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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
R
e
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h
he εΠ−⎟⎟⎠
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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
30000
40000
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
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
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
0.0
0.2
0.4
0.6
0.8-20 0 20 40 60 80 100
0.0
0.2
0.4
0.6
0.8
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
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Emission Intensity (arb. units)
T I M E (ns)
77K 296.1K
0 5 10 15 20
0.0
0.1
0.2
0.3
0.4
0.5
0.6
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
0.38
0.40
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
0.5
0.6
0.7
0.8
0.9
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
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
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
0.30
0.35
0.40
0.45
0.50
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
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