Enhanced thermoelectric performance of c-axis oriented Bi1−xPbxCuSeO single crystalline thin films.

Electrochemical properties of ruthenium oxide (Ru02) films with different degrees of crystallinity were studied. The Ru02 films were grown by pulsed laser deposition at substrate temperatures from 30 to 400°C. At low temperatures, the amorphous phase of Ru02 films was formed. At high temperatures, the crystalline phase of Ru02 films was obtained. From the cyclic voltammetric study in H2S04 solution, it was found that the current response for an electrode of the amorphous Ru02 film was higher than that of the crystalline film. A proton diffusion length of larger than 5.8 nm was obtained on the Ru02 film prepared at 3 0°C.Electrochemical properties of crystalline Ru02 films have been studied intensively. Ru02 films are usually prepared by thermal decomposition and oxidation of soluble precursor materials at higher temperatures (300 to 500°C).' The most common precursor is RuC13 xH2O, dissolved in water or alcohol. The Ru02 can be oxidized and reduced reversibly through a mechanism involving proton exchange as shown below23The Ru valence state changes from Ru2 at -0 V to Ru4 at -1.4 V vs. the standard hydrogen electrode, respectively. If every Ru ion in the bulk is involved in a reaction with two protons, the charge density should be 1450 C/g. The specific capacitance for Ru02 should be -1036 FIg based on the assumption that the proton exchange rate occurs evenly in the potential range of 0 to 1.4 V. However, values of specific capacitance reported in literature are always much lower than the theoretical estimation. The highest specific capacitance for crystalline Ru02 reported by Raistrick4 was 380 FIg for a specific surface area of 120 m2/g. The capacitance calculated was 317 F/cm2. If one assumes that two protons could have exchanged with each Ru ion located within the layer of a unit cell thickness at the surface, the capacitance estimated is 339 to 490 F/cm2 depending on the crystal orientation of the Ru02. The consistency between the experimental measurement and the theoretical projection indicates that redox reactions involved only the surface Ru available on a rutile structure oxide.Recently, we have demonstrated56 that the amorphous form of hydrous ruthenium oxide (Ru02 xH2O) powder has a much higher specific capacitance that that of crystalline Ru02. The Ru02 xH2O powder was prepared by a sol-gel method in aqueous solution. It was found that the proton exchange rate was strongly dependent on the crystalline structure of Ru02 xH2O. It is believed that the proton can be easily diffused into the bulk of Ru02 xH2O in amorphous phase but not into the material in crystalline phase. However, it is difficult to quantitatively determine the diffusion length of the proton inside the Ru02 xH2O, because of nonuniform powder size, irregular powder shape, and Teflon binder used in making electrodes. This binder could block the full utilization of the active material. We demonstrate here that the proton diffusion process can be semiquantitatively determined in Ru02 films which are prepared by pulsed laser deposition.

Single-phase, c-axis-oriented BiCuSeO thin films have been directly grown on the commercial silicon (001) wafers without any surface pretreatment by using pulsed laser deposition. Xray diffraction pole figure confirms that the film does not have any ab-plane texture, whereas cross-sectional transmission electron microscopy shows good crystallinity of the film even if there exists an amorphous native oxide layer on the wafers surface. At room temperature, the resistivity of the film is about 14 mΩ cm, which is much lower than that reported for corresponding single crystals as well as polycrystalline bulks. This work demonstrates the possibility of using the available stateof-the-art silicon processing techniques to create BiCuSeObased thin-film thermoelectric devices.

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