Nanoionics ? Engineering Nanostructured Crystals from Charged Nanopart
8 May, 2006 03:09 pm
 have been rather limited. Most methods currently in use rely on all-attractive potentials (vdW), and lead to relatively few types of close-packed arrangements. As we have recently reported [1], one way to circumvent these limitations is to mimic Nature's way of building inorganic crystals and to use electrostatic forces.)
Self-assembly of nanoscopic components into higher-order architectures defines the forefront of fundamental nanoscience research and is important for the development of new materials with applications in optoelectronics, high-density data storage, catalysis, and biological sensing. Despite widespread interest, however, the means of assembling nanoscopic objects into larger super-structures (crystals) have been rather limited. Most methods currently in use rely on all-attractive potentials (vdW), and lead to relatively few types of close-packed arrangements. As we have recently reported [1], one way to circumvent these limitations is to mimic Nature's way of building inorganic crystals and to use electrostatic forces.
To our surprise, the NPs arranged themselves into a diamond lattice (Figure 1), in which each particle is surrounded by four neighbors positioned at the vertices of a tetrahedron. Closer examination of the problem revealed that the diamond structure is a consequence of a peculiar nature of the electrostatic forces between nanoscopic objects. On one hand, efficient screening due to "atmospheres" of small counterions surrounding each NP make these forces short ranged; on the other, the "atmospheres" of like charged particles try to "avoid" one another and push the NPs apart. The sum of these two processes is that crystal structure is determined by local interactions only, and favors loosely packed structures (e.g., in a diamond crystal, only 34% of space is taken by NPs).
The screening effects have further counter-intuitive consequences for the quality of the grown crystals. When the co-crystallizing NPs are very monodisperse, they form only very poor-quality crystals. In contrast, when the solution contains a range of NP sizes, the crystals are significantly larger and have less defects. This enhancement results from the presence of smaller particles that shield strong electrostatic focres between the larger NPs (the ones we wish to crystallize). Whithout these smaller particles, the solutions are unstable and tend to flocculate instaed of crystallizing.
While the diamond crystals display several intriguing material/optical characteristics we are now studying, we view them only as an illustration of a wider concept, which we call "nanoionics". Indeed, the research we now pursue is aimed at making crystals from NPs bearing charges of different magnitudes, and at using these particles in ways analogous to those Nature uses build its crystalline forms from real ions. When full understanding of the phenomena accompanying electrostatic interactions at the nanoscale is achieved, "nanionics" might become a method of choice for the rational synthesis of the much anticipated nanostructured materials.
[1] Kalsin et
al. Science, 312, 420 (2006).
Figure 1. Nanoparticle supracrystals self-assembled from equal numbers of oppositely-charged AuNPs (5 nm) and AgNPs (5 nm). These crystals
