Gresback et al. Nanoscale Research Letters 2011, 6:68 http://www.nanoscalereslett.com/content/6/1/68 functionalization of the NCs with ligands and for grow- ing an inorganic zinc sulfide (ZnS) shell around the plasma-synthesized InP NCs. Experimental InP NCs were synthesized using a nonthermal radio-fre- quency (13.56 MHz) plasma operated at 2.5 Torr with nominal power ranging from 50 to 80 W applied through a matching network. A system schematic is shown in Figure 1. The reactor consists of two ring elec- trodes around a 9.4-mm-outer diameter quartz tube with a 6.25-mm inner diameter. Similar reactor designs were reported for the synthesis of Si [10] and Ge NCs [12]. The ground and powered electrodes were, respec- tively, located 3 and 4 cm from the grounded metal fit- ting. A precursor gas mixture of 15% phosphine (PH3) in hydrogen (H2) and trimethylindium (TMIn, In(CH3)3) vapor was introduced into the reactor. TMIn sublimes at room temperature allowed its vapor to be entrained in an argon carrier gas stream [13]. Additional argon is used to sustain the plasma and dilute the precursors. This study used a ratio of 90 Ar:17 H2:3 PH3:1 TMIn, with total mass flow rates being varied between 34 and 172 sccm, which resulted in residence times of the gas within the plasma zone to be between 2 and 10 ms. The precursors were dissociated in the plasma by the hot plasma electrons, leading to nucleation and growth of InP NCs. The NCs were transported out of the Page 2 of 6 plasma region by gas drag and deposited on a stainless steel mesh as a dry powder. The powder was extremely sensitive to air exposure; therefore, all the processes were performed under the exclusion of oxygen and moisture. After the synthesis, the NC material was transferred under nitrogen to a glove box or Schlenk line for handling and further reactions. The NCs pro- duced in the plasma were found to react readily with ligands and coordinating solvents, such as amines, phos- phine oxides, and fatty acids in the presence of non- coordinating solvents at temperatures less than 200°C in less than 1 h. Based on the article by Xie et al. [14], the ZnS shells were grown as follows: a known mass of dry InP NCs was transferred into a solution of octadecene (ODE) and myristic acid under nitrogen. Then, the solution was sonicated for 5 min and heated to 200°C for 30 min under a flow of nitrogen. Each monolayer of ZnS was grown by using preheated and deoxygenated solutions of 0.05 M zinc stearate in ODE and sulfur in ODE. The zinc and sulfur precursors were injected separately at 15-min intervals with stoichiometric ratios (based on a spherical volume thickness calcula- tion) and heated to 220°C. The dispersion in ODE was then cooled and repeatedly washed with methanol, precipitated by the addition of acetone, and separated by centrifugation. The InP/ZnS NCs were then re- dispersed in toluene. Figure 1 Nonthermal plasma synthesis of InP NCs. (left) Photograph and (right) schematic representation.