Author: Mario Kotlár

Anisotropic In-Plane Thermal Transport in Monolayer ReSe2 and Its Modulation through Layer Control and Selenium Vacancies: Experiment vs. Theory

Posted on by Mario Kotlár

Shipra Aswal, Sirsendu Ghosal, Himanshu Murari, Ravinder Chahal, Viliam Vretenár, Ravi Kumar Biroju, Ľubomír Vančo, Subhradip Ghosh and P. K. Giri

In: Nanoscale, 2025


https://doi.org/10.1039/D5NR03668B

Abstract

Understanding phonon transport and thermal anisotropy in two-dimensional (2D) materials is essential for their integration into electronic and thermal nanoscale devices. In this work, we achieve large-area, contamination-free monolayer Rhenium diselenide (ReSe2) growth via chemical vapor deposition, confirmed by atomic force microscopy and HAADF-STEM imaging. To probe its anisotropic thermal properties, we employ non-contact low-temperature Raman spectroscopy with unpolarized laser excitation to measure its thermal conductivity (κ). We report exceptionally low values of in-plane thermal conductivity κ~25.2 Wm-1K-1 for the pristine monolayer, the lowest among the TMDs. Critically, we find the introduction of more selenium vacancies further suppresses it to κ~20.7 Wm-1K-1. Polarization-dependent Raman analysis reveals a layer-dependent change in anisotropy ratio, with the ratio decreasing from 6.23 (pristine monolayer) to 4.42 (monolayer with vacancies) and further to 3.82 (trilayer), highlighting the distinct effects of both interlayer interactions and point defects on phonon transport. The suppression of κ with increasing thickness suggests enhanced phonon scattering from structural distortions and weak van der Waals coupling. These findings provide critical insights into how both layer thickness and intrinsic defects like Se vacancies can be used to modulate anisotropic transport in low-symmetry 2D materials. In addition, we employed Density Functional Theory and Boltzmann Transport Theory to elucidate the lattice phonon dynamics and thermal transport behaviour of monolayer ReSe2. The computed lattice thermal conductivity (κ_l) exhibits excellent agreement (κ_l^x ≈ 21.5 Wm-1 K-1 and κ_l^y ≈ 23.8 Wm-1 K-1) with the experimental data, thereby providing strong validation for our experimental approach. This work establishes ReSe2 as a strong candidate for thermoelectric and nanoelectronic applications where tunable thermal properties are paramount.

Tailoring the optoelectronic properties of MoOX nanoparticles: a novel microwave-assisted synthesis for near-infrared absorbing polyoxometallic clusters. 

Posted on by Mario Kotlár

Daniel Truchan, Adriana Hvizdošová Annušová, Guilhem Curé, Matej Mičušík, Vojtech Nádaždy, Mário Kotlár, Giulia C. Fadda, Peter Nádaždy, Matej Jergel, Claire Wilhelm, Aurore Van de Walle, Peter Šiffalovič & Yoann Lalatonne

In: Discover Nano 20, 197 (2025)

https://doi.org/10.1186/s11671-025-04378-5

Abstract

The emergence of novel catalytic, electrochemical, and biomedical applications of nanomaterials requires an understanding of their structural basis for stimuli-responsive performance. The chemistry of polyoxometallic nanomaterials, with a variety of interesting properties, remains poorly explored. In this study, microwave-assisted non-aqueous sol-gel synthesis was used for the first time to prepare nanoparticles based on polyoxomolybdates. Their optoelectronic properties, focusing on laser-triggered photothermal response, were investigated in detail depending on the synthesis temperature. Significant differences were observed between products prepared from the same precursor via a fast protocol by varying the synthesis temperature. Only low-temperature synthesis (≤ 90 °C) yielded near-infrared (NIR) photothermally active MoOX nanoparticles. The regular packing with large lattice defects of these clusters, along with a low reduction degree, allows water molecules to penetrate and interact with surface Mo = O bonds, forming intermediate electron states within the bandgap. These intermediate electron states are responsible for the NIR laser response suitable for photothermia. Additionally, the NIR response can be modulated in a controlled manner even after synthesis through electrochemical impedance spectroscopy. These findings have direct implications for MoOX photothermal therapy, targeted defect engineering of polyoxomolybdate structures, and their electrochemical and biological applications.

