Shell) Nanocrystals


Dec 25, 2018 - Copyright © 2018 American Chemical Society. *E-mail: [email protected], *E-mail: [email protected] Cite this:Nano Lett. 2019, ...
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Synthesis of Ag/Mn Co-Doped CdS/ZnS (Core/Shell) Nanocrystals with Controlled Dopant Concentration and Spatial Distribution, and Dynamics of Excitons and of Energy Transfer between Co-Dopants Wonseok Lee, Juwon Oh, Woosung Kwon, Sang Hyeon Lee, Dongho Kim, and Sungjee Kim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03923 • Publication Date (Web): 25 Dec 2018 Downloaded from http://pubs.acs.org on December 26, 2018

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Synthesis of Ag/Mn Co-Doped CdS/ZnS (Core/Shell) Nanocrystals with Controlled Dopant Concentration and Spatial Distribution, and Dynamics of Excitons and of Energy Transfer between Co-Dopants Wonseok Lee†, Juwon Oh||, Woosung Kwon§, Sang Hyeon Lee||, Dongho Kim*,||, and Sungjee Kim*,†,‡ †Department of Chemistry, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang 37673, South Korea ‡School of Interdisciplinary Bioscience and Bioengineering, Pohang University of Science and Technology, 77 Cheongam-ro, Nam-gu, Pohang 37673, South Korea ||Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea §Department of Chemical and Biological Engineering Sookmyung Women’s University 100 Cheongpa-ro 47-gil, Seoul 04310, South Korea

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ABSTRACT

We report lightly Ag/Mn co-doped CdS/ZnS (core/shell) nanocrystals (NCs) as a model system for studying interactions between co-dopants, and between NCs and dopants. The co-doped NCs were prepared with varying average number of Ag dopant atoms per CdS core of the NC from zero to eight; at the same time the depth profile of the Mn dopants in the ZnS shells was controlled to be either close to or far from the Ag dopants. Incorporation of average one to two Ag dopant atoms per NC increased the band-edge photoluminescence (PL), however, it was quenched at higher doping concentration. This alternation is attributed to change of the Ag ions’ occupancy from PLenhancing interstitial sites to PL-quenching substitutional sites. Mn PL increased as the number of Ag atoms per NC increased up to approximately seven, then decreased. For NCs doped only with Ag ions, the Ag dopants in substitutional sites acted as PL-quenching hole traps. In Ag/Mn codoped NCs, the Ag dopants acted as Dexter-type relay sites that enhanced the energy transfer from NC to Mn ions; this effect increased as the distance between Ag and Mn dopants decreased. This model study demonstrates that simultaneous control of dopant concentrations and spatial distributions in co-doped semiconductor NCs enables sophisticated control of their optical properties. KEYWORDS: Semiconductor nanocrystal; Transition metal doping; Co-doping; Energy transfer; Time-resolved spectroscopy; Exciton dynamics

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Doped colloidal semiconductor nanocrystals (NCs) have shown new functionalities in optoelectronics,1-3 spintronics,4,5 biological labels,6 and so forth.7 Co-doping is commonly performed for many optical and electrical functional materials. For example, Er-based amplifiers typically adopt co-dopants such as Yb to expand the absorption cross-sections and thus to increase the optical gain.8 For Er/Yb co-doped amplifiers, the optical gain is critically dependent on the energy transfer efficiency from Yb ions to Er ions. Interactions between co-dopants, for example between Er and Yb ions, have been investigated by varying the concentration and ratio of codopants.9 In NCs, co-doping has been mostly studied for metal oxides to tune their plasmonic properties.10-13 On the other hand, interactions between exciton and co-dopants in semiconductor NCs have not been well studied. Recently, many synthetic strategies have been developed to introduce dopants in semiconductor NCs.14 The strategies include use of co-precursors during NC growth,15 and cation exchange of pre-prepared NCs.16 Such strategies can afford quite precise control on the average number of dopants per NC with the number distribution following Poisson statistics. Recently, innovative strategies have been reported to obtain monodispersity of the doping level and realize ‘quantized doping’, for example, by employing metal magic clusters as growth seeds of NCs.17,18 Synthetic advances on NCs can also provide control over the radial distributions of dopants.15 Co-doped semiconductor NCs can be engineered with varying doping level and radial distribution for each dopant species, which can be a useful model to study inter-dopant interactions at nanoscales under the quantum confinement effect. Photo-generated excitons and co-dopants can interact strongly in the highly-confined structures of; as a result, they can show novel properties.4,19 To study inter-dopant interactions in semiconductor NCs, we used lightly doped II-VI NCs. Here, we define doping of NCs with the term of ‘light’ when the dopants induce a negligible

