1

Synthesis and Characterization of PbS Quantum Dots

Author: Supervisor:

Abdelrazek Mousa Dr. Mahmmoud Sayed

Submitted to the Department of Chemical Physics, Lund University, 2011 in

Partial fulfillment of the Requirements for the Degree of Master of Science in

Chemistry

2

Abstract

Colloidal semiconductor nanocrystals or quantum dots (QDs) have recently

attracted much attention because of their unique electronic and optical properties

and potential for a wide range of applications. Near-infrared (NIR) PbS QDs were

synthesized using oleic acid (OA) as a surfactant for stabilizing the nanocrystals.

PbS QD’s of different diameters were obtained by varying the injection

temperature from 90-120 o

C. The absorption and fluorescence measurements were

recorded and the fluorescence quantum yield relative to IR-125 dye has been

measured.

3

Table of contents

Abstract 2

List of Symbols 4

List of Figures 5

Introduction 6

Quantum dots …………………………………………………………………. 6

Quantum confinement …………………………………………………………. 6

Fluorescence ……………………………………………………………………. 7

Quantum dots synthesis …………………………………………………………. 8

Lead sulfide (PbS) quantum dots ……………………………………………….. 8

Experimental section and Results 10

Materials ……………………………………………………………………… 10

Synthesis of PbS quantum dots capped by oleic acid ………………………… 10

Characterization of PbS quantum dots ………………………………………… 10

Results and discussion 11

Calculation the size of PbS quantum dots 14

Determination the fluorescence quantum yield 14

Fluorescence quantum yield ………………………………………………….. 14

Determination the quantum yield of PbS quantum dots……………………… 15

Conclusion 18

Acknowledgement 19

References 20

4

List of symbols

Exciton Bohr radius…………………………………………………... 6

Dielectric constant of the material …………………………………….. 6

Rest mass of the electron……………………………………………….. 6

Mass of the particle (an electron or hole)………………………………. 6

Bohr radius of the hydrogen atom……………………………………... 6

Band gap energy……………………………………………………….. 14

The quantum yield for the PbS quantum dots…………………………. 16

The quantum yield for the standard dye (IR-125)…………………….. 16

The area under the emission spectrum of the standard dye (IR-125)….. 16

The area under the emission spectrum of the PbS quantum dots………. 16

The refractive index of Toluene…………………………………………. 16

The refractive index of Dimethyl sulfoxide (DMSO)………………….. 16

5

List of figures

Figure Page

1 Energy levels diagram for bulk semiconductor and quantum dot………. 7

2 Electronic transition for quantum dots …………………………………. 7

3 The experimental setup for synthesis of PbS quantum dots……………. 9

4 Absorption spectrum of PbS QDs in octadecene ……………………….. 11

5 Absorption spectrum of PbS QDs in toluene (after purification) ………. 11

6 Absorption and emission spectrum of PbS QDs in toluene at 90 oC ……. 12

7 Absorption and emission spectrum of PbS QDs in toluene at 100 oC …… 12

8 Absorption and emission spectrum of PbS QDs in toluene at 110 oC ……. 13

9 Normalized fluorescence spectrum of PbS QDs in toluene ………………. 13

10 TEM images of the synthesized PbS QDs…………………………………. 14

11 EDX spectra of the synthesized PbS QDs………………………………….. 14

12 Absorption and fluorescence spectra for PbS QDs and IR-125 dye ………. 15

6

Introduction

Quantum dots

Quantum dots (QDs) are semiconductors nanocrystals composed of II-VI, III-V or

IV-VI group elements; also known as zero-dimensional nanocrystals since the

electrons and holes within are confined in all three dimensions and with sizes

typically in the range 1-10 nm in diameter. Quantum dots have many applications

such as solar cells, optoelectronic devices, biological fluorescence labeling and

lasers1-2

.

Quantum confinement

This phenomenon occurs when the size of the semiconductor becomes comparable

to or smaller than the exciton Bohr radius ( ), the electron and hole are confined

by the boundaries of the materials. This phenomenon is also known as the quantum

size effect and leads to discrete energy levels3. The energies of the electron and

hole levels are sensitive to the amount of confinement. The Bohr radius of a

particle is defined as

Where:

is the dielectric constant of the material

is the rest mass of the electron

is the mass of the particle (such as an electron)

is the Bohr radius of the hydrogen atom

Strong confinement regime occurs when the nanocrystals radius, , is much smaller

than and (where is the Bohr radius of the electron, is the Bohr

radius of the hole and is the exciton Bohr radius ). In such case the electron

and hole are strongly confined by the nanocrystals boundary. The band gap of

semiconductor nanocrystals increases as their size decreases.

