A General Approach to Stabilize Nanocrystal Superlattices by Covalently Bonded Ligands.

Self-assembled inorganic nanocrystal (NC) superlattices are powerful material platforms with diverse structures and emergent functionalities. However, their applications suffer from the low structural stability against solvents and other stimuli, due to the weak interparticle interactions. Existing strategies to stabilize NC superlattices typically require the design and incorporation of special ligands prior to self-assembly and are only applicable to superlattices of certain NCs, ligands, and structures. Here we report a general method to stabilize superlattices of various NC compositions and structures via strong, covalently bonded ligands. The core is the use of light-triggered, nitrene-based cross-linkers that do not interfere the self-assembly process while nonspecifically and effectively bonding the native ligands of NCs. The stabilized 2D and 3D superlattices of metal, semiconductor, and magnetic NCs retain their structures when being exposed to solvents of different polarities (from toluene to water) and show high thermal stability and mechanical rigidity. This can further stabilize binary NC superlattices, beyond those achievable in previous methods. Stabilized superlattices show robust and reproducible functionalities, for instance, when serving as reusable substrates for surface enhanced Raman spectroscopy. These results create more possibilities in exploiting the impressive library of NC superlattices in a broad scope of applications.