Microfluidic Preparation of Air- and Moisture-Sensitive

Precursor Solutions and Semiconductor Nanocrystals

by

Parnian Saberi

A thesis submitted in conformity with the requirements

for the degree of Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

© Copyright by Parnian Saberi 2016

ii

Microfluidic Preparation of Air- and Moisture-Sensitive Precursor

Solution and Semiconductor Nanocrystals Parnian Saberi

Master of Applied Science

Graduate Department of Mechanical and Industrial Engineering

University of Toronto

2016

Abstract Syntheses of a wide range of air- and moisture-sensitive chemistries are conducted in

dedicated, capital-intensive setups that are not amenable to automation and offer only limited

process control and scalability. Here, we demonstrate a compact Schlenk-line-to go (SL2G)

platform that overcomes these limitations. Case studies of semiconductor precursor solutions and

colloidal nanomaterials are presented. The rapid and reproducible preparation of precursor

solutions and NCs is achieved by a microfabricated sparger that promotes rapid mixing and

temperature control. We expect the benchtop SL2G platform to enable air- and moisture-sensitive

syntheses to be conducted independent of a fume hood or a glovebox and in a manner that is

scalable, automatable, and compatible with flow chemistry.

iii

Acknowledgments

I am grateful for Prof. Axel Guenther for calming me down any time that the shear amount of work

would test my patience level, and to help me find the path in this project. His perspective changed

mine for the better. Dr. Yasser Hassan spent many hours in the lab explaining fundamentals of

semiconductor nanocrystals to me, while being overloaded with other responsibilities. For this and

many other lessons I learned from him, I thank him. As a groupmate, I enjoyed working with

Shashi Malladi and I am grateful for her company, general consult and help on coding. Ruoxi

Wang joined during the last two months of my work, and with her self-starter mindset,

determination and hard work, she helped the project progress to the finish line, and I would like to

thank her for travelling from the so-called heaven, California, to Toronto to work with me.

Above all, the support of my family helped me throughout my work; my father for always

believing in me, and my mother for being by my side during good and bad times. I hope to keep

making you proud and hopeful that you did not go so wrong with one of your children! And lastly,

I am thankful for Sahand Jafarian for being my second brain, ears to my mouth and for helping me

survive tough times since too long ago.

iv

Table of Contents

Acknowledgments.......................................................................................................................... iii Table of Contents ........................................................................................................................... iv List of Tables ...................................................................................................................................v List of Figures ................................................................................................................................ vi List of Appendices ........................................................................................................................ vii List of Symbols ............................................................................................................................ viii List of Abbreviations .......................................................................................................................x List of Chemical Formulae ........................................................................................................... xii 1. Introduction .................................................................................................................................1 1.1. Semiconductor Nanocrystals ...............................................................................................1 1.2. Evolution of Synthetic Methods ..........................................................................................1 1.3. Hot Injection Synthesis ........................................................................................................2 1.4. Reaction Platforms ...............................................................................................................4 2. Experimental ...............................................................................................................................6

2.1. Experimental Apparatus.......................................................................................................6 2.2. Materials and Methods .......................................................................................................12

3. Results and Discussions ............................................................................................................20 4. Conclusions and Outlook ..........................................................................................................39

4.1. Conclusions ........................................................................................................................39 4.2. Outlook ..............................................................................................................................39

References ......................................................................................................................................41 Appendices .....................................................................................................................................46

A Experimental Details ..........................................................................................................46 A.1 Device Design ....................................................................................................................46 A.2 Manifold Design ................................................................................................................50 A.3 Intensity Measurement .......................................................................................................56 B Microfabrication ................................................................................................................57 C Precursor Characterization .................................................................................................60 C.1 Te Precursor Characterization ...........................................................................................60 C.2 Cd Precursor Characterization ...........................................................................................65

v

List of Tables Table 1. Preparation composition of Cd-OA precursor solution for CdTe NC Synthesis ............ 12 Table 2. Preparation composition of Te-TBP precursor solution for CdTe NC Synthesis........... 13 Table 3. Properties of CdTe NCs prepared at 200°C and 220°C .................................................. 29 Table 4. Time-dependent control properties of precursor and CdTe NCs in SL2G and SL. ........ 33 Table 5. Initial and Final Controller Parameters for Tsp=300°C in SL and SL2G. ....................... 34 Table 6. Peak list for Cd precursor and NC solutions................................................................... 37 Table 7. O-ring/Gland Design for Sealing Glass Cylinder ........................................................... 52 Table 8. PTFE GORE Sheet Gasket Specifications/Design ......................................................... 52

Table A. 1. Precursor Preparation Device Design Specifications ................................................ 46 Table A. 2. Typical solution Properties of Cd-OA Mixture (Protocol 2)[57] .............................. 47 Table A. 3. Typical pressure calculations within the microfluidic device with 100 µm diameter holes. ............................................................................................................................................. 48

vi

List of Figures Figure 1. Comparison of Schlenk Line (SL) and Schlenk Line-to-Go (SL2G) platforms.. ........... 7 Figure 2. Illustration of SL2G platform operation with MF device.. ............................................. 8 Figure 3. Experimental setup and control systems.. ....................................................................... 9 Figure 4. Schematic of detailed parts of SL2G manifold ............................................................. 11 Figure 5. Electrical diagram of temperature control system ......................................................... 15 Figure 6. Comparison of NMR preparation setups.. ..................................................................... 17 Figure 7. Time-dependent SL2G platform operation.. ................................................................. 21 Figure 8. Characterization of Te-TBP precursor solution prepared in SL2G platform. ............... 23 Figure 9. 31P NMR of phospine-based ligands and Te precursor for CdTe NC synthesis. ........... 24 Figure 10. Characterization of Cd-OA precursor solution prepared in SL2G platform. .............. 26 Figure 11. 1H NMR of HOA and Cd-OA precursor comparing removal of acidic proton. .......... 27 Figure 12. Characterization of prepared CdTe NCs in microfluidic platforms. ........................... 29 Figure 13. Profile of lattice plane distances of 5.3 nm CdTe NCs synthesized at 220°C............. 30 Figure 14. PXRD pattern of CdTe NCs prepared at 220°C. ......................................................... 30 Figure 15. Temperature profiles of precursor and NC preparations.. ........................................... 32 Figure 16. Process response behavior in Cd-OA preparation in SL.. ........................................... 35 Figure 17. Process response behavior in Cd-OA preparation in SL2G ........................................ 36 Figure 18. FTIR spectra of precursors and NC solutions.. ........................................................... 38 Figure A. 1. Device Configuration for Precursor Preparation.. .................................................... 49 Figure A. 2. Sealing Design of SL2G. .......................................................................................... 51 Figure A. 3. Drawing of top Al part of manifold. ......................................................................... 53 Figure A. 4. Drawing of bottom Al part of manifold.................................................................... 54 Figure A. 5. Drawing of insulating part of manifold. ................................................................... 55

Figure B. 1. Microfabrication Process Flow Diagram .................................................................. 58 Figure B. 2. Side-wall angle analysis of microfluidic devices with DRIE protocol changes. ...... 59

Figure C. 1. 31P NMR of Te-TBP and commercial TBPO. .......................................................... 60 Figure C. 2. MS results for centrifuged Te-TBP Sample. ............................................................ 61 Figure C. 3. MS results for magnified region of Figure 8b. ......................................................... 62 Figure C. 4. MS results for filtered Te-TBP Sample. ................................................................... 63 Figure C. 5. MS results for crude Te-TBP/TOP/ODE/OLA Sample. .......................................... 64 Figure C. 6. FTIR Comparison of Te ligand and precursors. ....................................................... 65 Figure C. 7. 113Cd NMR of Cd(ClO4)2 in comparison with previously-reported values. ............. 66 Figure C. 8. Superimposed 1H NMR of Cd-OA ( ), CdTe ( ), and HOA ( ). ............................ 66

vii

List of Appendices

A. Experimental Details………………………………………………………………………….46 B. Microfabrication………………………………………………………………………………57 C. Precursor Characterization……………………………………………………………………60

viii

List of Symbols

Symbols

∆𝐏𝐏𝐜𝐜𝐜𝐜𝐜𝐜 Capillary pressure (Pa)

∆𝐏𝐏𝐆𝐆 Gas pressure drop across the channel (Pa)

0 Reference condition

a Dimensionless parameter

a, b, c Lattice parameters (Å)

C Sutherland's constant for gaseous material (K)

C=O Cabonyl group

COO- Carboxylate group

COOH Carboxylic acid

de Equivalent bubble diameter (m)

g Gravitational acceleration (m.s-2)

H Device channel height (m)

hl Liquid column height (m)

i ith component

I(t) Time-dependent intensity (arb.)

