Synthesis of Lead Sulfide Quantum Dots and their Ligands for

Photon Upconversion Applications

By

Philip Robin Sohn

A thesis submitted in conformity with the requirements

for the degree of Master of Science

Department of Chemistry

University of Toronto

© Copyright by Philip Robin Sohn, 2018

ii

Abstract

Synthesis of Lead Sulfide Quantum Dots and their Ligands for

Photon Upconversion Applications

Philip Robin Sohn

Master of Science

Department of Chemistry

University of Toronto

2018

Lead sulfide quantum dots (PbS QDs) are prime candidates for various optical applications

due to their size-dependent optical properties, tunability, narrow absorption peaks, and their spin-

mixing properties. Small PbS QDs have proven to be a synthetic challenge but are important

candidates for use in optical upconversion which is the process by which low energy photons are

combined to form a higher energy photon. We describe our attempts at optimizing the relatively

new synthetic method of using thioureas as precursors to make small PbS QDs, as well as

determining the mechanism of monomer formation. We also describe our synthesis and

characterization of 5-(p-carboxyphenyl)tetracene, a ligand that can act as a triplet extractor from

PbS QDs for use in optical upconversion.

iii

Acknowledgements

During my academic journey, I have met many people who have assisted, inspired, and

encouraged me in my research. First and foremost, I’d like to thank my mother for always being

there for me and fostering my education. Whether it was letting me be a nerd and buying me

university textbooks for my birthday (upon request) or not exhibiting any disappointment when I

initially left university to pursue unemployable passions, I could always count on her.

On the side of academia, I would like to thank Dr. Stanislaw Skoniecny and Dr. Andy

Dicks for giving me the wonderful opportunity to be involved in the chemistry Olympiad. It has

inspired me to pursue both chemistry and teaching. Their impact on my life is immeasurable and

I hope to impact others in the same way. I would like to thank Dr. Jik Chin and Dr. Sophie

Rousseaux for believing in my abilities and acting as guides during my return to university. I am

not sure if I would have pursued a graduate degree without them. In addition, I would also like to

acknowledge their academic contributions, as they helped guide me through a lot of my research

that focused on organic synthesis.

Thank you Aleks for lending me equipment and solvents, as well as your wisdom. You

have made my synthesis much shorter than it would have otherwise been. Dropping all formalities

in speech, thanks to the denizens of Sniffler’s Cave for keeping me sane throughout the years and

just overall being great people. You guys are the literal bomb (but not actually literal) and I’m glad

that you guys don’t yet have the life to abandon me. At least until I’m married and have kids, I

hope you guys remain hopelessly addicted to video games.

TJ, thank you so much for being my rock. You have been with me every step of the way

and I can’t imagine how any of this would have happened without you.

Finally, I’d like to thank Mark for being a great supervisor. You have fostered a wonderful

environment to conduct research, and to learn. You have inspired me to become a better scientist

and a better person and I hope that I can carry over everything that you have taught me to wherever

I move onto in life. Thank you for all your words of wisdom and your patience.

iv

Table of Contents

Acknowledgements ........................................................................................................................ iii

Table of Contents ........................................................................................................................... iv

List of Tables ................................................................................................................................. vi

List of Figures ............................................................................................................................... vii

Chapter 1: Optical Properties of Quantum Dots and Upconversion ............................................... 1

1.1 Lead Sulfide Quantum Dots .................................................................................................. 1

1.2 Synthesis of Lead Sulfide Quantum Dots ............................................................................. 3

1.3 Lead Sulfide Growth Mechanism ......................................................................................... 4

1.4 Upconversion ........................................................................................................................ 5

Chapter 2: Lead Sulfide Quantum Dot Synthesis Using Thiourea Derivatives.............................. 7

2.1 Small Lead Sulfide Quantum Dots........................................................................................ 7

2.2 Survey of Suitable Thioureas ................................................................................................ 8

2.3 Synthesis of Small Lead Sulfide Quantum Dots ................................................................. 11

2.4 Thiocarbamates and Cyclic Thioureas ................................................................................ 12

2.5 Mechanism of Lead Sulfide Quantum Dot Synthesis ......................................................... 14

2.6 Future Work ........................................................................................................................ 16

2.7 Appendix: 1H NMR of Chemical Compounds .................................................................... 18

Chapter 3: CPT as a Transmitter Ligand for Lead Sulfide Quantum Dots ................................... 23

3.1 Transmitter Ligands ............................................................................................................ 23

3.2 Synthesis of CPT ................................................................................................................. 25

3.3 CPT Ligand Exchange ........................................................................................................ 28

3.4 CPT Assisted Upconversion................................................................................................ 29

v

3.5 ACA Assisted Upconversion .............................................................................................. 29

3.6 Future Work ........................................................................................................................ 31

3.7 Appendix: 1H NMR of Chemical Compounds .................................................................... 33

Bibliography ................................................................................................................................. 37

vi

List of Tables

Table 1. List of selected substituted diphenylthioureas synthesized ............................................ 11

vii

List of Figures

Figure 1. Model of lead sulfide quantum dot with ligand coverage .............................................. 1

Figure 2. Absorption spectra of various lead sulfide quantum dots synthesized via injection of

thiourea precursors .......................................................................................................................... 2

Figure 3. LaMer Dinegar model of burst nucleation of quantum dots. I is the monomer creation

phase, II is the nucleation phase, and III is the quantum dot growth phase.................................... 4

Figure 4. Examples of thioureas showing the increased kinetics of reaction in the synthesis of lead

sulfide quantum dots based on steric hindrance and electron deficiency ....................................... 7

Figure 5. Proposed nucleophilic substitution mechanism for synthesis of lead sulfide quantum

dots via thiourea precursors ............................................................................................................ 8

