Electronic Supplementary Information

A Relationship Between Crystal Structures and

Spectroscopic Properties in Cesium Lead Bromide

(CsPbBr3) Perovskite Nanocrystals : Focusing on the

Photoluminescence Efficiency

Jumi Park,† Youngsik Kim,‡ Sujin Ham,† Ju Young Woo,§ Taehee Kim,† Sohee Jeong,‡,* and

Dongho Kim†,*

† Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722,

Republic of Korea.

‡ Department of Energy Science, Sungkyunkwan University, 2066 Seobu-Ro, Jangan-Gu,

Suwon, Gyeonggi-do 16419, Republic of Korea.

§ Micro/Nano Scale Manufacturing Group, Korea Institute of Industrial Technology, 143

Hanggaulro, Sangrok-gu, Ansan, Gyeonggi-do 15588, Republic of Korea.

Corresponding Author: dongho@yonsei.ac.kr (D.Kim); s.jeong@skku.edu (S.Jeong)

Electronic Supplementary Material (ESI) for Nanoscale.This journal is © The Royal Society of Chemistry 2019

Table of Contents

Experimental

1. Synthesis of CsPbBr3 nanocrystals (NCs)

2. UV-Visible & Photoluminescence spectroscopy

3. Femtosecond transient absorption spectroscopy (fs-TA)

4. Single molecule confocal microscopy

5. Auger ionization efficiency

6. PL intensity-lifetime scaling

Supporting Figures

1. Basic optical parameters

2. Photoluminescence (PL) spectra

3. Photoluminescence (PL) intensity trace in single crystals

4. Transient absorption spectra

5. Emission frequency shift

6. Multi-exciton Auger lifetime

7. X-ray photoelectron spectroscopy analysis

8. Energy diagram

9. Photoluminescence (PL) scan image

Reference

Experimental Section

1. Synthesis of CsPbBr3 nanocrystals (CPB)

CsPbBr3 nanocrystals were prepared following previously reported procedures with

modifications1,2

1) Preparation of cesium oleate solution: Cs2CO3 (0.163 g, Aldrich, 99.9 %), 1-

octadecene (8 mL, Uniam, 99%) and oleic acid (0.5 mL, Uniam, 99%) were loaded

into 100 mL of 3-neck flask. And then the solution was dried under vacuum at 110 oC

for 2 h. After drying, the temperature of the solution was further raised to 150 oC under

N2 and kept until the injection.

2) Synthesis of pristine-CPB and ZnBr2-CPB: We note that the samples are denoted as

“pristine-CPB” or “ZnBr2-CPB” depending on the presence of ZnBr2 in the synthesis.

0.138 g of PbBr2 (Aldrich, ≥98%), 1 mL of oleic acid (degassed, Uniam, 99%), and 1

mL of oleylamine (degassed, Aldrich, 70%) were placed into 100 mL 3-neck flask

(0,095 g of ZnBr2 (Aldrich, 99.999%) was added for the synthesis of ZnBr2-CPB). The

flask containing precursor solution is dried under vacuum at 110 oC for 2 h. And then,

the solution was further heated to 170 oC under N2, and 0.8 mL of prepared Cs-oleate

solution (150 oC) was swiftly injected. Approximately 10 s from the injection, the flask

was rapidly cooled down in an ice bath.

3) Isolation of CsPbBr3 nanocrystals: Reaction crude solution was centrifuged for 12

min at 10000 rpm. And then the precipitates were redispersed in 3 mL of anhydrous

toluene with strong vortexing. When the NCs were completely redispersed, the NC

solution was centrifuged for 32 min at 13000 rpm (temperature was set to 18 oC during

centrifugation). After the centrifugation, the precipitates were redispersed in 4 mL of

anhydrous hexane, and again centrifuged for 10 min at 6000 rpm. Finally, supernatant

was collected and filtered with syringe filter (pore size 0.45 µm) while the precipitates

were discarded. We note that all isolation procedures were performed under inert

condition.

