Synthesis and Characterization of CdSe Quantum
Dots
______________________________________________
Pre-Lab Assignment
Before coming to lab:
 Read the lab thoroughly and answer the pre-lab questions that appear at the end of this
document
 Background information can be found in the ‘Chemistry of Tomorrow Today:
Nanotechnology’ article that can be found on the website with this document
 Follow the guidelines in the “Lab Notebook Policy and Format for Lab Reports’ section
of the lab manual to complete the Title, Lab Purpose, Procedure and Data Tables.
Purpose
In this experiment you will learn to synthesize CdSe Quantum dots using surfactant techniques to
control the dots dimensions by controlling the reaction time. You will record absorption spectra
for Quantum Dots of different size, and measure the wavelength for the onset of absorption for
these samples. You will take existing data from “Measurement and assignment of the sizedependent optical spectrum in CdSe quantum dots” by D.J. Norris and M.G. Bawendi, Physical
Review B 33 (24) 16338-16346 (1996) to create a calibration curve that connects the size of the
quantum dot to the wavelength for the onset of absorption, and use it to calculate the size of the
quantum dots samples you have made. Finally you will use a theoretical model based on the idea
that the electrons excited in absorption spectrum behave like a particle-in-a-box to estimate the
size of the quantum dots, and compare the sizes calculated to those calculated using the
calibration curve derived from the experimental data.
National Nanotechnology Infrastructure Network
www.nnin.org
Copyright©2012 Nicholas Blake and the University of California Santa Barbara
Permission granted for printing and copying for local classroom use without modification
Development and distribution partially funded by the National Science Foundation
NNIN Document: NNIN-xxxx
Rev: date
Page 1
1. INTRODUCTION
Here we are going to make small crystals of CdSe (Quantum Dots or QDs) of different sizes. In
Fig. 1 you can see how the appearance of these dots change with their size. On the left the dots
are maybe 0.5 nm in radius, while on the right we have dots of 2-2.5 nm. Notice that the colors
of the QDs vary with the size of the dots. The reason for this is very interesting. It happens
because the electrons - which absorb
visible light - want to occupy a
volume that is bigger than the dot
itself; then the energy levels of the
excited electron depend on the size
of the dot.
The fly in the room analogy
To understand this, consider a fly in
a square room of dimensions L x L.
(see Fig. 2a) It is in the middle of
Figure 1: CdSe Quantum Dot solutions. The size of the
dots increases in size from left to right. The final dot radius
the room flying in a circle of radius
R (where R << L) at a constant speed is approximately 3 nanometers
v. This is like an excited electron in a large chunk of semiconductor (Fig. 2b). The fly doesn’t
seem to know the walls are there, as it never leaves the middle of the room and never collides
with the walls. If I change the size of the room by making it bigger, it makes no difference to the
fly. Similarly if I make the chunk of semiconductor bigger it does change the absorption
spectrum for the semiconductor,
as the excited electron remains
a
b
the same size and doesn’t ‘sense’
the edges of the crystal. But
what happens to the behavior of
the fly when the dimensions of
the room are shrunk to the size of
a jar? (See Fig 2c) Now L < R,
the fly would like to occupy a
volume much bigger than the
volume of the jar itself. Have
you ever seen a fly in a jar? The
d
c
fly flies from one wall to the
other, walks on the walls of the
jar never leaving. The smaller the
jar the more rapidly the fly
reaches the other wall, it seems
to bounce back and forth with a
higher frequency. So now the
size of the contain does affect the
fly’s trajectory, and the fly fully
explores the total volume jar only
confined by the walls. This is
like an excited electron in a QD.
The particle-in-a-box idea
Scientists can ascertain the
motion by solving Schrodinger’s
Figure 2. (a) a fly, flying in a circle of radius R in a room of
dimensions LxL, with R <L: (b) the orbital for an Exciton of
dimensions smaller than the size of the crystal it is localized in: (c)
a fly confined within a small jar flying from wall to wall, (d) an
Exciton confined within a QD
Page 2
equation (which is the equation for motion of quantum particles) for a particle-in-a-box. In this
model the electron is in the box, where its potential energy PE = 0. Outside the box PE= ¥, so
the most stable solutions have the electron confined in the box. Since the potential energy is zero
any energy the electron has is purely kinetic (due to its motion), and is
En =
n2h2
8mL2
(1)
here L is the length of the box, n = 1, 2, 3, ... and is a quantum number defining the state, m is
the electron mass and h Planck’s constant. Notice that the bigger L the smaller the En, and notice
that because the smallest n can be is 1, there is a minimum energy the electron can have, called
the zero point energy1. In this simple example the wavefunctions y n (x) are of the form
y n (x) =
2
æ p nx ö
sin ç
÷
L è L ø
(2)
for L ³ x ³ 0 .
The probability waves (orbitals) described in equation
(2) are shown in figure 3. These are the same as the
