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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/6

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    PublicationOpen Access
    Synthesis of stable gold nanoparticles using linear polyethyleneimines and catalysis of both anionic and cationic azo dye degradation
    (Royal Society of Chemistry (RSC), 2020) Abkenar, Sirous Khabbaz; Ow-Yang, Cleva W.; N/A; Department of Chemistry; Çavuşlar, Özge; Acar, Havva Funda Yağcı; Faculty Member; Department of Chemistry; Graduate School of Sciences and Engineering; College of Sciences; N/A; N/A; N/A; 178902
    Reduction of auric acid with polyethyleneimine (PEI) provides a simple, low-cost alternative for the production of cationic gold nanoparticles (GNPs). However, linear PEI (lPEI) failed to produce small, colloidally stable GNPs, so far. Since lPEI is a polyelectrolyte, pH should be an important factor both in reduction and stabilization of GNPs and may be optimized to produce small and stable lPEI/GNPs. Cationic GNPs were produced by the direct reduction of auric acid in water with lPEI utilizing two different methods to dissolve the polymer: by protonation or at high temperature. The influence of pH on the particle formation and properties was studied over a wide pH range (3.5 to 10). The impacts of the PEI/Au mass ratio, polymer molecular weight (2.5 and 25 kDa) and post-synthetic pH on the particle properties were also studied. Best is to dissolve lPEI by protonation and to clean the GNPs via controlled centrifugal precipitation. The MW did not influence the hydrodynamic size, stability or particle shape, but low MW lPEI provided faceted particles. This simple one pot synthesis of small, stable cationic GNPs in water is a valuable, simple alternative for producing new cationic GNPs with even low molecular weight lPEI. Additionally, these GNPs were evaluated as catalysts in the degradation of methyl orange (MO) (anionic-zwitterionic) and methylene blue (MB) (cationic) azo dyes at different pH values. The fastest degradation of MO and MB was recorded at pH 7.5 and 3.5, respectively. Overall, this is a rare case where a single catalyst quickly and effectively catalyzes the degradation of both cationic and anionic dyes.
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    PublicationOpen Access
    Advanced materials and device architectures for magnetooptical spatial light modulators
    (Wiley-VCH, 2019) N/A; Department of Electrical and Electronics Engineering; Kharratian, Soheila; Onbaşlı, Mehmet Cengiz; Ürey, Hakan; Faculty Member; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; N/A; 258783; 8579
    Faraday and Kerr rotations are magnetooptical (MO) effects used for rotating the polarization of light in transmission and reflection from a magnetized medium, respectively. MO effects combined with intrinsically fast magnetization reversal, which can go down to a few tens of femtoseconds or less, can be applied in magnetooptical spatial light modulators (MOSLMs) promising for nonvolatile, ultrafast, and high-resolution spatial modulation of light. With the recent progress in low-power switching of magnetic and MO materials, MOSLMs may lead to major breakthroughs and benefit beyond state-of-the-art holography, data storage, optical communications, heads-up displays, virtual and augmented reality devices, and solid-state light detection and ranging (LIDAR). In this study, the recent developments in the growth, processing, and engineering of advanced materials with high MO figures of merit for practical MOSLM devices are reviewed. The challenges with MOSLM functionalities including the intrinsic weakness of MO effect and large power requirement for switching are assessed. The suggested solutions are evaluated, different driving systems are investigated, and resulting device architectures are benchmarked. Finally, the research opportunities on MOSLMs for achieving integrated, high-contrast, and low-power devices are presented.
