Improving Photoluminescence Emission Efficiency of Nanocluster

Dec 12, 2016 - Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, California 908...
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Improving Photoluminescence Emission Efficiency of NanoclusterBased Materials by in Situ Doping Synthetic Strategy Jian Lin,† Dan-Dan Hu,† Qian Zhang,† Dong-Sheng Li,‡ Tao Wu,*,† Xianhui Bu,§ and Pingyun Feng*,∥ †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China College of Materials and Chemical Engineering, Hubei Provincial Collaborative Innovation Center for New Energy Microgrid, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion Materials, China Three Gorges University, Yichang 443002, China § Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Boulevard, Long Beach, California 90840, United States ∥ Department of Chemistry, University of California, Riverside, California 92521, United States ‡

S Supporting Information *

ABSTRACT: Solid-state red phosphors of Mn2+-doped nanocrystals usually suffer from poor intensity. While the d−d emission of Mn2+ in yellow window has been extensively studied, shift toward lower energy remains challenging. Typically, intrinsic surface defects and self-purification of dopants are two obstacles for enhancing the intensity of red emission. Moreover, for red phosphors Mn2+ ions also need an appropriate host matrix and environment. Through an in situ doping strategy and optimization of the Mn2+ doping level, intense red-emitting Mn2+ dopant emission is reported here for MnCdInS@InS host. The doping strategy allows doping of Mn2+ at the core and/or surface sites of supertetrahedral “core−shell” nanocluster (Mn@MnCdInS@InS), leading to the red emission (at 643 nm) with over 40% quantum yield. Moreover, systematic control of doping level results in a series of crystalline Mn2+-doped materials with tunable photoluminescence quantum yield. In addition to the synthesis of an important class of red-emitting materials rarely obtained from Mn2+ doping, details of the physical chemistry associated with the doping process are probed with the new fundamental findings reported here.

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tunable emission of Mn2+ is sensitive to various host materials, composition, and locations of dopants.17 It has been recognized that creating a large lattice strain on Mn2+ ions is helpful for the red-shifted emission of Mn2+-doped materials. This led to extensive efforts to realize the Mn2+-related red emission through in situ formed lattice strain on the Mn2+ ions.15,19−22 Still, PLQY of the reported Mn2+-doped red phosphors is still much lower than that of the yellow emissive one. Therefore, creating Mn2+-doped metal chalcogenide phosphors with highly efficient solid-state emission continues to be challenging. Similar to the initially studied Mn2+-doped host material, the low red PL efficiency is mainly ascribed to the traditional doping method and/or the nature of host materials.6,23−27 Intrinsic defects on nanocrystal surfaces are inevitably created during the traditional doping processes,28 which increase the possibility of nonradiative pathways; the “self-purification” mechanism to some extent limits the Mn2+ doping level in nanocrystals,7,29 and even for heavily Mn2+-doped nanocrystals

INTRODUCTION 2+ Mn -doped metal chalcogenide semiconductor materials have attracted great attention for several decades because the stable Mn2+-related photoluminescence (PL) properties endow them with potential applications in the realm of bioimaging and bioanalysis.1−4 Currently, there is an increasing interest in improving the optical functionality of Mn2+-doped phosphors in solid optoelectronic devices, such as LED.5 To meet the basic requirements of these devices, Mn2+-doped chalcogenide phosphors with high stability and high solid-state emission efficiency are necessary. Although much efforts on improving host materials have led to the successful synthesis of a series of Mn2+-doped semiconductor nanocrystals with good emission efficiency,4,6−13 most of these exhibit yellow or orange emissions that are unsuitable for applications in warm-white light emission devices that combine blue LED chip with red emissive phosphors. After years of development on the doping strategy and host materials, yellow−orange emission centered at around 585 nm (2.12 eV) originating from the 4T1(G) → 6 A1(S) d−d transition excited by energy transfer from host lattice to Mn2+ active centers has been shown to be tunable.14−19 As summarized by Pradhan in a minireview, red © 2016 American Chemical Society

Received: September 9, 2016 Revised: December 11, 2016 Published: December 12, 2016 29390

DOI: 10.1021/acs.jpcc.6b09126 J. Phys. Chem. C 2016, 120, 29390−29396

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create a large amount of so-called “killer” defects in both NCs and NC-based microcrystals, which provide additional nonradiative pathway (Scheme 1a). The limited Mn2+-related

the formed Mn2+ clusters or pairs in them may result in serious self-absorption. In our recent work, a crystalline Mn2+-doped phosphor with red emission peaked at around 630 nm was realized through postdoping Mn2+ ion into NC-based molecular crystals (denoted as ISC-10-CdInS). ISC-10-CdInS is composed of coreless supertetrahedral NC with ”core−shell” nanostructure (denoted as Cd6In28S56 or ○@CdInS@InS) (Figure 1a).16

Scheme 1. Proposed Doping Processesa

a For post Mn2+-doped sample (a) and in situ Mn2+-doped sample (b); proposed energy diagram in post Mn2+-doped sample (c) and in situ Mn2+-doped sample (d).

