Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO2-Si photoanode

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Science Advances  03 Apr 2020:
Vol. 6, no. 14, eaay6650
DOI: 10.1126/sciadv.aay6650
  • Fig. 1 Characterization and measurement of the Ni-TiO2-Si junction.

    (A) The energy level alignment for the metal, oxide, and semiconductor is shown, along with the expected photoexcited hole transfer in the p-type MOS junction. The Si is p-type–doped by boron at 1015 per cm3. The presence of oxygen defect levels (n-type) in the TiO2 layer was previously confirmed by photoemission spectroscopy of a Si-TiO2 junction (23). The band bending is calculated using the drift-diffusion equation (27). (B) A TEM measurement of the thickness of the TiO2 and Ni, which are 19 ± 0.6 nm and 5.6 ± 0.6 nm, respectively. The TiO2 is amorphous, and an ~1-nm SiO2 interface is measured where the TiO2 and Si contact. (C) The black line overlay is the ground-state XUV absorption. The regions that correspond to the Ti M2,3 edge, the Ni M2,3 edge, and the Si L2,3 edge are indicated by the colored boxes. The differential XUV absorption that results from photoexcitation is shown as the background color map, with the scale on the right of the graph. Each peak is observed to have a unique response to the photoinitiated charge transfer, which will be discussed in the main text.

  • Fig. 2 Photoexcited changes in the Si, TiO2, and Ni separately and in a junction.

    The differential absorption versus photon energy from Fig. 1C is plotted in the bottom row, while the top row shows the differential absorption scaled and added to the ground-state absorption. The solid lines represent the excited-state change for the elements in the junction for (A) Si 100 fs after photoexcitation, (B) Ni 100 fs after photoexcitation of the Si, and (C) TiO2 1 ps after the photoexcitation of Si. The same time differential absorption versus photon energy is shown for photoexcitation of each element alone as a dashed line. As discussed in the text, different excitation wavelengths and thicknesses were required to photoexcite the elements alone as compared to the junction. A representative error bar of the experimental few mOD noise is shown at key comparison energies.

  • Fig. 3 Comparing the fit kinetics separately and in a junction.

    The spectral signatures described by Fig. 2 are fit to extract the excited-state kinetics, shown here. In each case, the error bars correspond to the nonlinear fit standard error from a robust fit weighted by the experimental uncertainty. For each plot, the junction fit parameters are shown as the colored symbols, while the gray symbols are for the isolated material. The time scale is logarithmic with a 100-fs offset for visualization. (A) The amplitude of electron spectral signature in the junction (purple symbols) has a similar rise time compared to the isolated material but has an increased decay rate. In the isolated Si, the amplitude of the hole spectral feature (gray symbols in bottom panel) follows the same kinetics as the electrons. In the junction, the hole signature slowly grows until 10 ps, suggesting that the initial photoexcited hole population was transferred out from the Si. (B) Top: The fit Fermi level of Ni decreases when the Si in the junction is excited with 800-nm light or when a 20× thicker Ni film alone is excited with 800-nm light. The fit Fermi level then decays on a longer time scale in the junction. The amplitudes are scaled for comparison of the rise times. Bottom: When TiO2 alone is excited with 266-nm light, a decrease in the fitted edge energy is observed because of the ligand-to-metal charge transfer. In the junction, the Ti edge fit energy increases on a time scale that matches the decay of the Fermi level in Ni.

  • Fig. 4 Quantifying the photoinitiated hole tunneling and diffusion in the junction.

    (A) The square light red symbols represent the magnitude of the measured rise time of the Ni edge in the junction; their scatter represents the error of the experimental measurement. The gray circles represent the rise time of the Ni in a Si-Ni junction with no TiO2 spacer. Fitting the experimental data to an error function (solid lines) convoluted with the excitation pulse gives a delayed rise of 17 ± 5 fs for the Si-Ni and 50 ± 6 fs with the TiO2 layer. The transit time in the TiO2 is therefore obtained as 33 ± 8 fs. (B) The edge shift kinetics, which, as noted previously, indicate the hole kinetics, measured for the TiO2 (gray triangles) and the Ni (red squares) in the junction are compared to the increase in electron recombination (or decrease in electron signature at the Si edge) in the junction (purple squares). The solid line is a fit to the diffusion equation with a diffusion constant of 1.2 ± 0.1 cm2/s and a surface recombination velocity of 200 ± 50 cm/s. The decrease in electron density qualitatively tracks the diffusion of holes through the TiO2. The dashed line represents the predicted (Pred.) arrival of holes at the Si-TiO2 interface based on the fit diffusion kinetics.

Supplementary Materials

  • Supplementary Materials

    Layer-resolved ultrafast extreme ultraviolet measurement of hole transport in a Ni-TiO2-Si photoanode

    Scott K. Cushing, Ilana J. Porter, Bethany R. de Roulet, Angela Lee, Brett M. Marsh, Szilard Szoke, Mihai E. Vaida, Stephen R. Leone

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