vibrational waves that can be created on a plucked
guitar string. The n=1 state is the fundamental note,
n=2 is the first harmonic, n=3 the second harmonic and
so forth. Because the electron is a wave, it is constantly
in motion and reflects from the sides of the dot. The
wavelength of the electron is intimately related to the
dimensions of the box,
ln =
2L
n
Figure 3. Some of the lowest allowed states of a
particle in a box along with their energies Ei
Here the term l n is the wavelength of the nth state of the electron. Notice that as n increase the
wavelength decreases, meaning that the electron orbital has more nodes in it the higher n gets. In
addition to the wavelength of the electron decreasing, the velocity of the electron increases with
n. The average speed of the electron can be obtained from the de Broglie relationship2
The electron can never be at rest, to do so would contradict the Heisenberg uncertainty principle which
states you cannot know the position and the speed of the electron with complete certainty simultaneously.
2
Louise de Broglie proposed the idea of matter waves in 1924. It was the birth of the wave-particle duality of
matter. All matter of mass m traveling at a speed v has a wavelength 
1
l=
h
mv
we now know that this holds true for all matter and it is at the heart of Quantum Mechanics, and helps us to
understand the structure of the atom, and introduces ideas like electron orbitals, molecular orbitals and so in. In
short we cannot explain chemistry without this profound idea, that matter is a wave of probability.
Page 3
vn =
h
nh
=
mln 2mL
So the speed of the electron is proportional to n the electron’s quantum number. Just like the fly
in the jar, the smaller the jar, the more rapidly the fly goes from side to side, for the electron the
smaller the dot the higher is frequency of motion and the higher its kinetic energy. The minimum
energy the electron can have is called the zero point energy. The zero point energy is the kinetic
energy of the longest wavelength of wave motion it can have.
Particles in a Spherical Box
In a spherical dot, the allowed states of the electron look like the Hydrogen electron wave
functions with 1S, 2S, 2P, 3S, 3P, 3D ….. orbitals. In other words, a Quantum Dot is like a big
artificial atom. The smaller the dot, the smaller the wavelengths have to be to fit in the dot, and
so the higher the frequency of the electron. This is the same with the guitar string, the shorter you
make the string, the higher the pitch of the note it makes. In the case of CdSe the dots cannot
absorb red light, and as they get smaller they cannot absorb orange light, or yellow light. Hence
the colors seen in Fig. 1.
CdSe Quantum Dots
For CdSe, the Bohr radius of an excited electron is around 0.5nm. The Bohr radius is the
average distance of the electron from the center of positive charge. The true size of the orbital is
about 5 x Bohr radius or 2.5 nm. This means that the electron will be found within a spherical
region of diameter 5.0 nm. If the QD has a radius smaller than 2.5 nm, then the electron will feel
‘squeezed’ or confined, and it will act like a particle-in-a-box. It will act like a special type of
atom that has been made to exist in a volume that is smaller than its ‘natural’ volume. Under
these conditions it acts as a QD. The smaller the dot, the bigger the spacing between the energy
levels, blue shifting the spectrum. In a quantum dot the excited state of the electron is
delocalized throughout the particle but it is confined to the particle itself.
But a QD is not an empty box!
But hang on a minute you might ask. What about all the nuclei and other electrons in the QD
surely they must get in the way? Surely the electron isn’t free to move in the dot like it is in an
empty Box?!
Good question! It turns out, at room temperature, electrons in semiconductors are free the move
around so long as they stay in the semiconductor. They move around almost like there are no
other electrons or nuclei. However there are a few minor differences to electrons moving around
in free space:
1. First of all their mass appears to be different. Instead of the electron having a mass m0, it
becomes m0.me, where me is a number less than 1, (for example in CdSe me = 0.11).
2. Second, the electron is moving in a space that contains other electrons. If the electron is
excited into a new state, it leaves a hole h+ where it once was, and this kind of behaves
like a positively charged electron called a hole. This also has a mass mh
3. The electron is attracted to the hole, like the electron is attracted to the proton in
Hydrogen so there is a potential energy V of attraction
4. The other electrons in the dot can move, and surround the hole and move away from the
excited electron, this reduces the strength of the interaction of the hole and the electron so
that V is less than it would be in say Hydrogen. It takes the form
Page 4
V =-
1.8e2
4pe oe R
Amazingly, it turns out that an electron in a quantum dot behaves quite similarly to a particle-ina-box. The only difference is we are dealing with a 3-d spherical box of radius R. The energy it
takes to excite an electron from the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO) is given by the following equation.
Figure 4: A simplified energy level diagram for a QD. Below the blue rectangle we see some of the energy levels
