Antenna-like plasmon resonances of single gold nanowires in the. mid-infrared. Frank Neubrech. SERS roundtable Universität Heidelberg

Transcription

1 Antenna-like plasmon resonances of single gold nanowires in the mid-infrared SERS roundtable 2006 Frank Neubrech Kirchhoff-Institut für Physik Universität Heidelberg

2 Outline Introduction Experimental setup Nano-antennas Experimental results Outlook Summary

3 Outline Introduction Experimental setup Nano-antennas Experimental results Outlook Summary

4 Introduction: experimental setup Spectroscopic IR-microscopy at the synchrotron light source ANKA[*] IR-microscope (Bruker IRscope II): optical path of the synchrotron-beam camera aperture MCT FTIR-spectrometer [ *] ANKA : Angströmsource Karlsruhe, Forschungszentrum Karlsruhe Schwarzschild objective (N.A. 0.52) sample light source (visible) polarizer

5 Introduction: measurement principle synchrotron light source IR beam Relative transmittance measurements 8µm spot size MCT detector nanowire substrate transmittance T wavelength [µm] sample wavenumber [cm -1 ] spectrometer 1.02 wavelength [µm] wavelength [µm] relative transmittance ( T / T 0 ) wavenumber [cm -1 ] diameter D = 100nm, length L = 1.4µm, substrate: ZnS transmittance T reference wavenumber [cm -1 ]

6 Introduction: nano-antennas λ Easiest model: macroscopic ideal antenna L = j e.g. ω λ / 2 = From measurements [ * ] 2 n and light scattering simulations of gold nanowires in the visible spectral range deviations are observed πc j L n L λ 3 Boundary element method (BEM) simulations [#] Deviations due to retardation, skin effect, influence of substrate and surrounding material D D πc D ωres L, neff, = ωλ / 2( L, neff ) R = R L L L n L [ * ] G. Schider, J. R. Krenn et al., Phys. Rev. B 68, (2003); [#] J. Aizpurua, G.W Bryant et al., Phys. Rev. B 71, (2005) eff

7 Outline Introduction Experimental setup Nano-antennas Experimental results Outlook Summary

8 Experimental results Measured relative transmittance spectra of single nanowires with different lengths and diameters on different substrates [ * ] Picture of a nanowire taken with the IRmicroscope in the visible mode 10µm x 10µm [ * ] F. Neubrech, T. Kolb, R. Lovrincic, G. Fahsold, J. Aizpurua, S. Karim, T. W. Cornelius, M. E. Toimil-Molares, R. Neumann and A.Pucci, Applied Physics Letters (submitted)

9 Experimental results: Resonance wavelength (Γ) resonance wavelength [µm] Influence of the substrate D = 200nm (exp.) D = 210nm (exp.) dipole (n = 1) dipole (n = n eff = 1.29) substrate:kbr wire length [µm] [ * ] Kreibig, U. und M. Vollmer: Optical properties of metal clusters. Springer, 1995 Effective medium: substrate Rough approximation [ * ] : eff ( ε ε ) 1 ε + 2 sub sur Modified λ/2- relation L = Γ 2 n eff ε < ε < ε air, eff neff nanowire sub ε eff

10 Experimental results: Resonance wavelength (Γ) Resonance wavelength (Γ) vs. wire length (L) [ * ] resonance wavelength [µm] 14 BEM calculations (D = 200nm, n = 1) BEM calculations (D = 200nm, n eff = 1.29) dipole (n = 1) 12 dipole (n = n eff = 1.29) D = 200nm (exp.) 10 D = 210nm (exp.) wire length [µm] [ * ] F. Neubrech, T. Kolb, R. Lovrincic, G. Fahsold, J. Aizpurua, S. Karim, T. W. Cornelius, M. E. Toimil-Molares, R. Neumann and A.Pucci, Applied Physics Letters (submitted) Light scattering simulations (performed by J. Aizpurua) of gold nanowires considering wire aspect ratio material properties n eff =1 (air) Scattering simulations in considering n eff are in good accordance with our measured data

