Glen Junor Senior Thesis

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Stability of PbTe Quantum Dots

Glen Junor

Department of Chemistry

Senior Thesis

University of California Irvine 2015

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Abstract

Lead telluride quantum dots (PbTe QDs) exhibit promising electronic properties for use

in next-generation optoelectronic and photovoltaic devices. Unfortunately, the rapid oxidation of

PbTe in air impedes the development of this material in favor of the less labile lead

chalcogenides PbSe and PbS. In the reduction half reaction of O2 to H2O (½ O2 + 2H+ +2e

-

H2O) the reduction potential is 1.23 V; the corresponding reactions for S, Se, and Te have

potentials of 0.17, -0.37, and -0.72 V, respectively. The systematic decrease in reduction

potential explains the decreasing stability of PbX (X=S, Se, Te) in the presence of oxygen, where

Te2-

is unable to maintain a 2- oxidation state for any appreciable time, when compared to Se2-

and especially S2-

.

Various sizes of PbTe and PbSe QDs are measured over several days in air and nitrogen

by optical absorption spectroscopy and transmission electron microscopy (TEM). Upon air

exposure, PbTe exhibits blue-shifting and broadening of its first exciton absorption peak;

suggesting non-homogeneous oxidation of PbTe into PbO and other oxidized species over time.

The oxygen exposure results in complete loss of the PbTe characteristic UV/Vis absorption

feature after only 2 hours when in solution. On the other hand, PbSe exhibits appreciable blue-

shifting only after 48 hours of air exposure and does not show significant peak broadening even

after 100 hours in air, indicating a slower oxidation of all QDs in solution. Neither PbTe nor

PbSe showed visible changes by TEM, indicating that exposure to oxygen does not destroy the

QD structure. Films of PbTe QDs were studied at each stage of processing: “as-made” QDs with

oleate ligands; 1,2-ethanedithiol (EDT) ligand exchanged QDs; and an Al2O3 (alumina) over-

coated, EDT exchanged film. The alumina overcoated PbTe QDs showed no discernable change

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over 3.5 months in air, thus providing a means to protect these labile PbTe QDs and lead to more

development of this promising material.

Introduction

Quantum dot (QD) semiconductors have unique optical and electrical properties that

make them promising for use in many applications, such as high-performance field-effect

transistors (FETs)1,2

and solar cells.3,4

Quantum dots are particles with quantum confinement in

three dimensions, resulting in a quantized density of states.1-5

One effect of such quantization is a

size-tunable semiconductor band gap.1-5

As the QD continues to get smaller with respect to the

material’s exciton Bohr radius, the band gap widens. Such freedom allows a chemist to choose

synthetic conditions to grow a QD with the optimal band gap for a given device application.

Quantum confined particles, like QDs, have unique properties allowing them to exhibit

Multiple Exciton Generation (MEG) more efficiently than bulk semiconductors. 3,4,7,8,10

MEG,

also called inverse Auger recombination, occurs when an electron absorbs a photon that is an

integer multiple of the semiconductor band gap. For example, an electron absorbs a photon with

energy just above twice the band gap. This highly excited electron can transfer the additional

kinetic energy to another electron through a Coulombic interaction; resulting in two carriers at

the band edge rather than one carrier excited to twice the band edge.10

Thus, MEG enhances

photocurrents by using the high energy photons of the solar spectrum to produce multiple

electrons.10

In bulk semiconductors, crystal momentum must be accounted for, increasing the

energy required to produce multiple electrons from a single photon, and greatly limiting the

wavelengths that can produce multiple carriers.3,4,6,10

However, the strong confinement of charge

carriers in QDs allows Auger processes, MEG and Auger recombination, to begin taking place

when the incident energy is only slightly more than twice the band gap.

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MEG allows for a single-junction solar cell to surpass the theoretical maximum

efficiency of 33%; making the new single-junction limit 44% .3,4,6,10

The combination of higher

efficiency from MEG enhanced photocurrents and size tunable band gaps makes the realization

of near-ideal photocells a more attainable reality. Quantum dot solar cells have been termed

Third-Generation or Next-Generation photovoltaics due to such unique properties.

Lead (II) chalcogenide (PbX, X = S, Se, Te) quantum dots have shown particular promise

for use in photovoltaic applications. The large exciton Bohr radii for the lead chalcogenide QDs

allows for particularly strong confinement when compared to other QDs of similar sizes.9 It has

also been shown that PbX QDs have exhibited the first and most efficient observable MEG

processes.3,4,7,8,10

When engineering a new photovoltaic technology, large-scale device properties must also

be taken into account, rather than just the properties of the individual building blocks. Important

from the device perspective, the lead chalcogenide QDs have high dielectric constants, reducing

the effective masses for charge carriers in QD solids.3,4

High dielectrics and large exciton Bohr

radii make for strongly coupled, electrically conductive films needed for use in photovoltaics and

field-effect transitors (FETs).4

Additionally, PbX quantum dots can be synthesized through

colloidal chemistry; allowing for solution-based processing, cheapening film production. Finally,

the strong absorption coefficients of PbX QDs allow for the creation of thin-film solar cells.

