Category: Articles

Thermally induced structural evolution and age-hardening of polycrystalline V1–xMoxN (x ≈ 0.4) thin films

Posted on by Mária Čaplovičová

Marián Mikula, Stela Uzon, Tomáš Hudec, Branislav Grančič, Martin Truchlý, Tomáš Roch, Peter Švec, Leonid Satrapinskyy, Mária Čaplovičová, Grzegorz Greczynski, Ivan Petrov, Magnus Odén, Peter Kúš, Davide G. Sangiovanni

Surface and Coatings Technology, Volume 405, 2021, 126723

https://doi.org/10.1016/j.surfcoat.2020.126723

Abstract

Rocksalt-structure (B1) (V,Mo)N alloys are inherently hard and tough ceramics. However, the mechanical properties and thermal stability of (V,Mo)N solid solutions at temperatures ⪆ 700 °C of relevance for practical applications have not been previously investigated. In this work, we synthesize single-phase B1 polycrystalline V0.57Mo0.43N0.95 coatings to investigate the effects induced by temperature on the nanostructural evolution and hardness (H) of the material. Nanoindentation measurements show that the as-deposited film (H = 23 ± 3 GPa) becomes ≈30% harder (up to 31 ± 2 GPa) upon annealing at 730 °C. Experimental characterization and analyses, based on dispersive X-ray spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM), reveal that the age-hardening effect originates from decomposition of the solid solution into coherent strained cubic VN-rich/MoN-rich domains. The experimental results are complemented by the composition/temperature (V,Mo)N phase diagram – constructed upon ab initio molecular dynamics free-energies – which indicates that the separation observed in the solid solutions is of spinodal nature. Films annealed at temperatures exceeding 850 °C undergo structural coarsening, with formation of hexagonal MoxNy and cubic VN phases, which cause a decrease in hardness to ≈22 GPa. Our present findings indicate that (V,Mo)N coatings may offer outstanding mechanical performances during operation at elevated temperatures.

ZnO nanoparticles as photodegradation agent controlled by morphology and boron doping

ZnO nanoparticles as photodegradation agent controlled by morphology and boron doping

Posted on by Mária Čaplovičová

Daniel Furka, Samuel Furka, Mira Naftaly, Erik Rakovský, Mária Čaplovičová, Marián Janek

Catal. Sci. Technol., 2021,11, 2167-2185

https://doi.org/10.1039/D0CY01802C

Abstract

Photolytic degradation of model dyes on pure and boron-doped zinc oxide (ZnO) nanoparticles with pseudohexagonal or elongated spindle-like morphology was investigated. ZnO nanoparticles were prepared by spray-assisted co-precipitation solvothermal synthesis. The bandgap (Eg) of pure ZnO was decreased by boron doping. The prepared nanoparticles were tested for their photocatalytic activity by decomposition of different organic dyes, using phloxine, oxazine and rhodamine as test substances simulating environmental pollutants. The photolysis experiments were done in a 3D printed photoreactor using a photodiode with a wavelength of 365 ± 5 nm as a narrow-line radiation source. The observed first-order reaction kinetics revealed that higher reaction rates were achieved on pseudohexagonal nanoparticles. Increasing the boron content led to accelerated photolysis rates. A significant linear correlation was observed between the optical bandgap energy Eg and the residual dye concentration remaining after 6 hours from the start of the reaction. It was found that ZnO particles with pseudohexagonal morphology decompose organic dyes faster than elongated spindle-like particles, indicating dependence on the surface area as determined by BET analysis. Interestingly, the residual dye concentration varied more strongly with doping when elongated spindle-like nanoparticles were used. It was shown that the efficiency of a photolysis reaction occurring on a solid oxide/solution interface is affected by the crystallographic plane on which it takes place.