11 Experimental results: surrounding medium Influence of the surrounding medium on ω res before evaporating paraffin after evaporating paraffin 1 sample reference sample reference nanowire 2 paraffin nanowire substrate (ZnS) substrate (ZnS) surrounding medium: air surrounding medium: paraffin

12 Experimental results: surrounding medium Shift of ω res due to the polarizability of paraffin relative transmittance diameter D = 100nm, length L = 1.4µm, substrate ZnS par ω res ω res 2850 cm cm -1 before evaporation after evaporation Estimation of the ratio par D par πc /( Lneff ) R ω n res L = = ωres air D n πc /( Lneff ) R L using system 1 system ε eff ( ε sub + ε sur ), neff 2 air eff par eff ε system eff wavenumber [cm -1 ] Exp. Value: / ω ω par res res = n n air eff par eff = 0.92 ε ZnS 4.84, ε = 2.02 = par

13 Experimental results: extinction cross section From relative transmittance to the extinction cross section related to the geometric cross section relative transmittance diameter D = 210 nm length L = 2.37 µm parallel polarization σ ext (ω) / σ geo (ω) diameter 210 nm, length 2.37 µm measurement (p-polarized) wavenumber [cm -1 ] 5 σ σ ext geo = A (1 T ( ω)) 0 1 LD (1 + n 2 sub ) wavenumber [cm -1 ] spot size extinction geometric cross section substrate effect

14 Experimental results: extinction cross section σ ext (ω) / σ geo Enhancement of the extinction cross section wavelength [µm] diameter D = 210 nm, length L = 2.37 µm, p-polarized, substrate: KBr Ratio σ ext / σ geo of a single nanowire at the resonance maximum: / 25 σ ext σ geo wavenumber [cm -1 ] Indication of localfield enhancement in the vicinity of the wire

15 Experimental results: extinction cross section Light scattering simulations wavelength [µm] σ ext (ω) / σ geo 20,0 10,0 6,67 5,00 4,00 3,33 2,86 2,50 2, E k diameter 210 nm, length 2.37 µm measurement (p-polarized) BEM calculation (n eff =1.29) Light scattering simulations (boundary element method (BEM)) performed by J. Aizpurua n eff = wavenumber [cm -1 ] Good accordance with light scattering simulations performed by J. Aizpurua

16 Outline Introduction Experimental setup Nano-antennas Experimental results Outlook Summary

17 Outlook: SEIRA Surface enhanced infrared absorption Occurs for adsorbates on rough films, island films, nanowires, Enhancement due to electrical field enhancement (in the near field of metal nanostructures), SEIRA-studies of arrays of gold nanowires SEIRA-studies of slits in gold nanowires Enhancement up to a factor of is expected [ * ] [ * ] J. Aizpurua, G.W Bryant et al., Phys. Rev. B 71, (2005)

18 Outlook first SEIRA measurements Adsorbate: octadecanthiol (ODT) with characteristic CH stretching vibration bands (2848 cm -1 and 2915 cm -1 [ * ] ) Adsorbent: single gold nanowire deposited on CaF 2 ODT (1 monolayer) nanowire substrate (CaF 2 ) sample reference [ * ] D. Enders and A.Pucci, Applied Physics Letters, 88, (2006)

19 Outlook First SEIRA measurements Relative transmittance spectra relative transmittance cm cm % diameter D = 100nm length L = 1.7µm substrate: CaF 2 no polarizer used wavenumber [cm -1 ]

20 Outlook First SEIRA measurements Relative transmittance spectra relative transmittance cm cm % diameter D = 100nm length L = 1.7µm substrate: CaF 2 no polarizer used 0.4 % 2850 cm cm wavenumber [cm -1 ] wavenumber [cm -1 ] Possible enhancement

21 Outline Introduction Experimental setup Nano-antennas Experimental results Outlook Summary

22 Summary Spectroscopic IR microscopy at single gold nanowires: resonances ω res, Γ : influenced by shape, substrate and surrounding medium σ ext / σ geo : Enhanced far-field cross section, which indicates an enhanced local field in the vicinity of the nanowire SEIRA We performed primarily SEIRA studies of ODT (1 monolayer) on single gold nanowires In order to maximize the SEIRA, several SEIRA measurements with gold nanowires are planned

