Shell Quantum Dots with

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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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00183 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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Chemistry of Materials

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

ACS Paragon Plus Environment

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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 hightemperature growth” precisely controlled method. Alloyed ZnCdSe core with certain ratio of Cd and Zn have been pre-synthesized first. Following 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 these superb characteristics including nonblinking, single-exponential PL decay dynamics, and suppressed FRET can be beneficial to high quality QD-based lighting-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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After 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 high stability of QDs was generally the main focus of world-wide 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 as being 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 single-photon 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 electron and hole trapping.17 In the past, the synthesis of QDs with nonblinking behavior is mostly revolved around 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 color 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 exciton wave function. Due to the higher


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conduction band minimum and lower valence band maximum, ZnS could localize the wave function of excitons effectively. However, the relative large lattice mismatch of ~ 12% between CdSe and ZnS can cause accumulation of interfacial strain.18 Therefore, it is inevitable to introduce the defects due to the lattice mismatch between CdSe core and 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 high enough for practical applications because they had to be processed in water-free and oxygen-free environment.20, 21 So there is high demand on 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 alloyed ZnCdSe core QDs with certain ratio of Cd and Zn have been pre-synthesized precisely, but also well-controlled ZnS shell with accurate different thickness (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.) can be grown. 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 thick-shell are successfully fabricated and the defects are greatly reduced during the precisely controlled growth process of ZnS shell. The as-synthesized ZnCdSe/ZnS core/shell QDs have the 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 QDs ensemble are single-exponential and the PL for single QD is nonblinking with the nonblinking threshold volume of ~ 137 nm3. After being transferred to


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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 emitter in QLEDs, the highest external quantum efficiency (EQE) is reached as high as 17%, which could compare favorably with the highest efficiency green QLEDs with traditional multilayered structures

consisting

of

layers

(ITO)//poly(ethylenedioxythiophene):polystyrene

of

indium

sulphonate

dioctylfluorene-co-N-(4-(3-methylpropyl))diphenylamine)

tin

oxide

(PEDOT:PSS)//poly(9,9-

(TFB)//ZnCdSe/ZnS

core/shell

QDs//ZnO//aluminum (Al) cathode. RESULTS AND DISCUSSION The as-synthesized ZnCdSe/ZnS core/shell QDs are prepared by the method of “low temperature injection and high temperature growth” reported previously by our group.23, 24 “Low temperature injection and high temperature growth” refers to “low temperature nucleation and high temperature shell growth”, that is, the shell growth temperature is higher than the nucleation temperature. Due to the higher temperature shell growth procedure, the as-synthesized QDs will have higher chemical/photochemical stability than QDs prepared by low temperature shell growth method. Figure 1a shows the evolution of PL and absorption spectra of the QDs with different ZnS shell thickness. ZnCdSe/ZnS core/shell QDs with x MLs ZnS shell are referred to hereafter as ZnCdSe/xZnS (x stands for an integer from 1 to 24 unless specified). Alloyed ZnCdSe QDs with mean diameter of ~ 5 nm were used as cores to grow core/shell QDs. As shown in Figure 1a and 1b, the absorption and PL peaks all shifted towards the higher energy side during shell growth, which has also been reported by other groups previously.8, 9, 25 The PL peak shifts from 525 nm of ZnCdSe core to 512 nm of ZnCdSe/20ZnS core/shell QDs.


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Figure 1. (a) Evolution of UV-vis and PL spectra of ensemble zinc blende 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 (< 8%) during the entire shell-growth process, which is consistent with the results of optical measurements (Figure 1c). As shown in Figure 1c, the PL FWHM of ZnCdSe/ZnS QDs decreases substantially with the continuous growth of ZnS shell. The FWHM values for most of such QDs can be well controlled with a narrow threshold of 25 nm (~ 110 meV). Such narrow size distribution may be due to the slow shell precursor infusion and the low reactivity of the octanethiol, which may provide a constant and sufficient monomer


