ZnS shells enhance triplet energy transfer from CdSe nanocrystals for

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Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

ZnS Shells Enhance Triplet Energy Transfer from CdSe Nanocrystals for Photon Upconversion Zhiyuan Huang,† Pan Xia,‡ Narek Megerdich,§ Dmitry A. Fishman,∥ Valentine I. Vullev,†,⊥ and Ming Lee Tang*,†,‡ †

Department of Chemistry, University of California, Riverside, Riverside, California 92521, United States Materials Science & Engineering Program, University of California, Riverside, Riverside, California 92521, United States § Department of Chemical Engineering, University of California, Riverside, Riverside, California 92521, United States ∥ Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States ⊥ Department of Bioengineering, University of California, Riverside, Riverside, California 92521, United States

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‡

S Supporting Information *

ABSTRACT: Triplet energy transfer (TET) from semiconductor nanocrystals (NCs) to molecules is one of the bottlenecks that limits the efficiency of photon upconversion. While an inorganic shell can enhance the photoluminescence quantum yields (PLQYs), the role of the shell with respect to TET is still not clear. In this work, CdS and ZnS shells with different shell thickness are grown on 2.9 nm diameter CdSe NCs, resulting in nanostructures here that have increased radiative rates compared to the core. TET from these NCs to bound 9-anthracene carboxylic acid is investigated with linear photon upconversion measurements, time-resolved photoluminescence lifetime, and transient absorption spectroscopy. The ZnS shell enhances the photon upconversion QYs 1.6 times from 5.7% to 9.3%, with a concurrent increase of TET efficiency from 6.68% to 12.9% and the net rate of TET from 4.97 × 108 s−1 to 6.67 × 1010 s−1. In contrast, TET is barely observed for CdSe/CdS core−shell NCs. Considering the changes in PLQYs and upconversion QYs, a sub-monolayer ZnS shell enhances TET by removing surface traps and increasing exciton lifetime, while a thick shell creates an energy barrier that diminishes TET. The shorter exciton lifetime and increased exciton−phonon coupling due to CdS explain the drastically different effects of ZnS and CdS shells. KEYWORDS: semiconductor nanocrystal, core−shell, zinc sulfide, triplet energy transfer, upconversion fforts to harness the interesting optical and electronic properties in nanocrystals (NCs) are usually hampered by uncontrolled charge transfer or electron−phonon or exciton− phonon coupling.1−5 These loss mechanisms are typically mitigated with thick or “giant” shells that decrease nonradiative rates and minimize photoluminescence (PL) intermittency.6−8 This allows the tunable band gaps and photostability of semiconductor NCs to be used in biological labeling,9 solidstate lighting and displays.10,11 Although “giant” shells result in stable NC emitters, they serve as barriers impeding charge or energy transfer. For example, NCs used as the active material in photovoltaic devices eschew thick shells for thinner ones that allow simultaneous charge percolation and trap passivation to improve power conversion efficiency.12−14 The role of the shell has not been clearly articulated for triplet exciton transfer, a process critical for singlet fission and photon upconversion.15−18 Since these multiexcitonic processes have the potential of exceeding the Shockley−Queisser limit with diffuse sunlight,15−23 it is important that the fundamental parameters controlling triplet transfer between molecules and NCs be elucidated for the rational design of these hybrid platforms for energy conversion. In this work, the effect of core−shell architecture on the efficacy of triplet energy transfer from CdSe NC light

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absorbers is investigated for photon upconversion. In cadmium selenide NCs, thermally accessible processes at room temperature involve hole excitations and lattice vibrations and, when a shell is present, electron excitations.4 The presence of a shell may be a bane or a boon. At the core−shell interface, charge localization affects the rate of Auger recombination;24 electron and hole wave function overlap influences the efficiency of multiple exciton generation.25,26 Shell thickness and band offset affect the tunneling of charges or excitons from the core.27,28 Heterogeneity in material composition affects the nuclear fluctuations, which in turn modulate radiative rates and nonradiative processes such as triplet energy transfer.5,29−31 Many workers have shown that ZnS or CdS shells on CdSe NCs dramatically improve NC brightness, because these higher band gap materials spatially confine the excitons to the core, thus minimizing the probability of encountering surface trap states.6,7,32−34 Building on previous work, a whole range of CdS and ZnS shell thicknesses are examined here, the difference being a focus on thin shells such as those used for QD solar cells.12 Unlike the long-range energy transfer of singlet excitons through space, triplet excitons are transferred Received: March 14, 2018 Published: June 20, 2018 A

