Challenges to Thick-Shell Synthesis in Cdse/Znse and Znse/Cds Core/Shell Quantum Dots

Supporting Information

Elucidation of Two Giants: Challenges to Thick-shell Synthesis in CdSe/ZnSe and ZnSe/CdS Core/Shell Quantum Dots

Krishna P. Acharya†, Hue M. Nguyen†, Melissa Paulite,† Andrei Piryatinski‡, Jun Zhang§, Joanna L. Casson§§, Hongwu Xu§, Han Htoon†, and Jennifer A. Hollingsworth†*

†Materials Physics and Applications Division: Center for Integrated Nanotechnologies; ‡Theoretical Division: Physics of Condensed Matter & Complex Systems; §Earth & Environmental Science Division: Earth System Observations, §§Chemistry Division: Physical Chemistry & Applied Spectroscopy, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States.

*[email protected]

Table of contents 1 Experimental ...... 3 1.1 Chemicals ...... 3 1.2 CdSe/ZnSe QDs synthesis ...... 3 1.2.1 Synthesis of different types of CdSe core ...... 3 1.2.2 Zn and Se precursor synthesis ...... 4 1.2.3. CdSe/ZnSe synthesis using SILAR ...... 5 1.3 ZnSe/CdS QDs synthesis ...... 5 1.3.1 Synthesis of ZnSe core ...... 5 1.3.2 Preparation of cadmium and sulfur precursor ...... 6 1.3.3 Synthesis of ZnSe/CdS using SILAR ...... 6 1.3.4 Synthesis of single source precursor and CdS growth ...... 7 2 Characterization ...... 7 2.1 Optical, morphology, crystallography, and composition ...... 7 2.2 Single-dot optical properties ...... 8 2.2.1 Blinking and g(2) measurements ...... 8 2.2.2 Blinking statistics measurements ...... 9 3 Discussion ...... 9 3.1 Extended discussion of factors influencing alloying or core/shell growth in the CdSe/ZnSe system 3.2 Discussion pertaining to lower photoluminescence quantum yields (QYs) obtained for lower- temperature shell growth in the ZnSe/CdS system 4 Supporting Figures ...... 11 Figure S1: Composition of ZnCdSe alloy...... 11 Figure S2. XRD patterns of different-phase CdSe cores and their products ...... 12 Figure S3. High pressure X-ray diffraction patterns ...... 13 Figure S4. TEM and composition of ZnSe/CdS grown using single source precursor ...... 14 Figure S5: Single-dot optical properties of CdSe/ZnSe 15

Figure S6: Single-dot optical properties of ZnSe/CdS ...... 16 5 References ...... 17

1 Experimental

1.1 Chemicals

Octadecylamine (ODA, 90%, Aldrich), diethylzinc (Et2Zn, 1.5 M solution in toluene, Acros) cadmium oxide (99.95%, Alfa Aesar), zinc oxide (99.99%, Aldrich), sulfur (99.999%, Acros), selenium (99.999%, Aldrich), diphenylphosphine (DPP, 98%, Aldrich), oleic acid (OA, 90%, Alfa Aesar), trioctylphosphine (TOP, 90%, Fluka), trioctylphosphine oxide (TOPO, technical grade, 90% Acros), oleyl amine (technical grade, 70%, Aldrich), 1-octadecene (ODE, technical grade, 90% Acros), n-octadecane (OD, 90%, Alfa Aesar), oleylamine (OLAM, 70%, Aldrich), NaOH pellets (97%, Fisher), myristic acid (Aldrich, 99%), cadmium acetate dihydrate (98%, Acros), diethyldithiocarbamic acid sodium salt (ACS reagent, Acros), octadecylphosphonic acid (ODPA, Alfa Aeser), selenium (IV) oxide (99.99%, Acros), butanol (99%, extra dry, Acros), methanol (anhydrous, 99.8%, Acros), hexane (anhydrous, 99.9%, Acros), acetone (99.8%, Optima), ethanol (anhydrous, Fisher). All chemicals were used as received without any further purification, and all syntheses were performed under Ar atmosphere using standard air free Schlenk technique.

