The morphology (left column) of the prepared films: (a) and (b) studied by AFM including phase detection (right column) in two different zooms.
TEM figures of fragments of nanocrystalline films annealed at (a) and (b) .
X-ray diffraction patterns of the prepared samples: (unannealed), , and . The correspondence of selected peaks to anatase and rutile phases is marked by the letters “a” and “r,” respectively. The thin gray lines represent the results of the Rietveld method analysis.
Raman spectra of the prepared samples: (unannealed), TiO2() . The letters in brackets indicate the assignment of the observed modes to anatase and rutile phases. Inset: detail of the anatase mode, colors (shades) are same as in the main figure.
PL spectra of the prepared samples: (unannealed), , and measured under atmospheric air pressure at room temperature, under cw 325 nm excitation.
Comparison of normalized PL spectra of nanocrystalline porous titanium dioxide films annealed at different temperatures: , , excited by different laser excitation wavelengths: 325 nm, 442 nm, and in various air pressures: 5 Pa, . (a) PL of vs under 325 nm excitation and air pressure . (b) PL of vs under 442 nm excitation and air pressure . (c) PL spectra of the sample under 325 nm excitation for different values of ambient air pressure. (d) PL spectra of the sample under 442 nm excitation for different values of ambient air pressure. The comparison of the relative PL intensity can be read from the ratios displayed in small frames. The color of numbers is same as the color of the corresponding curve. The higher number means, how many times the PL spectrum is stronger.
Extinction spectrum of the nanoporous titanium dioxide film under different values of ambient air pressure: 5 Pa, . Insets: (a) change in extinction due to air pressure lowering [extinction (5 Pa) minus extinction ], (b) extinction of the sample measured by a commercial UV-Vis spectrophotometer in a broader spectral region.
Temperature dependence of the PL of the sample . (a) evolution of PL spectra under 325 nm excitation, the yellow dashed line is a theoretical fit according to Eq. (1) (b) evolution of PL spectra under 442 nm excitation, (c) comparison of temperature dependence 0 of maximal values of the PL intensity, lines are fits according to Eq. (2) (d) comparison of FWHMs, line is a fit according to Eq. (3) (e) evolution of spectral positions of maximal values of the PL intensity.
PL dynamics measured by upconversion technique at selected PL photon energies of the sample , excited by 3.06 eV, 70 fs laser pulses. The curves are normalized. Inset: Spectral dependence of the effective PL decay rate.
Basic scheme of radiative transitions leading to PL related to (a) phonon assisted STEs recombination represented by configuration coordinate diagram (b) surface states PL influenced by filling of subgap states. Notation: , —represent the excited, respectively, ground state of STE in configuration coordinate diagram, PL–photoluminescence, CB—conduction band, VB—valence band, DOS—density of states.
Proposed scheme of dominant electron processes under different conditions. Notation: CB—conduction band, VB—valence band, STE—self trapped exciton, SS–subgap states, PL—photoluminescence, Exc.—excitation. (a) 325 nm, , (b) 442 nm, , (c) 325 nm, 5 Pa, (d) 442 nm, 5 Pa.
Results of the Rietveld analysis of the x-ray diffraction patterns of the prepared nanocrystalline films. The obtained weight percentages and average diameters of the anatase and rutile nanocrystals in the films are displayed.
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