The Effect of Excitation Energies on the Photoluminescence Quantum

Yield of Lead-Selenide Quantum Dots of Various Diameters

by

Brent Godau

A thesis submitted in Partial Fulfillment of the

Requirements for the Degree of

BACHELOR OF SCIENCE

in the Department of Chemistry

©Brent Godau, 2014

University of Victoria

All rights reserved. This thesis may not be reproduced in whole or in part,

by photocopying or other means, without the expression of the author.

1

Table of Contents

Table of Contents 1

Abstract 2

Introduction 2

Experimental 6

Data Treatment 10

Results & Discussion 13

Conclusion 19

Bibliography 19

2

Abstract:

Three samples of colloidal lead-selenide quantum dots (QDs) were synthesized and

characterized by transmission electron microscopy (TEM), absorption spectroscopy, and

photoluminescence spectroscopy. The values obtained from these characterized samples were

used to design a study to determine if the photoluminescence quantum yield of the synthesized

samples decreased as the excitation wavelength used to trigger emission from the sample

increased in energy on the high energy side of the absorbance exciton. It was determined that the

photoluminescence quantum yield decreased as the excitation energy increased in energy likely

due to the formation of multi-exciton states within single quantum dots resulting in non-radiative

Auger processes.

Introduction:

The field of nanotechnology is already widely applied within technology and current

scientific research, and has plenty of opportunity for further advancement as more is learned as

scientists delve deeper into the depths of the unseen. Semiconducting materials are a major area

of study within materials chemistry, one subtopic of this area in particular, has crossed paths

with the fast-evolving field of nanotechnology. Quantum dots are a fairly recent addition to the

field of semiconducting materials, and are named as such due to their physical properties being

well described by quantum mechanical theory. The requirements for a material to be classified as

a semiconductor are related to the band gap of the material. The band gap is the distance in

energy between the valence band, or highest occupied energy band of the material, and the

conduction band, or lowest unoccupied energy level of the material. A band gap occurs when

there is a gap of energies where there are no molecular orbitals. In other words, there are no

3

quantum mechanical states allowed within the energy range of the band gap. Many physical

properties of materials are dependent upon the size of their band gap.1 The relative sizes of band

gaps for insulators, semiconductors, and metals is shown in figure 1 below.

Figure 1. Relative band gap energies (Eg) of insulators, semiconductors, and metals

Quantum dots are essentially tiny little particles of semiconducting material that exhibit

interesting and useful physical and optical properties due to their increased surface area to

volume ratio as compared to bulk semiconducting materials. They have several applications

within the realms of bio-imaging2, quantum computing

3, spintronics

4, and photovoltaic devices.

5

One of their more notable characteristics is their ability to emit light, or, in other words, their

photoluminescence. Their small volumes increase the efficiency in which they emit light relative

to bulk semiconductors because a bulk semiconductor has more volume to allow for faster

energy dispersal resulting in non-radiative processes.6 The high efficiency photoluminescence of

quantum dots is also in part due to their band gap structure in regards to the positioning of the

conduction and valence bands in momentum space. Increased efficiency is seen with a direct

4

band gap, in which the conduction band and valence band are positioned at the same value of

momentum, given by the directional wave-vector k, in momentum space.7

Figure 2. Optical transition of direct and indirect band gaps in momentum (k)-space

Not only are quantum dots efficient light emitters, their optical properties are

customizable and highly dependent upon their diameter.2 The customizable optical properties of

quantum dots are best described by the quantum confinement effect which applies to pieces of

semiconducting material having a radius under 10nm. The equation of the band gap energy of a

quantum dot is derived using the particle-in-a-sphere model application of the quantum

confinement effect to give the equation8:

(1)

Where Eg,0 is the band gap energy of the bulk semiconductor made of the same material as the

quantum dot, meh is the average effective mass of the electron and the electron hole(these

concepts will be described in a following paragraph), R is the radius of the quantum dot, and is

5

the reduced form of Planck’s constant divided by 2 . The only non-constant parameter in the band

gap equation is the radius of the quantum dot, giving the proportionality:

(2)

This proportionality shows that as the radius of a quantum dot decreases, the band gap energy

increases. This concept will be used in combination with the mechanism of emission to explain

the radial size effects on the optical properties of quantum dots.