The Cover Art in the ACS Applied Electronic Materials

Posted on by Mario Kotlár

Recently, the Supplementary Cover Art for manuscript: “Graphene-ZnO Thin-Film Heterostructure-Based Efficient  UV Photosensors” has been published in the ACS Applied Electronic Materials. Dr Ravi K. Biroju, who is the main author of this article explains, that this work basically demonstrates fabrication of CVD grown graphene/ZnO thin-film van der Waals heterostructures. When we combine a wide direct band gap semiconductor such as ZnO and a highly conductive layer of graphene, we get a unique platform for applications such as photosensors and gas sensors. Under light illumination, the effective separation of photo-generated electron−hole pairs by the local electric field suppresses carrier recombination rates and increases carrier lifetime. This leads to an increased free carrier density. It can reduce the Schottky barrier between graphene and the ZnO thin film, facilitating the transport and collection of photocarriers. As a result, a high Ilight/Idark ratio can be achieved, much higher than for bare ZnO thin films or ZnO nanostructures.

Incorporation of polyaniline and cobalt deposition to tune electrochemical performances of MoSe2 catalysts for overall electrochemical water splitting

Posted on by Mario Kotlár

Sadik Cogal, Matej Mičušík, Mário Kotlár, Mária Omastová

In: Materials Chemistry and Physics, Volume 333, 130354

https://doi.org/10.1016/j.matchemphys.2024.130354

Abstract

The development of affordable and plentiful electrocatalysts for electrochemical water splitting process has received considerable interest in recent years. Two-dimensional transition metal dichalcogenide (TMD)-based materials are highly promising catalysts for electrochemical water splitting because of their tunable physical and electrical properties. This study presents molybdenum diselenide (MoSe2) hybrid nanostructures prepared via a hydrothermal method in the presence of polyaniline (PANI) conductive supports with different morphologies. Different PANI supports were prepared via oxidative chemical polymerization in the presence of different surfactants. The MoSe2@PANI nanostructures obtained were also modified with cobalt by electrodeposition approach to enhance the electrocatalytic performance of the resulting catalysts. The resulting cobalt-modified MoSe2@PANI-sodium dodecyl sulphate (SDS) (Co–MoSe2@PANI-SDS) catalyst exhibited low overpotentials 339 mV and 134 mV at a current density of 10 mA cm−2 with low Tafel slopes 51 mV dec−1 and 110 mV dec−1 for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. The improved performance of Co–MoSe2@PANI-SDS can be attributed to the incorporation of PANI-SDS as an ideal conducting support and cobalt interactions with both the MoSe2 and the conducting polymer. This work would be insightful in developing new electrocatalysts with bifunctional activities for efficient production of hydrogen through water splitting.

Cobalt-doped WSe2@conducting polymer nanostructures as bifunctional electrocatalysts for overall water splitting

Posted on by Mario Kotlár

Sadik Cogal, Gamze Celik Cogal, Matej Mičušík, Mário Kotlár, Maria Omastová

In: International Journal of Hydrogen Energy, (2023)

https://doi.org/10.1016/j.ijhydene.2023.09.002

Abstract

Designing of high-performance, low-cost, and nonprecious metal-based bifunctional electrocatalysts is highly significant for the development of water splitting process and expanding the practical application of green hydrogen production. Transition metal dichalcogenides (TMDs) with intrinsic physical and chemical properties have been considered potential catalytic materials for electrode fabrication. However, it has remained challenging to develop TMD catalysts that have bifunctional properties for overall water splitting. Herein, WSe2, as a typical representative of TMDs, was utilized to design electrocatalysts using polypyrrole (PPy) or polyaniline (PANI) as a conducting polymer (CP) and cobalt doping. A facile hydrothermal preparation of WSe2 in the presence of CP enabled the construction of cobalt-doped WSe2@CP electrocatalysts. Morphological analysis indicated that the CP played an important role as a conductive template to enhance the distribution of WSe2 nanosheets, leading to higher surface area. In addition, cobalt doping led to the formation of defect structures and boosted the electrocatalytic activities of the catalysts for oxygen evolution reaction (OER).

