Honors Theses
Chemistry
Spring 2013
A study on the synthesis and characterization of
ternary semiconductor nanocrystals
Tyler J. Hurlburt
Penrose Library, Whitman College
Permanent URL: http://hdl.handle.net/10349/1219
This thesis has been deposited to Arminda @ Whitman College by the author(s) as part of their
degree program. All rights are retained by the author(s) and they are responsible for the content.
A STUDY ON THE SYNTHESIS AND CHARACTERIZATION
OF TERNARY SEMICONDUCTOR NANOCRYSTALS
by
Tyler J. Hurlburt
A thesis submitted in partial fulfillment of the requirements
for graduation with Honors in Chemistry.
Whitman College
2013
Certificate of Approval
This is to certify that the accompanying thesis by Tyler J. Hurlburt has been
accepted in partial fulfillment of the requirements for graduation with Honors in
Chemistry.
________________________
Steven Hughes
Whitman College
May 8, 2013
ii
Table of Contents
Introduction……………………………………………………………. 1
Semiconductors…………………………………………………...
2
Particle in a Box…………………………………………………... 5
Advantages of Nanocrystal………………………………………
8
Quantum Yield…………………………………………………….
9
Previous Research……………………………………………….
12
Experimental Section………………………………………………… 16
Results and Discussion………………………………………………
27
CIS Synthesis……………………………………………………... 27
CGS Shell on CIS…………………………………………………
29
CGSe Synthesis…………………………………………………..
31
Cation Exchange………………………………………………….
32
AIS Synthesis………….………………………………………….. 34
AGS Synthesis…………………………………………………….
34
Changing of Stoichiometry……………………………………….
38
Ligand Effects……………………………………………………..
40
Growth Time……………………………………………………….
43
Conclusions…………………………………………………………...
46
Acknowledgements…………………………………………………..
49
References…………………………………………………………….
50
iii
List of Figures
1. Stokes shift……………………………………………………….
2
2. Photoluminescence of a semiconductor………………........... 3
3. Indirect band gap………………………………………………..
4
4. Example of the particle in a box……………………………….. 7
5. Core/shell nanocrystal system…………………………...........
10
6. Type I and type II heterostructures……………………………. 11
7. Chalcopyrite crystal structure of I-III-VI semiconductors
14
8. Band gaps for several of the materials being studied
15
9. Photoluminescence of CIS at various stages of growth…….
27
10. UV/Vis absorption spectra of CIS at various stages of
growth…………………………………………………………….
27
11. Photoluminescence of CIS before and after shelling with
ZnS………………………………………………………………..
28
12. Photoluminescence of CIS/ZnS at various stages of
growth…………………………………………………………….
28
13. Photoluminescence of CGS……………………………………
29
14. XRD of one-pot synthesis of CIS/CGS………………………..
30
15. Photoluminescence of CGS shelling of small CIS seeds…...
31
16. Photoluminescence of CGS shelling of large CIS seeds…… 31
17. Photoluminescence of CGSe nanocrystals…………………..
32
18. Photoluminescence of nanoparticles before and after cation
exchange to cadmium…………………………………………..
32
19. Photoluminescence of AIS nanocrystals……………………...
33
20. Photoluminescence of AGS nanocrystals grown with
different amounts of time before sulfur injection……………..
35
21. Photoluminescence of AGS nanocrystals grown with and
without vacuum………………………………………………….. 36
22. Photoluminescence of AGS nanocrystals grown at different
temperatures……………………………………………………..
37
23. Photoluminescence of AGS nanocrystals with different
Ag:Ga ratios……………………………………………………...
38
24. XRD pattern for 0.5:1 silver to gallium AGS nanocrystals….
39
iv
25. Photoluminescence of AGS particles grown in the presence
of three different ligands………………………………………..
40
26. Photoluminescence of AGS particles grown in the presence
of different ratios of stearic acid and hexanoic acid…………. 41
27. TEM images of nanocrystals grown in the presence of
different ratios of stearic acid and hexanoic acid…………….
42
28. PL and absorption spectra for AGS particles taken at
various times of growth…………………………………………
43
29. XRD patterns for AGS particles grown for 5 min, 1 hour,
and 2 hours………………………………………………………
44
30. TEM images of AGS particles grown for 5 min, 1 hour, and
2 hours……………………………………………………………
45
v
Introduction
Currently, the lighting market is a billion dollar industry and there is a
move away from incandescent and fluorescent light bulbs to light emitting diodes
(LEDs) for general lighting purposes.1 When developing lighting for rooms, a
warm white light is desired. Unfortunately, most LEDs today have a blue hint to
them, which most consumers find to be cold.2 This is due to the fact that in the
spectrum of light emitted there is a large peak in the blue region with no emission
in the green and red regions.2
One way to fix this problem is to coat the LED with a phosphor (a material
that shows luminescence) that absorbs light in the ultraviolet and blue region
very well and then reemits that light at longer wavelengths.2 Typically a blue LED
will be coupled with either one material that emits in the yellow region or two
materials: one that emits in the green region and one that emits in the red.
Usually, the added red phosphor causes the two phosphor system to produce a
warmer white light than the single phosphor system. Unfortunately, adding an
additional phosphor also increases the heat generated by the system, making
these systems more inefficient. 3
The most common materials used to make these phosphors are rare-earth
metals which can be costly and inefficient.1 Additionally, many of the rare earth
oxides used to make the phosphors are experiencing worldwide shortages,
furthering the need to find an alternative material. An emerging trend in research
is to develop semiconductor nanocrystals to downshift the light.1,4,5
Figure 1. Stokes shift.
This downshifting of the blue light comes from the Stokes shift of the
material. Stokes shift is the difference between the peak wavelength of the
absorption and emission spectra of a substance.6 A typical Stokes shift has an
emitted photon with less energy than the absorbed photon (see Figure 1);
conversely a substance with an anti-Stokes shift would emit a photon with
greater energy than the absorbed photon, causing a blue-shift of the light.7 The
Stokes shift is the principle of all LED phosphors because of the ability to alter
the wavelength of the light.
Semiconductors:
A semiconductor is a material with properties between those of conductors
and insulators. All solids have a valence band, which the valence electrons
normally occupy, and a conduction band, which excited electrons occupy. The
energy between these two bands, where other energy states are forbidden, is
called the band gap. In a conductor, these two bands are touching or even
2
overlapping such that valence electrons can lie in the conduction band without
needing to be promoted, where they can then travel around the bulk material. In
an insulator the bands are different in energy, so a great amount of energy is
required to excite an electron form the valence band into the conduction band.8
Semiconductors have an energy gap between the bands (band gap) that
is commonly in the range of the energy held by photons in the visible light
spectrum. Shining light of sufficient energy on to a semiconductor can promote
an electron from the valence band to the conduction band leaving an electron
hole in the valence band.8 This electron-hole pair is commonly referred to as an
exciton. After excitation, the exciton is free to travel in the material in the
conduction band, allowing for conduction. The ability of an exciton to travel is
measured by both the exciton mobility and lifetime. The lifetime of an exciton is
the time between excitation and recombination, for CdSe nanocrystals the
exciton lifetime is on the range of 6 ns.9 A more mobile exciton with a longer
lifetime can conduct much better than a less mobile exciton.8 A long lifetime is
not always desirable for uses in LEDs as more time allows for a greater chance
of an exciton to travel to the surface of the nanocrystal.
Figure 2. Photoluminescence of a semiconductor. In this case: silver gallium sulfide
3
All semiconductors have a finite exciton lifetime, meaning that eventually
the excited electron will fall back down to the valence band. When this occurs,
the electron will drop in energy from the lowest energy state in the conduction
band to the highest energy state in the valence band, emitting a photon with
energy equivalent to the band gap.8 Thus, a semiconductor emits light with
energy equal to its band gap, as shown in Figure 2.
Figure 3. An indirect band gap. Energy is plotted against crystal momentum. In order
for an electron to be promoted to the conduction band it needs a photon of sufficient
energy coupled with a phonon to overcome the difference in crystal momentum.
