General Synthesis and White Light Emission of Diluted Magnetic

Jul 15, 2013 - Because of the fundamental properties and possible applications in spin-based electronics and photonics, diluted magnetic semiconductor...
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General Synthesis and White Light Emission of Diluted Magnetic Semiconductor Nanowires Using Single-Source Precursors Gaoling Yang,† Guangyuan Xu,† Bingkun Chen,† Shuangyang Zou,† Ruibin Liu,‡ Haizheng Zhong,*,† and Bingsuo Zou*,‡ †

Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing 100081, China ‡ Micro Nano Technology Center, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing 100081, China S Supporting Information *

ABSTRACT: Because of the fundamental properties and possible applications in spin-based electronics and photonics, diluted magnetic semiconductor nanowires are actively pursued. Here we report a general and facile solution synthetic strategy to prepare colloidal diluted magnetic semiconductor nanowires through solution-liquid−solid (SLS) doping approach using singlesource precursors. On the basis of this strategy, transition metal ions such as Mn and Eu doped CdS nanowires were successfully synthesized and characterized. The material characterizations demonstrated that the doping process is nucleation controlled. We further investigated the Mn doping effects on nanowire growth as well as their photoluminescence properties. The Mn doped CdS nanowires exhibit photoluminescence emission related to the excitonic magnetic polaron in CdS, single Mn2+ ion and Mn−S−Mn centers as well as trap states, evidenced by the time-resolved photoluminescence spectra and magnetic measurements. With the increase of Mn precursor that used in the doping process, the Mn2+ related emission becomes more pronounced. By tuning the doping concentration, white emissive doped CdS nanowires were achieved. KEYWORDS: diluted magnetic semiconductor, nanowires, solution synthesis, single-source precursor, Mn doped CdS, SLS doping

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effects on spin related physical properties.23,24 There now exists a few of versatile solution-based nanowires (NWs) prepared through well developed solution−liquid−solid (SLS) approach,25−33 which combines lower-cost solution processability with easy control of the size and shape and provides a suitable solution route toward colloidal NWs. Earlier works by Li and Lu et al. have demonstrated its adoption to be an alternative route to fabricate transition ions (Mn or Co) doped CdSe NWs,34,35 which facilitate the spectroscopic study and applications explorations in many cutting-edge devices.15 Herein, we present a simple and general SLS doping approach to synthesize transition ions doped CdS NWs using singlesource precursors. Transition metal ion (Mn, Eu, Cu) doped CdS nanostructures exhibit attractive luminescence properties and are potential candidates for many light-emitting applications.36 The synthetic chemistry of spherical colloidal doped CdS nanocrystals has been well developed based on the surfaceadsorption/diffusion growth mechanism.37,38 However, it is very difficult to synthesize doped CdS NWs using conventional

INTRODUCTION Because of the possible applications in magnetic memory, spinbased electronics and photonics, and transition metal ion doped semiconductors, have been actively pursued recently.1−3 Embedding transition metal ions creates diluted magnetic semiconductor (DMS) with intermediate energy states between the valence and conduction bands of host semiconductor, which enable us to manipulate their electronic states,4 exciton spin relaxation,5,6 magneto-optical properties,7,8 and transport phenomena,9−11 governing the sp-d exchange interaction between the carriers and the magnetic ions.12 Beyond the fundamental understanding, DMSs have been also widely utilized as essential building elements in various optoelectronic applications such as solar cells,13 field-effect transistors (FETs),14,15 photodetectors,16 and light-emitting diodes (LEDs).17 In the system of colloidal DMS nanocrystals, quantum size confinement effects and doped ion transitions are combined, resulting in new class of materials with particular optical, magnetic as well as magneto-optical properties.18,19 Recently, spherical colloidal nanocrystals doped with Mn or Cu ions have been widely studied,20−22 but anisotropic one- or twodimensional doped colloidal nanomaterials are rather exceptional and remain an obstacle to understand the dimensional © XXXX American Chemical Society