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Figure 1. (a) Illustration of the triplet energy transfer (TET) in this hybrid photon upconversion system. Green and blue arrows denote photoexcitation of the nanocrystals (NCs) and emission from diphenylanthracene (DPA), respectively. Dotted black and red curved arrows denote the triplet−triplet annihilation and TET. The absorption (solid line) and emission (dashed line) spectra of (b) 9-ACA (top), DPA (middle), 2.9 nm diameter CdSe NCs (bottom); (c) CdSe/ZnS and (d) CdSe/CdS core−shell NCs with different shell thickness (ML = monolayer). The black arrow in the bottom panel of (b) indicates the excitation wavelength used (488 nm). Measurements are performed in hexane at room temperature.

Table 1. Shell Thickness n in Monolayer (ML); Absorption and Emission Maxima, λabs and λems; Full Width at Half-Maximum of the Emission Peak, fwhm; Average Number of Bound Ligands per NC, N; Mole Ratio of Se:Cd:Zn from ICP-AES; Radiative Rate Constant, kr; Nonradiative Rate Constant, knr; Photoluminescence Quantum Yield, ΦPL; and Photon Upconversion Quantum Yield, ΦUC, for Each Nanocrystal NC

n/ML

λabsa/nm

λemsa/nm

fwhm/meV

Nb

mole ratio Se:Cd:Zn

krc/μs−1

knrc/μs−1

ΦPLa/%

ΦUCd/%

CdSe CdSe/ZnS

0 0.5 1 1.5 0.25 0.5 1.4 1.8 3.0 3.4 3.4

540 538 535 533 543 559 563 576 580 588 589

552 549 544 545 555 570 578 591 597 603 604

131.3 110.9 113.4 141.6 129.9 115.4 126.7 99.8 101.5 92.3 95.2

2.63 1.84 5.14 9.31 4.56 2.61 3.05 8.59 9.78 13.4 12.3

1:1.32:0 1:1.34:0.492 1:1.36:0.0447 1:1.36:0.597 1:1.42:0 1:1.26:0 1:3.33:0 1:4.10:0 1:6.93:0 1:8.56:0 1:8.54:0

0.43 6.21 4.84 8.15 2.54 1.53 2.06 8.11 7.27 15.3 9.03

16.0 6.01 5.22 6.40 12.6 16.2 24.0 21.5 20.9 11.5 13.3

2.72 50.8 48.1 56.0 16.8 8.63 7.91 27.4 25.8 57.1 40.4

5.7 9.3 8.1 5.6 0.59 0.31 2.3 2.5 1.9 1.0 0.91

CdSe/CdS

Hexanes, RT; ΦPL with a R6G standard. bGiven by UV−vis spectroscopy. cCalculated from ΦPL and nanocrystal lifetime (see SI) and does not include sub-nanosecond kinetics. d5 μM NC in 2.15 mM DPA in hexane at RT, excited with a CW 488 nm laser at 19.8 W/cm2.

