Researcher:
Önal, Asım

Loading...
Profile Picture
ORCID

Job Title

PhD Student

First Name

Asım

Last Name

Önal

Name

Name Variants

Önal, Asım

Email Address

Birth Date

Search Results

Now showing 1 - 10 of 12
  • Thumbnail Image
    PublicationOpen Access
    Emergence of near-infrared photoluminescence via ZnS shell growth on the AgBiS2 nanocrystals
    (American Chemical Society, 2024) Department of Chemistry; Department of Electrical and Electronics Engineering; Önal, Asım; Kaya, Tarık Safa; Metin, Önder; Nizamoğlu, Sedat; Department of Chemistry; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Sciences; College of Engineering
    AgBiS2 nanocrystals (NCs), composed of nontoxic, earth-abundant materials and exhibiting an exceptionally high absorption coefficient from visible to near-infrared (>105 cm(-1)), hold promise for photovoltaics but have lack of photoluminescence (PL) due to intrinsic nonradiative recombination and challenging shell growth. In this study, we reported a facile wet-chemical approach for the epitaxial growth of ZnS shell on AgBiS2 NCs, which triggered the observation of PL emission in the near-infrared (764 nm). Since high quality of the core is critical for epitaxial shell growth, we first obtained rock-salt structured AgBiS2 NCs with high crystallinity, nearly spherical shape and monodisperse size distribution (<6%) via a dual-ligand approach reacting Ag-Bi oleate with elemental sulfur in oleylamine. Next, a zincblende ZnS shell with a low-lattice mismatch of 4.9% was grown on as-prepared AgBiS2 NCs via a highly reactive zinc (Zn(acac)(2)) precursor that led to a higher photoluminescence quantum yield (PLQY) of 15.3%, in comparison with a relatively low reactivity precursor (Zn(ac)(2)) resulting in reduced PLQY. The emission from AgBiS2 NCs with ultrastrong absorption, facilitated by shell growth, can open up new possibilities in lighting, display, and bioimaging.
  • Placeholder
    Publication
    Efficient nanocrystal-based white LEDs with suppressed absorption losses
    (Optica Publishing Group, 2022) Department of Electrical and Electronics Engineering; N/A; N/A; N/A; Nizamoğlu, Sedat; Önal, Asım; Sadeghi, Sadra; Melikov, Rustamzhon; Faculty Member; PhD Student; PhD Student; PhD Student; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 130295; N/A; N/A; N/A
    We demonstrate efficient white LEDs by using the combination of green-emitting near-unity quantum dots with red-emitting nanorods. Stokes-shift in red via dot-to-rod transition reduced absorption losses and led to a high quantum efficiency of 42.9%.
  • Placeholder
    Publication
    High brightness illumination based on laser light diffusion with mie scattering
    (American Society of Mechanical Engineers (ASME), 2022) N/A; Department of Computer Engineering; Önal, Asım; Nizamoğlu, Sedat; PhD Student; Student; Department of Computer Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 130295
    Limited luminous flux per wafer area of light emitting diodes (LEDs) for high power solid state illumination causes some packaging real estate issues. This problem can be tackled with laser diodes (LDs). At high current densities, LDs offer higher efficiency, however with very low etendue and divergent angle. This significantly increases the complexity of color conversion for white light generation. Concentrated light can carbonize the color conversion unit and have high speckle contrast. These problems can be addressed by efficient diffusion of the laser beam and this paper is aimed to introduce the first laser diffusion system based on TiO2 Mie particles. Based on a series of ray tracing simulations, an idealized cost-effective system is modeled and results showed an almost lossless diffusion with a guiding system based on reflection resulting in an almost uniform irradiance level with only 17% power loss. Furthermore, offered design can reduce the challenges for the compact packaging of white LDs by eliminating the heat sink for color conversion coating and enabling a safe light intensity for utilizing quantum dots for color engineering.
  • Placeholder
    Publication
    Multiscale dynamics of lipid vesicles in polymeric microenvironment