Band gaps come in two categories: direct and indirect. In direct band gaps,
the excited electron and its corresponding hole have the same crystal momentum
in their respective bands.8 This means that the lowest energy state in the
conduction band is directly above the highest energy state in the valence band,
allowing for a photon with energy equal to the band gap to be sufficient to
promote an electron. In this case, crystal momentum is typically zero and
radiative recombination is common. For indirect band gaps, shown in Figure 3,
4
the highest state in the valence band and the lowest state in the conduction band
occur at different crystal momentums. This means that not only must an electron
gain sufficient energy from a photon to be promoted, but it must also gain the
right amount of momentum via a lattice vibration or phonon.8 This same principle
applies to the recombination of an electron and a hole, making a much slower
two-step process thereby decreasing the likelihood of radiative recombination.
For this reason, semiconductors with direct band gaps are much more widely
used for optical devices.8
Particle in a box:
Nanocrystals are crystalline particles with a diameter less than 100
nanometers.10 Due to their very small size, nanocrystals can exhibit a
phenomenon known as quantum confinement. Quantum confinement is a
phenomenon by which the size of the band gap for a semiconductor is
determined by the size of the particle.11 It effectively is a real-world example of
the “particle in a box.” The particle in the box refers an electron confined in a
potential well of length a. In the simplest model, inside the well there is a
potential energy of zero, while anywhere outside the box has an infinite potential
energy. This means that the probability of finding an electron outside the box is
zero and inside is 1. It follows that the probability of finding the particle at the
edges of the box is also zero. Consider the time-independent Schrödinger
equation for a one dimensional potential well defined in the x-direction:
-ħ2/2m · d2ψ/dx2 + V(x)ψ(x) = Eψ(x)
5
Where ħ is Planck’s constant, m is the mass of the particle, ψ(x) is the
wavefunction of the particle at location x, V(x) is the potential of the particle at x,
and E is the total energy. Applying the boundary conditions imposed
(ψ(0)=0,ψ(a)=0) to the general solution of the time-independent Schrödinger
equation
ψ(x) = Asin(k·x)+Bcos(k·x)
where A, B, and k are constants, gives the wavefunction of the particle as
ψ(x) = Asin(k·x).
Differentiating the wavefunction gives us
dψ/dx = kAcos(k·x)
and
d2ψ/dx2 = -k2 Asin(k·x).
We see that
d2ψ/dx2 = -k2 ψ
Where using the Schrodinger equation, k can be solved for
k = (8π2mE/h2)1/2
giving
ψ(x) = Asin((8π2mE/h2)1/2 ·x).
Applying the boundary conditions again gives us
ψ(x) = Asin(nπ/a·x)
where n is an integer. We can find A by normalizing the wavefunction. This gives
A = √(2/L)
6
and thus
ψ(x) = √(2/L) ·sin(nπ/a·x).
Solving for the allowed energies in the box, we find:
En = (n2h2)/(8ma2)
This shows that the energy levels in the box are defined by the size of the box, a.
Decreasing the size of the box causes the energy of the respective levels to
increase, also increasing the gaps between levels. In terms of nanocrystals, the
smaller particle, the smaller a, causes the gap between the valence and
conduction band to increase, resulting in a photon with greater energy being
necessary to promote an electron and a photon with greater energy being
emitted when the exciton recombines.8
Blue
>
Red
Figure 4. As an example of the particle in a box, the larger the nanocrystal, the redder
the light emitted.
Since the photon emitted has a greater energy for smaller particles, it
follows that smaller particles emit bluer light. This quantum confinement leads to
the tunability of the light emitted from nanocrystals. Previous research has shown
7
that it is quite easy to tune the emission of nanocrystals based on the size of the
particles with larger particles emitting redder light, as in Figure 4.4
Advantages of nanocrystals:
Nanocrystals exhibit several very significant advantages over other
materials used in LEDs. Perhaps the biggest advantage of nanocrystals is the
fact that they can be synthesized in a colloidal system.12 This bottom-up
approach allows for scalable synthesis.13 In a bottom-up synthesis, the particles
are formed by the nucleation of just a few atoms followed by the controlled
growth on to the outer surface of the crystal, until the final particle is formed. For
the sake of comparison, in a top-down approach, you start with a piece of
material and etch down until you have your desired particle. This top-down
method is far more inefficient and difficult to scale.
Nanocrystals are also highly absorbing, especially in the UV to blue
region,14 which is beneficial for downshifting applications, such as LEDs (and
also extremely useful for solar cells since much of the sun’s power lies in the
UV).8 For LEDs it is helpful for the nanocrystals to be highly absorbing so that
enough photons are captured from the shorter wavelength regions to allow for
adequate emission at the longer wavelengths. If they were not highly absorbing,
it would be necessary to add a greater number of the particles in order to get the
same effect, increasing the heat generated and making the LED more inefficient.
Due to the incredibly small size of nanocrystals, they have a very high
surface to volume ratio.15 This is not particularly beneficial for use in LEDs, but it
8
provides a tremendous advantage for use in catalysis. Very small nanocrystals
can almost be considered entirely surface, which is where catalysis occurs,
theoretically making nanocrystals efficient catalysts with promising results
towards water splitting in particular.16,17
Quantum yield:
One of the main measures of the efficiency of a semiconductor
nanocrystal is the photoluminescence quantum yield. Photoluminescence
quantum yield is defined as the ratio of the photons absorbed to the photons
emitted. The greater the quantum yield, the brighter the emitted light. Typically
quantum yields less than 1% are not visible to the naked eye, while yields in the
80% region are considered to be extremely good.4,18
The main reason that an exciton would fail to reemit a photon would be
due to “traps”.4 These traps are energy levels that occur in the normally forbidden
region of a band gap which then cause a multi-step recombination process
whereby no photon with the energy of the band gap is emitted. The most
significant source of these traps is from the energy levels associated with the
surface of the materials, both on the exterior of the nanocrystal and at any crystal
defects in the interior of the particle due to the presence of atoms that are not
bonded to the proper number of other atoms.8
An important factor for reducing these traps, and thereby increasing the
quantum yield of the nanocrystals, is surface passivation.1 The particles must
have a ligand bonded to the surface of the crystal that passivates the surface,
9
effectively keeping the exciton from seeing the surface. In terms of the particle in
a box, the passivating ligand increases the height of the walls of the well.
The same ligand can also act as a surfactant, allowing for better
solvation.1 Without passivation the nanocrystals will aggregate, making larger
particles, and then precipitate instead of staying suspended for long periods of
time due to the size of the particles. Another implication of aggregation is the
formation of more allowed energy levels in the forbidden region thereby creating
traps and reducing quantum yield.
Figure 5. A basic core/shell structure for nanocrystals.
Another way to increase quantum yield of a nanocrystal is to have a
core/shell system.4 This works by having a core of one semiconductor material
shelled by another as in Figure 5. For LED applications, the outer shell must
have a band gap that is larger than that of the core with the valence band of the
shell lower than that of the core and the conduction band of the shell higher than
that of the core.14 This allows for only photons with a higher energy to be
absorbed promoting an electron up to the level of the shell’s conduction band.
10
This sort of band gap pair is termed a type I heterostructure (also called a nested
heterostructures) as shown in Figure 6(a). Because electrons move toward
states at lower energies while holes tend to move towards higher energy states,
the exciton will travel into the shell where the conduction band is lower and the
valence band is higher. This increases the likelihood that a photon will be emitted
with the proper energy. This allows for excellent confinement, since the entire
surface of the particle will be composed of the material with a greater band gap.
Figure 6. (a) A type I heterostructure. In this case both the excited electron and the hole
would move toward the material on the right. (b) A type II heterostructure. In this case an
excited electron would move to the lower energy conduction band of the material on the
right, while the hole would move up in energy toward the material on the left.
In type II (or staggered) heterostructures, the band gap of the shell
material does not fully enclose that of the core material. Either the conduction
band of the shell is lower than that of the core or the valence band of the shell is
higher than that of the core, shown in Figure 6(b). Type II heterostructures lead
to exciton separation, since the hole or electron will remain in the shell material,
while the other particle will move toward the core material. This sort of charge
separation may be beneficial for making solar cells, but very detrimental towards
LEDs as the exciton pair would no longer recombine.
