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Abstract
High quality blue emitting quantum dots (QDs) are regarded as promising nanomaterials for lasers, photovoltaic cells and displays. However, few reports realize high photoluminescence quantum yield (PL QY), narrow emission band width and pure blue emitting (450∼460 nm) simultaneously. Herein we propose a facile one-step synthesis of thick shell blue emitting CdZnS/ZnS QDs. ZnS shell was overcoated on the prepared cores by directly introducing zinc oleate/S TBP solution (zinc oleate powder and S dissolved in TBP) into mixture without any purification steps and the thickness of ZnS shell was controlled by adjusting the adding amount of zinc oleate/S TBP solution. The optimal QDs with ten monolayers of ZnS shell exhibit pure blue light (∼455 nm) with narrow line width (full width half maximum, FWHM 17.2 nm) and high photoluminescent quantum yield (QY) (92%). Due to the thick ZnS shell, nonradiative recombination of the QD solids is suppressed efficiently.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Semiconductor QDs with sizes in the quantum confinement regime are considered as ideal materials in various applications, such as light-emitting diodes, solar cells, lasing [1–3]. QDs have advantages in their simple colloidal synthetic processes, solution process ability and unique optical properties including tunable band emission over the whole visible spectrum, efficient broadband absorption and narrow emission width [4]. In particular, blue light emitting QDs have been significantly optimized adopting CdZnS, CdZnSe, CdSeS, CdS and CdSe QDs as the emitting centers [5–9]. Such blue emitting cores are hard to guarantee high emitting PL QY, which is mainly because the optical properties of these materials are sensitive to their surface chemistry and chemical environment. This issue could be solved by passivating the surfaces though epitaxial growth of wide bandgap inorganic overlayers [10–12]. In the core/shell structured QDs, the shell enhances physical and chemical stabilities, as well as photostabilities to the core QDs [13–15]. More importantly, QDs with multi-shell or giant shell are not susceptible to repeated purification cycle due to the physical separation of organic ligands from emitting inner core and thus retaining the original QY. In recent years, high quality blue emitting QDs with core/shell structure have been achieved [16–20]. Seonghoon Lee’s group reported a facile method of highly luminescent blue emitting CdZnS/ZnS core/shell structured QDs. The prepared nanocrystals with PL emission wavelength at 433 nm have high PL QY (up to 80%) and narrow spectral bandwidth (FWHM < 25 nm) [20]. Subsequently, CdZnS/ZnS QDs were prepared with nearly 100% PL QY, and the emission band width (452 nm) is about 31 nm [18]. Hyeonseok Yoon’s group reported CdZnS@ZnS QDs with 99.5% PL QY and 20.1 nm FWHM with a modified method, the PL emission wavelength of which is 438 nm [16]. In addition, many high quality blue light QDs synthesis methods have been reported [21–23]. However, there are rarely reports of blue emitting QDs to realize high PL QY, narrow emission band width and pure blue emitting (450∼460 nm) simultaneously.
Herein, we report one-pot synthesis of highly luminescent blue emitting core-shell structured CdZnS/ZnS QDs. The prepared QDs exhibit high PL QY (up to 92%) with narrow line width (FWHM <18 nm) and the emission wavelength is modulated to about 455 nm. During the coating process, a thick ZnS shell is obtained with zinc oleate powder as the shell precursor instead of the traditional zinc oleate solution. We demonstrate that the non-radiative recombination of solid form is suppressed greatly due to the thick ZnS shell.
2. Experiment
2.1 Material synthesis
Cadmium oxide (CdO, 99.99%) and tributyl phosphate (TBP) was bought from Macklin. Zinc acetate (Zn(CH3COO)2, 99.99%), n-hexane (97%) and ethanol (99.7%) were purchased from Aladdin. Oleic Acid (OA), 1-octadecene (1-ODE) were purchased from Alfa Aescar. Sulfur (S, 99.99%) was purchased from Sigma Aldrich. Zinc oleate powder was prepared as follow: dissolving zinc acetate (30 mmol) in 20 mL of OA and 40 mL 1-ODE at 170 °C under N2 flow, the reaction solution was maintained for 30 min and then cooled to room temperature. Subsequently, the zinc oleate solution (0.5M) was filtered and cleaned with n-hexane to remove the excess organic solvent. Finally, the product was dried at 60 °C.