Received: June 7, 2013 Revised: July 11, 2013

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Bi droplets become saturated with reactant and dopant, CdS NWs begin nucleation and growth with the instantaneous decomposition of Cd(S2CNEt2)2 precursor. In the present study, we first demonstrated the incorporation of Mn or Eu precursors to obtain doped CdS NWs and that the doping ratio can be varied by controlling feed of raw materials. We further investigate the effect of dopant on their morphologies as well as photoluminescence (PL) emissions. An interesting white light emission was achieved when we regulate the doping concentration during reaction.

growth procedures possible due to the insufficient surface adsorption of doping ions. In the current work, we successfully synthesized high quality colloidal doped CdS NWs by introducing single-source precursors into SLS doping approach. The combination of metallic and nonmetallic semiconductor constituents ameliorates the growth issues stemming from the different decomposition kinetics and/or reactivity of separate metal/chalcogen precursors and induce a nucleation controlled doping process.39,40 As shown in Scheme 1, the SLS doping

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Scheme 1. Schematic Illustration of the SLS Doping Approach Using Single-Source Precursors

EXPERIMENTAL SECTION

Materials. Tri-n-octylphosphine (TOP, 90%), tri-n-octylphosphine oxide (TOPO, 99%), n-tetradecylphosphonic acid (TDPA, 98%), manganese acetate tetrahydrate [Mn (CH3COO)2·4H2O], europium acetate hydrate [Eu (CH3COO)3·H2O], and bismuth trichloride (BiCl3, 99.9%) were purchased from Alfa Aeser. Oleylamine (OLA, 70%), NaS2CN(C2H5)·3H2O (99%), and cadmium chloride (CdCl2, 99.99%) were purchased from Sigma Aldrich. All chemicals were used as purchased. The single-source Cd(S2CNEt2)2 precursor was prepared by following the method reported in the literature.41 Preparation of the Injection Solutions. For the preparation of manganese precursor, a mixture of (CH3COO)2Mn·4H2O (0.1 mmol) and OLA (5 mL) was loaded in a reaction container and heated at 140 °C under vacuum with magnetic stirring for 30 min to remove residual water and oxygen. To vary the doping ratio, we also prepared manganese precursors solution with higher concentrations (Table S1, Supporting Information). For the preparation of bismuth catalyst solution, 0.0126 mg (40 μmol) of BiCl3 was dissolved in 20 mL acetone to give a 2 mM solution, after that, the as-prepared solution was diluted to 1 mM. Separately, Cd(S2CNEt2)2 (40 mg, 0.1 mmol)

approach using single-source precursor usually involves three steps. First, the in situ generated Bi nanoparticles melt into droplets at high temperature and serve as catalysts; then, after

Figure 1. (a) Representative low-magnification TEM image of Mn doped CdS NWs. Example HRTEM images: (b) catalyst part, (c) middle, and (d) tip of the Mn doped CdS NWs. (e) Selected area electron diffraction pattern obtain from the HRTEM image in part b. (f) Powder XRD pattern of undoped and Mn doped CdS NWs. The standard patterns of the bulk CdS with wurtzite (ICDD-PDF No. 01-075-1545) structure is also shown in the graph. (g) Typical TEM images of Mn doped CdS NWs at different reaction stages. B

