Probing the Dependence of Electron Transfer on Size and Coverage in Carbon Nanotube−Quantum Dot Heterostructures

As a model system for understanding charge transfer in novel architectural designs for solar cells, double-walled carbon nanotube (DWNT)− CdSe quantum dot (QD) (QDs with average diameters of 2.3, 3.0, and 4.1 nm) heterostructures have been fabricated. The individual nanoscale building blocks were successfully attached and combined using a hole-trapping thiol linker molecule, i.e., 4-mercaptophenol (MTH), through a facile, noncovalent π−π stacking attachment strategy. Transmission electron microscopy confirmed the attachment of QDs onto the external surfaces of the DWNTs. We herein demonstrate a meaningful and unique combination of near-edge X-ray absorption fine structure (NEXAFS) and Raman spectroscopies bolstered by complementary electrical transport measurements in order to elucidate the synergistic interactions between CdSe QDs and DWNTs, which are facilitated by the bridging MTH molecules that can scavenge photoinduced holes and potentially mediate electron redistribution between the conduction bands in CdSe QDs and the C 2p-derived states of the DWNTs. Specifically, we correlated evidence of charge transfer as manifested by (i) changes in the NEXAFS intensities of π* resonance in the C K-edge and Cd M3-edge spectra, (ii) a perceptible outer tube G-band downshift in frequency in Raman spectra, as well as (iii) alterations in the threshold characteristics present in transport data as a function of CdSe QD deposition onto the DWNT surface. In particular, the separate effects of (i) varying QD sizes and (ii) QD coverage densities on the electron transfer were independently studied. ■ INTRODUCTION Carbon nanotube (CNT)−quantum dot (QD) heterostructures, which merge the favorable charge transport properties of CNTs with the interesting size-tunable optoelectronic properties of QDs into an integrated whole, represent not only a conceptually unusual architectural paradigm but also a practically functional nanocomposite in the field of photovoltaic cells. In our group, we have successfully demonstrated various synthetic methods including covalent attachment, π−π stacking, as well as an in situ route toward the simple, siteselective, and coverage-controllable synthesis of single-walled carbon nanotube (SWNT), double-walled carbon nanotube (DWNT), and multiwalled carbon nanotube (MWNT)− CdSe/CdTe quantum dot (QD) conjugates. In this work, we have specifically chosen to use DWNTs as opposed to SWNTs. One rationale is that the acid purification process developed to remove the metal catalysts and amorphous carbon impurities from the pristine carbon nanotubes often involves the breaking of the sp structure of the nanotube sidewall, thereby diminishing the attractive electronic transport characteristics we seek to exploit when utilizing SWNTs. Moreover, with DWNTs, consisting of two coaxial tubules, we can selectively functionalize the outer tube while retaining the desirable electronic properties of the inner tube. Nevertheless, the crucial step in understanding the nature of QD-based photovoltaic cells involves control over the effective interfacial charge transfer, which is often inefficient because of the spatial confinement of the electron and the hole to the interior of the QDs as well as to the unavoidable recombination Received: September 5, 2015 Revised: October 28, 2015 Published: November 16, 2015 Article