Figure 1. Mn2+-doped metal-chalcogenide supertetrahedral Cd−In−S NCs with well-defined core−shell structure and ordered composition distribution.

emission sites, additional nonradiative relaxation, and partially reserved donor−acceptor pair (DAP) have resulted in the low PL quantum efficiency of the postdoped materials (Scheme 1c). Here, through an in situ doping strategy and optimization of the Mn2+ doping levels, Mn2+-doped NCs with much improved photoluminescence emission efficiency has been obtained. The PLQY of Mn2+-doped samples can reach up to 43.68%, being 82-fold higher than that of the postdoped sample. The synthesized NC-based Mn2+-doped semiconductor materials with highly efficient solid-state emission could be a promising red phosphor in the field of optoelectronic devices.

Compared to the traditional Mn2+-doped metal chalcogenide nanocrystals, such NC-based Mn2+-doped material displays longer emission wavelength, which is due to the special tetrahedral coordination environment of Mn2+ ion at the core site of supertetrahedral Mn@CdInS@InS nanostructure (Figure 1b), providing a large crystal lattice strain and strong ligand field for Mn2+ dopant and correspondingly reducing the gap between 4T1(G) and 6A1(S). In principle, the NC-based doping method can effectively enhance the dispersity of Mn2+ ions because each NC can trap Mn2+ dopants that are automatically separated by NCs themselves. Such Mn2+doped NC could intrinsically enhance energy transfer efficiency between host lattice to Mn2+ dopants because electron−hole pair or exciton produced in the small size of NC (less than 2 nm) is pretty close to Mn2+ active center. Therefore, we predicted that this NC-based phosphor should have high red emission efficiency due to high dispersity. Unfortunately, this post Mn2+-doped sample exhibited quite low quantum efficiency (0.53%), likely caused by the associated postdoping process. The reasons could be (1) slow dynamic in ion diffusion to some extent limited the doping level, that is, not all coreless NCs are filled by Mn2+ ion at the core site; (2) corrosion from organic solvents during postdoping process may

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EXPERIMENTAL SECTION Materials. Indium (In, 99.99%, 200 mesh), sulfur powder (S, 99.99%), Cd(NO3)2·4H2O (99%, AR), Mn(Ac)2·4H2O (99%, AR), piperidine (PR, 99%), and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN, 99%, AR) were purchased from Aladdin Industrial, Inc. All chemicals were used as received without further purification. Synthesis of in Situ Mn2+-Doped Samples. The Mn2+doped microcrystals with different doping level were synthesized based on the literature method.30 Sulfur powder (120 mg, 3.74 mmol), indium metal (80 mg, 0.697 mmol), Cd(NO3)2· 4H2O (40 mg, 0.126 mmol), Mn(Ac)2·4H2O (1.4, 2.8, 5.5, 11.0, 16.5, and 22.0 mg, respectively), and deionized H2O (1.0 29391