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and TOP (0.8 g) were mixed in a small vial, placed in a water-filled ultrasonic bath until the single-source precursor completely dissolved to yield a clear solution. When the manganese precursor solution cooled to room temperature, 10 μL of Mn precursor in OLA solution and 25 μL of BiCl3 solution were added in the small vial as the injection solution. Synthesis of Mn Doped CdS NWs. A modified synthetic procedure was employed to prepare Mn doped CdS NWs.26 First, 4 g of TOP and 7 mg of TDPA were placed into a 50 mL three necked flask coupled with a condenser. The mixture was heated to 250 °C under N2 flow for 10 min. After temperature equilibration, the asprepared precursor was rapidly injected into the solution and reacted at 250 °C for 5 min. Afterward, the heat was turned off and the reaction mixture was allowed to cool with argon gas flow. When the temperature was reduced to 80 °C, 5 mL of toluene was added to the solution to prevent the TOPO from solidifying. The resulting NWs were precipitated using excess methanol (5 mL) and recovered by centrifuging the suspension, discarding the supernatant. The precipitate was redispersed in organic solvent such as chloroform, toluene, or hexane. The precipitation and redispersion was repeated until the samples were sufficiently purified. The purified wires were redispersed in toluene for further use. Characterization. Transmission electron microscopy (TEM) observations were performed on JEM-2100F transmission electron microscope. High-resolution TEM images (HRTEM), scanning transmission electron microscopy (STEM), energy-filtered TEM (EFTEM) maps, and Energy-dispersive X-ray spectroscopy (EDS) measurements were conducted on a FEI Tecnai G2 F20 TEM equipped with a DX-4 analyzer (EDAX) operating at an acceleration voltage of 300 kV. X-ray diffraction (XRD) measurements were carried out at room temperature with a Bruker D8 diffractometer using Cu Kα radiation (wavelength = 1.5406 Å). The percentage of dopant was determined by inductively coupled plasma-atomic emission spectrometer (ICP-AES, Leeman, U.S.A.). The doped NWs were repeatedly purified to remove excess precursors. The purified NWs were dissolved in toluene; then, the solution was evaporated, and the dried NWs were digested in concentrated HNO3. The nitric acid solution of the samples was diluted with double distilled water to do the measurements. UV−vis absorption spectra were recorded on a Hitachi U-3010 spectrophotometer. Fluorescence and excitation spectra were recorded by Hitachi F-4500 spectrophotometer. Magnetization measurements were performed on a superconducting quantum interference device (SQUID) magnetometer (MPMS Quantum Design). The hysteresis cycles were obtained at 5 K in a magnetic field varying from +3000 to −3000 kOe. The sample preparation was performed by dispersing the particles in paraffin to have a solid dispersion of noninteracting NWs, which was introduced then into gelatin capsules under ambient conditions.

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XRD measurements (Figure 1f). Compared with the bulk CdS and undoped CdS NWs, all the reflection peaks of doped NWs slightly shifted to the lower-angle region. This may arise from an enlargement in the lattice constant that induced by the replacement of large Cd atoms with Mn atoms. To confirm Mn doping, STEM-EDS mapping was applied to characterize the resulting materials. Figure 2 shows the

Figure 2. TEM image (a), bright-field STEM image (b), and the corresponding EDS elemental mapping of a typical Mn doped NW, including spatial distribution of Cd (c), S (d), Mn (e), and Bi (f) recorded on a doped NW.

mapping results of a typical NW with a Bi catalyst particle at its tip-end, the bright points indicate the high concentration of the elements. It is observed that Mn ions were uniformly embed into the NW, Cd and S were homogeneously distributed along the NW body, and Bi was located at the top end of the NW. The higher Mn and S concentration at the tip of the NW demonstrated that the SLS doping approach is nucleation controlled. As proposed in Scheme 1, Bi nanodroplets were continuously fed with a mixed solution of CdS precursor (Cd(S2CNEt2)2) and doping precursor (i.e., Mn2+) at a definite rate, maintaining the concentration of precursors above the nucleation limit. To demonstrate the generality of the SLS doping approach using single-source precursors, we further extend the method to synthesize Eu doped CdS NWs using Eu (CH3COO)3·H2O as doping precursor. Representative TEM images show that this SLS doping approach produced high-quality Eu doped CdS NWs with an average diameter of ∼20 nm; the presence of Eu elements in the resulting CdS NWs was verified by using selected EDS measurements (see Figure S2, Supporting Information). Doping Effects. It is well-known that many factors can influence the NW growth and affect the diameter and size distribution of resulting NWs, such as temperature, catalyst concentration, reaction mixture concentration, NWs and catalysts growth kinetics, and other processes.42 Doping process plays a key role in the synthetic process; therefore, doping ions should influence the NW growth. To investigate doping effect on the morphology of resulting NWs, we carried out a set of experiments with different doping concentration (Table S1, Supporting Information). Pure CdS NWs were first prepared at

