Oriented attachment process

Figure 10.3. PbSe nanowires synthesized by the oriented attachment and fusion of nanoparticles, (a) Schematic representation of the oriented attachment process.

Several papers have discussed the role of oriented attachment in the anisotropic growth of nanocrystals. A recent paper by Cho and coauthors [50] discusses the oriented attachment process as the main mechanism involved in the construction of anisotropic PbSe nanocrystals in several shapes. The authors used the proposition of... 

Fig. 14 Overview of the directed self-assembly process showing the evolution from PbS nucleates to star-shaped and octahedral nanocrystals and the subsequent oriented-attachment process that leads to the formation of radically branched and zigzag nanowires, respectively. ... 

Oriented attachment was first found for Ti02 particles generated in a hydrothermal process [94]. Chains of nanoparticles fused in crystallographic register were found and the fusion siufaces were identified to be the highest energy surfaces of the nanoparticles (Fig. 4). 

Fig.5 Classical (a) vs. non-classical crystallization (b,c). a Crystallization of hydroxyapatite (HAP) fibers from block copolymer aggregates [108], where the block copolymers adsorb to all faces parallel to the HAP c-growth axis resulting in whisker structures with occasional branches (see arrows). b,c Formation of single crystalline and defect-free BaS04 (210) oriented fiber bundles by the process of oriented attachment in experiments described in [109-111], Figure reproduced from [112] with kind permission of Editorial Universitaria, Universidad de Chile, Santiago, Chile...

The evidence for non-classical particle-mediated crystallization reactions has increased over the last few years. Oriented attachment, hquid precursors, as well as mesocrystal crystalhzation pathways have shown their potential to significantly expand the crystalhzation toolbox in materials chemistry. On the other hand, biominerahzation processes can be seen from a new perspective. 

Koh and co-workers have used tris(diethylamino)phosphine selenide (TDPSe) as a new source for selenium instead of the commonly used TOPSe for the synthesis of PbSe nanorods (NRs) by colloidal method (Fig. 9). The new phosphine selenide precursor (TDPSe) in tris(diethylamino)phos-phine (TDP) was rapidly injected to a mixture of PbO, OA in ODE at 170 °C to produce monodisperse single-crystalline PbSe NRs of ca. 4nm diameter with length of ca. 40 nm. The growth mechanism of the NRs was possibly by oriented attachment in addition to the Ostwald ripening process. These NRs showed absorption and emission peaks at 1360 and 1440 nm compared to the spherical PbSe NCs absorption and emission at 1375 and 1420nm respectively. The QY of the NRs was found to be 15 % which is close to the reported QY (20-40 %) of spherical PbSe NCs and notably high. 

But the chemical reactions provide only half of the information on the way to establishing a complete nanoparticle formation mechanism. The other half includes all types of crystallization processes such as prenucleation, nucleation and growth, and assembly and agglomeration. Size, shape, and size distribution of the nanoparticles are strongly dominated by crystallization processes. The tremendous variety of nanoparticle morphologies and architectures made it necessary to expand classical crystallization theory to new concepts such as oriented attachment, particle-based crystallization, or mesocrystal formation [154]. 

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