One Spot Synthesis of thick shell CdZnS/ZnS QDs: Ternary alloy CdZnS/ZnS QDs were synthesized following a published literature with some modification. Briefly, a 100 mL three-neck flask containing 7 mmol of Zn(CH3COO)2, 1 mmol of CdO, 5 mL of OA and 15 mL of 1-ODE was heated to 140 °C under vacuum for 1 h to obtain a translucent solution, then filled with N2, the mixture was heated to 310 °C. When the temperature was reached to 280 °C, 1.4 mmol of sulfur dissolved in 3 mL 1-ODE was quickly injected into the hot reaction mixture, and maintained at 310 °C for 12 min to allow the growth of the CdZnS core. Subsequently, 11.6 mmol of sulfur and 5.65 mmol zinc oleate powder dissolved in 8 mL TBP (as the shell precursor) was added dropwise into the solution with the speed of 5 mL/h for the shell coating. When finishing this step, the solution was cooled to room temperature. The product was precipitated using n-hexane/ethanol solution and separated by centrifugation. The supernatant was drained and the precipitate was dispersed in n-hexane, and then centrifuged to separate the insoluble solid. This step was repeated three times. CdZnS/ZnS QDs were precipitated again using ethanol, purified with n-hexane/ethanol, and finally stored in n-hexane at 4 °C. For the preparation of CdZnS/ZnS QDs with 5 monolayer ZnS shell, 6.22 mmol of sulfur dissolved in 3 mL TBP was added dropwise into the solution with the speed of 5 mL/h for the shell coating. For the prepared of CdZnS/ZnS QDs with other layers of ZnS shell, the methods are the same except that the content of zinc oleate powder in TBP is 1, 3 and 8.5 mmol for 6, 8 and 11 monolayer ZnS shell, respectively. For the amount of S in the TBP solution, we need to ensure that the total amount of cation in the solution is 1.05 times as that of S.
One Spot Synthesis of CdZnS/ZnS QDs using zinc oleate solution as ZnS shell precursor: The steps before the growth process of CdZnS core were exactly the same as the above operation procedure. After the core growth finishing, 11.3 mL as-prepared hot zinc oleate solution (0.5M) was injected into the solution quickly, subsequently 11.6 mmol of sulfur dissolved in 5 mL TBP was added dropwise into the solution with the speed of 5 mL/h for the shell coating. The following procedures of purify and storage are identical with mentioned above.
2.2 QDs Solid Films Fabrication
QDs solid films were fabricated with pre-patterned quartz glass substrates those were ultrasonic cleaned and then treated in an ultraviolet-ozone for 20 min. Subsequently, the QDs (QDs in n-octane, 30 mg/mL) were spin-coated onto the quartz glass substrates at 2000 r.p.m for 60 s, respectively, followed by baking at 80 ℃ for 30 min.
2.3 Characterizations
Absorption and PL emission spectra were recorded with UV visible absorption spectroscopy (Horiba, Duetta). Transmission electron microscopy (TEM) and high-resolution scanning transmission electron microscopy (HRTEM) images of CdZnS core, CdZnS/ZnS core/shell QDs were taken with a FEI Tecnai F20st operating at 200 kV. X-ray diffraction (XRD) patterns were recorded on a D/MAXRB X-ray diffractometer operated at 12 kW with Cu Kα radiation (l = 1.5418 Å). Fluorescence spectroscopy (Horiba, Fluoromax spectrofluorometer) was used to detect the 365 nm excitation light from 300 W Xenon lamps. An integrating sphere (Quanta-φ) was used to measure the efficiency data. According to the Eq. (1), the quantum yield of PL emission of the QDs was calculated.