7

Figure 1. Energy levels diagram for bulk semiconductor and quantum dot

Fluorescence

When an electron is excited into the conduction band, it is unstable due to the large

amount of energy and immediately returns to the lower energy valence band

(ground state). As the electron back down, it releases photon in the form of light

with an energy that matches the energy of the band gap. This emission is known as

fluorescence. Due to relaxation process, there is a slight difference in energy

between the absorption and emission spectrum known as Stock shift.

Figure 2. Electronic transition for quantum dots

Quantum dots synthesis

Bulk semiconductor

Conduction band

Valence band

Quantum dot

E

8

There are two routes for synthesis of quantum dots: one of these is bottom-up

method and the other is top-down method

1. Bottom-up method

Molecular or ionic precursors including the surfactant react together in

solution to produce colloidal quantum dots.

2. Top-down method

The bulk semiconductor is thinned to form quantum dots e.g. lithography

technique. A top down technique inherently includes material damage and

many quantum dots produced with this technique have defects that reduce

their effectiveness.

Lead sulfide (PbS) quantum dots

Due to their small direct band gap (0.41 eV) and larger exciton Bohr radius (18

nm)4,11

, PbS QD’s are important semiconductor material. They provide potential

for applications, such as telecommunications, tunable IR lasers and solar cells5. For

these advantages, PbS was selected during my research project.

Two organic synthetic methods exist for the synthesis of PbS QD’s. The first

method includes the injection of bis (trimethylsilyl) sulfide in octadecene to a hot

solution of lead oleate (Hines and Scholes).The second method in which

oleylamine is used as solvent and includes lead chloride (PbCl2) and elemental

sulfur as sulfur precursor (Cademartiri)6-7

.

In the present work, lead sulfide (PbS) QDs capped by oleic acid were synthesized

by a method similar to that described by Hines and Scholes2. In three necked flask,

a mixture of lead oxide (PbO) and oleic acid dissolved in octadecene were heated

to 90 oC for 1 hour to form lead oleate. Then the temperature was changed from

90-120 oC depending on the size of the quantum dots. At the desired temperature, a

solution of bis (trimethylsilyl) sulfide in octadecene was rapidly injected to lead

oleate to induce nucleation and growth of the PbS quantum dots as described in

Figure 3.

9

Figure 3. The experimental setup for synthesis of PbS quantum dots capped by oleic acid

Experimental section and results

Materials

Lead oxide (PbO, 99.0%), oleic acid (OA), octadecene (ODE, 90.0%), bis (tri

methylsilyl) sulfide (TMS), toluene (anhydrous, 99.8%), acetone (99.8%) and

methanol (99.8%) were purchased from Sigma-Aldrich.

Synthesis of PbS quantum dots capped by oleic acid

Lead sulfide (PbS) quantum dots were synthesized by a method that is similar to

that described by Hines and Scholes. 446 mg (2 m mol) lead oxide (PbO) was

dissolved in a mixture of 2 ml oleic acid and 20 ml octadecene then was heated to

Bis (trimethylsilyl) sulfide

Lead oleate

Nitrogen gas

10

90 oC under nitrogen in a three necked flask. After the lead oxide is completely

decomposed to form lead oleate (indicated by color change of the solution from

yellow to clear). The temperature of the solution was raised to 120 oC. Then 0.2 ml

of bis (tri methylsilyl) sulfide dissolved in 10 ml octadecene was rapidly injected

into the lead oleate solution. The rapid injection of TMS solution into the reaction

flask changed the colour of the reaction mixture from colorless to deep brown. The

reaction continued for 2 min so that PbS nanocrystals form. The solution in the

flask was dissolved in 10 ml of toluene and was precipitated with methanol and

acetone, followed by centrifugation.

To vary the size of the QDs, the injection temperature was changed to be (90, 100,

110 and 120 o

C) and by increasing the temperature, the size of the quantum dots

increase.

Characterization of PbS quantum dots

All absorption measurements were carried out on an Agilent 8453 UV-Visible-NIR

Spectrophotometer. A blank spectrum was taken before each measurement and

subtracted from the experimental spectrum. Fluorescence spectra were measured

using IR-laser and the excitation wavelength was 700 nm. All optical

measurements were performed at room temperature.

Results and discussion

Figure 4 shows the absorption spectrum of PbS QD’s in octadecene , the original

solvent of the experiments, these absorption spectra were recorded immediately

after the synthesis of the QDs.

11

500 600 700 800 900 1000 1100 1200

90 0C

1000C

1100C

1200C

Ab

so

rba

nce

Wavelength (nm)

Figure 4. Absorption spectrum of PbS QDs in octadecene.

500 600 700 800 900 1000 1100 1200

0

1

2

3

Ab

so

rba

nce

Wavelength (nm)

90 0C

1000C

1100C

1200C

Figure 5. Absorption spectrum of PbS QDs in toluene (after purification).

