Browsing by Subject "Characterization"
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Item Open Access Atomic force microscopy for the investigation of molecular and cellular behavior(Elsevier, 2016-10) Ozkan A.D.; Topal, A. E.; Dana, A.; Güler, Mustafa O.; Tekinay, A. B.The present review details the methods used for the measurement of cells and their exudates using atomic force microscopy (AFM) and outlines the general conclusions drawn by the mechanical characterization of biological materials through this method. AFM is a material characterization technique that can be operated in liquid conditions, allowing its use for the investigation of the mechanical properties of biological materials in their native environments. AFM has been used for the mechanical investigation of proteins, nucleic acids, biofilms, secretions, membrane bilayers, tissues and bacterial or eukaryotic cells; however, comparison between studies is difficult due to variances between tip sizes and morphologies, sample fixation and immobilization strategies, conditions of measurement and the mechanical parameters used for the quantification of biomaterial response. Although standard protocols for the AFM investigation of biological materials are limited and minor differences in measurement conditions may create large discrepancies, the method is nonetheless highly effective for comparatively evaluating the mechanical integrity of biomaterials and can be used for the real-time acquisition of elasticity data following the introduction of a chemical or mechanical stimulus. While it is currently of limited diagnostic value, the technique is also useful for basic research in cancer biology and the characterization of disease progression and wound healing processes.Item Open Access Bi-angular lens for material characterization(IEEE, 1994) Yaralıoğlu, Göksen Göksenin; Atalar, Abdullah; Köymen, HayrettinIn this paper a new lens design is proposed for characterization of layered materials. Lamb wave lens employs Lamb waves for this purpose since these waves propagate along interfaces. However, below cut-off angle, the critical angles of Lamb wave modes are low and the generated V(z) curves have small number of oscillations, which in turn causes measurement difficulties and accuracy degradation. Bi-angular lens described in this paper, generates an extra obliquely incident wave, instead of normally incident beam, in order to provide the reference specular reflection. Simulation results as well as experimental results are presented and it is shown that a high sensitivity can be obtained by using this new lens.Item Open Access Cell-graph mining for breast tissue modeling and classification(IEEE, 2007-08) Bilgin, C.; Demir, Çiğdem; Nagi, C.; Yener, B.We consider the problem of automated cancer diagnosis in the context of breast tissues. We present graph theoretical techniques that identify and compute quantitative metrics for tissue characterization and classification. We segment digital images of histopatological tissue samples using k-means algorithm. For each segmented image we generate different cell-graphs using positional coordinates of cells and surrounding matrix components. These cell-graphs have 500-2000 cells(nodes) with 1000-10000 links depending on the tissue and the type of cell-graph being used. We calculate a set of global metrics from cell-graphs and use them as the feature set for learning. We compare our technique, hierarchical cell graphs, with other techniques based on intensity values of images, Delaunay triangulation of the cells, the previous technique we proposed for brain tissue images and with the hybrid approach that we introduce in this paper. Among the compared techniques, hierarchical-graph approach gives 81.8% accuracy whereas we obtain 61.0%, 54.1% and 75.9% accuracy with intensity-based features, Delaunay triangulation and our previous technique, respectively. © 2007 IEEE.Item Open Access Characterization and imaging with lamb wave lens at gigahertz frequencies(IEEE, 1994-10-11) Bozkurt, Ayhan; Yaralıoğlu, Göksenin; Atalar, Abdullah; Köymen, Hayrettin; Kramer, K.Lamb wave lenses with conical refracting surfaces are fabricated for use at 400 MHz and 1 GHz. The conical surfaces are ground and polished with mechanical means and they are sufficiently smooth for the frequencies of interest. The wide bandwidth of transducers allow frequency tuning necessary for Lamb wave lenses. The fabricated lenses show the expected V(Z) performance. At high frequencies the attenuation in the coupling medium can be very high, but due to the smaller wavelength the resolution is better and defocus distance can be reduced. Inherently higher leaky wave sensitivity of Lamb wave lens enables a good V(Z) characterization