These solar cells can be made where only a few hundred nanometers of material are required to

absorb more than 99% of incident light.

Lead (II) telluride (PbTe) quantum dots were recently shown to have the highest MEG

efficiencies of all lead chalcogenide QDs to date.8 PbTe also has the highest exciton Bohr radius

(152 nm) and dielectric constant compared to other lead chalcogenides.4 These properties

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suggest the possibility of superior coupling of PbTe QD films for use in FETs and solar devices.

If PbTe QDs exhibit superior coupling, conduction, and MEG efficiencies the successful creation

of a PbTe photovoltaic cell is essential to pushing the limits of QD solar devices.

In literature, PbTe QDs have not only been successfully synthesized but have also been

attributed with the highest MEG efficiencies, yet there is no literature at this time employing

PbTe QDs in an actual solar cell device. Here the stability characteristics of PbTe QDs are

investigated both as thin films and in solution to shed light on some possible issues to overcome

before employing PbTe in electronic devices and testing possible solutions. We explore the

general stability of three sizes of PbTe QDs with diameters of 3.1 nm, 5.5 nm, and 7.1 nm in

solution and the stability of 5.5 nm QDs during each stage of thin film creation in both air and

nitrogen. Finally, we applied a protective overcoat of Al2O3 (alumina) by atomic layer deposition

(ALD). Alumina was able to protect the film from oxidation for 3.5 months and still counting.

Materials and Methods

Materials: Trioctylphospine (TOP, 97%) was purchase from Strem Chemical. Lead (II)

oxide (PbO, 99.9999%) was purchased from Alfa Aesar. Oleic acid (OA, technical grade 90%),

Tellurium and Selenium shot, 1-Octadecene (ODE, technical grade 90%), and anhydrous ethanol

(200 proof), hexanes (mixture of isomers, 99%), and tetrachloroethylene (TCE, 99%) were

purchased from Sigma-Aldrich.

Synthesis of 3.1 nm Diameter Oleate-Capped PbTe Nanocrystals: The synthesis of small

PbTe nanocrystals was performed in a three-neck round bottom flask. The reaction involved

Pb:OA ratios of 1:2 and Pb:Te ratios of 1:3. First, PbO and OA was stirred in ODE under

vacuum at 120 ̊C to form a lead (II) oleate solution. After degasing for an additional hour, to

ensure a dry solution, the solution was placed under Argon and heated to 140 ̊C. At 140 ̊C

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trioctylphosphine telluride (TOPTe) was rapidly injected into the mixture. The quantum dots

nucleate instantly, turning the solution black, so a rapid injection was important to obtain a

narrow size distribution. After growing 3 minutes, the QDs were quenched in a liquid nitrogen

bath and 10 mL of hexanes was added to assist initial cooling from high temperatures. Hexanes

must be injected slowly to avoid bumping and loss of NCs. PbTe is air-sensitive so all

subsequent purification was performed in a glovebox with anhydrous reagents.

A typical synthesis mixed 3 mmol PbO and 6 mmol oleic acid in 20 mL of 1-Octadecene.

At 140 ̊C, 12 mL of 0.75 M TOPTe was injected. After 3 minutes, the reaction vessel was placed

in liquid nitrogen and frozen to -10 ̊C before the round-bottom was removed from the Schlenk

line and brought into the glovebox. Purification involved three sequences of QD precipitation by

ethanol, decanting the supernatant, and re-suspending the QDs in hexanes. All optical

characterization was carried out while the dots were dissolved in TCE. Typical yield is roughly

70 mg (5%). See below for description of the calculation of theoretical yield.

Synthesis of 5.5 nm Diameter Oleate-Capped PbTe Nanocrystals: The synthesis of

medium sized PbTe quantum dots was carried out using the same methods as the small PbTe

quantum dots but with a few specific changes. The Pb:OA ratio was 1:2.6 and the Pb:Te ratio

was 1:5. Additionally, the TOPTe injection temperature was 180 ̊ C and the growth time was 30

seconds. Also, all precursors were more concentrated, see below.

A typical synthesis mixed 6.7 mmol PbO and 17.6 mmol oleic acid in 12.7 mL of 1-

Octadecene. At 180 ̊C, 10 mL of 1.5 M TOPTe was injected. After 30 seconds, the reaction

vessel was placed in liquid nitrogen and frozen to -10 ̊C before the round-bottom was removed

from the Schlenk line and brought into the glovebox. Purification followed the methods used on

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the small quantum dots. Yield was roughly 175 mg (7%). See below for description of the

calculation of theoretical yield.

Synthesis of 7.1 nm Diameter Oleate-Capped PbTe Nanocrystals: The synthesis of large

sized PbTe quantum dots was carried out using the same methods as the medium PbTe quantum

dots besides a few specific changes. The Pb:OA ratio remained1:2.6 and the Pb:Te ratio

remained 1:5. However, the TOPTe injection temperature was 200 ̊ C and the growth time was

45 seconds. The precursors were in the same concentrations as 5.5 nm PbTe synthesis.


