Effects of Local Dielectric Environment on Single-Molecular

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Effects of Local Dielectric Environment on Single-Molecular Spectroscopy of CdSe/CdS Core/Shell Quantum Dot Meiyi Zhu, Jianhai Zhou, Zhuang Hu, Haiyan Qin, and Xiaogang Peng ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00919 • Publication Date (Web): 09 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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Effects of Local Dielectric Environment on Single-Molecular Spectroscopy of CdSe/CdS Core/Shell Quantum Dot Meiyi Zhu, Jianhai Zhou, Zhuang Hu, Haiyan Qin*, Xiaogang Peng* Center for Chemistry of Novel & High-Performance Materials, and Department of Chemistry, Zhejiang University, Hangzhou, 310027, P. R. China Abstract: This work studies effects of local dielectric environment on both single exciton and high-order excitons in single CdSe/CdS core/shell quantum dot (QD) with nearly ideal photoluminescence (PL) properties. For typical single-molecular spectroscopy measurements, isolated QDs embedded in a thin polymer layer (sub-wavelength thickness) are deposited on top of a glass slide. This results in a spatial dependence of dielectric environment for the QDs along the vertical direction. PL peak positions of single-exciton and negative trion (two electrons and one hole) states of single QD are barely affected by the local dielectric environment, which can be applied as indicators of size distribution of the ensemble and size of a specific QD. Other PL properties of single-exciton and high-order excitons, including PL intensity of single-exciton, PL quantum yield of trion and bi-exciton, mono-exponential radiative decay lifetime of all states studied, and Auger recombination lifetime of high-order excitons, are all found to be sensitive to the local dielectric environment. Furthermore, by removing the heterogeneous dielectric environment, all spectroscopic properties of single QD are found to well correlate with the ensemble measurements. Keywords: Colloidal quantum dots, single molecular spectroscopy, dielectric environmental homogeneity, photoluminescence decay dynamics, trion state.

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Introduction Colloidal semiconductor nanocrystals with their sizes in quantum-confinement regime are known as quantum dots (QDs).

1-3

Their unique optical properties and solution processibility render

them as outstanding emissive materials in important applications at both ensemble and single dot levels, such as light-emitting diodes, molecular tracking.

11-13

4-6

lasing,

7, 8

single photon sources,

9, 10

and single

In nearly all applications, the dielectric environment surrounding QD

emitters is complex and heterogeneous, which can affect their emission properties significantly. It has been well established that the radiate properties of an emitter is dependent on its local dielectric environment.

14

Local dielectric environment usually refers to the dielectric

surroundings within sub-wavelength range of an emitter, which affects the emitter directly during the decay of its excited state. For instance, radiative decay rate of a simplified two-level system is not only determined by its own electric transition dipole moment, but also by the optical density of states and the electromagnetic field strength at the local dielectric field of the emitter. 15, 16

For QD emitters, the situation is complicated on account of that high-order excitons are

quite common in comparison with small molecular emitters, whose responses to local dielectric environment are largely unknown due to their involvement of Auger effects.

On the ensemble level, local dielectric effects have been extensively studied in organic molecules, 17

rare earth metal complex, 18 doped nanocrystals, 19 and QDs. 20, 21 While a number of intriguing

phenomena of environmental effects on single molecules have been unveiled by single-molecule


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spectroscopy,

22-27

further investigation on this issue is restricted due to troublesome

photo-bleaching of small molecules. Single-molecule detection technology has been applied for identification of intrinsic properties of QDs which could otherwise be obscured by their size inhomogeneity.

While

single-molecule

spectroscopy

has

confirmed

much

improved

anti-bleaching properties of single QD in comparison to small molecules, it has also uncovered interesting intensity blinking phenomenon,

28

namely random switching of PL intensity between

different brightness states under constant excitation due to photo-chemical events.

29, 30

Studies

on single QD reveal that charging of a neutral QD induces transition from the bright state to the dim/dark state, and discharging recovers its bright state from the dim/dark state. Several groups have studied local dielectric environmental effects on single QD.

