Emissive PbS/CdS core/shell nanosheets are synthesized using a cation-exchange method. A significant blue-shift of the photoluminescence is observed, indicating a stronger quantum confinement in the PbS core as its thickness is reduced. High resolution transmission-electron-microscopy images of the cross sections of the core/shell nanosheets show atomically sharp interfaces between PbS and CdS. Accurate analysis of the thickness of each layer reveals the relationship between the energy gap and the thickness in the extremely one-dimensionally confined nanostructure. Photoluminescence lifetime of the core/shell nanosheets is significantly longer than the core-only nanosheets, indicating better surface passivation.

Colloidal lead sulfide (PbS) nanoribbons are synthesized using organometallic precursors with chloroalkane cosolvents. The few-atom-thick nanoribbons have a typical width 20 nm and a length more than 50 nm. Different from a nanosheet where the quantum confinement energy is mainly determined by the thickness, the narrow width of the nanoribbon has an additional contribution to the increase of energy gap. In contrast to nanosheets, the nanoribbons are much brighter. At room temperatures, well-passivated nanoribbons have achieved more than 30% photoluminescence quantum yield in the infrared spectrum, competing with the well-developed colloidal lead chalcogenide quantum dots of the similar energy gap.

In the synthesis of colloidal PbS nanosheets, acetic acid – either injected externally or produced during the reaction – has a significant effect on the growth of the nanosheets. When the acetic acid to lead molar ratio is above 1:8, no nanosheets are observed in the product. By replacing lead acetate with lead oxide to prepare the lead precursor for the reaction, the effect of acetic acid is avoided, resulting in a robust synthesis with nearly 100% success rate. In the new synthesis, the purity of trioctylphosphine (the co‐solvent for sulfur precursor) has no significant effect on the formation of nanosheets. Thickness tunability is achieved in the acetate‐free synthesis by tuning the reaction temperature. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

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By changing the precursor lead-to-sulfur molar ratio in the synthesis of colloidal PbS nanosheets, the lateral size of the nanosheet can be tuned in a wide range from 100 nm to 1500 nm while keeping its thickness around 2.4 nm. Using chloroalkane molecules with a long carbon-chain as the capping ligands can further reduce the lateral size down to 20 nm. The concentration of the chloroalkane in the reaction solution and the reaction temperature also have significant effects on the lateral size. At room temperature, nanosheets with a small lateral size exhibit a narrow light-emission linewidth. The same nanosheets also show a sharp exciton peak near the band edge in the optical absorption spectrum.[a] Dr.