Ultrasmall Nanoplatelets: The Ultimate Tuning of Optoelectronic Properties

2D semiconducting nanoplatelets (NPLs) are an emerging class of photoactive materials. They can be used as building blocks in optoelectronic devices thanks to their large absorption coefficient, high carrier mobility, and unique thickness‐dependent optical transitions. The main drawback of NPLs is their large lateral size, which results in unfavorable band energy levels and low quantum yield (QY). Here, ultrasmall lead chalcogenide PbSe 1– x S x NPLs are prepared, which exhibit an unprecedented QY of ≈60%, the highest ever reported for this structure. The NPLs are applied as light absorber in a photoelectrochemical system, leading to a saturated photocurrent density of ≈5.0 mA cm −2 (44 mL cm −2 d −1 ), which is a record for NPL‐based photoelectrodes in solar‐driven hydrogen generation. Ultrasmall NPLs hold the potential for breakthrough developments in the field of optically active nanomaterials.