31-33

Since PL properties of the

QDs at both ensemble and single-dot levels have not been well controlled, these early studies have mostly focused on the single-exciton state (or without differentiating different emissive states of the single dot). As a result, local dielectric environmental effects on single-exciton state have not been fully understood and those on radiative and non-radiative decay channels of high-order excitons—including dim/dark and bi-exciton states—have barely been explored.

Recent synthetic development

34

allows us to access colloidal zinc-blende CdSe/CdS core/shell

QDs with nearly ideal optical properties at both ensemble and single-dot levels, i.e., near unity PL quantum yield (QY), mono-exponential PL decay dynamics, matched PL peak position and peak width between ensemble and single-dot measurements, and nearly non-blinking under general excitation conditions. This model QD system with well-defined PL decay rates and


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intensities

enables

us

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to

concentrate on studying local dielectric environment effects on single QD. Furthermore, the blinking behavior of these QDs can

be

atmosphere,

regulated 35

by

the

which enables us

to differentiate effects of local dielectric

environment

single-exciton,

trion

on (two

electrons and one hole), and

Figure 1. (a) PL and UV−vis spectra and (b) PL decay dynamics of typical CdSe/CdS core/shell QD solution. The red curve in (b) is the single-exponential decay fitting of experimental data. (c) Representative PL intensity time trace (black) of single CdSe/CdS core/shell QD with background (grey). The bin time is 50 ms. (d) PL spectrum of a representative single dot compared with ensemble measurement.

bi-exciton states.

Results and Discussion In this work, CdSe/CdS core/shell QDs with nearly ideal optical properties (see Figure 1 for example) are adopted as the model system. Isolated QDs dispersed in dilute PMMA (polymethyl methacrylate)-toluene solution are spin-casted onto a glass substrate (left panel in Figure 2a) following typical sample preparation procedure in previous studies.

36, 37

Embedding isolated

single QDs within thin polymer film (~40 nm in thickness) inherently introduces inhomogeneity of local dielectric environment for the QDs, given the refractive indexes of glass, PMMA, and air being respectively 1.50, 1.48 and 1.00.


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Following the general methods for studying effects of local dielectric environment of single organic emitter 24

and

single

millimeter-thick

QD,

31-33

layer

refractive-index-matching

a of

oil

(a

mixture of squalene and lens oil, refractive index 1.48) is added on top of the polymer layer (right panel in Figure 2a), which effectively converts the

inhomogeneous

dielectric

environment to a homogeneous one for the isolated QDs in polymer film. Control experiments show that addition of an appreciable amount of the index-matching oil (~10%) into the QD-toluene solution wouldn’t cause any measurable change in optical properties of QDs. For convenience,

Figure 2. (a) Illustration of experimental setup (side view) and corresponding fluorescence microscope images of single QDs. Color-coded circles indicate different single dots. (b) PL spectra and (c) PL decay dynamics of single QD before (labeled as Air) and after the deposition of oil (labeled as Oil) respectively. (d) PL lifetime alteration of 32 single QDs with air and oil on top of QD-PMMA layer. Red diamonds with error bars are the average lifetime values and standard deviations. (e) Lifetime alteration of three isolated QDs in PMMA thin layer facing air (n=1.00), oil (n=1.48), and diethyl ether (n=1.35). CdSe QDs with 8 monolayers of CdS shell are used in (b)-(d) and those with 16 monolayers of shell are used in (e).


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the configurations in Figure 2a left and right panels may be named as “air-polymer interface” and “oil-polymer interface” respectively, though the latter does not have a noticeable dielectric interface.

The home-built microscope system enables us to track and correlatively measure the optical properties of single dots before and after addition of the index-matching oil. Measurements on optical properties of single-exciton and bi-exciton states using the home-built system have been described previously.