Octadecene. At 200 ̊C, 10 mL of 1.5 M TOPTe was injected. After 45 seconds, the reaction

vessel was placed in a pool of liquid nitrogen until the temperature reached 50 ̊C before the

round-bottom was removed from the Schlenk line and brought into the glovebox. Purification

followed the methods used on the medium quantum dots. Yield is roughly 97 mg (4%). See

below for description of the calculation of theoretical yield.

Synthesis of 5 nm Diameter Oleate-Capped PbSe Nanocrystals: The synthesis of medium

sized PbSe quantum dots was carried out in an attempt to create the same QD-ligand

environment as the PbTe counterparts, to make comparison more reasonable. The Pb:OA mole

ratio for PbSe reactions was 1:2.46 and the Pb:Se ratio changed to 1:3.74. The TOPSe injection

temperature was 165 ̊C and the growth time was 2 minutes. The concentration of TOPSe was 2

M.


Octadecene. At 165 ̊C, 8.4 mL of 2 M TOPSe was injected. After 2 minutes, the reaction vessel

was placed in a pool of liquid nitrogen until the temperature reached -10 ̊C before the round-

bottom was removed from the Schlenk line and brought into the glovebox. Purification followed

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the methods used on the PbTe quantum dots. Yield is roughly 25 mg (1%). See below for

description of the calculation of theoretical yield.

Synthesis of 7 nm Diameter Oleate-Capped PbSe Nanocrystals: The synthesis of large

PbSe was carried out in a similar fashion to the synthesis of 5 nm PbSe. The Pb:OA mole ratio

for PbSe reactions was 1:2.46 and the Pb:Te ratio changed to 1:3.74. The TOPSe injection

temperature was 165 ̊C and the growth time was 4.5 minutes. The concentration of TOPSe was

2 M.


Octadecene. At 165 ̊C, 8.4 mL of 2 M TOPSe was injected. After 4.5 minutes, the reaction vessel

was placed in a pool of liquid nitrogen until the temperature reached -10 ̊C before the round-

bottom was removed from the Schlenk line and brought into the glovebox. Purification followed

the methods used on the PbTE quantum dots. Yield is roughly 17 mg (1%). See below for

description of the calculation of theoretical yield.

Synthesis of 3 nm Diameter Oleate-Capped PbSe Nanocrystals: Attempts were made to

synthesize 3 nm PbSe QDs using the same reagents and methods as described for 7 nm and 5 nm

PbSe but yields were less than 3 mg. Sufficient UV/Vis samples could not be made from one

synthesis with yields so low and thus the material was not characterized.

Calculation of Theoretical Yield for QDs: A rough model was used to approximate the

theoretical yield of the various QD syntheses. The following model was used to estimate the

mass of PbTe or PbSe present in one QD.

𝑚𝑎𝑠𝑠 𝑃𝑏𝑇𝑒 = 𝑉𝑄𝐷 ∙ 𝐷𝑃𝑏𝑇𝑒

Where VQD is the volume of the quantum dot in units of nm3 calculated by assuming the

QDs were spherical and all QDs in a sample had the same radius (i.e. 5 nm). DPbTe was the

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density of PbSe or PbTe found in the Sigma-Aldrich catalogue, but converted into g/nm3.11

Ligands are also present on each quantum dot and the mass was accounted for according to the

following.

𝑚𝑎𝑠𝑠 𝑂𝐴 = (𝐴𝑄𝐷

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𝐴𝑃𝑏𝑇𝑒

) ∙ (𝑚𝑜𝑙𝑒𝑐. 𝑚𝑎𝑠𝑠 𝑂𝐴)

AQD is the area of the, presumably, spherical QD. APbTe is the area of a single unit of

PbTe or PbSe calculated with ionic radii reported elsewhere12

. APbTe was halved under the

assumption that each surface-occupying formula unit would contribute half its surface area to the

surface of the quantum dot. The ratio of the areas was multiplied by the mass of a single

molecule of oleic acid (in grams), assuming one OA molecule per formula unit on the surface.

The percent yield can be estimated using the previous two models.

%𝑦𝑖𝑒𝑙𝑑 =(𝑚𝑎𝑠𝑠 𝑦𝑖𝑒𝑙𝑑) −

𝑚𝑎𝑠𝑠 𝑂𝐴𝑚𝑎𝑠𝑠 𝑃𝑏𝑇𝑒 (𝑚𝑎𝑠𝑠 𝑦𝑖𝑒𝑙𝑑)

𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑃𝑏𝑇𝑒 ∙

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𝑚𝑜𝑙 𝑃𝑏𝑂 ∙ 100%

“mass yield” is the mass of QDs purified from the reaction. “mass OA” and “mass PbTe”

are calculated according to the procedures above. “mol PbO” was calculated from the reagents

used in the reaction.

Preparation of PbTe QD thin films: To study the stability of PbTe QD thin films, samples

were made at every stage of processing and studied using FTIR or UV/Vis spectroscopy. PbTe

QDs were applied to cleaned glass and double-sided polished silicon substrates by spin coating.

A typical film was made by placing 2 drops of an octane solution of PbTe QDs with

concentration 100 mg/mL. The substrate was spun at 600 rpm for 2 minutes then 2000 rpm for 5

seconds. Oleate-capped films are 160 nm thick on average. If ligand exchange was performed,

the substrate was placed in a solution of 1,2-ethanedithiol (0.1 M) in acetonitrile for 15 minutes

to ensure complete ligand exchange. After ligand exchange, film thicknesses shrunk to 125 nm.

