SEIRA effects. Annemarie Pucci

Transcription

1 SEIRA effects Annemarie Pucci Kirchhoff Institute of Physics, Heidelberg University 1. Introduction 2. SEIRA with metal-island films 3. SEIRA with metal dots 4. Dynamic charge transfer 5. Effect of nanoantenna resonances

2 Introduction Poltersdorf October 2007 A. Pucci 2

3 SEIRA Surface enhanced infrared absorption, for metal-island films enhancement up to a factor of 2000 observed Analogous to SERS (surface enhanced Raman scattering) and complementary (other selection rules) Occurs for adsorbates on metal nanostructures (rough nanofilms, island films, grids,...) Enhancement due to field enhancement (in the nearfield of metal particles at resonance, 2 at hot spots of disordered metal-island films, enhancement! E local chemical effect (increase in vibrational polarizability by adsorption) first layer effects (dynamic charge transfer supported by atomic surface defects, effects from spill out of electron density at metal surface) E 2 Poltersdorf October 2007 A. Pucci 3

4 SEIRA Osawa 2000 Poltersdorf October 2007 A. Pucci 4

5 Silver-film morphologies percolation Atomic force-microscopy pictures (500 nm 500 nm) of silver films grown at 300K on MgO(001), average film thickness: (a) 1nm Ag; (b) 3.4nm Ag; (c) 5.3nm Ag; (d) 9.5nm Ag For different temperatures same structure at percolation, structure width determined by mean free path. Poltersdorf October 2007 A. Pucci 5

6 IR transmittance spectra percolation Island-like growth From the Drude-type fit ( ) follows that at about 9 nm the bulk conductivity is reached. F. Meng, thesis 2007, pss (b) 2007 Poltersdorf October 2007 A. Pucci 6

7 SEIRA with metal-island films Poltersdorf October 2007 A. Pucci 7

8 CO adsorption on Cu films at 100K 200nmx 200nm 5.5nmCu on Si(111) 7x7 with defects, dep. at 100 K Relative transmittance, reference is the pure Cu film E conductivity change p vib SEIRA G. Fahsold, M. Sinther, A. Priebe, S. Diez, and A. Pucci, Phys. Rev B 70 (2004) nm x 200nm 5.4 nm Cu on KBr grown at 100K Poltersdorf October 2007 A. Pucci 8

9 SEIRA as a probe of film morphology CO as "surfactant for smoothening of Cu films AFM pictures UHV cleaved MgO without CO 1.0 with CO 1nm 1nm 2nm 2nm Relative Transmission nm nm Cu Without CO Poltersdorf 3nm 3nm 3.64nm 4nm 4nm 4.25nm 5nm 5nm 6nm 6nm for CO for CO 6.94nm 6.14 nm Cu With CO nm2 6.14nm A. Priebe, F. Meng, A. Pucci, Asian J. Phys. 15, 239 (2006) PCO=4*10-8mbar October 2007 Wave Number(cm ) A. Pucci 9

10 SEIRA of CO as probe CO-modified Cu films: effect on SEIRA spectra of CO at 100 K Relative Transmission cm -1 without CO (UHV) 2.81L with CO (UHV) 2.83L SEIRA spectra of CO on Cu films grown at 300 K on MgO(001), CO saturation exposure, 100 K cm Wave Number(cm -1 ) 2077 cm -1 CO/Cu (111) 2100 cm -1 CO/Cu (110) For smaller Cu islands more IR intensity is transferred to the higher frequency (dipole-dipole interaction for dipoles perpendicular to the array plane) Poltersdorf October 2007 A. Pucci 10

11 SEIRA of CO as probe Silver-island film on MgO(001), p co = 10-8 mbar Boudouard reaction (?) at 100 K: 2 CO => C +CO 2 CO C CO 2. AFM picture, 500nm x 500nm Ag grown on MgO at 500K, ca. 40nm Ag on average. F. Meng, thesis 2007 Poltersdorf October 2007 A. Pucci 11

