(682b) Covalent Bonding of CdSe Quantum Dots to Carbon Nanotubes Without Ligands for Solar Energy Harvesting

Hybrid nanomaterials have a wide range of applications in modern technology; different functionalization strategies are being intensely sought for preparing nanocomposites with tunable properties and structures. Multi-Walled Carbon Nanotube (MWNT)/ CdSe Quantum Dot (QD) and Single-Walled Carbon Nanotube (SWNT)/ CdSe Quantum Dot (QD) heterostructures serve as an important example for an active component of solar cells. The attachment mechanism of CdSe QDs and carbon nanotubes is known to strongly affect the charge transfer between them and consequently to alter the efficiency of solar cell devices.  Previous authors [e.g. 1] have shown that short ligands improve charge transfer; we show that more direct bonding results in significant improvement.

In this study, we present a novel method that enables the exchange of some of the organic capping agents on the QDs with carboxyl functionalized MWNTs and SWNTs upon ultrasonication. This produces a ligand-free covalent attachment of the QDs to the carbon nanotubes. EXAFS characterization reveals direct bond formation between the Cd in the CdSe QDs and the nanotubes.

Additionally, we characterize the optical and structural properties of the QD-MWNT heterostructures and investigate how these properties are affected by the attachment. The steady state photoluminescence of QDs is quenched to the noise level of the instrument when the QDs are covalently bonded to an oxygen group on the nanotubes.

Time-resolved fluorescence spectroscopy was also used to reveal more information about the dynamics of charge carriers within the heterostructures probed.  We monitored the decay of exciton emission of CdSe after mixing with MWNT or SWNT using magnetic stirring and after reacting with MWNT or SWNT using ultrasonication. The normalized luminescence emissions of the materials when they are excited at 390 nm shows that the lifetime is <<3nm (the laser pulse width) for the QDs covalently bonded to the MWNT or the SWNT as no tail in the PL signal is observed.  We will also discuss photoconductivity data. 

[1] X. H. Peng, J. Y. Chen, J. A. Misewich and S. S. Wong, Chem Soc Rev, 2009, 38, 1076-1098.