1.2 CdSe/ZnSe quantum dot (QD) synthesis 1.2.1 CdSe core syntheses Zinc-blende CdSe QDs were synthesized using a published procedure with slight modifications.1 To make cadmium myristate, 2 mmol cadmium acetate dihydrate was dissolved in 20 mL anhydrous methanol. Sodium myristate was prepared by mixing 6 mmol sodium hydroxide and 6 mmol myristic acid in 200 mL anhydrous methanol. Then, cadmium acetate solution was added dropwise into the sodium myristate solution with vigorous stirring. The resultant white precipitate of cadmium myristate was separated by centrifugation and washed with methanol and finally dried in vacuum overnight. To prepare the CdSe QDs, 0.4 mmol cadmium myristate was mixed with 0.4 mmol selenium (IV) oxide and 25 mL 1-ODE in a three-neck flask fitted with a reflux condenser. The mixture was heated to 240º C. At this point, 0.4 mL oleic acid was added dropwise, and the reaction was monitored by following changes in absorption spectra with time. The reaction was thermally quenched once the desired CdSe QD size was reached. The reaction mixture was mixed with 40 mL acetone and QDs were separated using centrifugation followed by re-dissolution in hexane.

Wurtzite CdSe QDs were synthesized using a published procedure.2 0.180 g (1.401 mmol) CdO, 0.84 g ODPA and 9 g TOPO (99%) were mixed and degassed at 150 °C for 1 h under active vacuum. The atmosphere was then switched to Ar and the temperature raised to 320 °C. At this point, 3 mL TOP was added and the temperature increased to 380 °C. At 380 °C, 1.40 mL 1 M TOP-Se (1.401 mmol) was swiftly injected. The reactor was allowed to cool and the CdSe was cleaned using acetone/hexane precipitation, followed by re-dissolution of the QDs in hexane.

Mixed-phase CdSe QDs were synthesized using a published procedure.3 1 g TOPO, 8 mL 1- ODE and 0.38 mmol Cd-oleate in a three neck flask fitted with a reflux condenser were degassed at 80 °C for 1 h under active vacuum. The atmosphere was then switched to Ar and the temperature raised to 300 °C. At this point, Se precursor containing 4 mmol TOP-Se, 1 mL 1- ODE, and 3 mL oleylamine was swiftly injected into the hot cadmium precursor solution. The reaction was monitored by following changes in absorption spectra with time. The reaction was thermally quenched once the desired CdSe QD size was reached. The QD was purified using ethanol/hexane precipitation, followed by re-dissolution of the QDs in hexane.

Powder X-ray diffraction (XRD) patterns and transmission electron microscopy (TEM) images of all three CdSe QDs are shown in Figure S2.

1.2.2 Zn and Se precursor syntheses 0.2 M zinc oleate solution (ZnO:OA ratio of 1:10) was prepared by dissolving 1.6286 g (0.02 mole) ZnO in 63 mL oleic acid and 37 mL 1-ODE in a three-neck flask. The flask was degassed at 80 °C under active vacuum for 30 minutes and heated further to obtain an optically clear zinc oleate solution. The temperature was then raised to 100 ºC and the atmosphere switched to Ar.

Although tertiary phosphine chalcogenides are commonly used for II-VI and IV-VI QD synthesis, secondary phosphines may in fact be present as byproducts and are thought to be responsible for formation of reactive precursors to QD nucleation. It is also known that DPP-Se reacts with metal carboxylate even at room temperature and forms crystalline CdSe at only 200 °C and PbSe at 80 °C. For these reasons, we employed DPP-Se as the selenium source for ZnSe shelling. DPP-Se was synthesized using a previously reported method.4 20 mmol Se shot is mixed with 20 mmol DPP and 25 mL anhydrous toluene in a three-neck flask fitted with a condenser and stirred (inside an inert atmosphere glovebox) until the Se becomes a turbid, white

mixture. This product was then refluxed at 110 ºC for 7 h and, finally, left overnight at room temperature under continuous Ar. The liquid portion was vacuum evaporated and the solid dissolved in hot toluene. Refrigeration overnight facilitated crystallization. The supernatant was then removed and the DPP-Se vacuum dried. To prepare 0.2 M DPP-Se, 1.325 g DPP-Se was dissolved in 25 mL 1-ODE and degassed at room temperature for 30 minutes. Then, the temperature was raised to 100 ºC under Ar flow to obtain a clear solution. This temperature was maintained for the injection.

