Browsing by Subject "Conductance"

Now showing 1 - 13 of 13

Open Access
Antibacterial electrospun nanofibers from triclosan/cyclodextrin inclusion complexes
(Elsevier, 2014) Celebioglu A.; Umu, O. C. O.; Tekinay, T.; Uyar, Tamer
The 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.
Open Access
Conductance through atomic contacts created by scanning tunneling microscopy
(Elsevier, 1999) Kiliç, Ç.; Mehrez, H.; Çıracı, Salim; Batra, I. P.
We investigate conductance through contacts created by pressing a hard tip, as used in scanning tunneling microscopy, against substrates. Two different substrates are considered, one a normal metal (Cu) and another a semi-metal (graphite). Our study involves the molecular dynamics simulations for the atomic structure during the growth of the contact, and selfconsistent field electronic structure calculations of deformed bodies. We develop a theory predicting the conductance variations as the tip approaches the surface. We offer an explanation for a quasiperiodic variation of conductance of the contact on the graphite surface, a behavior which is dramatically different from contacts on normal metals.
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.
Open Access
Determination of current transport and trap states density in AlInGaN/GaN heterostructures
(Elsevier, 2019) Arslan, Engin; Ural, Sertaç; Altındal, Ş.; Özbay, Ekmel
The energy distribution and the relaxation time constant of the trap states with respect to conduction bands in the (Ni/Au) Schottky contact on AlInGaN/GaN heterostructures were investigated using the admittance technique. The potential dependent capacitance/conductance measurements were done in the frequency range of 5 kHz to 5 MHz at a temperature of 300 K. We found strong frequency dispersions at the accumulation regions and at the sharp transition regions (depletion region) in the capacitance curves. High frequency dispersion at the accumulation regions in C-V characteristics indicates that there is a high-density of surface traps between the metal–AlInGaN quaternary layer interfaces. Furthermore, the frequency dispersion at the sharp transition regions behavior can be attributed to the interface traps state between the AlInGaN quaternary layer and GaN layer. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming the models in which traps are located between the metal–AlInGaN interface (surface traps) and between AlInGaN/GaN interfaces (interface traps). The trap states density and time constants of the traps states were calculated as a function of energy separation from the conduction-band edge. The trap states' densities change between 1.3 × 1011 eV−1 cm−2 and 6.2 × 1011 eV−1 cm−2. Also, 4.8 to 5.3 μs time interval calculated for the relaxation times.
Open Access
High-conducting magnetic nanowires obtained from uniform titanium-covered carbon nanotubes
(American Physical Society, 2004) Daǧ, Sefa; Durgun, Engin; Çıracı, Salim
We have shown that a semiconducting single-wall carbon nanotube (SWNT) can be covered uniformly by titanium atoms and form a complex but regular atomic structure. The circular cross section changes to a squarelike form, and the system becomes metallic with high state density at the Fermi level and with high quantum ballistic conductance. Metallicity is induced not only by the metal-metal coupling, but also by the band-gap closing of SWNT's at the corners of the square. Even more interesting is that uniform titanium-covered tubes have magnetic ground state with significant net magnetic moment. Our results have been obtained by the first-principles pseudopotential plane-wave calculations within the density-functional theory.
Open Access
Investigation of trap states in AlInN/AlN/GaN heterostructures by frequency-dependent admittance analysis
(Springer US, 2010-09-17) Arslan, Engin; Bütün, Serkan; Şafak, Yasemin; Özbay, Ekmel
We present a systematic study on the admittance characterization of surface trap states in unpassivated and SiN x -passivated Al 0.83In 0.17N/AlN/GaN heterostructures. C-V and G/ω-V measurements were carried out in the frequency range of 1 kHz to 1 MHz, and an equivalent circuit model was used to analyze the experimental data. A detailed analysis of the frequency-dependent capacitance and conductance data was performed, assuming models in which traps are located at the metal-AlInN surface. The density (D t) and time constant (τ t) of the surface trap states have been determined as a function of energy separation from the conduction-band edge (E c - E t). The D st and τ st values of the surface trap states for the unpassivated samples were found to be D st≅ (4 - 13)× 10 12 eV - 1 cm - 2 and τ st ≈ 3 μs to 7 μs, respectively. For the passivated sample, D st decreased to 1.5× 10 12eV - 1cm - 2 and τ st to 1.8 μs to 2 μs. The density of surface trap states in Al 0.83In 0.17N/AlN/GaN heterostructures decreased by approximately one order of magnitude with SiN x passivation, indicating that the SiN x insulator layer between the metal contact and the surface of the Al 0.83In 0.17N layer can passivate surface states.
Open Access
Josephson effect in superconductive SNS heterostructures with barriers
(The American Physical Society, 2003) Cakir, O.; Kulik, I. O.
The dc Josephson effect in a planar superconductor-normal-metal-superconductor (SNS) junction is studied in the existence of a δ barrier in the normal region. The Green function of the structure is obtained by solving the Gorkov equations for the structure and then the current is calculated from the Green functions. The effect of the strength and position of the barrier is investigated. The current shows a weak dependence on the position of the barrier and it is seen to be maximum when the barrier is at the middle of the normal region. Also it is found that the current shows a stronger dependence on the strength of the barrier at low temperatures. A comparative discussion of three possible types of Josephson junctions, the SIS, SCS, and SNS contacts, is presented.
Open Access
Point normal metal-superconductor (NS) contact in nonballistic regime
(World Scientific Publishing, 2003) Askerzade, İ. N.; Kulik, Igor Orestovich
We analyze the point NS contact conductivity taking into account the depression of superconductivity at high-injection current density and Andreev reflection at the adaptive NS boundary. The dependence of the excess current on the voltage, as well as conductivity of contact at arbitrary voltage is obtained.
