Tag: diamond

First-principles investigation of hydrogen-related reactions on (100)–(2×1)∶H diamond surfaces

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>

 

Graphical Abstract for Carbon publication on the adsorption of H onto diamond.
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.

Abstract

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.

Materiomics Chronicles: week 11 & 12

After the exam period in weeks nine and ten, the eleventh and twelfth week of the academic year bring the second quarter of our materiomics program at UHasselt for the first master students. Although I’m not coordinating any courses in this quarter, I do have some teaching duties, including being involved in two of the hands-on projects.

As in the past 10 weeks, the bachelor students in chemistry had lectures for the courses introduction to quantum chemistry and quantum and computational chemistry. For the second bachelor this meant they finally came into contact with the H atom, the first and only system that can be exactly solved using pen and paper quantum chemistry (anything beyond can only be solved given additional approximations.) During the exercise class we investigated the concept of aromatic stabilization in more detail in addition to the usual exercises with simple Schrödinger  equations and wave functions. For the third bachelor, their travel into the world of computational chemistry continued, introducing post-Hartree-Fock methods with also include the missing correlation energy. This is the failure of Hartree-Fock theory, making it a nice framework, but of little practical use for any but the most trivial molecules (e.g. H2 for example already being out of scope). We also started looking into molecular systems, starting with simple diatomic molecules like H2+.

SnV split vacancy defect in diamond.

SnV split vacancy defect in diamond.

In the master materiomics, the course Machine learning and artificial intelligence in modern materials science hosted a guest lecture on Large Language Models, and their use in materials research as well as an exercise session during which the overarching ML study of the QM9 dataset was extended. During the course on  Density Functional Theory there was a second lab, this time on conceptual DFT. For the first master students, the hands-on project kept them busy. One group combining AI and experiments, and a second group combining DFT modeling of SnV0 defects in diamond with their actual lab growth. It was interesting to see the enthusiasm of the students. With only some mild debugging, I was able to get them up and running relatively smoothly on the HPC. I am also truly grateful to our experimental colleagues of the diamond growth group, who bravely set up these experiments and having backup plans for the backup plans.

At the end of week 12, we added another 12h of classes, ~1h of video lecture, ~2h of HPC support for the handson project and 6h of guest lectures, putting our semester total at 118h of live lectures. Upwards and onward to weeks 13 & 14.

Localized vibrational modes of GeV-centers in diamond: Photoluminescence and first-principles phonon study

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>

 

GeV split vacancy defect in diamond and the phonon modes near the ZPL.
Graphical Abstract: GeV split vacancy defect in diamond and the phonon modes near the ZPL.

Abstract

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.

Impact of methane concentration on surface morphology and boron incorporation of heavily boron-doped single crystal diamond layers

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>

 

Graphical Abstract B doped diamond
Graphical Abstract: Artist impression of B incorporation during CVD growth of diamond.

Abstract

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.

Partitioning the vibrational spectrum: Fingerprinting defects in solids

Authors:  Danny E. P. Vanpoucke
Journal: Computational Materials Science 181, 109736 (2020)
doi: 10.1016/j.commatsci.2020.109736
IF(2019): 2.863
export: bibtex
pdf: <ComputMaterSci>   (Open Access)
github: <Hive-toolbox>

 

Graphical abstract Computational Materials Science 181, 109736 (2020)
Graphical Abstract: Finger printing defects in diamond through the creation of the vibrational spectrum of a defect.

Abstract

Vibrational spectroscopy techniques are some of the most-used tools for materials
characterization. Their simulation is therefore of significant interest, but commonly
performed using low cost approximate computational methods, such as force-fields.
Highly accurate quantum-mechanical methods, on the other hand are generally only used
in the context of molecules or small unit cell solids. For extended solid systems,
such as defects, the computational cost of plane wave based quantum mechanical simulations
remains prohibitive for routine calculations. In this work, we present a computational scheme
for isolating the vibrational spectrum of a defect in a solid. By quantifying the defect character
of the atom-projected vibrational spectra, the contributing atoms are identified and the strength
of their contribution determined. This method could be used to systematically improve phonon
fragment calculations. More interestingly, using the atom-projected vibrational spectra of the
defect atoms directly, it is possible to obtain a well-converged defect spectrum at lower
computational cost, which also incorporates the host-lattice interactions. Using diamond as
the host material, four point-defect test cases, each presenting a distinctly different
vibrational behaviour, are considered: a heavy substitutional dopant (Eu), two intrinsic
point-defects (neutral vacancy and split interstitial), and the negatively charged N-vacancy
center. The heavy dopant and split interstitial present localized modes at low and high
frequencies, respectively, showing little overlap with the host spectrum. In contrast, the
neutral vacancy and the N-vacancy center show a broad contribution to the upper spectral range
of the host spectrum, making them challenging to extract. Independent of the vibrational behaviour,
the main atoms contributing to the defect spectrum can be clearly identified. Recombination of
their atom-projected spectra results in the isolated spectrum of the point-defect.

