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Cadmium-Free InP/ZnSeS/ZnS Heterostructure-Based Quantum Dot Light-Emitting Diodes with a ZnMgO Electron Transport Layer and a Brightness of Over 10 000 cd m−2

Hung Chia Wang, Heng Zhang, Hao Yue Chen, Han Cheng Yeh, Mei Rurng Tseng, Ren Jei Chung,* Shuming Chen,* and Ru Shi Liu*

in two types of devices: QDs for photoluminescence (PL; white QD-LEDs) and QDs for electroluminescence (EL, QLEDs). Photoluminescent white QD-LEDs can be used for lighting purposes due to the improvement of color rendering index, or as backlight for liquid-crystal display (LCD) due to the improvement of color saturation displays. In 2012, Yang et al.[9] synthesized InP/ZnS QDs using the heating-up method. In this method, InP/Zn core size was controlled by the In/Zn ratio and the reaction time. After the growth of InP/Zn core structure, Zn(DDTC)2 single precursor was added to further generate the outer ZnS shell. This method is facile in synthesizing high-quality QDs and is used in white QD-LEDs. In 2013, Lim et al.[10] synthesized InP@ZnSeS with gradient composition shell QDs via the hot-injection method. After growing the InP core, sele-nium–trioctylphosphine (Se-TOP), zinc acetate (Zn(OA)2), and 1-dodecanethiol (DDT) were added to grow the outer ZnSeS shell. It is difficult to control the emission wavelength of the InP core via the hot-injection method with a precursor injected at a high temperature. We modified the previous syn-thesis process and used the interfacial alloyed ZnSeS layer as the buffer layer. When we gradually increased the thickness of the additional ZnS, the strong lattice strain of the ZnS shell on the InP core crystal caused lattice defects and low PL. The interfacial alloyed ZnSeS shell can decrease the lattice mis-match between InP and ZnS and enhance the stability of InP/ZnSeS/ZnS QDs. The device efficiency of QLED is substan-tially limited because of the effect of nonradioactive relaxation (Förster resonant energy transfer) among the closely packed QDs,[11] the effect of water and oxygen, and Auger recombina-tion. These effects are more evident when the QDs are in the form of a thin film in QLEDs.[11] To suppress these effects, the most effective solution is to enlarge the outer passivated shell. In a previous study, Bae et al.[12] discovered that the interfa-cial alloyed layer of CdSe0.5S0.5 would reduce the Auger decay rate and improve QLEDs’ performance. In InP/ZnSeS/ZnS QLEDs, we assume that the ZnSeS interfacial alloy layer has the same function. Giant QDs have outer passivated ZnS shells that are thicker by more than ten monolayers.[11] Similar to that of cadmium-based QDs, the development of thick-shelled cadmium-free QDs is important for the application of QLEDs. Apart from improving the performance of QDs, the structure of the QLED should be carefully designed and optimized so as to achieve high efficiency. In designing a QLED device, the DOI: 10.1002/smll.201603962

Quantum Dots

H. C. Wang, Prof. R. S. LiuDepartment of ChemistryNational Taiwan UniversityTaipei 106, TaiwanE-mail: [email protected]

H. Zhang, Prof. S. M. ChenDepartment of Electrical and Electronic EngineeringSouthern University of Science and TechnologyShenzhen 518055, ChinaE-mail: [email protected]

H. Y. Chen, Prof. R. J. ChungDepartment of Chemical Engineering and BiotechnologyNational Taipei University of TechnologyTaipei 106, TaiwanE-mail: [email protected]

H. C. Yeh, M. R. TsengMaterial and Chemical Research LaboratoriesIndustrial Technology Research Institute (ITRI)Hsinchu 300, Taiwan

Prof. R. S. LiuDepartment of Mechanical Engineering and GraduateInstitute of Manufacturing TechnologyNational Taipei University of TechnologyTaipei 106, Taiwan

