Elena Vigil

TiO2 thin film deposition from solution using microwave heating

Abstract A new method has been devised for the deposition of TiO2 thin films on conducting glass using a microwave reactor. The substrates are immersed in a diluted homogeneous aqueous solution which was prepared by mixing equal volumes of a fluorine-complexed titanium(IV) solution ([Ti]=3.4×10−2 M) and 6.8×10−2 M boric acid solution. Low microwave power and short deposition time have been used. The TiO2 layers obtained are well-adhered, homogenous, with good specularity and colored by interference of reflected light. Their thickness is in the range of 100–500 nm. SEM experiments denote that films are formed by small crystallites having linear dimensions under 100 nm. Crystal dimensions depend on microwave power and deposition time. The layers show a high degree of crystallinity and the observed crystal phase is anatase. Microwave heating has proved to be an efficient and inexpensive method for solution growth of TiO2 films; it should also be of importance for other materials layers grown from solution.

Comparative study of nanocrystalline TiO2 photoelectrodes based on characteristics of nanopowder used

TiO2 photoelectrodes are made using the “dip-coating” technique with colloidal suspensions of TiO2 nanopowders. Two different commercial TiO2 powders are selected with nanocrystals having different porosity and somewhat different crystal structure. Photoelectrodes are characterized using a practical two-electrode photoelectrochemical cell arrangement. Differences in open-circuit photovoltage and short-circuit photocurrent are reported and explained based on initial powder characteristics. Differences in time response found are also explained.

Titanium(IV) oxide thin films obtained by a two-step soft-solution method

Abstract TiO 2 films were deposited on either glass or Si substrates by using a two-step soft-solution method. A submonolayer of anatase TiO 2 nanocrystals was first deposited on the substrate by a drain-coating process that was performed at 333 K from an aqueous TiO 2 colloidal solution. The substrate partially covered with the TiO 2 nanocrystals was immersed in an aqueous solution, containing a titania precursor [fluorine-complexed Ti(IV)], and the whole was treated with microwave irradiation. The nanocrystals deposited on the substrate acted as growth seeds in the subsequent formation of the TiO 2 film. The obtained TiO 2 films showed a high degree of crystallinity (anatase), even without further thermal treatment; however, they did not show photocatalytic activity. Thickness of the films was varied as a function of microwave power and irradiation time.

Preparation of anatase powders from fluorine-complexed titanium(IV) aqueous solution using microwave irradiation

The microwave radiation activated precipitation of titanium dioxide from homogeneous aqueous solutions of titanium–fluoro complexes and boric acid has been carried out in batch reactors. Crystalline anatase powders with narrow monodisperse size distribution have been obtained in a short time, at low temperature and atmospheric pressure. Microwave power, irradiation time and precursor concentration determine the properties of the obtained materials, i.e. crystal size, powder aggregate morphology and BET surface area. The obtained materials do not show photocatalytic activity, probably due to a considerable fluoride content that is in the range of 2–5%.

Structural analysis of TiO2 films grown using microwave-activated chemical bath deposition

Abstract TiO2 layer films were grown using the microwave (MW)-activated chemical bath deposition technique on two different indium tin oxide substrates. The TiO2 films are studied to determine their structural response when changing the MW heating power. Thickness (areal density), oxygen concentration profile, composition and surface homogeneity were determined using Rutherford backscattering spectrometry, nuclear reaction analysis and atomic force microscopy. The analysis showed that the composition, thickness and surface structure of the films are highly influenced by MW heating power. The substrate, acting as seed for nucleation, influences the layer thickness, indicating that a thinner layer of TiO2 is obtained for the more conducting substrates. The oxygen concentration profile is constant in the TiO2 layer at low MW heating, power (≈20%). The rugosity of the samples and the non-homogeneity increase with the MW heating power. If the MW heating power is high enough pinholes in the TiO2 layer of the order of the sample thickness are produced.

