Browsing by Subject "Electric conductivity"
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Item Open Access Antibacterial electrospun nanofibers from triclosan/cyclodextrin inclusion complexes(Elsevier, 2014) Celebioglu A.; Umu, O. C. O.; Tekinay, T.; Uyar, TamerThe electrospinning of nanofibers (NF) from cyclodextrin inclusion complexes (CD-IC) with an antibacterial agent (triclosan) was achieved without using any carrier polymeric matrix. Polymer-free triclosan/CD-IC NF were electrospun from highly concentrated (160% CD, w/w) aqueous triclosan/CD-IC suspension by using two types of chemically modified CD; hydroxypropyl-beta-cyclodextrin (HPβCD) and hydroxypropyl-gamma-cyclodextrin (HPγCD). The morphological characterization of the electrospun triclosan/CD-IC NF by SEM elucidated that the triclosan/HPβCD-IC NF and triclosan/HPγCD-IC NF were bead-free having average fiber diameter of 520±250nm and 1100±660nm, respectively. The presence of triclosan and the formation of triclosan/CD-IC within the fiber structure were confirmed by 1H-NMR, FTIR, XRD, DSC, and TGA studies. The initial 1:1molar ratio of the triclosan:CD was kept for triclosan/HPβCD-IC NF after the electrospinning and whereas 0.7:1molar ratio was observed for triclosan/HPγCD-IC NF and some uncomplexed triclosan was detected suggesting that the complexation efficiency of triclosan with HPγCD was lower than that of HPβCD. The antibacterial properties of triclosan/CD-IC NF were tested against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. It was observed that triclosan/HPβCD-IC NF and triclosan/HPγCD-IC NF showed better antibacterial activity against both bacteria compared to uncomplexed pure triclosan.Item Open Access Convection-reaction equation based magnetic resonance electrical properties tomography (cr-MREPT)(Institute of Electrical and Electronics Engineers Inc., 2014) Hafalir, F. S.; Oran, O. F.; Gurler, N.; Ider, Y. Z.Images of electrical conductivity and permittivity of tissues may be used for diagnostic purposes as well as for estimating local specific absorption rate distributions. Magnetic resonance electrical properties tomography (MREPT) aims at noninvasively obtaining conductivity and permittivity images at radio-frequency frequencies of magnetic resonance imaging systems. MREPT algorithms are based on measuring the B1 field which is perturbed by the electrical properties of the imaged object. In this study, the relation between the electrical properties and the measured B1 field is formulated for the first time as a well-known convection-reaction equation. The suggested novel algorithm, called 'cr-MREPT,' is based on the solution of this equation on a triangular mesh, and in contrast to previously proposed algorithms, it is applicable in practice not only for regions where electrical properties are relatively constant but also for regions where they vary. The convective field of the convection-reaction equation depends on the spatial derivatives of the B1 field, and in the regions where its magnitude is low, a spot-like artifact is observed in the reconstructed electrical properties images. For eliminating this artifact, two different methods are developed, namely 'constrained cr-MREPT' and 'double-excitation cr-MREPT.' Successful reconstructions are obtained using noisy and noise-free simulated data, and experimental data from phantoms.Item Open Access Current constrained voltage scaled reconstruction (CCVSR) algorithm for MR-EIT and its performance with different probing current patterns(Institute of Physics Publishing, 2003) Birgül, Ö.; Eyüboğlu, B. M.; İder, Y. Z.Conventional injected-current electrical impedance tomography (EIT) and magnetic resonance imaging (MRI) techniques can be combined to reconstruct high resolution true conductivity images. The magnetic flux density distribution generated by the internal current density distribution is extracted from MR phase images. This information is used to form a fine detailed conductivity image using an Ohm's law based update equation. The reconstructed conductivity image is assumed to differ from the true image by a scale factor. EIT surface potential measurements are then used to scale the reconstructed image in order to find the true conductivity values. This process is iterated until a stopping criterion is met. Several simulations are carried out for opposite and cosine current injection patterns to select