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Permanent URI for this collectionhttps://hdl.handle.net/20.500.14288/3
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Publication Metadata only Mechanical properties of silicon nanowires with native oxide surface state(Elsevier, 2024) Department of Mechanical Engineering; Zarepakzad, Sina; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; Department of Mechanical Engineering; n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced Research; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of EngineeringSilicon nanowires have attracted considerable interest due to their wide-ranging applications in nanoelectromechanical systems and nanoelectronics. Molecular dynamics simulations are powerful tools for studying the mechanical properties of nanowires. However, these simulations encounter challenges in interpreting the mechanical behavior and brittle to ductile transition of silicon nanowires, primarily due to surface effects such as the assumption of an unreconstructed surface state. This study specifically focuses on the tensile deformation of silicon nanowires with a native oxide layer, considering critical parameters such as cross-sectional shape, length -to -critical dimension ratio, temperature, the presence of nano -voids, and strain rate. By incorporating the native oxide layer, the article aims to provide a more realistic representation of the mechanical behavior for different critical dimensions and crystallographic orientations of silicon nanowires. The findings contribute to the advancement of knowledge regarding size -dependent elastic properties and strength of silicon nanowires.Publication Metadata only Objective-free ultrasensitive biosensing on large-area metamaterial surfaces in the near-IR(AMER CHEMICAL SOC, 2024) Department of Physics; Ramazanoğlu, Serap Aksu; Öktem, Evren; Department of Physics; College of Sciences; Graduate School of Sciences and EngineeringPlasmonic metamaterials have opened new avenues in medical diagnostics. However, the transfer of the technology to the markets has been delayed due to multiple challenges. The need of bulky optics for signal reading from nanostructures patterned on submillimeter area limits the miniaturization of the devices. The use of objective-free optics can solve this problem, which necessitates large area patterning of the nanostructures. In this work, we utilize laser interference lithography (LIL) to pattern nanodisc-shaped metamaterial absorber nanoantennas over a large area (4 cm(2)) within minutes. The introduction of a sacrificial layer during the fabrication process enables an inverted hole profile and a well-controlled liftoff, which ensures perfectly defined uniform nanopatterning almost with no defects. Furthermore, we use a macroscopic reflection probe for optical characterization in the near-IR, including the detection of the binding kinematics of immunologically relevant proteins. We show that the photonic quality of the plasmonic nanoantennas commensurates with electron-beam-lithography-fabricated ones over the whole area. The refractive index sensitivity of the LIL-fabricated metasurface is determined as 685 nm per refractive index unit, which demonstrates ultrasensitive detection. Moreover, the fabricated surfaces can be used multiple times for biosensing without losing their optical quality. The combination of rapid and large area nanofabrication with a simple optical reading not only simplifies the detection process but also makes the biosensors more environmentally friendly and cost-effective. Therefore, the improvements provided in this work will empower researchers and industries for accurate and real-time analysis of biological systems.Publication Metadata only Solar-light-driven photocatalytic hydrogen evolution activity of gCN/WS2 heterojunctions incorporated with the first-row transition metals(Elsevier Science Sa, 2023) Acar, Eminegul Genc; Aslan, Emre; Patir, Imren Hatay; Department of Chemistry; Yılmaz, Seda; Eroğlu, Zafer; Metin, Önder; 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 SciencesThe design of semiconductor-based heterojunctions is an effective strategy to build highly active photo-catalyst systems. In this study, tungsten disulfide (WS2) modified graphitic carbon nitride (gCN) hetero-junction (gCN/WS2) is incorporated with Co and Ni (gCN/WS2-Co and gCN/WS2-Ni) to enhance the photocatalytic hydrogen evolution reaction (HER) activity of gCN/WS2 via performing a chemical reduction method and characterized by advanced analytical techniques. The photocatalytic HER activities of gCN, gCN/ WS2, gCN/WS2-Ni and gCN/WS2-Co were measured as 0.126, 0.221, 0.237 and 0.249 mmol g-1h-1, respec-tively, under the visible light irradiation. The improvement of photocatalytic activity and stability of gCN/ WS2-Ni and gCN/WS2-Co nanocomposites could be attributed to the 2D/2D heterojunction structure, ex-tended light harvesting ability, increased electron-hole lifetime and decreased recombination rate of the charge carriers. Moreover, mechanistic studies revealed that a S-scheme heterojunction is attributed to the enhanced photocatalytic HER by the gCN/WS2-Ni and gCN/WS2-Co photocatalysts, which provides pro-moted efficiency by photocarrier transfer and separation.Publication