Synthesis and Characterization of Mn-Doped ZnO Nanocrystals Ranjani Viswanatha, ‡ Sameer Sapra, ‡ Subhra Sen Gupta, ‡,² B. Satpati, § P. V. Satyam, § B. N. Dev, § and D. D. Sarma* ,‡ Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012, India, and Institute of Physics, SachiValaya Marg, Bhubaneshwar-751005, India ReceiVed: January 5, 2004; In Final Form: February 28, 2004 We report the synthesis and characterization of several sizes of Mn-doped ZnO nanocrystals, both in the free-standing and the capped particle forms. The sizes of these nanocrystals could be controlled by capping them with polyvinylpyrollidone under different synthesis conditions and were estimated by X-ray diffraction and transmission electron microscopy. The absorption properties of PVP-capped Mn-doped ZnO exhibit an interesting variation of the band gap with the concentration of Mn. Fluorescence emission, electron paramagnetic resonance, and X-ray absorption spectroscopy provide evidence for the presence of Mn in the interior as well as on the surface of the nanocrystals. I. Introduction Dilute magnetic semiconductors (DMSs) have attracted a great deal of attention in the past few years as enabling materials in the emerging field of “spintronics”. 1 DMSs are semiconductor solid solutions, where a small percentage of cations are replaced by magnetic impurities such as Mn. Due to the host sp-Mn d interactions in these DMSs, unusual magnetotransport and magnetooptical phenomena like large Faraday rotations, giant negative magnetoresistances, and magnetic field induced metal- insulator transitions have been observed. 2 Among the DMSs, Mn-doped II-VI compounds have been extensively studied. 3 However, most of the II-VI compounds studied are chalco- genides; the corresponding oxides are comparatively less investigated. Bulk ZnO is a well-known wide band gap (3.3 eV) semi- conducting material. In recent times, bulk ZnO has attracted increasing attention as it is a potentially useful material for a wide range of applications, such as solar cells, luminescent devices, and chemical sensors. 4 Studies have been carried out to fine-tune the properties of ZnO to adopt it for different applications; for example, the band gap of ZnO is modified to use as UV detectors and emitters. 5 Doping Mn into the bulk ZnO matrix offers an interesting way to alter various proper- ties, 6,7 for example, the band gap of the host material can be tuned from 3.3 eV to 3.7 eV. Additionally, it also alters the emission properties by providing an efficient channel for the recombination of the electron and the hole via the dopant Mn d levels. The optical and electronic properties of semiconductors can be further tuned by varying the size of the particles in the range below 10 nm. Additionally, it is recently reported 8 that Mn-doped ZnO thin films as well as in bulk exhibit ferromag- netism at room temperature. Studies on various Mn-doped semiconductor nanocrystals 9 have revealed that the properties of these samples, like the band gap, are influenced by the quantum confinement of electronic states; accordingly, these properties of the doped nanocrystals are considerably different compared to those of the doped bulk system. 10 These interesting changes in confined DMSs have enlarged the scope of research activity in this field, prompting us to study the confinement effects in Mn-doped ZnO nanocrystals. While it has been known 11 for a while that it is possible to produce free-standing ZnO nanocrystals, synthesizing such free- standing ZnO nanocrystals doped with a transition metal ion has been realized only recently by doping with Co. 12 We have been pursuing the synthesis of good quality free-standing ZnO nanocrystals doped with other transition metal ions by exploring various synthetic routes. Here we report for the first time the synthesis and characterization of Mn-doped free-standing ZnO nanocrystals with a particle size of a few nanometers. While the diameter of these free-standing particles varied from ∼5 nm to ∼10.5 nm, to obtain smaller particles and a good control over the size, we have capped Mn-doped ZnO nanocrystals with polyvinylpyrollidone (PVP). These nanocrystals are character- ized using energy dispersive analysis of X-rays (EDAX), inductively coupled plasma atomic emission spectrometry (ICP- AES), X-ray diffraction (XRD), transmission electron micros- copy (TEM), and ultraviolet-visible (UV-vis) absorption spectroscopy. Fluorescence spectroscopy (PL), electron para- magnetic resonance (EPR), and X-ray absorption spectroscopy (XAS) have been used to probe the nature of doped Mn ions in the ZnO nanocrystal matrix. II. Experimental Section Materials. Zinc acetate dihydrate (Zn(OAc) 2 ‚2H 2 O) (Ranb- axy), manganese acetate tetrahydrate (Mn(OAc) 2 ‚4H 2 O) (Ran- baxy), PVP (Aldrich, MW 10 000), sodium hydroxide (NaOH) (Ranbaxy), and isopropyl alcohol (i-PrOH) (Merck) were used as received. Double-distilled water was employed for all experiments. Synthesis. In a typical synthesis of the Mn-doped ZnO nanocrystals, the required amount of Mn(OAc) 2 ‚4H 2 O was dissolved in 1 mL of water. This solution was added to 100 mL of the solvent, i-PrOH, under vigorous stirring. After * Corresponding author. E-mail: sarma@sscu.iisc.ernet.in. Also at the Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Ban- galore-560064, India. ‡ Indian Institute of Science. § Institute of Physics. ² Also at the Department of Physics, Indian Institute of Science, Bangalore-560012, India. 6303 J. Phys. Chem. B 2004, 108, 6303-6310 10.1021/jp049960o CCC: $27.50 © 2004 American Chemical Society Published on Web 04/23/2004