2022

Carbon dots (CDs) display photophysical features that appear similar, if not identical, despite the variety of synthesis protocols used to obtain them. Our calculations based on density functional theory helped to elucidate the structural sequence leading to the spontaneous formation of a compact network of molecular species. The thus obtained CDs are characterised for their optical fluorescence.
The work is a collaboration with the group of Prof. Lenka Zajíčková (MU/Ceitec Brno, Czech Republic).

J.I. Gómez, M. Sulleiro, N. Pizúrová, A. Bednařík, P. Lepcio, D. Holec, J. Preisler, L. Zajíčková: "Spontaneous formation of carbon dots helps to distinguish molecular fluorophores species". Appl. Surf. Sci., 610, 155536 (2023). DOI: 10.1016/j.apsusc.2022.155536

Our colleagues applied advanced diffraction techniques to entangle the impact of Zr on the γ-TiAl alloys. We supported their investigations with first-principles calculated compositionally-dependent structural parameters. Together we have constructed sections of the ternary Ti-Al-Zr phase diagram.
The work was coordinated by Michael Musi (MUL Leoben)

M. Musi, S. Kardos, L. Hatzenbichler, D. Holec, A. Stark, M. Allen, V. Güther, H. Clemens, P. Spoerk-Erdely: "The effect of zirconium on the Ti-(42-46 at.\%)Al system". Acta Mater., 241, 118414 (2022). DOI: 10.1016/j.actamat.2022.118414

Inspired by the intriguing interplay between chemical disorder and alloying, we applied ab initio methods to study the impact on bcc cubic-to-hexagonal phase transformations in TiAl+Mo alloys. We combined SQS (local relaxations) and CPA (effective long-range disorder) approaches. We can show that both chemical disorder and Mo alloying lead to a gradual charge of the phase preference from hexagonal/B19 (no Mo) to bcc/B2 (above approx. 10at. % Mo). Most of these transformations show barrier-less character, and marginal barriers emerge for scenarios when both cubic and hexagonal phases have the same energies.
The work was a collaboration with the group of Jürgen Spitaler (MCL Leoben).

N. Abdoshahi, M. Dehghani, A.V. Ruban, M. Friák, M. Šob, J. Spitaler, D. Holec: "On the energetics of the cubic-to-hexagonal transformations in TiAl+Mo alloys". Acta Mater., 240, 118268 (2022). DOI: 10.1016/j.actamat.2022.118268

Celine has successfully defended her MSc thesis titled “Development of a semi-automated molecular dynamics workflow for testing interatomic potentials against ab-initio data“. Also Lukas successfully defended his MSc thesis titled “Prediciting Phase Stability of TiAl-based High Entropy Alloys from First-Principles“. Today they both passed their final exams and thus have earned an MSc degree in Materials Science.
Congratulations!

We used ab initio calculations to study the impact of several alloying elements, including Al, Cr, Mg, Mn, and Si, on the stability of cementite. This was quantified using the change in formation energy between the alloyed cementite and the pure cementite. Additionally, we also considered multi-phase scenario and calculated partitioning partitioning energy between cementite, ferrite and austenite, in order to make statements about the effect of alloying elements on the cementite stability. Furthermore, our calculations of elastic properties of Al and Si alloyed to cementite show that both the elements increase its stability.

A. Sakic, C. Hofer, R. Schnitzer, D. Holec: "Ab initio study of alloying impact on the stability of cementite in transformation-induced plasticity-assisted advanced steels". Adv. Eng. Mater., , 2200532 (2022). DOI: 10.1002/adem.202200532

Our calculations focused on the stability of various (Mg, Al)Ca₂ Laves phases variants, C14, C15 and C36, as a function of the Al-to-Mg ratio. We could show that the energy differences between different variants are negligible for the intermediate compositions, thus suggesting extremely low stacking fault energies. In the second part, we simulated Mg/Al₈Mn₅ interface and showed, that the Al₈Mn₅ promotes segregation of Ca from the Mg matrix. Our calculations were a basis for discussing an extensive experimental study employing analytical high-resolution electron microscopy and atom probe tomography.
The work is a collaboration of several research groups and was coordinated by Jiehua Li (MUL Leoben).

J. Li, X. Zhou, A. Breen, Z. Peng, J. Su, P. Kürnsteiner, M. Correa, M. Chwałek, H. Wang, D. Holec, J. Mayer, G. Dehm: "Elucidation of formation and transformation mechanisms of Ca-rich Laves phase in Mg-Al-Ca-Mn alloys". J. Alloys Compd., 928, 167177 (2022). DOI: 10.1016/j.jallcom.2022.167177

Anna Paulik, Dominik Gehringer, Ganesh Nayak and David Holec attended 10th International conference on Materrials Structure & Mechanics of Fracture held September 11–14 2022 in Brno, Czech Republic. We all held oral presentations, enjoyed company of our colleagues from Brno and abroad, and found new friends.

