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Abstract
Alloyed CdZnSe/ZnSe quantum dots with a spectral region from 620 to 690 nm are synthesized by a facile one-pot method. The core shell structure for CdZnSe/ZnSe quantum dots are prepared through two Se precursor injections without any purification steps and Zn precursor injections. The emission color is tuned by reaction time, reaction temperature, and the amount of Se. The CdZnSe/ZnSe quantum dots (QDs) show narrow size distribution, wide absorption spectrum, and a high photoluminescence (PL) quantum yield (QY) of up to 50% at room temperature. Electroluminencent light emitting diodes based on the resultant QDs deliver an external quantum efficiency as high as 6.8%.
© 2017 Optical Society of America
1. Introduction
Colloidal semiconductor quantum dots (QDs) have been intensively studied as luminescent materials for lighting [1–4], laser [5, 6], display [7, 8] and biological imaging applications [9–11], owing to their unique features like wide size-controllable emission-wavelength tunability, narrow emission linewidths and high quantum yields. In particular, CdZnSe alloy quantum dots have attracted more and more attention, because the emission color can be tuned flexibly by adjusting the Cd:Zn ratio without changing the nanocrystal size [12–14]. However, at current stage of development, the emission wavelengths are limited, ranging from 360 to 620 nm, which restricts their further applications [13–20]. In particular, the CdZnSe based QLEDs cannot provide red color (> 620 nm) in displays and light devices. Therefore, it is hard to achieve high color rendering index (CRI) for a white light source without pure red emission component [21–23]. How to extend their emission wavelength beyond 620 nm still remains a challenging task. To achieve a whole red band CdZnSe QDs requires further constructions in synthetic strategies.
Generally, the preparation of CdZnSe QDs could be implemented in three ways: 1) Injection of the pre-prepared binary QDs (CdSe/ZnSe seeds) followed by injection of zinc or cadmium precursor to exchange the cations; 2) Simultaneous injection of both pre-prepared binary QDs (CdSe and ZnSe seeds); 3) Transforming the core/shell structured QDs into alloyed CdZnSe QDs by heat treatment. Most of these strategies focus on optimizing the pre-prepared CdSe or ZnSe seeds, whereas one-pot synthesis routes for alloy CdZnSe QDs are seldom reported [20]. One-pot synthesis method has advantages of improving the reaction efficiency, avoiding complex purification procedures to remove the intermediate chemical compounds and increasing chemical yield. Using one-pot synthesis, highly luminescent ternary alloyed QDs have been produced successfully, such as ZnCdS [24], CdSeTe [25] and ZnCdS/ZnS [26]. Although the alloyed core/shell/shell CdZnSe/ZnSe/ZnS QDs with emission peak at 543nm have been prepared by one-pot method and the PL QY is up to 63% [20]. It still calls further investigations for a facile synthetic method to produce high quality QDs with an extended emission wavelengths toward red region.
In this work, highly luminescent red emitting CdZnSe/ZnSe QDs are systhesized through a facile one-pot procedure. The synthesis is inspired by the work of Fitzmorris et al. [20], introducing more zinc acetate to avoid Zn procedures injection, abandoning oleylamine and introducing less oleic acid to develop a low-cost and hypotoxic synthetic approach, changing the reaction conditions to tune the emission wavelengths of QDs. CdZnSe core is produced by one step without pre-prepared QD seeds and purification process, then ZnSe as shell is subsequently overcoated by only adding Se precursor. Systematic characterization of their composition, morphology and photoelectric properties has been carried out. The CdZnSe/ZnSe core/shell QDs show a widely emission tunablility from 620 to 690 nm, a narrow size distribution, a wide absorption spectrum and a room temperature absolute PL quantum yield as high as 50%. Furthermore, two types of quantum dots light emitting diodes (QLEDs) using 635 and 672 nm CdZnSe/ZnSe core/shell QDs have also been fabricated. It is demonstrated that the 635 and 672 nm QLEDs achieve external quantum efficiencies (EQEs) of 5.9% and 6.8%, respectively.
