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Publication

Evidences of the evolution from solid solution to surface segregation in Ni-doped SnO2 nanoparticles using Raman spectroscopy

(John Wiley and Sons Ltd, 2011-05-01)

HERRERA ARAGON, FERMIN FIDEL

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HUAMANI COAQUIRA, JOSE ANTONIO

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Hidalgo P.

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Da Silva S.W.

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Brito S.L.M.

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Gouvêa D.

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Morais P.C.

Ni-doped SnO2 nanoparticles, promising for gas-sensing applications, have been synthesized by a polymer precursor method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data analyses indicate the exclusive formation of nanosized particles with rutile-type phase (tetragonal SnO2) for Ni contents below 10 mol%. The mean crystallite size shows a progressive reduction with the Ni content. Room-temperature Raman spectra of Ni-doped SnO2 nanoparticles show the presence of Raman active modes and modes activated by size effects. From the evolution of the A1g mode with the Ni content, a solubility limit at ∼2 mol% was estimated. Below that content, Raman results are consistent with the occurrence of solid solution (ss) and surface segregation (seg.) of Ni ions. Above ∼2 mol% Ni, the redshift of A1g mode suggests that the surface segregation of Ni ions takes place. Disorder-activated bands were determined and their integrated intensity evolution with the Ni content suggest that the solid-solution regime favors the increase of disorder; meanwhile, that disorder becomes weaker as the Ni content is increased. Copyright © 2011 John Wiley & Sons, Ltd.

Publication

Magnetic characterization of maghemite nanoparticles dispersed in surface-treated polymeric template

(Springer Nature, 2007-04-01)

HUAMANI COAQUIRA, JOSE ANTONIO

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Rodriguez A.F.R.

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Santos J.G.

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Silveira L.B.

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Oliveira A.C.

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Garg V.K.

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Soares F.Q.

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Rabelo D.

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Morais P.C.

Magnetic characterization of maghemite nanoparticles dispersed in a polymeric template and treated under different chemical processes is reported in this work. Particle size estimated from magnetic measurements, DM ≈ 10 nm, for the free-surfactant sample, is consistent with values determined from XRD analysis and TEM images. The magnetic collapse of sextets towards a quadrupole doublet as the temperature is increased reveals the thermal relaxation of smaller γ-Fe2O3 nanoparticles. Magnetic measurements show a strong irreversibility between ZFC and FC curves suggesting the occurrence of particle-particle interaction. © 2008 Springer Science+Business Media B.V.

Publication

Structural and magnetic properties of pure and nickel doped SnO2 nanoparticles

(Institute of Physics Publishing, 2010-12-15)

HERRERA ARAGON, FERMIN FIDEL

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HUAMANI COAQUIRA, JOSE ANTONIO

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Hidalgo P.

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Brito S.L.M.

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Gouvêa D.

;

Castro R.H.R.

Ni-doped SnO2 nanoparticles prepared by a polymer precursor method have been characterized structurally and magnetically. Ni doping (up to 10 mol%) does not significantly affect the crystalline structure of SnO 2, but stabilizes smaller particles as the Ni content is increased. A notable solid solution regime up to ∼3 mol% of Ni, and a Ni surface enrichment for the higher Ni contents are found. The room temperature ferromagnetism with saturation magnetization (MS) ∼1.2 × 10 -3 emu g-1 and coercive field (HC) ∼40 Oe is determined for the undoped sample, which is associated with the exchange coupling of the spins of electrons trapped in oxygen vacancies, mainly located on the surface of the particles. This ferromagnetism is enhanced as the Ni content increases up to ∼3 mol%, where the Ni ions are distributed in a solid solution. Above this Ni content, the ferromagnetism rapidly decays and a paramagnetic behavior is observed. This finding is assigned to the increasing segregation of Ni ions (likely formed by interstitials Ni ions and nearby substitutional sites) on the particle surface, which modifies the magnetic behavior by reducing the available oxygen vacancies and/or the free electrons and favoring paramagnetic behavior. © 2010 IOP Publishing Ltd.

Publication

Magnetic hyperfine field at highly diluted Ce impurities in the antiferromagnetic compound GdRh 2Si 2 studied by perturbed gamma-gamma angular correlation spectroscopy

(Elsevier, 2012-02-25)

Cabrera-Pasca G.A.

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Carbonari A.W.

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Saxena R.N.

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Bosch-Santos B.

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HUAMANI COAQUIRA, JOSE ANTONIO

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Filho J.A.

The magnetic properties of GdRh 2Si 2 compound were investigated by hyperfine interactions and magnetization measurements. The temperature dependence of the magnetic hyperfine field (mhf) at highly diluted 140La- 140Ce probe nuclei in the GdRh 2Si 2 compound was measured using perturbed gamma-gamma angular correlation spectroscopy. A well-defined magnetic interaction is observed at 140Ce substituting Gd atoms in the compound below the Néel temperature (T N). However, the temperature dependence of mhf shows a deviation from the expected Brillouin-like behavior and a sharp increase of mhf values is observed for temperatures below approximately half of T N. This behavior has been associated with an additional magnetic interaction which is ascribed to the polarization of Ce spin moment induced by the magnetic field coming from Gd ions. A model based on the molecular-field theory is used to fit the temperature dependence of the mhf. The mhf contributions from the probe atom itself and from the magnetic Gd ions of the host matrix are determined. The results are compared to previous measurements of mhf at 140Ce in Gd and GdAg compound. The contribution from the host to the mhf is discussed in terms of the f-s and f-d interactions. © 2011 Elsevier B.V. All rights reserved.

Publication

Criticality of electron and hole escape sequence in nano-structured photovoltaic devices

(Springer Nature, 2006-12-01)

Alemu A.

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HUAMANI COAQUIRA, JOSE ANTONIO

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Freundlich A.

It has been empirically established that for quantum confined p-i-n solar cells a high electric field across the i-region is necessary for an optimal extraction of carriers from the well. This restriction imposes an upper limit for the total thickness of the i-region beyond which severe performance degradations are reported. For a given material system, the best efficiency trade-off is often achieved in the vicinity of this critical i-region thickness where the open circuit voltage degradation remains minimal and a higher photocurrent is afforded by the larger number of wells. But, even for devices that satisfy this condition, occasionally severe open circuit voltage degradation occurs. Here, we show that this degradation is directly correlated to the carrier escape sequence from the wells and that a careful engineering of hole and electron confining potentials can be used to alleviate this shortcoming. © 2006 Materials Research Society.

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