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Efficient Heterotransfer Between Visible Quantum Dots Wenping Yin, Namhun Kim, Jaehak Jeong, Kil Suk Kim, Heeyeop Chae, and Tae Kyu Ahn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10640 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 9, 2017

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The Journal of Physical Chemistry

Efficient Heterotransfer between Visible Quantum Dots Wenping Yin,a,† Namhun Kim,b,† Jaehak Jeong,b Kil Suk Kim,a Heeyeop Chae, b,* Tae Kyu Ahn a,* a

Department of Energy Science, Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea.

b

School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon, 16419, Republic of Korea.

† These authors contributed equally to this work. Corresponding Author： *[email protected], *[email protected] Abstract: We fabricated Green-Red (GR) and Blue-Red (BR) bilayer stacked Quantum Dots (QDs)

using

electrospray

deposition.

Along

with

steady

state

and

time-resolved

photoluminescence (PL), sub-nanosecond donor PL decay and corresponding acceptor PL rise signals were observed, which are ascribed to the energy transfer between different visible QDs (heterotransfer). The heterotransfer rates were estimated as (0.57±0.01 ns)-1 and (0.65±0.02 ns)-1 for GR and BR systems, respectively, which agree well with theoretical calculations. Owing to their geometrical proximity, mixed QD layers with GR and BR showed qualitatively higher efficiencies of 64% and 81%, compared to stacked QD layers, which have efficiencies of 23% and 64%, respectively.

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Introduction Quantum dots (QDs) have been extensively used as light absorbers or emitters mainly via unobstructed or suppressing fluorescence resonance energy transfer (FRET) owing to the high extinction coefficient, multi-photon absorption, cost-effectiveness, and the capability for hot electron injection1. Among the various applications of QDs, quantum light emitting diodes (QLED) have received increasing attention due to high spatial resolution and color customizability, which can be achieved by geometrically separated blue/green/red (B/G/R) QDs2-6. Theoretically, it does not seem difficult to adjust each QD components in QLED; however, due to the undesirable energy transfer, obtaining a pure color emission in the visible range still presents challenges7. In this regards, understanding energy transfer between QDs is essential to improve the QLED efficiency progressively, especially the energy transfer between QDs of different colors, i.e., heterotransfer8. Most of the previous studies argued that they showed the heterotransfer based on the donor’s shortened PL lifetime in donor-acceptor complex compared to that of sole donor samples4-6. However, the findings could not exclude the possibility of Auger recombination, homotransfer (energy transfer between QDs of the same color), charge separation, and exciton-exciton annihilation, all of which disturb the donor-acceptor dynamics. Although some authors believed that the weak pulsed light source in time-resolved photoluminescence (TRPL) measurement could reduce the Auger recombination9, none of them systematically verified the PL dynamics at quantitative power so far. Besides, there are still other geometric and morphological factors that make it almost impossible to accurately evaluate the heterotransfer between donor and acceptor. Therefore, the photodynamic studies of heterotransfer between QDs of different colors remain unclear.


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In this work, the heterotransfer rates and efficiencies from G to R QDs and B to R QDs were selectively studied. The R QD is commonly used as the acceptor because it has the lowest transitions among B/G/R QDs. Herein, the laser power intensity was further attenuated to 0.35µW/cm2 using a neutral density (ND) filter, to reduce other power-dependent processes such as Auger recombination and/or exciton-exciton annihilation. Combined with the donor PL decay, and acceptor PL rise, the heterotransfer rates were confirmed as sub-nanosecond (0.57±0.01 ns and 0.65±0.02 ns for GR and BR, respectively), which is much slower than that of reported homotransfer (~6-23ps)9. In addition, geometrical effects were investigated by comparing the stacked QD layers (one homogeneous QD layer on top of another QD layer) and mixed QD layers (heterogeneous layers composed of two kinds of QDs) samples. The observed subnanosecond heterotransfer makes it much easier to modify and tune the emitting spectra in QLED. Beyond that, this work may also be applicable to the biomedical research, in the applications of DNA/RNA sensor10 and fluoro-immunoassay detectors11 particularly under complex and volatile cell environments12. Methods Synthesis of blue-emitting CdZnS/ZnS QDs (B).

