Nanocrystal Inks without Ligands: Stable Colloids of Bare Germanium Nanocrystals
Entry by Yuhang Jin, AP225 Fall 2011
Zachary C. Holman and Uwe R. Kortshagen, Nano Lett., 2011, 11, 2133.
nanocrystal, quantum dot, colloidal stability, germanium, benzonitrile
Colloidal semiconductor nanocrystals (NCs) have received increasing attention recently for their potential in novel optical and electronic devices, and in particular inexpensive printing of quantum-confined thin films. Methods have been developed to synthesize II-VI and IV-VI NCs in solution with ligands, which provide steric stabilization in nonpolar solvents so that aggregation of NCs due to van derWaals forces does not occur. However, the ligands also hinder charge carrier transport between NCs, resulting in insulating films. Exchanging the ligands for shorter molecules or removing the ligands after film deposition seem to be alternative approaches, but they both have intrinsic difficulties due to the formation of covalent bonds or other adverse properties of the materials involved. To avoid such problems, this paper reports stable Ge NC colloids formed by dispersing ligand-free NCs in select solvents. This approach offers new opportunities for NC processing and device fabrication in which semiconducting films may be printed from solution without postprocessing.
Experimental details and results
Germanium NCs were synthesized with a nonthermal, continuous-flow plasma process: germanium tetrachloride vapor and hydrogen gas were dissociated in the plasma, allowing nanometer-sized Ge crystallites to nucleate via chemical clustering of the dissociation products. The diameter of the NCs was kept at 6 nm by controlling the duration of their residence in the plasma. The chemistry at the surface of the NCs is a mixture of H and Cl passivation. The NCs studied here were shown by ion beam measurements to have three times as much H on their surfaces as Cl.