Olesya Yarema

Last Name

Yarema

First Name

Olesya

ORCID

0000-0002-1653-1338

Organisational unit

02140 - Dep. Inf.technologie und Elektrotechnik / Dep. of Inform.Technol. Electrical Eng.

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Publications1 - 10 of 49

Size, Ligand, and Defect-Dependent Electron-Phonon Coupling in Chalcogenide and Perovskite Nanocrystals and Its Impact on Luminescence Line Widths
Yazdani, Nuri; Volk, Sebastian; Yarema, Olesya; et al. (2020)
ACS Photonics
Unravelling the amorphous structure and crystallization mechanism of GeTe phase change memory materials
Wintersteller, Simon; Yarema, Olesya; Kumaar, Dhananjeya; et al. (2024)
Nature Communications
The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable ‘ideal glass’ state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature.
Phase transitions in germanium telluride nanoparticle phase-change materials studied by temperature-resolved x-ray diffraction
Michel, Ann-Katrin U.; Donat, Felix; Siegfried, Aurelia; et al. (2021)
Journal of Applied Physics
Germanium telluride (GeTe), a phase-change material, is known to exhibit four different structural states: three at room-temperature (one amorphous and two crystalline, α and γ) and one at high temperature (crystalline, β). Because transitions between the amorphous and crystalline states lead to significant changes in material properties (e.g., refractive index and resistivity), GeTe has been investigated as a phase-change material for photonics, thermoelectrics, ferroelectrics, and spintronics. Consequently, the temperature-dependent phase transitions in GeTe have been studied for bulk and thin-film GeTe, both fabricated by sputtering. Colloidal synthesis of nanoparticles offers a more flexible fabrication approach for amorphous and crystalline GeTe. These nanoparticles are known to exhibit size-dependent properties, such as an increased crystallization temperature for the amorphous-to-α transition in sub-10 nm GeTe particles. The α-to-β phase transition is also expected to vary with size, but this effect has not yet been investigated for GeTe. Here, we report time-resolved x-ray diffraction of GeTe nanoparticles with different diameters and from different synthetic protocols. We observe a non-volatile amorphous-to-α transition between 210 ∘C and 240 ∘C and a volatile α-to-β transition between 370 ∘C and 420 ∘C. The latter transition was reversible and repeatable. While the transition temperatures are shifted relative to the values known for bulk GeTe, the nanoparticle-based samples still exhibit the same structural phases reported for sputtered GeTe. Thus, colloidal GeTe maintains the same general phase behavior as bulk GeTe while allowing for more flexible and accessible fabrication. Therefore, nanoparticle-based GeTe films show great potential for applications such as in active photonics.
Nanocrystal superlattices as phonon-engineered solids and acoustic metamaterials
Yazdani, Nuri; Jansen, Maximilian; Bozyigit, Deniz; et al. (2019)
Nature Communications
Phonon engineering of solids enables the creation of materials with tailored heat-transfer properties, controlled elastic and acoustic vibration propagation, and custom phonon–electron and phonon–photon interactions. These can be leveraged for energy transport, harvesting, or isolation applications and in the creation of novel phonon-based devices, including photoacoustic systems and phonon-communication networks. Here we introduce nanocrystal superlattices as a platform for phonon engineering. Using a combination of inelastic neutron scattering and modeling, we characterize superlattice-phonons in assemblies of colloidal nanocrystals and demonstrate that they can be systematically engineered by tailoring the constituent nanocrystals, their surfaces, and the topology of superlattice. This highlights that phonon engineering can be effectively carried out within nanocrystal-based devices to enhance functionality, and that solution processed nanocrystal assemblies hold promise not only as engineered electronic and optical materials, but also as functional metamaterials with phonon energy and length scales that are unreachable by traditional architectures.
Engineering of Oxide Protected Gold Nanoparticles
Von Mentlen, Jean-Marc; Clarysse, Jasper; Moser, Annina; et al. (2022)
The Journal of Physical Chemistry Letters
Gold nanoparticles that are partially or fully covered by metal oxide shells provide superior functionality and stability for catalytic and plasmonic applications. Yet, facile methods for controlled fabrication of thin oxide layers on metal nanoparticles are lacking. Here, we report an easy method to reliably engineer thin Ga2O3 shells on Au nanoparticles, based on liquid-phase chemical oxidation of Au–Ga alloy nanoparticles. We demonstrate that, with this technique, laminar and ultrathin Ga2O3 shells can be grown with ranging thickness from sub- to several monolayers. We show how the localized surface plasmon resonance can be used to understand the reaction process and quantitatively monitor the Ga2O3 shell growth. Finally, we demonstrate that the Ga2O3 coating prevents sintering of the Au nanoparticles, providing thermal stability to at least 250 °C. This approach, building on dealloying of bimetallic nanoparticles by the solution-phase oxidation, promises a general technique for achieving controlled metal/oxide core/shell nanoparticles.
