Research ArticlePHYSICAL SCIENCES

Femtosecond visualization of oxygen vacancies in metal oxides

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Science Advances  06 Mar 2020:
Vol. 6, no. 10, eaax9427
DOI: 10.1126/sciadv.aax9427
  • Fig. 1 Schematic illustration of the fsV principles.

    Femtosecond laser pulses (pump) excite the oxygen vacancy (O-V)–contained metal oxides. Interaction of broadband supercontinuum pulses (probe) with the excited states characteristic of the oxygen vacancies induces a spectroscopic picture of TA as a function of photon energy (hν) and time delay (τ). Alignment of TA (hν, τ) with the distribution of the TDOS with photon energy E enables precise determination of the unique oxygen-vacancy arrangement for a given metal oxide. The slides present schematically the varied chemical structures and TDOS spectra for different O-V configurations of WO2.9 and WO2.72. The black dashed circle highlights a schematic O-V site. The dashed green ring on one of the slides highlights the defect state plotted as TDOS spectrum calculated on a specific O-V structure in WO2.72. The correlation between the tentative O-V structures, the TDOS spectra, and the measured TA dynamics enables determination of the true O-V configuration through precise matching between the TDOS and TA performance.

  • Fig. 2 Crystal structure models and TDOS calculation results.

    Local structural configurations of (A) WO3, (B) WO2.9, (C) WO2.72 and the corresponding TDOS distributions for (D) WO3 (top)/WO2.9 (bottom) and (E) WO2.72 (①, ②, ③, and ④).

  • Fig. 3 TA measurements on WO2.9 with pumping at 1.55 eV.

    (A) A 3D plot of the TA spectra at delays from 0 to 1 ps. (B) The TA dynamics at 1.50, 1.52, 1.61, and 2.22 eV for a time delay from 0 to 900 ps. Inset: An expanded view of the delay range of 0 to 2 ps. (C) TA dynamics at 1.52 eV used to determine the lifetimes of the involved transition processes. Inset: Related photoelectronic transitions. (D) Alignment of the calculated TDOS with the transitions resolved from the measured TA spectra. (E) Detailed analysis of the oscillatory TA dynamics at 1.61 eV. Inset: 2PA- and 2PE-enhanced En absorption, bleached E0 absorption.

  • Fig. 4 TA spectroscopic measurements on WO2.72 with pumping at 1.55 eV.

    (A) TA as a function of time delay and probe photon energy. (B) TA dynamics measured for WO2.72 at 1.51 and 1.60 eV and WO2.9 at 1.52 eV for a delay time range of 0 to 10 ps, using a pump fluence of 400 μJ/cm2. Inset: TA dynamics in a delay time range of 20 to 1000 ps. (C) Pump fluence dependence of the TA dynamics at 1.60 eV. Inset: Plots of the measured TA amplitude at a delay of 0.15 ps (red circles) and 2.5 ps (black circles) as a function of the pump fluence, together with fits to third-order (Y = A1X − B1X2 − C1X3) and second-order (Y = A2X − B2X2) polynomials, respectively, with A1 = 1.23, B1 = 1.65 × 10−3, C1 = 2 × 10−5, A2 = 0.35, and B2 = 1.08 × 10−3. (D) Calculated TDOS for WO2.72 (open black circles) and the fit (red solid curve) obtained by decomposing the band distribution into multiple subbands (I to VI, black solid curves). The TDOS for WO2.9 (green-filled circles) is included for comparison. (E) Proposed energy levels aligned with (D) and possible electronic transitions through excitation by the pump pulses and absorption of the probe pulses. (F) The TA spectrum at a delay of 2.5 ps between the pump and probe pulses, resolving the two energy bands I and II that include E0 and ED, respectively.

  • Fig. 5 Chemical bond configurations for WO3, WO2.9, and WO2.72 with different oxygen-vacancy distributions.

    Dashed arrows indicate weakest bonds (B5 and B6), according to the theoretical calculations.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/10/eaax9427/DC1

    Fig. S1. Comparison between the Gibbs free energies of WO3, WO2.9, and WO2.72 in a temperature range of 300 to 900 K.

    Fig. S2. The possible crystal structures and the corresponding TDOS distributions of WO2.9.

    Fig. S3. TA measurements on WO3 with pump photon energies of 3.1 and 1.55 eV and the corresponding alignments with the calculated TDOS distributions.

    Fig. S4. TA spectra at different time delays for measurements on WO2.9.

    Fig. S5. TA dynamics measured on WO2.9 at 1.51 and 1.6 eV for pumping at 3.1 eV.

    Fig. S6. Pump fluence dependence of the TA dynamics measured on WO2.9 using a pump at 1.55 eV.

    Fig. S7. Comparison between the TA spectra of WO2.72 (at a delay of 0.4 ps) and WO2.9 (at delays of 0.25 and 0.4 ps).

    Fig. S8. TA dynamics measured for WO2.72 at 1.50 and 1.61 eV and WO2.9 at 1.50 eV.

    Fig. S9. Diagrammatic summary of the logic relationships between different sections.

    Table S1. The calculated formation energies of WO2.72 corresponding to the four kinds of configurations with different distributions of oxygen vacancies.

    Table S2. Bond length and bond population for chemical bonds of WO3, WO2.9, and WO2.72.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Comparison between the Gibbs free energies of WO3, WO2.9, and WO2.72 in a temperature range of 300 to 900 K.
    • Fig. S2. The possible crystal structures and the corresponding TDOS distributions of WO2.9.
    • Fig. S3. TA measurements on WO3 with pump photon energies of 3.1 and 1.55 eV and the corresponding alignments with the calculated TDOS distributions.
    • Fig. S4. TA spectra at different time delays for measurements on WO2.9.
    • Fig. S5. TA dynamics measured on WO2.9 at 1.51 and 1.6 eV for pumping at 3.1 eV.
    • Fig. S6. Pump fluence dependence of the TA dynamics measured on WO2.9 using a pump at 1.55 eV.
    • Fig. S7. Comparison between the TA spectra of WO2.72 (at a delay of 0.4 ps) and WO2.9 (at delays of 0.25 and 0.4 ps).
    • Fig. S8. TA dynamics measured for WO2.72 at 1.50 and 1.61 eV and WO2.9 at 1.50 eV.
    • Fig. S9. Diagrammatic summary of the logic relationships between different sections.
    • Table S1. The calculated formation energies of WO2.72 corresponding to the four kinds of configurations with different distributions of oxygen vacancies.
    • Table S2. Bond length and bond population for chemical bonds of WO3, WO2.9, and WO2.72.

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