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U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat1 Chimica Fisica dei Materiali Avanzati Part 6.a – Size effects and applications of metal and semiconductor.

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Presentation on theme: "U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat1 Chimica Fisica dei Materiali Avanzati Part 6.a – Size effects and applications of metal and semiconductor."— Presentation transcript:

1 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat1 Chimica Fisica dei Materiali Avanzati Part 6.a – Size effects and applications of metal and semiconductor nanoparticles Laurea specialistica in Scienza e Ingegneria dei Materiali Curriculum Scienza dei Materiali

2 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat2 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

3 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat3 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.

4 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat4 Plasmons

5 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat5 Metal nanostructures & surface plasmons Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metals surface, the properties of surface plasmonsin particular their interaction with lightcan 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, Nature 424, 824 (2003)

6 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat6 Plasma Equations

7 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat7 Dielectric Constant Reflection Propagation

8 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat8 Volume EM Waves in Metals A bandgap exists for propagation due to the negative dielectric constant

9 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat9 Negative Dielectric Constant E & H out of phase, imaginary Dispersion of EM in Plasma

10 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat10 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. z x propagation Metal

11 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat11 Dispersion Curves Propagation curve in 1 (dielectric medium) does not cross the surface plasmon dispersion curve

12 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat12 SPR excitation techniques

13 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat13 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)

14 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat14 SPR Applications Chemical 1.Sensing 2.Reaction kinetics 3.Concentration measurement 4.Mass Spectrometry 5.Equilibrium properties … and many more Biological 1. Real time sensing 2. Detection of binding reactions 3. Proteomics 4. Plasma membrane studies 5. Drug delivery techniques … and many more

15 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat15 SPR-based commercial products SPR-based commercial products SPR biosensor Spreeta from Nomadics-Texas Instruments Applications Beverages Medical diagnostics Food safety Security Water quality Research

16 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat16 Localized Plasmon Resonance @ NPs Coherent excitation of conduction electrons driven by the E radiation field. Theoretically modeled by Mie Theory

17 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat17 Optical Properties Mie Theory(1908) Empirically Drude free electron model J.H. Hodak, et al. ; J. Phys Chem. B, 104(43), 9954, 2000. 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 << /10

18 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat18 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.

19 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat19 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

20 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat20 Surface Enhanced Raman Scattering Surface Enhanced Raman Scattering (SERS) Normal Raman Scattering

21 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat21 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.

22 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat22 Surface Plasmon Spectroscopy (a)Absorption spectrum of 6nm dia. Silver particles (0.1mM) (b) Predicted difference spectrum following injection of 5µM electrons (c)Experimental difference spectrum using pulse radiolysis (N 2 O, 2-propanol) Blue shift occurs due to Increased electron concentration.

23 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat23 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.

24 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat24 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!

25 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat25 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

26 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat26 Nanoshells for expanding SPRs 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.

27 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat27 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)

28 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat28 NIR photothermal tumor therapy

29 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat29 Single Electron Devices from NCs The capacitance of a small particle is C = Q/V = e/4πe o a in vacuum. To add an electron to the particle costs an energy, U = Q 2 /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.

30 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat30 Single Electron Devices from NCs

31 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat31 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 Inframat Corporation, Farmington, CT

32 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat32 Nanocrystals are of tremendous interest because they (i)Have size dependent optical, electronic, catalytic and magnetic properties. (ii)Quantum size effects provide direct insight into the validity of current models of bonding and structure. (iii)Numerous applications require miniaturisation. (iv)QSE may provide the fundamental limits to Moores law. (v)Such materials are fun to play with! Motivation for Studying Semiconductor Nanoparticles

33 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat33 Growing CdSe Quantum Dots Similar preps for: InAs, InP, CdS, ZnS, ZnSe, CdTe, PbS, PbSe, ZnO, alloys and core-shells of these materials.

34 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat34 Typical Absorption and Fluorescence Kinetic Profiles CdSe nucleation in octadecene at 275 o C; Growth at 250 o C; oleic acid/dodecylamine capping agents (L): Absorption (R): Luminescence vs time

35 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat35 Radius vs Time Particle growth is slow, distribution narrows 50mM monomer in Octadecene at 275oC

36 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat36 Tweaking… If you make the nucleation go fast enough… If you can stop the nanocrystals growing… If you can make them all the same size… If you can make them as single crystals with no defects… If you can stop them sticking together…

37 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat37 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. Quantum Size Effect produces Artificial Atoms

38 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat38 Size Control is Critical! 1 2 3 4 5 6 7 8 9

39 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat39 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

40 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat40 Exciton transitions in cuprous oxide

41 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat41 Weak Confinement a > a B 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)

42 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat42 Strong Confinement Regime ( a < a B ) 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

43 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat43 Summary of QSE heat R > 5 nm (bulk) R < 2 nm R < 3 nm Filled energy levels Unoccupied energy levels

44 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat44 Quantum Size Effects produces Artificial Atoms 1.5 nm 7 nm Nanocrystal Diameter

45 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat45 Applications - NCs Tunable lasers Tunable LEDs Biolabelling Smart Glasses

46 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat46 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.

47 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat47 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.

48 U NIVERSITA DEGLI S TUDI DI P ADOVA Corso CFMA. LS-SIMat48 Single Nanocrystal Spectroscopy Temperature (K)FWHM 300 K 20 K 10 K (low exct. Int 185W/cm 2 ) (15 – 18) nm (50 – 60) meV (0.06 – 0.3) nm (0.2 – 1) meV 0.04 nm 120 eV Confocal microscope


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