Browsing by Subject "Nanowires."

Now showing 1 - 9 of 9

Open Access
Chalcogenide micro and nanostructures and applications
(2014) Aktaş, Ozan
Chalcogenides, which are glasses consist of S, Se and Te elements, are promising materials for photonics as silicon for modern electronics, due to their extraordinary material properties such as high nonlinearity and wide mid-IR transparency. However, the biggest barrier before their full extend technological exploitation is the difficulty in utilization of these unique material properties within photonic devices with various forms of desired geometries including nanowires, microspheres, and microdisks as necessitated by unique optical functionalities for specific applications, some of which are optical microresonators, modulators, and photodetection devices. In this study, the author explore new routes for the fabrication of on-chip photonic elements with chalcogenides and consider a low cost high-yield production method with a compatible and extendable integration phase. The study illustrates production of chalcogenide optical cavities embedded in a polymer fiber, on-chip integration of the cavities having spherical, spheroidal, and ellipsoidal boundaries, and results of their optical characterizations. Besides the fabrication of active photonic devices with electro-optical capabilities, tapered chalcogenide fibers are also considered as evanescent couplers for the resonators of high index materials. In addition, a large area chalcogenide nanowire based photodetection device is demonstrated including fabrication of photoconductive pixels, design of an electronic readout circuit, development of a custom software for a pattern detection application. Keywords: Chalcogenides glasses, nanowires, optical microresonators, asymmetric resonant cavities, electro-optical Kerr effect, modulators, whispering gallery mode resonators, photonics, fiber drawing.
Open Access
Design and development of novel large scale applications in micro/nanophotonics and nanobiotechnology
(2014) Özgür, Erol
Developments in micro/nanophotonics and nanobiotechnology creates new opportunities regarding development of devices with unprecedented capabilities, which could improve human civilization substantially. On the other hand, a certain level of maturity in transforming these possibilities into reality still requires considerable efforts. One of the main problems of these novel technologies is that their practical know-how is so scarce that they could only be utilized within strictly determined laboratory conditions, and by highly sophisticated scientists. This thesis focuses on large scale applications at the intersection of microphotonics and nanobiotechnology, and also in nanophotonics. On microphotonics side, optical microresonators with toroidal shape were successfully fabricated and optically integrated. Having an extremely high sensitivity towards perturbations in their environments, these microcavities could be used as biological sensors; however, they are also very sensitive for nonspecific interactions. Thus, a novel surface chemistry enabling bioconjugation of molecular probes without compromising their sensitivity and enhancing their selectivity was developed, based on methylphosphonate containing silane modification of the microtoroid surface. After this functionalization, microtoroids were used in biodetection in complex media. Also, a macroscopic photodetection device composed on intrinsically aligned semiconducting selenium nanowires were demonstrated. This device could be considered as a novel and efficient demonstration of nanowire integration to the macroscopic world. Together with the research on biosensors, these are important large scale applications of emergent science of our age.
Open Access
Electronic and atomic processes in nanowires
(1996) Mehrez, Hatem
The variation of conductance of a nanowire which is pulled between two metal electrodes has been the subject of dispute. Recent experimental set-ups using a combination of STM and AFM show that changes in conductivity are closely related with modification of atomic structure. In this thesis electron transport in the nanoindentation and in the connective neck are studied and features of measured conductance are analyzed. Molecular Dynamics simulations of nanowires under tensile stress are carried out to reveal the mechanical properties in nanowires in the course of stretching. A novel type of plcistic deformation, which leads to the formation of bundles with “giant” yield strength is found. An extensive analysis on how abrupt changes in the conductance and the last plateau before the break are related with “quantization phenomena” and atomic structure rearrangements in the neck. By using ab-initio self-consistent field pseudopotential calculations we also investigated electron properties of nanowires and atomic chains and predicted the large yield strength observed in the center of connective neck.