for the hole, Above the blue region (Band Gap) we see the confined energy levels for the excited electron. When a
photon is absorbed an electron leaves one of the hole states and moves into an excited state. The energy of the
photon = change in the energy which is given by equation (3)
0.248e4 m0 æ me mh ö
h2
h2
1.8e2
DEn = Egap +
+
8m0 me R 2 8m0 mh R 2 4pe 0e R 8( hee )2 çè me + mh ÷ø
0
= band gap + e confinement energy + h confinement energy + exciton energy + e- - h + correlation
-
+
(3)
Egap is the gap in energy between the top of the valence band and the bottom of the conduction
band in bulk CdSe, me is the effective mass of the electron and mh is the mass of the hole. Other
terms include e the charge on a proton, and  and 0 which are the dielectric constant and
permittivity of free space3. In equation (2) the first term is the energy gap between the HOMO
(top of the valence band) and the LUMO (the bottom of the conduction band). The second and
third terms are the kinetic energy of the excited electron and hole in the spherical box, the fourth
3
In metals and semiconductors to accurately model the interaction of the electron with other electrons one
must include their spatial distribution resulting in a screened dielectric that depends on R when R is small but
constant otherwise here we use e iErf R / Rc where Rc = 9 nm
[
]
Page 5
term is the Coulombic attraction between the hole and the electron, and the last term is a term to
account for how the motion of the hole and the electron are correlated4.
The energy needed to excite the QD is absorbed from a photon of light. The lowest frequency
(the onset frequency)  = c/ (where c is the speed of light and  the onset wavelength of the
light) required to excite the QD is given by
DEif = Ei - E f = hn =
hc
DE fi = E f - Ei = hn =
hc
l
l
:for emission of a photon (E f <E i )
:for absorption of a photon (E f >E i )
(4)
So if we measure either the onset wavelength of absorption for the light we can determine the
dot’s radius R. In this experiment we will both make dots with different radii, measure the onset
wavelength, and use the above formula to calculate the size of the QDs we make.
B. Making Quantum Dots in Micelles
So how do we make such dots; and how can we make dots that are so small? We will be
synthesizing CdSe nanocrystals from CdO and SeO2 using a kinetic growth method. In such a
method, the particle size depends on the reaction time. The longer the reaction time the larger the
average particle size.
To be able to make nanoparticles we are going to need to grow these particles slowly enough that
we can stop the reaction when they are the size we desire. To do this we are going use a special
organometallic compound, called cadmium Myristate.
As you can see above, the Myristate is a C17H35
carbon chain attached to a carboxylate group. This
type of compound is called a surfactant. Surfactants
are amphillic, meaning that they contain both
hydrophobic groups (their tails, here the carbon
chain) and hydrophilic groups (their heads, the
carboxylic group and the cadmium ion). Surfactants
lower the interfacial tension between two liquids,
hydrophobic parts of the surfactant prefer the
environment of other non-polar molecules (organics,
oils, ethers, alkanes etc), while the hydrophyllic parts
of the molecule prefer the environment of polar
molecules, ions, water etc. When cadmium myristate
is dissolved in an organic non-polar solvent, the
molecules arrange themselves to form inverse
micelles. In this way the tails are in contact with one4
Figure 5: Schematic of Cadmium Myristate
and a Cadmium Myristate inverse micelle.
The head group comprises the cadmium ion
and the carboxylic groups while the tails are
the C17H35 alkyl chains
Y. Kayanuma, “Quantum-Size Effects of Interacting Electrons and Holes in Semiconductor Microcrystals with
Spherical Shape,” Physical Review B, Vol. 38, No. 14, 1988, pp. 9797-9805. doi:10.1103/PhysRevB.38.9797
Page 6
another and the solvent, while the solvent repelling hydrophyllic head groups aggregate at the
center of the micelle (see Fig. 3)
In the experiment we react cadmium myristate with 1-octadecene and selenium. Octadecene is a
long chain alkane with an alkene group at the end. It acts as a reducing agent reducing Se(IV) to
Se(0) and Cd(II) to Cd(0), while the alkene bond is oxidized to either a ketone an aldehyde or to a
longer chain alkene. The selenium dioxide is polar so it will be attracted to the center of the
micelle, and so the reduction of the Se(IV) and Cd(II) will occur in the center of the micelle, where
the nucleation of the CdSe particle will occur. Over time the CdSe particle will grow as will the
micelle as shown left, with the Myristate anion “capping” the nanoparticle.
C. Advantages to Growing CdSe Nanoparticles in a Micelle
So why is it that we choose to grow the CdSe particles in this way? What are the advantages to
such an elaborate synthesis? The answer is four-fold
1. The micelle stabilizes the quantum dot making it more stable, creating a narrower
distribution of particle sizes at a given instant, and this makes for better optical
properties.
2. The heavy organic tails slow down the rate of diffusion into the center of the micelle,
slowing down the rate of formation of the quantum dot, making it easier to control the