a

via an electron-exchange mechanism. Therefore, a decrease in the thickness of the shell is expected to enhance the efficiency of triplet energy transfer (TET) because of improved electronic coupling.23 In particular, we sought to investigate if epitaxial growth of a thin shell could ameliorate surface traps, and if exciton−phonon coupling could be suppressed by using shell material with a higher bulk modulus compared to the core. With the 2.9 nm diameter CdSe NCs used here, we find that only sub-monolayer ZnS shells enhance triplet energy transfer and photon upconversion. Conversely, CdS shells impeded triplet energy transfer regardless of thickness. We study triplet energy transfer from CdSe/ZnS and CdSe/CdS core/shell NCs to anthracene molecules with linear photon upconversion, ultrafast transient absorption (TA) spectroscopy, and timeresolved photoluminescence measurements. The upconversion quantum yield (QY) is enhanced 1.6 times by the growth of a ZnS shell from 5.7% to 9.3%, consistent with TA spectra that show an increase in the efficiency of triplet energy transfer from 6.68% to 12.9%. All the core−shell NCs here have an order of magnitude larger radiative rate than the core and a larger photoluminescence quantum yield (PLQY), indicating successful removal of a subset of surface traps. TA spectra shows that the electron localized at the CdS shell is rapidly quenched within nanoseconds in the presence of the anthracene acceptor with negligible triplet transfer. The

major difference in the efficiencies of triplet transfer arise from stronger exciton−phonon coupling with the CdS shell that dissipates the photogenerated exciton in the CdSe core, similar to the line width broadening observed in the photoluminescence of single CdSe/CdS core/shell NCs5 and the large line width in CdS nanoplatelets.35−38 In contrast, the efficacy of the ZnS shell arises from reduced exciton−phonon coupling, arising from the larger bulk modulus and the type I electronic structure introduced.

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RESULTS AND DISCUSSION The hybrid upconversion system investigated here is depicted in Figure 1a. It is composed of CdSe NCs as sensitizer, 9anthracenecarboxylic acid (9-ACA) as transmitter, and diphenylanthracene (DPA) as annihilator. During linear photon upconversion experiments, the NCs are photoexcited by a 488 nm CW laser. Triplet energy transfer occurs first from the NCs to the triplet state of 9-ACA, which is directly bound on the NC surface, then second to free DPA in solution. Two DPA molecules in their triplet excited state annihilate to form a singlet that emits the upconverted 430 nm photon. Figure 1b shows the absorption and emission spectra of 9-ACA, DPA, and the CdSe core NCs. In order to epitaxially grow sub-monolayer thick CdS shells on CdSe cores, slow deposition at high temperatures was performed with the injection of octanethiol and cadmium B

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Figure 2. (a) Photoluminescence QYs, ΦPL. (b) Radiative (kr) and nonradiative (knr) rate constants normalized relative to the CdSe core. (c) Photon upconversion QYs (solid curve) of NCs as sensitizers with surface-bound 9-ACA and 2.15 mM DPA with respect to shell thickness with error bars. CdSe/ZnS and CdSe/CdS core/shell NCs are denoted in red and black, respectively. Data in (b) are obtained from lifetime measurements (see SI) with a 406 nm pulsed laser and (c) with a 488 nm CW laser at 19.8 W/cm2. All samples are dissolved in hexane and measured at room temperature.