1.2.3. CdSe/ZnSe core/shell QD synthesis using SILAR The CdSe core QD diameter and molar extinction coefficient were determined using previously established procedure5 and used to obtain the QD concentrations. The ZnSe shell was grown on CdSe cores by successive ionic layer adsorption and reaction (SILAR) method.6 For this, the amount of shell precursor required for each injection (to achieve 1 monolayer, ML, of shell growth) was calculated according to a previously published method for cubic ZnS growth.7 In a typical SILAR process, a three-neck round bottom flask equipped with a condenser was charged with 1×10-7 mole CdSe core, 5 mL 1-ODE, and 5 mL OLAM. This mixture was degassed at 80 º C for 30 min (removing hexanes) and subsequently switched to an Ar atmosphere. Then, the reaction temperature was raised to 240 ºC and the calculated amount of zinc oleate required for the first ML of shell growth was injected. For so-called “long anneal” SILAR, the mixture was allowed to react for 2 h (10 min for “short anneal” SILAR) before the Se precursor was added. After DPP-Se injection, 1 h was allowed for “long” SILAR (10 min for “short”) before the Zn precursor for ML 2 was injected. This SILAR process was repeated to until the desired number of shell MLs had been added. Energy-dispersive x-ray spectroscopy (EDX) analysis was used to assess elemental composition of the products. EDX data for the product of “long anneal” SILAR for zinc blende CdSe cores are shown in Figure S1. XRD patterns of the products obtained for each of the three CdSe core types following addition of Zn and Se precursor sufficient for 12 MLs of ZnSe shell (240 °C, both long and short SILAR) are also shown in Figure S2.

1.3 ZnSe/CdS core/shell QD synthesis

1.3.1 Synthesis of ZnSe QDs ZnSe QDs were synthesized according to reported procedures with some modification.8 Briefly, 5.0 g octadecylamine is degassed for 1 h at 120 °C in a three-neck flask fitted with a reflux

condenser. The flask is switched to Ar and the temperature increased to 300 °C. At this point, a mixture containing 0.3 mmol TOP-Se (0.3 mL 1 M TOP-Se) and 0.8 mL TOP was injected. When the temperature of the reactor reached 300 °C, a mixture containing 0.3 mmol diethylzinc

(0.2 mL 1.5 M Et2Zn in toluene) and 0.8 mL TOP was injected. The reaction was allowed to proceed for 10 min at 300 °C, resulting in ZnSe QDs with the 1S absorption peak at ~366 nm. Larger ZnSe QD cores were obtained adding additional precursor in a controlled, continuous fashion. The precursors mixture in this case contained 1:1 moles Et2Zn and TOP-Se in TOP and

ODE [1.8 mL TOP-Se, 4 mL TOP (90%), 4 mL ODE, 1.2 mL 1.5 M Et2Zn in toluene]. The absorption spectra were monitored and the heating mantle removed when the desired-size ZnSe QDs were obtained. ZnSe cores were cleaned in an inert atmosphere glovebox to avoid any surface oxidation prior to shell growth. Specifically, the solidified reaction mixture was transferred to a glovebox, heated to liquify followed by the addition of 15 mL freeze-pump-thaw degassed butanol. ~10 mL of this mixture was then removed (handling air-free) and anhydrous hexane was added to obtain an optically clear solution. Sufficient anhydrous MeOH was then added to precipitate all of the ZnSe QDs followed by centrifugation. The ZnSe precipitate was dissolved in hexanes and stored inside a glovebox freezer.

1.3.2 Preparation of cadmium and sulfur precursors 0.2 M cadmium oleate with CdO:OA molar ratio of 1:10 was prepared by mixing 2.5 g CdO in 63.2 mL oleic acid and 32.8 mL octadecane. The mixture was degassed at 80 °C for 2 h, switched to Ar and the temperature raised to 280 °C. The mixture was heated until all of the CdO dissolved and an optically clear solution of cadmium oleate was obtained. The temperature was lowered to 80 °C and the flask contents again degassed for 40 min to remove water formed during the acid-base reaction. Similarly, 100 mL of 0.2 M S solution was prepared by degassing 0.641 g S in 100 mL 1-ODE for 1 h at 80 °C. Then, the temperature was increased to180 °C (under Ar) to dissolve all S, after which the solution was immediately cooled to room temperature.

