Under Construction
Permanent link to this article: https://dannyvanpoucke.be/diamond/
E. Aylin Melan, MSc. Materiomics’24
[BILA-UHasselt/UNamur, 2024-2028]
The optical properties of diamond and transition metal oxides (TMOs) are promising materials for high-tech applications. Further development, however, requires detailed control and thus understanding of their properties. Quantum mechanical atomic-scale modelling is ideally suited for this task.
The colour centres give diamond specific optical properties, sought after in single-photon-sources. Such single-photon sources are used in quantum cryptography and quantum information technology of the future. TMOs are investigated for electrochromic applications and the development of intelligent windows for dynamic management of light and heat in buildings.
The common point between these two applications (and many others) is the need to describe complex atomic systems whose crystal defects are at the origin of the expected properties. In this context, artificial intelligence offers approaches to reduce the necessary computing resources, by learning from systems studied with the most precise quantum mechanical methods. The contribution of AI also makes it possible to reduce the carbon footprint of the simulations by reducing the required computer resources.
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Permanent link to this article: https://dannyvanpoucke.be/ai-accelerated-modeling/
Mar 06 2024
| Authors: | Emerick Y. Guillaum, Danny E. P. Vanpoucke, Rozita Rouzbahani, Luna Pratali Maffei, Matteo Pelucchi, Yoann Olivier, Luc Henrard, & Ken Haenen |
| Journal: | Carbon 222, 118949 (2024) |
| doi: | 10.1016/j.carbon.2024.118949 |
| IF(2022): | 10.9 |
| export: | bibtex |
| pdf: | <Carbon> |
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| Graphical Abstract: (left) Ball-and-stick representation of aH adsorption/desorption reaction mediated through a H radical. (right) Monte Carlo estimates of the H coverage of the diamond surface at different temperatures based on quantum mechanically determined reaction barriers and reaction rates. |
Hydrogen radical attacks and subsequent hydrogen migrations are considered to play an important role in the atomic-scale mechanisms of diamond chemical vapour deposition growth. We perform a comprehensive analysis of the reactions involving H-radical and vacancies on H-passivated diamond surfaces exposed to hydrogen radical-rich atmosphere. By means of first principles calculations—density functional theory and climbing image nudged elastic band method—transition states related to these mechanisms are identified and characterised. In addition, accurate reaction rates are computed using variational transition state theory. Together, these methods provide—for a broad range of temperatures and hydrogen radical concentrations—a picture of the relative likelihood of the migration or radical attack processes, along with a statistical description of the hydrogen coverage fraction of the (100) H-passivated surface, refining earlier results via a more thorough analysis of the processes at stake. Additionally, the migration of H-vacancy is shown to be anisotropic, and occurring preferentially across the dimer rows of the reconstructed surface. The approach used in this work can be generalised to other crystallographic orientations of diamond surfaces or other semiconductors.
Permanent link to this article: https://dannyvanpoucke.be/2024-paper-hadsorption-emerick-en/
Apr 21 2022
| Authors: | Kirill N. Boldyrev, Vadim S. Sedov, Danny E.P. Vanpoucke, Victor G. Ralchenko, & Boris N. Mavrin |
| Journal: | Diam. Relat. Mater 126, 109049 (2022) |
| doi: | 10.1016/j.diamond.2022.109049 |
| IF(2020): | 3.315 |
| export: | bibtex |
| pdf: | <DRM> |
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| Graphical Abstract: GeV split vacancy defect in diamond and the phonon modes near the ZPL. |
The vibrational behaviour of the germanium-vacancy (GeV) in diamond is studied through its photoluminescence spectrum and first-principles modeled partial phonon density of states. The former is measured in a region below 600 cm−1. The latter is calculated for the GeV center in its neutral, charged, and excited state. The photoluminescence spectrum presents a previously unobserved feature at 248 cm−1 in addition to the well-known peak at 365 cm−1. In our calculations, two localized modes, associated with the GeV center and six nearest carbon atoms (GeC6 cluster) are identified. These correspond to one vibration of the Ge ion along with the [111] orientation of the crystal and one perpendicular to this direction. We propose these modes to be assigned to the two features observed in the photoluminescence spectrum. The dependence of the energies of the localized modes on the GeV-center and their manifestation in experimental optical spectra is discussed.
