Self-Organization of Semiconductor Quantum Nanocrystals on Carbon Single-Wall Nanotubes into Close-Packed Linear Arrays
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Self-Organization of Semiconductor Quantum Nanocrystals on Carbon Single-Wall Nanotubes into Close-Packed Linear Arrays
C. Engtrakul, J.M. Nedeljkovic, Y.-H. Kim, S.P. Ahrenkiel, K.E.H. Gilbert, J.L. Alleman, S.B. Zhang, O.I. Micic, A.J. Nozik, and M.J. Heben Center for Basic Sciences, National Renewable Energy Laboratory, Golden, CO 80401
ABSTRACT We report the synthesis of organized colloidal semiconductor nanocrystal / carbon singlewalled nanotube hybrid nanostructures. The synthetic protocol described here avoids the need for covalent chemical modification of carbon nanotube (CNT) surfaces. Specifically, InP quantum dots (QDs) and CdSe QDs were found to strongly adsorb onto the surfaces of carbon single-walled nanotubes (SWNTs) by gentle heating in organic solvents. Transmission electron microscopy (TEM) was used to characterize the semiconductor nanocrystal (NC) / SWNT assemblies, and revealed that the surfaces of the SWNT bundles template the adsorption of the NCs from solution. Small QDs were found to randomly absorb onto SWNTs, while larger QDs self-assembled into long linear chains. The nature of binding and ordering was investigated by simply considering van der Waals (vdW) forces for both NC-SWNT and NC-NC interactions. Quantum rods (QRs) were also found to adsorb along the nanotube surfaces. These findings have important implications for the synthesis of NC / SWNT hybrid nanostructures. INTRODUCTION There is a great deal of interest in using CNTs as building blocks for the fabrication of nanoelectronic devices due to their unique structural, electrical, and mechanical properties.1 SWNTs are cylinders of graphite, and depending on the nanotube diameter and chirality, SWNTs can be either metallic or semiconducting materials.2 It is clear, however, that in order to utilize the unique properties of CNTs, the attachment of these nanotubes to other nanostructured materials in order to form composites or devices will likely be required. A general challenge in the area of nanotube chemistry is finding methods for supporting and arranging individual nanostructures on the surfaces of CNTs, while preserving the structural, optical, and electrical properties of the nanotube in the resulting assembly. The integration of semiconductor NCs onto CNTs affords a method for combining a wider range of chemical systems with CNTs to obtain materials with distinctive properties. Semiconductor NCs have optical and electronic properties that are size- and shape-dependent. In fact, when the particle size decreases for a semiconductor NC, distinct quantum effects emerge and the band gap of this material increases.3 Accordingly, the optical and physical properties of these semiconductor NC / SWNT assemblies are expected to be useful for applications in photocatalysis and solar energy conversion. Currently, several chemical functionalization methods exist for decorating acid oxidized CNT surfaces with semiconductor NCs.4-10 These strategies have sought to attach semiconductor NCs to CNT sidewalls and defect sites through
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