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change of the band-edge photoluminescence (PL), which indicates that the introduction of dopants do not severely perturb the band-edge electronic structure of NCs. According to our definition, codoped NCs that emit white light, e.g., Cu/Mn co-doped ZnInS NCs,20 are heavily doped system. The white light emission originates from the severe deformations of the electronic band structure. Introduction of Ag co-dopants into Ag/Mn co-doped ZnSe NCs, selectively quenches the ZnSe band-edge PL over the Mn-originated phosphorescence.21 These Ag/Mn co-doped ZnSe NCs are considered to be lightly doped, but both dopants interact solely with NC excitons (energy transfer from NC to Mn vs. energy transfer from NC to Ag) and inter-dopant interactions were not reported. We observed sensitization of Mn dopant emission upon co-doping of Ag ions in CdS NCs, which has attempted us to further study CdS/ZnS (core/shell) NCs as a model host system. The core/shell structures afforded both spatial separation and precise control over the radial distributions of the two dopants, and interactions between exciton and dopants and between the co-dopants were investigated. In II-VI semiconductor NCs, Mn ions can act as an efficient energy acceptor due to the strong sp-d exchange interactions.15,22,23 Ag ions also interact strongly with NC excitons, and can be introduced into many II-VI NCs by cation exchange; furthermore, the mild reaction conditions of the cation exchange allow simple control over the average number of dopants per NC. Norris and co-workers reported Ag-doped CdSe NCs of which band-edge PL increased at very low Ag doping concentration but decreased at higher Ag concentrations.16 Inspired by this work, we varied the average number of Ag dopants {nAg} in CdS/ZnS NCs and also controlled the average distance between the Ag and Mn dopants. We prepared CdS/ZnS NCs in which the CdS cores were doped with zero, one, two, or up to eight Ag atoms. At the same time, Mn dopants were introduced into the ZnS shells at controlled radial positions. The number of Ag dopants was

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controlled by carefully tuning the dopant precursor concentration during the cation exchanges. The radial distribution of Mn was determined by sequential ZnS shell overcoatings which had the Mn co-introduction step at the proper sequence. This approach (Scheme 1) allows simultaneous control over the Ag and Mn dopants during synthesis of undoped CdS/ZnS NCs (UDNCs), shallow manganese-doped CdS/ZnS NCs (SMDNCs) and, deep manganese-doped CdS/ZnS NCs (DMDNCs). Ag ions were introduced after the NC synthesis by cation exchange, which respectively converted UDNCs to silver solely-doped CdS/ZnS NCs (SSDNCs), SMDNCs to fardistant co-doped CdS/ZnS NCs (FCDNCs), and DMDNCs to near-distant co-doped CdS/ZnS NCs (NCDNCs) (Scheme 1). Table 1 shows acronyms for all the NC samples used herein. FCDNCs have relatively long distance between the Ag and Mn dopants. In contrast, the co-dopants are closely located facing the core/shell interface in NCDNCs. We measured both steady-state and transient optical properties of Ag/Mn co-doped CdS/ZnS NCs and investigated the interactions between the co-dopants and between the NCs and each dopant. We have observed strong interaction between the two co-dopants, where Ag dopants acted as Dexter-type intermediates that transfer energy from NC excitons to Mn dopants. 3.7 nm Mn-doped CdS NCs was synthesized using a synthetic route previously reported by others (Figure S1a).24 Average 9.8 Mn dopants were incorporated per NC as confirmed by inductively coupled plasma atomic emission spectroscopy (ICP-AES). This NC ensemble showed the absorption peak at 420 nm and almost negligible band-edge emission but predominant emission from the Mn dopant d-d transition (4T1 to 6A1) centered at ~585 nm (Figure S1b). To the Mn-doped CdS NCs, Ag dopants were introduced as varying the number of Ag dopants per NC ({nAg}) from 0 to 10 by cation exchange reactions at 50 °C.16 The resultant Ag/Mn co-doped NCs showed the Mn emission intensities highly dependent on the Ag doping levels. Up to {nAg} of 4.1, the Mn

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emission increased up to 230% when compared to that of Mn solely-doped NCs (zero {nAg}) and decreased at higher {nAg}s (Table S1). To our surprise, co-doping of Ag in Mn-doped NCs effectively sensitized the Mn emission. To carefully study the underlying mechanisms, we have synthesized Ag/Mn co-doped CdS/ZnS core/shell NCs with varying doping depths and concentrations. CdS cores were prepared by a method following previous publications with slight modifications. Synthetic details can be found in the Supporting Information (SI). The CdS cores (Figure S2a) had an average diameter of 4.0 ± 0.3 nm (Figure S2b), and showed an excitonic absorption peak at 418 nm (Figure S2c). Thin ZnS shells were overcoated onto the CdS cores (Figures S3a and S3b) to endow stability for the subsequent steps, for example, to block Mn diffusions into the CdS cores or to the core/shell interfaces. The average shell thickness was 1.0 monolayers (Figure S3 for the transmission electron microscopy (TEM) image, size histograms and absorption spectrum). Subsequently, additional ZnS shells were overcoated until the shell thickness was 1.5 monolayers (Figure S4 for the TEM image, size histograms and absorption spectrum), then Mn was introduced (Figure S5 for the TEM image, size histograms and absorption spectrum). After completion of Mn doping, the sample was purified and ZnS shells were further overcoated to a thickness of 2.0 monolayers; this process yielded SMDNCs. Alternatively, after the initial overcoating with thin (1.0 monolayers) ZnS shell, Mn was introduced before the subsequent ZnS shell overcoatings to reach the same final size as SMDNCs; this process yielded DMDNCs. As a control, the initial thin ZnS shell sample was further ZnS overcoated to reach the same final size, but without the Mn doping step; this process yielded UDNCs. The absorption spectra of all UDNC, SMDNC and DMDNC samples had an excitonic peak at 430 nm (Figure 1a). SMDNCs and DMDNCs displayed dual emissions: band-edge PL at ~475 nm