In this equation, Lsample is the emission intensity; Ereference and Esample are the intensities of the excitation light not absorbed by the reference sample (n-hexane in this case) and the QDs, respectively. Time-resolved fluorescence measurement was performed by using a fluorescence lifetime measurement system based on time-correlated single photon counting (TCSPC). Excitation of the sample was achieved with a Nano-LED with 329 nm, 1 MHz repetition rate. The PL lifetime at peak wavelength of CdZnS/ZnS QDs was determined by double-exponential fit to the decays using the following Eq. (2).
3. Results and Discussion
Core/shell structured CdZnS/ZnS QDs were obtained through two times injection of S precursors directly into a hot mixture of zinc oleate and cadmium oleate (Fig. 1). For the CdZnS cores nucleating and growing, the first injection of S precursor is S-ODE (S powder dissolved in non-coordinating solvent 1-ODE), which facilitates the rapid completion of the nucleation process. Sequentially, sulfur and zinc oleate powder dissolved in TBP as the shell precursor were introduced into the reaction mixture to form ZnS outer layer on the surface of CdZnS cores. The thickness of ZnS shell could be controlled by adjusting the addition amount of shell precursor and make sure the total amount of cations is 1.05 times as that of S added in the two steps. The as-prepared CdZnS cores take uniform spherical shapes with the dimeter of 5.6 nm (Fig. 2(a)). Figure 2(b)-(f) show the TEM images of CdZnS/ZnS core/shell QDs, all of which are single crystal particles with good monodispersity. The CdZnS/ZnS core/shell QDs with different ZnS layers are polygonal structured and the average size increases to about 8.7 nm, 10 nm, 11.1 nm, 12.4 nm and 12.9 nm, respectively. The corresponding HRTEM images of CdZnS cores and CdZnS/ZnS core/shell QDs (insets in Fig. 2(a), (e)) revealed high crystallinity without obvious defects throughout the whole particles, and no evidence of interfaces between core and shell region were observed. All the core/shell QDs were synthesized from the same starting 5.6-nm CdZnS cores. Consider that the thickness of one monolayer ZnS is about 0.3 nm, the increased shell thickness indicates the formation of five, six, eight, ten and eleven monolayers (MLs) of ZnS shell on the core surface. The crystallinity of the CdZnS core and CdZnS/ZnS QDs with different shell thickness was investigated using XRD patterns (Fig. 2 g). The XRD patterns of bulk wurtzite structure CdS and ZnS are also shown at the bottom and top, respectively. The bare CdZnS core and CdZnS/ZnS QDs exhibit wurtzite crystal structure. The XRD peaks of CdZnS cores are closer to that of wurtzite CdS, which indicates that the Cd is the main component in the CdZnS core. With the shell precursors introduced into the reactor to form ZnS outer layer, the peaks of the QDs shift to higher angles as the thickness increases gradually, which corresponds to the growth of ZnS shell.
Fig. 1. Schematic illustration for the synthesis of CdZnS/ZnS QDs: S-ODE was first injected into the hot solution containing Cd2+ and Zn2+ precursors, oleic acid (OA) and 1-octadecene (1-ODE), followed by injecting (zinc oleate powder and S)-TBP into the reaction solution.