Ifit Fitting grayscale intensity

Io Incident light intensity

L Device channel length (m)

l Liquid component

N Nth component

PG,experimental Experimental inlet gas pressure (Pa)

PG,min Minimum inlet gas pressure (Pa)

PL Liquid column pressure (Pa)

PL Photoluminescence spectroscopy

P-O Phosphoryl group

P-Te Phosphorus telluride group

Q Liquid flow rate (m3.s-1)

r Device hole radius (m)

ix

R Fluidic resistance (kg.m-4.s-1)

T Temperature (°C, K)

t Time (min)

T(t) Time-dependent temperature (°C)

Tmin Minimum temperature (°C)

trxn Reaction time (min)

Tsp Temperature setpoint (°C)

u∞ Terminal gas velocity (m.s-1)

V Liquid volume (ml)

VBN Viscosity Blending Number

W Device channel width (m)

x Mass fraction (g/g)

γ surface tension (N.m-1)

δ Chemical shift (ppm)

θ Side-wall angle (°)

λ Wavelength (Å)

μ Dynamic viscosity (Pa.s)

ν Kinematic viscosity (m2.s)

νmixture Kinematic viscosity of mixture (cst)

ρ Density (kg.m-3)

τ Time constant (min)

χ2 Chi-squared test statistics

x

List of Abbreviations

Abbreviations 113Cd NMR Cadmium NMR 1H NMR Proton NMR 31P NMR Phosphorus NMR

Al Aluminum

AOT Sodium di-2-ethylhexyl sulfosuccinate

ASTM American Society for Testing and Materials

Cd-OA Cadmium oleate

CMOS Complementary metal–oxide–semiconductor

DART Direct Analysis in Real Time

DRIE Deep Reactive-Ion Etching

FFKM Kalrez® perfluoroelastomer

FTIR Fourier transform infrared spectroscopy

FWHM Full width at half maximum (nm)

HMDS Hexamethyldisilazane

HOA Oleic acid

HRTEM High resolution TEM

ICDD International Centre for Diffraction Data

ID Inner diameter (mm)

LC Liquid chromatography

MS Mass spectrometry

NC Nanocrystal

NMR Nuclear magnetic resonance spectroscopy

OD Outer diameter (mm)

ODE 1-Octadecene

O-H Hydroxy group

OLA Oleylamine

P63mc 6mm hexagonal space group

PEEK Polyether ether ketone

PFA Perfluoroalkoxy

xi

PID Proportional-integral-derivative

PLQY Photoluminescence quantum yield

PTFE Polytetrafluoroethylene

PVC Polyvinyl chloride

PXRD Powder x-ray diffraction

QD Quantum dot

r.t. Room temperature (°C)

RTD Resistance termperature detector

SEM Scanning Electron Microscopy

Si Silicon

SL Schlenk line

SL2G Schlenk-line-to-go

SS Stainless steel

STP Standard temperature and pressure

TBP Tri-n-butylphosphine

TBPO Tri-n-butylphosphine oxide

Te Tellurium

TEM Transmission electron microscopy

Te-TBP Tri-n-butylphosphine telluride

Toluene-d8 Deuterated toluene

TOP Tri-n-octylphosphine

TOPO Tri-n-octylphosphine oxide

UV-vis Ultra-violet-visible spectroscopy

v/v Volume fraction

xii

List of Chemical Formulae

Chemical Formulae

C12H27PTe Tri-n-butylphosphine telluride

C18H34O2 Oleic acid

C36H66CdO4 Cadmium oleate

C3H9P Trimethyl phosphine

Cd(CH3)2 Dimethylcadmium

Cd(ClO4)2.6H2O Cadmium perchlorate hexahydrate

Cd(SO4)2 Cadmium Sulfate

Cd2+ Cadmium ion

CdCl2 Cadmium chloride

CdO Cadmium oxide

CdS Cadmium sulfide

CdSe Cadmium selenide

CdTe Cadmium telluride

CH2 Methylene group

D2O Deuterium oxide

H2S Hydrogen sulfide

HgTe Mercury telluride

KBr Potassium bromide

ZnSe Zinc selenide

Si(CH3)4 Tetramethylsilane

1

Chapter 1

1. Introduction 1.1. Semiconductor Nanocrystals Nanocrystals (NCs) with an inorganic semiconductor core and an organic shelling surfactant layer

exhibit interesting size-dependent properties within nanometer size ranges. These colloidal NCs

differ from their bulk counterparts in their physicochemical properties, since their crystal size is in

the order of the materials’ exciton Bohr radii, where electron transfer resembles molecular

behavior as opposed to bulk.[1, 2] These NCs are also called quantum dots (QDs) due to their

quantum confinement effect and 0-dimensional discrete energy levels. Quantum confinement

effect can be translated to tunable electronic bandgaps between the occupied valence band and the

unoccupied conduction band in semiconductor NCs.[1] Since introduction of quantum

confinement in colloidal CdS NCs by Rossetti et al. in 1983, nanomaterial research has accelerated

with considerable efforts on controllable synthetic routes, characterization, and study of

fundamental properties of these materials.[3, 4] In addition, crystallinity of the inorganic core

makes it possible to controllably manipulate surface energies, and to consequently change

thermodynamic behaviors of the NCs.[1] Hence, precise control of the size, shape and composition

of semiconductor NCs has attracted several researchers. The high level of interest in such materials

is regarded to the tunable properties quantum confinement can result in. By increasing the size of

the NC, the bandgap energy decreases, leading to an increase in the absorption and emission

wavelengths. Therefore, by changing the size of particles, a wide range of spectral properties can

be achieved to cover the entire visible color spectrum. This property has attributed to various

applications, including biological imaging, detection and therapeutics, photovoltaics, sensing, and

optoelectronics.

1.2. Evolution of Synthetic Methods Although solid-state synthesis of nanoscale semiconductors was achieved in the 1970s using top-

down approaches such as molecular beam epitaxy [5] and metal-organic chemical vapor

deposition,[6] Kalyanasundaram et al. introduced the first bottom-up approach, arrested

precipitation, to form soluble semiconductor colloids in aqueous solutions, overcoming photo-

degradation in previous methods.[7] This method used slow injection of aqueous metal salts such

2

as Cd(SO4)2 into an alkaline solution containing chalcogenide salts such as (NH4)2S. After a

double-displacement reaction resulting in CdS NCs, the extremely low solubility of the product in

the aqueous solution led to precipitation, with styrene/maleic anhydride copolymer preventing

agglomeration.[8, 9] However, this method exhibited low photoluminescence quantum yield

(PLQY) of NCs due to charge trapping and photo-corrosion, as well as challenging size control

due to Ostwald ripening. Another method to improve the size distribution of various NCs

significantly was templated growth within structured media. In the original work by Meyer et al.,

within a reversed micelle of sodium di-2-ethylhexyl sulfosuccinate (AOT), an organic iso-octane

solvent was used containing CdCl2 and (NaPO3)6, into which CdS and H2S were introduced.[10]

Therefore, CdS NCs were entrained within the micelle. Similar approaches using different micelle

materials were implemented over the following few years, with sustained efforts to improve size

distribution. Combining reagents in arrested precipitation and the micelle material from templated

growth led to successful production of CdSe NCs by Steigerwald et al. In this method,

Cd(ClO4)2.6H2O solution was emulsified in AOT reversed micelles, and organometallic precursor

of bis(trimethylsilyl)selenide was injected into the solution.[11] With successful synthesis of

narrow size-distribution CdSe NCs, subsequent research was followed on organometallic

precursors solutions. Steigerwald et al. utilized tertiary phosphine chalcogenides for the first time

as highly-reactive chalcogen sources, with triethylphoshpine telluride and Hg metal to yield HgTe

NCs. Following this method, various NCs were synthesized benefiting from the instability of

tertiary phosphine chalcogenides. It was later shown by Brennan et al. that branched phosphine

complexes can be inserted in metal-carbon bond when metal alkyl compounds (eg. Cd(CH3)2) were

used as the cation source.[12]

1.3. Hot Injection Synthesis Inspired by the use of organometallic precursors in reversed micelles, Bawendi et al. used a melt

of high boiling mixtures of tri-n-butylphosphine (TBP) and tri-n-butylphosphine oxide (TBPO),

and annealed the intermediate nanoclusters leading to highly-luminescent NCs.[13] Consequently,

a method referred to as hot injection was developed by Murray et al., in 1993, which ever since

has proven to be the most controllable and reproducible approach for synthesis of metal

chalcogenide NCs.[4, 8] This method included injection of organometallic precursor solutions of

dimethyl cadmium (Cd(CH3)2) and selenium (Se) into a hot coordinating solvent and ligand (Tri-

n-octylphosphine oxide (TOPO), and Tri-n-octylphosphine (TOP)),[14] where the oxide sites

3

would bond to Cd2+ ions, and the phosphine sites would bond to the chalcogenide, in order to

create a neutrally-charged nanocrystal with an equal number of cations and anions. Organic ligands

were used to passivate the surface of the nanocrystal and to control the size and shape, confirming

Steigerwald’s hypothesis.[11, 15] In order to decrease safety risks of this toxic Cadmium salt and

to eliminate thermodynamic limitations of the rapid injection approach, Peng and Peng suggested

CdO, as the source of Cd2+ ion using a non-injection one-pot synthesis method, which has ever

since become the most common approach for synthesis of Cd-based nanocrystals.[16, 17] Wu and

Peng also synthesized high-quality CdS, CdSe and ZnSe nanocrystals with a non-coordinating

solvent (1-octadecene (ODE) and ligand (oleic acid (HOA)).[18] By introduction of one or both

of the reagents in the solvent at elevated temperatures, a burst of nuclei occurs followed by growth

of NCs at lower temperatures. In order to control the size of NCs, the reaction is terminated by

quenching. This method was developed to increase the potential for greener synthesis, scaling up

and fundamental studies of the crystallization phenomenon. Despite several studies for other

possible non-injection methods, the final nanocrystal quality is not comparable to abovementioned

methods.[12, 18, 19] Therefore, this work has pursued based on adaptations of hot injection

method for synthesis of CdTe NCs.[18, 20-22]

