Researcher:
Zarepakzad, Sina

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PhD Student

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Sina

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Zarepakzad

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Zarepakzad, Sina

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2021 - 20234

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Researcher Of Publications: search.filters.isAuthorOfPublication.ce19c17e-97c4-4afc-a068-9cc175db1f85

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    A new characterization approach to study the mechanical behavior of silicon nanowires
    (Springer, 2021) Esfahani, Mohammad Nasr; Taşdemir, Zuhal; Wollschlaeger, Nicole; Li, XueFei; Li, Taotao; Leblebici, Yusuf; N/A; N/A; Department of Mechanical Engineering; Zarepakzad, Sina; Yılmaz, Mustafa Akın; Alaca, Burhanettin Erdem; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; N/A; N/A; 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; College of Engineering; N/A; N/A; 115108
    This work proposes a new approach to characterize the mechanical properties of nanowires based on a combination of nanomechanical measurements and models. Silicon nanowires with a critical dimension of 90 nm and a length of 8 mu m obtained through a monolithic process are characterized through in-situ three-point bending tests. A nonlinear nanomechanical model is developed to evaluate the mechanical behavior of nanowires. In this model, the intrinsic stress and surface parameters are examined based on Raman spectroscopy measurements and molecular dynamics simulations, respectively. This work demonstrates a new approach to measure the mechanical properties of Si nanowires by considering the surface effect and intrinsic stresses. The presented technique can be used to address the existing discrepancies between numerical estimations and experimental measurements on the modulus of elasticity of silicon nanowires.
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    The role of native oxide on the mechanical behavior of silicon nanowires
    (Elsevier, 2023) Nasr Esfahani, Mohammad; Department of Mechanical Engineering; N/A; Alaca, Burhanettin Erdem; Zarepakzad, Sina; Faculty Member; PhD Student; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); N/A; College of Engineering; Graduate School of Sciences and Engineering; 115108; N/A
    Molecular dynamics simulations are employed to study the effect of native oxide on the size-dependent mechanical properties of silicon nanowires. Despite their immense potential as essential building blocks in nanoelectromechanical systems, mechanical behavior of silicon nanowires still needs further attention for a full understanding. The leading source of ambiguity can be traced back to the fact that the presence of native oxide on silicon nanowire surfaces is ignored when interpreting nanomechanical test data, when it comes, for example, to converting force and deflection measurements to stress and strain. This problem needs immediate attention, because, first, nanowires have a significant surface area, and second, native oxide is the dominant surface state. With prior work reporting conflicting dimensional and computational viewpoints regarding the effect of native oxide on silicon nanowires properties, size dependence of nanowire mechanical properties is investigated here with great attention placed on critical size and atomistic simulation perspectives. For this purpose, Tersoff-Munetoh and modified Stillinger–Weber potentials are employed in this intensive computational study to address the influence of size and crystal orientation on nanowire elastic behavior and tensile strength. As a result, a striking set of differences is obtained. First, the presence of native oxide layer is observed to decrease both the modulus of elasticity and the ultimate strength. The reduction in the modulus of elasticity is observed to be as much as 30% and 40% for <100> and <110>-oriented nanowires, respectively. Similarly, the reduction in the ultimate strength is estimated to be as much as 20% using the modified Stillinger–Weber potential, which proved to be more suitable for strength analysis compared to Tersoff-Munetoh potential. Finally, the failure behavior is studied through the ductile failure probability calculations, where a higher size-dependent failure probability is observed for decreasing nanowire width upon oxidation. These results shed light on the background of existing inconsistencies between experimental and numerical findings in the literature, as opposing trends for silicon nanowire stiffness and strength were reported with decreasing size. The study provides a guideline to quantify the scale effect in silicon nanowire mechanical behavior as a combined outcome of oxide thickness, nanowire size and crystal orientation and thus to reduce the extent of uncertainties originating from inadequate interpretation of nanomechanical test data.
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    PublicationOpen Access
    Molecular dynamics study of orientation-dependent tensile properties of Si nanowires with native oxide: surface stress and surface energy effects
    (Institute of Electrical and Electronics Engineers (IEEE), 2021) Esfahani, Mohammad Nasr; Department of Mechanical Engineering; Alaca, Burhanettin Erdem; Zarepakzad, Sina; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; Graduate School of Sciences and Engineering; 115108; N/A
    Molecular dynamics (MD) simulations are employed to investigate the influence of native oxide layer on the mechanical properties of Si nanowires (NWs) through analyzing surface stress and surface energy effect. This work studies the tensile response of Si NWs along <100> and <110> crystal orientations. MD results are compared with the traditional core-shell model on the estimation of the modulus of elasticity of Si NWs with a native oxide layer. Density functional theory (DFT) methods are used to verify MD results on the surface energy calculations. Surface stress and surface elastic constants are studied for native oxide surface using MD simulations and compared with unreconstructed surfaces. In this work, the role of native oxide is addressed to understand the difference between experimental and computational findings on the modulus of elasticity of Si NWs.
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    PublicationOpen Access
    Effect of native oxide on stress in silicon nanowires: implications for nanoelectromechanical systems
    (American Chemical Society (ACS), 2022) Esfahani, Mohammad Nasr; Li, Taotao; Li, XueFei; Tasdemir, Zuhal; Wollschlaeger, Nicole; Leblebici, Yusuf; Department of Mechanical Engineering; Alaca, Burhanettin Erdem; Zarepakzad, Sina; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); College of Engineering; Graduate School of Sciences and Engineering; 115108; N/A
    Understanding the origins of intrinsic stress in Si nanowires (NWs) is crucial for their successful utilization as transducer building blocks in next-generation, miniaturized sensors based on nanoelectromechanical systems (NEMS). With their small size leading to ultrahigh-resonance frequencies and extreme surface-to-volume ratios, silicon NWs raise new opportunities regarding sensitivity, precision, and speed in both physical and biochemical sensing. With silicon optoelectromechanical properties strongly dependent on the level of NW intrinsic stress, various studies have been devoted to the measurement of such stresses generated, for example, as a result of harsh fabrication processes. However, due to enormous NW surface area, even the native oxide that is conventionally considered as a benign surface condition can cause significant stresses. To address this issue, a combination of nanomechanical characterization and atomistic simulation approaches is developed. Relying only on low-temperature processes, the fabrication approach yields monolithic NWs with optimum boundary conditions, where NWs and support architecture are etched within the same silicon crystal. Resulting NWs are characterized by transmission electron microscopy and micro-Raman spectroscopy. The interpretation of results is carried out through molecular dynamics simulations with ReaxFF potential facilitating the incorporation of humidity and temperature, thereby providing a close replica of the actual oxidation environment-in contrast to previous dry oxidation or self-limiting thermal oxidation studies. As a result, consensus on significant intrinsic tensile stresses on the order of 100 MPa to 1 GPa was achieved as a function of NW critical dimension and aspect ratio. The understanding developed herein regarding the role of native oxide played in the generation of NW intrinsic stresses is important for the design and development of silicon-based NEMS.