSb.

    Fig. S7. 2D layered behavior of 2D-ZnSb.

    Fig. S8. Octet rule for ZnSb bidimensional polymorphs.

    Fig. S9. Band structure and PDOS.

    Fig. S10. Structural phase transformation from 3D-ZnSb to 2D-ZnSb.

    References (3947)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Crystal structure and synthetic pathway of 2D-AZnSb.
    • Fig. S2. 2D layered ZnSb polymorphs by the selective etching of alkali metals.
    • Fig. S3. Lithium detection for 2D-LiZnSb and 2D-ZnSb.
    • Fig. S4. Exfoliated nanosheets of Li-etched 2D-ZnSb.
    • Fig. S5. Structural confirmation of 2D-LiZnSb and determined structure of 2D-ZnSb.
    • Fig. S6. Crystal structure of 2D-ZnSb.
    • Fig. S7. 2D layered behavior of 2D-ZnSb.
    • Fig. S8. Octet rule for ZnSb bidimensional polymorphs.
    • Fig. S9. Band structure and PDOS.
    • Fig. S10. Structural phase transformation from 3D-ZnSb to 2D-ZnSb.
    • References (3947)

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