On the possibility of using

QDs as energy acceptors

Marc Font Molins

Bologna, 15/07/09

Semiconductor nanocrystals (quantum dots, QDs)

VALENCE

BAND

h = Eg + Ee- + Eh+

CONDUCTION

BAND

e-

a0

EXCITON

a0 < rBohr

EgEg >>kT

E 1/r2

h+

QDs are nanometer-sized fragments (smaller than the exciton Bohr radius)

of the corresponding bulk materials and they can be composed of a few

hundred to a few thousand of atoms.

A brief insight about QDs as donors: FRET sensors

When the concentration of maltose is over a limit the colour change of the solution is visible to

the human eye.

In this work both dark quenchers and luminescent organic dyes has been used.

I. L. Medinz, A. R. Clapp, H. Mattoussi, E. R. Goldman, B. Fisher and J.M. Mauro. Self-assembled nanoscale biosensors based on quantum dot FRET donors Nat. Mat.., 2003, 2, 6300.

Basic research of QDs have been widely studied the last years and as a result of this

intensive work applications as transistors, diode lasers, amplifiers, diagnostic imaging were

developed.

Innovations in QD research: QDs as acceptors

A few years ago the idea of using QDs as energy acceptors began to appear in the

literature.

In principle QDs are supposed to be excellent energy acceptors owing to their high in a

wide spectral range.

At the moment comparisons between different experiments and between steady-state and

time-resolved results have not been possible.

Titrations have never been used to test QD as acceptors.

QD

donor

FRET

300 400 500 600 700 800

A

b

s

o

r

b

a

n

c

e

/

P

L

i

n

t

e

n

s

i

t

i

e

s

(

u

.

a

)

(nm)

CdSe core/shell system with [Ru(phen)3](PF6)2

[Ru(phen)3](PF6)2 was chosen to test the acceptor capacity of CdSe core/shell system

because of its long lifetime.

15800cm-1

ET

18000cm-1

1MLCT

3MLCT

1BG

h h

21000cm-1

[Ru(phen)3](PF6)2 CdSe/CdSZnS

N N

N

NN

N Ru

2

Fluorophore:

CdSe/CdZnS properties

5,0 5,5 6,0 6,5 7,0 7,5 8,0 8,5

0

5

10

15

20

25

30

35

n

D (nm)

xc=7.37 nm

* W. William Yu, Lianhua Qu, Wenzhuo Guo and Xiaogang Peng.Experimental Determination of the Extinction Coefficient of CdTe, CdSe, and CdS nanocrystals Chem. Mater.,

2003, 15 (14), 2854-2860.

D=5,54+(3x0,7)=7,64 nm=623 nm

Theoretically calculated with the empirical formula reported by *X. Peng

exc=463 nm

500 550 600 650 700 750 800

P

L

I

n

t

e

n

s

i

t

y

(

a

.

u

)

(nm)

Titration of [Ru(phen)3](PF6)2 with CdSe core/shell

(nm)

300 400 500 600 700 800

A

b

s

o

r

b

a

n

c

e

0.0

0.5

1.0

1.5

2.0

2.5

3.0

There are no new bands in absorption spectra.

Contributions of ruthenium complex and QD cannot be separated.

We must search another system in order to appreciate the behaviour of QDs in presence of a

fluorophore.

Synthesis of CdTe core

CdO+OA +1-ODE

Ar

Te powder + TBP + 1-ODE T=300C

NCs growth temperature=260C,

growth time aproximately 9 min

1 11

2

3

45 6

7

8

9

1 10

2

3

45 6

7

8

9

1 10

400 500 600 700 800

A

b

s

o

r

b

a

n

c

e

/

P

L

I

n

t

e

n

s

i

t

i

e

s

(

a

.

u

)

0

2

4

6

8

10

12

14

16

(nm)

(nm)

400 500 600 700 800 900

A

b

s

o

r

b

a

n

c

e

s

1 min

3 min

4,5 min

9 min

585 nm

590 nm

CdTe properties

4.5 5.0 5.5 6.0 6.5 7.0 7.5

0

5

10

15

20

25

n

D (nm)

xc==5,90 nm

38100

(M-1cm-1)

1.48E71.83E6600.11713690

Knrad(s-1)krad (s

-1)PL (ns)PL (nm)0-0 (nm)

D=5,56nm=690nm

300 400 500 600 700 800

A

b

s

o

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b

a

n

c

e

/

P

L

I

n

t

e

n

s

i

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i

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s

(

a

.

u

)

Solution to the overlap of emission spectra: CdTe core

ET

1MLCT

1BG

14200cm-1

3MLCT

h h

21000cm-1

18000cm-1

[Ru(phen)3](PF6)2 CdTe

N N

N

NN

N Ru

2

Fluorophore:

exc=436nm

500 600 700 800

P

L

I

n

t

e

n

s

i

t

y

(

a

.

u

)

(nm)

520 540 560 580 600 620 640 660

500 600 700 800

A

b

s

o

r

b

a

n

c

e

0.0

0.1

0.2

0.3

0.4

(nm)

corr=575nm

500 600 700 800

P

L

I

n

t

e

n

s

i

t

y

(

a

.

u

)

(nm)

exc=436nm

500 600 700 800

P

L

I

n

t

e

n

s

i

t

y

(

a

.

u

)

(nm)

540 560 580 600 620 640 660

The linear regime in the emission spectra is not fulfilled due to the high absorbances reachead during the

tritation. We must correct for the Fg and the reabsorbance.

