Enhancement Mechanism of Photoluminescence Quantum Yield in

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C: Plasmonics, Optical Materials, and Hard Matter

Enhancement Mechanism of Photoluminescence Quantum Yield in Highly Efficient ZnS-AgInS Quantum Dots with Core/Shell Structures 5

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Seonghyun Jeong, Soyeon Yoon, So Yeon Chun, Hee Chang Yoon, Noh Soo Han, Ji Hye Oh, Seung Min Park, Young Rag Do, and Jae Kyu Song J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01774 • Publication Date (Web): 18 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Enhancement Mechanism of Photoluminescence Quantum Yield in Highly Efficient ZnS-AgIn5S8 Quantum Dots with Core/Shell Structures

Seonghyun Jeong,†,§ Soyeon Yoon,‡,§ So Yeon Chun,† Hee Chang Yoon,‡ Noh Soo Han,† Ji Hye Oh,‡ Seung Min Park,† Young Rag Do*,‡ and Jae Kyu Song*,†

†

Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea ‡

Department of Chemistry, Kookmin University, Seoul 136-702, Korea

* Corresponding authors. E-mail addresses: [email protected]; [email protected] §

S. J. and S. Y. equally contributed to this work.

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ABSTRACT The optical properties of ZnS−AgIn5S8 quantum dots (QDs) with core/shell structures are examined to clarify the enhancement mechanism of photoluminescence (PL) quantum yield (QY). Two types of QDs are synthesized by varying the concentration of zinc precursors, with the structures of alloyed-core (ZnS−AgIn5S8, ZAIS), inner-shell (ZnIn2S4, ZIS), and outer-shell (ZnS), such as ZAIS/ZIS/ZnS and ZAIS/ZnS. Upon alloying/shelling processes from the preformed AgIn5S8 QDs, the evolution of the band gap energy indicates the formation of the solid-solution of ZAIS. Due to the difference in the degree of alloying between ZAIS/ZIS/ZnS and ZAIS/ZnS QDs, the blue-shift of PL, Stokes shift, and QY are different. The alloying/shelling processes improve the QY of the intrinsic defect states more effectively than the QY of the surface defect states, while the time-resolved studies suggest that the enhanced radiative rate of the intrinsic states is responsible for the improvement of the QY, in addition to the reduced nonradiative rate. In ZAIS/ZIS/ZnS QDs, the QY increases to 85%, which is attributed to the existence of the ZIS layer, as well as the reduced nonradiative states and the enhanced radiative states by the alloying/shelling processes. The ZIS layer mitigates the lattice strains and provides the appropriate levels of the electronic structures in the QDs, which further reduces the nonradiative rate and enhances the radiative rate, respectively, leading to the unprecedentedly high PL QY of ZAIS/ZIS/ZnS QDs.


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1. INTRODUCTION The optical properties of semiconductor quantum dots (QDs) have been studied for applications in various fields, such as biolabeling, light emitting diodes, solar cells, and photocatalysts.1-5 Recently, ternary I-III-VI QDs, such as Ag-In-S and Cu-In-S, have attracted attention as alternatives to the toxic Cd- and Pb-based QDs,6-8 because the properties of I-III-VI QDs can be easily controlled by the chemical composition,9,10 in addition to the shapes and sizes as in typical QDs.11,12 Indeed, the I-III-VI QDs show the high potentials for applications in solar cells and photocatalysts.13-16 Furthermore, the photoluminescence (PL) quantum yield (QY) of I-III-VI QDs is usually high,17,18 although the PL of QDs is mainly from the donor–acceptor pair recombination of defect states.19,20 Over the last decade, the PL QYs of I-III-VI QDs have been further improved by alloyed-core structures coated with shell layers, such as Zn-Cu-In-S QDs of 92% and Zn-Ag-In-S QDs of 87%,21,22 which are comparable to the exciton emission of II-VI QDs.23,24 The PL QY of I-III-VI QDs depends on the size, because the concentration of defect states is certainly affected by the sizes of QDs. In general, the QY of small QDs is high, which is attributed to the high concentration of defect states.8 However, the decrease in crystallinity limits the QY enhancement in small QDs, which is overcome by forming the alloyed-core with the shell layer at high temperature.25-27 The high PL QY in the alloyed-core/shell structures is explained by the graded compositions in the core,12,28 which mitigate the lattice mismatch between the core and shell by the gradual strain release. Moreover, the interface layer between the core and shell can further reduce the lattice mismatch.28,29 Another role of the interface layer in the alloyed-core/shell structures is to sustain a type-I band configuration with the band offset between the core and shell,30,31 because the alloying influences the band gap energy (Eg) of the