Permanent link to this article: https://dannyvanpoucke.be/paper_gevpldft_vadim-en/
Oct 22 2020
| Authors: | Rozita Rouzbahani, Shannon S.Nicley, Danny E.P.Vanpoucke, Fernando Lloret, Paulius Pobedinskas, Daniel Araujo, Ken Haenen |
| Journal: | Carbon 172, 463-473 (2021) |
| doi: | 10.1016/j.carbon.2020.10.061 |
| IF(2019): | 8.821 |
| export: | bibtex |
| pdf: | <Carbon> |
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| Graphical Abstract: Artist impression of B incorporation during CVD growth of diamond. |
The methane concentration dependence of the plasma gas phase on surface morphology and boron incorporation in single crystal, boron-doped diamond deposition is experimentally and computationally investigated. Starting at 1%, an increase of the methane concentration results in an observable increase of the B-doping level up to 1.7×1021 cm−3, while the hole Hall carrier mobility decreases to 0.7±0.2 cm2 V−1 s−1. For B-doped SCD films grown at 1%, 2%, and 3% [CH4]/[H2], the electrical conductivity and mobility show no temperature-dependent behavior due to the metallic-like conduction mechanism occurring beyond the Mott transition. First principles calculations are used to investigate the origin of the increased boron incorporation. While the increased formation of growth centers directly related to the methane concentration does not significantly change the adsorption energy of boron at nearby sites, they dramatically increase the formation of missing H defects acting as preferential boron incorporation sites, indirectly increasing the boron incorporation. This not only indicates that the optimized methane concentration possesses a large potential for controlling the boron concentration levels in the diamond, but also enables optimization of the growth morphology. The calculations provide a route to understand impurity incorporation in diamond on a general level, of great importance for color center formation.
Permanent link to this article: https://dannyvanpoucke.be/paper_bdoping-en/
Feb 18 2020
| Authors: | Viraj Damle, Kaiqi Wu, Oreste De Luca, Natalia Ortí-Casañ, Neda Norouzi, Aryan Morita, Joop de Vries, Hans Kaper, Inge Zuhorn, Ulrich Eisel, Danny E.P. Vanpoucke, Petra Rudolf, and Romana Schirhagl, |
| Journal: | Carbon 162, 1-12 (2020) |
| doi: | 10.1016/j.carbon.2020.01.115 |
| IF(2019): | 8.821 |
| export: | bibtex |
| pdf: | <Carbon> (Open Access) |
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| Graphical Abstract: The preferential adsorption of biological matter on oriented diamond surfaces. |
Diamond has been a popular material for a variety of biological applications due to its favorable chemical, optical, mechanical and biocompatible properties. While the lattice orientation of crystalline material is known to alter the interaction between solids and biological materials, the effect of diamond’s crystal orientation on biological applications is completely unknown. Here, we experimentally evaluate the influence of the crystal orientation by investigating the interaction between the <100>, <110> and <111> surfaces of the single crystal diamond with biomolecules, cell culture medium, mammalian cells and bacteria. We show that the crystal orientation significantly alters these biological interactions. Most surprising is the two orders of magnitude difference in the number of bacteria adhering on <111> surface compared to <100> surface when both the surfaces were maintained under the same condition. We also observe differences in how small biomolecules attach to the surfaces. Neurons or HeLa cells on the other hand do not have clear preferences for either of the surfaces. To explain the observed differences, we theoretically estimated the surface charge for these three low index diamond surfaces and followed by the surface composition analysis using x-ray photoelectron spectroscopy (XPS). We conclude that the differences in negative surface charge, atomic composition and functional groups of the different surface orientations lead to significant variations in how the single crystal diamond surface interacts with the studied biological entities.
Permanent link to this article: https://dannyvanpoucke.be/paper_diamondromanaorientation2020-en/
Mar 29 2019
| 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(2019): | 2.650 |
| export: | bibtex |
| pdf: | <DiamRelatMater> |
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| Graphical Abstract: Spin polarization around the various Eu-defect models in diamond. Blue and red represent the up and down spin channels respectively. |
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.
Permanent link to this article: https://dannyvanpoucke.be/paper_eudopingdrmspecial2018-en/
Mar 18 2019
The participants to SBDD XXIV of 2019. (courtesy of Jorne Raymakers, SBDD XXIV secretary)
Last week the 24th edition of the Hasselt diamond workshop took place (this year chaired by Christoph Becher). It’s already the fourth time, since 2016, I have attended this conference, and each year it is a joy to meet up with the familiar faces of the diamond research field. The program was packed, as usual. And this year the NV-center was again predominantly present as the all-purpose quantum defect in diamond. I keep being amazed at how much it is used (although it has a rather low efficiency) and also about how many open question remain with regard to its incorporation during growth. With a little luck, you may read more about this in the future, as it is one of a few dozen ideas and questions I want to investigate.
A very interesting talk was given by Yamaguchi Takahide, who is combining hexagonal-BN and H-terminated diamond for high performance electronic devices. In such a device the h-BN leads to the formation of a 2D hole-gas at the interface (i.e., surface transfer doping), making it interesting for low dimensional applications. (And it of course hints at the opportunities available with other 2D materials.) The most interesting fact, as well as the most mind-boggling to my opinion, was the fact that there was no clear picture of the atomic structure of the interface. But that is probably just me. For experiments, nature tends to make sure everything is alright, while we lowly computational materials artificers need to know where each and every atom belongs. I’ll have to make some time to find out.
