Synthesis, structural, and optical properties of type-II ZnO–ZnS core–shell nanostructure M. Sookhakian a,n , Y.M. Amin a , W.J. Basirun b,c , M.T. Tajabadi b , N. Kamarulzaman d a Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysia b Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia c Nanotechnology & Catalysis Research Centre (NanoCat), Institute of Postgraduate Studies, University Malaya, 50603 Kuala Lumpur, Malaysia d Centre for Nanomaterials Research Institute of Science, Level 3 Block C (Old Engineering Building), Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia article info Article history: Received 11 October 2012 Received in revised form 7 May 2013 Accepted 10 July 2013 Available online 31 July 2013 Keywords: ZnO–ZnS core–shell Photoluminescence Type II band alignment Hetero-interface abstract We demonstrate a facile one-step method for the preparation of ZnO–ZnS core–shell type-II nanos- tructures, pure ZnS quantum dots and pure ZnO nanoparticles with different experimental conditions. Treatment with sodium hydroxide as a capping agent is investigated systematically during the synthesis of ZnS quantum dots (QDs). The thickness of the ZnS shell is controlled by the concentration of the sodium sulphide during the synthesis of ZnO–ZnS core–shell nanostructures. The morphology and structure of samples are verified by X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray analysis (EDX). The UV–vis absorption spectra of the pure ZnS QDs exhibit a blue shift in the absorption edge due to the quantum confinement effect. The PL emission spectra of the ZnO–ZnS core–shell nanostructure are compared with the ZnO nanoparticles. The ZnO–ZnS core–shell nanostructures show decrease in the UV and green emissions with the appearance of a blue emission, which are not found in the ZnO nanoparticles. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Semiconductor nanocrystals, particularly quantum dots (QDs), have attracted much attention in recent years due to their unique physical and chemical properties which are different from the bulk materials. They have a wide range of physical applications such as light-emitting diodes, biomedical labelling, photo-catalysis, optical waveguide, photo-conductive devices, solar cells, lasers and sen- sors [1–12]. The physical and chemical properties of semiconduc- tor nanoparticles (NPs) are controlled by a spatial three dimensional confinement of electrons and holes in a small box called quantum confinement effect [13]. Due to the quantum confinement effect, a significant increase in the band-gap is observed when the nanoparticle size is close to the exciton Bohr radius and results in a blue shift in the absorption spectra. The effective mass model [14] is used to analyse the size effects on luminescent properties of the nanostructures. Recently, II–VI semiconductors, particularly CdTe and CdS, has been extensively analysed for their size and shape control [15,16]. It was found that the nanoparticles encapsulated with a shell coating layer enhances the optical properties. ZnO is one of the most famous II–VI semiconductor nanoparticles due to a large exciton binding energy of 60 meV and a wide band gap of 3.3 eV at room temperature, and are used for various applications such as optoelectronics [17], field-effect transistors [18], sensors [19], transparent conducting films [20], light-emitting diodes [21] and catalysts [22]. It is reported that the optical properties of ZnO NPs could be signifi- cantly improved by the encapsulation of the ZnO with wide band- gap nanoparticles [23]. ZnS is a non-toxic semiconductor with a wide and direct band gap which can be observed naturally in two phases: first is the zinc blend structure with a cubic phase, and second is the wurtzite structure with a hexagonal phase. The band-gap energy of the bulk cubic and hexagonal phases of ZnS is 3.68 eV and 3.80 eV respectively. Zinc blend ZnS is more stable at lower tempera- ture and atmospheric pressure, but transforms to wurtzite ZnS at temperature higher than 1000 1C [24]. Recently, ZnS QDs are reported to show various luminescence properties such as photo-luminescence, electro-luminescence, mechano-lumines- cence, and thermal-luminescence [25–28]. Also ZnS QDs is a phos- phor material and are widely used in infrared windows, flat-panel displays and LED [29,30] due to their wide exciton binding energy of 40 meV. It is found that, ZnS nanoparticles (NPs) is an excellent shell coating layer on the ZnO, and enhances the optical properties of the ZnO–ZnS core–shell nanostructures. Li et al. [23] fabricated ZnO–ZnS core–shell nanowires by a self-assembling mechanism Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.07.032 n Corresponding author. Tel.: +60 146474432. E-mail address: m.sokhakian@gmail.com (M. Sookhakian). Journal of Luminescence 145 (2014) 244–252