core. However, the electronic structures in the core/shell structures have not been thoroughly understood in the alloyed I-III-VI QDs. Quite recently, state-of-the-art nanocomposites have been prepared with core/shell structures, which are composed of an alloyed-core (ZnS−AgIn5S8, ZAIS), inner-shell (ZnIn2S4, ZIS), and outer-shell (ZnS).22 Small-sized AgIn5S8 (AIS) QDs are synthesized with high concentration of the defect states at low reaction temperature.32,33 The first step to improve the PL QY is the incorporation of Zn into AIS to form the solid-solution of ZAIS at medium temperature.25-27 The second step is the development of the inner-shell at high temperature, where ZIS is proposed as a feasible material. At high temperature, the strong exothermic reaction supplies sufficient energy to form a ZIS shell on the surfaces of the alloyed-core.22 Then, two additional steps at medium temperature form the outer-shell of ZnS, which completes ZAIS/ZIS/ZnS QDs with the unprecedentedly high PL QY of 87%.22 Such multi-step alloying/shelling process is employed to fabricate CdSe-based QDs with several shell layers.34 In multi-step alloying/shelling process, the blue-shift is reduced after the first step due to the inhibited cation exchange at the core/shell interface in the Zn-Cu-In-S core/shell QDs.35 Although the ZAIS/ZIS/ZnS QDs show a promising PL QY for the applications of optoelectronic devices, the origin of the QY enhancement still remains unclear, due to the complicated alloying/shelling processes and compositions of QDs. In this study, we prepare two types of QDs with the high concentration zinc acetate dihydrate for the alloyed-core/innershell/outer-shell (ZAIS/ZIS/ZnS) structure of QDs and with the low concentration zinc acetate dihydrate for the alloyed-core/outer-shell (ZAIS/ZnS) structure of QDs,22 which shows the role of the inner-shell in the QDs. Two types of QDs are examined by steady-state and time-resolved PL spectroscopy, which reasonably explains the enhancement mechanism of the PL QY. AIS


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QDs show the dominant relaxation processes from the surface and intrinsic states, whose electronic structures were influenced by the alloying/shelling processes. The evolution of the electronic structures changes the optical properties of QDs by improving the radiative processes and suppressing the nonradiative processes, which leads to the strong enhancement of the PL QY. To the best of our knowledge, this is the first report on the detailed study of the QY enhancement mechanism with the alloying/shelling processes from AIS to ZAIS and ZAIS/ZIS/ZnS, where the main PL originates from the defect states in the alloyed-core, instead of the exciton recombination of typical QDs.

2. EXPERIMENTAL SECTION 2.1. Synthesis of ZAIS/ZIS/ZnS QDs. The detailed synthetic procedure has been explained elsewhere.22,32 To prepare AIS QDs, silver nitrate and indium acetylacetonate were dissolved in oleic acid (OA) and 1-octadecane (ODE) during purging with N2 gas at room temperature. After 20 min, the mixture was heated and 1-octanethiol (OTT) was injected at 90 oC to react for 30 min. The sulfur dissolved in oleylamine (OLA) was injected at 120 oC to react for 3 min. For the first alloying/shelling process, the zinc acetate dihydrate dissolved in OTT and OA was injected into the preformed AIS QDs, and the solution was maintained at 180 °C for 2 h. The second alloying/shelling process was carried out by injecting the zinc acetate dihydrate in OTT and OA at 230 °C for 2 h. An additional two-step outer-shelling process was carried out by injecting the zinc acetate dihydrate in OTT and OA at 180 °C for 2 h. Two types of QDs were synthesized using different concentrations of the zinc acetate dihydrate, i.e., high concentration (4.0 mmol) and low concentration (0.4 mmol). The synthesized QDs were purified by centrifugation and stored in hexane.