Fig. 2. TEM images (a-f) and XRD patterns (h) of corresponding ZnCdS core and ZnCdS@ZnS core/shell QDs with different ZnS shell thickness. Upper right insets in (a) and (d) are the HRTEM images of corresponding QDs, respectively. (Scale bar in inset of Fig. 2(a) and (e) is 5 nm)
The optical properties and composition changes were monitored during core growth process. As shown in Fig. 3, the room temperature UV-vis absorption and PL spectra of CdZnS core were obtained at various times (1, 6, 12, 20 and 30 min) after S-ODE injection. It is observed that the excitonic absorption peak gradually becomes weak with increasing shell thickness. The PL spectra of the CdZnS core show narrow Gaussian-shaped band edge emission (446-472 nm) and broad emission (500-750 nm) originating from the trap states around the particle surface (crystal defects, dangling bonds and so on). With the core growth process, the band edge emission is red-shifted and the emission width becomes narrower evidently in the first 12 min. The redshift is mainly caused by an increase in particle size considering the increase of the Zn2+ content in the core during this time. Meanwhile, the trap states emission is suppressed, which suggests the lattice defects decrease gradually [8]. After 12 min reaction, the fluorescence peak (∼466 nm) and emission width did not change significantly, indicating that the core size and composition tended to be uniform.
Fig. 3. Absorption (a) and PL spectra (b) of CdZnS core obtained at different reaction times (Excitation wavelength of the PL spectra is 365 nm).
Figure 4(a) shows the PL and absorption spectra of the core/shell QDs with different layers of ZnS shell. With the ZnS shell formation, the trap state emission was strongly suppressed and the band edge PL emission was enhanced drastically. The fluorescence peaks of all samples were blue shifted significantly, and the thicker ZnS shell, the bluer shifted peak wavelength. Figure 4(b) shows the PL QY of the core/shell QDs with different ZnS shell thickness. Among them, the PL QY of CdZnS/ZnS QDs with ten monolayers of ZnS shell was measured to be 92%. Further increasing the thickness of ZnS shell to eleven layers would lead to the decrease of the PL QY of CdS/ZnS core/shell QDs probably resulting from the increase of the mismatch stress [24]. Emission peak shift during ZnS shell coating was also investigated. As shown in Fig. 5, ten ZnS layers CdZnS/ZnS QDs as example, the PL peak of the QDs after 10 min reaction time is red-shifted by 3 nm to that of the core indicating that there is still a little amount of Cd2+ in the reaction solution to further grow CdZnS shell. As the reaction continues, Cd2+ in the solution is consumed completely and the emission peak starts to shift blue. The blue shift in PL emission originates from the intradiffusion of Zn from the ZnS shells into the core region at high temperature (310 °C).
Fig. 4. (a) Evolution of UV – Vis absorption and PL spectra of the CdZnS and CdZnS/ZnS QDs upon shell growth; (b) PL QY and emission wavelength of CdZnS and CdZnS/ZnS QDs with a given number of monolayers (MLs) of the ZnS shell. Inset in (b): photo images of QDs diluent solution upon 365 nm UV lamp irradiation (upper) and room light (lower).
Fig. 5. Evolution of (a) PL spectra and (b) PL emission peak wavelengths and emission width of CdZnS/ZnS QDs as a function of reaction time during the shell formation.
The remarkable enhancement in optical properties is due to the successful trap passivation of CdZnS core by ZnS shell with wider band gap. The ZnS shell structurally passivates the dangling bonds on the core surface and suppresses the leakage of excitons from the core into the shell. More importantly, shelling at a high temperature (310 °C) could induce the interfacial alloying and thus render the core/shell boundary less abrupt [18]. Besides, in this work for a thick ZnS shell overcoating, zinc oleate powder added into the S-TBP (sulfur dissolved in TBP), instead of conventional zinc oleate dissolved in oleic acid and 1-ODE, was introduced as the Zn source. Previous research found that high concentration of oleic acid molecules (OAH) in the reaction solution facilitated the dissolution of the core nanocrystals during epitaxial shell growth process [25]. More importantly, the different binding preferences of OAH to facets of a single crystal, such as wurtzite crystal structure here, usually induce non-uniform epitaxial growth during constructing core/shell structured nanomaterials [26–28]. If the conventional zinc oleate solution is adopted during ZnS shell coating, the free oleic acid molecules will bind to the bare cations on the surface of the nuclear CdZnS particles, which would prevent the deposition of the shell precursor on the particle surface and lead to a non-uniform ZnS shell eventually. That would be detrimental to isolate the wave function of the CdZnS core from the surface defects and environment [14]. As a comparison, CdZnS/ZnS core/shell QDs using zinc oleate solution as Zn source for the ZnS shell formation were prepared. The amount of zinc oleate is the same as that used for above mentioned ten ZnS layers QDs and there was no significant difference in particle size, but the PL QY of diluent solution is only about 71%.
