Researcher: Esfahani, Mohammad Nasr
Loading...
Name Variants
Esfahani, Mohammad Nasr
Email Address
Birth Date
10 results
Search Results
Now showing 1 - 10 of 10
Publication Metadata only Multiscale coupling based on quasicontinuum method in nanowires at finite temperatures(IEEE, 2015) Sonne, Mads Rostgaard; Hattel, Jesper Henri; N/A; Department of Mechanical Engineering; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 115108Nanoelectromechanical systems have been developed for ultra-high frequency oscillators because of their small size and excellent material properties. Using flexural modes and electrothermal features in nanowires for frequency tuning necessitates a sound modeling approach. The quasicontinuum method was developed to link atomistic models with the continuum finite element method in order to study the material behavior across multiple length scales. These significant efforts to develop a continuum theory based on atomistic models have so far been limited to zero temperature. The purpose of this work is to develop the theoretical framework needed to study the mechanical response in nanoscale components such as nanowires at finite temperatures. This is achieved up to a temperature of 1000 K by integrating Engineering Molecular Mechanics and the Cauchy-Born hypothesis. The proposed method is verified with Molecular Dynamics and Molecular Mechanics simulations reported in literature. Bending properties of nanowires at finite temperatures were studied with the proposed method. Thermomechanical properties were investigated by including surface effects.Publication Metadata only Thermo-coupled surface cauchy-born theory: an engineering finite element approach to modeling of nanowire thermomechanical response(Elsevier, 2016) Sonne, M. Rostgaard; Hattel, J. Henri; N/A; Department of Mechanical Engineering; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 115108There are remarkable studies geared towards developing thermomechanical analyses of nanowires based on quasiharmonic and Molecular Dynamics simulations. These methods exhibit limited applicability due to the associated computational cost. In this study an engineering finite-temperature model based on Surface Cauchy-Born theory is developed, where surface energy is accounted for in the prediction of the thermomechanical response. This is achieved by using a temperature-dependent interatomic potential in the standard Cauchy-Born theory with a surface energy contribution. Simultaneous calculation of thermal and mechanical stresses is achieved by eliminating the diagonalization matrix of entropy in the quasiharmonic system. This leads to a reduction in the degrees of freedom by more than 99% in comparison with equivalent Molecular Dynamics models. For the purpose of validation, results obtained on copper and nickel nanowires through the proposed method are compared with those of the more involved Molecular Dynamics simulations. This comparison verifies the significant reduction in the computational process with an acceptable accuracy. Hence, the proposed method provides a promising engineering tool without compromising the underlying physics of the problem and has potential implications in the effective modeling of the nanoscale thermomechanical behavior. (C) 2015 Elsevier Ltd. All rights reserved.Publication Metadata only A deformation-based approach to tuning of magnetic micromechanical resonators(2018) Yalçınkaya, Arda D.; Department of Mechanical Engineering; N/A; Department of Mechanical Engineering; Biçer, Mahmut; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; Researcher; PhD Student; 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; College of Engineering; N/A; N/A; 115108Resonance frequency tuning in magnetic micromechanical resonators remains a primary field of study for frequency reference applications. The use of magnetic micromechanical resonators for innovative timing, oscillator and sensing applications necessitates a platform for the precise control of the resonance frequency. The present work addresses a deformation based technique for tuning the resonance frequency of nickel micromechanical resonators. Frequency response is measured through magnetic actuation and optical readout. The tuning approach is based on a combination of flexural deformation and uniaxial strain. The bending deformation is achieved by using a DC current through the microbeam. This magnetomotive mechanism reduces the resonance frequency by about 13% for a maximum DC current of 80 mA. A substrate bending method is used for applying uniaxial strain to increase the resonance frequency by about 8%. A bidirectional