Mechanisms for electric field induced color change in coupled colloidal quantum dot molecules revealed by low temperatures single particle spectroscopy

Colloidal quantum dots (QDs), the building blocks of modern displays and optoelectronic devices, have reached the highest level of size and shape control, and stability during the last 30 years. However, full utilization of their potential requires integration or assembly of more than one nanocrystal as in the case of coupled quantum dots molecules (CQDM), where two core-shell QDs are fused to form two emission centers in close proximity. These CQDMs were recently shown to switch color under an applied electric field at room temperature. Here we use cryogenic single particle spectroscopy of single CQDMs under an electric field to show that various mechanisms can contribute to the spectrum change under an applied electric field at cryogenic temperatures. The first mechanism is the control of the delocalized electron wave function when the electric field is applied along the dimer axis. The electric field bends the conduction band and forces the electron wave function to localize in one of the QDs yielding preferential emission of that particular center. In addition, we found that QDs and CQDMs could become sensitive to surface traps under an electric field. In the case of CQDMs, that can result in decreasing the intensity of one of the QDs while increasing the other QD's intensity. Moreover, we show that there are surface charges which screen the applied electric field in some of the QDs. This as well can result in electric field-induced color-tuning of CQDMs. Understanding the underlying mechanisms responsible for spectral shifts under applied electric fields is critical for the development of color-tunable devices utilizing CQDMs, including efficient displays and single photon sources.