In addition, since introduction of hot injection method by Murray et al.[14] more than two decades

ago, several groups have investigated the synthesis, growth and characterization of semiconductor

NCs,[15, 17, 19, 23-26] and more recently, precursor reaction mechanisms and kinetics.[27-31]

The dependence of precursor conversion on concentration, solvent type and ligands has been

established in many colloidal synthesis reactions.[15, 29, 32] However, a more complete

fundamental understanding of precursor behavior is still limited by contradictory results and the

quest to expand the range of candidate chemistries available for the preparation of air-sensitive

materials. For example, conflicting literature reports on the effect of alkylamine ligands have been

attributed to a lack of mechanistic understanding of reactions.[30, 33, 34] In addition, the

continued discovery and improvement of air-sensitive precursor chemistries will remain a

necessary step to further improve consistent control over NC size and shapes at high yield, at

reduced cost and in environmentally-benign manners. As recently suggested for the case of metal

chalcogenide nanocrystal synthesis,[31] new chalcogen sources promise more controlled and

tunable conversion kinetics. Increasingly, limitations associated either with current precursor

reactions or preparatory approaches become bottlenecks in preparing semiconductor NCs with

more consistently defined size, shape, and charge transfer properties.[29]

4

1.4. Reaction Platforms Since the introduction of microfluidics-based synthesis of NCs in 2003,[35] several researchers

have gravitated towards flowable approaches to produce highly-controllable nanoparticles with

precursor solutions that were prepared ex situ.[36-47] In spite of the increased control over

nucleation and growth stages and the fidelity gained by employing flow chemical formats, the

upstream precursor preparation process so far remained unchanged, relying on a Schlenk line (SL)

apparatus introduced by Wilhelm Schlenk more than one century ago.[48] He invented a set of

glassware and Teflon valves which could create inert and vacuum environments efficiently and

within one platform. In addition to SLs, gloveboxes have found some popularity due to elimination

of handling dangerous glassware within an inert environment. Gloveboxes are continuously

controlled to remove any air and moisture from the closed apparatus. These methods have become

the standard method for handling air- and moisture-sensitive materials since 1913.[4] Nowadays,

depending on the available laboratory space, sensitivity of the material to exposure to air, and the

ease of handleability, said infrastructures find popularity. Both of these capital-intensive

approaches limit operation outside of closed environments, while enabling chemical synthesis of

materials with unprecedented qualities. The head space of SLs is connected to vacuum and inert

gas lines to ensure an inert and moisture-free gas atmosphere. The preparation of a precursor

solution begins by adding the starting materials to a flask. By increasing the temperature of the

continuously-stirred solution above the boiling point of water and by applying a vacuum below

300 mTorr, water vapor is removed. A subsequent further increase in the solution temperature

results in a color change that is indicative of the formation of ionic bonds and the dissociation of

solid starting materials. The reaction is thermally quenched and the prepared precursor solutions

are collected under inert conditions. The use of (SL) increasingly becomes the bottleneck that is

in part responsible for undesirable batch-to-batch variations, inflexibility and lack of scalability

due to the dependence on fume hoods, mass and heat transport limitations and lack of in-line

characterization. Removing this problem will therefore be a necessary requirement to

comprehensively study air-sensitive precursor conversion reactions and demonstrate the flow

chemical preparation of NCs all the way from the starting materials. Inspired by this Peng’s

approach, we have developed a platform to portably and controllably perform air-sensitive

processes.[16, 17] We combine the favorable heat and mass transfer associated with bubble

columns with the controlled fluid delivery of microfluidic devices in a platform that enables rapid

5

and reproducible preparation of precursor complexes. We hope to enable study of nanocrystal

synthesis without inherent limitations of Schlenk line techniques, towards more convergent

literature results, greener chemicals and conditions for nanocrystal synthesis. Schlenk Line-to-Go

enables rapid and controllable preparation of air-sensitive solutions.

6

Chapter 2

2. Experimental The rapid preparation of of moisture- and air-sensitive NC precursor complexes is presented in the

Schlenk Line-to-Go (SL2G) platform with a considerably smaller footprint, a high degree of

repeatability, and amenability to automation and scale-out. Dynamic monitoring of the absorbance

of the solution indicated complex formation. The chemical integrity of the precursor solutions was

validated using 1H, 31P and 113Cd NMR and Mass Spectrometry (MS). To demonstrate the in situ

preparation of NCs using adapted hot injection method, we selected CdTe NCs as a case study

with availability of well-documented protocols for the preparation of cadmium-oleate (Cd-OA)

and tri-n-butylphosphine telluride (Te-TBP) precursor solutions.[14, 18, 20-22, 27, 49] NCs were

characterized based on their UV-vis and photoluminescence (PL) spectra, Transmission Electron

Microscopy (TEM) and Powder X-ray Diffraction (PXRD). The details of experimental apparatus

and assembly, materials and methods for characterization are discussed further.

2.1. Experimental Apparatus Briefly, all materials were mixed via bubbling for at least 5 minutes at 130°C to ensure oxygen

and water vapor were removed from the system. The excess fumes were drained through the top

vent line. The liquid was heated above 200°C to increase solubility of solids, and to ensure

complex formation. Change of solution turbidity and optical intensity indicated reaction

termination point. Consequently, waste was drained, and the final product was pumped back into

the chip and collection vial. Figure 1 schematically compares the preparation of precursor

solutions between the SL and the SL2G approaches. To represent unit operations, Te-TBP

precursor was selected as the green solution shown in the following figures. As shown in Figure

1a, with an estimated volume of 1.8 L, the SL2G platform possesses nearly 2 orders of magnitude

smaller of a volume compared to a simplified SL with an estimated volume of 82.5 L. In addition,

based on nearly-identical Te-TBP preparation experiments in both systems, Figure 1b shows a

reduction of experiment time by more than two-fold. Both systems were used for preparation of

10 ml solution at the same temperatures. With a similar approach of addition of starting materials,

dissolution/complex formation and quenching, both Cd-OA and Te-TBP precursors were

prepared, followed by synthesis of CdTe NCs in SL and SL2G.

7

The cross-sectional image of the SL2G platform is shown in Figure 2a, where the starting

materials are sequentially introduced into the vertical glass cylinder from above, via fluid inlets

provided within a customized cap. The latter encloses the top of the cylinder and accommodates a

thermocouple for measurement of the time-dependent temperature of the precursor solution, T(t).

On the bottom, the cylinder is enclosed via a temperature-controlled mantle with embedded

cartridge heaters and provision for perfusion of cold water, and a silicon-based microfluidic device.

The microfluidic device contains at its bottom side two microchannel networks that are capped

with an anodically-bonded glass layer (see Appendix A for details). After all of the starting

materials are introduced into the continuously bubble-perfused cylinder and water vapor is

removed by increasing the solution temperature above 100°C, the temperature is further increased

to completely dissolve all solids. The solution’s change in color and absorbance as assessed by a

time-dependent intensity measurement from the side (camera 1) is indicative of the desired

complexes to be formed and allows to objectively determine when this step is completed. Upon

completion of the precursor preparation, the solution temperature is lowered by perfusing cooling

water through the mantle. The precursor is then collected into a nitrogen-purged vial by opening

the outlet and increasing the pressure applied to the nitrogen-filled head space of the column. A

Figure 1. Comparison of Schlenk Line (SL) and Schlenk Line-to-Go (SL2G) platforms. (a) Schematic of SL and SL2G platforms with 2 orders of magnitude reduced volume in the latter case. (b) Schematic comparison of precursor preparation methods using SL (top) and SL2G (bottom) approaches. Symbols indicate (1a) solvent and (1b) solid addition, (2) air/moisture removal via heating above 100°C, (3) solid dissolution and complex formation, (4) cooling and collection.

8

detailed instrumentation diagram and the assembled SL2G are illustrated in Figure 3 for

preparation of Te-TBP precursor. The process is monitored using cameras 1 and/or 2, the PID

temperature and pressure controllers to ensure leak-tight and controlled operation, while cycling

temperatures. To highlight the portable nature of the SL2G platform as well as its ability to prepare

air- and moisture-sensitive precursors outside of a fume hood, the experimental setup was

successfully operated within a secondary containment (commercially available metal lantern) as

shown in the photograph Figure 2c.

Figure 2. Illustration of SL2G platform operation with MF device. (a) Schematic illustration of sparged reservoir employed for precursor preparation in SL2G platform with thermally-conducting layers (gray), insulating layer (nude), and precursor liquid in column (green), cameras for two fields of view and bottom image of microfluidic device Scale bar: 10 mm. (b) Bright field photograph of microfluidic device obtained from camera 2. Scale bar: 1.5 mm. (c) Photograph of SL2G platform operated within portable secondary containment.

9

Figure 3. Experimental setup and control systems. (a) P&ID control system with gas ( ), liquid ( ), starting materials ( ) and (b) photograph of experimental setup used for precursor preparation, (c) Schematic of SL2G setup for Te precursor preparation.