Tailoring the electronic properties of the SnO2 nanoparticle layer for n-i-p perovskite solar cells by Ti3C2TX MXene

Posted on by Mario Kotlár

Muhammad Faraz Ud Din, Shima Sousani, Mario Kotlar, Sami Ullah, Maros Gregor, Tomas Scepka, Yaryna Soyka, Anastasiia Stepura, Ashin Shaji, Femi Igbari, Karol Vegso, Vojtech Nadazdy, Peter Siffalovic, Matej Jergel, Maria Omastova, Eva Majkova

In: Materials Today Communications, Volume 36, (2023)

https://doi.org/10.1016/j.mtcomm.2023.106700

Abstract

The effect of the Ti3C2Tx MXene modification of the SnO2 electron transport layer (ETL) was studied for the concentration range 0–7.4 wt% MXene. The electronic properties of the MXene-modified ETL were studied by the electrical conductivity measurements, density of states mapping by the energy-resolved electrochemical impedance spectroscopy, ultraviolet photoelectron spectroscopy, and photoluminescence. The structure and morphology of the MXene-modified ETL and the top perovskite layer were analyzed by the scanning electron microscopy (SEM), scanning transmission electron microscopy, grazing-incidence X-ray diffraction and in situ grazing-incidence wide-angle X-ray scattering (GIWAXS). The increased electrical conductivity and electron selectivity for the MXene-modified SnO2 ETL was confirmed up to 1 wt% MXene. For 7.4 wt% MXene, significant suppression of the hole blocking property of the ETL was found. The in situ GIWAXS was performed during the post-deposition annealing of the perovskite layer. The increased perovskite grain size on the SnO2 ETL modified by MXene compared to the pure SnO2 ETL visible by SEM was confirmed. The uniaxial texture of the perovskite crystals was revealed in both cases with an increased misorientation angle for the MXene-modified ETL. The grain size and misorientation angle do not exhibit any systematic temporal changes during the post-deposition annealing. The increasing number of the grains during the annealing was observed. These results are explained using the nucleation and growth model. The increased power conversion efficiency from 17.4% to 18.3% of the archetypal methylammonium-lead-iodide perovskite solar cell after the modification of the SnO2 ETL with 0.1 wt% MXene is the effect of two contributions – increased electrical conductivity of the ETL and improved crystallinity and larger grain size compared to the pure SnO2 ETL, which lowers the total boundary area and charge recombination at trap states typically formed at grain boundaries.

Thermoplastic starch/bentonite clay nanocomposite reinforced with vitamin B2: Physicochemical characteristics and release behavior

Posted on by Mario Kotlár

Abolfazl Heydari, Milad KhajeHassani, Haniyeh Daneshafruz, Sepideh Hamedi, Faeze Dorchei, Mário Kotlár, Fahimeh Kazeminava, Samahe Sadjadi, Farideh Doostan, Ivan Chodak, Hassan Sheibani

In: International Journal of Biological Macromolecules. Vol. 242, (2023)

https://doi.org/10.1016/j.ijbiomac.2023.124742

Abstract

This study presents the development and characterization of a nanocomposite material, consisting of thermoplastic starch (TPS) reinforced with bentonite clay (BC) and encapsulated with vitamin B2 (VB). The research is motivated by the potential of TPS as a renewable and biodegradable substitute for petroleum-based materials in the biopolymer industry. The effects of VB on the physicochemical properties of TPS/BC films, including mechanical and thermal properties, water uptake, and weight loss in water, were investigated. In addition, the surface morphology and chemical composition of the TPS samples were analyzed using high-resolution SEM microscopy and EDS, providing insight into the structure-property relationship of the nanocomposites. The results showed that the addition of VB significantly increased the tensile strength and Young’s modulus of TPS/BC films, with the highest values observed for nanocomposites containing 5 php of VB and 3 php of BC. Furthermore, the release of VB was controlled by the BC content, with higher BC content leading to lower VB release. These findings demonstrate the potential of TPS/BC/VB nanocomposites as environmentally friendly materials with improved mechanical properties and controlled release of VB, which can have significant applications in the biopolymer industry.