2. One-pot synthesis of alloyed core/shell CdZnSe/ZnSe QDs
The synthesis of CdZnSe/ZnSe is according to previous work [20], but with substantial modifications. The modification includes changing reaction temperatures and amounts of the Se precursors in order to tune the emission wavelengths from 620 to 690 nm. In detail, more zinc acetate is introduced in the synthesis of cores in order to avoid Zn procedures injection in the shell reaction to simplify the productive process. Less oleic and no oleylamine is used to be environmentally friendly. Since the reaction temperature is high here, we reduce the reaction time from 1 hour to 8 minutes for the core synthesis. This facile strategy develops a low-cost and hypotoxic synthetic preparation of alloyed core/shell QDs with bright red color for practical applications. As shown in Fig. 1, a certain amount of Cd precursors and an excess amount of Zn precursors are mixed together offering free Cd2+ and Zn2+ ion solution. After Se-TBP precursor is injected into the solution, ternary CdZnSe particles are formed with Zn2+ dangling bonds attached to their surfaces. And then Se-TBP precursor is injected again to react with the Zn2+ and produce a ZnSe shell onto the surface of CdZnSe core. Under the protection of ligands, core/shell CdZnSe/ZnSe QDs will grow to a certain size and then maintain stable.
3. Experimental: materials and methods
3.1 Synthesis of QDs
Cadmium oxide (CdO, 99.99%, powder) and selenium (Se, 99.99%, powder) are purchased from sinopharm chemical reagent Co., Ltd. Zinc acetate (Zn(Ac)2, 99.99%, powder), 1-octadecene (1-ODE, 90%) and Tributyl phosphine (TBP, 98%) are purchased from Shanghai Macklin Biochemical Co., Ltd. Oleic acid (OA, 90%) is purchased from Aladdin Industrial Corporation. All of the commercial reagents are used as received without further purification.
In detail, 0.5 mmol CdO, 5 mmol Zn(ac)2, 3 ml OA, and 8 ml ODE are added in a 50-ml three-necked round-bottomed flack. The mixture is heated at 100 °C under vacuum (<100 mTorr) for 30 min to remove oxygen and water. Then, the mixture is heated up to 290 °C under nitrogen protection to get free Cd2+ and Zn2+ ions. 0.5 mmol Se-TBP (1 mmol Se dissolved in 0.5 ml TBP and 0.5 ml ODE) is rapidly injected into the flask. The cores are allowed to grow for 8 min. Continue to heat up to 300 °C, and then 1 mmol Se-TBP is slowly injected into the flask to prepare the shell, followed by heat preservation for 30 min to achieve equilibrium. The reaction mixture is purified by dissolving in hexane/acetone (1:3) solution and centrifuging separation for 10 min. After this process is repeated three times, the precipitated CdZnSe/ZnSe QDs are redispensed in hexane for further characteristic measurements and QLED fabrication. With the variation of core reaction temperature (290-310 °C), shell reaction temperature (300-320 °C) and the second injected mass for Se-TBP (1-2.5 mmol), the emission wavelength of the CdZnSe/ZnSe QDs can be conveniently tuned from 618 to 688 nm.
3.2 Fabrication of QLEDs
The indium tin oxide (ITO) coated glass substrates are washed with detergent, deionized water, acetone and isopropanol. Poly(ethylenedioxythiophene):polystyrene sulphonate (PEDOT:PSS) solutions (4 wt.%) are deposited by spin-coating onto the substrates at 4000 r.p.m. for 50 s and baked at 150 °C for 20 min. poly(N,N’-bis(4-butylphenyl)N,N’-bis(phenyl)benzidine) (poly-TPD, in chlorobenzene, 8 mg/ml) and CdZnSe/ZnSe QDs (in octane, 15 mg/ml) are spin-coated at 2,000 r.p.m. for 50 s. The poly-TPD film is baked at 150 °C for 30 min. ZnO nanocrystals (in alcohol, 30 mg/ml) are deposited onto the QDs layer by spin-coating at 2,500 r.p.m. for 40 s and baked at 80 °C for 30 min. Finally, the Al anodes are deposited via a thermal evaporation at a rate of ≈0.1 nm/s under a high vacuum of 5 × 10−6 Torr.