10

Experimental Setup: The apparatus shown in Figure 3c, consisted of a thermal insulation layer

and two thermal conduction layers to help dissipate and conduct heat, respectively. A 2.25 cm2

square microfluidic device was embedded between an aluminum and a ceramic layer, and

compression-sealed using a PTFE sheet gasket (McMaster-Carr). A pyrex glass cylinder (OD: 14

m, ID: 11.6 mm, Wale Apparatus) was sandwiched between both conduction layers for liquid

holdup, and was sealed form both top and bottom using Paroflour (FFKM) O-rings (2-011/FF200

and 2-015/FF200, Parker). Starting materials were introduced through the top conduction layer,

and inert gas was injected through the bottom conduction layer, bubbled through the microfluidic

device into the bubble column. Liquid temperature was controlled with a PID temperature

controller, with an immersed Transition Joint thermocouple in liquid and a cartridge heater in the

bottom Aluminum layer.

Experimental Assembly: Standard PEEK and SS adaptors, PFA and SS tubing were used and

installed on both Aluminum layers of the manifold initially (IDEX Health and Science). As shown

in Figure 4a, after lubrication of O-rings (2, 4), they were placed in both top and bottom Aluminum

layers (1, 5). The glass tube (4) was diamond-filed and lubricated to be sealed by O-rings. The

microfluidic device (7) and laser-cut PTFE sheet gasket (6) were placed in the bottom Aluminum

layer (5) with through fluidic connections aligned. The quartz disc (8, McMaster-Carr) was placed

in the glass-mica ceramic layer (9, McMaster-Carr) and inverted to be attached to (5) by

compression sealing. After insertion of (3) into (4), (1) was pressed into (3). After addition of an

O-ring to (9), the pyrex petri-dish (10, vwr) was placed as a secondary container in case of

apparatus leakage. After the apparatus was tested and proven to be leak-tight, this peace was

removed to facilitate bottom-view imaging of the microfluidic device. Details of microfluidic

device and sealing designs with drawings of individual parts of the SL2G manifold are illustrated

in Appendices A. In addition, microfabrication of the device is discussed in Appendix B.

11

Figure 4. Schematic of detailed parts of SL2G manifold; (a) Exploded cross-sectional view with (1) Top conducting layer, (2) Top O-ring, (3) Pyrex Column, (4) Bottom O-ring, (5) Bottom Conducting layer, (6) PTFE sheet gasket, (7) Microfluidic device, (8) Quartz disc

12

2.2. Materials and Methods This section includes materials and their compositions, control approaches as well as

characterization methods for both precursor and NC syntheses.

Chemicals: All reagents were used as received without further purification. Te (99.8%), CdO

(99.99+%), tri-n-octylphosphine (TOP, 90%), tri-n-butylphosphine (TBP, 97%), Tri-n-

butylphosphine oxide (TBPO, 95%), oleic acid (HOA, 90%), oleylamine (OLA, 70%), 1-

octadecene (ODE, 90%) were purchased from Aldrich. Toluene-d8 (99.5%) was obtained from

Cambridge Isotope Laboratories Inc., and hexanes (ACS grade), chloroform (ACS grade), acetone

(ACS grade) and isopropanol (ACS Reagent grade, 99.5+%) were purchased from Caledon

Laboratory Chemicals. Ethanol (anhydrous, 100%) was purchased from Commercial Alcohols.

Precursor Compositions: Precursors were prepared using two protocols within SL (Protocol 1)

and SL2G (Protocol 2) platforms, followed by preparation of CdTe NCs in a segmented-flow

reactor, and SL2G, respectively. In Protocol 2, fixed molar concentrations from Protocol 1 were

used for reduced volume of 5 ml. Protocol 3 was prepared considering CdO:Te (2 mmol:4 mmol)

as compared with other literature.[27, 30, 50] This method was simplified to Protocol 4 in order to

study precursor coordination without interfering effects from organic ligands and solvents. Typical

values for all protocols are listed in Table 1and Table 2.

Table 1. Preparation composition of Cd-OA precursor solution for CdTe NC Synthesis

CdO ODE HOA Total

Protocol 1

[18, 20-22] Molar Concentration (M) 0.031 2.403 0.731

Volume (ml) 0.006 10.000 3.000 13.006

Protocol 2 Molar Concentration (M) 0.031 2.403 0.731

Volume (ml) 0.002 3.844 1.153 5.000

Protocol 3 Molar Concentration (M) 0.341 1.813 1.313

Volume (ml) 0.032 3.500 2.500 6.032

Protocol 4 Molar Concentration (M) 0.678 0.000 3.135

Volume (ml) 0.032 0.000 3.000 3.032

13

Table 2. Preparation composition of Te-TBP precursor solution for CdTe NC Synthesis

Te ODE TOP TBP OLA Total

Protocol 1

[18, 20-22]

Molar Concentration (M) 0.058 1.102 0.659 0.470 0.714

Volume (ml) 0.010 3.000 2.500 1.000 2.000 8.510

Protocol 2 Molar Concentration (M) 0.058 1.102 0.659 0.470 0.714

Volume (ml) 0.006 1.763 1.469 0.588 1.175 5.000

Protocol 3 Molar Concentration (M) 0.660 0.771 0.553 0.658 0.999

Volume (ml) 0.082 1.500 1.500 1.000 2.000 6.082

Protocol 4 Molar Concentration (M) 1.314 0.000 0.000 3.896 0.000

Volume (ml) 0.083 0.000 0.000 3.000 0.000 3.083

CdTe NC Synthesis via Segmented-Flow Reactor: Using Protocol 1, precursors were transferred

to glass syringes, and were run via two automated syringe pumps (high pressure OEM syringe

pump module, Harvard Apparatus). Syringes were attached to PFA tubing (1/16” ID, IDEX Health

and Sciences LLC.) and fluid was introduced to an Aluminum manifold. The design of the

microfluidic device and manifold was achieved by Dr. Milad Abolhasani during his PhD. studies

at Guenther laboratory. With an inherent separation of the cold and hot regions on the device, use

of hot injection was made possible through halo etches in the device. Therefore, the device was

not at a uniform temperature, preventing pre-mature nucleation and growth. Cd-OA and Te-TBP

precursors were introduced in the hot and cold regions, respectively, and Te-TBP was injected into

Cd-OA at high temperature. By introducing an inert gas in the T-junction of the device, the reaction

medium was divided into trains of liquid parcels separated by gas bubbles. Gas pressure was

controlled using a digital pressure regulator (Type 3410, full scale: 0-15 psig, accuracy of 0.5 %

full scale, Marsh Bellofram). Temperature of the reaction (hot) region was controlled using a PID

controller, with the help of a machined Aluminum heating jacket attached to the reactor via a high

temperature epoxy (OmegaBOND, Omega Engineering Inc.). The RTD temperature sensor was

inserted into the custom-made slit, and the cartridge heaters were inserted into through-holes of

the heating jacket. The reaction was monitored with a high-speed CMOS camera (pco 1200hs,

PCO), where image-based gas velocity was calculated and the product was characterized similar

to the SL2G platform. Residence times were calculated based on known reactor volume, liquid/gas

length ratio and gas velocity.

14

SL2G-prepared CdTe NC Experimental Conditions and Characterization: Based on Protocol 2

and 3, Te precursor was initially prepared, followed by Cd precursor. Te precursor was then

injected into Cd precursor at high temperatures and while bubbling. Gas pressure was controlled

similar to the segmented flow reactor with a range of 0.1-0.8 psig (see above). An apparatus was

dedicated to each operation in order to prevent contamination and crosstalk between precursors

causing undesired nucleation or growth. In order to characterize the system in real-time,

temperature measurements from the PID controller was interfaced with camera 1 to capture

images. Later, a MATLAB code was used for image-based intensity analysis, and temporal

behavior of both variables (T(t) and I(t)) is shown. In addition, after preparation of precursors,

samples of free ligands and solvents, precursors, and eventually NCs were characterized for

chemical integrity and quality. A variation of NMR (1H, 31P and 113Cd), MS, FTIR, UV-vis/PL,

TEM, and PXRD were utilized. The details of characterization methods will be discussed in this

section.

Temperature Control and Optimization: Using a Platinum PID temperature controller combined

with a solid-state relay (CN16DPT-304-C24, G3NA-205B, Omega), the diagram shown in Figure

5 was set-up for SL2G platform, and controller parameters were configured via the computer

interface. Later, similar configuration was used for SL, where the CSH cartridge heater was

replaced with a heat mantle (180W, Glas-Col). Both setups were initially autotuned for optimum

performance with different timeouts, followed by manipulation of P, I, D parameters and output

modes. A compromise between minimum overshoot percentage and rise time was selected for

further experiments.

