Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Single-phased emission-tunable Mg and Ce co-doped ZnO quantum dots for white LEDs
Graphical abstract
Introduction
Recently, white light-emitting diodes (LEDs) have attracted much attention [1,2], especially white LEDs based on semiconductor quantum dots (QDs), which are regarded as one of the most promising next generation solid-state light sources because of their large absorption coefficients, high chemical stability, and size dependent emissions [[3], [4], [5]]. Currently, either two-color (blue and yellow emitting QDs) or three-color (red, green, and blue emitting QDs) systems are generally used to achieve white light emission in QD-based White LEDs. The self-absorption between the QDs for different color emissions and subsequent undesired energy transfer results in a decrease of luminous efficiency [6]. Compared with multi-component systems, single-phased white light-emitting phosphors have a higher luminous efficiency, greater reproducibility, low-cost preparation, and higher color stability. Therefore, in recent years, single-phased white light-emitting QDs suitable with near-ultraviolet (NUV) excitation have attracted significant attention. Several white light-emitting QDs have been reported, such as CdS-QDs, CdSe-QDs, and alloyed ZnxCd1−xS-QDs [3,7,8]; however, they usually contain toxic heavy metals, and this greatly limits their practical applications.
ZnO QDs have received appreciable attention over the past decades as an efficient material for LED applications because of their exceptional optical and electronic properties. Furthermore, compared with traditional II–VI group QDs (such as CdS, CdSe, and CdTe QDs), ZnO QDs are cheap, non-toxic, and biocompatible. ZnO QDs exhibit tunable photoluminescence (PL) over a wide range from blue to green and further to red via chemical doping and particle size control [[9], [10], [11]]. As reported, the peak position for deep-level emission from ZnO QDs can be gradually tuned from 490 nm to 510 nm by Sn doping [9], from 480 to 501 nm by Li doping, from 547 to 562 nm by Na doping [10], and from 470 to 538 nm by Mg doping [11]. Therefore, by adjusting the luminescence properties via doping, white light emission from ZnO could be achieved. Recently, Kang et al. [12] reported white luminescence from terbium-doped ZnO nanowalls. White light emission was also observed in gallium and indium co-doped ZnO [13]. Our previously reported work demonstrated white light emission from Mg-doped ZnO bulk phosphors [14,15], however, those phosphors exhibited a high correlated color temperature (CCT, 8929 K) and low color rendering index (CRI, 76.5). To gain higher quality white emission, the exceptional luminescence properties of QDs were leveraged to realize a lower CCT and higher CRI by synthesizing Mg and Ce co-doped ZnO QDs. To the best of our knowledge, Mg and Ce co-doped ZnO QDs with white light emission have not been reported to date. In this work, Mg and Ce co-doped ZnO QDs were synthesized via a facile low-temperature route. The effects of varying the Ce ion concentration on the structure, morphology, and luminescence properties of Mg-doped ZnO nanoparticles was focused on, while the Mg doping concentration was kept constant. Additionally, we discuss the origin of visible-light emission in our samples in detail. Our results show that emissions from yellow to white and further to green could be obtained upon 370-nm excitation. Finally, by integrating the synthesized Mg and Ce co-doped ZnO QDs and a NUV chip, we successfully fabricated a white LED device so as to evaluate the potential application of Mg and Ce co-doped ZnO QDs. For the fabricated white LED device under a voltage of 3 V and a driven current of 200 mA, the Commission International de l'Eclairage (CIE) coordinates was (x = 0.32, y = 0.30), the CCT was 5733 K, and the CRI was 81, which make them potential candidates as single-phased QDs for white LEDs.
Section snippets
Preparation of samples
Mg and Ce co-doped ZnO QDs were synthesized via a facile low-temperature route. The related raw materials were purchased from Sinopharm Chemical Reagent Co. Ltd. Initially the concentration of Mg was optimized (10 mol%), and Ce ions were then doped at different concentrations (0, 0.4 mol%, 0.8 mol%, and 1 mol%) to find an adequate composition for white light emission. The details of the sample preparation are as follows: Zn(CH3COO)2·2H2O (2 mmol) and LiOH·H2O (6 mmol) were dissolved in absolute
Results and discussion
The XRD patterns of undoped ZnO and CexMg0.1Zn0.9−xO (x = 0, 0.004, 0.008, and 0.01) QDs are shown in Fig. 1. All the XRD reflections were consistent with the standard diffraction pattern of ZnO (JCPDS 80-0075). There was no evidence of peaks corresponding to doped Mg ions, Ce ions, or any other impurity, which confirms the synthesis of a single-phase hexagonal ZnO and also effective doping of Mg and Ce ions in the ZnO lattice. Furthermore, all the samples show broad diffraction peaks, and the
Conclusions
In summary, we have successfully synthesized Mg and Ce co-doped ZnO QDs via a simple low-temperature route. The XRD results indicated that the obtained samples were single-phase hexagonal ZnO. TEM results show that the average diameters of the particles were estimated to be ~10, 8, 7, 6, and 4 nm for undoped ZnO, Mg-doped ZnO, CexMg0.1Zn0.9−xO (x = 0.004), CexMg0.1Zn0.9−xO (x = 0.008), and CexMg0.1Zn0.9−xO (x = 0.01), respectively. The PL results show that the visible emission of Mg and Ce
CRediT authorship contribution statement
Qiang Shi: Conceptualization, Investigation, Writing - original draft. Kai Ling: Investigation, Writing - review & editing. Susu Duan: Investigation, Writing - review & editing. Xue Wang: Investigation, Writing - review & editing. Shengxiang Xu: Investigation, Writing - review & editing. Dong Zhang: Formal analysis, Validation. Qingru Wang: Formal analysis, Validation. Shuhong Li: Formal analysis, Validation. Ling Zhao: Resources. Wenjun Wang: Resources.
Declaration of competing interest
None.
Acknowledgements
This work was supported by National Natural Science Foundation of China (No. 61775089), Shandong Provincial Natural Science Foundation of China (Nos. ZR2018MA039, ZR2019MF068, ZR2017BF009 and ZR2018MA036), Projects of Shandong Province Higher Educational Science and Technology Program (No. J17KA175), Industrial Alliance Fund of Shandong Provincial Key Laboratory (No. SDKL2016038), Experimental Technology Research Fund of Liaocheng University (No. 26322170234), and the Special Construction
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