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Since films are made by spin coating one layer, rather than layer-by-layer dip coating, ligand

exchange causes extensive cracking of the film due to film contraction. If the film was infilled

and over-coated with protective Al2O3, atomic layer deposition was performed until the overcoat

reached 20 nm.

Absorption Spectroscopy: Absorption spectroscopy measurements were performed with a

PerkinElmer Lambda 950 UV/Vis Spectrometer. Scans were performed between 2000 nm and

800 nm at rates of approximately 289 nm/min. Air-exposed measurements were performed at six

minute intervals for the first 1.5 hours, then at increasingly long intervals, up to once per day, for

the remainder of the week. Air-free samples were measured from every few hours to once a day

over the course of the week.

Transmission Electron Microscopy: Transmission electron microscopy (TEM) images of

PbTe QDs were taken with a Philips CM 20 at a 200 kV accelerating voltage. The samples were

prepared by drop casting a dilute solution of QDs dissolved in TCE onto the amorphous carbon-

coated side of copper grids purchased from Ted Pella, Inc. The films were allowed to dry and

organize in the glovebox. QD diameters were measured with the “measuring tool” of GIMP 2.8

image manipulation software.

Analysis of UV/Vis Absorption spectra: All quantitative measurements of UV/Vis

absorption features (i.e. position and full-width at half maximum), were done with Igor Pro 6.36

Multi-peak fitting software.

Results and Discussion

Stability of 5.5 nm Diameter Oleate-capped PbTe QDs in Solution: After synthesis and

thorough drying, the QDs were suspended in enough TCE to create a 0.6 mg/mL stock solution.

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The stock solution was split into two samples, one meant for air exposure and one meant to be

stored in N2 conditions.

Both samples were placed in pyrex glass cuvettes having a 1 cm path length with a

Teflon-lined screw-cap lid. The cuvette was capped and never opened for the duration of the

study in the case of the QDs stored in nitrogen. The nitrogen sample remained inside the

glovebox for the entire study, except for 12 minute intervals required to measure the sample in

the UV/Vis spectrometer. The air exposed sample was stored in air with the cap off. A solvent

line was marked at the top of the cuvette so the TCE could be replenished before each

measurement to compensate for solvent evaporation.

To allow for TEM study of aged air-free QDs, a sample was stored in a glass vial with a

plastic screw cap lid alongside the cuvette. This sample was only opened after three days of

storage to create a TEM sample. This sample was never removed from the glovebox.

Figure 1 shows the results of air exposure over several days. During the first 90 minutes

most of the 1st exciton peak definition is lost with very little blue-shifting. From 90 minutes to

150 minutes, there is rapid blue-shifting that accompanies the continued loss of 1st exciton

definition. By 6 hours the peak was indistinguishable and the decreasing absorbance began to

taper off; changing only slightly over several days.

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Figure 1: Absorbance spectra of Air-exposed 5nm diameter PbTe QDs over time

Figure 2: As made QDs (top left), After 3 days of air-exposure (top right), 3 days in nitrogen

(bottom)

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Figure 1 suggests that the PbTe QDs are oxidizing in a non-uniform fashion. The

hypothesized mechanism can be visualized by imagining that the reaction rate is faster than the

rate that O2 can dissolve in the solvent. As a result, the PbTe QDs at the top of the cuvette

oxidize immediately partially turning into PbO or other oxidized species, preventing O2 from

reaching the rest of the sample. As a result, the uniform QD size distribution is lost because a

large amount of PbTe QDs have been oxidized while significant amounts remain purely PbTe, or

nearly so. Since the oxidation causes blue-shifting, as the effective size of PbTe begins to

decrease throughout the QDs, the UV/Vis features begin to broaden as some QDs retain their

original features while others shift towards higher energies. To test this hypothesis, the process

was repeated with the less labile PbSe. PbSe oxidation was hypothesized to be a slower process,

allowing the O2 to nearly saturate the solution before changes were observed. If this was true, not

only would we see slower oxidation rates, but the peaks would not broaden significantly because

QDs at all depths would have ample O2 to react with. Figure 2 contains TEM images of O2

exposed PbTe QDs. The images show that air exposed and non-exposed PbTe have no visible

differences. This figure confirms expectations that the UV/Vis blue-shifting is caused by the

decrease in effective size of PbTe by formation of PbO and other species rather than actual

destruction of the QD structure itself.

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Figure 3: PbSe QDs exposed to air (Left) and stored in TCE. (Right) Stored in N2

Figure 3 shows the expected result for oxidation of PbSe. First, no noticeable blue-

shifting occurred until 48 hours of air exposure. Second, measurements of the full-width at half-

maximum values for the peaks show that there is no significant feature broadening in the PbSe

samples. Similar to PbTe, PbSe QDs stored in nitrogen exhibit no significant changes. Another

important test was to repeat this measurement with PbTe QDs while stirring. Unfortunately, our

setup is not yet equipped to do this measurement. Future studies interested in testing the

hypothesis that PbTe reaction with O2 is faster than the diffusion rate of O2 into TCE may

consider stirring the sample during the UV/Vis measurement with a small stir bar and plate fixed

inside the spectrometer.