For the synthesis of Blue QDs, we

followed Prof. Yang’s work13, 1 mmol of cadmium oxide (CdO) and, 10 mmol of zinc oxide (ZnO) were dissolved in 7 mL of oleic acid (OA) under Ar flow, and were heated to 150℃ for 5 min. Immediately after, 1-octadecene (ODE) (15 mL) was added and the reaction temperature was increased to 310℃ within 10 min. Subsequently, 2 equivalent clear sulphur (S) ODE solution (2 mL) was rapidly introduced to generate CdZnS core QDs, while another 8 equivalent sulphur stock ODE solution (5 mL) was added to generate ZnS shell with refluxing for about 4 to 6 h. All the processes were conducted under the same temperature and was then quenched by


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rapid cooling to room temperature. The reaction mixture was centrifugally purified by hexane and hexane/acetone (1:4) successively. Synthesis of green-emitting CdSe@ZnS/ZnS (G)-one pot synthesis. In the synthesis of green QDs we modified a previously described protocol6.Gradient core@shell QDs were prepared as follows; mixture of Cd acetate (0.14 mmol ), ZnO (3.41 mmol), ODE (15 mL), and OA (7 mL) in a flask was vacuum degassed at 120 ℃ for 90 min with N2 flow, followed by heating to 310 ℃. Then, S and selenium (Se) stock solution, which was created by dissolving Se (2.2 mmol) and S (2.2 mmol) into trioctylphosphine (TOP, 2.0 mL), was injected into the above mixture, and the gradient core reaction was allowed to proceed at the same temperature for 10 min. After, S (1.6 mmol) dissolved in ODE (2.4 mL) was introduced into the CdSe@ZnS QD growth solution for 12 min. Subsequently, OA (1.66 mL) and ODE (8 mL) solution containing 2.86 mmol Zn acetate was quickly injected. Synthesis of red-emitting CdSe/Zn1-xCdxS QDs (R). The details of the synthesis can be found in the previous work14. The synthesis was completed using Schlenk line techniques. Essentially, a clear Cd(MA)2 solution was obtained using CdO (1 mmol) and myristic acid (MA) (3 mmol) mixed into ODE (15 mL) under 300℃ with Ar flow. 0.5 mmol Selenium in TOP was injected to generate the QD cores. 1.5 mmol zinc oleate [Zn(OA)2] and 1 mmol 1-dodecanethiol (DDT) were added into the reactor to generate inner shell. After that, 1 mmol of cadmium oleate [Cd(OA)2], 2 mmol of Zn(OA)2, and 3 mmol of TOP (1:2:3) were introduced into the reaction mixture for outer shell growth. Then the reaction mixture was centrifugally purified. All of the QDs were capped with aliphatic ligands. In this work, we used home-developed electrospray deposition (ESD) for large-scale, high throughput, uniform thin film fabrication without any annealing process15, which has recently


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been introduced to QD large-scale LED fabrication. We can easily control the morphology of the thin film by maintaining the factors, for instance, flow rate, voltage and the deposition time. In this study, appropriate QDs were dissolved in solvents (hexane:octane=4:1) and sprayed onto 1 cm2 quartz collectors layer by layer for the stack samples. However, for the mixed samples, equimolar B (or G) and R QDs were mixed into the solvents in advance. The voltage was maintained between 3.5 to 4.5 kV, the distance between electrodes was about 4.0 cm, and the deposition time here for each layer was 20 s. For the mixed samples, the deposition time was set as 40 s. The steady-state photoluminescence (PL) & excitation spectra were recorded with a fluorescence spectrophotometer (F-7000, HITACHI) equipped with a standard light source (P/N 250-0123) with a wavelength range between 200 to 900 nm. The size and shape of the QDs were monitored with a high-resolution transmission electron microscope (HR-TEM, JEOL, JEM 2100F). Quantum yield (QY) measurements were performed using an absolute PL QY spectrometer (C11347, Hamamatsu Photonics) in an integrating sphere. Time-resolved PL (TRPL) was measured using a commercial Time-Correlated Single Photon Counting (TCSPC) system (FluoTime 200, PicoQuant)16. Samples were excited by a picosecond diode laser of 405 nm (LDH-P-C-405, PicoQuant) with a variable repetition rate (1/2 MHz for this work). The excitation laser power was attenuated to 0.35 µW/cm2 by applying ND filters to avoid the unwanted power-dependent processes. The emitted PL was spectrally dispersed with a monochromator (ScienceTech 9030) for each PL signal and was collected by a fast photomultiplier tube (PMT) detector (PMA 182, PicoQuant GmbH) with a magic angle (54.7°) orientation to remove anisotropy effects. The incident angle of the excitation pulse was set to about 30° along the direction of the sample to avoid major scattering. The resulting instrumental