size.
3. The reaction can only happening at higher temperatures which ties in with (2) and means
that removing sample from the reaction vessel essentially quenches (stops) the reaction as
soon as it cools below 200 oC.
4. Because the reaction is relatively slow, thermodynamic considerations are important
meaning that the quantum dots are more spherical making for more consistent optical
properties (Spheres are shapes that minimize the ratio of surface area to volume which is
advantageous when you need to reduce the surface tension between two components in a
solution)
D. Growing QDs of Different Nanometer Dimensions
So the above will be our method, and because the rate of growth is comparatively slow, we will
be able to control the size of the dots. Our reactants are colorless so change in color of the
reaction mixture is because QDs are forming, and the color of the dots is directly related to the
size of the dots. As soon as our mixture develops color we remove aliquots from the reaction
using a Pasteur pipette. As soon as the mixture is drawn into the pipette its temperature is too
low for further reaction to occur and growth of the dots inside the pipette stops. This aliquots are
transferred to a test tube for later optical analysis using an absorption spectrophotometer.
By withdrawing aliquots at different moments in the reaction we will effectively have solutions
with dots of different average size and so different optical properties. The later we withdraw the
aliquot the bigger the dots will be.
Page 7
2. ESTIMATING THE SIZE OF QUANTUM DOTS USING ABSORPTION
SPECTROSCOPY
Using the equation from the introduction we have a way to assess the size of the dots from the
onset wavelength for absorption. The only problem with this is that the model simplifies the
exciton energy by treating it as a particle in a box, and we need to assess the accuracy of this
model first. To do this we will take data from a previous experiment that is able to measure the
size of the dots using an alternative method whereby the dots are imaged using an electron
microscope. Using this data we will create a calibration curve and we will compare the
experimental results with the results predicted from the equation in the introduction. We will use
both the calibration curve and our model to predict the sizes of the QDs in our samples.
A. Comparing absorption spectra with theory
To see how we can estimate the size of these QDs using absorption spectra look at Fig. 4 and
Fig. 5. In Figure 4 we show typical spectra obtained in this experiment. Electronic peaks are
very broad because of vibrational motion. The lowest energy electronic peaks are centered
around 545 nm and 556 nm.
0.035
0.03
absorbance
0.025
0.02
0.015
0.01
556 nm
0.005
545 nm
0
400
450
500
550
600
650
700
wavelength (nm)
Figure 6: Visible absorption spectra of the CdSe samples taken from earlier classes. The samples were
withdrawn from the reaction mixture at two different times, the sample with the electronic energy peak at 545
nm was withdrawn earlier than the one with the 556 nm
Page 8
Figure 7: The calculated absorption spectra for a particle-in-a-spherical-box model of CdSe of different radii, Top
1.25 and 1.75 nm, below, 1.50 nm, 2.00 nm and 2.50 nm
Figure 5 shows the anticipated spectra on obtains using the particle-in-a-spherical box model,
summing up over all the possible transitions between quantum states. Notice that the results are
similar to what is seen in Fig. 4. From Figure 5 we would estimate the two absorption spectra
are for QDs of approximately 1.5 nm in radius.
B. Comparing absorption spectra with previous data where QD sizes are known
In the above example we compared the experiment with theory. Another way we can compare
the data is to compare it with experiments where the absorption spectra were recorded for QDs of
known radius. (This can be done by studying the dots using a Scanning Tunneling Microscope
(STM).
Page 9
Onset wavelength for absorption vs. dot radius
800
Onset Wavelength (nm)
700
600
y = 156.38ln(x) + 432.97
500
Equation 3
400
Experiment
300
Log. (Equation 3)
200
100
0
0.00
2.00
4.00
6.00
8.00
Dot Radius (nm)
Figure 8: The calibration curve (red squares) and the model results (blue diamonds) for the data given should
( l -432.97 )
156.38 shows the relationship between
look like this. The equation l = 156.38 ln(R) + 432.97 or R = e
the radius R in nm and the wavelength also in nm for the experiment.
Here we plot experimental data of the energy for the threshold energy of transition as a function
of the inverse square of the radius of the QDs (in nm).
In Fig. 4 we saw we have threshold wavelengths of 545 nm and 556 nm, using equation of fit in
Fig. 6 - which is a fit to the experimental data - we can calculate the energy of transition using
equation (5)
R(nm) = eéë(
l -432.97 )/156.38 ùû
(5)
Inserting our values for  into this equation we get
( 545.0-432.97 )
R1 = e
= 2.05nm
156.38
( 556.0-432.97 )
R2 = e
156.38
= 2.20nm
C. Data Analysis for this Experiment
In this experiment you will be comparing your absorption spectra with both previous
experiments and the theoretical model proposed earlier to estimate the size of your QDs.
 In part I you will first make your quantum dots.
 In part II you will solubilize your QDs and record their absorption spectra.
Page 10

