Optical studies of colloidal nanocrystals (NCs) at cryogenic temperatures in high magnetic fields provide valuable information about their emission properties. Low temperatures (typically 1.7 – 4 K) are required to reduce the population of acoustic phonons and narrow the linewidth, and to reduce the population of the exited states. These conditions enable ensemble measurements. However, ensemble techniques generally give an “average” estimate of the underlying photophysics. Single NC spectroscopy, on the other hand, offers valuable additional insight into the properties of these materials, but has several disadvantages. First, the experimental complexity increases, and second, typically only NCs with bright and stable emission are selected for single-NC measurements, therefore, their properties not always represent the properties of all NCs in ensemble. Therefore, a combination of ensemble and single-NC measurements can provide a comprehensive understanding of the properties of these materials.

InP-based colloidal NCs are being developed as an alternative to cadmium-based materials. Since it is a relatively new material, understanding of their magneto-optical properties is still at an early stage. We have performed both ensemble and micro-photoluminescence measurements of InP/ZnSe/ZnS NCs [1]. The ensemble measurements revealed excitonic emission and provided information about hole ground state and exciton fine structure splitting. On a single-NC level, the trion emission has been discovered. The Zeeman splitting between the trion emission lines provided electron and hole Lande g-factors.

Colloidal CdSe-based nanoplatelets (NPLs) are quasi-two-dimensional nanocrystals with a strong quantum confinement in only one direction. They benefit from a lack of inhomogeneous broadening and have narrow and bright photoluminescence. To increase photostability of NPLs and enhance their quantum yield, it is desired to coat the CdSe core with a larger bandgap semiconductor shell. However, the shell growth leads to a large increase of the linewidth (>40 meV in core/shell NPLs versus ~10 meV in bare NPLs at cryogenic temperatures). Understanding of the broadening mechanisms is important, since narrow ensemble emission spectra are required in a lot of applications. We have demonstrated that at cryogenic temperatures the emission spectra of individual CdSe/CdZnS NPLs exhibit multiple isolated emission lines separated by several meVs spanning a range of 30 – 60 meV [2]. We believe that these features stem from trions localized in shallow (a few meV deep) traps. We have demonstrated that the population of occupied traps can be controlled optically.