Mid-Infrared Stimulated Emission from Bulk Lead Sulfide Nanocrystals Formed by Aggregative Growth

Stimulated emission based on colloidal nanocrystals (NCs) is well established in the visible and near-infrared spectrum, but equal impact could be reached in the mid-infrared (MIR, >2500 nm) range, for example in chemical sensing. The recently demonstrated concept of obtaining efficient optical gain via 'bulk' nanocrystals is however not directly translatable to the MIR, as NC sizes are required that are typically inaccessible by current synthesis methods. Here, we present a fast and scalable method to obtain bulk infrared nanocrystals (BINCs) relying on aggregative growth of small PbS NCs, thereby superseding commonly used hot injection and tedious layer-by-layer schemes. Growth is shown to be controlled by the addition of lead halides, allowing us to reach monodisperse NCs approaching the Bohr diameter (ca. 45 nm). These NC materials showcase stimulated emission in the MIR up to the bulk PbS gap of ca. 2950 nm with optical gain metrics in line with visible materials, an observation matching theoretical limits. Our results indicate the wider potential of chalcogenide NCs to push solution processable optoelectronics, and in particular lasing, into the infrared regime, and verify the general suitability of aggregative growth as an ideal synthetic route to obtain them.