Universiteit Van Vlaanderen

A bit over 1 month ago, I told you about my adventure at the film studio of “de Universiteit Van Vlaanderen“. Today is the day the movie is officially released. You can find it at the website of de Universiteit Van Vlaanderen: Video. The video is in Dutch as this is a science-communication platform aimed at the local population, presenting the expertise available at our local universities.

 

In addition to this video, I was asked by Knack magazine to write a piece on the topic presented. As computational research is my central business I wrote a piece on the subject introducing the general public to the topic. The piece can be read here (in Dutch).

And of course, before I forget, this weekend there was also the half-yearly daylight saving exercise with our clocks.[and in Dutch]

 

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(2019): 2.650
export: bibtex
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.

SBDD XXIV: Diamond workshop

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.

Science Figured out

Diamond and CPU's, now still separated, but how much longer will this remain the case? Top left: Thin film N-doped diamond on Si (courtesy of Sankaran Kamatchi). Top right: Very old Pentium 1 CPU from 1993 (100MHz), with µm architecture. Bottom left: more recent intel core CPU (3GHz) of 2006 with nm scale architecture. Bottom right: Piece of single crystal diamond. A possible alternative for silicon, with 20x higher thermal conductivity, and 7x higher mobility of charge carriers.

Diamond and CPU’s, now still separated, but how much longer will this remain the case?
Top left: Thin film N-doped diamond on Si (courtesy of Sankaran Kamatchi). Top right: Very old Pentium 1 CPU from 1993 (100MHz), with µm architecture. Bottom left: more recent intel core CPU (3GHz) of 2006 with nm scale architecture. Bottom right: Piece of single crystal diamond. A possible alternative for silicon, with 20x higher thermal conductivity, and 7x higher mobility of charge carriers.

Can you pitch your research in 3 minutes, this is the concept behind “wetenschap uitgedokterd/science figured out“. A challenge I accepted after the fun I had at the science-battle. If I can explain my work to a public of 6 to 12 year-olds, explaining it to adults should be possible as well. However, 3 minutes is very short (although some may consider this long in the current bitesize world), especially if you have to explain something far from day-to-day life and can not assume any scientific background.

Where to start? Capture the imagination: “Imagine a world where you are a god.

Link back to the real world. “All modern-day high-tech toys are more and more influenced by the atomic scale details.” Over the last decade, I have seen the nano-scale progress slowly but steadily into the realm of real-life materials research. This almost invisible trend will have a huge impact on materials science in the coming decade, because more and more we will see empirical laws breaking down, and it will become harder and harder to fit trends of materials using a classical mindset, something which has worked marvelously for materials science during the last few centuries. Modern and future materials design (be it solar cells, batteries, CPU’s or even medicine) will have to rely on quantum mechanical intuition and hence quantum mechanical simulations. (Although there is still much denial in that regard.)

Is there a problem to be solved? Yes indeed: “We do not have quantum mechanical intuition by nature, and manipulating atoms is extremely hard in practice and for practical purposes.” Although popular science magazines every so often boast pictures of atomic scale manipulation of atoms and the quantum regime, this makes it far from easy and common inside and outside the university lab. It is amazing how hard these things tend to get (ask your local experimental materials research PhD) and the required blood, sweat and tears are generally not represented in the glory-parade of a scientific publication.

Can you solve this? Euhm…yes…at least to some extend. “Computational materials research can provide the quantum mechanical intuition we human beings lack, and gives us access to atomic scale manipulation of a material.” Although computational materials science is seen by experimentalists as theory, and by theoreticians as experiments, it is neither and both. Computational materials science combines the rigor and control of theory, with access to real-life systems of experiments. It, unfortunately also suffers the limitations of both: as the system is still idealized (but to much lesser extend than in theoretical work) and control is not absolute (you have to follow where the algorithms take you, just as an experimentalist has to follow where the reaction takes him/her). But, if these strengths and weaknesses are balanced wisely (requires quite a few years of experience) an expert will gain fundamental insights in experiments.