RESULTS AND DISCUSSION

Synthesis and Characterizations. The SLS doping approach using single-source precursor was first applied to synthesize Mn ions doped CdS NWs. When using a small amount of Mn precursors, high quality diluted Mn doped CdS NWs can be synthesized following the SLS mechanism.32 Lowmagnification TEM image of a typical Mn doped CdS nanowire show that the wire has a length of ∼2.5 μm and diameter of ∼17 nm (Figure 1a). The crystalline structure of Mn doped CdS NWs was clearly seen in the HRTEM images of the tip, middle, and end part of the NWs (Figure 1b−d). The fast Fourier transform (FFT) pattern of the image can be indexed to the wurtzite (WZ) structure (Figure 1e), and a wire growth in the [0002] direction can be induced. The space of the lattice fringes perpendicular to the growth direction was determined to be 0.325 nm, which is in consistent with the lattice distance of (0002) face for CdS with WZ phase. The WZ structure of these Mn doped CdS NWs was further verified using powder C

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Optical Properties. Figure 4a shows the UV−visible absorption spectra of the Mn doped (red line) and undoped CdS NWs (black line). The Mn doped CdS NWs have an excitonic absorption peak that located at 405 nm with clearly broadening and band tail shift, and the broader absorption peaks may arise from the broader nanowire size distributions. It is obvious that both of the doped and undoped CdS NWs exhibit blue-shifted absorption from the band gap value of bulk CdS (517 nm). TEM detection shows that Mn doped CdS NWs have average diameters of 17−20 nm, which are three times that of the bulk exciton Bohr radius of pure CdS (aB = 2.9 nm). Therefore, the observed blue-shift cannot be explained by the well-known quantum confinement effects. Similar phenomenon have also been observed in Kuno’s work.43 According to the band-filling and particle-in-a-cylinder models,43,44 the blueshift can be explained by the reduction of size dependent carrier effective mass and carrier band-filling in both the conduction and valence bands. To understand the absorption features, photoluminescence excitation (PLE) spectra were applied. In comparison to the undoped CdS NWs, an obvious peak at ∼410 nm was observed (Figure S4, Supporting Information), corresponding to the band edge exciton absorption. We further determined the PL emission spectra of these NWs. The undoped CdS NWs exhibit a dual emissions peaked at ∼420 nm and ∼530 nm (Figure S5, Supporting Information). The observed emission centered at 420 nm is due to exciton recombination near the band edge, while the other emission centered at 530 nm has been attributed to the well-known trap defects.45 Figure 4b shows the PL spectra of several Mn doped CdS NWs samples with different Mn content from 0.181% to 0.816% (determined by ICP-AES, summarized in Table S2, Supporting Information). The emission spectra of all the samples show similar emission spectra with undoped CdS NWs. However, the blue emission due to exciton recombination, shifted from ∼420 nm for undoped NWs to ∼435 nm for Mn undoped CdS NWs. The observed shift may be related to the exciton magnetic polaron formation.46 With Mn doping, the emission that peaked at ∼530 nm for undoped CdS NWs shifted to 580 nm for doped CdS NWs. To understand the emission spectra shift, we further analyzed the PL emission spectra of Mn doped CdS NWs by fitting the broad emission that peaked at 580 nm. As shown in Figure S6, Supporting Information, the broad emission peaked at 580 nm can be simply fitted into three emission peaks: an intensive sharp peak located in the orange emission region centered at 581 nm (Em2), which can be attributed to normal single Mn ion emission in doped semiconductors;47 trap defects related emission centered at 530 nm (Em1), as that in pure CdS shown in Figure S5, Supporting Information; and the band at 645 nm (Em3) that may originate from the emission centers with a Mn ion next to Mn emission center (Mn−S−Mn ferromagnetic centers),48,49 which can be evidenced by the magnetic measurement (see Figure 5). It is also observed that the broad PL emission band around 580 nm was enhanced with Mn doping ratio increase. In addition, the ratio of PL emission at 435 and 580 nm can be significantly varied by the Mn doping concentration. Therefore, the orange emission at 580 nm is most likely attributed to the 4T1 → 6A1 transition of Mn2+ ion. Although, further study is needed to elucidate the recombination process, it is deduced that the energy transfer of band exciton to Mn ions should be more efficient than that to the defect states, which suggests that Mn doped CdS NWs exhibit

