Sequential Cation Exchange in Nanocrystals: Preservation of Crystal Phase and Formation of Metastable Phases

Abstract

We demonstrate that it is possible to convert CdSe nanocrystals of a given size, shape (either spherical or rod shaped), and crystal structure (either hexagonal wurtzite, i.e., hexagonal close packed (hcp), or cubic sphalerite, i.e., face-centered cubic (fcc)), into ZnSe nanocrystals that preserve all these characteristics of the starting particles (i.e., size, shape, and crystal structure), via a sequence of two cation exchange reactions, namely, Cd(2+) ⇒Cu(+) ⇒Zn(2+). When starting from hexagonal wurtzite CdSe nanocrystals, the exchange of Cd(2+) with Cu(+) yields Cu(2)Se nanocrystals in a metastable hexagonal phase, of which we could follow the transformation to the more stable fcc phase for a single nanorod, under the electron microscope. Remarkably, these metastable hcp Cu(2)Se nanocrystals can be converted in solution into ZnSe nanocrystals, which yields ZnSe nanocrystals in a pure hcp phase.

Cation Exchange: A Versatile Tool for Nanomaterials Synthesis

“…78 The 14 and Zn 2+ . 25,26 Furthermore, the mild reaction conditions attendant to most exchanges make it a particularly powerful strategy for accessing many nonequilibrium morphologies, crystal phases, and materials that are difficult to prepare by direct hot-injection methods. Control over the extent of the exchange reaction further extends the utility of cation exchange for accessing a wide variety of heterostructures and doped nanocrystals.…”

“…7,31 Through preferential ligation of trialkyl phosphines to soft Ag + and Cu + ions, subsequent exchanges with Pb 2+ or Zn 2+ can be used to further extend these complex morphologies to the lead and zinc chalcogenides ( Figure 4). 26,27 This cation exchange strategy, therefore, comprises a simple three-step synthetic route to anisotropic nanorod morphologies of isotropic crystal phases, such as rock salt PbS. 27 Since cation exchange is a low-temperature, template-based synthetic method, it is well suited to the preparation of kinetically trapped, metastable crystal phases.…”

New materials for tunable plasmonic colloidal nanocrystals

“…63 Imaging, sensing and photothermal therapy do not conclude the range of potential application for copper chalcogenide NCs. NCs of these materials, because of their intrinsic ability to undergo cation exchange, 64 29 This suggests that a large fraction of the holes in these copper-deficient chalcogenides is actually localized. It was therefore concluded that the shape of the NCs has little influence on their plasmonic response.…”

Synthesis and Thermoelectric Properties of Noble Metal Ternary Chalcogenide Systems of Ag–Au–Se in the Forms of Alloyed Nanoparticles and Colloidal Nanoheterostructures

“…Besides controlling size and shape of NPs, it allows sequential deposition of several materials in the form of a single hybrid NP, it fully exploits unconventional reactivity of nanomaterials such as cation or anion exchange, and it stabilizes metastable phases. [14][15][16][17][18][19][20][21][22][23] Thus, the formation of heterostructures with appropriate interfaces and the fine control over their chemical composition are reasonable aspects to be achieved by means of colloidal chemistry, and definitely key factors for further developments. 16,24,25 Just as a tiny fraction of numerous examples, type II-semiconductor heterostructures such as CdSe@CdTe multibranched NPs, metal-semiconductor hybrid systems such as Au(Pt)-CdSe nanodumbbells or Au(Pt)-Cu 2 ZnSnS 4 NPs, bimetallic core@shell Co@Cu or FePd@Pd nanostructures and narrow band gap semiconductor core-shell PbTe@PbS NPs have shown to be efficient systems for optoelectronic, catalytic and thermoelectric applications.…”