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Propiedades electrónicas y magnéticas de películas de ZnO y ZnO:Mn

Electronic and magnetic propierties of ZnO and ZnO:Mn films

CAROLINA BOHORQUEZ MARTINEZ (2017)

En este trabajo de tesis se han estudiado las propiedades ópticas, electrónicas y magnéticas de películas de ZnO y ZnO dopado con Mn usando las técnicas de catodoluminiscencia, espectroscopía túnel y microscopía de fuerza atómica. Las muestras de ZnO caracterizadas fueron de dos naturalezas, la primera un monocristal con la cara Zn pulida y orientación (0001), la segunda consiste en películas crecidas por el método de depósito por capas atómicas. Las películas dopadas con Mn fueron obtenidas mediante la técnica de depósito por baño químico (CBD). La morfología de las muestras se estudió a través de microscopía electrónica de barrido (SEM), microscopía de efecto túnel (STM) y microscopía de fuerza atómica (AFM) para las muestras dopadas. Para el estudio de las propiedades ópticas se utilizó la técnica de catodoluminiscencia (CL) a temperatura ambiente cuyo dispositivo se encuentra adaptado en el SEM. Las propiedades electrónicas estudiadas a través de espectros de conductancia fueron usados para medir del gap de superficie, para ellos se utilizó espectroscopia túnel de barrido a baja temperatura 77 K. Por último las muestras de ZnO:Mn fueron caracterizadas mediante microscopía de fuerza magnética (MFM) para observar los dominios magnéticos inducidos por la impurificación del Mn. De los resultados se obtuvo que hay dos emisiones que componen el espectro de CL del monocristal, una intensa centrada en 3.2 eV correspondiente al borde de banda del semiconductor y otra, no común en cristales de alta calidad cristalina, menos intensa centrada en 2.45 eV que corresponde a la emisión asociada a la presencia de vacancias de oxígeno, los cuales fueron generados por fronteras de grano identificadas por el STM. Los resultados de espectroscopía túnel (STS) revelaron una brecha de energía prohibida en la superficie del monocristal de aproximadamente 1.5 eV, atribuido a una alta densidad de portadores de carga libres posiblemente inducidos por la presencia de defectos nativos tipo vacancias de oxígeno, identificados por CL. Los estudios de AFM de la película de ZnO:Mn mostró que la película de ZnO sintetizada por ALD, usada como sustrato, promovió el crecimiento de granos de forma triangular que corresponden a la orientación (0001) del ZnO. Si bien la morfología de esta película dopada con Mn fue irregular, compuesta por numerosos aglomerados del mismo material adheridos a su superficie, su composición fue homogénea.

In this research, the optical, electronic and magnetic properties of ZnO and Mn-doped ZnO were studied using the techniques of cathodoluminescence, tunnel spectroscopy, and atomic force microscopy. The undoped samples studied were ZnO monocrystalline with a polished Zn face and orientation (0001), besides a ZnO film grown by atomic layer deposition (ALD). The Mn-doped sample was a film grown by chemical bath deposition (CBD) technique. The morphology of the samples was studied by scanning electron microscopy (SEM), scanning tunnel microscopy (STM) and atomic force microscopy (AFM) for the doped samples. For the study of the optical properties the cathodoluminescence (CL) technique, adapted in an SEM system, was used at room temperature. The electronic structure of samples was studied acquiring tunnel conductance curves using an STM in UHV and operated at 77 K. Finally, the magnetic response of the ZnO:Mn films was characterized by magnetic force microscopy (MFM). CL spectrum acquired from the ZnO monocrystal revealed two emissions centered at 3.2 eV that correspond to the ZnO band-edge besides a weak and broad emission of 2.45 eV assigned to the presence of oxygen vacancies, generated at the grain boundaries of the monocrystal, which were identified by STM. Tunnel spectroscopy measurements from the ZnO monocrystalline showed a surface band gap of about 1.5 eV, attributed to a high density of free electrons possibly induced by native oxygen vacancies. AFM images of the ZnO:Mn film exhibited the growth of triangular grains corresponding with the (0001) plane of the ZnO. These results also showed an irregular surface morphology composed of ZnO:Mn agglomerates adhered at the surface of the film. Tunnel spectroscopy of the ZnO:Mn films revealed the presence of donor states at 1.1 and 1.7 eV above the Fermi level, and acceptor states at 0.5 eV under the Fermi level. Finally, the MFM measurements of this film exhibited a strong magnetic interference in the oscillation phase of the cantilever at the agglomerates and the granular surface of the sample. This result was attributed to the homogeneous concentration of the Mn ion incorporated in the ZnO.

