Mark J. Schulz

Kinetics of Growing Centimeter Long Carbon Nanotube Arrays

Carbon nanotubes (CNTs) are fascinating materials with outstanding mechanical, optical, thermal, and electrical properties [1-4]. CNTs also have a huge aspect ratio and a large sur‐ face area to volume ratio. Because of their unique properties, vertically aligned centimeter long CNT arrays have generated great interest for environmental sensors, biosensors, spin‐ ning CNT into yarn, super-capacitors, and super-hydrophobic materials for self-cleaning surfaces [5-11]. Yun et al. studied a needle-type biosensor based on CNTs to detect dopa‐ mine. Their results showed advantages of using CNT biosensors for detecting neurotrans‐ mitters [11]. Most of the envisioned applications require CNTs with high quality, a long length, and well aligned vertical orientation. Although many researchers have studied the synthesis of vertically aligned CNT arrays, the CNT growth mechanism still needs to be bet‐ ter understood. In addition, CNT lengths are typically limited to a few millimeters because the catalyst lifetime is usually less than one hour [1216]. Many groups have studied the ki‐ netics of CNT growth trying to improve CNT properties. Different observation methods [17-22] were used to determine the effect of the catalyst, buffer layers, carbon precursor, and deposition conditions on nanotube growth. One of the suggested growth mechanisms pos‐ tulates several steps [23]. First, the carbon source dissociates on the surface of the substrate. Next, the carbon atoms diffuse to the molten catalyst islands and dissolve. The metal-carbon solution formed reaches a supersaturated state. Finally, the carbon nanotubes start to grow from the carboncatalyst solution. In situ observation of CNTs during their nucleation and growth is a useful method to understand the growth mechanism, which might help to over‐ come the limitation of the short length of nanotubes, and to control array growth and quali‐ ty. Various remarkable approaches of in situ observation have been performed to affirm the growth mechanism of vertically aligned CNTs and also to obtain kinetics data such as

MODELING THE ELECTRICAL IMPEDANCE OF CARBON NANOTUBE RIBBON

Carbon Nanotube (CNT) ribbon is a thin layer of aligned, partially overlapping CNTs drawn from a forest of CNTs grown on a substrate. The electrical properties of the ribbon must be understood to put this material into multifunctional applications. Measurements show that CNT ribbon exhibits interesting characteristics including frequency-dependent electrical impedance. The impedance is mainly a combination of resistive and capacitive impedance. The magnitude of the impedance of ribbon increases moderately with increasing frequency then decreases significantly at higher frequency, MHz and above. An electrical model was developed to approximate the electrical impedance of the CNT ribbon. Based on this model, some important properties of the CNT ribbon can be understood. The ribbon capacitance, CNT–CNT contact resistance and resistivity can be approximated using the model. This information is useful in determining the suitability of ribbon for different applications. Methods to improve the electrical conduction of CNT ribbon are also discussed.