Figure 6. Substituted diphenylthiourea derivatives used in synthesis of lead sulfide quantum dots

......................................................................................................................................................... 8

Figure 7. Click reaction of an isothiocyanate and amine to synthesize thioureas ......................... 9

Figure 8. Absorption spectra of PbS QDs synthesized by injection of thiourea A ...................... 10

Figure 9. Cyclic thiourea I ........................................................................................................... 13

Figure 10. General structure of thiocarbamates ........................................................................... 13

Figure 11. Thiocarbamates synthesized for lead sulfide quantum dot synthesis ......................... 14

Figure 12. Organic mechanism of lead sulfide quantum dot synthesis using thioureas as precursors

....................................................................................................................................................... 15

Figure 13. Structures of heterocyclic thioureas M and N ............................................................ 17

Figure 14. 1H NMR of thiourea A................................................................................................ 18

Figure 15. 1H NMR of thiourea C................................................................................................ 19

Figure 16. 1H NMR of thiourea B ................................................................................................ 20

Figure 17. 1H NMR of thiocarbamate K ...................................................................................... 21

Figure 18. 1H NMR of thiourea N................................................................................................ 22

Figure 19. TTA upconversion diagram using a lead based quantum dot as a sensitizer with CPT

as a transmitter ligand and rubrene as the annihilator ................................................................... 23

Figure 20. Structure of 5-(p-carboxyphenyl)tetracene, CPT ....................................................... 25

Figure 21. Bromination of tetracene using N-bromosuccinimide as the bromine source............ 25

viii

Figure 22. Suzuki coupling of 5-bromotetracene and boronic acid ester .................................... 26

Figure 23. Hydrolysis of tetracene ester to CPT .......................................................................... 26

Figure 24. Absorption/emission spectra of CPT and absorption spectra of 765 nm PbS QDs .... 27

Figure 25. Absorption spectra of PbS QDs with CPT ligands after 20, 60 minutes of exchange 28

Figure 26. Structures of 9-anthracenylcarboxylic acid, ACA, and diphenylanthracene, DPA .... 30

Figure 27. Absorption spectra of cadmium selenide quantum dots after ligand exchange for 13

hours with carboxyanthracene ...................................................................................................... 31

Figure 28. synthetic scheme of rubrene derivative where R1 would be a carboxy group or

carboxyphenyl group in order to act as a ligand onto lead sulfide quantum dots ......................... 33

Figure 29. 1H NMR of 5-(4-ethyl(4-benzoate))tetracene............................................................. 34

Figure 30. 1H NMR of crude CPT ............................................................................................... 35

Figure 31. 1H NMR of CPT after column .................................................................................... 36

Chapter 1

Optical Properties of Quantum Dots and Upconversion

1.1 Lead Sulfide Quantum Dots

Colloidal quantum dots are semiconducting nanocrystals covered in ligands as shown in

Figure 1.1 They exhibit size-dependent optical properties due to wavefunction confinement: in

general, the larger the quantum dot, the smaller the optical gap resulting in the absorption of lower

energy photons.2 In particular, small (2.5–3.5 nm) lead sulfide quantum dots, PbS QDs are

important candidates for applications where a near-infrared emitter (λ: 700–1000nm) is required,

such as solar cells and excitonic upconversion.1,3

Figure 1. Model of lead sulfide quantum dot with ligand coverage1

A valuable property of PbS QDs is that they typically have narrow absorption peaks, as

shown in Figure 2. This indicates good electronic monodispersity, and—when the QDs are cast

into close-packed films for optoelectronic devices—ensures a flat energy landscape, which is

conducive to carrier transport.4 Further, also apparent in Figure 2, is that the optical gap is readily

tuned throughout the short-wave infrared by controlling the final size during synthesis. PbS QDs

also have a large molar absorption cross-section and generally high photoluminescent quantum

yields compared to infrared-emitting organic fluorophores.5,6 Additionally, as recently

demonstrated, even though these nanocrystals have low-lying states that can directly absorb

2

photons, spin dephasing of these excitations via spin-orbit coupling is rapid,7 and the resulting

exciton can undergo energy transfer to generate spin-triplet excitons on conjugated organic

molecules.1,8,9 This allows PbS QDs to be used in applications as a sensitizer in photon

upconversion which will be described in section 1.4.

Figure 2. Absorption spectra of various lead sulfide quantum dots synthesized via injection of

thiourea precursors8

Many factors can influence the absorption linewidth of PbS QDs. First, features can be

heterogeneously broadened in ensemble samples, because quantum confinement10 causes any size-

polydispersity resulting from synthetic imperfection to result in electronic polydispersity.8,11,12

Then, the homogenous linewidth at room temperature for all PbS QDs has important contributions

from exciton-phonon coupling as well as the excitonic fine structure.11 In contrast, the optical

properties for the smallest PbS nanocrystals are more unusual, including the common observation

that the absorption features broaden markedly.11 The origin of this effect remains uncertain, but

was recently attributed to the emission of two states, the band-edge exciton, and a thermally

accessed defect.11 To fully optimize the synthesis of PbS QDs for narrow bandwidths, all of these

3

factors must be addressed in principle, but the most important task is to achieve good size

monodispersity.

