Experimental Evidence for Two-Dimensional Ostwald Ripening in Semiconductor Nanoplatelets

Abstract

Colloidal semiconductor nanoplatelets are rectangular, quasi-two-dimensional crystallites that are only a few atomic layers thick. In the most heavily studied systems (such as CdE with E = S, Se, or Te), the highly anisotropic nanoplatelet shape forms from an isotropic cubic crystal lattice. This has been difficult to reconcile with standard nanocrystal growth models. Previously, we proposed that nanoplatelets arise due to an intrinsic kinetic instability that enhances growth on narrow crystal facets under surface-reaction-limited conditions. Here, we test this model experimentally by synthesizing small "baby" CdS nanoplatelets and performing ripening experiments. During heating, we observe transitions to thicker nanoplatelet populations that are consistent with two-dimensional Ostwald ripening. We then heat CdSe nanoplatelets in the presence of "baby" CdS nanoplatelets to form CdSe-CdS core-crown nanoplatelets. This indicates that individual nanoplatelets dissolve and transfer their material to thicker nanoplatelets, consistent with the kinetic instability model. Finally, we grow thin films of CdS nanoplatelets by combining "baby" CdS nanoplatelets with cadmium carboxylate and heating on a substrate. This shows that Ostwald ripening may also be exploited as a facile and versatile approach to nanoplatelet growth.

Manipulation of the Optical Properties of Colloidal 2D CdSe Nanoplatelets

“…The exposed Cd‐terminated basal facets of colloidal zinc blende CdSe NPLs are composed of structurally identical polar [ 28 ] planes and thus are stabilized by various types of ligands [ 75 ] to assure charge neutrality. Although there are several competing formation mechanisms of CdSe NPLs, including oriented attachment with specific precursors, [ 76,77 ] anisotropic Ostwald ripening of small NPLs, [ 78 ] and kinetic instability caused by insoluble precursors, [ 79 ] the driving force of anisotropic growth for CdSe NPLs with isotropic zinc blende lattice structure is still under debate. However, ligands are believed to play an important role during the anisotropic growth of CdSe NPLs.…”

Unraveling the Growth Mechanism of Magic-Sized Semiconductor Nanocrystals

“…The fast generation of CdSe monomers from the reaction of bis(acyl) selenides (sulfides) and cadmium carboxylates was previously utilized to form CdSe (CdS) nanoplatelets. 45,46 To assess the importance of highly reactive Se precursors for MSNCs, we repeated our reaction with two less-reactive Se precursors that are often exploited in nanocrystal syntheses, elemental Se and trioctylphosphine selenide (Figure S8). Neither produced MSNCs.…”

“…Such a process has already been invoked to explain ripening of CdSe nanoplatelets. 46,51 For the MSNCs, these monomers add to one of the facets of the tetrahedral-shaped particles and complete a layer to form the next bigger size. Note that the addition of a layer to any of the four facets of a tetrahedron results in the next bigger tetrahedron in a series, consistent with MSNC growth.…”

“…Rather, nanoplatelets of increasing thickness appear and disappear due to lateral expansion of small nuclei followed by lateral dissolution. 46,49,51 Nevertheless, the similarities between the mechanisms for MSNCs and nanoplatelets suggest that synthetic strategies that have extended the thickness range of nanoplatelets could similarly impact the achievable size range of MSNCs. In particular, previous studies have shown that thicker nanoplatelets can be grown when chloride (or a halide more generally) is added to standard nanoplatelet protocols.…”

Nanomaterial Shape Influence on Cell Behavior

“…For instance, Gao et al reported the formation of hollow bundle-shaped NaYF 4 microparticles, where multi-stage shape evolution occurred due to Ostwald ripening [ 53 ]. Furthermore, Knusel et al showed that two-dimensional materials with tunable thickness could be produced due to this effect [ 54 ]. Zhang and Wang have also demonstrated that complex shell-in-shell morphologies can be obtained through this process [ 55 ].…”