Open Access
Single-and double-qubit gates by manipulating fourfold degeneracy
(American Physical Society, 2002) Hakioǧlu, Tuğrul; Anderson, James Robert; Wellstood, F. C.
We propose a mechanism for single- and double-qubit state manipulations in quantum computation with an inseparable double-qubit structure. The principle is based on manipulating the degeneracy of the four energy levels. The degeneracy of levels is allowed to be fourfold, doubly degenerate in pairs and nondegenerate, and these configurations are tuned by two coupling parameters. The quantum gate operations are performed at the nondegenerate whereas the data are stored in the degenerate configurations. The fact that the center of the four energy levels is invariant at all parameter values is exploited, and this particularly increases the performance of the controlled operations. A particular realization of the proposed mechanism is suggested by using the recently proposed inductively coupled rf superconducting quantum interference device loops in the macroscopic quantum tunneling regime where the energy eigenlevels are directly connected with the measurable flux states. This very fact also allows precise read-in and read-out operations by measurement of the flux which can also be done by existing techniques. That other physical realizations of the mechanism than that examined here are likely to exist is also discussed briefly.
Open Access
Structure of aluminum atomic chains
(American Physical Society, 2001) Sen, P.; Çıracı, Salim; Buldum, A.; Batra, I. P.
First-principles density-functional calculations reveal that aluminum can form planar chains in zigzag and ladder structures. The most stable one has equilateral triangular geometry with four nearest neighbors; the other stable zigzag structure has wide bond angle and allows for two nearest neighbors. An intermediary structure has the ladder geometry and is formed by two strands. While all these planar geometries are more favored energetically than the linear chain, the binding becomes even stronger in nonplanar geometries. We found that by going from bulk to a chain the character of bonding changes and acquires directionality. The conductance of zigzag and linear chains is 4e2/h under ideal ballistic conditions.
Open Access
Systematic study of adsorption of single atoms on a carbon nanotube
(American Physical Society, 2003) Durgun, Engin; Dag, S.; Bagci, V. M. K.; Gülseren, O.; Yildirim, T.; Çıracı, Salim
We studied the adsorption of single atoms on a semiconducting and metallic single-wall carbon nanotube from first principles for a large number of foreign atoms. The stable adsorption sites, binding energy, and the resulting electronic properties are analyzed. The character of the bonding and associated physical properties exhibit dramatic variations depending on the type of the adsorbed atom. While the atoms of good conducting metals, such as Cu and Au, form very weak bonding, atoms such as Ti, Sc, Nb, and Ta are adsorbed with relatively high binding energy. Most of the adsorbed transition-metal atoms excluding Ni, Pd, and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects, etc.
Open Access
Theoretical analysis of effects of π-conjugating substituents on building blocks for conducting polymers
(American Chemical Society, 1999) Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A.
Geometries of 4-dicyanomethylene-4H-cyclopenta[2,1-b:3,4-b'] dithiophene 1 and its C=O, C=S, C=CH2, C=CF2, and C=C(SR)2 analogues were optimized using density functional theory. Three of the above groups, C=C(CN)2, C=O, and C=S, were also examined on dipyrrole, difuran, dicyclopentadiene, and diborole. Electronic structures were analyzed with respect to their suitability as building blocks for conducting polymers with the natural bond orbital (NBO) method. All bridging groups investigated decrease HOMO-LUMO gaps compared to the unsubstituted parent dimers. Substitution affects HOMO and LUMO energies. Energy gap reduction is caused by a stronger decrease of LUMO energies compared to HOMO energies. The C=S group leads to even smaller energy gaps than the dicyanomethylene group since the HOMO is lowered less in energy with C=S. Compared to unsubstituted dimers, the strongest substituent effects are found with pyrroles and furans. Boroles and thiophenes are least affected. The smallest HOMO-LUMO gaps are obtained for electron-poor systems such as boroles followed by cyclopentadienes. This is analogous to the trend for the unsubstituted parent systems. All of the bridging groups are potential π-acceptors due to their low-lying π*-orbitals, and the corresponding polymers are predicted to be n-dopable. In aromatic structures, the LUMO is localized around the bridging substituent and the coefficients at the α-carbon atoms that reflect electron density are small. This might contribute to the poor conductivity of the n-doped form of poly-1. Electron- poor monomers and polymers tend to switch to quinoid structures. In quinoid repeat units, the HOMO is localized but not as strongly as the LUMO in the aromatic repeat units. The LUMO in quinoid repeat units is delocalized with large coefficients at the α-carbon atoms. Quinoid polymers could therefore be good conductors in the n-doped state.
Open Access
Theoretical study of crossed and parallel carbon nanotube junctions and three-dimensional grid structures
(American Physical Society, 2004) Dag, S.; Senger, R. T.; Çıracı, Salim
This work presents a first-principles study of parallel and crossed junctions of single-wall carbon nanotubes (SWNT). The crossed junctions are modeled by two-dimensional grids of zigzag SWNTs. The atomic and electronic structure, stability, and energetics of the junctions are studied for different magnitudes of contact forces pressing the tubes towards each other and hence inducing radial deformations. Under relatively weak contact forces the tubes are linked with intertube bonds which allow a significant conductance through the junction. These interlinking bonds survive even after the contact forces are released and whole structure is fully relaxed. Upon increasing contact force and radial deformation the tube surfaces are flattened but the interlinking bonds are broken to lead to a relatively wider intertube spacing. The intertube conductance through such a junction diminish because of finite potential barrier intervening between the tubes. The linkage of crossing tubes to form stable junctions is enhanced by a vacancy created at the contact. The three-dimensional grid structure formed by SWNTs is also investigated as a possible framework in device integration.