Master thesis

ZnO, Mn, Defectos puntuales, Catodoluminiscencia, Espectroscopía túnel, Microscopía de fuerza magnética, Estados profundos Point defects, Cathodoluminescence, Tunnel spectroscopy, Magnetic force microscopy, Deep states CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA FÍSICA DEL ESTADO SÓLIDO SEMICONDUCTORES

Influence of low-temperatue nucleation on the crystallization process of poly(L-lactide)

FERNANDO HERNANDEZ SANCHEZ (2005)

The crystallization kinetics of poly(L-lactide), PLLA, is slow enough to allow a quasi-amorphous polymer to be obtained at low temperature simply by quenching from the melt. The PLLA crystallization process was followed by differential scanning calorimetry and optical microscopy after nucleation isothermal treatments at temperatures just below (53 °C) and just above (73 °C) the glass transition temperature. The crystallization exotherm shown in the heating thermograms shifts toward lower temperatures as the annealing time at 73 °C increases. The same effect is shown to a lesser extent when the sample nucleates at 53°C, showing the ability to nucleate in the glassy state, already shown in other polymers. The shape of the DSC thermograms is modeled by using Avrami's theory and allows an estimation of the number of crystallization germs formed. The results of optical microscopy are converted to thermograms by evaluating the average gray level of the image recorded in transmission mode as a function of temperature and calculating its temperature derivative. The shape of such optical thermograms is quite similar to that of the DSC traces but shows some peculiarities after long nucleation treatments. Atomic force microscopy was used to analyze the crystal morphology and is an additional proof of the effect of nucleation in the glassy state. The crystalline morphology observed in samples crystallized after nucleation in the glassy state is qualitatively different from that of samples nucleated above the glass transition temperature, and the number of crystals seems to be much greater than what could be expected from the crystallization kinetics.

Article

 ANNEALING ATOMIC FORCE MICROSCOPY CRYSTALLIZATION DIFFERENTIAL SCANNING CALORIMETRY INGENIERÍA Y TECNOLOGÍA

PHOTOCHEMICAL SYNTHESIS OF HEPARIN-BASED GOLD NANOPARTICLES AND THEIR USE IN SURFACE ENHANCED RAMAN SPECTROSCOPY

María del Pilar Rodríguez-Torres (2015)

"This dissertation deals with gold nanoparticle green synthesis in aqueous solution using heparin, as a reducing and stabilizing agent, without other reagents through photochemistry, and their usage as surface enhanced Raman colloidal substrates. This work is divided in four parts: The first one is about proposing the photochemical synthesis method used to prepare the heparinbased gold nanoparticles in aqueous solution using UVA light (λ=366 nm). Two types of experiments were carried out: The nanoparticles were synthesized using reactive and pharmaceutical-grade heparins. Nanoparticle characterization was carried out with UV-Vis spectroscopy and Scanning Electron Microscopy. In the second one, a nanoparticle formation mechanism is proposed with the aid of UV-Vis and FTIR spectroscopies and pH measurements.

