SUPPLEMENTARY MATERIAL

Enhanced Biexciton Emission from Single Quantum Dots Encased in N-type Semiconductor Nanoparticles

Zhijie Li, Guofeng Zhang*, Bin Li, Ruiyun Chen, Chengbing Qin, Yan Gao, Liantuan Xiao*, and Suotang Jia

1State Key Laboratory of Quantum Optics and Quantum Optics Devices, Institute of Laser Spectroscopy, Shanxi

University, Taiyuan, 030006, China

2Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi, 030006, People’s Republic of

China.

1. Sample Preparation

The NIR CdSeTe/ZnS3ML core/multishell QDs (Qdot®800ITKTM Organic QDs) were ordered from

Thermo Fisher Scientific. Absorption and emission spectra for the CdSeTe/ZnS3ML core/multishell QDs

are shown in Fig. S1. Their maximum fluorescence emission wavelength is at ~800 nm. A transmission

electron microscope (TEM) image of QDs is shown in Fig. S2. The size of the QDs is 8.3±2.1 nm and

the diameter of core is ~6.4 nm. The indium tin oxide (ITO) nanoparticles (<100 nm particle size (DLS),

30 wt. % in isopropanol) were ordered from Sigma-Aldrich. The ITO was purified with a filter with pore

size of 0.45 m. After centrifuging it at 3000 rpm for 5 minutes, then the supernatant was wiped out, and

the precipitate was dispersed in toluene with a concentration of ~15 wt.％. At last, single QDs solution

of 10-8 mol/L in toluene was added to the dispersion and formed a mixture with single QDs

concentration of ~10-9 mol/L. The mixture was spin-coated onto a cleaned glass coverslip with a

rotational speed of 3000 rpm to form a ~350 nm ITO film encasing single QDs. The samples were

placed in vacuum at 315 K for 5 hours to remove the residual solvent. A scanning electron microscope

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(SEM) image and an atomic force microscope (AFM) image of ITO nanoparticles encasing single QDs

are shown in Fig. S3. Single QDs were prepared onto glass coverslips for a reference experiment

synchronously.

FIG. S1. Absorption and emission spectra of CdSeTe/ZnS3ML core/multishell QDs in solution.

FIG. S2. Transmission electron microscope (TEM) image of single near-infrared emitting CdSeTe/ZnS3ML core/multishell QDs.

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FIG. S3. Scanning electron microscope (SEM) image and atomic force microscope (AFM) image of indium tin oxide (ITO) nanoparticles spin-coated onto glass coverslip.

2. Experimental Setup

FIG. S4. Schematic depiction of experimental setup.

The photoluminescence (PL) behavior of single QDs was measured under a sample-scanning

confocal microscope system consisting of an oil-immersion objective lens (Nikon, 100, 1.3NA) in

combination with picosecond-pulsed diode laser excitation at 635 nm (PDL800-D PicoQuant, 55 ps).

Schematic depiction of experimental setup is shown in Fig. S4. The laser light, passing through

polarization beam splitter (PBS), a /2 and a /4 wave-plate, become circular polarization light. The oil

immersion objective was used to focus laser light onto the sample and collect fluorescence

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simultaneously. The fluorescence, passing through a dichroic mirror (Semrock) and a high pass filter

(Semrock), was focused into a 100 μm pinhole to reject out-of-focus photons. After the pinhole, the

fluorescence was split by a 50/50 beam splitter cube (BS) into two beams and finally detected by a pair

of single-photon avalanche diode detectors (SPCM-AQR-15, PerkinElmer). The synchronization of

pulse laser and the outputs of detectors were fed into a time-tagged, time-resolved and time-correlated

single photon counting (TTTR-TCSPC) data acquisition card (HydraHarp 400, PicoQuant), by which

the time-dependence information for all of the detected photons can be recorded simultaneously. A

piezo-scan stage (Tritor 200/20 SG) with an active x-y-z feedback loop mounted on the inversion

microscope was used to scan the sample over the focused excitation spot. The time resolution of the

system is approximately 350 ps. All measurements were performed at room temperature.

3. Selection of the pump fluence

FIG. S5. The pump fluence dependence of g(2)(0) in the range of <N> from 0.1 to 1.0.

The <N> of ~ 0.25 as a weak excitation is selected by the pump fluence dependence of g(2)(0). The

pump fluence dependences of g(2)(0) for single QDs on glass and in ITO are shown in Fig. S5, which

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are obtained from the average values of ~50 single QDs on glass and in ITO, respectively. The values

of g(2)(0) are measured with <N> in the range of 0.1 to 1.0. When <N> is smaller than 0.1, it is difficult

to measure the g(2)(0) for single QDs in ITO due to the extremely low PL intensity. From the Fig. S5,

we can see that the g(2)(0) values in the two cases approach two constant values when <N> is reduced to

0.1. For single QDs on glass, the constant value of g(2)(0) is ~0.12. For single QDs in ITO, the constant

value of g(2)(0) is ~0.57. When <N> is bigger than 0.3, the g(2)(0) value increases along with the

increases of <N>. When <N> is between 0.1 and 0.3, the g(2)(0) values are little changed. Therefore, we

set <N> to be ~0.25 to measure the PL intensity, decay curves, and the second-order correlation

function in the Letter.