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    PublicationOpen Access
    Biocompatible quantum funnels for neural photostimulation
    (American Chemical Society (ACS), 2019) N/A; Department of Chemical and Biological Engineering; N/A; Department of Electrical and Electronics Engineering; Department of Molecular Biology and Genetics; N/A; Jalali, Houman Bahmani; Doğru-Yüksel, Itır Bakış; Eren, Güncem Özgün; Nizamoğlu, Sedat; Karatüm, Onuralp; Melikov, Rustamzhon; Dikbaş, Uğur Meriç; Kavaklı, İbrahim Halil; Sadeghi, Sadra; Yıldız, Erdost; Ergün, Çağla; Şahin, Afsun; PhD Student; Faculty Member; PhD Student; Master Student; Faculty Member; PhD Student; PhD Student; Faculty Member; Department of Chemical and Biological Engineering; Department of Electrical and Electronics Engineering; Department of Molecular Biology and Genetics; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Sciences and Engineering; College of Engineering; College of Sciences; School of Medicine; N/A; N/A; N/A; 130295; N/A; N/A; N/A; 40319; N/A; N/A; N/A; 171267
    Neural photostimulation has high potential to understand the working principles of complex neural networks and develop novel therapeutic methods for neurological disorders. A key issue in the light-induced cell stimulation is the efficient conversion of light to bioelectrical stimuli. In photosynthetic systems developed in millions of years by nature, the absorbed energy by the photoabsorbers is transported via nonradiative energy transfer to the reaction centers. Inspired by these systems, neural interfaces based on biocompatible quantum funnels are developed that direct the photogenerated charge carriers toward the bionanojunction for effective photostimulation. Funnels are constructed with indium-based rainbow quantum dots that are assembled in a graded energy profile. Implementation of a quantum funnel enhances the generated photoelectrochemical current 215% per unit absorbance in comparison with ungraded energy profile in a wireless and free-standing mode and facilitates optical neuromodulation of a single cell. This study indicates that the control of charge transport at nanoscale can lead to unconventional and effective neural interfaces.
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    PublicationOpen Access
    A diversity combination model incorporating an inward bias for interaural time-level difference cue integration in sound lateralization
    (Multidisciplinary Digital Publishing Institute (MDPI), 2020) N/A; Department of Computer Engineering; Mojtahedi, Sina; Erzin, Engin; Ungan, Pekcan; Faculty Member; Faculty Member; Department of Computer Engineering; Graduate School of Sciences and Engineering; College of Engineering; School of Medicine; N/A; 34503; N/A
    A sound source with non-zero azimuth leads to interaural time level differences (ITD and ILD). Studies on hearing system imply that these cues are encoded in different parts of the brain, but combined to produce a single lateralization percept as evidenced by experiments indicating trading between them. According to the duplex theory of sound lateralization, ITD and ILD play a more significant role in low-frequency and high-frequency stimulations, respectively. In this study, ITD and ILD, which were extracted from a generic head-related transfer functions, were imposed on a complex sound consisting of two low- and seven high-frequency tones. Two-alternative forced-choice behavioral tests were employed to assess the accuracy in identifying a change in lateralization. Based on a diversity combination model and using the error rate data obtained from the tests, the weights of the ITD and ILD cues in their integration were determined by incorporating a bias observed for inward shifts. The weights of the two cues were found to change with the azimuth of the sound source. While the ILD appears to be the optimal cue for the azimuths near the midline, the ITD and ILD weights turn to be balanced for the azimuths far from the midline.
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    PublicationOpen Access
    Photovoltaic neurointerface based on aluminum antimonide nanocrystals
    (Springer Nature, 2021) Department of Electrical and Electronics Engineering; N/A; Han, Mertcan; Nizamoğlu, Sedat; Jalali, Houman Bahmani; Yıldız, Erdost; Qureshi, Mohammad Haroon; Şahin, Afsun; Master Student; Faculty Member; PhD Student; PhD Student; PhD Student; Faculty Member; Department of Electrical and Electronics Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Health Sciences; School of Medicine; N/A; 130295; N/A; N/A; N/A; 171267
    Light activated modulation of neural activity is an emerging field for the basic investigation of neural systems and development of new therapeutic methods such as artificial retina. Colloidal inorganic nanocrystals have great potential for neural interfaces due to their adjustable optoelectronic properties via high-level structural, compositional, and size control. However, toxic heavy metal content (e.g., cadmium, mercury), electrochemical coupling to the cells and low photon-to-current efficiency limit their effective use. Here, we introduce the use of aluminum antimonide (AlSb) nanocrystals as the cell interfacing layer for capacitive neural stimulation in the blue spectrum. We demonstrate successful photostimulation of primary hippocampal neurons below ocular safety limits. In addition, our device shows high biocompatibility in vitro and passive accelerated ageing tests indicate a functional lifetime over 3 years showing their feasible use for chronic implants. We demonstrate that nanocrystal biointerfaces hold high promise for future bioelectronics and protheses.
