Enhanced visible photoluminescence in ZnO quantum dots by promotion of oxygen vacancy formation† Adersh Asok, a Mayuri N. Gandhi a and A. R. Kulkarni * b Received 30th April 2012, Accepted 18th June 2012 DOI: 10.1039/c2nr31044a We report on the synthesis of ZnO quantum dots (QDs) rich in oxygen vacancies by inducing an oxygen deficient environment. The precise tunability of particle size is achieved by counter ion capping of the precursor used for synthesis. The prepared QDs show size tunable visible emission with high quantum yield. Wide band gap semiconductor ZnO nanostructures have gained a great deal of attention due to their exceptional optical, magnetic and electronic properties, compared with their bulk counterparts. These unique properties arise from the quantum confinement effect and rich defect chemistry in this material system. 1 Further improvement in these physical properties would make commercial utility a reality in photonics, spintronics, energy applications, catalysis, sensors, UV protection films, biocidal coating, in vitro and in vivo diagnostics and therapeutic applications. 2 The direct band gap (E g ¼ 3.37 eV) and large exciton binding energy (60 meV) make ZnO one of the brightest emitters among available wide band gap semiconductors. 3 The other added advantage of ZnO QDs is its biocompatibility while consid- ering toxic heavy metal based semiconductor QDs (e.g., CdTe, CdSe, etc. ). 4 A wide variety of solution based techniques have been reported to prepare undoped multi-colour emitting ZnO QDs, 5 unfortunately these QDs exhibit low quantum yield (QY), when compared with their doped counterparts. 2d,6 Methods to improve the QY of undoped ZnO QDs still remain less explored. In order to devise a suitable synthesis method for processing highly luminescent ZnO QDs a thorough understanding of the photophysics behind the visible emission is required. A theoretical model by Guisbiers suggests that at the nanoscale, the high surface area to volume ratio of nano- particles will favour the formation of more vacancy defects on the surface of nanoparticles. 7 In the case of ZnO QDs, the origin of visible photoluminescence is linked to the presence of defects due to oxygen vacancy (V o ). Based on V o , different plausible luminescence mechanisms have been proposed. 8 Zhang et al. reported recently on the nature of size tunable visible emission in ZnO QDs, which showed that both the defect energy state (due to single ionized oxygen vacancy (V o _ )) and the quantum confinement are the major factors contributing to tunable visible emission. 9a The emission peaks for defect related transitions are broad and the reason for broadening of emission peaks is not clearly understood; it may be due to (1) poly- dispersity in particle-size distribution or (2) the result of several closely spaced transitions inherent within the nanocrystal. 9b However, the correlation between the role of surface defects in the luminescence intensity and the broadening observed in the visible emission is not yet fully understood. From the available literature it can be inferred that inducing more point defects along with precise control of particle size will result in size tunable visible photoluminescence with sharp emission bands. ZnO nanoparticles prepared at room temperature (RT) through the sol–gel method show strong visible fluorescence, attributed to rich surface defects. 10 On the other hand metal oxide nanoparticles, syn- thesised under inert atmosphere, promote the formation of more oxygen defects resulting in enhanced luminescence. 11 It is well known that controlled growth of ZnO nanoparticles can be achieved by varying the [Zn]/[OH] molar ratio resulting in different particle sizes, due to the attraction of counter ions by the hydroxyl ions around the QDs forming a virtual capping layer. 12 Based on this understanding, we hypothesise that ZnO QDs synthesised at RT and in oxygen free ambient conditions along with varying [Zn]/[OH] molar ratios could result in the formation of defect rich QDs of different sizes. In this communication, we report an ethanolic precipitation synthesis designed at RT for defect rich ZnO QDs. An inert oxygen deficient environment was created by ultrasonic degasification of the precursor solutions and purging nitrogen to the reaction vessel during synthesis. The size tunability was achieved by varying the [Zn]/[LiOH] molar ratio. This synthesis procedure resulted in defect rich ZnO QDs with size tunable visible emission. A detailed procedure for the synthesis of defect rich ZnO QDs is given in the ESI.† This synthesis procedure promotes oxygen vacancy formation on the surface of QDs resulting in high QY as listed in Table 1. All the diffraction peaks in X-ray diffraction patterns (Fig. S2†) of different size QDs were indexed to the wurtzite crystal structure of ZnO (JCPDS card no. 36-1451). Impurity and residue related peaks were not observed, which confirmed the phase purity of the synthe- sized ZnO QDs. The broadening in the diffraction peaks was observed. The high resolution TEM image (Fig. 1a) shows high a Centre for Research in Nanotechnology and Science, Indian Institute of Technology Bombay, Mumbai 400076, India b Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India. E-mail: ajit. kulkarni@iitb.ac.in; Fax: +91-22-2572-3480; Tel: +91-22-25767636 † Electronic supplementary information (ESI) available: Experimental procedures, XRD patterns, additional TEM images, UV-Vis absorption data, PL intensity comparison and quantum yield are included. See DOI: 10.1039/c2nr31044a This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 4943–4946 | 4943 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 4943 www.rsc.org/nanoscale COMMUNICATION Published on 20 June 2012. Downloaded by INDIAN INSTITUTE OF TECHNOLOGY BOMBAY on 10/11/2013 05:32:49. View Article Online / Journal Homepage / Table of Contents for this issue