Quantum dots (QDs) can be used in many applications, such as light-emitting diodes (LEDs),[1–4] solar cells,[5] and bioim-aging.[6] QD-based LEDs are excellent candidates for next-generation displays because of their advantages of high color saturation, simple fabrication, and good stability. Colloidal QD-based display industries have gradually flourished in recent years. At present, QDs can be classified into three types: cadmium-based types (CdSe, CdS, and CdTe), Cd-free types (InP and CuInS2), and the new type perovskite (CsPbBr3

[7] and CH3NH3PbBr3

[8]) QDs. Traditional cadmium-based QDs have been developed for the past 20 years and have many commercial applications in the LED industry. However, cad-mium-based QDs can negatively affect the environment, and they are difficult to recycle. In recent years, cadmium-free QDs have gained considerable attention because of their environ-mental friendliness. In a cadmium-free based QD, InP is con-sidered as an ideal cadmium alternative material because of its narrower emission wavelength (45–65 nm) and higher color purity for displays. Cadmium-free QD-based LEDs are applied

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following conditions should be considered: (i) type of device structure, namely, normal structure or inverted architecture; (ii) band structure of hole transport and electron transport layers; (iii) mobility of hole transport and electron transport layers. In a recent study, the best electron transport material for QLEDs was determined to be ZnO nanoparticles, which exhibited high electron mobility and good band structure (−4.3 to −7.5 eV) for QD-emitting layer. However, cadmium-free QDs (e.g., InP[13] and CuInS2

[14,15]) have higher conduc-tion band minimum (CBM) level than the ZnO electron transport layer, which indicates that electron injection from the ZnO electron transport layer to the QD-emitting layer is difficult. Consequently, QLEDs with ZnO exhibited high turn-on voltage and lower efficiency. (iv) The neighboring layers intermix during the spin-coating process.[15,16] Once the afore-mentioned issue has been overcome, cadmium-free QDs can be suitable for the inverted device. In the inverted device, we can prevent the layer from intermixing with neighboring layers, and the thickness of the multilayer can easily be controlled using the mature thermal evaporation technique. Moreover, reducing the electron injection barrier is an important factor in improving the performance of cadmium-free QLED device. In a previous research,[17] a new type of ZnMgO-nanoparticle-based electron transport layer was developed for application in the CuInS2 QLED device. The ZnMgO electron transport layer has a higher CBM band structure than the ZnO electron transport layer, which means that the electron injection bar-rier between the electron transport layer and the QD-emitting layer is reduced. This Cd-free QLED device can reach a bright-ness of 2785 cd m−2 and a current efficiency of 5.75 cd A−1. The magnesium-alloyed electron transport layer can dramatically improve the performance of cadmium-free QLED. In 2013, Lim et al.[18] adopted a thin polymer layer such as poly[(9,9-bis(3′-(N,N dimethylamino) propyl)-2,7-(9,9-ioctyl-fluorene) (PFN) in the InP QLED device. The PFN layer functioned as an intermediate layer between the QD-emitting layer and the ZnO layer. Like the ZnMgO electron transport layer, the PFN polymer layer deposited on the ZnO electron transport layer also has the higher band structure than the original ZnO elec-tron transport layer. These cadmium-free QLEDs can reach a brightness of 3900 cd m−2 and an external quantum efficiency (EQE) of 3.43%. How-ever, using the solution process will cause the intermixing of neighboring layers and cause difficulty in controlling the thickness of the PFN layer. In the previous research, the performance of cadmium-free QLEDs was dominated by the band structure of the electron transport layer and the quality of QDs. At present, the brightness and power efficiency of cadmium-free QLEDs remain low. Thus, we propose the heating-up method to synthesize thick-shelled cadmium-free InP/ZnSeS/ZnS QDs. High-performance Cd-free QLEDs are demon-strated by using thick-shelled InP/ZnSeS/ZnS QDs and ZnMgO commercial elec-tron transport layer in an inverted struc-ture. The outer ZnS shell can improve the

stability of the InP QDs, and the ZnMgO electron transport layer can reduce the electron injection barrier. The brightness of this improved QLED device is higher than 10 000 cd m−2, which is the highest value observed for cadmium-free QLEDs.