In the third part, surface enhanced Raman activity is investigated, finding that reactive-grade heparin gold nanoparticles yield good Raman signals and the pharmaceutical-grade ones do not. The probe molecules used were differently charged dyes: Methylene Blue (-), Rosebengal (+) and Neutral Red (+-); as well as proteins (GP120 and Bovine Serum Albumin). There are no reports on heparin-gold nanoparticles being used for this purpose to our knowledge. In the final section, this dissertation´s contributions are presented as a closure to the research that was performed."

Doctoral thesis

UV-VIS SPECTROSCOPY, SCANNING ELECTRON MICROSCOPY, NANOPARTICLES, RAMAN SPECTROSCOPY CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ÓPTICA

COMPARACIÓN NUMÉRICA Y EXPERIMENTAL DE LA FUERZA GENERADA SOBRE LAS SECCIONES DE UN ASPA A BAJOS NÚMEROS DE REYNOLDS

NIDIANA ROSADO HAU MAURICIO GAMBOA MARRUFO MAURICIO ALBERTO ESCALANTE SOBERANIS (2017)

Durante el estado inicial del giro del rotor de una turbina, el n´umero de Reynolds que experimentan las

aspas es bajo y el ´angulo de ataque presente es alto. La estimaci´on de la fuerza generada por el aspa en este

estado es importante para conocer el momento producido y para saber si ´este ser´a suficiente para vencer

el torque resistivo del generador el´ectrico. En este trabajo se calculan experimental y num´ericamente

las fuerzas que se generan sobre 2 secciones de un aspa dise˜nada con el perfil aerodin´amico s1210. Las

pruebas en t´unel de viento se realizaron a 20 y 15 m/s, permitiendo n´umeros de Reynolds promedio de

1.15E5, 8.6E4, 5.5E4 y 4.1E4 sobre las secciones. Las secciones se probaron a los ´angulos de inclinaci´on

de 0, 15, 30, 45, 60, 70, 80, 85 y 90 grados; posteriormente, se realiz´o una estimaci´on num´erica que

emple´o el programa xfoil para la obtenci´on de los coeficientes de sustentaci´on del perfil en un rango de

n´umeros de Reynolds entre 3.4E4 y 1.3E5, y en un rango de ´angulo de ataque entre 0 y 90 grados. Los

resultados muestran que la estimaci´on num´erica presenta un error relativo menor al 15 % con respecto

a la estimaci´on experimental a n´umeros de Reynolds de 4.1E4 y 5.5E4 y en un rango de ´angulo de

inclinaci´on entre 15 y 70 grados. El perfil s1210 presenta coeficientes de sustentaci´on mayores a 1 en el

rango de 1 a 30 grados de ´angulo de ataque a n´umeros de Reynolds menores de 1.35E5.

Article

BIOLOGÍA Y QUÍMICA MEDICINA Y CIENCIAS DE LA SALUD Xfoil Wind turbine Tunnel High attack angle

Diseño e implementación de un sistema de análisis de películas delgadas en frecuencias de microondas usando microscopio de fuerza atómica y analizador de redes vectorial

Design and implementation of a microwave system for thin-film analysis using atomic-force microscope and vector-network analyzer

ULISES AUGUSTO PONT DE LA TORRE (2020)

En este reporte de tesis se presenta la implementación de un sistema de microscopía de microondas utilizando un Microscopio de Fuerza Atómica (AFM por sus siglas en inglés) y un Analizador de Redes Vectorial (VNA por sus siglas en inglés). Se diseñaron y fabricaron sondas resonantes y no resonantes que terminan en una punta conductiva, y que operan a frecuencias que van desde 500 MHz hasta 6 GHz utilizando únicamente las pérdidas por reflexión. El sistema propuesto muestra la capacidad de determinar cambios en las propiedades de muestras de películas delgadas a escala nanométrica, tales como altura del dieléctrico, altura de las capas conductivas, y permitividad dieléctrica de la capa aislante. Se presentan los resultados simulados y medidos experimentalmente de las pruebas resonante y no resonante, y son comparados y discutidos.

