Spectroscopy at nanometer scale

Transcription

1 Spectroscopy at nanometer scale 1. Physics of the spectroscopies 2. Spectroscopies for the bulk materials 3. Experimental setups for the spectroscopies 4. Physics and Chemistry of nanomaterials

2 Various spectroscopic methods Electrons EELS AES IPS APS Neutrals He Scattering n 0 Scattering Sample Ions ISS RBS SIMS PIX Photons PL FTIR Raman XPS UPS

3 Born-Oppenheimer Approximation

4 Electron Spectroscopy 1. Photons in, photons out PL 2. Photons in, electrons out UPS, XPS 3. Electrons in, electrons out EELS

5 Binding energy and effective radius for the exciton E e = (m*/m e )(ε/ε 0 ) -2 (13.6 ev) a eff = (ε/ε 0 )(m*/m e ) -1 ( nm) For GaAs, ε/ε 0 ~ 13.2 and m*~ 0.067m e then E e ~ 5 mev and a eff ~ 10 nm

6 Concentric Hemispherical Analyzer (CHA)

7 Prism S Grating α β = mλ s = d (sinα sinβ) d

8 Vibrational Spectroscopy 1. Photons in, photons out IR, Raman 2. Electrons in, electrons out EELS

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11 The Theory of Raman Spectroscopy

12 TIGP Introduction technology (I) November 17, 2006 Properties of individual nanoparticles Clusters

13 Particle nature of photons Einstein s proposal: E = hν P = h/λ m e v ϑ λ φ Compton Scattering λ

14 Wave nature of electrons de Broglie s proposal: λ = h/p ν = h/e For electrons: λ (nm) = 1.22/E 1/2 (ev) Grids e-gun ev Crystal LEED

15 Fundamentals of quantum mechanics 1. Quantization 2. Tunneling ρ kt 3. Statistics 1 ε

16 Critical Length scale C. Joachim et al., Nature 408, 541 (2000).

17 One dimensional size effect Ψ(x) V(x) a/2 0 a/2 x 2 n = 1 -a/2 0 a/2 sin(nπx/a), n even Ψ(x) = { cos(nπx/a), n odd E = n 2 π 2 h 2 /2ma 2, n = 1,2,3 x Atomic Levels

18 NanoSci Nano Lab Size effect Size

19 Ratio of surface atoms ratio (%) /d Diameter of particle d (nm)

20 Enhanced catalytic effect

21 Au nanoparticle as an example E F = (ħ 2 /2m) (3π 2 n) 2/3 g(e F ) = (3/2) (n/e F ) δ = 2/[g(E F )V] = (4/3) (E F /N) 10 nm Number of valence electrons (N) contained in the particles is roughly 40,000. Assume the Fermi energy (E F ) is about 7 ev for Au, then δ ~ 0.22 mev ~ 2.5 K

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23 Electronic Structure of Single-wall Nanotubes Nature 391, 59 (1998).

24 Optical properties of nanoparticles (in the infrared range) (1) Broad-band absorption: Due mainly to the increased normal modes at the surface. (2) Blue shift: Due mainly to the bond shortening resulted from surface tension.

25 Optical properties of nanoparticles (in the visible light range) (1) Blue shift: Due mainly to the energy-gap widening because of the size effect. + (2) Red shift: Bond shortening resulted from surface tension causes more overlap between neighboring electron wavefunctions. Valence bands will be broadened and the gap becomes narrower. Excitons + (3) Enhanced exciton absorption: Due mainly to the increased probability of exciton formation because of the confining effect.

26 Optical properties

27 Semiconductor quantum dots (Reproduced from Quantum Dot Co.)

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29 Mass Analyzer B r qv = ½ mv 2 F = qvb = mv 2 /r m/q = ½ B 2 r 2 /V

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33 Reactivity of nanoclusters

34 Magic clusters

35 Mackay icosahedra P = 1 20 fcc(111) faces P = 2 P = 3 Shell model N = 1 + Σ (10p 2 + 2)

36 Nanopucks of same size and shapes (a) (b) Deposited at 110 K and annealed to 175 K Flux = 0.1 ML/min

37 Atomically resolved Ag nanopucks

38 Size distribution of Ag nanopucks Counting rate (%) Ag atom number N

39 di/dv curves of Ag 34 nanopucks Ag 34 di / dv Sample Bias (V)

40 Energy levels of some magic clusters (a) Ag 24 (b) Ag di / dv E (ev) (c) Ag 34 (d) Ag di / dv E (ev) (e) Ag 40 (f) Ag di / dv E (ev) Sample Bias (V) Experiment Theory -2.0

41 Binding energy per atom Binding energy per atom (ev) Ag atom number N (Ag atom number N) -1/2 2-D magic nanopucks 2-D hexagonal magic nanopucks 1-layer Ag thin film Guided line of geometrical effect

42 E = (E n+1 +E n-1 )-2E n Stability of Ag nanopucks Counting rate Ag atom number N Ag atom number Nan o 127 Sci La La b

43 Size-,, site- & shape-controlled self-organized growth 10 nm Ratio (%) Ag atom number N

44 STS of Si(111)-(7x7) Science 234, 304 (1986). UPS IUPS

45 STS of Si(111)-(7x7) topograph 1. Science 234, (1986). 2. Phys. Rev. Lett. 56, (1986).

46 Density of states of various dimensions 3D 2D D(ε) D(ε) ~ ε 1/2 D(ε) D(ε) = m * /πh 2 1D ε F ε 0D ε F ε D(ε) D(ε) ~ (ε E n ) 1/2 D(ε) E 1 E 2 E 3 ε F ε δε ε F ε

47 Quantum size effect λ = de Broglie wavelength of electron a = thickness of metal film a >> λ a λ M a M Substrate Substrate a k z Fermi surface k z n=5 n=4 k n=3 F n=2 n=1 k y k y k x k x

48 Spectroscopic images 4 4 V = -0.36V V = -0.56V Topography di/dv ( arb. unit) 11 ε 11 n =10 F /e Sample bias (Volt) (N) (4) (6) (8) (9) (5) (7) V = 1.28V V = 0.56V di/dv mapping

49 Inelastic Tunneling

50 Single Molecule Vibrational Spectroscopy and Microscopy B.C. Stipe, M.A. Rezaei, and W. Ho, Science 280, (1998).

51 Atomic Scale Coupling of Photons to Single-Molecule Junctions S.W. Wu and N. Ogawa and W. Ho, Science 312, (2006)

52 NanoSci Nano Lab Quantum corral 5 nm D.M. Eigler, IBM, Amaden

53 Artificial atom