nanoparticles, nanocrystals, and quantum dotspeter/334a/nanomaterials1.pdf · dabbousi, b.o. et al....
TRANSCRIPT
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Nanoparticles,nanocrystals, and
quantum dotsWhat they are, why
they’re interesting, andwhat we can do with them
J. Nadeau, Department of Biomedical Engineering
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Colloidal nanocrystals of different materialsColloidal nanocrystals of different materials……
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…And differentgeometries
From: Science. 2005 January 28;
307(5709): 538 544.
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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!
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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.
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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
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Synthesis
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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”
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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”
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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!
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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
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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
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AFM image of a cluster of CdSe nanocrystals
(3.3 nm). Image size 70nm x70 nm
Characterization
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HR TEM shows latticestructure
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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
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Interesting physics!
• Trap states
• Stokes shift
• Stark Effect
• Blinking
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The importance of surface statesMore than half the atoms are
at the surface
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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
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What causes the Stokesshift?
•Exciton fine structure
•Independent of surface
Norris and Bawendi, JOURNAL OF CHEMICAL PHYSICS 103 (13): 5260-5268 OCT 1 1995
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Blinking
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“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
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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)
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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
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Uses of Stark Effect
Becker et al., Nature Materials 5, 777 - 781 (2006)
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Interestingapplications!
• Biological labels
• Single-particle tracking
• Biosensors
• Memory
• Solar cells
• Etc…
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Copyright ©2006 Society for Neuroscience
Pathak, S. et al. J. Neurosci. 2006;26:1893-1895
Biological labeling: neurons and glia
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Single-particle tracking
From: Science. 2005 January 28;
307(5709): 538 544.
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QDs as biosensors
Doxorubicin (adriamycin)
Dopamine
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QD-dopamine as aredox sensor
CB
h
VB
hO
R
O, R
Energy
Dopamine is an excellent electron donor
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Normal conditions
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Reducing conditions
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Uptake into cells
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With antioxidants
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Redox dependence
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More oxidizing…
Addition of theglutathionesynthesis inhibitorBSO (10 mM)affects theintracellular redoxpotential withoutaltering that ofthe medium
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Or more reducing
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Photoenhancement
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Quantum dot memory
APPLIED PHYSICS LETTERS 86 (19): Art. No. 193106 MAY 9 2005
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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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1. Aldana, J., Wang, Y.A. & Peng, X.G. Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols. Journal of the
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
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To come
• Toxicity
• Stability and alternativecoatings
• Metal particles
• Insulator particles