oleate at 240 °C over 2 h, following the methodology recently reported by Bawendi and Chen.32 Shell growth was only reproducible with CdSe cores larger than 2.9 nm in diameter, presumably because strain at the core−shell interface is not prohibitive beyond this size. This core size is still small enough to maintain a driving force of 0.47 eV for triplet energy transfer to the anthracene acceptor. However, since the lattice mismatch between ZnS and CdSe is 12% (compared to 4.5% between CdS and CdSe),39 no epitaxial growth can occur. Therefore, the ZnS shell is grown on the CdSe core33 using the more reactive precursors of diethylzinc and hexamethyldisilathiane, injected at 160 °C over 10 min. In order to make highquality core−shell NCs, these CdSe/ZnS core/shell NCs were annealed for 3 h at 90 °C. ZnS and CdS shells blue and red shift the absorption maxima of the CdSe NCs, respectively, as seen in Figure 1c and d. The 2−7 nm blue shift as the ZnS shell increases from 0.5 to 1.5 ML indicates contraction of the CdSe core and is interpreted as formation of a Zn−Cd alloy structure at the surface and a type I electronic structure.40 On the other hand, growth of a CdS shell leads to exciton delocalization with a red shift as large as 49 to 589 nm (3.4 monolayers), suggesting a pseudo type II electronic structure.40 Delocalization of the electron into the shell arises from the small band offsets between both materials (300 meV in the bulk).40 The shell thickness was determined by the ratios of Se, Cd, and Zn obtained from inductively coupled plasma atomic emission spectroscopy (Table 1), the size of the CdSe core (determined by the first exciton absorption peak), and the bulk density of CdSe, CdS, and ZnS.41 The shell thickness is represented as average thicknesses in terms of monolayers (MLs) of shell materials, where 1 ML is 3.4 Å42 and 3.1 Å33 for wurtzite CdS and ZnS, respectively. For the CdSe core, all CdSe/ZnS core/shell NCs, and the 0.25 and 0.5 ML CdSe/ CdS core−shell NCs, the molar ratios of Cd/Se are 1.3−1.4, indicating that the composition of the NC does not change significantly when the shell is very thin. The estimated shell thickness and the first exciton absorption peaks of CdSe/CdS core−shell NCs in this work match the calibration curve reported by Mulvaney and co-workers well.43 Transmission electron micrograph (TEM) images of these NCs are in Figure S1 in the Supporting Information (SI). As shown in Figure 2a and b, the core/shell NCs here have larger photoluminescence QYs, ΦPL, and radiative rate constants, kr, compared with the core, implying surface traps

detrimental to emission are removed with shell growth. For example, ΦPL is 2.72%, 56.0%, and 57.1% and kr is 0.43, 8.15, and 15.3 μs−1 for the CdSe core and the CdSe/(1.5 ML)ZnS and CdSe/(3.4 ML)CdS core/shell NCs, respectively (see Table 1). This increase in PLQY for NCs with shell thickness less than 1 ML suggests that shell growth may initiate at trap states or high-energy facets. The intensity-weighted average lifetimes, τ̅, calculated from a triexponential fit of time-resolved PL data show that the original CdSe NC exciton lifetime is increased with the ZnS shell and decreased with the CdS shell (Table 1, Figures S3, S4). As shown in Tables S1 and S2, the lifetimes and radiative and nonradiative rate constants, kr and knr, respectively (Figure S4), calculated using only the lifetime with the largest weight match the trend given by τ̅ (the intensity-weighted average). This indicates that the increase (decrease) in exciton lifetime with ZnS (CdS) is real and independent from the method of calculating the rate constants, which can be complicated when there are three components, as in the case here. Growth of both CdS and ZnS shells partially removes the surface traps, as shown by the increase in PLQYs discussed above. Presumably thicker shells would lead to nearunity PLQY CdSe/CdS core−shell NCs reported in the literature.32 Typically, CdSe NCs with CdS shell thickness exceeding 3 ML show decreasing radiative rate constants with increasing shell thickness, due to reduced wave function overlap between the hole and electron, with the electron sampling the shell and the hole confined to the core. This trend is also expected for single-component CdSe NCs, but unexpected from a pure type II core−shell structure.44 However, the core−shell NCs here, all with less than 3 ML thick shells, have higher radiative rates compared to the CdSe core, indicating surface passivation outcompetes exciton delocalization (indicated by the bathochromic shift with a CdS shell), as opposed to the trend observed when thick CdS shells are grown on CdSe NCs.27,43,45 The photon upconversion quantum yield, ΦUC, of the NC photosensitizers is enhanced by the ZnS shell, reduced by the CdS shell, and inversely proportional to the nonradiative rate constants, knr. As seen in Table 1, the CdSe/ZnS core/shell NCs have an order of magnitude lower knr compared to the other NCs. The ZnS shell enhances ΦUC from 5.7% (CdSe core) to 8.1% and 9.3% for the CdSe/(1 ML)ZnS and CdSe/ (0.5 ML)ZnS core/shell NCs, respectively. However, NCs with CdS shells ranging from 0.25 to 3.4 MLs have lower ΦUC, with the highest ΦUC less than half that of the CdSe core. ΦUC C

