Single-walled carbon nanotubes metallicity discrimination via chemical force microscopy
Thong, Ya Xuan
Date of Issue2015
School of Chemical and Biomedical Engineering
Single walled carbon nanotubes (SWNTs) have been known to possess a unique combination of electrical, thermal, and mechanical properties that affect applications ranging from nanoelectric circuits and biosensors to field effect transistors. However, a key challenge in the fabrication of good electrical devices involves the separation of carbon nanotubes (CNTs) mixture into its component metallic and semiconducting parts since the production of CNT is in a ratio of 1:2 naturally. Of all the established methods, separation based on non-covalent selective chemistry is attractive because it does not greatly contaminate or modify the nanotubes. In addition, it is affordable and relatively easy to scale up. However, the purity achieved with this method is still insufficient for high end applications and this is compounded by the absence of methods to directly measure the interactions between various functional groups for the different types of SWNTs. Hence, little is known about the preferential affinity of various functional groups for the different SWNT metallicity. Therefore, by identifying functional groups that bind selectively to either the metallic or semiconducting SWNTs, it would aid in the total separation of SWNTs based on metallicity. In this study, a novel technology platform based on a recently developed atomic force microscopy (AFM) mode which directly quantifies the adhesion forces between the chosen functional group (tethered on the AFM probe) and individual nanotubes of known metallicity (deposited on the substrate) is first introduced. Validation of this method comes in the form of being able to discriminate the subtle adhesion force differences of several functional groups with known preferential affinity for pure metallic as opposed to pure semiconducting nanotubes. After which, the electron-donating/withdrawing effect of the functional moieties were studied. It was found that the electron donating groups on alkanethiols exhibit distinct selectivity for semiconducting tubes. In particular, alkyl -OH is able to distinguish between same sized tubes of different metallicities. In addition, the same functional group can have different effects on the selectivity or discrimination power depending on whether aromatics are present. Further, the discrimination power of the para-substituted naphthylamine was found to increase with derivatization; and two novel molecules (sodium 4-amino-1-naphthalenesulfonate, and 1-amino-4-nitronaphthalene) were discovered to be able to provide complete metallicity discrimination. With this technology platform providing a route towards rapid advances in understanding of non-covalent interactions of large libraries of compounds with nanotubes of varying metallicity and diameter, the discovery of more effective metallicity-based SWNT separation agents can be better accomplished.