Behind the buzz and beyond the hype:
Our Nanowerk-exclusive feature articles
Posted: Sep 12, 2007
Superlattices are key to advanced nanocrystal applications
(Nanowerk Spotlight) Zinc Oxide (ZnO) has long been used in its powdered form as pigments in paints, coatings for papers, in the commercial manufacture of rubber goods as well as UVA and UVB blocker and mild antimicrobial in cosmetics. ZnO is also one of the most important semiconductor compounds and numerous reports have been documented in the literature about the preparation and characterization of ZnO nanocrystals. While polycrystalline forms of ZnO have been used for technical uses such as piezoelectric transducers, light emitting diodes, and transparent conducting films, the progress in developing single crystal bulk ZnO have brought its promise as a wide band gap semiconductor to the fore. Superstructures formed from ZnO nanocrystal quantum dots may find applications in various areas such as optics, electronics and magnetism. For these 2D and 3D superstructures to be useful they need to be well-ordered. Usually, nanocrystals without any surface modification are less stable and they usually undergo aggregation or crystal growth, and consequently it is rather hard for bare nanocrystals to self-assemble into 2D, and especially into 3D, ordered structures. So far, most well-ordered assemblies of nanocrystals have been prepared through a surface modification approach. Efforts have been made to prepare superstructures composed of ZnO nanocrystals but it is rather challenging to obtain well-ordered 3D ZnO superlattices. Researchers in China have now found that ZnO nanocrystals capped with ionic liquids spontaneously assemble into a three-dimensional lattice. Apparently, simply drying a solution of the modified ZnO nanocrystals is all that is needed for the superlattice to form. The presence of the ionic liquid prevents the nanocrystals from aggregating.
"Ionic liquid molecules had been used previously for capping of nanocrystals, but no superstructures which show characteristic XRD patterns were obtained from these capped nanocrystals" Dr. Jiesheng Chen explains to Nanowerk. "Previously, the ionic liquid molecules were grafted onto nanocrystals which were formed prior to grafting. In contrast, we used ionic liquid zinc salt as a precursor to form ZnO nanocrystals and in this way the ZnO nanocrystal size may be tuned through adjusting the reaction conditions. The ionic liquid components render the ZnO nanocrystals very stable and highly luminescent."
Chen, a Professor of Chemistry at the State Key Laboratory of Inorganic Synthesis & Preparative Chemistry at Jilin University in PR China, and his team were able to observe the spontaneous superlattice formation of ZnO nanocrystals – which are capped by the ionic liquid components – and, for the first time, record superlattice-related X-ray diffraction patterns for these self-assembled ZnO nanocrystal materials.
"The superlattice materials we obtained exhibit intense stable visible photoluminescence" says Chen. "Initially, we had hoped to prepare stable ZnO nanocrystals with efficient photoluminescent properties. We used ionic liquid molecules to protect the ZnO nanocrystals because an ionic liquid molecule may bear charges that prevent the nanoparticles from aggregation due to charge repulsion. Later on, we found that after simple evaporation of the solvent from the as-prepared product, superlattices form very easily through self-assembly of the ionic liquid capped ZnO nanocrystals."
Schematic representation of the superlattice structures of IL-Zn, IL-ZnO-A and -B/C (Reprinted with permission from RSC Publishing)
It is interesting to note that the superlattice symmetry varies with the nanocrystal size. Chen's team worked with three ZnO samples of different particle sizes (2, 2.5 and 4 nm) and found that they give rise to rather different XRD patterns. Chen believes that the difference in X-ray diffraction arises from the distinction in nanocrystal packing in the superlattice for the three samples. "Moreover" he says, "the XRD indicates that the relative amounts of the 'soft' and 'hard' components of
the ZnO nanocrystals play an important role in determining the symmetry of the self-assembled superlattices, suggesting that it is feasible to fabricate different 3D organizations just by controlling the nanocrystal particle size of the inorganic core."
Chen points out that the ionic liquid components are not always suitable for capping on nanocrystals. "This means we need to design and to synthesize ionic liquid molecules with specific functional groups to suit various nanocrystals" he says. "In addition, a certain nanocrystal size (less than 4 nm) is also necessary because as the nanocrystal size increases, it becomes difficult to cap larger ionic liquid molecules onto the nanocrystal surface, and as a result, the self-assembly to form superlattices becomes difficult. Through careful selection of ionic liquid molecules, it becomes reliably possible to prepare various nanocrystals which self-assemble to superlattices."
ZnO nanocrystals with efficient luminescence could be promising candidates for use as light emitting materials because they are less toxic and more stable towards air than other semiconducting compounds such as for instance cadmium selenide.
Chen's team at Jilin University are now trying to cap other types of nanocrystals such as noble and magnetic metals using ionic liquid molecules.
"In principle, these particles may also form superlattices, and new properties may be exhibited by these superlattices" Chen hopes. "The challenges lie in that it is not always possible to cap a nanocrystal using ionic liquid molecules, and the properties of the superlattices may not always be optimized."