Rok: 2018

Structure of superconducting MgB2 thin films prepared by vacuum evaporation and ex-situ annealing in Ar and O-2 atmospheres

ROCH, Tomáš – GREGOR, Maroš – ŠVEC, Peter, Jr. – PLECENÍK, Tomáš – SATRAPINSKYY, Leonid – ČAPLOVIČOVÁ, Mária – BYSTRICKÝ, Roman – KÚŠ, Peter – PLECENÍK, Andrej

In Applied Surface Science. Vol. 461, (2018)


Superconducting MgB2 thin films were fabricated on c-cut sapphire substrates by vacuum co-evaporation of magnesium and boron followed by an ex situ post-annealing. We show that annealing in oxygen atmosphere can significantly improve the superconducting properties of the MgB2 thin films compared to the typical annealing in argon atmosphere. In this work, influence of the ex-situ annealing atmosphere on the structure, texture and morphology of the superconducting MgB2 thin films has been studied by transmission electron microscopy, X-ray diffraction and pole figure measurements. Samples annealed at 800 °C in Ar, at 800 °C in O2 and at 500 °C in O2 have been compared. The annealing in O2 at 800 °C produces MgB2 thin films with the highest superconducting transition temperature and critical current density. We show that this is thanks to the thickest MgO layer at the surface produced in this case, which acts as a protecting barrier against out-diffusion of Mg during the annealing and leads to better stoichiometry and larger MgB2 grains compared to the samples annealed in Ar. Our method can be alternative to the customary ex-situ post annealing of Mg-B precursor in sealed vapor cell. In all samples, the MgB2 phase showed single axis texture with the (0 0 0 1) planes slightly inclined by 0° to 15° with respect to the Al2O3(0 0 0 1) substrate surface incurred with the initial precursor evaporation geometry. During the annealing, the excess Mg also reacts with the Al2O3 substrate and minor MgAl2O4 and MgO phases are produced at the substrate-layer interface.

Raman spectroscopy of porous silicon substrates

KADLEČÍKOVÁ, Magdaléna – BREZA, Juraj – VANČO, Ľubomír – MIKOLÁŠEK, Miroslav – HUBEŇÁK, Michal – RACKO, Juraj – GREGUŠ, Ján

In Optik. Vol. 174, (2018)


We have investigated the effect of the etching time on the Raman spectra of porous silicon prepared by anodic etching. Electrochemical destruction of the substrate increasing with the etching time and the correlation between the microstructure of the silicon wafer and the shape and position of their Raman spectra have been observed. Raman analysis has shown that the intensity of the Raman dominant silicon band decreases and the bandwidth is shifted to lower frequencies, depending on the morphology of the sample. Therefore we believe that the electrochemical destruction of the surface of Si substrates leads to surface amorphization.

Degradation of Al4C3 Due to Atmospheric Humidity

NÝBLOVÁ, Daniela – BILLIK, Peter – NOGA, Jozef – ŠIMON, Erik – BYSTRICKÝ, Roman – ČAPLOVIČOVÁ, Mária – NOSKO, Martin

In JOM. Vol. 70, iss. 10 (2018)


The degradation of Al4C3 was investigated by exposing powder samples to atmospheric humidity at laboratory temperature for 150 days. Samples were monitored using powder x-ray diffraction analysis, scanning and transmission electron microscopy, thermal analysis, and energy-dispersive x-ray spectroscopy. Initially, the degradation resulted in an amorphous Al–O–OH network, which gradually recrystallized as low-crystalline boehmite (γ-AlOOH), and, later, partially, as Al(OH)3. Low-crystalline γ-AlOOH consisted of plate-like particles with diameter of 5 µm to 40 µm. The 020 diffraction position of γ-AlOOH was close to that of well-crystalline γ-AlOOH, which may be explained by accumulation of compressive stress along the b-axis of the γ-AlOOH structure.

Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications

Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications

TIAN, Yilei – VISMARA, Elena – VAN DOORENE, Steve – ŠUTTA, Pavol – VANČO, Ľubomír – VESELÝ, Marián – VOGRINČIČ, Peter – ISABELLA, Olindo – ZEMAN, Miro

In ACS Applied Energy Materials. Vol. 1, iss. 7 (2018)


Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.