Our colleagues revisited the mechanical properties of Al-rich AlCrN coating, known for its good hard but brittle mechanical response. They could show that when wurtzite-structured precipitates appear after annealing, the mechanical properties are enhanced, and the cleavage mode changes from intergranular to transgranular mode. Our DFT calculations supported this scenario by providing interface strength energies for discussing the feasibility of such a phase-transformation-induced toughening mechanism.
The work was coordinated by Michael Meindlhumer and it is a collaboration with the groups of Jozef Keckes (MUL Leoben) and Jochen Schneider (RWTH Aachen).

M. Meindlhumer, T. Ziegelwanger, J. Zalesak, M. Hans, L. Löfler, S. Spor, N. Jäger, A. Stark, H. Hruby, R. Daniel, D. Holec, J.M. Schneider, C. Mitterer, J. Keckes: "Precipitation-based grain boundary design alters Inter- to Trans-granular Fracture in AlCrN Thin Films". Acta Mater., 237, 118156 (2022). DOI: 10.1016/j.actamat.2022.118156

Dominik Gehringer, Ganesh Nayak and David Holec attended 4th International Congress of Chemists and Chemical Engineers of Bosnia and Herzegovina held June 30–July 2 2022 in Sarajevo. We all held oral presentations, found new friends and got a brief feeling of local hospitality, culture and history.

Marcus and Zsolt performed a comprehensive study of thermal stability in (V, Al)N coatings using a correlation of transmission electron microscopy and atom probe tomography data. They concluded that the spinodal decomposition is linked to the formation of Al-rich regions which are precursors for wurtzite AlN. Our DFT calculations support their appearance by pointing to a faster Al diffusion caused by 25% lower activation energy for bulk diffusion than V.
The work was coordinated by Marcus Hans and it is a collaboration with the group of Jochen Schneider (RWTH Aachen).

M. Hans, Z. Czigány, D. Neuß, J.A. Sälker, H. Rueß, J. Krause, G.K. Nayak, D. Holec, J.M. Schneider: "Probing the onset of wurtzite phase formation in (V, Al) N thin films by transmission electron microscopy and atom probe tomography". Surf. Coat. Technol., , 128235 (2022). DOI: 10.1016/j.surfcoat.2022.128235

Though it has been over a year since Lukas has started as a PostDoc in Aachen, today he finally made a big thick line behind his PhD studies! He presented his thesis titled “Defects and their influence on mechanical properties in nitrides: an atomistic study” which had been positively reviewed and passed with distinction the oral exam.
Congratulations!

Ganesh Nayak and David Holec have attended this year's ICMCTF in San Diego. It was a refreshing experience to meet our friends and collaborators again in person, enjoy their talks in the lecture halls, and afterward get engaged in live discussions. Ganesh presented a talk on diffusion in HENs and a poster on pressure impact on diffusion in TiN. At the same time, David showed results on phase transformations in TM diborides.

In this work, we contributed by showing the charge transfer between a functionalized graphene quantum dot (GQD) and a flat graphene substrate. The GQD was modelled as a graphene flake with its dangling bonds saturated with H, NH₂, and CONH₂ groups. We could show that some charge is transferred from the graphene to the QGD, thus effectively leading to p-doping of the graphene. Our colleagues used sophisticated methods, primarily atomic force microscopy, Kelvin probe force microscopy, and Raman spectroscopy, to prove this charge transfer experimentally.
The work was coordinated by Rajarshi Roy and it is a collaboration with the groups of Lenka Zajíčková (CEITEC Brno, Czech Republic) and Christian Teichert (MUL, Leoben).

R. Roy, D. Holec, M. Kratzer, P. Muenzer, P. Kaushik, L. Michal, G. Kumar, L. Zajíčková, C. Teichert: "Probing the charge transfer and electron-hole asymmetry in graphene-graphene quantum dot heterostructure". Nanotechnology, 33(32) (2022). DOI: 10.1088/1361-6528/ac6c38

Using DFT we have investigated bonding of graphene on different Si surfaces. We could reveal that while C-Si covalent bonds are formed on the (111) and (100) orientated surfaces, only van der Waals bonding state exists on the (110) orientated one. Next, the form of the substrate bonding leads to modifications of the electronic structure near the Fermi level, resulting in an unconventional doping proposed in this work.
The work is a collaboration with the groups of Colin Humphreys and David Dunstan, Queen Mary University of London, London, UK.