the best current injection pattern for a 2D thorax model. The contrast resolution and accuracy of the proposed algorithm are also studied. In all simulation studies, realistic noise models for voltage and magnetic flux density measurements are used. It is shown that, in contrast to the conventional EIT techniques, the proposed method has the capability of reconstructing conductivity images with uniform and high spatial resolution. The spatial resolution is limited by the larger element size of the finite element mesh and twice the magnetic resonance image pixel size.Item Open Access Electrochemically obtained insulating and conducting polymers and composites of acrylonitrile(1998) Yilmaz, B.Y.; Akbulut, U.; Toppare L.Electrochemically obtained polyacrylonitrile and a commercial polyacrylonitrile were heat treated to improve their conductivities. The parameters chosen for heat treatment conditions were the temperature, treatment medium (vacuum or air) and, doping agent. The conductivity of all heat treated polymers was measured. The characterization of the heat treated polymers was made by IR analysis. The composite films of polyacrylonitrile with polypyyrole and polythiophene were electrochemically prepared at different compositions. The change in the conductivity of composites was analyzed as a function of the percent composition of the insulating component. IR, DSC, TGA and SEM analyses were used to characterize the polymer composites.Item Open Access Experimental results for 2D magnetic resonance electrical impedance tomography (MR-EIT) using magnetic flux density in one direction(Institute of Physics Publishing, 2003) Birgül, Ö.; Eyüboğlu, B. M.; İder, Y. Z.Magnetic resonance electrical impedance tomography (MR-EIT) is an emerging imaging technique that reconstructs conductivity images using magnetic flux density measurements acquired employing MRI together with conventional EIT measurements. In this study, experimental MR-EIT images from phantoms with conducting and insulator objects are presented. The technique is implemented using the 0.15 T Middle East Technical University MRI system. The dc current method used in magnetic resonance current density imaging is adopted. A reconstruction algorithm based on the sensitivity matrix relation between conductivity and only one component of magnetic flux distribution is used. Therefore, the requirement for object rotation is eliminated. Once the relative conductivity distribution is found, it is scaled using the peripheral voltage measurements to obtain the absolute conductivity distribution. Images of several insulator and conductor objects in saline filled phantoms are reconstructed. The L2 norm of relative error in conductivity values is found to be 13%, 17% and 14% for three different conductivity distributions.Item Open Access Fabrication of supramolecular n/p-nanowires via coassembly of oppositely charged peptide-chromophore systems in aqueous media(American Chemical Society, 2017-07) Khalily, M. A.; Bakan, G.; Kucukoz, B.; Topal, A. E.; Karatay, A.; Yaglioglu, H. G.; Dana, A.; Güler, Mustafa O.Fabrication of supramolecular electroactive materials at the nanoscale with well-defined size, shape, composition, and organization in aqueous medium is a current challenge. Herein we report construction of supramolecular charge-transfer complex one-dimensional (1D) nanowires consisting of highly ordered mixed-stack π-electron donor-acceptor (D-A) domains. We synthesized n-type and p-type β-sheet forming short peptide-chromophore conjugates, which assemble separately into well-ordered nanofibers in aqueous media. These complementary p-type and n-type nanofibers coassemble via hydrogen bonding, charge-transfer complex, and electrostatic interactions to generate highly uniform supramolecular n/p-coassembled 1D nanowires. This molecular design ensures highly ordered arrangement of D-A stacks within n/p-coassembled supramolecular nanowires. The supramolecular n/p-coassembled nanowires were found to be formed by A-D-A unit cells having an association constant (KA) of 5.18 × 105 M-1. In addition, electrical measurements revealed that supramolecular n/p-coassembled nanowires are approximately 2400 and 10 times more conductive than individual n-type and p-type nanofibers, respectively. This facile strategy allows fabrication of well-defined supramolecular electroactive nanomaterials in aqueous media, which can find