Estudio correlativo de propiedades electrónicas locales de GaSb mediante microscopías electrónica y túnel de barrido

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 05-03-2001Depto. de Física de MaterialesFac. de Ciencias FísicasTRUEpu

The role of surface properties in the cathodoluminescence of Zn2GeO4/SnO2 nanowire heterostructures

Herein, we report the influence of the surface conditions on the cathodoluminescence (CL) emissions from Zn_2GeO_4 nanowires and Zn_2GeO_4/SnO_2 heterostructures obtained by thermal evaporation technique. A Zn_2GeO_4 nanowire surrounded by a discontinuous shell of SnO_2 crystals composed the Zn_2GeO_4/SnO_2 heterostructures. Local CL measurements at different acceleration voltages allow monitoring the emission bands originated at the interface region, showing an additional deep-ultraviolet (UV) emission at 4.40 eV, which has not been previously reported. CL spectra from SnO_2 coated Zn_2GeO_4 nanowires also show this deep-UV emission. The results would confirm the presence of a shallow energy level close to the conduction band, which becomes active by passivation of Zn_2GeO_4 nanowires surface by the SnO_2 coating

Kinetic study of the thermal quenching of the ultraviolet emission in Zn_2GeO_4 microrods

Zn_2GeO_4 microrods obtained by thermal evaporation of a compacted powder mixture of ZnO and Ge exhibit quite intense UV luminescence at low temperatures. Herein, the luminescence properties of Zn_2GeO_4 microrods are studied for 2:1 and 1:1 ZnO:Ge ratio in the precursor mixture. In both cases, Zn2GeO4 microrods of high crystal quality produce a 355 nm emission under aforementioned bandgap excitation conditions at low temperatures. However, this emission vanishes at room temperature (RT) in the 1:1 samples while it is kept in the 2:1 ones. Herein this work, the thermal quenching of the UV luminescence is studied by means of steady and time-resolved photoluminescence techniques from 4 K up to RT for both Zn_2GeO_4 microrods. The analysis of the results leads us to conclude that although the luminescence mechanisms are the same in both cases, a higher decay rate is observed in the 1:1 in both intensity and lifetime, which explain the observed thermal quenching at RT

High aspect ratio GeO_2 nano- and microwires with waveguiding behaviour

High aspect ratio GeO_2 nano- and microwires have been grown by thermal treatment at 600 degrees C of compacted Ge powder under argon flow. The wires have cross-sectional dimensions from less than 100 nm to about 1 mu m, depending on the duration of the treatment, and lengths of up to about 2000 mu m. Waveguide behaviour of the wires was demonstrated for visible light, which shows the potential applications of these structures for optical nanodevices

Study of the incorporation of the Er and Nd Ions in Gallium Antimonide grown by the Bridgman method

(c) Sociedad Española de Cerámica y Vidrio, Consejo Superior de Investigaciones Científicas. Licencia Creative Commons 3.0 España (by-nc). Este trabajo ha sido subvencionado por CICYT a través del proyecto ESP-98 1340.En este trabajo se ha realizado el estudio de cristales de GaSb crecidos por el método Bridgman y dopados con Er y Nd con distintas concentraciones. Se han realizado análisis de absorción atómica pudiéndose obtener el coeficiente de segregación efectivo de ambos dopantes a lo largo de la dirección de crecimiento del material. Mediante medidas de efecto Hall se ha determinado el tipo de portadores mayoritarios (huecos) así como los valores de la movilidad, la densidad de portadores y la resistividad para cada una de las concentraciones. Los análisis de dispersión de energías de rayos X (EDAX) y de microscopio electrónico de barrido (SEM) han demostrado la presencia de agregados formados por los iones de tierras raras y Sb para las concentraciones de dopante más altas. La reducción de los defectos nativos tales como las vacantes de Ga y Ga en posición de Sb por los iones de Er ha sido también demostrado a través de análisis de catodoluminiscencia.In this work the study of GaSb crystals grown by the Bridgman method doped with Er and Nd with different concentrations has been carried out. Atomic absorption analysis have been developed for obtaining the effective segregation coefficient of the two dopants along the growth direction of the material. The resistivity, mobility and density of carriers were obtained by the van der Pauw technique fur the different dopant concentrations. The Energy dispersive X ray analysis (EDX) and the scanning electron microscope (SEM) have demonstrated the presence of precipitates made from Sb and rare earth elements for the highest dopant concentrations. Cathodoluminescence (CL) analysis have proved that the rare earth elements have a reduction effect of native defects in GaSb.Depto. de Física de MaterialesFac. de Ciencias FísicasTRUECICYTpu

