Cvd Growth, Optical and Thermal Characterization of Vertically- Aligned Single-walled Carbon Nanotubes

Vertically aligned single-walled carbon nanotubes (VASWNTs) is expected to be an extra-ordinal material for various optical, electrical, energy, and thermal devices. The recent progress in growth control and characterization techniques will be discussed. The CVD growth mechanism of VA-SWNTs is discussed based on the in-situ growth monitoring by laser absorption during CVD. The growth curves are characterized by an exponential decay of the growth rate from the initial rate determined by ethanol pressure. The initial growth rate and decay of it are discussed with carbon over-coat on metal catalysts and gas phase thermal decomposition of precursor ethanol. For the precisely patterned growth of SWNTs, we recently propose a surface-energy-difference driven selective deposition of catalyst for localized growth of SWNTs. For a self assembled monolayer (SAM) patterned Si surface, catalyst particles deposit and SWNTs grow only on the hydrophilic regions. The proposed all-liquid-based approach possesses significant advantages in scalability and resolution over stateto-the-art techniques, which we believe can greatly advance the fabrication of nano-devices using high-quality as-grown SWNTs. The optical characterization of the VA-SWNT film using polarized absorption, polarized Raman, and photoluminescence spectroscopy will be discussed. Laserexcitation of a vertically aligned film from top means that each nanotube is excited perpendicular to its axis. Because of this predominant perpendicular excitation, interesting crosspolarized absorption and confusing and practically important Raman features are observed. The extremely high and peculiar thermal conductivity of single-walled carbon nanotubes has been explored by non-equilibrium molecular dynamics simulation approaches. The thermal properties of the vertically aligned film and composite materials are studied by several experimental techniques and Monte Carlo simulations based on molecular dynamics inputs of thermal conductivity and thermal boundary resistance. Current understanding of thermal properties of the film is discussed. INTRODUCTION Synthesis of vertically aligned single-walled carbon nanotubes (VA-SWNTs) was first reported (1) in early 2004 using a technique based on alcohol catalytic chemical vapor deposition (ACCVD) (2). Many other methods were developed soon after, such as water-assisted (3), oxygen-assisted (4), microwave plasma (5), and molecular beam (6) synthesis. Among these methods, the ACCVD method is arguably the simplest, and unique in that the catalyst can be applied by various methods such as the original dip-coating method (7) and combinatorial sputtering deposition (8, 9). In this paper, we use a solution-based dip-coating method (7) that has been shown to produce monodisperse nanoparticles (10) with diameters of approximately 2 nm. Some advantages of this wet approach include deposition of a very small amount of metal catalyst, as well as excellent potential for low-cost scalability. We also extend this wet process to tailor the key structural parameters of the SWNTs array, including diameter, length, and growth location of the SWNTs. Specifically, the patterned growth of SWNTs by patterning of SiO2 layer (11) and by patterning self-assembled monolayer (SAM) film (12). The first approach (11) is the conventional concept of using SiO2 patterned Si substrates to selectively grow 3D carbon nanotube structures. High-quality VA-SWNT patterns can be easily obtained by this protocol. Apart from the sintering of catalyst into Si at high temperature, the difference in surface wettability between Si and SiO2 also plays an important role in this selective growth, which leads us to a novel method of patterning the growth on chemically modified surfaces. The latter approach (12) is based on the substrate wettability, which is found to be critical for the yield of SWNTs. On an OHterminated hydrophilic Si/SiO2 surface, the growth can be promoted by 10 times, but can be completely suppressed on a CH3-terminated hydrophobic surface. Selective surface modification is utilized to localize the growth of SWNTs. The