Figure 5 shows the absorption spectrum of PbS QD’s in toluene after purification.

To vary the size of the nanoparticles, we changed the injection temperature from

90-120 oC. In this method, we grew PbS nanocrystals of four different sizes, their

absorption maxima are 875 nm, 903 nm, 935 nm and 960 nm respectively.

12

600 800 1000 1200

0.00

0.05

0.10A

bso

rba

nce

Wavelength (nm)

90 0C

800 850 900 950 1000 1050

0

200

400

600

800

1000

1200

1400

1600

1800

Flo

ure

sce

nce

Wavelength (nm )

90 0C

930

Figure 6. Absorption and emission spectrum of PbS QDs in toluene at 90 oC

600 800 1000 1200

0.00

0.05

0.10

Ab

so

rba

nce

wavelength (nm)

100 0C

800 850 900 950 1000 1050 1100

0

200

400

600

800

1000

1200

1400

Flu

ore

sce

nce

Wavelength (nm)

100 0C

950

Figure7. Absorption and emission spectrum of PbS QDs in toluene at 100 oC

13

600 800 1000 1200

0.00

0.05

0.10

Ab

so

rba

nce

Wavelength (nm)

QD 110

800 850 900 950 1000 1050 1100

0

200

400

600

800

1000

1200

Flu

ore

sce

nce

Wavelength (nm)

110 0C

965

Figure 8. Absorption and emission spectrum of PbS QDs in toluene at 110 oC

Figures 6, 7 and 8 show the absorption and emission spectra for three diluted

solutions of PbS QDs in toluene. The fluorescence maxima are 930 nm, 950 nm

and 965 nm for the PbS QDs at different injection temperatures (90 o

C, 100 o

C and

110 oC) respectively. A red shift occurs in the absorption and the emission spectra

by increasing the injection temperature.

800 850 900 950 1000 1050

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d F

luo

resce

nce

wavelength (nm)

90 0C

100 0C

110 0C

Figure 9. Normalized fluorescence spectrum of PbS QDs in toluene

Calculation the size of the PbS QDs

14

From the optical band gap ( in eV), we calculated the diameter (d in nm) of PbS

QDs following the empirical equation developed by Iwan Moreels et al9.

Where is the optical band gap of the quantum dots

Table 1. The optical band gaps and the diameters of PbS quantum dots at different injection

temperatures.

Injection temperature (oC) (nm) (eV) The diameter of QD

(nm)

90 875 1.41 2.8

100 903 1.38 2.9

110 935 1.34 3

120 960 1.31 3.1

The band gap increase by decreasing the size of the quantum dots due to increase

the degree of the confinement.

TEM analysis of the synthesized PbS quantum dots

15

Figure 11.TEM images of the synthesized PbS QDs

EDX analysis of PbS quantum dots

Figure 12. EDX spectra of the synthesized PbS QDs

Figure 11. Shows two transmission electron microscopy (TEM) images of PbS

quantum dots which reflect the aggregation of the synthesized nanocrystals. While

16

Figure 12. Shows the elemental composition of the nanocrystals using energy

dispersive X-ray spectroscopy (EDX) which confirm the presence of Pb and S.The

peak of S is overlapped with the peak of the Pb and the two peaks appear at 2.3

KeV12

.peaks of C, Cu and O due to the carbon coated copper grid used for TEM.

Determination the fluorescence quantum yield

Fluorescence quantum yield is defined as the number of the emitted photons

to the number of the absorbed photons.

There are two methods for determining the fluorescence quantum yield

(1) Absolute method

It is not easy to determine the absolute quantum yield because this method

requires sophisticated instrumentation.

(2) Comparative method

The relative quantum yield is determined by comparison to a reference dye

with known quantum yield.

Determination the fluorescence quantum yield of PbS QD’s

Experimental

A number of diluted solutions of PbS QDs were prepared and the

absorption and the emission spectra were recorded. The fluorescence quantum

yields of PbS QDs were measured using a relative optical method. IR-125 dye

( in Dimethyl sulfoxide) was used as reference and the excitation

wavelength for the samples was 700 nm using IR-laser.

Results

17

600 800 1000

0.00

0.05

0.10

Ab

so

rba

nce

Wavelength (nm)

90 0C

100 0C

110 0C

600 800 1000 1200

0.00

0.05

Ab

so

rba

nce

Wavelength (nm)

IR-125

800 850 900 950 1000 1050 1100

0

200

400

600

800

1000

1200

Flu

ore

sce

nce

in

ten

sity

Wavelength (nm)

90 0C

100 0C

110 0C

780 800 820 840 860 880 900 920 940 960 980

200

400

600

800

1000

1200

1400

Flu

ore

sce

nce

in

ten

sity

Wavelength (nm)

IR 125

Figure 10. Absorption spectra for diluted solution of PbS QDs (a), Fluorescence spectra for PbS

QDs (b), absorption spectra for a diluted solution of IR-125 dye in DMSO(c), and Fluorescence

spectrum of IR-125 dye.

