Electron Microscopy of Mesoscale Arrays of Quantum-Confined CdS Nanoparticles Formed on DNA Templates
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Hall effect. 2 The MRS symposium, Advances in Microcrystalline and Nanocrystalline Semiconductors - 1996, is a continuation of the search for improved routes for the synthesis of quantum-confined materials with unusual properties. The fabrication method and initial characterization of self-assembled mesoscale arrays of quantum-confined CdS nanoparticles are described in this paper. A DNA template is used to control the location of nanoparticle formation, and hence the overall shape of the resulting structure. The plasmid DNAs used as templates resulted in rings less than 0.5 pm in diameter with linewidths less than 10 nm.3' 4 Previously described methods for forming quantum-confined structures are based on the lithography of thin continuous films,' or on gas,' liquid,' or solide phase synthesis of nanocrystalline materials from atomic precursors. The lithographic methods require expensive equipment and highly experienced personnel. Most synthetic methods result in nanoparticles that have no spatial relationship with each other: the individual nanoparticles are randomly distributed and oriented. While it is possible to fabricate thin films 9 and even three-dimensional arrays of nanoparticles'0 using volumetric restriction techniques, these methods result in unpatterned, large-scale materials with overall macroscopic dimensions that are equivalent to thin films or bulk. The difference is that they are composed of 591 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
nanoparticles instead of individual atoms. The approach described here differs significantly from the recent studies of Alivisatos et al." and Mirkin et al. 2 who reported the use of polynucleotides to assist in the packaging of pre-formed gold nanoparticles into clusters. We use the polynucleotide as the template for actual nanoparticle synthesis, not as a linking agent for pre-formed particles. The nanoparticles in our method are semiconductors, not metals, and they are much smaller than the gold particles used by Mirkin et al. (5 nm as compared to 13 nm). The mesoscale structures fabricated using our method are also much smaller than the clusters reported in their research (< 8 nm wide compared to 50 nm). Our methodology has already succeeded in making the large nanocrystal molecules envisioned by Alivisatos." 3 METHODOLOGY There are four concepts which form the basis of our methodology: (1) the synthesized material must demonstrate quantum-confinement effects, meaning that the individual nanoparticles must be less than about 6 nm in diameter for CdS; (2) the nanoparticles should be semiconductors for their unique optical and electrical properties, and for possible integration with other semiconductor technologies; (3) the array shapes should be controllable; and (4) the structures should self-assemble, that is, lithography should not be required. Previous work at TCU and UNT conclusively demonstrated that DNA 5 1 1 4 1can stabilize the formation of quantum-confined semiconductor nanoparticles in solution. ' ' 6 This concept was ex
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