2. UV-Visible & Photoluminescence spectroscopy

Steady-state absorption spectra were measured on a UV/Vis spectrometer (Cary5000, Varian)

and photoluminescence spectra were recorded by using a fluorescence spectrophotometer (F-

2500, Hitachi). Photoluminescence spectra are spectrally corrected by using correction factor

of the fluorescence spectrophotometer. All steady-state measurements carried out by using a

quartz cuvette with a pathlength of 1 cm at ambient temperatures.

3. Femtosecond transient absorption spectroscopy

The femtosecond time-resolved transient absorption (fs-TA) spectrometer consists of an optical

parametric amplifier (OPA; Palitra, Quantronix) pumped by a Ti:sapphire regenerative

amplifier system (Integra-C, Quantronix) operating at 1 kHz repetition rate and an optical

detection system. The generated OPA pulses have a pulse width of ~ 100 fs in the range of 280-

2700 nm, which are used as pump pulses. White light continuum (WLC) probe pulses were

generated using a sapphire window (4 mm thick) by focusing a small portion of the

fundamental 800 nm pulses which was picked off by a quartz plate before entering the OPA.

The time delay between pump and probe beams was carefully controlled by making the pump

beam travel along a variable optical delay (ILS250, Newport). Intensities of the spectrally

dispersed WLC probe pulses are monitored by a High Speed Spectrometer (Ultrafast Systems)

for both visible and near-infrared measurements. To obtain the time-resolved transient

absorption difference signal (ΔA) at a specific time, the pump pulses were chopped at 500 Hz

and absorption spectra intensities were saved alternately with or without pump pulse. Typically,

4000 pulses excite the samples to obtain the fs-TA spectra at each delay time. The polarization

angle between pump and probe beam was set at the magic angle (54.7°) using a Glan-laser

polarizer with a half-wave retarder in order to prevent polarization-dependent signals. Cross-

correlation fwhm in pump-probe experiments was around 200 fs and chirp of WLC probe

pulses was measured to be 1.2 ps in the 450-800 nm region. To minimize chirp, all reflection

optics were used in the probe beam path. A quartz cell of 2 mm path length was employed.

After completing each set of TA experiments, the absorption spectra of all samples were

carefully checked to rule out the presence of artifacts or spurious signals arising from, for

example, degradation or photo-oxidation of the samples in question.

4. Single molecule confocal microscopy

Samples for single molecule confocal measurements were prepared by spin-coating NC

solutions on rigorously cleaned quartz coverslips at 3000 rpm for 30s. The NC solutions were

composed of NCs in hexane containing 20 mg∙mL-1 polystyrene (M.W. = 40000, Aldrich). The

confocal microscope (TE2000-U, Nikon) was equipped with a sample scanning stage at RT.

Circular polarized light from a picosecond pulsed diode laser (LDH-D-C-450, Picoquent, 1

MHz repetition rate, prepared using a Berek compensator (5540, New Focus)) excited the

samples. It was passed through a laser line filter (FF01-420/10-25, Semrock) and collimating

lens. Then, it subsequently focused on the sample via an oil immersion objective (Plan Fluor,

1.3 NA, 100×, Nikon) with a power density corresponding to an average number of excitons

per pulse, ⟨𝑁𝑥⟩ = 0.1. Fluorescent signals were passed through a dichroic mirror (T425lpxr,

Chroma Technology), spectrally filtered using a notch filter (HNPF-420.0AR-1.0, Kaiser

optical systems) and a band pass filter (FF-01-430/LP-25, Semrock), and then split by using a

non-polarizing 50:50 beam splitter. Half of the fluorescence was dispersed via a spectrograph

(SpectraPro 2150i, Princeton Instruments) and projected onto an EMCCD camera (PL

PROEM:512B EMCCD, Princeton Instruments). The other half was detected by an avalanche

photodiode (APD) module (SPCM-AQR-16-FC, EG&G). The fluorescent signal detected by

the APD was registered by a time-correlated single photon counting (TCSPC) PC (SPC 830,

Becker & Hickl). The TCSPC was operated in first-in-first-out regime in which the arrival time

after the beginning of acquisition and the time lag with respect to the excitation pulse were

stored for each detected photon. The full-width half maximum (FWHM) of the overall

instrumental response function approximately corresponded to 500–600 ps. The data were

processed by using a BIFL data analyzer software (Scientific Software Technologies Center)

to obtain fluorescence intensity trajectories and the time-resolved fluorescence decays.