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response function was about 200 ps in full-width-half-maximum (FWHM). A cut-off filter (GG495 nm, Edmund) was additionally applied to block the residual scattering. For timeresolved emission spectra (TRES) experiments that were conducted at every 10 nm step (total 23 different wavelengths) from 480 to 700 nm, we set all the acquisition times to 20 s identically. Results The stacked QD layers were fabricated from dried and uniform QDs by electrospray deposition without orthogonal solvent with a roughness of only ca. 1.82 nm (Fig. S1). The average diameters of B, G, and R QDs were measured as 9.13, 13.36, and 8.01 nm by HR-TEM, respectively, the lengths of ligands were included too (Fig. 1a, 1b, 1c). It is noteworthy that the average size of multi-shell G QDs was significantly larger than those of B and R QDs, so that the PL quantum yield (QY) achieved for G QDs was as high as 90%. PL QY for R and B QDs were 84% and 38.9%, respectively. The PL emission peaks seemed to have largely Gaussian shapes centered at 455, 520, and 635 nm for B, G, and R QD layers with FWHMs of 23, 22, and 34 nm, respectively (Fig. 1d). B and G QDs had less than 30 nm FWHMs, which means the size variation of B and G QDs was less than 5%17. On the other hand, a time-resolved spectral redshift within 30 nanoseconds was observed for R QDs by using time-resolved emission spectra (TRES) (Fig. S2). Owing to larger size QDs had longer lifetime in the red-side spectrum while the smaller QDs showed shorter lifetime in the blue spectrum, the slightly red-shift in TRES implied the size variation of R QDs18.


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Figure 1. High-resolution transmission electron microscope (HR-TEM) images of (a) blue QDs; (b) green QDs; and (c) red QDs, respectively. All the images show close proximity of QDs; and (d) the steady state photoluminescence (PL) of each QDs with an excitation at 405 nm.

There are two main models of energy transfer between donor and acceptor: 1) FRETspontaneously occurs when the donor dipole moment electrostatically interacts with the acceptor dipole moment and it is dominant when the distance between donor and acceptor is within the Fӧrster radius (typically between 1 to 10 nm)19; 2) Dexter energy transfer-prevail when the donor and acceptor are in proximity closer than 1 nm, as such, the orbital overlap between donor and acceptor cannot be negligible20. In our QDs system, the latter Dexter model can be drastically excluded, because the QDs were encapsulated by aliphatic ligands with tails longer than 1 nm which will block the orbital overlap. By definition, the efficiency (E) of FRET is /

(1)


