From the excitation of surface plasmon polaritons (SPP s) by evanescent waves to SERS active sites Andreas Otto

Transcription

1 From the excitation of surface plasmon polaritons (SPP s) by evanescent waves to SERS active sites Andreas Otto 1) History of plasmons, surface plasmon polaritons (SPP) and Ottoconfiguration 2) Roughness spectrum of a smooth silver film obtained by SPP-SPP scattering. 3) Collecting light from SPP-cone with the Weierstraß-prism (WP) 4) Proof of the excitation of SPP s by hot electrons in metal-insulatormetal junctions, using the WP 5) Using the WP in surface Raman spectroscopy from single crystal copper electrodes 6) SERS active sites

2 Bulk-plasma oscillations (plasmons) 1) Oscillations in ionized gases: Tonks & Langmuir ) Bulk plasma oscillations in free electron metal: Pines & Bohm, Phys.Rev.85(1952)338 3) Bulk plasmon in Silver, Fröhlich & Pelzer 1955 Proc.Phys.Soc.A68(1955)525 ε(ω) = 0 at ca.3.75ev

3 Surface plasmon polariton of silver hω ( ev ) ω/k = c bulk plasmon (Fröhlich, Pelzer 1955) radiative SPP-dispersion relation nonradiative Surface plasmon-polariton of plane silver with phase velocity parallel surface < c Dielectric theory with retardation: plasmaradiation by Ritchie & Eldridge 1962, SPP dispersion in thin film by Otto 1965 (first?) k parallel ω εω ( ) ( ω) = ( ) c εω ( ) + 1 Surface Plasmon: Ritchie, Phys.Rev.106 (1957) 874, Ferrell and Stern 1958, 1960 unretarded 1/2 k parallel surface

4 c α c Problem and idea phase velocity length surface v phase,parallel surface = c/sin α >c silver c c v phase,parallel surface = c/n sin α α α total reflection c/n c/n total reflection: n sin α > 1 evanescent field with v phase,parallel surface < c PUT the silver sample in about a wavelength distance BELOW the prism!

5 The realization (1968) A. Otto, Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection, Z. Physik 216 (1968) 398, download from Want to know more about history, priority, who invented and introduced the names X-configuration? Look for uni-duesseldorf.de/ lecturexiamen/lecturei measuring D by intererence fringes of white light gap width d ~λ SPP resonance only observed for p-polarized light (equivalent to TH polarization)

6 Experimental dispersion ω k( hω)= sinα c minimum of reflectivity

7 In 1968, the sensitivity of the ATR SPP resonance was clear, but real surface diagnostic needs were not known, at least to me. +2nm AgS +1nm AgS calculated experiment error bar

8 2) Roughness spectrum of a smooth silver film obtained by SPP-SPP scattering

9 Scattered intensity Fluid prism (refractive index of BK7 glass) Laser in Quantitative measurement of the roughness spectrum of silver films, J. Bodesheim, A. Otto Surf. Sci. 45 (1974) 441 k = k SPP k = ω c out in k x SPP(in) SPP(out) scattering with fluid-prism k = n c ω

10 roughness spectrum of a smooth silver film without ATR-prism with ATR-prism, perpendicular incidence from SPP SPP scattering

11 3) Collecting light from SPP-cone with the Weierstraß-prism (WP)

12 Invention of the Weierstraß-prism : Integration over all emitted SPP s from an emitting point k = k SPP k ω = c out out k x k = n c ω W. Wittke, A. Hatta, A. Otto Efficient use of the surface plasmon polariton resonance in light scattering from adsorbates. Applied Physics A 48 (1989)

13 Adjusting the gap sample-weierstraßprism Peter Borthen, Diplomarbeit Düsseldorf 1988 R=1 d=2λ L R=0.94 d=3/2 λ L R=0.45 d=3/4 λ L R=0.61 d=1/2 λ L gap modes SPP R=0.71 d=5/4 λ L R=0.35 d=λ L R=0.81 d< 1/2 λ L

14 4) Proof of the excitation of SPP s by hot electrons in metal-insulatormetal junctions, using the WP

15 Light emission from Al/AlOx/Ag tunnelling junctions D. Diesing, G. Kritzler, A. Otto, Surface reactions of hot electrons at metal-liquid interfaces, in Solid Liquid Interfaces, Macroscopic Phenomena and Microscopic Understanding, eds. S. Thurgate and K. Wandelt, Topic in Applied Physics 85, p Springer 2003 Ag hot electrons excitation of slow modes? excitation of SPP s? Reversed bias: No excitation of SPP s? d

16 Inset: integrated normalized emission as function of d 200nm Diesing, G. Kritzler, A. Otto D. Surface reactions of hot electrons at metal-liquid interfaces Topic in Applied Physics 85, p Springer 2003 Reversed bias: No light, no excitation of SPP s! 400 nm 1000nm Proof of hot electron SPP mechanism 2000nm