The synthesis of cadmium-free InP/ZnSeS/ZnS QDs is shown in Figure 1. The detailed protocol is shown in the Sup-porting Information. In this synthesis, we can control the emission wavelength of the InP core by changing the amount of zinc source. When we decreased the amount of zinc source, the emission wavelength of the InP core is expected to have a redshift. After the formation of the InP/Zn core, we inject the sulfur source to form the ZnS shell that protects the InP/Zn core. To increase the thickness of the ZnS shell, we should consider the problem of lattice mismatch. Given the higher lattice mismatch between the InP core and ZnS shell, the thick shell of ZnS will cause the lattice to collapse and decrease the stability of QDs. Thus, we inject the selenium-TOP precursor to the InP/ZnS solution to form a ZnSe buffer layer. Finally, we can increase the thickness of the ZnS shell. We optimize the thickness of the ZnS layer to improve the quality of the InP/ZnSeS/ZnS QDs. After purifying InP/ZnSeS/ZnS for fourth times, the QD precipitate is redispersed into the octane solution for device fabrication. The PL and UV–vis spectra of the InP/Zn core, the InP/ZnSeS core/shell, and the InP/ZnSeS/ZnS core/shell/shell are shown in Figure 2. The original emission wavelength of the In/Zn core is 499 nm. After the formation of the ZnSeS shell, the emission wave-length shifts to 506 nm. The emission wavelength of the green QDs can shift from 506 to 525 nm because of the thick ZnS shell formation process. In the UV–vis spectra, the first absorption maximum peak is unobvious because of the thick outer ZnS shell (spectra in the inset of Figure 2b). The thick outer shell reduces the absorption peak of the InP core. This phenomenon is also observed in the cadmium-based thick shell QD system. The quantum yield of InP/ZnSeS/ZnS was measured using an absolute PLQY spectrometer (c11347, Hamamatsu). An excitation wavelength of 460 nm was set to calculate the absolute quantum yield. The absolute quantum yields of InP/Zn, InP/ZnSeS, and InP/ZnSeS/ZnS were 20%, 55%, and 70%, respectively. In this work, we synthesized


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Figure 1. The synthesis of green InP/ZnSeS/ZnS Cd-free quantum dots.


green (525 nm) InP/ZnSeS/ZnS QDs with an emission band-width of 65 nm. Compared with other cadmium-free QDs, the synthesized QDs are most favorable for use in display. The average particle size of the InP/ZnSeS QDs was estimated to be 3.3 ± 0.4 nm using a high-resolution transmission electron microscope (HR-TEM), as shown in Figure S1a (Supporting Information). The average particle size of the InP/ZnSeS/ZnS QDs was estimated to be 7.4 ± 0.5 nm, as shown in Figure S1b (Supporting Information). The thickness of the overgrown ZnS shell was ≈2.1 nm. The thick ZnS shell enabled the InP QDs to maintain their high fluorescent stability and lifetime against environmental changes on the surface of QDs. For crystal characterization, the lattice fringe result of InP/ZnSeS/ZnS is shown in Figure S2 (Supporting Information). InP/ZnSeS/ZnS QDs were blended with zinc using a d-spacing of 3.37 Å. This value matched the interspacing of the (111) planes of the InP bulk crystal. The X-ray diffraction (XRD)