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Figure 3. Femtosecond transient absorption (TA) spectra of (a) CdSe; (b) CdSe/(0.5 ML)CdS; and (c) CdSe/(0.5 ML)ZnS nanocrystals (NCs); (d−f) TA spectra of the same NCs with surface-bound 9-anthracenecarboxylic acid. Samples are dissolved in hexane and excited with a 540 nm pulsed laser. The black dashed curves in (a) and (c) are the linear absorption spectra of CdSe core and 0.5 ML CdSe/ZnS core−shell NCs, and the magenta and black dashed curves in (b) are that of CdSe core and 0.5 ML CdSe/CdS core−shell NCs, respectively.

Figure 4. (a) Initial ground state bleach at 523 nm for CdSe, CdSe/(0.5 ML)ZnS, and CdSe/(0.5 ML)CdS with and without surface-bound 9ACA. The curves for NCs only are normalized with the initial minimum OD to be −1, and curves for NCs capped with 9-ACA are normalized based on the NC concentration, i.e., the OD at 540 nm in the linear absorption spectra. Green curves are the global fits to the recovery of the GSB at 433 nm (averaged over 430−435 nm) and 523 nm (averaged over 520−525 nm) for CdSe//9-ACA and 0.5 ML CdSe/ZnS//9-ACA. Measurements were performed in hexane at room temperature, with the excitation of a 540 nm pulsed laser. (b) Energy diagram describing the physical processes during TA measurements.

for each NC is optimized by varying N, the average number of surface-bound 9-ACA ligands per NC (see Figure S2). The highest ΦUC with the corresponding N are presented in Table 1 and Figure 2c. ΦUC reaches a maximum at 0.5 ML of ZnS and 1.8 ML of CdS and then decreases as the shell thickness increases because the tunneling barrier slows down the triplet energy transfer from the CdSe core to 9-ACA. A similar trend is observed for triplet transfer from PbS/CdS core−shell NCs to rubrene.23 In our hands, it was challenging to grow ZnS shells less than 0.5 ML on these 2.9 nm diameter cores; thus the highest reported ΦUC is for the CdSe/(0.5 ML)ZnS NCs at 9.3%. Note that in this work ΦUC is defined as

ΦUC = 2Φref × ×

(photons absorbed by reference) (photons absorbed by UC sample)

PL signal(UC sample) PL signal(reference)

where Φref is the fluorescence QY of the reference rhodamine 6G. The factor of 2 in the equation means ΦUC = 100% when 50 upconverted photons are produced for every 100 photons absorbed. Transient absorption studies reveal the dynamics of the photoinduced electronic transitions in the presence of the CdS and ZnS shells. Our TA investigation focuses on the CdSe core D