2.2. Characterization of ZAIS/ZIS/ZnS QDs. The chemical compositions of the synthesized QDs were examined by energy dispersive spectroscopy (EDS, JSM7401F). The crystal structures of QDs were determined by X-ray diffraction (XRD, D-max 2500) with Cu Kα radiation. The size and lattice of QDs were examined by transmission electron microscopy (TEM, JEM-2100F). The absorption spectra of QDs were measured by UV-visible spectrometry (S3100). The PL QY of QDs was estimated by comparison with rhodamine 6G at the optical density of 0.05 centered at 450 nm. For time-resolved PL spectroscopy, QDs were excited by a second harmonic (355 nm) of a cavity-dumped oscillator (Mira/PulseSwitch, 710 nm, 150 fs). The collected emission was spectrally resolved using a monochromator, detected using a photomultiplier, and recorded using a time-correlated single photon counter (PicoHarp).33,36

3. RESULTS AND DISCUSSION The QD of interest is the alloyed-core/inner-shell/outer-shell (ZAIS/ZIS/ZnS) structure with a type-I band configuration (Figure 1a). From the preformed AIS QDs with an average diameter of 2.8 nm and PL QY of 31%,32,33 the core/shell structures were synthesized by typical hot-injection methods with four-step reactions, of one-step medium-temperature, one-step high-temperature, and two-step medium-temperature reactions. Two types of QDs were synthesized by injecting the different concentration of Zn precursors, i.e., the high concentration zinc acetate dihydrate (4.0 mmol, HZAD) and the low concentration zinc acetate dihydrate (0.4 mmol, LZAD), at every reaction step of the alloying/shelling processes. Both types of QDs were prepared at the same reaction temperature and time for consistent comparison. The crystal structures of QDs were analyzed by XRD patterns. Despite the broad diffraction peaks due to the small sizes of QDs, the XRD pattern indicated the cubic phase of AIS (Figure


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S1).20,32 Upon the alloying/shelling process, three peaks of AIS were shifted to higher angles towards the cubic phase of ZnS,22,32 which indicated the formation of the solid-solution between AIS and ZnS. In the alloying/shelling processes with HZAD, the (006) plane in the hexagonal phase of ZIS was observed at 22°,37,38 although other peaks of ZIS were not clearly detected, due to the close diffraction peaks of ZIS to those of AIS and ZnS, in addition to the broad nature of the peaks. Accordingly, the XRD pattern suggested the formation of a ZIS layer between the alloyed-core and shell, such as ZAIS/ZIS/ZnS.22 However, in the alloying/shelling processes with LZAD, the diffraction peaks of ZIS were not observed, which indicated the absence of the inner-shell, i.e., QDs could be denoted as ZAIS/ZnS. TEM images exhibited the crystalline nature of ZAIS/ZIS/ZnS (Figure 1b) and ZAIS/ZnS (Figure 1c) with the inter-planar spacing of 0.30 and 0.32 nm, respectively. The difference in the lattice spacing supported the formation of a different crystal phase, such as the ZIS interface layer, in the alloying/shelling processes with HZAD. The average size of the ZAIS/ZIS/ZnS QDs was slightly smaller than that of the ZAIS/ZnS QDs, when the size was averaged over 130 QDs (Figure S2). Indeed, the size of the ZAIS/ZIS/ZnS QDs included the thickness of the inner-shell and outer-shell, in addition to the core, while the size of the ZAIS/ZIS/ZnS QDs was determined by the core and outer-shell. Accordingly, the smaller size of the ZAIS/ZIS/ZnS QDs indicated that the composition of QDs prepared with HZAD changed more dynamically, because the inner-shell and outer-shell were formed and etched simultaneously at a faster rate than the conventional shelling process with LZAD. In this regard, the formation of the ZIS layer was supported by the XRD patterns and TEM images.22 The color and brightness of ZAIS/ZIS/ZnS QDs (inset of Figure 1b) were also different from those of ZAIS/ZnS QDs (inset of Figure 1c). The alphabet numbers (0, 1, 2, 3, and 4) in Figures indicated the number of the alloying/shelling process from the preformed AIS QDs.



