Tag: Doping

Can Europium Atoms form Luminescent Centres in Diamond: A combined Theoretical-Experimental Study

Authors: Danny E. P. Vanpoucke, Shannon S. Nicley, Jorne Raymakers, Wouter Maes, and Ken Haenen
Journal: Diam. Relat. Mater 94, 233-241 (2019)
doi: 10.1016/j.diamond.2019.02.024
IF(2017): 2.232
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pdf: <DiamRelatMater>

 

Spin polarization around the various Eu-defect models in diamond. Blue and red represent the up and down spin channels respectively
Graphical Abstract: Spin polarization around the various Eu-defect models in diamond. Blue and red represent the up and down spin channels respectively.

Abstract

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.

Mechanochemical synthesis of nanostructured metal nitrides, carbonitrides and carbon nitride: A combined theoretical and experimental study

Authors: Seyyed Amin Rounaghi, Danny E.P. Vanpoucke, Hossein Eshghi, Sergio Scudino, Elaheh Esmaeili, Steffen Oswald and Jürgen Eckert
Journal: Phys. Chem. Chem. Phys. 19, 12414-12424 (2017)
doi: 10.1039/C7CP00998D
IF(2017): 3.906
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pdf: <Phys.Chem.Chem.Phys.>

Abstract

Nowadays, the development of highly efficient routes for the low cost synthesis of nitrides is greatly growing. Mechanochemical synthesis is one of those promising techniques which is conventionally employed for the synthesis of nitrides by long term milling of metallic elements under pressurized N2 or NH3 atmosphere (A. Calka and J. I. Nikolov, Nanostruct. Mater., 1995, 6, 409-412). In the present study, we describe a versatile, room-temperature and low cost mechanochemical process for the synthesis of nanostructured metal nitrides (MNs), carbonitrides (MCNs) and carbon nitride (CNx). Based on this technique, melamine as a solid nitrogen-containing organic compound (SNCOC) is ball milled with four different metal powders (Al, Ti, Cr and V) to produce nanostructured AlN, TiCxN1-x, CrCxN1-x, and VCxN1-x (x~0.05). Both theoretical and experimental techniques are implemented to determine the reaction intermediates, products, by-products and finally, the mechanism underling this synthetic route. According to the results, melamine is polymerized in the presence of metallic elements at intermediate stages of the milling process, leading to the formation of a carbon nitride network. The CNx phase subsequently reacts with the metallic precursors to form MN, MCN or even MCN-CNx nano-composites depending on the defect formation energy and thermodynamic stability of the corresponding metal nitride, carbide and C/N co-doped structures.

Mechanochemical route to the synthesis of nanostructured Aluminium nitride

Authors: S. A. Rounaghi, H. Eshghi, S. Scudino, A. Vyalikh, D. E. P. Vanpoucke, W. Gruner,
S. Oswald, A. R. Kiani Rashid, M. Samadi Khoshkhoo, U. Scheler and J. Eckert
Journal: Scientific Reports 6, 33375 (2016)
doi: 10.1038/srep33375
IF(2016): 4.259
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pdf: <Sci.Rep.> (open access)

Abstract

Hexagonal Aluminium nitride (h-AlN) is an important wide-bandgap semiconductor material which is conventionally fabricated by high temperature carbothermal reduction of alumina under toxic ammonia atmosphere. Here we report a simple, low cost and potentially scalable mechanochemical procedure for the green synthesis of nanostructured h-AlN from a powder mixture of Aluminium and melamine precursors. A combination of experimental and theoretical techniques has been employed to provide comprehensive mechanistic insights on the reactivity of melamine, solid state metalorganic interactions and the structural transformation of Al to h-AlN under non-equilibrium ball milling conditions. The results reveal that melamine is adsorbed through the amine groups on the Aluminium surface due to the long-range van der Waals forces. The high energy provided by milling leads to the deammoniation of melamine at the initial stages followed by the polymerization and formation of a carbon nitride network, by the decomposition of the amine groups and, finally, by the subsequent diffusion of nitrogen into the Aluminium structure to form h-AlN

First-Principles Study of Antisite Defect Configurations in ZnGa2O4:Cr Persistent Phosphors

Authors: Arthur De Vos, Kurt Lejaeghere, Danny E. P. Vanpoucke, Jonas J. Joos, Philippe F. Smet, and Karen Hemelsoet
Journal: Inorg. Chem. 55(5), 2402-2412 (2016)
doi: 10.1021/acs.inorgchem.5b02805
IF(2016): 4.857
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pdf: <Inorg.Chem>
Graphical Abstract: (left) Ball-and-stick model of zinc gallate (right) density of states of Cr doped zinc gallate.
Graphical Abstract: First-principles simulations on zinc gallate solid phosphors (ZGO) containing a chromium dopant and antisite defects (left) rationalize the attractive interactions between the various elements. A large number of antisite pair configurations is investigated and compared with isolated antisite defects. Defect energies point out the stability of the antisite defects in ZGO. Local structural distortions are reported, and charge transfer mechanisms are analyzed based on theoretical density of states (right) and Hirshfeld-I charges.