Electrochemical modified Pt nanoflower @ rGO for non- enzymatic electrochemical sensing of glucose

Posted on by Mario Kotlár

Saravanan Gengan, R.M. Gnanamuthu, Sanjay Sankaranarayanan, Venumbaka Maneesh Reddy, Bhanu Chandra Marepally, Ravi Kumar Biroju

Sensors and Actuators A: Physical, Volume 353, (2023)

https://doi.org/10.1016/j.sna.2023.114232

Abstract

Since lower danger of biorecognition element degradation, enzymes-less glucose have the potential for more reliable in vivo activity, but it suffers due to lack of linear response and poor selectivity. We made attempt to improve selectivity, linear response and stability, environmentally benign electrochemical method adopted to fabricate Pt nanoflowers (PtNF) anchored on rGO modified GCE (PtNF-rGO/GCE). The PtNF-rGO/GCE electrode demonstrated good glucose electrooxidation in alkaline solution, with a linear range, sensitivity and detection limit are up to 3.5 mM, 335.5 μA mM−1 cm−1 and 53 μM (S/N = 3) respectively. The PtNF-rGO/GCE electrode is not only selective also inhibit interfering molecules like uric, dopamine, ascorbic acid. This allows for broadly sensitive, work at low-potential, stable, and quick glucose current detection, which is capable for the expansion of non-enzymatic glucose detectors.

Defective Graphene/Plasmonic Nanoparticle Hybrids for Surface-Enhanced Raman Scattering Sensors

Posted on by Mario Kotlár

Biroju Ravi K., Marepally Bhanu Chandra, Malik Pariksha, Dhara Soumen, Gengan Saravanan, Maity Dipak, Narayanan Tharangattu N., Giri Pravat K.

In: ACS Omega 2023, 8, 4, 4344–4356, (2023)

https://doi.org/10.1021/acsomega.2c07706

Abstract

Two-dimensional–zero-dimensional plasmonic hybrids involving defective graphene and transition metals (DGR-TM) have drawn significant interest due to their near-field plasmonic effects in the wide range of the UV–vis–NIR spectrum. In the present work, we carried out extensive investigations on resonance Raman spectroscopy (RRS) and localized surface plasmon resonance (LSPR) from the various DGR-TM hybrids (Au, Ag, and Cu) using micro-Raman, spatial Raman mapping analysis, high-resolution transmission electron microscopy (HRTEM), and LSPR absorption measurements on defective CVD graphene layers. Further, electric field (E) mappings of samples were calculated using the finite domain time difference (FDTD) method to support the experimental findings. The spatial distribution of various in-plane and edge defects and defect-mediated interaction of plasmonic nanoparticles (NPs) with graphene were investigated on the basis of the RRS and LSPR and correlated with the quantitative analysis from HRTEM, excitation wavelength-dependent micro-Raman, and E-field enhancement features of defective graphene and defective graphene-Au hybrids before and after rapid thermal annealing (RTA). Excitation wavelength-dependent surface-enhanced Raman scattering (SERS) and LSPR-induced broadband absorption from DGR-Au plasmonic hybrids reveal the electron and phonon interaction on the graphene surface, which leads to the charge transfer from TM NPs to graphene. This is believed to be responsible for the reduction in the SERS signal, which was observed from the wavelength-dependent Raman spectroscopy/mappings. We implemented defective graphene and DGR-Au plasmonic hybrids as efficient SERS sensors to detect the Fluorescein and Rhodamine 6G molecules with a detection limit down to 10–9 M. Defective graphene and Au plasmonic hybrids showed an impressive Raman enhancement in the order of 108, which is significant for its practical application.

Correlated reflectance and Raman spectroscopy in substrates with coherent transparent layers

Posted on by Mario Kotlár

VANČO, Ľubomír – KOTLÁR, Mário – VRETENÁR, Viliam – KADLEČÍKOVÁ, Magdaléna – VOJS, Marian – VOGRINČIČ, Peter

In Surfaces and Interfaces. Vol. 34, (2022)

https://doi.org/10.1016/j.surfin.2022.102309

Abstract

Intensity of Raman bands in substrates covered with transparent overlayers can be enhanced due to optical interference, leading to incorrect quantitative interpretation of Raman signals. If thickness and optical properties of the overlayer are known, correction can be done using appropriate models. We theoretically discuss and experimentally evaluate a model where thickness and refractive index of the overlayer remain unknown and determination of enhancement factor is possible via linear relationship to reflectance-related response of the whole structure. Correct interpretation of the spectra is then possible since refractive index and thickness of the transparent layer are implicitly introduced in the measured reflectance. For experimental evidence we exploit SiNx/Si and SiO2/Si structures to find a significant correspondence with the model, aiming toward correlative reflectance and Raman spectroscopy.