The decrease in absorption over time is difficult to explain due to confounding of

variables. To measure the sample over several days while exposed to air, the cuvette was left

open to the atmosphere allowing solvent evaporation. A line was carefully drawn on the cuvette

beforehand so TCE could be replenished before each measurement. UV/Vis is very sensitive to

concentration, so perhaps the line was drawn slightly inaccurate. More exploration is necessary

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to determine if the decrease in absorbance was due to measurement error or if another

mechanism is occurring.

Figure 4: Absorbance spectrum of air-free samples over time.

Figure 3 shows the effect of storing QDs in TCE under air-free conditions. The change is

minimal but there is small blue-shifting, increasing absorbance at higher energies, and decreasing

absorbance at lower energies. The behavior of the air-free QDs also indicates a small amount of

QD shrinking over time. The air-free shrinking appears to be more uniform than the air-exposed

samples since the peak does not appear to broaden, thus the size distribution does not seem to

increase. The oxidation could be due to a slow etching of the QDs in TCE rather than a rapid,

non-uniform oxidation in air, or some other more-controlled mechanism. The air-free samples

also show no significant differences from the “as made” samples by TEM. To test the hypothesis

that the QDs undergo a small amount of etching over time in TCE, the experiment was

performed in a different solvent.

Stability of 5 nm Diameter QDs Stored in Octane: To confirm the etching of QDs when

stored in TCE, QDs were also stored in octane both in air and in nitrogen. Since octane absorbs

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in the same near-IR range as the QDs the octane was subtracted out as a background. In Figure 4,

it is apparent that the QDs are more susceptible to oxidation while stored in octane. Overall, the

same behavior is seen in both octane and TCE where the QDs are rapidly oxidized and the 1st

exciton rapidly disappears. Perhaps the more rapid oxidation in octane compared to TCE also

supports the earlier hypothesis that the O2 solubility is the limiting factor in PbTe oxidation. To

test this, O2 solubility data should be measured for both solvents and could be another avenue for

future testing of this hypothesis.

Figure 5: (Left) 5.5 nm QDs in air stored in TCE. (Right) 5.5 nm QDs in air stored in octane.

Note: these are two different samples of 5.5 nm QDs with slightly different first exciton energy.

After confirming that the PbTe QDs have the same oxidation behavior when exposed to

air, whether stored in TCE or octane, the QDs were compared in nitrogen. As shown by Figure 5,

PbTe QDs stored in TCE slowly change over time, with small amounts of blue shifting,

indicating a decrease in size, or etching. PbTe QDs in octane do not share this behavior. Even

after several days in octane there is no change in absorbance, indicating that PbTe QDs are stable

when stored in air-free conditions.

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Figure 6: (Left) QDs in nitrogen stored in TCE. (Right) QDs in air stored in octane. Note: these

are two different samples of 5.5 nm PbTe QDs.

Stability of 7 nm Diameter Oleate-capped PbTe QDs in Solution: To have a more

complete understanding of the oxidation behavior of PbTe QDs oxidation was studied as a

function of QD size. Two additional sizes were synthesized, 7.1 nm and 3.1 nm QDs. The

samples were prepared and stored in the same manner as the 5.5 nm QDs. Specific synthetic

conditions are discussed in Materials and Methods.

Figure 7: (Left) UV/Vis spectrum of 7.1 nm diameter PbTe QDs stored in TCE and air exposed

(Right) TEM image of 7.1 nm PbTe QDs

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Figure 7 shows that 7.1 nm QDs oxidize in a similar fashion as 5.5 nm QDs. The 1st

exciton features in the UV/Vis rapidly blue shift and broaden. To compare the oxidation

behavior of PbTe to that of PbSe, 7 nm PbSe QDs were studied as well.

Figure 8: 7 nm PbSe QDs stored in air (left) and nitrogen (right)

Similar to 5 nm PbSe, 7 nm PbSe QDs oxidize more slowly in air than their PbTe

counterparts. Interestingly, 7 nm PbSe QDs oxidize faster than 5 nm PbSe and have a much more

pronounced decrease in absorbance over time. The decrease in absorbance is not accompanied by

a broadening, since the full width at half maximum does not change significantly, and appears to

level off after two days of air-exposure. The sudden increase in absorbance may be due to

measurement error when refilling solvent after 20 days of evaporation. It is also important to

note that no significant changes in the absorbance spectrum occurred when 7 nm PbSe QDs were

stored in nitrogen.

Stability of 3 nm Diameter Oleate-capped PbTe QDs in Solution: The absorbance

spectrum and TEM images for the 3 nm diameter QDs were prepared in the same manner as the

5.5 nm diameter ones.

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Figure 9: Absorbance of air-exposed 3 nm diameter PbTe QDs over time.

