A method offering precise control over the synthesis conditions to obtain carbon nanotube (CNT) samples of a single chirality (metallic or semi-conducting) is presented. Using this nanolithographic method of catalyst deposition, the location of CNT growth is also precisely defined. This technique obviates three significant hurdles that are preventing the exploitation of CNT in micro- and nano-devices. Microelectronic applications (e.g., interconnects, CNT gates, etc.) require precisely defined locations and spatial density, as well as precisely defined chirality for the synthesized CNT. Conventional CVD synthesis techniques typically yield a mixture of CNT (semi-conducting and metallic types) that grow at random locations on a substrate in high number density, which leads to extreme difficulty in application integration. Dip Pen Nanolithography (DPN) techniques were used to deposit the catalysts at precisely defined locations on a substrate and to precisely control the catalyst composition as well as the size of the patterned catalyst. After deposition of catalysts, a low temperature Chemical Vapor Deposition (CVD) process at atmospheric pressure was used to synthesize CNT. Various types of catalysts (Ni, Co, Fe, Pd, Pt, and Rh) were deposited in the form of metal salt solutions or nano-particle solutions. Various characterization studies before and after CVD synthesis of CNT at the location of the deposited catalysts showed that the CNT were of a single chirality (metallic or semiconducting) as well as a single diameter (with a very narrow range of variability). Additionally, X-ray photoelectron spectroscopy (XPS) was used to characterize the deposited samples before and after the CVD, as was lateral force microscopy (LFM) for determination of the successful deposition of the catalyst material immediately after DPN as well as following the CVD synthesis of the samples. The diameter of the CNT determines the chirality. The diameter of the CNT measured by TEM was found to be consistent with the chirality measurements obtained from Raman Spectroscopy for the different samples. Hence, the results showed that CNT samples of a single chirality can be obtained by this technique. The results show that the chirality of the synthesized CNT can be controlled by changing the synthesis conditions (e.g., size of the catalyst patterns, composition of the catalysts, temperature of CVD, gas flow rates, etc.).

Thesis advisor: Lawrence T. Scott / Carbon nanotubes are theorized to possess many extraordinary properties. To a certain extent, these properties have been demonstrated using the products of current nanotube growth technologies; however, the specific characteristics of distinct nanotube topographies remain untapped on the industrial scale. Carbon vaporization and “flame” methods produce mixtures of various nanotube chiralities and diameters. Although progress has been made, separation techniques are limited. Currently, organic synthesis and subsequent elongation of a select hydrocarbon template is the only approach that promises significant access to specific nanotube topographies without the need for separation. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

The primary goal of this research was to develop the laser chemical vapor deposition (LCVD) process to be able to directly deposit carbon nanotubes onto substrates selectively. LCVD has traditionally been used to directly deposit complex geometries of other materials, including many metals and ceramics. Carbon nanotube deposits were formed using codeposition and other techniques. Multiwall carbon nanotubes as small as 7 nm were synthesized. Utilizing electron microscopy, deposits were characterized to determine the effects of laser power, catalyst and hydrocarbon concentration, time, pressure, and other variables on the number of nanotubes formed, their size, and their spatial location. The most important variables were shown to be hydrocarbon and catalyst concentration and laser power. These results were analyzed and statistics based models were developed to express these trends. Additionally, the process was also used successfully to deposit linear patterns of carbon nanotubes. Carbon nanotube deposits were also carried out in the presence of an electric field. It was demonstrated that a field of sufficient strength could be used to orient tube growth. LCVD is a thermally driven process and a thermal feedback and control system is typically employed to allow for real time control of the reaction zone temperatures. The current thermal imaging system installed on the LCVD reactor is limited to operation at temperatures above which nanotube deposition occurs. A heat and mass transport model was therefore developed to simulate deposition temperatures and provide an estimate of the desired laser power needed to achieve a desired reaction temperature. This model included all significant modes of heat transport including conduction, natural convection and radiation. Temperature dependant material properties were also employed to help achieve greater accuracy. Additionally, the model was designed to be able to simulate a scanning laser beam which was used to deposit linear patterns of carbon nanotubes. Modeling calculations of laser heating compared favorably with experimental data. The results of this work show that LCVD has potential for use in the commercial market for selective direct deposition of patterns of aligned carbon nanotubes on multiple substrate materials.

