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An ever growing demand for efficient energy conversion, for instance inluminescent lamps and solar cells, has resulted in the current significantgrowth of research on functionalized nanomaterials for these applications.Recently it has been found that reduction of size of some transition metals, like gold(Au) and silver (Ag), below 1 nm results in emergence of properties, otherwiseabsent in their bulk counterparts. These properties include, but are notlimited to, bright photoluminescence. The latter can be tuned by changing thesize of these metal particles or the chemical composition of the host matricescontaining them.Luminescent few-atom Ag nanoclusters can be synthesized in various forms, such as incolloids or stabilized within flexible polymer matrices, or solid hosts. Inthis work we have proposed a simple method, based on a widely used glassmelt-quenching technique, for preparation of Ag nanoclusters stabilized withinbulk transparent glass hosts. The glass host provides many useful benefits,e.g. relatively easy fabrication en massand long-term chemical stability (optical and structural properties of oursamples has shown no apparent degradation over the course of several years).As-prepared glass samples doped with silver demonstrate a bright (quantum yield up to 20 %at room temperature) white luminescence that can be efficiently excited using abroad range of near-ultraviolet (UV) and some visible (VIS) wavelengths.In the course of the work we have employed numerous experimental techniques suchas continuous-wave (CW), time-resolved and polarization-resolvedphotoluminescence methods, optical absorption spectroscopy, x-ray diffraction,transmission electron microscopy (TEM) and energy-filtered TEM (EFTEM), andelectron spin resonance (ESR). Using the above techniques we have been able tounambiguously assign the detected luminescence to tiny (smaller than 1 nm)silver nanoclusters. Implementation of the theoretical modelling, inparticular, density functional theory (DFT), has allowed us to identifydiamagnetic Ag-tetramers (Ag42+) as the most commonluminescent Ag species in our glass, and to calculate energy levels thataccount for the luminescence.We have deepened our understanding of the mechanism of the luminescence bystudying dependence of Ag nanoclusters emission on the glass chemicalcomposition and temperature, when both cooling a sample down to few Kelvin orheating it up to several hundredth degree Celsius. We have found out that thestrong temperature dependent interaction of Ag nanoclusters with their localatomic surrounding (glass host) limits the quantum yield of luminescence. Whilethe inter-cluster energy transfer accounts for some temperature dependentspectral features of the emission.We have demonstrated that the above energy transfer can be used to spectrallybroaden excitation windows of some rare-earth ions, particularly, Yb3+.Here the energy is transferred from Agnanoclusters excited in UV to Yb3+ions, emitting at about 1 µm, due to the spectral overlap of the Agnanoclusters emission band with the excitation band of Yb3+-ions.

academic collection --- 538.91 <043> --- Structures, including transitions--Dissertaties --- Theses --- 538.91 <043> Structures, including transitions--Dissertaties

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The study of multiferroics and in particular the magnetoelectric couplin g is nowadays a new focus among the many hot topics in physics. The prom ising future of multiferroic materials lies in the coexistence of tradit ionally uncorrelated properties and the existence of a sizable coupling between them. Multiferroics provide a possibility to control one primary order by means of another primary order and create devices that are sui table for special conditions or can create multifunctionality. However, it is difficult to find multiferroicity due to the incompatiblity of fer romagnetism and ferroelectricity. The mechanism to establish magnetoelec tric coupling is always exotic and varying. Although there have been imp ortant research findings in multiferroics, such as lone-pair based multi ferroics, spin-driven ferroelectricity and multiferroic composites, etc, the field is far from mature yet, and the limitations lie both in the f abrication process as well as the complex measurement methods for charac terizing the magnetoelectrical properties. In this thesis, we achieved a structure consisting of Co atoms and Co na noparticles embedded in a crystalline ZnO thin film, aiming at finding m ultiferroicity and novel magnetotransport in such a nanocomposite system . We fabricated the system by pulsed laser deposition of ZnO followed by high-fluence Co ion implantation. The structural properties were charac terized by X-ray diffraction, transmission electron microscopy and X-ray absorption fine structure measurements. The majority of Co exists in th e matrix in an oxidized state (i.e. can be treated as a so-called (Zn,Co )O matrix). On the other hand, crystallized nanosize Co particles were f ound to be embedded in the (Zn,Co)O thin films, in which the thin film s till keeps good crystallinity. In such a complex nanocomposite system, t he electrical properties were studied by means of a helium-4 flow cryost at with a superconducting magnet down to 2.5 K. The low-temperature resi stance of the films followed the Efros-Shklovskii type variable-range-ho pping. Magnetoresistance revealed a spin-dependent signature that surviv es up to 250 K. Negative magnetoresistance exceeding 10% was observed at a magnetic field of 1 T. The magnetic properties were characterized by superconducting quantum interference device magnetometry. The composite reveals a characteristic superparamagnetic behavior, which is consistent with the presence of metallic Co nanoparticles. In addition, in some ca ses we observed exchange bias in the ZnO-Co system. This exchange bias e ffect constitutes, to the best of our knowledge, the first time observat ion of this effect in the ZnO-Co system. Temperature dependent dielectric constant measurements revealed relaxor ferroelectricity, which was further confirmed by polarization measuremen ts. The combination of the polarization measurement with a cryomagnetic system allowed us to determine the magnetoelectric coupling properties. A remarkable magnetoelectric coupling showed up when measuring the polar ization curve under an applied magnetic field. The unexpected relaxor fe rroelectricty is believed to be the result of the local lattice distorti on induced by the incorporation of the Co nanoparticles.

538.97 <043> --- 538.91 <043> --- Special geometry and interaction with particles and radiation--Dissertaties --- Structures, including transitions--Dissertaties --- Theses --- Academic collection --- 538.91 <043> Structures, including transitions--Dissertaties --- 538.97 <043> Special geometry and interaction with particles and radiation--Dissertaties