Y.W. Sun, D. Holec, D. Gehringer, L. Li, O. Fenwick, D.J. Dunstan, C.J. Humphreys: "Graphene on silicon: Effects of the silicon surface orientation on the work function and carrier density of graphene". Phys. Rev. B Condens. Matter, 105(16), 165416 (2022). DOI: 10.1103/PhysRevB.105.165416

The low intrinsic fracture toughness of transition metal nitride thin films critically restrains their applicability as protective coatings. We therefore investigated the Ti_1−xW_xN_y system to provide detailed theoretical and experimental insight into simultaneous hardening and toughening effects induced by heavy-element-alloying via an enhanced metallic bonding character. The combination of structural and chemical analyses, supported by density functional theory (DFT) calculations. demonstrates that the addition of W progressively increases the concentration of nitrogen vacancies in rocksalt (rs) structured Ti_1−xW_xN_y. DFT-calculated charge density maps revealed that the superior toughness-related performance of Ti_1−xW_xN_y (with respect to TiN, which showed a pronounced radial crack formation) is linked to a metallisation of the interatomic bonds, being most pronounced for balanced W and Ti contents and N vacancies.
The work is a collaboration with the group of Paul Mayrhofer (TU Wien).

J. Buchinger, N. Koutná, A. Kirnbauer, D. Holec, P.H. Mayrhofer: "Heavy-element-alloying for toughness enhancement of hard nitrides on the example Ti-W-N". Acta Mater., , 117897 (2022).

Anna Paulik has been awarded "Wissenschaftspreis für Montanistinnen 2022" in the category Junior Scientist for her scientific work on novel NiTiH phases in the framework of her MSc thesis. This prize is awarded to female researches at the Montanuniversiät Leoben also in PreaDoc (PhD) and PostDoc categories.
Congratulations!

Interfaces between components of a material govern its mechanical strength and fracture resistance. Here we combine ab initio and classical molecular dynamics simulations, nanoindentation, and transmission electron microscopy to reveal atomistic mechanisms underlying plasticity and crack growth in B1 AlN(001)/TiN(001) superlattices under loading. Our simulations demonstrate an anisotropic response to uniaxial tensile deformation in principal crystallographic directions due to different strain-activated plastic deformation mechanisms. Superlattices strained orthogonal to (001) interfaces show modest plasticity and cleave parallel to AlN/TiN layers. Contrarily, B1-to-B3 or B1-to-B4(Bk) phase transformations in AlN facilitate a remarkable toughness enhancement upon in-plane [110] and [100] tensile elongation, respectively.
The work is a collaboration with the groups of Paul Mayrhofer (TU Wien), Davide Sangiovani (Linköping University) and Zaoli Zhang (ESI-OeAW Leoben).

N. Koutná, L. Löfler, D. Holec, Z. Chen, Z. Zhang, L. Hultman, P.H. Mayrhofer, D.G. Sangiovanni: "Atomistic mechanisms underlying plasticity and crack growth in ceramics: a case study of AlN/TiN superlattices". Acta Mater., , 117809 (2022). DOI: 10.1016/j.actamat.2022.117809

In this work use ab initio calculations top show how does the vacancy concentration impact the core electron binding energies. Our predictions on the core electron energy shifts in cubic titanium oxynitride (Ti_1-δO_xN_1-x) are beautifully correlating with measured XPS spectra. This work unequivocally demonstrates that the formation and population of Ti vacancies in cubic Ti_1-δO_xN_1-x thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts.
The work is a collaboration with the group of Jochen Schneider group, RWTH Aachen, Germany.

P. Ondračka, M. Hans, D.M. Holzapfel, D. Primetzhofer, D. Holec, J.M. Schneider: "Ab initio-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in Ti_1-δO_xN_1-x thin films". Acta Mater., 230(117778), 117778 (2022). DOI: 10.1016/j.actamat.2022.117778

XPS spectrum of amorphous W-B-C material was previously analyzed by fitting it using three components corresponding to the relative amount of W-W, W-B, and W-C bonds. However, using ab initio calculations we could show that the amount W-W bonds substantially influences positions of the peak components. Our calculations enabled us to formulate the relationship between the W 4f electron binding energies (BE) and the local atomic environments of W atoms. Our analysis confirms the expected W 4f chemical shifts in W-B-C caused by W-B and W-C bonds and reveals that W-W bonds shift the W 4f electronic states in the same direction as W-B bonds, which has substantial implications for the correct interpretation of the measured XPS spectra.
The work is a collaboration with the group of Petr Vašina (MU Brno).