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With the advent of microelectronics and micromechanical systems, the benefits of miniaturized technology became evident. With the discovery of carbon nanotubes by Iijima in 1991, a material has been found that offers superior porperties such as high tensile strength, excellent electrical and heat conductivity while being lightweight, flexible and tunable by the specific atomic arrangement in its structure. The first part of this thesis deals with a new synthesis approach, which combines the known routes of chemical vapour deposition and laser ablation. The results concerning diameter and yield fit well within an established model for the nucleation and growth of carbon nanotubes and extend it by considering a larger parameter space. Furthermore, conventional laser ablation has been used to synthesize C-13 augmented carbon nanotubes, whose diameters depend among the usual synthesis parameters also on the C-13 content, an influence which is in line with the changed thermal conductivities of isotope mixtures. Manipulation of carbon nanotubes inside a transmission electron microscope forms the second part of this thesis. With the help of an in-situ nanomanipulator, several experiments involving the mechanical and electrical properties of carbon nanotubes have been performed. Two-probe resistances of individual nanotubes have been measured and the observation of individual shell failures allowed for the determination of current limits per carbon shell. With the help of electrical current, a nanotube was modified in its electrical characteristics by reshaping its structure. By application of DC-currents or square current pulses, the filling of iron- or cementite-filled multi-wall carbon nanotubes has been found to move in a polarity-defined direction guided by the nanotube walls. Depending on the current, nanotube shape, and composition of the filling different regimes of material transport have been identified, including the reworking of the inner nanotube shells. The application of a high driving current leads to a complete reworking of the host nanotube and the current-induced growth of carbonaceous nanostructures of changed morphology. Utilizing the obtained results, a transport mechanism involving momentum transfer from the electron wind to the filling atoms and a solid filling core during transport is developed and discussed. Finally, measurements of mechanical properties using electrically induced resonant or non-resonant vibrations inside the transmission electron microscope have been observed and important mechanical parameters have been determined with the help of a modified Euler-Bernoulli-beam approach. / Mit dem Aufkommen von Mikroelektronik und mikromechanischen Systemen wurden die Vorteile miniaturisierter Geräte augenscheinlich. Mit der Entdeckung von Kohlenstoff-Nanoröhren durch Iijima 1991 wurde ein Material gefunden, welches überlegene Eigenschaften wie hohe Festigkeit, exzellente elektrische und Wärmeleitfähigkeit zeigt, während es zeitgleich leicht und flexibel ist. Diese Eigentschaften können durch eine Änderung der spezifischen atomaren Anordnung in der Nanoröhrenhülle beeinflusst werden. Der erste Teil dieser Dissertationsschrift behandelt einen neuartigen Syntheseansatz, welche die bekannten Syntheserouten der chemischen Gasphasenabscheidung und Laserablation kombiniert. Die Ergebnisse bezüglich des Durchmessers und der Ausbeute lassen sich gut mit einem etablierten Modell der Nukleation und des Wachstums von Kohlenstoff-Nanoröhren beschreiben - sie erweitern es, indem sie einen größeren Parameterraum berücksichtigen. Des Weiteren wurde konventionelle Laserablation benutzt, um C-13 angereicherte Kohlenstoff-Nanoröhren herzustellen, deren Durchmesser nicht nur von den üblichen Parametern, sondern auch vom C-13 Anteil abhängt. Diese Abhängigkeit geht mit der veränderten thermischen Leitfähigkeit von Isotopenmischungen einher. Die Manipulation von Kohlenstoff-Nanoröhren in einem Transmission-Elektronenmikroskop formt den zweiten Teil der Dissertationschrift. Mit Hilfe eines in-situ Manipulators wurden vielfältige Experimente durchgeführt, um die mechanischen und elektrischen Eigenschaften der Kohlenstoff-Nanoröhren zu bestimmen. Zweipunktmessungen des Widerstands einzelner Nanoröhren und die Beobachtung des Versagens einzelner Kohlenstoffschichten erlaubte die Bestimmung der Stromtragfähigkeit einzelner Hüllen. Mit Hilfe eines elektrischen Stromes konnte eine Nanoröhre durch die veränderung der Struktur in ihren elektrischen Eigenschaften verändert werden. Unter Verwendung dauerhaften oder gepulsten Gleichstroms konnte die Eisen- oder Zementit-Füllung der Kohlenstoff-Nanoröhren in eine polaritätsabhängige Richtung bewegt werden. Die Füllung wurde dabei durch die Wände der Nanoröhre geführt. Abhängig von Strom, Form der Nanoröhre und Zusammensetzung der Füllung ließen sich verschiedene Bereiche des Materialtransports identifizieren, u.a. das Umarbeiten einiger innerer Kohlenstoffschichten. Ein hoher Strom hingegen bewirkt eine Umarbeitung der kompletten Nanoröhre und strominduziertes Wachstum von Kohlenstoff-Nanostrukturen mit veränderter Morphologie. Mit Hilfe der gewonnenen Resultate wurde ein Transportmodell entwickelt, welches den Impulstransfer von Elektronen an Füllungsatome sowie einen festen Füllungskern während des Transports diskutiert. Messungen der mechanischen Eigenschaften, welche mit Hilfe von resonanter oder nicht-resonanter elektrischer Anregung von Schwingungen im Transmissions-Elektronenmikroskop durchgeführt wurden bilden den Abschluss der Arbeit. Durch die Beobachtungen konnten mit einem modifizierten Euler-Bernoulli-Balkenmodell wichtige mechanische Eigenschaften bestimmt werden.