a variety of applications in optoelectronics, photovoltaics, organic chromophore arrays, and bioelectronics.Item Open Access Frequency responses of ground-penetrating radars operating over highly lossy grounds(IEEE, 2002) Oğuz, U.; Gürel, LeventThe finite-difference time-domain (FDTD) method is used to investigate the effects of highly lossy grounds and the frequency-band selection on ground-penetrating-radar (GPR) signals. The ground is modeled as a heterogeneous half space with arbitrary background permittivity and conductivity. The heterogeneities encompass both embedded scatterers and surface holes, which model the surface roughness. The decay of the waves in relation to the conductivity of the ground is demonstrated. The detectability of the buried targets is investigated with respect to the operating frequency of the GPR, the background conductivity of the ground, the density of the conducting inhomogeneities in the ground, and the surface roughness. The GPR is modeled as transmitting and receiving antennas isolated by conducting shields, whose inner walls are coated with absorbers simulated by perfectly matched layers (PML). The feed of the transmitter is modeled by a single-cell dipole with constant current density in its volume. The time variation of the current density is selected as a smooth pulse with arbitrary center frequency, which is referred to as the operating frequency of the GPR.Item Open Access Highly doped silicon micromachined photonic crystals(IEEE, Piscataway, NJ, United States, 2000) Temelkuran, B.; Bayındır, Mehmet; Özbay, Ekmel; Kavanaugh, J. P.; Sigalas, M. M.; Tuttle, G.Summary form only given. Photonic crystals are periodic structures with the property of reflecting the electromagnetic (EM) waves in all directions within a certain frequency range. These structures can be used to control and manipulate the behaviour of EM waves. Although earlier work concentrated on building these crystals with dielectric materials, there are certain advantages of introducing metals to photonic crystals. First, metals offer a high rejection rate when compared to the dielectric crystals. Second, for microwave applications, the dimensions of metallic crystals can be kept much smaller than the minimum dimensions needed for a typical dielectric crystal. In the paper, we propose a method for the fabrication of layer-by-layer metallic photonic crystals. A similar method had been used by Ozbay et al. to fabricate dielectric photonic crystals using silicon wafers. We fabricated a new layer-by-layer photonic crystal using highly doped silicon wafers.Item Open Access ITO-schottky photodiodes for high-performance detection in the UV-IR spectrum(IEEE, 2004) Bıyıklı, Necmi; Kimukin, I.; Butun, B.; Aytür, O.; Özbay, EkmelHigh-performance vertically illuminated Schottky photodiodes with indium-tin-oxide (ITO) Schottky layers were designed, fabricated, and tested. Ternary and quarternary III-V material systems (AlGaN-GaN, AlGaAs-GaAs, InAlGaAs-InP, and InGaAsP-InP) were utilized for detection in the ultraviolet (UV) (λ < 400 nm), near-IR (λ ∼ 850 nm), and IR (λ ∼ 1550 nm) spectrum. The material properties of thin ITO films were characterized. Using resonant-cavity-enhanced (RCE) detector structures, improved efficiency performance was achieved. Current-voltage, spectral responsivity, and high-speed measurements were carried out on the fabricated ITO-Schottky devices. The device performances obtained with different material systems are compared.Item Open Access Laser-micromachined millimeter-wave photonic band-gap cavity structures(American Institute of Physics, 1995) Özbay, Ekmel; Tuttle, G.; McCalmont, J. S.; Sigalas, M.; Biswas, R.; Soukoulis, C. M.; Ho, K. M.We have used laser-micromachined alumina substrates to build a three-dimensional photonic band-gap crystal. The rod-based structure has a three-dimensional full photonic band gap between 90 and 100 GHz. The high resistivity of alumina results in a typical attenuation rate of 15 dB per unit cell within the band gap. By removing material, we have built defects which can be used as millimeter-wave cavity structures. The resulting quality ~Q! factors of the millimeter-wave cavity structures were as high as 1000 with a peak transmission of 10 dB below the incident signal. © 1995 American Institute of Physics.Item Open Access Many-body effects in the Coulomb drag between low density electron