11
For this core/shell system to work properly, the two materials must have
lattice structures that are similar enough to allow for proper bonding at the
interface between the two materials.18 If the two materials have a lattice
mismatch greater than about 10%, there will be poor shelling. Poor shelling is the
result of the formation of cracks and gaps in the crystal structure due to strain.
This leads to an increase in the amount of traps and thus decreasing the
quantum yield of the nanocrystals.4 However, if the lattices match up sufficiently,
they will be able to stretch slightly at the interface, decreasing the strain and to
allow for good deposition of the shell material.
Previous research:
Previous research has shown that it can be relatively easy to tune the
emission of nanocrystals by controlling the size of the particles. One of the main
ways to control the size of the nanoparticles synthesized is to limit their growth
time.4 The longer the particles are grown, the bigger they will get and thus the
longer the wavelength of emission will be.
The vast majority of previous research on semiconductor nanocrystals has
centered on studying binary nanocrystals, or crystals with two different
elements.2,14,15 These materials are generally either II-VI semiconductors such as
CdSe and PbS or III-V semiconductors such as InAs and GaAs.2,14,19 There has
also been research done on the ternary nanocrystals, mostly of the I-III-VI form
such as CuInS2 (CIS) and AgInS2 (AIS).20-23 All of these materials have band
gaps in the 1-3 eV range making them suitable semiconductors for light
12
applications. However, most of the binary semiconductors contain toxic elements,
particularly the II-VI’s which contain cadmium and lead.5 It is more difficult to
make and market practical LEDs containing these materials due to their
environmental effects, especially since they are banned in many regions.5 Luckily,
the ternary semiconductors tend to be made of far less toxic elements.20
Recent research has shown that the band gap of a material, particularly
for materials with at least three elements, can also be tuned by varying the
stoichiometry of the constituent elements in the material.5,24-26
These materials are all commonly used as the core material in a core/shell
system. The most popular shelling material for all of the various nanocrystals is
ZnS due to its wide band gap (3.8 eV)27 which allows for nested heterostructures
in almost all cases, its favorable lattice match to a wide range of materials
(a=5.409 Å)28, and its relatively inexpensive cost due to the natural abundance of
zinc and sulfur.4,5,14
Because ZnS has a very large band gap, only photons in the UV region
have enough energy to promote an electron to the conduction band. This means
that any nanocrystal with a ZnS shell would not be able to absorb blue light.
Since we are looking to make a material that can absorb well in the blue region
and then emit at longer wavelengths, our research looks to find an efficient way
to shell a CIS core with another ternary compound instead of ZnS. These ternary
I-III-VI materials typically have a chalcopyrite structure (Figure 7) with unit cell
constants of a=5.52 Å and c= 11.12 Å for CIS.
13
In
Cu
Cu
Cu
In
S
Cu
S
S
S
In
Cu
Cu
In
Cu
In
Cu
S
S
S
Cu
In
Cu
In
S
Cu
Cu
In
In
In
Cu
Figure 7. Chalcopyrite structure of I-III-VI semiconductors. For
CIS, copper is in pink, indium in blue, and sulfur in yellow.
We will look into using both CuGaS2 (CGS) and AgGaS2 (AGS) as a
shelling material. CGS shows promise because of a very good lattice match with
CIS and the possibility of a graded shell where the interior of the nanocrystal is
CIS and the shell is CGS, but there is also a region between the two with a mix of
indium and gallium. For the AGS shell, the band gaps for these two materials line
up very nicely, as seen in Figure 8, and they have a decent lattice match which is
better than that of CIS and ZnS. By using a system that has two ternary materials,
there is also that added tunability of the band gaps based on stoichiometry of the
constituent elements. In order to achieve this, we will first find a good way to
synthesize and control the CIS cores. We will then look into synthesizing CGS
and AGS on their own, with a greater focus on the AGS particles due to a lack of
previous research on the material, before moving on to attempting to shell the
CIS particles in CGS and AGS.
14
Figure 8. Band gaps for several of the materials being studied.
15
Experimental Section
Materials:
Gallium(III) acetylacetonate (99.99%), copper(I) iodide (99.999%),
indium(III) acetate (99.99%), indium(III) acetylacetonate (99.99%), indium(III)
chloride (99.999%), zinc stearate (90%), trioctylphosphine oxide (99%),
trioctylphosphine (97%), 1-dodecanethiol (96%), oleylamine (70%), octylamine
(99%), 1-octadecene (90%), tert-dodecanethiol (98.5%) were purchased from
Sigma-Aldrich. Lead(II) nitrate (99.9%), lead(II) chloride (100.0%), zinc acetate
(99.9%), zinc chloride (99.1%), silver acetate (99%), sulfur powder (99.98%), and
hexanoic acid (98%) were all purchased from J.T. Baker Chemical Co. Silver
nitrate (99%) was purchased from EMD. Pyridine (99%) and toluene was
purchased from Mallinckrodt. Toluene (99.5%) was also purchased from VWR,
as was methanol (99.8%) and isopropanol (99%). Methanol (99.8%) was also
purchased from Alfa Aesar. Selenium powder (99.99%) was purchased from the
General Chemical Company. Stearic acid and cadmium chloride were provided
by the Whitman College Chemistry Department Stockroom. All chemicals were
used as received.
Methods:
One-pot synthesis of CIS/ZnS: A 25 mL three-neck round bottom flask was
loaded with indium(III) acetate (In(Ac)3) (0.5 mmol), CuI (0.25-0.75 mmol), and 1dodecanethiol (1-DDT) (5 mL). A 20 mL scintillation vial was loaded with zinc
stearate (2 mmol), 1-DDT (0.5 mL), and 1-octadecene (ODE) (2 mL). The zinc
stearate solution was heated to 190°C to promote dissolution. The copper and
16
indium mixture was heated to 100°C with stirring and under a vacuum (<100
mTorr) and then held at 100°C for 45 min. The solution was then backfilled with
argon and heated to above 200°C. Starting around 200°C, the solution began to
change color from yellow to orange and then to red as the temperature increased.
When the reaction mixture reached the desired color, the zinc stearate solution
was added dropwise at a rate which kept the temperature nearly constant (~1
mL/min). After the addition of the zinc stearate solution, the reaction mixture was
heated at 240°C for 1 hour. The solution was then cooled, cleaned, and collected
as described in the cleaning section; all syntheses followed this same cleaning
process.
Cleaning process: The reaction mixture was cooled by squirting the flask with
water. Once at room temperature, the solution was added to centrifuge tubes and
an equal part of 50:50 methanol/isopropanol (IPA) was added. The mixture was
shaken and then centrifuged at 12,500 rpm for 5 min. The supernatant was
discarded and the solid product was collected in toluene.
CIS synthesis (method A): CIS nanoparticles were synthesized by using the
method described above, but instead of injecting a zinc stearate solution when
the desired color was achieved, the solution was cooled.
CIS synthesis (method B): A 25 mL three-neck round bottom flask was loaded
with In(Ac)3 (1 mmol), CuI (1 mmol), and ODE (10 mL). The mixture was stirred
under a vacuum (<100 mTorr) for 30 min and then backfilled with argon and
heated to 150°C. 1-DDT (2 mL) was then rapidly injected causing the solution to
turn yellow. The solution was then heated to 280°C. During this heating the
17
solution went from yellow to orange to red to black. The solution was held at
280°C for 30 min.
Two-pot synthesis of CIS/ZnS: A portion of the final product of CIS from
method A (1.5 mL) was loaded in a 25 mL three-neck round bottom flask. The
toluene was blown off with argon and light heating (50°C). Once the toluene was
blown off, ODE (5 mL) was added to the flask (4 g of trioctylphosphine oxide
(TOPO) were alternatively used) and the solution was degassed for 30 min
(vacuum <100 mTorr). Meanwhile, a 20 mL scintillation vial was loaded with zinc
stearate (2 mmol), 1-DDT (0.5 mL), and ODE (2 mL). The zinc stearate solution
was heated to 190°C to aid in dissolution. The CIS solution was heated to 100°C
while still under vacuum and then backfilled with argon and heated to 230°C. At
230°C, the zinc stearate solution was added dropwise at a rate which kept the
temperature nearly constant (~1 mL/min). After the addition of the zinc stearate
solution, the reaction mixture was heated at 240°C for 1 hour.
