Centrum pre Nanodiagnostiku Materiálov

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.

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.

Ravi K. Biroju, Dipak Maity, Viliam Vretenár, Ľubomír Vančo, Rahul Sharma, Mihir Ranjan Sahoo, Jitendra Kumar, G. Gayathri, Tharangattu N. Narayanan, Saroj Kumar Nayak

In: Materials Today Advances, Volume 22, 2024, 100504

https://doi.org/10.1016/j.mtadv.2024.100504

Abstract

Engineering transition metal dichalcogenides-based semiconducting two-dimensional (2D) layered materials for photo(electro)chemical (PEC) hydrogen evolution reaction (HER) by water splitting is an enduring challenge. Here, alloy-assisted photoconductivity and photoresponse from CVD-grown MoS2(1-x)Se2x (MSSE) 2D ternary atomic layered alloy-based photodetector device is presented for the realization of PEC HER. The explicit role of ‘S–Se’ and ‘Se2’ atomic alloy sites including chalcogen-induced vacancy defects on the photoconductivity/photoresponse and PEC HER performance of MSSE 2D alloy is investigated. Alloy formation, atomic site-by-site ‘Se’ composition and atomic structure are characterized using Raman/Photoluminescence (PL) spectroscopy, high-angle annular dark field (HAADF)- scanning transmission electron microscopy (STEM) extensively and supported with Auger Electron Spectroscopy (AES) mapping. Further, the local density and concentration of S–Se, Se2 atomic sites and defects were quantitatively estimated using HAADF-STEM image analysis in correlation with AES and it is found between the range of ∼15–20 % in MSSE alloy. A 10-fold high photoresponsivity in the case of MSSE concerning as-grown MS having fast photocurrent growth time and the prolonged decay time originates from the ‘Se’ and this alloy assisted states to enhance the PEC performance of MSSE alloy. The enhanced PEC HER activity of MSSE alloy was identified in terms of overpotential and current density. In addition, increased density of states as a function of ‘Se’ alloying, shifts in a p-band centre and lowers ΔGH* according to density functional theory calculations, which makes MSSE alloy an efficient HER activity. Further, the PEC stability and presence of the ‘S–Se’ and ‘Se2’ alloying and their role towards HER have been correlated by the spectral line shape analysis of PL and Raman spectra from post-PEC HER catalysts. These experimental and theoretical findings establish the role of chalcogen, and transition metal-based 2D alloy, leading to the design of new PECs of engineered 2D atomic layer interfaces.

Andrii Lys, Viktor Zabolotnii, Mária Čaplovičová, Iryna Tepliakova, Agris Berzins, Martin Sahul, Ľubomír Čaplovič, Alexander Pogrebnjak, Igor Iatsunskyi, Roman Viter

In: Journal of Alloys and Compounds,Volume 984, 2024, 173885

https://doi.org/10.1016/j.jallcom.2024.173885

Abstract

Recent research places significant importance on the development of innovative nanocomposites for photoelectrochemical applications. This paper presents the fabrication, characterization, and possible photoelectrochemical applications of novel ZnFe₂O₄/ZnO core-shell nanofibers. These core-shell nanofibers were fabricated through co-axial electrospinning using PVP solutions containing iron and zinc nitrate precursors for the core and shell. The structural and optical properties of ZnFe₂O₄/ZnO core-shell nanofibers were examined through TEM, SEM, XRD, FTIR, Raman spectroscopy, and diffuse reflectance spectroscopy. This comprehensive analysis unveiled that the development of core and shell characteristics was notably influenced by the interdiffusion of [Fe]/[Zn] during the annealing process. The photoelectrochemical properties of ZnFe₂O₄/ZnO core-shell nanofibers were assessed through electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and the Mott-Schottky method. These core-shell nanofibers demonstrated a robust electrochemical response to visible light. Photocurrent and photoconversion efficiency of the core-shell nanofibers were calculated and compared with the corresponding values for core-shell nanoparticles. The mechanisms underlying the structural, optical, and photoelectrochemical properties of ZnFe₂O₄/ZnO core-shell nanofibers were discussed. These advanced nanofibers hold potential applications in photocatalysis, photovoltaics, and energy storage, making this research timely and crucial for advancing sustainable energy technologies and environmental remediation efforts.

