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Publications in 2024

ABSTRACT: This study explores a novel approach to surface-enhanced Raman scattering (SERS) substrate fabrication through the heat-induced fragmentation of gold nanowires (Au NWs) and its impact on gold nanoparticle adhesion/static friction using atomic force microscopy manipulations. Controlled heating experiments and scanning electron microscopy measurements reveal significant structural transformations, with NWs transitioning into nanospheres or nanorods in a patterned fashion at elevated temperatures. These morphological changes lead to enhanced Raman signals, particularly demonstrated in the case of Rhodamine B molecules. The results underscore the critical role of NW shape modifications in augmenting the SERS effect, shedding light on a cost-effective and reliable method for producing SERS substrates.

ChemEngineering 2024, 8(1), 15
DOI: 10.3390/chemengineering8010015
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ABSTRACT: The development and testing of antimicrobial coatings continues to be a crucial approach, considering the ongoing emergence of antibiotic-resistant bacteria and the rapid transmission of highly pathogenic viruses. In this study, three types of coatings—pure metallic copper (Cu), zinc oxide (ZnO), and a three-layer zinc oxide and copper mixed coating (ZnO/Cu/ZnO)—were deposited by magnetron sputtering on polyethylene terephthalate substrates to evaluate their antimicrobial potential using various microorganisms, including viruses. Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli bacteria were used for the assessment of antibacterial properties. Antiviral testing was performed using MS2 bacteriophage and replication-deficient Semliki Forest virus, both representing single-stranded RNA-containing viruses. The samples’ ability to cause reactive oxygen species formation was measured, and the effect on bacterial metabolic activity was evaluated. Cu-coated samples showed high inhibitory activity (>95%) against E. coli and S. aureus bacteria, as well as against tested viruses (SFV and MS2). The antibacterial and antiviral properties of ZnO/Cu/ZnO and ZnO coatings were not significant. Although ZnO/Cu/ZnO and ZnO caused inhibition of the metabolic activity of the bacteria, it was insufficient for complete bacteria eradication. Furthermore, significant reactive oxygen species (ROS) production was detected only for single Cu-coated samples, correlating with the strong bacteria-killing ability. We suppose that the ZnO layer exhibited a low release of Zn ions and prevented contact of the Cu layer with bacteria and viruses in the ZnO/Cu/ZnO coating. We conclude that current ZnO and Cu-ZnO-layered coatings do not possess antibacterial and antiviral activity.

Coatings 2024, 14(1), 14
DOI: 10.3390/coatings14010014
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ABSTRACT: This study introduces a novel approach for fabricating ZnS/Al2O3/TaSe2 heterostructured core/shell nanowires (NWs) through the selenization of a metallic Ta thin film precursor. The synthesis process involves a meticulously designed four-step protocol: (1) generating ZnS NWs on an oxidized silicon substrate, (2) encapsulating these NWs with a precisely controlled thin Al2O3 layer via atomic layer deposition (ALD), (3) applying a Ta precursor layer by magnetron sputtering, and (4) annealing in a Se-rich environment in a vacuum-sealed quartz ampoule to transform the Ta layer into TaSe2, resulting in the final core/shell structure. The characterization of the newly produced NWs using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) was validated using the integrity and composition of the heterostructures. Our method not only establishes a new pathway for the synthesis of TaSe2-based core/shell NWs but also extends the potential for creating a variety of core/shell NW systems with chalcogenide shells by adapting the thin film metal precursor approach. This versatility opens the way for future advancements in nanoscale material applications, particularly in electronics and optoelectronics where core/shell geometries are increasingly important.

ChemEngineering 2024, 8(1), 25
DOI: 10.3390/chemengineering8010025 (Open Access)
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ABSTRACT: We investigate the effective oxidation state and local environment of yttrium in photochromic YHO thin film structures produced by e-beam evaporation, along with their chemical structure and optical properties. Transmission electron microscopy images reveal the oxidized yttrium hydride thin film sample exhibiting a three-layered structure. X-ray photoelectron spectroscopy (XPS) measurements manifest that the oxidation state of yttrium is modified, dependent on the film’s composition/depth. Furthermore, Ion beam analysis confirms that this variability is associated with a composition gradient within the film. X-ray absorption spectroscopy at the Y K-edge reveals that the effective oxidation state of yttrium is approximately +2.5 in the transparent/bleached state of YHO. Spectroscopic ellipsometry investigations showed a complex non-linear optical depth profile of the related sample confirming the dominant phase of YHO and the presence of Y2O3 and Y towards the middle of the film. The first evidence of n; k) dispersion curves for e-beam sputtered photochromic YHO thin films are reported for transparent and dark states.

