Category: publications

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.


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.

On the influence of water on THz vibrational spectral features of molecular crystals

Authors: Sergey Mitryukovskiy, Danny E. P. Vanpoucke, Yue Bai, Théo Hannotte, Mélanie Lavancier, Djamila Hourlier, Goedele Roos and Romain Peretti
Journal: Physical Chemistry Chemical Physics 24, 6107-6125 (2022)
doi: 10.1039/D1CP03261E
IF(2020): 3.676
export: bibtex
pdf: <PCCP>


Graphical Abstract: Comparison of the measured THz spectrum of 3 phases of Lactose-Monohydrate to the calculated spectra for several Lactose configurations with varying water content.


The nanoscale structure of molecular assemblies plays a major role in many (µ)-biological mechanisms. Molecular crystals are one of the most simple of these assemblies and are widely used in a variety of applications from pharmaceuticals and agrochemicals, to nutraceuticals and cosmetics. The collective vibrations in such molecular crystals can be probed using terahertz spectroscopy, providing unique characteristic spectral fingerprints. However, the association of the spectral features to the crystal conformation, crystal phase and its environment is a difficult task. We present a combined computational-experimental study on the incorporation of water in lactose molecular crystals, and show how simulations can be used to associate spectral features in the THz region to crystal conformations and phases. Using periodic DFT simulations of lactose molecular crystals, the role of water in the observed lactose THz spectrum is clarified, presenting both direct and indirect contributions. A specific experimental setup is built to allow the controlled heating and corresponding dehydration of the sample, providing the monitoring of the crystal phase transformation dynamics. Besides the observation that lactose phases and phase transformation appear to be more complex than previously thought – including several crystal forms in a single phase and a non-negligible water content in the so-called anhydrous phase – we draw two main conclusions from this study. Firstly, THz modes are spread over more than one molecule and require periodic computation rather than a gas-phase one. Secondly, hydration water does not only play a perturbative role but also participates in the facilitation of the THz vibrations.

The 0.5THz finger-print mode of alpha-Lactose Monohydrate.

The 0.5 THz finger-print mode of alpha-lactose monohydrate.

Deep Eutectic Solvents as Non-flammable Electrolytes for Durable Sodium-ion Batteries

Authors: Dries De Sloovere, Danny E. P. Vanpoucke, Andreas Paulus, Bjorn Joos, Lavinia Calvi, Thomas Vranken, Gunter Reekmans, Peter Adriaensens, Nicolas Eshraghi, Abdelfattah Mahmoud, Frédéric Boschini, Mohammadhosein Safari, Marlies K. Van Bael, An Hardy
Journal: Advanced Energy and Sustainability Research 3(3), 2100159 (2022)
doi: 10.1002/aesr.202100159
IF(2022): ??
export: bibtex
pdf: <AdvEnSusRes> (OA)


Graphical Abstract: Understanding the electronic structure of Na-TFSI interacting with NMA.


Sodium-ion batteries are alternatives for lithium-ion batteries in applications where cost-effectiveness is of primary concern, such as stationary energy storage. The stability of sodium-ion batteries is limited by the current generation of electrolytes, particularly at higher temperatures. Therefore, the search for an electrolyte which is stable at these temperatures is of utmost importance. Here, we introduce such an electrolyte using non-flammable deep eutectic solvents, consisting of sodium bis(trifluoromethane)sulfonimide (NaTFSI) dissolved in N-methyl acetamide (NMA). Increasing the NaTFSI concentration replaces NMA-NMA hydrogen bonds with strong ionic interactions between NMA, Na+, and TFSI. These interactions lower NMA’s HOMO energy level compared to that of TFSI, leading to an increased anodic stability (up to ~4.65 V vs Na+/Na). (Na3V2(PO4)2F3/CNT)/(Na2+xTi4O9/C) full cells show 74.8% capacity retention after 1000 cycles at 1 C and 55 °C, and 97.0% capacity retention after 250 cycles at 0.2 C and 55 °C. This is considerably higher than for (Na3V2(PO4)2F3/CNT)/(Na2+xTi4O9/C) full cells containing a conventional electrolyte. According to the electrochemical impedance analysis, the improved electrochemical stability is linked to the formation of more robust surface films at the electrode/electrolyte interface. The improved durability and safety highlight that deep eutectic solvents can be viable electrolyte alternatives for sodium-ion batteries.