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production rate and conform to the model reported recently.26 Unfortunately, in the QD synthesis procedure, the ratio between Zn and Cd in every ZnCdSe core still cannot be controlled to be homogeneous. Therefore, the ensemble PL spectrum is inhomogeneously-broadened,27, 28 and the FWHM of ensemble QDs is broader than that of single QD (Figure S2). With the introduction of the first several MLs of ZnS shell (Figure 1d), the PL QYs of QDs increase substantially due to the passivation effect of ZnS shell which could reduce the PL quenching centers (dangling bond). As the shell thickness increases to 6-10 MLs, the PL QYs of core/shell QDs run up to ~ 100%. The PL QYs then decrease by further increasing the shell thickness. Such superior optical properties can be attributed to the successful surface passivation of the ZnCdSe cores with wider bandgap ZnS shell. It is more essential for the growth of shell material at high temperature, which can promote the diffusion of S and Zn atoms into the ZnCdSe cores. The formation of ZnCdSeS/ZnSeS interfacial alloyed layer between ZnCdSe core and ZnS shell can be proven by the blue-shift of PL peaks and the results of energy dispersive spectroscopy (EDS) elemental mapping. As shown in Figure S3, the cores are alloyed with the elements of Zn, Cd, and Se and the diameter of Cd distribution is consistent with the size of ZnCdSe core (~ 5 nm). Due to the diffusion of S and Zn atoms at high temperature, the elements of Zn, Se, and S are detected both in the core and shell of ZnCdSe/4ZnS QDs. Cd distribution in ZnCdSe/4ZnS QDs with the diameter of ~ 6.4 nm is broader than that in ZnCdSe cores (~ 5 nm). This indicates the diffusion of Cd element into the shell. Comparing the element distribution of Se, S, and Zn with Cd in ZnCdSe/4ZnS core/shell QDs, both the core and shell contain the elements of Se, S, and Zn. While, the diameter of Cd distribution is only 6.4 nm, which is less than the size of ZnCdSe/4ZnS core/shell QDs (7.5 nm). Therefore, it can be deduced that in the process of growing 4 MLs ZnS shell, the ZnCdSe cores are covered by the ZnCdSeS/ZnSeS


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alloyed layer. The gradual decrease of Se and Cd components in ZnCdSeS/ZnSeS shells can energetically confine the excitons in the cores and also can form consecutive transition of lattice parameters from core to shell, which are critical for the reduction of structural defects. High resolution transmission electron microscopy (HR-TEM) images (Figure 2) reveal that the lattice fringes run through the whole particles of ZnCdSe/ZnS QDs, indicating the good crystallinity of as-prepared QDs. This is distinct from the non-gradient-alloy core/shell QDs, which inevitably possess some stacking faults.29 The formation of defect-free zinc-blende ZnCdSe/ZnS core/shell QDs indicates the perfect epitaxial growth of ZnS shell onto the alloyed cores. We speculate that the significantly reduced defect observed here is at least partially caused by the highly crystalline shell due to the high-temperature slow shell-growth condition. X-ray diffraction (XRD) patterns confirm the zinc-blende structure with characteristic (111), (220), and (311) Bragg peaks (Figure S4), in good agreement with the zinc-blende crystal structure of the ZnCdSe core, demonstrate the formation of epitaxial shells. It is also clearly shown that XRD peaks get narrower along with the increase of shell thickness, which indicates the increase of the QD size.


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Figure 2. HR-TEM images of ZnCdSe core and ZnCdSe/ZnS core/shell QDs with a given number of MLs of the ZnS shell (Scale bar: 5 nm). Combining with the narrow FWHM of PL and near 100% QYs, the ZnCdSe/(6-10)ZnS QDs may have low optical polydispersity. As is known, the word “monodispersity” is often used to describe a collection of subjects with the same variable feature, such as size, shape, and mass, etc. In the past, monodispersity usually refers to size monodispersity, i.e., particles with the same size. Similar to size monodispersity, low optical polydispersity refers to a collection of subjects with very identical optical properties. Most recently, many researchers find that low optical polydispersity is even more important than size monodispersity in QD-based practical applications. Low optical polydispersity for QDs generally means that all the QDs have almost the same luminescent properties, such as the emission peak position, intensity, and FWHM of PL as well as the ways of excitons recombination, and so on.30 As for QDs ensemble, such low optical polydispersity represents the narrow FWHM, near-unity QYs, and single-channel PL decay dynamics. Size monodispersity is, in fact, part of the foundation of low optical polydispersity, since the luminescent properties of QDs are partly determined by their dimensions. The QDs with low optical polydispersity can be very beneficial for the preparation of nonblinking QDs. Previously reported nonblinking red CdSe/CdS core/shell QDs with low optical polydispersity were prepared by a low temperature (150 ℃) shell growth process, but such QDs do not have high stability and generally require water-free and oxygen-free environment for ligand exchange and dark storage environment.21 Therefore, it is more important to synthesize QDs with outstanding optical properties and high stability. By adopting high temperature (310 ℃) growth process, our as-synthesized core/shell QDs do show high