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shell NC of Mn@CdInS@InS or unoccupied to form coreless NC when Mn2+ doping level is low. The NCs are subsequently assembled into molecular crystal (Scheme 1b). It can be expected that, with the increasing Mn2+ doping level, more Cd2+ sites on the tetrahedral faces of Cd−In−S NC, instead of In3+ sites, could be replaced by Mn2+ ions because they have the same ion charge. Finally, with enough Mn2+ in the synthesis the core−shell NC of Mn@MnCdInS@InS with some Mn2+ ions residing on the cluster surface could be formed, leading to possible Mn···Mn interactions (Figure 1c). In actual synthesis, when divalent Cd2+ source was completely replaced by Mn2+ source during in situ doping process, we obtained an isostructure (denoted as ISC-10-MnInS) composed of Mn7In28S56 NCs, which was characterized by PXRD and EDS measurements. In the core−shell NC of Mn@MnInS@InS (Figure 1d), one Mn2+ ion resides at core site and six at the surface of NC. Although we cannot determine the precise location of Mn2+ ions in the NC through single-crystal X-ray diffraction, the basic composition distribution predicted above is reasonable and obeys the Pauling electrostatic valence sum rule. The following PL and EPR characterizations also strongly support our prediction. A series of Mn2+-doped samples with different doping level were obtained. To conveniently describe these samples, we labeled these as MnxCdyIn28S56 (x ≤ 1, y = 6; x ≥ 1, y = 7 − x). The actual doping level (x-value) was investigated by EDS measurements and calculated as 0.58, 1.01, 1.50, 1.83, 2.36, 3.20, and 7, respectively (details in Table S1). PXRD patterns of in situ Mn2+-doped samples were shown in Figure S2. All these doped samples show PXRD patterns similar to the one simulated from ISC-10-CdInS, which indicates that the in situ doping method produced the same structure as the original structure of ISC-10-CdInS. The PXRD pattern of ISC-10MnInS also matches well with that of ISC-10-CdInS but with a slight shift to larger angles due to the size contraction of Mn− In−S NC (Figure S3). Note that any given batch of Mn2+doped sample may contain NCs with different doping types as discussed above but the average doping level of the sample can reflect which type of NCs is predominant in final crystal sample. Such NC-based in situ doping method greatly improves the Mn2+ doping level. As shown in our recent work, the Mn2+ ions can in situ replace Zn2+ sites in other supertetrahedral Zn4Ga14Sn2S35 NC.31 Highly efficient red emission is expected because good crystalline samples effectively suppress the original DAP emission and reduce the nonradiative relaxation pathway. In addition, a substantial amount of dopants could serve as additional Mn2+-related emission sites (Scheme 1d). Optical Properties of in Situ Mn2+-Doped Samples. Tauc plots of in situ Mn2+-doped samples derived from UV−vis DRS were shown in Figure 2. All these samples exhibit two obvious absorption edges. The predominant adsorption edge at around 3.25 eV is caused by Mn2+-doped host material, and the minor absorption edge at around 2.75 eV is ascribed to direct d−d transition of Mn2+ dopants. With the increasing Mn2+doping level, the absorption intensity of d−d transition gradually increases, consistent with the color change of Mn2+doped samples (inset of Figure 2). Figure 3 shows the PL spectra of in situ Mn2+-doped samples under the excitation of 400 nm. They all give a single broad red emission band (550−750 nm) peaked at 634, 643, 639, 638, 631, and 629 nm, respectively, corresponding to x = 0.58, 1.01, 1.50, 1.83, 2.36, and 3.20. The emerging Mn2+-related red emission, similar to that of the post Mn2+-doped samples,

mL) were mixed with DBN (1.0 mL) and PR (1.2 mL) in 25 mL Teflon cup, and the mixture was magnetically vigorously stirred for half an hour. The Teflon liner was then sealed in stainless steel autoclave and heated at 200 °C for 10 days. The autoclave was subsequently cooled to room temperature. The raw products were taken out of autoclave and ultrasonically washed by ethanol three times. Pale yellow octahedral or pillared crystals were obtained after drying in air (Figure S1). Synthesis of ISC-10-MnInS. The microcrystals of ISC-10MnInS were synthesized through a similar method with ISC10-CdInS by replacing Cd(NO3)2·4H2O with Mn(Ac)2·4H2O. Structure Characterization. Powder X-ray diffraction (PXRD) data were collected on a desktop diffractometer (D2 PHASER, Bruker, Germany) using Cu−Kα (λ = 1.54056 Å) radiation operated at 30 kV and 10 mA. Optical Absorption Measurements. Room-temperature solid-state UV−vis diffuse reflectance spectra of crystal samples were measured on a SHIMADZU UV-3600 UV−vis-NIR spectrophotometer. The band gap of the obtained samples was determined from the Tauc plot with [F(R)*hν]2 versu hν by extrapolating the linear region to the abscissa. Photoluminescence (PL) Measurements. PL spectra, PL excitation (PLE) spectra, and PLE versus PL 2D mapping spectra were measured by an HORIBA scientific Fluorolog-3 steady state fluorescence spectrophotometer equipped with a 450 W xenon lamp. PL dynamic decays were recorded using an HORIBA scientific Fluorolog-3 time-resolved fluorescence spectrophotometer with a time-correlated single-photon counting (TCSPC) spectrometer and a pulsed xenon lamp as the excitation source. PL quantum yield (PLQY) and lowtemperature PL spectra were recorded on a HORIBA scientific Fluorolog-3 spectrophotometer with a quantum-yield and a low-temperature accessory, respectively. Morphology and Composition Measurements. The morphology and semiquantitative elemental analysis of Mn2+doped ISC-10-CdInS and ISC-10-MnInS crystals were characterized by using a HITACHI S-4700 field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectroscopy (EDS) detector. EDS results for different batches of Mn2+-doped samples were summarized in Table S1. Electron Paramagnetic Resonance (EPR). EPR measurements were carried out on powder samples at 9.06 GHz using a JES-FA 200 EPRJEOL spectrometer at room temperature. Thermogravimetric Measurements. Thermogravimetric analyses were performed on a Shimadzu TGA-50 thermal analyzer by heating sample from room temperature to 800 °C with ramping rate of 10 °C/min in nitrogen flow.

