Study of the tip bending process of Vertically Aligned Carbon Nanotubes

Abstract

Vertically Aligned Carbon Nanotubes (VACNTs), or better known as CNT forests have many potentials in engineering applications due to its exceptional properties. Patterning of the CNT forest is important for its useful application in Micro-Electromechanical System (MEMS) and sensor fabrication. One of the most common method in this regard is in situ patterning of VACNTs during production. However, this method is limited to produce 2-D patterns with a uniform height. In order to realize true 3-D micropatterning of the CNT forest tool based technique was introduced using Micro-Electro Discharge Machining (µEDM). However, this method lacks of high resolution in the resultant features due to the excessive spark gap. This project aims to mitigate the problem of the spark gap in the patterning of the CNT forest by introducing tip bending process. In this process, the individual tip of the nanotubes is bent and flattened by a rigid cylindrical micro tool either by using its bottom surface (the process is called micromechanical bending, M2B) or the peripheral surface (the process is called micromechanical rolling, M2R). Both M2B and M2R method were conducted using a high precision micro-CNC machine, where all key parameters such as rotational speed of the spindle, lateral speed of the tool, step size in Z-direction, total depth in Z-direction and surface roughness of the tool - were varied. Followed by the tip bending process, morphological study and optical characterization of the samples were carried out to explain the influence of these parameters. The best average surface roughness value for M2B and M2R were found to be 15 nm and 4 nm respectively. In this thesis, an analytical model has been proposed to predict the surface roughness of the patterned CNT forest carried out by the M2B process. The model was verified with experimental data and observed to be in good agreement with experimental data. Optical characterization (with the help of a polarized monochromatic green laser with 532 nm wavelength) of the M2B and M2R processed patterns revealed that the reflectances of these samples are sensitive to the rotational angle (of either the sample itself or the optical polarizer’s orientation). The range of change in reflectance with rotation of sample was found to be 4.2% for the M2R-processed sample at 45o incident angle and the range of change in reflectance with polarizer’s orientation is 10.4% for the M2B-processed sample at 40o incident angle.