pattern further confirmed the structure of InP/ZnSeS/ZnS. The XRD patterns of the InP core, the InP/ZnSeS core/shell, and the InP/ZnSeS/ZnS core/shell/shell are shown in Figure S1c (Supporting Information). Three distinct reflection planes with values of (111), (220), and (311) were in accord-ance with the zinc-blended structure (InP; JCPDS No. 10-0216). The reflection peaks of InP/ZnSeS/ZnS shifted to a larger 2θ compared with the InP core and the InP/ZnSeS QDs because of the additional ZnS (JCPDS No. 77-2100) layer deposited on the InP/ZnSeS QDs. The chemical compo-sitions of the thin-shelled InP/ZnS and the thick-shelled InP/ZnSeS/ZnS were assessed via energy-dispersive X-ray spec-troscopy (EDS). The EDS spectra of the InP/ZnS QDs are shown in Figure S3 (Supporting Information). Compared with the EDS spectra of the InP/ZnSeS/ZnS QDs in Figure 3d, a higher elemental ratio of zinc to sulfur was observed. The EDS spectra also confirmed the overcoating of


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Figure 2. a) PL spectra and b) UV–vis spectra of the InP/Zn core, the InP/ZnSeS core/shell, and the InP/ZnSeS/ZnS core/shell/shell QDs. The inset photograph in part (a) shows that the QD solution is under UV irradiation. The spectra in the inset of part (b) shows the characteristic absorption of InP QDs.

Figure 3. HR-TEM images of a) thin-shelled InP/ZnSeS QDs, b) thick-shelled InP/ZnSeS/ZnS QDs with average sizes of 3.3 ± 0.4 and 7.4 ± 0.5 nm, respectively. c) XRD spectra of the InP/Zn core, the InP/ZnSeS core/shell, and the InP/ZnSeS/ZnS core/shell/shell structure. d) EDS spectra of the InP/ZnSeS/ZnS core/shell/shell structure.

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the ZnS shell. The PL decay time result was shown in Figure 4. In Figure 4a, the thin-shelled InP/ZnSeS QDs show a PL decay time of 62 ns in solution state and 17 ns in film state. In contrast, the thick-shelled InP/ZnSeS/ZnS QDs show a PL decay time of 76 ns in solution state and 28 ns in film state. The PL decay time of thin-shelled InP/ZnSeS has decreased 72% from solution state to film state because of the nonradiative decay and Auger recombination. However, the thick-shelled InP/ZnSeS/ZnS decreases 63% during the spin-coating process. It can be confirmed that the outer ZnS shell can effectively decrease the influence of nonradiative decay and Auger recombination. It also functioned as a phys-ical barrier to suppress Förster resonant energy transfer between closely packed QD films in QLEDs. To further con-firm the higher stability of InP/ZnSeS/ZnS, we collected QD powder through centrifugation. For the thermal stability test, we used the thermal controller system to test the thermal sta-bility of the InP QDs with different thickness values. The results are shown in Figure S5 (Supporting Information). When the temperature was raised to 150 °C, the relative PL intensity of the thick-shelled InP/ZnSeS/ZnS was ≈75%. The PL intensity of InP/ZnS decreased by 50% when the temper-ature was raised to 150 °C. The thick-shelled InP/ZnSeS/ZnS QDs exhibited higher stability compared with the InP/ZnS

QDs. The InP/ZnSeS/ZnS cadmium-free QLED devices were fabricated with the inverted structure using the spin-coating and the thermal evaporation technique. We controlled the concentration of the material and the rotation speed to tune the thickness of the multilayer. The thickness of the electron transport layer (ETL), emitting layer (EML), and hole trans-port layer (HTL) strongly affected the performance of the QLED device. If the thickness of the QD EML is too thin, then electrons and holes will recombine in adjacent layers and cause parasitic emission in the electroluminescent (EL) spectra. The parasitic emission reduces the efficiency and color purity of the QLEDs. Thus, the thickness of each layer should be carefully optimized. The schematic illustration of the device structure and the cross-sectional TEM image are shown in Figure 5a,b. We introduce the function of each layer in the following. In this multilayered structure, patterned indium–tin oxide (ITO) was used as a cathode. The ZnMgO nanoparticle was spin-coated onto the ITO cathode with a thickness of 45 nm and served as ETL. The QDs dispersed into octane solution could be spin-coated onto the ZnMgO layer without intermixing with the QD layer. The thickness of the InP/ZnSeS/ZnS film was ≈20 nm. Tris(4-carbazoyl-9-yl-phenyl)amine (TcTa, 30 nm) and N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB, 30 nm) were


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Figure 4. PL decay time of a) thin-shelled InP/ZnSeS QDs and b) thick-shelled InP/ZnSeS/ZnS QDs in solution state versus film state.