Two-pot synthesis of CIS/CGS (method A): A portion of the final product of
CIS from method A (1.5 mL) was loaded in a 25 mL three-neck round bottom
flask. The toluene was blown off with argon and light heating (50°C). Once the
toluene was blown off, gallium(III) acetylacetonate (Ga(acac)3) (0.5 mmol),
copper(II) acetylacetonate (Cu(acac)2) (0.5 mmol), TOPO (1.75 mmol), and ODE
(5 mL) were all added to the flask. The mixture was then heated to 100°C under
argon, degassed for 30 min, and then heated to 230°C under argon. The solution
was held at 230°C for 1 hour.
18
Two-pot synthesis of CIS/CGS (method B): A portion of the final product of
CIS from method A (1.5 mL) was loaded in a 25 mL three-neck round bottom
flask. The toluene was blown off with argon and light heating (50°C). Once the
toluene was blown off, Ga(acac)3 (0.5-1 mmol), Cu(acac)2 (0.5-1 mmol), TOPO
(1.75-3.5 mmol), and ODE (5 mL) were all added to the flask. The mixture was
degassed for 30-60 min at 40°C, and then heated to 150°C under argon. A
mixture of 1-DDT (0.25 mL) and t-DDT (1.75 mL) heated to 150°C was rapidly
injected. The solution was heated to 245°C and held there for 30-60 min.
CGS synthesis: A 25 mL three-neck round bottom flask was loaded with
Ga(acac)3 (1 mmol), Cu(acac)2 (1 mmol), TOPO (3.5 mmol), and ODE (10 mL).
The mixture was stirred under vacuum at room temperature for 30 min and then
backfilled with argon and heated to 150°C. A mixture of 1-DDT (0.25 mL) and tDDT (1.75 mL) was rapidly injected. The solution was then heated to 280°C for
30 min.
Two-pot synthesis of CIS/CGS (method C): A portion of the final product of
CIS from method A (1.5 mL) was loaded in a 25 mL three-neck round bottom
flask. The toluene was blown off with argon and light heating (50°C). Once the
toluene was blown off, Ga(acac)3 (0.5 mmol), Cu(acac)2 (0.5 mmol), TOPO (1.75
mmol), and ODE (5 mL) were all added to the flask and the synthesis for CGS
described above was followed.
One-pot synthesis of CIS/CGS: A 25 mL three-neck round bottom flask was
loaded with In(Ac)3 (0.5 mmol), CuI (0.5-1.5 mmol), and 1-DDT (5 mL) and
degassed at room temperature. A 20 mL scintillation vial was loaded with
19
Ga(acac)3 (1 mmol) and 1-DDT (20 mL). The copper and indium solution was
heated to 100°C under vacuum and then backfilled with argon and heated to
280°C. During heating to 280°C, the solution changed color from orange to black
at 210°C. At this point the gallium solution (10 mL) was added dropwise. The
solution was heated at 280°C for 30 min.
Two-pot synthesis of CIS/AGS: A portion of the final product of CIS from
method A (1.5 mL) was loaded in a 25 mL three-neck round bottom flask. The
toluene was blown off with argon and light heating (50°C). Once the toluene was
blown off, Ga(acac)3 (1 mmol), Ag(ac) (1 mmol), and 1-DDT (5 mL) were all
added to the flask. The solution was degassed at room temperature for 30 min
(<100 mTorr) and then backfilled with argon and heated to 150°C. A mixture of 1DDT (0.25 mL) and tert-dodecanethiol (t-DDT) (1.75 mL) heated to 150°C was
rapidly injected. The solution was heated at 150°C for 2 hours.
CGSe synthesis: Oleylamine (20 mL) was pumped under vacuum (<100 mTorr)
at 80°C for 16 hours turning slightly golden in color. A 25 mL three-neck round
bottom flask was loaded with selenium powder (0.4 mmol) and the degassed
oleylamine (8 mL). The solution was heated at 120°C under vacuum for 30 min
turning golden brown and then backfilled with argon and heated at 250°C for 30
min which turned the solution orange. A separate three-neck flask was loaded
with Cu(acac)2 (0.2 mmol), Ga(acac)3 (0.2 mmol), and degassed oleylamine (5
mL). This solution was heated at 80°C under vacuum for 1 hour giving a dark
blue-green color. This copper and gallium solution (5 mL) was rapidly injected
20
into the selenium solution causing the solution to immediately turn black. The
solution was heated at 250°C for 1 hour.
Cation exchange: Stock solutions of 0.15 mmol CuCl2·2H2O, 0.15 mmol AgNO3,
0.15 mmol CdCl2, 0.15 mmol PbCl2, 0.15 mmol Pb(NO3)2, 0.15 mmol CuI, 1.5
mmol AgNO3 and 0.25 mmol TOP in methanol were each prepared. A stock
solution of CIS nanoparticles as prepared by method A was diluted to have an
absorbance at 350 nm of 0.53. A series of 8 mL scintillation vials were loaded
with the CIS stock solution (0.1-0.5 mL) and toluene (1 mL). The cadmium stock
solution or one of the lead stock solutions (1-4 mL) and an equal part of the TOP
solution were added to the vial and the solution was then centrifuged with
methanol. The small amount of product was collected in toluene (2 mL). A series
of 8 mL scintillation vials were loaded with toluene (2 mL), methanol (0-0.3 mL),
and one of the copper or silver stock solutions (2-6 mL). This solution was added
to the cadmium exchanged product from before. The solution was centrifuged
with methanol and the product was collected in toluene.
AGS synthesis (method A): A 25 mL three-neck round bottom flask was loaded
with Ga(acac)3 (0.4 mmol), Ag(ac) (0.4 mmol), and 1-DDT (5 mL). The mixture
was heated at 150°C under argon for 2 hours at which time the solution was
black. At this point a solution of sulfur powder (3 mmol) in 1-DDT (5 mL) heated
to 170°C was rapidly injected. The solution was then heated at 150°C for another
2 hours.
AGS synthesis (method B): A 25 mL three-neck round bottom flask was loaded
with Ga(acac)3 (0.4 mmol), Ag(ac) (0.4 mmol), and 1-DDT (5 mL). The mixture
21
was stirred under vacuum (<200 mTorr) at room temperature for 15 min and then
heated to 100°C under vacuum. The solution was then backfilled with argon and
heated at 150°C for 30 min. At this point a solution of sulfur powder (3 mmol) in
1-DDT (5 mL) heated to 170°C was rapidly injected. The solution was then
heated at 230°C for 15 min.
AGS synthesis (method C): A 25 mL three-neck round bottom flask was loaded
with Ga(acac)3 (0.4-0.8 mmol), AgNO3 (0.1-0.8 mmol), and 1-DDT (5 mL). The
mixture was stirred under vacuum (<200 mTorr) at room temperature for 30 min
and then heated to 100°C under vacuum. The solution was then backfilled with
argon and heated to above 150°C. At the point where the solution reached a light
yellow or orange color, a separate solution of sulfur powder (3 mmol) in 1-DDT (5
mL) heated to 170°C was rapidly injected. The solution was then heated at either
150°C or 250°C for 0.5-4 hours.
AGS synthesis (method D): AGS nanoparticles were synthesized using method
B, but replacing Ag(ac) with AgNO3.
AGS synthesis (method E): A 25 mL three-neck round bottom flask was loaded
with Ga(acac)3 (0.4 mmol), AgNO3 (0.4 mmol), and 1-DDT (5 mL). The mixture
was heated to 150°C under argon at which time the solution was light yellow. At
this point a solution of sulfur powder (3 mmol) in 1-DDT (5 mL) heated to 170°C
was rapidly injected turning the solution black. The solution was then heated at
150°C for another 2 hours and the solution had turned to a dark red color.
22
AGS synthesis (method F): AGS nanoparticles were synthesized following
method C but with the addition of ligands (0.8-1.6 mmol). Ligands used were
octylamine, or a mixture or hexanoic acid and stearic acid of varying ratios.
