Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali

1. Chimica Fisica dei Materiali AvanzatiPart 6.a – Size effects and applications of metal and semiconductor nanoparticles Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali Corso CFMA. LS-SIMat

2. Technological Interest in Metal Nanocrystals Novel optical effects Nanoelectronics Biolabelling Single electron devices (capacitors, memory storage) Polarizers Shape control SERS - molecular detection Catalysis Plasmonics and Optical Chips Corso CFMA. LS-SIMat

3. HRTEM Ag Nanoparticles “Monodisperse” 6nm diameter, crystalline silver nanoparticles in water produced by radiolytic reduction. Solution is bright yellow! Inset: Shows metal atoms with lattice spacing of 2.36Å - same as bulk. Corso CFMA. LS-SIMat

4. Plasmons Corso CFMA. LS-SIMat

5. Metal nanostructures & surface plasmons Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal’s surface, the properties of surface plasmons—in particular their interaction with light—can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics. WL Barnes, A Dereux and TW Ebbesen, Nature424, 824 (2003) Corso CFMA. LS-SIMat

6. Plasma Equations Corso CFMA. LS-SIMat

7. Dielectric Constant Reflection Propagation Corso CFMA. LS-SIMat

8. Volume EM Waves in Metals • A “bandgap” exists for propagation due to the negative dielectric constant Corso CFMA. LS-SIMat

9. Dispersion of EM in Plasma Negative Dielectric Constant E & H out of phase, imaginary b b Corso CFMA. LS-SIMat

10. x propagation z Metal Surface Plasmon • Coherent excitation of plasma near the surface of a metal – coupled to surface EM wave • These are essentially light waves that are trapped on the surface because of their interaction with the free electrons of the conductor (i.e., guided waves) • SPs help us to concentrate and channel light using subwavelength structures. Corso CFMA. LS-SIMat

11. Dispersion Curves • Propagation curve in e1 (dielectric medium) does not cross the surface plasmon dispersion curve Corso CFMA. LS-SIMat

12. SPR excitation techniques Corso CFMA. LS-SIMat

13. SPP at planar metal surfaces • Plasmon-polariton excitation produces absorbance peak at specific frequency • Shift in the absorbance spectrum indicates presence of analyte molecules (change in dielectric refractive index) Corso CFMA. LS-SIMat

14. Chemical Sensing Reaction kinetics Concentration measurement Mass Spectrometry Equilibrium properties … and many more Biological Real time sensing Detection of binding reactions Proteomics Plasma membrane studies Drug delivery techniques … and many more SPR Applications Corso CFMA. LS-SIMat

15. SPR-based commercial productsSPR biosensor Spreeta from Nomadics-Texas Instruments Applications • Beverages • Medical diagnostics • Food safety • Security • Water quality • Research Corso CFMA. LS-SIMat

16. Localized Plasmon Resonance @ NPs Coherent excitation of conduction electrons driven by the E radiation field. Theoretically modeled by Mie Theory Corso CFMA. LS-SIMat

17. Optical Properties Mie Theory(1908) Drude free electron model Empirically • Surface Plasmon Resonance is invariant with respect to the size on the nanoparticle. • The FWHM scales with the radius of the particles. • Assumes spherical particle • Particle diameter <<l/10 Corso CFMA. LS-SIMat J.H. Hodak, et al. ; J. Phys Chem. B, 104(43), 9954, 2000.

