ALL-INORGANIC PEROVSKITE CESIUM LEAD BROMIDE QUANTUM

DOTS/POLYMER NANOCOMPOSITE PLASTIC SCINTILLATOR FOR GAMMA-RAY

DETECTION

By

JING XIE

A THESIS PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2018

© 2018 Jing Xie

To my family, friends, and everyone who supported me

4

ACKNOWLEDGMENTS

First and foremost, I would like to express my sincere gratitude to my advisor, Dr.

Jiangeng Xue, for his guidance and generous support over the past one years. The completion of

this thesis would not be possible without his insight and mentorship.

I would also like to thank Dr. James Baciak and Dr. Andreas Enqvist for serving on my

committee. Their instruction and guidance are indispensable for the completion of this thesis.

I am grateful for the help received from past and current Xue group members. To my

fellows Dr. Mike Sexton, Dr. Daken Starkenburg, Dr. Sihan Shen, Zhengtao Xu, Jiangyong Pan

and Ying Xue, I will miss the moments doing synthesis and polymerization with you guys’ help.

From other research groups and facilities, I would also like to extend my gratitude to Ph.

D candidate Yuan Gao in Dr. Baciak’s group and Dr. Xianfei Wen, Dr. Yinong Liang in Dr.

Enqvist’s group for their discussion and help during the gamma-ray scintillation measurements. I

am also indebted to Dr. Lilong Zhu in Dr. Manuel’s group for providing the polisher.

And finally, to my mother, old brother and relatives, words cannot express my gratitude

for your steadfast love and support. Thank you.

5

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...............................................................................................................4

LIST OF TABLES ...........................................................................................................................7

LIST OF FIGURES .........................................................................................................................8

LIST OF ABBREVIATIONS ........................................................................................................10

ABSTRACT ...................................................................................................................................11

1 INTRODUCTION ..................................................................................................................13

Overview of Gamma-Ray Detectors ......................................................................................13

Semiconductor Detectors .................................................................................................13

Scintillators ......................................................................................................................14

Conventional Scintillators ........................................................................................14

Nanocomposite Plastic Scintillators .........................................................................15

Gamma-Matter Interaction Mechanisms and Detecting Device ............................................16

Gamma Ray-Matter Interaction .......................................................................................16

Photoelectric absorption ...........................................................................................16

Compton scattering ..................................................................................................17

Pair production .........................................................................................................19

Motivation and Research Scope of the Thesis ........................................................................20

2 FABRICATION OF CSPBBR3 QDS/POLY (METHYL METHACRYLATE)

NANOCOMPOSITE SCINTILLATORS ..............................................................................23

Background .............................................................................................................................23

Experimental Section ..............................................................................................................24

Materials ..........................................................................................................................24

Synthesis of CsPbBr3 QDs in MMA ...............................................................................25

Fabrication of CsPbBr3 / PMMA Nanocomposite Monoliths .........................................25

Characterization ...............................................................................................................25

Results and Discussion ...........................................................................................................26

Synthesis of CsPbX3 (X = Cl, Br) QDs in MMA ............................................................26

Fabrication of CsPbX3 (X = Cl, Br)/ PMMA Nanocomposite Monoliths .......................29

Conclusion ..............................................................................................................................32

3 FABRICATION OF CSPBBR3 QDS/ POLY (BUTYL METHACRYLATE)

NANOCOMPOSITE SCINTILLATOR ................................................................................33

Background .............................................................................................................................33

Experimental Section ..............................................................................................................33

Materials ..........................................................................................................................33

6

Synthesis of CsPbBr3 QDs in BMA ................................................................................34

Fabrication of CsPbBr3 / PBMA Nanocomposite Monoliths ..........................................34

Characterization ...............................................................................................................34

Results and Discussion ...........................................................................................................35

Synthesis of CsPbBr3 QDs in BMA ................................................................................35

Synthesis of CsPbBr3 in PMMA .....................................................................................36

Influence of Thickness and Concentration on Transmittance .........................................38

Conclusion ..............................................................................................................................41

4 SCINTILLATING PROPERTIES OF CSPBBR3/PMMA AND CSPBBR3/PBMA

NANOCOMPOSITE PLASTIC SCINTILLATORS .............................................................42

Background .............................................................................................................................42

Experimental Section ..............................................................................................................42

Results and Discussion ...........................................................................................................45

Conclusion ..............................................................................................................................48

5 CONCLUSION, FUTURE WORK AND PERSPECTIVE ...................................................49

Conclusion ..............................................................................................................................49

Future Work ............................................................................................................................50

Perspective ..............................................................................................................................54

LIST OF REFERENCES ...............................................................................................................56

BIOGRAPHICAL SKETCH .........................................................................................................59

7

LIST OF TABLES

Table page

3-1 Absorbance and PL data of monolith in Figure 3-2 (b) .....................................................37

4-1 Different gamma ray nuclides and their corresponding gamma ray energy. .....................47

8

LIST OF FIGURES

Figure page

1-1 Schematic illustration of the photoelectric absorption process (a) and the production

of K x-ray (b) (Copyright 2017, UCLA) ...........................................................................17

1-2 Schematic illustration of a Compton scattering process (Copyright 2017, UCLA) ..........19

1-3 Schematic illustration of a pair production process. (Copyright 2017, UCLA) ................19

1-4 Schematic illustration of the detection of gamma photon using a scintillator coupled

to a photomultiplier tube. (Copyright 2017, UCLA) .........................................................20

2-1 Absorbance and photoluminescence spectra of typical CsPbBr3 in MMA solution.

Inset: Cubic perovskite structure of CsPbBr3 inorganic perovskite quantum dots ............27

2-2 Photographs of perovskites CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 solutions in MMA

under ambient light (Left three vials) and under 7 W 365 nm UV-light (Right three

vials). ..................................................................................................................................28

2-3 Normalized photoluminescence spectra of CsPbBr3, CsPbBr2Cl, and CsPbBrCl2

solutions in MMA (Excitation wavelength: 350 nm) ........................................................28

2-4 Linear fitting of integrated PL intensities vs. absorbance for (a) dilute quinine sulfate

solution; (b) dilute CsPbBr3/MMA solution; .....................................................................29

2-5 Normalized photoluminescence spectra of CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 in

PMMA monoliths (Excitation wavelength: 350 nm).........................................................30

2-6 Normalized photoluminescence spectra comparisons of CsPbBr3, CsPbBr2Cl, and

CsPbBrCl2 before and after polymerization ......................................................................30

2-7 CsPbBr3/PMMA samples with different concentrations prepared for solid QY

measurement ((a): under ambient light; (b): under 365 nm UV light; samples

dimension: 25 mm diameter, 12 mm thickness). (c) and (d) show polished two

samples ...............................................................................................................................31

2-8 Linear fitting of integrated PL intensities versus absorbance for CsPbBr3/PMMA

nanocomposite monoliths ..................................................................................................32

3-1 Photograph of CsPbBr3 perovskites quantum dots form in BMA (left) and MMA

(right) under the ambient light. ..........................................................................................35

3-2 Linear fitting of Integrated PL intensities versus absorbance of CsPbBr3 in BMA

dilute solution.....................................................................................................................36

9

3-3 Photographs of diluted different concentrations of CsPbBr3 in PBMA bulk (vials 1,

2, 3 in (a)) and cut and polished monolith from vial 2 in (a). ............................................37

3-4 (a) Photographs comparison between CsPbBr3-PBMA nanocomposite (left) and

CsPbBr3-PMMA (right) nanocomposite under ambient light, and (b) Normalized PL

spectra comparison between CsPbBr3 in MMA, BMA and CsPbBr3 in PMMA,

PBMA ................................................................................................................................38

3-5 Transmittance spectra of CsPbBr3-PBMA monoliths with different thicknesses

(concentration: 9 mL-150 μL; diameter: 25 mm) ..............................................................39

3-6 Transmittance of CsPbBr3-PBMA composite with different thicknesses

(concentration: 9 ml-300 μL) .............................................................................................40

3-7 Transmittance comparison CsPbBr3-PBMA composite with the same thickness but

with different concentrations .............................................................................................41

4-1 Photographs of every single part of the measuring system for detecting the gamma

photon by using the plastic scintillators coupled to photomultiplier tube. ........................44

4-2 Schematic diagram of the detection of gamma photon using a scintillator coupled to

photomultiplier tube and signal-processing electronics .....................................................44