One-pot synthesis of CIS/AGS: A 25 mL three-neck round bottom flask was
loaded with In(Ac)3 (0.5 mmol), CuI (0.5-1.5 mmol), and 1-DDT (5 mL) and
degassed at room temperature. A 20 mL scintillation vial was loaded with
Ga(acac)3 (0.4 mmol), Ag(NO)3 (0.4 mmol), and 1-DDT (5 mL) and another was
loaded with sulfur powder (3 mmol) and 1-DDT (5 mL); both of these solution
were heated to 130°C. The copper and indium solution was heated to 100°C
under vacuum and then backfilled with argon and held at 100°C for 30 min. The
solution was then heated to 230°C. Around 210°C, the solution had turned red.
At this point, 2.5 mL of the other two solutions were added dropwise over two
minutes. The solution was then heated at 230°C for 30 min.
Two-pot synthesis of CIS/AGS (Method A): A portion of the final product of
CIS from method A (1.5 mL) was loaded in a 25 mL three-neck round bottom
flask. The toluene was blown off with argon and light heating (50°C). The product
was redissolved in 1-DDT (5 mL) and heated to 125°C giving a red solution.
Another three-neck flask was loaded with Ga(acac)3 (0.4 mmol), AgNO3 (0.4
mmol), and 1-DDT (5 mL). The mixture was heated to 150°C under argon at
which time the solution was light yellow. At this point, the CIS solution was
rapidly injected, immediately turning the solution black. The solution was heated
at 150°C for 2 hours.
23
Two-pot synthesis of CIS/AGS (Method B): A portion of the final product of
CIS from method A (1.5 mL) was loaded in a 25 mL three-neck round bottom
flask. The toluene was blown off with argon and light heating (50°C). The product
was redissolved in 1-DDT (2 mL) and heated to 125°C giving a red solution.
Another three-neck flask was loaded with Ga(acac)3 (0.4 mmol), AgNO3 (0.4
mmol), and 1-DDT (5 mL). The mixture was heated to 150°C under argon at
which time the solution was light yellow. At this point, the CIS solution was
rapidly injected, immediately turning the solution black. Once the solution
reached 150°C again, a solution of sulfur powder (3 mmol) in 1-DDT (5 mL)
heated to 170°C was rapidly injected. The solution was heated at 150°C for 2
hours.
AIS synthesis: A 25 mL three-neck round bottom flask was loaded with In(ac)3
(0.4 mmol), silver(I) acetate (0.4 mmol), and 1-DDT (5 mL). The mixture was
heated at 150°C under argon for 2 hours at which time the solution was yellow.
At this point a solution of sulfur powder (3 mmol) in 1-DDT (5 mL) heated to
170°C was rapidly injected turning the solution an opaque red-orange. The
solution was then heated at 150°C for another 2 hours.
One-pot AGS/ZnS synthesis (Method A): Method C for synthesizing AGS
particles was followed using 0.2 mmol AgNO3 and 0.4 mmol Ga(acac)3, but once
250°C was reached, 1.25-2.5 mL of a solution of zinc stearate (3 mmol) in 1-DDT
(1 mL) and ODE (4 mL) heated to 170°C was added dropwise at a rate that kept
the temperature constant. The solution was then heated at 250°C for 2 hours.
24
Two-pot AGS/ZnS synthesis (Method B): A portion of the final product of AGS
from method C (1.5 mL) was centrifuged with methanol and the product was
collected in ODE (5 mL). The solution was stirred under vacuum (<100 m Torr) at
room temperature for 30 min and then backfilled with argon and heated to 80°C
or 220°C. A solution of zinc stearate (0.4 mmol) in 1-DDT (1 mL) and ODE (4
mL) heated to 90°C was added dropwise (~1 mL/min). The solution was then
heated at 220°C for 1 hour.
Ligand Exchange: A series of 24 scintillation vials (8 mL) were each half filled
with toluene. Each vial had a portion of AGS nanoparticles as prepared by
method C added to them: six vials had a low concentration of AGS with a 1:1
Ag/Ga ratio added; six had a low concentration of AGS with a 1:2 Ag/Ga ratio
added; six had a high concentration of AGS with a 1:1 Ag/Ga ratio added; six had
a high concentration of AGS with a 1:2 Ag/Ga ratio added. Each of these vials
had three drops of one of the following ligands added: hexanoic acid, pyridine, 1DDT, trioctylphosphine (TOP), octylamine, or oleylamine. The samples were
analyzed by PL after being left to sit for a month.
Photoluminescence Measurements: Approximately 1 mL of the final
nanocrystal solution was diluted in approximately 5 mL toluene. This diluted
nanocrystal solution was analyzed by a Jasco FP-6200 Fluorescence
Spectrophotometer. The excitation wavelength was set to 340 nm and a scan
rate of 125 nm/min was used.
25
UV/Vis Measurements: The same diluted solution used for the
photoluminescence measurements was used. UV/Vis spectra were obtained
using a StellerNet EPP2000.
Elemental Analysis: A series of silver standards were made using silver nitrate
in 2% HNO3 with silver concentrations ranging from 0.12 ppm to 1.00 ppm. A
series of gallium standards were made using gallium acac in 2% HNO3 with
gallium concentrations ranging from 50 ppm to 200 ppm. These standards were
analyzed using a Perkin-Elmer AAnalyst 400. Nanocrystals were dried by gentle
heating while having argon blown over the solvent. The solid nanocrystals were
then dissolved in 1 mL water with 5 mL concentrated HNO3. These nanocrystal
solutions were diluted to fall within the concentration range for each metal being
studied and then analyzed on the same FAAS.
Powder X-Ray Diffraction: Nanocrystal solutions were centrifuged with
methanol to crash out the particles and then blown over with argon to remove
any remaining solvent. The XRD pattern was obtained using an Oxford
Diffraction Nova X-ray diffractometer with Cu Kα source and an Onyx CCD
detector using the powder diffraction setting. A 100 second integration time was
used.
TEM Imaging: TEM imaging of the nanocrystals was provided by the Portland
State CEMN.
26
Results and Discussion
CIS synthesis:
Previous research has shown that CIS/ZnS nanoparticles could be
produced with a high photoluminescence quantum yield.3 We were able to
replicate these results synthesizing particles with a photoluminescence quantum
yield (71%) similar to that seen in the literature.
It was also found that we could readily synthesize nanocrystals with
varying band gaps by altering their growth time. CIS nanocrystals could be grown
with photoluminescence either in a range around 420 nm or around 620 nm. As
seen in Figure 9, as the growing time increases the 420 nm peak diminishes and
the 620 nm peak gets bigger. By cooling the reaction immediately once the
desired highest temperature was reached (205-240°C), it was possible to tune
the ratio of these peaks. During heating of the reaction mixture in this range, the
color of the solution would change from yellow to orange to red and then to a
very dark blood red showing an increase in the size of the particles.
Figure 10. UV/Vis absorption spectra of
CIS at various stages of growth.
Figure 9. Photoluminescence of CIS at
various stages of growth.
27
CIS particles on their own could be tuned by stopping the growth when
desired or a ZnS shell could be added by adding a zinc stearate solution
dropwise when the desired color (and thus desired size) was reached. The
majority of the red shifting of the CIS particles was found to occur while the
temperature was increasing, although the crystals did continue to grow while the
temperature was held constant. The longer growth also resulted in a greater
intensity of photoluminescence, as shown in Figure 9. During this growth, the
emergence of a slight shoulder in the absorption was found to appear around
470 nm as in Figure 10 as the band gap of the particles decreased with the
increasing particle size.
Figure 12. Photoluminescence of
CIS/ZnS at various stages of growth.
Time is after ZnS injection.
Figure 11. Photoluminescence of CIS
before and after shelling with ZnS.
It was seen that adding the ZnS shell to the CIS nanocrystals always
resulted in particles that emitted near 570 nm, with no change based on the size
of the CIS cores. Since the resulting shelled materials always had
photoluminescence near 570 nm, we sometimes found a blue shift of the emitted
light even though the shelled particle would be bigger than the unshelled CIS
(Figure 11). This blue shifting would become more pronounced as the reaction
28
time increases as shown in Figure 12. Again, the photoluminescence became
more intense the longer the growth was allowed to run (Figure 12). The
absorption did not change significantly upon the addition of the ZnS shell or
during the growth of this shell as shown in Figure 10.
