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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Characterization of structural and mechanical properties of HfNbTaTiZr refractory high entropy alloy after gas nitriding(Elsevier, 2024) Alphan, Yunus; Kaba, Mertcan; Cimenoglu, Huseyin; N/A; Motallebzadeh, Amir; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/AThis study was initiated to improve surface hardness and wear resistance of a HfNbTaTiZr refractory high entropy alloy (RHEA) by gas nitriding at a medium temperature (600 °C) for 3 h. Structural characterizations conducted by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) equipped scanning electron microscope (SEM) revealed that nitriding led to formation of a 1.5 μm thick surface layer containing precipitates of oxides and nitrides of the alloying elements. Detection of oxides within the surface layer was attributed to the presence residual oxygen in the nitriding atmosphere. Nevertheless, the employed gas nitriding provided remarkably higher scratch resistance compared to the untreated state, as the results of increment in the surface hardness and development of larger compressive residual stress. © 2024 Elsevier LtdPublication Metadata only Synthesis of oleic acid – coated zinc – doped iron boride nanoparticles for biomedical applications(Elsevier Sci Ltd, 2024) Paksoy, Aybike; Somer, Mehmet; Balcı-Çağıran, Özge; N/A; Aydemir, Duygu; Ulusu, Nuriye Nuray; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); Graduate School of Health Sciences; School of MedicineAlthough various iron-based magnetic materials have been extensively studied in biomedical field for many years, iron boride compounds with interesting chemical and magnetic properties are relatively less explored, and their potential applications are not as widely known. In this study, the synthesis, coating, surface modification, and cytotoxicity tests of the Fe–Zn–B system were presented. Iron boride-based nanoparticles (NPs) containing elemental zinc (Zn) were developed by using a direct chemical synthesis of FeCl3, ZnCl2 and NaBH4, and investigated for potential use in biomedical applications. Powders having the phases of pure FeB with small amount of elemental Zn were obtained with a uniform morphology and an average particle size of 68 nm. The NPs were then coated with oleic acid (OA) and surface modified with sodium tricitrate, to increase their stability and biocompatibility, and well-dispersed NPs were obtained with sizes below 30 nm. TEM investigations revealed the presence of hybrid clusters with nanoparticle – OA structures, indicating that FeB nanoparticles were stabilized by being embedded in OA clusters, forming both agglomerated sub-micron and free nano-sized structures. Obtained NPs showed ferromagnetic property, with a saturation magnetization of 25.9 emu/g and a low coercivity of 90 Oe. As a result of testing different types of healthy and cancer cell lines with NPs, Zn-doped-FeB@OA NPs exhibited a high biocompatibility. Results suggested that highly biocompatible and magnetic OA-coated Zn-doped FeB particles can be potential candidates for biomedical applications such as medical imaging or drug delivery systems.Publication Metadata only Targeting cancer cells via tumor-homing peptide CREKA functional PEG nanoparticles(Elsevier, 2016) N/A; N/A; N/A; Department of Chemical and Biological Engineering; Okur, Aysu Ceren; Erkoç, Pelin; Kızılel, Seda; Master Student; PhD Student; Faculty Member; Department of Chemical and Biological Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; N/A; 28376Targeting cell microenvironment via nano-particle based therapies holds great promise for treatment of various diseases. One of the main challenges in targeted delivery of nanoparticles for cancer therapy includes reduced localization of delivery vehicles at tumor site. The therapeutic efficacy of drugs can be improved by recruiting delivery vehicles towards specific region of tumorigenesis in the body. Here, we demonstrate an effective approach in creating PEG particles via water-in-water emulsion technique where tumor-homing peptide CREKA was used for functionalization. Simultaneous conjugation of laminin peptide IKVAV into hydrogel network and influence of altered combinations of ligands on intracellular uptake of anticancer drugs by HeLa cells were investigated. CREKA conjugated hydrogel nanoparticles were more effective to improve apoptotic effects of the model drug Doxorubicin (DOX) compared to that of particles conjugated with other peptides. Fluorescence intensity analysis on confocal micrographs suggested significantly higher cellular uptake of CREKA conjugated PEG particles than internalization of nanoparticles in other groups. We observed that fibrin binding ability of PEG particles could be increased up to 94% through CREKA conjugation. Our results suggest the possibility of cancer cell targeting via CREKA-functional PEG nanoparticles.Publication Metadata only Silk as a biodegradable resist for field-emission scanning probe lithography(Institute of Physics (IOP) Publishing, 2020) Sadeghi, Sadra; Rangelow, Ivo W.; Department of Mechanical Engineering; Department of Electrical and Electronics Engineering; N/A; N/A; Department of Electrical and Electronics Engineering; Alaca, Burhanettin Erdem; Kumar, Baskaran Ganesh; Melikov, Rustamzhon; Doğru-Yüksel, Itır Bakış; Nizamoğlu, Sedat; Faculty Member; Other; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; Department