A second extremely interesting presentation was given by Anke Krueger (who will be the chair of the 25th edition of SBDD next year), showing of her groups skill at creating fluorine terminated diamond…without getting themselves killed. The surface termination of diamond with fluorine comes with many different hazards, going from mere poisoning, to fire and explosions. The take-home message: “kids don’t try this at home”. Despite all this risky business, a surface coverage of up to 85% was achieved, providing a new surface termination for diamond, with a much stronger trapping of negative charges near the surface, ideal for forming negatively charged NV centers.
On the last day, Rozita Rouzbahani presented our collaboration on the growth of B doped diamond. She studied the impact of growth conditions on the B concentration and growth speed of B doped diamond surfaces. My computational results corroborate her results and presents the atomic scale mechanism resulting in an increased doping concentration upon increased growth speed. I am looking forward to the submission of this nice piece of research.
And now, we wait another year for the next edition of SBDD, the celebratory 25th edition with a focus on diamond surfaces.
Permanent link to this article: https://dannyvanpoucke.be/sbdd-xxiv-diamond-workshop/
Sep 23 2017
| Authors: | Danny E. P. Vanpoucke and Ken Haenen |
| Journal: | Diam. Relat. Mater 79, 60-69 (2017) |
| doi: | 10.1016/j.diamond.2017.08.009 |
| IF(2017): | 2.232 |
| export: | bibtex |
| pdf: | <DiamRelatMater> |
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| Graphical Abstract: Combining a scan over possible values for U and J with reference electronic structures obtained using the hybrid functional HSE06, DFT+U can be fit to provide hybrid functional quality electronic structures at the cost of DFT calculations. |
The neutral C-vacancy is investigated using density functional theory calculations. We show that local functionals, such as PBE, can predict the correct stability order of the different spin states, and that the success of this prediction is related to the accurate description of the local magnetic configuration. Despite the correct prediction of the stability order, the PBE functional still fails predicting the defect states correctly. Introduction of a fraction of exact exchange, as is done in hybrid functionals such as HSE06, remedies this failure, but at a steep computational cost. Since the defect states are strongly localized, the introduction of additional on site Coulomb and exchange interactions, through the DFT+U method, is shown to resolve the failure as well, but at a much lower computational cost. In this work, we present optimized U and J parameters for DFT+U calculations, allowing for the accurate prediction of defect states in defective
diamond. Using the PBE optimized atomic structure and the HSE06 optimized electronic structure as reference, a pair of on-site Coulomb and exchange parameters (U,J) are fitted for DFT+U studies of defects in diamond.
Poster-presentation: here
DFT+U series (varying J) for a specific spin state of the C-vacancy defect.
Permanent link to this article: https://dannyvanpoucke.be/paperdrm-diamond-dftu-en/
Mar 08 2017
For a recent project, attempting to investigate Eu dopants in bulk diamond, I ended up simplifying the problem and investigating the C-vacancy in diamond. The setup is simple: take a super cell of diamond, remove 1 carbon atom and calculate. This, however, ended up being a bit more complicated than I had expected.
Removing the single carbon atom gives rise to 4 dangling bonds on the neighboring carbon atoms. The electrons occupying these bonds will gladly interact with one another, giving rise to three different possible spin states:
Starting the calculations without any assumptions gives nice results. Unfortunately they are wrong, and we seem to have ended up in nearby local minima. Including the spin-configurations above as starting assumptions solves the problem, luckily.
The electronic structure is, however, still not a perfect match for experiment. But this is well-known behavior for Density Functional Theory with local functionals such as LDA and PBE. A solution is the use of hybrid functionals (such as HSE06). Conservation of misery kicks in hard at this point, since the latter type of calculations are 1000x as expensive in compute time (and the LDA and PBE calculations aren’t finished in a matter of seconds or minutes, but need several hours on multiple cores). An old methodology to circumvent this problem is the use of Hubbard-U like correction terms (so-called DFT+U). Interestingly for this defect system the two available parameters in a DFT+U setup are independent, and allow for the electronic structure to be perfectly tuned. At the end of the fitting exercise, we now have two additional parameters, which allow us to get electronic structures of hybrid functional quality, but at PBE computational cost.
The evolution of the band-structure as function of the two parameters can be seen below.
DFT+U series (varying U) for a specific spin state of the C-vacancy defect.
DFT+U series (varying J) for a specific spin state of the C-vacancy defect.
Permanent link to this article: https://dannyvanpoucke.be/diamond-dftu-en/