Carbon nanotube alignment, pH sensor application and nanocomposite micromachining

Pengfei Li

Dielectrophoresis has been used in the controlled deposition of carbon nanotubes with the focus on the alignment of nanotube films and their applications in the last decade. In this work we extend the research from the selective deposition of carbon nanotube films to the alignment of nanotube bundles and individual nanotubes. Electrodes with "teeth" -like patterns are fabricated to study the influence of the electrode width on the deposition and alignment of carbon nanotubes. Due to the fact that only photolithography-based techniques are used in the process, the fabrication cost is low and the devices are inexpensive. The alignment of carbon nanotube films, bundles, and individual nanotubes is achieved under the optimized experimental conditions. The microscopy inspection of the samples demonstrates that the alignment of nanotube bundles and individual nanotubes can be achieved using narrow electrodes and low-concentration nanotube solutions. With the assistance of dielectrophoresis, we fabricate and characterize pH sensors using aligned single-walled carbon nanotubes (SWNTs). Electrodes with "teeth" -like patterns are used to generate concentrated electric fields and strong dielectrophoretic forces for the SWNTs to deposit and align in desired locations. The devices are used as pH sensors with the electrodes as the conducting pads and the dielectrophoretically captured SWNTs as the sensing elements. When exposed to solutions with various pH values, the SWNTs change their resistance accordingly. The SWNT-based sensors demonstrate a linear relationship between the sensor resistance and the pH value in the range of 5-9. The characterization of multiple sensors proves their pH sensitivity is highly repeatable. We investigate the machinability of polymethylmethacrylate (PMMA)/multi-walled carbon nanotube (MWNT) nanocomposites in focused ion beam (FIB) micromachining. PMMA/MWNT nanocomposites are fabricated with a solution casting method. Microscale pockets are created on the PMMA/MWNT nanocomposites to study the material removal mechanism using FIB. It is observed that the material removal rate increases with increasing input current and decreasing overlap%. Soft lithography is used to translate the ion-milled pockets from nanocomposites into microscale posts on polydimethylsiloxane (PDMS). A scanning electron microscope (SEM) is used to investigate samples. Our results demonstrate an effective method for producing microscale patterns on nanotube-based nanocomposites.

Carbon nanotube alignment, pH sensor application and nanocomposite micromachining

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