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, where R0 is the Fӧrster radius, which is the radius at which the energy transfer efficiency is about 50%, and R is the center-center distance between the donor and acceptor. Eq. 1 is used as an alteration for QDs simplified from the standard R6-dependent model8,18. Since R can be calculated as κ 2 ,where κ2 is the orientation factor (which contains the angular distribution of dipole moments of donor and acceptor, it is normally defined as 2/3 in the random distribute system), is the refractive index, is the quantum yield of the donor in the absence of acceptor, and J means spectral overlap of acceptor absorption and donor emission21. Based on the above equations, the efficiency of FRET in multicolored QD systems depends on the proximity between QDs, spectral overlap, and the quantum yield of donor. In this study, not solely GR-mix/stack samples but also BR-mix/stack samples were designed to compare energy transfer rate and efficiency. Fig. 2a and 2b show excitation spectra by following the R QDs emission center of 630 nm, which corresponded to the absorption spectra of QDs. Considering that QD layers were fabricated with almost the same amount of QDs, the enhanced excitation absorption under 400 nm corresponded to the enhanced emission from R, i.e. energy transfer from donor G (or B) to acceptor. Fig. 2c and 2d show the PL spectra normalized to PL intensity of R QDs. The quenched donor PL of mixed samples (GR-mix and BR-mix) compared to those of stacked samples (GR-stack and BR-stack) indicated the stronger energy transfer from G and B to R QDs, respectively. Fig. 2e shows the zoomed R emission peaks which slightly shifted to blue from R, GR-stack, to GR-mix sample. Moreover, Fig. 2f reveals the same blue-shift in the PL spectra from R, BR-stack, to BR-mix sample. We believe the blue-


8

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shift of steady state PL can be attributed to the reduced interaction between R QDs themselves of different sizes when fabricating with B or G QDs22. Furthermore, detailed kinetics were investigated using TRPL, which includes donor-acceptor interaction rates. Two FRET processes in QDs system can generally occur: homotransfer and heterotransfer. The former occurs between nominally monochromatic populations while the later prevails between QDs of different colors8. Again, in this work we mainly focused on heterotransfer by attenuating the excitation laser source to avoid any other power-dependent

Figure 2. The excitation spectra (a) of R only (black), GR-stack (green), and GR-mix (red); (b) of R only (black), BR-stack (blue), and BR-mix (red) monitored at 630 nm. The normalized PL spectra (c) of GR-stack (green) and GR-mix (red); (d) of BR-stack (blue) and BR-mix (red). The zoomed PL spectra (e) of R only (black), GR-stack (green), and GR-mix (red); (f) of R only (black), BR-stack (blue), and BR-mix (red) with the excitation at 405 nm. (Other conditions were the same during the measurements, e.g. excited/emission slit width and PMT voltage.)


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ultrafast processes. Fig. S3a compares different TRPL decays of G reference under various excitation power intensities. The fitting result showed increased average lifetime as the pump fluence decreased (Fig. S3b). In the attenuated power condition (approximately 0.35 µW/cm2), we were able to avoid redundant ultrafast kinetics that were observed at higher power excitation (45.8 µW/cm2), which allowed us to focus on the heterotransfer. All the TRPL measurements were done under the attenuated excitation. Using bi-exponential fitting, TRPL decay of G reference revealed two components: a fast component of ca. 4 ns and a long component of ca. 10 ns (Table. 1), which are consistent with previous results13. When G was combined with R, the fast components newly generated were fitted to 0.53 ns and 0.52 ns for the GR-stack and GR-mix, respectively (Fig. 3a). By definition, the efficiency of heterotransfer is the proportion of heterotransfer over the summation of rates in all other processes, and it can be estimated using average lifetime of each sample as 1−

3

, where τDA and τD are the donor average fluorescence lifetimes in the presence and absence of acceptor. We assigned the energy transfer time as 0.57 ns and 0.56 ns for GR-stack and GR-mix by reciprocal of heterotransfer rates (KET), respectively (Table. 1 & Fig. 3a). The KET values were derived from 1/τDA(1)-1/τD(avg), namely 1/τ1(GR-mix/stack)-1/τavg(G) for each sample here. While for B reference, there were originally three components of 4 ns, 18 ns, and 100 ns, that we speculate originated from the trap states (Fig. 3b)17. The overall fitting results of BR-mix and BR-stack are listed in Table 1. The obvious fast PL decays of 0.62 ns and 0.61 ns were converted to 0.65 ns and 0.64 ns of energy transfer time for BR-stack and BR-mix. All values above demonstrated the heterotransfer processes from G (or B) to R. The reason for the similarity of energy transfer rates (KET) and the discrepancy of heterotransfer efficiency (E) is


10

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firstly originated from the different equations to estimate them. KET was calculated only by the fast components (τ1 in table 1) for each donor-acceptor complex, which emphasizes the dynamics between donor (G/B) and acceptor (R). However, we estimated the heterotransfer efficiencies (E) by the calculation using average lifetime with and without acceptor, which includes all components from τ1 to τ4 in Table 1.