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corresponding to excitons delocalized in the shell is 68% quenched within the 3 ns window, without sensitization of the molecular triplet state. No rise channel is observed at 433 nm, the pseudoisosbestic point for these NCs that coincides with the T1−Tn transition of bound 9-ACA.46 This is consistent with the low ΦUC of 0.31%. The presence of 9-ACA quenches the exciton on the CdS shell more efficiently than that on the core, probably by better coupling of the CdS shell to molecular vibrations. TA measurements confirm that the ZnS shell removes the surface-based trap states. The TA difference spectrum of CdSe/ZnS core/shell NCs (Figure 3c and f) has similar ESA and GSB features to the core CdSe NCs (Figure 3a and d), with the exception of the most red-shifted positive absorption peak (560 nm). This feature is typically associated with trap states47−50 and is decreased with the ZnS shell (Figure 4b). For both the CdSe core and the CdSe/ZnS core/shell NCs, the presence of the 9-ACA transmitter ligand results in a faster recovery of the GSB and decay of the ESA that can be correlated with triplet energy transfer. For both, ΦUC from linear upconversion measurements correlates with the rate constant (kTET) and efficiency (ΦTET) of triplet energy transfer obtained from TA spectroscopy. Since the T1−Tn transition of bound 9-ACA is centered at 433 nm,46 we monitor the kinetics at 433 nm to extract the kTET and ΦTET (see SI for details). Global fitting of the kinetics at 433 nm and GSB at 523 nm for CdSe//9-ACA and CdSe/ZnS//9-ACA (Figure 4b) provides kTET, kTET per 9-ACA ligand, and ΦTET (Table 2). Analysis at 523 nm was performed to eliminate artifacts from the scatter of the 540 nm excitation beam at that GSB. Note that ΦTET is calculated from the OD and extinction coefficient of the 9ACA triplet and GSB of NCs. ΦTET could be underestimated due to the limited accuracy of the extinction coefficients. The ZnS shell enhances kTET from 4.97 × 108 s−1 to 6.67 × 1010 s−1 and ΦTET 1.9 times from 6.68% to 12.9%, which is close to the 1.6 times increase of ΦUC from 5.7% to 9.3%. This makes sense because ΦUC = ΦTET × ΦTTA(DPA) × ΦF(DPA) where ΦTTA(DPA) and ΦF(DPA) are the efficiency of triplet−triplet annihilation and fluorescence quantum yield of DPA, respectively. As ΦTTA(DPA) and ΦF(DPA) are constants, ΦUC is proportional to ΦTET. The surface coverage of 9-ACA is about the same for both the CdSe core and CdSe/ZnS core/shell NCs, and when kTET is normalized by N (N = the number of bound 9-ACA), kTET/N is 1.89 × 108 s−1 and 3.63 × 1010 s−1, respectively (Table 2). Besides trap state passivation, the ZnS shell reduces exciton−phonon coupling when 0.5 and 1 ML are applied.5 The CdSe NC’s exciton is less available for triplet transfer when it couples with phonon relaxation pathways, which may be minimized in the presence of a sub-monolayer thick shell of ZnS. Considering the bulk modulus of wurtzite ZnS, CdS and CdSe is 82.3, 54.0, and 44.1 GPa,51−54 respectively, the relative incompressibility of ZnS compared to CdS results in NCs that have a higher PLQY for core/shell NCs with the same shell thickness (Figure 2a). The same trend is routinely observed in molecules where a more rigid aromatic framework results in a higher fluorescence QY due to reduced exciton−vibrational coupling. In our previous work, we showed that the growth of CdS shell on PbS NCs enhances TET to molecular acceptors by eliminating exciton decay pathways associated with surface trap states.23,55 The passivation of trap states on the PbS core with a CdS shell there increased both the photon upconversion and photoluminescence quantum yields. Here, a CdS shell on a CdSe core increases photoluminescence but decreases photon

(ΦUC = 5.7%), CdSe/(0.5 ML)ZnS (ΦUC = 9.3%), and CdSe/ (0.5 ML)CdS (ΦUC = 0.31%) NCs capped with 9-ACA dissolved in hexanes. For these three samples, there are on average two 9-ACA transmitter molecules bound per NC. We chose CdSe/(0.5 ML)ZnS core−shell NCs because they have the highest upconversion QY and chose its Cd counterpart with the same shell thickness for comparison. Samples were excited with a 540 nm femtosecond laser at low power (220 nJ) to avoid the creation of more than one exciton per NC (Figure S5). Figure 3 shows the TA difference spectra of the CdSe core, CdSe/(0.5 ML)ZnS, and CdSe/(0.5 ML)CdS core−shell NCs with and without surface-bound 9-ACA. The positive features are assigned to the excited state absorption (ESA), and the negative features to the ground state bleach (GSB) of the NCs. The latter corresponds to maxima in the linear absorption spectra (dashed lines in Figure 3a,b,c). No evidence of charge transfer corresponding to the 9-ACA radical cation or anion was observed. TA spectra show that the CdSe/CdS core/shell NCs have virtually no excited state absorption, indicating sub-picosecond depletion of the exciton (Figure 3b and e). This is clearly seen in the 3 ps window in Figure 4a, where the kinetics of the GSB at 523 nm for the three sets of NCs with and without the surface-bound 9-ACA are shown. In principle, the population of photoexcited CdSe NCs should be proportional to the concentration of NCs. Therefore, the initial amplitude at time zero for the GSB at 523 nm should be the same after normalizing by the linear absorption at 540 nm (the excitation wavelength). However, it is considerably smaller with the CdS shell. If we define the decrease of the initial amplitude, ΔXB, as ΔXB =

