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Register a new userCan Europium Atoms form Luminescent Centres in Diamond: A combined Theoretical-Experimental Study
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Added by
Danny E. P. Vanpoucke Dr.
Publication date
2019
fields
Physics
and research's language is
English
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The incorporation of Eu into the diamond lattice is investigated in a combined theoretical-experimental study. The large size of the Eu ion induces a strain on the host lattice, which is minimal for the Eu-vacancy complex. The oxidation state of Eu is calculated to be 3+ for all defect models considered. In contrast, the total charge of the defect-complexes is shown to be negative -1.5 to -2.3 electron. Hybrid-functional electronic-band-structures show the luminescence of the Eu defect to be strongly dependent on the local defect geometry. The 4-coordinated Eu substitutional dopant is the most promising candidate to present the typical Eu3+ luminescence, while the 6-coordinated Eu-vacancy complex is expected not to present any luminescent behaviour. Preliminary experimental results on the treatment of diamond films with Eu-containing precursor indicate the possible incorporation of Eu into diamond films treated by drop-casting. Changes in the PL spectrum, with the main luminescent peak shifting from approximately 614 nm to 611 nm after the growth plasma exposure, and the appearance of a shoulder peak at 625 nm indicate the potential incorporation. Drop-casting treatment with an electronegative polymer material was shown not to be necessary to observe the Eu signature following the plasma exposure, and increased the background luminescence.
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The effects of rhenium doping in the range 0 to 10 atomic percent on the static and dynamic magnetic properties of Fe65Co35 thin films have been studied experimentally as well as with first principles electronic structure calculations focusing on the change of the saturation magnetization and the Gilbert damping parameter. Both experimental and theoretical results show that the saturation magnetization decreases with increasing Re doping level, while at the same time Gilbert damping parameter increases. The experimental low temperature saturation magnetic induction exhibits a 29 percent decrease, from 2.31 T to 1.64 T, in the investigated doping concentration range, which is more than predicted by the theoretical calculations. The room temperature value of the damping parameter obtained from ferromagnetic resonance measurements, correcting for extrinsic contributions to the damping, is for the undoped sample 0.0027, which is close to the theoretically calculated Gilbert damping parameter. With 10 atomic percent Re doping, the damping parameter increases to 0.0090, which is in good agreement with the theoretical value of 0.0073. The increase in damping parameter with Re doping is explained by the increase in density of states at Fermi level, mostly contributed by the spin-up channel of Re. Moreover, both experimental and theoretical values for the damping parameter are observed to be weakly decreasing with decreasing temperature.
The defect chemistry of perovskite compounds is directly related to the stoichiometry and to the valence states of the transition metal ions. Such relations are of high interest as they offer the possibility to influence the catalytic activity of perovskites for the application in solid-oxide fuel- and electrolyser cells. Combining theoretical and experimental approaches, we explore the feasibility of actively manipulating the valence state of Fe and the concentration of point defects by synthesizing non-stoichiometric LaFeO$_3$ (LFO). In the theoretical part, formation energies and concentrations of point defects were determined as a function of processing conditions by first-principles DFT+U calculations. Based on the DFT+U results, significant compositional deviations from stoichiometric LFO cannot be expected by providing rich or poor conditions of the oxidic precursor compounds (Fe$_2$O$_3$ and La$_2$O$_3$) in a solid-state processing route. In the experimental part, LFO was synthesized with a targeted La-site deficiency. We analyze the resulting phases in detail by X-ray diffraction and dedicated microscopy methods, namely scanning electron microscopy (SEM) and (scanning) transmission electron Microscopy ((S)TEM) in combination with energy dispersive X-ray spectroscopy (EDS) and electron energy-loss spectrometry (EELS). Instead of a variation of the La/Fe ratio, a mixture of two phases, Fe$_2$O$_3$/LaFeO$_3$, was observed resulting in an invariant charge state of Fe, which is in line with the theoretical results. We discuss our findings with respect to partly differing assumptions made in previously published studies on this material system.
A combined experimental and theoretical study is presented of the band gap of AlInN, confirming the breakdown of the virtual crystal approximation (VCA) for the conduction and valence band edges. Composition dependent bowing parameters for these quantities are extracted. Additionally, composition dependent band offsets for GaN/AlInN systems are provided. We show that local strain and built-in fields affect the band edges significantly, leading to optical polarization switching at much lower In composition than expected from a VCA approach.
The structural behaviour of CsCdF3 under pressure is investigated by means of theory and experiment. High-pressure powder x-ray diffraction experiments were performed up to a maximum pressure of 60 GPa using synchrotron radiation. The cubic $Pmbar{3}m$ crystal symmetry persists throughout this pressure range. Theoretical calculations were carried out using the full-potential linear muffin-tin orbital method within the local density approximation and the generalized gradient approximation for exchange and correlation effects. The calculated ground state properties -- the equilibrium lattice constant, bulk modulus and elastic constants -- are in good agreement with experimental results. Under ambient conditions, CsCdF3 is an indirect gap insulator with the gap increasing under pressure.
Lanthanides are vital components in lighting, imaging technologies and future quantum memory applications due to their narrow optical transitions and long spin coherence times. Recently, diamond has become a preeminent platform for realization of many experiments in quantum information science. In this work, we demonstrate a promising approach to incorporate Eu ions into single crystal diamond and nanodiamonds, providing a means to harness the exceptional characteristics of both lanthanides and diamond in a single material. Polyelectrolytes are used to electrostatically assemble Eu(III) chelate molecules on diamond and subsequently chemical vapor deposition is employed for the growth of a high quality diamond crystal. Photoluminescence, cathodoluminescence and time resolved fluorescence measurements show that the Eu atoms retain the characteristic optical signature of Eu(III) upon incorporation into the diamond lattice. Computational modelling supports the experimental findings, corroborating that Eu3+ in diamond is a stable configuration within the diamond bandgap. The versatility of the synthetic technique is further illustrated through the creation of the well-studied Cr defect center. Together these defect centers demonstrate the outstanding chemical control over the incorporation of impurities into diamond enabled by the electrostatic assembly together with chemical vapour deposition growth.
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