Analytical Techniques

3.3.1 X-ray Diffraction (XRD)

X-ray diffraction (XRD) is a non-destructive tool for analyzing material properties such as crystallinity, the phase identification, and orientation. In addition, grain size and strain can also be easily analyzed with this technique.

XRD was utilized to analyze the crystallinity, the phase identification of TNAs/Ti samples. The samples were scanned by Siemens Diffractometer D5000 (NCTU) with Cu Kα (λ=1.5405Å) source and using synchrotron beamline 17 B1 at National Synchrotron Radiation Research Center in Hsinchu, Taiwan (NSRRC). During scanning period, X-ray beam of wavelength λ was irradiated to the sample at an angle θ, and the diffracted intensity at an angle 2θ was recorded by a detector as illustrated in Figure 3-7. All of scans use θ-2θ mode with 2θ ranging from 20^o to 50^o.

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Figure ⁹ 3-7 Definition of the angle of incidence and diffraction in an XRD experiment

3.3.2 X- ray Absorption

The XANES measurement were performed using the beam line 20 A of National Synchorotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. The XANES spectra were collected in the vicinity of titanium L-edge (445-480 eV) and oxygen K-edge (520-570 eV) regions. All spectra in this thesis were measured in total electron yield mode (TEY) using a high-energy spherical grating monochrometer with energy resolution of about 1/8000 [70], by monitoring the total sample photocurrent as a function of photon energy scanned through Ti L2,3-edge. The overall experimental resolution around the Ti L2,3 edge was 100 meV. All spectra were collected at room temperature and the chamber pressure was about 2x10^-8 Torr or better. The incoming radiation flux was monitored by the total photocurrent produced in a clean Au mesh inserted in the beam.

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During XANES measurement, the incident X-ray beam was irradiated directly to the surface of samples. Moreover, X-ray beam was only absorbed by the surface layer sample.

Therefore, the XANES results will describe the characteristics of the surface layer of sample.

With the TNAs were annealed by furnace, the heating sample was homogeneously leading to the crystalline structure of sample was uniformly. In this case, the properties in entire sample can be the analyzed basing on XANES results. On the other hand, when the TNAs were irradiated by laser in parallel mode, the surface layer of sample was annealed by laser first and following by internal parts of sample. Hence, the XANES results will only explain the properties of the surface layer of sample annealed by laser.

The incident photo intensity (I_o) was calibrated by aligning the Ti L_2,3 and O K-edges of a SrTiO3. All the X-ray absorption spectra were then normalized to Io. To perform the fitting for the amount of crystalline, amorphous phases and impurities in TNAs grown in NH4F solutions and after using excimer laser annealing with specific conditions, we used the intensity ratios of orbitals in the O K-edge according to the published spectra [71-75] to determine the oxidation states of Ti. The fitting of a mode with various orbitals performed with a program with the Maximum Likelihood (ML) estimator to match spectra for experimental spectra [76]. Then the edges from TiO, Ti2O3, TiO2 (anatase) and TiO2

(amorphous) were added to the background. The fitting mode comprises a power-law background AF^-r, where E is the photon energy, A and r are tow parameters to be estimated the amount of oxidation states, crystalline and amorphous phases [76]. More details about creating a model and its fitting procedure can be found in the literature [76-77].

3.3.3 Raman Spectroscopy

Raman spectroscopy is a spectroscopic technique used in condensed matter physic and chemistry to study vibrational, rotational, and other low-frequency modes in a system. It relies

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on inelastic scattering, or Raman scattering, of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range. The laser light interacts with phonons or other excitations in the system resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system. The Raman Effect occurs when light impinges upon a molecule and interacts with the electron cloud of the bonds of that molecule. The incident photon excites the molecule into a virtual state. For the spontaneous Raman Effect, the molecule will be excited from the ground state to a virtual energy state, and relax into a vibrational excited state, which generates Stokes Raman scattering. If the molecule was already in an elevated vibrational energy state, the Raman scattering is then called anti-Stokes Raman scattering as shown Figure 3-8. A change in the molecule polarization potential or amount of deformation of the electron cloud with respect to the vibrational coordinate is required for the molecule to exhibit the Raman Effect. The amount of the polarizability change will determine the Raman scattering intensity, whereas the Raman shift is equal to the vibrational level that is involved.

In this research Jobin Yvon Horiba Scientific Raman Spectrometer was used. All the Raman spectra were recorded with a double grating spectrometer in backscattering geometry, at room temperature; the He–Ne laser (632.8 nm) was used for the excitation. The diameter of the spot in the sample was typically 1 µm.

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Figure¹⁰3-8 Energy level diagrams showing the states involved in Raman signal [78]

3.3.4 Scanning Electron Microscopy (SEM)

Field emission scanning electron microscopy (FESEM) (JEOL 6700F) was employed to examine the surface structure and the cross sectional morphology of the TiO₂ nano-tubes films annealed by the furnace and excimer laser. The JEOL 6700F was operated at 15kV, while the emission current was set at 10 µA and the operating pressure was under 9.6 x10^-5 Torr.

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