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and energy-transfer-induced Mn d-d transition at ~600 nm (Figure 1b). Actually the Mn emission wavelengths differed slightly for SMDNCs and DMDNCs as 600.7 and 602.5 nm, which should attribute to the different crystal fields applied to the Mn dopants at different radial positions. Lattice strains applied to dopants should differ by the radial positions, which resulted in different dopant local environments. TEM images of UDNCs, SMDNCs, and DMDNCs confirm that they are of the same size (Figures 1c-e). UDNCs showed only band-edge emission. The Mn doping concentrations were measured using ICP-AES, and were controlled to be ~4 Mn ions per NC for both SMDNC and DMDNC samples (Table 2). Both SMDNC and DMDNC samples showed similar Mn PL quantum efficiencies of ~15%. The Mn emission lifetimes exhibited 3.21 and 3.40 ms, respectively (Table 2), and the difference is also attributed to the different dopant local environments. Given the Mn doping concentration, Poisson distributions predict that ~2% of NCs should remain undoped. When comparing the band-edge PL quantum efficiencies of SMDNCs and DMDNCs over that of UDNCs, most of the band-edge PL from SMDNCs and DMDNCs originates dominantly from doped NCs and not from undoped NCs.25 Both Mn emission wavelength and lifetimes indicate well-controlled radial distributions of Mn dopants for SMDNC and DMDNC. To compare how the depth of Mn affects the efficiency of energy transfer from CdS core to Mn, transient absorption (TA) spectroscopy was performed for SMDNCs and DMDNCs (Figure 2 and Table 3). To calculate the energy transfer rates, the recovery curves were fitted using sums of exponential functions. In the TA decay profiles, UDNCs, SMDNCs and DMDNCs showed the similar initial fast decay, where the measured proportion A1 and lifetime τ 1 were similar among UDNCs (A1 = 0.191, τ 1 = 6 ps), SMDNCs (A1 = 0.187, τ 1 = 6 ps), and DMDNCs (A1 = 0.177, τ 1 = 6 ps). The initial fast decay shared by UDNCs, SMDNCs and DMDNCs was independent of the Mn-doping. This proportion is attributed to trapped excitons in

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NCs, presumably due to insufficient surface passivation.26 For UDNCs, the subsequent slow decay (τ2 > 1000 ps) was measured, which was also observed for SMDNCs and DMDNCs. This common slow decay pathway is assigned as the radiative and nonradiative recombination processes in association with band-edge PL of the CdS/ZnS NCs. The TA decay profiles of SMDNCs and DMDNCs displayed additional decay features, which were fitted with time constants in the order of hundreds ps (τ3 = 305 ps for SMDNCs and τ3 = 140 ps for DMDNCs). Considering the very long τ2 for UDNCs, SMDNCs and DMDNCs, Mn doping is thought to open an additional channel which has a lifetime that is represented by τ3. According to previous studies, Mn doped II-VI NCs typically show Dexter type energy transfer process from II-VI NCs to Mn dopant sites with rates in the order of tens to hundreds ps.4,25,27 In this regard, the additional decay dynamics in SMDNCs and DMDNCs are interpreted as the energy transfer process from CdS cores to Mn dopants. DMDNCs exhibited faster energy transfer dynamics than that of SMDNC. This can be comprehended by their Mn locational difference. In DMDNCs, the Mn dopants sit near the core/shell interface and close to the CdS cores, whereas SMDNCs the Mn dopants are located relatively far from the CdS cores. Since photoexcitation mainly produces excitons in the CdS core, the shorter distance between Mn dopants and CdS cores in DMDNCs triggers the faster and more efficient energy transfer process. Consequently, these different TA decay profiles of SMDNCs and DMDNCs confirm our synthetic capability for the radial distribution control of Mn dopants either close to or far from the CdS cores. FCDNCs and NCDNCs were respectively prepared from SMDNCs and DMDNCs by introducing Ag dopants to the CdS cores by a simple cation exchange method.16 The cation exchange reactions were carried out under mild conditions below 50°C to prevent Mn ion migrations. As discussed earlier, the Mn dopants were carefully placed as sandwiched between the