1.2 Synthesis of Lead Sulfide Quantum Dots

Though a variety of synthetic methods for PbS QDs introduced,13 we briefly review three

leading families all involving injection: 1) the Hines-Scholes prep involving the injection of TMS-

sulfur into lead (II) oleate,12 2) syntheses following Cademartiri and Ozin using elemental sulfur

and oleylamine injected into lead (II) chloride,14–16 and 3) the recent work from Hendricks and

Owens using lead (II) oleate and a library of thioureas as sulfur sources.8

The Hines-Scholes prep involves the injection of TMS-sulfur into lead (II) oleate at high

temperatures (>80oC).12 The size of the resulting PbS QD is influenced by the stoichiometry of the

reactants, the reaction temperature, as well as how long the reaction is allowed to proceed before

quenching (typically by removing the reaction vessel from heat, or immersion in a cooling

bath).12,17,18 With minor variations, this synthetic procedure continue to be used to make the PbS

QD used in present-day record-efficiency photovoltaic devices.19

The lead (II) chloride prep involves the injection of elemental sulfur into a heated solution

of lead (II) chloride in oleylamine.14–16 The size of the resulting PbS QD is influenced by the

stoichiometry of the precursors.14–16 This synthetic procedure has been shown to have high

monodispersity for large PbS QDs, but the synthesis of small PbS QDs has not been reported using

this method.14–16

The most recent method of PbS QD synthesis involves the use of thioureas as precursors.8

In this synthesis, a solution of lead (II) oleate is injected with a thiourea at 60-100oC.8 Here, the

size of the PbS QD is determined by the electronic and steric properties of the thiourea, and is

more robust to other synthetic conditions.8,20 Consequently, the use of the same thiourea would

produce relatively the same size of PbS QDs which allows for consistency.8 It has also been

reported in literature to resist Ostwald-Ripening at room temperature.8 Thus, thiourea injection

could be a reliable method for the synthesis of small PbS QDs.

4

1.3 Lead Sulfide Growth Mechanism

The LaMer and Dinegar model of burst nucleation describes the growth of nanoparticles

from supersaturated solutions.21 Although this model does not apply to all types of quantum dots,

it is a reliable model for the growth of PbS QDs.8,21 This model, as shown schematically in Figure

3, describes the synthesis with respect to the time-dependent concentration of a ‘monomer’—a

synthetic intermediate formed by the reaction of the formal precursors and considered to be the

chemical species immediately preceding the formation of a nanocrystals.21 Considering classical

nucleation theory, where the free energy benefit scaling with the volume of the nucleated particle

must overcome a surface-energy penalty, LaMer and Dinegar proposed that there was a critical

size above which nuclei would be stable against dissociation21. Then, they observed that such

nuclei would begin to be formed once the amount of monomers reached a critical concentration,

and nucleation would continue until the concentration of monomers dropped back below this

critical concentration (due to monomer consumption via nucleation events as well as growth of

seed nuclei.)21 This effect created a burst of nucleation that was short in time, and would be

followed by further growth of the nuclei by monomer addition.21 These are favourable conditions

for the creation of nanostructures with good size monodispersity.21

Figure 3. LaMer Dinegar model of burst nucleation of quantum dots. I is the monomer creation

phase, II is the nucleation phase, and III is the quantum dot growth phase22

In many nucleation-and-growth reactions, the ultimate size of the synthesized species is

controlled by temperature quenching—cooling the reaction vessel such that the growth phase is

halted.12,21 By contrast in reactions that proceed to completion, (for example, PbS nanocrystals

synthesized using thioureas as shown by Hendricks), the ultimate size is governed by monomer

5

exhaustion.8 In essence, the more nucleation events occur, the smaller the final quantum dots, since

the available monomers are spread across more seeds. According to the LaMer and Dinegar model,

the number of seed nuclei is based on how long and how high the concentration of monomers was

above the critical concentration.8,12,21 With more reactive thioureas, the rate of monomer synthesis

is higher, which results in a higher supersaturation above the critical concentration as well as a

longer duration.8,21 Thus, it was shown that the size of the PbS QDs could be controlled by the

reactivity of the thioureas.8

Although burst nucleation is advantageous for creating an initially-monodisperse ensemble, an

additional challenge in nanoparticle synthesis is to preserve this monodispersity throughout the

growth phase. A key concern is Ostwald Ripening (very common in the Hines-Scholes prep) where

an solution of quantum dots becomes more polydisperse due to smaller quantum dots

disassembling and dissolving into solution and larger quantum dots continuing to grow.8,22,23 One

approach to address this is to employ a continuous injection of precursors to keep the monomer

concentration above the stability threshold for the smallest nanocrystals.24 By contrast, Hendricks

et al. notably claimed that Ostwald Ripening was not observed in their thiourea-based syntheses.8

1.4 Upconversion

Photon upconversion is the process by which photons are absorbed and combined to emit

higher energy photons.25 In quantum dot sensitized upconversion, a quantum dot absorbs a low

energy photon and transfers the resulting exciton to the spin-triplet excited state of an annihilator

molecule.1,25 Two of these excited annihilator molecules can then undergo triplet-triplet

annihilation to form a single higher energy spin-singlet exciton which can then emit a higher

energy photon.1,25 The use of quantum dots as a sensitizer is due to their spin-mixing properties,

their size-tunable optical gaps, their high photoluminescence quantum yields compared to infrared

organic emitters, and the ease of gram-scale synthesis compared to organometallic complexes.1,25

Since the triplet energy levels of most organic molecules are dark states, quantum dots essentially

act as a triplet generator for the annihilators.1 Upconversion has been reported in solution and

solid-state but with a low quantum yield.1,25–27 For example, the upconversion quantum yield of

PbS QDs in solution with rubrene as the annihilator was initially reported to be 0.021%.25 Though