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RESULTS AND DISCUSSION Structure Analysis and Characterization of in Situ Mn2+-Doped Samples. In the coreless supertetrahedral NC of Cd6In28S56, there are one vacancy at the core site and six Cd2+ ions randomly distributed at 12 sites on the faces of NC. The fact that the core site of Cd6In28S56 is not occupied by Cd2+ during the crystal growth and that such core site can be filled by Mn2+ ion during postdoping process clearly indicates that Mn2+ ion, instead of Cd2+ ion, is preferred at the core site of NC. This is largely because the Mn2+ ionic radius (0.80 Å) is much smaller than that of Cd2+ ion (0.95 Å), making it a better fit at the small space of the core area. For the in situ doping process, with all the precursors in the reaction and through selfassembly process the cores are either occupied to form core− 29392

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related emission, which demonstrates that such in situ doping is a facile method to realize doping of Mn2+ ion into the core site and/or surface site of Cd−In−S NC. Optical Properties of ISC-10-MnInS. In comparison with other in situ Mn2+-doped samples, ISC-10-MnInS exhibits some entirely different features on absorption, PL and PLE spectra. First, ISC-10-MnInS shows broad absorption and excitation region from 200 to 550 nm, and it also gives off orange-reddish emission centered at 620 nm (Figure 4).

Figure 2. Tauc plot of in situ Mn2+-doped samples with different doping level derived from UV−vis DRS. Inset: optical photographs of undoped and in situ Mn2+-doped samples under ambient light.

Figure 4. Adsorption (black spot line), PLE (red spot line), and PL (blue spot line) spectra of ISC-10-MnInS.

Notably, the maximum excitation wavelength of ISC-10-MnInS is around 510 nm, instead of 400 nm as observed in other Mn2+-doped samples. The maximum absorption wavelength of ISC-10-MnInS is close to 400 nm (∼3.1 eV), but its maximum emission wavelength remains unchanged when excited at different wavelength (Figure S7a). These results demonstrate that direct excitation on Mn2+ ion is the predominant mode in realizing PL emission of ISC-10-MnInS, which is also proved by excitation-depended PL mapping spectra (Figure S7b). On the basis of above results, it is suggested here that the maximum PL emission peak of in situ Mn2+-doped sample is influenced by the ratio of Mn2+ ions between core site and surface site. For instance, for lightly Mn2+-doped samples (x = 0.58, 1.01), the maximum emission is around 634 and 643 nm, respectively. However, for the heavily Mn2+-doped ones (x = 1.50, 1.83, 2.36 and 3.20), the maximum emission gradually blue-shifts to 630 nm, and even further shifts to 620 nm for ISC-10-MnInS with the highest doping level. Therefore, ISC10-MnInS can serve as a good reference for evaluating the effect of distribution of Mn2+ ions in the NC on PL property of in situ Mn2+-doped samples because it unambiguously comprises two types of Mn2+ ions: one at core site and other six at surface site of NC. As more Mn2+ ions occupy surface sites of NC, blueshifted emission will be observed. It is worth noting that in situ doping method generally causes the coexistence of several types of Mn2+-doped NCs in the final crystal. Even for one specific type of NC with the same Mn2+ doping level and doping site, the ligand field around each Mn2+ ion is still different due to random distribution of In3+ and Cd2+. In our opinion, this usually leads to different crystal lattice strain for different NCs and correspondingly causes the different gap between 4T1 and 6A1 for the excited Mn2+ ions.

Figure 3. PL spectra of in situ Mn2+-doped samples with different doping level upon the excitation at 400 nm.

convincingly demonstrates that Mn2+ ion is successfully inserted into the Cd−In−S NC during the in situ doping process. For in situ lightly doped samples (x = 0.58, 1.01), they only have a single narrow excitation band (370−450 nm) (Figure S4), similar to that observed in post Mn2+-doped sample. However, the in situ heavily doped samples (x = 1.50, 1.83, 2.36, and 3.20) display broadened excitation band (350− 450 nm) together with a minor one (450−550 nm), which happens to be in the absorption range of Mn2+ d−d transition. This means that PL emission from the heavily doped samples can be realized by directly exciting Mn2+ ions. Considering the emission intensity upon the excitation at short wavelength is much stronger than that excited at long wavelength, it is concluded that indirect excitation by virtue of energy transfer from host lattice to Mn2+ is predominant, which is also embodied in 2D photoluminescence excitation (PLE) spectra (Figure S5). To check the repeatability of in situ Mn2+-doping process, PL properties of different batches of Mn2+-doped samples were recorded (Figure S6). These different batches of Mn2+ doped samples give unambiguous and completely repeatable Mn2+29393