CGS shell on CIS:
Copper gallium sulfide (CGS) was picked as a potential shelling material
for CIS due to the very good lattice match (a=5.523 Å for CIS and a=5.35 Å for
CGS) between the two materials.27 CGS particles were first synthesized on their
own. The CGS nanoparticles were found to emit in the 400-500 nm range and
have a peak near 420 nm and a very distinct shoulder around 500 nm as seen in
Figure 13.
Figure 13. Photoluminescence of CGS nanocrystals
All attempts at making a CIS/CGS nanoparticle system via one pot
syntheses resulted in nanocrystals with no photoluminescence in the visible
range. This is likely due to the poor alignment of the band gaps of the two
materials, as CGS has a higher valence band and a higher conduction band than
29
CIS,28 leading to a separation of the electron-hole pair. While the
photoluminescence was very poor, the x-ray diffraction pattern for CGS particles
grown using a one pot synthesis fit an average of the CIS and CGS patterns as
shown in Figure 14. This would indicate that an alloy of CIS and CGS could have
been produced instead of a core/shell system; more research on this is needed.
Figure 14. XRD of one-pot synthesis of CIS/CGS. Experimentally found pattern for
CIS/CGS is in red, pattern for bulk CIS is in blue and to the left of the CIS/CGS peaks,
and bulk CGS is in green and to the right of the CIS/CGS peaks.
Shelling CIS seeds with a CGS shell via a two pot synthesis gave mixed
results. When the shelling was done on small CIS seeds (CIS particles that were
grown for a short amount of time, which gave a photoluminescence peak at 420
nm and a shoulder at 470 nm), it was found that adding a CGS shell resulted in
blue shifting of the main peak and red shifting the shoulder. This effectively
30
extended the distance between these two peaks, see Figure 15. However, when
the shelling was done on CIS seeds which were allowed to grow to where they
were emitting at 640 nm, the resulting photoluminescence was quenched as
shown in Figure 16. This is likely due to the fact that the larger particles had a
more rigid lattice, so there would be a greater amount of strain at the interface.
XRD analysis of the two-pot shelling did not give a pattern indicating an alloy, as
it did for the one-pot shelling, but a pattern indicating a third structure.
Figure 15. Photoluminescence of CGS
shelling of small CIS seeds, before and
after shelling.
Figure 16. Photoluminescence of CGS
shelling of large CIS seeds, before and
after shelling.
CGSe Synthesis:
Because copper gallium selenide (CGSe) has a band gap that would form
a nested heterostructure as the core material when shelled with either AGS or
CGS, we looked at synthesizing CGSe nanocrystals as an alternative to CIS. The
resulting particles gave no emission in the visible region as shown in Figure 17.
Coupling this with the fact that the synthesis called for pumping the solvent under
31
a high vacuum for 16 hours, we decided to not look into this material any farther
at this time.
Figure 17. Photoluminescence of CGSe nanocrystals.
Cation exchange:
Cation exchange is a process where a metal cation in a crystal structure is
replaced by a different metal cation. This often involves metals of different
charges such as Ag+ replacing Cd2+, for this to happen two silver cations take the
place of one cadmium ion.29 This is possible for nanocrystals because of the
small size.26 Attempts were made at using cation exchange on CIS nanoparticles
to potentially incorporate silver into the particles. First, a solution of nanocrystals
was added to a solution containing either cadmium or lead ions which would give
structures of Cd0.5InS2 and Pb0.5InS2. It was found that this process quenched the
emission of the nanocrystals (Figure 18), indicating that cadmium and lead may
have moved into the crystal structure. We expect the cadmium and lead ions to
exchange with the softer copper ions instead of the harder indium ions, due to
the fact that two In3+ ions would need to be replaced with three Cd2+ ions.
32
Figure 18. Photoluminescence of nanoparticles before and after
cation exchange to cadmium. Prior to cation, particles are CIS.
Unfortunately, subsequent cation exchange from either of the cadmium or
lead containing particles to the corresponding silver or original copper
nanocrystals did not show any increase in photoluminescence. There has been
very little research done on cation exchange with ternary nanocrystals, so the
effectiveness of these processes is questionable and difficult to measure. Since
another cation is present (indium) it is possible that the process which works well
for binary nanocrystals for both reaction directions will not work for ternary
nanocrystals, especially at the usually easy conditions seen in cation exchange
of binary nanocrystals.29-31
33
AIS synthesis:
Figure 19. Photoluminescence of AIS nanocrystals.
Due to the fact that silver indium sulfide (AIS) is a more well-studied
material than AGS (which we are very interested in for shelling of CIS) and
because it should behave similarly, we looked at synthesizing AIS nanocrystals
as a jumping off point. The resulting particles were found to have
photoluminescence with a sharp narrow peak at 390 nm and a wide peak at 490
nm (Figure 19). The XRD pattern matched very well to that of bulk AIS. Due to
the odd emission spectra of AIS and a reported band gap around 2 eV,27,28 which
is smaller than what we are looking for, this material was not explored further at
this time.
AGS synthesis:
Based on the fact that silver gallium sulfide has a valence band lower than
that of CIS and a conduction band higher than that of CIS (band gap of 1.53 eV
for CIS and 2.7 eV for AGS)27,28, AGS was chosen as a potential shelling
34
material for CIS nanoparticles. Additionally, the lattice mismatch between the two
materials is fairly low at 4% (lattice constant a=5.52 Å for CIS and 5.75 Å for
AGS).27,28
Attempts at first shelling the CIS particles in AGS following a one-pot
synthesis very similar to the shelling of CIS in ZnS resulted in particles with no
photoluminescence. Since there has been little previous research done on AGS
nanoparticles, before using it as a shelling material, we wanted to better
understand the synthetic conditions for growing AGS nanoparticles on their own
in order to better understand how to grow it as a shell. Over the course of this
study many different syntheses were run to varying degrees of success.
Figure 20. Photoluminescence of AGS nanocrystals grown for no time and for 2
hours before sulfur injection
In the first syntheses, the metal precursors in 1-DDT were heated to
150°C during which the solution turned clear red-orange and then light yellow.
The solution was then held at 150°C for 2 hours, during which time the solution
turned dark brown, before the sulfur in 1-DDT was injected into the solution. This
35
resulted in photoluminescence with many different peaks. By letting the metal
precursors in 1-DDT heat at 150°C for an extended period of time it is likely that
some AGS particles were being formed by using the sulfur from the solvent. This
likely lead to the formation of many differently sized particles upon the injection of
the sulfur as well as possibly created other compositions such as AgS and GaS.
When the sulfur was instead injected immediately when the reaction mixture
turned light yellow around 150°C only one photoluminescence peak emerges as
shown in Figure 20. For this reason it is important to not let the metal precursors
heat for too long prior to the nucleation injection.
Figure 21. Photoluminescence of AGS nanocrystals grown with and
without vacuum at the beginning of the synthesis
Due to suspicion that residual moisture and oxygen may play a role in the
reaction kinetics, degassing of the reaction mixture prior to heating and during
the first part of the heating process was then added to the synthesis. The
reaction mixture was degassed at room temperature for 15-30 min. and then held
36
under vacuum during heating until 100°C. At this point the flask was backfilled
under a flow of argon. It was found that doing so resulted in a more narrow
emission peak that was slightly blue-shifted for similar reaction conditions as can
be seen in Figure 21. As hypothesized, this is likely due to a smaller presence of
water and other oxidizers.
Figure 22. Photoluminescence of AGS nanocrystals grown at
different temperatures.
The next syntheses included heating the reaction mixture to higher
temperatures. In the earlier syntheses, after injecting the sulfur, the solution was
heated at 150°C. It was found that by increasing the temperature after the
injection to 250°C a fairly large blue-shift in the emission of the particles was
seen as shown in Figure 22. The reason for this blue-shift is not understood and
requires further study.