Image-based Intensity Measurements and Analysis: With a possibility to image the apparatus

from two views using cameras 1 and 2 (see Figure 2a), a high-speed CMOS camera (pco 1200hs,

PCO) was used. An image of the microfluidic device is shown in Figure 2b in order to monitor

fluid behavior within the channels and ports of the device (view from camera 2). In order to

perform intensity measurement over the entire length of experiment, framerate of the camera was

reduced drastically to 0.94 fps (corresponding to maximum 1.5-hour experiment) and view from

camera 1 was used with the side-view of the bubble column as the region of interest. The apparatus

was enclosed within two perpendicular white foam slabs to prevent environmental intensity

artifacts and reflections from the glass cylinder. A white light illuminator (Doal 75, Microscan

Systems Inc.) was used either in-line with the camera lens or under the apparatus to illuminate the

15

solution in the column without any background shadows (see Figure 3b). Temperature and images

were recorded simultaneously throughout the experiment. The MATLAB code (see Appendix

A.3.) was used to measure grayscale intensity and plot it as a function of time. It needs to be

mentioned that the intensity measurement in this code was relative, and the value was normalized

with respect to the global maximum and minimum. Spikes in the data correspond to noise in the

images captured caused by inevitable change of background or reflected light from the cylinder.

Figure 5. Electrical diagram of temperature control system

16

NMR Sample Preparation and Characterization: In 5 mm standard NMR tubes, two methods for

preparation of air-free samples were used in order to substitute J Young NMR tubes. In both

systems, at least 3 degas/purge cycles were completed using a vacuum pump and flowing nitrogen

within a double-manifold Schlenk line. Method 1 is illustrated in Figure 6a, where a tightly-fitted

combination of Tygon 33, Nylon 6, PVC and PEEK tubes and fittings and a PEEK four-way valve

were used. It was ensured that all connections to the sample and toluene-d8 were through PEEK

and pneumatic air connections in order to minimize contamination or damage. The Tygon tube

was connected to the Schlenk line from one end, and through a pneumatic air tube, an adaptor and

PEEK tubing was connected to port #1. Port #2 was connected to PEEK tubing and a flexible PVC

tube to seal around the NMR tube. With a pressure relief/vent connection to port #3, all samples

were introduced through port #4. After initial degassing and purging of the setup, the NMR tube

were filled with 1 ml of toluene-d8, degassed and purged with nitrogen multiple times, followed

by adding test sample, mixing, degassing and purging multiple times. Method 2 (see Figure 6b)

included use of a Stainless Steel needle (6” long 18 G, Aldrich), NMR septum (Kimble™

Kontes™, Fischer) attached to the NMR tube and sealed using Parafilm tape (Aldrich). The Tygon

tube was connected to the Schlenk line, and the apparatus was degassed and purged with nitrogen

initially, with a vent needle with the same diameter. The samples were added in sequence under

nitrogen flow, degassed, and the process was repeated at least three times. TBP samples were

prepared in both systems to study their applicability based on the degree of oxidation as a result of

exposure to air. With 31P NMR, it was determined that Method 2 was more reproducible, with no

detectable source of air contamination (formation of TBPO), and therefore was selected for further

NMR studies. Confirmation of oxide formation was concluded through 31P NMR of commercial

TBPO as shown in Figure C. 1. As shown in Figure 6c, Method 1 exhibits a peak corresponding

to TBPO at 42.65 ppm, while Method 2 indicates both peaks associated with TBP. All NMR

characterizations were performed using Agilent DD2 600MHz and O500MHz spectrometers at

room temperature and in toluene-d8. 31P NMR spectra were referenced to trimethyl phosphine

(C3H9P), acquired at 242.81 and 202.35 MHz, with a relaxation delay of 0.1 s and 1H decoupling. 113Cd NMR was referenced to dimethyl cadmium (Cd(CH3)2) at 110.87 MHz, with relaxation delay

of 0.2 s, and 1H decoupling. 1H NMR was referenced to tetramethylsilane (Si(CH3)4) at 599.82

MHz, with relaxation delay of 1.0 s and 13C decoupling.

17

Figure 6. Comparison of NMR preparation setups. (a) Method 1 with ports to (1) SL, (2) NMR tube, (3) Vent, and (4) Sample injection. (b) Method 2 while being purged with nitrogen, (c) 31P NMR results of TBP samples prepared by (a) and (b), with magnified inset showing oxide peaks at 52.98, 42.65, 28.34 ppm for Method 1 ( ), and 52.67, 41.45, 27.72 pm for Methods 2 ( ).

18

MS Sample Preparation and Characterization: In order to remove undissolved Te solid Te-TBP,

crude sample was characterized in by centrifugation and filtration. For the former method, as-

prepared samples of Te-TBP precursor were collected in nitrogen-filled centrifuge tubes,

centrifuged at 6000 rpm for 5 min, and the supernatant was collected and transferred into nitrogen-

filled vials, sealed with silicone septa. In the latter, as-prepared sample was filtered through a 0.2

μm filter and transferred into a similar vial. Positive Direct Analysis in Real Time (DART)

Ionization was used in ambient environment at 250°C, where the compound mass was calculated.

With raw data of both methods shown in Appendix C, centrifuged sample was deemed less

contaminated.

Fourier Transform IR (FTIR) Sample Preparation and Characterization: A Perkin Elmer is50

ATR spectrometer was used equipped with KBr crystal, where crude samples were deposited and

analyzed within two sets of ranges between 550-4000 cm-1 and 100-1800 cm-1, with a number of

scans of 8. Lower range of samples was aimed for characterization of phosphorus groups, while

the higher range focused on organic groups in Cd-based samples. Additional spectrum for Te-

based samples is shown in Appendix C.

Spectroscopy Sample Preparation and Characterization: Using a Cary Eclipse and Varian 50 Bio

fluorescence and UV-vis spectrophotometers (Agilent Technologies), samples were dissolved in

n-hexane or toluene and studied for size estimation and quality, quantum confinement effect with

respect to different residence times and precursor preparation approaches.

TEM Sample Preparation and Characterization: CdTe samples were dissolved in equal volumes

of isopropanol and chloroform in centrifuge tubes (Falcon), mixed with a vortex mixer, and

centrifuged for 5 min at 6000 rpm. Supernatant was removed and the sample was re-dissolved in

the same mixture, and centrifuged at least twice more. The samples were prepared by drop casting

the nanoparticles in acetone and isopropanol solutions on a carbon/formvar coated 400-mesh TEM

copper grid. (Prod #: 01824, Ted Pella, USA). Microscopy analysis was carried out using the

Hitachi HF-3300 TEM/STEM/SEM at 300 kV.

XRD Sample Preparation and Characterization: 2.5 ml of CdTe samples was dissolved in 7.5 ml

of methanol, centrifuged for 2 × 5 min at 6000 rpm, after which the supernatant was drained, the

sample was re-dissolved in toluene and filled to 10 ml with methanol, and the process was repeated

7 times. The final sample was dried under nitrogen pressure. PXRD was performed using a Rigaku

19

MiniFlex 600 diffractometer, with a graphite monochromatized Cu-Kα radiation (λ=1.5406 Å).

Sample was loaded on zero-background Silicon single crystal, with a scanning interval of 10°< 2θ

< 80° and a scan speed of 0.4°/min (see Figure 12b). In addition, in order to study the effect of

temperature on crystal structure, after centrifugation of the sample for 5 times with above

conditions, 1 ml of the solution was inserted in a nitrogen-filled glass vial, while being heated to

250°C for 2 hours (see Figure 14).

20

Chapter 3

3. Results and Discussions Precursor Real-Time Analysis: Figure 7a shows time-dependent measurements of T(t) and I(t)

associated with Cd-OA precursor preparation in the SL2G platform during steps that resemble the

ones schematically shown in Figure 7b (bottom). The temperature was controlled using a PID

controller during each of the two steps in the heating process. Cooling was achieved using cold

water circulation. Bright field images of the purged precursor solution were continuously captured

using camera 1 (frame rate 0.94fps). During step (1a), i.e. for times 0

21

Figure 7. Time-dependent SL2G platform operation. (a) Measured time-dependent changes in solution temperature and visible light transmission during Cd-OA precursor preparation. (b) Bright field photographs (camera 1) of column after addition of CdO powder to HOA (top), and after formation of Cd-OA complex and cooling (bottom). (c) Photograph of SL2G platform operated within portable secondary containment. Scale bars: 10 mm.

22

Te-TBP Characterization: In order to examine coordination of Te precursor using SL2G, the

simplified protocol was used with Te being dissolved and bound to TBP. Protocols 2,3 and 4 were

used for this precursor as shown in “Materials and Methods”. Figure 8 illustrates 31P NMR and

MS results of Te-TBP precursor prepared at 280°C. Figure 8a compares 31P NMR (242.81 MHz,

toluene-d8, r.t.) resonances of TBP (top) and Te-TBP (bottom), with a downfield shift and line

width broadening of the resonance from -32.51 ppm to -16.24 ppm. This 16.27 ppm downfield

shift indicates the exchange between TBP and Te in close agreement with previously-reported

values.[27, 28, 30] The difference between aforementioned and reported values of -31 ppm and -

13.2 ppm by Anderson et al. and Liu et al. is attributed to characterization conditions, such as the

deuterated solvent.[27, 28] Therefore, our results confirm the coordination of the phosphorus atom

where the electron density decreases when Te bonds to phosphorus, leading to deshielding and

broadening of the resonance peak.[51] After centrifugation of Te-TBP precursor, Te-TBP was

studied using DART positive ion MS at 250°C, resulting in confirmation of Te pattern and

fundamental structural formula of C12H27PTe, with abundance of 130Te. In Figure 8b, MS results

validate the formation of the Te-TBP complex by detection of the tellurium intensity pattern, with

the most pronounced peak of 130Te at 333.1 m/z. In addition, bands have been observed at 219.2

m/z and 437.4 m/z (see Figure C. 2) that correspond to [TBPO+H]+ and [2TBPO+H]+ and are

attributed to the MS measurement being performed at high temperature and under non-inert

conditions. Also, results of Te-TBP MS from preparation by filtration are shown in Figure C. 4,

with assigned peaks for 203.2, 219.2. 333.1, 421.4, 437.4 m/z, 551.3 m/z corresponding to

[TBP+H]+, [TBPO+H]+, [Te-TBP+H]+, [TBP+TBPO+H]+, [2TBPO+H]+, [Te-TBP+TBP+H]+.