18. Different Plasmon Modes • Bulk Plasmons: • Surface Plasmons on Flat Surface: • Surface Plasmons on a Small Sphere: • Surface Plasmons on a Small Ellipsoid: • Intrinsically sensitive to surface perturbations. • “Small” means < 30nm. • L depends on aspect ratio. Corso CFMA. LS-SIMat

19. Dielectric confinement and local field amplification • The value of the local field near the metal nanocrystal is • A huge amplification of the local field occurs near the SP resonance at the pole Corso CFMA. LS-SIMat

20. Surface Enhanced Raman Scattering (SERS) Surface Enhanced Raman Scattering Normal Raman Scattering Corso CFMA. LS-SIMat

21. Overview of Novel Effects in Metal Nanocrystals • Surface plasmon changes due to electron density. • Surface plasmon changes due to shells or adsorbates e.g. biomolecules. • Surface plasmon changes with particle size. • Surface plasmon changes with particle shape. • Single Electron Effects: Coulomb Blockade. Corso CFMA. LS-SIMat

22. Surface Plasmon Spectroscopy Absorption spectrum of 6nm dia. Silver particles (0.1mM) (b) Predicted difference spectrum following injection of 5µM electrons Experimental difference spectrum using pulse radiolysis (N2O, 2-propanol) Blue shift occurs due to Increased electron concentration. Corso CFMA. LS-SIMat

23. Shape Effects: Metal Nanorods (Left) Nanorods absorb two colours. The colour depends on the length of the rod. For silver, almost all colours of the rainbow are predicted to be possible. Gans predicted this effect in 1911. First proper gold rods made in 1994 to test this. Corso CFMA. LS-SIMat

24. Shape Control - Polarizers, Liquid Crystals, Nanomechanics Aims: Use shape control to create new materials with unusual optical properties Color of Gold Nanorods depends on aspect ratio and orientation! Corso CFMA. LS-SIMat

25. Primitive Alignment of Ag Rods • Focus a laser onto the silver rods and they melt back into yellow nanospheres Stretch a polymer film with silver rods (10nm x 40nm) and hold it under a polarizer Corso CFMA. LS-SIMat

26. Nanoshells for expanding SPR’s • More sensitive than simple nanoparticles. (gain ~r2/r1) • Resonance frequency is a strong function of geometry • More resonant frequencies possible Optical resonances of gold-silica core nanoshells as a function of their core/shell ratio. Corso CFMA. LS-SIMat

27. Nanoshells in bio-applications Properties: • Optical activity in bio-compatible wavelengths. • Strong tunable absorption in NIR region (700-1300 nm) (maximum light penetration through tissue in NIR) • Easy conjugation with specific proteins • Chemical / Photochemical stability • Biocompatible, non-toxic to tissue (gold) Corso CFMA. LS-SIMat

28. NIR photothermal tumor therapy Corso CFMA. LS-SIMat

29. Single Electron Devices from NCs The capacitance of a small particle is C = Q/V = e/4πeoa in vacuum. To add an electron to the particle costs an energy, U = Q2/2C. If U >>kT, then the usual I-V curve will show jumps. If these jumps can be distinguished then the presence of single electrons can be confirmed, and a storage or logic device created. Low temps are usually needed. An STM can be used to study the flow of electrons through a single Gold nanoparticle. Corso CFMA. LS-SIMat

30. Single Electron Devices from NCs Corso CFMA. LS-SIMat

31. Metal-ceramic nanocomposite materials • For inductive components in high-frequency electronic devices • Chemical synthesis of Ni-Fe/SiO2, Co/SiO2, Fe-Co/SiO2, Fe/nickel-ferrite, Ni-Zn-ferrite/SiO2, Fe-Ni/ polymer, and Co/polymer composite magnetic materials • Exchange coupling between nanoparticles  large magnetic permeability • Cancellation of magnetic anisotropy • Small parasite currents Corso CFMA. LS-SIMat Inframat Corporation, Farmington, CT

32. Motivation for Studying Semiconductor Nanoparticles Nanocrystals are of tremendous interest because they Have size dependent optical, electronic, catalytic and magnetic properties. Quantum size effects provide direct insight into the validity of current models of bonding and structure. Numerous applications require miniaturisation. QSE may provide the fundamental limits to Moore’s law. Such materials are fun to play with! Corso CFMA. LS-SIMat

33. Growing CdSe Quantum Dots Similar preps for: InAs, InP, CdS, ZnS, ZnSe, CdTe, PbS, PbSe, ZnO, alloys and core-shells of these materials. Corso CFMA. LS-SIMat

