Article Cite This: Chem. Mater. 2018, 30, 3668−3676
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Synthesis and Evaluation of Ideal Core/Shell Quantum Dots with Precisely Controlled Shell Growth: Nonblinking, Single Photoluminescence Decay Channel, and Suppressed FRET Zhaohan Li,† Fei Chen,† Lei Wang,† Huaibin Shen,*,† Lijun Guo,‡ Yanmin Kuang,‡ Hongzhe Wang,† Ning Li,† and Lin Song Li*,† †
Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng 475004, China Institute of Photo-biophysics, School of Physics and Electronics, Henan University, Kaifeng 475004, China
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S Supporting Information *
ABSTRACT: Due to the unique optical properties, colloidal quantum dots (QDs) are excellent candidates for developing next-generation display and solid-state lighting technologies. However, some factors including photoluminescence blinking and Förster resonance energy transfer (FRET) still affect their practical applications. Herein, a series of ZnCdSe-based core/ shell QDs with low optical polydispersity have been successfully synthesized by a “low-temperature injection and high-temperature growth” precisely controlled method. The alloyed ZnCdSe core with a certain ratio of Cd and Zn was presynthesized first. Followed by accurate ZnS shell growth, the assynthesized core/shell QDs are nonblinking with the nonblinking threshold volume of ∼137 nm3. The PL decay dynamics are all single-exponential for both QDs in solutions and close-packed solid films when ZnS shell thickness varying from 2 to 20 monolayers. FRET can be effectively suppressed after growing 10 monolayers of ZnS shell. All of these superb characteristics including nonblinking, single-exponential PL decay dynamics and suppressed FRET can be beneficial to high-quality QD-based light-emitting devices (QLEDs). By applying the ZnCdSe-based core/shell QDs with 10 monolayers ZnS shell, the highest external quantum efficiency of ∼17% was reached, which could compare favorably with the highest efficiency of green QLEDs with traditional multilayered structures.
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electron and hole trapping.17 In the past, the synthesis of QDs with nonblinking behavior has mostly revolved around the CdSe core-based core/shell QDs. Because of only 4% lattice mismatch between CdSe and CdS, the CdSe/CdS core/shell QDs are one of the best candidates for the synthesis of nonblinking QDs.13 However, the emission colors of CdSe/ CdS core/shell QDs are limited to the long wavelength range from orange to red, owing to the small conduction-band offset between CdSe and CdS. For green/blue nonblinking QDs, QD cores should be realized by the shell materials with stronger spatial confinement of the exciton wave function. Due to the higher conduction-band minimum and lower valence-band maximum, ZnS could localize the wave function of excitons effectively. However, the relatively large lattice mismatch of ∼12% between CdSe and ZnS can cause an accumulation of interfacial strain.18 Therefore, it is inevitable to introduce the defects due to the lattice mismatch between the CdSe core and the ZnS shell in the previous studies.9,19 Only a few nonblinking green QDs were reported most recently, but the stability of these QDs may not be high enough for practical
fter three decades of rapid development, colloidal semiconductor quantum dots (QDs) have already been synthesized with narrow band-edge emissions, size- and composition-dependent properties in electronic structure, good photostability, easy solution processability, and high photoluminescence (PL) quantum yields (QYs), and so on.1−3 As the most promising luminescent materials, QDs have successfully extended their original fundamental research into many practical applications, including color enhancement film for display, active-matrix light-emitting diodes, and in vitro diagnostics, etc.4−7 In the past, how to obtain monodisperse particles, high QYs, and QD high stability was generally the main focus of worldwide researchers.8−12 However, most recent studies indicate that QDs with these good optical properties are still not enough to satisfy the demands of practical applications.13,14 Because such QDs may still show PL blinking behavior when continuously excited by excitation sources like laser and applied bias in electroluminescence (EL) devices.14,15 The existence of PL blinking of QDs may affect its applications in QD-based light-emitting devices (QLEDs) and a singlephoton light source, etc.14,16 When one or both of the photoexcited electron−hole pairs are trapped by the defects, PL blinking will occur. Therefore, nonblinking QDs should be defect-free and generally avoid © 2018 American Chemical Society
Received: January 14, 2018 Revised: April 25, 2018 Published: May 7, 2018 3668
DOI: 10.1021/acs.chemmater.8b00183 Chem. Mater. 2018, 30, 3668−3676
Article
Chemistry of Materials applications because they had to be processed in a water-free and oxygen-free environment.20,21 So there is high demand for green or blue QDs which can enjoy the advantages of PL nonblinking and high stability. Herein, we have successfully synthesized a series of nonblinking ZnCdSe-based core/shell QDs by a “low-temperature injection and high-temperature growth” precisely controlled method. Not only have alloyed ZnCdSe core QDs with a certain ratio of Cd and Zn been presynthesized precisely, but well-controlled ZnS shells with accurate different thicknesses can be grown as well. (1−20 ZnS monolayers; monolayer is abbreviated to ML hereafter. According to the lattice constant of zinc-blende ZnS,22 it can be deduced that the average thickness of 1 ML ZnS is 0.31 nm. Then, the ML thickness can be estimated by the TEM images.) Due to the small lattice mismatch between ZnCdSe and ZnS, the trivial accumulation of interfacial strain between ZnS and CdSe is almost ignorable. The green ZnCdSe-core-based QDs with a thick shell are successfully fabricated, and the defects are greatly reduced during the precisely controlled growth process of the ZnS shell. The as-synthesized ZnCdSe/ZnS core/shell QDs have a uniform dimension and shape, high QYs (the highest QY of ∼100%), and PL emissions with narrow full width at half-maximum (fwhm). More importantly, for the QDs with ≥2 MLs of ZnS shell, the PL decay curves of the QD ensemble are single-exponential, and the PL for single QD is nonblinking with the nonblinking threshold volume of ∼137 nm3. After being transferred to solid films, the nonblinking QDs have been found to retain high optical characteristics and can suppress Förster resonance energy transfer (FRET) effectively. By using such ZnCdSe/10ZnS core/shell QDs with nonblinking and single-exponential decay characteristics as an emitter in QLEDs, the highest external quantum efficiency (EQE) is reached as high as 17%, which could compare favorably with the highestefficiency green QLEDs with traditional multilayered structures consisting of layers of indium tin oxide (ITO)//poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)//poly(9,9-dioctylfluorene-co-N-(4-(3methylpropyl))diphenylamine) (TFB)//ZnCdSe/ZnS core/ shell QDs//ZnO//aluminum (Al) cathode.
Figure 1. (a) Evolution of UV−vis and PL spectra of ensemble zincblende ZnCdSe core and ZnCdSe/ZnS core/shell QDs upon shell growth. PL peak position (b), fwhm (c), and PL QY (d) of the ZnCdSe core and ZnCdSe/ZnS core/shell QDs as functions of ML number of ZnS shells, respectively.
TEM images of ZnCdSe core and ZnCdSe/ZnS core/shell QDs (Figure S1) show that the diameter of QDs can be tuned continuously from 5 to 22 nm by increasing the shell thickness from 0 to 24 MLs. Due to precisely controlled shell growth, the size distribution of the core/shell QDs remains exceptionally narrow (