Steady-state Titration [Ru(phen)3](PF6)2 with CdTe core

bAemFg

AexcIobsIcorr

*10

1**=

More corrections are needed to separate the contribution of [Ru(phen)3](PF6)2 and eliminate the contribution of

increasing concentrations.

= nmncexIreabsorba

nmobsI

obsIncexIreabsorbaIcorr 575.*

575.0)](PFRu(phen)[

0)](PFRu(phen)[

263

263

=

1

*QD

QDx

RutheniumC

CIIcorr

A. Credi, L. Prodi.From observed to corrected luminescence intensity of solution systems: an easy-to-apply correction method for standard spectrofluorimeters , Spectrochimica

Acta Part A, 1998, 54, 159-170.

em=600nm

0

50

100

150

200

250

300

350

400

0.0E+00 1.0E-07 2.0E-07 3.0E-07 4.0E-07 5.0E-07

[C dT e] (M )

1

(

n

s

)

em=600nm

0

50

100

150

200

250

300

350

400

0.0E+00 1.0E-07 2.0E-07 3.0E-07 4.0E-07 5.0E-07

[C dT e] (M )

em=600 nm

0

1

2

3

4

5

6

7

0.0E+00 1.0E-07 2.0E-07 3.0E-07 4.0E-07 5.0E-07

[C dT e] (M )

Time resolved titration [Ru(phen)3](PF6)2 with CdTe core

Decay-times at the ruthenium complex emission zone.

KQ

=

=600nm averagetime

y = 6.781E+05x + 1.014E+00

R2 = 9.614E-01

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0.0E+00 1.0E-07 2.0E-07 3.0E-07 4.0E-07

[CdTe] (M)

0

/

1

=600nm average time

y = 1.430E+06x + 9.664E-01

R2 = 9.907E-01

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0.E+00 1.E-07 2.E-07 3.E-07 4.E-07

[CdTe] (M)

0

/

1

KQ=1.87E+12 KQ=3,94E+12

Time resolved titration [Ru(phen)3](PF6)2 with CdTe core

em=715 nm

41

41

42

42

43

43

44

44

0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08 3.0E-08 3.5E-08

[CdTe] (M)

2

(

n

s

)

em=715 nm

0

5

10

15

0.0E+00 1.0E-08 2.0E-08 3.0E-08 4.0E-08

[CdTe] (M)

3

(

n

s

)

em=715 nm

0

50

100

150

200

250

300

350

400

0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08 3.0E-08 3.5E-08

[CdTe] (M)

a

v

e

r

a

g

e

t

i

m

e

(

n

s

)

Decay-times at the CdTe core emission zone.

em=715 nm

0

50

100

150

200

250

300

350

400

0.0E+00 5.0E-09 1.0E-08 1.5E-08 2.0E-08 2.5E-08 3.0E-08 3.5E-08

[CdTe] (M)

a

v

e

r

a

g

e

t

i

m

e

(

n

s

)

550 600 650

0

2

4

6

8

10

P

L

I

n

t

e

n

s

i

t

y

(

a

.

u

)

(nm)

em=436nm

Steady-state reverse titration: CdTe core with

[Ru(phen)3](PF6)2 Only exciting the QDExciting both QD and [Ru(phen)3 ](PF6)2

The emission spectra were corrected for the

dilution effects occurred in every addition,

multiplying each spectrum by the coefficient

Vx/Vi. Otherwise the spectra of the ruthenium

complex contribution were corrected also for the

increment of concentrations.

700 750 800 850

0

5

10

15

20

25

30

35

40

45

50

55

60

P

L

I

n

t

e

n

s

i

t

y

(

a

.

u

)

(nm)

exc=650nm

500 550 600 650 700 750 800 850

0

10

20

30

40

50

60

70

80

90

100

110

P

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I

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y

(

a

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l (nm)

exc=436nm

Contribution of the O2 effects in the CdTe core

[Ru(phen)3](PF6)2 system

Deareated measurements could give us some useful clues to find appropriate

mechanistic hypothesis or to refuse any of them.

Steady-state measurements:

The PL spectra were corrected for the Fg, the reabsorption, and normalized for the

concentration influence.

300 400 500 600 700 800 900

0

1

2

A

b

s

o

r

b

a

n

c

e

(nm)

exc=436nm

(nm)

500 600 700 800

P

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n

t

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s

i

t

y

(

a

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u

)

1

2

3

4

5

Front Face Titration

exc=436nm

(nm)

500 600 700 800

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(

a

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1

2

3

4

5

0.00 2.50x10-7

5.00x10-7

[CdTe] (M)

P

L

I

n

t

e

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s

i

t

y

(

a

.

u

.