1.3.3 Synthesis of ZnSe/CdS using SILAR To calculate the concentration of the ZnSe cores, the molar extinction coefficient (ε) for 3.7 nm ZnSe QDs (absorption peak at 390 nm) was adapted from literature (ε = 1.8 ×105 L .mol-1 cm-1).9 Then, ε values for ZnSe QDs of similar size were approximated using a linear dependence of size and ε. Synthesis of ZnSe/CdS core/shell QDs was carried out using the SILAR method. For

this, the thickness of one ML of CdS was taken as 0.35 nm, which is half of the wurtzite lattice parameter along the c axis. The amount of precursors required for various MLs were determined using the calculated shell volume and known CdS density. In a typical ZnSe/CdS synthesis using SILAR, a mixture containing 5 mL OLAM and 5 mL 1-ODE was degassed for 1 h at 80 °C, then the flask was switched to Ar followed by injection of 2×10-7 mole ZnSe cores in hexane. The mixture was further degassed (20 min) to remove hexane. The temperature was then increased to 240 °C. The pre-determined quantity of 0.2 M cadmium oleate was injected dropwise followed by a 2 h anneal (again, 10 min for short SILAR) before the S precursor injection. After another 1 h (10 min for short SILAR), the second ML of Cd precursor was added. This process was repeated to obtain the desired number of CdS MLs. Note: overnight, reactions were maintained at room-temperature, with stirring and under continuous-flow Ar.

1.3.4 Synthesis of single source precursor and CdS growth

Cadmium diethyldithiocarbamate [Cd(DEDTC)2 ] single-source (SS) precursor was synthesized by drop-wise addition of aqueous sodium diethyldithiocarbamate (20 mmol in 60 mL water) into an aqueous solution of cadmium acetate (10 mmol in 100 mL water) with vigorous stirring. The white precipitate was separated by centrifugation, washed and vacuum dried.10 To prepare

ZnSe/CdS QDs using the SS precursor, an amount of 0.2 M Cd(DEDTC)2 in 1-ODE was added to ZnSe cores in OLAM and 1-ODE equivalent to obtain a desired number of shell MLs. The reaction mixture was annealed for 20 min for each ML-equivalent of CdS precursor, i.e., to mimic the total time allowed for short-anneal SILAR. Similarly, to replicate long-anneal SILAR times, the SS precursor can be added very slowly over the equivalent total time (3 h per ML). TEM and EDX data with elemental quantifications are shown in Figure S4.

2 Characterization 2.1 Optical, morphology, crystallography, and composition Absorption spectra were recorded using a CARY UV-Vis-NIR spectrophotometer and Thermo Scientific NanoDrop UV-Vis Spectrophotometers, emission spectra were recorded using a Horiba Nanolog and NanoDrop Fluorospectrometers. Transmission electron microscopy (TEM) images were acquired using a JEOL 2010. Ultra-dilute QD solutions in hexane were dropped onto a carbon-coated Cu grid and dried in air for TEM analyses. XRD measurements were obtained using a Rigaku Ultima III diffractometer that employs a Cu Kα (λ = 1.5406 Ǻ) X-ray

source. The powder sample was kept on an off-axis cut backgroundless silicon wafer and the data were collected in continuous scan mode using parallel beam slit geometry over the 2Ɵ from 10 to 90º. Elemental analysis was carried out by energy dispersive X-ray spectroscopy (EDX analysis) using an FEI Quanta 400 FEG scanning electron microscope (SEM). Some of the ZnSe/CdS core/shell samples were also analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES; Galbraith Laboratories). High-pressure X-ray diffraction experiments were carried out on wurtzite CdSe cores and CdSe/ZnSe (5 ML) core/shell samples using a diamond anvil cell.

