Evanescent-wave coupled right angled buried waveguide: Applications in carbon nanotube mode-locking

In this paper we present a simple but powerful subgraph sampling primitive that is applicable in a variety of computational models including dynamic graph streams (where the input graph is defined by a sequence of edge/hyperedge insertions and deletions) and distributed systems such as MapReduce. In the case of dynamic graph streams, we use this primitive to prove the following results: -- Matching: First, there exists an $\tilde{O}(k^2)$ space algorithm that returns an exact maximum matching on the assumption the cardinality is at most $k$. The best previous algorithm used $\tilde{O}(kn)$ space where $n$ is the number of vertices in the graph and we prove our result is optimal up to logarithmic factors. Our algorithm has $\tilde{O}(1)$ update time. Second, there exists an $\tilde{O}(n^2/\alpha^3)$ space algorithm that returns an $\alpha$-approximation for matchings of arbitrary size. (Assadi et al. (2015) showed that this was optimal and independently and concurrently established the same upper bound.) We generalize both results for weighted matching. Third, there exists an $\tilde{O}(n^{4/5})$ space algorithm that returns a constant approximation in graphs with bounded arboricity. -- Vertex Cover and Hitting Set: There exists an $\tilde{O}(k^d)$ space algorithm that solves the minimum hitting set problem where $d$ is the cardinality of the input sets and $k$ is an upper bound on the size of the minimum hitting set. We prove this is optimal up to logarithmic factors. Our algorithm has $\tilde{O}(1)$ update time. The case $d=2$ corresponds to minimum vertex cover. Finally, we consider a larger family of parameterized problems (including $b$-matching, disjoint paths, vertex coloring among others) for which our subgraph sampling primitive yields fast, small-space dynamic graph stream algorithms. We then show lower bounds for natural problems outside this family

Single-Walled Carbon Nanotube Arrays for High Frequency Applications

This dissertation presents a thorough analysis of semiconducting Single-Walled Carbon Nanotube-based devices, followed by a test structure fabrication and measurements. The analysis starts by developing an individual nanotube model, which is then generalized for many nanotubes and adding the parasitic elements. The parasitic elements appear when forming the device electrodes degrade the overall performance. The continuum model of an individual nanotube is developed. A unique potential function is presented to effectively describe the electron distribution in the carbon nanotube subsequently facilitating solving Schrödinger\u27s equation to obtain the energy levels, and to generalize the model for many nanotubes. It is shown that the overall energy band gap is inversely proportional to the number of nanotubes due to the coupling between the nanotubes. The coupling is then enhanced by applying an external transverse electric field, which controls the energy band gap. The electric field is represented as a function of the number of nanotubes per device showing that the higher the number of nanotubes, the lower the value of the electric field needed to alter the energy band gap. An electromagnetic model is developed for the contact where a detailed parametric study of the length, thickness, and conductivity of the contact area is presented. The overlap length between the nanotube and the metal of the contact appears to be the dominating factor.There is a clear inverse proportionality between overlap length and contact resistance to reach a minimum value after an effective overlap length. An equation is developed to describe the conductance as a function of the number of nanotubes per device. A four-electrode test structure is fabricated using both photolithography and electron-beam-lithography. The carbon nanotubes are deposited using the dielectrophoresis method for many devices simultaneously to provide a sheet resistance as low as 10 K/. The I-V characteristics are measured with and without change in the transverse electric field. It shows a change in the current reflecting the changes in the energy band gap discussed earlier. There are many applications for the results presented in this dissertation such as optimizing devices operating in the THz frequency range

Single-walled carbon nanotube device fabrication using spin coating of dispersions

This research looks at ways to utilize already synthesized carbon nanotubes (CNT) to manufacture electrical connections using current tools and fabrication methods employed in the semiconductor industry. Purchased single-walled carbon nanotubes (SWNT) are separated and placed in suspension using poly(sodium styrene sulfonate) (PSS). The PSS non-covalently bonds to the SWNTs, causing them to repel each other due to the negative charge of the PSS. The suspension of SWNTs is spin coated over a processed silicon (Si) wafer with fabricated trenches. A Si wafer with a top silicon dioxide (SiO 2) layer is spin coated with Shipley 1827 photoresist. UV light is used to expose areas to the photoresist, creating trench areas. After removal of the exposed areas of the photoresist, trenches are etched into the SiO 2 layer with a buffered oxide etch (BOE) solution of hydrofluoric acid. The suspension of SWNTs is spin coated over the processed Si wafer. The wafer is placed on a hot plate at 115° C to slowly evaporate the water from the SWNT suspension. As the water evaporates, the SWNTs remain on the surface of the Si wafer or gather in the trenches. Finally, the photoresist is removed, lifting off all of the SWNTs that are not in the trenches. Several trenches have a sufficient fill rate to allow IV characteristics to be performed. A Keithley probe station is used to measure the resistance of the SWNT composite material in the trench. These results, 47.3 kΩ, are similar to other fabricated SWNT/polyelectrolyte thin films, showing that the method presented can be used to simplify the process of fabricating SWNT composite wires. Raman spectroscopy is also used to determine if the SWNTs in the SWNT composite structure are aligned in any direction. There is no preferential orientation of the SWNTs in the structure, rather the SWNTs appeared to be randomly oriented in all directions