5. Auger ionization efficiency

The probability (P) of creating multiple excitons within a given bin time should be considered

to understand the on-time kinetics using Auger ionization processes. The probabilities of

creating n excitons in a single pulse and multiexciton (MX, n ≥ 2) were calculated by Equation

1 and 2.3

P⟨Nx⟩(n) = e-⟨Nx⟩∙⟨Nx⟩n/n! (1): the probability of the average number of created exciton on <Nx>

PMX = 1‒ e-⟨Nx⟩‒ ⟨Nx⟩·e-⟨Nx⟩ (2): the probability of the multiexciton on <Nx>, PMX

Here, the average number of generated excitons ⟨Nx⟩ can calculated by multiplying the

absorption cross section (σ, cm-2) at the excitation wavelength and the intensity of laser (j,

photons/cm2∙pulse) (⟨Nx⟩=jσ).

The Auger ionization efficiency influences the fall-off time of on-time distribution since Pfall-

off is the probability of forming the multiexciton state (PMX) and the ionization probability

(Pionize).3,4

Pfall off = ΔTrep/τfall-off = PMX × Pionize (3)

Pionize = Pfall off/PMX (Auger ionization probability, Pionize), where ΔTrep is a repetition rate of the

pulsed laser.

6. PL intensity-lifetime scaling

Intensity-lifetime scaling or the ratio of radiative rates between two distinct intensity levels ()

β =𝜏𝑜𝑛𝐼𝑜𝑛

×𝐼𝑜𝑓𝑓

𝜏𝑜𝑓𝑓

where and I are the PL lifetime and intensity of the corresponding “on” and “off” states.

In Figure 4(a), the radiative lifetime ratio of the the brightest state and dark state is calculated

as

𝜏𝑟𝑜𝑛𝜏𝑟𝑜𝑓𝑓

=𝜏𝑜𝑛𝐼𝑜𝑛

𝐼𝑜𝑓𝑓

𝜏𝑜𝑓𝑓=

4.24

95.2 − 2(𝑏𝑎𝑐𝑘𝑔𝑟𝑜𝑢𝑛𝑑)×15.6 − 2(𝑏𝑎𝑐𝑘𝑔𝑟𝑜𝑢𝑛𝑑)

0.49≈ 1.3

Thus, the dark state emission arises from trion.

In Figure 4(d), the radiative lifetime ratio of the brightest state and dark state is calculated as

𝜏𝑟𝑜𝑛𝜏𝑟𝑜𝑓𝑓

=𝜏𝑜𝑛𝐼𝑜𝑛

𝐼𝑜𝑓𝑓

𝜏𝑜𝑓𝑓=

3.54

35.8 − 2(𝑏𝑎𝑐𝑘𝑔𝑟𝑜𝑢𝑛𝑑)×14.2 − 2(𝑏𝑎𝑐𝑘𝑔𝑟𝑜𝑢𝑛𝑑)

1.38≈ 0.9

Thus, the dark state emission results from the trapping of band-edge electrons and holes.

Supporting Figures

Table S1 Basic optical parameters of CsPbBr3 NCs.

a The band gap energy yielded from first excitonic absorption peak Abs. b The Stokes shift is

defined as the energy difference between the first minimum in the 2/2 spectrum and the

emission peak. c The transition line width. d Absorption cross section at 3.1 eV. e Biexciton

binding energy.

Fig. S1 PL spectra and PL quantum yield of fresh Pristine- and ZnBr2-CPB.

3.1 2.5 2.1 1.8

400 500 600 700

Wavelength (nm)

PL

In

tensity (

a.u

.)