    (Mdpi, 2022) N/A; N/A; N/A; N/A; N/A; Department of Electrical and Electronics Engineering; Department of Chemical and Biological Engineering; Department of Chemical and Biological Engineering; Karaz, Selcan; Han, Mertcan; Akay, Gizem; Önal, Asım; Nizamoğlu, Sedat; Kızılel, Seda; Şenses, Erkan; Master Student; Master Student; PhD Student; PhD Student; Faculty Member; Faculty Member; Faculty Member; Department of Electrical and Electronics Engineering; Department of Chemical and Biological Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; College of Engineering; College of Engineering; N/A; N/A; N/A; N/A; 130295; 28376; 280298
    Understanding dynamic and complex interaction of biological membranes with extracellular matrices plays a crucial role in controlling a variety of cell behavior and functions, from cell adhesion and growth to signaling and differentiation. Tremendous interest in tissue engineering has made it possible to design polymeric scaffolds mimicking the topology and mechanical properties of the native extracellular microenvironment; however, A fundamental question remains unanswered: that is, how the viscoelastic extracellular environment modifies the hierarchical dynamics of lipid membranes. in this work, we used aqueous solutions of poly(ethylene glycol) (PEG) with different molecular weights to mimic the viscous medium of cells and nearly monodisperse unilamellar DMPC/DMPG liposomes as a membrane model. Using small-angle X-ray scattering (SaXS), dynamic light scattering, temperature-modulated differential scanning calorimetry, bulk rheology, and fluorescence lifetime spectroscopy, we investigated the structural phase map and multiscale dynamics of the liposome-polymer mixtures. the results suggest an unprecedented dynamic coupling between polymer chains and phospholipid bilayers at different length/time scales. the microviscosity of the lipid bilayers is directly influenced by the relaxation of the whole chain, resulting in accelerated dynamics of lipids within the bilayers in the case of short chains compared to the polymer-free liposome case. at the macroscopic level, the gel-to-fluid transition of the bilayers results in a remarkable thermal-stiffening behavior of polymer-liposome solutions that can be modified by the concentration of the liposomes and the polymer chain length.
  • Placeholder
    Publication
    Highly efficient white LEDs by using near unity emitting colloidal quantum dots in liquid medium
    (Optica Publishing Group, 2022) Department of Electrical and Electronics Engineering; Department of Chemistry; Department of Chemistry; N/A; N/A; N/A; N/A; N/A; N/A; N/A; N/A; Department of Electrical and Electronics Engineering; Nizamoğlu, Sedat; Yılgör, İskender; Metin, Önder; Özer, Melek Sermin; Önal, Asım; Eren, Güncem Özgün; Sadeghi, Sadra; Melikov, Rustamzhon; Jalali, Houman Bahmani; Doğru-Yüksel, Itır Bakış; Han, Mertcan; Karatüm, Onuralp; Faculty Member; Faculty Member; Faculty Member; Researcher; PhD Student; PhD Student; PhD Student; PhD Student; PhD Student; PhD Student; Master Student; Other; Department of Chemistry; Department of Electrical and Electronics Engineering; College of Engineering; College of Sciences; College of Sciences; N/A; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; 130295; 24181; 46962; N/A; N/A; N/A; N/A; N/A; N/A; N/A; N/A; N/A
    We developed quantum dot (QD) based color-conversion white LEDs that reach over 150 lumens per electrical Watt. For that we synthesized alloyed ZnCdSe/ZnSe QDs with 94% of quantum efficiency and injected QD-liquids on blue LEDs.
  • Placeholder
    Publication
    Past, present and future of indium phosphide quantum dots
    (Tsinghua Univ Press, 2022) N/A; N/A; Department of Electrical and Electronics Engineering; N/A; N/A; Jalali, Houman Bahmani; Nizamoğlu, Sedat; Sadeghi, Sadra; Önal, Asım; Doğru-Yüksel, Itır Bakış; PhD Student; Faculty Member; PhD Student; PhD Student; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; 130295; N/A; N/A; N/A
    Indium phosphide (InP) colloidal quantum dots (QDs) have been drawn significant attention as a potentially less toxic alternative to cadmium-based QDs over the past two decades. The advances in their colloidal synthesis methods have allowed for the synthesis of a wide variety of compositions, heterojunctions, dopants, and ligands that enabled spectral tunability from blue to near-infrared, narrow emission linewidths, and perfect quantum yields approaching unity. Furthermore, it has higher covalency compared to cadmium chalcogenides leading to improved optical stability. The state-of-the-art InP QDs with appealing optical and electronic properties have excelled in many applications such as light-emitting diodes, luminescent solar concentrators (LSCs), and solar cells with high potential for commercialization. This review focuses on the history, recent development, and future aspect of synthesis and application of colloidal InP QDs.
