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 jay.nadeau@mcgill.ca
Colloidal nanocrystals of different materials
And different geometries From: Science. 2005 January 28; 307(5709): 538 544.
Medieval Nanotechnology! The colors in some stainedglass 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 Nanocrystallites. 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? Interesting physics Applications in optoelectronics Applications in biology
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 Energy 0 e- h+ E e = h2 2 nl 2m* +E gap ( ) e E h = h2 2 nl 2m* h E CB = m h * E VB m* e = 3.2 (wurzite ) Bulk CdSe Q dot Because of these quantized energy 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 A 0.38 0.18 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) -0.02 300 350 400 450 500 550 600 650 700 WL/nm
250 Emission CdSe nanocrystals 200 2.7 nm 3.0 nm 3.2 nm 3.6 nm Intensity 150 100 50 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? 3 to 10 nm CdSe, CdS, ZnS,CdTe, etc Emission wavelength is related to the size of the crystal Slow to photobleach and radiation resistant 1Normalized intensities Emission can be quenched/modulated by attaching electron donors or acceptors to the surface Absorption Emission Can be suspended in aqueous and nonaqueous environments Many colors obtained with a single UV excitation source 0 450 500 550 600 650 700 (nm) Surface can be conjugated to chemically and biologically important molecules
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 E 2 QDs show both aspects, but E dependence is only seen in singledot 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 Copyright 2006 Society for Neuroscience
Single-particle tracking From: Science. 2005 January 28; 307(5709): 538 544.
QDs as biosensors Doxorubicin (adriamycin) Dopamine
QD-dopamine as a redox sensor Energy CB h O, R h O R VB Dopamine is an excellent electron donor
Normal conditions
Reducing conditions
Uptake into cells
With antioxidants
Redox dependence
Addition of the glutathione synthesis inhibitor BSO (10 mm) affects the intracellular redox potential without altering that of the medium More oxidizing
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