6

this efficiency has reached 8.4%28 in solution, and 7%29 in the solid state, this number remains too

low for solar cell applications.30

However, if higher efficiencies were achieved, upconversion would be very useful for solar

cell applications.1,3,25 This is because the Shockley-Queisser limit sets the theoretical limit of the

efficiency of single junction p-n silicon solar cells at 33%,31 due to many factors including

thermodynamics and the range of wavelengths silicon can absorb.32 In particular, commercially

dominant silicon solar cells do not absorb in the infrared range which is about a third of the energy

in solar light.32 Using materials that can undergo photon upconversion to absorb sub-bandgap

photons and convert it into photons in the visible range for the silicon solar cells to absorb, the

theorized efficiency of p-n junction solar cells rises to 43%.25,32 Thus, a solid-state upconversion

device that can efficiently convert infrared photons to visible photons would be of value.1 In

addition, photon upconversion has also found use in a variety of biomedical applications such as

bioimaging.33

7

Chapter 2

Lead Sulfide Quantum Dot Synthesis Using Thiourea Derivatives

2.1 Small Lead Sulfide Quantum Dots

Small PbS QDs are quite difficult to synthesize compared to larger ones.34 In the TMS-

sulfur injection method, the synthesis of small PbS QDs requires precision and speed with very

little room for error which is typically difficult to achieve whilst using a glove box.12 In the thiourea

injection method, the synthesis of small PbS QDs seemed to be limited by the availability of

suitable thioureas.8 Since the use of thioureas as precursors is a relatively new and unexplored

method of synthesizing PbS QDs, we decided to try to make small PbS QDs by synthesizing and

testing different thioureas.8

Figure 4. Examples of thioureas showing the increased kinetics of reaction in the synthesis of

lead sulfide quantum dots based on steric hindrance and electron deficiency

It is reported in literature that the two parameters for the resulting size of PbS QDs via

thiourea injection is how electron rich the thiourea is, and how sterically hindered it is.8 Hendricks

showed that the more electron deficient and the less sterically hindered a thiourea is, the faster the

rise in monomer concentration of PbS QD synthesis which in turn results in smaller QDs as shown

in Figure 4.8 Using these parameters, we hypothesized that the reaction went through a simple

nucleophilic substitution reaction to create monomers as shown in Figure 5. This was consistent

with the parameters because a nucleophilic substitution mechanism would have faster kinetics with

a more electrophilic thiourea and faster kinetics with less steric bulk for the oleate to attack. The

8

rest of the chapter outlines what we’ve learned about synthesizing small PbS QDs and its

mechanism via the use of substituted thioureas and its derivatives.

Figure 5. Proposed nucleophilic substitution mechanism for synthesis of lead sulfide quantum

dots via thiourea precursors

2.2 Survey of Suitable Thioureas

In general, suitable thioureas were found to be both too costly difficult to source

commercially. Thus, we synthesized them for our purposes using isothiocyanates and amines

which were commercially available. Though there was interest in making small PbS QDs, there

was a greater interest in finding trends and optimizing the synthesis of them. Thus, substituted

N,N’-diphenylthioureas were used as shown in Figure 6, in order to keep the steric bulk

approximately consistent while changing the electronic parameters.

Figure 6. Substituted diphenylthiourea derivatives used in synthesis of lead sulfide quantum dots

9

To synthesize these disubstituted thioureas, a click reaction as shown in Figure 7 from

literature between an isothiocyanate and amine was used as follows.8 The amine (1 eq) was

dissolved in toluene to which the isothiocyanate (1 eq) was added slowly. Once a white precipitate

finished forming, the solvent was removed via vacuo. If the product was white, it was assumed

that it was pure, and no purification steps were taken.

Figure 7. Click reaction of an isothiocyanate and amine to synthesize thioureas

However, this method did not succeed in making thioureas with electron withdrawing

groups on both the isothiocyanate and the amine, which we expected to be necessary in order to

make the smallest PbS QDs. We suspected that an amine with electron withdrawing substituents

might be too weak of a nucleophile causing the reaction to be too sluggish, as was also noted by

Hendricks.8 To achieve strongly electron deficient thioureas, we considered the possible solution

of adding only weakly electron-withdrawing groups onto the amine and compensating with strong

electron withdrawing groups on the isothiocyanate. However, we concluded that, practically, the

electron deficiency needed to be near-symmetric due to the high cost and low catalogue availability

of electron deficient isothiocyanates. Instead, the solutions were either stirred overnight or gently

heated which resulted in white precipitates, with the exception of the nitro containing derivatives

as discussed below.

During the search for suitable thiourea precursors, it was found that the trifluoromethyl

substituent on thioureas made PbS QDs successfully and, of the substituents used by Hendricks, it

was the strongest electron withdrawing group.8 The absorption spectra of the resulting PbS QDs

is shown in Figure 8. Thus, we found that this was a good starting point to attach functional groups.

The ideas for different moieties included nitro groups, cyano groups, and halogens, since these

were readily available for sale.

10

Figure 8. Absorption spectra of PbS QDs synthesized by injection of thiourea A

The synthesis of thioureas using cyano groups, halogens, and trifluoromethyl groups were

straightforward and usually resulted in a white product with near unity yield. Table 1 below lists

the various thioureas synthesized. By contrast, every attempt at synthesizing thioureas with nitro

groups resulted in a yellow product. Given that the target product is colourless, we suspected an

impurity. To assess whether these particular materials were worth pursuing further, we attempted

to use the unpurified product to produce PbS QDs as described below. This was based on our

assumption that an impurity in small concentration was unlikely to wholly disrupt the process of

PbS nucleation and growth. Thus, if QDs were formed, we would purify the thiourea in a future

synthesis so that we could have consistent results and optimize the reaction. If not, it would indicate

a deeper issue.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

400 500 600 700 800 900 1000 1100 1200

Ab

sorp

tio

n

Wavelength (nm)

11

Table 1. List of selected substituted diphenylthioureas synthesized

Thiourea X Y Z X’ Y’ Z’

A CF3 H CF3 H H H

B CF3 H CF3 CF3 H CF3

C CF3 H CF3 Cl H H

D H CN H H H H

E H CN H CF3 H CF3

F H NO2 H H H H

G H NO2 H CF3 H CF3

H CF3 H CF3 OCH3 H H

2.3 Synthesis of Small Lead Sulfide Quantum Dots

To synthesize PbS QDs, a solution of lead (II) oleate was injected with thioureas using the

following procedure adapted from literature.8 Lead (II) oleate (840 mg, 1.09 mmol) was dissolved

in octadecene (12.5 mL) under N2 and heated to 95oC. Thiourea (1.64 mmol) was dissolved in

diglyme (2 mL) and injected into the lead (II) oleate solution after heating to 95oC. If PbS QDs

were formed, the solution would turn black and solution was concentrated under vacuo. The

concentrated solution was dispersed in toluene and then washed with acetone and centrifuged. The

supernatant was removed and the precipitate was redispersed in toluene and stored in inert

atmosphere.

