Gálvez-Saldaña, Marco A.

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    Synthesis and characterization of nanostructured ZnO- and BiFeO3-Based Multifunctional Materials
    (2012) Gálvez-Saldaña, Marco A.; Perales-Pérez, Oscar J.; College of Arts and Science - Science; Radovan, Henri; Fernández, Félix; Department of Physics; Macchiavelli, Raúl E.
    The effective incorporation of dopant species into ZnO host structure usually induces changes in its physical and chemical properties enabling the establishment of novel multi-functional properties. In the case of doping with transition metal ions, the subsequent exchange interaction between available spins of the magnetic species would induce a ferromagnetic behavior in the so-called ZnO-based diluted magnetic semiconductor. This ferromagnetic functionality will enable the application of this material in data storage and spintronics-based devices. On this basis, a systematic study was carried out to determine the effect of composition and crystal size on the structural, optical and magnetic properties of pure and doped ZnO [Zn1-x(M)xO], where M: Co (II), Sc(III) or V(III), nanocrystalline powders and films. The Co, Sc and V doping levels varied in the 0.0 at% to 10 at% range. Powders and films were synthesized via a sol-gel approach, where ethanolamine was used to increase the viscosity of the precursor solutions and promote the adhesion of precursor and final oxides onto quartz and Silicon (100) substrates. Thermogravimetric analysis in air suggested the complete formation of the oxide structure at temperatures above 350oC. X-ray diffractometry verified the development of the ZnO host structure after annealing of the precursors. The average crystallite size of ZnO powders varied from 25 nm to 30 nm when the samples were annealed in air at 400oC and 500oC, respectively, for 1hour. Sc and V dopants produce a decrease in the average crystallite size. This behavior is also observed in the samples of doped ZnO films, in this case the average crystallite size decreases from 24 nm to 15 nm annealed at 500oC and from 27 nm to 14 nm in the films annealed at 550oC. UV-vis and photoluminescence (PL) measurements corroborated the formation of high-quality ZnO host structure. PL measurements evidenced an intense emission peak in the UV region around 390 nm, for an excitation wavelength of 342 nm. The intensity of this main emission peak was strongly dependent on the level of Co, Sc and V species in both powders and thin films. Furthermore, it was found that the dopant concentration and the annealing temperature play an important role in the ferromagnetic behavior of the material. On the other hand, multiferroic materials are the focus of intensive research because of their expected improved performance in data storage and processing devices. Bismuth Ferrite, BiFeO3 or BFO, is a multiferroic with a non-centro symmetric rhombohedral perovskite structure that exhibits antiferromagnetic and ferroelectric behaviors. Challenges associated with the synthesis of this material are the co-existence of BFO with impurity compounds. The incorporation of specific dopant species (transition metal or rare earths) in the BFO structure can enhance the ferroelectric properties and produce a weak ferromagnetic response. Accordingly, the present study was focused on evaluating the effect on the physical properties of the partial substitution of Praseodymium species in Bi-site in BFO structure as well as the substitution of Cobalt ions in Fe-site in BFO films. The level of the doping species varied from 0 at% to 4 at% BFO thin films were synthesized by a sol-gel approach, where glycol was aggregated to the main solvent to increase the viscosity of the precursor solutions and promote their adhesion onto platinum substrates. The development of the host BFO structure was confirmed by XRD analyses of samples annealed at 500oC for two hours in air, all samples showed single phase corresponding to rhombohedral BFO. The average crystallite size varied from 28 nm to 40 nm and 28nm to 31nm for Pr doped BFO and Co doped BFO films, respectively, with arise of the doping level. In turn, the corresponding coercivity value was increased from 166 Oe to 428 Oe in pure and 4 at% doped Pr-doped samples respectively and there was no variation in coercivity with respect to the pure films in the Cobalt system. Furthermore a weak ferroelectric behavior was observed, with predominant paraelectric response in pure and doped films in both systems.