From Highly Monodisperse Indium and Indium Tin Colloidal Nanocrystals to Self-Assembled Indium Tin Oxide Nanoelectrodes
Yarema, Maksym; Pichler, Stefan; Kriegner, Dominik; et al. (2012)
ACS Nano
Synthesis of Small Ag-Sb-Te Nanocrystals with Composition Control
Moser, Annina; Yarema, Olesya; Yarema, Maksym; et al. (2020)
Journal of Materials Chemistry C
Ternary telluride nanocrystals have gained increasing interest as materials for thermoelectric, optoelectronic, and phase-change memory applications. Synthetic approaches for colloidal multicomponent tellurides, however, remain sparse. Here, we report a convenient, amide-promoted synthesis for Ag-Sb-Te nanocrystals with small sizes and narrow size distributions (e.g., nanocrystal diameters of 3.5 ± 0.8 nm). We focus on achieving composition control for Ag-Sb-Te nanocrystals by adjusting the ratio of cationic precursors and find a broad solid solution range for AgxSb1-xTe1.5-x nanocrystals (x is from 0.3 and 0.6), which extends beyond that measured in Ag-Sb-Te thin films. The ability to produce size- and composition-controlled Ag-Sb-Te nanocrystals is a first step in achieving bottom-up assembled Ag-Sb-Te semiconductors for device applications.
Recombination Dynamics in PbS Nanocrystal Quantum Dot Solar Cells Studied through Drift–Diffusion Simulations
Lin, Weyde; Yazdani, Nuri; Yarema, Olesya; et al. (2021)
ACS Applied Electronic Materials
The significant performance increase in nanocrystal (NC)-based solar cells over the last decade is very encouraging. However, many of these gains have been achieved by trial-and-error optimization, and a systematic understanding of what limits the device performance is lacking. In parallel, experimental and computational techniques provide increasing insights into the electronic properties of individual NCs and their assemblies in thin films. Here, we utilize these insights to parameterize drift–diffusion simulations of PbS NC solar cells, which enable us to track the distribution of charge carriers in the device and quantify recombination dynamics, which limit the device performance. We simulate both Schottky- and heterojunction-type devices and, through temperature-dependent measurements in the light and dark, experimentally validate the appropriateness of the parameterization. The results reveal that Schottky-type devices are limited by surface recombination between the PbS and aluminum contact, while heterojunction devices are currently limited by NC dopants and electronic defects in the PbS layer. The simulations highlight a number of opportunities for further performance enhancement, including the reduction of dopants in the nanocrystal active layer, the control over doping and electronic structure in electron- and hole-blocking layers (e.g., ZnO), and the optimization of the interfaces to improve the band alignment and reduce surface recombination. For example, reduction in the percentage of p-type NCs from the current 1–0.01% in the heterojunction device can lead to a 25% percent increase in the power conversion efficiency.
Deep Level Transient Spectroscopy (DLTS) on Colloidal-Synthesized Nanocrystal Solids
Bozyigit, Deniz; Jakob, Michael; Yarema, Olesya; et al. (2013)
ACS Applied Materials & Interfaces
Phase-Controlled Synthesis and Phase-Change Properties of Colloidal Cu-Ge-Te Nanoparticles
Kumaar, Dhananjeya; Can, Matthias; Weigand, Helena; et al. (2024)
Chemistry of Materials
Phase-change memory (PCM) technology has recently attracted a vivid interest for neuromorphic applications, in-memory computing, and photonic integration due to the tunable refractive index and electrical conductivity between the amorphous and crystalline material states. Despite this, it is increasingly challenging to scale down the device dimensions of conventionally sputtered PCM memory arrays, restricting the implementation of PCM technology in mass applications such as consumer electronics. Here, we report the synthesis and structural study of sub-10 nm Cu-Ge-Te (CGT) nanoparticles as suitable candidates for low-cost and ultrasmall PCM devices. We show that our synthesis approach can accurately control the structure of the CGT colloids, such as composition-tuned CGT amorphous nanoparticles as well as crystalline CGT nanoparticles with trigonal alpha-GeTe and tetragonal Cu2GeTe3 phases. In situ characterization techniques such as high-temperature X-ray diffraction and X-ray absorption spectroscopy reveal that Cu doping in GeTe improves the thermal properties and amorphous phase stability of the nanoparticles, in addition to nanoscale effects, which enhance the nonvolatility characteristics of CGT nanoparticles even further. Moreover, we demonstrate the thin-film fabrication of CGT nanoparticles and characterize their optical properties with spectroscopic ellipsometry measurements. We reveal that CGT nanoparticle thin films exhibit a negative reflectivity change and have good reflectivity contrast in the near-IR spectrum. Our work promotes the possibility to use PCM in nanoparticle form for applications such as electro-optical switching devices, metalenses, reflectivity displays, and phase-change IR devices.

Publications1 - 10 of 49

Olesya Yarema

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