Towards “artificial molecules” of semiconductor nanocrystals

1 Dec 2016. NUS chemists have developed a synthesis approach to link semiconductor nanoparticles in a site-specific, stoichiometrically controlled manner analogous to molecular synthesis.

The synthesis of molecules with higher order structural complexity is made possible because functional groups on individual molecules can be made to react with high specificity and controlled stoichiometry. In the case of inorganic nanoparticles, which are approximately 10 to 100 times larger, methods to covalently link them together facet-to-facet with the precision and control afforded by functional group chemistry are currently not available. The ability to build structural complexity and functionality by linking individual nanoparticles together in a manner analogous to molecular synthesis would be a significant milestone in colloidal nanoscience because size, shape and composition critically determine the properties of a nanoparticle.

A team led by Prof Yin Thai CHAN from the Department of Chemistry, NUS in collaboration with the Institute of High Performance Computing, Agency for Science, Technology and Research (A*STAR), developed a method that allows anisotropically shaped semiconductor nanoparticles to be linked together facet-to-facet in a stoichiometrically controlled manner. The team demonstrated that rod-like semiconductor nanoparticles whose length is on the order of 10 nanometres (nm) can be joined end-to-end, resulting in a linear chain of rods longer than 1 µm. Surprisingly, the linking behaviour can be modelled very well by standard polymer theory that is generally applied to molecules. When used as the active material in photodetectors, the long chain of semiconductor nanorods achieved excellent photodetection performance due to good charge percolation across the length of the chains.

The team also showed that it is possible to link nanorods with different-sized ends into ordered dimers (akin to a molecular complex consisting of two structurally similar molecules) in yields as high as 75%, with 24% of the particles remaining unlinked and 1% forming higher order complexes (trimers, tetramers, etc). Extending the synthetic protocols to other anisotropically shaped nanocrystals allowed for the fabrication of hierarchical structures not achievable by direct synthesis. For example, the team showed that a highly cross-linked network of semiconductor tetrapods or tetrapods whose ends are linked to asymmetric nanorods can be readily synthesised.

The findings of this work suggest that inorganic nanoparticles can be linked to each other, facet-to-facet, in a site-specific and stoichiometrically controlled manner. A next step is to control the angles between individual particles that are linked. Achieving this would provide strong evidence that the synthesis of highly complex nanoparticles can be carried out with the precision, control and diversity of their molecular counterparts.

Figure above illustrates how various semiconductor nanoparticle building blocks within the centre circle can be linked to each other, facet-to-facet, to form hierarchically complex nanostructures that cannot be produced via direct synthesis.

Reference

Chakrabortty S; Guchhait A; Ong X; Mishra N; Wu WY; Jhon MH*; Chan Y*, “Facet to Facet Linking of Shape Anisotropic Inorganic Nanocrystals with Site Specific and Stoichiometric Control” NANO LETTERS Volume: 16 Issue: 10 Pages: 6431-6436 DOI: 10.1021/acs.nanolett.6b02875 Published: 2016.