Organization of nanoparticles Spontaneous Formation of Ordered 2D and 3D Superlattices of ZnO Nanocrystals** Myrtil L. Kahn,* Miguel Monge, Etienne Snoeck, AndrØ Maisonnat, and Bruno Chaudret* A large amount of research is presently devoted to the syn- thesis of nanoparticles of zinc oxide. [1–11] Zinc oxide is a lu- minescent, wide-bandgap semiconductor material that may find applications in a wide range of domains. [12] However, many applications require the organization of the nanoparti- cles into two- or three-dimensional (2D or 3D) superlatti- ces. [12] Recently, Yang and co-workers reported a 2D organi- zation of nanorods that display lasing properties in the ul- traviolet region. [13] This very efficient organization was ob- tained by a physical method, but chemical approaches to- wards such organizations would also be of interest since they are easy to perform and allow for a facile scale-up pro- cedure. In this respect, we recently reported a very simple synthetic method for the preparation of well-defined crystal- line zinc oxide nanoparticles. [14] The process takes advantage on one hand of the very exothermic reaction of the organo- metallic precursor bis(cyclohexyl) zinc (1) with water to produce crystalline zinc oxide and, on the other hand, of the kinetic control of the decomposition by using long-alkyl- chain amine ligands. However, if the use of amine ligands allows access to ZnO nanoparticles of low size dispersity, these particles are not monodisperse and consequently, no 2D or 3D organizations were observed. A lot of effort has recently concerned the formation of crystalline 2D or 3D superlattices of metals, [15–20] alloys, [21–23] or metal oxides. [24–26] In this field, we recently reported the formation of superlattices of tin nanoparticles [27] or iron nanocubes [28] using a combination of two types of ligands. In these two examples, the nanocrystal superlattices result from a crystallization process in solution involving both the particles and their ligand shells. This phenomenon has proven to be efficient for different zero-valent metallic nanoparticles. For metal-oxide nanoparticles, a few 3D or- ganizations resulting from particle self-assembly on a sur- face have been described. [24–26] The ZnO nanoparticles we recently reported [14] are produced by evaporating the sol- vent of the reaction solution, a method used in molecular chemistry to form monocrystals. Providing we could pro- duce monodisperse particles, a similar method seemed at- tractive to form 2D and 3D ordered superlattices. We report here the synthesis of ZnO nanoparticles stabilized by a com- bination of amine and acid ligands and their spontaneous organization into 2D- and 3D ordered superlattices from colloidal solutions. The synthesis of the ZnO nanoparticles is carried out by slow hydrolysis in air of a THF solution containing the pre- cursor ZnCy 2 (1) as well as one equivalent of a long-alkyl- chain amine and half an equivalent of a long-alkyl-chain acid. Thus, when the reaction mixture was exposed to air and left standing at room temperature, the solvent slowly evaporated and left a white and luminescent product, which was characterized by X-ray diffraction (XRD) and transmis- sion electron microscopy (TEM) as crystalline, isotropic ZnO nanoparticles ( 3.5 nm mean diameter) and very low size dispersity. This procedure was adapted from that previ- ously described elsewhere. [14] Different ligand systems were studied, combining hexadecylamine, dodecylamine, and oc- tylamine on one side (HDA, DDA, and OA) and octanoic acid, oleic acid, and lauric acid on the other (OcA, OlA, and LcA). Table 1 summarizes the results obtained under various reaction conditions and Figure 1 illustrates some ex- amples of the materials obtained. All new materials were characterized by TEM as well as by XRD and/or selected- area electron diffraction (SAED). In all cases the nanoparti- cles were of isotropic shape and consistently displayed the hexagonal zincite phase (space group P6 3 mc). It is notewor- thy that the presence of a carboxylic acid is essential to obtain monodisperse isotropic nanoparticles but that the nature of the amine and acid ligands has only a very limited influence on the size of the particles. If the synthesis is carried out in similar conditions but in the absence of solvent, a liquid fraction is formed even if the reagents are initially solids. Whatever the proportions of the components, the reaction yields isotropic nanoparticles, in contrast to the long nanorods obtained when only long- alkyl-chain amines are present, [14] which is probably a result of modification of the self-organization of the ligands. The size of the particles obtained under these conditions is inde- pendent of the reaction conditions and similar to those re- sulting from the reaction in THF, but with a broader disper- [*] Dr. M. L. Kahn, Dr. M. Monge, Dr. A. Maisonnat, Dr. B. Chaudret Laboratoire de Chimie de Coordination CNRS, UPR 8241 205 route de Narbonne, 31077 Toulouse Cedex 4 (France) Fax: (+ 33) 561553003 E-mail: mkahn@lcc-toulouse.fr chaudret@lcc-toulouse.fr Dr. E. Snoeck Centre d’Elaboration de MatØriaux et d’Etudes Structurales 29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4 (France) [**] This research was supported by the Centre National de la Recher- che Scientifique (CNRS). M. M. thanks the Spanish M.E.C.D. (Fel- lowship: EX2002-0319) for financial support. small 2005, 1, No. 2, 221 –224 DOI: 10.1002/smll.200400007  2005 Wiley-VCH Verlag GmbH & Co. KGaA, D-69451 Weinheim 221