Photophysical Properties of CdSe/ZnS Quantum Dots Embedded in Polymer Films and Solubilized in

Toluene

Final PresentationJamie Golden

CHEM 49604/30/10

Introduction to Quantum Dots• QDs are semiconductor particles; size = 1-99nm

• Photophysical properties (absorption & emission spectra) controlled by size and shape

• Ex = Atkin’s P. Chem Book p.(307)

• Use in applications such as LEDs, flat screen monitors, and solar cells.– Must be suspended in solid (polymer) matrix for

applications to be materialized

• Interaction between QDs and polymer matrix is of interest to investigate

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Selection of polymers to be used

• PMMA polymer of choice– Optically transparent– Water resistant– Chemically stable

• Amorphous thermoplastic– Convenient rheological properties– High strength-weight ratio

Previous Studies• Previous studies involving CdSe/ZnS quantum dots involved the use

of a thermal lens to measure the quantum yield of QDs in PMMA suspended in three different solvents– toluene, tetrahydrofuran, and chloroform.

• Pilla et al found that the quantum yield of the QDs exhibiting fluorescence ranged from 0.60-0.85 at room temperature when suspended in organic solvents.4 – were not able to explain the quenching mechanisms involved of the

quantum yield in function of increasing concentration because it was not completely understood.

– Proposed behavior could be due to the formation of a cluster or due to particle agglomerations.4

• In another study involving the same QDs and polymer film (PMMA) by Tamborra et al, optical and physical properties of nanocomposties were investigated. – It was evident from fluorescence microscopy images that there is a

presence of larger aggregates in CdSe/ZnS in PMMA than for CdS.5 4 Pilla V, Alves LP, Munin E, Pacheco MTT. Radiative quantum efficiency of CdSe/ZnS quantum dots suspended in different solvents. Opt. Comm 2007; 280: 225-229.5 Tamborra M, Striccoli M, Curri ML, Agostiano A. Hybrid Nanocomposites Based on Luminescent Colloidal Nanocrystals in Poly(methyl methracrylate): Spectroscopical and Morphological Studies. J Nanoscience and Nanotechnology 2008; 8: 628-634.

Experimental

• QDs from Evident Technology; no further modification

• Temperature resolved laser photolysis setup using 3ns laser pulse

• Polymer film placed in quartz dewar to cool to 77 K using liquid nitrogen

• Fluoresceien used as a standard in order to calculate quantum yield of QDs in toluene

• Used simple formula: QY1/QY2 = I1/I2

Emission Spectra

480 500 520 540 560 580 600

0

10000

20000

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50000

Luminescence spectraOcean Optics Spec337 nm laser excitation12/1/07

W A V E L E N G T H (nm)

QD520 Toluene (RT) QD520 Toluene (77K) QD520/PMMA (RT) QD520/PMMA (77K)

Emission spectra of QDs

solubilized in toluene and

embedded in PMMA at RT and 77 K

No significant change in toluene

and PMMA film

Indicates no change in QD

size in film process

Slight change may be due to

close proximity

Luminescence Quantum Yield of Quantum Dots Suspended in Toluene Solution

Fluorescein used as a standard QY = 0.92

Absorption and Emission Spectra of Fluorescein and QDs in toluene

excitation wavelength chosen to ensure identical spatial distribution of excited molecules in cell

Relative QY ratio = areas under emission spectra ratio

QY1/QY2 = (Area)1/(Area)2

QY of QDs in toluene = 0.52

λx

450 500 550 600 6500.00

0.02

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Luminescence Intensity (arb. units)

Absorbance

W A V E L E N G T H (nm)

Fluorescein (Absorption) QDs in Toluene (Absorption) QDs in Toluene (Emission) Fluorescein (Emission)

Quantum Yield of QDs in PMMA Film

-20 0 20 40 60 80 100

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0.8-20 0 20 40 60 80 100

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Quantum Yield Measurement

AreaQD in Toluene

= 12.01752AreaQD in PMMA = 5.67362

QYQD in Toluene = .53QY

QD in PMMA = .25

T I M E (ns)

QD520 in Toluene (RT) QD520 in PMMA (RT)

Laser photolysis used in order to avoid technical difficulties to determine QY of QDs in PMMA film

Compared areas under emission decay curves of QDs in toluene and embedded in polymer film

QY1/QY2 = (Area)1/(Area)2

QY of QDs in PMMA = 0.25 which is only 46% of that measured in toluene

Fig. Emission decays of QDs in toluene and embedded in PMMA at RT

Emission Decays of QDs in PMMA Film at RT and 77 KInsert: Laser Pulse Profile

0 20 40 60 80

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Emission Intensity (arb. units)

T I M E (ns)

77K 296.1K

0 5 10 15 20

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0.7 Laser photolysis experimentQD 520 in PS filmRoom Temperature2/1/2010

Emission Intensity (arb. units)

T I M E (ns)

QD Luminescence Laser pulse profile ExpDec1 fit 2 t

1/2 = 1ns

ExpDec1 fit 1 t1/2

= 7.5 ns

•Normalized RT quantum yield of QDs in PMMA film. From this value, can get QY at any temp.