4-3 Photographs of standard reference plastic scintillator EJ212 ordered from EJ

Company and as-fabricated CsPbBr3-PBMA nanocomposite plastic scintillator. (a) is

under the ambient light, and (b) is under 7W 365 nm UV-light. .......................................45

4-4 (a)–(e) Pulse height spectra of standard reference plastic scintillator EJ212 ordered

from EJ Company with different gamma ray sources; (f) linear fitting of Compton

edge energy of different gamma sources versus integral energy. ......................................46

4-5 Pulse height spectrum of CsPbBr3-PBMA nanocomposite with Cs137 gamma ray

source .................................................................................................................................48

5-1 Schematic illustration of partial ligand exchange and the molecular structures of

surface ligands and monomers ...........................................................................................51

5-2 Photographs of PMMA (left) and PBMA (right) with surface modified CsPbBr3

embedded (~ 2 wt%; 25 mm diameter; ~2 mm thickness) ................................................51

5-3 (a) Diagrammatic sketch of an all-inorganic perovskite CsPbBr3@A-CsPbBrx

composite; (b) structural characterization of perovskite CsPbBr3 (Copyright 2018

from the authors) ................................................................................................................53

5-4 (a) Normalized UV-vis absorbance and PL spectra (excQD = 350 nm, excFBtF = 420

nm) of CdxZn1-xS/ZnS core/shell quantum dot and FBtF dye in dilute solutions. (b)

FRET process between QD and dye. (Copyright 2017 from the authors) .........................54

10

LIST OF ABBREVIATIONS

BMEP

BMA

DMF

EGDMA

FRET

LY

MMA

OA

OAm

PBMA

PMMA

Bis[2-(methacryloyloxy)ethyl] phosphate

Butyl methacrylate

Dimethylformade

Ethylene glycol dimethacrylate

Forster resonance energy transfer

Light yield

Methyl methacrylate

Oleic acid

Oleylamine

Poly (butyl methacryalte)

Poly (methyl methacrylate)

PL

PLQY

Photoluminescence

Photoluminescence quantum yield

QDs

SR

TPO

Quantum dots

Supersaturation recrystallization

Diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide

11

Abstract of Thesis Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Master of Science

ALL-INORGANIC PEROVSKITE CESIUM LEAD BROMIDE QUANTUM

DOTS/POLYMER NANOCOMPOSITE PLASTIC SCINTILLATOR FOR GAMMA-RAY

DETECTION

By

Jing Xie

December 2018

Chair: Jiangeng Xue

Major: Materials Science and Engineering

The development of scintillators with high performance and low cost has been an

important research filed because of their wide application in national securities and medicine.

Conventional scintillators are limited by the cost and their intrinsic properties, the researches in

nanocomposite plastic scintillators have great promises because it can combine advantages from

both the organics and inorganics. However, the increase complexity of blending nanoparticles

into a polymer matrix brings challenges from both science and engineering aspects, impeding the

advancing of this field.

This thesis is dedicated to the engineering development and scientific understandings of

perovskite quantum dots-based nanocomposite plastic scintillators. In Chapter 2, all inorganic

perovskite quantum dots CsPbX3 (X = Cl, Br) were successfully synthesized in methyl

methacrylate solution, and the PLQY of CsPbBr3 in MMA was determined to be about 45%.

The transparent CsPbBr3/PMMA nanocomposites scintillators have been fabricated via

photoinitiated polymerization. The PLQY of CsPbBr3/PMMA nanocomposites scintillators was

determined to be 21%. In Chapter 3, CsPbBr3 perovskite nanoparticles were successfully mixed

into PBMA, and the PLQY of CsPbBr3 in BMA and PBMA were determined to be 66% and

12

25%. The influence of thickness and two different concentrations of CsPbBr3 nanoparticles on

transparency of CsPbBr3-PBMA monoliths were investigated. In Chapter 4, gamma scintillation

measurement was performed using a home-built system. Unfortunately, only the pulse height

spectra of standard reference EJ212 plastic scintillator were obtained by using different gamma

ray sources. The pulse height spectra, the Compton plateau and Compton edge of as-fabricated

CsPbBr3-PBMA nanocomposite were not observed. The reason could be the extremely low

loading of CsPbBr3 nanoparticles, which leads to the very low gamma ray stopping power and

cannot detect the gamma ray. The method to solve this low loading and aggregation problem is

to apply the surface ligand exchange.

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CHAPTER 1

INTRODUCTION

Overview of Gamma-Ray Detectors

Semiconductor Detectors

Low cost, high performance gamma ray radiation detectors are being extensively

investigated over many decades due to their wide potential applications in the fields of high

energy physics, homeland security (non-proliferation of nuclear materials), medical imaging and

fundamental scientific research.1-3 In this context, numerous detectors could be used for gamma

ray detection, such as semiconductor detectors, scintillators. For semiconductor, it converts the

energy of the incident gamma photons into charger carriers, electron and hole pairs, and then

collect and detect these charger carriers, giving the energy information of the incident gamma

photons. The most important advantage of semiconductor detector over scintillator is the greatly

improved resolution. A typical energy resolution of semiconductor detectors is small than 1%,

whereas for scintillators it is 3-10%.1, 4 Such advantage can be attributed to factors that

semiconductor detectors have the capability of producing much more charger carriers under

lower energy required for the production of an electron hole pair (around 3 eV) than that of

scintillation photons (tens of eV) and that Fano factors for semiconductor detectors are usually

less than one compared to the near unity Fano factor for scintillators. Hence, semiconductor

detectors are always applied for high precision in energy resolutions (< 3%).56-7 However,

semiconductor detectors are much more expensive to fabricate and operate, which require a large

detector with extremely high purity, such as high purity germanium (HPGe) detector. Some new

materials such as CdTe, Cd1-xZnxTe, HgI2, and cesium lead halide perovskites have been

proposed and investigated as the high-Z room temperature substitutes of HPGe semiconductor

14

detectors, but their performance is still substantially inferior to the HPGe, mainly because of the

difficulties in achieving a high purity level at large detector size.5656, 7-9

Scintillators

Conventional Scintillators

Inorganic scintillators have been paid attention for scintillation gamma photon detection

since Robert Hofstadter discovered that the thallium doped sodium iodide crystal has prominent

scintillation response.5656 The doped high-Z activator, which always have a high Z higher than

50, provides inorganic scintillator high density, high attenuation power, and high photoelectric

cross-section of gamma photons, improving the spectroscopic detection of gamma ray. However,

the production and purification process are time and energy consuming, and most salts used are

hygroscopic. These disadvantages lead to a high price of inorganic single-crystal scintillators,

limiting their wide application in which large volumes are needed. Organic scintillators are

another important class of scintillation detector.5611, 12 It contains two classes: single crystals,

such as pure anthracene or stilbene, and amorphous, which consists both liquid scintillator and

plastic scintillator. While single crystals have high light yields up to 17,000 photons/MeV,

however, it is hard to grow crystals with large sige and the inferior mechanical properties prevent

them from widespread application. Alternatively, plastic scintillators and liquid scintillators

become the mainstream organic scintillators because of their great advantage in large-scale

production (especially for large-size detection systems), easy fabrication, and cost-effectiveness.

The structures of plastic and liquid scintillators are almost the same, with former being a “solid

solution” version of the liquid scintillators.5

15

Nanocomposite Plastic Scintillators

In the past two decades, much attentions have been paid in the development of new high-

Z nanoparticle, such as BaF2, HfO2, rare earth fluorides and oxide, II-VI semiconducting

quantum dots.13-17 Due to the strong excluded volume effect, directing mixing nanoparticles into

polymer is impractical in fabricating transparent nanocomposites.18

In 2003, Vasil’chenko and Solov’ev first reported 10 μm diameter particles mixed with