29, 37

For measurements of dim-state emission, the excitation power is

increased to an appropriate level to generate sufficient dim-state photon counts and avoid too much contribution from bi-exciton emission to either bright or dim state.

29

Specifically, the

average number of photons absorbed by a single dot (〈N〉) per pulse for pulsed laser excitation (or per lifetime duration for CW laser excitation) is ~0.2 for the dim-state measurements.

Figure 2b-2e summarize results for correlation measurements of the optical properties of single-exciton state. Removal of the air-polymer interface by deposition of the thick oil layer has no observable influence on the PL peak position and spectral contour of single QD (see Figure 2b). The single-exciton PL lifetime however is significantly shortened, though the PL decay dynamics remains mono-exponential (one example in Figure 2c). Diethyl ether (n = 1.35) is found to readily dissolve the index-matching oil without affecting optical properties of the QDs in solution. By heavily diluting the oil layer with diethyl ether, the QD-polymer layer is exposed to nearly pure ether. The correlation measurements indicate that replacement of the


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index-matching oil by diethyl ether increases the mono-exponential PL lifetime of the single-exciton state to a level between those of the corresponding QD facing air-polymer interface and oil-polymer interface (Figure 2e). Correlation measurements further reveal that, addition of the thick oil layer on top of the polymer film would generally narrow the distribution of mono-exponential PL decay lifetime of single QDs (Figure 2d).

Results in Figure 2 and to be described below are qualitatively reproducible for CdSe/CdS core/shell QDs with different core sizes and/or different shell thicknesses. Thick CdS shell would result in relatively long PL decay lifetime for the single-exciton state of a QD. 34 For example, Figure 2e illustrates results for the QDs with 16 monolayers of the CdS shells while Figure 2b-2d are related to the QDs with 8 monolayers of the CdS shells. Evidently, relatively long PL decay lifetime of QDs would result in more pronounced change upon dielectric interface replacement.

Figure 3. Statistical results of (a) PL peak positions, (b) PL lifetimes, (c) PL intensities of 91 QDs before and after oil deposition. Sum of (d) PL spectra and (e) PL decay curves of 38 individual dots compared with their corresponding ensemble measurements in toluene.

The statistical data of PL peak energy, mono-exponential PL lifetime, and PL peak intensity of single-exciton emission for 91 randomly


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selected QDs before and after addition of the thick oil layer are shown in Figure 3a-3c. Results in Figures 3 not only statistically confirm the results in Figure 2 but also offer a comparison between single-dot and ensemble measurements (Figure 3d and 3e). Specifically, the PL peak positions of single QDs with either types of interfaces only fluctuate slightly (< 1%) around the ensemble value (Table 1, first row).

Upon replacing the air-polymer interface by the oil-polymer interface, distribution of the mono-exponential PL decay lifetime of single exciton is narrowed by ~60%

Table 1. The average values and standard deviations of PL peak positions, lifetimes and intensities of 91 individual QDs in Figure 3a-3c and ensemble QD sample in Figure 3d and 3e.

and its average value is practically identical to that of the ensemble value (Table 1, second row). The similar distribution of the PL peak energy under inhomogeneous and homogeneous dielectric environments (Figure 3a) implies that, instead of the local dielectric environment, there should be some other sources to cause the slight variation of the PL peak energy in Figure 3a. These sources seem to affect the PL lifetime of single-exciton state as well because the PL lifetime correlates with the PL peak energy negatively for the QDs within homogeneous dielectric environment (Figure S1, Supporting Information). In principle, if the core size and/or shell thickness varies slightly in these nearly ideal QDs,

34

it

would also result in small variations of their PL peak and decay dynamics.


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While the PL peak and PL decay lifetime are correlated each other negatively for the QDs under homogeneous dielectric environment, such correlation is not valid for the QDs facing the air-polymer interface (Figure S1, Supporting Information). This suggests that the large variation of PL lifetime in Figure 3b for the QDs with the air-polymer interface should mainly be a result of inhomogeneity of the dielectric environment, i.e., different distances to the air-polymer interface. In principle, the local dielectric environment of those QDs embedded relatively deep in the polymer film (or close to the glass-polymer interface) would not change much upon addition of the thick index-matching oil. This phenomenon can be explained according to the theoretical models.

