Terahertz and Microwave Detection Using Metallic Single Wall Carbon Nanotubes

Carbon nanotubes (CNTs) are promising nanomaterials for high frequency applications due to their unique physical characteristics. CNTs have a low heat capacity, low intrinsic capacitance, and incredibly fast thermal time constants. They can also exhibit ballistic transport at low bias, for both phonons and electrons, as evident by their fairly long mean free paths. However, despite the great potential they present, the RF behavior of these nanostructures is not completely understood. In order to explore this high frequency regime we studied the microwave (MW) and terahertz (THz) response of individual and bundled single wall nanotube based devices. This thesis is an experimental study which attempts to understand the high frequency characteristics of metallic single walled carbon nanotubes, and to develop an ultra-fast and sensitive direct THz detector. First, the appropriate high frequency detector background is introduced. CNTs previously measured behavior draws similarities to two types of detectors: diode and bolometer. Therefore, our CNT devices are geared towards those designs. Second the fabrication process of devices is reviewed. UV lithography is used to pattern THz coupling log periodic antennas, on top of which CNTs are deposited by using a dielectrophoretic process. Third, the fabricated devices are tested at DC, MW, and THz frequencies. All of these measurements are done as a function of temperature, power, and frequency. Finally, the physical processes that give rise to the diode and bolometric detections at MW and THz detection at different temperatures and under different bias regimes (i.e. low and high) are explained

DEVELOPMENT OF INFRARED AND TERAHERTZ BOLOMETERS BASED ON PALLADIUM AND CARBON NANOTUBES USING ROLL TO ROLL PROCESS

Terahertz region in the electromagnetic spectrum is the region between Infrared and Microwave. As the Terahertz region has both wave and particle nature, it is difficult to make a room temperature, fast, and sensitive detector in this region. In this work, we fabricated a Palladium based IR detector and a CNT based THz bolometer. In Chapter 1, I give a brief introduction of the Terahertz region, the detectors already available in the market and different techniques I can use to test my detector. In Chapter 2, I explain about the Palladium IR bolometer, the fabrication technique I have used, and then we discuss the performance of the detector. In Chapter 3, I explained about the Roll to Roll based THz bolometer, its working and fabrication techniques, and at the end we discussed its performance

Synthesis of Thermal Interface Materials Made of Metal Decorated Carbon Nanotubes and Polymers

This thesis describes the synthesis of a low modulus, thermally conductive thermal interface materials (TIM) using metal decorated nanotubes as fillers. TIMs are very important in electronics because they act as a thermally-conductive medium for thermal transfer between the interface of a heat sink and an electronic package. The performance of an electronic package decreases with increasing operating temperature, hence, there exists a need to create a TIM which has high thermal conduction to reduce the operating temperature. The TIM in this study is made from metal decorated multi-walled carbon nanotubes (MWCNT) and Vinnapas®BP 600 polymer. The sample was functionalized using mild oxidative treatment with nitric acid (HNO3) or, with N-Methly-2-Pyrrolidone (NMP). The metals used for this experiment were copper (Cu), tin (Sn), and nickel (Ni). The metal nanoparticles were seeded using functionalized MWCNTs as templates. Once seeded, the nanotubes and polymer composites were made with or without sodium dodecylbenzene sulfonate (SDBS), as a surfactant. Thermal conductivity (k) measurement was carried out using ASTM D-5470 method at room temperature. This setup best models the working conditions of a TIM. The TIM samples made for this study showed promise in their ability to have significant increase in thermal conduction while retaining the polymer’s mechanical properties. The highest k value that was obtained was 0.72 W/m-K for a well dispersed aligned 5 wt percent Ni@MWCNT sample. The Cu samples underperformed both Ni and Sn samples for the same synthesis conditions. This is because Cu nanoparticles were significantly larger than those of Ni and Sn. They were large enough to cause alloy scattering and too large to attach to the nanotubes. Addition of thermally-conductive fillers, such as exfoliated graphite, did not yield better k results as it sunk to the bottom during drying. The use of SDBS greatly increased the k values of the sample by reducing agglomeration. Increasing the amount of metal@MWCNT wt percent in the sample had negative or no effect to the k values. Shear testing on the sample shows it adheres well to the surface when pressure is applied, yet it can be removed with ease

Growth and Electrical Properties of Chemical Vapour Deposited Low Dimensional sp2 Carbons.