Figure 3. Time-resolved Photoluminscence (TRPL) result (a) of G-only (black), GR-stack (red), and GR-mix (blue); (b) of B-only (black), BR-stack (red), and BR-mix (blue) under the excitation of 405 nm. The PL wavelength was set at 520 nm for the PL decay of G and 460 nm for the PL decay of B. Table 1. TRPL fitting results for GR and BR series (the values of the goodness-of-fit parameters (χ2) are all less than 1.2).


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samples kinetics

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G

GR-stack

GR-stack

GR-mix

GR-mix

R

520em

520em

630em

520em

630em

630em

τrise/ns

0.54

0.54

A1/%

12.08

32.99

τ1/ns

0.53

0.52

A2/%

47.69

58.41

88.93

63.35

88.64

86.89

τ2/ns

3.85

3.92

11.00

3.49

14.02

8.91

A3/%

52.31

29.52

11.07

3.60

11.36

13.11

τ3/ns

9.73

9.98

28.92

10.00

30.00

25.33

τavg/ns

6.92

5.30

12.98

2.52

15.83

11.06

E/% KET/109s-1 samples kinetics

23.41

63.58

1.74 (0.57±0.01ns)

1.78 (0.56±0.01ns)

B

BR-stack

BR-stack

BR-mix

BR-mix

R

460em

460em

630em

460em

630em

630em

τrise/ns

0.62

0.63

A1/%

42.16

65.05

τ1/ns

0.62

0.61

A2/%

62.63

45.48

79.85

30.46

73.09

86.89

τ2/ns

3.71

3.03

11.06

3.21

12.94

8.91

A3/%

31.55

10.69

20.15

3.91

26.91

13.11

τ3/ns

17.69

16.34

26.70

17.45

27.62

25.33

A4/%

5.82

1.67

0.58

τ4/ns

101.98

95.70

96.00

τavg/ns

13.84

4.98

16.89

11.06

14.21

2.61

E/%

64.01

81.10

KET/109s-1

1.54

1.57


12

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(0.65±0.02ns)

(0.64±0.02ns)

where, I(t) = Arise(1-exp(-t/τrise)) + A1exp(−t/τ1) + A2exp(−t/τ2) + A3exp(−t/τ3)+A4exp(−t/τ4); E=1-τDA(avg)/τD(avg); KET= 1/τDA(1)-1/τD(avg) (we used 7 ns and 13 ns as average lifetimes of G and B dnonor for KET calculation here, respectively). Discussion As expected, the efficiency of heterotransfer in a mixture QDs is more significant than that in stacked samples: 63.58% (mix) to 23.41% (stack) for GR and 81.10% (mix) to 64.01% (stack) for BR series. Besides, the heterotransfer rate of BR for both stacked and mixed samples is somewhat slower than that of GR. For further theoretical calculations, the refractive index () and orientation factors (κ2=2/3) in Eq. 2 were assumed as the same for different samples21. Then K is proportional to / . The donor quantum yield ( ) (38.9% vs 90%) and donor lifetimes ( ) (Table. 1) for B and G have been stated above. The distance between donor and acceptor QDs was calculated by center to center proximity as 8.57 nm and 10.685 nm for BR and GR, respectively. Different from general organic dyes, when QDs are used as energy transfer acceptor, that heterotransfer can be achieved by direct excitation, and also can be realized by more efficient two-photo excitation of acceptor23-25. In view of the foregoing, the spectral overlap between donor emission and acceptor absorption was calculated in the whole broad wavelength range of R absorption, which showed (BR) is about 3.38 times larger than (GR). Apart from the aforementioned parameters, one more assignable factor has to be considered, i.e., the number ratio between donors and acceptors. Due to the heterotransfer rate strongly depends on the possibility of donor and acceptor reaching each other within the energy transfer domain8, 26, we assessed the number of donors per acceptor by simulation based on QDs diameters in the TEM image. Altogether, the ratio between K (BR) and K (GR) was