12 SEIRA Usually normal transmittance with adsorbate is normalized to normal transmittance without adsorbate. For ultrathin metals with T 0 close to transmittance of the substrate: T T 0! ( 1+ n ) s + 2 " c d Im# eff eff,0 ( 1+ n ) s + 2 " c d Im# eff eff Due to the mixture with the metal the effective dielectric function ε eff does not show a Lorentzian imaginary part! Peak positions deviate from resonance frequencies! Poltersdorf October 2007 A. Pucci 12

13 Line shape modification SEIRA due to interaction between vibration dipoles and plasmonic dipoles of metal islands Maximum signal and asymmetry of SEIRA lines at percolation! O. Krauth, G. Fahsold, N. Magg, A. Pucci, J. of. Chem. Phys. 113, 6330 (2000) percolation Calculation, 2D Bruggeman, one oscillator, filling factor F as parameter A. Priebe, M. Sinther, G. Fahsold, and A. Pucci, J. of Chemical Physics. 119 (2003) 4887 Poltersdorf October 2007 A. Pucci 13

14 Enhancement factor 2000 Wet-chemically prepared goldnanoparticle layers for SEIRA of org. molecules (ODT), nanoparticles close to coalescence. D. Enders and A. Pucci, APL Poltersdorf October 2007 A. Pucci 14

15 SEIRA with metal dots Poltersdorf October 2007 A. Pucci 15

16 Cu on MgO(001) 200nm x 200nm 5nm Cu on MgO grown at 300K Poltersdorf October 2007 A. Pucci 16

17 SEIRA of CO on small Cu islands rel. transmitttance Effect of island size on CO bonding Cu island on MgO(001) formed by Cu deposition at 300K, island density ~ 8 to 2 x cm -2 Islands size increase with average Cu thickness without significant shape change up to 0.3 nm Cu is highly mobile at 300 K - no atomic roughness CO exposure at 100 K up to saturation, CO sticks on Cu only SEIRA factor far below resonance frequency A. Priebe, thesis 2002, F. Meng thesis 2007 Poltersdorf October 2007 A. Pucci 17

18 Charge-transfer effects Poltersdorf October 2007 A. Pucci 18

19 Charge-transfer effects C 2 H 4 on an atomically rough Cu film compared to adsorption on a smooth film 50 K 65 K M. Sinther, thesis, Heidelberg M. Sinther, A. Pucci, A. Otto, et. al., physica status solidi (a) 188 (2001) A.Priebe, M.Sinther, G. Fahsold, A. Pucci, JCP 119 (2003) nm, 300 K Additional IR lines occur at positions of A g modes. This indicates the "first layer effect". 5.4 nm, 50 K, atomic roughness Poltersdorf October 2007 A. Pucci 19

20 Charge-transfer effects C 2 H 4 and C 2 H 6 layers on different copper-island films at 50K Cu grown at 300 K Cu grown at 50 K Raman lines of C 2 H 4 appear on atomically rough Cu! A g deformation modes of C 2 H 4 : No monolayer signal in the CH-stretching region: according to flat lying adsorbate geometry. Poltersdorf October 2007 A. Pucci 20

21 Charge-transfer effects in IR reflectance IRRAS at 80, ca. 20 nm cold-dep. Cu on Cu plate, A. Priebe, A. Pucci, A. Otto, J. Phys. Chem B Poltersdorf October 2007 A. Pucci 21

22 Artificial surface roughening IRRAS of cold-deposited Cu on Cu(111) Cu deposition at 90 K onto a well prepared surface. In-situ measured broadband change of relative reflectance of p-polarized light at 85 incidence. M. Binder, diploma thesis 2006, τ eh according to B.N.J. Persson s theory of adsorbate induced surface friction at 2460 cm -1 Three regions A,B,C Linear range! eh " 1#10 $10 s (similar to Ag on Ag). A B C constant From 0.3 ML defects dominate Poltersdorf October 2007 A. Pucci 22