The calculated and found masses of 333.09908 and 333.09995 with 12C121H2831P1130Te chemical

formula is the same for both preparation methods confirming the chemical nature of the Te-TBP

sample. However, the higher number of peaks found in Figure C. 4 are associated to exposure to

air and filter material while processing the sample. Therefore, further analysis is pursued using

centrifugation of Te-TBP precursors.

To characterize the crude Te precursor for CdTe NC synthesis, Te-TBP/TOP/OLA/ODE was

prepared and characterized using DART MS at 400°C due to the added solutes and solvent. As

expected, various peaks were observed, which might exhibit interfering effects as well (see Figure

C. 5). The highest intensities were attributed to [TBP+H]+, [TOP+H]+ and [2TBPO+H]+. A

magnified region of the spectrum is shown confirming existence of Te-TBP complex, while other

complexes were not all characterized due to complexity of the sample components. Liquid

23

chromatography (LC) is suggested for future work to separate components and characterize

individual compound patterns.

In order to further verify coordination of the phosphine-based ligand to Te for preparation of CdTe

NCs, samples of TBP, TOP, a mixture of TBP/TOP/OLA/ODE and the prepared Te-

TBP/TOP/OLA/ODE (Te precursor for CdTe synthesis) were characterized using 31P NMR and

are shown in Figure 9. Dashed lines indicate defined peaks corresponding to the vicinity of free

ligands of TBP and TOP and bound Te-TBP and Te-TOP as well as TBPO/TOPO. The common

peak of phosphorus vicinity is dominant at -32.62 ppm for all free ligand mixtures (a, b, c). The

resonance peaks of TBP in (a) and TOP in (b) are at -18.88ppm and at -19.56ppm. These peaks

can be determined similarly in the mixture of TBP/TOP/OLA/ODE (c) with -18.83 ppm and -

19.56 ppm corresponding to TBP and TOP, respectively.

As can be seen in Figure 9, the resonances all shift downfield and broaden, and (1), (2) and (3)

indicate the shift of resonance peaks with respect to coordination of Te to phosphine sites in Te-

TBP/TOP/OLA/ODE mixture (d). With resonance peaks at -24.43 ppm, -7.74 ppm and -8.85 ppm,

sample (d) exhibits downfield shifts in resonance peaks of (1) 8.19 ppm, (2) 11.09 ppm and (3)

10.71 ppm, all of which are consistent in direction and magnitude. Therefore, coordination of Te

Figure 8. Characterization of Te-TBP precursor solution prepared in SL2G platform (Protocol 4). (a) 31P NMR (242.81 MHz, toluene-d8, δTBP, δTe-TBP, ppm): -32.51, -16.24. (b) MS of Te-TBP using positive polarity DART Ionization at 250°C.

24

to both TBP and TOP is concluded. Small peaks for oxidation are present within 52.66 ppm and

27.72 ppm for all samples and visible in the magnified inset, in agreement with previously-reported

values for TOPO and TBPO impurities.[27, 28]

Figure 9. 31P NMR of phospine-based ligands and Te precursor for CdTe NC synthesis (Protocol 3). (A) and commercial TBPO indicating partial oxidation of the precursor. (a) 31P NMR TBP (202.35 MHz, toluene-d8, δ, ppm): -19.56, -32.64, -37.74. (b) 31P NMR spectrum of TOP (202.35 MHz, toluene-d8, δ, ppm): 52.78, 41.39, 27.74, -18.88, -32.58, -37.70. (c) 31P NMR spectrum of TBP/TOP/OLA/ODE (202.35 MHz, toluene-d8, δ, ppm): -18.83, -19.56, -32.62. (d) 31P NMR spectrum of Te-TBP/TOP/OLA/ODE (202.35 MHz, toluene-d8, δ, ppm): -7.74, -8.85, -24.43.

25

Cd-OA Characterization: As mentioned in “Materials and Methods”, crude Cd-OA precursor for

CdTe NC synthesis included ODE as the non-coordinating solvent (Protocol 2 and 3). However,

similar to Te-TBP precursor, in order to study the direct behavior of complex formation and the

quality of the precursor, the simplified protocol was used with CdO being dissolved and bound to

HOA (Protocol 4). Figure 10 illustrates 1H and 113Cd NMR results of the crude Cd-OA precursor

prepared at 200°C, both of which demonstrate coordination of the acidic proton and the Cd2+ in

the precursor, indicative of complex formation and in agreement with previous literature.[28, 30,

31] According to 1H NMR (599.82 MHz, toluene-d8, r.t.) of HOA and Cd-OA solutions in Figure

10a, the resonance peak of the carboxylic acid proton (-COOH) is shifted upfield from 12.43 ppm

in HOA (top) to 11.36 ppm in Cd-OA (bottom), and the intensity of the resonance decreases by

49% due to near-stoichiometric consumption of two HOA molecules by one CdO.[28, 30] Since

excess amount of HOA was used in the reaction, the acidic proton is detected in the precursor as

well as the solvent (HOA) (2 mmol CdO: 16 mmol HOA). Excess acid was successfully removed

and the sample was characterized further (see Figure 11). Broadening of the peak line width

indicates coordination of the HOA to Cd2+ resulting in a total of 66.24 protons, which is within

0.36% of the Cd-OA chemical formula (C36H66CdO4). This value is consistent with the total

number of protons in HOA, which was determined to be 33.91 within 0.26% of the chemical

formula of HOA (C18H34O2). These values were derived from integration of the 1H NMR data with

toluene-d8 internal lock, and normalization with respect to the ligand vinyl resonances. The loss of

two protons corresponds to the reaction between CdO and HOA towards formation of water.

However, lack of a resonance attributed to water indicates successful removal of moisture using

SL2G. The small peak around 0.21 ppm corresponds to silicone impurity from the liner of the vial

caps, as shown in the magnified inset of Figure 10a.[31] As suggested by Figure 10b, 113Cd NMR

of Cd-OA in toluene-d8 indicates a resonance peak at -703.06 ppm, referenced at 0 ppm using

Cd(CH3)2. This result is in agreement with the previously-reported values with a difference of

references between Cd(CH3)2 and Cd(ClO4)2, relatively.[30, 50] García-Rodríguez and Liu

reported chemical shifts corresponding to coordination of carboxyl Cd2+ at -9.1 and -28.1 ppm in

absence and at -48.7 ppm in presence of excess HOA, with a reference at -641.5 ppm.[30, 50] In

addition, to compare our direct results of 113Cd NMR with abovementioned references, Cd(ClO4)2

was used as a second reference for 113Cd NMR, with the peak at 644.95 ppm (see Figure C. 7.),

leading to a 58.11 ppm chemical shift of Cd peak for Figure 10b, coherent with previous reports.

26

Using 1H NMR, it was confirmed that the excess amount of HOA was removed from Cd-OA

sample by dissolving in 5 ml of acetone, centrifuging at 6000 rpm for 5 min, repeating the

procedure for 6 times, and drying the white precipitate under nitrogen overnight. The sample was

then dissolved in 1 ml toluene-d8 and transferred to a NMR tube with the characterization shown

in Figure 11. The calculated number of protons in HOA and Cd-OA are 34.79 and 65.71, matching

empirical formulas of HOA (C18H34O2) and Cd-OA (C36H66CdO4), respectively. Evident in the

magnified region, there is no resonance corresponding to the acidic proton indicating successful

removal of HOA from the sample.

Figure 10. Characterization of Cd-OA precursor solution prepared in SL2G platform (Protocol 4) (a) HOA: 1H NMR (599.82 MHz, toluene-d8, ppm, δ): 12.43 (s, 1H; OH), 5.39, 2.00-2.08, 1.44-1.47, 1.13-1.36, 0.88-0.90; Cd-OA: 1H NMR (599.82 MHz, Toluene-d8, δ): 5.40-5.45, 1.53-1.55, 1.19-1.38, 0.89-0.92. Δ is attributed to silicone impurity from vial caps at 0.20-0.22 ppm, as shown in the magnified inset. (b) 113Cd NMR (110.87 MHz, toluene-d8, δCd-OA, ppm): -703.06, referenced to CdCl2 in D2O at 0 ppm.