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    PublicationOpen Access
    A new class of porous materials for efficient CO2 separation: ionic liquid/graphene aerogel composites
    (Elsevier, 2021) Department of Chemical and Biological Engineering; N/A; Department of Chemistry; Zeeshan, Muhammad; Yalçın, Kaan; Keskin, Seda; Uzun, Alper; Öztuna, Feriha Eylül Saraç; Ünal, Uğur; PhD Student; Faculty Member; Faculty Member; Department of Chemical and Biological Engineering; Department of Chemistry; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Engineering; College of Sciences; N/A; N/A; 40548; 59917; N/A; 42079
    Here, we report a new post-synthesis modification strategy for functionalizing reduced graphene aerogels (rGAs) towards an exceptional CO2 separation performance. 1-N-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) was impregnated on a rGA, prepared by reducing GA at 700 degrees C, at various ionic liquid (IL) loadings of 5, 10, 30, and 50 wt%. The resulting composites were characterized in deep detail by X-ray photoelectron spectroscopy, X-ray diffraction, N-2 physical adsorption measurements, scanning electron microscopy, Fourier transform infrared and Raman spectroscopies, and thermogravimetric analysis. Results indicated the presence of interactions between the rGA surface and the anion of the IL, potentially improving the CO2 affinity. Volumetric gas adsorption measurements using these materials showed that the deposition of [BMIM][PF6] on rGA surface at an IL loading of 50 wt% boosts the CO2/CH4 selectivity by more than 20-times, exceeding an absolute value of 120, a remarkably higher CO2/CH4 selectivity compared to that of other functionalized materials under similar operating conditions. Tunability of both the IL structure and the surface characteristics of rGA offer a tremendous degree of flexibility for the rational design of these IL/rGA composites towards high performance in gas separation applications.
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    PublicationOpen Access
    Key properties of inorganic thermoelectric materials - tables (version 1)
    (Institute of Physics (IOP) Publishing, 2022) Freer, R.; Ekren, D.; Ghosh, T.; Biswas, K.; Qiu, P.; Wan, S.; Chen, L.; Han, S.; Fu, C.; Zhu, T.; Ashiquzzaman Shawon, A.K.M.; Zevalkink, A.; Imasato, K.; Snyder, G.J.; Cardoso-Gil, R.; Svanidze, E.; Funahashi, R.; Powell, A.V.; Mukherjee, S.; Tippireddy, S.; Vaqueiro, P.; Gascoin, F.; Kyratsi, T.; Sauerschnig, P.; Mori, T.; N/A; Department of Chemistry; Aydemir, Umut; Sağlık, Kıvanç; Özen, Melis; Faculty Member; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences; 58403; N/A; N/A
    This paper presents tables of key thermoelectric properties, which define thermoelectric conversion efficiency, for a wide range of inorganic materials. The twelve families of materials included in these tables are primarily selected on the basis of well established, internationally-recognized performance and promise for current and future applications: tellurides, skutterudites, half Heuslers, Zintls, Mg-Sb antimonides, clathrates, FeGa3-type materials, actinides and lanthanides, oxides, sulfides, selenides, silicides, borides and carbides. As thermoelectric properties vary with temperature, data are presented at room temperature to enable ready comparison, and also at a higher temperature appropriate to peak performance. An individual table of data and commentary are provided for each family of materials plus source references for all the data.
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    PublicationOpen Access
    High-quality MgB2 nanocrystals synthesized by using modified amorphous nano-boron powders: study of defect structures and superconductivity properties
    (American Institute of Physics (AIP) Publishing, 2019) Erdem, Emre; Hassler, Wolfgang; Department of Chemistry; N/A; Somer, Mehmet Suat; Bateni, Ali; Faculty Member; PhD Student; Department of Chemistry; College of Sciences; Graduate School of Sciences and Engineering
    Nano sized magnesium diboride (MgB2) samples were synthesized using various high-quality nano-B precursor powders. The microscopic defect structures of MgB2 samples were systematically investigated using X-ray powder diffraction, Raman, resistivity measurements and electron paramagnetic resonance spectroscopy. A significant deviation in the critical temperature T-c was observed due to defects and crystal distortion. The symmetry effect of the latter is also reflected on the vibrational modes in the Raman spectra. Scanning electron microscopy analysis demonstrate uniform and ultrafine morphology for the modified MgB2. Defect center in particular Mg vacancies influence the connectivity and the conductivity properties which are crucial for the superconductivity applications.
