Luis De Los Santos Valladares | University of Cambridge (original) (raw)
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Papers by Luis De Los Santos Valladares
Journal of Applied Physics, 1997
We have investigated the change in crystallographic and magnetic properties of sendust thin films... more We have investigated the change in crystallographic and magnetic properties of sendust thin films before/after annealing. Films were deposited by both rf diode and rf magnetron sputtering. rf diode-sputtered films did not possess soft magnetic properties in the as-deposited state. Films were subsequently annealed in both a conventional oven and a rapid thermal annealing ͑RTA͒ system. The coercivity of the films decreased from over 10 Oe to a minimum value of 0.6 Oe after annealing and a systematic shift in the ͑110͒ x-ray diffraction ͑XRD͒ peak position towards that of bulk sendust was observed. No additional XRD peaks were observed after annealing. The optimum annealing conditions to minimize coercivity was found to be 1-2 h at 550°C. By comparison, films processed by RTA resulted in minimum coercivity of 0.9 Oe obtained after annealing for 2 min at 550°C. XRD for films annealed by both RTA and oven annealing showed a similar decrease in the d spacing of the ͑110͒ plane, indicating a release of tensile strain in the films that had been introduced during deposition. rf magnetron-deposited samples possessed coercivities as low as 1.4 Oe as deposited and showed in-plane anisotropy. XRD again showed a ͑110͒ texture, however, the peak intensity of magnetron-sputtered samples was a factor of 20 less than the diode-sputtered films of the same thickness. The position of the ͑110͒ peak, however, was close to that of rf diode deposited films after annealing indicating that the magnetron-sputtered films were relatively stress-free.
Revista de Investigación de Física, 2015
En este trabajo se estudia las propiedades estructurales y morfológicas de las películas delgadas... more En este trabajo se estudia las propiedades estructurales y morfológicas de las películas delgadas de plata de 60 nm de espesor, depositadas sobre sustrato de SiO2 y tratadas térmicamente a distintas temperaturas desde 250 hasta 1000℃. Se ha encontrado que al incrementar la temperatura de recocido, la película de plata se cristaliza en la dirección [111] aumentando la rugosidad superficial y formando islas. Sin embargo, entre la temperatura de recocido de 800 a 900°C se observa un cambio en la orientación preferencial a la dirección [200]. Se discute este cambio de orientación mediante los procesos de difusión.
Microstructure and tensile properties of fine-grained copper fabricated by thermomechanical conso... more Microstructure and tensile properties of fine-grained copper fabricated by thermomechanical consolidation of copper nanopowder/micron-sized powder blend
Revista de la Sociedad Química del Perú
En la presente investigación presentamos un composito de polisiloxano funcionalizado con nanopart... more En la presente investigación presentamos un composito de polisiloxano funcionalizado con nanopartículas de plata (NPs-Ag). Se sintetizó NPs-Ag, por reducción de una solución de nitrato de plata (AgNO3) con borohidruro de sodio (NaBH4), las NPs-Ag se caracterizaron por el método espectrofotométrico UV-visible, encontrando que entre 400 – 410 nm se formó el típico pico de las NPs-Ag por la resonancia de plasmón superficial. En el análisis por Dispersión de Luz Dinámica (DLS) de las NPs-Ag se encontró una distribución polidispersa con dos poblaciones de 59,78 nm en un 70 % y de 4,33 nm en un 30%. Seguidamente, se sintetizó el gel de polisiloxano por hidrolización de clorosilano, usando como disolvente éter dietílico, el polisiloxano obtenido se caracterizó por espectrofotometría infrarroja con transformada de Fourier (FTIR). En el espectro se observan picos en la región de 1258,88 cm-1 correspondientes al metilo, entre 1000 - 1100 cm-1 un pico correspondiente al enlace Si- O-Si y una b...