2.2 Single-QD optical properties 2.2.1 Blinking and g(2) measurements Single-QD optical measurements were conducted on a home-built confocal microscope with a 40x, numerical aperture (NA) 0.55 air objective. Single-QD-level samples were prepared by drop-casting very dilute hexane solutions of QDs onto cleaned glass coverslips. The laser beam was focused to a nearly diffraction limited spot and single QD emission was collected by a 100x, NA 1.3 oil immersion objective (for better collection efficiency). QD emission was detected by a pair of fast avalanche photodiodes in Hanbury Brown–Twiss detection scheme. Besides using a long pass filter to reject laser scatter, 775 nm short pass filters were placed in front of each detector to minimize cross-talk. Signals from the detectors were recorded by a HydraHarp400 time-correlated single-photon counter (TCSPC) module for time-tagged time-resolved (TTTR) data with 32 ps time resolution. TTTR data was processed by a custom-written IgorPro routine to obtain the second-order photon-correlation function g(2)(τ), fluorescence intensity trajectories (100 ms binning time) and photoluminescence decay curves of the QD. To ensure that g(2) values are mainly contributed to by bi-exciton emission, we employed sufficiently low pump-excitation powers to maintain average carrier populations below 0.2 electron-hole pairs per dot.11 Furthermore, to verify that we were investigating single gQDs rather than small clusters of emitters, we used a time-gated g(2) approach as described previously.12 Finally, shell thickness dependent blinking traces of CdSe/ZnSe QDs synthesized at 240 °C are shown in Figure S5.

Explanatory note: g(2)(0) (second-order photon-correlation function at zero time delay) measures the distribution of time intervals between two sequential photon detection events. For single-

photon emitters, g(2)(0) is 0, while for multi-photon emitters g(2) values are higher. This value is a reflection of the relative bi-exciton quantum yield (i.e., the ratio of bi-exciton to single-exciton quantum yield). The observed g(2) for the new CdSe/ZnSe gQDs of 0.4 implies efficient bi- exciton emission and, thereby, suppressed non-radiative Auger recombination that would otherwise quench charge-state and multiple-exciton-state emissions.

2.2.2 Blinking statistics determination To obtain blinking statistics for many QDs simultaneously, standard wide-field micro- photoluminescence experiments were conducted. Freshly diluted QDs in hexane solution were dispersed onto a clean glass slide. Sample was excited by a 405 nm 40 mW continuous wave (cw) laser focused to an ~100 µm diameter spot. Multiple excited QDs over an area of ~40 x 40 µm were imaged onto a liquid-nitrogen-cooled CCD camera. A long pass filter was placed in the beam path in front of the CCD camera to block laser scatter and reduce background. A series of 18000 picture frames were recorded in 1 h. After that, intensity trajectories with temporal resolution ~ 200 ms (100 ms integration time + 100 ms readout time) of each QD were extracted using a custom-written IgorPro routine. From this data “on-time” statistics of the QDs were determined. The percentages of the total QD population that exhibit on-times ≥20, 50, 80 or 99% were calculated, respectively. QDs with on-time fraction >99% are considered to be non-blinking (Figure S5).

3 Discussion 3.1 Extended discussion of factors influencing alloying or core/shell growth in the CdSe/ZnSe system The similarity in results obtained for the different cubic- and hexagonal-phase cores (main text, Figure 2e) suggests that core crystal structure is not the determining factor influencing the extent of cation diffusion and mixing during SILAR shell addition. Furthermore, the experimentally determined Cd:Se ratio for all three cores is ~1:1. For this reason, it also does not appear that QD stoichiometry is playing a significant role in promoting or suppressing alloying during shell growth. We propose, instead, that surface effects may dominate in the alloying process. In addition to having different crystal structures, the various CdSe cores are prepared using different coordinating/stabilizing ligands. The distinguishing feature of the mixed-phase core is the use of a primary amine in its preparation. Ligand-mediated surface-driven crystalline-phase transformations13 and particle shape control3 are known, and have been described previously in

