Centrum pre nanodiagnostiku materiálov

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.

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.

Tadas Paulauskas, Vaidas Pačebutas, Viktorija Strazdienė, Andrejus Geižutis, Jan Devenson, Mindaugas Kamarauskas, Martynas Skapas, Rokas Kondrotas, Mantas Drazdys, Matas Rudzikas, Benjaminas Šebeka, Viliam Vretenár, Arūnas Krotkus

In: Discover Nano. Vol. 18, iss. 1 (2023)

DOI: 10.1186/s11671-023-03865-x

Abstract

Group III-V semiconductor multi-junction solar cells are widely used in concentrated-sun and space photovoltaic applications due to their unsurpassed power conversion efficiency and radiation hardness. To further increase the efficiency, new device architectures rely on better bandgap combinations over the mature GaInP/InGaAs/Ge technology, with Ge preferably replaced by a 1.0 eV subcell. Herein, we present a thin-film triple-junction solar cell AlGaAs/GaAs/GaAsBi with 1.0 eV dilute bismide. A compositionally step-graded InGaAs buffer layer is used to integrate high crystalline quality GaAsBi absorber. The solar cells, grown by molecular-beam epitaxy, achieve 19.1% efficiency at AM1.5G spectrum, 2.51 V open-circuit voltage, and 9.86 mA/cm² short-circuit current density. Device analysis identifies several routes to significantly improve the performance of the GaAsBi subcell and of the overall solar cell. This study is the first to report on multi-junctions incorporating GaAsBi and is an addition to the research on the use of bismuth-containing III-V alloys in photonic device applications.

PAULAUSKAS, Tadas – PACEBUTAS, Vaidas – BUTKUTE, Renata – ČECHAVIČIUS, Bronislovas – NAUJOKAITIS, Arnas – KAMARAUSKAS, Mindaugas – SKAPAS, Martynas – DEVENSON, Jan – ČAPLOVIČOVÁ, Mária – VRETENÁR, Viliam – LIU, Xiaoyan – KOCIAK, Mathieu – KROTKUS, Arunas

In Nanoscale Research Letters. Vol. 15, iss. 1 (2020)

https://doi.org/10.1186/s11671-020-03349-2

Abstract

The distribution of alloyed atoms in semiconductors often deviates from a random distribution which can have significant effects on the properties of the materials. In this study, scanning transmission electron microscopy techniques are employed to analyze the distribution of Bi in several distinctly MBE grown GaAs_1−xBi_x alloys. Statistical quantification of atomic-resolution HAADF images, as well as numerical simulations, are employed to interpret the contrast from Bi-containing columns at atomically abrupt (001) GaAs-GaAsBi interface and the onset of CuPt-type ordering. Using monochromated EELS mapping, bulk plasmon energy red-shifts are examined in a sample exhibiting phase-separated domains. This suggests a simple method to investigate local GaAsBi unit-cell volume expansions and to complement standard X-ray-based lattice-strain measurements. Also, a single-variant CuPt-ordered GaAsBi sample grown on an offcut substrate is characterized with atomic scale compositional EDX mappings, and the order parameter is estimated. Finally, a GaAsBi alloy with a vertical Bi composition modulation is synthesized using a low substrate rotation rate. Atomically, resolved EDX and HAADF imaging shows that the usual CuPt-type ordering is further modulated along the [001] growth axis with a period of three lattice constants. These distinct GaAsBi samples exemplify the variety of Bi distributions that can be achieved in this alloy, shedding light on the incorporation mechanisms of Bi atoms and ways to further develop Bi-containing III-V semiconductors.

GUARDIA, Laura – SUÁREZ, Loreto – QUEREJETA, Nausika – VRETENÁR, Viliam – KOTRUSZ, Peter – SKÁKALOVÁ, Viera – CENTENO, Teresa A.

In Electrochimica Acta. Vol. 298, iss. 3 (2019)

Abstract

We present a simple and effective alternative which optimizes electrodes based on low-cost carbons for high-performance supercapacitors. The combination with reduced graphene oxide (rGO) greatly improves the operation of microporous carbons easily produced by one-pot activation of grape seeds. The use of composite electrodes with rGO lowers the supercapacitor resistance and enables a much higher rate capability.

The mixture of rGO flakes and particles of a highly porous carbon obtained by KOH activation allows retaining the high capacitance of 260 F g⁻¹ of the standard electrodes at 1 mA cm⁻² in aqueous H₂SO₄ whereas the value at 200 mA cm⁻² is increased by around 2.4 times. Consequently, at high current density, the capacitor assembled with these composites stores eight times more energy and the power density is multiplied by four.

The synergy between rGO and an ultramicroporous carbon produced by CO₂-activation results extremely profitable, the cell assembled with composite electrodes reaching three times more energy and power at 200 mA cm⁻² than the best performance of the standard counterpart.

More importantly, the higher density of the composite electrodes leads to a capacitance of around 200 F cm⁻³ which translates into a remarkable improvement in the supercapacitor operation normalized to volume.