Engineering nanocarbon interfaces for electron transfer

• Main Author: Hilmer, Andrew J. (Andrew Joseph)
• Other Authors: Michael S. Strano.
• Format: Others
• Language: English
• Published: Massachusetts Institute of Technology 2014
• Subjects: Chemical Engineering.
• Online Access: http://hdl.handle.net/1721.1/83783

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2013. === Cataloged from PDF version of thesis. === Includes bibliographical references (pages 131-141). === Electron-transfer reactions at nanometer-scale interfaces, such as those presented by single-walled carbon nanotubes (SWCNTs), are important for emerging optoelectronic and photovoltaic technologies. Electron transfer also governs a primary means by which these interfaces are chemically functionalized and subsequently manipulated. This thesis explores several chemical approaches to understanding and controlling charge transfer at nanocarbon interfaces. In the first part of this thesis, we explore ground-state electron transfer via the chemical reaction of SWCNTs with selected diazonium salts as a means of controlling the number of moieties attached to a given nanotube. We initially explore this reaction theoretically using a kinetic Monte Carlo simulation, with rate parameters evaluated using Gerischer-Marcus theory, in order to examine the extent to which these reactions can be controlled stoichiometrically. These modeling results indicate that heterogeneities in SWCNT chiral population result in a large variance in the number of covalent defects, even at low conversions, thereby limiting the ability to control these reactions through stoichiometry. We then experimentally examine the ability to impart an additional degree of control over these reactions through utilization of the adsorbed surfactant layer. Surfactants are commonly employed in the processing of nanoparticles to impart colloidal stability to otherwise unstable dispersions. We find that the chemical and physical properties of adsorbed surfactants influence the diazonium reaction with SWCNT in several ways. Surfactants can impose electrostatic attraction or repulsion, steric exclusion, and direct chemical modification of the reactant. Electrostatic effects are most pronounced in the cases of anionic sodium dodecyl sulfate and cationic cetyltrimethylammonium bromide, where differences in surfactant charge can significantly affect the ability of the diazonium ion to access the SWCNT surface. For bile salt surfactants, with the exception of sodium cholate, we find that the surfactant wraps tightly enough that exclusion effects are dominant. Here, sodium taurocholate exhibits almost no reactivity under the explored reaction conditions, while for sodium deoxycholate and sodium taurodeoxycholate, we show that the greatest extent of reaction is observed among a small population of nanotube species, with diameters between 0.88 and 0.92nm. The anomalous reaction of nanotubes in this diameter range implies that the surfactant is less effective at coating these species, resulting in a reduced surface coverage on the nanotube. Contrary to the other bile salts studied, sodium cholate enables high selectivity toward metallic species and small band-gap semiconductors, which is attributed to surfactant-diazonium coupling to form highly reactive diazoesters. We subsequently move on to examine excited-state electron transfer events between SWCNTs and fullerenes. This electron transfer system is distinct from the diazonium system since it does not result in the formation of a covalent bond between the donor and acceptor species. To study this interface, we synthesized a series of methanofullerene amphiphiles, including derivatives of C60 , C70, and C84, and investigated their electron transfer with SWCNT of specific chirality, generating a structure/reactivity relationship. In the cases of lipid-C61-PEG and lipid-C 71-PEG, which are predicted to similar surfactant surface coverages, band-gap dependent, incomplete quenching was observed across all semiconducting species, indicating that the driving force for electron transfer from SWCNT is small. This is further supported by a Marcus theory model, which predicts that the energy offsets between the SWCNT conduction bands and the fullerene LUMO levels are less than the exciton binding energy of the SWCNT in these two systems. In contrast, the lipid-C 85-PEG derivative shows complete quenching of all SWCNT species utilized in this work. This enhancement in quenching efficiency is consistent with the fact that the LUMO level of C85 methanofullerene is approximately 0.35eV lower than that of the smaller fullerene adducts, resulting in energy offsets which exceed the exciton binding energy. This result, combined with the fact that C8 5 has much higher photo-stability than C61 and C71, makes this larger fullerene adduct a promising candidate for SWCNT-based sensors and photovoltaics. Finally, we design and synthesize fullerene derivatives that self-assemble into onedimensional arrays. We find that a dendritic fullerene, which possesses a Boc-L-Ser-L-Ala-OMe dipeptide sequence at its apex, selectively forms S-oriented, helical, one-dimensional nanowires upon cooling from an isotropic state in cyclohexane. These nanowires possess diameters of 3.76 ± 0.52nm, and can be several microns in length. Control molecules, which do not possess the dipeptide sequence, only produce poorly formed aggregates under identical conditions, indicating that dipeptide-dipeptide interactions are integral to assembly. These nanorods open new opportunities in the chiral assembly of novel electron acceptor materials for optoelectronic and photovoltatic applications. === by Andrew J. Hilmer. === Ph.D.