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crystallinity and stability. And the excellent optical stability could be retained after the process of ligand exchange and repeated purification in atmospheric environment. As shown in Figure S5, after 12 times of purification process, the PL intensity only quenches by < 20% and < 10% for CdSe/8ZnS and CdSe/14ZnS core/shell QDs, respectively. After exposure to UV light for 500 hours, their PL intensities are all kept at ~ 90% of the initial PL intensity. More importantly, the batch to batch reproducibility in synthesis of our CdSe/ZnS core/shell QDs can be ensured precisely, with an acceptable PL peak position deviation within ±1 nm. Not only the peak position, but also the PL QYs of the QDs are of good repeatability.

Figure 3. (a) The schematic illustration of ZnCdSe core and ZnCdSe/xZnS (x = 2, 5, 8, 14, from top to bottom) core/shell QDs. (b) Blinking behavior of single ZnCdSe core and ZnCdSe/xZnS (x = 2, 5, 8, 14) core/shell QDs in 1200 s, with the binning time of 50 ms. The red and black traces are the PL intensities of single QD and the corrected background noise intensities, respectively. (c) Second-order autocorrelation function measurements for the single ZnCdSe core and ZnCdSe/xZnS (x = 2, 5, 8, 14) core/shell QDs, respectively.


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It is well known that every QD is continuously excited in the operating QLEDs. Therefore, it is critical to study the optical property of QDs under continuous excitation for better understanding of how to fabricate high quality QLEDs. Until now many kinds of QDs with various compositions have been reported with high PL QYs, but only a few kinds of QDs were found to be nonblinking under continuous excitation.13,

31

Here total internal reflection fluorescence

microscope was used to study the PL blinking behavior of as-synthesized single ZnCdSe cores and ZnCdSe/ZnS core/shell QDs under continuous excitation. Diluted QDs were immobilized in a poly(methyl methacrylate) (PMMA) matrix, and their single-dot fluorescence traces in 1200 s were recorded. Second-order photon correlation experiments were carried out to confirm that all the measurements were associated with a single QD. The left panel in Figure 3b shows the fluorescence traces of five representative samples, and the corresponding photon counting histograms are illustrated in the right panel. As shown in Figure 3c, the average area ratios of the central to the side peaks are all < 0.5, this indicates that the measured emission light comes from a single QD. As it can be seen in Figures 3b and S6, PL blinking behavior of all the ZnCdSe cores and ZnCdSe/xZnS (x = 2, 5, 8, 14, 20) core/shell QDs does not occur frequently. The nonblinking threshold volume,32 which is the smallest volume of the nonblinking QDs, is ~ 137 nm3, corresponding to the single-particle volume of ZnCdSe/2ZnS core/shell QDs. This threshold volume is commensurate with the reported nonblinking green QDs.20 The suppressed PL blinking behavior of QDs can be attributed to the following points. First, low reactivity octanethiol is used as the ligand and sulphur source, meanwhile, it could modify and decrease the surface defects of the QDs.33 Second, the formation of alloyed core and the insertion of typical alloyed intermediate layer of ZnCdSe/ZnCdSeS at core/shell interface are an even more powerful strategy for Auger recombination suppression. That is because the alloyed


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intermediate layer can effectively smooth core/shell interfacial potential, and thereby decreases the wave function overlap of the initial and final states of the carriers in Auger decay process.3437