The implementation of a Microwave Microscopy (MM) system using an Atomic-Force Microscope (AFM) and a Vector Network Analyzer (VNA) is presented in this thesis report. Resonant and non-resonant probes are designed and fabricated to operate at frequencies from 500 MHz to 6 GHz using reflection losses data only and are integrated into the AFM. A conductive tip is configured at the end of the probes for the measurements. The proposed system shows the capability to determine changes in thin-films properties, such as height of dielectric, height of conductors in the nanometer scale, and dielectric permittivity of insulator layers. Both methods, resonant and non-resonant, are compared and discussed.

Master thesis

RF, microondas, permitividad, microscopio de fuerza atómica, películas delgadas RF, microwave, permittivity, atomic force microscopy, thin films. INGENIERÍA Y TECNOLOGÍA CIENCIAS TECNOLÓGICAS OTRAS ESPECIALIDADES TECNOLÓGICAS OTRAS OTRAS

Influence of structural defects on the electrical properties of carbon nanotubes and their polymer composites

GUSTAVO DOMINGUEZ RODRIGUEZ JORGE ALEJANDRO TAPIA GONZALEZ G.D. SEIDEL FRANCIS AVILES CETINA (2016)

The influence of carbon nanotube (CNT) structural damage (CNTSD) on the axial and transverse electrical conductivities of CNT/polymer composites is explored through a hierarchical multiscale modeling strategy. The composite cylinder and Mori–Tanaka's methods are used to model effective representative volume elements of CNT/polymer composites containing different fractions of defects. The axial and transverse CNT conductivities are adversely influenced by CNTSD, with the decrease being more pronounced for small radius CNTs. The predictions indicate that an 8% fraction of CNTSD decreases the axial and transverse conductivities of composites containing randomly oriented CNTs by 25–30%. Similar reductions in conductivity are found for both random and clustered damage.

Article

CARBON DEFECTS NANOTUBES YARN INGENIERÍA Y TECNOLOGÍA INGENIERÍA Y TECNOLOGÍA

Influence of structural defects on the electrical properties of carbon nanotubes and their polymer composites

GUSTAVO DOMINGUEZ RODRIGUEZ JORGE ALEJANDRO TAPIA GONZALEZ G.D. SEIDEL FRANCIS AVILES CETINA (2016)

The influence of carbon nanotube (CNT) structural damage (CNTSD) on the axial and transverse electrical conductivities of CNT/polymer composites is explored through a hierarchical multiscale modeling strategy. The composite cylinder and Mori–Tanaka's methods are used to model effective representative volume elements of CNT/polymer composites containing different fractions of defects. The axial and transverse CNT conductivities are adversely influenced by CNTSD, with the decrease being more pronounced for small radius CNTs. The predictions indicate that an 8% fraction of CNTSD decreases the axial and transverse conductivities of composites containing randomly oriented CNTs by 25–30%. Similar reductions in conductivity are found for both random and clustered damage.

Article

CARBON DEFECTS NANOTUBES YARN INGENIERÍA Y TECNOLOGÍA

Influence of protons on reduction degree and defect formation in electrochemically reduced graphene oxide

JAVIER ALAN QUEZADA RENTERIA Ania Conchi O. Luis Felipe Cházaro Ruiz JOSE RENE RANGEL MENDEZ (2019)

"The electrochemical reduction of GO was investigated in aqueous, at both acid and basic pH, and organic media, to identify the possible role of protons (H+) in the reduction mechanism of this material. The obtained rGO films were characterized by FTIR, electrochemical methods, Raman and XPS spectroscopy. Data showed that the reduction was more efficient in acid and basic media due to the presence of protons and the capacity of water that works as a proton donor, resulting in C/O ratios of 3.8 and 7.8, respectively. Mostly hydroxyl, epoxide and carbonyl moieties were removed. In a proton-free organic electrolyte, a C/O ratio of 1.8 was obtained for most of the samples; nevertheless, the graphitic carbon sp2 domains were restored to a large extent in the absence of H+. The characterization of the material showed that the presence of protons, during the electrochemical reduction, caused hydrogenation reactions, which targeted the graphitic domains in rGO and resulted in the loss of sp2 hybridization. The presence of such defects modified the electrochemical properties of the rGO films, where, despite of exhibiting higher C/O ratio, the films reduced in aqueous electrolytes displayed lower electron transfer (e.g. ferrocyanide redox-probe) than those reduced in organic electrolyte."