Energy (eV)

Pristine-CPB

ZnBr2-CPB

37.6%

71.5%

CsPbBr3 Ega [eV] s

b [meV] c [meV] absd

[10-15 cm2] xxe [meV] QY [%]

Pristine 2.46 45 14 9.27 77 72

ZnBr2 2.47 56 67 8.50 81 38

Fig. S2 Representative blinking dynamics for individual NCs : (a) fresh Pristine-, (b) 7-days

Pristine-, (c) fresh ZnBr2- and (d) 7-days ZnBr2-CPB.

Fig. S3 Femtosecond transient absorption (fs-TA) spectra of (a) Pristine-CPB and (b)

ZnBr2-CPB at early time before aging.

0 20 40 60 80 1000

50

100

150

200

250

300

350

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50 60

0

20

40

60

80

100

120

140

160

180

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50 60

0

50

100

150

200

250

300

350

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50 60

0

20

40

60

80

100

120

140

Time (s)

PL inte

nsity (

count/20m

s)

(a)

(b)

(c)

0 10 20 30 40 50 600

50

100

150

200

250

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50 60

0

50

100

150

200

250

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50 60

0

20

40

60

80

100

120

140

160

180

Time (s)

PL inte

nsity (

count/20m

s)

0 10 20 30 40 50 60 700

50

100

150

200

250

300

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50 60

0

20

40

60

80

100

120

140

160

PL inte

nsity (

count/20m

s)

Time (s)

(d)

0 10 20 30 40 50 600

20

40

60

80

100

120

140

160

180

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50 60 70

0

20

40

60

80

100

120

140

160

180

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50

0

10

20

30

40

50

60

PL inte

nsity (

count/20m

s)

Time (s)0 5 10 15 20 25 30 35

0

10

20

30

40

50

60

70

80

PL inte

nsity (

count/20m

s)

Time (s)

0 10 20 30 40 50 600

20

40

60

80

100

120

140

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50

0

20

40

60

80

100

120

Time (s)

PL inte

nsity (

count/20m

s)

0 10 20 30 40 500

20

40

60

80

100

PL inte

nsity (

count/20m

s)

Time (s)0 10 20 30 40 50

0

10

20

30

40

50

60

70

Time (s)

PL inte

nsity (

count/20m

s)

0 20 40 60 800

20

40

60

80

100

120

PL inte

nsity (

count/20m

s)

Time (s)

0 10 20 30 40 50 600

50

100

150

200

250

300

PL inte

nsity (

count/20m

s)

Time (s)

0 20 40 60 80 1000

20

40

60

80

100

PL inte

nsity (

count/20m

s)

Time (s)

480 500 520 540 560

2.58 2.48 2.38 2.30 2.21

-80

-40

0

40

80

Energy (eV)

m

A

Wavelength (nm)

0 ps 0.15 ps

0.35 ps 0.55 ps

0.75 ps 0.9 ps

1.5 ps 2 ps

3 ps 4 ps

5 ps

480 500 520 540 560

2.58 2.48 2.38 2.30 2.21

-80

-40

0

40

80

Energy (eV)

m

A

Wavelength (nm)

0 ps 0.15 ps

0.35 ps 0.55 ps

0.75 ps 0.9 ps

1.5 ps 2 ps

3 ps 4 ps

5 ps

Pristine-CPB(a) (b) ZnBr2-CPB

Fig. S4 Histograms of the net spectral shifts of a single CsPbBr3 NCs for (a) fresh Pristine-

CPB (b) Pristine-CPB after aging (c) fresh ZnBr2-CPB and (d) ZnBr2-CPB after aging.

Fig. S5 The Auger lifetime obtained from fitting the multiexciton dynamics by mono-

exponential decay for (a) fresh Pristine-CPB (b) fresh ZnBr2-CPB (c) Pristine-CPB after

aging and (d) ZnBr2-CPB after aging.