Protection of Metals and Physical Chemistry of Surfaces
Titanium dioxide (TiO2) was deposited on polymethylmethacrylate (PMMA) substrates by magnetron sp... more Titanium dioxide (TiO2) was deposited on polymethylmethacrylate (PMMA) substrates by magnetron sputtering using a Compact Planar Magnetron sputtering device at different flow rate ratios of O2/Ar. The deposited TiO2 on PMMA substrates were characterized using X-ray Diffraction analysis, X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, UV-Visible Spectroscopy, and Scanning Electron Microscopy (SEM). These techniques confirm the deposition of a chemically stable amorphous TiO2 layer on the PMMA surface. Photocatalytic activity of the amorphous TiO2 layers were tested via photodegradation of Rhodamine 6G (Rh6G) dye solution. The samples were able to degrade 18–27% of the Rh6G solution after the initial 25 minutes of UV irradiation and complete degradation of Rh6G was observed after 7 hours of UV irradiation.
Advances in Science and Technology, Apr 27, 2006
Revista De La Sociedad Quimica Del Peru, Oct 1, 2007
Matéria (Rio de Janeiro), 2016
We report the fabrication and characterization of copper microelectrodes obtained by a homemade l... more We report the fabrication and characterization of copper microelectrodes obtained by a homemade lithography technique and after gold electroless deposition. For the fabrication, planes consisting of arrays of electrodes (black in color) with bow tie shape were designed and printed on a transparent paper (Canson ltd.). Using an embroidery frame with a silk fabric, a photographic emulsion was spread on the silk and simultaneously pressing the Canson paper on it. The system was introduced into a closed box and exposed with a UV light. The designed electrode templates prevented direct exposition of the UV light over copper films and indelible ink was spread over it. After the ink was dried, the copper film is immersed into ferric acid to attack the uncovered copper parts (where there is no ink). In this way, we obtained copper electrodes with initial gap separation of ~142μm and subsequently, they followed electroless deposition of gold to make the copper electrodes to contact. For the characterization, electrical measurements were performed. They present ohmic resistance values in the order of 10 6 Ω produced by surface scattering of the electrons within the gold microwire and enhanced by oxidation of the copper electrodes.
this chapter, the fabrication of metal nano-spaced electrodes for electronic nanodevices by elect... more this chapter, the fabrication of metal nano-spaced electrodes for electronic nanodevices by electro and electroless plating is discussed. The necessary reagents, conditions, and processes required to obtain nano and atomic gaps between soft and clean surfaces electrodes are described. In the electroless method, the plating process is performed catalytically after immersing the sample in a solution which contains the same metal ions. In the electrodeposition technique, metal ions of an electrolyte move towards the sample under an applied voltage. Both techniques are explained with examples, the first technique
We present the synthesis of YBa2Cu3O7-x superconductor oxide with critical temperature - 92 K, by... more We present the synthesis of YBa2Cu3O7-x superconductor oxide with critical temperature - 92 K, by Sol-Gel method. The synthesis of this superconducting compound was performed by using oxalate precursor solution which were prepared by dissolving stoichiometric mixtures of the Y(OOCCH3)•4H2O; Ba(OOCCH3)2 and Cu(OOCCH3)2•H2O acetates in an aqueous solution of oxalic acid, HCOO-COOH. The gel obtained was calcined at 860 oC and then fired at 920 oC in oxidizing atmosphere. The characterization of this superconducting ceramic was performed by x-ray diffraction, micro-Raman spectroscopy and SQUID techniques.