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initial ZnS shells and post-deposited ZnS shells to minimize possible diffusions into the CdS cores or to the core/shell interfaces. Restricted Mn diffusions upon the introduction of Ag ions was corroborated by the Mn d-d transition wavelength, lifetime, and energy transfer rate from NC excitons (vide infra). Ag ions were introduced into core/shell NCs doped with Mn ions in the shells. Upon the Ag co-doping, high mobility of Ag ions and relatively thin overall ZnS shells allowed preferential location of the Ag dopants in the cores. As will be further discussed below, very small doping concentration (~1.0 Ag per NC) significantly brightened the band-edge PL intensity up to ~150%. This is not easily explained if the Ag dopant was located in the shell or in the core/shell interface. The Ag ion visitors are assumed to first take a position near the center of CdS cores because of the strong Coulomb repulsion interaction between the charged Ag impurity ion and the image charge at the dielectrically-polarized interface between the NC and the surrounding medium.28 As the number of Ag ions per NC increases, they begin to repel one another, so they should occupy sites near the core/shell interface.28 To further prove the Ag dopant location at the cores in the Ag/Mn co-doped CdS/ZnS core/shell NCs, the cation exchange reactions were studied with excess Ag ions to verify if the incoming Ag ions preferentially replace Cd ions but not Zn ions. UDNCs which have the CdS/ZnS core/shell structure with identical CdS core size and ZnS shell thickness as SMDNC and DMDNC were cation-exchanged with Ag ions using the amounts up to Ag+/Cd2+ = 0.5 and Ag+/Cd2+ = 1.0. As doping the UDNCs with Ag ions, the absorption and X-ray diffraction (XRD) patterns were followed (Figure S6). CdS cores used for the synthesis of UDNCs were also investigated to compare the CdS cores and CdS/ZnS core/shell UDNCs. In comparison to the CdS cores, UDNCs showed broader absorption profile with stiffer increase at the short wavelengths (Figure S6a). As Ag ions were introduced to the UDNCs, the band-edge peak was broadened and tailing appeared at long wavelengths reaching

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~700 nm, which is indicative of conversion of CdS to Ag2S. The CdS band-edge broadening and red-tail emergence were proportional to the amount of Ag ions used for the cation-exchange. On the other hand, the absorbance at the UV region was not notably attenuated upon the cationexchange, which suggests the retention of ZnS shells. The XRD patterns showed more conclusive results. Upon the cation exchange by Ag ions, acanthite Ag2S phase started to appear and the diffraction intensity was intensified as more Ag ions were introduced. Concurrently, diffraction peaks from zinc-blende CdS and ZnS shifted to higher angles, which is indicative of simultaneous disappearance of CdS and retention of ZnS (Figure S6b). For an example, CdS cores showed the (111) peak at 26.64°. The (111) peak (now a merged (111) peak from both CdS and ZnS (111) diffractions) shifted to 27.28° for CdS/ZnS UDNCs, and further shifted to 27.70° and 28.08° for the cases of Ag+/Cd2+ = 0.5 and Ag+/Cd2+ = 1.0. Incoming Ag ions preferentially exchanged CdS in the cores and the phase of the NCs is thought to have transformed to Ag2S:CdS/ZnS where acanthite Ag2S and zinc-blende CdS domains co-existed in the cores while ZnS shells remained pretty much unchanged. During such phase change, no noticeable change in size or shape was observed under TEM (Figure S7). Further cation exchanges with Ag ions are expected to result in morphological change in NCs because of the accompanying volume change from CdS to Ag2S. However, such change was not observed within the degree of cation exchange used herein up to the condition of Ag+/Cd2+ = 1.0. As Ag ions become incorporated into SMDNCs or DMDNCs, they should initially occupy interstitial sites rather than substitutional sites of CdS cores, because the cation substitution reaction has a higher energy cost than the interstitial reaction.28 To quantify how {nAg} affected the Ag/Mn interaction in our NCs, up to ~8 Ag ions were introduced per FCDNC and NCDNC. This range of {nAg} did not noticeably affect the XRD patterns (Figure S8a). This indicates that neither Mn nor Ag ion incorporation noticeably altered the CdS/ZnS