Abstract

Zinc gallate doped with chromium is a recently developed near-infrared emitting persistent phosphor, which is now extensively studied for in vivo bioimaging and security applications. The precise mechanism of this persistent luminescence relies on defects, in particular, on antisite defects and antisite pairs. A theoretical model combining the solid host, the dopant, and/or antisite defects is constructed to elucidate the mutual interactions in these complex materials. Energies of formation as well as dopant, and defect energies are calculated through density-functional theory simulations of large periodic supercells. The calculations support the chromium substitution on the slightly distorted octahedrally coordinated gallium site, and additional energy levels are introduced in the band gap of the host. Antisite pairs are found to be energetically favored over isolated antisites due to significant charge compensation as shown by calculated Hirshfeld-I charges. Significant structural distortions are found around all antisite defects. The local Cr surrounding is mainly distorted due to a ZnGa antisite. The stability analysis reveals that the distance between both antisites dominates the overall stability picture of the material containing the Cr dopant and an antisite pair. The findings are further rationalized using calculated densities of states and Hirshfeld-I charges.

Doping of CeO2 as a Tunable Buffer Layer for Coated Superconductors: A DFT Study of Mechanical and Electronic Properties

Authors: Danny E. P. Vanpoucke,
Journal: Developments in Strategic Ceramic Materials:
Ceramic Engineering and Science Proceedings 36(8), 169-177 (2016)
(ICACC 2015 conference proceeding)
Editors: Waltraud M. Kriven, Jingyang Wang, Dongming Zhu,Thomas Fischer, Soshu Kirihara
ISBN: 978-1-119-21173-0
webpage: Wiley-VCH
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pdf: <preprint> 

Abstract

In layered ceramic superconductor architectures, CeO2 buffer layers are known to form micro cracks during the fabrication process. To prevent this crack formation, doping of the CeO2 layer has been suggested. In this theoretical study, the influence of dopants (both tetravalent and aliovalent) on the mechanical and structural properties of CeO2 is investigated by means of density functional theory. Group IVa and IVb dopants show clearly distinct stability, with the former favouring interface and surface doping, while the latter favour uniform bulk doping. This behaviour is linked to the dopant electronic structure. The presence of charge compensating vacancies is shown to complicate the mechanical and structural picture for aliovalent dopants. We find that the vacancies often counteract the dopant modifications of the host material. In contrast, all dopants show an inverse relation between the bulk modulus and thermal expansion coefficient, independent of their valency and the presence of oxygen vacancies. Based on the study of these idealized systems, new dopants are suggested for applications.

Comment on ‘Europium doping induced symmetry deviation and its impact on the second harmonic generation of doped ZnO nanowires’

Authors: Danny E. P. Vanpoucke
Journal: Nanotechnology 25(45), 458001 (2014)
doi: 10.1088/0957-4484/25/45/458001
IF(2014): 3.821
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pdf: <Nanotechnology>

Abstract

In Dhara et al. 2014 Nanotechnology 25 225202, the authors reported on the synthesis of Eu-doped ZnO nanowires (NWs) and investigated the influence of Eu doping on the second harmonic generation (SHG). Maximum SHG was found to correlate strongly with the structural deformation attributed to Eu3+ doping. In this comment, we show the deformation of interest is due to the presence of Eu2+ dopants, based on both the experimental data presented by Dhara et al. and ab-initio density functional theory calculations.