Polarization dependent photoluminescence and optical anisotropy in CuPtB-ordered dilute GaAs1-xBi xalloys

Posted on by Viliam Vretenár

PAULAUSKAS, Tadas – ACEECHAVIČIUS, Bronislovas – KARPUS, Vytautas – JOČIONIS, Lukas – TUMĖNAS, Saulius – DEVENSON, Jan – PAČ̌EBUTAS, Vaidas – STANIONYTE, Sandra – STRAZDIENE, Viktorija – GEIAUTIS, Andrejus – ČAPLOVIČOVÁ, Mária – VRETENÁR, Viliam – WALLS, Michael G. – KROTKUS, Arunas

In Journal of Applied Physics. Vol. 128, iss. 19 (2020)

https://doi.org/10.1063/5.0030091

The GaAs1–xBix semiconductor alloy allows one to achieve large bandgap reduction and enhanced spin–orbit splitting energy at dilute Bi quantities. The bismide is currently being developed for near- to mid-infrared lasers, multi-junction solar cells, and photodetectors. In this structure–property relationship study of GaAsBi alloys, we report polarization dependent photoluminescence that reaches a polarization ratio up to 2.4 at room temperature. Polarization dependence is also presented using transmittance spectra, birefringence, and linear dichroism. The optical anisotropy observations agree with the predictions of point symmetry reduction in the CuPtB-type ordered GaAsBi phase. The structural ordering is investigated experimentally from the atomic scale in molecular-beam epitaxy (MBE) grown samples on exact and miscut (001) GaAs substrates, as well as on (001) Ge. The latter sample is composed of anti-phase domains in which the ordering axes are rotated by 90° angles. Since the conditions stabilizing the CuPtB ordered phase fall within the typical MBE growth regime of dilute bismides, the optical anisotropy in GaAsBi alloys is expected to be ubiquitous. These findings are important for the future development of GaAsBi-based optoelectronics and also provide new means to analyze structurally complex bismide alloys.

Dehydroaromatization of methane over Mo/ZSM-5 zeolites: influence of aluminum distribution in the crystals

Posted on by Mária Čaplovičová

HORŇÁČEK, Michal – HUDEC, Pavol – JORÍK, Vladimír – ČAPLOVIČOVÁ, Mária – ČAPLOVIČ, Ľubomír – KALIŇÁK, Michal – SMIEŠKOVÁ, Agáta

https://doi.org/10.1007/s11144-020-01887-5

Abstract

Mo/ZSM-5 type catalysts based on two ZSM-5 zeolites with comparable crystal morphology, shape and size, Si/Al in bulk (22–23), acidity and amount of Mo (~ 4.7 wt.%) but having a different (homogeneous and non-homogeneous) Al distribution in particles and different acid sites type distribution were investigated in methane dehydroaromatization. From the results follows that Mo is associated with both BAS and LAS in the samples with almost homogenous Al distribution (MoZSM-5/AH). In the case of both samples with non-homogenous Al distribution (MoZSM-5/BH and MoZSM-5/BNH4) Mo preferably associates with BAS. Evidently lower activity and selectivity of Mo/ZSM-5/AH catalyst in methane transformation compared with Mo/ZSM-5/BH we attribute to the fact that from Mo species which interact with Lewis acid sites in sample ZSM-5/AH less active carburized species are formed as from the Mo associated with Brønsted sites in sample ZSM-5/BH.

Ag-modified LiMn2O4 cathode for lithium-ion batteries: Coating functionalization

Posted on by Mario Kotlár

ABBAS, Somia M. – HASHEM, Ahmed M. – ABDEL-GHANY, Ashraf E. – ISMAIL, Eman H. – KOTLÁR, Mário – WINTER, Martin – LI, Jie – JULIEN, Christian M.

In Energies [Open access]. Vol. 13, iss. 19 (2020)

https://doi.org/10.3390/en13195194

Abstract

In this work, the properties of silver-modified LiMn2O4 cathode materials are revisited. We study the influence of calcination atmosphere on the properties of the Ag-coated LiMn2O4 (Ag/LMO) and highlight the silver oxidation. The effect of the heat treatment in vacuum is compared with that in air by the characterization of the structure, specific surface area, Li transport properties and electrochemical performance of Ag/LMO composites. Surface analyses (XPS and Raman spectroscopy) show that the nature of the coating (~3 wt.%) differs with the calcination atmosphere: Ag/LMO(v) calcined in vacuum displays Ag nanospheres and minor AgO content on its surface (specific surface area of 4.1 m2 g−1), while Ag/LMO(a) treated in air is mainly covered by the AgO insulating phase (specific surface area of 0.6 m2 g−1). Electrochemical experiments emphasize that ~3 wt.% Ag coating is effective to minimize the drawbacks of the spinel LiMn2O4 (Mn dissolution, cycling instability, etc.). The Ag/LMO(v) electrode shows high capacity retention, good cyclability at C/2 rate and capacity fade of 0.06% per cycle (in 60 cycles).