Figure 10: As made 3 nm diameter QDs (Left) 3 days in nitrogen (Right)

Quantum dots with 3 nm diameters show similar behaviors to the 5.5 nm QDs. First,

there is a loss in 1st exciton definition and decrease in absorbance. It is difficult to see any

definitive blue-shifting since the 1st exciton is close to the absorption take-off at the edge of the

visible spectrum.

Unlike the 5.5 nm QDs, the 3 nm diameter QDs showed a partial precipitation after

several hours of air-exposure as indicated by a black powder at the bottom of the cuvette after 9

hours of exposure. The absorbance drop is noticeably different than the normal drop caused by

oxidation because the y-intercept changes more than it did at previous intervals. Due to the

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sudden precipitation, TEM samples made from the air-exposed samples after 3 days of exposure

would not show the precipitate and would not be indicative of the true effect of air-exposed

aging. Thus, a TEM sample was not created for 3 days of air-exposure.

Air-free samples exhibited slow blue-shifting and an increase in absorbance similar to the

5.5 nm QDs, in Figure 8. The peak width does not appear to broaden, similar to the etching of

5.5 nm QDs in TCE. A change in this manner would likely not be visible by TEM on such a

short time interval, again confirmed by TEM in Figure 7.

Figure 11: Absorbance of 3 nm QDs stored under nitrogen over time.

For comparison of PbTe QDs to PbSe QDs, 3 nm PbSe syntheses were attempted but

offered no significant yields under similar reaction conditions. Many QD studies use small doses

of highly reactive phosphine precursors (i.e. diphenylphosphine) to increase the yields of PbSe.1,2

Since the use of diphenylphospine causes uncontrolled growth of PbTe, diphenylphospine was

avoided for all synthesis procedures to allow PbTe and PbSe to be compared in similar ligand

environments. Unfortunately, the lack of diphenylphosphine during the small PbSe QD synthesis

prevented yields from being significant enough to facilitate measurement.

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Figure 12 contains a quantitative summary of the results of the solution phase stability

studies. PbTe (Blue) is compared with PbSe (Red) as a function of QD diameter. Additionally,

Figure 12 contains plots of the 1st exciton absorption maximum position as a function of air

exposure. The full-width at half maximum (FWHM) for the 1st exciton feature was measured as a

function of air exposure. Here, FWHM is a measure of the size distribution for each type of QD.

It is important to note that the last values of FWHM calculated for PbTe samples have somewhat

erratic behavior because the program used for peak analysis (Igor Pro 6.36 Multi-Peak fitting)

had great difficulty identifying the peak after significant air exposure. Due to the erratic behavior

quantitative measurements of PbTe samples could no longer be performed after relatively short

air exposure times.

Figure 12: (Left) Plot of 1st exciton peak position vs time in air (Right) Plot of full-width at half

maximum values for 1st exciton features vs time in air. Both plots contain comparisons of PbTe

(Blue) to PbSe (Red) and contain all available sizes for each QD. PbTe plots have fewer points in

regions of long air exposure because the 1st exciton features become very broad and difficult to

identify, preventing accurate measurement.

Stability of PbTe QD thin films: The goal of research of PbTe QDs is to explore their

viability as solar cells, field-effect transistors (FETs), and other solid-state electronics. Thus, it is

paramount to characterize the stability of this material in the solid state in addition to the solution

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phase. Stability in the solution phase allows us to understand the decay mechanisms that will be

faced by solutions of PbTe QDs that are likely to be used for roll-to-roll printing and other

manufacturing techniques so that proper precautions may be taken to protect the material during

pre-manufacturing stages. Similarly, it is important to understand the decay that thin films may

experience during the manufacturing process. Understanding the sensitivity of each stage can

provide guidance towards what stages must be protected and how much. Our current method of

field-effect transistor production involves three stages: spin coating of oleate-capped QDs, ligand

exchange with 1,2-ethanedithiol (EDT), and atomic layer deposition of Al2O3 to infill and

overcoat the thin film.

To study each stage of manufacturing with fresh, non-air-exposed samples seven films

were made from the same batch of PbTe QDs (all QDs were made during the same synthesis).

Two films were made on double-sided polished silicon and five were made on glass. All seven

substrates were spin-coated with the same conditions. A few samples were removed at each stage

of processing for air-exposure and characterization, resulting in one sample continuing through

all stages. It should be noted that the entire yield of the reaction was used to make all seven films

from one synthesis and no solution phase UV/Vis was taken. As made QD size was determined

using the 1st exciton absorption peak of an oleate-capped film stored in nitrogen.

Stability of 5 nm Diameter Oleate-capped PbTe QD thin films: Optical characterization

and film stability was measured using UV/Vis absorption spectroscopy by simple absorption

measurements. Integrating sphere was not used to determine reflectance changes. Oleate-capped

PbTe films prepared in this manner are 160 nm thick, see Figure 15.

23

Figure 13: (Left) Absorption spectrum of Oleate-capped PbTe thin film stored in nitrogen

(Right) Image of film. The coloring is a result of the angle when the picture was taken. Straight-

on views show the film is black.