DOI: 10.1021/acs.jpcc.6b09126 J. Phys. Chem. C 2016, 120, 29390−29396

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The Journal of Physical Chemistry C This could explain the wide full width of Mn2+-related emission from doped sample. PL Quantum Efficiency of In-situ Mn 2+ -Doped Samples and ISC-10-MnInS. Figure 5 shows the quantum

spin interaction in an individual Mn2+ ion and accordingly give rise to broad EPR signals. Figure 6 shows the EPR signals of

Figure 6. EPR spectra of Mn1.01Cd5.99In28S56 sample and ISC-10MnInS.

ISC-10-MnInS and Mn1.01Cd5.99In28S56 sample. ISC-10-MnInS has a broad EPR signal, whereas the Mn1.01Cd5.99In28S56 sample shows some additional hyperfine splitting signals on a broad EPR signal. The observed broad background in ISC-10-MnInS indicates the presence of a large amount of Mn2+ pairs in Mn7In28S56NC, in agreement with the structure analysis by XRD. The observed hyper-fine splitting pattern also indicates that there exist a lot of Mn@CdInS@InS NCs with isolated Mn2+ ion at its core site in Mn1.01Cd5.99In28S56 sample. Spin and orbitally forbidden transition from 4T1 (spin 3/2) to 6A1 (spin 5/2) in Mn2+ ion usually results in a very long PL lifetime in the range of several hundreds of microseconds to several milliseconds. In addition, magnetic coupling between neighboring Mn2+ pairs can also shorten decay time because they partially lift the spin selection rule in exchange coupled Mn2+ pairs. ISC-10-MnInS gives ∼60.56 ± 0.12 μs PL lifetime at 620 nm when excited at 400 nm, which is much shorter than that of Mn0.58Cd6In28S56 (∼1050.16 ± 5.07 μs) (Figure 7).

Figure 5. Plot of PLQY versus doping level. Inset: PLQY of ISC-10MnInS under different excitation wavelength. Top: in situ Mn2+-doped sample under UV light (365 nm).

efficiency of in situ Mn2+-doped samples and ISC-10-MnInS. It is worth noting that the in situ Mn2+-doped samples exhibit high quantum efficiency, and the highest PLQY (43.68%) for one (Mn1.01Cd5.99In28S56) sample is 82-fold higher than that of the post Mn2+-doped sample (0.53%) (Figure S8). When Mn2+ doping level is increased from x = 0.58 to 3.20, the average PLQY of in situ Mn2+-doped samples is changed from 30% to 38%, 26%, 25%, 23%, and 21%, respectively. Obviously, lightly doped sample (x = 0.58) gives a relatively high quantum efficiency, which reaches to the highest value for x = 1.01, When Mn2+ doping level increases further, more self-absorption caused by Mn2+ ions will occur, which could reduce emission efficiency. This trend was also observed in the case reported by Sarma et al.32 For ISC-10-MnInS with the highest level of Mn2+, PLQY even reduces to 17.09% when excited at maximum e x c i t a t io n w a v e l e n g t h o f 5 1 0 n m ( F i g u r e S 9) . Mn1.01Cd5.99In28S56 is the first tetrahedral Mn2+-doped metal chalcogenide sample possessing such high solid-state PL quantum efficiency, comparable to recently reported perovskite-type compounds AMnX 3 (A = pyrrolidinium or pyrrolinium; X = Br, Cl), exhibiting intense red emission (around 640 nm, PLQY 53.6%) arising from (t2g)3(eg)2− (t2g)4(eg)1 electronic transition in octahedrally coordinated Mn2+ ion.33−35 Impressive PLQY and unusual red emission as well as thermal stability (Figure S10) make Mn1.01Cd5.99In28S56 sample a potential candidate of rare-earth-free red phosphors. EPR, PL Lifetime, and Temperature-Dependent PL Spectra of In-Situ Mn2+-Doped Samples and ISC-10MnInS. It is generally accepted that hyperfine splitting signals can be observed from a sample with an individual and isolated Mn2+ ion, originating from the interaction between the spin (S = 5/2) of the unpaired 3d-electrons and spin (I = 5/2) of the 55 Mn nucleus. Strong Mn···Mn interactions between adjacent Mn2+ ions usually lead to a reduction of the electron−nuclear

Figure 7. PL dynamics of Mn0.58Cd6In28S56 sample and ISC-10-MnInS monitored at 620 nm and excited at 400 and 470 nm respectively at room temperature. Inset: plot of lifetime versus doping level. 29394