Open Access
Magnetism of transition metal nanowires
(2008) Ataca, Can
In this thesis we investigated structural, electronic and magnetic properties of 3d (light) transition metal (TM) atomic chains and Cr nanowires using firstprinciples pseudopotential plane wave calculations. Infinite periodic linear, dimerized linear and planar zigzag chain structures, as well as their short segments consisting of finite number of atoms and chromium nanowires have been considered. For most of the infinite periodic chains, neither linear nor dimerized linear structures are favored; to lower their energy the chains undergo a structural transformation to form planar zigzag and dimerized zigzag geometries. Dimerization in both infinite and finite chains are much stronger than the usual Peierls distortion and appear to depend on the number of 3d-electrons. As a result of dimerization, a significant energy lowering occurs which, in turn, influences the stability and physical properties. Metallic linear chain of vanadium becomes half-metallic upon dimerization. Infinite linear chain of scandium also becomes half-metallic upon transformation to the zigzag structure. The end effects influence the geometry, energetics and the magnetic ground state of the finite chains. Structure optimization performed using noncollinear approximation indicates significant differences from the collinear approximation. Variation of the cohesive energy of infinite and finite-size chains with respect to the number of 3d-electrons are found to mimic the bulk behavior pointed out by Friedel. Furthermore, we considered Cr nanowires, which have cross section comprising a few (4,5 - 9,12) atoms. Chromium nanowires are found to be in a local minimum in the Born-Oppenheimer surface and are ferrimagnetic metals. The type of coupling, as for ferromagnetic or antiferromagnetic, between neighboring Cr atoms depends on their interatomic distances. The spin-orbit coupling of finite chains are found to be negligibly small for finite molecules and Cr nanowires.
Open Access
Mechanical and electronic properties of metal chain nanowires
(2003) Çakır, Deniz
The fabrication of stable gold monoatomic chains suspended between two gold electrodes is one of the milestones in nanoscience and technology, since miniaturization of the electronic components is one of the great importance in development and improvement of new devices in nanoelectronic. Monoatomic chain nanowires show unusual mechanical and electronic properties such as quantized conductance and much stiff bonds compare to the ones in bulk. Ohnishi et al. [1], has visualized the monoatomic chains by using transmission electron microscopy (TEM). At the same time, Yanson et al [2], have produced the monoatomic chains and they measured its conductance. In the bond length measurement of monoatomic chains, unusually long interatomic lengths have been observed compare to interatomic distances in the bulk and dimer. In order to understand the nature of bonding and unusual structural properties, in this thesis, mechanical and electronic properties of metal chain nanowires are investigated from first principles by using pseudopotential plane wave calculations. Six different metals (Au, Ag, Al, Cu, Pt and Na) are studied in detail. All metals under study show two wire structure which are linear and zigzag structure. Au, Al and Pt show two different zigzag structure. All the wires are metallic. Relative stabilities are investigated by calculating the tension corresponding to apply force to keep the wire at a specific length. Au and Pt have bigger breaking force at breaking point relative to other metallic wires. In this thesis, effect of H, H2 and C impurities on mechanical and electronic properties of Au monoatomic chains are also studied. In wires with H and C impurities, wire under tension break from Au-Au bond away from the impurity. However, wire break from Au-H bond in H2 system. Except from Au-H system, wire become insulator when it contain C or H2 impurities. Before breaking, Au-impurity-Au bond length is in the range of long interatomic distance observed in experiment. So, the presence of an impurity can explain the observed long interatomic distances. However, changing of bond lengths and breaking bond during the stretching of wire depends on the type of impurity. If one stretch the Au-H system, all bond lengths increase in the same amount before breaking. However in Au-C system, Au-Au bond length away from the C impurity increase much more than other bonds. It is shown that absorption of impurity atoms modify the stiffness of the bonds in the wire. This related to the charge transfer from Au to impurity (for H and C). In H and C systems, wire break from Au-Au bond away from the impurity. However in H2 system, wire break from Au-H bond.
Open Access
Silicon and carbon based nanowires
(2004) Tongay, Sefaattin
Nanowires have been an active field of study since last decade. The reduced dimensionality end size allowing electrons can propagate only in one direction has led to quantization which are rather different from the bulk structure. As a result, nanowires having cross section in the range of Broglie wavelength have shown stepwise electrical and thermal conductance, giant Young modulus, stepwise variation of the cross-section etc. Moreover, the atomic structure of nanowires have exhibited interesting regularities which are not known in two or three dimensions. These novel properties of nanowires have been actively explored since last decade in order to find an application in the rapidly developing field of nanotechnology. In the present thesis, we investigated the atomic and electronic structure of a variety of Si and C atom based very thin nanowires starting from linear chain including pentagonal, hexagonal and tubular structures. We found that the C and Si linear chains form double bonds and have high binding energy. Although bulk carbon in diamond structure is an insulator, carbon linear chain is metal and has twice conductance of the gold chain. We carried out an extensive analysis of stability and conductance of the other wires. Our study reveals that Si and C based nanowires generally show metallic properties in spite of the fact that they are insulator or semiconductor when they are in bulk crystal structure. Metallicity occurs due to change in the character and order of bonds.
