Highly concentrated zinc oxide nanocrystals sol with strong blue emission M. Vafaee a , M. Sasani Ghamsari a,b,n , S. Radiman b a Solid State Lasers Research Group, Laser and Optics Research School, NSTRI, 11365-8486 Tehran, Iran b School of Applied Physics, Faculty of Science and Technology, National University of Malaysia (UKM), 43600 Bangi, Selangor, Malaysia article info Article history: Received 22 May 2010 Received in revised form 4 August 2010 Accepted 22 September 2010 Available online 1 October 2010 Keywords: ZnO nanocrystals Sol–gel Blue emission abstract Highly concentrated ZnO sol was synthesized by an improved sol–gel method. Water was used as a modifier to control the sol–gel reaction and provide a way to increase the sol concentration. Concentration of ZnO in the prepared sol is higher than from other methods. Optical absorption and photoluminescence were used to investigate optical properties of the prepared sol. FTIR test was performed to study the influence of water on the compounds of as-prepared sol. The size and morphology of ZnO nanoparticles have been studied by HRTEM. The prepared colloidal ZnO nanocrystals have narrow size distribution (5–8 nm) and showed strong blue emission. The prepared sol has enough potential for optoelectronic applications. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Unique optical and electronic properties of ZnO make it an attractive nanomaterial for use in different applications such as thin film transistors, solar cells, sensors, detectors and so on [1,2]. Among wide-band gap semiconductors, ZnO with a band gap of 3.37 eV at room temperature and a large exciton binding energy (60 meV) is a promising luminescent material for application in UV and blue wavelength regions. In photoluminescence (PL) spectrum of the ZnO nanomaterial two different emission peaks have been recorded. Due to band edge emission, the first peak appeared in the ultraviolet (UV) region. The second peak is observed in the visible region and it is accepted that crystalline point defects are the main sources of visible peak. The emission of ZnO nanocrystals in two different wavelengths (UV and blue) is very important for integrated optoelectronics and biological fluorescence labeling [3]. In the visible range, the blue emission of ZnO nanocrystals is normally not as bright as the green–yellow one [4]. Therefore, producing ZnO nanoparticles with strong blue emission and understanding PL mechanisms in such nano- materials are highly anticipated. To date, several physical and chemical methods have been used to prepare ZnO nanocrystals with strong blue emission [5–8]. In chemical methods, several different strategies have been used to increase the intensity of blue emission. Surface modification as a prevalent strategy for improving the optical properties of semiconductor nanocrystals was used and ZnO nanomaterial was not excluded from this strategy. Various surface modifiers such as organic molecules or inorganic shells (SiO 2 ) and in situ initiating polymerization on surfaces of ZnO nanocrystals have been employed to prepare stable ZnO nanocrystals with blue emission [9–12]. On the other hand, the control of shape, morphology and size of synthesized ZnO nanocrystals was selected as the second strategy for this purpose [13–15]. Besides the control of size and mor- phology, easy band gap engineering (doping) of prepared ZnO nanocrystals is an alternative useful approach [16–18]. Since, the crystal defects have a critical role on the strong blue emission from ZnO nanocrystals, the control of these defects can help us to have a strong blue luminescent material [19]. Therefore, we need to find a way for introducing crystalline defects inside the prepared ZnO nanocrystals in liquid phase. In our last report [20], it has been shown that optical properties of prepared ZnO sols can be affected by concentration of ZnO sol, which is in agreement with obtained result by others [21]. We have motivated to prepare highly concentrated ZnO sol and check the possibility of increasing the blue emission intensity. The first sol– gel derived zinc oxide was reported by Spanhel and Anderson [22] in 1991. They controlled the size of ZnO nanocrystals by pH adjustment, which was based on the point zero charge (PZC) theory. Many other research groups have used such a system concerning their different features [23–25]. They used a solution of 0.1 M Zn 2+ concentration to synthesize ZnO nanoparticles. Simultaneously, Bahnemann et al. [26] and other researchers [27–29] employed a similar low concentration solution with different solvents and hydrolyzing agents. If, in such sol concen- tration, Zn 2+ increases and enriches to 1 M or more, the zinc oxide nanoparticles will precipitate as uncontrollable aggregated par- ticles. Consequently, for the preparation of highly concentrated Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.09.042 n Corresponding author at: School of Applied Physics, Faculty of Science and Technology, National University of Malaysia (UKM), 43600 Bangi, Selangor, Malaysia. Tel.: + 6089213344; fax: + 6089269470. E-mail address: msghamsari@yahoo.com (M. Sasani Ghamsari). Journal of Luminescence 131 (2011) 155–158