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  • 1. Semiconductor Quantum Dots:CdSe, ZnSe, ZnS, ZnOGroup members:Trn Phc ThnhCao Vn PhcHong Vn Tin

2. Outline Introduction What is semiconductor quantum dots Why quantum dots Properties Synthesis Applications, challenges, and potentials Conclusions 3. Introduction:Image courtesy of Dr. D. Talapin, University of Hamburg 4. What is Quantumdots? Quantum dots are semiconductornanocrystals. They are made of many of the samematerials as ordinary semiconductors (mainlycombinations of transition metals and/ormetalloids). Unlike ordinary bulk semiconductors, whichare generally macroscopic objects, quantumdots are extremely small, on the order of a fewnanometers. They are very nearly zero-dimensional. 5. Exciton Bohr Diameter Material Dependent Parameter The same size dot of different materials may not both bequantum dots The Bohr Diameter determines the type ofconfinement 3-10 time Bohr Diameter: Weak Confinement Smaller than 3 Bohr Diameter: Strong Confinement 6. Experimental Observation ofConfinement Just imaging a small dot is not enough to say it isconfined Optical data allows insight into confinement Optical Absorption Raman Vibration Spectroscopy Photoluminescence Spectroscopy 7. Optical Absorption Optical Absorption is atechnique that allows one todirectly probe the band gap The band gap edge of amaterial should be blueshifted if the material isconfined Bukowski et al. present theoptical absorption of Gequantum dots in a SiO2matrix. As the dot decreases in sizethere is a systematic shift ofthe band gap edge towardshorter wavelengths 8. Raman Vibrational Spectroscopy Raman vibrationalspectroscopy probes thevibrational modes of asample using a laser As the nanocrystal becomesmore confined the peak willbroaden and shrink Here we see a peak shifttoward the laser line Various Ge dots of differentsizes on an Alumina film 9. Direction of Raman Shift Here we see the samebroadening and shrinking ofthe Raman Peak We see a peak shift awayfrom the laser line No systematic shift of theRaman line Shifts toward the laser line are due to confinement Shifts away from the line are due to lattice tension due to film miss-match 10. Photoluminescence Spectroscopy Photoluminescencespectroscopy is atechnique to probe thequantum levels ofquantum dots Here we see dots ofvarious size in aquantum well (a) is quantum well spectrum (d) is smallest particles 80 nm 11. Properties of Quantum Dots Compared to Organic Fluorphores? High quantum yield; often 20 times brighter Narrower and more symmetric emission spectra 100-1000 times more stable to photobleaching High resistance to photo-/chemical degradation Tunable wave length range 400-4000 nm CdSe CdTehttp://www.sussex.ac.uk/Users/kaf18/QDSpectra.jpgJ. Am. Chem. Soc. 2001, 123, 183-184 12. Why 13. Excitation in a SemiconductorThe excitation of an electron from the valance bandto the conduction band creates an electron hole pair E ECB h E =g ( ) + )h BhV e C+ ( BCreation of an electron hole pairwhere h is the photon energyEVBsemiconductor Band Gap optical detector (energy barrier)E=h exciton: bound electron and hole pair usually associated with an electron trapped in a localized state in the band gap 14. Recombination of Electron Hole Pairs Recombination can happen two ways: radiative and non-radiativeEECBrecombination processes EVBE band-to-band recom bination recom bination atinterband trap statesECB (e.g. dopants, impurities)E = hradiative recombination photonEVBnon-radiative recombination phonon (latticevibrations) ) + ) ra d ia tiven o n -ra d ia tive re co m b in a tio nre c o m b in a tio ne C+ ( B h ( BhV 15. Effective Mass ModelDeveloped in 1985 By Louis BrusRelates the band gap to particle size of a sphericalquantum dotBand gap of spherical particlesThe average particle size in suspension can be obtained from the absorptiononset using the effective mass model where the band gap E* (in eV) can beapproximated by: .e 1 h2 1 1 1 03 1 1 2 8 .42e 1E + + = + *b u l k E 2 m h h 2 0 hm ( 0m m ) m 4 2e m g 0 2 eer 0 m 04 r m0Egbulk - bulk band gap (eV), h - Planks constant (h=6.626x10-34 Js)r - particle radiuse - charge on the electron (1.602x10-19 C)me - electron effective mass - relative permittivitymh - hole effective mass 0 - permittivity of free space (8.854 x10-14 F cm-1)m0 - free electron mass (9.110x10-31 kg)Brus, L. E. J. Phys. Chem. 1986, 90, 2555 16. Term 1The second term on the rhs is consistent with the particle in abox quantum confinement modelAdds the quantum localization energy of effective mass meHigh Electron confinement due to small size alters the effectivemass of an electron compared to a bulk materialConsider a particle of mass m confinedP oten tia l E nergyin a potential well of length L. n = 1, 2, For a 3D box: n2 = nx2 + ny2 + nz2 2n h2 2 n h22 E=n 2=x2 L 8 L mm20 L h 11 1e. 2 0 2e 1 1 21 8. 44 1E = g lk+ 2 * Eub + + r m 0 m 0 4 0 h ( 0 2 e 0 m 0 8 emhm r 2 2 ) m m hm Brus, L. E. J. Phys. Chem. 1986, 90, 2555 17. Term 2 The Coulombic attraction between electrons and holes lowersthe energyAccounts for the interaction of a positive hole me+ and a negativeelectron me-Electrostatic force (N) between two charges (Coulombs Law):qq F= 1 2 2 Work, w = Fdr 4 0rConsider an electron (q=e-) and a hole (q=e+)The decrease in energy on bringing a positivercharge to distance r from a negative charge is: 22ee =E d= r 4 0 r24 0 r h 11 1e . 2 0 2e 1 1 218. 441 E = g l + 2*Euk b+ + r m 0 m 0 4 0 h ( 0 2 e 0 m 08 emhm r 2 2 ) m m hm Brus, L. E. J. Phys. Chem. 1986, 90, 2555 18. Term InfluencesThe last term is negligibly smallTerm one, as expected, dominates as the radius is decreased Energy (eV )Modulus 1 term 1term 2term 300510 d (nm)Conclusion: Control over theparticles fluorescence is possibleby adjusting the radius of theparticle 19. Quantum Confinement of ZnO ZnO has small effective masses quantum effects can be observed for relatively large particle sizes Confinement effects are observed for particle sizes