We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticl... more We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticles. Structural characterizations, carried out using X-ray diffraction, confirm the formation of magnetite phase with a mean size of ∼7 and ∼8 nm for the uncoated and gold covered magnetite nanoparticles, respectively. The value of the gold coated Fe 3 O 4 nanoparticles is consistent with the mean physical size determined from transmission electron microscopy images. Mössbauer spectra at room temperature are consistent with the thermal relaxation of magnetic moments mediated by particle-particle interactions. The 77 K Mössbauer spectra are modeled with four sextets. Those sextets are assigned to the signal of iron ions occupying the tetrahedral and octahedral sites in the core and shell parts of the particle. The room-temperature saturation magnetization value determined for the uncoated Fe 3 O 4 nanoparticles is roughly ∼60 emu/g and suggests the occurrence of surface effects such as magnetic disorder or the partial surface oxidation. These surface effects are reduced in the gold-coated Fe 3 O 4 nanoparticles. Zero-field-cooled and
Applied Physics Letters, 2009
Here we report on the preparation of transparent and flexible polymerized graphite oxide, which i... more Here we report on the preparation of transparent and flexible polymerized graphite oxide, which is composed of carbons with sp 3 -hybridized orbitals and a nonplanar ring structure, and which demonstrates dispersion in its dielectric constant at room temperature. This frequency dependence renders the material suitable for creating miniaturized, flexible, and transparent variable capacitors, allowing for smaller and simpler integrated electronic devices. We discuss this polarizability in terms of space charge effects.
Hyperfine Interactions, 2011
In this work, we study the long-term aging effect caused by Fe atoms in the superconductor CaLaBa... more In this work, we study the long-term aging effect caused by Fe atoms in the superconductor CaLaBa{Cu 1−x Fe x } 3 O 7−δ with 0 ≤ x ≤ 0.07. XRD confirms that this system has a YBCO-like structure. The critical temperature (T c ) is strongly affected by aging and depends on the amount of Fe in the structure. Room temperature Mössbauer spectroscopy reveals the presence of the typical species A, B-B , C and new species E and F. Interestingly; A, which corresponds to the Fe 3+ atom located in the Cu(1) of the chains with spin S z = 3/2, shows a drastic reduction which means migration to the species B, B and C. Species B and B correspond to the Fe 3+ in the Cu(2) site forming planar quasi-octahedral and planar square pyramidal, while the C specie is a square pyramidal with O(5) respectively (spin S z = 3/2 in all these cases). Aging causes loss of superconductivity in the samples with 5 and 7% of iron content.
Journal of Applied Physics, 1997
We have investigated the change in crystallographic and magnetic properties of sendust thin films... more We have investigated the change in crystallographic and magnetic properties of sendust thin films before/after annealing. Films were deposited by both rf diode and rf magnetron sputtering. rf diode-sputtered films did not possess soft magnetic properties in the as-deposited state. Films were subsequently annealed in both a conventional oven and a rapid thermal annealing ͑RTA͒ system. The coercivity of the films decreased from over 10 Oe to a minimum value of 0.6 Oe after annealing and a systematic shift in the ͑110͒ x-ray diffraction ͑XRD͒ peak position towards that of bulk sendust was observed. No additional XRD peaks were observed after annealing. The optimum annealing conditions to minimize coercivity was found to be 1-2 h at 550°C. By comparison, films processed by RTA resulted in minimum coercivity of 0.9 Oe obtained after annealing for 2 min at 550°C. XRD for films annealed by both RTA and oven annealing showed a similar decrease in the d spacing of the ͑110͒ plane, indicating a release of tensile strain in the films that had been introduced during deposition. rf magnetron-deposited samples possessed coercivities as low as 1.4 Oe as deposited and showed in-plane anisotropy. XRD again showed a ͑110͒ texture, however, the peak intensity of magnetron-sputtered samples was a factor of 20 less than the diode-sputtered films of the same thickness. The position of the ͑110͒ peak, however, was close to that of rf diode deposited films after annealing indicating that the magnetron-sputtered films were relatively stress-free.
Revista de Investigación de Física, 2015
En este trabajo se estudia las propiedades estructurales y morfológicas de las películas delgadas... more En este trabajo se estudia las propiedades estructurales y morfológicas de las películas delgadas de plata de 60 nm de espesor, depositadas sobre sustrato de SiO2 y tratadas térmicamente a distintas temperaturas desde 250 hasta 1000℃. Se ha encontrado que al incrementar la temperatura de recocido, la película de plata se cristaliza en la dirección [111] aumentando la rugosidad superficial y formando islas. Sin embargo, entre la temperatura de recocido de 800 a 900°C se observa un cambio en la orientación preferencial a la dirección [200]. Se discute este cambio de orientación mediante los procesos de difusión.