The optical properties of the QDs were examined to explain the difference in color and brightness. In the absorption spectrum of AIS QDs, a hump was observed (Figure 2a), suggesting the exciton absorption.8,10 However, with the alloying/shelling processes, the excitonic absorption became smeared out, and only the monotonic increase of the absorbance was observed from the second alloying/shelling step. Thus, the Eg of QDs was estimated by extrapolating the linear part of (αhν )2 in the absorption spectra to consistently investigate the change of Eg.17,18 The AIS QDs showed a larger Eg (2.50 eV) than the bulk (1.80 eV),39 due to the quantum confinement effects. During the first and second steps, the increase in Eg suggested the alloying effect, such as the chemical composition change in the solid-solution of ZAIS, as the monotonous increase in Eg was observed with the ratio of Zn in the solid-solution of ZnS−AgInS2.40-42 On the other hand, the increase in Eg was considerably reduced at the third and fourth steps, which indicated that the main reaction was the shelling with ZnS. The optical spectra of the QDs were also presented with the wavelength scale (Figure S3) for better comparison. In the PL spectrum of AIS QDs (Figure 2c), the peak was observed at 2.06 eV with a Stokes shift of 0.44 eV, which indicated defect emission, such as the donor–acceptor pair recombination, as typically observed in Ag–In–S QDs.7,8,33 Like the change of Eg, the first and second steps resulted in the blue-shift of PL, which was much reduced at the third and fourth steps in ZAIS/ZIS/ZnS QDs. Indeed, the blue-shift of absorbance with the alloying/shelling processes in Figure 2a could be from the disappearance of the Urbach tail due to the annealing effect in a high temperature,43 because the Urbach tail was related to the absorption of defect states. However, the blue-shift of absorbance was mainly observed above the energy of the defect emission. In


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addition, the increase in Eg coincided with the blue-shift of PL, which ruled out the possibility of the Urbach tail in the absorption spectra of QDs. A similar propensity was observed in ZAIS/ZnS QDs (Figure 2b). However, the increase in Eg of ZAIS/ZIS/ZnS QDs was larger than that of ZAIS/ZnS QDs, which indicated the high degree of alloying by HZAD, i.e., the increase in the ratio of Zn in ZAIS due to the high concentration of Zn precursors.22 Likewise, the blue-shift of the PL in ZAIS/ZIS/ZnS QDs (0.31 eV) was larger than that in ZAIS/ZnS QDs (0.19 eV) in Figure 2d, which supported the concentration effect of Zn precursors for the degree of alloying (Figure 3a). Therefore, the increase in Eg and the following blue-shift of PL were responsible for the color change with the alloying/shelling processes. On the other hand, the blue-shift of PL (0.31 eV) was larger than the increase in Eg (0.09 eV), which reduced the Stokes shift from 0.44 eV at AIS QDs to 0.22 eV at ZAIS/ZIS/ZnS QDs (Figure 3b). The variation of the Stokes shift implied that the chemical composition influenced the electronic structures of the defect states, as well as Eg. Indeed, the defect levels of the alloyed QDs (ZnS–AgInS2 and ZnS–AgIn5S8) were shallower than those of unalloyed ones (AgInS2 and AgIn5S8),32,42,44 indicating that the electronic structures of the defect states changed with the chemical compositions by the alloying. Moreover, the Stokes shift of the ZAIS/ZIS/ZnS QDs (0.22 eV) was smaller than that of the ZAIS/ZnS QDs (0.30 eV), which was related to the extent of the electronic structure changes by the degree of alloying, i.e., shallower levels of defect states by the higher degree of alloying in the ZAIS/ZIS/ZnS QDs. In addition to the change of Eg and the electronic structures, the PL QY was greatly improved by the alloying/shelling processes (Figure 3c). The QY increased from 31% to 60% by the alloying/shelling processes with LZAD, which was related to the reaction temperatures and



