Gresback et al. Nanoscale Research Letters 2011, 6:68 http://www.nanoscalereslett.com/content/6/1/68 Page 4 of 6 Figure 4 Raman spectra of InP NCs passivated with myristic acid and synthesized using various plasma residence times; NC diameters from TEM. UV-Vis absorption was performed on colloidal InP NCs with myristic acid as the ligand after being heated to 190°C for 1 h. Figure 5 shows that the onset of the absorption changes as a function of the residence time of the NCs in the plasma. Shorter residence times lead to a blue shift in the onset of the absorption, corre- sponding to smaller NCs. The first exciton peak is not as clearly defined as in solution-synthesized InP NCs, likely because of the broad size distribution. The onset of the absorption correlates well with sizes observed in TEM. Both the as-produced NCs and the NCs solely functio- nalized with organic ligands show little or no PL. The NCs that were allowed to oxidize exhibit a small increase in PL; however, the quantum yield (QY) of these samples was still less than 1%. Low QY in InP NCs is often attributed to phosphorus-dangling bonds at the surface which lead to non-radiative recombination of photo-generated carriers [17]. Several methods were proposed to eliminate non-radiative recombination sites, including fluorine etches [17] and capping of NCs with a wideband gap semiconductor such as ZnS [14]. In this study, we used the method of growing a ZnS layer. Figure 6 shows the representative PL spectra of several samples of InP/ZnS core-shell NCs. Peak emission from the InP/ZnS NCs varies from 508 to 635 nm with the full-width half-maximum of the PL varying between 70 and 100 nm. The peak emission, like the absorption Figure 5 UV-Vis absorption spectra of InP NCs passivated with myristic acid synthesized using various residence times in the plasma. onset, is strongly dependent on the residence times of the NCs in the plasma. Figure 7 shows the relationship between the residence time and peak emission. Unfortu- nately, owing to experimental setup limitations, a wider range of residence time could not be explored. However, there is no indication that a continuous range of emission from bulk emission at 1.37 eV to quantum- confined NC emission at 2.5 eV could not be achieved. As is shown in the inset of Figure 7, it is remarkable to see that a single monolayer of ZnS leads to a significant increase of the PL QY. The addition of more ZnS monolayers does not lead to a further increase in PL, likely because all non-radiative states were passivated by the first monolayer. Quantum yields between 10 and 15% were measured for InP/ZnS structures. This is lower than the best results reported by Xie et al. [14] for solution-grown InP capped by ZnS using zinc stearate and elemental sulfur, and the cause for this difference is unknown. In future studies, the authors will explore the use of more reactive precursors, such as diethylzinc and bis(tri- methylsilyl)sulfide, which have also been found effective for coating solution-grown InP NCs resulting in higher emission QYs [18]. Conclusion In summary, InP NCs with controllable size were suc- cessfully synthesized from the flow-through nonthermal plasma. Size control was achieved through adjusting the