Figure 13 shows the stability of oleate-capped PbTe thin films stored in nitrogen. The

film was made in the glovebox and sealed in a conflat flange and never removed for the

remainder of the study to prevent air exposure. As expected, the films do not appear to change

significantly during the course of the study. Taking a careful look at the traces, it is clear that

there is no pattern to the changing spectra over time. Since the film had to be carefully

repositioned for every measurement, the random changes in the spectrum can be attributed to

small deviations in film position. For convenience, the full-width at half-maximum (FWHM) and

peak position are noted in eV on the figure for quick comparison to other films.

Time 0: Position: 0.826 eV

FWHM: 62.3 meV

24

Figure 14: (Left) Absorption spectrum of Oleate-capped PbTe thin-film exposed to air.

During the measurement of air exposed films, the films were not moved for the first hour.

Thus, the relative peak positions are accurate for at least those time frames. The oleate-capped

thin films appear to degrade in the presence of air in the same manner as the QDs in solution.

The films decay significantly faster, showing appreciable changes to the absorption features after

only 6 minutes of air exposure. Six minutes is the fastest spectra can be taken using this

spectrometer in this wavelength range. Faster oxidation is expected because the solubility and

diffusion rate of O2 in various solvents is no longer a factor and films are exposed directly to

atmospheric levels of oxygen. It is important to notice the shape of the UV/Vis 1st exciton feature

in these films. The spectra resemble the solution phase spectra because the QDs are capped with

insulating ligands, preventing electronic communication of QDs and giving rise to spectra that

are similar to a collection of isolated QDs in solution. Notice that though the peak position did

not change significantly, the air exposure during the 10 second sample transfer, from the

glovebox to the spectrometer, caused peak broadening of 20.21 meV. It is clear that PbTe QD

thin films are significantly more susceptible to oxidation than solutions.


FWHM: 84.4 meV

25

Figure 15: Scanning electron microscope images of PbTe QD thin films. (Top Left) Top-down

image zoomed out. (Top Right) Top-down image zoomed in. (Bottom) Cross-section with

average thickness of 160 nm

Stability of 5 nm Diameter 1,2-ethanedithiol-capped PbTe QD thin films: The next stage

of processing, ligand exchange, was explored after soaking fresh oleate-capped thin films in a

0.1 M solution of 1,2-ethanedithiol (EDT).

26

Figure 16: (Left) FTIR spectrum of thin films made from oleate-capped (OA) and 1,2-

ethanedithiol capped (EDT) PbTe QDs to determine successful ligand exchange. Spectra were

taken on different films. (Right) image of ligand-exchanged film. The coloring is a result of the

angle when the picture was taken. Straight-on views show the film is black

Figure 16 shows an FTIR spectrum of oleate-capped (OA) and 1,2-ethanedithiol (EDT)

capped thin films. After soaking an OA capped thin-film in 0.1 M EDT solution a nearly

complete removal of the C-H stretching mode at ~3000 cm-1

was noted implying complete ligand

exchange from oleate to 1,2-ethanedithiol. Figure 17 shows the extensive cracking resulting from

film contraction. The cross section shows how the film thickness drops to 125 nm as a result of

ligand exchange.

27

Figure 17: (Top Left) Zoomed out Top-down image. (Top Right) zoomed in Top-down image.

(Bottom Left) closer look at cracking Top-down image. (Bottom Right) Cross section with

average film thickness of 125 nm.

The UV/Vis spectra of films of EDT-capped PbTe QDs exhibit some interesting changes

when compared to solution phase and thin films of QDs with insulating ligands. Inspection of the

UV/Vis spectra of EDT-capped films stored in nitrogen show that ligand exchange causes

significant red shifting (70 meV) and broadening (8.7 meV) of the 1st exciton compared to OA-

capped films. Since EDT is a very short ligand, it allows the PbTe QDs to couple to each other

electronically. This coupling may cause the observed red shifting because the electron can now

sample a larger amount of semiconductor material. Broadening can be explained by the

distribution of nearest neighbors in the disordered arrays of QDs that form during the ligand

exchange. Since not all QDs have coupling to the same number of neighbors, due to the long-

28

range film disorder, the 1st exciton feature broadens. Currently, further experiments are being

pursued to be certain that red shifting is caused by coupling and not something else, like fusing

of neighboring QDs.

Another interesting feature comes from the comparison of air exposed and non-exposed

EDT-capped films in Figure 18. The 10 seconds of air exposure during sample transfer from the

glovebox to the spectrometer caused a blue shifting of 65 meV and broadening of 28.1 meV.

This change is significantly larger than the difference between oleate-capped films that were and

were not exposed to air. It is evident that PbTe QD thin films that are capped with 1,2-

ethanedithiol are much more sensitive to oxidation that their oleate-capped counterparts. Once

again, it is important to note that variation in absorbance for EDT-capped films stored in nitrogen

is likely due to small deviations in the position of the sample between measurements. Air

exposed samples were not moved for the first hour, and thus the relative positions are accurate.

Normalizing the spectra at 1600 nm is evidence since all spectra lay on top one another,

indicating no significant changes over time, see Figure 18 below.