Therefore, the soft confinement of excitons could greatly suppress the Auger recombination

and QD blinking.14,

34, 35

The beneficial effect of the alloyed layer on Auger recombination

suppression should be more appreciable in ZnCdSe/ZnS core/shell QDs with thicker shell. As the thicker ZnS shell will make the alloyed layer being positioned in a more inner side compared to the thinner ones, affording a more solid and complete structure of alloyed core/alloyed intermediate layer/outer shell. Moreover, the perfect lattice matching between the core, the gradient alloy intermediate layer, and the outer shell can afford a well-defined epitaxial growth of shell and greatly reduce interfacial defects. Consequently, the behavior of QD blinking is greatly suppressed with the growth of ZnS shell layer. However, due to the increasing defects induced by the ever-thickening shells, the frequency of PL blinking increases as the continuous increase of the shell thickness in ZnCdSe/xZnS (x > 20) core/shell QDs(Figure S6).


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Figure 4. (a) Ensemble PL decay dynamics of ZnCdSe core and ZnCdSe/xZnS (x = 2, 5, 8, 10, 20) core/shell QDs. Shell-thickness dependence of average PL decay lifetime of QD solution (b) and film (c). Transient PL can indirectly provide information of excited-state decay dynamics of QDs. This makes transient PL as one of the most powerful tools to monitor exciton behavior during the growth of QDs. In order to study excited-state decay dynamics of the as-synthesized ZnCdSe/ZnS core/shell QDs during the process of shell growth, the PL decay dynamics of ensemble QDs with different shell thickness are studied and the data of six representative samples are presented in Figure 4a. It is known that complex multi-exponential PL decay behavior implies multiple decay channels for the excitons. The bi-exponential PL decay dynamics of ZnCdSe core solution shows fast (τ1 = 11.6 ns) and slow (τ2 = 28.0 ns) components of PL lifetime, representing two different exciton radiative recombination processes, respectively. Except for the intrinsic radiative decay, the electron traps, mainly referring to excess or unpassivated Cd/Zn surface sites on ZnCdSe QDs, lead to the slow component in PL dynamics. The electron traps are very shallow, and can be readily isolated from the electron wave function of the excitons with certain layers of ZnS shell.38 Consequently, with the increase of the ZnS shell up to 2-20 MLs, the PL decay curves turned into characteristics of nearly single-exponential with the lifetime of ~ 11 ns for all QD solutions. This indicates that two MLs of ZnS shell could have already suppressed the recombination of long-lived defect states due to the surface passivation effect of ZnS shell.17 The PL decay channel with ~ 11 ns lifetime is possibly originated from the intrinsic radiative decay of the ZnCdSe/ZnS core/shell QDs. Further increase of shell thickness will induce lattice strain accumulation and oriented growth. As can be seen in Figures 2 and S1, the morphology of


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ZnCdSe/24ZnS core/shell QDs is no longer spherical shape. As a consequence, the PL decay dynamics goes back to bi-exponential decay when the shell thickness is increased to 24 MLs (Figure S7). The PL decay dynamics of the close-packed QD film is similar to that of QD solution. For the QD films composed of either ZnCdSe cores or ZnCdSe/24ZnS core/shell QDs, their PL decay dynamics are dominated by bi-exponential decay. And single-exponential decay dynamics has been observed for the close-packed films consisting of ZnCdSe/(2-20)ZnS core/shell QDs. Combined with the analysis mentioned above (QDs with the QYs of ~ 100% and narrow FWHM), the obtained ZnCdSe/(5-10)ZnS core/shell QDs can be categorized as a series of QDs with representative features of low optical polydispersity, i.e. QDs with nonblinking behavior and single PL decay channel. Figure 4b shows the variation tendency of average PL lifetimes of ZnCdSe/ZnS core/shell QD solutions. The average PL lifetime of QD solution is greatly reduced by adding two MLs of ZnS shell onto ZnCdSe cores, and then it reaches a plateau with ~ 11 ns PL lifetime for ZnCdSe/(220)ZnS core/shell QDs. Consequently, ZnCdSe/ZnS core/shell QDs with nearly intrinsic radiative decay and single PL decay channel are obtained. Finally, the average PL lifetime increases to 12.4 ns for ZnCdSe/24ZnS core/shell QD solution. This is quite different from the tendency of PL lifetimes of CdSe/CdS core/shell QDs reported recently.39 That is because radiative recombination rate is known to be related to the overlap of electron and hole wave functions. As for the quasi-type ℃ CdSe/CdS core/shell QDs, the electron wave function delocalizes in the entire QD. That is to say, with the increase of shell thickness, the overlap of electron and hole wave function decreases. Therefore, the radiative decay lifetimes of CdSe/CdS QDs increase monotonically with the shell thickness. While, as for ZnCdSe/ZnS core/shell QDs,