In order to synthesize lead (II) oleate, the following procedure from literature was used.8

Lead (II) oxide (10.00g, 44.8 mmol) was suspended in acetonitrile (20 mL) and cooled to 0oC for

10 min. Trifluoroacetic acid (0.7 mL, 8.96 mmol) and trifluoroacetic anhydride (6.2 mL, 44.8

mmol) was then added. After the lead (II) oxide fully dissolved, the solution was allowed to warm

to room temperature. Oleic acid (25g, 90.05 mmol), isopropanol (180 mL), and triethylamine (10.2

g, 101 mmol) were mixed and then the lead (II) oxide solution was added which formed a white

precipitate. The solution was refluxed until the precipitate dissolved into solution and then was

12

cooled to room temperature and allowed to sit overnight. White precipitate reformed and was

vacuum filtered and washed with methanol.

Thioureas A-H were all used as precursors of PbS QD synthesis. Amongst these thioureas,

D-G were unsuccessful. Thioureas D and E seemed to make QDs as, during injection, the solution

made the characteristic colour change from clear to black. However, the resulting PbS QDs were

found to be colloidally unstable and the absorption spectra did not show an excitonic feature in the

visible range, consistent with aggregation and extreme polydispersity. The nitro containing

thioureas F and G did not yield PbS QDs as when injected into lead oleate under usual conditions,

the solution would turn bright yellow, and the product has no absorption features of quantum dots.

This was consistent with the original thiourea being contaminated by a yellow impurity.

Thiourea A was used and shown in literature to produce PbS QDs with a first excitonic

feature around 980 nm. Thioureas C and H were assumed to make smaller PbS QDs due to their

stronger electron withdrawing substituents, but both were found to consistently give larger PbS

QDs. This led us to question the proposed mechanism as more electron deficient thioureas did not

produce smaller PbS QDs in this case.

2.4 Thiocarbamates and Cyclic Thioureas

In order to check whether the nucleophilic substitution mechanism proposed was correct,

we chose to broaden our library of thiourea derivatives. One candidate was a cyclic thiourea, I, as

shown in Figure 9. Cyclic thiourea I which was chosen as we hypothesized the ring strain and

bond angle would favour an sp3 hybridized carbon center which would speed up the kinetics of the

reaction. If a simple nucleophilic substitution mechanism were to occur, the carbon-sulfur double

bond breaking would be thermodynamically favoured. However, when we tried to synthesize PbS

QDs with I, no PbS QDs were formed. The solution did not turn black upon addition, just a pale

brown.

13

Figure 9. Cyclic thiourea I

We then explored the use of thiocarbamates, as shown in Figure 10, to produce smaller

PbS QDs as they are more electron deficient than comparable thioureas, and the proposed

nucleophilic substitution was expected to work the same similarly.

Figure 10. General structure of thiocarbamates

In order to synthesize thiocarbamates, multiple approaches were taken. The simplest

approach used was a click reaction analogous to the one for thioureas. This click reaction, however,

would use an isothiocyanate and an alcohol. The procedure was as follows. The alcohol (1 eq) was

dissolved in toluene to which the isothiocyanate (1 eq) was added. After stirring overnight, the

solvent was removed via vacuo. Other attempts involved using triethylamine as a base and heating

the solution but none of these methods seemed to improve the synthesis so the click reaction was

used instead.

Using their respective precursors, thiocarbamates J-L as shown in Figure 11 were

synthesized and used for PbS QD synthesis. When injected, the thiocarbamates were found to

produce no PbS QDs. The solution never turned black upon injection and only turned a similar

brown when thiourea I was injected. These experimental results were further evidence at odds with

the simple proposed nucleophilic substitution mechanism.

14

Figure 11. Thiocarbamates synthesized for lead sulfide quantum dot synthesis

2.5 Mechanism of Lead Sulfide Quantum Dot Synthesis

Hendricks reported in that selenoureas could be used to synthesize lead selenide quantum

dots.20 The organic byproduct from this reaction was found to be an N-acylurea which is not

consistent with the nucleophilic substitution mechanism. Using the identity of this byproduct, a

mechanism using thioureas and lead acetate was found which resulted in PbS QDs and N-acylureas

as a product.35 The proposed mechanism based on literature is shown in Figure 12.35

The mechanism of PbS QD synthesis using a thiourea as a precursor starts off with a

deprotonation of an amine which, in concerted fashion, results in the carbon-sulfur pi bond

attacked the lead acetate and kicking off an acetate ligand. Then, the same deprotonation happens

on the remaining amine and results in the carbon sulfur bond breaking as the sulfur localizes onto

the lead, resulting in PbS. This creates a carbodiimide which eventually forms an N-acylurea.