Animation representing the buildup of a diamond surface in computational work.

Animation representing the buildup of a diamond surface in computational work.

As a computational materials scientist, you build a real-life system, atom by atom, such that you know exactly where everything is located, and then calculate its properties based on the rules of quantum mechanics, for example. In this sense you have absolute control as in theory. This comes at a cost (conservation of misery 🙂 ); where nature itself makes sure the structure is the “correct one” in experiments, you have to find it yourself in computational work. So you generally end up calculating many possible structural combinations of your atoms to first find out which is the one most probable to represent nature.

So what am I actually doing?I am using atomic scale quantum mechanical computations to investigate the materials my experimental colleagues are studying, going from oxides to defects in diamond.” I know this is vague, but unfortunately, the actual work is technical. Much effort goes into getting the calculations to run in the direction you want them to proceed (This is the experimental side of computational materials science.). The actual goal varies from project to project. Sometimes, we want to find out which material is most stable, and which material is most likely to diffuse into the other, while at other times we want to understand the electronic structure, to test if a defect is really luminescent, this to trace the source of the experimentally observed luminescence. Or if you want to make it more complex, even find out which elements would make diamond grow faster.

Starting from this, I succeeded in creating a 3-minute pitch of my research for Science Figured out. The pitch can be seen here (in Dutch, with English subtitles that can be switched on through the cogwheel in the bottom right corner).

Some external links:

 

VSC User Day 2018

Today, I am attending the 4th VSC User Day at the “Paleis de Academiën” in Brussels. Flemish researchers for whom the lifeblood of their research flows through the chips of a supercomputer are gathered here to discuss their experiences and present their research.

Some History

About 10 years ago, at the end of 2007 and beginning of 2008, the 5 Flemish universities founded the Flemish Supercomputer Center (VSC). A virtual organisation with one central goal:  Combine their strengths and know-how with regard to High Performance Compute (HPC) centers to make sure they were competitive with comparable HPC centers elsewhere.

By installing a super-fast network between the various university compute centers, each Flemish researcher has nowadays access to state-of-the-art computer infrastructure, independent of his or her physical location. A researcher at the University of Hasselt, like myself, can easily run calculations on the supercomputers installed at the university of Ghent or Leuven. In October 2012 the existing university supercomputers, so-called Tier-2 supercomputers, are joined by the first Flemish Tier-1 supercomputer, which was housed at the brand new data-centre of Ghent University. This machine is significantly larger than the existing Tier-2 machines, and allows Belgium to become the 25th member of the PRACE network, a European network which provides computational researchers access to the best and largest computer facilities in Europe. The fast development of computational research in Flanders and the explosive growth in the number of computational researchers, combined with the first shared Flemish supercomputer (in contrast to the university TIER-2 supercomputers, which some still consider private property rather than part of VSC) show the impact of the virtual organisation that is the VSC. As a result, on January 16th 2014, the first VSC User Day is organised, bringing together HPC users from all 5 universities  and industry. Here the users share their experiences and discuss possible improvements and changes. Since then, the first Tier-1 supercomputer has been decommissioned and replaced by a brand new Tier-1 machine, this time located at the KU Leuven. Furthermore, the Flemish government has put 30M€ aside for super-computing in Flanders, making sure that also in the future Flemish computational research stays competitive. The future of computational research in Flanders looks bright.

Today is User Day 2018

During the 4th VSC User Day, researchers of all 5 Flemish universities will be presenting the work they are performing on the supercomputers of the VSC network. The range of topics is very broad: from first principles materials modelling to chip design, climate modelling and space weather. In addition there will also be several workshops, introducing new users to the VSC and teaching advanced users the finer details of GPU-code and code optimization and parallelization. This later aspect is hugely important during the use of supercomputers in an academic context. Much of the software used is developed or modified by the researchers themselves. And even though this software can present impressive behavior, it doe not speed up automatically if you provide it access to more CPU’s. This is a very non-trivial task the researchers has to take care of, by carefully optimizing and parallelizing his or her code.

To support the researchers in their work, the VSC came up with ingenious poster-prizes. The three best posters will share 2018 node days of calculation time (about 155 years of calculations on a normal simple computer).

Wish me luck!

 

Single-slide presentation of my poster @VSC User Day 2018.

Single-slide presentation of my poster @VSC User Day 2018.