3.3 Characterizations
The absorption spectra of QDs are characterized by UV-vis spectrophotometer (Cary 300, Varian) with spectral resolution set to 5 nm. The emission spectra are recorded using a steady-state photoluminescence system (Cary Eclipse Varian) that utilized a xenon discharge light source and R-928 photomultiplier tube. Time resolved photoluminescence (TR-PL) experiments are performed on a spectrometer (Bruker Optics 250IS/SM) equipped with an intensified charge coupled device CCD detector (Andor, IStar 740). The infrared spectra of QD samples are recorded using a Fourier transform infrared (FTIR) spectrometer (Bruker,ALPHA-T).
Absolute PL QYs of the QDs are measured using an absolute QY spectrometer (Quantaurus-QY C11347-11, Hamamatsu Photonics Co., Ltd.) with spectral resolution set to 2 nm at room temperature. Specifically, the absolute PL QY is defined by the ratio of photons emitted () to absorbed photons () (Eq. (1). Five different concentrations of QDs are measured to get five pairs of data (,), and the PL QY is calculated by the slope of linear fit of them.
The current density–voltage characterizations for QLEDs are tested using an electrometer (Keithley 2400). Light output performances (EL, number of photons, etc.) are measured using an integrating sphere (3P-GPS-033-SL, Labsphere) coupled with a spectrometer (Maya 2000, Ocean Optics) with 0.2 V driving voltage intervals from 0 V to 10 V. The EQE is the ratio of the number of photons () escaped to the outside to the number of charge carriers () injected into the QLED device. Because the number of photons is recorded as all of the photons emitted by QLED in the integration time (), the number of charge carriers is calculated by determining the charge in the integration time, current () multiply by , and dividing it by the charge of an electron ().
4. The properties of as-prepared CdZnSe/ZnSe QDs
The specific growing conditions for alloy core/shell CdZnSe/ZnSe QDs are listed in Table 1. Quantum dots with different emission wavelengths are prepared by changing the Se-TBP injection temperature and the second Se-TBP injection amount. The resultant QDs show good emission wavelength tunability, ranging from 620 to 690 nm with moderate absolute QYs from 20% up to 50%. Different growing conditions lead to QDs with different impurities and defect densities, which will affect the stabilities of the QDs. As a result, the resulting QDs exhibit different PL QYs depending on the sythetic conditions.
Table 1. The Growth Conditions, Emission Wavelangths, and QYs of core/shell CdZnSe/ZnSe QDs.
Figures 2(b) and 2(c) show the absorption and emission spectra for as-prepared CdZnSe/ZnSe QDs. The temperature and the amount of injection Se-TBP can make significant red shifts of the absorption peaks. With the increase of termperatures or the decrease of Se-TBP, the PL peak postions shift from 688 to 618 nm, which is consistent with the absorption peaks shift. The terperatures and the amounts of Se-TBP injection also give notable influences on the full width at half maximum of the mission (FWHM). Lower temperatures produce a narrower FWHM, while the FWHM decreases for more Se-TBP at a specific temperature. The minimum FWHM is about 35 nm when Se-TBP is 2.5 mmol, suggesting narrow size distribution for 672 nm QDs. The highest PL QY of the 672 nm QDs agrees well with the FWHM results. In contrast, over 50 nm FWHMs of both 618 and 688 nm QDs reveal wide size distributions.
Fig. 2 (a) Picture of the core/shell CdZnSe/ZnSe QDs in the solution under the uv lamp light. (b) Absorption spectra, (c) emission spectra and (d) normalized photoluminescence decay traces of core/shell CdZnSe/ZnSe QDs.
In order to study the fluorescence decay dynamic behaviors of the core/shell CdZnSe/ZnSe QDs, the QD samples dispersed in hexane are tested. Because the absorption peaks for samples extend from 590 to 690 nm and the emission peaks are located at the range from 620 to 690 nm, the transient fluorescence measurement is performed using an excitation wavelength of 590 nm. The PL decay traces of QDs are shown in Fig. 2(d). Usually, single-exponential functions are used to fit the decays and fluorescence lifetimes. When the goodness-of-fit (χ2) of single-exponential model is greater than 1.3, the radiative decay lifetimes of QDs are calculated by multi-exponential fitting. Lifetime and fractional contribution of different decay channels for core/shell CdZnSe/ZnSe QDs are listed in Table 2. The PL decay of 618, 672 and 688 nm QDs fit well with the calculated curves by single-exponential model. In contrast, the PL decays of the 635 and 650 nm QDs are decomposed to bi-exponential contributions. The short lifetime channel of 650 nm QDs (~7 ns) should responsible for the lower QY of them than 635 nm QDs, because the fast channels usually implies existence of middle-gap states [27]. The middle-gap states would capture the holes and quench the PL [28]. Although the PL decay of 688 nm QDs evolved to single channel decay, the QY of them is the minimum in all of the QDs. Because of a strong dependence of the decay dynamics on NC diameter [29], the wide size distribution might result in low QY of 688 nm QDs.
