Semiconductor quantum dots are nanometer-sized crystals containing around 1000 atoms meaning they transcend the size regime between isolated molecules and bulk solids. Consequently quantum dots display many unique electronic and photochemical properties which are size dependent that could open up new routes towards superfast computing or highly efficient cell probes through conjugation with peptides and nucleic acids.
Silicon dots are leading candidates for use in living systems since they display a robust surface chemistry and, in contrast to those nanocrystals based upon heavy metals (such as CdSe), present fewer cytotoxic effects[1]. As a consequence, information about their structure and composition will have a critical bearing on the performance of any future dot-based device and so reliable details of their structural and chemical characteristics are essential.
For the first time electron energy loss spectroscopy (EELS) has been employed in conjunction with aberration-corrected scanning transmission electron microscopy (STEM) at atomic resolution to determine chemical composition profiles over a range of individual undecyl-capped silicon nanocrystals. Covalent alkyl passivation is known to render the silicon surfaces resistant to ambient oxidation. Our initial results show a significant amount of silicon oxide surrounding the crystalline silicon core which suggests the presence of many unalkylated species that could have undergone subsequent oxidation. This evidence is more pronounced for smaller dots (3 – 9 nm in diameter) than for larger (25 nm) clusters. Theoretical studies[2] have shown how the alkyl chain length can determine the eventual stability of the capped dot. The work presented here seemingly confirms this result namely that the degree of passivation is influenced by steric effects.
[1] Derfus A M, Chan W C W & Bhatia S N Nano Lett 2004, 4, 11-18
[2] Reboredo F A & Galli G J. Phys. Chem. B. 2005, 109, 1072-1078
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