frequency modulation is thus demonstrated by utilizing both deformation techniques. The interpretation of results is carried out by finite element analysis and electromechanical analogy in an equivalent circuit. Using deformation techniques, this study provides a rigorous approach to control the resonance frequency of magnetic micromechanical resonators.Publication Metadata only A monolithic approach to downscaling silicon piezoresistive sensors(Ieee-Inst Electrical Electronics Engineers Inc, 2017) Leblebici, Yusuf; N/A; Department of Mechanical Engineering; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; PhD Student; Faculty Member; Department of Mechanical Engineering; Koç University Surface Science and Technology Center (KUYTAM) / Koç Üniversitesi Yüzey Teknolojileri Araştırmaları Merkezi (KUYTAM); Graduate School of Sciences and Engineering; College of Engineering; N/A; 115108Multi-scale integration remains the primary challenge in the fabrication of miniature piezoresistive sensors, as the co-fabrication of a silicon nanowire along with a microscale shuttle is the main architecture facilitating high-sensitivity transduction. The efforts in this field are marred by the lack of batch techniques compatible with semiconductor manufacturing. A technology is introduced here that leads to the fabrication of a piezoresistive silicon nanowire sharing the same single-crystalline device layer of a thick silicon-on-insulator wafer as the microscale component. The approach is based on a combination of high-resolution lithography with a two-stage etching process. The demonstration is carried out by spanning an electrostatic comb-drive actuator and a micromechanical amplifier by a single nanowire. A gage factor range of 135-145 is obtained, corresponding to an almost 20% resistance change for a nanowire strain of 1.26 x 10(-3). The technique is shown to generate a two-order-of-magnitude scale difference within the same silicon crystal. It also provides ease of electrical access to the nanowire, as the nanowire does not remain buried underneath the thick micromechanical system. With the associated lack of high-temperature processes and its CMOS-compatibility, the technique is a promising enabler for future miniaturized piezoresistive sensors. [2017-0007]Publication Metadata only Selecting the optimum engineering model for the frequency response of fcc nanowire resonators(Elsevier Science Inc, 2017) Sonne, M. R.; Hattel, J. H.; N/A; N/A; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; PhD Student; PhD Student; Faculty Member; 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; N/A; N/A; 115108The full potential of the nanoelectromechanical systems, NEMS, as one of the leading examples among the new-generation sensing technologies, is yet to be realized. One of the main challenges on the road is the mechanical modeling of their core elements, the tiny mechanical building blocks such as the nanowire resonators. The success of the engineering design of such miniaturized systems will depend heavily on the availability of accurate mechanistic models with the least possible computational cost. Although a variety of models are available for this purpose, the boundaries between their admissible domains remain rather vague. For example, analytical approaches including EUler-Bernoulli and Timoshenko beam theories provide closed-form solutions and work reasonably well for moderate nanowire geometries, and hence, they are frequently utilized in the literature. However, their validity in the case of extreme surface-to-volume ratios remains questionable. Classical finite element method can partially be used to address these deficiencies. on the other hand, molecular dynamics provide accurate results, while nanowire geometries studied with this computationally demanding technique usually remain confined to dimensions below those of practical interest. To address these issues, a benchmarking study among analytical and numerical techniques is carried out, where Surface Cauchy Born theory serves as the reference. Using gold nanowires with different dimensions and boundary conditions, it is observed that analytical models are applicable within a length to-thickness ratio range of 7-11 in the fixed-fixed configuration, whereas they can be used safely within a length-to-thickness ratio range of less than 25 in the fixed-free configuration. Deviations as high as 50% are encountered for length-to-thickness ratios exceeding 11 for both the analytical approach and the classical finite element method in the fixed-fixed structure. The deviations are quantitatively linked to the dominance