PbSe quantum dots: Finite, off-stoichiometric, and structurally distorted

PbSe quantum dots with sizes of 2.8(1), 3.7(1), and 6.2(1) nm, freely suspended in solution, have been studied by high-energy x-ray diffraction coupled to atomic pair-distribution-function analysis and computer simulations. The atomic-scale structure of the quantum dots is best described by 371-, 763-, and 4084-atom configurations, respectively, that are substantially off-stoichiometric and structurally distorted. It is argued that such finite-size, ``large-molecule-type'' structure models and not quasi-infinite ones based on periodic crystalline lattices are better to be used when the properties of a few nanometer-sized materials, in particular, those of PbSe quantum dots, are to be explained in full detail.