of Electrical and Electronics Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştirmalari Merkezi (KUYTAM); N/A; N/A; N/A; N/A; College of Engineering; College of Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; College of Engineering; 115108; N/A; N/A; N/A; 130295The patterning of silk allows for manufacturing various structures with advanced functionalities for optical and tissue engineering and drug delivery applications. Here, we propose a high-resolution nanoscale patterning method based on field-emission scanning probe lithography (FE-SPL) that crosslinks the biomaterial silk on conductive indium tin oxide (ITO) promoting the use of a biodegradable material as resist and water as a developer. During the lithographic process, Fowler-Nordheim electron emission from a sharp tip was used to manipulate the structure of silk fibroin from random coil to beta sheet and the emission formed nanoscale latent patterns with a critical dimension (CD) of similar to 50 nm. To demonstrate the versatility of the method, we patterned standard and complex shapes. This method is particularly attractive due to its ease of operation without relying on a vacuum or a special gaseous environment and without any need for complex electronics or optics. Therefore, this study paves a practical and cost-effective way toward patterning biopolymers at ultra-high level resolution.Publication Metadata only Antibacterial silicone-urea/organoclay nanocomposites(Springer, 2009) Department of Chemistry; N/A; N/A; Department of Chemistry; Yılgör, Emel; Nugay, Işık Işıl; Bakan, Murat; Yılgör, İskender; Researcher; Undergraduate Student; Undergraduate Student; Faculty Member; Department of Chemistry; College of Sciences; College of Engineering; College of Engineering; College of Sciences; N/A; N/A; N/A; 24181Montmorillonite modified with distearyldimethyl ammonium chloride (C18-QAC) (Nanofil-15) (NF15) was incorporated into polydimethylsiloxane-urea (silicone-urea, PSU) copolymers. PSU was obtained by the reaction of equimolar amounts of aminopropyl terminated polydimethylsiloxane (PDMS) oligomer (= 3,200 g/mol) and bis(4-isocyanatohexyl) methane (HMDI). A series of PSU/NF15 nanocomposites were prepared by solution blending with organoclay loadings ranging from 0.80 to 9.60% by weight, corresponding to 0.30 to 3.60% C18-QAC. Colloidal dispersions of organophilic clay (NF15) in isopropanol were mixed with the PSU solution in isopropanol and were subjected to ultrasonic treatment. Composite films were obtained by solution casting. FTIR spectroscopy confirmed that the organoclay mainly interacted with the urea groups but not with PDMS. XRD analysis showed that nanocomposites containing up to 6.40% by weight of organoclay had fully exfoliated silicate layers in the polymer matrix, whereas 9.60% loading had an intercalated structure. Physicochemical properties of nanocomposites were determined. PSU/NF15 nanocomposites displayed excellent long-term antibacterial properties against E. coli.Publication Metadata only Nanoheterogeneous ZrTa metallic glass thin films with high strength and toughness(Elsevier, 2022) Behboud, Ali B.; Ozerinc, Sezer; N/A; Motallebzadeh, Amir; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/AThis study investigated the mechanical behavior of ZrxTa1-x (x = 21-79 at%) thin films and nanolayered films of ZrTa with modulated composition as model systems to gain insight into the hardness and toughness of metallic glasses and metallic glass nanocomposites. The monolithic films exhibit two primary micro-structures, namely, a fully amorphous form (Zr = 35-70at%.) and an amorphous-crystalline composite (21-30 at% Zr). The amorphous films show a monotonic hardness variation with composition over a wide range of 5.5 - 9 GPa. The partial crystallization of the films results in a further jump in hardness, as opposed to the general trend of softening upon crystallization. The emergence of the crystalline phase also improves the ductility of the films, as verified by nanoindentation-based fracture toughness measurements. The indentation pile-up exhibits several shear bands in the fully amorphous films, replaced by a featureless pileup zone for the case of Zr25Ta75, further verifying the superior toughness of the composite. The second part of the analysis pursued obtaining a similar toughening through fully amorphous nanolayered films of Zr35Ta65 / Zr70Ta30. The results indicate that these films provide a balanced combination of high hardness and enhanced ductility, providing an alternative route to the development of tough metallic glass coatings. Data Availability: The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.Publication Metadata only Structural changes in a Schiff base molecular assembly initiated by scanning tunneling microscopy tip(Institute of Physics (IOP) Publishing, 2016) Tomak, A.; Bacaksiz, C.; Mendirek, G.; Sahin, H.; Hur, D.; Gorgun, K.; Senger, R. T.; Peeters, F. M.; Zareie, H. M.; N/A; Birer, Özgür; Researcher; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/AWe report the controlled self-organization and switching of newly designed Schiff base (E)-4-((4-(phenylethynyl) benzylidene) amino) benzenethiol (EPBB) molecules on a Au (111) surface at room temperature. Scanning tunneling microscopy and spectroscopy (STM/STS) were used to image and analyze the conformational changes of the EPBB molecules. The conformational change of the molecules was induced by using the STM tip while increasing the tunneling