In part III you will find the onset wavelength for absorption, and then use the onset
wavelength to estimate the size of the dots in your various samples using the methods
outlined above.
Page 11
3. PROCEDURE
Part 1: Synthesis of the Cadmium Myristate
Cadmium is toxic when ingested above 30 micrograms / day. Wear gloves to avoid
absorption of Cadmium myristate through the skin, and wash hands after the lab. SeO2 is
toxic by inhalation of absorption through the skin and effects of ingestion are cumulative.
Octadecene vapor should not be inhaled and contact with the skin and eyes should be
avoided. The synthesis must be performed in a fume hood, and the students engaged in
the synthesis must wear nitrile gloves and eye protection. Left over solutions are to be
disposed of in the proper disposal bottles set aside in the fume hoods, and cleaned in
toluene
Materials











10 ml Octadecene (ODE)
0.32 g Cadmium Myristate
0.06 g SeO2
25 ml or 50 ml round
bottom flask
40mL Hexane, Decane
or Toluene
heating mantle
400oC Hg Thermometer
9 in Glass Pasteur
pipette
UV/VIS spectrometer
(here we use a Vernier
SpectroVis UV/VIS
spectrometer)
10 small test tubes or
vials
Access to a UV light
source
1. A 25 or 50 ml round bottom flask is clamped in place
in a heating mantle, (here we are using a 3-necked flask with
2 septa, but this is not necessary - see the figure on the right)
in a fume cupboard.
2. 0.06 g of Selenium Dioxide (SeO2) and 0.30 g of
cadmium myristate are added to the flask.
3. Clamp a 400 oC Hg thermometer in place inserting it
through the neck of the flask.
4. Measure out 10 ml of 1-Octadecene in a measuring
cylinder.
5. Prepare 12 clean small test tubes in a rack, and make
sure you have your Pasteur pipette handy.
6. Carefully add the SeO2, the Cadmium Myristate and
the Octadecene to the round bottom flask and start heating.
7. Pay attention. At around 120oC the cadmium
myristate will start to dissolve and as the temperature rises
above 140oC the color of the mixture may start to change
color, get ready with the pipette ….
8. Insert a Pasteur pipette and remove aliquots of the
reaction mixture as you see the color changing. Each time
carefully discharge the pipette into the test tubes sequentially having one person noting
the temperature of the reaction mixture as it is removed. Each sample should appear a
different hue from the previous samples.
9. Once either each test tube contains a sample or else the color has stopped evolving in the
flask stop the experiment.
10. When the flask is cool enough to handle dispose of the remaining mixture in the waste
container provided (do not let the mixture come into contact with your skin, and if it does
immediately wash it off) clean the glassware first in a toluene bath in a hot soapy water
Page 12
mixture using a test tube brush to help remove the myristate. Pour the contaminated
toluene inot the disposal flask.
Part 2: Analyzing the QDs Using UV-VIS Absorption Spectroscopy
Using the Vernier SpectroVis Spectrometer
Figure 9. Vernier SpectroVis Spectrometer
In this section we are going to measure the
absorbance of the samples using a UV/VIS
spectrophotometer (Fig. 7). This device measures
how many photons of varying wavelength across the
ultra violet and visible light spectrum are absorbed by
the sample, and makes a graph of the absorbance of
the light at each wavelength versus the wavelength in
nanometers.
A. Solubilizing the QDs
Before you can record the spectra for your dots you will need to solubilize them
For this you will need a hot plate with a 250 mL beaker ¾ filled with water and heated to 75oC.
•
•
•
•
Take 1/2 mL of your QD solution
Add 3-4 mL hexane
Place test tube in hot water bath at 75oC
QD should dissolve and make a clear solution
Remove the stoppers from the test tubes containing the
QDs and insert the test tubes into the hot water bath, (see
right). With time the solutions should clear resembling that
seen in the picture on the right (Fig. 8.).
Take a picture on your smart phone to include with the
report.
B. Starting and Calibrating the Spectrometer
Plug in the SpectroVis via a USB port to a computer that
already has the Logger Pro 3 software installed, (You will
know this is the case because the device will be recognized
by Windows). Start Logger Pro.
Figure 10: Top the set up for
solubilizing the QDs, bottom, how
the solutions should look after
solubilizing
Calibrate the spectrophotometer by choosing Calibrate > Spectrometer from the Experiment
Menu.
Page 13
Insert a clean empty small test tube, (like the ones used to contain the solutions of your QDs).
Follow the instructions in the dialog box to complete the calibration then click OK. In what
follows DO NOT USE THE PLASTIC CUVETTES THAT COME WITH THE
SPECTROMETER AS THEY REACT WITH THE SOLVENTS IN WHICH THE QDS ARE
SUSPENDED
C. Collecting the Absorption Spectrum
Select Absorbance vs. Wavelength, and insert one of your solubilized solutions into the cuvette
holder (make sure that the solution is transparent), then click
and then
to end
data collection. Remove that sample from the spectrometer and replace it with another sample
(make sure this too is transparent) and collect the spectrum for this sample. Save the spectra, and
print out copies to include with your report making sure each absorbance curve is labeled
according to temperature at which the sample had been removed from the reaction vessel, and
the apparent color of the sample.
D. Fluorescence Spectra under UV light
Expose your QDs to a UV light (on the long wavelength setting) in the darkened lab, and take a
photo using a smart phone. All going well you will end up with something like that shown in Fig.
9 below
Figure 11: QD samples under UV light
Part 3: Data Analysis – The Determination of the Size of the QDs
Since the wavelength for the onset of absorption is dependent on the size of the QDs, we should
be able to determine the size of the dots for each absorption spectrum recorded. In this section
we will be using existing data to come with an equation that relates the size of the dot to the
threshold wavelength or frequency of absorption.

