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 exchangesinvolving octahedrally coordinated ions (i.e., with coordination number6), such as Pb2+ or Sn2+, yielded rock-saltpolycrystalline PbTe or SnTe nanocrystals with Cu2–xTe@PbTe or Cu2–xTe@SnTe core@shell architectures at the early stages of the exchangeprocess. In this case, the octahedrally coordinated ions are probablytoo large to diffuse easily through the Cu2–xTe structure: their limited diffusion rate restricts theirinitial reaction to the surface of the nanocrystals, where cationexchange is initiated unselectively, leading to core@shell architectures.Interestingly, these heterostructures were found to be metastableas they evolved to stable Janus-like architectures if annealed at200 °C under vacuum.
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