K. Smyrnova, M. Sahul, M Haršáni, Ľ. Čaplovič, V. Beresnev, Mária Čaplovičová, M. Kusy and A. Pogrebnjak

In: J. Phys.: Conf. Ser. 2712 012014, 2024

https://doi.org/10.1088/1742-6596/2712/1/012014

Abstract

In this work, WN/NbN nanolaminate coatings were synthesized by cathodic-arc physical vapor deposition (CA-PVD) technique on a stainless-steel substrate. The paper reports the microstructure, cross-sectional morphology, surface roughness, and adhesion strength changes caused by variations in the absolute values of the negative substrate bias voltage, U_s, in the 50-200 V range. Synthesized coatings were analyzed by Grazing incidence X-ray diffraction (GI-XRD), scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM), and Daimler-Benz test. The phase analysis revealed that multilayer coatings had complex polycrystalline microstructure. They consisted of face-cantered cubic (fcc) β-W₂N, fcc δ-NbN, and hexagonal ε-NbN phases. The total thickness and surface roughness had a descending trend with an increase in the absolute value of the negative bias voltage. Moreover, the WN/NbN coating deposited at U_s = -50 V demonstrated the best adhesion strength to the substrate, suitable for protective coatings.

Fahimeh Gholami, Arash Ghazitabar, Malek Naderi, Aylar Hoviatdoost, Delasa Ali Jani Ashna, Kiarash Ghazitabar, Bogumił Brycki, Viliam Vretenár

In: Surfaces and Interfaces, (2024)

https://doi.org/10.1016/j.surfin.2024.103936

Abstract

In this study, N-doped cobalt-decorated graphene/carbon nanotube/cellulose microfiber composite aerogels were synthesised and used as oil sorbent. In the current approach, graphene aerogel nanocomposites containing carbon nanotubes and cellulose microfibres were synthesised through a chemical reduction process using ascorbic acid and sodium bisulphite as reducing agents. Subsequently, N-doping was performed using ammonium treatment. The microstructure of these composites was characterised by field emission and conventional scanning electron microscopy, and transmission electron microscopy measurements. The molecular bonding and composition of the composites were analysed using energy-dispersive spectroscopy, Fourier Transform Infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The hydrophobicity of the composites was characterised by a water contact angle test. Finally, the oil sorption performance of the samples was evaluated through a self-design procedure based on a combination of ASTM F716 and 726 standards and Bazargan et al. report. The N-doped Co decorated graphene/carbon nanotube/cellulose microfiber sample with a GO:CNT:CMF mass ratio of 4:2:1 (CGCMF-24) demonstrated the highest hydrophobicity and oil sorption capacity in comparison with other samples. CGCMF-24 adsorbed up to 30, 44, and 54 g/g toluene, cyclohexane, and hexane, respectively. Meanwhile, the most durable adsorbent was CGCMF-24, with 74% durability after five sorption-desorption cycles, and it maintained this capacity for up to 10 cycles.

Anamul Haque, Filip Zechel, Viliam Vretenár, Mrinmoy Roy, Milan Sýkora

In: Adv. Funct. Mater. 2023, 2305296

https://doi.org/10.1002/adfm.202305296

Abstract

A synthesis and characterization of luminescent nano-heterostructures consisting of CdSe nanorod (NR) cores and a ZnO shell with up to three monolayers of ZnO is reported. The core/shell heterostructures show a tunable, dual photoluminescence (PL) in visible and Near Infrared (NIR) spectral ranges. Upon shelling the visible PL band attributed to the carrier recombination within the CdSe core shifts to lower energy by ≈0.05 to 0.15 eV relative to the bare CdSe NRs, due to a reduced quantum confinement. A NIR band, observed ≈0.4 – 0.5 eV below the PL energy of the CdSe core, is attributed to a type-II carrier recombination across the CdSe/ZnO interface. The total PL quantum yield (PLQY) in the brightest heterostructures reaches ≈20%, increasing ≈100-fold over the PLQY of the corresponding bare CdSe NRs. The average lifetimes of the visible PL in some heterostructures exceeds 100 ns, compared to ≈5 ns lifetime typical for bare CdSe NRs. The average PL lifetimes attributed to the type-II charge separated states exceed one microsecond. Strong NIR PL, tunable in the 800–900 nm spectral range and the long-lived charge separated state make the CdSe/ZnO core-shell NRs appealing materials for exploitation in applications such as bioimaging, photocatalysis and optoelectronics.