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the electron and hole wave functions are all localized in the ZnCdSe core. With the shell thickness increasing, the electron and hole wave functions are both highly confined in the cores of the QD, which means that the wave function do not extend too much into the shell. Therefore, with the shell thickness increasing, the overlap of electron and hole wave function is nearly unchangeable. Consequently, with the shell thickness varying from 2 MLs to 20 MLs, the radiative decay lifetimes are all ~ 11ns. PL QY equals to the ratio of radiative rate (kr) and the sum of radiative rate and nonradiative rate (kn). Generally, the non-radiative decay is very fast with the typical lifetime of several to hundreds ps. The increase of PL QY generally corresponds to the decrease of non-radiative decay rate, which will lead to longer PL lifetime. The instrument response function (IRF) of the fluorescence spectrometer is shown in Figure S7, and the FWHM of IRF is ~200 ps. As shown in Figure 1d and Figure 4, the PL lifetime is almost unchanged with the increase of PL QYs. Therefore, it is possible that there exist other non-radiative recombination process modulating the PL QYs, but it is too fast to be detected by current spectrometer setup. As shown in Figure 4c, the variation tendency of average PL lifetimes of ZnCdSe/ZnS core/shell QD film is similar to that of ZnCdSe/ZnS core/shell QD solution. For the ZnCdSe/(220)ZnS core/shell QD films, the PL decay dynamics is single exponential. This indicates that no other radiative recombination channel is introduced. And most importantly, these as-synthesized QDs could not only retain single exponential PL decay dynamics very well, but also suppress FRET effectively. FRET is a nonradiative transfer process without net transport of charges. It is a one-step process wherein deexcitation of the donor and excitation of the acceptor occurs simultaneously. It is distinct from radiative transfer in which an intermediate photon is first emitted from the


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donor and then reabsorbed by the acceptor with no direct donor acceptor interaction.40 For closepacked QDs, an exciton in QD with larger energy gap (smaller QD) can transfer to QD with smaller energy gap (larger QD). Therefore, in the QD-based EL devices, QY of solid film, spectroscopic stability, and device efficiency are substantially limited by nonradiative processes of inter-QD FRET within a closely packed QD ensemble.40, 41 Consequently, the radiative lifetime decreases and the PL peak shifts to the lower energy side. Thereafter, such FRET process is often accompanied by red-shift of PL peak, reduced QY of solid film, the addition of decay channels, and finally deteriorate the efficiency and lifetime of QLEDs. So the FRET process could be revealed by the comparison between the steady PL and/or transient PL results of QD solution and close-packed QD film. Up to now, great efforts have been made to reduce the side effect of FRET in practical application by engineering shell thickness. For instance, Pal et al. and Lim et al. observed that light-emitting diodes based on “giant” QDs afforded enhanced performance compared to their counterparts incorporating thin-shell systems due to the suppression of FRET.37, 42 Nevertheless, most of the reported thick-shell QD’s PL QYs were still not very high and the PL decay dynamics were multiple exponential.17,

35, 37

The PL decay dynamics of QD films are generally more

complicated than that of QD solutions. Multiple recombination channels in QD films should be avoided in high quality QD-based PL and EL devices. Therefore, it is critical to obtain QDs with high PL QY, nonblinking, single exponential PL decay dynamics, and suppressed FRET for high-end QD-based practical applications. FRET, which follows the well-known R-6 scaling (R is the donor-acceptor separation), depends strongly on the distance between the adjacent QDs.43 Due to FRET strongly dependent on the distance and the small size of ZnCdSe core QDs (~ 5 nm), the PL lifetime of ZnCdSe cores in


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film is much shorter than that of ZnCdSe core QDs in solution. But it only shows slight difference for ZnCdSe/ZnS core/shell QDs in forms of films and solutions. Along with the increase of shell thickness, the PL lifetime ratio between the close-packed QD films and solutions (τf:τs) increases monotonically (Figure S8a). This indicates that the ZnS shell can localizes electron and hole wave functions in the ZnCdSe core and suppresses the FRET process effectively.