the same conditions in the absence of Mn precursor simultaneously. TEM images (Figures S3a and c, Supporting Information) and the diameter distribution (Figure S3d, Supporting Information) of as-prepared CdS NWs show that the NWs were uniform in diameter (∼15 nm) with typical NW length of ∼2.5 μm.26 When the fed Mn precursor increased from 0 to 400 μL, the average diameter of NWs increased from 15.5 to 20 nm, and the diameter distribution notably became broad (Figure 3 and Figure S3, Supporting Information). It is a

Figure 3. (a−c) TEM images and corresponding (d−f) diameter distribution of Mn doped CdS NWs prepared with different Mn precursor concentrations.

challenge to explain how the doped ions influence the growth of NWs due to the mechanism behind the decomposition of these single-source precursors is not fully understood, and the exact role of Bi salts in catalyzing NW growth is not totally clear. Here, based on these results and crystal growth theory, a possible mechanism was proposed. Generally, dopant reduces the supersaturation degree, resulting in a higher ratio of crystal growth rate to nucleate rate. So, the crystallization rate becomes slower. Due to the competitive growth of nanocatalysts and NWs, a corresponding rapid growth of nanocatalysts leads to large Bi particles and thicker NWs.42 In our single-source precursor SLS doping system, the supersaturation degree of CdS precursors was lower than that of undoped NWs systems because of the presence of Mn precursor. This induced that Bi nanodroplets have enough time to grow bigger and produce thicker NWs. The broad diameter distribution of the NWs is attributed to the competition among doped NWs growth, Bi nanocatalyst growth, and other processes.42 D

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Figure 4. (a) UV−visible absorption spectra of the undoped and Mn doped CdS NWs. (b) PL emission spectra of Mn doped CdS NWs formed with different Mn precursor concentrations, as indicated in the figure. All spectra were measured at room temperature, and the excitation wavelength was 380 nm. (c) PL decay of CdS NWs (530 nm) and Mn doped CdS NWs (580 nm) excited at 365 nm. (d) CIE diagram showing the color chromaticity calculated coordinates of the PL spectra of part b. Insert is the photograph of white light emitting Mn doped NWs in toluene under the irradiation of a 365 nm UV lamp.

Table 1. Lifetimes for Various Decay Components in Mn Doped and Undoped CdS NWs sample

τ1 (ns)

τ2 (ns)

τ3 (ns)

Mn:CdS CdS

2.74 1.53

48.59 16.49

256.86 84.85

The average PL lifetime (190 ns) of Mn doped CdS NWs at 580 nm have been observed to be much higher than the trap state emission peak at 530 nm of the undoped CdS NWs (55 ns). Hence, it is concluded that the origin of the emissions obtained from the doped CdS NWs involves the Mn impurity states from the dopant. The spectroscopic results revealed that Mn doped CdS NWs exhibit dual emissions (blue and orange lights) and can be tuned by varying the Mn concentration (Figure S7, Supporting Information). This allowed us to modify the emission spectrum for white light generation, which may be single phosphor as potential candidates for light-emitting applications. The color properties of Mn doped CdS NWs were determined by measuring their International Commission on Illumination (CIE) coordinate. The resulting CIE color coordinates calculated from corresponding PL spectra are shown in Figure S8, Supporting Information. The PL spectra corresponding to 0.181% Mn doped NWs has chromaticity coordinate of (0.37, 0.35), which is within the white light region of the 1931 CIE diagram as shown in Figure 4d. The CIE coordinate confirms the generation of white lights, which is also visible in the photographs. Magnetic Properties. The magnetic properties of the Mn doped CdS NWs were investigated by using a SQUID magnetometer. The magnetization (M) versus an applied

