Nanoparticles, Nanocrystals, and Quantum Dots What They Are, Why They’Re Interesting, and What We Can Do with Them J

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Nanoparticles, nanocrystals, and quantum dots What they are, why they’re interesting, and what we can do with them J. Nadeau, Department of Biomedical Engineering [email protected] Colloidal nanocrystals of different materials… …And different geometries

From: Science. 2005 January 28; 307(5709): 538544. Medieval Nanotechnology!

The colors in some stained- glass windows from medieval cathedrals are probably due to nanocrystals of compouds of Zn, Cd, S, and Se. 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

WhatWhat isis itit goodgood for?for?

••InterestingInteresting physicsphysics ••ApplicationsApplications inin optoelectronicsoptoelectronics ••ApplicationsApplications inin biologybiology Synthesis Quick review of semiconductors • A semiconductor has a forbidden zone or “band gap” between the conduction and valence band • When an electron is excited into the conduction band, there is a hole left in the valence band; this pair is an “exciton pair” • When the size of the crystal is comparable to the exciton Bohr radius, the confinement energy becomes signficant… at this point we have a “quantum dot”

Quantum mechanics of QDs

h22 E = nl +E ( ) e * gap e- 2m e Energy –h2 2 E = nl h * 0 2m h h+ E m* CB = h = 3.2 (wurzite ) * EVB me

Because of these quantized energy Bulk CdSe Q dot levels, QDs are more like atoms than like bulk materials--earning them the name “artificial atoms” This is an oversimplification… • “Box” wells are not infinite • Particles aren’t spherical • Boundary conditions must be considered • We assume only a single electron • However--the approximation is surprisingly good! Size-dependent spectra

Temporal evolution of CdSe nanocrystals

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) A

0.38

0.18

-0.02 300 350 400 450 500 550 600 650 700 WL/nm Emission CdSe nanocrystals

2.7 nm 3.0 nm 3.2 nm 3.6 nm

250

200

Intensity 150

100

0 450 500 550 600 650 Wavelength (nm) Characterization

AFM image of a cluster of CdSe nanocrystals (3.3 nm). Image size 70nm x70 nm HR TEM shows lattice structure So what is it good for?

•Emission wavelength is related to the size CdSe, CdS, of the crystal ZnS,CdTe, 3 to 10 nm etc •Slow to photobleach and radiation resistant

•Emission can be quenched/modulated by attaching electron donors or acceptors to 1 the surface

•Can be suspended in aqueous and non- aqueous environments Absorption Emission Normalized intensities Normalized •Many colors obtained with a single UV excitation source 0 •Surface can be conjugated to chemically 450 500 550 600 650 700 and biologically important molecules (nm) Interesting physics!

• Trap states • Stokes shift • Stark Effect • Blinking The importance of surface states More than half the atoms are at the surface How to probe surface states Transient absorption spectroscopy

Electron and hole acceptors quench PL ==> PL results from exciton recombination Burda et al, J. Phys. Chem. B, 105 (49), 12286 -12292, 2001 What causes the Stokes shift? •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 the immediate vicinity of, but external to, the QD. Tunneling of the electron out of the QD results in a charged particle, quenching emission (Kuno et al. 2003, Phys. Rev. B 67, 125304). • Internal hole traps, presumably at surface states or crystal imperfection sites. Energetic diffusion of the electronic states results in a time-dependent resonance condition in which Auger-assisted trapping of the hole results in an 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 E dependence 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) Interesting applications!

• Biological labels • Single-particle tracking • Biosensors • Memory • Solar cells • Etc… Biological labeling: neurons and glia

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

From: Science. 2005 January 28; 307(5709): 538544. QDs as biosensors

Dopamine

Doxorubicin (adriamycin) QD-dopamine as a redox sensor

EnergyCB O, R O h h R

Dopamine is an excellent electron donor Normal conditions Reducing conditions Uptake into cells With antioxidants Redox dependence More oxidizing…

Addition of the glutathione synthesis inhibitor BSO (10 mM) affects the intracellular redox potential without altering that of the 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 atomic physics at the almost macroscopic scale • However, there are always complications due to surface states, solvent interactions, etc that make them more than a particle in a box • A lot has been done, but a lot more remains to be done before we understand these particles and can use them in complex media Les incontournables 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). To come

• Toxicity • Stability and alternative coatings • Metal particles • Insulator particles