Toward BaSi2/Si Heterojunction Thin-Film Solar Cells: Insights into Heterointerface Investigation, Barium Depletion, and Silicide-Mediated Silicon Crystallization

Posted on by Lubomír Vančo

TIAN, Yilei – MONTES, Ana Rita Bento – VANČO, Ľubomír – ČAPLOVIČOVÁ, Mária – VOGRINČIČ, Peter – SUTTA, Pavol – SATRAPINSKYY, Leonid – ZEMAN, Miro – ISABELLA, Olindo

In Advanced Materials Interfaces. Vol. 7, iss. 19 (2020)

https://doi.org/10.1002/admi.202000887

Abstract

The knowledge of the structural and compositional details of Si/BaSi2/Si heterostructure annealed at high temperature is a prerequisite for BaSi2 application in heterojunction thin-film solar cells. For this purpose, Si/BaSi2/Si heterostructures deposited by magnetron sputtering with different Si layer thickness are submitted to systematic structural and compositional characterizations. Results reveal a BaSi2/Si heterointerfacial variation caused by surface oxidation and Ba diffusion at the high temperature. Its effects on the optical and electrical properties of Si/BaSi2/Si heterostructure are presented. The outcomes of this work can be extended to BaSi2 deposited by other techniques, and generate substantial advantages in BaSi2 development ranging from improvement on material qualities and eventual deployment in thin-film solar cells.

Collapse Mechanism in Few-Layer MoS2 Langmuir Films

Posted on by Mario Kotlár

BODÍK, Michal – DEMYDENKO, Maksym – SHABELNYK, Tetiana – HALAHOVETS, Yuriy – KOTLÁR, Mário – KOSTIUK, Dmytro – SHAJI, Ashin – BRUNOVÁ, Alica – VEIS, Pavel – JERGEL, Matej – MAJKOVÁ, Eva – ŠIFFALOVIČ, Peter

In Journal of Physical Chemistry C. Vol. 124, iss. 29 (2020)

https://doi.org/10.1021/acs.jpcc.0c02365

Abstract

Recent advances in the liquid-phase exfoliation enabled large-scale production of two-dimensional (2D) materials, including few-layer graphene and transition metal dichalcogenides. The exfoliated flakes of 2D materials allow cost-effective deposition of continuous films for various applications ranging from optoelectronics to lubrication technology. The self-assembly of 2D materials on water subphase and subsequent transfer of such a Langmuir film onto a solid substrate offers an unprecedented layer quality in terms of spatial homogeneity as it proceeds in thermodynamic equilibrium. However, while the formation of conventional organic molecular Langmuir films has been widely studied, the application of the Langmuir technique to rigid inorganic 2D materials is still rather unexplored. Here, we study the underlying mechanism behind the formation and collapse at the critical surface pressure of the Langmuir film composed of few-layer MoS2 flakes. The in situ wide-angle X-ray scattering measured in real time and other supportive techniques applied ex situ after the film transfer onto a Si/SiO2 substrate were employed. We identify all principal compression stages up to the Langmuir monolayer collapse and beyond, relying on the texture, surface pressure, and elastic modulus temporal evolution. The results obtained and the conclusions drawn can be extended to a large family of the inorganic Langmuir films of other 2D materials to optimize the deposition process for envisaged application.

Atomic-Resolution EDX, HAADF, and EELS Study of GaAs1-xBix Alloys

Posted on by Viliam Vretenár

PAULAUSKAS, Tadas – PACEBUTAS, Vaidas – BUTKUTE, Renata – ČECHAVIČIUS, Bronislovas – NAUJOKAITIS, Arnas – KAMARAUSKAS, Mindaugas – SKAPAS, Martynas – DEVENSON, Jan – ČAPLOVIČOVÁ, Mária – VRETENÁR, Viliam – LIU, Xiaoyan – KOCIAK, Mathieu – KROTKUS, Arunas