•Like to explore temp dependence on QY because electron transfer quenching is expected to slow on lowering the temp

Fig showing 2 normalized emission decays at RT and 77 K of QDs in PPMA film. Insert : laser pulse profile and QD luminescence

Laser Pulse Profile and QDs Luminescence

• Insert is laser pulse profile and QDs luminescence – Wanted to determine if laser pulse is short enough

and detection system fast enough to accurately monitor QD’s luminescence decays

– Decay part of laser pulse profile was compared to actual decay of QD luminescence

– Found decay part of QD luminescence had a half-lifetime ~ 7.5ns

– Found decay part of laser pulse profile had half-lifetime ~ 1ns

– This proves that the laser photolysis apparatus is fast enough to accurately measure the decays

Quantum Yield of QDs in PMMA Film

• QY at 77K is still less than that of toluene at RT

• Why is that?

50 100 150 200 250 300

0.28

0.30

0.32

0.34

0.36

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Absolute Quantum Yield (arb. units)

T E M P E R A T U R E (K)

QDs in PMMA film

PMMA and PP Structures

H2C C

CH3

C

OO

CH3

n

CH2C

H

CH3

n

PMMA

PP

Temperature Dependence of QY Temperature resolved laser photolysis technique

Relative QY of QDs in PMMA film and relative QY of QDs in PP film as a function of temperature

Found as temperature decreases, the QY increases continuously

50 100 150 200 250 300

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1.0

QD520 in PMMA (1st run) QD520 in PMMA (2nd run) QD520 in PP Area (2nd run) QD520 in PP Area (1st run)

T E M P E R A T U R E (K)

Summary of PMMA & PP Study• Found that both PMMA and PP matrices reduce

the QY of QDs to the same extent• Conclusions about decrease in QY (from QDs in

toluene– Neither energy or electron transfer observed– PP is an inert polymer matrix where energy or

electron transfer cannot happen• Decided to see QDs go through liquid to solid

phase (Freeze toluene)• Compare using another polymer to further

investigate interaction; using polyestyrene (PS)

Introduction to PS• Amorphous polystyrene

– Similar chemical composition as toluene– Soluble in toluene

• Interest to study– Effects of a liquid to solid phase transition (QDs

suspended in toluene)

Polystyrene: Toluene:

Quantum Yield of QDs in PS Film• Fluorescein used as a standard QY = 0.92• Experimental parameters kept same• Relative QY ratio = areas under emission spectra

ratio• QY of QDs in toluene = 0.52• Laser photolysis used in order to avoid technical

difficulties to determine QY of QDs in PS• Ratio of Area under emission decay curves =

Ratio of QY (QDs in PS and QDs in toluene)– QY of QDs in PS film equals 0.27

Emission Decays of QDs in Toluene and Embedded in PS at RT Insert: Excitation Wavelength

0 20 40 60 80

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Emission Intensity (arb. units)

T I M E (ns)

RT (toluene) 298.5 K; PS film

300 350 400 450 500 550 600

0.0

0.2

Excitation Wavelength = 472.7nm

A B S O R B A N C E

W A V E L E N G T H (nm)

Fluorescein (2/15/10) QDs in Toluene (2/15/10)

Results/Discussion

• To further gain insight in this difference, QY’s of QDs in PS/Toluene solution measured as PS concentration increased

• Found:– No change in emission decays for QDs in toluene

upon adding 10% PS addition to solution– Continuous decrease of QY as the weight by

weight percent of PS increased

QY as a Function of PS Concentration

0 20 40 60 80 1000.25

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0.35

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0.55

0.60 QD520 in Polystyrene/toluene solutionLaser photolysis dataQuantum yield was calculated bycomparing the emission decaysof the solution used in fluorimetryand PS liquid or solid solutions.

Quantum Yield

PS Concentration (w/w%)

Results/Discussion• Study liquid to solid phase transitions using

emission decays of QDs in PS at 77 K and 298.1 K• Comparing QY’s for QDs in Toluene and QDs in PS

film as a function of temperature• QY increases continuously as the temperature

decreases for QDs in PS film• DSC analysis did not show phase transition• Different behavior observed for QDs in Toluene:

– QY constant, then decreases abruptly as temperature increases at approximately 250 K

– QY increases as temperature decreases at ~ 200 K– Note melting point of toluene is -93C (180 K)

Temperature Dependence of QY for QDs in Toluene and QDs in PMMA

50 100 150 200 250 3000.25

0.30

0.35

0.40

0.45

0.50

0.55

Quantum Yield

Temperature (K)

QD's in toluene QD's in PS

Melting Point: 180 K

Conclusions

• Quantum yield of quantum dots in polymer films is lower than that of quantum dots in toluene (by about half)

• Faster emission decay for QDs in polymer film at RT than 77 K

• Study liquid to solid phase transitions QDs in polymer film from RT to 77 K– QY of QDs in Polymer film increases as temperature

decreases– QY of QDs in Toluene decreases as temperature increases

and around melting point, QY begins to increase as temperature decreases

• DSC analysis did not show phase transition