layers(Elsevier Science, 2003) Yurtsever, A.; Moldoveanu, V.; Tanatar, BilalRecent Coulomb drag experiments in low-density double-layer electron systems have the power of distinguishing various many-body formulations of the effective interactions. In this work we theoretically study the correlation effects on the drag resistivity in these systems within various models. The effective inter-layer interactions are best described by the generalization to the double-layer case of the Kukkonen-Overhauser approach which differs significantly from the self-consistent field approach of Singwi et al. [Phys. Rev. 176 (1968) 589]. Following the formulation of Vignale and Singwi [Phys. Rev. B 32 (1985) 2156] we derive an expression for the effective inter-layer interaction which embodies the many-body correlations through the local-field corrections. The drag resistivity is calculated within this approach together with the Hubbard approximation for the intra-layer local-field factor and a simple model for the inter-layer correlations. Comparison with the recent measurements of Kellogg et al. [Solid State Commun. 123 (2002) 515] yields very good agreement. Our results are also contrasted with the corresponding drag resistivities given by the Singwi et al. theory, the dynamic random-phase approximation and the Hubbard approximation. The significant differences found between these theories emphasize the strong sensitivity of the drag resistivity to the effective inter-layer interactions.Item Open Access Microfluidics for reconfigurable electromagnetic metamaterials(AIP Publishing, 2009) Kasirga, T. S.; Ertas, Y. N.; Bayındır, MehmetWe propose microfluidics as a useful platform for reconfigurable electromagnetic metamaterials. Microfluidic split-ring resonators (MF-SRRs) are fabricated inside a flexible elastomeric material by employing rapid prototyping. The transmission measurements performed for mercury-injected MF-SRR exhibits sharp magnetic resonances at microwave wavelengths. We further calculate transmission properties of the MF-SRR array and the effect of electrical conductivity of the liquid inside the channel on the magnetic resonance. The measured results agree well with numerical calculations. Our proposal may open up directions toward switchable metamaterials and reconfigurable devices such as filters, switches, and resonators.Item Open Access On the profile of frequency and voltage dependent interface states and series resistance in (Ni/Au)/Al0.22Ga0.78N/AlN/GaN heterostructures by using current-voltage (I-V) and admittance spectroscopy methods(Elsevier, 2011-06-08) Demirezen, S.; Altindal, S.; Özelik, S.; Özbay, EkmelIn order to explain the experimental effect of interface states (N ss) and series resistance (Rs) of device on the non-ideal electrical characteristics, current-voltage (I-V), capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of (Ni/Au)/Al 0.22Ga0.78N/AlN/GaN heterostructures were investigated at room temperature. Admittance measurements (C-V and G/ω-V) were carried out in frequency and bias voltage ranges of 2 kHz-2 MHz and (-5 V)-(+5 V), respectively. The voltage dependent Rs profile was determined from the I-V data. The increasing capacitance behavior with the decreasing frequency at low frequencies is a proof of the presence of interface states at metal/semiconductor (M/S) interface. At various bias voltages, the ac electrical conductivity (σac) is independent from frequencies up to 100 kHz, and above this frequency value it increases with the increasing frequency for each bias voltage. In addition, the high-frequency capacitance (C m) and conductance (Gm/ω) values measured under forward and reverse bias were corrected to minimize the effects of series resistance. The results indicate that the interfacial polarization can more easily occur at low frequencies. The distribution of Nss and R s is confirmed to have significant effect on non-ideal I-V, C-V and G/ω-V characteristics of (Ni/Au)/Al0.22Ga0.78N/AlN/ GaN heterostructures.Item Open Access The operation of a novel hot electron vertical cavity surface emitting laser(SPIE, 1998) Balkan, N.; O'Brien-Davies, A.; Thoms, A. B.; Potter, R. J.; Poolton, N.; Adams, M. J.; Masum, J.; Bek, Alpan; Serpengüzel, Ali; Aydınlı, Atilla; Roberts, J. S.The hot Electron Light Emission and Lasing in Semiconductor Heterostructures devices (HELLISH-1) is novel surface emitter consisting of a GaAs quantum