J. Ženíšek, P. Ondračka, J. Čechal, P. Souček, D. Holec, P. Vašina: "W 4f electron binding energies in amorphous W-B-C systems". Appl. Surf. Sci., 586, 152824 (2022). DOI: 10.1016/j.apsusc.2022.152824

The formation of Al₃Zr particles within dilute aluminum alloys can contribute effectively to controlling microstructure evolution and enhancing material properties. However, the possible transformation of Al₃Zr from its initial metastable crystal structure L1₂ into its stable, tetragonal structure D0₂₃ is associated with faster coherency loss and the coarsening of Al₃Zr particles. In this regard, our study aims at identifying ternary elements that can disrupt this mechanism. In parallel to experimental investigations, we conducted ab initio calculations to investigate fundamental properties of ternary AlZrX-particles (X=Er, Sc, Hf, Y, Nb, Mn, Cu, Zn and Si) such as substitution likeliness, heat of formation and transformation mechanisms. These calculations helped to elucidate the partitioning and overall effect of alloying observed in experiments.
The work is a collaboration with the group of Stefan Pogatscher (MUL).

F. Schmid, D. Gehringer, T. Kremmer, L. Cattini, P.J. Uggowitzer, D. Holec, S. Pogatscher: "Stabilization of Al₃Zr allotropes in dilute aluminum alloys via the addition of ternary elements". Materialia, 21, 101321 (2022). DOI: 10.1016/j.mtla.2022.101321

In the framework of the departmental seminar series "Materials Science Colloquium", Pavel Ondračka (Masaryk University, Brno, Czech Republic) has presented a new combined theoretical and experimental approach for quantifying point defect contents on oxides and oxynitrides from the X-ray photoelectron spectroscopy (XPS) measurements and ab initio calculations.

An in-depth study on interactions in ternary TiAl+Mo intermetallic alloy allowed us to identity the importance of multi-site as well as some unexpectedly long-range pair-interactions. Subsequently we used them in a Monte Carlo simulation to show that Mo alloying dramatically increases the order–disorder transition temperatures in bcc and hcp Al-deficient Ti_0.5Al_0.5−xMo_x alloys.
The work is a collaboration with the group of Jürgen Spitaler (MCL Leoben).

M. Dehghani, A.V. Ruban, N. Abdoshahi, D. Holec, J. Spitaler: "Stability and ordering of bcc and hcp TiAl+Mo phases: An ab initio study". Comput. Mater. Sci., 205, 111163 (2022). DOI: 10.1016/j.commatsci.2021.111163

Within this work the effect of the B content on the microstructure, phase composition and mechanical properties of CVD Ti(B,N) coatings is investigated. With increasing B content, a transition from an fcc to an hex-dominated structure via a dual-phase fcc/h-Ti(B,N) microstructure was observed, which was accompanied by a decrease in grain size from the μm to nm range. Complementary ab initio calculations predicting the phase formation helped to interpretat the experimental results. We could conclude, that both hex-TiB₂ and dual-phase fcc/hex-Ti(B,N) coatings withmaximized B content yield superior mechanical properties over TiN and consequently improved performance.
The work is a collaboration with many groups, initiated and coordinated by Michael Tkadletz (MUL).

M. Tkadletz, N. Schalk, A. Lechner, L. Hatzenbichler, D. Holec, C. Hofer, M. Deluca, B. Sartory, A. Lyapin, J. Julin, C. Czettl: "Influence of B content on microstructure, phase composition and mechanical properties of CVD Ti(B,N) coatings". Materialia, 21, 101323 (2022). DOI: 10.1016/j.mtla.2022.101323

This paper solves a longstanding debate: Raman measurements on double-walled carbon nanotubes appear to show that significantly more pressure than expected can be transmitted to the inner tube. We reinterpret those Raman spectra consistently reported in the literature, by assigning the Raman peaks to coupled vibrational modes of both walls, instead of individual contributions from the inner and outer tubes. These coupled vibrational modes are important for the correct interpretation of the Raman shift from strained layered 2D materials (we demonstrate it on bilayer graphene as an example), for researching the mechanical properties, thermal expansion, and strain engineering of two-dimensional materials.
The work is a collaboration with the groups of Colin Humphreys and David Dunstan, Queen Mary University of London, London, UK.

Y.W. Sun, D. Gehringer, D. Holec, D.G. Papageorgiou, O. Fenwick, S.M. Qureshi, C.J. Humphreys, D.J. Dunstan: "Significant interlayer coupling in bilayer graphene and double-walled carbon nanotubes: A refinement of obtaining strain in low-dimensional materials". Phys. Rev. B Condens. Matter, 105(2), 024103 (2022). DOI: 10.1103/PhysRevB.105.024103