The mechanism shown is in disagreement with Hendricks’ proposed thiourea parameters

for PbS QD size.8 Although steric bulk is a factor in the deprotonation of the amines, in the case

where the amine are more substituted, there are less protons to deprotonate which reduces the

speed of reaction. When there are no protons to deprotonate, the thiourea would form an iminium

moiety instead which would be much slower than if there were a proton. This makes tertiary

amines much less reactive than a primary amine. For the parameter governed by the electron

density of thioureas, electron deficiency is generally a good measure of kinetics but a potential

factor that influences the size of PbS QDs upon injection is the localization of electron density

from the sulfur to the carbon in the thiourea. If the electron density is much higher on the carbon

due to the electron withdrawing substituents, then the electrons may not participate in lead-sulfur

15

Figure 12. Organic mechanism of lead sulfide quantum dot synthesis using thioureas as

precursors

16

bond formation. Consequently, extremely electron deficient thioureas may produce comparatively

larger PbS QDs.

The experimental results found are consistent with this new mechanism.

Trifluoromethylated thiourea A is smaller than more electron deficient thioureas C and H due to

the localization of electron density on the carbon away from the sulfur. Cyclic thiourea I has too

much ring strain to form a carbodiimide which would prevent PbS QD synthesis. Thiocarbamates

would not form a carbodiimide (or an isocyanate with a trivalent oxygen) since the trivalent oxygen

would be unfavoured. However, both the thiocarbamates and cyclic thiourea I could be

deprotonated once and form a single lead-sulfur bond which would explain the light brown

discolouration of the solutions upon injection into lead oleate.

2.6 Future Work

The commercial availability of electron deficient precursors for thioureas is moderate, but

common electron withdrawing substituents—such as the cyano and nitro groups—have been found

not to work. To continue the investigation into the thiourea electron density dependence of PbS

QD size, more electron withdrawing substituents on thioureas would need to be found. It does,

however, appear that just using substituted N,N’-diphenylthioureas would not give enough data

points to make a reliable Hammett plot. The main point of investigation would be to test whether

the size of PbS QDs increases upon injection of increasingly electron deficient thioureas or if the

injection of thiourea A was an outlier. If the size of PbS QDs did indeed increase with the electron

withdrawing power of its substituents at a certain point, it would imply that there is a fundamental

limit to the size of PbS QDs that could be synthesized by tuning electronic parameters of thioureas.

An alternative method of making smaller PbS QDs would be to tune the steric bulk or increase the

number of available protons on the amines. To expand the library of thioureas, heterocyclic

thioureas M and N as shown in Figure 13 were synthesized but not yet tested.

17

Figure 13. Structures of heterocyclic thioureas M and N

The unification of knowledge between the organic and materials chemistry communities is

a further future goal. Presently, the intricacies of the mechanism of thiourea injection to synthesize

PbS QDs do not appear to be known to the materials chemistry community, based on the absence

of citations by nanocrystal synthetic chemists to the work of Ghosh et al describing the formal,

arrow-pushing mechanisms of monomer formation.8,20 Thus, bridging this gap in knowledge may

lead to further optimization of the reaction, including the determination of the fundamental limit

in PbS QD size based on the parameters of thioureas, as well as finding analogous precursors to

make other types of quantum dots. Recently in literature, it was reported that thiocarbamates and

thionates could be used as precursors to synthesize cadmium sulfide quantum dots, since the

kinetics of thiourea injection into cadmium oleate was too fast.36 With a proper knowledge of the

mechanism, many novel syntheses and methods could potentially be introduced to create a variety

of quantum dots.

18

2.7 Appendix: 1H NMR of Chemical Compounds

Figure 14. 1H NMR of thiourea A

19

Figure 15. 1H NMR of thiourea C

20

Figure 16. 1H NMR of thiourea B

21

Figure 17. 1H NMR of thiocarbamate K

22

Figure 18. 1H NMR of thiourea N

23

Chapter 3

CPT as a Transmitter Ligand for Lead Sulfide Quantum Dots

3.1 Transmitter Ligands

Traditionally, the purpose of ligands in PbS QDs is to change their physical properties.

Arguably the most important role of a ligand is to give a quantum dot colloidal stability to prevent

aggregation and allow solubilization in common solvents.37 Recently, since the discovery of triplet

exciton transfer between conjugated organic molecules and quantum dots,38,39 the use of ligands

to also extract excitons from quantum dots has been exhibited.25,26 This class of ligands has been

named “transmitter ligands.”

Transmitter ligands ideally have their first excited triplet energy level below the valence

band energy level of the quantum dot they’re chelating. This way, the extraction of triplet excitons

is thermodynamically favourable. In excitonic upconversion devices, the annihilator would have a

first excited triplet energy level below that of the transmitter ligand for the same reason as shown

in Figure 19.1,27,28

Figure 19. TTA upconversion diagram using a lead based quantum dot as a sensitizer with CPT

as a transmitter ligand and rubrene as the annihilator25

It is known that the first excited triplet energy level of tetracene is slightly higher than that

of rubrene.25 As a result, it has been demonstrated that in solution, tetracene derivatives can be

24

used as transmitter ligands to increase the upconversion quantum yield of rubrene sensitized by

PbS QDs.25 However, for practical applications, this would need to be demonstrated in solid-state.