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calculated as 0.79, which is approximately close to the real experimental result of about 0.88±0.05. On the other hand, both BR-mix and GR-mix samples showed significant rise components in acceptor (R) PL traces, which matched well with corresponding fastest components of donor (G or B) PL decays. The heterotransfer kinetics for GR sample was fitted as 0.54 ns (Fig. 4a), and that for BR sample was 0.63 ns (Fig. 4b), with a K (BR): K (GR) ration of about 0.86. Although that rise components for stacked samples were concealed by a larger proportion of essential absorption of R QDs themselves, the result of BR-mix and GR-mix samples adequately proved the heterotransfer rates, and this calculation method can be applicable to calculate the

Figure 4. TRPL decays (a) of GR-mix; (b) of BR-mix, and TERS 2D graphs (c) of GR-mix; (d) of BR-mix.


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heterotransfer rates. TRES was measured for QD samples for every 10 nm step (Fig. S4), which showed a clear red-shift in the mixed samples compared to the stacked samples, similar with steady state PL (Fig. 2c&2d). What’s more, in both the GR and BR systems, the PL lifetime for R QDs increased after contacting with donor (Fig. 5)5, 27-28, which illustrated the less interaction between R QDs again. In order to prove that heterotransfer exists in electroluminescence (EL) devices, we made GRstack and GR-mix QLED with a structure of ITO/PEDOT:PSS/PVK/QDs/ZnO nanoparticles/Al (S.I.). The emission results of different samples are shown in Fig. 6, in which the Commission Internationale de I’Eclairege (CIE) color coordinates are used. As shown in the GR-stack schematically depicted in Fig. 6a, the emitting light appears at the red field at low voltage (4.0 V) while gradually moves to the green field at high voltage (10.0 V), as measured by spectroradiometer. The emitting properties were less effected by the depressed heterotransfer in

Figure 5. The PL decay of R QDs (acceptor) in reference (balck), stack (red) and mixture state (blue) (a) for GR series and (b) for BR series.


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the stacked sample. However, for the GR-mix sample (Fig. 6b), the emitting light remained in the red region under high voltage, which revealed that a considerable number of G QDs had lost the ability to emit light due to the inevitable grievous heterotransfer to R QDs, which is consistent with our spectral experiment in the emissive QD layers.

Figure 6. Commission Internationale de l’Eclairage (CIE) coordinates for the colors (a) of GR-stack and (b) of GR-mix. Additionally, to monitor the intrinsic dynamics of QDs, we measured PL lifetime in toluene and in acetonitrile solvent (Fig. S5). We inserted solvent as several drops onto the QD film samples. Both PL decays of B QDs and G QDs were slowed in shorter time region. We speculate that is due to either isolation of QDs from the arranged layer or the stabilization of the surface trap states. Further studies that investigate the origin of traps, including temperature dependent measurements,29-31 are ongoing. Conclusions Heterotransfer between QDs of different colors was clearly shown by coincident acceptor PL rise and donor PL decay. By exploring donor dependent (B/R) and morphology (mix/stack) dependent heterotransfer using PL traces, we were able to observe clear sub-nanosecond


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heterotransfer between G (or B) and R QDs. As sub-nanosecond energy transfer is much easier to tune and modify, we anticipate that this work could be exploited in multiple strategies for storing solar energy in light-harvesting assemblies, next-generation displays, along with the potential to make new types chemiluminscence, and even engineering probes. Moreover, multicolored QDs can be used as energy transfer donors or acceptors in chosen QDs systems for a certain target by monitoring the heterotransfer kinetics. Supporting Information. A brief statement listing the contents of the atomic force microscope image and other optical results supplied as Supporting Information. Acknowledgements This work was supported by Woo Jang Chun Special Project (PJ009106022013) by RDA, Degree and Research Center Program (2015) by NST, the Basic Science Researcher Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education,

Science

and

Technology

(NRF-2013R1A1A2012829)

and

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