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The Journal of Physical Chemistry C With the increasing Mn2+ doping level from x = 0.58 to 3.20, PL lifetime at 620 nm decreases from ∼1 to ∼0.3 ms (inset of Figure 7, Figure S11, and Table S2). The downward trend of PL lifetime suggests that there should be enhanced Mn···Mn interactions occurring in the heavily doped samples. The higher Mn2+-doping level leads to more Mn···Mn interactions, and a shorter lifetime. The highest Mn-doping level was observed in the ∼2 nm of Mn−In−S NC in ISC-10-MnInS. Temperature-dependent PL measurements have also been made to investigate the possible effect of Mn2+ doping level or doping site on Mn2+-related emission position. The maximum emission peak for Mn1.01Cd5.99In28S56 sample only shifts from 643 to 647 nm when the temperature decreases from room temperature to 10 K (Figure 8a). However, for ISC-10-MnInS,

temperature possibly causes a big lattice strain in the asymmetric ligand field of surface Mn2+ ion. In our opinion, this correspondingly lowers the level of 4T1 excited state and leads to emission peak at long wavelength section.

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CONCLUSION In summary, crystal phosphors with impressively high-efficient red emission are realized through in situ doping Mn2+ ions into coreless supertetrahedral chalcogenide NCs, which subsequently assemble into NC-based semiconductor molecular crystals. Samples with different Mn2+ doping levels were prepared and their optical and physical properties were studied. Sample with the highest PLQY was identified and the interpretation for the trend of the properties was presented. The prepared Mn1.01Cd5.99In28S56 sample gives off red emission at 643 nm with a maximum PLQY of 43.68%. These crystalline red phosphors may be used as promising candidates in the optoelectronic devices. In addition, the developed in situ doping strategy provides a new avenue for creating metal-ion doped phosphors with high PL efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09126. Additional Information including powder X-ray diffraction, optical absorption, extra figures of PL spectra, tables for elemental analysis, and PL lifetime (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86 512 6588 2675. *E-mail: [email protected]. Phone: +951 827 2042. ORCID

Dong-Sheng Li: 0000-0003-1283-6334 Tao Wu: 0000-0003-4443-1227 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. 21271135, 21671142), a start-up fund (Q410900712) from Soochow University, Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20160006), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Young Thousand Talented Program, Graduate Student Fund (KYZZ15_0323) from Scientific Research Innovation Projects of Jiangsu Province and National Science Foundation (DMR-1506661, P.F.).

Figure 8. Temperature-dependent PL spectra of Mn1.01Cd5.99In28S56 sample (a) and ISC-10-MnInS (b) with the excitation wavelength at 400 nm.

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it exhibits a large red shift from 621 to 646 nm (Figure 8b). Such distinct difference in temperature-dependent PL properties between Mn1.01Cd5.99In28S56 sample and ISC-10-MnInS may be caused by Mn2+ doping sites. In Mn1.01Cd5.99In28S56 sample, the predominant NC is Mn@CdInS@InS, in which there is only a single Mn2+ ion located at the core of NC. The symmetric ligand field of Mn2+ ion may be not drastically tuned by low-temperature induced size contraction of NC. Whereas in ISC-10-MnInS, most of Mn2+ ions are located on the face of Mn@MnInS@InS NC and contraction of NC at low

REFERENCES

(1) Sharma, V. K.; Gokyar, S.; Kelestemur, Y.; Erdem, T.; Unal, E.; Demir, H. V. Manganese Doped Fluorescent Paramagnetic Nanocrystals for Dual-Modal Imaging. Small 2014, 10, 4961−4966. (2) Thakar, R.; Chen, Y. C.; Snee, P. T. Efficient Emission from Core/(Doped) Shell Nanoparticles:Applications for Chemical Sensing. Nano Lett. 2007, 7, 3429−3432. (3) Wu, P.; Yan, X. P. Doped Quantum Dots for Chemo/Biosensing and Bioimaging. Chem. Soc. Rev. 2013, 42, 5489−5521.

