Centrum pre Nanodiagnostiku Materiálov

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.

Ľubomír Vančo, Ravi K. Biroju, Mário Kotlár, Viliam Vretenár, Dipak Maity, Tharangattu N. Narayanan

In: J. Phys. Chem. C 2025, 129, 47, 20995–21004

https://doi.org/10.1021/acs.jpcc.5c05299

Abstract

Molybdenum disulfide (MoS₂) monolayers have emerged as promising materials for a variety of applications. Their behavior depends critically on surface composition; therefore, careful characterization is necessary to describe their properties accurately. Although Auger electron spectroscopy (AES) is a standard method capable of addressing this issue, it suffers from beam-induced damage and variation of spectral features in complex samples. To overcome these limitations, we employed correlative analysis to examine MoS₂ and V-doped MoS₂ 2D surfaces by using Auger scatterplots. As we demonstrate, this method enables the nondestructive imaging and assessment of the lateral and depth distributions of the elements and provides a remarkably convenient way to estimate S-rich/depleted regions. The scatterplot technique indicates that V doping in MoS₂ retards desulfurization in an Ar/H₂ plasma environment. By reducing the electron dose, the analysis using scatterplots can improve the accuracy of AES by up to 30%. The Auger scatterplot method provides insight into the affinity or independence of surface constituents through quantitative relationships, enabling separate analysis of the characteristic areas within a complex sample. These findings are supported by Raman spectroscopy and transmission electron microscopy, which highlight the effectiveness of the Auger scatterplots and their potential for examining the surfaces of 2D materials. Auger maps also show a strong correlation with photoluminescence features in MoS₂ monolayers, thereby illustrating the overlap with practical applications.

Dipak Maity, Ravi K. Biroju, Viliam Vretenár, Mihir Ranjan, Sahoo, L’ubomír Vančo, Matej Mičušík, Tharangattu N. Narayanan, Kalpataru Pradhan

In:  ACS Appl. Nano Mater. 2025, 8, 47, 22573–22585

https://doi.org/10.1021/acsanm.5c03601

Abstract

Two-dimensional semiconductor-transition-metal dichalcogenide (2D-STMD) based semiconductors have emerged as promising materials for future spintronic and optoelectronic applications, including photodetectors and transistors. Transferring high-quality chemical vapor deposition (CVD)-grown monolayer 2D-STMDs and their alloys to the target substrate is very challenging for fabricating efficient devices. Unfortunately, current post-transfer methods struggle to completely remove unwanted contamination residues during wet-transfer processes, which adversely affects material quality and intrinsic properties. In this work, the effect of ethanol cleaning on the qualitative and quantitative assessment of molecular adsorbates is demonstrated, based on atomic-resolution high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image analysis supported by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and density functional theory (DFT) calculations, which showcases ultraclean material structures. We estimate the unidentified molecular adsorbates in the proximity of molybdenum (Mo) and chalcogen (S, Se) atomic sites, which are tentatively assigned as ‘C₂H₅OH (EtOH)’, ‘H₂O’, and ‘O₂’ related adsorbates. The attribution is based on HAADF-STEM Gaussian line shape fitting of atomic intensity columns and corresponding computed adsorption energy values after ethanol treatment of the MoS_2(1–x)Se_2x (MSSE) alloy. In line with experimental observations of persistent OH-containing residues on the surface, DFT simulations show that EtOH has better adsorption on both pristine and sulfur-vacancy MSSE monolayers than H₂O and O₂. Photodetector device measurements revealed a remarkable ∼90% enhancement in photocurrent values for ultraclean samples, significantly boosting the material’s photoresponsivity. DFT calculations on the adsorption energy and density of electronic states were also conducted to validate our experimental findings.

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 MoO_X 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 MoO_X photothermal therapy, targeted defect engineering of polyoxomolybdate structures, and their electrochemical and biological applications.