The quantum yields of the PbS quantum dots can be calculated from the following

equation:

Where

a c

b d

18

is the quantum yield for the PbS quantum dots.

is the quantum yield for the standard dye(IR-125),

is the area under the emission spectrum of the standard dye (IR-125).

is the area under the emission spectrum of the PbS quantum dots.

is the refractive index for toluene,

is the refractive index for DMSO,

Table 2. Quantum yields of PbS quantum dots at different injection temperatures.

Sample name FWHM

QD 90 106.8 0.36

QD 100 94.5 0.29

QD 110 88.0 0.23

The value of the quantum yield decrease by increasing the size of nanocrystals due

to increasing the size distribution.

19

Conclusion

PbS quantum dots with different sizes were synthesized using lead oxide as lead

source and bis (tri methylsilyl) sulfide as sulfur source (Pb: S=2:1). The quantum

dots were characterized by different techniques including absorption, fluorescence,

transmission electron microscopy and energy dispersive x-ray spectroscopy. The

quantum yield was measured relative to IR-125 dye as a reference dye.

20

Acknowledgement

I would like to offer my sincerest gratitude to Prof. Tõnu Pullerits for guidance and

support, Dr. Mahmmoud Sayed Abd El-sadek, Nanomaterial Lab., Physics

Department, Faculty of Science, South Valley University, Qena-Egypt, for his

inspiring guidance, continuous encouragement, stimulating discussions and

scholarly advice to carry out the present investigations and Dr. Kaibo Zheng, who

has support me throughout my project with his useful discussions and knowledge.

Then I want to thank everyone within our group and all people within the chemical

physics department for help and respect.

Special thanks to Mohamed Abdellah, Sumera Tubasum and Alice Corani for help

and discussion. I would like to thank my family and friends. Without their love and

support, none of this would have been possible.

21

References

1- H. Du, C. Chen, R. Krishnan, T. Krauss and J. Silcox. Optical properties of colloidal

PbSe nanocrystals. Nano letters.2004, 2, 1321-1324.

2- A. Guchhait, A. Rath and A. Pal. To make polymer: quantum dot hybrid solar cells NIR-

active by increasing diameter of PbS nanoparticles. Solar energy materials and solar cells,

2011, 95, 651-656.

3- V. Klimov. Nanocrystal quantum dots. 2nd edition. New Mexico, P.66.

4- A. Phuruangrat, T. Thongtem and S. Thongtem. Preparation of ear-like, hexapod and

dendritic PbS using cyclic microwave-assisted synthesis. Material letters, 2009, 63, 667-

669.

5- E. Sargent. Infrared quantum dots. Adv. Mater. 2005, 17, 515-522.

6- I. Moreels, Y. Justo, B. Geyter, K. Haustraete, J. Martine and Z. Hens. Size-tunable,

bright, and stable PbS quantum dots: a surface chemistry study. ACS NANO. 2004, 5,

2004-2011.

7- M. Hines and G. Scholes. Colloidal PbS nanocrystals with size-tunable Near-Infrared

emission: observation of post-synthesis self-narrowing of the particle size distribution.

Adv. Mater. 2003, 15, 1844-1849

8- M. Grabolle, M. Spieles, V. Lesnyak, N. Gaponik, A. Eychmuller, and U. Resch-Genger.

Determination of the fluorescence quantum yield of quantum dots: suitable procedures

and achievable uncertainties. Anal. Chem.2009, 81, 6285-6294.

9- I. Moreels, K. Lambert, D. Smeets, D. De Muynk, T. Nollet, J. Martins, F. Vanhaecke, A.

Vantomme,C. Delerue, G. Allan, and Z. Hens. Size-dependent optical properties of

colloidal PbS quantum dots.ACS NANO.2009, 3, 3023-3030.

10- J.Tang, L. Brzozowski, D. Aaron, D. Jamakosmanovic, and E.H. Sargent. Quantum dot

photovoltaics in the extreme quantum confinement regime: The surface-chemical origins

of exceptional air-and light-stability. ACS NANO.2010, 4,869-878.

11- k.K.Nada, F.E.Kruis, and H.Fissan. Effective mass approximation for two extreme

semiconductors: Band gap of PbS and CuBr nanoparticles. Journal of Applied

Physics.2004, 95, 5035-5041.

12- J. Warner, A. Watt and R. Tilley. Controlling PbS nanocrystals aggregation in conducting

polymers. Nanotechnology.2005, 16, 2381-2384.