Structural, surface and magnetic properties of chalcogenide Co9S8 nanoparticles prepared by mechanochemical synthesis


In Journal of Alloys and Compounds. Vol. 745, (2018)


In this study, the mechanochemical synthesis of Co9S8 nanoparticles from cobalt and sulphur by high-energy milling in a planetary mill in an argon atmosphere is reported. Structural characterization of the synthesized nanoparticles by XRD was performed. Co9S8 nanoparticles crystallize in the cubic structure with the crystallite size of about 16 nm. The microstructure of the Co9S8 nanoparticles was further studied using TEM, HRTEM and EDS-HAADF-STEM techniques. Co9S8 nanoparticles consist of nanocrystals exhibiting size in the range of 10–30 nm that are densely aggregated into spherical-like objects. The highest specific surface area value observed was 4 m2g-1 and the pore properties of this sample are quite poor. The magnetic properties of mechanochemically synthesized nanoparticles were investigated using SQUID magnetometer. The room temperature magnetic data supported the paramagnetic spin structure of Co9S8 nanoparticles. The transition from paramagnetic to weak ferromagnetic or ferrimagnetic behaviour was indicated from temperature dependence of magnetization at cryogenic temperatures. It is demonstrated that mechanochemical synthesis can be successfully employed in the one-step solid-state preparation of Co9S8 nanoparticles.

Adhesive-deformation relationships and mechanical properties of nc-AlCrN/a-SiNx hard coatings deposited at different bias voltages

Adhesive-deformation relationships and mechanical properties of nc-AlCrN/a-SiNx hard coatings deposited at different bias voltages

M. Haršáni, N. Ghafoor, K. Calamba, P. Zacková, M. Sahul, T. Vopát, L. Satrapinskyy, M. Čaplovičová, Ľ. Čaplovič

Thin Solid Films, Volume 650, 2018, Pages 11-19


A series of Al-Cr-Si-N hard coatings were deposited on WC-Co substrates with a negative substrate bias voltage ranging from −50 to −200 V using cathodic arc evaporation system. A Rockwell-C adhesion test demonstrated that excellent adhesion was observed at lower bias voltages of −50 V and −80 V, while further increases in bias voltage up to −200 V led to severe delamination and worsening of the overall adhesion strength. X-ray diffraction and transmission electron microscopy analysis revealed a single phase cubic B1-structure identified as an AlCrN solid solution with a nanocomposite microstructure where cubic AlCrN nanocrystals were embedded in a thin continuous amorphous SiNx matrix. Coatings exhibited a 002-texture evolution that was more pronounced at higher bias voltages (≥−120 V). Stress-induced cracks were observed inside the coatings at high bias voltages (≥−150 V), which resulted in stress relaxation and a decline in the overall residual stresses.

On the formation of hydrophobic carbon quantum dots Langmuir films and their transfer onto solid substrates

BODIK, Michal – ŠIFFALOVIČ, Peter – NÁDAŽDY, Peter – BENKOVIČOVÁ, Monika – MARKOVIČ, Zoran – CHLPÍK, Juraj – CIRÁK, Július – KOTLÁR, Mário – MICUŠÍK, M. – JERGEL, Matej – MAJKOVÁ, Eva

In Diamond and Related Materials. Vol. 83, (2018)


In this work, we present a study of the Langmuir film formation composed of hydrophobic carbon quantum dots (hCQDs). We studied the kinetics of hCQDs pyrolysis and optimized the reaction time for maximum photoluminescence. The resulting hCQDs were analyzed by transmission electron microscopy, X-ray photoemission spectroscopy and grazing-incidence X-ray diffraction. The process of monolayer formation was studied by monitoring surface pressure, surface potential, elastic modulus and Brewster angle microscopy in real time under continuous compression conditions. The monolayer transferred onto a silicon wafer was examined on the nano- and micro-scales by means of atomic force microscopy and confocal fluorescence microscopy, respectively.