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    PublicationOpen Access
    Seed-mediated synthesis of plasmonic gold nanoribbons using cancer cells for hyperthermia applications
    (Royal Society of Chemistry (RSC), 2018) Singh, Ajay Vikram; Alapan, Yunus; Jahnke, Timotheus; Laux, Peter; Luch, Andreas; Aghakhani, Amirreza; Bill, Joachim; Sitti, Metin; N/A; Department of Electrical and Electronics Engineering; Kharratian, Soheila; Onbaşlı, Mehmet Cengiz; Faculty Member; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 258783
    A surfactant-less, seed mediated, biological synthesis of two dimensional (2-D) nanoribbons in the presence of breast cancer cells (MCF7) is demonstrated. The diameter and yield of nanoribbons are tunable via seeds and gold precursor concentration. Such crystalline nanoribbons serve to enhance the Raman signals over MCF7 cells. The side and slopes of the triangular nanoplatelets fused as nanoribbons exhibit plasmon excitement in quadrupole resonance modes in the infrared region. Consequently, when irradiated with an infrared laser they show an excellent photothermal effect and rapid rise in temperature. The experimental results verified by finite-difference time-domain (FTDT) calculations reveal the presence of wedge-plasmon polaritons propagating along the edges of the nanoribbons. These simulations confirm that long aspect ratio nanoribbon's edges and vertices act as an active optical waveguide, allowing for heat propagation along the long axis, killing cancer cells in the process at lower power doses.
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    PublicationOpen Access
    Guide to water free lithium bis(oxalate) borate (LiBOB)
    (American Chemical Society (ACS), 2021) Zor, Ceren; Afyon, Semih; N/A; Department of Chemistry; Haciu, Durata; Subaşı, Yaprak; Somer, Mehmet Suat; Teaching Faculty; Researcher; Department of Chemistry; Koç University AKKİM Boron-Based Materials _ High-technology Chemicals Research _ Application Center (KABAM) / Koç Üniversitesi AKKİM Bor Tabanlı Malzemeler ve İleri Teknoloji Kimyasallar Uygulama ve Araştırma Merkezi (KABAM); Graduate School of Sciences and Engineering; College of Sciences
    Lithium bis(oxalate) borate, LiB(C2O4)(2) (LiBOB), is one of the most important electrolyte additives for Li-ion batteries (LIBs) due to its numerous advantages such as thermal stability, good solubility in organic solvents, high conductivity, and low cost as well as providing safer operations with superior electrochemical performance compared to conventional electrolyte combinations. However, the use of LiBOB is limited due to slight instability issues under ambient conditions that might require extra purification steps and result in poorer performances in real systems. Here, we address some of these issues and report a high purity water free LiBOB synthesized with fewer processing steps, employing lithium carbonate, oxalic acid, and boric acid as low-cost starting materials, and via ceramic processing methods under protective atmosphere. The physical and chemical characterizations of both anhydrous and monohydrate phases are performed with X-ray powder diffraction (XRPD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy (SEM) analyses to determine the degree of the purity and the formation of impurities, such as LiBOB center dot H2O, HBO2, and Li2C2O4, as a result of the aging investigations where the as-synthesized salt was exposed to ambient conditions for different durations. Differential thermal analysis (DTA) is applied to determine the optimum synthesis conditions for anhydrous LiBOB and to analyze the water loss and the decomposition of LiBOB center dot H2O. Aging experiments with the water free LiBOB are carried out to evaluate the effect of humidity on the phase changes and resulting impurities under various conditions. The detrimental effect of even slightest humidity conditions is shown, and protective measures during and after the synthesis of LiBOB are discussed. Anhydrous LiBOB could be widely used as an electrolyte additive to improve the overall electrochemical performances for LIBs through development of a protective solid electrolyte interface (SEI) on the surface of high voltage cathodes and by bringing about superior electrochemical properties with increased cycling stability, rate capability, and Coulombic efficiency, if synthesized, purified, and handled properly before use in real electrochemical systems.