BC-600 epoxy adhesive to fabricate the composite scintillator, but the large size nanoparticles

lead to severe optical loss.19 In 2006, Campbell and Crone mixed the quantum dots with

semiconducting polymer to fabricate scintillator detectors.20 They reported that the light yield

can be enhanced by the addition of the quantum dots into the organic matrix due to the high-Z

properties. Nevertheless, they only fabricated thin film, which is not good enough for stopping

high energy gay. Many works were done on nanocomposites with quantum dots, but the loadings

of nanoparticles are very low, less than 0. 5wt%, which is because of the severe aggregation

problem of nanoparticles at high loadings.21,22

From 2012-2018, the Pei group made a breakthrough on fabricating the transparent high-

loading nanoparticle-polymer nanocomposite plastic scintillators.23-27 By grafting methacrylate

groups onto the nanoparticles surface, the nanoparticles can be covalently bonded onto the

growing polymer chain. In situ polymerization, the monomer solution containing these surface-

modified nanoparticles was polymerized to produce highly transparent nanocomposite monoliths

containing up to 20 wt% of Gd2O3 nanoparticles.23 With the addition of blue or green fluorescent

dye with large Stocks shift, the detection of a 662 KeV gamma photopeak resolution was

determined to be 11.4% by using their transparent Gd2O3-loading nanocomposites. In 2016, Pei

group used the same methodology successfully fabricated YbF3-PVT nanocomposite monolith

for gamma ray detection. The loading can get to 80 wt%, ligands included, and the light yield is

16

8600 photos/MeV, but no photopeak resolution reported.25 In 2016, they fabricated HfO2-PVT

nanocomposite with 40 wt% (net hafnium wt% up to 28.5%). The full energy photopeak for 662

KeV gamma ray resolution is less than 8%.24 In 2017, quantum dot (CdZnSe/ZnS)-polymer

nanocomposite monoliths were made with 60 wt% loading and with 9.8% resolution for 662

KeV Cs-137 gamma rays.26 However, this resolution is still far from the 6% resolution of NaI

(Tl).

Gamma-Matter Interaction Mechanisms and Detecting Device

Gamma Ray-Matter Interaction

Interaction between gamma photons and matter has three types: photoelectric absorption,

Compton scattering, and pair production.1, 5

Photoelectric absorption

The photoelectric absorption typically happens when the incident gamma photon energy

is relatively low, with just a few hundred KeV. As shown in Figure 1-1, in the photoelectric

absorption process, an incident gamma photon comes in, interacting with the electrons (always

the inner-most K shell electrons of the atom) of the absorber in which the gamma photon

completely disappears, and the photoelectron is given all of the gamma photon energy and is

ejected from the atom. The photoelectron energy is given by:

𝐸𝑒− = 𝐸𝑟 − 𝐸𝑏 (1-1)

where 𝐸𝑒− , 𝐸𝑟 , 𝐸𝑏 represents the photoelectron energy, incident gamma photon energy and

binding energy of the photoelectron, respectively. After this, the vacancy in K shell will be

quickly refilled by an electron from the outer shell, resulting in a characteristic K-shell X-ray

produced. The photoelectric absorption cross-section could be significantly improved by

increasing the Z of absorber. A rough approximation for the relationship between the

photoelectric absorption cross-section and Z is:

17

𝜎 𝑍𝑛

𝐸𝑟3.5 (1-2)

where the exponent n varies between 4 and 5 over the gamma ray energy region of interest. So,

many detectors used for gamma ray detection are chosen from high-Z components for the same

reason.

Figure 1-1. Schematic illustration of the photoelectric absorption process (a) and the production

of K x-ray (b) (Copyright 2017, UCLA)

Compton scattering

The Compton scattering takes place between the incident gamma photon and the

electrons in the absorbing material. Typically, it is the predominant interaction mechanism for

gamma energy of radioisotope sources. In Compton scattering, the incoming gamma photon is

18

deflected through an angle θ with respect to its original direction and transfer part of its energy to

the electron. Assuming the original electron is at rest, it is given momentum and kinetic energy.

This electron is called recoil electron, as shown in Figure 1-2. For Compton scattered gamma

photons, the energy detected by the detector is essentially the energy transferred from incident

gamma photon to the recoil electron, if no other process with energy deposition happens. The

Energy of the scattered gamma photon and the recoil electron can be derived by solving the

equations obeying the conversation of energy and momentum:

𝐸′ =𝐸𝛾

1+(1−cos 𝜃)𝐸𝑟

𝑚𝑒𝑐2

(1-3)

𝐸𝑒 = 𝐸𝑟 − 𝐸′ (1-4)

where 𝐸′, 𝐸𝛾, θ, 𝑚𝑒𝑐2, and 𝐸𝑒 are the energy of scattered gamma photon, energy of incident

gamma photon, scattering angle of the gamma photon, rest-mass energy of an electron (0.511

MeV), and the energy of the recoil electron, respectively. It is obvious that for a fixed incident

gamma photon energy, the energy deposited in the absorber through Compton scattering is only

determined by the scattering angle θ, which can vary from zero to 180˚. As a consequence, the

detected energy profile is a continuum from 0 to the highest recoil electron energy, for which the

scattering angle is 180˚. This continuum is called Compton continuum, which decreases abruptly

to zero at the highest recoil electron energy, creating a Compton edge.

19

Figure 1-2. Schematic illustration of a Compton scattering process (Copyright 2017, UCLA)

Pair production

Pair production takes place when the incident gamma energy is larger than 1.02 MeV,

twice of the rest-mass energy of an electron. As shown in Figure 1-3, the interaction takes place

under the strong coulomb field near nucleus. The gamma photon with high energy disappears

and the electron-position pair is created. All the excess energy above 1.02 MeV are shared by

electron and position pair to form the kinetic energy, then they deposit their kinetic energy on

their path. The position will annihilate with an electron to produce two annihilation gamma

photons with 511 KeV as the position is slowed down in the absorber.

Figure 1-3. Schematic illustration of a pair production process. (Copyright 2017, UCLA)

20

Therefore, the fast electrons produced in the gamma-matter interaction carry the kinetic

energy converted from incident gamma photons, and the detection of gamma ray energy is then

converting the energy of fast electrons into readable signals, such as visible photons. The fast

electron deposits its energy in a continuous way via ionization or excitation of other electrons of

atoms on its trajectory; in the meanwhile, the energy of fast electron could also be converted into

electromagnetic waves, such as Kα X-ray or Bremsstrahlung. Finally, the electron-hole pair

(excitons) produced in these interaction process recombine together to create visible photons,

which are finally detected by photomultiplier tube and converted into readable signals, as shown

in Figure 1-4.

Figure 1-4. Schematic illustration of the detection of gamma photon using a scintillator coupled

to a photomultiplier tube. (Copyright 2017, UCLA)

Motivation and Research Scope of the Thesis

As mentioned before, semiconductor detector and conventional scintillators suffer from

either high price, limited scale or from poor light yield and low gamma-ray attenuation power.

The idea of mixing inorganic high-Z nanoparticles into plastic matrix has been studied to

leverage high Z of the inorganics with the high gamma ray stopping power and fast emission,

and low cost and easy to fabricate of plastics, nevertheless current researches are still developing

21

in this field. Major obstacles are: 1) limited methods in synthesizing uniformly distributed small-

size, typically less than 10 nm diameter, high-Z nanoparticles with good solubility in low-

polarity organic solvent, such as hexanes, octanes and chloroform; 2) difficulty in preventing

nanoparticles aggregation in the process of forming nanoparticle/polymer matrix

nanocomposites, which has limited loadings of nanoparticles and then impact the improvement

of Z; 3) Limited researches and understanding of the photophysical properties of emitter and

matrix, which leads to the poor choice of materials.

Admitting these shortcomings in nanocomposite scintillators, the major job of this thesis

is then focusing on understandings in chemical synthesis, polymer chemistry and photophysics to

solve these tricky problems and improve the performance of nanocomposite plastic scintillators.

Based on the work completed during my master study, this thesis is divided into four chapters:

Chapter 1, the current chapter, provides an overview of gamma ray detectors, gamma ray-

matter interaction mechanisms and research topic covered in this thesis, followed by research

motivation and layout of this thesis.

Chapter 2 presents a novel synthesis of CsPbBr3 quantum dots and the embedment of

quantum dots into widely used poly (methyl methacrylate) (PMMA) matrix to fabricate the

CsPbBr3/PMMA nanocomposite scintillators. The quantum yield of CsPbBr3 quantum dots was

investigated in liquid organic solvent and sold PMMA matrix, as well as the photophysics study

of this nanocomposite plastic scintillators.

Chapter 3 shows an extend work based on Chapter 2. The synthesis of CsPbBr3 quantum

dots in different plastic matrix poly (butyl methacrylate) (PBMA) to improve the quantum yield

of CsPbBr3 in organic solvent and sold PBMA matrix. The obtained CsPbBr3/PBMA

22

nanocomposite plastic scintillators have higher transparency than that of CsPbBr3/PMMA

nanocomposite.

