Gresback et al. Nanoscale Research Letters 2011, 6:68 http://www.nanoscalereslett.com/content/6/1/68 N A N O E X P R E S S Open Access Combined plasma gas-phase synthesis and colloidal processing of InP/ZnS core/shell nanocrystals Ryan Gresback1, Ryan Hue2, Wayne L Gladfelter2, Uwe R Kortshagen1* Abstract Indium phosphide nanocrystals (InP NCs) with diameters ranging from 2 to 5 nm were synthesized with a scalable, flow-through, nonthermal plasma process at a rate ranging from 10 to 40 mg/h. The NC size is controlled through the plasma operating parameters, with the residence time of the gas in the plasma region strongly influencing the NC size. The NC size distribution is narrow with the standard deviation being less than 20% of the mean NC size. Zinc sulfide (ZnS) shells were grown around the plasma-synthesized InP NCs in a liquid phase reaction. Photoluminescence with quantum yields as high as 15% were observed for the InP/ZnS core-shell NCs. Introduction Over the last two decades, semiconductor nanocrystals (NCs) have attracted significant attention because of their various unique properties. Semiconductor NCs provide size-tunable optical and electrical properties, based on quantum confinement of charge carriers within them, high surface-to-volume ratios, and other attributes that have led to the development of a new generation of materials and devices [1,2]. A significant amount of study has focused on compound semiconduc- tor NCs of the II-VI and IV-VI systems because of the relative ease of synthesizing high quality materials using colloidal techniques [3,4]. While synthesis and surface functionalization methods for these NC materials are well established and NCs have shown impressive optical and electronic properties, there exists considerable inter- est to produce systems consisting of NCs of other mate- rials whose constituent elements pose less of an environmental concern, which are more radiation resis- tant, and less prone to photo-oxidation. Compound semiconductor NCs of the III-V systems may offer some of these desirable attributes. While III-V semiconductors are known to be “radiation-hard,” they also are direct bandgap semiconductors, which is a big advantage for optical applications compared to, for * Correspondence: kortshagen@umn.edu 1Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN, USA Full list of author information is available at the end of the article instance, group IV NC materials [5]. Hence, there has been considerable interest in the synthesis of high qual- ity III-V compound NCs, such as indium phosphide (InP). While synthesizing methods similar to those of II- VI semiconductor NCs can be applied to InP NCs, these have turned out to be very difficult and time consuming, often requiring days to produce NCs of high quality [6,7]. More recently, synthesis of high quality InP NCs has been achieved with non-coordinating solvents [8] and weak-coordinating solvents [9]. However, as these synthesizing methods require organic surfactants to sta- bilize NCs in solution during synthesis, they provide challenges for applications of the synthesized NCs in devices because of the often electrically insulating nature of the ligands. Synthesis of NCs using nonthermal plasmas has become a viable method even for materials, which need to be produced in crystalline form requiring high tem- peratures, such as Si [10,11] and Ge [12]. An added advantage of plasma synthesis as compared to liquid phase routes is that the resulting material is free of ligands or surfactants, and that the ligands can be added later depending on the targeted application. This makes integration of plasma-produced NCs into devices easier, as it eliminates intermediate steps of ligand exchanges often required for II-VI and IV-VI NCs. In this article, we propose a method to synthesize InP NCs with a nonthermal plasma. In addition, we show that solution chemistry routes can be used for post-synthesis © 2011 Gresback et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.