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crystal structure. No noticeable size or shape change was observed under TEM measurements upon the Ag incorporation (Figures S8b and S8c). Despite no observable change in the crystal structure and size of CdS/ZnS NCs, the increase of Ag dopant concentration is known to change the doping site in NCs. In CdSe NCs, the change of Ag doping site occurred at doping concentration of ~2 per NC.16,28-30 In interstitial sites of the host crystal framework, Ag ions may form positively-charged field centers that impede abrupt trapping of charge carriers.16,31 As the result, doped Ag ions in interstitial sites can intensify the band-edge PL of CdSe NC. As additional Ag ions are introduced, they migrate to substitutional sites, where they quench band-edge PL.32 Ag ions in substitutional sites were considered to act as charge-carrier traps because Ag ions are not isovalent with Cd ions in the CdSe NCs. This change in preferred site of the incoming Ag ions is also expected in our series of CdS/ZnS-based FCDNCs and NCDNCs that have different {nAg}. To scrutinize the optical properties of UDNCs, FCDNCs and NCDNCs upon the increase of {nAg}, a series of samples of each kind were prepared with {nAg} from zero to approximately eight. For each series, the doping process was carefully controlled to obtain {nAg} = 0, 0.5, 1, 2, 3, 5, or 8; seven samples of each {nAg} were prepared for each kind of NC. Actual {nAg}s were determined by ICP-AES; names used for the samples append measured {nAg} to the sample type; e.g., FCDNC2.9 represents an FCDNC with {nAg} = 2.9. Figure 3 shows a representative data set of absorption and emission spectra of UDNC and SSDNC series (a and b), SMDNC and FCDNC series (c and d), DMDNC and NCDNC series (e and f). UDNCs, FCDNCs, and NCDNCs with {nAg} in the range of 0~5 showed similar absorption spectra, which confirms that Ag co-doping did not severely perturb the electronic structure of the CdS/ZnS NC host, either in the Mn-undoped or the Mn-doped NCs with different depth profiles. In the cases of FCDNCs, and NCDNCs, the

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Mn emission peak wavelengths remained almost constant regardless of {nAg}, which suggests that Mn positions were not severely perturbed by Ag co-doping. TEM images showed no noticeable morphological changes in NCs as the Ag doping process proceeded (Figure S9). These results mean that all the samples, {nAg} in the range of 0~5, are within the lightly Ag-doped regime as defined above. However, the absorption spectra of SSDNC7.9, FCDNC8.2, NCDNC8.3 showed slight broadening compared to the other samples in its series (insets of Figures 3a, 3c and 3e); this broadening is indicative of Urbach tailing, and means that SSDNC7.9, FCDNC8.2, and NCDNC8.3 are in the heavily Ag-doped regime.19 Our {nAg} spans for each series cover the entire range of the lightly Ag-doped regime that we are interested in. In the SSDNC series, the PL intensity was 1.3 times and 1.5 times higher than that of UDNC for SSDNC0.5 and SSDNC0.9, respectively (Table 4). However as {nAg} increased beyond these levels, the PL intensity decreased consistently; the PL intensities of SSDNC4.6, and SSDNC7.9 dropped 0.55 and 0.18 times compared with UDNC, respectively. Here, trioctylphosphine(TOP), which was used as the complexing ligand for Ag ion precursor in the Ag doping process, does not affect the PL properties of SSDNC, FCDNC, and NCDNC series (Figure S10). The increase then decrease in PL intensity with increase of {nAg} in SSDNCs was similar to that observed when Ag ions are doped into CdSe NCs.16 In the Ag-doped CdSe NCs, the largest PL intensity was observed at {nAg} = 2.3 for 3.9-nm CdSe NCs. In this regard, the emissive features in UDNC series suggest the migration of Ag dopants from interstitial sites to substitutional sites.16,19,30 Ag ions in interstitial sites may increase the band-edge PL by generating a local electric field that suppresses abrupt trapping of excitons in NC cores, whereas Ag ions in substitutional sites may quench the bandedge PL by trapping charge carriers. Thus, when Ag ions were doped into the CdS/ZnS NCs, the change in Ag dopant location is expected to occur in 1 < {nAg} < 2.

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PL quantum yields (QYs) of the SSDNC series and PL QYs for band-edge and Mn PL of the FCDNC and NCDNC series were strongly affected by {nAg} (Table 4). Figure 4 shows the changes of PL intensities for band-edge and Mn emissions upon the increase of {nAg}. The error bars in Figure 4 are based on our three times repetition of the entire sample preparations of UDNCs, SMDNCs, DMDNCs, six kinds of SSDNCs, six kinds of FCDNCs, and six kinds of NCDNCs and their optical measurements and elemental analyses. The most striking feature is that Mn PL increased monotonically as {nAg} increased, except in the samples that were most heavily doped with Ag (i.e., FCDNC8.2 and NCDNC8.3). In general, excitons in any trap states are more localized than untrapped excitons, so energy transfer from any trapped exciton to Mn dopant should be impeded by the reduced wavefunction overlap between the donor and acceptor. Surprisingly, our FCDNC and NCDNC series in the lightly Ag-doped regime showed opposite PL features where the Mn PL was enhanced upon the increase of {nAg}. This suggests that introduction of additional charged exciton trapped states by incoming Ag ions help transferring energy from initially photo-generated excitons to Mn dopants, and thereby contribute to Mn PL. To further prove the mediation of energy transfer by Ag dopants, we have prepared Mn-doped CdS/ZnS NCs with slightly higher Mn concentrations (15.0 Mn ions per NC on average) (Figure S11a). The higher Mn concentration resulted in increase in the Mn emission PL QY to 32.3 % (Figure S11b). A series of NCDNCs were prepared using the bright Mn-doped CdS/ZnS NCs as varying the {nAg} up to 8.0. Similar to the case of NCDNCs in Figure 4, the Mn PL increased monotonically as the {nAg} increased except in the sample that was most heavily doped with Ag ({nAg} of 8.0). The Mn emission was maximally sensitized by the Ag co-doping showing the QY of 47.3% when 5.0 Ag ions were co-doped per NC (Figure S11c). The Mn emission sensitization by Ag co-doping showed the very similar Ag concentration dependence regardless of the Mn concentrations within our

