Tag Archive: DFT

Sep 23

A combined experimental and theoretical investigation of the Al-Melamine reactive milling system: a mechanistic study towards AlN-based ceramics

Authors: Seyyed Amin Rounaghi, Danny E.P. Vanpoucke, Hossein Eshghi, Sergio Scudino, Elaheh Esmaeili, Steffen Oswald and Jürgen Eckert
Journal: J. Alloys Compd. 729, 240-248 (2017)
doi: 10.1016/j.jallcom.2017.09.168
IF(2017): 3.779
export: bibtex
pdf: <J.Alloys Compd.>

 

Graphical Abstract: Evolution of the end products as function of Al and N content during ball-milling synthesis of AlN.
Graphical Abstract: Evolution of the end products as function of Al and N content during ball-milling synthesis of AlN.

Abstract

A versatile ball milling process was employed for the synthesis of hexagonal aluminum nitride (h-AlN) through the reaction of metallic aluminum with melamine. A combined experimental and theoretical study was carried out to evaluate the synthesized products. Milling intermediates and products were fully characterized via various techniques including XRD, FTIR, XPS, Raman and TEM. Moreover, a Boltzmann distribution model was proposed to investigate the effect of milling energy and reactant ratios on the thermodynamic stability and the proportion of different milling products. According to the results, the reaction mechanism and milling products were significantly influenced by the reactant ratio. The optimized condition for AlN synthesis was found to be at Al/M molar ratio of 6, where the final products were consisted of nanostructured AlN with average crystallite size of 11 nm and non-crystalline heterogeneous carbon.

Apr 28

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
export: bibtex
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.

Mar 30

Linker Functionalization in MIL-47(V)-R Metal–Organic Frameworks: Understanding the Electronic Structure

Authors: Danny E. P. Vanpoucke
Journal: J. Phys. Chem. C 121(14), 8014-8022 (2017)
doi: 10.1021/acs.jpcc.7b01491
IF(2017): 4.484
export: bibtex
pdf: <J.Phys.Chem.C>
Graphical Abstract: Evolution of the electronic band structure of MIL-47(V) upon OH-functionalization of the BDC linker.
Graphical Abstract: Evolution of the electronic band structure of MIL-47(V) upon OH-functionalization of the BDC linker. The π-orbital of the BDC linker splits upon functionalisation, and the split-off π-band moves up into the band gap, effectively reducing the latter.

Abstract

Metal–organic frameworks (MOFs) have gained much interest due to their intrinsic tunable nature. In this work, we study how linker functionalization modifies the electronic structure of the host MOF, more specifically, the MIL-47(V)-R (R = −F, −Cl, −Br, −OH, −CH3, −CF3, and −OCH3). It is shown that the presence of a functional group leads to a splitting of the π orbital on the linker. Moreover, the upward shift of the split-off π-band correlates well with the electron-withdrawing/donating nature of the functional groups. For halide functional groups the presence of lone-pair back-donation is corroborated by calculated Hirshfeld-I charges. In the case of the ferromagnetic configuration of the host MIL-47(V+IV) material a half-metal to insulator transition is noted for the −Br, −OCH3, and −OH functional groups, while for the antiferromagnetic configuration only the hydroxy group results in an effective reduction of the band gap.

Nov 16

Modern art in research.

Which combination to take?

Although it looks a bit like a modern piece of art, it is one more attempt at trying to find an optimum combination of parameters.

I’m currently trying to find “the best choice” for U and J for a DFT+U based project… DFT??? Density Functional Theory. This is an approximate method which is used in computational materials science to calculate the quantum mechanical behavior of electrons in matter. Instead of solving the Schrödinger equation, known from any quantum mechanic course, one solves the Hohenberg-Kohn-Sham equations. In these equations it are not the electrons which play a central role (which they do in the Schrödinger equations) but the electron density. Hohenberg, Kohn and Sham were able to show that their equations give the exact same results as the Schrödinger equations. There is, however, one small caveat: you need to have an “exact” exchange-correlation functional (a functional is just a function of a function). Unfortunately there is no known analytic form for this functional, so one needs to use approximated functionals. As you probably guessed, with these approximate functionals the solution of the Hohenberg-Kohn-Sham equations is no longer an exact solution.

For some molecules or solids the error is much larger than average due to the error in the exchange-correlation functional. These systems are therefore called “strongly-correlated” systems. Over the years, several ways have been devised to solve this problem in DFT. One of them is called DFT+U. It entails adding additional coulomb interactions (Hubbard-U-potential) between the “strongly interacting electrons”. However this additional interaction depends on the system at hand, so one always needs to fit this parameter against one of more properties one is interested in. The law of conservation of misery, however, makes sure that improving one property goes hand in hand with a deterioration of another property.

Since actual DFT+U has two independent parameters (U and J, though for many systems they can be dependent reducing to a single parameter) I had quite some fun running calculations for a 21×21 grid of possible pairs. Afterward, collecting the data I wanted to use for fitting purposes took my script about 2h! 😯 Unfortunately the 10 properties of interest I wanted to fit give optimum (U,J)-pair all over the grid. In the picture above, you see my most recent attempt at trying to deal with them. It shows for the entire grid how many of the 10 properties are reasonably well fit.There are two regions which fit 6 properties; One around (U,J)=(5,10) and another around (U,J)=(8.5,17.5). There will be more work before this gives a satisfactory result, the show will go on.

