Stoichiometry-Controlled InP-Based Quantum Dots: Synthesis

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Stoichiometry-controlled InP-based quantum dots: synthesis, photoluminescence, and electroluminescence Yang Li, Xiaoqi Hou, Xingliang Dai, Zhenlei Yao, Liulin Lv, Yizheng Jin, and Xiaogang Peng J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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Journal of the American Chemical Society

Stoichiometry-controlled InP-based quantum dots: synthesis, photoluminescence, and electroluminescence Yang Li†, Xiaoqi Hou†, Xingliang Dai†,‡, Zhenlei Yao†,‡, Liulin Lv†, Yizheng Jin†,‡*, and Xiaogang Peng†* † Center for Chemistry of Novel & High-Performance Materials and Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China. ‡ State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou, 310027, P. R. China. Supporting Information ABSTRACT: We introduce stoichiometry control within both core and shell regions of InP/ZnSe/ZnS core/shell/shell quantum dots (QDs) to drastically advance their properties, approaching those of state-of-the-art CdSe-based QDs. The resulting QDs possess near-unity photoluminescence (PL) quantum yield (QY), mono-exponential decay dynamics, narrow linewidth, and nonblinking at a single-dot level. Quantum-dot light-emitting diodes (QLEDs) with the InP/ZnSe/ZnS core/shell/shell QDs as emitters exhibit a peak external quantum efficiency (EQE) of 12.2% and a maximum brightness of > 10,000 cd m-2, greatly exceeding those of the Cd/Pb-free QLEDs reported in literature. These results pave the way towards Cd/Pb-free QDs as outstanding optical and optoelectronic materials.

Colloidal quantum dots promise an unprecedented class of emissive materials due to their unique combination of sizedependent emission color, narrow linewidth, convenient photo- and electro-excitation, good photo-stability, and excellent solution processibility. 1 These intriguing properties have motivated extensive efforts on pursuing various applications of QD emitters. 2-9 State-of-the-art CdSe-based QDs possess nearly ideal luminescence properties 10,11, dominating all promising applications. Lead-halide perovskites QDs possess high PL QY and narrow PL linewidth. 12 However, both Cd and Pb are restricted for use in industry. 13 InP QDs are considered as the most promising candidates as Cd/Pb-free QDs with emission colors covering most part of visible and near-infrared window. 14 Studies 15-25 demonstrate wide-bandgap zinc chalcogenides (ZnSe and ZnS) can be epitaxially grown onto InP QDs to confine photo- and electrogenerated electron-hole pair (exciton) mostly within the InP cores. Though synthesis of InP QDs started ~25 years ago, 26,27 nearly every parameter of their luminescence properties is much less developed than that of CdSe-based QDs. 14 Here we propose a new synthetic concept for InP/ZnSe/ZnS core/shell/shell QDs to drastically boost their luminescence properties, i.e., stoichiometry control of both core (III-V) and shell (II-VI). In and P are known to be electronic dopants for II-VI semiconductors (such as ZnSe), and vice verse, 28,29 which inevitably generate intra-gap states severing as non-radiative recombination centers. 28-30 Results here reveal that it is possible to retain ideal stoichiometry between In and P within the core QDs and avoid indium incorporation during epitaxial growth of either ZnSe or ZnS shells, which results in QDs with excellent luminescence properties. The stoichiometry-controlled synthesis scheme consists of four stages (Figure 1a). InP and ZnSe possess zinc-blende structures with ~3% lattice mismatch, facilitating epitaxy of ZnSe shells onto InP cores. With a relatively small lattice mismatch of 5% between ZnSe and ZnS, additional ZnS shells are epitaxially grown onto the InP/ZnSe core/shell QDs to improve chemical stability of the QDs.

Figure 1. (a) Stoichiometry-controlled synthesis scheme. (b) TEM image of the resulting InP/ZnSe/ZnS core/shell/shell QDs with size-distribution histogram (top-right) and a high-resolution TEM image (bottom-left). (c) PL (red) and absorption (black) spectra of the InP/ZnSe/ZnS core/shell/shell QDs, (inset) stability of their PL QY under ambient conditions. (d) Transient PL spectra of ensemble (black) and single-dot (red) InP/ZnSe/ZnS core/shell/shell QDs. (e) Representative PL intensity trajectory of single QD with the background. (f) Histogram of the “on” time fraction for 50 QDs.

Figure 1b illustrates that our InP/ZnSe/ZnS core/shell/shell QDs (6 monolayers of ZnSe and 2 monolayers of ZnS shells) are nearly monodisperse in size (7.8 ± 0.5 nm). High-resolution transmission electron microscope (TEM) (inset, Figure 1b) and X-ray powder diffraction (Figure S1) reveal single-crystalline zinc-blende structure of the QDs. These red-emitting QDs show a PL peak at ~618 nm with a full width at half-maximum (FWHM) of 42 nm and near-unity PL QY (Figure 1c). PL QY of a sample stored in solution under ambient conditions for one month (inset, Figure 1c) is 93 ± 3 %. Figure 1d shows that, at both ensemble and single-dot levels, the transient PL spectra of the QDs can be well fitted by a monoexponential function (1000 counts, goodness-of-fit 90%), and mono-exponential decay dynamics (Figure S11), which are superior to those of other green-emitting InP-based QDs reported in literature. 17,19,21,25,34 With our InP/ZnSe/ZnS core/shell/shell QDs as emitters in QLEDs, a device structure (Figures 4a and S12) consisting of indium tin oxide (ITO) as anode, poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) and poly(N,N’-bis(4-butylphenyl)N,N’bis(phenyl)-benzidine) (poly-TPD) as hole-transport layer, QDs, Zn0.9Mg0.1O nanocrystals as electron-transport layer and silver as cathode is used. Figure 4b shows the current density-luminancevoltage characteristics of a device with optimal efficiency, which gives a turn-on voltage of 1.8 V. The electroluminescence peak is at 630 nm (Figure 4b, inset), corresponding to Commission Internationalede l’Eclairage (CIE) color coordinates of (0.69, 0.31). EQE of this device reaches 12.2%, corresponding to a high current efficiency of 14.7 cd A-1 (Figure 4c). The maximum brightness exceeds 10,000 cd m-2. A histogram of 35 devices shows an average peak EQE as 11.1% with a low relative standard deviation (Figure 4d), indicating excellent reproducibility. The overall performance of these red-emitting QLEDs, including EQE, turn-on voltage, current efficiency, maximum brightness, and reproducibility, greatly exceed that of the red-emitting QLEDs with InP-based QDs or other Cd/Pb-free QDs reported in literature 20,25,35-37 and are close to those of the red-emitting QLEDs with CdSe-based QDs. 2,6,7,38-46

Our work demonstrates stoichiometry control as a new concept for InP-based QDs with outstanding PL and electroluminescence properties, highlighting the feasibility of replacing CdSe-based emitters by Cd/Pb-free QDs. At present, zinc chalcogenides are the most probable material choices as epitaxial outer shells for nearly all types of application-relevant QDs, especially those targeting Cd/Pb-free products. Thus, the key design principle, i.e., controlling the stoichiometry of the elements within each portion—

We are grateful for the National Program on Key Research and Development Project (2016YFB0401600), and the National Natural Science Foundation of China (91833303).

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