Figure 5. a) Inverted device structure, b) cross-sectional TEM image of Cd-free QLED, and c) overall energy level diagram of Cd-free QLED.


used as the electron-blocking layer (EBL) and HTL, res-pectively. Dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN, 15 nm) was used as the hole injection layer (HIL), and aluminum (Al, 100 nm) was used as the anode. To determine the energy band structure of the cadmium-free QLEDs, we performed UV photoelectron spectroscopy (Figure S4, Supporting Information) to measure the CBM of the InP/ZnSeS/ZnS NC film. The Tauc plot of the InP/ZnSeS/ZnS was measured via UV–vis absorption spectra. These spectra can be used to calculate the bandgap value of InP/ZnSeS/ZnS (2.29 eV). The CBM and valence band maxi mum values of the InP/ZnSeS/ZnS QDs were −3.52 and −5.81 eV, respectively. The energy levels of cadmium-free QLEDs, such as ITO, ZnMgO/ZnO, TcTa, NPB, HATCN, and Al, were obtained from refs. [17,19,20]. The energy band structure of the cadmium-free QLED device is shown in Figure 2. The Schottky–Mott rule states that the injection effi-ciency of the OLED device is dominated by the potential barrier height.[15] Injecting the electrons from ZnO was diffi-cult because of the high CBM value of InP/ZnSeS/ZnS. Thus, we used ZnMgO, which had a higher CBM value because of the addition of magnesium ions, as the ETL has to reduce the electron injection barrier. On the other hand, TcTa with low highest occupied Molecular orbital (HOMO) level of 5.7 eV

was employed as HTL to reduce the hole injection barrier. Also, TcTa can serve as effective electron blocking layer because of the low electron mobility and high lowest unoccu-pied molecular orbital (LUMO) level.[20] The NPB HTL and HATCN HIL improved hole transport efficiency and charge balance. The device characteristics are shown in Figure 6. In Figure 6a, luminance, as a function of the applied bias (L–V) characteristic, is shown to have turn-on voltages that are nearly the same at different ETLs. Also the current density–voltage (J–V) curve shown in Figure S7 (Supporting Informa-tion) can further indicate the improvement of the electron injection efficiency of ZnMgO. For example, we can compare the current density under the same voltage (6 V), the current density of the device with ZnO ETL is 93.6 mA cm−2, and the device with ZnMgO is 52.9 mA cm−2. The reduction of the current density indicates that the ZnMgO electron transport layer can inject the electron into the EML more effectively. The cadmium-free QLED device with ZnO ETL started to decay when the applied bias was over 10 V. By contrast, the cadmium-free QLED device with the ZnMgO layer main-tained a brightness of over 10 V. This result showed that the ZnMgO layer also increased the charge balance and improved the stability at high voltages. The current efficiency, EQE, and power efficiency of this device are shown in


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Figure 6. Device characteristic of the inverted InP/ZnSeS/ZnS QLED with different ETLs: a) luminance as a function of applied bias, b) current efficiency vs J, c) EQE vs J, and d) power-efficiency–J spectra of this device.

Table 1. Device characteristics of the inverted InP/ZnSeS/ZnS QLED with different ETLs.

