SEM images of samples A, B, and C obtained at a magnification M = 120 kX.
Photoluminescence spectra of samples A, B, and C obtained for above-bandgap (upper panel) and below-bandgap excitation (lower panel, the vertical arrow indicates the position of the excitation photons energy). Curves were vertically shifted and the same abscissa scales were used for the two graphs to help comparison. Vertical dashed-dotted lines evidence the PL emission bands centered at about 3.0 eV, 2.35 eV (observed at both above and below-gap excitation), and 1.5 eV (exhibited by sample C). Dotted vertical line (lower panel) evidences the PL peak positioned in the red region of the spectrum obtained for below-bandgap excitation.
XRD data obtained for samples A (top panel), B (middle panel), and C (bottom panel) after annealing treatment. The graphs show the diffraction peaks associated with (110) planes of the rutile phase (R-TiO2) and to (101) planes of the anatase phase (A-TiO2). The solid curve represents the result of XRD simulations. In the inset, the XRD pattern obtained on as-grown samples A and C are shown.
Left panel: Comparison of PL spectra obtained at above-bandgap excitation (Eexc = 3.82 eV) on as-grown samples (dashed, red line) and after annealing (solid, black line). The dashed black vertical lines evidence thered-band emission enhanced by the annealing process. The dotted blue vertical line indicates the NIR resonance at 1.5 eV exhibited by sample C (both as-grown and after annealing) and slightly observable also in sample A after annealing. Right panel: Comparison of PL spectra obtained at below-bandgap excitation (Eexc = 2.80 eV) on as-grown samples (dashed, red line) and after annealing (solid, black line).
Peak-normalized photoluminescence spectra obtained for sub-bandgap excitation (2.80 eV photon energy) on as-grown and annealed samples. Spectra vertically shifted in order to facilitate comparison. Dashed blue curves represent individual spectral components of the “red band” PL emission, as obtained by a double-Gaussian fit.
Numerical parameters obtained from multi-peak Gaussian fit of PL spectra of as-grown TiO2 NPs samples. Quantities Ei, σi, wi, represent peak position, spectral width and spectral weight of the ith Gaussian function, according to Eq. (1) .
Comparison between numerical parameters obtained from two-peak Gaussian fit of RB-PL spectra obtained at below-bandgap excitation on as-grown vs. annealed samples. Quantities Ei, σi, wi, are defined in the same manner is in Table I .
Morphological and structural characteristics of the TiO2 nanostructured films produced by fs PLD, and main PL features observed before and after formation of TiO2 crystalline phases by samples annealing. The relative content of rutile (R) and anatase (A) phases in the samples after annealing, obtained by means of a simulation of XRD patterns based on kinematical theory, is also reported in the forth column.
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