Jingjing Wei

Beyond Entropy: Magnetic Forces Induce Formation of Quasicrystalline Structure in Binary Nanocrystal Superlattices

Here, it is shown that binary superlattices of Co/Ag nanocrystals with the same size, surface coating, differing by their type of crystallinity are governed by Co-Co magnetic interactions. By using 9 nm amorphous-phase Co nanocrystals and 4 nm polycrystalline Ag nanocrystals at 25 °C, triangle-shaped NaCl-type binary nanocrystal superlattices are produced driven by the entropic force, maximizing the packing density. By contrast, using ferromagnetic 9 nm single domain (hcp) Co nanocrystals instead of amorphous-phase Co, dodecagonal quasicrystalline order is obtained, together with less-packed phases such as the CoAg13 (NaZn13-type), CoAg (AuCu-type), and CoAg3 (AuCu3-type) structures. On increasing temperature to 65 °C, 9 nm hcp Co nanocrystals become superparamagnetic, and the system yields the CoAg3 (AuCu3-type) and CoAg2 (AlB2-type) structures, as observed with 9 nm amorphous Co nanocrystals. Furthermore, by decreasing the Co nanocrystal size from 9 to 7 nm, stable AlB2-type binary nanocrystal superlattices are produced, which remain independent of the crystallinity of Co nanocrystals with the superparamagnetic state.

Ag Nanocrystals: 1. Effect of Ligands on Plasmonic Properties

Silver nanocrystals (NCs) stabilized using amine-terminated coating agents (oleylamine or dodecylamine), their size ranging between 2 and 12 nm in diameter, are synthesized by hot injection methods. Their dispersion in size is relatively low (typically below 10%) without the need for a postsynthesis size segregation process. The amine-terminated coating agents are replaced by thiol-terminated molecules (dodecanethiol or hexadecanethiol) by ligand exchange, allowing the formation of alkanethiol coated Ag colloids. All NCs with various surface coatings are dispersed in toluene. Regardless of the nature of the coating agent, the surface plasmon resonance (SPR) is red-shifted with decreasing the NC size. For a given size, the SPR peak of thiol-stabilized NCs is shifted to lower energies compared to that of amine-stabilized NCs. Furthermore, with thiol-stabilized Ag NCs, the position of the SPR peak was found to be sensitive to the length of the alkyl chains of the coating agent, whereas minor differences are detected for Ag NCs coated with amines terminated with differing alkyl chain lengths.

3D superlattices of uniform metal nanocrystals differing by their sizes called binary supracrystals

By mixing uniform large and small nanocrystals (NCs), three-dimensional (3D) crystalline structures referred to as supracrystals (SCs) are produced. The small NCs fill in the interstices of large NCS self-assembled in well-defined patterns. Various binary structures are produced, in specific conditions, as predicted by a hard-sphere model. It is demonstrated that the ligands used to stabilize NCs play the role of a key parameter in the final structure, as well as the crystalline structure of the NCs, called nanocrystallinity, with production of one-component SCs characterized by vicinal surfaces. Quasi-binary SCs are produced either by a ligand exchange process during the SC growth or through magnetic forces. Using magnetic NCs having two different diameters shows unexpected destructive magnetic property.