37
Changing of stoichiometry:
Previous research has shown that the band gap of ternary nanocrystals
can be slightly tuned based on the stoichiometry of the metals in the material.4 In
typical AGS the ratio of silver to gallium is 1:1. AGS nanoparticles with a higher
relative concentration of gallium were first synthesized by reducing the amount of
silver precursor in the synthesis. It was seen that having more gallium present
than silver resulted in a final product that was more yellow, even approaching
green, than the typical orange of normal AGS. Additionally, these particles
resulted in the broad, low photoluminescence peaks shown in Figure 23. In a
similar fashion, AGS particles with a greater relative amount of silver were also
synthesized by increasing the amount of silver precursor present in the synthesis.
These particles resulted in a final solution that was more dark red or black, also
with very broad and low photoluminescence.
Figure 23. Photoluminescence of AGS nanocrystals with different Ag:Ga ratios.
38
These particles were analyzed via flame atomic absorption spectroscopy
in order to determine if the ratio of starting material actually resulted in
nanoparticles with a corresponding ratio. It was found that the ratio of the metal
precursors used in the synthesis fairly closely leads to the desired ratio of metals
in the resulting nanoparticles as shown in Table 1. XRD analysis of nanocrystals
with 0.5:1 silver to gallium shows the same pattern as bulk AGS as shown in
Figure 24. This means that in the chalcopyrite structure the more prevalent metal
must be taking up the lattice locations of the less common metal. This is possible
on the nanoscale because the small size of the particles allows for crystals with
less rigid lattices giving way to intermediate crystal structures.
Precursor Ratio
Ratio in particles
Error
1:1
1.02:1
0.02
2:1
1.91:1
0.04
0.5:1
0.52:1
0.05
Table 1. Experimentally determined Ag:Ga ratios (and error) of
various AGS nanocrystals with different ratios of metal precursors.
Figure 24. XRD pattern for 0.5:1 silver to gallium AGS
particles with the pattern for bulk AGS overlaid.
39
Ligand effects:
In the early syntheses of AGS, the dodecanethiol used as the solvent for
the synthesis was also the passivating ligand and surfactant for the nanocrystals.
Because the AGS particles had trouble staying suspended, additional
surfactants/ligands were added to the system. Octylamine was the first ligand
used for synthesis. It was found that adding octylamine resulted in extreme
bubbling of the reaction mixture during the heating process. While the
photoluminescence of these particles was found to be decent (Figure 25), the
extreme bubbling resulted in a loss of precursor as they were deposited on the
sides of the condenser, making octylamine a poor choice for these syntheses.
Figure 25. Photoluminescence of AGS particles grown in the presence of three
different ligands: octylamine in blue, hexanoic acid in red, and stearic acid in green.
We then looked at using carboxylic acids as the surfactants because they
are cheap, commonly used for binary nanocrystals, and come in a variety of
chain lengths. Both hexanoic acid and stearic acid were used individually and
40
together in various ratios to get a mix of short and long chains. A series of
reactions were carried out with: only stearic acid; 80% stearic acid, 20%
hexanoic acid; 60% stearic acid, 40 % hexanoic acid; 40% stearic acid, 60%
hexanoic acid, 20% stearic acid, 80% hexanoic acid, and only hexanoic acid. In
these reactions the total amount of ligand was equal to the total amount of metal
precursor. The 100% hexanoic acid resulted in particles with broad and weak
photoluminescence with a peak at 540 nm most likely due to poor size control.
The 100% stearic acid resulted in a more narrow emission centered on 535 nm.
The mixtures of hexanoic acid and stearic acid all gave somewhat similar broad
emissions with peaks between 460 and 480 nm as seen in Figure 26. Out of all
of these, only the 80/20 and 20/80 ratios gave particles that were able to stay
suspended in solution. All of the reactions run with stearic acid resulted in what
appears to be noisy emission curves, but previous research has shown that
these regular and repetitive smaller peaks may be due to phonon assisted
transitions.32
Figure 26. Photoluminescence of AGS particles grown in the presence of
different ligands systems with stearic acid and hexanoic acid.
41
a
b
c
d
e
f
Scale bar is at 20 nm
Figure 27. TEM images with inset picture of nanocrystals in solution. Nanocrystals were
prepared with: (a) 100% stearic acid; (b) 80% stearic acid, 20% hexanoic acid; (c) 60%
stearic acid, 40% hexanoic acid; (d) 40% stearic acid, 60% hexanoic acid; (e) 20% stearic
acid, 80% heaxanoic acid; (f) 100% hexanoic acid. Increased scattering is observed in
samples (a), (c), (d), and (f), due to increased aggregation.
Each of the products from these reactions was analyzed by TEM, shown
in Figure 27. It was seen that the 100% stearic acid, 80% stearic acid, and 100%
hexanoic acid all gave very poor size and shape control. The 60/40, 40/60, and
20/80 ratios all gave significantly improved shape and size control. Based on the
size (about 5 nm in diameter) and shape control seen in the TEM images, as well
as the ability to stay suspended (which indicates less aggregation), it was
determined that the 20% stearic acid and 80% hexanoic acid was the optimal
ligand system to be used for further modifications.
The total amount of ligand used was then doubled to see if the amount of
ligand has a significant effect as having a greater amount of the ligands present
could result in a decrease in aggregation. This resulted in particles that gave a
42
decent emission centered at 525 nm with the same phonon assisted transitions
seen. Oleic acid, a commonly used surfactant, replaced the stearic acid giving
similar results. While no TEM images of these particles have been taken yet,
there is no appreciable improvement or detriment in the photoluminescence of
these nanocrystals.
Growth time:
Figure 28. PL and absorption spectra for AGS particles with 20% stearic acid, 80%
hexanoic acid taken at various times of growth. Absorption did not change with time of
growth.
The time of growth during the synthesis of AGS nanoparticles was studied
to find if there was an optimal growth time by taking aliquots of the reaction
mixture at various times during the synthesis. A ligand system of 20% stearic
acid and 80% hexanoic acid was used. The reaction was allowed to run for 2
hours at 250°C. Aliquots of the reaction were taken at 5 min., 15 min., 30 min.,
1hour, and a last aliquot at 2 hours right before cooling the solution.
43
A general red shifting of the emission for the particles from 490 nm to 520
nm was seen during the first hour of growth (Figure 28). This is consistent with
what is typically seen, as the particles would be getting larger as time goes on,
leading to a smaller band gap and thus redder emission. This would allow for a
slight tunability of the emission wavelength of AGS nanoparticles based on
growth time.
Figure 29. XRD patterns for AGS particles with 20% stearic acid, 80% hexanoic acid for growth
of 5 min, 1 hour, and 2 hours.
From the first hour to the second hour, there was a blue shift from 520 nm
to 500 nm, shown in Figure 28. The reason for this shift is not understood at this
time. The XRD patterns for the particles grown for 5 min. and 1 hour show
significant Scherrer broadening which is to be expected for nanoparticles, while
44
the XRD pattern for 2 hours of growth shows very sharp and distinct peaks that
match bulk AGS (Figure 29). This is also consistent with TEM images for these
particles shown in Figure 30. At 5 min. of growth, the particles are about 4 nm in
diameter or smaller and very spherical. At 2 hours, many of the particles are at
least 10 nm in size with most of the particles having non-spherical shapes and a
large amount of aggregation between particles. At these sizes and shapes the
particles begin to exhibit some bulk properties as seen in the XRD pattern.
Scale bar at 20 nm.
Figure 30. TEM images of AGS particles with 20%
stearic acid, 80% hexanoic acid grown for: (a) 5 min; (b)
2 hours.
45
Conclusions
This research was done to better understand several different I-III-VI
semiconductor nanocrystal systems in order to create a core/shell system
consisting of two different ternary materials which could be used to down-shift
light from LEDs in order to make a warm, white light. Having a core/shell system
of these materials should allow for unique tunability based on the stoichiometry of
the metal cations in both of the materials. These materials would also provide for
less toxic nanocrystals as compared to the commonly used binary systems.