Chapter 4 continues the exploration on scintillating properties of nanocomposite plastic

scintillators by using the Cs-137 gamma ray source, but, unfortunately, the gamma photons are

failed to be detected. The reasons are analyzed and the improvements are provided.

Finally, Chapter 5 summarizes the work and provides the future work and perspectives.

23

CHAPTER 2

FABRICATION OF CSPBBR3 QDS/POLY (METHYL METHACRYLATE)

NANOCOMPOSITE SCINTILLATORS

Background

Perovskites have been widely investigated in the recent years due to their special

advantages: good luminescent ability, narrow emission full width at half maximum (FWHM) and

tunable emission wavelength in the visible light region.1528, 29 Besides, perovskites have been

widely applied in many devices, such as solar cell, light-emitting diodes (LEDs), photon

detectors, lasing and bioimaging.30-33 For the cubic APbX3-type perovskite lattice (where A is

methylammonium (MA, CH3NH3+), formamidinum (FA, CH (NH2)2

+) or cesium (Cs+), and X is

one or more halides (Cl-, Br-, or I-)), the all inorganic cesium lead halides perovskites (CsPbX3)

are much more stable than the hybrid organic-inorganic lead halides and they have higher Z for

the gamma ray detection purpose.33, 35 Besides, though no surface shelling is applied, the

quantum yields (QY) of all inorganic halide perovskites could be as high as 90%, as reported in

many literatures, making all inorganic perovskite family a promising candidate for optical and

optoelectronic applications.31 However, despite those advantages, this perovskite is still

moisture- and air-sensitive. The practical utilization of perovskites requires additionally

protective strategy from decomposition from the oxygen and water.36 Blending with commercial

polymer has been studied in recent decades to obtain stable perovskites devices. Blending with

polymer is also very easy to handle when compared with conventional shelling of quantum dots.

Recently, many reports have utilized different kinds of commercial polymers to improve the

perovskite stability for advanced applications, such as poly (methyl methacrylate) (PMMA), poly

(vinyl toluene) (PVT), polystyrene (PS), so on and so forth.23-27, 36 However, direct blending

perovskites with polymer monomer is unfeasible due to the fact that the matrix host generate a

strong van der Waals force at the nanoscale coupled with excluded volume interaction and

24

deplete force, which push the oleic acid (OA) and oleylamine (OAm) capped nanoparticles

towards each other and form the “excluded” clusters, resulting in the opaque monolith, which is

useless for the gamma ray detection, unless the ligand exchange is implemented, in which some

of the surface ligands (OA or OAm) are exchanged by other ligands that could tightly anchor on

the particles surface on one end and covalently copolymerize with matrix monomer chains on the

other end to prevent aggregation and facilitate high transparency.5, 25

In this Chapter, we utilize the supersaturated recrystallization method to prepare

perovskite-polymer nanocomposite: the CsPbBr3 was prepared in methyl methacrylate (MMA)

monomer in one-pot, avoiding the redundant synthesis, separation and protection of the

perovskite quantum dots.36 PMMA is very common and cheap in commercial and it has wide

application. Pure PMMA is very transparent. The transparency is even higher than glasses. After

the formation of perovskite, the mixture was polymerized under 365 nm UV-light. The

transparent CsPbBr3/PMMA nanocomposite plastic scintillator was obtained. The quantum yield

of CsPbBr3 in MMA was measured as well as the quantum yield CsPbBr3/PMMA

nanocomposite.

Experimental Section

Materials

Cesium bromide (99.999% metals basis, Alfa Aesar), lead bromide (99.999% trace

metals basis, Sigma-Aldrich), lead chloride (99.999% trace metals basis, Sigma-Aldrich), N,N-

dimethylformamide (DMF, anhydrous, 99.8%, ACROS organics), oleic acid (90%, technical

grade, Sigma-Aldrich), oleylamine (70% technical grade, Sigma-Aldrich), methyl methacrylate

(MMA, 99%, contains 30 ppm MEHQ as inhibitor, Sigma-Aldrich), diphenyl (2,4,6-

trimethylbenzoyl) phosphine oxide (TPO, 97%, Sigma-Aldrich), ethylene glycol dimethacrylate

25

(EGDMA, 99%, Frontier Scientific), quinine sulfate (dihydrate, 1 mM in 0.1 M H2SO4, Thermo

Fisher Scientific), aluminum oxide (99.5% trace metals basis, Sigma-Aldrich)

Synthesis of CsPbBr3 QDs in MMA

The CsPbBr3 QDs were synthesized via supersaturated recrystallization (SR) strategy

developed by Haibo Zeng’s group31. Monomer MMA and crosslinking monomer EGDMA were

purified aluminum oxide columns before using. Other reagents were used directly without

further purification. In a typical synthesis of CsPbBr3 in MMA solution, CsBr (0.04 mmol) and

PbBr2 were dissolved in DMF (1 mL). Oleylamine (50 μL) and oleic acid (100 μL) were added

to stabilize the precursor solution in sequence. Then, 150 μL of the precursor solution was

quickly injected into 9 mL MMA under vigorous stirring. Strong green emission could be

observed immediately after the injection. All above operations were implemented at room

temperature.

Fabrication of CsPbBr3 / PMMA Nanocomposite Monoliths

Once the CsPbBr3 QDs were synthesized in MMA solution, the UV-light photoinitiator

TPO (29.4 mg) and cross linker EGDMA were added into CsPbBr3 /MMA solution in 25 mm

diameter glass vials, the molar ratio of monomer: photoinitiator: cross linker = 100: 0.1: 1,

followed by UV-light curing under 365 nm UV-light bulb for 12 h. After curing, the resulting

monoliths were removed from the glass vials and polished for further characterizations.

Characterization

Photoluminescence (PL) spectra were obtained via Model FP-6500 spectrofluorometer.

Solution-sample emissions were acquired by using 10 mm standard glass cuvette, whereas solid-

samples were cut and polished to form the same size as cuvette for tests in this

26

spectrofluorometer. Photoluminescence quantum yields (PLQY) of the QDs in MMA and

PMMA were measured by the relative method as described in an IUPAC technical report, with

quinine sulfate in 0.1 M H2SO4 solution (PLQY = 0.58) serving as the reference standard.37 In

brief, photoluminescence (under 350 nm excitation) and absorbance spectra were recorded for a

series of dilute CsPbBr3 / MMA solution (or CsPbBr3 /PMMA monoliths) with different

concentrations. The integrated PL intensities were plotted against the absorbance at 350 nm for

CsPbBr3 /MMA solutions, CsPbBr3 /PMMA monoliths and quinine sulfate solutions. PLQY of

the solution and solid samples were given by:

Фs = Фr (𝐾𝑠

𝐾𝑟) (

𝑠

𝑟)2 (2-1)

where Ф, K, are the PLQY, slope of the curve for integrated PL intensity vs. absorbance, and

refractive index of the solvent, respectively (subscript “s” means sample and “r” means

reference).

Results and Discussion

Synthesis of CsPbX3 (X = Cl, Br) QDs in MMA

The synthesis of perovskites is designed according to supersaturated recrystallization

method, which is operated at room temperature, within few second, free from inert gas and

injection operation. The UV-vis absorption and photoluminescence (PL) spectra of CsPbBr3 in

MMA solution under 350 nm excitation are present in Figure 2-1. The inset in Figure 2-1

illustrates the cubic perovskite structure of CsPbBr3 inorganic perovskite quantum dots. Crystals

of these perovskites comprise Cs+ ions site at the corner of cubic and PbBr6 octahedra in the

middle. The peak position is centered around 505 nm with a full-width of half maximum 21 nm,

which is accord with the data reported in many literatures.

27

Figure 2-1. Absorbance and photoluminescence spectra of typical CsPbBr3 in MMA solution.

Inset: Cubic perovskite structure of CsPbBr3 inorganic perovskite quantum dots

The photographs of perovskites CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 solutions in MMA

under ambient light and under 7 W 365 nm UV-light were shown in Figure 2-2. As we can see,

the solution with light yellow, green and almost transparent color were obtained. Under 7W 365

nm UV-light, the perovskites CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 solutions in MMA emit

green, blue, and deep blue color, respectively. Figure 2-3 represents normalized

photoluminescence spectra of CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 solutions in MMA. The

excitation wavelength is 350 nm. The emission peaks of CsPbBr3, CsPbBr2Cl, and CsPbBrCl2

are centered at 503 nm, 459 nm and 435 nm, respectively,

28

Figure 2-2. Photographs of perovskites CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 solutions in MMA

under ambient light (Left three vials) and under 7 W 365 nm UV-light (Right three

vials).