In Nanoscale Research Letters. Vol. 15, iss. 1 (2020)

https://doi.org/10.1186/s11671-020-03349-2

Abstract

The distribution of alloyed atoms in semiconductors often deviates from a random distribution which can have significant effects on the properties of the materials. In this study, scanning transmission electron microscopy techniques are employed to analyze the distribution of Bi in several distinctly MBE grown GaAs1−xBix alloys. Statistical quantification of atomic-resolution HAADF images, as well as numerical simulations, are employed to interpret the contrast from Bi-containing columns at atomically abrupt (001) GaAs-GaAsBi interface and the onset of CuPt-type ordering. Using monochromated EELS mapping, bulk plasmon energy red-shifts are examined in a sample exhibiting phase-separated domains. This suggests a simple method to investigate local GaAsBi unit-cell volume expansions and to complement standard X-ray-based lattice-strain measurements. Also, a single-variant CuPt-ordered GaAsBi sample grown on an offcut substrate is characterized with atomic scale compositional EDX mappings, and the order parameter is estimated. Finally, a GaAsBi alloy with a vertical Bi composition modulation is synthesized using a low substrate rotation rate. Atomically, resolved EDX and HAADF imaging shows that the usual CuPt-type ordering is further modulated along the [001] growth axis with a period of three lattice constants. These distinct GaAsBi samples exemplify the variety of Bi distributions that can be achieved in this alloy, shedding light on the incorporation mechanisms of Bi atoms and ways to further develop Bi-containing III-V semiconductors.

Green synthesis of stable nanocolloids of monodisperse silver and gold nanoparticles using natural polyphenols from fruits of Sambucus nigra L

Posted on by Mária Čaplovičová

MARIYCHUK, Ruslan – PORUBSKÁ, Jana – OSTAFIN, Marek – ČAPLOVIČOVÁ, Mária – ELIÁŠOVÁ, Adriana

In Applied Nanoscience. Vol. 10, iss. 12 (2020)

https://doi.org/10.1007/s13204-020-01324-y

Abstract

In the present work, we report, for the first time, the production of stable nanocolloids of highly monodisperse silver and gold nanoparticles (AgNPs and AuNPs) using phytosynthesis approach. With the aim to eliminate misinterpretations associated with the use of usually applied crude plant extracts, the presented study was focused on the use of polyphenolic fraction isolated from elderberry fruits (Sambucus nigra L.) for the synthesis. Obtained AgNPs and AuNP were characterized by UV–Vis and FTIR spectroscopy, DLS and TEM. We observed unexpectedly high capability of the natural polyphenols not only to form monodisperse nanoparticles but also to stabilise the concentrated nanocolloids (539 mg/L AgNPs and 739 mg/L AuNPs) for at least two years. Moreover, AgNPs showed strong antibacterial properties against pathogenic bacterial strains Staphylococcus aureus and Escherichia coli (inhibited after 15 min and totally stopped growth after 3 and 5 h at 3.125 mg/L concentration).

Ni-mediated reactions in nanocrystalline diamond on Si substrates: the role of the oxide barrier

Posted on by Mária Čaplovičová

TULIČ, Semir – WAITZ, Thomas – ROMANYUK, Oleksandr – VARGA, Marián – ČAPLOVIČOVÁ, Mária – HABLER, Gerlinde – VRETENÁR, Viliam – KOTLÁR, Mário – KROMKA, Alexander – REZEK, Bohuslav – SKÁKALOVÁ, Viera

In RSC Advances. Vol. 10, iss. 14 (2020)

https://doi.org/10.1039/D0RA00809E

Abstract

Nanocrystalline diamond (NCD) films grown on Si substrates by microwave plasma enhanced chemical vapor deposition (MWPECVD) were subjected to Ni-mediated graphitization to cover them with a conductive layer. Results of transmission electron microscopy including electron energy-loss spectroscopy of cross-sectional samples demonstrate that the oxide layer on Si substrates (∼5 nm native SiO2) has been damaged by microwave plasma during the early stage of NCD growth. During the heat treatment for graphitizing the NCD layer, the permeability or absence of the oxide barrier allow Ni nanoparticles to diffuse into the Si substrate and cause additional solid-state reactions producing pyramidal crystals of NiSi2 and SiC nanocrystals. The latter are found impinged into the NiSi2 pyramids but only when the interfacial oxide layer is absent, replaced by amorphous SiC. The complex phase morphology of the samples is also reflected in the temperature dependence of electrical conductivity, where multiple pathways of the electronic transport dominate in different temperature regions. We present models explaining the observed cascade of solid-state reactions and resulting electronic transport properties of such heterostructures.