Nanocrystal Inks without Ligands: Stable Colloids of Bare Germanium Nanocrystals

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Entry by Yuhang Jin, AP225 Fall 2011

Reference

Zachary C. Holman and Uwe R. Kortshagen, Nano Lett., 2011, 11, 2133.

Key words

nanocrystal, quantum dot, colloidal stability, germanium, benzonitrile

Introduction

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

File:Nl200774y 1
Fig.1 Bare Ge NCs dispersed in benzonitrile, with colloid concentrations 5, 0.5, and 0.04 mg/mL from left to right.

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.

Germanium NCs were collected as a powder downstream of the plasma reactor, and transferred air-free to vials. Solubility of Ge NCs was investigated in a host of solvents whose properties were similar to 1,2-dichlorobenzene in dielectric constant, chemical structure, or Hansen Solubility Parameters. Many of the solvents contained benzene rings, and halogens or nitrile groups. Common solvents such as toluene, hexanes, and water were also tried. Ultrasonication was performed to break up the powder and suspend the NCs after solvents were added to the vials. Of all the solvents studied, benzonitrile, and to a lesser extent acetonitrile, solubilized bare Ge NCs upon contact: a brief shake alone could yield an optically transparent colloid (shown in Fig. 1).