Figure 2-3. Normalized photoluminescence spectra of CsPbBr3, CsPbBr2Cl, and CsPbBrCl2

solutions in MMA (Excitation wavelength: 350 nm)

29

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0

5000

10000

15000

20000

25000

Inte

gra

ted P

L I

nte

nsi

ties

/a.u

.

Absorbance

R2 = 0.9916

K= 2.5

(a)

0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 0.055

6000

7000

8000

9000

10000

11000

12000

13000

Inte

gra

ted

PL

In

ten

sity

/a.u

.

Absorbance

R2 = 0.9907

K = 1.5 105

(b)

Figure 2-4. Linear fitting of integrated PL intensities vs. absorbance for (a) dilute quinine sulfate

solution; (b) dilute CsPbBr3/MMA solution;

Photoluminescence quantum yields (PLQY) of the perovskites quantum dots in MMA

were measured by the relative method as described in an IUPAC technical report, with quinine

sulfate in 0.1 M H2SO4 (PLQY = 0.52) serving as the reference standard. Figure 2-4 shows the

linear fitting of integrated PL intensities of dilute quinine sulfate and dilute CsPbBr3 in MMA

solution. The refractive indices of MMA and 0.1 M H2SO4 are 1.48 and 1.33, respectively.

According to the Equation 2-1 mentioned before, we can easily obtain the quantum yield of

CsPbBr3 in MMA solution is about 45%.

Fabrication of CsPbX3 (X = Cl, Br)/ PMMA Nanocomposite Monoliths

Once the perovskites were prepared in MMA monomer, the photoinitiator and crosslinker

were added into the mixtures in small vials. The polymerization processes were conducted under

7W 365 nm UV-light for 12 h. Figure 2-5 shows the normalized photoluminescence spectra of

CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 in PMMA monoliths. The excitation wavelength is 350 nm.

The peak positions and the FWHM slightly change before and after the polymerization process.

The Figure 2-6 presents the normalized photoluminescence spectra comparisons of CsPbBr3,

CsPbBr2Cl, and CsPbBrCl2 before and after polymerization. The emission peak of CsPbBr3 has a

30

small red shift from being in MMA to PMMA, but for chlorine doped CsPbBr3, the emission

peaks slightly shift to blue region.

Figure 2-5. Normalized photoluminescence spectra of CsPbBr3, CsPbBr2Cl, and CsPbBrCl2 in

PMMA monoliths (Excitation wavelength: 350 nm)

Figure 2-6. Normalized photoluminescence spectra comparisons of CsPbBr3, CsPbBr2Cl, and

CsPbBrCl2 before and after polymerization

After the CsPbBr3 quantum dots were embedded into PMMA, these monoliths were

removed from vials and polished. In order to measure the PLQY of CsPbBr3 in PMMA by using

31

relative method described in the IUPAC technical report, four transparent monoliths with

different concentrations of CsPbBr3 were fabricated and polished, as we can see in Figure 2-7

(a). Under 365 nm UV-light, all of these monoliths show bright green emissions. Subsequently,

these monoliths were cut and polished into the cuvette-like shape (10 mm length and width, 12

mm height), and then measure the absorbance and PL intensity of these monoliths. Figure 2-8

exhibits linear fitting of integrated PL intensities versus absorbance for CsPbBr3/PMMA

nanocomposite monoliths. By using the slope of fitted line, the refractive index of PMMA (1.49)

and the Equation 2-1, the PLQY of CsPbBr3 in PMMA can be determined to be 21%. The

decrease of PLQY of CsPbBr3 from being in liquid to solid probably could be attributed to the

slight aggregation of perovskite nanoparticles during the polymerization process.

Figure 2-7. CsPbBr3/PMMA samples with different concentrations prepared for solid QY

measurement ((a): under ambient light; (b): under 365 nm UV light; samples

dimension: 25 mm diameter, 12 mm thickness). (c) and (d) show polished two

samples (Left: under ambient light; Right: under 365 nm UV light; samples

dimension: 10 mm length and width, 12 mm height)

32

Figure 2-8. Linear fitting of integrated PL intensities versus absorbance for CsPbBr3/PMMA

nanocomposite monoliths

Conclusion

In summary, all-inorganic perovskite quantums dots CsPbX3 (X = Cl, Br) were

successfully synthesized in methyl methacrylate solution, and the PLQY of CsPbBr3 in MMA

was determined to be about 45%. The transparent CsPbBr3/PMMA nanocomposites scintillators

have been fabricated via photoinitiated polymerization. The PLQY of CsPbBr3/PMMA

nanocomposites scintillators was determined to be 21%.

33

CHAPTER 3

FABRICATION OF CSPBBR3 QDS/ POLY (BUTYL METHACRYLATE)

NANOCOMPOSITE SCINTILLATOR

Background

As mentioned in Chapter 2, CsPbBr3 nanoparticles were successfully embedded into

PMMA. For plastic scintillator, the choice of polymer matrix is very important. The monomer

polarity would influence the formation of CsPbBr3 nanoparticle via supersaturation

recrystallization method, and the reactivity ratio of monomer would also influence the uniformity

of the final monolith. Therefore, we also tried to form CsPbBr3 nanoparticle in another

monomer: butyl methacrylate (BMA). Although the CsPbBr3 nanoparticles could be formed in

MMA, the high polar MMA could be still unfavorable for the formation and stability of

perovskites, especially when the perovskites were formed in this polar environment, the PLQY

could be deteriorated. A possible way to solve this problem is to find another monomer with

comparable polymerization rates but with less polarity than MMA. Butyl methacrylate (BMA)

with long alkyl chains should meet these requirements.36, 38

In this chapter, the MMA monomer was replaced by BMA. The quantum yield of

CsPbBr3 in BMA and CsPbBr3-PBMA nanocomposite monolith were determined. The influence

of monolith thickness and different concentration of CsPbBr3 in PBMA were investigated as

well.

Experimental Section

Materials

Cesium bromide (99.999% metals basis, Alfa Aesar), lead bromide (99.999% trace

metals basis, Sigma-Aldrich), N,N-dimethylformamide (DMF, anhydrous, 99.8%, ACROS

organics), oleic acid (90%, technical grade, Sigma-Aldrich), oleylamine (70% technical grade,

Sigma-Aldrich), butyl methacrylate (BMA, 99%, contains 10 ppm monomethyl ether

34

hydroquinone as inhibitor, Sigma-Aldrich), diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide

(TPO, 97%, Sigma-Aldrich), ethylene glycol dimethacrylate (EGDMA, 99%, Frontier

Scientific), quinine sulfate (dihydrate, 1 mM in 0.1 M H2SO4, Thermo Fisher Scientific),

aluminum oxide (99.5% trace metals basis, Sigma-Aldrich)

Synthesis of CsPbBr3 QDs in BMA

The CsPbBr3 QDs were synthesized via supersaturated recrystallization (SR) strategy.

Monomer BMA and crosslinking monomer EGDMA were purified aluminum oxide columns

before using. Other reagents were used directly without further purification. In a typical

synthesis of CsPbBr3 in BMA solution, CsBr (0.04 mmol) and PbBr2 were dissolved in DMF (1

mL). Oleylamine (50 μL) and oleic acid (100 μL) were added to stabilize the precursor solution

in sequence. Then, 150 μL of the precursor solution was quickly injected into 9 mL BMA under

vigorous stirring. Strong green emission could be observed immediately after the injection. All

above operations were implemented at room temperature.

Fabrication of CsPbBr3 / PBMA Nanocomposite Monoliths

Once the CsPbBr3 QDs were synthesized in BMA solution, the UV-light photoinitiator

TPO (29.4 mg) and cross linker EGDMA were added into CsPbBr3 /BMA solution in 25 mm

diameter glass vials, the molar ratio of monomer: photoinitiator: cross linker = 100: 0.1: 1,

followed by UV-light curing under 365 nm UV-light bulb for 12 h. After curing, the resulting

monoliths were removed from the glass vials and polished for further characterizations.

Characterization

Photoluminescence (PL) spectra were obtained via Model FP-6500 spectrofluorometer.

