Time-evolution of photoluminescence properties of ZnO/MgO core/shell quantum dots

In situ and ex situ time-evolution of photoluminescence data during growth of ZnO/MgO core/shell quantum dots (Qdots) were used to study the stability of the green–white (CIE: 0.32, 0.41) luminescence from ZnO defects. The ZnO/MgO Qdots, synthesized by a sol–gel process, exhibited a quantum yield of 13% compared with less than 5% for ZnO Qdots. The growth of ZnO Qdots was arrested and the defect emission was stabilized by formation of the MgO shell. UV–Vis absorption data verified the quantum confinement of ZnO/MgO Qdots. Photoluminescence emission from defects in the ZnO/MgO Qdots was stable ∼70 days at room temperature. M Supplementary information is available in the online edition at http://stacks.iop.org/JPhysD/41/182002 (Some figures in this article are in colour only in the electronic version) Nanoparticles of wurzite ZnO are of great interest because of several promising applications [1, 2]. Zinc oxide has a wide band gap of ∼3.37 eV (∼365 nm) at 300 K, but also exhibits a strong and broad visible emission peak centred at ∼520 nm [3]. Several approaches have been investigated to improve the efficiency of this visible emission. One approach is the addition of either cationic or anionic foreign ions to dope or segregate in ZnO nanostructure [4, 5]. Bang et al [4] reported that an enhanced and stable deep level defect emission at ∼520 nm from ZnO quantum dots (Qdots) could be achieved by surface segregation of Mg. Hexagonal ZnO (a: 3.25 Å, c: 5.31 Å) and cubic MgO (a: 4.13 Å) have previously been studied for wide band gap engineering of ternary alloy MgxZn1−xO in heterostructure optical devices [4, 6–8]. A shell of MgO on ZnO is expected to provide a potential step at the interface both in the valence (3.67 eV) and conduction (0.46 eV) bands, thereby localizing the electrons 1 Author to whom any correspondence should be addressed. and holes in the ZnO Qdots core. Based on its band gap and electron affinity, MgO should be an ideal candidate shell material for ZnO passivation despite the ∼18% (in-plane) lattice mismatch [8]. Good confinement of charge carriers to the ZnO core should result in both enhanced photostability as well as improved luminescent efficiency from defect emission. In the present communication, we report data consistent with these expectations. Herein, ZnO/MgO Qdots are synthesized by a sol–gel process using acetate precursors of Zn and Mg and NaOH and without using any surfactant unlike other synthesis methods of Qdots, such as microemulsion [9–11] and high and low temperature synthesis methods [12–15]. For a typical synthesis process, an appropriate amount of Zn-acetate and excess amount of NaOH were dissolved in ethanol at 70 ◦C and room temperature, respectively [3]. 10 ml of a 0.5 M OH− solution was added dropwise to 30 ml of Zn2+ solution with vigorous stirring in an ice-bath. This was followed by very slow addition of a 0.033 M Mg2+ solution in ethanol. 0022-3727/08/182002+04$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK J. Phys. D: Appl. Phys. 41 (2008) 182002 Fast Track Communication Figure 1. A transmission electron micrograph of ZnO/MgO core/shell Qdots (representative Qdots are marked with arrows). Inset: selected area diffraction pattern (SAED) showing a wurzite structure. The atomic ratio of Mg to Zn was kept in the range 7–13%. The precipitation of ZnO/MgO Qdots from the transparent sol was carried out by addition of n-heptane (n-heptane-tosol volumetric ratio was 3 : 2) followed by vigorous stirring of the solution and centrifugation. After multiple washing with ethanol andn-heptane, the precipitated Qdots were redispersed with ethanol for characterizations. Control samples of pristine ZnO Qdots (without Mg) were also prepared using the above process. Figure 1 shows a high-resolution transmission electron micrograph (HRTEM) (JEOL 2010F operated at 200 kV) of ∼3.5 nm ZnO/MgO core/shell dispersed on a carbonfilm. The HRTEM micrograph shows mostly agglomerated nanoparticles after washing with a few segregated Qdots (shown with the arrows). Note that Qdots are synthesized by the sol–gel method without using any surfactant, unlike most of the other synthesis methods. Therefore, agglomeration of Qdots occurs both before and after washing of Qdots. It is, however, possible to synthesize monodispersed, water soluble Qdots by using surfactant in sol–gel method [16]. The structure and particle size were further characterized using selected area electron diffraction (SAED) and x-ray diffraction (XRD) analyses. The inset in figure 1 shows a SAED pattern consisting of concentric rings from the labelled planes of hexagonal ZnO. The size and crystalline phases of ZnO/MgO Qdots from XRD analyses (not shown here) were consistent with TEM size and structure data. The cubic phase of MgO was not detected. Based on the Auger electron spectroscopy (see supplementary information available in the online edition), the atomic percentages of Mg in the samples were approximated to be 6–9%, suggesting a thin shell. The shell thickness was kept below the critical value for relaxation from the hexagonal to the cubic system, and only a hexagonal crystal structure 3000 3750 4500 5250 6000 6750 7500 ZnO Qdots