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(iii). Nanotube-Nanocrystal Heterostructures:
1. Wet-chemical synthesis of molecular heterostructures consisting of carbon nanotubes and semiconductor nanocrystals (CdSe and TiO2) with demonstration of charge transfer.
2. Synthesis of a seamless nanotube-nanocrystal junction interface. In situ nucleation and controlled gr owth of crystalline CdTe and CdSe quantum dots on the surfaces of oxidized multi-walled and single-walled carbon nanotubes. The work demonstrates the creation of a sharp junction interface, whose properties are manipulable and hence, predictable. The implication is that the direction of charge and electron transfer can be modulated. This is of importance to the development of photoelectrochromic materials, in particular optical storage devices.
3. Recent demonstration of a covalent route towards site-selective synthesis of multiwalled carbon nanotube (MWNT)-nanoparticle conjugates containing two different types of nanoscale species, i.e. Au nanoparticles (NPs) and CdSe quantum dots (QDs). We systematically and quantitatively probed the effects of varying oxidation treatments, precursor concentrations, and incubation times in order to rationally affect the spatial coverage and distribution of either Au NPs or semiconducting QDs on the MWNT sidewalls and tips. Interesting charge-transfer, electromagnetic enhancement, as well as energy transfer behavior between CNTs and the corresponding nanoparticles/quantum dots have been observed, and will likely render such conjugates as key components in a range of nanoscale devices important for photocatalytic and solar applications.
4. The unique electronic structure and optical properties of double-walled carbon nanotubes (DWNTs) have rendered them as a key focus material of research in recent years. However, the incorporation of DWNTs with quantum dots (QDs) into nanocomposites via a covalent chemical approach as well as the optical properties of the composites has rarely been explored. In particular, we have been interested in this model system as to whether nanomaterial heterostructures can provide efficient pathways for charge separation relative to loss mechanisms such as recombination. In this specific work, the synthesis of DWNT-CdSe quantum dot (QD) heterostructures using a conventional covalent protocol has been demonstrated. CdSe QDs with terminal amino groups have been conjugated onto the surfaces of oxidized DWNTs by the formation of amide bonds. The observed trap emission of CdSe is thought to arise from the presence of 2-aminoethanethiol capping ligands, and is effectively quenched upon conjugation with the DWNT surface due to the charge transfer from CdSe to DWNTs.
Refs.: J. Phys. Chem. C, v.114, 8766 (2010); Chem. Mater., v.21, 682 (2009); Chem. Commun., 1866 (2004); Adv. Mater. (inside cover), v.16, 34 (2004); J. Am. Chem. Soc., v.125, 10342 (2003); and Nano Letters, v.2, 195 (2002).
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