  • Placeholder
    Publication
    Quantum dot enabled efficient white leds for wide color gamut displays
    (Wiley-Blackwell, 2023) Melikov, Rustamzhon; Department of Electrical and Electronics Engineering; N/A; N/A; N/A; Nizamoğlu, Sedat; Eren, Güncem Özgün; Önal, Asım; Kaya, Lokman; Faculty Member; PhD Student; PhD Student; Master Student; Department of Electrical and Electronics Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; 130295; N/A; N/A; N/A
    In recent years quantum dots (QDs) emerge as a powerful building block for liquid-crystal display (LCD) technology, and they are successfully used in the form of color enhancement films. However, their transition toward efficient and wide color gamut LEDs for backlighting remain as an important challenge, which can decrease the required amount of QDs per TV for orders of magnitude. Here, QD-based white LEDs are reported that can simultaneously operate with a high external quantum efficiency (EQE) of 39.8% and a wide NTSC color gamut coverage of 133.3%. For red-emitting spectral range, giant CdSe/CdS core/shell QDs are synthesized via hot injection method, and it is observed that 20 monolayers of CdS shell can lead to a near-unity photoluminescence quantum yield (PLQY) of 98%. In addition, green-emitting CdZnSeS/ZnS alloyed core/shell QDs show a PLQY of 97% with a narrow full width at half maximum (FWHM) of 20 nm. To maintain the optical properties of QDs in device architecture, QDs are injected in liquid matrix on blue LED chips and used as the backlighting source for LCD TV that lead to bright and vivid colored images. QD-based white LEDs with high efficiency and wide color gamut have significant potential for next-generation display technology.
  • Placeholder
    Publication
    Optical neuromodulation at all scales: from nanomaterials to wireless optoelectronics and integrated systems
    (Royal Society of Chemistry (RSC), 2023) Gwak, Min-Jun; Hyun, Junghun; Koirala, Gyan Raj; Kim, Tae-Il; N/A; N/A; Department of Electrical and Electronics Engineering; Karatüm, Onuralp; Önal, Asım; Nizamoğlu, Sedat; PhD Student; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 130295
    Light-based neuromodulation systems offer exceptional spatiotemporal resolution combined with the elimination of physical tether to communicate with neurons. Currently, optical neuromodulation systems ranging from the nano to the centimeter scale enable neural activity control from the single cell to the organ level in retina, heart, spinal cord, and brain, facilitating a wide range of experiments in intact and freely moving animals in different contexts, such as during social interactions and behavioral tasks. Nanotransducers (e.g., metallic nanoparticles, silicon nanowires, and polymeric nanoparticles) and microfabricated photodiodes convert light to electrical, thermal, and mechanical stimuli that can allow remote and non-contact stimulation of neurons. Moreover, integrated devices composed of nano and microscale optoelectronic components comprise fully implantable and wirelessly powered smart optoelectronic systems that exhibit multimodal and closed-loop operation. In this review, we first discuss the material platforms, stimulation mechanisms, and applications of passive systems, i.e., nanotransducers and microphotodiodes. Then, we review the use of organic and inorganic light-emitting diodes for optogenetics and implantable wireless optoelectronic systems that enable closed-loop optogenetic neuromodulation through the use of light-emitting diodes, wireless power transfer circuits, and feedback loops. Exploration of materials and mechanisms together with the presented applications from both research and clinical perspectives in this review provides a comprehensive understanding of the optical neuromodulation field with its advantages and challenges to build superior systems in the future.
  • Thumbnail Image
    PublicationOpen Access
