Influence of the Ion Coordination Number on Cation Exchange Reactions with Copper Telluride Nanocrystals

Abstract

Cu2–xTe nanocubes were used as starting seeds to access metal telluride nanocrystals by cation exchanges at room temperature. The coordination number of the entering cations was found to play an important role in dictating the reaction pathways. The exchanges with tetrahedrally coordinated cations (i.e., with coordination number 4), such as Cd2+ or Hg2+, yielded monocrystalline CdTe or HgTe nanocrystals with Cu2–xTe/CdTe or Cu2–xTe/HgTe Janus-like heterostructures as intermediates. The formation of Janus-like architectures was attributed to the high diffusion rate of the relatively small tetrahedrally coordinated cations, which could rapidly diffuse in the Cu2–xTe NCs and nucleate the CdTe (or HgTe) phase in a preferred region of the host structure. Also, with both Cd2+ and Hg2+ ions the exchange led to wurtzite CdTe and HgTe phases rather than the more stable zinc-blende ones, indicating that the anion framework of the starting Cu2–xTe particles could be more easily deformed to match the anion framework of the metastable wurtzite structures. As hexagonal HgTe had never been reported to date, this represents another case of metastable new phases that can only be accessed by cation exchange. On the other hand, the exchanges involving octahedrally coordinated ions (i.e., with coordination number 6), such as Pb2+ or Sn2+, yielded rock-salt polycrystalline PbTe or SnTe nanocrystals with Cu2–xTe@PbTe or Cu2–xTe@SnTe core@shell architectures at the early stages of the exchange process. In this case, the octahedrally coordinated ions are probably too large to diffuse easily through the Cu2–xTe structure: their limited diffusion rate restricts their initial reaction to the surface of the nanocrystals, where cation exchange is initiated unselectively, leading to core@shell architectures. Interestingly, these heterostructures were found to be metastable as they evolved to stable Janus-like architectures if annealed at 200 °C under vacuum.

Plasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives

“…A particularly interesting characteristic of copper chalcogenide NCs is their ability to engage in cation exchange reactions, making them an attractive tool to synthesize novel materials, such as alloys based on copper chalcogenides [85,155,351] or to create more complex heterostructures composed of more than one material. [44,85,352] As an example, De Trizio et al [85] showed that, starting from The octahedrally coordinated cations instead are probably slower diffusers, due to their larger size, and this limits their reaction to the surface of the NCs, thereby forming core/shell nanostructures. Notably, an annealing process of these core/shells led to the transformation into Janus like structures, as obtained by cation exchange with the tetrahedrally coordinated cations (Figure 33c and d The role of copper vacancies on the cation exchange reaction was investigated by Lesnyak et al, [356] in which the reactivity of stoichiometric and non-stoichiometric copper selenide NC with two different divalent cations (Zn 2+ and Cd 2+ ) was studied.…”

“…d) nano heterostructures prepared by partial cation exchange of Cu 2-x Te NCs with Hg 2+ and Pb 2+ cations, respectively. Adapted with permission from ref[352] Copyright 2016 American Chemical Society. HRTEM images of Cu 2-x Te/PbTe core/shell NCs before the in situ annealing treatment at 200 °C, which causes the transition from a core/shell architecture to a more stable Janus-like morphology, as a consequence of the minimization of the interfacial energy.…”

Semiconductor Nanocrystal Engineering by Applying Thiol‐ and Solvent‐Coordinated Cation Exchange Kinetics

“…[8] Despite the progress made with regards to composition and crystallinity control, [9] efficient aliovalent doping and highly curved heterostructuring to metal in the nanoscale are critical and still remain abig challenge.Cation exchange conversion of CSNCs has been exploited as ap otential crystal engineering strategy for precisely controlling the morphology,c omposition and crystallinity. [1][2][3][4] The groups of Alivisatos,M anna, Schaak, Zhang,K im, and Pradhan [7,8,[10][11][12][13][14][15][16] have taken such advantages to synthesize CSNCs.Wehave reported that tertiary phosphines (R 3 P) with different Rg roups could initialize cation exchange reactions to precisely tune the targeted CdX (X = S, Se etc. )N Cs crystallization, dopant concentration, and hetero-interface to Au NCs.…”

“…)N Cs crystallization, dopant concentration, and hetero-interface to Au NCs. [5,9a, 17] While cation exchange by utilizing phosphine ligands has gone through astonishing developments, [5,[10][11][12][13][14][15][16] it is anticipated that the chemistry of ligand-coordinated cation exchange reactions will enable new nanocrystal engineering.I np articular, so far, efficient doping and heterointerface control in the environmentally friendly CSNCs,s uch as ZnS with intrinsic defects,h as not been achieved. [5,9a, 17] While cation exchange by utilizing phosphine ligands has gone through astonishing developments, [5,[10][11][12][13][14][15][16] it is anticipated that the chemistry of ligand-coordinated cation exchange reactions will enable new nanocrystal engineering.I np articular, so far, efficient doping and heterointerface control in the environmentally friendly CSNCs,s uch as ZnS with intrinsic defects,h as not been achieved.…”

“…[5,17] In particular,t he stable deep-position Ag + or Cu + doping in CdX CSNCs has been achieved by reverse cation exchange between Ag 2 X, Cu 2 X, and phosphine-coordinated Cd 2+ ions,thus leading to the unprecedented discovery of an excitonic pathway to photoinduced magnetism in our Ag-doped CdSe NCs. [5,9a, 17] While cation exchange by utilizing phosphine ligands has gone through astonishing developments, [5,[10][11][12][13][14][15][16] it is anticipated that the chemistry of ligand-coordinated cation exchange reactions will enable new nanocrystal engineering.I np articular, so far, efficient doping and heterointerface control in the environmentally friendly CSNCs,s uch as ZnS with intrinsic defects,h as not been achieved. Furthermore,t he understanding of the fundamental chemistry of cation exchange is expected to create more general and constructive approaches towards versatile nanoscale building blocks and the desired multi-functionality.…”