Synthesis and Growth Mechanism of Lead Sulfide Quantum Platelets in Lamellar Mesophase Templates

Thin (1 nm) PbS quantum platelets are prepared from lead acetate and thiourea in n-octylamine solvent at and near room temperature. Mechanistic studies establish that the quantum platelets are formed within lamellar amine-bilayer mesophase templates. UVvisible extinction spectra contain discrete features assigned to the n = 1 and n = 2 quantum-well transitions in the quantum platelets. The observed shifting of the spectral features to lower energies during growth of the quantum platelets is ascribed to increasing multi-quantum-well coupling between adjacent quantum platelets in the template stacks. Re-examination of a previously published higher-temperature synthesis of PbS nanosheets provides strong evidence for an analogous oleate-bilayer templated pathway. The lamellar mesophases investigated here undergo elastic expansion in the high-vacuum environment of the TEM, which appears to be a general phenomenon. Thus, lamellar spacings measured by TEM are larger than the actual spacings at ambient pressure.