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experimental range. Such mediators of energy transfer to Mn-doped NCs have been reported for surface ligands. Thiol surface ligands on NCs are typically surface hole traps, but Son and coworkers reported that octanethiol surface ligands on Mn-doped CdS/ZnS NCs acted as an intermediate of energy transfer process.33 Ag dopants in CdSe NCs are reported to show holeaccepting behavior.34,35 We hypothesize that the Ag dopants in FCDNCs and NCDNCs created hole-charged exciton-trapped states that mediated the energy transfer to Mn dopants. For this process, lifetimes of the hole-charged exciton-trapped states should lie in between those of the photo-generated exciton and excited Mn states, which can lead the exciton-trapped states to be optimal for the energy relay intermediate acting like a reservoir. However, the lifetime of the Mn d-d transition did not change noticeably as {nAg} increased; this inconsistency may have occurred because of the huge mismatch in the timescales of Mn d-d transition and the trapped state mediated by Ag dopants, regardless of the trap density used herein. The unchanged lifetimes of Mn d-d transition for different {nAg} also suggest that the local environment around the Mn dopants were not perturbed by the Ag co-doping (Figure S12). Within the entire lightly doped regime, the increase in Mn PL by Ag doping was consistently 1.2 to 1.3 times larger in the NCDNC series than in the FCDNC series, except at the highest Ag doping level, at which Mn PL was larger in FCDNC8.2 than in NCDNC8.3. We hypothesize that the energy transfer from Ag to Mn dopant is Dexter type, which depends on quantum tunneling, and therefore should decrease rapidly with increase in distance between the dopants. Ag dopants are closer to Mn in NCDNCs than in FCDNCs at any given {nAg}. This distance-dependent decrease in the energy transfer rate is consistent with our hypothesis that Ag dopant helps to relay energy from trapped excitons at Ag ions to Mn dopants. In heavily Ag-doped FCDNCs and NCDNCs, we do not have a clear idea how the Ag-doped sites impeded the exciton energy transfer

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to Mn dopants. However, this inefficiency of Mn PL is not unusual in NCs that have many trap sites. At high {nAg}, the hole traps may become too deep, so they become inefficient for the energy transfer process. The increased {nAg} may also promote transfers between Ag-mediated hole traps themselves; this process would decrease the efficiency of energy transfer to Mn dopants.19 PL decays were measured for the UDNC and SSDNC series (Figures 5a and 5b). Each decay was fitted using a biexponential curve with fitting parameters Af, As, τf, and τs (Table 5). The fast and slow decay features represent predominantly the core-state and surface-state related dynamics, respectively.36-39 As the {nAg} increased for the SSDNC series, their PL intensity showed two stages; (i) the PL decays became slower from UDNC to SSDNC0.9 (Figures 5a) and (ii) then became gradually faster with the increase of Ag doping concentration (Figures 5b). For the PL enhancement stage, lifetime for the fast decay, τf, increased from 2.61 ns to 3.32 ns and 4.14 ns from UDNC to SSDNC0.5 and SSDNC0.9, respectively, which attributes to suppression of fast nonradiative dynamics such as Auger recombination at the CdS core states by the positive impurity field from the interstitially doped Ag ions.16,28 For the PL decrease stage, the increase in Af and decrease in τf were observed, which is indicative of decrease in the surface-originated states and addition of faster decay pathway by charge trapping to heterovalent dopants.21,33,40-42 The most heavily Ag-doped sample SSDNC7.9 showed exceptionally fast PL decay, which accords well with the strong quenching of PL states. Increased interactions among Ag ions such as energy transfers among them may have resulted in the abrupt PL quenching.19 In SSDNC, NCDNC and FCDNC series, the band-edge PL intensity initially increased when {nAg} < 2, then decreased at higher {nAg}. Figures 5c-e showed the TA decay profiles of DMDNC and NCDNC series (NCDNC0.6 to NCDNC5.1) at their band-edge peaks. In the slow time scale over >1000 ps, the TA decay profiles of the NCDNC series showed the slower recovery features