27

CdTe NC Characterization: As mentioned in “Materials and Methods”, both available platforms

of the segmented-flow microreactor and SL2G were used in order to prepare CdTe NCs. The

segmented-flow microreactor was primarily utilized for preparation of CdTe NC cores, and CdSe

shells in a two-pass approach. Therefore, precursors were prepared in conventional SLs (Protocol

1), transferred to syringes and pumped into the reactor manifold. The relevant spectral results of

CdTe core synthesis in this platform are compared with the second platform, where SL2G was

used to prepare CdTe NCs in situ following precursor preparation. CdTe NC preparation was

carried out according to previously-reported procedures (Protocol 2).[24, 27, 30, 52] As mentioned

earlier, Cd precursor was prepared from dissolution of CdO (0.15 mmol) in a mixture of ODE (3.9

ml) and HOA (1.2 ml). Te precursor prepared from dissolution of Te (0.3 mmol) in a mixture of

ODE (1.8 ml), TBP (1.0 ml), TOP (1.5 ml) and OLA (1.2 ml). The Te precursor was injected into

the Cd-OA precursor at 220°C. NC nucleation and growth took place during periods of 2.0 min

(A) and 0.8 min (B), which was accompanied by a gradual increase in the visible light absorbance

by the solution: from optically-clear to black. The normalized UV-vis and PL (n-hexane) spectra

for both prepared NCs in situ from precursors in our SL2G platform were compared with CdTe

NCs from conventionally-prepared precursors at 200°C ((C), see Figure 12a). The detailed reaction

Figure 11. 1H NMR of HOA and Cd-OA precursor comparing removal of acidic proton (Protocol 4). 1H NMR HOA (599.82 MHz, toluene-d8, δ, ppm): 12.43 (s), 5.41, 2.06-2.09, 1.14-1.46, 0.90. 1H NMR Cd-OA (499.88 MHz, toluene-d8, δ, ppm): 5.47, 2.08, 1.29-1.56, 0.93.

28

conditions and results for NC synthesis are listed in Table 3. Identical Cd and Te precursors were

prepared at 240°C and 280°C in a SL, respectively. They were then fed into a silicon-based

microfluidic device at 70 µl min-1, for Cd-OA/ODE, and 5 µl min-1, for Te-TBP/TOP/OLA/ODE.

The mixture was heated to 200°C and segmented by nitrogen bubbles at 1.0 psi. The absorption

and emission wavelengths of CdTe NCs red-shift from 683 and 714 nm in (A) to 637 and 662 nm

in (B), and to 598 and 625 nm in (C), respectively. Accordingly, the average NC diameters were

predicted based on reported fitting functions as 5.3 nm in (A), 4.1 nm in (B) and 3.6 nm in (C).[53]

The inset in Figure 12a represents a high resolution TEM (HRTEM) image of CdTe NCs with an

average NC diameter of 5.3 nm. Using HRTEM images from condition (A), an averaged lattice

spacing of 0.25 nm was determined by ImageJ software, where grayscale intensity of image pixels

was plotted within a distance of 5 nm. The distances between each two consecutive peaks and

valleys were determined and averaged (see Figure 13).

As shown in Figure 12b, PXRD patterns ( ) of these NCs exhibit prominently wurtzite crystal

structure, with lattice constants of a=b=4.5911(11) Å and c=7.575(2) Å. The overlapped refined

pattern ( ) shows agreement of the observed and theoretical plots using Rietveld refinement

method with χ2 of 2.84, and the broadening and attenuation of (102) and (103) peaks are associated

with structural defects such as stacking faults.[14] Diffraction peaks are located at 22.44°, 23.57°,

25.38°, 32.73°, 39.31°, 42.47°, 45.69°, 46.31°, 47.33° and 52.00°. The corresponding diffraction

planes of bulk CdTe crystals with space group of P63mc are shown ( ), and agree well with

literature values of 22.34°, 23.47°, 25.28°, 32.63°, 39.21°, 42.37°, 45.59°, 46.21°, 47.23° and

51.90° according to the ICDD database (DB card number 01-079-3179). These values are in

agreement with CdTe NCs prepared for PXRD via annealing indicating wurtzite structure (see

Figure 14). PXRD pattern for this method exhibit wurtzite crystalline structure prominently with

lattice constants of a=b=4.5698(12) Å and c=7.516(3) Å. The overlapped refined pattern ( ) shows

agreement of the observed and ideal plots using Rietveld refinement method with χ2 of 2.28, and

similar stacking faults.

29

Table 3. Properties of CdTe NCs prepared at 200°C and 220°C

(A) (B) (C)

Precursor Preparation Protocol (see Table 1and Table 2) 2 3 1

Reaction Temperature (°C) 220 220 200

Nitrogen Gas Pressure (psi) 0.2 0.3 1.0

Residence Time (min) 2.0 0.8 1.6

Absorption Peak Wavelength (nm) 683 637 598

Emission Peak Wavelength (nm) 714 662 625

FWHM (nm) 46 46 44

Predicted NC size (nm) 5.3 4.1 3.6

Figure 12. Characterization of prepared CdTe NCs in microfluidic platforms (a) Normalized UV-vis (solid line; A,B,C) and PL spectra (dashed line; A’,B’,C’) of CdTe NCs, with Te- and Cd-precursors, and CdTe NCs at 220°C all prepared in SL2G platform (A, A’, B, B’); conventionally-prepared Te- and Cd-precursors and downstream microfluidic CdTe synthesis at 200°C (C,C’,( )). CdTe NCs prepared at 220°C have residence times of 0.8 ( ) and 2.0 min ( ). Inset: HRTEM image of CdTe NCs with an average diameter of 5.3 nm (A). (b) PXRD pattern of CdTe NCs prepared at 220°C ( ) with Retvield refinement ( ) indicating χ2 of 2.84. Bulk PXRD pattern of CdTe ( ) is shown with observed planes.

30

Figure 13. PXRD pattern of CdTe NCs prepared at 220°C ( ) with Retvield refinement ( ). Bulk PXRD pattern of CdTe ( ) is shown with observed planes.

Figure 14. Profile of lattice plane distances of 5.3 nm CdTe NCs synthesized at 220°C and 2.0 min residence time, with an average lattice spacing of 0.25 nm.

31

Precursor and NCs Thermal Behavior: Another set of experiments was designed to observe

temperature and intensity changes with subsequent reactions. In order to prevent unreacted solids

adhering to the column wall, material addition sequence was changed. It was ensured that after

addition of solids, the remainder of the liquid was added with unchanged molar concentration.

Results of a sequence of precursor and NC preparation are presented in Figure 15 with controller

output values and temperature changes. Each step of the reaction is observed in the system

response, and has been coordinated in the same fashion as shown in Figure 7a. Based on the step

response assumption of the control system, thermal behavior of the above system is shown in

Table 4. In addition, temperature profiles with intensity measurement shown in Figure 16 and

Figure 17 followed thermal behavior shown in Table 4d.

Based on Figure 15a, ODE was first injected, bubbled and heated to 130°C, followed by addition

of CdO and OA at t=13, 28 min, respectively, with observed temperature drops of 51 and 29°C.

Set point of the temperature controller was changed to 200°C at t=35 min, and the reaction was

terminated at 5=39 min with visual determination of steady-state clarity of the Cd-OA precursor.

The setup was then cooled using water circulation. The kink in the cooling region is due to

withdrawal of the sample using a syringe from the head space, rather than head pressurization.

Both Cd-OA and Te-TBP/TOP/OLA/ODE sample exhibited this complication, where in order to

preserve sample quality, it was quickly withdrawn and transferred to a nitrogen-filled vial. For Te-

TBP/TOP/OLA/ODE precursor shown in Figure 15b, after initial addition, mixing and heating of

ODE and TOP, Te was added at t=17 min, TBP and OLA were added at t=25 min. Temperature

drops of 76 and 18°C were observed, respectively, with a quick rise as soon as the controller was

engaged.Set point of the temperature controller was changed to 280°C at 30 min, and the reaction

was terminated at t=55 min with visual determination of steady-state clarity of the Te precursor.

Finally, for CdTe synthesis, 3ml of as-prepared Cd-OA precursor was bubbled and sequentially

heated to 130, 200 and 220°C, until addition of 3 ml of Te precursor at t=14.73 min. trxn was

determined starting with this point and running until t=15.53 min, resulting in 48 s (0.8 min)

residence time reported earlier in the text. Reaction was then terminated, cooled via water

circulation, and the sample was collected in an inert vial using head pressurization. The change in

cooling slope is regarded to the quick pumping of CdTe, resulting in nitrogen-filled glass cylinder

at t=18 min.

32

Figure 15. Temperature profiles of precursor and NC preparations. (a) Cd-OA at 200°C. (b) Te-TBP/TOP/OLA/ODE at 280°C. (c) CdTe at 220°C.

33

Table 4. Time-dependent control properties of precursor and CdTe NCs in SL2G and SL. (a) Cd-OA @ 200°C

(i) Heating from

Tmin to 130°C

(ii) Heating from

130°C to 200°C

(iii) Cooling from

200°C to Tmin

Time constant (min) 2.22 2.53 3.49

Total required time (min) 11.09 12.65 17.47

Percentage Overshoot (%) 7.58 -4.30 N/A

(b) Te-TBP-TOP-OLA-ODE @ 280°C

(i) Heating from

Tmin to 130°C

(ii) Heating from

130°C to 200°C

(iii) Cooling

from 200°C to

Tmin

Time constant (min) 1.98 4.67 3.67

Total required time (min) 9.92 23.36 18.33

Percentage Overshoot (%) 13.23 2.94 N/A

(c) CdTe @ 220°C

(i) Heating

from Tmin to

130°C

(ii) Heating

from 130°C

to 200°C

(iii) Heating

from 200°C

to 220°C

(iv) Cooling

from 200°C

to Tmin

Time constant (min) 1.81 1.40 0.99 3.17

Total required time (min) 9.03 6.99 4.94 15.87

Percentage Overshoot (%) 12.42 10.56 30.42 N/A

(d) Cd-OA in Conventional SL

(a) Heating from

Tmin to 130°C

(b) Heating from

130°C to 200°C

(c) Cooling from

200°C to Tmin

Time constant (min) 3.39 1.06 6.33

Total required time (min) 16.97 5.28 31.65

Percentage Overshoot (%) 65.18 -62.38 N/A

An apparent advantage of the SL2G over SL is the enhanced heat transfer and temperature control,

as seen by heating time requirements and overshoot (Table 4a and d). Temperature plots of Cd-

OA preparation are shown in Figure 16 and Figure 17 for optimized performance of SL and SL2G,

respectively.