the surface passivation. The reconstructed lattice structures at high temperature could reduce the nonradiative states,25,40 which were further suppressed by the surface passivation, such as shelling.32,44 Moreover, the radiative states could be enhanced by the changes of electronic structures related to the chemical composition in ZAIS.25-27 Notably, QY increased up to 85% by the alloying/shelling processes with HZAD, which was attributable to the formation of the ZIS interface layer between ZAIS and ZnS,28-31 in addition to the reduced nonradiative states and enhanced radiative states. Indeed, the improved QY of the ZAIS/ZIS/ZnS QDs was mainly related to the inner-shell structure, rather than the possible formation of the independent ZIS QDs, although the formation of the ZIS QDs was not totally ruled out during the reactions with HZAD. The full-width at half-maximum (FWHM) of PL also changed by the alloying/shelling processes (Figure 3d). The FWHM of AIS QDs was 0.32 eV, which increased to 0.41 eV and decreased slightly to 0.40 eV with the alloying/shelling processes by HZAD. Since the FWHMs might afford a clue to the QY enhancement, the PL of AIS QDs was deconvoluted by two Gaussian bands (Figure 4a), which were assigned as the surface defect states at high energy (D1) and the intrinsic defect states at low energy (D2).17,33 The intensity of D1 was higher than that of D2, indicating the high density of the surface states in the AIS QDs, due to the high surface-tovolume ratio.17,44 The PL spectrum of the ZAIS/ZIS/ZnS QDs was also deconvoluted into two Gaussian bands with similar FWHMs and energy differences between D1 and D2 (Figure 4b). The relative contribution of D2 increased with the alloying/shelling processes (Figure 4c), which was responsible for the changes in the FWHM of the PL spectra (Figure S4). Moreover, the intensity of D2 increased more steeply than that of D1 (Figure 4d), when the PL QY was taken into account, which showed that the alloying process enhanced the QY of the intrinsic states


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more effectively than that of the surface states. The PL spectra of the ZAIS/ZnS QDs were also deconvoluted into two Gaussian functions with similar FWHMs and energy differences (Figure S5), which exhibited comparable results to the ZAIS/ZIS/ZnS QDs. To further investigate the optical properties of the defect states, the QYs of the surface states (QYD1) and the intrinsic states (QYD2) were calculated from the PL QYs. The densities of the radiative surface and intrinsic states were estimated from the surface-to-volume ratio (see Supporting Information for details), while the relative contribution of D1 and D2 was taken into account. The obtained values of QYD1 (0.40) and QYD2 (0.23) in the AIS QDs explained the high PL QY of the AIS QDs (0.31), although the PL was from the defect states, instead of the exciton recombination.33 Indeed, the surface states have been considered as nonradiative relaxation centers.18,32 However, the QYD1 of the AIS QDs was higher than QYD2, which led to the high PL QY of the AIS QDs, because the surface states were not the nonradiative centers, but the effective radiative ones. Under the assumption that the core size of ZAIS was not much affected by the alloying/shelling processes,12,31 the QYD1 and QYD2 of the QDs were similarly estimated (Figure 5a). Notably, the QYD2 increased more steeply than QYD1, which was primarily responsible for the intensity change of D1 and D2 (Figure 4d). The steep increase in the QYD2 indicated that the QYs of the intrinsic states were more effectively enhanced than those of the surface states by the alloying/shelling process. Likewise, the QYD1 and QYD2 of the ZAIS/ZnS QDs were also estimated (Figure 5b), which indicated comparable results to those of the ZAIS/ZIS/ZnS QDs, except for the lower values of the QYD1 and QYD2. To explain the QY enhancement in more detail, the emission of QDs was measured by timeresolved PL spectroscopy. The decay profiles of PL were fitted by the double-exponential model (Figure 5c).



I (t ) = A1 exp( −t / τ 1 ) + A2 exp( −t / τ 2 )

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(1)

where I(t) is the intensity, τ1 and τ2 are time constants, and A1 and A2 are relative magnitudes. The decay profile of the AIS QDs showed time constants of τ1 = 50 ns and τ2 = 340 ns, which were assigned as the lifetimes of D1 and D2, respectively.33 The emissive states were relaxed by the radiative and nonradiative decay, while the lifetime depended on the total relaxation process.

τ=

1 kr + k nr

(2)

where kr is the radiative decay rate and knr is the nonradiative decay rate. The QY of emissive states was determined by the radiative and nonradiative decay rates.