To do this, a solid-state upconversion device could be devised with a layer of quantum dots

attached to a layer of the annihilator.1

In a solid state upconversion device, the addition of a transmitter ligand could potentially

increase the upconversion quantum yield in various ways. In a device sensitized by quantum dots,

the ligands can extract the excitons and have them live longer to reach the annihilator. Tetracene

in particular has a triplet exciton lifetime of 100µs which is 1-2 orders of magnitude larger than

that of PbS QDs which allows a higher probability of the triplet exciton transferring to the

annihilator.1,9 Additionally, the ligand can create a network so that the exciton can hop from one

ligand to another until it reaches the annihilator. The donation of the exciton back into the quantum

dot would be unfavourable thermodynamically. We also speculate the possibility of the structure

of the ligands allowing the interface between the quantum dot layer and the annihilator to be more

rigid since, for example, rubrene crystallizes readily, and may reorganize at the interface.40

5-(p-Carboxyphenyl)tetracene, CPT, is a tetracene molecule with a carboxylic acid moiety

bound via a phenyl ring bridge as shown in Figure 20. The carboxyphenyl moiety changes the

first excited triplet energy level from 1.21 eV to 1.16 eV such that it is still higher than that of

rubrene which is 1.14 eV.25 Adding a carboxylic acid moiety to the tetracene allows it to be used

as a ligand on PbS QDs. CPT as a ligand on PbS QDs has been demonstrated to improve the

upconversion quantum yield of rubrene in solution compared to without a ligand.25 Hence, we

wanted to synthesize CPT and see if we could reproduce these results in the solid state.

25

Figure 20. Structure of 5-(p-carboxyphenyl)tetracene, CPT

3.2 Synthesis of CPT

CPT was synthesized from tetracene procedures adapted from literature as shown

below.25,41 It is of note that a portion of the yield of the final product was lost to an excessive

washing of the product as in solution, CPT is yellow but was mistaken to potentially be a N-

bromosuccinimide, NBS, impurity.

Figure 21. Bromination of tetracene using N-bromosuccinimide as the bromine source41

To synthesize 5-bromotetracene as shown in Figure 21, tetracene (1 g) was dissolved and

stirred in a solution of dry dichloromethane (300 mL) under N2. The solution was heated to 50oC

and N-bromosuccinimide (776 mg) dissolved in dimethylformamide (50 mL) was added dropwise

over 2.5 h. The reaction was allowed to proceed overnight after which it the solvent was removed

in vacuo. The residue was diluted in methanol after which the precipate was filtered by vacuum.

The precipate was washed twice with water to remove excess NBS. 65.2% yield.

26

Figure 22. Suzuki coupling of 5-bromotetracene and boronic acid ester25

To synthesize 5-(4-ethyl(4-benzoate))tetracene as shown in Figure 22, 5-bromotetracene

(877 mg), 4-(ethoxycarbonyl)phenylboronic acid (667 mg), and cesium carbonate (3.72 g) was

mixed in a biphasic solution of toluene (34.0 mL) and water (14.7 mL). The solution was stirred

and heated at 60oC overnight. Then, the solution was cooled and washed with water, extracted

with ethyl acetate four times, and then given a final wash with brine. The organic layer was dried

with anhydrous magnesium sulfate and gravity filtered. The solvent was removed in vacuo and

then characterized by 1H NMR. 82.9% yield.

Figure 23. Hydrolysis of tetracene ester to CPT25

To synthesize CPT as shown in Figure 23, 5-(4-ethyl(4-benzoate))tetracene (891 mg) was

suspended in a solution of tetrahydrofuran and methanol (275 mL, 1:1). An aqueous solution of

potassium hydroxide (2M, 5.92 mL) was added to the suspension after which it was N2 was in for

20 min and then stirred and refluxed for 3 h. The mixture was put under vacuo for 1 h after which

an excess of hydrochloric acid was added to form a precipitate. The precipitate was filtered under

vacuum and washed with water and hot chloroform. The crude product was then characterized by

1H NMR. 85.0% yield. The overall yield was 45.9%.

27

From the 1H NMR, the crude product still contained some unhydrolyzed ester so flash

column chromatography was performed on aliquots. Various mixtures of hexanes and ethyl acetate

were used but it was found that pure ethyl acetate gave the best separation. However, once purified

using ethyl acetate, a new impurity was found via 1H NMR but the origin could not be identified.

This impurity was found in the 1H NMR of the crude product but in such a small amount that it

could have been attributed to signal noise but became prominent once the ester impurity

disappeared.

Figure 24. Absorption/emission spectra of CPT and absorption spectra of 765 nm PbS QDs

The absorption and emission spectra of CPT were taken in toluene as shown in Figure 24.

Both the absorption and emission spectra of CPT matched that of literature.25 The characterization

gave enough confidence that the right compound was synthesized, and ligand exchange could be

performed.

28

3.3 CPT Ligand Exchange

In order to get CPT on PbS QDs as a ligand, the ideal condition would be to have CPT

chelate the PbS during the QD synthesis because it is believed that the ligand exchange can

introduce surface defects.25,26 However, this synthesis is not present in literature, and it is shown

that a high surface density of CPT on PbS QDs causes colloidal instability which is not desirable.25

Thus, a ligand exchange can be performed in which a solution of CPT is mixed with PbS chelated

by oleate.25,29,42 The CPT is allowed to swap with oleate over time. It should be noted that the

longer the ligand exchange goes on, the lower the quality of the QDs as it is believed that a

perturbation in the equilibrium of the QDs to ligands in solution can etch the surface of the QDs.25

Figure 25. Absorption spectra of PbS QDs with CPT ligands after 20, 60 minutes of exchange

Using 2.4 nm PbS QDs prepared by TMS-sulfur injection of lead (II) oleate, the oleate

ligands were exchanged with CPT using the following procedure adapted from literature.25 PbS in

hexanes (5 uL, 594 uM) was added to CPT in toluene (45 uL, 1 mM). Then, toluene (250 uL) was

added. The ligand exchange was allowed to go on for various times (20, 40 or 60 minutes). Acetone

(2.7 mL) was added to crash the PbS QDs and was centrifuged at 6000 rpm for 10 min. The

supernatant was removed and the precipate was dispersed in toluene (0.3 mL).

29

The absorption spectra of the PbS QDs after ligand exchange were taken as shown in

Figure 25. It can be seen that both the PbS QD and CPT features are present in the absorption

spectra which likely indicates that ligand exchange has occurred. Different ligand exchange

durations were tested to optimize the reaction, but the solutions used for absorption spectra were

too dilute to get meaningful comparisons.