PL QY and fluorescence lifetime measurements of about 30 nm-thick QDs films spinning on the bare glass substrates have been carried out. The dependence of the PL QY on shell layer can be seen in Fig. 6(a). The CdZnS/ZnS QDs solid films exhibited reduced QY values compared to the results from QDs dilute solution due to the existence of Förster resonance energy transfer (FRET) between the QDs [29]. With more ZnS layers shell the PL QY of the solid QDs film gradually increases. Figure 6(b) illustrates the PL decay results of CdZnS/ZnS core/shell QDs with different ZnS shell thickness. The PL decay dynamics of the QDs exhibits a trend that the dramatically declining lifetime along with the increasing shell thickness. According to the fitting results, the ensemble PL decay dynamics behave in a double-exponential way, suggesting at least two decay pathways for the excitons (Table 1). The short lifetime decay pathway belongs to the intrinsic PL decay channel and the long-lived lifetime components from the surface defect state recombination [19]. With thin ZnS shell the surface sites on the CdZnS core were passivated incompletely, leading to the slow process dominating the PL decay. As the shell thickness is creased, the recombination of long-lived defect states is suppressed and thus the ZnS shell could provide spatial confinement efficiently. As a result, the lifetime of the intrinsic fast process was shortened from 43.95 ns to 13.74 ns and the ratio of fast process increases from 84% to 99%. PL spectra of solid QDs film were all found to be red-shifted compared with those of QDs dilute solution (Fig. 7). And the thicker the ZnS shell is the less red shift. Since the red shifted in wavelength originate from the FRET process in close-packed QDs solid film, the results indicate that thick ZnS shell could confine electron and hole wave functions for good luminescent properties efficiently [30,31].
Fig. 6. (a) PL QY of solid QDs films and (b) Time-resolved PL decay of CdZnS/ZnS QDs with different ZnS layers shell.
Fig. 7. PL spectra of diluent solution and solid film for CdZnS/ZnS QDs with different ZnS shell thickness in 365 nm UV lamp irradiation. The amounts of redshift are labelled in the images.
Table 1. Fitting Results of the Decay Curves for CdZnS/ZnS QDs with Different ZnS Shell Layersa
4. Conclusion
High quality blue emitting CdZnS/ZnS core/shell structured QDs with thick ZnS out layers were prepared by using one pot synthesis method. The QDs possess high color purity (λmax. =455 nm, FWHM=17.2 nm) and high PL QY (92%). These results demonstrate that adopting zinc oleate powder as shell precursor instead of zinc oleate solution could guarantee a high PL QY. The obtained thick shell CdZnS/ZnS QDs could confine excitons within the CdZnS cores and facilitate the intrinsic radiative recombination.
Funding
Natural Science Foundation of Guangdong Province (2018A030307011, 2019A1515011228, 2019A1515011461); Program of Young Creative Talents in Universities of Guangdong Province (2018KQNCX153); Natural Science Foundation of Jiangxi Province (20192ACBL21045); National Natural Science Foundation of China (61705095); Innovation Team of Guangdong Higher Education Institutes (2019KCXT012); Special Projects in Key Areas of “Service for Rural Revitalization Plan” of the Ordinary Colleges in Guangdong Province (2019KZDZX2008).
Acknowledgments
The authors also thank the State Key Laboratory of Materials Processing and Die & Mould Technology of HUST for the characterizations.
Disclosures
The authors declare no conflicts of interest.
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