Microstructure and tensile properties of fine-grained copper fabricated by thermomechanical conso... more Microstructure and tensile properties of fine-grained copper fabricated by thermomechanical consolidation of copper nanopowder/micron-sized powder blend
Revista de la Sociedad Química del Perú
En la presente investigación presentamos un composito de polisiloxano funcionalizado con nanopart... more En la presente investigación presentamos un composito de polisiloxano funcionalizado con nanopartículas de plata (NPs-Ag). Se sintetizó NPs-Ag, por reducción de una solución de nitrato de plata (AgNO3) con borohidruro de sodio (NaBH4), las NPs-Ag se caracterizaron por el método espectrofotométrico UV-visible, encontrando que entre 400 – 410 nm se formó el típico pico de las NPs-Ag por la resonancia de plasmón superficial. En el análisis por Dispersión de Luz Dinámica (DLS) de las NPs-Ag se encontró una distribución polidispersa con dos poblaciones de 59,78 nm en un 70 % y de 4,33 nm en un 30%. Seguidamente, se sintetizó el gel de polisiloxano por hidrolización de clorosilano, usando como disolvente éter dietílico, el polisiloxano obtenido se caracterizó por espectrofotometría infrarroja con transformada de Fourier (FTIR). En el espectro se observan picos en la región de 1258,88 cm-1 correspondientes al metilo, entre 1000 - 1100 cm-1 un pico correspondiente al enlace Si- O-Si y una b...
Protection of Metals and Physical Chemistry of Surfaces
Titanium dioxide (TiO2) was deposited on polymethylmethacrylate (PMMA) substrates by magnetron sp... more Titanium dioxide (TiO2) was deposited on polymethylmethacrylate (PMMA) substrates by magnetron sputtering using a Compact Planar Magnetron sputtering device at different flow rate ratios of O2/Ar. The deposited TiO2 on PMMA substrates were characterized using X-ray Diffraction analysis, X-ray Photoelectron Spectroscopy, Fourier Transform Infrared Spectroscopy, UV-Visible Spectroscopy, and Scanning Electron Microscopy (SEM). These techniques confirm the deposition of a chemically stable amorphous TiO2 layer on the PMMA surface. Photocatalytic activity of the amorphous TiO2 layers were tested via photodegradation of Rhodamine 6G (Rh6G) dye solution. The samples were able to degrade 18–27% of the Rh6G solution after the initial 25 minutes of UV irradiation and complete degradation of Rh6G was observed after 7 hours of UV irradiation.
Advances in Science and Technology, Apr 27, 2006
Revista De La Sociedad Quimica Del Peru, Oct 1, 2007
Matéria (Rio de Janeiro), 2016
We report the fabrication and characterization of copper microelectrodes obtained by a homemade l... more We report the fabrication and characterization of copper microelectrodes obtained by a homemade lithography technique and after gold electroless deposition. For the fabrication, planes consisting of arrays of electrodes (black in color) with bow tie shape were designed and printed on a transparent paper (Canson ltd.). Using an embroidery frame with a silk fabric, a photographic emulsion was spread on the silk and simultaneously pressing the Canson paper on it. The system was introduced into a closed box and exposed with a UV light. The designed electrode templates prevented direct exposition of the UV light over copper films and indelible ink was spread over it. After the ink was dried, the copper film is immersed into ferric acid to attack the uncovered copper parts (where there is no ink). In this way, we obtained copper electrodes with initial gap separation of ~142μm and subsequently, they followed electroless deposition of gold to make the copper electrodes to contact. For the characterization, electrical measurements were performed. They present ohmic resistance values in the order of 10 6 Ω produced by surface scattering of the electrons within the gold microwire and enhanced by oxidation of the copper electrodes.