Solution-sample emissions were acquired by using 10 mm standard glass cuvette, whereas solid-

samples were cut and polished to form the same size as cuvette for tests in this

spectrofluorometer. Photoluminescence quantum yields (PLQY) of the QDs in BMA and PBMA

35

were measured by the relative method as described in an IUPAC technical report, with quinine

sulfate in 0.1 M H2SO4 solution (PLQY = 0.58) serving as the reference standard.

Results and Discussion

Synthesis of CsPbBr3 QDs in BMA

As mentioned in Chapter 2 that CsPbBr3 perovskite quantum dots were form in MMA.

Although the CsPbBr3 nanoparticles could be form in MMA, the high polar MMA could be still

unfavorable for the high stability of perovskites, especially when the perovskites were formed in

this polar environment, the PLQY could be deteriorated. A possible way to solve this problem is

to find alternative monomer with similar polymerization rates but with less polarity than MMA.

BMA with long alkyl chains should satisfy these requirements.36 Figure 3-1 shows the

photograph comparison of CsPbBr3 perovskites quantum dots form in BMA (left) and MMA

(right) under the ambient light. The CsPbBr3 perovskites quantum dots prepared in BMA

solution is much clearer than that of MMA solution. What is more, the CsPbBr3 – BMA solution

is light green color but the CsPbBr3 – MMA solution is yellow color, which mean CsPbBr3

nanoparticles have already aggregated to some extend in MMA solution.

Figure 3-1. Photograph of CsPbBr3 perovskites quantum dots form in BMA (left) and MMA

(right) under the ambient light.

36

Therefore, the PLQY of CsPbBr3 in BMA solution was measured. Figure 3-2 presents the

linear fitting of Integrated PL intensities versus absorbance of CsPbBr3 in BMA dilute solution.

By utilizing the method mentioned in Chapter 2, the PLQY of CsPbBr3 in BMA solution was

determined to be 66%, higher than the CsPbBr3 in MMA solution measured in Chapter 2.

Figure 3-2. Linear fitting of Integrated PL intensities versus absorbance of CsPbBr3 in BMA

dilute solution

Synthesis of CsPbBr3 in PMMA

In order to measure the PLQY of CsPbBr3 in PBMA, dilute different concentrations of

CsPbBr3 were embedded into PBMA, as shown in Figure 3-3 (a) vials 1,2, and 3. But due to the

violent polymerization of pure BMA or with small concentration of CsPbBr3 nanoparticles,

many small bubbles were produced inside the bulk and make them opaque, resulting in only

small portion of the bulk transparent, as shown in Figure 3-3 (b), which is cut and polished from

vial 2 in Figure 3-3 (a). Therefore, the PLQY of CsPbBr3 in PMMA was roughly estimated to be

25%. Table 3-1 shows the absorbance, peak position, FWHM, and integrated area of this

CsPbBr3-PBMA nanocomposite plastic scintillator.

37

Figure 3-3. Photographs of diluted different concentrations of CsPbBr3 in PBMA bulk (vials 1, 2,

3 in (a)) and cut and polished monolith from vial 2 in (a).

Table 3-1. Absorbance and PL data of monolith in Figure 3-2 (b)

Abs. at 350 nm Peak Pos. FWHM Area

0.0478 510 nm 19 nm 4150

The CsPbBr3 perovskite quantum dots were successfully embedded into polymer matrix

PMMA and PBMA. However, due to the synthesizing environment of perovskite nanoparticles

and the polymerization rate of polymer matrix are both different, the PL spectra of CsPbBr3-

PMMA nanocomposites and CsPbBr3-PBMA nanocomposites should be influenced by these

different factors. Therefore, we made a comparison of PL spectra between CsPbBr3-PMMA

nanocomposite and CsPbBr3-PBMA nanocomposite plastic scintillators. Figure 3-4 (a) shows the

photographs comparison between CsPbBr3-PBMA nanocomposite (left) and CsPbBr3-PMMA

(right) nanocomposite under ambient light. CsPbBr3-PBMA nanocomposite high transparency

and shows yellow-green color, but the CsPbBr3-PMMA nanocomposite exhibit yellow color,

which means CsPbBr3 nanoparticles aggregate in PMMA to some extent. Figure 3-3 (b) presents

normalized PL spectra comparison between CsPbBr3 in MMA, BMA and CsPbBr3 in PMMA,

PBMA. As we can see, the CsPbBr3 emission peaks shift to right and the FWHM become small

after polymerization.

38

Figure 3-4. (a) Photographs comparison between CsPbBr3-PBMA nanocomposite (left) and

CsPbBr3-PMMA (right) nanocomposite under ambient light, and (b) Normalized PL

spectra comparison between CsPbBr3 in MMA, BMA and CsPbBr3 in PMMA,

PBMA

Influence of Thickness and Concentration on Transmittance

High transparency is a very important property for plastic scintillator to detect gamma

ray. Therefore, the influence of different thicknesses and concentrations of CsPbBr3-PBMA

nanocomposites plastic scintillators on the transmittance were investigated. As mentioned in the

experimental part before, the more amount of precursor was injected into BMA, the more

CsPbBr3 perovskite nanoparticles would be synthesized in it. Hence, for convenience, the

volume of precursor injected will be used to indicate the different loading concentration of

CsPbBr3 in BMA or PBMA. In Figure 3-5, the CsPbBr3-PBMA nanocomposites were fabricated

by injecting 150 μL precursor into 9ml BMA to form CsPbBr3-BMA solution in one pot, so the

39

concentration of CsPbBr3 in PBMA was denoted as 9 mL-150 μL. Hence, when the

concentration is 9 mL-150 μL, most of the UV and blue light can be absorbed (from 400 – 500

nm), but cannot completely be absorbed (curves tilt from 400-500 rather than flat). As the

thickness decreases from 14 mm to 10 mm, small amount of light in 400 – 500 nm region goes

through these nanocomposites because the concentration of the absorber CsPbBr3 in PBMA is

too low. Transmittance of light from 500 nm to 520 nm or so increases significantly. For 10 mm

thickness nanocomposite, it shows the transmittance of 85% at 520 nm. The slight decrease of

transmittance at shorter wavelength is due to intensified Rayleigh scattering caused by the

addition of QDs as thickness increases.

Figure 3-5. Transmittance spectra of CsPbBr3-PBMA monoliths with different thicknesses

(concentration: 9 mL-150 μL; diameter: 25 mm)

When the injected precursor was doubled, Figure 3-6 shows transmittance of CsPbBr3-

PBMA composite with different thicknesses with concentration of 9 ml-300 μL. As the

concentration increases, CsPbBr3-PBMA nanocomposites with 10 mm to 14 mm thicknesses can

fully absorb light from 400-500 nm region, but nanocomposite with 8 mm thickness cannot fully

40

absorb light in this region, and because of more QDs loaded, Rayleigh scattering significantly

intensified at short wavelength region.

Figure 3-6. Transmittance of CsPbBr3-PBMA composite with different thicknesses

(concentration: 9 ml-300 μL)

Figure 3-7 shows the transmittance comparison CsPbBr3-PBMA composite with the same

thickness but with different concentrations. When the concentration of CsPbBr3 in PBMA was

increased, more light in 400-500 nm region was absorbed, but at the same time, more sever

Rayleigh scattering happens in the short wavelength region. According to Rayleigh scattering

equation:

𝑇 =𝐼

𝐼0= exp [−

32𝜋4𝑉𝑃𝑥 ⅆ3𝑛𝑚4

𝜆4 ((

𝑛𝑃𝑛𝑚

)2

−1

(𝑛𝑃𝑛𝑚

)2

+2)

2

] (3-1)

Where Vp: volume fraction

x: optical path length (thickness)

d: diameter of embedded nanoparticles

: wavelength of light

41

np: refractive index of nanoparticles

nm: refractive index of matrix

once the concentration was increased, the CsPbBr3 nanoparticles in PBMA aggregate together to

form large particles, which will significantly decrease the transparency. The refractive index

mismatch of nanoparticles and matrix would also significantly decrease the transparency of

monolith.