This thesis describes the growth of sp2 carbon materials - namely graphene and carbon nanotube (CNT) materials using a chemical vapour deposition (CVD) process. A novel CVD process tool based on a photothermal process (PT-CVD) that differs from standard thermal CVD has been developed. This thesis reports the investigations into the properties of the deposited carbon nanomaterials and applications that exploit their electronic properties. The first investigation is into the growth of vertically aligned MWCNT forests. Growth of CNTs at 370°C by a one-step PT-CVD method was demonstrated. The growth rate can reach ~1.3 μm/min, which is faster than most other reported thermal CVD methods. The use of bimetallic catalyst (Fe/Ti) and the use of rapid thermal process are the keys to this process. AFM topography studies showed that the fast top-down heating mode of the PT-CVD leads to the formation of a Fe/Ti uniform solid solution, which is believed to improve the CNT growth. These CNTs are composed of a few layer crystalline graphene sheets with a 5-6 nm diameter. Raman scattering provides supporting evidence that the as-grown CNTs are of high quality, better than some CNTs grown at higher temperatures by traditional CVD methods. CVD growth of graphene was investigated using Cu foils as substrate, with the field-effect in the graphene subsequently demonstrated by transferring it to a back-gate bottom contact transistor arrangement using poly-4-vinyl-phenol gate dielectric as an alternative to oxide based insulators. This graphene transistor showed a simple, inexpensive fabrication method that is completely compatible to large scale fabrication of organic devices, to demonstrate a field effect hole mobility of 37 cm2/Vs. Despite the mobility being lower than that found in exfoliated graphene, it demonstrates the potential of a graphene based all carbon transistor for large area electronics. The fabrication and electrical performance of a 3 terminal graphene device is further reported. This device displayed characteristics similar to a p-type graphene FET. While past investigations of distortion and saturation in transfer characteristics of graphene FET indicated that metal-graphene interaction may be the controlling mechanism, this device operation is based on the design of transferring graphene onto a Diamond-like-carbon DLC/p-Si heterostructure with Si as the back contact and with the DLC acting as the dielectric support in contact to graphene. Thus, this provided a mechanism for the DLC/p-Si heterojunction to moderate the I-V characteristics of this device, resulting in a p-type only conduction process in graphene that is also saturable. Following the work on using conventional thermal CVD (T-CVD) for graphene growth, we demonstrated the possibility of using the PT-CVD to develop a graphene growth process. It is found that the non-thermal equilibrium nature of PT-CVD process resulted in a much shorter duration in both heating up and cooling down, thus allows the reduction of the overall growth time for graphene. The choice of performing growth on Ni also allows for the alleviation of hydrogen blister damage that is commonly encountered during growth on Cu substrates. To characterize the film’s electrical and optical properties, pristine PT-CVD grown graphene was used as the transparent electrode material in an organic photovoltaic devices (OPV) and is found to be comparable to that reported using pristine graphene prepared by conventional CVD

Nanocarbons as charge carriers in organic solar cells

Organic photovoltaics (OPV) are an alternate to conventional silicon and thin film solar cells. They have several advantages such as simple fabrication, low material cost, light weight, and the ability to cover large areas. Typical OPV are based on bulk heterojunction structure consisting of a blend of an electron-donating semiconductor polymer and an electron-accepting molecule. Various approaches have been pursued to improve the performance of OPV: low bandgap polymers, new electron acceptors, processing additives, and tandem cell architecture. Nanocarbons possess unique physical and chemical properties that can contribute to the enhancement of OPV performance, such as charge carrier mobility, thermal conductivity, mechanical strength, increased optical absorption and solubility in organic solvents. Nanocarbons also can improve the morphology of the active layer, which is critical to the optimization of OPV performance. The objective of this study is to utilize nanocarbons namely carbon nanotubes, nanodiamond and carbon black as additives and to synthesize composites with fullerenes (C60, C70) as charge carriers in OPV to improve power conversion efficiency (PCE). Functionalized multi-walled carbon nanotubes (MWNTs) are used as additives to C70 to form a weak bonded composite, which result in better absorption and better phase separation. Consequently, PCE increases 70%. In another set of experiments, MWNTs are size sorted and MWNTs of different sizes are tested as an additive to the OPVs to improve PCE. It has been found that the shorted MWNTs provide larger surface area for exciton dissociation and better phase separation and probably lead to less charge recombination. Therefore, they show higher PCE. Functionalized nanodiamonds (ND) are also introduced to C60 to synthesize C60-ND-COOH composite by taking the advantage of their small size distribution (4-5 nm) and modest charge mobility. The NDs have shown the ability to enhance short circuit current density and PCE. Finally carbon black (CB) is implemented into the P3HT/C60 OPV system and the effect of concentration is investigated. The addition of low concentration (12.5 ppm) of CB results in 35% improvement in short circuit current density, and 79% improvement in PCE.In conclusion, the nanocarbons (MWNT, ND, CB) are promising additives for the performance enhancement of polymer photovoltaic cells and may work with diverse donor-acceptor systems