3- D superlattices

An Au/Ag mixture, the particles of which are approximately the same size, gives only randomly oriented colloids. However, an Au Ag mixture (10 1) with 8 nm Au and 5 nm Ag particles results in an AB2 superlattice, in agreement with the Laves alloy rules. It should be stated that these comparisons of tme alloys with mixtures of nanopartides of different sizes are not really permissible, as the conditions do not correspond. For instance, the fact that the nanopartides are ligand-stahiUzed means that the metal cores do not actually touch each other however, as a simple model to understand the different stmctures of the 2-D superlattices, this is quite satisfactory. 

Well-ordered and largely extended (up to 1 pm) 2-D superlattices by the selforganization of silver and gold nanopartides, protected by alkylthiolates, were identified by Whetten et al. in connection with the formation of 3-D assemblies (see Section 4.2.1) [6, 7]. The Ag particles were produced using a special aerosol technique in which elementary silver was evaporated at 1200-1500 K into an atmosphere of helium, followed by a cooling step to produce 4-5 nm Ag particles that were finally co-condensed with an excess of alkylthiols (C12 chains) [7]. 

The wetting properties of the substrate can infiuence the superlattice morphology [19[. For example, if the colloidal solution of nanoparticles wets the substrate, then a 2-D superlattice will grow preferentially, forming a monolayer. As the surface coverage increases, however, the nanocrystals vfill adsorb to the ledges and kinks of the growing structure, so as to form terraces that extend laterally across the substrate. 

L. O. Brown and J. E. Hutchison. Eormation and electron diffraction studies of ordered 2-D and 3-D superlattices of amine-stabilized gold nanocrystals. J. Phys. Ghent B, 105 8911-8916, 2001... 

A magnetic field, when applied perpendicularly to the substrate, induces the formation of 2-D hexagonal superlattices of individual Co nanoparticles [30] Pileni et al. [28] observed the formation of a hexagonal network of about 1 [tm dot-shaped aggregates made from 8nm cobalt nanopartides. CoPts nanociystals (4nm), when deposited under a magnetic field of 0.9 Tapplied perpendicularly to the substrate, can... 

In an alternative approach, membranes prepared from a monolayer of dodeca-nethiol-stabilized gold nanopartides with a diameter of 6 nm were obtained via a simple evaporation process on a silicon nitride window area [87] (Figure 5.59). Another example showed that nanopartides could seF-organize in 2-D or 3-D superlattices so as to form supra crystals [88] (Figure 5.60). 

Fig. 5.3-lr2 TEM image showing a superlattice of thiol-passivated Au clusters with a bimodal size distribution ( 4.5 and 7.8 nm diameter). The 2-D supra-crystallization shows a regular self-organized bimodal packing. (After 3.144)... 

Mucic and co-workers [15] and Alivisatos et al. [16] successfully constructed supramolecular self-assemblies of QDs and oligonucleonucleotides by precise molecular recognition of nucleotides and Watson-Crick base-pairing interactions, respectively. These methods can precisely control the coupling distance of two quantum dots of a species. Similarly, artificial molecules of homostructures may carry out the coupling of quantum dots of heterostructures to form donor-acceptor assemblies, even 2-D or 3-D QD superlattices. 

Subject to the caveat that there can be a significant shift in peak position when the total layer thickness is sub-micrometre we can determine the average composition of the MQW using the zeroth order, or average mismatch, peak. Asymmetric reflections are often used, both to determine any relaxation and to enhance the diffraction from thin layers. Let the period of the superlattice in real space be A, and the thickness of layers of A B i of composition x i and x 2 be D 1 and D 2 respectively. Then... 

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