Assigning probabilities to non-Lipschitz mechanical systems

Authors:  Danny E. P. Vanpoucke and Sylvia Wenmackers
Journal: Chaos 31, 123131 (2021)
doi: 10.1063/5.0063388
IF(2021): 3.642
export: bibtex
pdf: <Chaos>   (Open Access) <ArXiv v1>
github: <NortonDomeExplorer>


Malament's Mounds
Graphical Abstract: Examples of Malament’s Mounds, of which the Norton Dome is the special case where α=½.


We present a method for assigning probabilities to the solutions of initial value problems that have a Lipschitz singularity. To illustrate the method, we focus on the following toy-example: \ddot{r} = r^{a}, r(t = 0) = 0, and \dot{r}|_{r(t=0)}= 0, with a∈]0; 1[. This example has a physical interpretation as a mass in a uniform gravitational field on a dome of particular shape; the case with a = ½ is known as Norton’s dome. Our approach is based on (1) finite difference equations, which are deterministic; (2) elementary techniques from alpha-theory, a simplified framework for non-standard analysis that allows us to study infinitesimal perturbations; and (3) a uniform prior on the canonical phase space.  Our deterministic, hyperfinite grid model allows us to assign probabilities to the solutions of the This allows us to assign probabilities to the solutions of the initial value problem in the original, indeterministic model.

Phase space vector field for all Malament's mounds

Phase space vector field for all Malament’s mounds

Review of 2020

Happy New Year

2020 will forever be the year of viruses for me and a lot of us. At Maastricht University, the year started with a university wide cyber-attack with ransomware. After the computer-viruses came the human viruses, with COVID-19 shutting down one country after the other, and shutting down education systems as well.

Hopefully 2021 will be better behaved, though we know already some of the hurdles which will make life interesting the coming year. COVID-19 is far from over, and it will take at least a year to vaccinate everyone. Furthermore, as of the first of today, the United Kingdom is no longer a part of the EU, making travel inside Europe a little harder again.

But before we launch into these new and interesting times, lets look back at 2020 one last time, keeping up with  tradition. What have I done during the last year of academic merit.

1. Publications: +6 (and currently a handful in progress)

2. Completed refereeing tasks: +17

  • Applied Physics Letters
  • Journal of Physical Chemistry (2x)
  • Computational Materials Science (2x)
  • Materials Chemistry and Physics
  • Journal of Physics: Condensed Matter (5x)
  • Diamond and Related Materials (6x)

3. Conferences & workshops in times of Corona: +3/+1 (Attended & Organised), >+4 internal 

ACOS poster prize 2020

ACOS poster prize 2020

With regard to conferences, 2020 was the year everyone came into contact with the concept of the online conference. Many conferences and events got canceled: such as TEDx@UHasselt (which will return in 2021)

  • ACOS 2020, Online, Oktober 28th, 2020 [poster presentation and video-pitch, 2nd poster prize]
  • RSC Chemical Science Symposium 2020, Online, September 29th-30th, 2020 [iposter presentation]
  • D-NL-HIT project meetings [oral presentations]
    • Virtual Partner Meeting, April 8th, 2020
    • Adhesives Pilot Branch meeting, October 7th, 2020
    • Virtual Partner Meeting, October 15th, 2020
    • UV-Curing Branch meeting, October 22nd, 2020
  • SBDD XXV, Hasselt University, Belgium, March 11th-13th, 2020 [(invited) oral presentation, poster presentation] …On Friday13th Belgium went into it’s first lock-down.
  • Pilot Branch meeting adhesives D-NL-HIT project, Maastricht University, Brighlands campus, February 26th, 2020 [Organised]

4. Science Communication & Social media:   

  • In February 2020, I finally caved and joined Twitter as @DelocalizedD .
  • Added several new repositories to my github account, with the most important ones being:
  • Started a YouTube channel (for the ACOS video pitch)

5. Current size of HIVE:

  • Continued work on a public version of HIVE at github: HIVE 4.x   (26K lines, 9 commands available)
  • 61K lines of program (code: 69 %)
  • ~100 files
  • 49 (command line) options

6. Hive-STM program:

And now, upward and onward, a new year, a fresh start.

