LETTERS nature materials | VOL 3 | DECEMBER 2004 | www.nature.com/naturematerials 891 Ultra-large-scale syntheses of monodisperse nanocrystals JONGNAM PARK 1 , KWANGJIN AN 1 , YOSUN HWANG 2 , JE-GEUN PARK 2 , HAN-JIN NOH 3 , JAE-YOUNG KIM 3 , JAE-HOON PARK 3 , NONG-MOON HWANG 4 AND TAEGHWAN HYEON 1 * 1 National Creative Research Center for Oxide Nanocrystalline Materials and School of Chemical Engineering, Seoul National University, Seoul 151-744, Korea 2 Department of Physics and Institute of Basic Science, Sungkyunkwan University, Suwon 440-746, Korea 3 Department of Physics and Pohang Light Source, Pohang University of Science and Technology, Pohang 790-784, Korea 4 School of Materials Science & Engineering and Nano-Systems Institute (NSI-NCRC), Seoul National University, Seoul 151-744, Korea *e-mail: thyeon@plaza.snu.ac.kr Published online: 28 November 2004; doi:10.1038/nmat1251 T he development of nanocrystals has been intensively pursued, not only for their fundamental scientifc interest, but also for many technological applications 1–3 . Te synthesis of monodisperse nanocrystals (size variation <5%) is of key importance, because the properties of these nanocrystals depend strongly on their dimensions. For example, the colour sharpness of semiconductor nanocrystal-based optical devices is strongly dependent on the uniformity of the nanocrystals 3–6 , and monodisperse magnetic nanocrystals are critical for the next-generation multi-terabit magnetic storage media 7–9 . For these monodisperse nanocrystals to be used, an economical mass-production method needs to be developed. Unfortunately, however, in most syntheses reported so far, only sub-gram quantities of monodisperse nanocrystals were produced. Uniform-sized nanocrystals of CdSe (refs 10,11) and Au (refs 12,13) have been produced using colloidal chemical synthetic procedures. In addition, monodisperse magnetic nanocrystals such as Fe (refs 14,15), Co (refs 16–18), γ-Fe 2 O 3 (refs 19,20), and Fe 3 O 4 (refs 21,22) have been synthesized by using various synthetic methods 23 . Here, we report on the ultra-large-scale synthesis of monodisperse nanocrystals using inexpensive and non-toxic metal salts as reactants. We were able to synthesize as much as 40 g of monodisperse nanocrystals in a single reaction, without a size-sorting process. Moreover, the particle size could be controlled simply by varying the experimental conditions. Te current synthetic procedure is very general and nanocrystals of many transition metal oxides were successfully synthesized using a very similar procedure. The process conditions required for the synthesis of monodisperse particles of micrometre size 24 are relatively well established, and a similar principle could be applied to the synthesis of uniform-sized nanocrystals. The inhibition of additional nucleation during growth, in other words, the complete separation of nucleation and growth, is critical for the successful synthesis of monodisperse nanocrystals. Our research group developed new procedures for the synthesis of monodisperse nanocrystals of metals 19,23,25 , metal oxides 19,26,27 , and metal sulphides 28 without a laborious size-sorting process. In particular, during the direct synthesis of monodisperse iron and iron oxide nanocrystals 19,23 , we were able to ascertain that the iron–oleate complex, which is generated in situ from the reaction of iron pentacarbonyl and oleic acid, is decomposed and acts effectively as a growth source in synthesizing monodisperse nanocrystals with increased particle size. From these results, we reasoned that a metal–surfactant complex would make an effective growth source for the synthesis of monodisperse nanocrystals. The overall synthetic procedure is depicted in Fig. 1 and the detailed experimental procedures are described in the Methods section. Instead of using toxic and expensive organometallic compounds such as iron pentacarbonyl, we prepared the metal– oleate complex by reacting inexpensive and environmentally friendly compounds, namely metal chlorides and sodium oleate. The Fourier transform infrared spectrum of the iron–oleate complex, which was prepared by reacting iron chloride (FeCl 3 ·6H 2 O) and sodium oleate, shows a C=O stretching peak at 1,700 cm –1 , which is a characteristic peak for a metal–oleate complex (see Supplementary Information, Fig. S1). The iron–oleate complex in 1-octadecene was slowly heated Metal chloride Na–oleate Metal–oleate complex Metal–oleate complex + + NaCl Thermal decomposition in high boiling solvent Monodisperse nanocrystals 20 nm Figure 1 The overall scheme for the ultra-large-scale synthesis of monodisperse nanocrystals. Metal–oleate precursors were prepared from the reaction of metal chlorides and sodium oleate. The thermal decomposition of the metal–oleate precursors in high boiling solvent produced monodisperse nanocrystals. ©2004 Nature Publishing Group