Micro- y nanoestructuras de Zn2GeO4 : síntesis, caracterización y estudio de las propiedades ópticas

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 05-11-2021In this PhD thesis work, the results obtained from the research on micro- and nanostructuresof zinc germanate (Zn2GeO4) are shown and discussed. This material is a wide bandgapsemiconductor, member of the transparent conducting oxide (TCO) family, which presentsseveral physical properties that make it a potential candidate for its integration in differentoptoelectronic or energy storage devices.The scope of applications of Zn2GeO4 can be extended or improved by controllingthe dimensions and morphology, as well as intrinsic defects and doping with impurities.Therefore, in this work, different synthesis or growth methods were employed in orderto achieve Zn2GeO4 structures with the desired dimensions, morphologies and physicalproperties. Thus, by means of the physical growth method of thermal evaporation, usingthe vapor-solid (VS) mechanism, both elongated Zn2GeO4 micro- and nanostructures wereobtained. This method was also used to dope the Zn2GeO4 structures by diffusion, includingthe dopant impurities during the growth stage. On the other hand, Zn2GeO4 nanocrystalswere synthesized via the chemical precipitation method...En el presente trabajo de tesis doctoral se muestran y discuten los resultados obtenidos durante la investigación realizada con micro- y nanoestructuras de germanato de zinc(Zn2GeO4). Este material es un semiconductor de gap ancho que forma parte de la familia de los óxidos conductores transparentes (TCO), y el cual presenta una serie de propiedades físicas que lo convierten en un potencial candidato para su incorporación en diversos dispositivos optoelectrónicos o de almacenamiento energético. El campo de aplicaciones del Zn2GeO4 se puede ampliar o mejorar mediante el control delas dimensiones y morfología, así como de los defectos intrínsecos y dopado con impurezas. Por ello, en este trabajo se emplearon diferentes métodos de síntesis o crecimiento, con el fin de lograr estructuras de Zn2GeO4 con las dimensiones, morfologías y propiedades físicas deseadas. Así, mediante el método de crecimiento físico de evaporación térmica, empleado el mecanismo vapor-sólido (VS), se obtuvieron tanto micro- y nanoestructuras alargadas de Zn2GeO4 como estructuras complejas. Este método fue también utilizado para doparlas estructuras de Zn2GeO4 mediante difusión, al incluir las impurezas dopantes durante la etapa de crecimiento. Por otro lado, se sintetizaron nanocristales de Zn2GeO4 a través del método de precipitación química...Fac. de Ciencias FísicasTRUEunpu

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

Transcranial static magnetic stimulation reduces seizures in a mouse model of Dravet syndrome

[Abstract] Dravet syndrome is a rare form of severe genetic epilepsy characterized by recurrent and long-lasting seizures. It appears around the first year of life, with a quick evolution toward an increase in the frequency of the seizures, accompanied by a delay in motor and cognitive development, and does not respond well to antiepileptic medication. Most patients carry a mutation in the gene SCN1A encoding the α subunit of the voltage-gated sodium channel Nav1.1, resulting in hyperexcitability of neural circuits and seizure onset. In this work, we applied transcranial static magnetic stimulation (tSMS), a non-invasive, safe, easy-to-use and affordable neuromodulatory tool that reduces neural excitability in a mouse model of Dravet syndrome. We demonstrate that tSMS dramatically reduced the number of crises. Furthermore, crises recorded in the presence of the tSMS were shorter and less intense than in the sham condition. Since tSMS has demonstrated its efficacy at reducing cortical excitability in humans without showing unwanted side effects, in an attempt to anticipate a possible use of tSMS for Dravet Syndrome patients, we performed a numerical simulation in which the magnetic field generated by the magnet was modeled to estimate the magnetic field intensity reached in the cerebral cortex, which could help to design stimulation strategies in these patients. Our results provide a proof of concept for nonpharmacological treatment of Dravet syndrome, which opens the door to the design of new protocols for treatment.Instituto de salud Carlos III; PI21/00151Xunta de Galicia; ED431C 2022/05 (CR)Ministerio de Ciencia e Innovacion (España); PID2019-108250RJ-10

Zn_2GeO_4/SnO_2 nanowire heterostructures driven by plateau-rayleigh instability

Herein, we report the formation of a particular core-shell structure, with a zinc germanate (Zn_2GeO_4) nanowire core and a discontinuous shell of SnO_2 nanocrystals, obtained in a single-step process. We propose a growth model that combines the Plateau-Rayleigh mechanism to produce a pattern of amorphous germanium oxide (a-GeO_2) particles along the Zn_2GeO_4 nanowire and the subsequent growth of well-faceted SnO_2 crystals when the nanowire orientation meets good lattice matching conditions. In this latter case, the linear array of a-GeO_2 particles acts as nucleation sites for the SnO_2 crystallites, leading to a skewer-like morphology that retains the periodicity of the Plateau-Rayleigh process. Otherwise, nanowires with different orientations appear decorated with a pattern of a-GeO_2 beads mimicking a necklace. Atomic resolution electron microscopy has been used to characterize the Zn_2GeO_4/SnO_2 nanoheterostructures. In addition, optical confinement effects have been observed in the luminescence maps and spectra, which have potential for further exploitation in the design of optical microcavities

Brain Structural and Functional Changes in Cognitive Impairment Due to Alzheimer's Disease.

Cognitive neuropsychology seeks a potential alignment between structural and functional brain features to explain physiological or pathological processes, such as Alzheimer's disease (AD). Several structural and functional brain changes occurring during the disease, including cognitive impairment, are found at the end of the patient's life, but we need to know more about what happens before its onset. In order to do that, we need earlier biomarkers at preclinical stages, defined by those biomarkers, to prevent the cognitive impairment. In this minireview, we have tried to describe the structural and functional changes found at different stages during AD, focusing on those features taking place before clinical diagnosis