When a quantum dot is excited with a photon, an electron is promoted from the valence

band to the conduction band, but when the electron leaves its valence band, it leaves an open

“hole” in the quantum state that the electron previously existed in. The two states are often called

an “electron-hole pair.” When the electron and the electron hole are in close proximity to one

another in space, they form a bound state known as an exciton due to the Coulombic interactions

between their opposing charges. The recombination of the two states often results in the emission

of a single photon, similar to the emission of a photon by a single atom. A quantum dot has

energy states that are often described using spherical quantum numbers with a subscript that

denotes whether the energy state is a hole or an electron.8

Figure 3. The defined energy states, S, P, and D, of a quantum dot within the valence band,

containing electrons and conduction band, containing electron holes

6

An absorption maximum is seen at the 1ShSe optically allowed transition, suggesting that

the most efficient conversion of absorbed light into emitted light may occur at this energy. The

correct term for this conversion is the quantum yield, given by the formula9:

(

) (3)

It is expected that as the excitation source light increases in energy relative to the absorption

maximum, the quantum yield will decrease due to an increased likelihood of non-radiative decay

processes, such as Auger processes.10

The quantum dots synthesized and used to determine the validity of this expectation are

colloidal lead-selenide (PbSe) quantum dots with oleic acid as a surfactant ligand layer making

the semiconductor core of PbSe dispersible in several solvents including tetrachloroethylene,

hexanes, and chloroform. These quantum dots are termed “colloidal” because the inorganic

semiconducting core is surrounded by an organic surfactant layer, much like a colloid.11

Colloidal quantum dots are a very practical type of nanoparticle to use in regards to the cost of

their synthesis, their relatively high quantum yields, and their photostability at room

temperature.12

Experimental:

Synthesis of PbSe Quantum dots with an Oleate Surfactant Layer:

The synthesis to make colloidal PbSe quantum dots was first reported by Murray et al.

The synthesis used to make the PbSe quantum dots required for the research explained in this

thesis was adapted from the Murray synthesis.11

0.4M Selenium in trioctylphosphine (TOP-Se)

was first prepared by dissolving selenium in TOP with heat and stirring inside of a nitrogen

7

atmosphere glove box. A solution of lead oleate in 1-octadecene was then prepared by adding

~0.45g of lead(II) oxide and 1.3mL of oleic acid into 30mL of 1-octadecene. The lead oleate

solution was heated to 130oC under vacuum , and kept under vacuum for 1.5 hours while stirring

to dissolve the lead oleate and remove water from the reaction flask.

Figure 4. Reaction set up for synthesis of colloidal PbSe quantum dots

Subsequently, the vacuum was turned off and the reaction flask was flushed with argon. The

temperature was changed to the preferred setting (see table 1 for reaction temperatures used).

Once the temperature equilibrated, 5mL of TOP-Se was obtained from the glove box and

injected into the reaction mixture with vigorous stirring. After the preferred reaction time, the

reaction was quickly quenched in 130mL of cold ethanol (~0-4oC) for 1.5 minutes. This

quenching step is also the precipitation step. The quantum dots were washed four times with

fresh ethanol each time. The wash method was centrifugation at 1717RCF for 5 minutes per

wash. After the final wash, the ethanol was removed and the quantum dots were dried using

8

argon gas and dispersed in tetrachloroethylene for storage/characterization. The quantum dots are

also dispersible in hexanes and chloroform. The samples were stored in an inert atmosphere by

bubbling with argon and sealing the vials with Teflon tape.

Table 1. Reaction conditions of characterized samples

Sample Temperature

/oC

Reaction Time

/min

Approximate*

Particle Size

/nm

Absorbance

Maximum /nm

A 130 2 3.0 1217

B 135 3 3.6 1334

C 140 3 5.0 1438

*Sizes are approximate due to the state of the transmission electron microscope (TEM) when the

images were taken. The images are not up to par with the normal standard of clarity.

