3, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
2, Center for Integrated Nanotechnologies, Albuquerque, New Mexico, United States
Time correlated single photon counting spectroscopy coupled with fluorescence microscopy has emerged as a powerful tool that reveals the complex photophysics of colloidal quantum dots. Although the nanocrystals can be interrogated individually, their behavior is often linked to an ideal structure derived from ensemble optical spectroscopy and conventional high resolution transmission electron microscopy (HRTEM). Further, dim or dark particles can go completely undetected while small aggregates may appear spectroscopically as a single particle. Recently, we’ve developed an intuitive and reproducible method to correlate the atomic and chemical structure of individual colloidal nanocrystals with the same particle’s fluorescence dynamics.1 Nanocrystal samples for correlation are prepared by first spin coating a solution of 1 mm polystyrene spheres onto an insulating 8 nm thick SiO2 TEM grid followed by drop-casting a pM concentration of quantum dots. The polystyrene forms unique arrays that are visible in both the fluorescence microscope and in the TEM allowing for unambiguous identification of single quantum dots. Utilizing a Tecnai Osiris in HRSTEM mode in conjunction with its advanced SuperX energy dispersive spectroscopy (EDS) system, high quality lattice images and chemical maps can be obtained and paired with that same quantum dots’ fluorescence dynamics. It can now be possible to identify sub-populations of structures that exhibit the desired photophysics then use that knowledge to direct chemistry to produce quantum dots with specifically chosen optical behavior. We have used this method to directly identify dark structures in commercial quantum dots while identifying shell defects and stacking faults which result in low on-time intermittency.1 In contrast, we found no significant dark fraction in giant-shelled CdSe/CdS quantum dots and instead confirmed charge-state emission and the culprit for low ensemble quantum yields.2 HRSTEM, advanced STEM-EDS and initial correlation results will be presented for thick shelled InP/ZnSe colloidal quantum dots. Additionally, correlation data of seeded CdSe/CdS nanorods and thick-shelled green emitting quantum dots will be presented.
1. Orfield, N.J.; McBride, J.R.; Keene, J.D.; Davis, L.M.; Rosenthal, S.J. Correlation of Atomic Structure and Photoluminescence of the Same Quantum Dot: Pinpointing Surface and Internal Defects That Inhibit Photoluminescence ACS Nano 2015, 9 (1), 831-839.
2. Orfield, N.J.; McBride, J.R.; Wang, F.; Buck, M.R.; Keene, J.D.; Reid, K.R.; Htoon, H.; Hollingsworth, J.A.; Rosenthal, S.J. Quantum Yield Heterogeneity among Single Nonblinking Quantum Dots Revealed by Atomic Structure-Quantum Optics Correlation ACS Nano 2016, 10 (2), 1960-1968.