of the surface effect through the use of the Surface Cauchy-Born model. For length-to-thickness ratios less than 7, the lack of cross-sectional deformations in analytical treatment is also observed to lead to high deviations for the fixed-fixed configuration through the comparison with higher order beam theories. Results are verified with silver nanowires as well. The work provides a guideline for selecting the optimum mechanical model given the nanowire resonator dimensions and boundary conditions. (C) 2016 Elsevier Inc. All rights reserved.Publication Metadata only Superplastic behavior of silica nanowires obtained by direct patterning of silsesquioxane-based precursors(Iop Publishing Ltd, 2017) Wollschlaeger, Nicole; Oesterle, Werner; Leblebici, Yusuf; N/A; N/A; Department of Mechanical Engineering; Yılmaz, Mustafa Akın; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; College of Engineering / 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; 115108Silica nanowires spanning 10 mu m-deep trenches are fabricated from different types of silsesquioxane-based precursors by direct e-beam patterning on silicon followed by release through deep reactive ion etching. Nanowire aspect ratios as large as 150 are achieved with a critical dimension of about 50 nm and nearly rectangular cross-sections. In situ bending tests are carried out inside a scanning electron microscope, where the etch depth of 10 mu m provides sufficient space for deformation. Silica NWs are indeed observed to exhibit superplastic behavior without fracture with deflections reaching the full etch depth, about two orders of magnitude larger than the nanowire thickness. A large-deformation elastic bending model is utilized for predicting the deviation from the elastic behavior. The results of forty different tests indicate a critical stress level of 0.1-0.4 GPa for the onset of plasticity. The study hints at the possibility of fabricating silica nanowires in a monolithic fashion through direct e-beam patterning of silsesquioxane-based resins. The fabrication technology is compatible with semiconductor manufacturing and provides silica nanowires with a very good structural integrity.Publication Metadata only Monolithic technology for silicon nanowires in high-topography architectures(Elsevier, 2017) Wollschlager, Nicole; Rangelow, Ivo W.; Leblebici, Yusuf; Department of Mechanical Engineering; Esfahani, Mohammad Nasr; Yılmaz, Mustafa Akın; Alaca, Burhanettin Erdem; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; 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; 115108Integration of silicon nanowires (Si NWs) in three-dimensional (3D) devices including integrated circuits (ICs) and microelectromechanical systems (MEMS) leads to enhanced functionality and performance in diverse applications. The immediate challenge to the extensive use of Si NWs in modern electronic devices is their integration with the higher-order architecture. Topography-related limits of integrating Si NWs in the third dimension are addressed in this work. Utilizing a well-tuned combination of etching and protection processes, Si NWs are batch-produced in bulk Si with an extreme trench depth of 40 gm, the highest trench depth obtained in a monolithic fashion within the same Si crystal so far. The implications of the technique for the thick silicon-on-insulator (S01) technology are investigated. The process is transferred to SOI wafers yielding Si NWs with a critical dimension of 100 nm along with a trench aspect ratio of 50. Electrical measurements verify the prospect of utilizing such suspended Si NWs spanning deep trenches as versatile active components in ICs and MEMS. Introducing a new monolithic approach to obtaining Si NWs and the surrounding higher-order architecture within the same SOI wafer, this work opens up new possibilities for modem sensors and power efficient ICs. (C) 2017 Elsevier B.V. All rights reserved.Publication Metadata only Piezoresistivity characterization of silicon nanowires through monolithic MEMS(Institute of Electrical and Electronics Engineers (IEEE), 2017) Leblebici, Yusuf; N/A; Department of Mechanical Engineering; Esfahani, Mohammad Nasr; Alaca, Burhanettin Erdem; PhD Student; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; College of Engineering; N/A; 115108This paper presents a monolithic approach for the integration of silicon nanowires (Si NWs) with microelectromechanical systems (MEMS). The process is demonstrated for the case of co-fabrication of Si NWs with a 10-μm-Thick MEMS on the same silicon-on-insulator (SOI) wafer. MEMS is designed in the form of a characterization platform