Measurement of Photoluminescence Quantum Yield using the Integrating Sphere Method:

An adapted method of measuring quantum yield using an integrating sphere was used to

produce the photoluminescence quantum yield values observed in this thesis.13

An integrating

sphere is a spherical sample chamber in which the sample is positioned in the center of the

sphere. The sphere’s interior walls are a reflective white surface, and there is an entrance hole for

the excitation source and an exit hole for light to hit the detector. A baffle was used to block

directly scattered light from entering the detector. First, a scan of pure solvent, in this case TCE,

was taken from 15nm on either side of the excitation wavelength. The scan was repeated 3 times

additively in order to achieve a high signal to noise ratio. The same scan was repeated with the

sample dispersed in TCE in order to calculate the difference in the amount of excitation light

absorbed by the sample from the amount of light absorbed by TCE. Subsequently, an emission

9

scan of the sample was taken with at least 50nm on either side of the emission to give a decent

baseline reading. This scan was repeated additively depending on how strong the emission was

from the sample. Typically, emissions seen from the samples inside of the integration sphere

were weak due to lost intensity from the number of reflections made in the sphere before hitting

the detector. The method of quantum yield measurement was previously calibrated by another

member within the research group using quinine bisulfate.14

The excitation light source used was

an Edinburgh Instruments Xe900 xenon lamp paired with a TM300 monochromator of the same

brand. A UC900 sample chamber also made by Edinburgh Instruments was used and the detector

was a Hamamatsu R5509 PMT detector with a detection range of 700-1700nm. F900 software

made by Edinburgh Instruments was used to record the spectra obtained for quantum yield

measurements as well as emission spectra for each sample.

Figure 5. Schematic diagram of the integrating sphere used to measure the photoluminescence

quantum yield.

10

Data Treatment:

In order to calculate the quantum yield for each sample, a number of steps were followed

to obtain an accurate quantum yield measurement. Since the number of photons absorbed or

emitted needs to be measured in energy, the wavelength axis of each spectrum was converted

into wavenumbers with the units m-1

.

Figure 6. Example conversion of emission spectrum of sample B to energy units. This

conversion was completed using the same method for all spectra used to calculate quantum yield.

The high peaks seen at the low energy side of the spectra are due to the Xe lamp correction file.

Once converted into energy units, the area under the curve in each spectra was obtained and used

as the amount of light absorbed or emitted respectively to the type of spectra being integrated.

In some of the spectra, higher order artifact signals of the excitation light, Xe lamp, from

the monochromator grating were seen in some of the signals (see figure 7). In this case, a line

was drawn across the artifact to connect either side of the emission curve, and the area under the

xenon artifact was calculated and subtracted from the total area under the curve. This does not

have a significant effect, if any at all, on the quantum yield as these artifacts were close to the

edges of the emission spectra where the emission curves approached the baseline.

11

Figure 7. Example of artifacts seen in the emission spectrum of sample A excited at 756nm

The xenon lamp used as the excitation source for the quantum yield measurements and

the emission spectra of all of the samples has an intensity profile that decreases on either end of

the monochromator range due to the grating of the monochromator. A correction file is used to

correct for this profile, and the effects of the correction file are seen in the emission spectra

around 1600nm or 630000m-1

(see figure 5). For sample C, which emits at 1485nm, the low

energy side of the emission spectra is interfered with by the correction file. For all of the

absorption and emission spectra of sample A, B, and C, the curves are Gaussian in shape, and the

emission and absorption spectra are extremely similar in shape. Due to this extreme similarity,

the portion of the emission spectrum that was affected by the xenon lamp correction file was

drawn in to replicate the shape of the absorption spectra low energy side. The accuracy of the

quantum yield calculated from these corrected spectra was not significantly affected since the

correction file interference was on the edge of the emission spectra where the curve approached

the baseline.