ΔA 0(NC) − ΔA 0(NC/9‐ACA) ΔA 0(NC)

where ΔA0 is the absorption at 523 nm at time zero for different samples, then ΔXB ≈ 5−8% for CdSe and CdSe/ZnS core/shell NCs, but is significantly higher at ΔXB = 25% for the CdSe/CdS core/shell NCs. As can be seen in Table 2, ΔXB Table 2. Rate Constant of Triplet Energy Transfer (TET), kTET; TET Efficiency ΦTET; and the Relative Decrease of Initial Exciton Bleach of CdSe NCs Monitored at 523 nm Due to 9-ACA, ΔXB NC

kTET (s−1)

kTET/N (s−1)

ΦTET (%)

ΔXB (%)

CdSe 0.5 ML CdSe/ZnS 0.5 ML CdSe/CdS

4.97 × 10 6.67 × 1010

1.89 × 108 3.63 × 1010

6.68 12.9

4.85 7.71 24.5

8

negatively correlates with ΦUC. ΔXB reflects the sub-picosecond recovery of the exciton bleach that might arise from ultrafast charge transfer from NCs to 9-ACA. The resolution of our TA measurements (200 fs) is not high enough to capture the trajectory of this ultrafast decay. Compared to CdSe and CdSe/ZnS NCs, CdSe/CdS NCs have a dramatically different TA spectrum with the absence of ESAs at 471 nm and GSBs at 501 nm and the presence of a new GSB bathochromically shifted from the core at 560 nm. Electronic transitions corresponding to excitons confined to the CdSe core and delocalized to the CdS shell are observed (Figure 3b). The CdSe core has a GSB at 446 and 540 nm (linear absorption in magenta), while the CdS shell has a GSB at 470 and 560 nm (linear absorption in black). In the presence of the 9-ACA transmitter ligand, the GSB at 560 nm E

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explained by relatively strong exciton−phonon coupling and decreased exciton lifetimes that deplete the density of excited states responsible for triplet sensitization. This work provides a guide for designing core−shell NCs that have enhanced triplet energy transfer for purposes of imaging and solar energy conversion.

upconversion. Clearly, with a CdS shell, certain surface trap states are passivated for both the PbS and CdSe core, thus increasing PL, but exciton−phonon coupling dissipates the photogenerated excited states with triplet character for CdSe− CdS core−shell NCs. Our data show that the CdS shell is detrimental for photon upconversion because its exciton is rapidly depleted at subpicosecond time scales, most likely thorough rapid charge transfer, and then again at nanosecond time scales when bound with the 9-ACA anthracene transmitter, without triplet energy transfer. Cui and Bawendi et al.5 have reported stronger exciton−longitudinal optical phonon coupling in CdSe/CdS compared with CdSe/ZnS core−shell NCs. This is because the exciton in CdSe/CdS core−shell NCs is delocalized to the shell, as observed in the linear absorption and TA difference spectrum here. This decreases electron−hole wave function overlap compared to the CdSe core and CdSe/ZnS core−shell NCs, polarizes the NC, and enhances exciton−phonon coupling. In addition, delocalization of excitons in CdSe/ CdS NCs to the shell increases the chance of sampling the surface-based defects, inducing more polarization and stronger exciton−phonon coupling at the expense of triplet energy transfer. At the nanoscale, coupling between electronic excited states and lattice fluctuations can change the emission maxima by as much as 90%, as shown by temperature-dependent measurements on CdSe/CdS core/shell NCs.4 While the reduction in driving force for triplet energy transfer arising from exciton delocalization results in lower upconversion QYs,56 since the photon upconversion QYs from these CdSe/ CdS NCs are up to an order of magnitude lower than with CdSe NCs having the same absorption/emission maxima,57 losses due to exciton−phonon coupling explain the diminished photon upconversion QY. Although all the core/shell NCs have increased radiative rates, only sub-monolayer ZnS shells improve photon upconversion QYs. We propose that CdSe/ZnS core/shell NCs with thicker shells that have more surface ligands (i.e., N > 9, the maximum number here) may show increased photon upconversion QYs in the presence of more triplet acceptors. This hypothesis is based on the observation that the net rate of hole transfer from CdSe/CdS core−shell NCs to surfacebound ferrocene for 7 ML CdS shells is higher than that for 3 and 5 MLs because of the higher surface loading of ferrocene acceptors.27 In terms of molecular design, bulky transmitters may be necessary to avoid triplet−triplet annihilation between neighboring ligands for photon upconversion.

