Abstract: Highly luminescent nanostructures, particularly highly luminescent quantum dots, comprising a nanocrystal core and thick shells of ZnSe and ZnS, are provided. The nanostructures may have one or more gradient ZnSexS1-x monolayers between the ZnSe and ZnS shells, wherein the value of x decreases gradually from the interior to the exterior of the nanostructure. Also provided are methods of preparing the nanostructures comprising a high temperature synthesis method. The thick shell nanostructures of the present invention display increased stability and are able to maintain high levels of photoluminescent intensity over long periods of time. Also provided are nanostructures with increased blue light absorption.