34

Temperature Control:

For both conditions of SL and SL2G, an output mode of “ON-OFF” was chosen with a deadband

of 2 s for all conditions after other possibilities were ruled out. During controller calibration,

maximum overshoots of 1.3% and 3.4% were achieved for SL (P:4.3, I:0.1, D:76.1) and SL2G

(P:7.0, I:0.0, D:70.0) for Tsp of 300°C respectively. However, in operation of both systems at their

optimum conditions and sequential heating, overshoot was higher, with maximum observed

overshoots of 65.18% for Sl and 13.23% for SL2G. The overshoot percentage is within acceptable

range (

35

Intensity Measurements:

As can be seen in Figure 16, for the case of conventional SL Cd-OA preparation, intensity of the

liquid was measured as before, and for an interval of 0 to 18 min of the experiment, the intensity

curve was fitted with a sigmoidal function with least squares fit, as shown below, with 95%

confidence interval. The response behavior was categorized as first-order-plus-delay.

Ifit = 4.4615 × 10−2 +3.3894 × 10−1

1 + exp �− t − 8.33066.4377 × 10−1�

Figure 16. Process response behavior in Cd-OA preparation in SL. (a) Temperature and scaled intensity plots with respect to time evolution. (b) Magnified region of intensity plot in (a) for maximum intensity change corresponding to complex formation. Sigmoidal function fit indicates first-order-plug-delay behavior of the solution.

36

As can be seen in Figure 17, for the case of Cd-OA preparation in SL2G, intensity of the liquid

was measured as before, and for an interval of 20 to 38 min of the experiment, the intensity curve

was fitted with a sigmoidal function with least squares fit, as shown below, with 95% confidence

interval. Similar to SL, the response behavior was categorized as first-order-plus-delay.

Ifit = 4.8440 × 10−2 +6.6390 × 10−1

1 + exp �− t − 7.7409 × 101

1.007 × 101 �

Figure 17. Process response behavior in Cd-OA preparation in SL2G. (a) Temperature and scaled intensity plots with respect to time evolution. (b) Magnified region of intensity plot in (a) for maximum intensity change corresponding to complex formation. Sigmoidal function fit indicates first-order-plug-delay behavior of the solution

37

FTIR Results for Functional Groups in Precursors and NCs: To identify functional groups

involved in precursors and NCs, FTIR was performed, with a list of assigned peaks shown in Table

6, followed by stacked spectra shown in Figure 18.

Table 6. Peak list for Cd precursor and NC solutions Peak (cm-1) Stretch Sample

2500-3500 O-H (broad) [54] HOA

2921 Asymmetric CH2 [54] HOA, Cd-OA, CdTe, TBP, Te-TBP

2853 Symmetric CH2 [54] HOA, Cd-OA, CdTe, TBP, Te-TBP

1707-1716 C=O [55] HOA, Cd-OA, CdTe

1554-1569 Asymmetric COO- [29] CdTe, Cd-OA

1169 P-O [56] TBP, Te-TBP

459 P-Te [56] Te-TBP

Figure 18 compares FTIR bands of Cd NCs (i, vii), free ligands (iii, iv) and precursors (ii, v, vi).

In Figure 18a, the wavenumber range of 550-4000cm-1 was focused on to determine respective

organic functional groups pertaining to Cd-related samples. It was observed that both (ii) and (iii)

exhibit sharp peaks at 2853 cm-1 and 2921 cm-1, corresponding to symmetric and antisymmetric

CH2, respectively.[54] The broad peak between 2500 and 3500cm-1 corresponds to the O-H stretch

of HOA (iii).[54] CdTe (i), Cd-OA (ii) and HOA (iii) experience a peak at 1707cm-1, 1712 cm-1

and 1716 cm-1 associated with C=O stretch. In addition, samples (i) and (ii) both exhibit a broad

peak within 1554 cm-1 and 1569 cm-1 to symmetric COO- stretch in formation of oleate and NC.

This observation agrees with a study by Wu et al., indicating HOA is chemisorbed as a carboxylate

onto the CdTe NCs.[54] As shown in Figure C. 6, similar sharp peaks between all spectra at

2921cm-1 and 2851cm-1 correspond to asymmetric and symmetric CH2. Furthermore, P-O stretch

from TBP (A.iv) and Te-TBP (A.v) causes a relatively small but sharp peak at 1169cm-1,[55]

indicating deposited samples TBP and Te-TBP were oxidized leading to TBPO formation due to

exposure to the atmosphere during characterization. In order to study phosphorus coordination for

Te-related samples, the lower range of 100-1800 cm-1 was selected. Although the band at 459 cm-

1 is not sharp, it is attributed to the double bond of Te-P according to a similar value of 472 cm-1

reported by Zingaro et al..[56] This peak is less pronounced for more complex samples such as

(vi) and (vii), possibly due to typical interfering effects of other functional groups below 1000 cm-

1.

38

Figure 18. FTIR spectra of precursors and NC solutions. (a) Comparison of CdTe NCs (i), Cd-OA precursor (ii) and free HOA (iii), with enlarged area indicating coordination of O-H band to form carboxylate. (b) Comparison of free TBP (iv) and Te-TBP (v), Te-TBP/TOP/OLA/ODE (vi), and CdTe NCs (vii), with similar behavior except for possible band for P-Te coordination in vii.

39

Chapter 4

4. Conclusions and Outlook 4.1. Conclusions This work was inspired by colloidal synthesis of semiconductor NCs, and focused on preparation

of air- and moisture-sensitive precursor solutions for such NCs. In summary, starting with Chapter

1, semiconductor NCs, their synthesis methods and precursor solutions were introduced. Chapter

2 focused on the experimental aspect of the introduced platform, Schlenk Line-To-Go (SL2G),

materials and methods used for preparation and characterization of Cd, Te precursors and CdTe

NCs. Finally, results and discussions on the prepared materials and the capabilities of SL2G was

presented in Chapter 3. It was shown that SL2G allows the rapid and controllable preparation of

air- and moisture-sensitive precursor solutions as well as the in situ preparation of semiconductor

NCs. The SL2G platform is expected to be highly scalable as it requires 2 orders of magnitude less

footprint compared with a conventional SL: In a standard 8’ fume hood only up to three SL systems

can be installed and concurrently operated, compared with, based on our estimates, up to 600 SL2G

platforms. The SL2G platform provides for improved process control via in situ temperature and

turbidity measurements and reduces preparation times >1.5 fold for the fastest conventional

syntheses, and up to 15 fold for samples which require overnight preparation. This advantage is

due to the enhanced mixing and heat transfer behavior. We envision SL2G platforms to enable

highly scalable and fully automated approaches for the discovery and consistent preparation of

new precursor chemistries. Their seamless integration with flow chemical synthesis may help

overcome a current bottleneck in the broad adaptation of flow chemical approaches in preparing

colloidal nanostructures with high fidelity.

4.2. Outlook With our introduction of a miniaturized, fast and reproducible platform for air- and moisture-

sensitive materials, a great amount of potential is envisioned for versatile applications of SL2G.

These can include study of mechanistic behaviors of known chemistries, synthesis of new

materials, as well as automation and portable operation for scalable material synthesis. Some of

the recommended next steps are followed for future work using SL2G. A suggestion for operating

this platform under vacuum is application of negative relative pressure from inlet gas pressure to

40

the vent line, where a feedback control can be implemented. It is envisioned that based on an

image-based bubble velocimetry, gas velocity and gas pressure can be determined at the sparging

holes, followed by head space pressure readout. Dynamic behavior of gas flow considering

capillary forces and temperature-dependence need to be taken into account. In addition, another

suggestion for the system is to utilize the chip-based imaging possibility to determine bubble

frequency, and hence, gas velocity. Similar approach can be taken to both automate the system

and to operate at variable pressure levels. In order to confirm these suggestions, a series of pressure

and velocity control platforms are to be developed.

In order to characterize more complex precursors, separation processes such as Liquid

Chromatography are recommended, whereby free ligands and coordinated precursors can be

characterized without interfering results or the risk of degradation. In addition, intensity

measurements of the liquid column will benefit from calorimetric analyses in order to determine

reaction termination point based on the available library of desired product colors. Also, to better

define the control system, both temperature and intensity measurements are to be taken as second-

order systems, where damping parameters also play an important role. Therefore, by understanding

the solution intensity/color change based on temperature change, mass transfer through bubbling

and the required times, SL2G platform can be fully-automated.

41

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