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METHODS Synthesis of Nanocrystals. CdSe cores with a diameter of 2.9 nm were synthesized following the procedure published by Carbone et al.58 CdSe/CdS32 and CdSe/ZnS33 core−shell NCs were synthesized by following the methods published by the Bawendi group. The amount of precursor was calculated based on the size and concentration of the CdSe core,59 the expected shell thickness, and the bulk density of CdS and ZnS.41 The extinction coefficient of core−shell NCs at absorption maxima are taken from that for the CdSe core.59 Sample Preparation for Photon Upconversion, Lifetime, and Transient Absorption Measurements. 9-ACA transmitter ligands were bound on NCs via ligand exchange in a glovebox: NCs and 9-ACA were dissolved in tetrahydrofuran, resulting in a mixture with a total volume of 500 μL containing 10 μM NCs. The concentration of 9-ACA was varied as shown in Figure S2. The mixture was stirred for 12 h at room temperature. A 1 mL amount of acetonitrile as bad solvent was added to the ligand exchange solution to crash out the NC/9ACA complex by centrifugation at 7830 rpm for 15 min. Then the clear supernatant was discarded. For photon upconversion measurements, the pellet was redispersed in 1 mL of 2.15 mM DPA in hexanes. For lifetime and transient absorption measurements, the pellet was redispersed in hexane. The photon upconversion and lifetime measurements were done with a 1 × 1 cm cuvette, and transient absorption measurements with a 2 mm path length cuvette. All samples were prepared in an argon glovebox.

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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/acsphotonics.8b00338.

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CONCLUSIONS In conclusion, triplet energy transfer from the CdSe exciton competes with both radiative and nonradiative loss mechanisms. Shell growth improves triplet energy transfer from CdSe donor to anthracene acceptors for three reasons. First, the ZnS shell minimizes the contribution of the fastest decay component comprising the NC lifetime, allowing triplet transfer to compete. Second, it decreases the nonradiative rate, knr, by chemically passivating the surface. Lastly, it minimizes exciton−phonon coupling with its larger bulk modulus and promotes wave function overlap between electron and hole with the type I electronic structure. The best performing CdSe/ZnS core−shell NCs with an average of 0.5 ML shell thickness enhance the photon upconversion QY 1.6 times from 5.7% to 9.3%. In contrast, the poor photon upconversion QYs in the presence of the CdS shell can be

Experimental details, photon upconversion quantum yields, time-resolved photoluminescence and transient absorption spectra, and fitting parameters (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhiyuan Huang: 0000-0003-4180-0234 Pan Xia: 0000-0001-8615-3590 Ming Lee Tang: 0000-0002-7642-2598 Author Contributions

M.L.T. conceived the project. Z.H. performed the synthetic experimental work and linear optical experiments. D.F. performed the transient absorption experiment. Z.H., P.X., and N.M. analyzed transient absorption data. M.L.T., Z.H., and V.V. wrote the manuscript. Notes

The authors declare no competing financial interest. F

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ACS Photonics

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ACKNOWLEDGMENTS M.L.T. is grateful to the Alfred P. Sloan Foundation. M.L.T., D.F., and V.V. acknowledge the National Science Foundation (1351663, 1532125, and 1465284, respectively).

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