the context of amine effects on the core and core/shell product when different types of amines (comparing 1°, 2° and/or 3°) are used in the shell-growth medium. In our case, for which identical conditions (e.g., ligand identities and concentrations) are employed for SILAR shell growth, any ligand effect would have to result from differences in the ligands employed during QD core synthesis. It is less clear how such ligand/surface effects could carry through from the QD core preparation to the SILAR shell-growth reaction. Nevertheless, all else being the same— including the use of a primary amine for shell growth—the mixed-phase CdSe cores are uniquely robust to cation-mixing. The mechanism by which the inclusion of a primary amine in the core QD synthesis might influence the shell growth process is not currently known, but may result from the ability of the amine to tune the binding mode of the other ligands in the core synthesis reaction. Namely, as we discussed in SI Ref. 3, the amine can serve two roles: coordinating ligand or base. As a base, the amine deprotonates the oleic acid (or other fatty acid) to form oleate, which can form a strong covalent bond with the QD surface Cd atoms. It is possible that this base-assisted strong ligand coordination in the case of the mixed-phase CdSe cores contributes to the relatively lesser tendency of this core type to form alloys. We also investigated the “post-synthesis” stability of the core/shell products to alloying – both at high temperature and high pressures (main text Fig. 2f,g). We found that once formed the core/shell structures were relatively stable to both conditions (temperature and pressure). Thus, with respect to temperature, ‘post synthesis’ core/shell stability was enhanced compared to ‘during growth’ stability. During shell growth, the shell-precursor ad-atoms interact with the QD surface and they do so in the presence of surface-bound and solvated ligands, where the ligands can influence this process by, e.g., competing with ad-atom addition, stabilizing the surface- bound ad-atom, and/or mediating reversible addition/re-dissolution processes. In contrast, ‘post synthesis,’ the core/shell (Cd/Zn) interface is buried within the structure. It appears that in that condition, i.e., an ‘internal interface,’ Cd-Zn mixing (or alloying) is less favored. Hence, we hypothesize that surface-related processes active during shell growth can strongly impact whether shell addition or alloying occurs. 3.2 Discussion pertaining to lower photoluminescence quantum yields (QYs) obtained for lower-temperature shell growth in the ZnSe/CdS system Use of lower reaction temperatures during shell growth may limit “defect annealing,” i.e., temperature-induced healing (re-positioning of ad-atoms into preferred crystallographic sites) of

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the core-shell interface during growth. However, lower QYs may also simply be characteristic of the core/shell structure, with higher QYs favored by a multishell product (ZnSe/CdSe/CdS) or an alloyed-core product (ZnCdSe/CdS), perhaps due to reduced lattice mismatch in the latter cases.

4 Supporting Figures

Figure S1. Composition of ZnCdSe alloy. EDX spectrum of ZnCdSe alloy formed after SILAR addition of 12 monolayers of Zn and Se shell precursors at 240 °C and using long anneal times. Starting core was zinc-blende CdSe. Inset shows the elemental composition of the alloy.

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Figure S2. Powder X-ray diffraction (XRD) patterns of CdSe cores and the products formed after SILAR addition of 12 monolayers (MLs) of ZnSe shell precursors (240°C): (a) zinc blende CdSe core (black trace), short-anneal-time SILAR (red trace), long-anneal-time SILAR (blue trace). Bottom red and top blue stick patterns are bulk zinc blende CdSe and ZnSe, respectively. (b) Wurtzite CdSe core (bottom black trace); short SILAR (red) and long-SILAR (blue). Bottom red and top blue stick patterns are bulk wurtzite CdSe and ZnSe, respectively. (c) Mixed-phase CdSe core (bottom black trace), short SILAR (red) and long SILAR (blue). Bottom red and top blue stick patters are bulk zinc blende CdSe and ZnSe, respectively. Insets show the corresponding TEM images of the CdSe cores.

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(a) (b) (c) Decompressed Decompressed 0.4 GPa

3.0 GPa 2.1 GPa 10.7 GPa 3.8 GPa

5.5 GPa 4.8 GPa

9.1 GPa 6.2 GPa 5.9 GPa

9.5 GPa 7.0 GPa 10.7 GPa 7.6 GPa 8.5GPa 9.1 GPa

5.6 GPa 8.5GPa 7.1GPa 7.1GPa 4.8 GPa

6.0GPa Intensity (a. u.) Intensity (a. u.) Intensity (a. u.) 3.8 GPa 5.1 GPa 6.0GPa 3.0 GPa 1.6 GPa