34. Typical Absorption and Fluorescence Kinetic Profiles CdSe nucleation in octadecene at 275oC; Growth at 250oC; oleic acid/dodecylamine capping agents (L): Absorption (R): Luminescence vs time Corso CFMA. LS-SIMat

35. Radius vs Time 50mM monomer in Octadecene at 275oC Particle growth is slow, distribution narrows Corso CFMA. LS-SIMat

36. Tweaking… • If you make the nucleation go fast enough… • If you can stop the nanocrystals growing… • If you can stop them sticking together… • If you can make them all the same size… • If you can make them as single crystals with no defects… Corso CFMA. LS-SIMat

37. Quantum Size Effect produces“Artificial Atoms” CdSe Nanocrystals ranging from 1nm to 6nm in diameter - to distinguish so many colours, the size distributions must be very narrow. The growth kinetics must be carefully controlled. Corso CFMA. LS-SIMat

38. Emission Intensity (a.u.) CdSe Diameter (nm) Wavelength (nm) 600 9 8 7 6 300 5 4 3 2 0 1 400 450 500 550 600 650 700 Size Control is Critical! Corso CFMA. LS-SIMat

39. Wannier excitons in bulk semiconducors • Bound electron-hole pairs that split off the conduction band due to mutual attractive (Coulomb) interaction • Binding energy and degree of localization depend on electrostatic screening with and Bohr radius Corso CFMA. LS-SIMat

40. Exciton transitions in cuprous oxide Corso CFMA. LS-SIMat

41. Weak Confinement a > aB • We will not discuss this region. For NCs small, but larger than the exciton radius, the first evidence of QSE is a slight blue shift of the exciton due to its feeling trapped. • The exciton energy increases until it reaches the band edge..at this point, “normal” excitons no longer exist in the particle.. • The shift is not as dramatic as in the strong regime… Example: CdSe Nanocrystals HRTEM CdSe Nanocrystal (Courtesy M Bawendi, MIT) Corso CFMA. LS-SIMat

42. Strong Confinement Regime (a < aB) • Smaller than exciton radius in bulk crystal. • Treat e,h as separate entities. • Spherical box. • Coulomb attraction is less than confinement energy. Varies as -1/a. Brus showed that lowest optical transition obeys: since Corso CFMA. LS-SIMat

43. Summary of QSE Unoccupied energy levels heat Filled energy levels R > 5 nm R < 3 nm R < 2 nm (bulk) Corso CFMA. LS-SIMat

44. Quantum Size Effects produces“Artificial Atoms” 7 nm 1.5 nm Nanocrystal Diameter Corso CFMA. LS-SIMat

45. Applications - NCs Tunable LEDs Tunable lasers Smart Glasses Biolabelling Corso CFMA. LS-SIMat

46. Biolabelling Advantages of QDs as biolabels • More photostable under laser irradiation. Dyes bleach very quickly. • Narrower spectra so more colours in any spectral region. • Simple excitation spectra, so readily multiplexed with simple optics. Dyes require multiple lasers in microscopes or cytometers. • Common chemistry for all labels. Note that gold NCs have long been used as biolabels! Today, interest in all types of NCs as labels: magnetic,metallic, fluorescent, mixtures thereof. Corso CFMA. LS-SIMat

47. Challenges and Conclusions Challenges for Nanocrystal Engineers • Shape Control, Core-Shell Synthesis and Passivation. • Monodispersity and Scale-Up. • Luminescence and Quantum Yield from UV to NIR. • QSE theory: matching with computational models. Challenges for NanoTechnologists • Ordering of NCs on different lengthscales. • Integration with Top-Down NT processes. • Creating Functional nanoscale materials and devices. Corso CFMA. LS-SIMat

48. Single Nanocrystal Spectroscopy Confocal microscope Corso CFMA. LS-SIMat