ability at higher frequencies as compared to the conventional spherical lens. Subsurface imaging with these Lamb wave lenses gives satisfactory results for layered structures. Chosen object has leaky wave modes within the angular coverage of the lens. The images exhibit a resolution close to the diffraction limit. Experimental V(Z) curves obtained with these lenses along with images are presented.Item Open Access Conducting polymer composites of polythiophene with natural and synthetic rubbers(Elsevier, 1996) Yigit, S.; Hacaloglu, J.; Akbulut, U.; Toppare, L.Electrochemical synthesis of conducting polymer composites of polythiophene was achieved. Synthetic and natural rubbers were used as the insulating polymer matrices. FT-IR, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and mass spectrometry (MS) were utilized to characterize the composite blends. The conductivity measurements were done by using a standard four-probe technique. The above-mentioned methods show that the resultant composites have different properties compared to polythiophene due to interaction of the rubbers with electrochemical polymerization of thiophene, whereas the same argument is not valid for the polypyrrole synthesis via the same procedure.Item Open Access Developing a transducer based on localized surface plasmon resonance (LSPR) of gold nanostructures for nanobiosensor applications(Trans Tech Publications, 2013) Turhan, Adil Burak; Ataman, D.; Çakmakyapan, S.; Mutlu, M.; Özbay, Ekmel; Vlachos, D. S.; Hristoforou, E.In this work, we report the nanofabrication, optical characterization, and electromagnetic modeling of various nanostructure arrays for localized surface plasmon resonance (LSPR) based biosensing studies. Comparison of the experimental results and simulation outputs of various nanostructure arrays was made and a good correspondence was achieved.Item Open Access Development of a rapid-scan fiber-integrated terahertz spectrometer(Springer New York LLC, 2014) Keskin, H.; Altan, H.; Yavas, S.; Ilday, F. O.; Eken, K.; Sahin, A. B.Scientists in terahertz (THz) wave technologies have benefited from the recent developments in ultrafast laser technologies and RF technologies and applied these new gained techniques into characterizing a wide variety of phenomena. Undoubtedly, the most successful of these applications has been in the development of time-domain terahertz spectroscopic and imaging systems which has been utilized in the characterization of dielectrics and semiconductors. This pulsed technique has allowed users to characterize dynamical behavior inside materials under illumination with picosecond resolution. Typically pump/probe or similar dynamical measurements require the use of amplified pulses derived from free-space solid state lasers in the μJ-mJ range and since interferometric techniques are typically used in pulsed measurements the measurement time of a THz spectrum can last at least tens of minutes. Better systems can be realized based on fiber laser technologies. Here we discuss the advantages of a THz spectrometer driven by an ultrafast Ytterbium doped fiber laser whose repetition rate can be tuned rapidly allowing for rapid dynamical measurements. The efficient gain medium, robust operation and compact design of the system opens up the possibility of exploring rapid detection of various materials as well as studying dynamical behavior using the high brightness source.Item Open Access Extracting the temperature distribution on a phase-change memory cell during crystallization(American Institute of Physics Inc., 2016-10) Bakan, G.; Gerislioglu, B.; Dirisaglik, F.; Jurado, Z.; Sullivan, L.; Dana, A.; Lam, C.; Gokirmak A.; Silva, H.Phase-change memory (PCM) devices are enabled by amorphization- and crystallization-induced changes in the devices' electrical resistances. Amorphization is achieved by melting and quenching the active volume using short duration electrical pulses (∼ns). The crystallization (set) pulse duration, however, is much longer and depends on the cell temperature reached during the pulse. Hence, the temperature-dependent crystallization process of the phase-change materials at the device level has to be well characterized to achieve fast PCM operations. A main challenge is determining the cell temperature during crystallization. Here, we report extraction of the temperature distribution on a lateral PCM cell during a set pulse using measured voltage-current characteristics and thermal modelling. The effect of the thermal properties of materials on the extracted cell temperature is also studied, and a better cell design is proposed for more accurate temperature extraction. The demonstrated study provides promising results for characterization of the temperature-dependent crystallization process within a cell.Item Open Access High-performance solar-blind AlGaN photodetectors(SPIE, 2005) Özbay, Ekmel; Tut, Turgut; Bıyıklı, N.Design, fabrication, and characterization of high-performance Al xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA leakage currents at 6 V and 12 V reverse bias were measured on p-i-n and Schottky photodiode samples respectively. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W-1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.15 A/W and 0.11 A/W peak responsivity values at 267 nm and 261 nm respectively. All samples displayed true solar-blind response with cut-off wavelengths smaller than 280 nm. A visible rejection of 4×104 was achieved with Schottky detector samples. High speed measurements at 267 nm resulted in fast pulse responses with >GHz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.Item Open Access Investigation of the adsorption behavior of cesium, barium and phenol onto modified humic acid and iron nanopraticles(Bilkent University, 2007) Çelebi, OğuzhanItem Open Access Linear programming via a quadratic penalty function(Physica - Verlag, 1996) Pınar, M. Ç.We use quadratic penalty functions along with some recent ideas from linear l1 estimation to arrive at a new characterization of primal optimal solutions in linear programs. The algorithmic implications of this analysis are studied, and a new, finite penalty algorithm for linear programming is designed. Preliminary computational results are presented.Item Open Access Nested metamaterials for wireless strain sensing(IEEE, 2009-12-28) Melik, R.; Unal, E.; Perkgoz, N. K.; Santoni, B.; Kamstock, D.; Puttlitz, C.; Demir, Hilmi VolkanWe designed, fabricated, and characterized metamaterial-based RF-microelectromechanical system (RF-MEMS) strain sensors that incorporate multiple split ring resonators (SRRs) in a compact nested architecture to measure strain telemetrically. We also showed biocompatibility of these strain sensors in an animal model. With these devices, our bioimplantable wireless metamaterial sensors are intended, to enable clinicians, to quantitatively evaluate the progression of long-bone fracture healing by monitoring the strain on the implantable fracture fixation hardware in real time. In operation, the transmission spectrum of the metamaterial sensor attached to the implantable fixture is changed when an external load is applied to the fixture, and from this change, the strain is recorded remotely. By employing telemetric characterizations, we reduced the operating frequency and enhanced the sensitivity of our novel nested SRR architecture compared to the conventional SRR structure. The nested SRR structure exhibited a higher sensitivity of 1.09 kHz/kgf operating at lower frequency compared to the classical SRR that demonstrated a sensitivity of 0.72 kHz/kgf. Using soft tissue medium, we achieved the best sensitivity level of 4.00 kHz/kgf with our nested SRR sensor. Ultimately, the laboratory characterization and in vivo biocompatibility studies support further development and characterization of a fracture healing system based on implantable nested SRR.Item Open Access New characterizations of ℓ1 solutions to overdetermined systems of linear equations(Elsevier, 1994) Madsen, K.; Nielsen H. B.; Pınar, M. Ç.New characterizations of the ℓ1 solutions to overdetermined system of linear equations are given. The first is a polyhedral characterization of the solution set in terms of a special sign vector using a simple property of the ℓ1 solutions. The second characterization is based on a smooth approximation of the ℓ1 function using a "Huber" function. This allows a description of the solution set of the ℓ1 problem from any solution to the approximating problem for sufficiently small positive values of an approximation parameter. A sign approximation property of the Huber problem is also considered and a characterization of this property is given. © 1994.Item Open Access Nickel nanoparticles decorated on electrospun polycaprolactone/chitosan nanofibers as flexible, highly active and reusable nanocatalyst in the reduction of nitrophenols under mild conditions(Elsevier, 2017-04) Karakas, K.; Celebioglu A.; Celebi, M.; Uyar, Tamer; Zahmakiran, M.Today, the reduction of nitro aromatics stands a major challenge because of the pollutant and detrimental nature of these compounds. In the present study, we show that nickel(0) nanoparticles (Ni-NP) decorated on electrospun polymeric (polycaprolactone(PCL)/chitosan) nanofibers (Ni-NP/ENF) effectively catalyze the reduction of various nitrophenols (2-nitrophenol, 2,4-dinitrophenol, 2,4,6-trinitrophenol) under mild conditions. Ni-NP/ENF nanocatalyst was reproducibly prepared by deposition-reduction technique. The