Metadata only Anisotropic wettability induced by femtosecond laser ablation(Wiley-V C H Verlag Gmbh, 2023) Yetisen, Ali K.; Department of Mechanical Engineering; Shojaeian, Mostafa; Taşoğlu, Savaş; Department of Mechanical Engineering; Koç University Research Center for Translational Medicine (KUTTAM) / Koç Üniversitesi Translasyonel Tıp Araştırma Merkezi (KUTTAM); KU Arçelik Research Center for Creative Industries (KUAR) / KU Arçelik Yaratıcı Endüstriler Uygulama ve Araştırma Merkezi (KUAR); Graduate School of Sciences and Engineering; College of EngineeringLaser ablation has been utilized for locally and selectively modifying the surface wettability of materials in situ and enabling on-demand microfabrication. The anisotropic wettability has been observed on chemical and/or topographical patterns, such as an array of laser-inscribed strips with spacings, created on surfaces during the fabrication process. Herein, the effectiveness of the femtosecond laser ablation is evaluated in selectively modifying surface wettability. The areas processed by laser ablation exhibit anisotropic wetting behavior, even after the laser strips are overlapped. The laser-induced anisotropic surface wettability is present in space governed by laser scanning speed, scan/strip overlap, laser fluence, scan repetition, and bidirectional scanning angle. Moreover, the femtosecond laser ablation process is optimized to enhance the conventional laser inscription, leading to a modified and consistent methodology to achieve cost-effective fabrication. Herein, an approach for locally and selectively modifying surface wettability of materials in situ induced by femtosecond laser ablation is described. The laser-induced anisotropic surface wettability is found to appear in space governed by laser scanning speed, scan/strip overlap, laser fluence, scan repetition, and bidirectional scanning angle.Publication Metadata only Development and characterization of skin substitutes from electrospun polycaprolactone/silk fibroin(Sage Publications Ltd, 2023) Yildiz, Gulsah; Arslan, Yavuz Emre; Derkus, Burak; Menceloglu, Yusuf Ziya; Bayar, Gurkan Rasit; Sezgin, Billur; School of MedicineTissue-engineered skin substitutes have great potential to treat chronic wounds and high-degree burns. Existing solutions, such as Integra Dermal Template, are extensively used for skin defects. However, these templates are still lacking in terms of recreating the functionality of the native tissue and providing scarless healing. In this study, polycaprolactone/silk fibroin (PCL/SF)-based nanofibers with varying blends were fabricated and characterized to develop a novel skin substitute. Morphological analysis showed that the nanofiber distribution of each sample was homogenous without showing any beads. In terms of mechanical properties, all the samples other than SF showed sufficient mechanical strength. It was observed that adding a specific amount of SF into the PCL nanofiber improves the tensile strength of the samples due to the introduction of intermolecular interactions from the functional groups of SF. In addition, incorporating SF into PCL improved Young's modulus of the PCL nanofibers since SF provides stiffness and structural integrity to the overall structure. Water contact angle analysis was performed as the hydrophilicity of a biomaterial is a significant factor in cell functionality. Each sample had a contact angle between 33 degrees and 48 degrees, indicating the adequate hydrophilicity of nanofibers for advanced cell proliferation other than PCL. Cell proliferation and viability studies were conducted with the seeding of primary human keratinocytes on the samples. It was examined that scaffolds containing blends of PCL and SF resulted in higher cell proliferation and viability after 7 days compared to pure PCL and SF nanofibers.Publication Metadata only Bidisperse magnetorheological fluids with strong magnetorheological response, long-term stability and excellent in-use performance(IOP Publishing Ltd, 2024) Department of Chemistry;Department of Mechanical Engineering; Nejatpour, Mona; Saleh, Mostafa Khalil Abdou; Ulasyar, Abasin; Ünal, Uğur; Lazoğlu, İsmail; Acar, Havva Funda Yağcı; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Manufacturing and Automation Research Center (MARC); Graduate School of Sciences and Engineering; College of Sciences; College of EngineeringThere is a critical demand for magnetorheological fluids (MRFs) with high particle loading, long-term stability, and high magneto-viscous properties to be used in industrial MRF devices. Bidisperse MRFs composed of highly magnetizable micron-sized carbonyl iron particles and poly(acrylic acid) coated superparamagnetic iron oxide nanoparticles (SPIONs-PAA) that can chemically interact are proposed to achieve such MRFs, here. Coating bare, commercial CI with lauric acid (LA) enhanced its dispersibility in a hydrophobic carrier fluid, allowed high magnetic loading and significantly prevented the sedimentation of the particles when mixed with 9-12 wt% SPION. Different carrier fluids (mineral oil, paraffin, and hydraulic oil) were tested, and hydraulic oil was determined as the best for this particle combination. The most stable bidisperse MRF was achieved at 83%-84% magnetic content with 12 