Figure 5. PL spectra of QD solution (black line) and close-packed film (blue line) and EL spectra of QLEDs (green line) for ZnCdSe core (a), ZnCdSe/5ZnS (b), ZnCdSe/10ZnS (c), and ZnCdSe/20ZnS (d) core/shell QDs. The existence of FRET process can also be revealed by the comparison between the steady PL spectra of QD solutions and QD films. As shown in Figure 5a and Table S1, the PL spectrum of ZnCdSe core QD film shifts 8 nm to the lower energy side in comparison with that of QD solution. The key criterion for QLEDs is to preserve high QD’s PL QYs even if the QDs were transferred to close-packed films, and it often means how to avoid and/or reduce the FRET


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process between close-packed QDs.42 With the increase of ZnS shell thickness, the extent of redshift between QD film and QD solution decreases from 3 nm (ZnCdSe/5ZnS) to 1 nm (ZnCdSe/10ZnS and ZnCdSe/20ZnS). The ratios of PL QYs between QD film and QD solution (QYf:QYs) increase monotonically with the increase of shell thickness (Figure S8b). For the ZnCdSe/2ZnS core/shell QDs, QYf:QYs is 74%. This value has been further increased to >81% and 85% for ZnCdSe/10ZnS and ZnCdSe/20ZnS core/shell QDs, respectively. This indicates that FRET process in the close-packed core/shell QD films can be substantially suppressed by simply increasing the shell thickness, which is consistent with the result reported by Pal and coworkers.37 To further evaluate device performance of as-synthesized ZnCdSe core and ZnCdSe/ZnS core/shell QDs, traditional solution-processed multilayer device structure has been employed (Figure S9). It consists of the patterned ITO transparent anode on glass substrate, PEDOT:PSS hole injection layer, TFB hole transport layer, QD emitting layer, ZnO nanoparticles electron transport layer, and Al cathode. As QD-based EL devices, the emission peaks of QDs in EL spectra usually shift towards longer wavelength side versus PL spectra. In addition, with the increase of the applied bias, a gradual EL shift to lower energy side occurs. This can be attributed to the following factors. One is the FRET within QDs, especially closely packed QD solids, and it is also the main reason of the red-shift observed in Figure 5. Another reason is the reduction of the energy of exciton recombination through the quantum confined Stark effect due to the strong electric field.44, 45 Under an electric field, the valance and conduction bands are tilted and the polarization of electron-hole pairs occurs, which lowers the energetic difference between electron and hole states. It leads to a red-shift of emission peak. Finally, thermodynamic effect caused by the charge injection imbalance in the QLEDs is also an important factor as well.


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In a charge imbalanced QLED, excess electrons can accumulate in QD layer, flow through all layers of devices without recombination, and generate substantial Joule heat. Consequently, this thermodynamic effect can lead to EL’s red-shift. Consistent with the result of most QLEDs, the EL spectra of ZnCdSe core based device shifted to the lower energy with the applied bias varying from 3 V to 7 V (Figure 5a and Figure S10a). Such driving bias related emission variation should be avoided in the practical applications of QLEDs like high quality displays. The thick ZnS shell (≥ 10ML) presumably functions as a physical spacer that may screen the external field and effectively impedes the exciton polarization. Therefore, QLEDs with high color stability have been obtained successfully by using ZnCdSe/xZnS (x = 10, 20) core/shell QD emitters (Figure S10). The devices based on ZnCdSe/xZnS (x = 10, 20) core/shell QDs showed almost the same peak wavelength during the increase of applied bias. ZnCdSe/10ZnS core/shell QD-based device has reached the highest EQE of ~ 17% (Figure 6b), which could compare favorably with the highest efficiency green QLEDs with traditional multilayered structures.46 However, the EQEs of the devices start to decline by further increasing the thickness of ZnS shell to 20 MLs. The decline of EQE is likely due to the following factors: First, the QYs of QD film begin to decrease after the thickness of ZnS shell is > 10 MLs. Second, because of the injection path for carriers and large band offset between the charge transport layer and QD layer (Figure 6a), the thicker the ZnS shell thickness is, the harder the carriers can be effectively injected into the QDs.


