Colloidal quantum dots (QDs) with infrared (IR) absorption and emission are promising candidates for next-generation optoelectronic applications, including machine vision (e.g., object, food, pharmaceutical, and plastic-recycling inspection), anticounterfeiting, biometric authentication, fog and smoke visors, light detection and ranging (LIDAR), theranostics, and optical communications. Among RoHS-compliant IR materials, InAs QDs are particularly attractive thanks to the tunability of their optical bandgap, which can span from 700 to more than 1700 nm, thus covering a relevant spectral range for the aforementioned applications.

Despite their potential, the use of InAs QDs in optoelectronic devices is still largely unexplored, mainly because their synthetic approaches still need optimization. In particular, efficient control over their size and size distribution has been achieved only recently by employing tris(trimethylsilyl)arsine (TMS-As), which enables the production of InAs QDs with narrow excitonic absorption peaks tunable down to 1600 nm and efficient photoluminescence up to ~950 nm. Another emerging synthetic route is based on tris(dimethylamino)arsine (amino-As), recently introduced as an alternative to TMS-As, because the latter is a pyrophoric, expensive, and commercially limited arsenic precursor, making it non suitable for large-scale production of InAs QDs for consumer applications.

The current challenges are to further advance the optical properties of amino-As-based InAs QDs via: i) the use of innovative ad-hoc reducing agents capable of tuning the reduction rate of As³⁺, present in amino-As, to As^3-, which is essential for improving control over QDs size and size distribution and, thus, over the excitonic peak position and width; ii) improved control over the overgrowth of shell materials capable of delivering InAs@shell QDs with enhanced PL efficiency and, ideally, reduced Auger recombination. Achieving this requires the identification of appropriate shell materials and a detailed understanding of their overgrowth on InAs cores.

In our recent work, we began addressing these challenges by employing a novel reducing agent, trioctylamino-alane, synthesized in our laboratory. This enabled improved control over the growth of amino-As-based InAs QDs, whose absorption could be tuned up to ~1300 nm. Careful optimization of the ZnSe shelling procedure yielded samples exhibiting PLQYs as high as 75% at 905 nm, 60% at 1000 nm, 46% at 1160 nm, 38% at 1250 nm, 32% at 1335 nm, and 23% at 1430 nm. These results were made possible by the use of ZnCl₂ as a crucial additive in the synthesis of the InAs cores. We elucidated the role of ZnCl₂ through combined experimental characterization and computational modeling. Our study indicates that ZnCl₂ is not simply passivating the surface of InAs QDs by acting as a Z-type ligand, but that its use leads to the inclusion of Zn atoms into the InAs QD surface, thereby passivating surface trap states, which are mainly located on As-rich (–1 –1 –1) facets.

These results pave the way for further advancements in InAs QDs, which will necessarily require a deeper understanding of how shell materials grow onto pre-formed InAs cores and how the band alignment between core and shell materials can be possibly optimized.