Abdul Kaium Mia, Sourav Dey, Lubomir Vanco, Viliam Vretenar, P. K. Giri

In: Materials Today Nano 31, 100638 (2025)

https://doi.org/10.1016/j.mtnano.2025.100638

Abstract:

The semiconducting 2D transition metal dichalcogenides (TMDs) have gained substantial attention, though the progress in their lateral heterostructures (HS) and in-situ growth for electronic and optoelectronic applications has been very limited. Herein, we report a single-step in-situ chemical vapor deposition growth of bilayer WS2-MoS2 lateral HS, which ensures a clean diffused interface between WS2 and MoS2, enabling efficient charge transport. The spatial Raman, photoluminescent (PL), and Auger mapping of in-situ WS2-MoS2 lateral HS shows a clear transition from pure WS2 to pure MoS2 region through a graded WS(1-x)MoxS2 alloy interface. The composition and the width of the alloy interface could be tuned by careful choice of the proportion of precursor materials and by tuning the growth parameters. Spatially resolved PL spectra and PL mapping reveal a strongly enhanced (more than one order of magnitude) PL intensity in the HS interface attributed to the strain-induced bandstructure modification in the alloyed interface. Interestingly, the alloyed interface in the lateral HS also dramatically improves the electronic properties, resulting in an on-off ratio of 108 in the fabricated field effect transistor, which is two orders of magnitude higher than their individual counterpart. These results on lateral HS are significant, and they pave the way to synthesize other different HSs for future electronic devices and integrated circuits.

Sreekant Anil, Dipak Maity, Snehith Adabala, Arpan De, Nagendra S. Kamath, Raheel Hammad, Janmey Jay Panda, Ravi K. Biroju, Viliam Vretenar, Rajalakshmi G, Suman Kalyan Pal, Soumya Ghosh, and Tharangattu N. Narayanan

In: Nano Lett. 2025

https://doi.org/10.1021/acs.nanolett.5c03877

Abstract:

MoS2 monolayers (MS) having magnetic impurities as dopants can bring about time-reversal asymmetry and hence room temperature magnetism. Here, we demonstrated the synthesis of Cr substitutionally doped (∼1%) MS (CrMS) along with its vanadium-doped MS counterpart (VMS) and investigated their suitability for valleytronics by studies based on chirality-selective photoluminescence, time-resolved transient absorption spectroscopy, and spin Hall effect of light (SHEL). While VMS showed room temperature valley splitting, no such shift was observed in CrMS although with their expected similarity. Density-functional-theory-based electronic structure calculations indicate a Cr-induced flat band below the Fermi level, even at ∼0.5 atom % doping, which masks the splitting in the energies of the K-point valleys. This finding is in tune with the experimental studies while in contrast to the theoretical and experimental data of VMS. Hence, this study establishes band valley tunabilities of MoS2, and SHEL as a powerful tool for valley polarization studies.

Ravi K. Biroju*, Sanat Nalini Paltasingh, Mihir Ranjan Sahoo, Soumen Dhara, Dipak Maity, Viliam Vretenár, P. K. Giri, Tharangattu N. Narayanan, Saroj Kumar Nayak

In: ACS Applied Electronic Materials

https://doi.org/10.1021/acsaelm.5c00348

Abstract

Graphene-based ZnO thin-film hybrids (GR-ZnO) have shown interesting properties for electronic and optoelectronic applications, such as enhanced UV photodetection and photocatalysis. The interaction and explicit role of large-area single-layer chemical vapor deposition (CVD)-grown graphene in the improved photophysical properties in such a kind of GR-ZnO hybrids have not been well-understood in recent reports. In the present work, we fabricated a photosensor made with large-area monolayer CVD GR-ZnO thin-film hybrids, which showed improved UV photodetection with high values of UV sensitivity and responsivity compared to bare ZnO films. The GR-ZnO thin-film hybrid photosensors demonstrated about a 20 time improvement in photoresponsivity (9.87 × 103 A/W) compared to the bare ZnO thin film (4.93 × 102 A/W). We investigated the origin of the high photosensitivity of GR-ZnO, and it is explained based on a comparatively large absorption coefficient, enhancement of the number of photogenerated carriers, and a reduction of the recombination rates of these carriers based on density functional theory (DFT) calculations. The high mobility of the graphene layer provides an efficient and faster charge transfer pathway for photogenerated carriers at the interface between ZnO and the graphene layers.