Sep 25

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
export: bibtex
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

Feb 11

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
export: bibtex
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.

Feb 04

Virtual Winterschool 2016: Computational Solid State Physics & Chemistry

In just an hour, I’ll be presenting my talk at the virtual winterschool 2016. In an attempt to tempt fate as much as possible I will try to give/run real-time examples on our HPC in Gent, however at this moment no nodes are available yet to do so. Let’s keep our fingers crossed and see if it all works out.

Abstract

Modern materials research has evolved to the point where it is now common practice to manipulate materials at nanometer scale or even at the atomic scale (e.g. Intel’s skylake architecture with 14nm features, atomic layer deposition and surface structure manipulations with an STM-tip). At these scales, quantum mechanical effects become ever more relevant, making their prediction important for the field of materials science.

In this session, we will discuss how advanced quantum mechanical calculations can be performed for solids and indicate some differences with standard quantum chemical approaches. We will touch upon the relevant concepts for performing such calculations (plane-wave basis-sets, pseudo-potentials, periodic boundary conditions,…) and show how the basic calculations are performed with the VASP-code. You will familiarize yourself with the required input files and we will discuss several of the most important output-files and the data they contain.

At the end of this session you should be able to set up a single-point calculation, a structure optimization, a density of states and band structure calculation.

Additional Files/Info

Feb 01

Computational Materials Science: Where Theory Meets Experiments

Authors: Danny E. P. Vanpoucke,
Journal: Developments in Strategic Ceramic Materials:
Ceramic Engineering and Science Proceedings 36(8), 323-334 (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
export: bibtex
pdf: <preprint> 

Abstract

In contemporary materials research, we are able to create and manipulate materials at ever smaller scales: the growth of wires with nanoscale dimensions and the deposition of layers with a thickness of only a few atoms are just two examples that have become common practice. At this small scale, quantum mechanical effects become important, and this is where computational materials research comes into play. Using clever approximations, it is possible to simulate systems with a scale relevant for experiments. The resulting theoretical models provide fundamental insights in the underlying physics and chemistry, essential for advancing modern materials research. As a result, the use of computational experiments is rapidly becoming an important tool in materials research both for predictive modeling of new materials and for gaining fundamental insights in the behavior of existing materials. Computer and lab experiments have complementary limitations and strengths; only by combining them can the deepest fundamental secrets of a material be revealed.

In this paper, we discuss the application of computational materials science for nanowires on semiconductor surfaces, ceramic materials and flexible metal-organic frameworks, and how direct comparison can advance insight in the structure and properties of these materials.

Nov 16

A Flexible Photoactive Titanium Metal-Organic Framework Based on a [TiIV33-O)(O)2(COO)6] Cluster

Authors: Bart Bueken, Frederik Vermoortele, Danny E. P. Vanpoucke, Helge Reinsch, Chih-Chin Tsou, Pieterjan Valvekens, Trees De Baerdemaeker, Rob Ameloot, Christine E. A. Kirschhock, Veronique Van Speybroeck, James M. Mayer and Dirk De Vos
Journal: Angew. Chem. Int. Ed. 54(47), 13912-13917 (2015)
doi: 10.1002/anie.201505512
IF(2015): 11.705
export: bibtex
pdf: <Angew.Chem.Int.Ed.> 

Abstract

The synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Here, we present an innovative approach to Ti-based MOFs using titanocene dichloride to synthesize COK-69, the first breathing Ti-MOF built up of trans-1,4- cyclohexanedicarboxylate linkers and an unprecedented [TiIV33-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in-depth by proton-coupled electron transfer experiments, which revealed that up to one TiIV per cluster can be photoreduced to TiIII, while preserving the structural integrity of the framework. From molecular modeling, the electronic structure of COK-69 was determined and a band gap of 3.77 eV was found.

Nov 13

Cover Image of Crystal Engineering Communications: Fine-tuning the theoretically predicted structure of MIL-47(V) with the aid of powder X-ray diffraction

Authors: Thomas Bogaerts, Louis Vanduyfhuys, Danny E.P. Vanpoucke, Jelle Wieme, Michel Waroquier, Pascal Van Der Voort, and Veronique Van Speybroeck,
Journal: CrystEngComm. 17(45), 8565 (2015)
doi: 10.1039/C5CE90198G
IF(2015): 3.849
export: bibtex
pdf: <CrystEngComm>

Abstract

The cover image depicts an X-ray beam hitting a sample of MIL-47(V) Metal-Organic Framework (reddish powder), resulting in an X-ray diffraction pattern. This leads to the atomic structure on the left, Where the spin-densities are indicated for the anti-ferromagnetic ground state.  (The related paper can be found here.)


Cover of CrystEngComm: Volume 17, Issue 45, dec. 7, 2015

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