1.8 GPa 0.8 GPa

0 GPa 5.1 GPa 0 GPa

0 GPa

4 6 8 10 12 14 16 18 6 8 10 12 14 16 18 6 8 10 12 14 16 2Θ (degree) 2Θ (degree) 2Θ (degree)

Figure S3. High-pressure X-ray diffraction patterns of (a) wurtzite CdSe core, (b) wurtzite CdSe/ZnSe (5 monolayer) core/shell QDs, (c) magnified pattern of (b) from 0 to 10.7 GPa. The weak and broad peaks shown in the green rectangle in (c) can be attributed to the rock salt (200) reflection of the high-pressure CdSe core. Green stick pattern is the bulk wurtzite CdSe reference pattern, and the red stick pattern is bulk wurtzite ZnSe.

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(a) (a)

Figure S4. ZnSe/CdS QDs grown using a single-source precursor: (a) TEM image of ZnSe/CdS

QDs prepared using a 4.6 nm diameter ZnSe core at 215 °C with the Cd(DEDTC)2 single-source precursor, (b) corresponding EDX spectrum, with elemental composition shown in the inset.

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Figure S5. Single-dot optical properties of CdSe/ZnSe QDs for different shell thicknesses. (a) 5 monolayer (ML) QDs photobleach rapidly at the single-dot level. No blinking statistics obtained. (b) Range of 9 ML on-time fractions: 0.30-0.55. Event statistics for time trace shown in inset represents on-time fraction of 0.45. Yellow line is location of threshold cutoff (140 Hz), below which “events” are counted as “off” and above which events are counted as “on.” (c) Range of 12 ML on-time fractions (as reported in main text): 0.40-0.80. Event statistics for time trace shown in inset represents on-time fraction of 0.71. Yellow line is location of threshold cutoff (140 Hz).

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Figure S6. Single-dot optical properties of ZnSe/CdS QDs. (a) Blinking traces for ZnSe/CdS QDs for different CdS monolayers (synthesized at 240 °C), (b) Blinking trace for long (50 min) interrogation time reveals both suppressed blinking and photobleaching, (c) Percent on-time histogram derived from assessment of 77 QDs over 50 min.

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5. References

1. Chen, O.; Chen, X,; Yang, Y.; Lynch, J.; Wu, H.; Zhuang, J.; Cao,Y. C. Angew. Chem., Int. Ed., 2008, 47, 8638. 2. Chen, O.; Zhao, J.; Chauhan, V. P; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H.-S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G.; Nat. Mater. 2013, 12, 445. 3. Ghosh, Y.; Mangum, B. D.; Casson, J. L.; Williams, D. J.; Htoon, H.; Hollingsworth, J. A. J. Am. Chem. Soc. 2012, 134, 9634. 4. Evans, C. M.; Evans, M. E.; Krauss, T. D. J. Am. Chem. Soc., 2010, 132, 10973. 5. Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854. 6. Li, J. J.; Y. Wang, Y. A.; Guo, W.; Keay, J. C; Mishima, T. D.; Johnson, M. B.; Peng, X. J. Am. Chem. Soc. 2003, 125, 12567. 7. Dong, B.; Cao, L.; Su, G.; Liu, W. Chem. Commun. 2010, 46, 7331. 8. Zhong, X.; Xie, R.; Zhang, Y.; Basche, T.; Knoll, W. Chem. Mater. 2005, 17, 4038. 9. Dorfs, D.; Salant, A.; Popov, I.; Banin, U. Small 2008, 4, 1319. 10. Nan, W.; Niu, Y.; Qin, H.; Cui, F.; Yang, Y.; Lai, R.; Lin, W.; Peng, X. J. Am. Chem. Soc. 2012, 134, 19685. 11 Park, Y.-S.; Malko, A. V.; Vela, J.; Chen, Y.; Ghosh, Y.; García-Santamaría, F.; Hollingsworth, J. A.; Klimov, V. I.; Htoon, H. Phys. Rev. Lett. 2011, 106, 187401-1-4. 12 Mangum, B. D., Ghosh, Y., Hollingsworth, J. A.; Htoon, H. Opt. Express 2013, 21, 7419. 13. Mahler, B.; Lequeux, N.; Dubertret, B. J. Am. Chem. Soc. 2010, 132, 953.

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