38

According to theory, total radiated intensity and radiative decay rate of an emitter

would be modulated by an approaching interface within the distance of the emitter’s wavelength. 38

Since the emitting wavelength is much larger than the film thickness (~40 nm, less than 10%

of the emission wavelength), influence of the interface on transition dipole in a QD should decline upon increase of the distance between the QD and the interface. The isolated QDs in this work should be distributed along the thickness direction of the PMMA film. Approximately, the closer a QD is to the PMMA-air interface, the greater its lifetime would depart from that of the QD within a homogeneous dielectric environment with refractive index around 1.5 (Figure S2).

For organic dyes, dependence of their radiative PL lifetime on the refractive index of local dielectric environment has been reported for both ensemble and single-molecule measurements. 24, 39

For single-molecule measurements, sensitivity to a given refractive interface depends on

both distance and orientation of the transition dipole with respect to the interface. Theory


40

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predicts that the transition dipole of spherical zinc-blende CdSe QD is a 3-dimensional one. Thus, distance to the interface is likely the sole factor in this case. The alteration of PL lifetime observed here qualitatively agrees with Lukosz’s calculation but is larger in magnitude. The reason could be that spontaneous emission rate of emitters in dielectric medium is not simply proportional to the medium refractive index. Instead, it follows a more complex relationship that usually illustrated by introducing local-field correction factor into the formula.

18, 41

In addition,

the calculation is for point emitters; whereas the geometric size of QDs (~10 nm) is not negligible for small distance (~40 nm) to the interface with regard to contribution of the evanescent waves.

In literature, the radiative decay lifetime of QDs dispersed in nonpolar solvents has been reported to decrease as the refractive index of the solvent increases.

21

In contrast, UV-vis and PL peak

position of wurtzite CdSe QDs in solution displays rather minor response to the alteration of refractive index of local dielectric environment.

42

Our measurements of the QDs in solution

confirm these effects (Figure S3, Supporting Information). Results in Figure S3 (Supporting Information) reveal that, consistent with Figure 3a and 3b, PL decay lifetime of single-exciton state of QDs depends on the refractive index of the solution, but the PL peak position is insensitive to the change of refractive index of the solvent. Although solvatochromism exists in QDs,

42

it does not lead to significant changes in the present case because the exciton is isolated

from the local environment by the ligands and CdS shells.


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Figures 3c and Table 1 (the third row) indicate that the change trend of PL intensity of the single-exciton state of single QD upon removing the air-polymer interface by addition of the thick index-matching oil layer is quite similar to that of the PL lifetime (Figure 3b). Specifically, the average PL intensity decreases by ~55% and the intensity distribution narrows by ~65%. To further illustrate this large change, Figure S4 (Supporting Information) illustrates results of correlation measurements, including the PL intensity, bin-averaged PL lifetime, PL spectra, and the PL decay dynamics, for a representative QD with two types of interfaces. Though both statistic results and correlation measurements confirm that PL intensity varies dramatically upon changing the dielectric environment, results below shall reveal that it is not due to change of the PL QY.

Figure 3d depicts the sum PL spectra of 38 randomly selected dots with either air-polymer or oil-polymer interface. Given the PL spectra (Figures 3d) and mono-exponential lifetime of the PL decay dynamics (Figure 3e) of single QDs with the thick oil layer being identical to those of the QDs in the solution with the same refractive index, it is safe to conclude that the PL QY of single-exciton (or the bright state under low excitation intensity) of single QD with the oil-polymer interface is the same as that for the ensemble value, i.e., practically unity. Furthermore, measurements in the solution with different refractive indices in Figure S3 (Supporting Information) confirm that the PL QY is independent of the refractive index. Thus, the higher intensity observed for the QDs with the air-polymer interface in Figure 3c cannot be a result of higher PL QY.