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.

Dipak Maity, Rajesh Kumar Yadav, Adi Levi, Rahul Sharma, Emanuel Ber, Eilam Yalon, Ravi. K. Biroju, Viliam Vretenár, Tharangattu N. Narayanan, Doron Naveh

In: Small Methods 2024, 2401938

https://doi.org/10.1002/smtd.202401938

Abstract

2D transition-metal dichalcogenide semiconductors such as MoS₂ are identified as a platform for next-generation electronic circuitries. However, the progress toward industrial applications is still lagging due to imperfections of wafer-scale deposition techniques and in-contact parasitic impedance affecting device integration in large circuits and systems. Here, on contact engineering of large-scale, chemical vapor deposition (CVD) grown monolayer MoS₂ films is reported, leading to improved performance of field effect transistors. The transistor performance of monolayer pure MoS₂ is initially characterized by its I_ON/I_OFF ratio (10⁶), carrier density (≈10¹² cm⁻²), and mobility (≈10 cm² Vs⁻¹), and the Schottky barrier height (SBH) of conventional metallic Au contact of MoS₂ (≈215 meV). Then, a CVD-grown degenerately-doped monolayer of alloy V_0.25Mo_0.75S₂ is introduced between Au and MoS₂ of a modified transistor, reducing the SBH to ≈100 meV. The reduced contact resistance (≈50%) of the device with an atomically thin contact interface complies with the theoretical model and is free from Fermi-level pinning effects. It is resilient to the high temperatures that are characteristic of physical metallization methods and is readily scalable.

Rahul Sharma, Henry Nameirakpam, David Muradas Belinchón, Prince Sharma, Ulrich Noumbe, Daria Belotcerkovtceva, Elin Berggren, Viliam Vretenár, Ľubomir Vančo, Matúš Maťko, Ravi K. Biroju, Soumitra Satapathi, Tomas Edvinsson, Andreas Lindblad, M. Venkata Kamalakar*

In: ACS Applied Materials & Interfaces, Vol 16, Issue 29

https://doi.org/10.1021/acsami.4c07028

Abstract

Two-dimensional (2D) van der Waals heterostructures combine the distinct properties of individual 2D materials, resulting in metamaterials, ideal for emergent electronic, optoelectronic, and spintronic phenomena. A significant challenge in harnessing these properties for future hybrid circuits is their large-scale realization and integration into graphene interconnects. In this work, we demonstrate the direct growth of molybdenum disulfide (MoS2) crystals on patterned graphene channels. By enhancing control over vapor transport through a confined space chemical vapor deposition growth technique, we achieve the preferential deposition of monolayer MoS2 crystals on monolayer graphene. Atomic resolution scanning transmission electron microscopy reveals the high structural integrity of the heterostructures. Through in-depth spectroscopic characterization, we unveil charge transfer in Graphene/MoS2, with MoS2 introducing p-type doping to graphene, as confirmed by our electrical measurements. Photoconductivity characterization shows that photoactive regions can be locally created in graphene channels covered by MoS2 layers. Time-resolved ultrafast transient absorption (TA) spectroscopy reveals accelerated charge decay kinetics in Graphene/MoS2 heterostructures compared to standalone MoS2 and upconversion for below band gap excitation conditions. Our proof-of-concept results pave the way for the direct growth of van der Waals heterostructure circuits with significant implications for ultrafast photoactive nanoelectronics and optospintronic applications.