Article

Graphene oxide Electrochemical reduction Reduced graphene oxide Defects BIOLOGÍA Y QUÍMICA QUÍMICA QUÍMICA

Spectroscopic and mechanical studies on the Fe-based amorphous alloy 2605SA1

AGUSTIN CABRAL PRIETO JUAN ANTONIO CONTRERAS VITE IRMA MARIA ENGRACIA GARCIA SOSA NOEL NAVA PERALTA FEDERICO GABRIEL GARCIA SANTIBAÑEZ SANCHEZ (2010)

The Vickers micro-hardness of this alloy was unusually dependent on the heat treatment from 300 to 634K, inferring important micro-structural changes and the presence of amorphous grains before its phase transition. Once the alloy is crystallized, the micro-hardness is characteristic of a brittle alloy, the main problem of these alloys. Within the amorphous state, other properties like free-volume, magnetic states and Fe-Fe distances were followed by PALS and MS, respectively, to analyze those micro-structural changes, thermally induced, which are of paramount interest to understand their brittleness problem.

Article

química amorphous alloys Mössbauer pectroscopy annihilation of positrons free-volume defects micro hardness brittleness BIOLOGÍA Y QUÍMICA

Rectificación del movimiento Browniano usando Ratchets ópticos

JAVIER SILVA BARRANCO (2010)

Particle transport phenomena far from thermal equilibrium in periodic systems but asymmetric spatially, has become an important topic research issues, mainly in disciplines where it plays an important role such as biology a nanotechnology.

This technique called ratchet, allows to rectify the Brownian motion of particles induced with asymmetric periodic potential structures. In particular, we consider the flashing ratchet model, where the perturbation involves switching on and off a spatial sawtooth potential. The diffusion governed by Brownian motion allows the directed transport of the particle as a function of temporal and spatial variation of potential. The asymmetrical pattern is generated with phase modulation which is transformed into an intensity pattern using the phase contrast method. In this context, we study an ensemble of micro-latex particles of diameter d0=0.5 µm, where is characterized the average velocity of the ensemble, which is exposed a different spatial periods as well as a different time periods of switching potential. The dynamics is described by the Langevien equation.

El fenómeno de transporte de partículas en sistemas periódicos, pero espacialmente asimétricos fuera del equilibrio térmico, ha despertado gran interés en temas de investigación principalmente en disciplinas como biología y nano tecnología. Esta técnica llamada ratchet, consiste en direccionar el movimiento de partículas Browniananas inducido con estructuras de potencial periódico asimétrico. En particular consideramos el modelo flashing ratchet, la perturbación involucra una conmutación de encendido y apagado del potencial óptico en forma de diente de sierra. La difusión gobernada por el movimiento Browniano permite el transporte direccionado de la partícula en función de la variación temporal y especial del potencial.

El patrón asimétrico es generado con un modulador de fase y transformado en un patrón de intensidad usando el método de contraste de fase. En este contexto, se estudia un ensamble de micropartículas de látex de diámetro d= 0.5 µm, donde se caracteriza la velocidad promedio del ensamble a diferentes periodos espaciales y tiempos de conmutación del potencial. La dinámica es descrita por la ecuación de Langevin.

Master thesis

Transport processes Force Fluctuations CIENCIAS FÍSICO MATEMÁTICAS Y CIENCIAS DE LA TIERRA FÍSICA ÓPTICA