4. Distinguishing Single QDs from QD Clusters

In order to confirm the central peak in correlation function is mainly caused by BX emission of a

single QD instead of QD clusters, we reconstruct the second-order correlation function by applying a

suitable time gate based on the different decay rates of BX and SX states.1,2 Figure. S6(a) and S6(b)

show a typical PL decay curve and its corresponding second-order correlation function for QDs in ITO.

Here we select the photons with arrival times between 11 ns and 200 ns (blue shaded area in Fig. S6(c))

to reconstruct the correlation function. By this way, we can effectively remove the photons caused by

BX emission. Obviously, a pure photon antibunching is obtained as shown in Fig. S6(d).

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FIG. S6. (a) Photoluminescence (PL) decay curve of a single QD encased in ITO and (b) the corresponding second-order correlation function. (c) PL decay curve of a single QD encased in ITO with a blue shaded area from 11 to 200 ns and (d) the second-order correlation function with a time gate to remove BX emission.

5. Estimating QYs of SX and BX

The QY of the QD ensemble is measured to be ~0.7. Therefore, the average ηSX of single QDs on

glass is considered to be ~0.7. According to ηBX/ηSX g(2)(0) = 0.12 in the main text, the average ηBX for

single QDs on glass is considered to be 0.08. The average intensity values of histograms in Fig. 1(c) are

9.1 and 6.3 for single QDs on glass and in ITO, respectively. Therefore, the average PL intensity drops

by a factor of 0.69 (=6.3/9.1). According to the change of PL intensity in the two cases, we can estimate

the average QY to be 0.48 for single QDs in ITO. Therefore, according to ηBX/ηSX g(2)(0) = 0.57, the

average ηSX and the average ηBX for single QDs in ITO are estimated to be ~0.31 and ~0.17, respectively.

The ηSX reduces from 0.7 on glass to 0.31 in ITO due to the nonradiative (NR) processes that

strongly quench SX emission, while ηBX increases from 0.08 on glass to 0.17 in ITO, because BX

emission is more dominant than the NR processes due to decrease of BX AR. Therefore, the increase in

g(2)(0) results from both the increase of ηBX and the decrease of ηSX.

In addition, ηBX /ηSX can be independently estimated by ηBX /ηSX = 4τBX/τSX with the ensemble lifetimes

of SX and BX.3 The single QDs are prepared on glass and in ITO, so it should be more appropriate to

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use the average lifetimes in Fig. 2(a) as the ensemble lifetimes. For the single QDs on glass, ηBX/ηSX

0.12 with τSX = 78 ns and τBX = 2.3 ns, and for the single QDs in ITO, ηBX/ηSX 0.56 with τSX = 20 ns and

τBX = 2.8 ns. Therefore, the ηBX/ηSX estimated by the average lifetimes are similar to that estimated by the

correlation function (the values of g(2)(0) are ~0.12 and ~0.57 for the QDs on glass and in ITO,

respectively).

6. Detailed Analysis about ITO Suppressing AR of BX

ITO has a higher Fermi level than that of QDs, therefore the electrons in ITO will be transferred to

QDs.4-6 The dielectric screening of QD is enhanced due to the extra charges induced, thus reducing

Coulomb interactions between carriers. Quantitative electrostatic force microscopy measurements have

shown about 2~3 electrons on single QD surface as single QDs are in contact with ITO. 4 Wang has

demonstrated that the electric field set up by a classic point charge located near the surface of a QD can

substantially alter the distribution of the electron and hole wave functions.7 For example, in the case of a

negative charge, the holes are attracted by the surface charge, while the electrons move in the opposite

direction, as a result of the reduced overlap between electron and hole wave functions.8

The reduction of the electron and hole overlap leads to a significant lengthening of both the SX

lifetime and the BX lifetime.9,10 In this Letter, the averaged BX lifetime is changed from 2.3 ns to 2.8 ns,

which seems to be consistent with the expectation. However, the averaged SX lifetime is reduced from

78 ns to 20 ns due to the more dominant NR processes introduced by ITO.

In addition, the reduced overlap also leads to a significant lengthening of BX AR lifetime,

indicating a suppression of AR of BX.9,10 Indeed, the is increased from 0.08 to 0.17 in this Letter,

because BX emission is more dominant than the NR processes due to decrease of BX AR.

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