(102d) Transformers: Colloidal Nanocrystals Converting Into Globules On Graphene Oxide

Because of its high carrier mobility and mechanical strength, graphene, a one-atom thick carbon sheet, is a promising two-dimensional (2-D) microcontact for nanometer length scale objects, such as nanocrystals. However, tethering nanocrystals on graphene remains challenging, as graphene has few functional sites to adhere nanocrystals to. Graphene oxide (GO), an oxidized form of graphene, has oxygen-rich moieties that can anchor the nanocrystals on the surface of the 2-D microcontact. We exchanged the ligand on the surface of nanocrystals so that the nanoparticles can bind electrostatically to GO. Interestingly, when blended with GO in aqueous solution for 12 h, the nanocrystals undergo substantial size increase and crystalline transformation. For example, 3.5 nm CdSe nanocrystals turn into ~500 nm amorphous Se globules. The crystalline transformation was investigated with nanocrystals of PbSe, Co, and CoPt₃ each blended with GO; notably, the crystalline transformation by acidic sites of GO turn out to be dictated by the bonding strength in the nanocrystals. We also identified the reaction conditions that allow uniform dispersion of intact nanocrystals on GO.