Zeitschrift für Physikalische Chemie 2024, February 28
DOI: 10.1515/zpch-2023-0507 (Open Access)
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ABSTRACT: Nanoparticles in microscopy images are usually analyzed qualitatively or manually and there is a need for autonomous quantitative analysis of these objects. In this paper, we present a physics-based computational model for accurate segmentation and geometrical analysis of one-dimensional deformable overlapping objects from microscopy images. This model, named Nano1D, has four steps of preprocessing, segmentation, separating overlapped objects and geometrical measurements. The model is tested on SEM images of Ag and Au nanowire taken from different microscopes, and thermally fragmented Ag nanowires transformed into nanoparticles with different lengths, diameters, and population densities. It successfully segments and analyzes their geometrical characteristics including lengths and average diameter. The function of the algorithm is not undermined by the size, number, density, orientation and overlapping of objects in images. The main strength of the model is shown to be its ability to segment and analyze overlapping objects successfully with more than 99 % accuracy, while current machine learning and computational models suffer from inaccuracy and inability to segment overlapping objects. Benefiting from a graphical user interface, Nano1D can analyze 1D nanoparticles including nanowires, nanotubes, nanorods in addition to other 1D features of microstructures like microcracks, dislocations etc.

Ultramicroscopy, Volume 261, July 2024, 113949
DOI: 10.1016/j.ultramic.2024.113949


ABSTRACT: The structural properties of photochromic yttrium oxyhydride powder in its transparent state were examined using x-ray diffraction and temperature-dependent extended x-ray absorption fine structure spectroscopy (EXAFS) combined with reverse Monte Carlo (RMC) simulations. The refinement of the x-ray powder diffraction pattern, employing the Rietveld method, indicates that yttrium oxyhydride crystallizes in the nanocrystalline phase with the cubic space group Fm-3m (225), at room temperature. The lattice parameter was determined as a = 5.404(3) Å, and the nanocrystallite size was estimated at d = 16(2) nm. The partial radial distribution functions (RDFs) g(r) for Y–O, O–O, and Y–Y atom pairs were obtained from the results of the RMC simulations of the Y K-edge EXAFS spectra measured at three temperatures (10, 150, and 300 K). The analysis of the RDFs reveals a subtle impact of the thermal disorder and splitting of the second coordination shell of yttrium atoms (the Y–Y RDF), remaining at all temperatures. This observation, also supported by our density functional theory calculations, suggests the presence of local structural distortions associated with yttrium sites, which do not affect the long-range crystal order.

Applied Physics Letters, Volume 124, 151901 (2024)
DOI: 10.1063/5.01999349 (Open Access)


ABSTRACT: The local atomic structure and lattice dynamics of two isostructural layered transition metal dichalcogenides (TMDs), 1T-TiSe and 1T-VSe , were studied using temperature-dependent X-ray absorption spectroscopy at the Ti, V, and Se K-edges. Analysis of the extended X-ray absorption fine structure (EXAFS) spectra, employing reverse Monte Carlo (RMC) simulations, enabled tracking of the temperature evolution of the local environment in the range of 10–300 K. The atomic coordinates derived from the final atomic configurations obtained using the RMC method were used to calculate the partial radial distribution functions (RDFs) and the mean-square relative displacement (MSRD) factors for the first ten coordination shells around the absorbing atoms. Characteristic Einstein frequencies and effective force constants were determined for Ti—Se, Ti—Ti, V—Se, V—V, and Se—Se atom pairs from the temperature dependencies of MSRDs. The obtained results reveal differences in the temperature evolution of lattice dynamics and the strengths of intralayer and interlayer interactions in TiSe2 and VSe2.

Physica B: Condensed Matter, Volume 685, 15 July 2024, 415995
DOI: 10.1016/j.physb.2024.415995


ABSTRACT: Metallic nanowires (NWs) are sensitive to heat treatment and can split into shorter fragments within minutes at temperatures far below the melting point. This process can hinder the functioning of NW-based devices that are subject to relatively mild temperatures. Commonly, heat-induced fragmentation of NWs is attributed to the interplay between heat-enhanced diffusion and Rayleigh instability. In this work, we demonstrated that contact with the substrate plays an important role in the fragmentation process and can strongly affect the outcome of the heat treatment. We deposited silver NWs onto specially patterned silicon wafers so that some NWs were partially suspended over the holes in the substrate. Then, we performed a series of heat-treatment experiments and found that adhered and suspended parts of NWs behave differently under the heat treatment. Moreover, depending on the heat-treatment process, fragmentation in either adhered or suspended parts can dominate. Experiments were supported by finite element method and molecular dynamics simulations.

Beilstein J Nanotechnol 2024 Apr 22:15:435-446.
DOI: 10.3762/bxiv.2024.4.v1 (Open Access)


ABSTRACT: Transition metal dichalcogenides (TMDs), specifically those involving V and Ti, possess fascinating material properties, making them interesting candidates for scientific studies. The existing growth methods of these materials are typically limited by scalability – either low yield or high cost. Here, we propose an alternative 2-step method valid for scalable production. In the first step, precursor films of Ti / V are deposited using magnetron sputtering, followed by the second step of selenization of these samples using elemental Se in a vacuum-sealed quartz ampoule for conversion to their respective diselenide material. Synthesized films are char-acterised using scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and X-ray photoelectron (XPS). The method demonstrated here can be used to increase the active surface area of TiSe2 and VSe2 for their potential catalytic and HER applications using nanostructured substrates, while also providing an opportunity for scalable synthesis of films that can be extended to synthesize other TMDs as well.

Latvian Journal of Physics and Technical Sciences, VOLUME 61 (2024): ISSUE 2 (APRIL 2024)
DOI: 110.2478/lpts-2024-0009 (Open Access)
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Institute of Solid State Physics, University of Latvia, Thin Films Laboratory