Aliovalent Doping of CeO2: DFT study of oxidation state and vacancy effects

Authors: Danny E. P. Vanpoucke, Patrick Bultinck, Stefaan Cottenier, Veronique Van Speybroeck, and Isabel Van Driessche,
Journal: J. Mater. Chem. A 2(33), 13723-13737 (2014)
doi: 10.1039/C4TA02449D
IF(2014): 7.443
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pdf: <JMaterChemA> <arXiv>

Abstract

The modification of CeO2 properties by means of aliovalent doping is investigated within the ab initio density functional theory framework. Lattice parameters, dopant atomic radii, bulk moduli and thermal expansion coefficients of fluorite type Ce1-xMxO2-y (with M = Mg, V, Co, Cu, Zn, Nb, Ba, La, Sm, Gd, Yb, and Bi) are presented for 0.00 ≤ x ≤ 0.25. The relative stability of the doped systems is discussed, and the influence of oxygen vacancies is investigated. It is shown that oxygen vacancies tend to increase the lattice parameter, and strongly decrease the bulk modulus. Defect formation energies are correlated with calculated crystal radii and covalent radii of the dopants, and are shown to present no simple trend. The previously observed inverse relationship between the thermal expansion coefficient and the bulk modulus in group IV doped CeO2 [J. Am. Ceram. Soc. 97(1), 258 (2014)] is shown to persist independent of the inclusion of charge compensating vacancies.

Tetravalent Doping of CeO2: The impact of valence electron character on group IV dopant influence

Authors: Danny E. P. Vanpoucke, Stefaan Cottenier, Veronique Van Speybroeck, Isabel Van Driessche, and Patrick Bultinck
Journal: J. Am. Ceram. Soc. 97(1), 258-266 (2014)
doi: 10.1111/jace.12650
IF(2014): 2.610
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pdf: <J.Am.Ceram.Soc.> <arXiv>

Abstract

Fluorite CeO2 doped with group IV elements is studied within the density functional theory (DFT) and DFT + U framework. Concentration-dependent formation energies are calculated for Ce1−xZxO2 (Z = C, Si, Ge, Sn, Pb, Ti, Zr, Hf) with 0 ≤ x ≤ 0.25 and a roughly decreasing trend with ionic radius is observed. The influence of the valence and near valence electronic configuration is discussed, indicating the importance of filled d and f shells near the Fermi level for all properties investigated. A clearly different behavior of group IVa and IVb dopants is observed: the former are more suitable for surface modifications and the latter are more suitable for bulk modifications. For the entire set of group IV dopants, there exists an inverse relation between the change, due to doping, of the bulk modulus, and the thermal expansion coefficients. Hirshfeld-I atomic charges show that charge-transfer effects due to doping are limited to the nearest-neighbor oxygen atoms.

Cover Image of Journal of Computational Chemistry : Extending Hirshfeld-I

Authors: Danny E. P. Vanpoucke
Journal: J. Comput. Chem. 34(5), i-ii (2013)
doi: 10.1002/jcc.23239
IF(2013): 3.601
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pdf: <J.Comput.Chem.>

Abstract

The image shows an isosurface of Hirshfeld-I “atoms in molecules” for Ti-doped CeO2, taken at an electron density of 0.03e/Å3, as presented by Danny E. P. Vanpoucke, Patrick Bultinck, and Isabel Van Driessche on page 405. The cubic Ce0.75Ti0.25O2 unit cell is shown along the 111 direction. The different atoms are still clearly distinguishable at this iso-surface level, and show the Ti atom in the corners to be much smaller than the Ce atoms on the sides. In this issue, this implementation of the Hirshfeld- I method for solids is published back to back with a Comment from Thomas A. Manz and the authors’ Reply.


Cover of Journal of Computational Chemistry: Volume 34, Issue 5, feb. 15, 2013

Tuning of CeO2 buffer layers for coated superconductors through doping

Authors: Danny E. P. Vanpoucke, Stefaan Cottenier, Veronique Van Speybroeck, Patrick Bultinck, and Isabel Van Driessche,
Journal: Appl. Surf. Sci. 260, 32-35 (2012)
doi: 10.1016/j.apsusc.2012.01.032
IF(2012): 2.112
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pdf: <Appl.Surf.Sci.> <arXiv>

Abstract

The appearance of microcracks in cerium oxide (CeO2) buffer layers, as used in buffer layer architectures for coated superconductors, indicates the presence of stress between this buffer layer and the substrate. This stress can originate from the differences in thermal expansion or differences in lattice parameters between the CeO2 buffer layer and the substrate. In this article, we study, by means of ab initio density functional theory calculations, the influence of group IV doping elements on the lattice parameter and bulk modulus of CeO2. Vegard’s law behavior is found for the lattice parameter in systems without oxygen vacancies, and the Shannon crystal radii for the doping elements are retrieved from the lattice expansions. We show that the lattice parameter of the doped CeO2 can be matched to that of the La2Zr2O7 coated NiW substrate substrate for dopant concentrations of about 5%, and that bulk modulus matching is either not possible or would require extreme doping concentrations.