23 Acknowledgement Kirchhoff-Institute for physics Our Group, Kirchhoff-Institut für Physik, Universität Heidelberg, Germany A. Pucci (group leader) G. Fahsold (former member) T. Kolb (former member) O. Skibbe R. Lovrincic M. Klevenz F. Meng M. Binder Materials Research Department, GSI, Darmstadt, Germany R. Neumann (group leader) E. Toimil-Molares (former member) S. Karim T. Cornelius Donostia International Physics Center, San Sebastian, Spain J. Aizpurua Employees at the ANKA IR-beamline, Forschungszentrum Karlsruhe, Germany Y.L. Mathis M. Suepfle

24 END

25 Introduction: preparation The gold nano-wires are prepared by electrochemical deposition in polymeric etched ion track membranes at the Material Research Department at the GSI [ * ] Preparation process: irradiation etching metallic layer electro-deposition dissolution [ * ] GSI : Gesellschaft für Schwerionenforschung, Darmstadt L ~ 1.7µm SEM image of a gold nano-wire (diameter D=100nm, length L=1.7µm substrate: silicon)

26 SEM images Au- nanowires (100nm) at Si HRTEM and SAED images 50 nm 20 nm Liu, J., Karim. S et al. Synthesis of gold nanowires in nanoporous ion track membranes L ~ 1.7µm Investigated nanowires 1-5µm long diameter nm single- and polycristalline

27 EM- Scattering at small particles consider a small spherical particle with D << λ and D < δ skin : D damping of the charge carrier oszillation: radiative decay: emission of a photon (scattering) nonradiative decay: intrabandtransisions (absorption) Quasistatic limit: description within the framework of Mie theory first only for spheres 1 resonance peak extention to ellipsoid particles 3 resonance peaks

28 Quasistatic Limit ω ω P L D aspectratio q=l/d 1 3 nanoparticle (Mie resonance) 1 L/D Bohren, C. und D. Huffmann: Absorption an scattering of light by small particles. John Wiley and Sons, No longer quasistatic, we have to regard retardation

29 Introduction: nano-antennas λ π Easiest model: macroscopic ideal antenna L = j e.g. ω λ / 2 = j 2 n L From measurements and light scattering simulations of nanowires in the visible spectral range deviations are observed c n L = λ 2 n L k E Measurements at gold nano-rods [ * ] Boundary element method (BEM) simulations [#] Deviations due to retardation, skin effect, influence of substrate and surrounding material D D πc D ωres L, neff, = ωλ / 2( L, neff ) R = R L L L n L [ * ] G. Schider, J. R. Krenn et al., Phys. Rev. B 68, (2003); [#] J. Aizpurua, G.W Bryant et al., Phys. Rev. B 71, (2005) eff

30 Experimental results: extinction cross section Enhancement of the extinction cross section σ ext (ω) / σ geo E k diameter 210 nm, length 2.37 µm measurement (p-polarized) MWS calculation (p. c. cylinder) MWS calculation (lossy metal) BEM calculation (n = 1) BEM calculation (n eff =1.29) BEM calculation (L = 2.15 µm, neff= 1.29) wavenumber [cm -1 ] Finite difference time domain (FDTD) simulations light scattering simulations (boundary element method (BEM)) performed by J. Aizpurua n eff = 1 (vacuum) n eff = 1.29 different lengths Good accordance with exact light scattering simulations performed by J. Aizpurua

31 Basics: Experimental determination of optical properties - thin films relative transmittance at normal incidence: T film/substrate T substrate ε film ε = 1 2 d ω Imε IIfilm(ω) c (1+n substrate ) iσ( ω) ωε 0 For films much thinner than the wavelength in the material d << λ / nfilm, simple relations for the transmitted intensity of a film on a transparent substrate can be derived from the Fresnel coefficients of the transmitted amplitude., Conductivity measurement without electrical contacts ( ) Local optics was supposed. It is valid in the middle IR also for metals, as more as shorter the mean free path l.