Figure 3-7. Transmittance comparison CsPbBr3-PBMA composite with the same thickness but

with different concentrations

Conclusion

In this chapter, CsPbBr3 perovskite nanoparticles were successfully mixed into PBMA,

and the PLQY of CsPbBr3 in BMA and PBMA were determined to be 66% and 25%. The

influence of thickness and two different concentrations of CsPbBr3 nanoparticles on transparency

of CsPbBr3-PBMA monoliths were investigated

42

CHAPTER 4

SCINTILLATING PROPERTIES OF CSPBBR3/PMMA AND CSPBBR3/PBMA

NANOCOMPOSITE PLASTIC SCINTILLATORS

Background

For the national security and nuclear nonproliferation purpose, the need for high

performance, low cost and reliable radiation detectors has motivated significant researches in

new radiation detector materials.1-3 Plastic scintillators materials are widely utilized for detecting

nuclear materials, primarily in radiation portal monitors that are applied to scan vehicles and

cargo containers at borders or other ports of entry to the United States.27 They offer a good

approach to fabricate large volume detectors due to the easy-fabrication properties, but the

performance of gamma ray stopping power is significantly hindered due to the low organic

atomic number. Conventional detection technologies such as gas ionization or semiconductor

detectors can provide better performance than scintillators detectors, but the major disadvantages

are their high cost and overall complexity. Reconciliation of contradictory materials

requirements requires entirely new approaches to design and materials synthesis of new

scintillating plastics. Recently, heavy element sensitized plastic scintillators have been paid more

and more attentions because this approach provides plastic scintillators with a high effective

atomic number (Zeff), and at the same time, have a high light yield under ionizing radiation, in

large volumes and high optical transparency.

Therefore, in this chapter, based on the studies of previous chapters, the as-fabricated

CsPbBr3-PMMA and CsPbBr3-PBMA nanocomposites plastic scintillators were applied to try to

detect the gamma ray by using Cs-137 gamma ray source.

Experimental Section

Figure 4-1 shows the photographs of every single part of the measuring system for

detecting the gamma photon by using the plastic scintillators coupled to photomultiplier tube.

43

First, the as-fabricated scintillator was coupled with PMT via EJ550 optical grease (Figure 4-1

(a)). The PTFE thread seal was utilized as reflective coatings, and black vinyl electrical tape

works as black box to prevent light outside coming into the scintillator (Figure (b) and Figure

(c)). Then, the PMT with scintillator was wired into a high voltage power supply with 1400 V

and channel digitizer (Figure (d) and (e)). Finally, the signals were collected and shown in

computer system (Figure 4-1 (f)). Figure 4-2 exhibits schematic diagram of the detection of

gamma photon using a scintillator coupled to photomultiplier tube and signal-processing

electronics, as described in Figure 4-1. Figure 4-3 exhibits photographs of standard reference

plastic scintillator EJ212 ordered from EJ company and as-fabricated CsPbBr3-PBMA

nanocomposite plastic scintillator. (a) is under the ambient light, and (b) is under 7W 365 nm

UV-light. These two plastic scintillators were measured in the measuring system described in

Figure 4-1. A key performance metric of plastic scintillator is its light yield, which is defined as1:

𝐿𝑖𝑔ℎ𝑡 𝑦𝑖𝑒𝑙𝑑 =# 𝑜𝑓 𝑠𝑐𝑖𝑛𝑡𝑖𝑙𝑙𝑎𝑡𝑖𝑜𝑛 𝑝ℎ𝑜𝑡𝑜𝑛𝑠

energy of particles (MeV) (4-1)

The detection of a single 1MeV gamma ray will typically only produce 103-105 photons per

event. Light yields of the scintillators were obtained by first comparing the sample’s Compton

edge channel number to that of a standard EJ212 reference sample tested under the same

conditions, followed by correction with regard to the PMT’s spectral sensitivity using the as-

obtained transmission-mode PL spectra. In this manner, the light yields were calculated using the

following formula39:

LYNC = LYEJ-212 (𝐶𝐸𝑁𝐶

𝐶𝐸𝐸𝐽−212) (

Ф𝐸𝐽−212

Ф𝑁𝐶) (4-2)

where LY, CE, and Ф represent the light yield, channel number of the Compton edge, and the

PMT-sensitivity correction factor, respectively. A light yield of 10,000 photons /MeV for EJ212

44

was used for the product specification. The PMT-sensitivity correction factor Фx for sample x

was calculated using the following formula:

Фx = ∫ 𝜑()𝐼𝑥()𝑑

∫ 𝐼𝑥()𝑑 (4-3)

where φ() and Ix () are the PMT’s quantum efficiency and the sample’s emission intensity at

wavelength , respectively.

Figure 4-1. Photographs of every single part of the measuring system for detecting the gamma

photon by using the plastic scintillators coupled to photomultiplier tube.

Figure 4-2. Schematic diagram of the detection of gamma photon using a scintillator coupled to

photomultiplier tube and signal-processing electronics

45

Figure 4-3. Photographs of standard reference plastic scintillator EJ212 ordered from EJ

Company and as-fabricated CsPbBr3-PBMA nanocomposite plastic scintillator. (a) is

under the ambient light, and (b) is under 7W 365 nm UV-light.

Results and Discussion

In order the calculate the light yield of CsPbBr3-PBMA nanocomposite plastic

scintillator, the pulse height spectra of standard reference EJ212 were measured by using

different gamma ray sources. Figure 4-4 (a)–(e) show pulse height spectra of standard reference

plastic scintillator EJ212 with different gamma ray sources. The Compton edges at different

energy position could be clearly observed. Figure 4-4 (f) shows (f) linear fitting of Compton

edge energy of different gamma sources versus integral energy for EJ212. Through this fitted

line, the horizontal energy values in Figure (a) to (e) can be determined.

Table 4-1 shows the different gamma ray nuclides and their corresponding gamma ray energy,

which are used to calculate the Compton edge energy by using Equation 1-3 and Equation 1-4

described before.

46

Figure 4-4. (a)–(e) Pulse height spectra of standard reference plastic scintillator EJ212 ordered

from EJ Company with different gamma ray sources; (f) linear fitting of Compton

edge energy of different gamma sources versus integral energy.

47

Table 4-1. Different gamma ray nuclides and their corresponding gamma ray energy.

Nuclides Gamma-ray energy

Na22 1274.5 KeV

Co60 1170 KeV

Mn54 835 KeV

Cs137 662 KeV

Na22 511 KeV

Ba133 356 KeV

Unfortunately, by using the same testing method, the Compton plateau and Compton

edge on CsPbBr3-PBMA nanocomposite cannot be observed, as shown in Figure 4-5. Only a

single peak centered around zero, which corresponds to the high sharp peak near zero KeV in

Figure 4-4 (a) to (e). These sharp peaks could be attribute to the X-ray produced by the Cs137

gamma source. The reason why the fabricated nanocomposite cannot detect the gamma ray could

be attributed to the extremely low loading of CsPbBr3 perovskite nanoparticles, resulting in very

low gamma ray stopping power. The possible solution and future work about this all inorganic

perovskite nanocomposite plastic scintillator will be illustrated in the following chapter.

48

Figure 4-5. Pulse height spectrum of CsPbBr3-PBMA nanocomposite with Cs137 gamma ray

source

Conclusion

In this chapter, gamma scintillation measurement was performed using a home-built

system. Unfortunately, only the pulse height spectra of standard reference EJ212 plastic

scintillator were obtained by using different gamma ray sources. The pulse height spectra, the

Compton plateau and Compton edge of as-fabricated CsPbBr3-PBMA nanocomposite were not

observed. The reason could be the extremely low loading of CsPbBr3 nanoparticles, which leads

to the very low gamma ray stopping power and cannot detect the gamma ray.

49

CHAPTER 5

CONCLUSION, FUTURE WORK AND PERSPECTIVE

Conclusion

The development of scintillators with high performance and low cost has been an

important research filed because of their wide application in national securities and medicine.

Conventional scintillators are limited by the cost and their intrinsic properties, the researches in

nanocomposite plastic scintillators have great promises because it can combine advantages from

both the organics and inorganics. However, the increase complexity of blending nanoparticles

into a polymer matrix brings challenges from both science and engineering aspects, impeding the

advancing of this field.

This thesis is dedicated to the engineering development and scientific understandings of

perovskite quantum dots-based nanocomposite plastic scintillators. In Chapter 2, all inorganic

perovskite quantum dots CsPbX3 (X = Cl, Br) were successfully synthesized in methyl

methacrylate solution, and the PLQY of CsPbBr3 in MMA was determined to be about 45%. The

transparent CsPbBr3/PMMA nanocomposites scintillators have been fabricated via photoinitiated

polymerization. The PLQY of CsPbBr3/PMMA nanocomposites scintillators was determined to

be 21%. In Chapter 3, CsPbBr3 perovskite nanoparticles were successfully mixed into PBMA,

and the PLQY of CsPbBr3 in BMA and PBMA were determined to be 66% and 25%. The

influence of thickness and two different concentrations of CsPbBr3 nanoparticles on transparency

of CsPbBr3-PBMA monoliths were investigated. In Chapter 4, gamma scintillation measurement

was performed using a home-built system. Unfortunately, only the pulse height spectra of

standard reference EJ212 plastic scintillator were obtained by using different gamma ray sources.