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.


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.

Small Data Materials Design with Machine Learning: When the Average Model Knows Best

Authors:  Danny E. P. Vanpoucke, Onno S. J. van Knippenberg, Ko Hermans, Katrien V. Bernaerts, and Siamak Mehrkanoon
Journal: Journal of Applied Physics 128, 054901 (2020)
doi: 10.1063/5.0012285
IF(2019): 2.286
export: bibtex
pdf: <JApplPhys>   (Open Access)
github: <Amadeus>


Graphical Abstract: Correlation plot of the RMSE of the validation set and the intercept value for linear model instances trained on 1000 subsets of a 25 point data set. The distribution of the correlation data is indicated by the black curve.


Machine Learning is quickly becoming an important tool in modern materials design. Where many of its successes are rooted in huge data sets, the most common applications in academic and industrial materials design deal with data sets of at best a few tens of data points. Harnessing the power of Machine Learning in this context is therefore of considerable importance. In this work, we investigate the intricacies introduced by these small data sets. We show that individual data points introduce a significant chance factor in both model training and quality measurement. This chance factor can be mitigated by the introduction of an ensemble-averaged model. This model presents the highest accuracy while at the same time it is robust with regard to changing data set size. Furthermore, as only a single model instance needs to be stored and evaluated, it provides a highly efficient model for prediction purposes, ideally suited for the practical materials scientist.

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.


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.

UV-Curable Biobased Polyacrylates Based on a Multifunctional 2 Monomer Derived from Furfural

Authors: Jules Stouten, Danny E. P. Vanpoucke, Guy Van Assche, and Katrien V. Bernaerts
Journal: Macromolecules 53(4), 1388-1404 (2020)
doi: 10.1021/acs.macromol.9b02659
IF(2019): 5.918
export: bibtex
pdf: <Macromolecules> (Open Access)



Grapgical abstract ACS Macromolecules 2020
Graphical Abstract: The formation of biobased polyacrylates.


The controlled polymerization of a new biobased monomer, 4-oxocyclopent-2-en-1-yl acrylate (4CPA), was
established via reversible addition−fragmentation chain transfer (RAFT) (co)polymerization to yield polymers bearing pendent cyclopentenone units. 4CPA contains two reactive functionalities, namely, a vinyl group and an internal double bond, and is an unsymmetrical monomer. Therefore, competition between the internal double bond and the vinyl group eventually leads to gel formation. With RAFT polymerization, when aiming for a degree of polymerization (DP) of 100, maximum 4CPA conversions of the vinyl group between 19.0 and 45.2% were obtained without gel formation or extensive broadening of the dispersity. When the same conditions were applied in the copolymerization of 4CPA with lauryl acrylate (LA), methyl acrylate (MA), and isobornyl acrylate, 4CPA conversions of the vinyl group between 63 and 95% were reached. The additional functionality of 4CPA in copolymers was demonstrated by model studies with 4-oxocyclopent-2-en-1-yl acetate (1), which readily dimerized under UV light via [2 + 2] photocyclodimerization. First-principles quantum mechanical simulations supported the experimental observations made in NMR. Based on the calculated energetics and chemical shifts, a mixture of head-to-head and head-to-tail dimers of (1) were identified. Using the dimerization mechanism, solvent-cast LA and MA copolymers containing 30 mol % 4CPA were cross-linked under UV light to obtain thin films. The cross-linked films were characterized by dynamic scanning calorimetry, dynamic mechanical analysis, IR, and swelling experiments. This is the first case where 4CPA is described as a monomer for functional biobased polymers that can undergo additional UV curing via photodimerization.

Influence of diamond crystal orientation on the interaction with biological matter

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)


Graphical Abstract Carbon paper with Romana
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.