17 5) Using the Weierstraß-prism in surface Raman spectroscopy from single crystal copper electrodes

18 Advantage of Otto configuration, when using flat single crystals of different orientations: Variation of gap width, no excessive heating of the sample A. Bruckbauer, A. Otto,, J. Raman Spectrosc., 29 (1998) M pyridine + 0.1M KClO 4 aqueous electrolyte Hg/Hg 2 SO 4 reference ring breathing Raman band of pyridine at optimal electrode potential Raman light Laser beam

19 Reversible potential dependance of SERS of pyridine at Cu electrodes of different crystallographic orientation electrolyte: 0.01 M pyridine in 0.1 M KClO 4, (Hg/Hg 2 SO 4 ) reference electrode Capacity measurements: pyridine stays adsorbed at all electrode potentials E. The dependence of intensity on potential reflects tuning in and out of the transient electron transfer resonance. (metal-adsobate) (metal + -adsorbate - ) (Socalled first layer chemical effect in SERS) Cu-electrode potential

20 SERS of pyridine at Cu electrodes originates at defects (SERS active sites) Cu(110) vicinal at E = -1000mV Cu(110) vicinal at E=-500 mv Cu(110) vicinal at E=-1700 mv Electromagnetic enhancement by SPP resonance is not enough to observe pyridine at atomic smooth facets Cu(111) at E=-1100 mv SERS of pyridine on Cu films deposited at T substrate 120K 240K 300K pyr. at defects pyridine at Cu(111) facets

21 Concentration of surface defects is unknown in this experiment 83 Average enhancement of SERS of pyridine at Cu electrodes of different crystallographic orientation electrolyte: 0.01 M pyridine in 0.1 M KClO 4, (Hg/Hg 2 SO 4 ) reference electrode Average enhancement with respect to pyridine in the liquid gap (assuming SERS originates from all adsorbed pyridine molecules, rather than only from species adsorbed at defects)

22 6) SERS active sites

23 pyridine on silver in Ultra-High-Vacuum, influence of atomic scale roughness Ü.Ertürk,D.Gherban, A.Otto, Surf.Sci.203, 554(1988) (a) Top: Raman spectrum of a silver film, deposited at room temperature, exposed at about 40 K to 1 L of pyridine in the range of the C-C breathing mode, 1 W, integration time 2000 s. Bottom: Raman spectrum of liquid pyridine. (b) Raman spectra of the sample described in (a) for the indicated average thickness d cold of additional silver deposited on top at about 40 K. 1 W, integration times 800,400, 400,400 s. SERS active sites normal sites

24 SERS intensity and thermo-desorptionspectroscopy of CO on cold-deposited Ag exposures CO-stretch SERSintensity W. Akemann, A. Otto Vibrational modes of CO adsorbed to disordered copper films investigated with Raman spectroscopy. J. Raman Spectrosc. 22 (1991) Thermal desorption SERS only from a minority of sites, where CO is most tighly bound and settles first desorption from defects desorption temperature (K) multilayer desorption

25 STM on Cu(211) Ball model of the Cu(211) surface. The distance between intrinsic steps is 0.625nm. Horizontal chains from left to right are along the (0, -1, 1) direction. Copper atoms in the step edge are labeled A, at the kink site: B CO binds preferentially to kink sites

26 Clean Cu(211), CO settles first at kink sites A. Otto, M. Lust, A. Pucci, G. Meyer Proceedings of SERRS 2006, in press in Canadian Journal of Analytical Sciences and Spectroscopy SERS active sites, facts and open questions

27 A.Otto, M.Futamata, Electronic Mechanisms of SERS, in Surface enhanced Raman scattering, physics and applications, eds K.Kneipp, M.Moskovits, H.Kneipp, Topics inapplied Physics 103 (2006) No CT-SERS at smooth surfaces resonant Raman effect of the complex by internal charge transfer

28 SERS active sites = sites, where electrons are trapped for some fs A. Otto, The chemical (electronic) contribution to SERS, J. Raman Spectr. 36 (2005)497 acts for a short time of about 5fs as an isolated metalmolecule complex ( hole does not run away ). Internal resonance Raman effect by charge transfer for molecules with π* orbital becomes possible. There is no first layer SERS-effect at a smooth surface.

29 Acknowledgments to my collaborators visitors, now at SPP3: Lopez-Rios, Futamata to Prof. Annemarie Pucci and her people (e.g. Lust, Sinther, Priebe), M.Futamata (Tsukuba) and G.Meyer (IBM Zürich) and my first SPP-students in München ( ): Sohler, Bodesheim, Huber.

30 Merci á Alain Dereux