)

RA

FF

FF spectra were corrected only for

the fraction of absorbed light

by the ruthenium

Contribution of the O2 effects in the CdTe core

[Ru(phen)3](PF6)2 system

Time-resolved measurements:

em=585nm

0

1000

2000

3000

4000

5000

0.0E+00 2.0E-07 4.0E-07 6.0E-07 8.0E-07

[CdTe] (M)

t

(

n

s

)

em=750nm

0

500

1000

1500

0.0E+00 2.0E-07 4.0E-07 6.0E-07 8.0E-07

[CdTe] (M)

t

(

p

s

)

Flash Photolysis measurements done confirmed the values of the deareated lifetimes measured

in single photon.

Mechanistic Hypothesis

The ruthenium complex may transfer the electron delocalized in its ligand, to the QD

that loses it due to the reduction of the oxygen.

h

3

O2 O2-

2

N N

N

NN

N Ru 3

O2 O2-

2

N N

N

NN

N Ru

h

Exciting both compounds: Only exciting the QD:

When the QD is irradiated the conduction and the valence band are formed, implying

a charge separation that provokes the transfer of an electron from the phenanthroline

to the QD.

h

h

(nm)

300 400 500 600 700 800

A

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(

a

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)

0

2

4

6

8

Solution for the low absorption coefficients and the nature

of the excited states: Zinc tetraphenylporphyrin (ZnTPP)

1S

1BG

h h

16400cm-1

14200cm-1

ET

ZnTPP CdTe

As previously seen, the molar extinction coefficient of [Ru(phen)3](PF6)2 is not large

enough to see clearly its supposed enhancement by QD effects.

Fluorophore:

450 500 550 600 650 700 750 800 850 900

exct=423nm

P

L

I

n

t

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s

i

t

y

(

a

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)

( nm)

Steady-state reverse titration: CdTe core with ZnTPPExciting both QD and [Ru(phen)3 ](PF6)2Only exciting the QD

550 600 650

P

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I

n

t

e

n

s

i

t

y

(

a

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(nm)

exc=423nm

700 750 800 850

exc=650nm

P

L

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n

t

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s

i

t

y

(

a

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)

(nm)

CdTe CdSe

Assays using close systems: QD+QD.

300 400 500 600 700 800

0

2

4

6

8

10

A

b

s

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b

a

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c

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/

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L

I

n

t

e

n

s

i

t

i

e

s

(

a

.

u

)

(nm)

1BG

1BG

h h

16800cm-1

14200cm-1

ET

CdSe CdTe

Steady-state reverse titration: CdTe core with CdSe core.

700 750 800 850

P

L

I

n

t

e

n

s

i

t

y

(

a

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)

(nm)

Blank exc

=650 nm

700 750 800 850

P

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(

a

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)

(nm)

Titration exc

=650 nm

700 750 800 850

P

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s

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y

(

a

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u

)

(nm)

exc

=659nm

600 660 720 780 840 900

P

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y

(

a

.

u

)

(nm)

exc

=575 nm

Conclusions

Ru-CdTe system has been interpreted in terms of an

electron reservoir mechanism.

Ru-CdTe and ZnTPP-CdTe apparently do not show

energy transfer, most likely because electron transfer is

the preferential process.

QDs can be use as energy acceptors, as demonstrated

in the CdSe-CdTe tritation.

Conclusions

An ideal donor should posses:

High quantum yield

Large extinction coefficient in a zone where the QD has a low

one

Large overlap of the emission sprectrum with the absorption

spectrum of the QD

Equal multiplicity of spin with the acceptor one

Good separation between its PL spectrum and the QD one

Future studies involving QDs as energy acceptors should

consider the following:

Methods:

Front Face tritations

Oxygen-free tritations

Steady-state and time-resolved measurements

Sphere of action corrections on quenching constants

Outlooks

1 Synthesizing CdTe/ZnS core/shell systems with different number

of layers

2 Preparing a CdSe-CdTe coupled system and changing the length

of the spacer

Discuss how distance affects the non radiative energy transfer,

which is achievable by:

Discuss how the QD bandgap modify the proposed electron transfer

mechanism, by:

1 Using different CdTe core dimensions

2 Coupling the Ru with QDs composed of different materials

Work in progress

Synthesis of CdTe/ZnS core/shell

5,5 6,0 6,5 7,0 7,5 8,0 8,5 9,0

0

5

10

15

20

25

30

35

40

n

D (nm)

Xc=7,06nm

Theoretical:

=670nm

D=4.85+(0.7x3)=6.95 nm

CdTe core solution in hexanes+TOP+TOPO +TBP+ODE

Zn(Et)2+TMS+TBP injection

solution. 3 injections (20 min).

Temperature of the additions=160C

1 11

2

3

45 6

7

8

9

1 10

2

3

45 6

7

8

9

1 10

1) vacuum

2) Ar