Open Access
Silicon nanowire-based complex structures : A Large-scale atomistic electronic structure and ballistic transport
(2014) Keleş, Ümit
While the hierarchical assembling as well as the dramatic miniaturization of Si nanowires (NWs) are on-going, an understanding of the underlying physics is of great importance to enable custom design of nanostructures tailored to specific functionalities. This work presents a large-scale atomistic insight into the electronic properties of NW-based complex structures, starting from the subsystem level up to the full assembly, within the framework of pseudopotential-based linear combination of bulk bands method. Laying the groundwork by grasping single Si NWs, we get into a large extent an unexplored territory of NW networks and kinked NWs. As one end product, a versatile estimator is introduced for the band gap and band-edge lineups of multiply-crossing Si NWs that is valid for various diameters, number of crossings, and NW alignments. Aiming for an exploration of the low-lying energy landscape, real space wave function analysis is undertaken for tens of states around band edges which reveal underlying features for a variety of crossings. Predominantly, the valence states spread throughout the network, in contrast the conduction minima are largely localized at the crossings. Given the fact that substantial portion of the band edge shift drives from the confined conduction states, branched Si NWs and nanocrystals have quite close band gap values as the networks of similar wire diameters. Further support to wave function analysis is provided via quantum ballistic transport calculations employing the Kubo-Greenwood formalism. The intriguing localization behaviors are identified, springing mainly at the crossings and kinks of NWs. The ballistic transport edge set apart the conducting extended states from the localized-band gap determining ones. Our findings put forward useful information to realize functionality encoded synthesis of NW-based complex structures, both in the bottom-up and top-down fabrication paradigms.
Open Access
Size and composition modulated superlattices of silicon based nanowires
(2008) Cahangirov, Seymur
Mechanical properties, atomic and energy band structures of bare and hydrogen passivated SinGen nanowire superlattices have been investigated by using firstprinciples pseudopotential plane wave method. Undoped, tetrahedral Si and Ge nanowire segments join pseudomorphically and can form superlattice with atomically sharp interface. Upon heterostructure formation, superlattice electronic states form subbands in momentum space. Band lineups of Si and Ge zones result in multiple quantum wells, where specific states at the band edges and in band continua are confined. The electronic structure of the nanowire superlattice depends on the length and cross section geometry of constituent Si and Ge segments. Also we showed that hydrogen saturated silicon nanowires of different diameters having different band gaps can form stable junctions. Superlattices formed by the periodically repeated junctions of silicon nanowire segments having different lengths and diameters exhibit electronic states which can be confined in regions having either narrow or wide parts of superlattice. A point defect, such as a missing atom or substitutional impurities with localized states near band edges can make modulation doping possible. Since bare Si and Ge nanowires are metallic and the band gaps of hydrogenated ones varies with the diameter, these superlattices offer numerous options for multiple quantum well devices with their leads made from the constituent metallic nanowires. Finally, we have considered the junction between bare and hydrogenated nanowires to realise metalsemiconductor heterostructure. We have treated this heterostructure within the supercell geometry and deduced the formation of Schottky barrier. We have shown that Si and Ge nanowires can bring about a novel concept in nanocircuit, where interconnects, devices etc are produced on a single rode.
Open Access
Thermoelectric efficiency in model nanowires
(2013) Badalov, Sabuhi
Nowadays, the use of thermoelectric semiconductor devices are limited by their low efficiencies. Therefore, there is a huge amount of research effort to get high thermoelectric efficient materials with a fair production value. To this end, one important possibility for optimizing a material’s thermoelectric properties is reshaping their geometry. The main purpose of this thesis is to present a detailed analysis of thermoelectric efficiency of 2 lead systems with various geometries in terms of linear response theory, as well as 3 lead nanowire system in terms of the linear response and nonlinear response theories. The thermoelectric efficiency both in the linear response and nonlinear response regime of a model nanowire was calculated based on Landauer-B¨uttiker formalism. In this thesis, first of all, the electron transmission probability of the system at the hand, i.e. 2 lead or 3 lead systems are investigated by using R-matrix theory. Next, we make use of these electron transmission probability of model systems to find thermoelectric transport coefficients in 2 lead and 3 lead nanowires. Consequently, the effect of inelastic scattering is incorporated with a fictitious third lead in the 3 lead system. The efficiency at maximum power is especially useful to define the optimum working conditions of nanowire as a heat engine. Contrary to general expectation, increasing the strength of inelastic scattering is shown to be a means of making improved thermoelectric materials. A controlled coupling of the nanowire to a phonon reservoir for instance could be a way to increase the efficiency of nanowires for better heat engines.