Journal of Alloys and Compounds 487 (2009) 466–471 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Tuning of ultra-violet to green emission by choosing suitable excitation wavelength in ZnO: Quantum dot, nanocrystals and bulk L. Robindro Singh a , R.S. Ningthoujam b,∗ , S. Dorendrajit Singh c a Department of Physics, PU College, Mizoram University, Aizawal, 796001, India b Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India c Department of Physics, Manipur University, Canchipur, 795 003, India article info Article history: Received 2 June 2009 Received in revised form 28 July 2009 Accepted 29 July 2009 Available online 5 August 2009 Keywords: Nanostructures Optical materials Chemical synthesis Luminescence abstract ZnO particles (quantum dots to bulk) have been prepared using urea hydrolysis and are dispersible in methanol. In photoluminescence study, as-prepared sample shows the main emission peaks at 375 and 394 nm, which are assigned to the exciton and band-edge emissions, respectively. The former peak disappears when as-prepared sample is heated at 300, 500, 700 and 900 ◦ C due to particle size effect. Interestingly, higher heated samples shows emission peak at 505 nm and it is attributed to oxygen vacancy created in ZnO lattice. The intensity ratio of UV-light at 394 nm to green light at 505 nm can be adjusted by choosing suitable excitation wavelengths and also particle size or heat-treatment. Optimum emission intensities at 394 and 505 nm can be obtained on 360–370 and 320–330 nm excitations, respectively. These materials will be useful in optoelectronic applications. © 2009 Elsevier B.V. All rights reserved. 1. Introduction In recent years, study on the low dimensional semiconduc- tors has been one of unabated topics because, these materials will find more and more applications in the technological devel- opment for future industries [1–26]. As a typical quantum solid, nano-semiconductor materials have more novel quality than their bulk materials. The surface defects, either intrinsic or due to unintentional doping (impurities), are often found to reduce the performance for such applications. For example, surface impurities such as hydroxyl are known to quench the exciton luminescence in ZnO [27]. At nanoscale the size not only directly impacts the defects contents but also alters the optical properties of materials. As a matter of fact, unlike the luminescence properties of bulk crystals which have been well characterized the photoluminescence (PL) spectra of nanostructured ZnO largely depend on synthesis meth- ods, crystallite size and structure, and probably the most important, the defect contents in core and surfaces [27–31]. Group II–VI semi- conductor materials have many new properties that attract both fundamental and technological interest to many researchers. The development of advanced display and lighting technology such as field-emission displays and plasma display panels require phos- phors which has a high efficient and low degradation [32,33]. Since sulphide is known to easily degrade at high current densities, the ∗ Corresponding author. Tel.: +91 22 25505151; fax: +91 22 25592321. E-mail address: rsn@barc.gov.in (R.S. Ningthoujam). research and development of oxide-based phosphors have become very important. Wide band-gap semiconductor materials are usually used for full color display [34]. ZnO is a II–VI semiconductor which crys- tallizes in the hexagonal wurtzite structure with wide band of 3.2 eV [35,36] and with large exciton binding energy of 60 meV at room temperature and it gives versatile applications. The numerous advantages of ZnO over other wide band-gap materials such as GaN or ZnSe are its higher chemical and thermal stability, higher radi- ation hardness and lower growth temperature. Study on bulk and nanoparticles is one of the active areas of research due its wide tech- nological application on optoelectronic, catalytic, display devices, gas sensors, photonic crystals, photodetectors, optical waveguides, electrical devices, and solar cells [37–43]. It has been reported that ZnO would be an alternative to TiO 2 nanostructured electrode in Grutzel-type photocells [44]. Moreover ZnO can be made as trans- parent and highly conductive or piezoelectric components as well [45,46]. Many synthetic methods have been reported to prepare ZnO nanoparticles during the couple of decades. Among the synthetic methods, sol–gel method [47], chemical precipitation [48–50], solvothermal/hydrothermal reaction [51–53] and gas-phase reac- tion [54] are commonly used to prepare ZnO bulk and nanosized particles. In the present preparation of ZnO nanoparticles we have used ethylene glycol as capping agent. As-prepared ZnO nanoparticles can be dispersed in organic solvents such as methanol and this will help in incorporation of ZnO particles in polymer based thin 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.166