32 Resonance width (ω Γ ) width ω Γ [cm -1 ] nm (Au@ZnS) 160 nm (Au@CaF) 330 nm (Au@CaF) 100 nm (Cu@KBr) 200 nm (Cu@KBr) 80 nm (Au@KBr) 100 nm (Au@KBr) 200 nm (Au@KBr) 210 nm (Au@KBr) 200 nm Sim (pc. cyl.) 100 nm Sim (pc. cyl.) resonance frequency [cm -1 ] in accordance with RCS- calculations (scattering at a perfect conducting wire) no difference between Au and Cu wires materialproperties are not dominant

33 Polarisationeffects experimental setup: polarizer nanowire MCT ideal case: (not this wire) E Per 1.00 substrat E Par 1 µm rel. transmittance parallel polarized perpendicular polarized real case: (not this wire) 5 µm wavenumber [cm -1 ] SEM images: Au- nanowires at Si done by S. Karim

34 What is SEIRA? Surface enhanced infrared absorption, enhancement up to a factor of 2000 observed, much more is theoretically predicted Analogous to SERS and in certain cases complementary (because of different selection rules)

35 What is SEIRA? Occurs for adsorbates on metal nanostructures (rough nanofilms, island films, grids,...) Enhancement due to field enhancement (in the nearfield of metal particles, at hot spots of disordered metal-island films, at plasmon resonances of periodic structures - see photonics,...) chemical effect (increase in polarizability by adsorption) first layer effect (non-adiabatic interaction between vibrations and electron-hole-pair excitations)

36 SEIRA Effect The phenomenon surface enhanced IR absorption (SEIRA) concerns molecules adsorbed on metal surfaces IR absorption, 2 µ 2 2 A ~ E Q cos Enhancement mechanism: -Electromagnetic field enhancement: Incident photon interaction with metal surface enhances local electric field E local at surface θ ; E µ = Q = = Electric Dipole Normal field moment Coordinate J.P.Kottmann, O.J.F. Martin, OPTICS EXPRESS 8 (2001) 655

37 SEIRA: enhancement factor 0.4 % 2850 cm cm wavenumber [cm -1 ] Our case: ~500

38 Resonance enhancend IR-spectroscopy the idea IR- spectra of a single gold nanowire: nearfield calculation: E(1666cm -1 ) /E 0 (1666cm -1 ) relative transmittance diameter 210 nm length 2.37 µm L=3.0µm, D=50nm wavenumber [cm -1 ] absorptionbands of paraffin (measured at anka): x [µm] model: y [µm] y x thickness: unknown the absorptionbands of paraffin have to be in the range of the resonance frequency of the nanowire

39 Resonance enhanced IR- spectroscopyexperimental setup experimental setup: IR- spectra: NW,par par,kbr KBr paraffin nanowire substrate (KBr) procedure: 1. rel. Transmision measurement at position [nw, par] and [par,kbr] 2. calcutate the ratio p = I(1470cm -1 ) / I(2930cm -1 ) 3. comparison between p [nw,par] and p [nw,kbr] we are not able to detect any enhancement

40 Resonance enhanced IR- spectroscopy el. Field enhancement is spacial limited L=3µm, D=50nm E(1667cm -1 ) /E 0 (1667cm -1 ) ratio of enhanced signal to background signal is small (~7%) L=2.7µm D=200nm A spot =54.4µm 2 only a small fraction of the evaporated paraffin perceive the enhanced field only a small fraction of the signal is enhancend we are not able to detect any enhancement with this experimental setup

41 At the sharp nanowire ends and particularly in a small gap between such ends a huge nearfield-field enhancement is expected. CO adsorption on Cu films at 100K SEIRA Island-film example: E p vib G. Fahsold, M. Sinther, A. Priebe, S. Diez, and A. Pucci, Phys. Rev B 70 (2004) E local enhanced