Device L [cd m−2]

Von [V]

CE (Max) [cd A−1]

CE at 1000 cd m−2 [cd A−1]

PE (Max) [Im w−1]

PE at 1000 cd m−2 [lm W−1]

Device 1 (ITO/ZnO/InP-QDs/TcTa/NPB/HATCN/AI) 8742 2.2 2.93 2.86 2.84 1.79

Device 2 (ITO/ZnMgO/InP-QDs/TcTa/NPB/HATCN/AI) 10 490 2.2 4.44 3.86 4.32 3.63

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Figure 6b–d. For the device with the ZnMgO ETL, these values were nearly twofold higher than the device with the ZnO ETL. The optimized inverted InP/ZnSeS/ZnS QLEDs exhibited low turn-on voltage (2.2 V) and a maximum lumi-nescence of 10 490 cd m−2. These turn-on voltage and lumi-nescence values are the highest values observed for the cadmium-free QLED devices.[15] The maximum values of power efficiency and current efficiency are 4.32 lm w−1 and 4.44 cd A−1, respectively. The power efficiency and current efficiency values are 3.63 lm w−1 and 3.86 cd A−1, respectively, at 1000 cd m−2. These results indicated that the proposed cad-mium-free QLEDs also performed well under high bright-ness. The complete device results are presented in Table 1. All the aforementioned results indicate that the thick-shelled InP/ZnSeS/ZnS QDs combined with the ZnMgO layer can decrease the injection barrier and subsequently improve the performance of the device. Figure 7 shows the EL spectra and the photograph of the optimized InP/ZnSeS/ZnS inverted QLEDs operated at 5 V. The EL emission wave-length was 545 nm. The optimized InP/ZnSeS/ZnS inverted device showed a redshift compared with the original PL spectra, which had an emission wavelength of 525 nm. This Stoke-shifted phenomenon was evident in the QLED device because of the energy transfer of closely packed QD EL and Stark shift. The Stark effect occurred under the presence of an external electric field. The CIE point of the InP/ZnSeS/ZnS QLEDs was (0.35, 0.63; Figure 7b). The commission interna-tional de’Eclairage (CIE) position indicates the high color purity of the cadmium-free device for display application.

In conclusion, the high photoluminescence quantum yield (PLQY) and stability of the thick-shelled green InP/ZnSeS/ZnS cadmium-free QDs were used in the inverted QLED device. We used the ZnSeS interlayer as buffer to decrease the lattice mismatch between InP and ZnS. Through the increase in the ZnS shell, we prevented Förster resonant energy transfer and Auger recombination of closely packed QD films. To further improve the performance of the QLED device, we adopted ZnMgO as the ETL to improve electron injection. This method can solve the higher CBM problem of cadmium-free InP/ZnSeS/ZnS QDs and improve device per-formance. Through the good combination of QD materials

with a suitable device, the brightness of the ITO/ZnMgO/InP/QD/TcTa/NPB/HATCN/Al inverted QLED device can reach over 10 000 cd m−2. To date, this value is the highest observed for a cadmium-free QLED device. The turn-on voltage and power efficiency were also improved to 2.2 V and 4.32 lm W−1, respectively. This breakthrough in environment friendly cadmium-free QLEDs will significantly enhance their application for display in the future.

Experimental Section

The typical synthesis process for colloidal InP/ZnSeS/ZnS QDs and inverted device fabrication process are provided in the Supporting Information.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

H.C.W. and H.Z. contributed equally to this work. The authors would like to thank the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2113-M-002-012-MY3), Guang-dong Natural Science Funds for Distinguished Young Scholars (2016A030306017), and the National Natural Science Foundation of China for financially supporting this research. The authors also express their gratitude to C. Y. Chien of the Precious Instrument Center (National Taiwan University) for her assistance in the TEM and FIB experiments.

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Figure 7. a) EL spectra of the inverted InP/ZnSeS/ZnS QLED device. The inset photograph in part (a) shows the working device and b) the CIE coordinate of the InP/ZnSeS/ZnS EL spectra under 5 V.


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Received: November 26, 2016Revised: December 26, 2016Published online: January 31, 2017


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