Investigation on Site Density of Carbon Nanotube Forests
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1018-EE05-12
Investigation on Site Density of Carbon Nanotube Forests Zhengchun Liu1, Sang Hwui Lee1, Navdeep Bajwa2, Lijie Ci2, Swastik Kar2, Pulickel M. Ajayan2, and Jian-Qiang Lu1 1 Center for Integrated Electronics, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY, 12180 2 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY, 12180
ABSTRACT We report on a method for direct measurement of site density of vertically-aligned carbon nanotubes (CNTs). Site density is an important parameter of vertically-aligned carbon nanotube forests for various applications. By freezing the CNT forests in a polymer matrix and exposing the CNT ends, we obtained the site density of vertically-aligned multi-walled CNTs through SEM observation and particle counting. Site densities of multi-walled CNTs grown by two different CVD processes, ferrocene/xylene process and Fe-Al/ethylene process, were measured to be ~10 tubes/µm2 and ~53 tubes/µm2, respectively. The results of site density distributions indicate non-uniform growth of carbon nanotubes at the micrometer scale in both processes. INTRODUCTION Single wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs) can be grown in a vertically-aligned manner [1-3]. These vertically-aligned carbon nanotubes (CNTs) are promising materials for various electronic applications, such as IC interconnects, supercapacitors, heat-sinks and field emission materials. CNT Pillars [4] and vertical interconnect assemblies (vias) [5] comprising of CNT forest have been demonstrated. CNT site density (i.e., nanotubes per unit area) is a very important factor determining the properties of the whole structure. For applications such as CNT interconnects and heat-sinks, the higher the site density, the more the conduction channels, the higher the performance if we assume that CNT quality and dimension remain the same. In addition, theoretical works [6, 7] have revealed that high quantum impedances inherent in CNTs are responsible for high RC delays in signal transfer, and packed nanotube bundles give lower RC delay in interconnect structures. Hence the control, characterization and improvement in CNT site density are extremely important for future interconnect and other applications. CNT site density can be calculated from the measured mass of CNT forest film [8]. This method is very useful for SWCNTs with narrow diameter distribution. However, it cannot give any information about the distribution of CNT site densities. For carbon nanotube interconnects the narrow distribution of site density is critical because it directly related to the uniformity of
electrical properties. In this paper, we demonstrate an alternative direct-observation method, which is capable of obtaining the distribution of CNT site densities. We think it is not only useful for CNT growth feedback, but also for calculation and evaluation of CNT properties. CARBON NANOTUBE GROWTH Vertically-aligned MWCNTs are grown using two different process recipes. Two di
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