detailed characterization of these Ni-NP/ENF based nanocatalyst have been performed by using various spectroscopic tools including ICP-OES, P-XRD, XPS, SEM, BFTEM, HRTEM and BFTEM-EDX techniques. The results revealed the formation of well-dispersed nickel(0) NP (dmean = 2.71–2.93 nm) on the surface of electrospun polymeric nanofibers. The catalytic activity of the resulting Ni-NP/ENF was evaluated in the catalytic reduction of nitrophenols in aqueous solution in the presence of sodium borohydride (NaBH4) as reducing agent, in which Ni-NP/ENF nanocatalyst has shown high activity (TOF = 46.2 mol 2-nitrophenol/mol Ni min; 48.2 mol 2,4-dinitrophenol/mol Ni min; 65.6 mol 2,4,6-trinitrophenol/mol Ni min). More importantly, due to the nanofibrous polymeric support, Ni-NP/ENF has shown a flexible characteristics along with reusability property. Testing the catalytic stability of Ni-NP/ENF revealed that this new catalytic material provides high reusability performance (at 3rd reuse 86% for 2-nitrophenol, 83% 2,4-dinitrophenol and 82% 2,4,6-trinitrophenol) for the reduction of nitrophenols even at room temperature and under air. The present study reported here also includes the compilation of wealthy kinetic data for Ni-NP/ENF catalyzed the reduction of nitrophenols in aqueous sodium borohydride solution depending on temperature and type of support material (Al2O3, C, SiO2) to understand the effect of the support material and determine the activation parameters.Item Open Access Non-interior piecewise-linear pathways to l-infinity solutions of overdetermined linear systems(Bilkent University, 1996) Elhedhli, SamirIn this thesis, a new characterization of solutions to overdetermined systems of linear equations is described based on a simple quadratic penalty function, which is used to change the problem into an unconstrained one. Piecewiselinear non-interior pathways to the set of optimal solutions are generated from the minimization of the unconstrained function. It is shown that the entire set of solutions is obtained from the paths for sufficiently small values of a scalar parameter. As a consequence, a new finite penalty algorithm is given for fx, problems. The algorithm is implemented and exhaustively tested using random and function approximation problems. .A comparison with the Barrodale-Phillips algorithm is also done. The results indicate that the new algorithm shows promising performance on random (non-function approximation) problems.Item Open Access One-Step Fabrication of Biocompatible Multifaceted Nanocomposite Gels and Nanolayers(American Chemical Society, 2017) Topuz, F.; Bartneck, M.; Pan, Y.; Tacke, F.Nanocomposite gels are a fascinating class of polymeric materials with an integrative assembly of organic molecules and organic/inorganic nanoparticles, offering a unique hybrid network with synergistic properties. The mechanical properties of such networks are similar to those of natural tissues, which make them ideal biomaterial candidates for tissue engineering applications. Existing nanocomposite gel systems, however, lack many desirable gel properties, and their suitability for surface coatings is often limited. To address this issue, this article aims at generating multifunctional nanocomposite gels that are injectable with an appropriate time window, functional with bicyclononynes (BCN), biocompatible and slowly degradable, and possess high mechanical strength. Further, the in situ network-forming property of the proposed system allows the fabrication of ultrathin nanocomposite coatings in the submicrometer range with tunable wettability and roughness. Multifunctional nanocomposite gels were fabricated under cytocompatible conditions (pH 7.4 and T = 37 °C) using laponite clays, isocyanate (NCO)-terminated sP(EO-stat-PO) macromers, and clickable BCN. Several characterization techniques were employed to elucidate the structure-property relationships of the gels. Even though the NCO-sP(EO-stat-PO) macromers could form a hydrogel network in situ on contact with water, the incorporation of laponite led to significant improvement of the mechanical properties. BCN motifs with carbamate links were used for a metal-free click ligation with azide-functional molecules, and the subsequent gradual release of the tethered molecules through the hydrolysis of carbamate bonds was shown. The biocompatibility of the hydrogels was examined through murine macrophages, showing that the material composition strongly affects cell behavior.Item Open Access A penalty continuation method for the ℓ∞ solution of overdetermined linear systems(Springer Netherlands, 1998) Pınar, M. Ç.; Elhedhli, S.A new algorithm for the ℓ∞ solution of overdetermined linear systems is given. The algorithm is based on the application