Gregory J. Exarhos

Atomic force microscopy of laser-conditioned and laser-damaged amorphous TiO 2 sol-gel thin films

Amorphous TiO2 films are spin-cast from highly acidic ethanol precursor solutions of titanium ethoxides onto fused silica and single crystal silicon substrates at 2000 to 3000 rpm. Immediately after spinning, the resulting homogeneous film is approximately 350 nm thick and has a refractive index of 1.6 as determined from UV-VIS-NIR transmission measurements. After 20 hours at room temperature, the refractive index increases to 1.8 and is accompanied by a decrease in film thickness. Mild heat-treatment of the film results in a further increase of the refractive index to 2.0. Thicker, homogenous films can be prepared by depositing subsequent layers. Raman spectra of these films indicate the presence of the amorphous phase of titania. Prior to mild heat-treatment, the amorphous titania films are observed to crystallize to the anatase phase upon exposure to cw radiation from an Argon-ion laser at a fluence of 2.5 Mw cm-2. Crystallization is initiated approximately 1.2 seconds after exposure and the illuminated spot becomes fully crystalline after 3.6 seconds. However, the laser fluence can be increased to 6.0 Mw cm-2 without resulting in crystallization if the amorphous film is conditioned by exposure to progressively greater laser fluence beginning below the damage threshold. Similarly, films that have experienced a mild heat-treatment prior to irradiation have a damage threshold greater than 6.0 Mw cm-2. In this study, atomic force microscopy is used to evaluate the microstructure of the laser-conditioned and laser-damaged titania films at the micron to submicron scale.

Transient stress evolution and crystallization in laser-irradiated amorphous titania sol-gel films

Amorphous Ti02 sol-gel films are irreversibly transformed to a crystalline anatase phase when heated to temperatures in excess of 575 K or subjected to intense pulsed or CW laser irradiation. The laser-induced transformation is initiated as a result of impurity absorption and subsequent heating, and results in densification and relative changes in compressive stress of the film. Isothermally annealed films exhibit a decrease in compressive stress as crystallization proceeds while an increase in compressive stress followed by a decrease in stress is observed when crystallization is laser-induced. Raman spectroscopy has been used to characterize the crystallization ingrowth kinetics and is used in this work as a real time probe of both film temperature and localized stress which can be evaluated from shifts in lattice phonon frequencies measured in real time during laser irradiation. The laser not only induces the phase transformation but excites inelastic Raman scattering from which film stress and temperature can be estimated. A second approach for the determination of these parameters requires incorporation of a thin ruby film between the titania and silica substrate. Here, the wavelength shift of the laser-induced ruby fluorescence can be used to quantify interfacial stress; the fluorescence lifetime measurements are used to determine temperature. The advantages and limitations of these techniques for evaluating transient stress and temperature evolution in thin titania films subjected to CW laser irradiation will be discussed.

Surface morphology, phase characterization, and stability of solution-deposited zirconia films

Deposition of zirconia films and multilayers from aqueous solution involves complexation of the precursor metal nitrate with a water soluble organic complexant. Following spin casting and associated water evaporation, an amorphous organic film is formed. Heat treatment initiates oxidation of the organic phase by nitrate, leaving behind a high index stoichiometric oxide film (n equals 2.01). Small grain sizes (approximately equals 50 nm) observed by AFM analysis stabilize the cubic and tetragonal crystalline phases of zirconia which are identified using XRD and Raman spectroscopy. Annealing above 700 degree(s)C causes recrystallization of these films to the monoclinic phase. A fluorescence method based upon introduction of a Sm+3 dopant into the film is shown to be a sensitive probe of the inherent crystalline phase and phase homogeneity and is used to evaluate crystalline phases in laser damaged coatings. These films exhibit visible damage areas upon irradiation at 532 nm with pulse energies in excess of 10 J/cm2. An increase in grain size accompanied by the presence of a disordered phase of zirconia is observed. Results are compared with damage regions induced in similarly irradiated PVD zirconia films.

Novel Electrosynthesis of Poly(cyclo‐organophosphazenes)

A new polymerization process for the synthesis of cyclomatrix polymers directly from the hexachlorocyclophosphazene trimer, (NPCl[sub 2])[sub 3], at an electrode surface is described. This electrochemical process is governed by the reductive coupling of an electrochemically generated reactive species from p-benzoquinone to the (NPCl[sub 2])[sub 3] through a nucleophilic replacement of chlorine. Based on the results of elemental analysis, infrared spectroscopy, and Auger electron spectroscopy, the molecular structure of the synthesized polymer has been inferred. The unique polymerization route makes this process viable for in situ synthesis of materials having a wide range of important physical and chemical properties.