QY =

kr k r + k nr

(3)

Accordingly, kr and knr could be estimated from τ and QY. In the AIS QDs, kr was 8.0 × 106 s-1 and knr was 12.0 × 106 s-1 for the surface states (D1).33 For the intrinsic states (D2), kr was 0.71 × 106 s-1 and knr was 2.24 × 106 s-1, which were comparable to the recombination rates of the AgInS2 QDs.8 With the alloying/shelling processes, decreasing A1 was observed with increasing A2 (Figure S6), which agreed with the change of the relative contribution between D1 and D2. In addition, τ1 and τ2 increased after the first and second alloying/shelling steps (Figure 5d), while τ1 and τ2 did not change much at the third and fourth steps, which was similar to the QY and the blue-shift of the PL. The lifetime change indicated that the alloying/shelling processes influenced the relaxation rates. In the ZAIS/ZIS/ZnS QDs, kr and knr of D1 were estimated to be 10.5 × 106 and 2.32 × 106 s-1, respectively, from τ1 (78 ns) and QYD1 (82%). Compared to the AIS QDs, kr of D1 became slightly enhanced in the ZAIS/ZIS/ZnS QDs (130%), suggesting that the alloying/shelling processes less affected the radiative rate of the surface states. On the other hand, 12 Environment ACS Paragon Plus

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knr of D1 was strongly suppressed (19%), compared to the AIS QDs. Indeed, the high PL QY of the AIS QDs was related to the nontrivial value of QYD1, which further increased with the alloying/shelling processes (Figure 5a). Since the nonradiative rate of the surface states was significantly suppressed in the ZAIS/ZIS/ZnS QDs, the increase in QYD1 was attributed to the reduced nonradiative surface states by the surface passivation as well as the alloying, which led the surface states to be more radiative. For D2 of the ZAIS/ZIS/ZnS QDs with τ2 of 460 ns and QYD2 of 87%, kr and knr were 1.89 × 106 and 0.28 × 106 s-1, respectively. Remarkably, kr of D2 became considerably enhanced in the ZAIS/ZIS/ZnS QDs (270%), compared to that in the AIS QDs. The intrinsic state was another important radiative center in the AIS QDs with the high value of QYD2, which increased more steeply than QYD1 with the alloying/shelling processes. Since the suppressed knr of D2 (13%) was similar to that of D1 (19%), the significantly improved kr of D2 was mostly responsible for the steep increase in QYD2. In other words, the alloying/shelling processes more effectively enhanced the radiative rate of the intrinsic states (270%) than did the surface states (130%), because the radiative rate of the intrinsic states was closely correlated to the chemical composition and the electronic structures of ZAIS.45-48 Although both QDs showed alloyed-core structure, the PL QY of the ZAIS/ZIS/ZnS QDs was higher than that of the ZAIS/ZnS QDs. In addition to the high ratio of Zn, the key difference would be the ZIS interface layer, which mitigated the lattice strain of unmatched crystal structures between the core and shell.28,29 The ZIS layer could afford gradual strain release between the ZAIS and ZnS, which reduced the nonradiative relaxation of charge carriers. In the time-resolved PL spectra of the ZAIS/ZnS QDs, the decay profiles showed a fast decay component (Figure S7a), which was not observed in the ZAIS/ZIS/ZnS QDs (Figure 5c). The