well, within the depletion region, on the n side of Ga 1-xAlxAs p- n junction. It utilizes hot electron transport parallel to the layers and injection of hot electron hole pairs into the quantum well through a combination of mechanisms including tunnelling, thermionic emission and diffusion of 'lucky' carriers. Super Radiant HELLISH-1 is an advanced structure incorporating a lower distributed Bragg reflector (DBR). Combined with the finite reflectivity of the upper semiconductor-air interface reflectivity it defines a quasi- resonant cavity enabling emission output from the top surface with a higher spectral purity. The output power has increased by two orders of magnitude and reduced the full width at half maximum (FWHM) to 20 nm. An upper DBR added to the structure defines HELLISH-VCSEL which is currently the first operational hot electron surface emitting laser and lases at room temperature with a 1.5 nm FWHM. In this work we demonstrate and compare the operation of UB-HELLISH-1 and HELLISH-VCSEL using experimental and theoretical reflectivity spectra over an extensive temperature range.Item Open Access Oxygenation of carbon nanotubes: Atomic structure, energetics, and electronic structure(American Physical Society, 2003) Dag, S.; Gülseren, O.; Yildirim, T.; Çıracı, SalimThis paper presents an extensive and systematic analysis of the oxygenation of semiconducting and metallic single-wall carbon nanotubes by using the first principles pseudopotential plane wave method. Our study involves the physisorption of oxygen molecules, chemisorption of oxygen atoms and formation of an oxide, and deals with the equilibrium binding geometry and corresponding electronic energy structure. The binding energies of an oxygen molecule physisorbed at different sites are determined by calculating short and long range interactions. The triplet state of the physisorbed oxygen molecule is energetically favorable, whereas the nonmagnetic (spin paired) state yields a relatively stronger binding energy. An oxygen atom is adsorbed on top of the carbon-carbon bond. The zigzag bonds of the nanotubes are weakened and eventually are replaced by a carbon-oxygen-carbon bridge bond. Chemisorption of atomic oxygen and physisorption of an oxygen molecule modify the electronic energy structure of the bare tube in different ways. For a different coverage and pattern, self-consistent field electronic energy structure calculations using the optimized physisorption geometry corresponding to the triplet ground state result in a small energy gap between unoccupied oxygen levels and the top of the valence band of the semiconducting carbon nanotube. These results invalidate the hole doping of the semiconducting carbon nanotube upon the physisorption of oxygen.Item Open Access Practical multi-featured perfect absorber utilizing high conductivity silicon(Institute of Physics Publishing, 2016) Gok, A.; Yilmaz, M.; Bıyıklı, N.; Topallı, K.; Okyay, Ali KemalWe designed all-silicon, multi-featured band-selective perfect absorbing surfaces based on CMOS compatible processes. The center wavelength of the band-selective absorber can be varied between 2 and 22 μm while a bandwidth as high as 2.5 μm is demonstrated. We used a silicon-on-insulator (SOI) wafer which consists of n-type silicon (Si) device layer, silicon dioxide (SiO2) as buried oxide layer, and n-type Si handle layer. The center wavelength and bandwidth can be tuned by adjusting the conductivity of the Si device and handle layers as well as the thicknesses of the device and buried oxide layers. We demonstrate proof-of-concept absorber surfaces experimentally. Such absorber surfaces are easy to microfabricate because the absorbers do not require elaborate microfabrication steps such as patterning. Due to the structural simplicity, low-cost fabrication, wide spectrum range of operation, and band properties of the perfect absorber, the proposed multi-featured perfect absorber surfaces are promising for many applications. These include sensing devices, surface enhanced infrared absorption applications, solar cells, meta-materials, frequency selective sensors and modulators. © 2016 IOP Publishing Ltd.Item Open Access Protein-releasing conductive anodized alumina membranes for nerve-interface materials(Elsevier Ltd, 2016) Altuntas, S.; Buyukserin, F.; Haider, A.; Altinok, B.; Bıyıklı, Necmi; Aslim, B.Nanoporous anodized alumina membranes (AAMs) have