Browsing by Author "Conde, Olinda"
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- Co and (Co,Mo) doping effects on the properties of highly reduced TiO2 anatase thin filmsPublication . Silvestre, António Jorge; Rout, S.; Dalui, S.; Pereira, Laura; Viana, Ana; Conde, OlindaThis work reports on the structural, optical, electrical and magnetic properties of Co:TiO2 and (Co,Mo):TiO2 anatase thin films grown onto (0001) sapphire substrates by Pulsed Laser Deposition in highly reducing conditions and at a low temperature of 350 _C. Undoped TiO2_d as well as doped films with nominal compositions of Ti0.95Co0.05O2_d, Ti0.94Co0.05Mo0.01O2_d, and Ti0.92Co0.05Mo0.03O2_d were studied. They all show similar microstructures, with smooth surfaces and RMS roughness values less than 0.5 nm. The optical band gap energies of the doped films are red shifted with respect to that deduced for the undoped TiO2_d. The correlation between the band gaps and the Urbach energies of the films is discussed. All samples show semiconductor behavior with n-type conduction. The Co:TiO2 sample is ferromagnetic with a saturation magnetization of 1.3 mB/Co, a high electrical conductivity of 123 S cm_1 and a carrier density of 1.88 _ 1021 cm_3 at room temperature. The ferromagnetic order of the Co:TiO2 system is suppressed when carriers are added by codoping with Mo.
- CVD of CrO(2): Towards a lower temperature deposition processPublication . Sousa, Pedro M.; Dias, Sonia A.; Silvestre, António Jorge; Conde, Olinda; Morris, Benjamin; Yates, Karen A.; Branford, William R.; Cohen, Lesley F.The deposition of highly oriented a-axis CrO(2) films onto Al(2)O(3)(0001) by atmospheric pressure (AP)CVD at temperatures as low as 330 C is reported. Deposition rates strongly depend on the substrate temperature, whereas for film surface microstructures the dependence is mainly on film thickness. For the experimental conditions used in this work, CrO(2) growth kinetics are dominated by a surface reaction mechanism with an apparent activation energy of (121.0 +/- 4.3) kJ mol(-1). The magnitude and temperature dependence of the saturation magnetization, up to room temperature, is consistent with bulk measurements.
- Influence of growth temperature and carrier flux on the structure and transport properties of highly oriented CrO2 on Al2O3 (0001)Publication . Sousa, Pedro M.; Dias, Sonia A.; Conde, Olinda; Silvestre, António Jorge; Branford, William R.; Morris, Benjamin; Yates, Karen; Cohen, Lesley F.In this work we report on the structure and magnetic and electrical transport properties of CrO2 films deposited onto (0001) sapphire by atmospheric pressure (AP)CVD from a CrO3 precursor. Films are grown within a broad range of deposition temperatures, from 320 to 410 degrees C, and oxygen carrier gas flow rates of 50-500 seem, showing that it is viable to grow highly oriented a-axis CrO2 films at temperatures as low as 330 degrees C i.e., 60-70 degrees C lower than is reported in published data for the same chemical system. Depending on the experimental conditions, growth kinetic regimes dominated either by surface reaction or by mass-transport mechanisms are identified. The growth of a Cr2O3 interfacial layer as an intrinsic feature of the deposition process is studied and discussed. Films synthesized at 330 degrees C keep the same high quality magnetic and transport properties as those deposited at higher temperatures.
- Investigation of the microstructure, chemical composition and lateral gorwth of TiN films deposited by laser-induced chemical vapour depositionPublication . Silvestre, António Jorge; Paramês, M. L. G. F.; Conde, OlindaA study of the laser direct writing of titanium nitride on mild steel substrates by pytolytic laser-induced chemical vapour deposition was carried out under different operating conditions. Lines of TiN were deposited from a reactive gas mixture of TiC1₄, N₂ and H₂ using a continuous wave TEM₀₀CO₂ laser beam as heat source. The deposited material was analysed by optical and scanning electron microscopies, wavelength-dispersive X-ray microanalysis and profilometry techniques. Golden coloured TiN films, close to the stoichiometric composition, were produced exhibiting good adherence, very fine grain size (~100nm) and broad gaussian, profiles. A time kinetics study based on the lateral growth rate of the deposited lines is also presented. From the transiente growth conditions, an empirical relation between the interaction time needed to achieve film deposition and the laser irradiance was established, which led to a value for the incubation period of 5.4 ns. This relation also gave a value for the threshold laser irradiance of 1.54 x 10ᶣ W cm-², in good agreement with the value found experimentally.