3.4 CPT Assisted Upconversion

To test for upconversion, PbS QDs that were ligand exchanged with CPT for 40 minutes

were used since it was reported in literature to give the highest upconversion quantum yields.25

Instead of dispersing the precipate after centrifugation in toluene (0.3 mL) from 3.3, the precipitate

was dispersed in a solution of rubrene in toluene (0.3 mL, 20 mM). This solution was then put into

a cuvette and a 720 nm laser was passed through and it could be seen that it was emitting yellow

rather than scattering red. This is qualitative evidence of upconversion which confirmed that the

synthesis and ligand exchange of CPT was at least partially successful.

3.5 ACA Assisted Upconversion

Since the synthesis of CPT is cumbersome and the reagents, expensive, 9-

anthracentylcarboxylic acid, ACA, which is a carboxylated anthracene derivative was seen as a

potential candidate for solid-state upconversion device. This was because ACA is expensive and

commercially available. As a parallel experiment to using CPT assisted upconversion, ACA, as

shown in Figure 26, was exchanged onto cadmium selenide quantum dots, CdSe QD, which were

synthesized by colleague Christian Imperiale, according to the following procedures adapted from

literature.26 CdSe in toluene (571 uL, 1.85 × 10-5 M) was mixed with toluene (529 uL). Then,

additional toluene (1.3 mL) and acetonitrile (4.7 mL) were added to the mixture. ACA (30 mg)

was added to the mixture and stirred vigorously for 14 h. Acetone (14 mL) was added to the

solution and then centrifuged at 6000 rpm for 15 min. The supernatant was removed, and the

precipitate was dispersed in minimal hexanes. Diphenylanthracene, DPA, was added in excess and

would not dissolve in the solvent which resulted in an unsalvageable mixture.

30

Figure 26. Structures of 9-anthracenylcarboxylic acid, ACA, and diphenylanthracene, DPA

Instead of adding DPA immediately, the CdSe QDs were characterized first in subsequent

experiments. The ligand exchange between CdSe QDs and ACA was performed in a procedure

modified from literature as follows.26 CdSe in toluene (1.1 mL, 1.85 * 10-5 M) was mixed with

toluene (529 uL). Then, additional toluene (1.3 mL) and acetonitrile (4.7 mL) were added to the

mixture. ACA (30 mg) was added to the mixture and stirred vigorously for 19 h and 13 h (two

separate exchanges). Acetone (7 mL) was added to the solution and then centrifuged at 6000 rpm

for 15 min. The supernatant was removed, and the precipitate was dispersed in minimal hexanes.

For the CdSe QDs exchanged at 19 h, the exchange duration was too long, and it was found

that there was almost no visible excitonic feature. For the CdSe QDs exchanged at 13 h, a first

excitonic feature at 552 nm was found in the absorption spectra as shown in Figure 27. The

absorption spectra shows both the features of carboxyanthracene at 300-400 nm and a broader

peak from the CdSe QD core at 500-600 nm which shows that both the anthracene and CdSe QDs

are present after washing. The next step would be to reproduce upconversion by adding DPA as

the annihilator and finding solvent conditions in which enough would dissolve.

31

Figure 27. Absorption spectra of cadmium selenide quantum dots after ligand exchange for 13

hours with carboxyanthracene

3.6 Future Work

In continuation of this work, transient photoluminescent decays would be taken of the QDs

with CPT on them at different concentrations and exchange durations. Since CPT could extract

the excitons from the PbS QDs, if it were acting as a ligand, the transient photoluminescent decay

of the PbS QDs with CPT would have a shorter life-time compared to without CPT. This would

be compared to NMR studies on the surface to be sure of the ligand coverage to see if there is a

calibration curve that could be made based on the life time versus the actual number of ligands on

the surface of PbS QD.

Since the observed upconversion is only qualitative, a quantitative measurement of the

quantum yield would be taken in addition to emission spectra. Once it is confirmed that

upconversion is indeed occurring, we would attempt to observe upconversion in the solid state. If

this was observed, we would optimize the device by changing parameters such as the number of

layers of QDs. This is particularly attractive since it was shown that a single layer of quantum dots

was optimal without transmitter ligands and a thicker layer of quantum dots—which transmitter

ligands might allow—would increase the amount of photons absorbed for upconversion.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

300 400 500 600 700 800

Ab

sorp

tio

n

Wavelength (nm)

32

Analogous to the work done on demonstrating tetracene as a transmitter ligand in the solid

state, we also plan on trying other potential ligands on various systems in the solid state. More

work using anthracene as a ligand of CdSe QDs would be done to show that transmitter ligands

can work in the solid state for a variety of quantum dots. A proposed synthetic scheme has also

been devised for the use of a rubrene derivative as a potential transmitter ligand, as it may be

advantageous to test if the annihilator being attached directly to the quantum dot results in greater

upconversion quantum yields. A synthetic scheme for the synthesis of a rubrene derivative to act

as a ligand is shown in appendix 3.7, Figure 28. As rubrene is an annihilator, we wanted to see if

chelating rubrene onto the sensitizer would improve the upconversion quantum yield. However,

we have yet to try the synthesis.

33

3.7 Appendix: 1H NMR of Chemical Compounds

Figure 28. synthetic scheme of rubrene derivative where R1 would be a carboxy group or

carboxyphenyl group in order to act as a ligand onto lead sulfide quantum dots

34

Figure 29. 1H NMR of 5-(4-ethyl(4-benzoate))tetracene

35

Figure 30. 1H NMR of crude CPT

36

Figure 31. 1H NMR of CPT after column

37

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