Now a day, advanced optical spectroscopy studies at the level of individual nanocrystals can be perform routinely. In parallel, advances in Hi-Res transmission electron microscopy allow imaging not only the size and shape of the nanocrystals but also their internal composition. However, for most of the times, these two powerful characterization experiments were applied on two different sets of a nanocrystal ensemble. As a result, establishing direct optical-structural property correlation becomes impossible. Here in this talk we will review our recent experiments, in which single nanocrystal, time tagged, time correlated photon counting experiments and z contrast scanning TEM experiments were performed on the same set of individual core/thick-shell nanocrystals quantum dots (i.e. giant QD or g-QDs). Direct correlation of individual g-QDs’ emission characteristics (i.e. lifetimes and photon emission statistics etc.) with their size, shape and composition allows us to identify photo-charging of the g-QDs, not the existence of dark g-QD subpopulation, as the physical origin of the imperfect QY of the core/thick-shell nanocrystals. Furthermore, by extending the experiment to high temperature and high light flux regime, we investigated PL bleaching issue of the nanocrystals that is hindering the application of nanocrystals in solid-state lighting. The experiment allows us to eliminate heat and light induced changes in shell-thickness and composition of the g-QD as a mechanism responsible for the PL bleaching. The experiment further shows that while the charging of g-QDs is partly responsible, the creation of hot carrier traps that intercept the excitons before they relax to the ground state is mainly responsible for permanent PL bleaching. More interestingly, we also observed that the g-QDs that are prone to charging are more resistant to the creation of hot carrier traps and hence permanent PL bleaching. These fundamental understandings on structure-function relationships open path toward nano-engineering fine internal structure of g-QDs for higher emission efficiency and thermal stability.