Controlled Positioning of Carbon Nanotubes for Mass Sensing

The motivation for this Ph.D. study is the realization of carbon nanotube (CNT) based mass sensors. Suspended CNTs bridges can be used for zepto-gram detection, which offers the possibility to detect single molecules. In order to achieve these mass sensors several critical fabrication technologies need to be developed and implemented.

These are shortly listed below: Controlled positioning of the nanotubes . In order to realize CNT based devices it is crucial that the positioning of the nanotubes can be controlled in a reproducible manner. This is the main focus of this Ph.D. study and three different positioning methods have been developed. 1.

Nanomanipulation and nanosoldering. The CNT is positioned on free hanging electrodes by means of an x, y, z nanomanipulator, and electromechanically connected to the cantilevers through nanosoldering by electron beam induced deposition (EBID) from a gold containing organometallic compound. Several multi-walled CNT (MWCNT) bridges are made with this technique.

The EBID connections are mechanically strong compared to the MWCNT, stabile and electrically conducting [conductivity ≈ 104(Ωcm)−1]. 2. Electric field assisted chemical vapor deposition (E-CVD). The CNT is grown at the desired place by CVD of a hydrocarbon over a catalyst particle. The nanotube direction is guided by an external electric field, which is present during CVD.

Several bridges made of bundles of singlewalled CNTs (SWCNTs) are grown by CVD of ethylene over iron nitrate or iron nitrate and molybdenum acetylacetonate catalyst solutions. The SWCNTs are connected to the electrodes on the test structures through the catalyst particles. Comparison with 3D Finite Element Simulations of the electrical field distribution between the electrodes of the test structures show that the location, as well as the orientation of the grown CNTs correspond to the regions of maximum electrical field. 3.

Self assembly of CNTs onto chemically modified substrates. The CNTs are made hydrophilic by dispersion into dimethylformamide (DMF) and then selectively assembled onto hydrophilic areas surrounded by hydrophobic areas. The hydrophobic areas are patterned by self assembly of octadecanthiol molecules onto gold surfaces.

The octandecanthiol molecules are then selectively removed from small areas by nanoshaving the gold substrate with the tip of an atomic force microscope (AFM) operating in contact mode. Hydrophilic areas are then patterned on the cleaned areas by self assembly of 2-mercaptoethanesulfonic acid. The CNTs connections to the underlying gold surface are made through van der Waals forces.

A MWCNT was positioned on an approximately 100 Å × 1 µm hydrophilic area patterned on a ≈ 1 × 1 cm2 hydrophobic area. Compatibility with cleanroom processing . The method of CNT positioning needs to be compatible with the fabrication of micrometer sized electrodes in order to interface to the nanometer sized structures.

The positioning methods have all been chosen so that they are compatible with an integration with microfabricated interfacing electrodes for the CNTs. The nanomanipulation and the E-CVD are post processing methods, where the nanotubes are positioned as the final fabrication step. The suggested self assembly method requires planar substrates and it is therefore most suitable that the electrodes are defined after the placement of the nanotubes.  Actuation of the CNTs .

A method of actuating the CNT needs to be developed in order to bring the CNT into resonance. In this project electrostatic excitation has from the beginning been envisioned and the actuation has been tested on CNTs suspended between two electrodes. These preliminary experiments indicate that it is possible to displace the free hanging nanotube bridges in a uniform electrical field.

Suggestions are presented for further experiments on CNTs resonating in an alternating electrical field.  Detection of the resonant frequency of the CNTs . A method for monitoring the electrostatic deflections of the CNTs needs to be chosen. In this project the electrostatic deflections of the CNT bridges have been observed directly in a scanning electron microscope (SEM).

A future goal would be to integrate the read-out on the device itself.