Role of the conducting layer substrate on TiO2 nucleation when using microwave activated chemical bath deposition

Nanostructured TiO2 is used in novel dye sensitized solar cells. Because of their interaction with light, thin TiO2 films are also used as coatings for self-cleaning glasses and tiles. Microwave activated chemical bath deposition represents a simple and cost-effective way to obtain nanostructured TiO2 films. It is important to study, in this technique, the role of the conducting layer used as the substrate. The influence of microwave–substrate interactions on TiO2 deposition is analysed using different substrate positions, employing substrates with different conductivities, and also using different microwave radiation powers for film deposition. We prove that a common domestic microwave oven with a large cavity and inhomogeneous radiation field can be used with equally satisfactory results. The transmittance spectra of the obtained films were studied and used to analyse film thickness and to obtain gap energy values. The results, regarding different indium–tin oxide resistivities and different substrate positions in the oven cavity, show that the interaction of the microwave field with the conducting layer is determinant in layer deposition. It has also been found that film thickness increases with the power of the applied radiation while the gap energies of the TiO2 films decrease approaching the 3.2 eV value reported for bulk anatase. This indicates that these films are not crystalline and it agrees with x-ray spectra that do not reveal any peak.

Two-layer TiO2 nanostructured photoelectrode with underlying film obtained by microwave-activated chemical bath deposition (MW-CBD)

A photoelectrode structure is proposed in which a dense and thin TiO2 film is grown on the conducting support using microwave activated chemical bath deposition (MW-CBD) before depositing the thicker and porous TiO2 layer. IPCE and open-circuit photovoltage spectra of the two-layer TiO2 nanostructured photoelectrode are presented. Better characteristics for the two-layer structure were found when compared to those of a single layer without the MW-CBD TiO2 film. Possible factors leading to parameter improvements are discussed. Use of MW-CBD underlying films can contribute to the development of liquid- and solid-dye-sensitized solar cells, as well as facilitate the use of other redox couples.

Rutherford backscattering spectrometry analysis of TiO2 thin films

Abstract TiO 2 layers grown by microwave-activated chemical bath deposition (MW) and dip coating (DC), as well as by the combination of both techniques, were studied by Rutherford backscattering spectrometry (RBS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). RBS analysis allows the determination of the stoichiometry and the thickness (in atoms/cm 2 ) of the TiO 2 layers. TiO 2 layers grown by DC have higher growth rates on a TiO 2 film obtained by MW compared to deposition directly onto an indium–tin oxide (ITO) substrate. TiO 2 layers grown by MW on a film obtained by DC have higher growth rates when compared to layers deposited onto ITO substrates. In this case, AFM analysis shows that the surface is rough and RBS reveals the presence of holes in TiO 2 films.

Nanostructured Solar Cells

Novel types of solar cells based on nanostructured materials are intensively studied because of their prospective applications and interesting new working principle – essentially due to the nanomaterials used They have evolved from dye sensitized solar cells (DSSC) in the quest to improve their behavior and characteristics. Their nanocrystals (ca. 10-50 nm) do not generally show the confinement effect present in quantum dots of size ca. 1-10nm where electron wave functions are strongly confined originating changes in the band structure. Nonetheless, the nanocrystalline character of the semiconductor used determines a different working principle; which is explained, although it is not completely clear so far,. Different solid nanostructured solar cells are briefly reviewed together with research trends. Finally, the influence of the photoelectrode electron-extracting contact is analyzed.

Women’s Presence in the Development of Semiconductor Physics and Semiconductor Devices Research in Cuba (abstract)

Physics research did not exist in prerevolutionary Cuba. In 1962, Cuban university programs were reformed and Havana University’s School of Physics was created. Equal opportunities for women and science development were made high priorities during university reform. In the early 1960s the growth of physics research began, particularly in semiconductors. This research is reviewed, emphasizing the presence and accomplishments of the women involved. Women physicists havr authored papers regarding the first alloyed semiconductor diode in 1967; IREs and LEDs (discrete, as well as integrated digits) in the 1970s; epitaxial growth in space; and LED and IRE technology transfer to the Cuban semiconductor industry in the 1980s. Women’s current active role in solar cell research is also reviewed.