-100 -50 0 50 1000

500

1000

1500

2000

2500

Count

Emission Frequency Shift (meV)

Model Gauss

Equation y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*((x-xc)/w)^2)

Reduced Chi-Sqr

107.46567

Adj. R-Square 0.99836

Value Standard Error

B

y0 1.18472 0.65746

xc 1.30571E-4 1.4801E-5

w 0.00833 2.97482E-5

A 27.45813 0.08575

sigma 0.00417 1.48741E-5

FWHM 0.00981 3.50259E-5

Height 2629.95644 8.10687

-100 -50 0 50 1000

500

1000

1500

2000

2500

Count

Emission Frequency Shift (meV)

Model Gauss

Equation y=y0 + (A/(w*sqrt(PI/2)))*exp(-2*((x-xc)/w)^2)

Reduced Chi-Sqr

197.87171

Adj. R-Square 0.99647

Value Standard Error

B

y0 1.47948 0.97212

xc 9.02056E-5 2.12087E-5

w 0.00746 4.26445E-5

A 22.04829 0.11023

sigma 0.00373 2.13223E-5

FWHM 0.00879 5.02101E-5

Height 2357.23614 11.62373

FWHM 12

FWHM 12

FreshPristine-CPB

-100 -50 0 50 1000

200

400

600

800

1000

1200

Count

Emission Frequency Shift (meV)

-100 -50 0 50 1000

200

400

600

800

1000

1200

Count

Emission Frequency Shift (meV)

Model Gauss

Equationy=y0 + (A/(w*sqrt(PI/2)))*exp(-2*((x-xc)/w)^2)

Reduced Chi-Sqr

57.95302

Adj. R-Square 0.99578

Value Standard Error

B

y0 0.85218 0.46664

xc 1.03956E-4 3.35035E-5

w 0.01226 6.74613E-5

A 15.89984 0.07679

sigma 0.00613 3.37307E-5

FWHM 0.01443 7.94296E-5

Height 1034.96319 4.91078

FWHM 8

FWHM 7

(a) (b)

(c) (d)

7-daysPristine-CPB

FreshZnBr2-CPB

7-daysZnBr2-CPB

0 20 40 60

0.1

1 <Nx>

1.5

2.5

3.7

7.4

14.2

0 20 40 60

0.1

1 <Nx>

1.5

2.5

3.7

7.4

14.2

0 20 40 60

<Nx>

1.4

2.2

3.4

6.8

13.0

-A

Time (ps) Time (ps)

-A

0 20 40 60

<Nx>

1.4

2.2

3.4

6.8

13.0

(a) (b)

(c) (d)Time (ps) Time (ps)

31 ps28 ps

31 ps26 ps

Fig. S6 (a) Pb/Cs and (b) Br/(Cs+Pb) atomic ratios of Pristine- and ZnBr2-CPB from XPS

data.

Fig. S7 Energy diagrams of Pristine-CPB (left) and ZnBr2-CPB (right). The bandgap energy

is obtained from first excitonic absorption peak (Table S1). The subband gap energy is

measured as the difference between the first transition energy and the second transition

energy. The intraband gap energy changes depending on the type of ligands adsorbed on the

substrate surface.

1.0

1.1

1.2

1.3

1.4

1.5

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Pristine-CPB ZnBr2-CPBPristine-CPB ZnBr2-CPB

Pb

/Cs

Br/

(Cs+

Pb

)

(a) (b)

0.0840.073

Pri

stin

e-

Cs

Pb

Br 3

Zn

Br 2

-

CsPb

Br 3

2.4

6

2.47

E (eV)

Cs-o

leat

e

Br-

ole

yla

mm

on

ium

Fig. S8 Typical PL scan images of CsPbBr3 NCs in single crystal measurements.

Fresh 7-days

Reference

1 L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R.

X. Yang, A. Walsh and M. V. Kovalenko, Nano Lett., 2015, 15, 3692–3696.

2 J. Y. Woo, Y. Kim, J. Bae, T. G. Kim, J. W. Kim, D. C. Lee and S. Jeong, Chem.

Mater., 2017, 29, 7088–7092.

3 J. J. Peterson and D. J. Nesbitt, Nano Lett., 2009, 9, 338–345.

4 Y. Kim, S. Ham, H. Jang, J. H. Min, H. Chung, J. Lee, D. Kim and E. Jang. ACS Appl.

Nano Mater., 2019, 2, 1496–1504.