Table 2. Lifetime and Fractional Contribution of Different Decay Channels for core/shell CdZnSe/ZnSe QDs.
The infrared (IR) spectra of OA, ODE and core/shell CdZnSe/ZnSe QDs are shown in Fig. 3. Because TBP is too easy to oxidize in the air, it does not have an infrared test. The IR spectra show that the –C = C band at 1640 cm−1 in the spectrum of ODE is missing in the spectra of QDs. Simultaneously, the strong –C = O stretching vibration band at 1709 cm−1 in the spectrum of OA is absent, indicating that the synthetic grafting and passivation procedure should be successful.
5. Properties for 672 nm CdZnSe/ZnSe QDs
For CdZnSe/ZnSe quantum dots with emission peak at 672 nm, the morphology, structure, and optical properties are characterized respectively. The transmission electron microscopy (TEM) images of QDs are shown in Fig. 4(a). It can be seen that 672 nm CdZnSe/ZnSe QDs prepared by one-pot method display spherical particles with mean size of about 3.5 nm. The red and black curves in Fig. 4(b) show the X-ray diffraction (XRD) spectrum of 672 nm CdZnSe/ZnSe QDs and the CdZnSe cores, respectively. The black and red XRD curves are in good agreements with cards 65-8875 and 89-2940, respectively. This indicates that the cores should be Cd3Zn7Se10 with cubic crystal structure, and these cores are capped with ZnSe shells with six-party crystal structure.
Fig. 4 (a) Transmission electron microscopy of 672 nm CdZnSe/ZnSe QDs. (b) X-ray diffraction, (c) normalized photoluminescence decay traces and (d) absorption and emission spectra of 672 nm CdZnSe/ZnSe QDs and their cores (CdZnSe QDs).
Transient fluorescence spectrum, absorption spectrum and emission spectrum are used to characterize the optical properties of the QDs. As shown in Fig. 4(c), the fluorescence lifetimes of cores and core/shell QDs are 16 and 24 ns, respectively. Compared with cores, the fluorescence intensity decay traces of core/shell QDs are much closer to a straight line. In other words, after introducing the outer shell ZnSe, fluorescent decay trace tend to be single channel attenuation, implying that other channels such as non-radiative decays are greatly reduced. The absorption and emission spectra properties of cores and core/shell QDs are shown in Fig. 4(d). The absorption spectrum of cores has broad range from 500 to 700 nm with absorption peak at ~650 nm. The absorption range of core/shell CdZnSe/ZnSe QDs is as broad as cores, but a small red shift to 660 nm. The emission spectra of QDs are obtained under the 500 nm wavelength excitation, which is the first absorption peak both for cores and core/shell QDs. The fluorescent intensity of core/shell QDs (red solid line) is about two times larger than that of the cores. This suggests that the QY of CdZnSe QDs is significantly improved by capping with ZnSe shell. The emission peak of core/shell QDs (~672 nm) has only a tiny red shift (2 nm) compared with the emission peak of cores (~670 nm). Moreover, the emission spectra of both cores and core/shell QDs are almost symmetrical without trailing phenomenon. It reveals that both of them have small defect density and uniform size distribution. This may be ascribed to the similar lattice parameters between shell ZnSe and core CdZnSe. The lattice matched ZnSe shells on CdZnSe cores cause little material defects, which benefits the high PL QYs of the core/shell QDs.
