M2 Bottom-Up Synthesis: Nucleation and Growth (LaMer Model)

Topic

In this bottom-up synthesis, the concentration of dissolved precursor—as it is generated or released into the solution—continues to rise until it surpasses the LaMer threshold; only upon exceeding this threshold does the solution become sufficiently supersaturated to trigger an explosive nucleation event (a "burst"), during which a vast number of new nanoparticle nuclei appear almost simultaneously. If the precursor concentration rises more slowly toward this LaMer threshold, the resulting supersaturation upon crossing it is lower; consequently, the explosive nucleation event produces fewer new nuclei.

Because this explosive nucleation event consumes a large amount of the dissolved precursor almost instantly, the precursor concentration drops just as rapidly back below the LaMer threshold. Once the concentration falls below this threshold, no new nuclei form; instead, any precursor subsequently generated or released deposits onto the existing nuclei, causing them to grow via diffusion-controlled growth. Maintaining the precursor concentration below the LaMer threshold for the remainder of the synthesis ensures that no further explosive nucleation events are triggered.

Thus, this model—characterized by the initial explosive nucleation followed by diffusion-controlled growth—reveals that the temporal separation of these two stages is the key to obtaining monodisperse nanoparticles. Since all nuclei from the explosive nucleation event appear almost simultaneously, they all have roughly the same amount of time to grow during the subsequent diffusion-controlled phase; furthermore, the nature of diffusion-controlled growth causes smaller nuclei to grow slightly faster than larger ones, creating a "size-focusing" effect that further narrows the resulting size distribution. The fact that this temporal separation between explosive nucleation and diffusion-controlled growth is the key to such monodispersity places this model in direct contrast to another model in the set—the one describing Ostwald ripening—a process that, conversely, destroys that very monodispersity if allowed to proceed beyond the diffusion-controlled growth stage.

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