CIS nanocrystals were found to emit light primarily in two general regions
depending on their growth, around 420 nm and around 640 nm, with some
tunability in those regions. Adding a ZnS shell to the CIS particles was found to
increase the photoluminescence quantum yield and consistently produce
particles that would emit around 570 nm no matter what the size of the CIS
seeds was. Adding a CGS shell to CIS particles may have formed, but showed
no significant improvement in photoluminescence, but when put on larger CIS
seeds, the resulting particles gave no photoluminescence.
AIS nanoparticles were synthesized with some photoluminescence in the
visible region and a good crystal structure, but the band gap was not desirable
for our research. The synthesis of CGSe nanoparticles was found to be time
consuming and give particles with no photoluminescence in the visible region.
Study of these systems was therefore not pursued.
Cation exchange with CIS nanoparticles and cadmium and lead ions was
found to only proceed in the forward direction. Other experiments with greater
46
concentrations, more time, higher temperature, and other conditions may provide
better results for the backwards direction.
We then looked at studying AGS because of its potential as a shelling
material for CIS nanoparticles due to its band gap alignment and lattice match
with CIS. This was accomplished by synthesizing AGS nanocrystals on their own
and optimizing the synthesis.
It was found that changing the relative amount of metal precursors in the
starting reaction mixture leads to a different ratio of the metal cations in the final
product. Having a ratio of these metals with roughly twice as much of either of
the metals leads to a decrease in the photoluminescence of the AGS
nanoparticles. More research on ratios closer to 1:1 could provide interesting
results.
We have found an efficient way to synthesize AGS nanocrystals and
found optimal conditions for this synthesis. The optimal ligand system uses a mix
of short and long chain ligands with a greater amount of the shorter chain ligand
(20% stearic acid/80% hexanoic acid). This mix of short and long chain ligands
does a good job of passivating the surface to control growth and improve
photoluminescence. In addition, it also works well as a surfactant to keep the
particles suspended. In doing so, this helps avoid aggregation of the particles
which can result in the formation of trap states.
Other modifications were made to the synthesis by optimizing various
parameters. It is very beneficial to degas the reaction mixture prior to heating up
and during the early part of the heating; doing so results in a more narrow
47
emission. Injecting the sulfur solution immediately once a high enough
temperature is reached is important to give only one peak. Growing the particles
at 250°C, as opposed to 150°C, gave a blue-shifted emission. A greater amount
of optimization of these parameters could lead to a better synthesis for AGS
nanoparticles.
AGS has shown to be a promising semiconductor material. If the
photoluminescence quantum yield could be increased significantly, possibly by
the addition of a ZnS shell, AGS nanocrystals could make for a viable option for
down-shifting light for blue/green LEDs. Continuing on the original goal of the
work, research should also be spent on using AGS as a shelling material for CIS
for red/orange LEDs, as it is a good match based on the band gap alignment and
lattice match of the two materials.
48
Acknowledgements
I would like to thank Steve Hughes for advising me through this project,
reading and editing several copies of this thesis, and teaching me about nano
scale materials. I would also like to thank Portland State CEMN for their imaging
of our nanocrystals, Doug Juers in the Whitman College Physics Department for
extensive help with the X-ray diffractometry, and Frank Dunnivant in the Whitman
College Chemistry Department for his assistance in the elemental analysis of our
particles. I would also like to acknowledge Allison Calhoun for also reading and
editing this thesis. A great deal of thanks must also go out to all of my friends and
family for supporting me and encouraging me during this research. I must also
extend a great deal of gratitude to you for taking the time to read this work that I
spent countless hours working on. This work was supported through funding from
Pacific Light Technologies and a Whitman College Perry Research grant.
49
References
(1) Kundu, J.; Ghosh, Y.; Dennis, A.M.; Htoon, H.; Hollinsworth, J.A. Nano Lett.
2012, 12, 3031-3037
(2) Ziegler, J.; Xu, S.; Kucur, E.; Meister, F.; Batentschuk, M.; Gindele, F.; Nann,
T. Adv. Mater. 2008, 20, 4068-4073
(3) The IES Nomenclature Committee and American National Standards Institute,
Nomenclature and Definitions for Illuminating Engineering, ANSI / IES RP-16-10,
New York: Illuminating Engineering Society of North America, 2010.
(4) Li, L.; Pandey, A.; Werder, D.J.; Khanal, B.P.; Pietryga, J.M.; Klimov, V.I. J.
Am. Chem. Soc. 2011, 133, 1176-1179
(5) Song, W.S.; Yang, H. Chem. Mater. 2012, 24, 1961-1967
(6) Gispert, J.R. Coordination Chemistry, Wiley, New York 2008
(7) Kitai, A. Luminescent Materials and Applications, John Wiley and Sons, West
Sussex UK 2008
(8) Green, M.A. Solar Cells; The University of New South Wales, Kensington NZ
1998
(9) Califano, M.; Franceschetti, A.; Zunger, A. Phys. Rev. B 2007, 75, 115401
(10) Fahlman, B. D. Materials Chemistry; Springer, Mount Pleasant, Michigan,
2007
(11) Haug, H.; Koch, S. W. Quantum Theory of the Optical and Electronic
Properties of Semiconductors, World Scientific Publishing Company 1994
(12) Junqing, H.; Bin, D.; Kaibin, T.; Qingyi, L.; Rongrong, J.; Yitai, Q. Solid State
Sci. 2001, 3, 275-278
(13) Norako, M.E.; Brutchey, R.L. Chem. Mater. 2010, 22, 1613-1615
(ccqx)
(14) Deka, S.; Quarta, A.; Lupo, M.G.; Falqui, A.; Boninelli, S.; Giannini, C.;
Morello, G.; De Giorgi, M.; Lanzani, G.; Spinella, C.; Cingolani, R.; Pellegrino, T.;
Manna, L. J. Am. Chem. Soc. 2009, 131, 2948-2958
(15) Scher, E.C.; Manna, L.; Alivisatos, A.P. Phil. Trans. R. Soc. Lond. 2003,
361, 241-257
(16) Jang, J.S.; Hong, S.J.; Kim, J.Y.; Lee, J.S. Chem. Phys. Lett. 2009, 475, 7881
(17) Kudo, A. I. J. Hyd. Ene. 2006, 31, 197-202
(18) Uematsu, T.; Doi, T.; Torimoto, T.; Kuwubata, S. J. Phys. Chem. Lett. 2010,
1, 3283-3287
(19) Smith, A.M.; Mohs, A.M.; Nie, S. Nature Nanotechnol. 2009, 4, 56-62
(20) Hamanaka, Y.; Ogawa, T.; Tsuzuki, M. J. Phys. Chem. 2011, 115, 17861792
(21) Booth, M.; Brown, A.P.; Evans, S.D.; Critchley, K. Chem. Mater. 2012, 24,
2064-2070
(22) Xie, R.; Rutherford, M.; Peng, X. J. Am. Chem. 2009, 131, 5691-5697
(23) Tang, J.; Hinds, S.; Kelley, S.O.; Sargent, E.H Chem. Mater. 2008, 20,
6906-6910
50
(24) Ford, G.; Guo, Q.; Agrawal, R.; Hillhouse, H.W. Chem. Mater. 2011, 23,
2626-2629
(25) Li, P.W.; Zhou, W.H.; Hou, Z.L.; Wu, S.X. Mater. Lett. 2012, 78, 131-134
(26) Wang, Y.A.; Pan, C.; Bao, N.; Gupta, A. Solid State Sci. 2009, 11, 19611964
(27) Chen, S.; Gong, X.G.; Wei, S.H. Phys. Rev. B 2007, 75, 205209
(28) Wei, S.H.; Zunger, A. J. Appl. Phys. 1995, 6, 3846-3856
(29) Luther, J.M.; Zheng, H.; Sadtler, B.; Alivisatos, A.P. J. Am. Chem. Soc. 2009,
131, 16851-16857
(30) Robinson, R.D.; Sadtler, B.; Demchenko, D.O.; Erdonmez, C.K.; Wang,
L.W.; Alivisatos, A.P. Science 2007, 317, 355-358
(31) Son, D.H.; Hughes, S.M.; Yin, Y.; Alivisatos, A.P Science 2004, 306, 10091012
(32) Yu, P.W.; Park, Y.S. J. Appl. Phys. 1974, 45, 823-827
51