Increasing the coverage of photoactive ligands on the surface of nanocrystals through a hybrid metal-oxide-ligand shell
Ligand exchanges on the surface of nanocrystals are ubiquitously employed for applications in photocatalysis, optoelectronics and biological imaging [1]. Particularly the positioning of polyaromatic hydrocarbons (PAH) ligands on semiconductor nanocrystals (NCs) has been crucial in harvesting or sensitizing molecular excited states obtained from multiexcitonic processes such as singlet fission and triplet fusion [3].
To maximally benefit from these processes the PAH density on the surface of NCs needs to be maximized. This is typically achieved through a one-for-one exchange driven by mass action between the PAH and native ligands [3]. However, the steric hindrance and limited solubility of the PAH-NC hybrids restricts the amount of PAH ligands that can be placed on NC surfaces.
Here, we demonstrate how to bypass this inherent limit by growing an alumina shell by colloidal atomic layer deposition (c-ALD) [4]. This is achieved by noting that the deposition of alumina locks the native ligands into the shell, and subsequently, new ligands can be added and are observed to selectively bind to new exposed binding sites on the alumina shell without displacing native ligands [5]. This process effectively increases the ligands density around the surface of NCs. We leverage this observation by growing alumina shells that embed a large number of PAH ligands allowing to significantly increase the PAH density. The formation of this metal-oxide-ligand shell promises to synergistically boost multiexcitonic processes while endowing enhanced stability to the NC core.