Local Structures of Zn 1-x TM x O (TM ) Co, Mn, and Cu) Nanoparticles Studied by X-ray Absorption Fine Structure Spectroscopy and Multiple Scattering Calculations Tao Liu,* ,† Hairuo Xu, ‡ Wee Shong Chin, ‡ Ping Yang, § Zhihua Yong, † and Andrew T. S. Wee † Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117542, Department of Chemistry, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117543, and Singapore Synchrotron Light Source, National UniVersity of Singapore, 5 Research Link, Singapore 117603 ReceiVed: January 7, 2008; ReVised Manuscript ReceiVed: July 17, 2008 Transitional metal doped ZnO is a good candidate for dilute magnetic semiconductors possessing high Curie temperature ferromagnetism. The local atomic configuration of dopant elements in ZnO is an important issue for understanding their ferromagnetic mechanism. In this work Co, Mn, and Cu doped ZnO nanoparticles with particle size of about 5 nm were prepared by the coprecipitation method. X-ray absorption fine structure spectra were measured at doppant metal K-edges for the as-prepared and calcinated samples. The results show significantly different local structural evolutions for various dopant element doping and heat treatment. Co-doped nanoparticles are stable up to high temperature calcinations, while Mn and Cu in ZnO exhibit complex interatomic diffusion and reduction behavior activated by modest calcinations, and this is explained by either a charge transfer from ZnO to doppant element or the reduction induced by thermal decomposition products of surfactants. Multiple scattering calculations were performed on Co substituted ZnO clusters to simulate the Co clustering in ZnO and its effect on the measured X-ray absorption fine structure spectra. Introduction Because of its direct band gap and large excitation energy, ZnO is a potential host material for realizing wide band gap dilute magnetic semiconductors (DMSs), particularly for sup- porting high Curie temperature ferromagnetism by doping with a variety of 3d transition metal (TM) ions. 1-3 The magnetic properties due to the doping of TM (Mn, Co, Fe, Cu, etc.) in ZnO prepared by a variety of methods have been reported. 4-10 However, some conflicting experimental results have been noted, and it has been argued that the observed ferromagnetism is not truly charge-mediated, but due to segregation or precipitates of ferromagnetic clusters. The difficulties experienced are the preparation or synthesis of uniformly TM-substituted ZnO DMS at the atomic level because of the insolubility or instability of impurities in the host materials, and also the lack of detailed microstructural characterization of atomic inhomogeneity. There- fore, despite numerous experimental and theoretical reports in the literature, the origin of DMS is still not clear. Because of the strong Zn-O interaction, it is difficult to break up this bond and realize uniform and reliable doping in ZnO. For example, much effort has been made to prepare p-type doped ZnO semiconductors by replacing either O or Zn by other elements such as N, P, Li, Na, etc.; however, no reproducible high-quality p-type ZnO has yet been successfully synthesized. 11 The solid state reaction method may not be a good choice as it involves high temperature calcinations, and since the doped DMS are metastable materials, high temperature treatment is not favorable for achieving uniform doping. Wet chemical synthesis is expected to provide better control over chemical homogeneity and morphology at an atomic or nanoparticle level. By capping particles with either second phase compounds or surfactant molecules, the functions of nanoparticles can be tuned. It is expected that the temperature required for sample post- treatment in nanoparticle form can be much reduced. Schwartz et al. 12 and Norberg et al. 13 synthesized colloidal Co 2+ -, Ni 2+ -, and Mn 2+ -doped ZnO quantum dots using a hydrolysis and condensation reaction method, and room temperature ferromag- netism was observed. Most studies of nanoparticles have so far focused on pure metals, pure metal oxides, or sulfides. The doping or mixing of impurities in nanoparticles can be very different from their bulk counterparts, which have been studied intensively. 13 An important issue for nanoparticles is the high interface or surface to volume ratio, which dominates the overall free energy of system and therefore affects the nucleation and precipitation of impurities as a second phase. In a previous article, we reported the in situ QEXAFS study of Co-doped ZnO (Zn 0.93 Co 0.07 O) nanoparticles, and the local structural evolution around Co and Zn atoms upon calcination were investigated and compared to the corresponding bulk materials. 15 In the present work, we extended our study to various dopant elements in ZnO, Co, Mn, and Cu, varied the doping concentrations, and calcinated the samples at two temperatures. Synchrotron radiation X-ray absorption fine structure (XAFS) experiments including both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements were performed at metal K-edges of the dopants. As a local probe, the XAFS technique is useful for investigating the local atomic configuration of a specific element and for providing local electronic and geometric information of the element investigated. The element specific character allows it to investigate the local structures of dopant elements in ZnO, particularly the occupation sites, defects and distortion, segregation, diffusion, etc. 16 To interpret the XANES features and explore the initial clustering and nucleation, we performed one electron multiple scattering calculations. The ferromagnetic measurement of the nanoparticles, however, is not the issue of the present context, which will be reported later. * Corresponding author. E-mail: tao.liu@iss.fzk.de. † Department of Physics. ‡ Department of Chemistry. § Singapore Synchrotron Light Source. J. Phys. Chem. C 2008, 112, 13410–13418 13410 10.1021/jp803908a CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008