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DOI: 10.1021/acs.jpcc.6b09126 J. Phys. Chem. C 2016, 120, 29390−29396

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The Journal of Physical Chemistry C (4) Pradhan, N.; Battaglia, D. M.; Liu, Y. C.; Peng, X. G. Efficient, Stable, Small, and Water-Soluble Doped ZnSe Nanocrystal Emitters as Non-cadmium Biomedical Labels. Nano Lett. 2007, 7, 312−317. (5) Nag, A.; Sarma, D. D. White Light from Mn2+-Doped CdS Nanocrystals: A New Approach. J. Phys. Chem. C 2007, 111, 13641− 13644. (6) Cao, S.; Zheng, J. J.; Zhao, J. L.; Wang, L.; Gao, F. M.; Wei, G. D.; Zeng, R. S.; Tian, L. H.; Yang, W. Y. Highly Efficient and Wellresolved Mn2+ Ion Emission in MnS/ZnS/CdS Quantum Dots. J. Mater. Chem. C 2013, 1, 2540−2547. (7) Zheng, J. J.; Ji, W. Y.; Wang, X. Y.; Ikezawa, M.; Jing, P. T.; Liu, X. Y.; Li, H. B.; Zhao, J. L.; Masumoto, Y. Improved Photoluminescence of MnS/ZnS Core/Shell Nanocrystals by Controlling Diffusion of Mn Ions into the ZnS Shell. J. Phys. Chem. C 2010, 114, 15331−15336. (8) Zhang, W. J.; Li, Y.; Zhang, H.; Zhou, X. G.; Zhong, X. H. Facile Synthesis of Highly Luminescent Mn-doped ZnS Nanocrystals. Inorg. Chem. 2011, 50, 10432−10438. (9) Cao, S.; Zhao, J. L.; Yang, W. Y.; Li, C. M.; Zheng, J. J. Mn2+doped Zn-In-S Quantum Dots with Tunable Bandgaps and High Photoluminescence Properties. J. Mater. Chem. C 2015, 3, 8844−8851. (10) Srivastava, B. B.; Jana, S.; Karan, N. S.; Paria, S.; Jana, N. R.; Sarma, D. D.; Pradhan, N. Highly Luminescent Mn-doped ZnS Nanocrystals: Gram-Scale Synthesis. J. Phys. Chem. Lett. 2010, 1, 1454−1458. (11) Zeng, R. S.; Zhang, T. T.; Dai, G. Z.; Zou, B. S. Highly Emissive, Color-tunable, Phosphine-free Mn:ZnSe/ZnS Core/Shell and Mn:ZnSeS Shell-alloyed Doped Nanocrystals. J. Phys. Chem. C 2011, 115, 3005−3010. (12) Pradhan, N.; Peng, X. G. Efficient and Color-Tunable Mndoped ZnSe Nanocrystal Emitters: Control of Optical Performance via Greener Synthetic Chemistry. J. Am. Chem. Soc. 2007, 129, 3339− 3347. (13) Cao, S.; Li, C. M.; Wang, L.; Shang, M. H.; Wei, G. D.; Zheng, J. J.; Yang, W. Y. Long-lived and Well-resolved Mn2+ Ion Emissions in CuInS-ZnS Quantum Dots. Sci. Rep. 2014, 4, 7510−7517. (14) Huang, F.; Zhou, J. C.; Xu, J.; Wang, Y. S. Formation of AgGaS2 Nano-pyramids from Ag2S Nanospheres through Intermediate Ag2SAgGaS2 Heterostructures and AgGaS2 Sensitized Mn2+ Emission. Nanoscale 2014, 6, 2340−2344. (15) Zuo, T. S.; Sun, Z. P.; Zhao, Y. L.; Jiang, X. M.; Gao, X. Y. The Big Red Shift of Photoluminescence of Mn Dopants in Strained CdS: A Case Study of Mn-Doped MnS-CdS Heteronanostructures. J. Am. Chem. Soc. 2010, 132, 6618−6619. (16) Lin, J.; Zhang, Q.; Wang, L.; Liu, X. C.; Yan, W. B.; Wu, T.; Bu, X. H.; Feng, P. Y. Atomically Precise Doping of Monomanganese Ion into Coreless Supertetrahedral Chalcogenide Nanocluster Inducing Unusual Red Shift in Mn2+ Emission. J. Am. Chem. Soc. 2014, 136, 4769−4779. (17) Pradhan, N. Red Tuned Mn d-d Emission in Doped Semiconductor Nanocrystals. ChemPhysChem 2016, 17, 1087−1094. (18) Hazarika, A.; Layek, A.; De, S.; Nag, A.; Debnath, S.; Mahadevan, P.; Chowdhury, A.; Sarma, D. D. Ultranarrow and Widely Tunable Mn2+-Induced Photoluminescence from Single Mndoped Nanocrystals of ZnS-CdS Alloys. Phys. Rev. Lett. 2013, 110, 267401−267405. (19) Hazarika, A.; Pandey, A.; Sarma, D. D. Rainbow Emission from an Atomic Transition in Doped Quantum Dots. J. Phys. Chem. Lett. 2014, 5, 2208−2213. (20) Chen, H. Y.; Maiti, S.; Nelson, C. A.; Zhu, X. Y.; Son, D. H. Tuning Temperature Dependence of Dopant Luminescence via Local Lattice Strain in Core/Shell Nanocrystal Structure. J. Phys. Chem. C 2012, 116, 23838−23843. (21) Ithurria, S.; Guyot-Sionnest, P.; Mahler, B.; Dubertret, B. Mn2+ as a Radial Pressure Gauge in Colloidal Core/shell Nanocrystals. Phys. Rev. Lett. 2007, 99, 265501−265504. (22) Yang, Y. A.; Chen, O.; Angerhofer, A.; Cao, Y. C. Radialposition-controlled Doping in CdS/ZnS Core/shell Nanocrystals. J. Am. Chem. Soc. 2006, 128, 12428−12429.