FWHM: 93.1 meV


FWHM: 122 meV

29

Figure 18: (Left) UV/Vis spectra of air exposed EDT-capped PbTe. (Right) EDT-capped PbTe

stored in nitrogen. (Bottom) Normalized EDT-capped stored in nitrogen

Stability of 5 nm Diameter Al2O3-infilled and overcoated PbTe QD thin films: One film

was carrier through the entire manufacturing process: deposition of oleate-capped film, ligand

exchange with 1,2-ethanedithiol to remove the insolating oleate ligands, and atomic layer

deposition of Al2O3 to infill and overcoat with a protective oxide layer. After using atomic layer

deposition to infill the PbTe QD thin film and provide a 20 nm overcoat, the film was

characterized in the same UV/Vis spectrometer over the course of several months, and is

continuing to be measured at the time of this writing.

Figure 19 exhibits several interesting features. Most importantly, the thin film of PbTe

QDs has remained optically stable over the course of 3.5 months, and counting. The application

of a 20 nm overcoat of Al2O3 has managed to stabilize the film against oxidation. It has now

been shown that PbTe films, similar to PbSe devices1, can be made air stable by atomic layer

deposition, a technique already used in industry.

It is also notable that the peak red shifted by 21 meV but did not broaden significantly

(1.9 meV) by simple infilling and overcoating with Al2O3. Perhaps filling the space between

30

quantum dots with an inorganic matrix in addition to an organic matrix increased the coupling

between PbTe quantum dots, or some other effect could have occurred as the result of the atomic

layer deposition technique. More tests would be necessary to determine the cause definitively.

Figure 19: (Left) UV/Vis spectra of EDT-capped ALD-overcoated PbTe QD film stored in air

for 3.5 months. (Right) Image of film. The gold color is a result of the angle when the picture

was taken. Straight-on views show the film is black.

Conclusions

This study sought to expand the knowledge of the PbTe quantum dot stability by

characterizing three sizes of PbTe quantum dots with 3.1 nm, 5.5 nm, and 7.1 nm diameters. All

PbTe quantum dots exhibit blue-shifting, loss in absorbance, and broadening of the 1st exciton

peak upon air exposure. This behavior indicates rapid oxidation kinetics comparable to the rate

of O2 diffusion into the solution or, in the solid state, diffusion through the thin film. These

oxidation reactions occurred more rapidly than the oxidation of similar PbSe samples in solution.

All QDs show little change when stored in nitrogen. Most changes can be attributed to slow

etching of QDs by particular solvents or small measurement errors during the analysis of films.

Thus, PbTe QD stability is significantly improved in the absence of air.


FWHM: 95.0 meV

31

As PbTe films proceed through various processing conditions, the air stability and overall

optical characteristics of these films change. After the film is freshly made the optical spectra

look similar to QDs in solution, due to the large insolating oleate ligands preventing coupling of

neighboring QDs. Oleate-capped QDs exhibit appreciable oxidation through changes in the

optical spectra after 6 minutes in air.

Exchanging oleate ligands for 1,2-ethanedithiol (EDT) is the next step in device creation.

These films are soaked in 0.1 M EDT solution for 15 minutes, ligand exchange is confirmed by

FTIR. Ligand exchange creates large cracks in the film, visible by electron microscopy. These

films exhibit 70 meV red shifting and 8.7 meV increase in full-width at half-maximum upon

ligand exchange, suggesting increased coupling between QDs in the film. Air stability is greatly

reduced after ligand exchange and significant changes in optical spectra are seen immediately

upon air exposure (at least 10 seconds).

Infilling and overcoating a 1,2-ethanedithiol capped film with 20 nm of Al2O3 further

changes the properties of the film. The 1st exciton peak red shifts by 21 meV and broadens by 1.9

meV, again suggesting increased coupling of the QDs in the film. Additionally, the film shows

stability against oxidation, measured over 3.5 months and counting. Atomic layer deposition of

Al2O3 is now a viable option to stabilize PbTe QD thin films for long-term use.

Future Studies

Since the goal is to use PbTe QDs for high performance field-effect transistors and solar

cells, future studies will explore the electrical properties of these films through the various

processing stages to determine carrier type, carrier density, carrier mobility, and other important

properties for electronic devices. These electrical studies will determine if PbTe maintains its

32

unique and promising properties throughout the stages of film production; and if so, lead us

towards the next step in the use of this promising material.

Acknowledgements

I would like to express my gratitude towards all the members of my group for the

contributions they have made towards the progress of my project. First and foremost, I would

like to thank my principal investigator Professor Matt Law for allowing me to join the group and

providing me with an interesting and fulfilling project along with the freedom to explore the

aspects I found most interesting. I would also like to thank Juliette Micone, who is currently

focusing on the electrical characterization of these PbTe QD thin films. We have worked

extensively together on the synthesis of high quality PbTe QDs with usable yields. She

discovered the key to synthesis of 3.1 nm PbTe QDs was the rapid cooling of the reaction vessel

with liquid nitrogen. I would also like to thank Jason Tolentino, Sam Keene, and Juliette Micone

for taking the SEM and TEM images of my QDs. I would like to thank all present and former

members of the Law group for our valuable discussions. Finally, I would like to thank the

University of California Leadership Excellence through Advanced Degrees scholarship program

for funding me and allowing me to focus and pursue this fascinating research.

References

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Glen Junor Senior Thesis