new component implied fast nonradiative relaxation pathways in the ZAIS/ZnS QDs, due to the absence of the ZIS layer, i.e., a large lattice strain between the core and shell. When the decay profiles were normalized by excluding the fast decay component, the decay profiles of the ZAIS/ZnS QDs were reasonably fitted by the double-exponential model (Figure S7b) with comparable lifetimes to the ZAIS/ZIS/ZnS QDs (Figure S7c). Therefore, the time-resolved PL spectra indicated that the relaxation dynamics in the ZAIS/ZnS QDs was not much different from that in the ZAIS/ZIS/ZnS QDs, except for the fast nonradiative pathways. Indeed, the fast decaying component in the ZAIS/ZnS QDs (30%) was not negligible (Figure S7d), which explained the lower PL QY of the ZAIS/ZnS QDs than the ZAIS/ZIS/ZnS QDs. Furthermore, the ZIS layer could induce the electron to be more confined inside the core. In a typical type-I band configuration of the QD, the wavefunctions of electron and hole were confined inside the core by the band offset between the core and shell,30,31 which enhanced the radiative recombination of electron and hole by the large overlap of wavefunctions. However, with increasing degree of alloying, the conduction band edge of the ZAIS core became similar to that of the ZnS shell,42,49 which could lead to quasi-type-II structure with the delocalized electron over the whole QD, due to the small energy difference of the conduction band edge between the core and shell.30,31 Although the Eg of ZIS was between that of ZAIS and ZnS,37,38 the exact energies of the conduction and valence band edge have not been known in the thin layer of ZIS. Presumably, the thin layer might elevate the conduction band edge of ZIS, due to the strong quantum confinement effect, which possibly sustained the band offset between the core and shell.30,31 Indeed, kr and knr of D2 in the ZAIS/ZnS QDs with τ2 of 440 ns and QYD2 of 62% were 1.41 × 106 and 0.86 × 106 s-1, respectively, which indicated that kr of D2 in the ZAIS/ZIS/ZnS QDs was larger (130%) than that in the ZAIS/ZnS QDs, feasibly due to the more confined


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wavefunctions of charge carriers. Overall, the ZIS layer mitigated the lattice strains and provided the appropriate levels of electronic structures in QDs, which reduced the nonradiative rate and enhanced the radiative rate of the intrinsic states, respectively, leading to the unprecedentedly high PL QY of the ZAIS/ZIS/ZnS QDs.

4. SUMMARY The enhancement mechanism of the PL QY in ZAIS/ZIS/ZnS QDs was investigated by steadystate and time-resolved PL spectroscopy. ZAIS/ZIS/ZnS QDs with type-I band configuration were synthesized from the preformed AIS QDs by four-step reactions with HZAD. During the first and second alloying/shelling steps, the increase in Eg and blue-shift of PL indicated the formation of a solid-solution, while much reduced blue-shift at the third and fourth steps showed that the main reaction was the shelling. With increasing the alloying/shelling processes, the PL intensity of the intrinsic states increased more steeply than that of the surface states. From the time-resolved PL studies, the improvement of the QY was attributed to the enhanced radiative rate of the intrinsic states, as well as the reduced nonradiative rates, because the radiative rate was closely correlated to the chemical composition and electronic structures of ZAIS. The high QY in the ZAIS/ZIS/ZnS QDs was also attributable to the presence of the ZIS interface layer between ZAIS and ZnS, in addition to the high ratio of Zn. The ZIS layer in the ZAIS/ZIS/ZnS QDs mitigated the lattice strains and provided the appropriate levels of electronic structures in the QDs, leading to the significantly high PL QY.



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SUPPORTING INFORMATION The XRD patterns, size distributions of quantum dots, optical spectra, PL spectra fitted by Gaussian functions, the relative contribution of two time-components, and the time-resolved PL spectra are presented. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2015R1A2A2A01002805). This work was also supported by the National Research Foundation

of

Korea

(NRF)

Grant

Funded

by the

(No.2016R1A5A1012966).


Korean

Government

(MSIP)

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(a)

core

alloyed-core

inner-shell

outer-shell

AgIn5S8

ZnS-AgIn5S8

ZnIn2S4

ZnS

(b)

(c) 2 nm

0 10 nm

1

2 nm

0

2 3 4

1

2 3

4

10 nm

Figure 1. (a) A schematic of the type-I band configuration in the ZAIS/ZIS/ZnS QDs. TEM images of (b) ZAIS/ZIS/ZnS QDs and (c) ZAIS/ZnS QDs. The lower insets show the photographs of the PL from QDs with increasing alloying/shelling process. The alphabet numbers (0, 1, 2, 3, and 4) in the insets indicate the number of the alloying/shelling process from the preformed AIS QDs.