(23) Norris, D. J.; Efros, A. L.; Erwin, S. C. Doped Nanocrystals. Science 2008, 319, 1776−1779. (24) Erwin, S. C.; Zu, L. J.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J.Doping Semiconductor Nanocrystals. Nature 2005, 436, 91−94. (25) Buonsanti, R.; Milliron, D. J.Chemistry of Doped Colloidal Nanocrystals. Chem. Mater. 2013, 25, 1305−1317. (26) Thanh, N. T. K.; Maclean, N.; Mahiddine, S. Mechanisms of Nucleation and Growth of Nanoparticles in Solution. Chem. Rev. 2014, 114, 7610−7630. (27) Yang, J.; Fainblat, R.; Kwon, S. G.; Muckel, F.; Yu, J. H.; Terlinden, H.; Kim, B. H.; Iavarone, D.; Choi, M. K.; Kim, I. Y.; et al. Route to the Smallest Doped Semiconductor: Mn2+-Doped (CdSe)13 Clusters. J. Am. Chem. Soc. 2015, 137, 12776−12779. (28) Weaver, A. L.; Gamelin, D. R. Photoluminescence Brightening via Electrochemical Trap Passivation in ZnSe and Mn2+-Doped ZnSe Quantum Dots. J. Am. Chem. Soc. 2012, 134, 6819−6825. (29) Nag, A.; Cherian, R.; Mahadevan, P.; Gopal, A. V.; Hazarika, A.; Mohan, A.; Vengurlekar, A. S.; Sarma, D. D. Size-Dependent Tuning of Mn2+ d Emission in Mn2+-Doped CdS Nanocrystals: Bulk vs Surface. J. Phys. Chem. C 2010, 114, 18323−18329. (30) Wu, T.; Zhang, Q.; Hou, Y.; Wang, L.; Mao, C. Y.; Zheng, S. T.; Bu, X. H.; Feng, P. Y. Monocopper Doping in Cd-In-S Supertetrahedral Nanocluster via Two-Step Strategy and Enhanced Photoelectric Response. J. Am. Chem. Soc. 2013, 135, 10250−10253. (31) Zhang, Q.; Lin, J.; Yang, Y.-T.; Qin, Z.-Z.; Li, D. S.; Wang, S.; Liu, Y. P.; Zou, X. X.; Wu, Y.-B.; Wu, T. Exploring Mn2+-LocationDependent Red Emission from (Mn/Zn)-Ga-Sn-S Supertetrahedral Nanoclusters with Relatively Precise Dopant Position. J. Mater. Chem. C 2016, 4, 10435−10444. (32) Nag, A.; Chakraborty, S.; Sarma, D. D. To Dope Mn2+ in a Semiconducting Nanocrystal. J. Am. Chem. Soc. 2008, 130, 10605− 10611. (33) Zhang, Y.; Liao, W. Q.; Fu, D. W.; Ye, H. Y.; Chen, Z. N.; Xiong, R. G.Highly Efficient Red-Light Emission in An OrganicInorganicHybridFerroelectric:(Pyrrolidinium)MnCl3. J. Am. Chem. Soc. 2015, 137, 4928−4931. (34) Ye, H. Y.; Zhou, Q.; Niu, X.; Liao, W. Q.; Fu, D. W.; Zhang, Y.; You, Y. M.; Wang, J.; Chen, Z. N.; Xiong, R. G.High-Temperature Ferroelectricity and Photoluminescence in a Hybrid Organic-Inorganic Compound: (3-Pyrrolinium)MnCl. J. Am. Chem. Soc. 2015, 137, 13148−13154. (35) Zhang, Y.; Liao, W. Q.; Fu, D. W.; Ye, H. Y.; Liu, C. M.; Chen, Z. N.; Xiong, R. G.The First Organic-Inorganic Hybrid Luminescent Multiferroic: (Pyrrolidinium)MnBr3. Adv. Mater. 2015, 27, 3942− 3946.

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DOI: 10.1021/acs.jpcc.6b09126 J. Phys. Chem. C 2016, 120, 29390−29396