with an electrostatic actuator and a mechanical amplifier spanned by a single Si NW. This integrated platform is utilized for the successful measurement of Si NW piezoresistive gauge factor (GF) under a uniform uniaxial stress. Available techniques in this field include: i) Indirect (substrate) or direct (actuator) bending of Si NW necessitating rigorous models for the conversion of load to stress, ii) nanomanipulation and attachment of Si NW on MEMS, a non-monolithic technique posing residual stress and alignment issues, and iii) heterogeneous integration with separate Si layers for Si NW and MEMS, where a single SOI is not sufficient for the end product. Providing a monolithic solution to the integration of micro and nanoscale components, the presented technique successfully addresses the shortcomings of similar studies. In addition to providing a solution for electromechanical characterization, the technique also sets forth a promising pathway for multiscale, functional devices produced in a batch-compatible fashion, as it facilitates co-fabrication within the same Si crystal.Publication Metadata only Piezoresistive silicon nanowire resonators as embedded building blocks in thick SOI(Iop Publishing Ltd, 2018) Karakan, M. Çağatay; Orhan, Ezgi; Hanay, M. Selim; Leblebici, Yusuf; N/A; N/A; Department of Mechanical Engineering; Esfahani, Mohammad Nasr; Kılınç, Yasin; Alaca, Burhanettin Erdem; PhD Student; PhD Student; Faculty Member; Department of Mechanical Engineering; 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 Engineering; N/A; N/A; 115108The use of silicon nanowire resonators in nanoelectromechanical systems for new-generation sensing and communication devices faces integration challenges with higher-order structures. Monolithic and deterministic integration of such nanowires with the surrounding microscale architecture within the same thick crystal is a critical aspect for the improvement of throughput, reliability and device functionality. A monolithic and IC-compatible technology based on a tuned combination of etching and protection processes was recently introduced yielding silicon nanowires within a 10 mu m-thick device layer. Motivated by its success, the implications of the technology regarding the electromechanical resonance are studied within a particular setting, where the resonator is co-fabricated with all terminals and tuning electrodes. Frequency response is measured via piezoresistive readout with frequency down-mixing. Measurements indicate mechanical resonance with frequencies as high as 100 MHz exhibiting a Lorentzian behavior with proper transition to nonlinearity, while Allan deviation on the order of 3-8 ppm is achieved. Enabling the fabrication of silicon nanowires in thick silicon crystals using conventional semiconductor manufacturing, the present study thus demonstrates an alternative pathway to bottom-up and thin silicon-on-insulator approaches for silicon nanowire resonators.Publication Metadata only A numerical simulation for the stress effect in flexural micro/nano electromechanical resonators(American Scientific Publishers, 2015) N/A; N/A; N/A; Department of Mechanical Engineering; Yılmaz, Mustafa Akın; Esfahani, Mohammad Nasr; Biçer, Mahmut; Alaca, Burhanettin Erdem; PhD Student; PhD Student; Researcher; Faculty Member; Department of Mechanical Engineering; Graduate School of Sciences and Engineering; Graduate School of Sciences and Engineering; N/A; College of Engineering; N/A; N/A; N/A; 115108Resonance frequencies and quality factors of micro/nano electromechanical resonators are known to differ significantly from target values in the presence of intrinsic stresses. This stress effect is modeled for a two-port system with electrostatic actuation and capacitive read-out. A methodology is proposed to compute equivalent electrical parameters for a double-clamped beam resonator under stress. The model is verified with finite element analysis, and a number of case studies are conducted in addition. Increase in resonance frequency with increasing intrinsic tensile stress is observed under mechanical and electrical effects, while a deterioration of quality factor is evident in cases with pronounced parasitic effects. Related challenges associated with the transition to the nanoscale are computationally captured. Finally, a short formulation is provided with relevant error margins for the direct estimation of equivalent circuit parameters. The proposed approach serves as a useful tool for layout design, where all involved dimensions are considered in addition to operational variables such as bias voltage and unloaded quality factor.