The pulse height spectra, the Compton plateau and Compton edge of as-fabricated CsPbBr3-

PBMA nanocomposite were not observed. The reason could be the extremely low loading of

50

CsPbBr3 nanoparticles, which leads to the very low gamma ray stopping power and cannot detect

the gamma ray.

Future Work

In order to solve the aggregation problem with high loading nanoparticles, especially

larger than 10 wt%, the surface modification of nanoparticles is necessary. Bis[2-

(methacryloyloxy)ethyl] phosphate (BMEP) is a good candidate reported in literatures. Figure 5-

1 shows the schematic illustration of partial ligand exchange and the molecular structures of

BMEP, surface ligand OA/OAm, and monomers. The commercially available ligand BMEP with

a strong binding head P-OH and a short polymerizable tail, was employed to partially replace the

original ligand OA on the surface of CsPbBr3 nanoparticles. The remain OA ligands guaranteed

the good solubility of nanoparticles in the monomer, whereas BMEP provided a steady

connection between nanoparticles and the polymer matrix to further facilitate nanoparticles

dispersion in the final nanocomposite via the strong binding of the head group P-OH to the

nanoparticle surface and the copolymerization of the tail end, methacrylate, with the polymer

chain. Based on this idea from literature, a series of ligand exchange processes was adopted, and

the physically mixing modified CsPbBr3 into MMA and BMA. After polymerization, the

photographs of monoliths were shown in Figure 5-2. In Figure 5-2, the left is CsPbBr3 in

PMMA; right one is CsPbBr3 in PBMA. The weight percentage of CsPbBr3 is estimated about 2

wt%. When the modified CsPbBr3 nanoparticles were ultrasonically dispersed in MMA or BMA,

the modified CsPbBr3 nanoparticles do not have good solubility in these two monomers, so we

observed that the mixture solutions are unclear and opaque. However, after UV-light curing,

transparent CsPbBr3-PMMA monolith was obtained, but the CsPbBr3-PBMA was opaque.

Obviously, the ligands BMEP work well in CsPbBr3-PMMA nanocomposite because BMEP and

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MMA have similar reactivity ratio, which are 0.46 and 0.52 at 60˚, while the reactivity ratio of

BMA is even higher than 1. Therefore, copolymerization process could happen in CsPbBr3-

MMA mixture.

Figure 5-1. Schematic illustration of partial ligand exchange and the molecular structures of

surface ligands and monomers

Figure 5-2. Photographs of PMMA (left) and PBMA (right) with surface modified CsPbBr3

embedded (~ 2 wt%; 25 mm diameter; ~2 mm thickness)

In order to achieve the highest performance of the plastic scintillator, the photons

produced should be opitimized. The mass attenuation coefficient of CsPbBr3 at 662 KeV of Cs-

137 is μ/ρ = -0.0867 cm2/g. According to Beer-Lambert Law:

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It =I0*exp [-(μ/ρ)* ρ *t] (5-1)

in which It, I0, ρ, and t is the transmittance gamma-ray intensity, incident gamma-ray intensity,

density of CsPbBr3 in PMMA, and the thickness of monolith. Therefore, the attenuated gamma-

ray intensity is Ia, which follows the equation below:

Ia = I0 - It =I0 {1-exp [(-μ/ρ)* ρ*t]} (5-2)

For the function:

y = 1 - exp (-x) (5-3)

when the value of x is 4, the y value is about 0.982. Therefore, when (μ/ρ)* ρ*t = 4, then 98.2%

of incident gamma ray can be attenuated. Then, we obtain this limitation:

ρ*t = 46 g/cm2 (5-4)

Hence, if we make a monolith with 1 cm thickness and 2.5 cm diameter, then the density of

CsPbBr3 in PMMA is ρ = 46 g/cm3. The volume of monolith is Vm = 4.9 cm3. Then, weight

percentage of CsPbBr3 in PMMA matrix could be determined to be 97.5 wt%. If 90% of gamma-

ray is attenuated, through the same calculation, the loading of CsPbBr3 in PMMA with 1 cm

thickenss and 2.5 cm diameter is 95.8 wt%, and 130 g CsPbBr3 is needed. The mass attenuation

coefficient of pure PMMA at 662 KeV of Cs-137 is μ/ρ = -0.0833 cm2/g and the density of pure

PMMA is 1.17 g/cm2. Then, when the thickness is 1 cm, the pure PMMA could attenuate {1-exp

[(-μ/ρ) * ρ*t]} = 9.3% of incident gamma ray. Therefore, the gamma ray stopping power could

be significantly improved by embedding CsPbBr3 nanoparticles into PMMA with high loading

concentration.

Once high loading is achieved, self-absorption problem would come up. There are two

possible solutions for the self-absorption problem. One solution is to synthesize core-shell

structure CsPbBr3 perovskite. S. X. Wang et al reported the core-shell structure of cubic

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CsPbBr3@Amorphous CsPbBrx perovskite quantum dots with a high blue photoluminescence

quantum yield of over 80%, as you can see in Figure 5-340. On one hand, the shell could cover

the surface defects of emissive core and decrease the trapping center of excitons. On the other

hand, the excitons produced in shell could degenerate into core and recombine at core to emit

photons, which could create a large Stocks shift between the absorption and emission spectra to

solve the self-absorption problem, but the requirement for shell is thick enough.

Figure 5-3. (a) Diagrammatic sketch of an all-inorganic perovskite CsPbBr3@A-CsPbBrx

composite; (b) structural characterization of perovskite CsPbBr3 (Copyright 2018

from the authors)

Another solution for the self-absorption problem is that by using a suitable organic dye

(wavelength shifter) which has a strong absorption at the quantum dots’ emissive wavelength,

the excitons could be transferred from quantum dots to the lower-band-gap dye through Forster

resonance energy transfer (FRET) to suppress QDs self-absorption and promotes the extraction

of QD-borne excitons to dye sites for photon production, which prevents the drastic LY

deterioration, as you can see in Figure 5-426, which is reported by C. Liu et al. For this method, it

requires high quantum yield of dye and efficient FRET process.

54

Figure 5-4. (a) Normalized UV-vis absorbance and PL spectra (excQD = 350 nm, excFBtF = 420

nm) of CdxZn1-xS/ZnS core/shell quantum dot and FBtF dye in dilute solutions. (b)

FRET process between QD and dye. (Copyright 2017 from the authors)

Perspective

For development of high-Z, high-light-yield, fast-response, and scalable nanocomposite

plastic scintillators, the quantum-dot/polymer nanocomposite system should be the most

promising option, as both Z and light yield can be improved with the addition of quantum dots.

For future development, the requirements in the materials perspective are listed as follows39:

a) A high Z element with substantial weight percentage;

b) High photoluminescence quantum yields at least 80%, even after surface modification.

c) Small sizes with radius less than 4 nm

d) Good solubility in monomer solution

e) Capability of being copolymerized with the monomer solution.

To solve the self-absorption problem, the dye needs to be:

a) Large Stokes shift

b) Good spectral match with the quantum dot’s emission, i.e. strong absorption with molar

extinction coefficient larger than 20,000 M-1cm-1

c) A high photoluminescent quantum yield at least 80%

For the core-shell structure solution, the thick enough shell and good lattice match are required.

55

In addition to the development of materials, it is also important to consider the advanced

photodetectors with improved sensitivities into the green or red spectral regions, since most

quantum dots with higher Z and good quantum yield tend to emit in the long wavelength. The

recently developed silicon photomultiplier (SiPM) is a potential candidate due to its high

quantum efficiency in the green and red region (around 40%), high gain, and fast response.

However, there are also issues regarding its use such as the small detector size and thermal

fluctuation. New electronics and accessories will need to be designed and fabricated for its

integration with the scintillators.

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BIOGRAPHICAL SKETCH

Jing Xie graduated from China University of Geosicence, Beijing with a Bachelor of

Engineering in Materials Science and Engineering. He began his graduate studies at the

University of Florida in Materials Science and Engineering Department under the guidance from

Dr. Jiangeng Xue in May 2017.