current. The switching of a domain or island of molecules was shown to be induced by the STM tip during scanning. Unambiguous fingerprints of the switching mechanism were observed via STM/STS measurements. Surface-enhanced Raman scattering was employed, to control and identify quantitatively the switching mechanism of molecules in a monolayer. Density functional theory calculations were also performed in order to understand the microscopic details of the switching mechanism. These calculations revealed that the molecular switching behavior stemmed from the strong interaction of the EPBB molecules with the STM tip. Our approach to controlling intermolecular mechanics provides a path towards the bottom-up assembly of more sophisticated molecular machines.Publication Metadata only Colloidal aluminum antimonide quantum dots(Amer Chemical Soc, 2019) Sahin, Mehmet; Öztürk, Hande; Ow-Yang, Cleva W.; N/A; N/A; Department of Electrical and Electronics Engineering; Jalali, Houman Bahmani; Sadeghi, Sadra; Nizamoğlu, Sedat; PhD Student; 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; 130295AlSb is a less studied member of the III-V semiconductor family, and herein, we report the colloidal synthesis of AlSb quantum dots (QDs) for the first time. Different sizes of colloidal AlSb QDs (5 to 9 nm) were produced by the controlled reaction of AlCl3 and Sb[N(Si(Me)(3))(2)](3) in the presence of superhydride. These colloidal AlSb quantum dots showed excitonic transitions in the UV-A region and a tunable band edge emission (quantum yield of up to 18%) in the blue spectral range. Among all III-V quantum dots, these quantum dots show the brightest core emission in the blue spectral region.Publication Metadata only Interstitial nitrogen enhances corrosion resistance of an equiatomic cocrni medium-entropy alloy in sulfuric acid solution(Elsevier Science Inc, 2021) Moravcik, Igor; Moravcikova-Gouvea, Larissa; Liu, Chang; Prabhakar, J. Manoj; Dlouhy, Ivo; Li, Zhiming; N/A; Peighambardoust, Naeimeh Sadat; Motallebzadeh, Amir; Researcher; Researcher; Koç University Boron and Advanced Materials Application and Research Center (KUBAM) / Koç Üniversitesi Bor ve İleri Malzemeler Uygulama ve Araştırma Merkezi (KUBAM); Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; N/A; N/AThe corrosion resistance of the equiatomic CoCrNi medium-entropy alloy (MEA) and its 0.5 atomic % nitrogen alloyed variant in 0.1 M H2SO4 solution was investigated and compared with that of the 316 L stainless steel as a reference material. All of the investigated materials showed single-phase face centered cubic (FCC) microstructures, and nitrogen was fully dissolved in the solid solution structure of the CoCrNi MEA. Both the nitrogen-free and nitrogen-doped MEAs showed significantly higher corrosion resistance (lower corrosion currents and rates) than the 316 L steel. Compared to the nitrogen-free CoCrNi, the interstitial nitrogen dissolved in solid solution causes a significant improvement in the pitting corrosion resistance of the CoCrNiN. Under the same testing condition, pitting corrosion was not observed in the CoCrNiN alloy, while the CoCrNi MEA and 316 L steel showed distinct pitting cavities. The better anti-corrosion performance of the CoCrNiN compared to that of the CoCrNi is correlated with a higher fraction of chromium oxide in the passive films.Publication Metadata only Analytical modeling and experimental validation of a structurally integrated piezoelectric energy harvester on a thin plate(Iop Publishing Ltd, 2014) Erturk, A.; N/A; Department of Mechanical Engineering; Arıdoğan, Mustafa Uğur; Başdoğan, İpek; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 179940Vibration-based energy harvesting using piezoelectric cantilevers has been extensively studied over the past decade. As an alternative to cantilevered harvesters, piezoelectric patch harvesters integrated to thin plates can be more convenient for use in marine, aerospace and automotive applications since these systems are often composed of thin plate-like structures with various boundary conditions. In this paper, we present analytical electroelastic modeling of a piezoelectric energy harvester structurally integrated to a thin plate along with experimental validations. The distributed-parameter electroelastic model of the thin plate with the piezoceramic patch harvester is developed based on Kirchhoff's plate theory for all-four-edges clamped (CCCC) boundary conditions. Closed-form steady-state response expressions for coupled electrical output and structural vibration are obtained under transverse point force excitation. Analytical electroelastic frequency response functions (FRFs) relating the voltage output and vibration response to force input are derived and generalized for different boundary conditions. Experimental validation and extensive theoretical analysis efforts are then presented with a case study employing a thin PZT-5A piezoceramic patch attached on the surface of a rectangular aluminum CCCC plate. The importance of positioning of the piezoceramic patch harvester is discussed through an analysis of dynamic strain distribution on the overall plate surface. The electroelastic model is validated by a comparison of analytical and experimental FRFs for a wide range of resistive electrical boundary conditions. Finally, power generation performance of the structurally integrated piezoceramic patch harvester from multiple vibration modes is investigated analytically and experimentally.