Crystal growth nanoparticle aggregation

Coarser biogenic ZnS nanoparticles are commonly twinned on very fine scales. We suggest that these microstructures arise as the result of an aggregation-based crystal growth mechanism (see below), probably commencing with clusters analogous to those described by Luther et al. (1999). 

Martin, this volume Waychunas, this volume) lead to the speculation that nanoparticle aggregation is a pathway for crystal growth from molecules to microcrystals in many systems under some conditions. However, processes involving particles with sizes between those of molecules and 2 nm are very difficult to study with the level of detail needed to clearly resolve reaction mechanisms (though, of course, the existence of particle aggregates can be documented fairly easily). 

ZnS occurs as very small, crystalline nanoparticles and oriented aggregation-based crystal growth involving few nanometer-diameter ZnS nanoparticles occurs. Thus, we postulate that sphalerite nanoparticles form due to the aggregation of sphalerite-like Zri4S6(H20)4 clusters whereas wurtzite nanoparticles form by oriented aggregation of wurtzite-like Zn6S6(H20)9 clusters. These consideration may be relevant to many important minerals that are isostructural with sphalerite and wurtzite. 

Shen and Lee (2001) report formation of 111 twins in Zr02 as the result of aggregation-based crystal growth in nanoparticles formed by laser ablation condensation. They directly observed the tetragonal to monoclinic phase transformation induced by the electron beam and showed that reaction is initiated at these twins. 

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