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In comparison to a QD under the millimeter-thick index-matching oil, the QD under the air-polymer interface would significantly improve the PL photon collection efficiency, given the comparatively low refractive index of air. This effect has been well-explained in Ref 32. Consequently, PL intensities for the QDs facing air-polymer interface is substantially increased as its distribution is broadened.

As expected, the sum PL decay dynamics for the 38 QDs with the oil-polymer interface is nearly identical to that of the ensemble in solution with the same refractive index of the polymer/oil matrix (Figure 3e). In comparison, the sum mono-exponential PL lifetime of single-exciton state of QDs with the air-polymer interface is significantly longer than that of the ensemble measurements in the solution (Figure 3e).

Overall, the results in Figure 3 statistically confirm the effects of local dielectric environment to PL spectrum and PL decay dynamics of the single-exciton state of single QD shown in Figures 1, 2, and S1 (Supporting Information).

In order to study effects of local dielectric environment on the trion state of single QD, CdSe/CdS core/shell QDs with a relatively large core (5 nm in diameter) are used because of their relatively high dim-state PL QY.

34

PL QY of a trion state is much reduced due to

non-radiative Auger recombination, which is thus observed as a “dim” state. 28, 43, 44 Trion can be


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considered as exciton with an additional charge—either positive or negative. In this case the extra charge has been verified to be a negative one via photon statistic approach in our previous work.

45

For CdSe/CdS core/shell QDs, the most common trion state has been confirmed as

negative trion state in literature by other approaches, such as spin dynamics

46

and electrostatic

force microscopy. 47

Single QD of this sample is non-blinking under regular measurement conditions—low excitation power and ambient environment—as reported in literature. 34 However, in argon atmosphere and with relatively increased excitation power, the negative trion state 46, 48, 49 occurs quite frequently and can last for long durations (Figure S5), which allows accurate extraction of photons for either bright or dim state (Figure 4).


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Correlation measurements (Figure 4a) reveal that PL spectra of both bright and dim states of a representative QD are unaltered by replacement of the air-polymer interface by the oil-polymer interface. Consistent with assignment of negative trion,

50-53

the dim-state PL spectra possess

lower peak energy than that of the corresponding single-exciton PL spectra (or bright state). Figure 4b shows that the PL decay dynamics of both bright and dim states is mono-exponential

with

either type of interfaces. Similar to the bright-state emission, PL decay of the dim-state

emission

is

accelerated by replacing the air-polymer interface with the oil-polymer interface.

Using the peak counts of each intensity histogram for

Figure 4. (a) PL spectra of bright state and dim state of a single QD in PMMA facing air or oil. (b) Alteration of PL decay dynamics of bright state and dim states in (c) and (f). PL intensity trajectories of a single QD with (c) air and (f) oil on top of PMMA layer, and (d, e) the corresponding histograms (time bin 30 ms). The trajectories are subtracted by background and normalized by their bright-state intensity. (g) Experimental recombination rates of bright state (kX,r), experimental recombination rates of dim state (kX*,exp), radiative rates of trion state (kX*,r), and Auger recombination rates of trion state (kX*,A) with two types of interfaces. Note: all results in this Figure are obtained under Ar atmosphere.

the bright state (Figures 4c and 4d) as the normalization standard, one can determine the PL QY of the corresponding dim


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state (QYX*) of the QD with either type of interfaces. For the QD in Figure 4, its QYX* with oil-polymer interface is 42%, which is noticeably higher than that with the air-polymer interface (QYX* = 37%). With the dim-state PL QY as QYX* = kX*,r /(kX*,r + kX*,A) and dim-state decay rate as kX*,exp = kX*,r + kX*,A, both radiative rate (kX*,r) and non-radiative Auger recombination rate (kX*,A) of the trion state are calculated (Figure 4g).