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Figure 6. (a) The schematic energy level diagram of QLEDs. (b) Luminance dependence of EQE for the devices based on ZnCdSe core and ZnCdSe/xZnS (x = 5, 10, 20) core/shell QDs. As mentioned above, the thicker shell can suppress FRET and stabilize the EL peak position, but extra thick shell will result in the decline of device efficiency. Due to the larger QD volume and larger interspace between the adjacent QDs, ZnCdSe/20ZnS core/shell QD-based devices showed higher leakage current (Figure S11). That is to say, the efficiency of the injection of electrons and holes into QDs for the ZnCdSe/20ZnS core/shell QD-based devices is much lower than that of ZnCdSe/10ZnS core/shell QD-based devices. The volume of QD expands greatly by the increase of shell thickness. This can lead to the increase of interspace between adjacent QDs and reduce the number of QDs per unit area in the active layer.37 Therefore, the overall effective emission of photons will be reduced and device EQE is decreased. CONCLUSIONS A series of ZnCdSe-based core/shell QDs were precisely synthesized by “low temperature injection and high temperature growth” method. The as-synthesizd ZnCdSe/ZnS core/shell QDs have good crystalline with reduced defects due to the low reactivity of octanethiol and the constant epitaxial growth of ZnS shell. By continuously increasing the shell thickness, all of the PL decay dynamics of ZnCdSe/(2-20)ZnS core/shell QDs solution and close-packed film are


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single exponential, and the ZnCdSe/(2-20)ZnS core/shell QDs are nonblinking with the nonblinking threshold volume of ~ 137 nm3. Moreover, the ZnCdSe/xZnS (x ≥ 10) core/shell QDs can suppress FRET effectively. QLEDs have been fabricated with such ZnCdSe/ZnS core/shell QDs, which show the superior characteristics including nonblinking, single exponential decay dynamics, and FRET suppression. The highest EQE of ~ 17% has been reached successfully based on ZnCdSe/10ZnS core/shell QDs, which can compare favorably with the highest efficiency green QLEDs with traditional multilayered structures.

ASSOCIATED CONTENT Acknowledgements The authors gratefully acknowledge the financial support from the research project of the National Natural Science Foundation of China (61474037, 21671058, and 61504040). The authors want to thank Professor Xiaoyong Wang (National Laboratory of Solid State Microstructures, School of Physics, and Collaborative Innovation Center of Advanced Microstructures, Nanjing University) providing anti-bunching (g2) measurements. Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org. Synthesis of ZnCdSe/ZnS core/shell QDs, PL intensity traces measurement for single QD, fabrication of QLEDs, TEM images of ZnCdSe core and ZnCdSe/ZnS core/shell QDs, PL spectra of single ZnCdSe/10ZnS QD and ensemble ZnCdSe/10ZnS QDs, EDS elemental mapping of ZnCdSe core, ZnCdeSe/4ZnS core/shell QDs, evolution of the relative PL stability of ZnCdSe/8ZnS and ZnCdSe/14ZnS core/shell QDs, XRD patterns of ZnCdSe core and


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ZnCdSe/ZnS core/shell QDs, blinking behavior of ZnCdSe/xZnS (x = 20, 24) core/shell QDs, ensemble PL decay dynamics of ZnCdSe/xZnS (x = 14, 18, 24) core/shell QD solutions and films and the instrument response function, the variation tendencies of the PL lifetime ratios of ZnCdSe/ZnS core/shell QDs close-packed films to solutions and the absolute PL QY of QDs solution and QDs film, schematic illustration of the QLEDs, evolution of device’s EL spectra with increasing bias from 3 V to 7 V, current density and luminance as a function of bias for the devices based on ZnCdSe core and ZnCdSe/xZnS (x = 5, 10, 20) core/shell QDs, the elemental composition of ZnCdSe cores detected by EDS, summary of QD solution PL, QD film PL, and the devices EL spectra emission peak positions and FWHM, and the QYs of ZnCdSe/ZnS core/shell QDs upon different precipitation times. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]. Author Contributions Zhaohan Li and Fei Chen contributed equally to this paper. The manuscript was written through contributions of all authors.


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Shell Quantum Dots with

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