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as the {nAg} increased (Figure 5c). The slow recovery with time scale of >1000 ps could be ascribed to hole trapped excitons which avoided energy transfer to Mn ions.33,43 Since electron subpopulations which couple with trapped holes relax very slowly (over tens of nanoseconds), the full recovery time of hole trapped exciton could not be measured exactly due to limitations of temporal time window of our TA experiments. However, the pathway attributed to the exciton energy transfer to Mn exhibited the constant decay time of ~140 ps regardless of the change in {nAg}. To better visualize this, the TA spectra were normalized by treating the persistent slow component as variations in the amplitude of the TA signals (Figures 5d and 5e). The observation of unchanged energy transfer lifetimes for samples of different {nAg}s is consistent with our hypothesis that Ag ions in CdS cores acted as hole traps. Although hole trapping is considered to occur at time scale of sub picosecond to tens of picoseconds, hole-trap-related behaviors are typically optically silent at ps time range in band-edge bleach recovery of CdS or CdSe NCs due to the large degeneracy of the valence band.44,45 The decay of Mn d-d transition emission was also independent on the change of {nAg} (Figure S12). The unchanged TA energy transfer lifetimes and Mn emission lifetimes for samples of different {nAg}s also suggest retention of Mn distribution upon Ag co-doping. In the case of Mn d-d transition emission, the rise kinetics at early time scale should be dependent on the {nAg}. However, the huge mismatch in time scale (sub nanosecond energy transfer Vs. milliseconds Mn emission) made it difficult to obtain enough signals to fully characterize the kinetics. The unchanged energy transfer rate to Mn dopants suggest that the Ag dopants played a role of intervention as a hole-related third relay party. In other words, Ag ions doped near the Mn ions in NCs act as energy-transfer intermediates; this hypothesis explains the increase of Mn PL upon the increase of {nAg} in the lightly doped regime.

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We synthesized lightly Ag/Mn co-doped CdS/ZnS (core/shell) NCs in which average Ag dopant concentrations {nAg} in the CdS cores were varied from zero to eight, and at the same time the depth profile of the Mn dopants in the ZnS shells was controlled to be either near to or far from the core/shell interface. Steady-state and transient optical properties were investigated for the various NCs. The band-edge PL was intensified when {nAg} < 2, then quenched at {nAg} > 2; this change is attributed to migration of Ag ions from interstitial to substitutional sites as a result of increasing Coulombic repulsion among Ag ions as their concentration increased. Ag dopants in interstitial sites may suppress fast nonradiative dynamics such as Auger recombination at CdS core states by acting as positive impurity field sites; in contrast, Ag dopants in substitutional sites may have acted as hole traps. When the NCs were co-doped with Ag and Mn, dopants in substitutional sites mediated hole traps and acted as Dexter-type sites to relay energy transfers. Mn PL was intensified within the entire lightly doped regime approximately up to {nAg} = 7. In addition, the Mn PL intensification mediated by Ag dopant was dependent on the distance between co-dopants. NCDNCs in which distance between Ag and Mn ions is smaller than in FCDNCs showed up to 30% increase in Mn PL compared to FCDNCs. These results demonstrate that co-doping affects the properties of semiconductor NCs; this strategy will pave a way to achieve novel properties and functionalities.

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FIGURES

Figure 1. (a) Absorption and (b) photoluminescence spectra for undoped CdS/ZnS NCs (UDNCs, black lines), shallow manganese-doped CdS/ZnS NCs (SMDNCs, red lines), and deep manganesedoped CdS/ZnS NCs (DMDNCs, blue lines). TEM images of (c) UDNC, (d) SMDNC, and (e) DMDNC.

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Figure 2. TA decay profiles (semi-log plot) of UDNC (black), SMDNC (red), and DMDNC (blue) at the band-edge bleaching peak (432 nm) with photoexcitation at 400 nm. Circles are for the raw data and lines are their exponential fitting line.

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Figure 3. Absorption and emission spectra of (a and b) UDNC and SSDNC series, (c and d) SMDNC and FCDNC series, and (e and f) DMDNC and NCDNC series Vs. {nAg}

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Figure 4. PL intensity ratios of band-edge emission (solid line) and d-d transition (dotted line) that are divided by the PL intensity at their {nAg} = 0 counterparts for (a) SSDNC, (b) FCDNC, and (c) NCDNC. The horizontal and vertical error bars display the standard deviations of Ag doping level and emission intensity, respectively.

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Figure 5. Time-resolved photoluminescence decay curve of UDNC and SSDNC series (a) UDNC (black), SSDNC0.5 (brown), and SSDNC0.9 (red) and (b) SSDNC2.3 (orange), SSDNC3.1 (yellow), SSDNC4.6 (green) and SSDNC7.9 (blue). The signals were probed at 475 nm. (c) TA decay profiles of DMDNC and NCDNC series in the time windows of 1000 ps. The band-edge bleach recovery signals were probed at 432 nm with photoexcitation at 400 nm. Normalized TA

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decay profiles of DMDNC and NCDNC series in the time windows of (d) 3000 ps and (e) 100 ps. The band-edge bleach recovery signals were probed at 432 nm with photoexcitation at 400 nm.