of quadratic penalty functions to a primal linear programming formulation of the ℓ∞ problem. The minimizers of the quadratic penalty function generate piecewise-linear non-interior paths to the set of ℓ∞ solutions. It is shown that the entire set of ℓ∞ solutions is obtained from the paths for sufficiently small values of a scalar parameter. This leads to a finite penalty/continuation algorithm for ℓ∞ problems. The algorithm is implemented and extensively tested using random and function approximation problems. Comparisons with the Barrodale-Phillips simplex based algorithm and the more recent predictor-corrector primal-dual interior point algorithm are given. The results indicate that the new algorithm shows a promising performance on random (non-function approximation) problems.Item Open Access S-band GaN high power amplifier design and implementation(Bilkent University, 2019-02) Kavuştu, MuhammetHigh power RF Microwave amplifiers are becoming more important as the telecommunications, defense and aerospace industries' demands develop. GaN on SiC technology offers higher power and better form factors for these applications compared to GaAs. In addition, SiC provides better mechanical properties and thermal performance. Design, manufacturing and measurements of a S-Band Power Amplifier by using a GaN discrete bare die transistor are presented. GaN on SiC technology, fabrication process, amplifier fundamentals and design steps are explained in detail. PCB laminate properties, manufacturing, wire bonding and importance of heat management are explained. Design, tapeout, characterization of a fabricated HEMT and its packaging are also mentioned. Power amplifier's small-signal gain of 14.5 dB is measured at center frequency. 41.5 dBm RF power at P6dB is measured at 200 μs pulse width 10% duty cycle at 3 GHz, reaching a power density of 5:4W=mm. Small-signal gain, IP3 measurements under different biases, AMAM and AM-PM distortions are also investigated in detail. EM simulations are performed in Keysight ADS design environment. Amplifier design is based on small-signal and loadpull measurements. De-embedding of fixture effects during HEMT characterization and their models are also investigated. Another hybrid amplifier design by using a packaged commercial GaN on SiC bare die power HEMT is also presented. Small-signal and power measurements are also offered.Item Open Access Sanal anlatıda eksik kılınmış bir portre: temel ve hikâyesi(Geleneksel Yayıncılık, 2007) Gür, N.Bu çalışmada, sözelleştirme geleneği içinde canlılık kazanan ve Temel fıkrası olarak yöresel bir bağlama yerleştirilmiş anlatıların, fıkra türüyle bütünleşmesi, bu bütünlüğün yazının hapsediciliğiyle eşzamanlı olarak varlığını sürdürmesi ve nihayetinde internet ortamında “sanal” paylaşımla buluşması, işlevsellik, söylem ve bağlam açısından incelenmeye çalışılacaktır. Öncelikle, Walter J. Ong’un, Sözlü ve Yazılı Kültür: Sözün Teknolojileşmesi adlı kitabında, sözelleştirme ve zihinsel bellek ilişkisiyle ilgili öne sürdüğü görüşler esas alınarak Temel’in bir “tip” olarak ortaya çıkışı ve sözelleştirme gücüyle yaratılan bu “tip”in, günümüz fıkra anlatılarında nerede yer aldığı üzerinde durulmuştur. Bu noktada, yazılı söylemin hapsediciliği ve sanal iletişimin yaygınlık kazanan kuşatıcılığıyla fıkra türünde meydana gelen çeşitlenme içerisinde, yöresel söylemin sentezi olarak tipleştirilen Temel’in, hâlâ bir hikâyesinin olup olmadığı sorgulanmıştır.Item Open Access Self-assembled peptide nanostructures for functional materials(Institute of Physics Publishing, 2016) Ekiz, M. S.; Cinar, G.; Khalily, M. A.; Güler, Mustafa O.Nature is an important inspirational source for scientists, and presents complex and elegant examples of adaptive and intelligent systems created by self-assembly. Significant effort has been devoted to understanding these sophisticated systems. The self-assembly process enables us to create supramolecular nanostructures with high order and complexity, and peptide-based self-assembling building blocks can serve as suitable platforms to construct nanostructures showing diverse features and applications. In this review, peptide-based supramolecular assemblies will be discussed in terms of their synthesis, design, characterization and application. Peptide nanostructures are categorized based on their chemical and physical properties and will be examined by rationalizing the influence of peptide design on the resulting morphology and the methods employed to characterize these high order complex systems. Moreover, the application of self-assembled peptide nanomaterials as functional materials in information technologies and environmental sciences will be reviewed by providing examples from recently published high-impact studies.