You will need to identify the lowest energy transition in the spectrum. This will be the
peak at highest wavelength in the spectrum as has been done in Fig. 4. The wavelength at
which the peak is centered we call threshold.
Page 14
Table 1: measured threshold wavelengths in nanometers for your QD samples along with the Temperature that
the sample was on extraction from the reaction vessel
Sample number
Temperature at
Extraction of
sample (oC)
threshold (nm)
1
2
3
4
5
6
7
8
9
10
Assessing the accuracy of the Particle-in-a-Box Model and Creating a Calibration
Curve
To ascertain the size of the quantum dots mono-dispersed in your samples it is necessary to be
able to calculate the size of the QD that is needed for a given threshold . We are going to estimate
the size of the QDs by two methods, one that constructs a calibration curve from previously
measured size vs wavelength data, and one that uses the following equation
DE fi = Egap +
0.248e4 m0 æ me mh ö
h2
h2
1.8e2
+
8m0 me R 2 8m0 mh R 2 4pe 0e (R)R 8( he (R)e )2 çè me + mh ÷ø
0
= 2.9 ´ 10 -19 J +
5.97 ´ 10 -37 JM 2 4.14 ´ 10-28 5.43 ´ 10 -20 JC 4
e (R)R
R2
e (R)2
= band gap + electron confinement energy + hole confinement energy + exciton energy
=
hc
(3)
lthreshold
where
e (R) = e iErf (R / 7nm)
with the parameters that are pertinent to CdSe (these are listed in Table 2 below).



