Synthesis and Characterization of Carbon Nanotubes to be Used in the Development of New Ionizing Radiation Sensors

Abstract

Detection of ionizing radiation are crucial in different fields including energy, national security, biological and nuclear research, and in applications such as monitoring the attrition of materials in space travel. In general, the systems for the detection of ionizing radiation usually have one or several of the following drawbacks: incapability to produce stable signals, expensive and complicated manufacturing, operation at low or very low temperatures, low sensitivity or even voluminous size, as is the case of Geiger counters. Single-walled carbon nanotubes (SWNTs) are attracting much attention as promising materials for application in nano-devices due to their excellent electrical conductivity, optical, thermal and mechanical properties arising from their quasi-one-dimensional structure. One of these potential applications is the use of SWNTs as radiation sensor. For this purpose, the critical steps in the design and fabrication of devices are focused on the growth of SWNTs into controlled architectures and onto appropriate substrates. Until now, the conventional way to obtain patterned vertically aligned (VA) carbon nanotubes (CNTs) is based in using a careful positioning of the metal catalyst (e.g. by evaporation, use of photoresist masks, or even Ar+ sputtering) in localized positions, from where the CNTs grow. Such catalyst patterning requires a precise control of the process with a long list of complicated steps. An easier method for depositing the catalyst consists in using a wet-based dip-coating approach, which provides some advantages in cost and scalability. However, in this wet-based method the catalyst deposition at desired locations cannot use conventional lithographic techniques. A similar method has been used for patterning the growth of high-quality VA-SWNTs on Si and SiO2 based on the difference of surface wettability of the catalyst in both materials. In the literature, TiN has been chosen as a suitable electrically conductive supporting material for CNTs growth. This includes the use of TiN as the selected substrate, or as a barrier layer on crystalline Si. When using a Si substrate, the formation of metal-silicides during the thermal treatment (annealing step) complicates the synthesis process. In this way, dense mats of vertically aligned multiwall carbon nanotubes have been grown on TiN substrates by using different precursors, including ferrocene or different metals as Fe, Ni, or Co as catalysts. Besides its electrically conductive properties, TiN seems to increase the quality of the grown CNTs (i.e. much thinner CNTs with higher density as compared with those obtained on Si substrates). In this research, SWNTs have been synthesized by CVD, using ethanol at low pressure as carbon source. The characterization of the synthesized materials has been carried out by electron microscopies (SEM and TEM), X-ray diffraction (XRD), and Raman. This material is currently being used for the design and development of a new prototype of radiation sensor.