Seedless Continuous Injection Synthesis of Indium Phosphide Quantum Dots As a Route to Large Size and Low Size Dispersity

Because of their lower toxicity, indium phosphide (InP) quantum dots (QDs) are replacing cadmium selenide QDs in applications that require narrowband luminescence across the visible spectrum. However, InP QDs generally suffer from lower size tunability and broader ensemble emission linewidths because of imperfect synthetic control of nucleation and growth. Despite evidence that InP QDs do not follow the mechanisms that hot-injection and heat-up syntheses were designed to facilitate, these methods are still used, at least as the first step, in most syntheses of InP QDs. In this article, we describe a new method to synthesize large InP QDs with low size dispersity that does not require a hot-injection or heat-up step. Instead, nucleation, growth, and size focusing all take place continuously during a slow syringe-pump-mediated continuous injection of precursors into the reaction mixture. In refining our method, we monitored the photoluminescence (PL) and absorption spectra to find indirect evidence for the role of persistent intermediates in the formation of a secondary population of smaller QDs. We then developed a growth model that is consistent with our observations, and we used it to improve our method with a variable-rate injection that controls the concentration of intermediates to extend growth and prevent secondary nucleation. The result was InP QDs with an absorption peak at 631 nm with a half width at half-maximum of 81 meV. Growth of a thin zinc selenide shell on these InP QDs revealed a PL peak centered at 655 nm with a full width at half-maximum of 53 nm. These values are indicative of large QDs with low size dispersity. This work demonstrates a new method to synthesize large InP QDs that may be attractive to large-scale industrial processes because of its simplicity and potential scalability.