numerous biomedical applications spanning from biosensors to controlled drug delivery and implant coatings. Although the use of AAM as an alternative bone implant surface has been successful, its potential as a neural implant coating remains unclear. Here, we introduce conductive and nerve growth factor-releasing AAM substrates that not only provide the native nanoporous morphology for cell adhesion, but also induce neural differentiation. We recently reported the fabrication of such conductive membranes by coating AAMs with a thin C layer. In this study, we investigated the influence of electrical stimulus, surface topography, and chemistry on cell adhesion, neurite extension, and density by using PC 12 pheochromocytoma cells in a custom-made glass microwell setup. The conductive AAMs showed enhanced neurite extension and generation with the electrical stimulus, but cell adhesion on these substrates was poorer compared to the naked AAMs. The latter nanoporous material presents chemical and topographical features for superior neuronal cell adhesion, but, more importantly, when loaded with nerve growth factor, it can provide neurite extension similar to an electrically stimulated CAAM counterpart.Item Open Access Quantum effects of thermal conductance through atomic chains(2001) Ozpineci, A.; Çıracı, SalimWe present a formalism for an atomic scale study of phononic heat transfer. The expression of thermal energy current can be cast in the Landauer form and incorporates the transmission coefficient explicitly. Calculation of the thermal conductance of a monoatomic chain of N atoms between two reservoirs shows interesting quantum features. The conductance density appears as Lorentzian type resonances at the eigenfrequencies of the chain. At low-temperature limit the discrete vibrational frequency spectrum of a "soft" chain may reflect on the thermal conductance by giving rise to a sudden increase. At room temperature, the conductance through a "stiff" chain may oscillate with the number of chain atoms. The obtained quantum features are compared with similar effects found in the quantized electrical conductance.Item Open Access A self-consistent microscopic model of Coulomb interaction in a bilayer system as an origin of Drag Effect Phenomenon(Elsevier B.V., 2008) Güven, K.; Siddiki, A.; Krishna, P. M.; Hakioǧlu T.In this work we implement the self-consistent Thomas-Fermi model that also incorporates a local conductivity model to an electron-electron bilayer system, in order to describe novel magneto-transport properties such as the Drag Phenomenon. The model can successfully account for the poor screening of the potential within the incompressible strips and its impact on the inter-layer Coulomb interaction. An externally applied current in the active layer results in the tilting of the Landau levels and built-up of a Hall potential across the layer, which, in turn, induces a tilted potential profile in the passive layer as well. We investigate the effect of the current intensity, temperature, magnetic field, and unequal density of layers on the self-consistent density and potential profiles of the bilayer system.Item Open Access Simulations of ground-penetrating radars over lossy and heterogeneous grounds(IEEE, 2001) Gürel, Levent; Oğuz, U.The versatility of the three-dimensional (3-D) finite-difference time-domain (FDTD) method to model arbitrarily inhomogeneous geometries is exploited to simulate realistic groundpenetrating radar (GPR) scenarios for the purpose of assisting the subsequent designs of high-performance GPR hardware and software. The buried targets are modeled by conducting and dielectric prisms and disks. The ground model is implemented as lossy with surface roughness, and containing numerous inhomogeneities of arbitrary permittivities, conductivities, sizes, and locations. The impact of such an inhomogeneous ground model on the GPR signal is demonstrated. A simple detection algorithm is introduced and used to process these GPR signals. In addition to the transmitting and receiving antennas, the GPR unit is modeled with conducting and absorbing shield walls, which are employed to reduce the direct coupling to the receiver. Perfectly matched layer absorbing boundary condition is used for both simulating the physical absorbers inside the FDTD computational domain and terminating the lossy and layered background medium at the borders.