    Cadmium-free and efficient Type-II InP/ZnO/ZnS quantum dots and their application for LEDs
    (American Chemical Society (ACS), 2021) Ritter, Maximilian; Şahin, Mehmet; Ow-Yang, Cleva W.; Lechner, Rainer T.; Department of Electrical and Electronics Engineering; Department of Physics; N/A; N/A; Nizamoğlu, Sedat; Sennaroğlu, Alphan; Eren, Güncem Özgün; Sadeghi, Sadra; Jalali, Houman Bahmani; Han, Mertcan; Toker, Işınsu Baylam; Melikov, Rustamzhon; Önal, Asım; Öz, Fatma; Faculty Member; Faculty Member; PhD Student; PhD Student; Master Student; PhD Student; Department of Electrical and Electronics Engineering; Department of Physics; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); College of Engineering; College of Sciences; Graduate School of Sciences and Engineering; 130295; 23851; N/A; N/A; N/A; N/A; N/A; N/A; N/A; N/A
    It is a generally accepted perspective that type-II nanocrystal quantum dots (QDs) have low quantum yield due to the separation of the electron and hole wavefunctions. Recently, high quantum yield levels were reported for cadmium-based typeII QDs. Hence, the quest for finding non-toxic and efficient type-II QDs is continuing. Herein, we demonstrate environmentally benign type-II InP/ZnO/ZnS core/shell/shell QDs that reach a high quantum yield of similar to 91%. For this, ZnO layer was grown on core InP QDs by thermal decomposition, which was followed by a ZnS layer via successive ionic layer adsorption. The small-angle Xray scattering shows that spherical InP core and InP/ZnO core/ shell QDs turn into elliptical particles with the growth of the ZnS shell. To conserve the quantum efficiency of QDs in device architectures, InP/ZnO/ZnS QDs were integrated in the liquid state on blue light-emitting diodes (LEDs) as down-converters that led to an external quantum efficiency of 9.4% and a power conversion efficiency of 6.8%, respectively, which is the most efficient QD-LED using type-II QDs. This study pointed out that cadmium-free type-II QDs can reach high efficiency levels, which can stimulate novel forms of devices and nanomaterials for bioimaging, display, and lighting.
  • Thumbnail Image
    PublicationOpen Access
    High performance white light-emitting diodes over 150 lm/W using near-unity-emitting quantum dots in a liquid matrix
    (American Chemical Society (ACS), 2022) N/A; N/A; Department of Electrical and Electronics Engineering; Department of Chemistry; Önal, Asım; Eren, Güncem Özgün; Sadeghi, Sadra; Melikov, Rustamzhon; Han, Mertcan; Karatüm, Onuralp; Özer, Melek Sermin; Jalali, Houman Bahmani; Doğru-Yüksel, Itır Bakış; Yılgör, İskender; Metin, Önder; Nizamoğlu, Sedat; PhD Student; PhD Student; Master Student; PhD Student; Faculty Member; Faculty Member; Faculty Member; Department of Electrical and Electronics Engineering; Department of Chemistry; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); Graduate School of Sciences and Engineering; College of Sciences; College of Engineering; N/A; N/A; N/A; N/A; N/A; N/A; N/A; N/A; N/A; 24181; 46962; 130295
    In the next decade, we will witness the replacement of a majority of conventional light sources with light-emitting diodes (LEDs). Efficient LEDs other than phosphors can enhance their functionality and meet different lighting needs. Quantum dots (QDs) have high potential for future LED technology due to their sensitive band-gap tuning via the quantum confinement effect and compositional control, high photoluminescence quantum yield (PLQY), and mass-production capacity. Herein, we demonstrate white LEDs using QDs that reach over 150 lumens per electrical Watt. For that we synthesized green-and red-emitting ZnCdSe/ZnSe core/shell QDs by low-temperature nucleation, high-temperature shell formation, and postsynthetic trap-state removal. Their cadmium concentration is lower than 100 ppm, satisfying the current EU RoHS regulations, and their PLQY reaches a high level of 94%. The PLQY of QDs is maintained within the device on blue LED via liquid injection, and their integration at optimized optical densities leads to 129.6 and 170.4 lm/W for red-green-blue (RGB)-and green-blue (GB)-based white LEDs, respectively. Our simulations further showed that an efficiency level of over 230 lm/W is achievable using ultraefficient blue LED pumps. The simple fabrication and high performance of white LEDs using QD liquids show high promise for next-generation lighting devices.