12

600000 650000 700000 750000 800000 850000

-2000

0

2000

4000

6000

8000

Inte

nsity /a

.u.

Wavenumber /m-1

Figure 8. Example removal of correction file interference from 600000m-1

to 625000m-1

for

sample C emission curve with an 836nm excitation source

Once all of the spectra generated to calculate quantum yields were properly converted to

the energy scale and corrected by methods discussed above, the area’s under the curve were

taken and equilibrated by dividing by the number of additive scans completed for each sample

emission and source light spectra. The equilibrated values were then used to calculate the

quantum yield for each sample measurement. An example of the non-equilibrated and

equilibrated values is shown in table 2 below.

13

Table 2. Example conversion of non-equilibrated peak areas to equilibrated values for sample B

excited at 1175nm

Spectrum Type: Pure TCE at

Excitation

Wavelength

PbSe Quantum Dots

in TCE at Excitation

Wavelength

Emission scan from

1000-1650nm

Number of Additive

Scans:

3 3 5

Area Under Non-

Equilibrated Curve:

/arbitrary units

2.82x109 2.66x10

9 1.03x10

8

Area Under

Equilibrated Curve:

/arbitrary units

9.40x108

8.88x108

2.06x107

Results & Discussion:

Three lead selenide colloidal quantum dot samples were synthesized with oleate used as

the surfactant for dispersal in various organic solvents. The aim was to make three PbSe quantum

dot samples evenly spaced in their absorption/emission peaks with relatively low dispersity.

Once synthesized, transmission electron microscope (TEM) images were taken using a JEOL

JEM-1400 TEM by a grad student within the research group.14

14

(a) (b)

(c) (d)

(e) (f)

Figure 9. TEM images of: sample A at (a) 300K magnification and (b) 600K magnification,

Sample B at (c) 200K magnification and (d) 300K magnification, and Sample C at (e)300K

magnification and (f) 600K magnification.

15

The TEM images obtained were not up to the usual standard quality due to the current working

state of the instrument. If the images were up to par, the edges of each quantum dot would be

more defined and the background of the images would not appear to be so spotted. Due to the

low quality of the images, the diameters of each sample characterized were deemed approximate,

and the dispersity of the samples was not reported. The approximate sizes were provided to

exemplify the effect of a decreasing radius on the absorption and emission maxima of a quantum

dot, but they were not relatively important to the purpose of the quantum yield study with varied

excitation energies. As discussed in the introduction, the absorbance and emission energies as the

radius of the samples decreased resulted in higher energy absorption and emission, following the

proportionality posed in equation (2).

Before determining the quantum yield of each sample, the absorption and emission

spectra were taken in preparation of the quantum yield measurements.

400 600 800 1000 1200 1400 1600

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Absorbance

Emission

Ab

so

rba

nce

& E

mis

sio

n /a

.u.

Wavelength /nm

(a) (b)

400 600 800 1000 1200 1400 1600

0.0

0.2

0.4

0.6

0.8

1.0

Absorbance

Emission

Abso

rba

nce

& E

mis

sio

n /a

.u.

Wavelength /nm

16

400 600 800 1000 1200 1400 1600

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Absorbance

Emission

Ab

so

rba

nce

& E

mis

sio

n /a

.u.

Wavelength /nm

(c)

Figure 10. Absorbance and emission spectra of (a) sample A, (b) sample B, and (c) sample C

Once all of the absorption and emission maxima were determined, excitation wavelengths for

each sample were determined using equal energy values from each samples respective

absorbance maximum.

Table 3. Determined excitation wavelengths for quantum yield measurements at three different

energy values higher in energy than the absorption maximum of each sample.

Sample Absorption

Maximum

/nm

Emission

Maximum

/nm

Excitation

Wavelength

at

E1=79keV

/nm

Excitation

Wavelength

at

E2=158keV

/nm

Excitation

Wavelength

at

E3=390keV

/nm

A 1217 1270 1083 976 756

B 1334 1380 1175 1050 800

C 1438 1485 1255 1113 836

17

The excitation energies determined for the quantum yield measurements were determined at

equal energy distances rather than equal wavelength distances because equal wavelength

distances at different positions on the electromagnetic spectrum are not equal in energy.