Figure 5. Hysteresis loops (M vs H) of doped CdS NWs measured at 5 K.

strong coupling effects between Mn d levels and host excitons.50 We further studied the PL decay of the resulting Mn doped CdS NWs by using time-resolved PL spectra. Figure 4c presents a comparison of the PL decays of the Mn doped CdS NWs determined at the 580 nm and the undoped CdS NWs determined at 530 nm. Both of PL decays are nonexponential and can be fitted by a triexponential function, I(t ) = A1exp( −t /τ1) + A 2 exp(−t /τ2) + A3exp(−t /τ3)

where τ1, τ2, and τ3 are the time constants and A1, A2, and A3 are the normalized amplitudes of the components, respectively. The time constants are summarized in Table 1. E

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magnetic field (H) of the heavily doped NWs measured at 5 K is shown in Figure 5. The magnetic hysteresis loops indicate that the Mn doped CdS NWs are ferromagnetic at low temperature. The inset displays the curve in the vicinity of H = 0, indicating that the coercive field (HC) is 30.7 Oe and the remanence (Mr) is 0.00367 emu/g at 5 K. Therefore, the existence of ferromagnetism at 5 K is clearly proven by the coercivity and remanence, and relatively low saturation field. The ferromagnetic behavior of the Mn doped NWs can be explained based on exchange interactions between Mn2+ ions and host nanoparticles. The half-filled 3d electrons of manganese will be formed an orderly arrangement through spin-exchange interaction within the atom, the spin direction of itinerate electron remain unchanged during cruise, so longrange Mn2+−Mn2+ ferromagnetic coupling will formed through itinerate electrons spin coupling, which also originate from Mn−Mn center. This is in good agreement with the observation of Mn−Mn related red emission.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported under National Basic Research Program of China (No.2011CB933600 and 2013CB328804), NSFC Grants (No.51003005 and 512002011) and the BIT Basic Research Funds (20120942006). The authors would like to thank Dr. Zhen Li and Dr. Yunchao Li for help discussions and Prof. Haiyan Xie for the spectroscopic measurements.

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CONCLUSION In summary, we present a simple, facile, and controllable solution synthesis of Mn or Eu doped CdS DMS NWs through SLS doping approach by using single-source precursors. The excellent controllability and processability are superior to the previous synthetic methods. Furthermore, by varying the doping concentration used during the reaction, we investigated the Mn ions doping effect on the morphology and PL emission. We demonstrated doping Mn ions have an obvious influence on the NW growth, resulting in the broad size distribution. Meanwhile, with the Mn precursor increase, the Mn2+ emission becomes more pronounced. Through varying the Mn doping concentration, the PL spectra of resulting Mn doped NWs can be finely tuned. It is noted that the Mn doped CdS NWs with Mn ion concentration of 0.181% can generate white light with a chromaticity coordinate of (0.37, 0.35), indicating their potential to be promising candidates for light-emitting applications. The magnetic moment measurement using SQUID reveals that Mn doped CdS NWs also exhibit ferromagnetic behaviors at low temperature. It is our hope that this general SLS doping approach using single-source precursors will open up the way to synthesize other DMS NWs and motivate the spectroscopic investigations of solution-based DMS NWs, in turn, prompting the realization of advanced spindependent optoelectronic nanodevices.

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ASSOCIATED CONTENT

S Supporting Information *

TEM, HRTEM, and PL spectra of CdS NWs, EDS spectra and PLE of doped and undoped CdS NWs, optical images of Mn doped CdS NWs tuned by varying the Mn concentration. The Gaussian fitting of a typical PL spectrum of Mn doped CdS NWs. TEM image and corresponding EDS spectrum of Eu doped CdS NWs. CIE coordinates calculated from PL emission spectra of Mn doped CdS NWs formed with different Mn precursors concentrations. Preparative parameters and ICP measurement of Mn doped CdS NWs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: Haizheng Zhong, [email protected]; Bingsuo Zou, [email protected]. F

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dx.doi.org/10.1021/cm401864d | Chem. Mater. XXXX, XXX, XXX−XXX