Experiments on different individual dots show the same trends as that shown in Figure 4. Upon the interface replacement, degree of increase for kX,r roughly correlates with that of the kX*,r and kX*,A (Table S1, Supporting Information). Interestingly, the radiative and Auger non-radiative decay rates (kX*,r and kX*,A) of the trion state seems to be correlated positively (Figure 4g and Table S1, Supporting Information), yet increase of radiative decay rate (kX*,r) is always greater than that of the Auger non-radiative decay (kX*,A) upon replacing the air by the index-matching oil. This is mainly because Auger recombination is a non-radiative process and depends little on the density of states of the local dielectric environment.

The PL quantum yield of bi-exciton of single QD can be calculated from the antibunching curve measured using a Hanbury Brown and Twiss setup.

54

Under low excitation limit, the PL QY

ratio between bi-exciton and single-exciton of a QD equals to the ratio of the integral areas of the center and the side peaks of the antibunching curve.

55

Correlation measurements (Figure 5a)

reveal that, by replacing the air-polymer interface with the oil-polymer interface, the PL QY of bi-exciton state increases noticeably. Table S2 (Supporting Information) shows that this increase


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is qualitatively reproducible for different QDs. This result is reasonable if the radiative and Auger non-radiative decay pathways of both negative and positive trions

44, 56

follow the same trend

described above, and the Auger non-radiative decay is less sensitive to the change of refractive index than the corresponding radiative decay (Figures 4g).

Bi-exciton PL QY measured by single-dot spectroscopy in literature has been noticed with extremely large distribution, i.e., from nearly zero to nearly unity for single QD even from the same sample.

57, 58

Results in Figure 5a and Table S2 (Supporting Information) indicate that, in

addition to the inhomogeneous optical properties of the specific samples revealed recently,

59

inhomogeneous

dielectric environment encountered in single-dot spectroscopy might have also made some contributions.

Antibunching curve at low excitation power,

in

principle,

55

contains

information of PL decay lifetime of both single-exciton and bi-exciton. However, photon counts similar to

Figure 5. (a) Second order photon correlation measurements of single QD with either air-polymer or oil-polymer interface. (b) Bright state PL decay dynamics of a representative single QD under high power excitation. The inset represents bi-exciton decay dynamics, which are obtained by subtracting the single exponential fitting curves of the same QD under low power excitation with different dielectric interfaces respectively. (c) Bi-exciton lifetime of 5 single QDs with either air-polymer or oil-polymer interface.


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those in Figure 5a are too low to ensure accurate measurements of the lifetime values. To further study the dielectric environment dependence of bi-exciton PL decay dynamics, we obtain transient PL spectra of single QD under relatively high excitation power to garner significant counts for the bi-exciton PL. Correlation measurements (Figure 5b) show that, upon replacing the air-polymer interface by the thick oil layer, the mono-exponential lifetime of both single-exciton and bi-exciton PL decay decreases. This decrease is found to be qualitatively reproducible for multiple dots (Figure 5c). The correlation measurement results in Figure 5c further imply that, similar to that of the PL decay lifetime of single-exciton (Figure 2d), the shorter the bi-exciton PL decay lifetime under inhomogeneous dielectric environment—closer to the air-polymer interface—is, approximately the less decrease of bi-exciton PL decay lifetime would have upon addition of the thick oil layer.

Conclusion In conclusion, under typical measurement configurations for single QD spectroscopy, different PL properties of single QD can be altered in different manner/degree. Addition of index-matching, chemically inert, and millimeter-thick oil layer on top of the single QD sample—usually embedded in either thin polymer film or directly deposited on glass (Figure S6, Supporting Information)—can quantitatively correlate single-dot measurements with ensemble measurements. Without the index-matching oil layer, PL spectrum of single QD—either bright state or dim state—is found to be insensitive to local dielectric environment. Thus, PL spectrum can be applied as a marker for both size distribution of the ensemble and the intrinsic indicator of