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(b)

0 1 2 3 4

1.8

0 1 2 3 4

Absorbance

Absorbance

(a)

2.0

2.2

2.4

2.6

2.8

1.8

3.0

2.0

Energy (eV)

2.4

2.6

2.8

3.0

(d)

0 1 2 3 4

2.6

2.8

3.0

0 1 2 3 4

PL intensity

(c)

2.2

Energy (eV)

PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.8

2.0

2.2

2.4

Energy (eV)

2.6

2.8

3.0

1.8

2.0

2.2

2.4

Energy (eV)

Figure 2. (a) The absorption spectra of QDs with increasing alloying/shelling process by HZAD. The alphabet numbers (0, 1, 2, 3, and 4) indicate the number of the alloying/shelling process from the preformed AIS QDs. (b) The absorption spectra of QDs with increasing alloying/shelling process by LZAD. (c) The PL spectra of QDs with increasing alloying/shelling process by HZAD. (d) The PL spectra of QDs with increasing alloying/shelling process by LZAD.



2.6

(b)

Eg

Energy (eV)

HZAD LZAD

2.4

emission peak 2.2

0.5

HZAD LZAD

0.4

Stokes shift (eV)

(a)

0.3

0.2

0.1

2.0 0

1

2

3

0

4

(c)

1

2

3

4

Number of process

Number of process 1.0

(d)

0.5 HZAD LZAD

0.8

0.6

0.4

0.4

FWHM (eV)

HZAD LZAD

QY

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3

0.2

0.2 0

1

2

3

Number of process

4

0

1

2

3

4

Number of process

Figure 3. (a) The Eg and peak energy of the PL with increasing alloying/shelling process by HZAD and LZAD. (b) The Stokes shift of the PL with increasing alloying/shelling process. (c) The PL QY with increasing alloying/shelling process. (d) The FWHM of the PL with increasing alloying/shelling process.


Page 25 of 27

D1 D2 sum

(b) 1.0 PL intensity

PL intensity

(a) 1.0

0.5

0.0

D1 D2 sum

0.5

0.0 1.8

2.0

2.2

2.4

2.6

2.8

1.8

2.0

Energy (eV)

2.2

2.4

2.6

2.8

Energy (eV)

(c)

(d) 0.6

D2 D1

0.5

Intensity (a.u.)

0.6

Ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

D2 D1

0.2

0.4 0.0 0

1

2

3

Number of process

4

0

1

2

3

4

Number of process

Figure 4. The PL spectra of (a) AIS QDs and (b) ZAIS/ZIS/ZnS QDs are deconvoluted into two Gaussian bands (D1 and D2). (c) The relative contribution between D1 and D2 in PL spectra changes with increasing alloying/shelling process by HZAD. (d) The intensities of D1 and D2 in PL are estimated by considering the PL QY with increasing alloying/shelling process by HZAD.



(a)

(b)

1.0

1.0 QYD2

0.8

QYT QYD1

QYD2

0.6

QY

QY

0.8

QYT

0.6

QYD1 0.4

0.4

0.2

0.2 0

1

2

3

0

4

Number of process 1

1

2

3

4

Number of process 4 2 1 0

(d)

0.1

500

τ2 400

Lifetime (ns)

(c) Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

D2 D1

300 80

τ1

60 40

0.01 0

500

1000

1500

2000

0

1

2

3

4

Number of process

Time (ns)

Figure 5. (a) The QY of QDs (QYT), QY of surface states (QYD1), and QY of intrinsic states (QYD2) increase with increasing alloying/shelling process by HZAD. (b) QYT, QYD1, and QYD2 with increasing alloying/shelling process by LZAD. (c) The decay profiles of PL with increasing alloying/shelling process by HZAD are fitted by double exponential model. The yellow lines indicate the fitted lines of the decay profiles. (d) The estimated τ1 and τ2 from the decay profiles with increasing alloying/shelling process by HZAD.


Page 27 of 27

TOC GRAPHICS

1.0

AgIn5S8

ZnS-AgIn5S8/ZnIn2S4/ZnS ZnS-AgIn5S8

0.8

Quantum Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

AgIn5S8 0

0.4

1000

Time (ns)

0.2 0.0 0

1

2

3

Number of process

4

2.0

2.4

2.8

Energy (eV)


3.2

Enhancement Mechanism of Photoluminescence Quantum Yield in

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