Use Excel and Table 2 to calculate Equation (3’) for R values ranging from 0.5 – 10 nm.
Use the generated data from Excel to complete table 3 and make a graph of wavelength
vs R(nm) showing both the data from equation (3) and the experimental data from Table
3.
Attempt to fit the experimental data with a logarithmic trendline and display the equation
and the R2 value on the graph. Your end graph should look similar to Fig. 6.
Page 15


Use the trendline to come up with an expression for the radius R of the dot in nm in terms
of threshold
Verify that your graph looks like the one shown in Fig. 6, and then complete Table 4.
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Table 2: Useful physical constants for CdSe along with some useful unit conversions
Constant
m0
me
mh

0
e
h
c
Egap
1 eV
Value
9.11E-31kg
0.13
0.45
10.72
8.85E-12 C2/Nm2
1.601E-19 C
6.626E-34 Js
2.998E8 m/s
1.8125eV
1.601E-19 J
Table 3: Experimental Data for the radius of mono-dispersed Quantum dots versus the threshold transition
energy. (data taken from “Measurement and assignment of the size-dependent optical spectrum in CdSe quantum
dots” by D.J. Norris and M.G. Bawendi, Physical Review B 33 (24) 16338-16346 (1996))
R (m)
4.00E-09
3.00E-09
2.50E-09
2.00E-09
1.50E-09
 
