What happens if some of the particles of a superlattice — an array of identical nanoscale crystals — are replaced with foreign ones? It emerges that the properties of superlattices can be radically altered in this way. See Letter p.450

For two decades, researchers have been preparing solid materials that consist of ordered arrays of nanometre-scale particles. By analogy with atomic lattices, the incorporation of traces of particular nanocrystals into these superlattices might allow the materials' properties to be tailored. On page 450 of this issue, Cargnello et al.1 report the formation of two-dimensional and thin-film superlattices of cadmium selenide (CdSe) or lead selenide (PbSe) nanocrystals that have been 'substitutionally doped' with nanoparticles of gold or gold–silver alloy — that is, controlled amounts of these nanoparticles have been incorporated so that they occupy lattice sites in the host material. The authors find that this does indeed alter the superlattices' properties in potentially useful ways.

The incorporation of foreign atoms into a host material's atomic lattice is an essential step in the fabrication of materials for communications technology, opto-electronic devices and construction. A classic example is the introduction of arsenic or phosphorus into a silicon crystal — each doped atom forms four bonds with neighbouring silicon atoms and donates its remaining valence electron to silicon's conduction band. This transforms silicon from an electrical insulator to the most widely used semiconductor. But the effects of analogous substitutional doping in superlattices have not been explored.

Nanocrystal superlattices emerged not long after the discovery2 that chemical synthesis could be used to produce suspensions of nanocrystals in solvents, in which the nanocrystals all have the same size and shape. Superlattices can form from these suspensions through crystallization induced by evaporation of the solvent under controlled conditions3. In their work, Cargnello and co-workers mixed a suspension of CdSe (or PbSe) with gold nanocrystals in hexane, and cast it on an immiscible liquid (ethylene glycol), which acted as a substrate on which superlattices could form. In this way, they prepared monolayers, bilayers and thin films of nanocrystals as ordered arrays, which exhibited astonishing regularity over hundreds to thousands of unit cells (the smallest units of a crystal lattice).

Using a careful statistical analysis, the authors demonstrated that dopant gold nanocrystals occupy random positions in the superlattice provided that they are the same size as the host's nanocrystals (Fig 1a). The chance of each lattice position being occupied by a dopant gold particle can be thought of as a fixed probability, determined by the ratio of the number of gold and semiconductor particles in the suspension from which the superlattice was made. This random occupation results in a uniform distribution, on average, through the superlattice (similar to the atomic dopants in a silicon crystal), and allows the concentration of the dopant to be gradually increased without causing changes to the structure of the host lattice. Figure 1: Alternative outcomes from superlattice doping. Superlattices are arrays of identical nanometre-scale particles. When a few of the particles are replaced by foreign 'dopant' particles, different structural arrangements can occur. a, Cargnello et al.1 report substitutional doping of a cadmium selenide (CdSe) superlattice by gold nanocrystals — random replacement of host particles by dopants — when the gold nanocrystals are identical in size to the CdSe particles. b, The authors also observed phase segregation (clustering of the different particle types) when the gold dopants were a different size from the CdSe particles. c, Different-sized particles can also form binary superlattices — well-defined crystal structures consisting of two sublattices, in which the two types of nanocrystal occupy specific positions. Binary superlattices did not form in the authors' system. Full size image

Obtaining such a random lattice occupation is far from a trivial achievement, because two other types of structure could have formed. The first alternative could have occurred through phase segregation: the formation of separate arrays of host and dopant nanocrystals (Fig. 1b). This can occur if the attraction between gold nanocrystals is stronger than that between the host nanocrystals, resulting in crystallization of the former before that of the latter. Cargnello et al. observed that phase segregation does indeed occur if the two types of nanocrystal differ in size. The second alternative is the formation of a binary superlattice — a well-defined crystal structure consisting of two sublattices, in which the two types of nanocrystal occupy specific positions4,5 (Fig. 1c). In fact, the ordered arrangement of dopants in such binary superlattices can be thought of as being the structural opposite of the random dopant arrangements in the solid solutions reported by Cargnello and co-workers.

Perhaps the best way to understand the observed randomness is to start from a model that considers the nanocrystals as spherical particles that do not attract each other in the suspension. This is a realistic assumption, because the particles are capped with organic molecules that are similar to the hexane molecules used in the starting nanocrystal suspension — which means that the solvent screens the interactions between the capping molecules of two adjacent nanocrystals, leaving only weak attractions between the particles' cores6. If these core–core attractions are either very small or similar in magnitude for the host and the gold nanocrystals, then random mixing without segregation might be expected, provided that both types of nanocrystal are the same size. However, the authors note that a subtle competition between random doping and segregation occurs, depending on the length of the capping molecules, for instance. This suggests that the model described above is too simple.

One of the forces driving the extensive research on nanocrystal superlattices is the prospect of designing materials that have currently unavailable optical, electronic or magnetic properties arising from quantum mechanical7,8,9 or dipolar interactions10 between their building blocks. Cargnello et al. find that that the electronic conductivity of the PbSe material increases by a factor of 106 when the concentration of dopant gold nanocrystals exceeds 16.5% of the total number of nanocrystals. Their work therefore adds to the growing knowledge in the field by demonstrating for the first time that substitutional doping can indeed enable the manipulation of such properties.Footnote 1

Notes

References 1 Cargnello, M. et al. Nature 524, 450–453 (2015). 2 Murray, C. B., Kagan, C. R. & Bawendi, M. G. Science 270, 1335–1338 (1995). 3 Vanmaekelbergh, D. Nano Today 6, 419–437 (2011). 4 Shevchenko, E. V., Talapin, D. V., Kotov, N. A., O'Brien, S. & Murray, C. B. Nature 439, 55–59 (2006). 5 Kiely, C. J., Fink, J., Brust, M., Bethell, D. & Schiffrin, D. J. Nature 396, 444–446 (1998). 6 Evers, W. H. et al. Nano Lett. 10, 4235–4241 (2010). 7 Schliehe, C. et al. Science 329, 550–553 (2010). 8 Boneschanscher, M. P. et al. Science 344, 1377–1380 (2014). 9 Zolotavin, P. & Guyot-Sionnest, P. ACS Nano 6, 8094–8104 (2012). 10 Talapin, D. V. MRS Bull. 37, 63–71 (2012). Download references

Author information Affiliations Daniel Vanmaekelbergh is in the Department of Chemistry, Debye Institute for Nanomaterials Science, University of Utrecht, Utrecht 3584 CC, the Netherlands. Daniel Vanmaekelbergh Authors Daniel Vanmaekelbergh View author publications You can also search for this author in PubMed Google Scholar Corresponding author Correspondence to Daniel Vanmaekelbergh.

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