Nanoparticles,nanocrystals, and

quantum dotsWhat they are, why

they’re interesting, andwhat we can do with them

J. Nadeau, Department of Biomedical Engineering

jay.nadeau@mcgill.ca

Colloidal nanocrystals of different materialsColloidal nanocrystals of different materials……

…And differentgeometries

From: Science. 2005 January 28;

307(5709): 538 544.

The colors in some stained-

glass windows from

medieval cathedrals are

probably due to nanocrystals

of compouds of Zn, Cd, S,

and Se.

Medieval Nanotechnology!

History of nanoparticles

1980 Ekimov observed quantum confinement on a sample of glass

containing PbS.

1982 Brus L.’s group conducted CdS colloid preparation and investigation

of band-edge luminescence properties.

1993 Murray C., Norris D., Bawendi M., Synthesis and Characterization of

Nearly Monodisperse CdE (E=S, Se, Te) Semiconductor Nano-

crystallites.

1995 Hines M., Guyot-Sionnest P., reported synthesis and Characterization

of Strongly Luminescent ZnS-Capped CdSe Nanocrystals

1998 Alivisatos and Nie independently reported Bio-application for core

shell dots.

2001 Nie’s group described Quantum dot-tagged microbeads for

multiplexed optical coding of biomolecules.

2003 T. Sargent at UOT observed electroluminescence spanning 1000 – 1600

nm originating from PbS nanocrystals embedded in a polymer matrix.

What is a quantum dot?

• Synthesis

• Quantum mechanics

• Optical properties

What is it good for?What is it good for?

••Interesting physicsInteresting physics

••Applications in optoelectronicsApplications in optoelectronics

••Applications in biologyApplications in biology

Synthesis

Quick review ofsemiconductors

• A semiconductor has a forbiddenzone or “band gap” between theconduction and valence band

• When an electron is excited into theconduction band, there is a hole leftin the valence band; this pair is an“exciton pair”

• When the size of the crystal iscomparable to the exciton Bohrradius, the confinement energybecomes signficant… at this pointwe have a “quantum dot”

Quantum mechanics of QDs

Ee =

h2nl2

2me*

+ Egap( )

Eh =–h2

nl2

2mh*

ECBEVB

=mh

*

me*

= 3.2 (wurzite )

Bulk CdSe Q dot

Energy

0

h+

e-

Because of these quantized energy

levels, QDs are more like atoms than

like bulk materials--earning them the

name “artificial atoms”

This is anoversimplification…

• “Box” wells are not infinite

• Particles aren’t spherical

• Boundary conditions must beconsidered

• We assume only a singleelectron

• However--the approximation issurprisingly good!

Temporal evolution of

CdSe nanocrystals

-0.02

0.18

0.38

300 350 400 450 500 550 600 650 700WL/nm

A

2.3 nm (5 s)

2.6 nm (20 s)

3.0 nm ( 1 min)

3.3 nm (1.5 min)

3.6 nm (2 min)

4.2 nm (30min,rt)

Size-dependent spectra

EmissionCdSe nanocrystals

0

50

100

150

200

250

450 500 550 600 650

Wavelength (nm)

Inten

sit

y2.7 nm 3.0 nm 3.2 nm 3.6 nm

AFM image of a cluster of CdSe nanocrystals

(3.3 nm). Image size 70nm x70 nm

Characterization

HR TEM shows latticestructure

So what is it good for?

CdSe, CdS,

ZnS,CdTe,

etc

•Emission wavelength is related to the size

of the crystal

•Slow to photobleach and radiation

resistant

•Emission can be quenched/modulated by

attaching electron donors or acceptors to

the surface

•Can be suspended in aqueous and non-

aqueous environments

•Many colors obtained with a single UV

excitation source

•Surface can be conjugated to chemically

and biologically important molecules450 500 550 600 650 700

0

1

Norm

aliz

ed inte

nsitie

s

(nm)

Absorption Emission

3 to 10 nm

Interesting physics!

• Trap states

• Stokes shift

• Stark Effect

• Blinking

The importance of surface statesMore than half the atoms are

at the surface

How to probe surface states

Electron and hole

acceptors quench PL

==> PL results from

exciton recombination

Transient absorption spectroscopy

Burda et al, J. Phys. Chem. B, 105 (49),

12286 -12292, 2001

What causes the Stokesshift?

•Exciton fine structure

•Independent of surface

Norris and Bawendi, JOURNAL OF CHEMICAL PHYSICS 103 (13): 5260-5268 OCT 1 1995

Blinking

“On” and “off” states

•Many groups have found that “off” states follow a power

law

•“On” times more controversial; perhaps power law,

perhaps power law convoluted with exponential

Two Models

• Fluctuating distribution of electron traps in theimmediate vicinity of, but external to, the QD.Tunneling of the electron out of the QD resultsin a charged particle, quenching emission(Kuno et al. 2003, Phys. Rev. B 67, 125304).