wt-%SPION-PAA, LA-coated-CI and 3 wt% polyvinyl alcohol. Such MRFs outperformed the commercial benchmark, 140-CG (R) from Lord Corp., in long-term stability (4 months) and stability under dynamic loading. Bidisperse MRFs were stable between 20 degrees C and 60 degrees C. Most importantly, the excellent performance of the bidisperse MRFs in dampers designed for washing machines suggests that these MRFs may provide comparable damping forces with much better stability, ensuring longer shelf-life and longer lifetime in use.Publication Metadata only Optimizing photonic devices under fabrication variations with deep photonic networks(SPIE-Int Soc Optical Engineering, 2024) Department of Electrical and Electronics Engineering; Görgülü, Kazım; Vit, Aycan Deniz; Amiri, Ali Najjar; Mağden, Emir Salih; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of EngineeringWe propose a deep photonic interferometer network architecture for designing fabrication-tolerant photonic devices. Our framework incorporates layers of variation-aware, custom-designed Mach-Zehnder interferometers and virtual wafer maps to optimize broadband power splitters under fabrication variations. Specifically, we demonstrate 50/50 splitters with below 1% deviation from the desired 50/50 ratio, even with up to 15 nm over-etch and under-etch variations. The significantly improved device performance under fabrication-induced changes demonstrates the effectiveness of the deep photonic network architecture in designing fabrication-tolerant photonic devices and showcases the potential for improving circuit performance by optimizing for expected variations in waveguide width.Publication Metadata only Chern numbers for the two-body Hofstadter-Hubbard butterfly(American Physical Society, 2024) Department of Physics;Department of Chemistry; Işkın, Menderes; Alyürük, Doruk Can; ; College of Sciences;We analyze the two -body spectrum within the Hofstadter-Hubbard model on a square lattice through an exact variational ansatz and study the topological properties of its low-lying two -body bound -state branches. In particular, we discuss how the Hofstadter-Hubbard butterfly of the two -body branches evolves as a function of onsite interactions and how to efficiently calculate their Chern numbers using the Fukui-Hatsugai-Suzuki approach. Our numerical results are fully consistent with the simple picture that appears in the strong -coupling limit, where the attraction between fermions forms a composite boson characterized by an effective hopping parameter and an effective magnetic -flux ratio.Publication Metadata only Cooper pairing, flat-band superconductivity, and quantum geometry in the pyrochlore-Hubbard model(American Physical Society, 2024) ; Department of Physics; Işkın, Menderes; Department of Physics; ; College of Sciences;We investigate the impacts of the quantum geometry of Bloch states, specifically through the band -resolved quantum -metric tensor, on Cooper pairing and flat -band superconductivity in a three-dimensional pyrochloreHubbard model. First we analyze the low-lying two -body spectrum exactly, and show that the pairing order parameter is uniform in this four -band lattice. This allows us to establish direct relations between the superfluid weight of a multiband superconductor and (i) the effective mass of the lowest -lying two -body branch at zero temperature, (ii) the kinetic coefficient of the Ginzburg-Landau theory in proximity to the critical temperature, and (iii) the velocity of the low -energy Goldstone modes at zero temperature. Furthermore, we perform a comprehensive numerical analysis of the superfluid weight and Goldstone modes, exploring both their conventional and geometric components at zero temperature.Publication Metadata only Stabilization and adiabatic control of antiferromagnetically coupled skyrmions without the topological hall effect(Royal Soc Chemistry, 2023) Department of Electrical and Electronics Engineering; Yağan, Rawana; Cheghabouri, Arash Mousavi; Onbaşlı, Mehmet Cengiz; Department of Electrical and Electronics Engineering; Graduate School of Sciences and Engineering; College of EngineeringSynthetic antiferromagnetically coupled (SAF) multilayers provide different physics of stabilizing skyrmions while eliminating the topological Hall effect (THE), enabling efficient and stable control. The effects of material parameters, external current drive, and a magnetic field on the skyrmion equilibrium and propagation characteristics are largely unresolved. Here, we present a computational and theoretical demonstration of the large window of material parameters that stabilize SAF skyrmions determined by saturation magnetization, uniaxial anisotropy, and Dzyaloshinskii-Moriya interaction. Current-driven SAF skyrmion velocities reach & SIM;200 m s(-1) without the THE. The SAF velocities are about 3-10 times greater than the typical ferromagnetic skyrmion velocities. The current densities needed for driving SAF skyrmions could be reduced to 10(8) A m(-2), while 10(11) A m(-2) or above is needed for ferromagnetic skyrmions. By reducing the SAF skyrmion drive current by 3 orders, Joule heating is reduced by 6 orders of magnitude. These results pave the way for new SAF interfaces with improved equilibrium, dynamics, and power savings in THE-free skyrmionics.