this chapter, the fabrication of metal nano-spaced electrodes for electronic nanodevices by elect... more this chapter, the fabrication of metal nano-spaced electrodes for electronic nanodevices by electro and electroless plating is discussed. The necessary reagents, conditions, and processes required to obtain nano and atomic gaps between soft and clean surfaces electrodes are described. In the electroless method, the plating process is performed catalytically after immersing the sample in a solution which contains the same metal ions. In the electrodeposition technique, metal ions of an electrolyte move towards the sample under an applied voltage. Both techniques are explained with examples, the first technique
We present the synthesis of YBa2Cu3O7-x superconductor oxide with critical temperature - 92 K, by... more We present the synthesis of YBa2Cu3O7-x superconductor oxide with critical temperature - 92 K, by Sol-Gel method. The synthesis of this superconducting compound was performed by using oxalate precursor solution which were prepared by dissolving stoichiometric mixtures of the Y(OOCCH3)•4H2O; Ba(OOCCH3)2 and Cu(OOCCH3)2•H2O acetates in an aqueous solution of oxalic acid, HCOO-COOH. The gel obtained was calcined at 860 oC and then fired at 920 oC in oxidizing atmosphere. The characterization of this superconducting ceramic was performed by x-ray diffraction, micro-Raman spectroscopy and SQUID techniques.
We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticl... more We report on the synthesis and characterization of uncoated and gold coated magnetite nanoparticles. Structural characterizations, carried out using X-ray diffraction, confirm the formation of magnetite phase with a mean size of ∼7 and ∼8 nm for the uncoated and gold covered magnetite nanoparticles, respectively. The value of the gold coated Fe 3 O 4 nanoparticles is consistent with the mean physical size determined from transmission electron microscopy images. Mössbauer spectra at room temperature are consistent with the thermal relaxation of magnetic moments mediated by particle-particle interactions. The 77 K Mössbauer spectra are modeled with four sextets. Those sextets are assigned to the signal of iron ions occupying the tetrahedral and octahedral sites in the core and shell parts of the particle. The room-temperature saturation magnetization value determined for the uncoated Fe 3 O 4 nanoparticles is roughly ∼60 emu/g and suggests the occurrence of surface effects such as magnetic disorder or the partial surface oxidation. These surface effects are reduced in the gold-coated Fe 3 O 4 nanoparticles. Zero-field-cooled and
Applied Physics Letters, 2009
Here we report on the preparation of transparent and flexible polymerized graphite oxide, which i... more Here we report on the preparation of transparent and flexible polymerized graphite oxide, which is composed of carbons with sp 3 -hybridized orbitals and a nonplanar ring structure, and which demonstrates dispersion in its dielectric constant at room temperature. This frequency dependence renders the material suitable for creating miniaturized, flexible, and transparent variable capacitors, allowing for smaller and simpler integrated electronic devices. We discuss this polarizability in terms of space charge effects.
Hyperfine Interactions, 2011
In this work, we study the long-term aging effect caused by Fe atoms in the superconductor CaLaBa... more In this work, we study the long-term aging effect caused by Fe atoms in the superconductor CaLaBa{Cu 1−x Fe x } 3 O 7−δ with 0 ≤ x ≤ 0.07. XRD confirms that this system has a YBCO-like structure. The critical temperature (T c ) is strongly affected by aging and depends on the amount of Fe in the structure. Room temperature Mössbauer spectroscopy reveals the presence of the typical species A, B-B , C and new species E and F. Interestingly; A, which corresponds to the Fe 3+ atom located in the Cu(1) of the chains with spin S z = 3/2, shows a drastic reduction which means migration to the species B, B and C. Species B and B correspond to the Fe 3+ in the Cu(2) site forming planar quasi-octahedral and planar square pyramidal, while the C specie is a square pyramidal with O(5) respectively (spin S z = 3/2 in all these cases). Aging causes loss of superconductivity in the samples with 5 and 7% of iron content.