Scalable Synthesis of InAs-CdSe Core-Shell QDs

For CSS QD synthesis InAs QDs were synthesized using (TMGe)₃As or (TMSi)₃As as a precursor and purified as described above. The diameter and the concentration of the synthesized particles in solution was determined by using a sizing curve46 (link). Based on QD size a rough estimate of shell precursor material that is necessary to grow a certain amount of shell monolayers was calculated5 (link). However, as InAs QDs do not exhibit a perfectly spherical shape the actual amount of deposited shell material per shell monolayer was found to slightly vary from the calculated values. The shell growth process was monitored by taking frequent aliquots throughout the growth process to determine absorption and fluorescence spectra, as well as QY values such that the final emission peak could be tuned to the desired wavelength. In a typical reaction, purified InAs QDs (96 nmoles, diameter of 4.7 nm, PL emission at 1,039 nm) in hexanes were transferred to a mixture of octadecene (3 ml) and oleylamine (3 ml) in a four neck flask. The mixture of octadecene and oleylamine was previously degassed at 115 °C for at least 1 h. The solution was switched to vacuum to remove the hexanes for 30 min at room temperature and another 10 min at 110 °C. Subsequently, the solution was heated to 280 °C for the shell growth. As soon as the mixture reached 240 °C, shell precursor injection was started using 0.05 M shell precursor solutions in octadecene. Cd(Ol)₂ (111 μmoles) and TOPSe (111 μmoles) were added over the course of 67 min (injection speed 2 ml h⁻¹). InAsCdSe CS QDs were purified using the above described procedure (however using acetone and methanol as non-solvents) and stored in hexanes. In contrast to bare InAs cores, the InAs core-shell QDs were not transferred to a glovebox but stored in air. The size was measured to be 5.1 nm (TEM) and the QDs exhibited a PL peak at 1,296 nm. The resulting InAsCdSe (roughly 86 nmoles, 10% loss through aliquots and purification) were redispersed in octadecene (3 ml) and oleylamine (3 ml), and degassed at room temperature for 30 min and at 100 °C for 10 min. To that solution 3 ml of a 0.05 M Cd(Ol)₂ and 3 ml of a 0.045 M sulfur solution in octadecene were added (150 μmoles Cd and 135 μmoles S) at 240 °C within 1 h. The final QD diameter was determined to be 6.9 nm (TEM) with a PL emission at 1,307 nm.