Excitation at exactly the absorption maximum for each sample was not taken due to the Gaussian

shape of emission spectra and interference that would have occurred with the emission curve and

excitation wavelength. The emissions observed occur at a slightly lower energy than the

absorption maxima due to the relaxation of the band structure via Coulombic interactions

between the electron and its paired electron hole.10

The photoluminescence quantum yield was determined for each sample at each energy

distance from the absorbance maximum using the quantum yield measurement method

previously discussed in the experimental section.

830000 835000 840000 845000 850000 855000 860000 865000 870000

0

200000

400000

600000

800000

PbSe QDs in TCE

Pure TCE

Inte

nsity /a

.u.

Wavenumber /m-1

600000 650000 700000 750000 800000 850000

-2000

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

Inte

nsity /a

.u.

Wavenumber /m-1

(a) (b)

Figure 11. Sample plots of (a) 1175nm excitation light of sample B pure TCE measurement and

PbSe quantum dots in TCE and (b) emission spectrum of sample B excited at 1175nm. Both

plots have been converted into the wavenumber energy scale for quantum yield determination.

18

Table 4.Determined photoluminescence quantum yields at E1, E2, and E3 for samples A, B,

and C

Sample Quantum Yield at

E1 /%

Quantum Yield at

E2 /%

Quantum Yield at

E3 /%

A 19 12 4.8

B 40 13 3.5

C (Trial 1) 20 11 1.5

C (Trial 2)* 14 9.6 1.4

C (Average) 17+/-4 10+/-1 1.4+/-0.1

*A second trial was conducted at a later date to see if the results were reproducible.

A common trend of decreased quantum yield was observed as the excitation wavelength

increased in energy and energy distance from the absorbance maximum. To give reason to the

decreasing quantum yield trend, one must consider what effects a higher energy excitation would

have on an excited quantum dot. Firstly, excitation by energies greater than twice the band gap

can result in bi- or multi-exciton formation.6 Multi-exciton formation is an occurrence when

more than one electron-hole pair is formed on a single quantum dot.12

In the case when a multi-

exciton is formed, the likelihood of non-radiative decay via the Auger process is increased. The

Auger process is when an electron is ejected from an outer energy level of a semiconducting

material. In atom-like systems of semiconductors such as quantum dots, electron-electron

coupling is larger than electron-photon coupling, which results in an increased rate for Auger

processes. The mechanism of the Auger process is believed to be a 4 particle process with the

involvement of a phonon.10

The Auger process has been proved to occur at a faster rate in atom-

like semiconducting materials, suggesting that the rate of Auger processes is size dependent.10

This may also give reasoning to varying quantum yield values for quantum dots excited at the

same energy distances from their absorption maxima. The main reasoning for varied quantum

19

yields at the same excitation energy distances from their respective absorption maxima in this

thesis is likely to be the varying quality of PbSe quantum dots synthesized. The loss of energy

due to reduced quality of nanoparticles would most likely occur via defects in the crystal

structure or surface defects producing energy states that are higher in energy than the conduction

band edge, but lower in energy than Pe and De state energies (see figure 3). In multi-exciton

formation, the Auger process rate is higher than the rate of radiative decay, resulting in a reduced

quantum yield as the excitation energy increases in distance from the absorption maximum and

in energy. The loss of an electron via the Auger process leaves an electron hole in the valence

band of the quantum dot that was excited. In order to neutralize the quantum dot, the ejected

electron must come back in a redox mechanism that is unknown.

Conclusion:

Three lead-selenide colloidal quantum dot samples were synthesized and their

photoluminescence quantum yields were determined at three different energy distances on the

high energy side of their respective absorption excitons, and it was concluded that the quantum

yield decreases as the excitation energy distance from the absorption exciton energy increases.

This effect is due to an increased likelihood of multi-exciton formation resulting in an increased

rate of non-radiative Auger processes.

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