23 IRRAS of C 2 H 4 /Cu/Cu(111) at saturation exposure 1 L, 90 K In range A the E sites have developed...cu atoms at step edges? A g modes of E species C B A Cu cov. [ML] Poltersdorf October 2007 A. Pucci 23

24 Tailoring SEIRA... Nanoantenna resonances Poltersdorf October 2007 A. Pucci 24

25 Resonances of (ideal) metal particles and particle shape!! 1 P D L E 1 3 IR { E 1 L/D corrections due to retardation size dependent antenna-like excitations, e.g. ideal antenna: m! n = 2L, m = 1,3,5,... Poltersdorf October 2007 A. Pucci 25

26 Field enhancement at resonance Calculation with Finite Differential Time Domain method based on local optics, Au wire length 3 microns, diameter 50 nm, fundamental resonance, Au treated as lossy metal, i.e. Re ε is not considered. E F. Neubrech, 2006 Lightning rod effect Poltersdorf October 2007 A. Pucci 26

27 Single-wire extinction-cross section in the far field experiment & calculations > σ ext / σ geo 30 at resonance > farfield enhancement > SEIRA is expected F. Neubrech et al, Appl.Physics Letters 2006, BEM=Boundary element method calc. by J. Aizpurua Poltersdorf October 2007 A. Pucci 27

28 Single-wire IR spectroscopy with IR microscope coupled to an FTIR spectrometer A0 8 µm spot size focused IR beam synchrotron light source nanowire substrate relative transmittance: Poltersdorf October 2007 A. Pucci Trel = TSubstrate+NW TSubstrate 28

29 Single-wire IR spectroscopy Measured relative transmission spectra of a single Cu nanowire with approximately 2.2 µm length and 200 nm diameter, on KBr Antenna resonance SEM picture NWs made and SEM done at the GSI, in R.Neumann s group, IR experiments done by G. Fahsold, T. Kolb, thesis 2006, F. Neubrech, diploma thesis 2006, APL Poltersdorf October 2007 A. Pucci 29

30 IR spectroscopy of one ML octadecanthiole on Au nanowires Set up T rel = T Substrate+NW with ODT T Substrate F. Neubrech, 2007 Poltersdorf October 2007 A. Pucci 30

31 IR spectra of one ML octadecanthiole on individual Au nanowires Vibration signal of 1% (rel. transmittance) observed from ODT molecules with low IR activity! F. Neubrech, thesis in preparation, 2007 Poltersdorf October 2007 A. Pucci 31

32 Conclusions SEIRA with metal islands for probing film morphology, surface structure, chemical surface species. Average electromagnetic enhancement up to three orders of magnitude. Charge transfer-effects lead to electronic dipole contributions to non-ir active modes. Considerable signal enhancement due to antenna resonances. Poltersdorf October 2007 A. Pucci 32

33 Acknowledgements - group members, cooperation partners, and funding O. Skibbe doctorate students: R. Lovrincic, F. Neubrech, M. Binder M. Klevenz Hoang V. Chung diploma student: M. Möller Former members: G. Fahsold, F. Meng, T. Kolb, D. Enders, A. Priebe, M. Lust, M. Sinther,... Cooperations: A. Otto, Düsseldorf, P. Zahn, Halle, J. Wolfrum and H. R. Volpp, Heidelberg D. Lattard and M. Trieloff, Heidelberg, T. Nagao and D. Enders, Tsukuba M. Jalochowski, Lublin, J. Aizpurua, San Sebastian, R. Neumann,E. Berdermann, GSI Darmstadt, M. Lamy de la Chapelle, Troyes->Paris R. Hillenbrand, Martinsried W. Yakovlev, Troitzk... ANKA Funding: DFG, DAAD, GSI Hochschulförderung, G. Daimler-and C. Benz Foundation, BASF Poltersdorf October 2007 A. Pucci 33