• Internal hole traps, presumably at surfacestates or crystal imperfection sites. Energeticdiffusion of the electronic states results in atime-dependent resonance condition in whichAuger-assisted trapping of the hole results inan off state (Frantsuzov and Marcus 2005, Phys.Rev. B 72, 155321)

Stark Effect

• Shift in energy with electric field

• Permanent dipole moment:dependence as E

• Polarizability: as E2

• QDs show both aspects, but Edependence is only seen in single-dot studies (not ensembles)

Empedocles and Bawendi, Science 19 December 1997: Vol. 278. no. 5346, p 2114

Uses of Stark Effect

Becker et al., Nature Materials 5, 777 - 781 (2006)

Interestingapplications!

• Biological labels

• Single-particle tracking

• Biosensors

• Memory

• Solar cells

• Etc…

Copyright ©2006 Society for Neuroscience

Pathak, S. et al. J. Neurosci. 2006;26:1893-1895

Biological labeling: neurons and glia

Single-particle tracking

From: Science. 2005 January 28;

307(5709): 538 544.

QDs as biosensors

Doxorubicin (adriamycin)

Dopamine

QD-dopamine as aredox sensor

CB

h

VB

hO

R

O, R

Energy

Dopamine is an excellent electron donor

Normal conditions

Reducing conditions

Uptake into cells

With antioxidants

Redox dependence

More oxidizing…

Addition of theglutathionesynthesis inhibitorBSO (10 mM)affects theintracellular redoxpotential withoutaltering that ofthe medium

Or more reducing

Photoenhancement

Quantum dot memory

APPLIED PHYSICS LETTERS 86 (19): Art. No. 193106 MAY 9 2005

Summary

• QDs allow us to observe atomicphysics at the almost macroscopicscale

• However, there are alwayscomplications due to surface states,solvent interactions, etc that makethem more than a particle in a box

• A lot has been done, but a lot moreremains to be done before weunderstand these particles and canuse them in complex media

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American Chemical Society 123, 8844-8850 (2001).

2. Burda, C., Green, T.C., Link, S. & El-Sayed, M.A. Electron shuttling across the interface of CdSe nanoparticles

monitored by femtosecond laser spectroscopy. Journal of Physical Chemistry B 103, 1783-1788 (1999).

3. Chan, W.C. & Nie, S. Quantum dot bioconjugates for ultrasensitive nonisotopic detection. Science 281, 2016-2018.

(1998).

4. Cho, S.J. et al. Long-term exposure to CdTe quantum dots causes functional impairments in live cells. Langmuir 23,

1974-1980 (2007).

5. Derfus, A.M., Chan, W.C.W. & Bhatia, S.N. Probing the cytotoxicity of semiconductor quantum dots. Nano Letters 4,

11-18 (2004).

6. Empedocles, S.A. & Bawendi, M.G. Quantum-confined stark effect in single CdSe nanocrystallite quantum dots.

Science 278, 2114-2117. (1997).

7. Empedocles, S.A., Norris, D.J. & Bawendi, M.G. Photoluminescence Spectroscopy of Single CdSe Nanocrystallite

Quantum Dots. Physical Review Letters 77, 3873-3876. (1996).

8. Hagfeldt, A. & Gratzel, M. Light-Induced Redox Reactions in Nanocrystalline Systems. Chemical Reviews 95, 49-68

(1995).

9. Haram, S.K., Quinn, B.M. & Bard, A.J. Electrochemistry of CdS nanoparticles: A correlation between optical and

electrochemical band gaps. Journal of the American Chemical Society 123, 8860-8861 (2001).

10. Bruchez, M., Jr., Moronne, M., Gin, P., Weiss, S. & Alivisatos, A.P. Semiconductor nanocrystals as fluorescent

biological labels. Science 281, 2013-2016 (1998).

11. Klimov, V.I. et al. Optical gain and stimulated emission in nanocrystal quantum dots. Science 290, 314-317. (2000).

12. Murray, C.B., Norris, D.J. & Bawendi, M.G. Synthesis and Characterization of Nearly Monodisperse Cde (E = S, Se,

Te) Semiconductor Nanocrystallites. Journal of the American Chemical Society 115, 8706-8715 (1993).

13. Dabbousi, B.O. et al. (CdSe)ZnS core-shell quantum dots: Synthesis and characterization of a size series of highly

luminescent nanocrystallites. Journal of Physical Chemistry B 101, 9463-9475 (1997).

14. Leatherdale, C.A. & Bawendi, M.G. Observation of solvatochromism in CdSe colloidal quantum dots. Physical Review

B 6316, art. no.-165315 (2001).

15. Nirmal, M. et al. Observation of the Dark Exciton in Cdse Quantum Dots. Physical Review Letters 75, 3728-3731

(1995).

16. Shimizu, K.T. et al. Blinking statistics in single semiconductor nanocrystal quantum dots. Physical Review B 63,

205316 (2001).

17. Kuno, M., Fromm, D.P., Hammann, H.F., Gallagher, A. & Nesbitt, D.J. Nonexponential "blinking" kinetics of single

CdSe quantum dots: A universal power law behavior. Journal of Chemical Physics 112, 3117-3120 (2000).

Les incontournables

To come

• Toxicity

• Stability and alternativecoatings

• Metal particles

• Insulator particles