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an individual dot under typical measurement configurations for single-dot spectroscopy. Conversely, the PL decay dynamics for single-exciton, negative trion, or bi-exciton state is sensitive to the local dielectric environment. Despite near unity PL QY of single-exciton emission, the measured PL intensity of single-exciton emission is sensitively dependent on the distance to the air-polymer interface under typical measurement configurations for single-dot spectroscopy. However, PL QY of both trion and bi-exciton emission is sensitive to the local dielectric environment, given the radiative decay being more sensitive to the dielectric environment than the corresponding non-radiative Auger decay. These results are not only important for understanding single-dot spectroscopy but also offer hints for developing advanced applications of colloidal QDs, such as lasers and single-photon sources.

METHODS AND EXPERIMENTAL Materials. Toluene (99%) was purchased from Sinopharm Reagents. PMMA was from Sigma Aldrich. Squalane (98%) was from Alfa Aesar. Glass coverslips were from Electron Microscopy Sciences. Immersion oil (Type-F) was from Olympus. The index-matching oil was a mixture of squalane (n = 1.45) and microscope immersion oil (n = 1.52). The refractive index of oil mixture was determined to be 1.48 by an Abbe refractometer.

Sample Preparations. The zinc-blende CdSe/CdS core/shell QDs with different core sizes and/or shell thicknesses were synthesized using the published procedures.34 All QD samples were


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purified using the methods reported previously 60 before used. The QD sample was diluted in 1.5% toluene-PMMA solution by weight and spin-coated on a clean glass coverslip to form a uniform QD-PMMA film. The film thickness was determined to be ~40 nm by a step profiler (KLA-Tencor P7). For measurements with isolated QDs on bare glass, the QDs were spin-coated onto a clean cover slip from a toluene solution without PMMA. The distance between the measured QD and neighboring dots in all single-dot measurements was at less 1 µm.

Optical measurements for single QDs. Single QD spectroscopy was performed on a far-field epi-fluorescence inverted microscopy system (Olympus IX 83) with a 60× (NA = 1.49) oil immersion objective (Figure S7). For steady-state PL measurements, a 445 nm CW laser (A.L.S. PiL044X) was used as an excitation light source. The emission signal from individual QDs was directed into an EMCCD camera (Andor iXon Ultra 897) for imaging. A spectrometer (Andor Shamrock 303i) was equipped before the camera for PL spectra measurements. Sequences of single QD spectra with bin time of 100 ms were recorded. PL intensity time traces were obtained by integrating the PL spectrum in each bin, and thus bright and dim states could be identified. PL spectrum of each brightness state (bright or dim state) was obtained by summing all spectra in the corresponding bins. For time-resolved measurements, the excitation laser was switched to a pulsed mode with 1MHz repetition rate. The emission signal from the same QD was collected with an avalanche photodiode (PicoQuant, τ-SPAD). The PL decay dynamics were carried out with a time-correlated single-photon counting (Becker & Hickl DPC-230) system in the time-tagged time resolved (TTTR) mode, with which PL intensity and lifetime time traces of a


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single QD could be obtained simultaneously. For the second-order photon correlation measurements, a Hanbury Brown and Twiss setup with two avalanche photodiodes was used. For all experiments 〈N〉 was kept below 0.1 unless otherwise mentioned.

Optical measurements for ensemble QDs. Absorption spectra were taken by an Agilent Technologies

Cary

4000

UV-vis

spectrophotometer.

Steady-state

and

time-resolved

photoluminescence spectra were measured using an Edinburgh Instruments FLS920 spectrometer. The absolute PL QY of ensemble QD sample was measured using an Ocean Optics FOIS-1 integrating sphere coupled with a QE65000 spectrometer. All optical measurements were performed at room temperature. All PL lifetime values were obtained by single exponential fit.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional figures and tables. (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] and [email protected]. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21573194), the National Key Research and Development Program of China (2016YFB0401600), and Science and Technology Planning Project of Guangdong Province, China (Grant 2015B090913001).

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Effects of Local Dielectric Environment on Single-Molecular

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