Room temperature polymorphism in WO_(3) produced by resistive heating of W wires

Polymorphous WO_(3) micro- and nanostructures have been synthesized by the controlled Joule heating of tungsten wires under ambient conditions in a few seconds. The growth on the wire surface is assisted by the electromigration process and it is further enhanced by the application of an external electric field through a pair of biased parallel copper plates. In this case, a high amount of WO_(3) material is also deposited on the copper electrodes, consisting of a few cm^(2) area. The temperature measurements of the W wire agrees with the values calculated by a finite element model, which has allowed us to establish the threshold density current to trigger the WO_(3) growth. The structural characterization of the produced microstructures accounts for the gamma-WO_(3) (monoclinic I), which is the common stable phase at room temperature, along with low temperature phases, known as delta-WO_(3) (triclinic) on structures formed on the wire surface and e-WO_(3) (monoclinic II) on material deposited on external electrodes. These phases allow for a high oxygen vacancies concentration, which is interesting in photocatalysis and sensing applications. The results could help to design experiments to produce oxide nanomaterials from other metal wires by this resistive heating method with scaling-up potential

Using machine learning to link the influence of transferred Agrobacterium rhizogenes genes to the hormone profile and morphological traits in Centella asiatica hairy roots

Hairy roots are made after the integration of a small set of genes from Agrobacterium rhizogenes in the plant genome. Little is known about how this small set is linked to their hormone profile, which determines development, morphology, and levels of secondary metabolite production. We used C. asiatica hairy root line cultures to determine the putative links between the rol and aux gene expressions with morphological traits, a hormone profile, and centelloside production. The results obtained after 14 and 28 days of culture were processed via multivariate analysis and machine-learning processes such as random forest, supported vector machines, linear discriminant analysis, and neural networks. This allowed us to obtain models capable of discriminating highly productive root lines from their levels of genetic expression (rol and aux genes) or from their hormone profile. In total, 12 hormones were evaluated, resulting in 10 being satisfactorily detected. Within this set of hormones, abscisic acid (ABA) and cytokinin isopentenyl adenosine (IPA) were found to be critical in defining the morphological traits and centelloside content. The results showed that IPA brings more benefits to the biotechnological platform. Additionally, we determined the degree of influence of each of the evaluated genes on the individual hormone profile, finding that aux1 has a significant influence on the IPA profile, while the rol genes are closely linked to the ABA profile. Finally, we effectively verified the gene influence on these two specific hormones through feeding experiments that aimed to reverse the effect on root morphology and centelloside contentAgencia Estatal de Investigación | Ref. PID2020-113438RB-I00Gobierno de Cataluña | Ref. 2017SGR98

Electrical and compositional properties on Bridgman-grown Gd-doped GaSb substrates

In this work pure and Gd-doped GaSb ingots with different dopant concentrations were grown by the Bridgman Method. The compositional and electrical analysis carried out on these materials have shown the dependence of these results on the initial dopant concentration. The analysis have revealed the presence of inclusions which have been identified as Gd-Sb precipitates. The doped material was found to be p-type with a maximum value of the carrier density at the central region of the ingots. It has been also demonstrated that the Gd enhances the A band emission reducing the band-gap emission

Mentoría y recursos educativos para el Trabajo Fin de Grado del Grado en Física y del Grado en Ingeniería de Materiales

Depto. de Física de MaterialesFac. de Ciencias FísicasFALSEsubmitte