6. Fabrication of QLEDs using CdZnSe/ZnSe core/shell QDs as emitters
In order to investigate whether red CdZnSe/ZnSe QDs synthesized by one-pot method can be used in practical applications, and then further explore the performance of them, we prepare the QLED devices using the as-prepared QDs as the emitters. A cross-sectional transmission electron microscopy (TEM) image and schematic structure of the QLED devices are shown in Fig. 5. The QLED devices are fabricated by sequentially depositing different layers onto the ITO layer (180 nm) coated glass substrate (1.1 mm), including a PEDOT:PSS layer (35 nm), a poly-TPD layer (15 nm), CdZnSe/ZnSe QD emitter layer (10 nm), a ZnO layer (60 nm) and Al electrode (100 nm). In this architecture, photons emit from the glass surfaces. PEDOT:PSS and poly-TPD are used as hole injection layer (HIL) and hole transport layer (HTL), respectively. ZnO acts as the electron transport layer (ETL). The injections of holes and electrons from HTL and ETL to QD layer lead to electron-hole recombination in QDs, thus emitting photons. Using 635 and 672 nm QDs, QLED devices are prepared by same procedures at identical experimental conditions.
The normalized electroluminescence (EL) spectra of QLEDs working at 6 V are shown in Fig. 6(a). The EL peak wavelength of QLED with 635 nm emitters is 639 nm, which is slightly red-shifted as compared to the PL peak position (635 nm). While the EL peak wavelength for 672 nm emitter QLEDs is 673 nm, which is 1 nm red shift to their PL peak. Such a small redshift between the EL and PL peak indicates that the inter-dot interaction enhancement from the close-packed film or dielectric dispersion of solvent [30] is greatly suppressed owing to the screening effect of the ZnSe shell. The chromaticity coordinates defined by Commission International de l’Eclairage (CIE) are (0.63, 0.37) for 635 nm emitter QLEDs and (0.69, 0.31) for 672 nm emitter QLEDs, respectively, as shown in Fig. 6(e). The two coordinates are both located at the edge of chromaticity diagram, which shows pure orange and red color.
Fig. 6 The performance of 635 and 672 nm emitter QLED devices. (a) EL spectra, (b) EQE and luminance efficiency, (c) current intensity and luminance, (d) luminous efficacy and (e) CIE color coordinates of 635 and 672 nm emitter QLEDs with increasing driving voltage. Insets: the pictures of QLED devices working at 6 V.
The EQE, luminance efficiency, luminance, current intensity and luminous efficacy of QLED devices with increasing driving voltage are shown in Figs. 6(b), 6(c) and 6(d), respectively. For 672 nm emitter QLEDs, the turn on voltage is about 2.2 V, and peak EQE (6.8%) is achieved when driving voltage up to 3 V. Further increasing the driving voltage to 8.8 V, luminance reaches maximum value (1801 cd/m2). The peak luminance efficiency (0.68 Cd/A) and luminous efficacy (4.8 lm/W) are achieved under 3 and 2.2 V driving voltage, respectively. The efficiency roll-off at high driving voltage is in common with the reported literatures [2, 8, 31]. It might be resulting from non-radiative recombination [32, 33]. Increasing carrier intensity in QD emitter layer results in QDs are charged and have critical Auger recombination processes. The increasing rate of current intensity is significantly increased when voltage exceeds 4 V, which result in EQE reaches maximum first than luminance. The 635 nm QLED shows a turn on voltage of 2.0 V with a maximum EQE of 5.9% at voltage of 3.8 V. The EQE of 672 nm emitter QLEDs are superior to 635 nm emitter QLEDs. This should be because the QY of 635 nm QDs is lower than the 672 nm QDs (Table 1).
7. Conclusion
In conclusion, a facile synthesis method for the preparation of red CdZnSe/ZnSe core/shell alloy QDs is developed. By varying the reaction temperatures and amounts of the Se precursors, the emission wavelengths can be tuned from 620 to 690 nm. This synthesis method can be easily extended to prepare alloy QDs with various compositions. The room temperature PL QY of CdZnSe/ZnSe core/shell alloy QDs reaches up to 50%. The EQE up to 6.8% for CdZnSe/ZnSe emitter QLED devices indicates these QDs could be very applied to photoelectric devices such as light-emitting devices or flat-panel displays.
Funding
Natural National Science Foundation of China (NSFC) (11564026, 61366003); Natural Science Foundation of Jiangxi Province (20151BBE50114, 20151BAB212001, 20171BAB202036, 20161BAB212035); Outstanding Youth Funds of Jiangxi Province (20171BCB23051, 20171BCB23052); Science and Technology Project of the education department of Jiangxi Province (GJJ150727, GJJ160681).
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