High Frequency Characterization of Carbon Nanotube Networks for Device Applications

This work includes the microwave characterization of carbon nanotubes (CNTs) to design new CNTs-based high frequency components. A novel developed method to extract the electrical properties over a broad microwave frequency band from 10 MHz to 50 GHz of carbon nanotubes (CNTs) in a powder form is performed. The measured scattering parameters (S-parameters) with a performance network analyzer are compared to the simulated one obtained from an in-house computed mode matching technique (MMT). An optimized first order gradient method iteratively changes the unknown complex permittivity parameters to map the simulated S-parameters with the measured one until convergence criteria are satisfied. The mode matching technique accurately describes waveguide discontinuities as both propagating and evanescent modes are considered allowing an error less than 5% on the extracted permittivity over a broad frequency range. The very large values obtained at low frequencies of carbon nanotubes permittivity are explained theoretically and experimentally based on the percolation theory. The powder composed of semiconducting and conducting CNTs illuminated by an electromagnetic field is seen as series of nano-resistance-capacitance which significantly increase the real and imaginary parts of the complex effective permittivity until the percolation threshold is reached. Based on experimental results different CNTs-based composites material are engineered to design novel microwave components for possible electromagnetic compatibility (EMC) applications. As the extraordinary properties of the carbon nanotubes exist along their axis, the second part of this work is oriented on the alignment and the deposition of carbon nanotubes using a dielectrophoresis (DEP) technique. Micro/nano-electrodes are fabricated using a lift-off process consisting of photo-lithography and electron-beam lithography techniques where the carbon nanotubes suspended in an aqueous solution are attracted in the gap between the electrodes by applying an AC bias voltage. After burning the conducting carbon nanotubes an observed photocurrent with aligned semiconducting CNTs is used to develop high frequency photo-device prototypes

Synthesis of carbon nanotube and the study of transfer characteristics of fet configurred CNT device

Due to the nanoscale structures of carbon nanotubes with highly appreciable optical, thermal and electrical properties which has lead it into a challenging purpose for many potential applications in the nano channel FET, MOSFET, MISC, biosensors, magnetic storage, memory devices etc. The preparation techniques of CNTs by conventional CVD or ARC plasma Discharge method is substituted by the low cost and easy pyrolysis technique as referred in a paper is used for the preparation of CNTs and the prepared CNT is characterized by XRD, SEM . Then a device using the millimetre long CNT, synthesized by CVD technique, as channel is fabricated in a simple manner and the substrate induced biasing effect has studied by the current -voltage characterization by using the source meter, showing the metallic behaviour of multi walled CNT channel. The change in resistance due to the exposure of fabricated device towards different external factors like LASER beam and the water vapour is observed, which shows the change in surface activity due to the influence of foreign molecules

A Review of Double-Walled and Triple-Walled Carbon Nanotube Synthesis and Applications

Double- and triple-walled carbon nanotubes (DWNTs and TWNTs) consist of coaxially-nested two and three single-walled carbon nanotubes (SWNTs). They act as the geometrical bridge between SWNTs and multi-walled carbon nanotubes (MWNTs), providing an ideal model for studying the coupling interactions between different shells in MWNTs. Within this context, this article comprehensively reviews various synthetic routes of DWNTs’ and TWNTs’ production, such as arc discharge, catalytic chemical vapor deposition and thermal annealing of pea pods (i.e., SWNTs encapsulating fullerenes). Their structural features, as well as promising applications and future perspectives are also discussed. Keywords: carbon nanotubes; double-walled carbon nanotubes; triple-walled carbon nanotubes; synthesis; catalytic chemical vapor deposition; arc discharge; fullerenes; pea pod

Graphene/carbon nanotube-based conductive materials

This project is basically an investigation of graphene and carbon nanotube (CNT) based material’s electrical properties. In its first part, graphene (G) and graphene oxide (GO)/carbon nanotubes (CNTs) hybrid films were successfully fabricated as highperformance electrode materials for an energy storage application using a simple water solution casting method and with an assistance of strong ultra-sonication. This was done with different contents of G, GO, single-wall CNT (SWCNT), multi-wall CNT (MWCNT) and multi-wall CNT with a hydroxyl group (MWCNT-OH). The films with MWCNTs showed well interconnected layered structures at the nanoscale range where GO worked as support insulated plates for the CNTs. [Continues.]</div