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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 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 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.Publication Metadata only Metal-support interaction in PT nanodisk-carbon nitride catalyst: insight from theory and experiment(MDPI, 2024) Doustkhah, Esmail; Kotb, Ahmed; Assadi, Mohammad Hussein Naseef; Balkan, Timuçin; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM)Metal-support interaction plays a critical role in determining the eventual catalytic activity of metals loaded on supporting substrates. This interaction can sometimes cause a significant drop in the metallic property of the loaded metal and, hence, a drop in catalytic activity in the reactions, especially in those for which low charge carrier transfer resistance is a necessary parameter. Therefore, there should be a case-by-case experimental or theoretical (or both) in-depth investigation to understand the role of support on each metal. Here, onto a layered porous carbon nitride (g-CN), we grew single crystalline Pt nanodisks (Pt@g-CN) with a lateral average size of 21 nm, followed by various characterisations such as electron microscopy techniques, and the measurement of electrocatalytic activity in the O-2 reduction reaction (ORR). We found that intercalating Pt nanodisks in the g-CN interlayers causes an increase in electrocatalytic activity. We investigated the bonding mechanism between carbon support and platinum using density functional theory and applied the d-band theory to understand the catalytic performance. Analysis of Pt's density of states and electronic population across layers sheds light on the catalytic behaviour of Pt nanoparticles, particularly in relation to their thickness and proximity to the g-CN support interface. Our simulation reveals an optimum thickness of similar to 11 angstrom, under which the catalytic performance deteriorates.Publication Metadata only Active sites and their individual turnover frequencies for ethylene hydrogenation on reduced graphene aerogel(Amer Chemical Soc, 2024) Department of Chemistry;Department of Chemical and Biological Engineering; Yalçın, Kaan; Öztulum, Samira Fatma Kurtoğlu; Öztuna, Feriha Eylül Saraç; Kanat, Gizem Hasibe; Ünal, Uğur; Uzun, Alper; 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 Sciences; College of EngineeringGraphene aerogel (GA) was reduced at various temperatures to prepare a series of reduced graphene aerogels (rGAs) with different surface characteristics. Detailed characterization demonstrated that an increase in the thermal reduction temperature leads to an increase in surface area accompanied by an increase in surface density of defect sites formed by the removal of the oxygen-containing functional groups. rGA samples were then tested for ethylene hydrogenation under identical conditions. A comparison of catalytic performances of each catalyst demonstrated that the rGA sample prepared by reduction in Ar at 900 degrees C (rGA-900) provides the highest performance compared with others prepared at lower temperatures. Next, we analyzed the per-gram activity of each catalyst as a sum of individual contributions from different defect sites quantified by Raman spectroscopy and CHNS-O analysis to determine the individual turnover frequencies (TOFs) of each active site. This analysis identified polyene-like structures and interstitial defects associated with amorphous sp(2) bonded carbon atoms as the dominant active sites responsible for hydrogenation. A comparison of their TOFs further indicated that the polyene-like structures provide approximately ten times higher TOF compared to those associated with the amorphous carbon defects. These results, identifying the dominant active centers and quantifying their corresponding TOFs, provide opportunities toward the rational design of GA-based carbocatalysts.Publication Metadata only An analytical-atomistic model for elastic behavior of silicon nanowires(IOP Publishing Ltd, 2024) Esfahani, Mohammad Nasr; Department of Mechanical Engineering; Zarepakzad, Sina; Alaca, Burhanettin Erdem; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced Research; Graduate School of Sciences and Engineering; College of EngineeringSilicon nanowires entail significant potential as sensors in nanoelectromechanical systems. Despite its crucial impact in such applications, inconsistent trends in mechanical behavior reported in computational and experimental studies remain unexplained. Hence, scale effect in even the most fundamental elastic properties requires clarification. This work introduces a multiscale model to bridge the existing gap between atomistic simulations and experimental observations encountered around a critical dimension of 10 nm. The combined approach of this work is based on molecular dynamics and modified core-shell model and captures the scale effect over a substantial size range. The evolution of the modulus of elasticity is thus studied and linked to nanowire critical dimension through the parameterization of surface inhomogeneity. The developed method is also validated through an analysis of native oxide revealing an average modulus of elasticity of 75 GPa. The method's applicability can be extended to similar one-dimensional structures with unique surface states.Publication Metadata only Step-edge decoration and clustering of Pt atoms on a Cu(211) stepped surface(Amer Chemical Soc, 2024) Department of Chemistry; Mohammadpour, Amin; Kaya, Sarp; Department of Chemistry; Koç University Tüpraş Energy Center (KUTEM) / Koç Üniversitesi Tüpraş Enerji Merkezi (KÜTEM); Graduate School of Sciences and Engineering; College of SciencesThe atomic manipulation of the low-coordination sites of metal catalysts can give rise to activity enhancement;however, it is rather challenging to locally probe the dynamic changes and activities of these sites. Herein, step-edge/terrace site decoration and site exchange of Pt atoms with a stepped Cu(211) surface were investigated by a combination of infrared reflection absorption spectroscopy (IRRAS) and temperature-programmed desorption (TPD) of carbon monoxide (CO). For a low coverage of Pt, step decoration and site exchange with Cu were found to be two pathways to isolate Pt as single atoms. CO preferentially adsorbs near the Cu step sites on the lower terrace, and the binding energies of CO show strong Pt coverage dependence. The presence of Pt on terrace and step sites modifies the binding energy of CO absorbed on Cu in the proximity. Increased Pt-Pt lateral coordination changes the site preference;however, the reduced binding energy of CO to Pt is attributed to heteroatom bond formation rather than the strain induced by the lattice mismatch.Publication Metadata only Correction: anticancer drug doxorubicin (DOX) loading performance of functionalized polyaniline (PANI) surface with active carbon (journal of materials science, (2023), 58, 11, (4726-4738), 10.1007/s10853-023-08291-z)(Springer, 2023) Can, Musa Mutlu; Kalindemirtas, Ferdane Danisman; Erdemir, Gokce; Kuruca, Durdane Serap; Kaneko, Satoru; Aktas, Zerrin; Oncul, Oral; Shawuti, Shalima; n2STAR-Koç University Nanofabrication and Nanocharacterization Center for Scientifc and Technological Advanced ResearchThe following acknowledgment was omitted from the original article: ‘‘The authors thank A. R. Sherwani for technical assistance with sample preparation during his work as a Ph.D. student.’’ © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2023.