Experiment
650
604
578
540
497
 
From Eqn (3)
Use the 
vs. R plot as a calibration curve to ascertain the approximate size of the
particles in your samples, and complete Table 4
Table 4: The interpolated size of the spherical QDs using the threshold for absorption using Equation (3) and the
calibration curve derived from Bawendi’s group data
Sample
number
threshold
(nm)
(from Spectra)
R from
equation (3)
(nm)
1
2
3
4
5
6
7
8
9
10
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R from
equation (5)
(nm)
Color of
sample
Color of
Fluorescing
sample
Draw Conclusions
Write a concluding paragraph about what you discovered in this experiment, and complete the
Post Lab Questions.
Comment on the following issues in your report

By comparing the experimental data of Bawendi et al given in Table 3 with Equation (3)
comment on how well the particle-in-a-spherical-box model works. If the results are
similar then we can conclude that CdSe QDs can be described an exciton ‘particle’ in a
box.

Were you successful is creating QDs? Did you make QDs of different colors? If so then
you have verified that this method of self-assembling QDs using a kinetic method can be
used to make dots of different colors.

What color QDs did you make? How small and how large were the dots you made?

What colors were the dots under UV light?

When the solution is cloudy it contains solids that scatter light on all visible wavelengths
in all directions. How would that affect the absorbance spectrum? Why do we therefore
need to solubilize the QDs?

How could the experiment be improved? What do you see as weaknesses in the current
experimental design?

The color of QDs is due to the structure of the dot we call this structural coloration. Do
you think that pigment due to structural coloration is more or less stable than pigment due
to a dye? Explain your reasoning.
Page 18
Prelab Questions – to be completed before the experiment is conducted
1. How small does a particle of CdSe need to be to behave as a quantum dot?
2. What are quantum dots normally made of?
3. Why does Cadmium Myristate form an inverse micelle in a non-polar solvent?
4. Why would the inverse micelle be spherical in shape?
5. Give 3 advantages to growing CdSe nanoparticles in an inverse micelle.
6. What role does the 1-octadecene play in the synthesis?
7. How do we create QDs of different sizes?
8. Calculate the onset wavelength (in nm) for absorption for a CdSe dot that has a radius
of 2.1nm using equation (5). What is the color of light absorbed? What would the
color of the solution appear if all wavelengths less that the onset wavelength are
absorbed also?
9. Name 3 technological applications for QDs.
10. Where does the synthesis need to be conducted?
11. What special precautions are needed for this synthesis?
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Post Lab Questions – to be handed in with the report
1. How large is a nanometer?
2. Give the length of a C-C bond and the Bohr radius for H in nanometers.
3. Define a semiconductor
4. What is a hole?
5. What is an exciton?
6. What is the definition of the Bohr radius, and what is the Bohr radius for a 1S electron?
7. What makes the Bohr radius of an exciton in a semiconductor so much larger than the Bohr
radius of a H atom?
8. How large or small does a CdSe dot need to be to behave as a QD?
9. Why do you think that the particle in a box approximation works at all for a Quantum dot?
10. Define the (a) the valence band, (b) the conduction band, (c) the band gap.
11. Name three applications of Quantum Dots
12. Name two other quantum dot compositions other than CdSe.
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13. In your calculation of the data for Table 2 which term is dominant, the confinement energy or
the exciton energy?
14. If the spectrum blue shifts does this mean the wavelength increases or decreases? Explain.
Toxicity of Cd
Need to Keep Cd levels below 1 microgram Cd/ kg / day
For a 50kg person = 2.5E-4 grams of Cd Myristate intake = 4.44E-7 moles (0.25 mg)
Moles Cd Myristate = 0.36g / 567 g/mol = 6.349E-4 moles
Mass Cd = 6.349E-4*112.41 g = 0.071 g
Page 21