SCHEMES

Scheme 1. Schematic illustration showing the synthesis routes for undoped CdS/ZnS (core/shell) NCs (UDNCs) (top panel), shallow manganese-doped CdS/ZnS NCs (SMDNC) (middle panel) and deep manganese-doped CdS/ZnS NCs (DMDNCs) (bottom panel). Subsequent cation exchange processes for UDNC, SMDNC and DMDNC yield respectively silver solely-doped CdS/ZnS NCs (SSDNCs), far-distant co-doped CdS/ZnS NCs (FCDNCs) and near-distant codoped CdS/ZnS NCs (NCDNCs).

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TABLES Table 1. List of acronyms stated in the main text Acronym

Full name

UDNC

Undoped CdS/ZnS NC

SMDNC

Shallow manganese-doped CdS/ZnS NC

DMDNC

Deep manganese-doped CdS/ZnS NC

SSDNC

Silver solely-doped CdS/ZnS NC

FCDNC

Far-distant co-doped CdS/ZnS NC

NCDNC

Near-distant co-doped CdS/ZnS NC

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Table 2. Mn doping concentration per NC {nMn}, luminescence quantum yield of band-edge emission QYBE (%), and quantum yield of Mn d-d transition QYMn (%), and phosphorescence lifetime τMn (ms) for UDNCs, SMDNCs and DMDNCs.

Sample ID

{nMn}

QYBE

QYMn

τMn

UDNC



15.3





SMDNC

4.0

2.41

15.2

3.21

DMDNC

3.8

0.57

13.7

3.40

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Table 3. Fitting parameters for the TA decay profiles of UDNC, SMDNC, and DMDNC probed at 432 nm.

Sample ID

A1

A2

A3

τ1 (ps)

τ2 (ps)

τ3 (ps)

UDNC

0.191

0.809



6

> 1000



SMDNC

0.187

0.583

0.230

6

> 1000

305

DMDNC

0.177

0.478

0.345

6

> 1000

140

*Each decay curves were fitted with sums of exponential functions, ΣAi exp(-t/τi).

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Table 4. Band-edge quantum yield QYBE (%) and d-d transition quantum yield QYMn (%) of UDNC, FCDNC and NCDNC upon different Ag doping concentrations {nAg} (numeral at end of Sample ID).

Sample ID

QY

Sample ID

QYBE

QYMn

Sample ID

QYBE

QYMn

UDNC

15.3

SMDNC

2.41

15.2

DMDNC

0.57

13.7

SSDNC0.5

19.4

FCDNC0.5

2.63

16.1

NCDNC0.6

0.70

16.8

SSDNC0.9

22.6

FCDNC0.9

2.89

16.5

NCDNC1.1

0.83

18.5

SSDNC2.3

18.3

FCDNC2.0

2.61

17.0

NCDNC1.9

0.64

19.4

SSDNC3.1

13.4

FCDNC2.9

2.43

18.1

NCDNC3.4

0.52

20.2

SSDNC4.6

8.42

FCDNC5.6

2.20

19.2

NCDNC5.1

0.49

21.4

SSDNC7.9

2.72

FCDNC8.2

1.06

10.8

NCDNC8.3

0.17

6.22

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Table 5. Fitting parameters of PL decay curves of UDNC and SSDNC series upon different {nAg}. Af, As, τf, and τs respectively stand for amplitude of fast and slow pathways and corresponding time parameters.

Sample ID

Af

As

τf (ns)

τs (ns)

UDNC

0.381

0.619

2.61

38.6

SSDNC0.5

0.383

0.617

3.32

38.4

SSDNC0.9

0.387

0.613

4.14

38.1

SSDNC2.3

0.444

0.556

2.69

37.9

SSDNC3.1

0.479

0.521

2.67

37.2

SSDNC4.6

0.506

0.494

2.68

37.5

SSDNC7.9

0.544

0.456

1.20

36.8

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ASSOCIATED CONTENT Supporting Information. TEM images, size histograms, and absorption and PL spectraof synthesized NCs in intermediate steps. Synthesis details, characterization methods, and PL spectra comparing the intensity of the Mn-doped NC before and after treating with trioctylphosphine, band-edge PL decay data for 200 ns time window and Mn d-d transition emission decay spectra for FCDNC and NCDNC series. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Prof. S. Kim. Department of Chemistry, Pohang University of Science and Technology (POSTECH),

77

Cheongam-ro,

Nam-gu,

Pohang

37673, South

Korea,

E-mail:

[email protected] * Prof. D. Kim. Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea, E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and Future Planning(NRF2016R1E1A1A01941427 and NRF-2015M3C1A3056411). The work at Yonsei University was

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supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System) and the Graduate School of YONSEI University Research Scholarship Grants in 2018. We acknowledge Center for Selfassembly and Complexity, Institute for Basic Science for the elemental analysis using ICP-AES.

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