Enhanced photocatalytic activity of hydrogenated and vanadium doped TiO2 nanotube arrays grown by anodization of sputtered Ti layers


In Applied Surface Science. Vol. 434, 15 March 2018 (2018)


TiO2 nanotube (TiNT) arrays were grown on silicon substrate via electrochemical anodization of titanium films sputtered by magnetron. To improve the photocatalytic activity of arrays annealed in air (o-TiNT), doping of o-TiNT with vanadium was performed (o-V/TiNT). These non-doped and doped TiNT arrays were also hydrogenated in H2/Ar atmosphere to r-TiNT and r-V/TiNT samples, respectively. Investigation of composition and morphology by X-ray diffraction (XRD), electron microscopy (SEM and TEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of well-ordered arrays of anatase nanotubes with average diameter and length of 100 nm and 1.3 μm, respectively. In both oxidized and reduced V-doped samples, vanadium is partly dissolved in the structure of anatase and partly deposited in form of oxide on the nanotube surface. Vanadium-doped and reduced samples exhibited higher rates in the photodegradation of organic dyes (compared to non-modified o-TiNT sample) and this is caused by limitation of electron-hole recombination rates and by shift of the energy gap into visible region. The photocatalytic activity was measured under UV, sunlight and visible irradiation, and the corresponding efficiency increased in the order (o-TiNT) < (r-TiNT) < (o-V/TiNT) < (r-V/TiNT). Under visible light, only r-TiNT and r-V/TiNT showed significant photocatalytic activity.

Antibacterial and Antibiofouling Properties of Light Triggered Fluorescent Hydrophobic Carbon Quantum Dots Langmuir-Blodgett Thin Films

STANKOVIČ, Nenad K – BODIK, Michal – ŠIFFALOVIČ, Peter – KOTLÁR, Mário – MICUŠÍK, M. – ŠPITÁLSKY, Zdenko – DANKO, Martin – MILIVOJEVIČ, Dušan D. – KLEINOVÁ, Angela – KULBAT, Pavel – CAPÁKOVÁ, Zdenka – HUMPOLIČEK, Petr – LEHOCKY, Marian – MARKOVIČ, Biljana – MARKOVIČ, Zoran

In ACS Sustainable Chemistry and Engineering. Vol. 6, iss. 3 (2018)


Inimitable properties of carbon quantum dots as well as a cheap production contribute to their possible application in biomedicine especially as antibacterial and antibiofouling coatings. Fluorescent hydrophobic carbon quantum dots are synthesized by bottom-up condensation method and used for deposition of uniform and homogeneous Langmuir–Blodgett thin films on different substrates. It is found that this kind of quantum dots generates singlet oxygen under blue light irradiation. Antibacterial and antibiofouling testing on four different bacteria strains (Escherichia coliStaphylococcus aureusBacillus cereus, and Pseudomonas aeruginosa) reveals enhanced antibacterial and antibiofouling activity of hydrophobic carbon dots thin films under blue light irradiation. Moreover, hydrophobic quantum dots show noncytotoxic effect on mouse fibroblast cell line. These properties enable potential usage of hydrophobic carbon quantum dots thin films as excellent antibacterial and antibiofouling coatings for different biomedical applications.

Chemical Oxidation of Graphite: Evolution of the Structure and Properties

SKÁKALOVÁ, Viera – KOTRUSZ, Peter – JERGEL, Matej – SUSI, Toma – MITTELBERGER, Andreas – VRETENÁR, Viliam – ŠIFFALOVIČ, Peter – KOTAKOSKI, J. – MEYER, Jannik C. – HULMAN, Martin

In Journal of Physical Chemistry C. Vol. 122, iss. 1 (2018)


Graphene oxide is a complex material whose synthesis is still incompletely understood. To study the time evolution of structural and chemical properties of oxidized graphite, samples at different temporal stages of oxidation were selected and characterized through a number of techniques: X-ray photoelectron spectroscopy for the content and bonding of oxygen, X-ray diffraction for the level of intercalation, Raman spectroscopy for the detection of structural changes, electrical resistivity measurements for probing charge localization on the macroscopic scale, and scanning transmission electron microscopy for the atomic structure of the graphene oxide flakes. We found a nonlinear behavior of oxygen uptake with time where two concentration plateaus were identified: Uptake reached 20 at % in the first 15 min, and after 1 h a second uptake started, reaching a highest oxygen concentration of >30 at % after 2 h of oxidation. At the same time, the interlayer distance expanded to more than twice the value of graphite and the electrical resistivity increased by seven orders of magnitude. After 4 days of chemical processing, the expanded structure of graphite oxide became unstable and spontaneously exfoliated; more than 2 weeks resulted in a significant decrease in the oxygen content accompanied by reaggregation of the GO sheets. These correlated measurements allow us to offer a comprehensive view into the complex oxidation process.