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Theoretical calculations based on density functional theory have found (PbS)(32) to be the smallest cubic cluster for which its inner (PbS)(4) core enjoys bulk-like coordination. Cubic (PbS)(32) is thus a "baby crystal," i.e., the smallest cluster, exhibiting sixfold coordination, that can be replicated to obtain the bulk crystal. The calculated dimensions of the (PbS)(32) cluster further provide a rubric for understanding the pattern of aggregation when (PbS)(32) clusters are deposited on a suitable surface, i.e., the formation of square and rectangular, crystalline nano-blocks with predictable dimensions. Experiments in which mass-selected (PbS)(32) clusters were soft-landed onto a highly ordered pyrolytic graphite surface and the resulting aggregates imaged by scanning tunneling microscopy provide evidence in direct support of the computational results.

The Er-doped preperovskite and perovskite singlecrystal PbTiO 3 (PTO) nanofibers with a series of doping concentration (0−4 mol %) were successfully synthesized by hydrothermal method and solid state phase transformation method, respectively. The XRD, XPS, and Raman results indicated that Er 3+ occupied Pb 2+ site in preperovskite PTO and Ti 4+ site in perovskite PTO nanofibers. The photoluminescence (PL) measurements indicated that only the strong green emission corresponding to the preperovskite PTO host matrix was observed, whereas up-conversion (UC) emission under infrared excitation (980 nm) was absent. In contrast, the strong green emission (at 524 and 554 nm) and the weak red emission around 670 nm of Er 3+ occurred in the PL and UC PL processes of Er-doped perovskite PTO nanofibers. The distinguished difference in these two kinds of nanofibers might be attributed to the substitution sites and chemical environments of Er 3+ . It is suggested that such Er-doped single-crystal perovskite PTO nanofibers could be interesting objects for applications in mechanical−electrical luminescence such as electric field modulation luminescence and mechanoluminescence.