Laboratoire de Physique des Interfaces et des Couches Minces (LPICM) CNRS, Ecole Polytechnique

1. Polarization properties of oblique incidence Tip enhanced Raman spectroscopy Gennaro Picardi Laboratoire de Physique des Interfaces et des Couches Minces (LPICM) CNRS, Ecole Polytechnique 91128 Palaiseau France

2. SPM (PSIA) Optical coupling Raman (HORIBA JY) Grating Detector Microscope Laser Analyzer Half-wave plate Notch filter Piezo z Piezos x, y Experimental set-up Feedback control Confocal Raman Oblique back-scattering configuration Side illumination of the tip

3. STM Au tip etching 1-2 min 7-8 min Break of the tip (circuit closes) Oscillatory electrodissolution of gold e- Applied Voltage: ~2.4 V Passive region AuOH Au*-H20 Anodic oxidation (passivation) H+, 2 Cl- Rtip = 20-30 nm Au dissolution H20, Cl- ½ e-, Cl- Au*-Cl- AuCl2- + H20 Au deposition Disproportion Cl- Electrochemical etching of a gold wire (0.125 mm) in a solution of 1:1 ethanol and conc. HCl (37%). Active region Au electrodissolution in HCl is diffusion limited AuCl4- Z.L. Li,T.H. Wu, Z.J. Niu, W. Huang, H.D. Nie ; Electrochem. Comm. 6 (2004) 44 B. Pettinger, B. Ren, G. Picardi, R. Schuster and G. Ertl; Rev. Sci. Instrum. 75 (4) (2004) 837 L. Billot, L. Berguiga, M.L. de la Chapelle, Y. Gilbert and R. Bachelot; Eur. Phys. J. Appl. Phys. 31 (2005) 139 X. Wang, Z. Liu, H. Zhang, X. Wang, Z. Xie, D. Wu, B. Ren and Z. Tian; Appl. Phys. Lett. 91 (2007) 101105

4. STM-TERS on dyes lexc 633 nm 20 s D1 filter 585 cm-1 Brilliant Cresyl Blue on Au (111) Au tip (oblique incidence) G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Europ. Phys. J. Appl. Phys. in press (2007)

5. TERS for structured materials nano-characterization Micro-Raman Spectroscopy provide mapping of stresses in Si structures ( well defined Raman shift due to strain ) 380 nm 300 nm Near-field Raman mapping at 521 cm-1 ( wSi-Si ) (using aperture-less probe) 3700 nm W.X. Sun and Z.X. Shen Near-field scanning Raman microscopy using apertureless probes J. Raman spectrosc. 2003; 34: 668-676

6. Introducing polarized TERS (p) - pol (p) - pol no analyzer analyzer at 90° « … polarization of the ligth scattered by the particle will differ from the polarization of the incident light. The light partly depolarized by the particle then inelastically scatters with the optical phonon in Si thereby producing allowed Raman signal. The allowed Si Raman signal should be associated with a local area around the particle. » V. Poborchii, T. Tada and T. Kanayama, Jpn. J. Appl. Phys. 44 (2005)

7. Scattered light Incident light incident light- tip interaction scattered light- tip interaction Analyzer Half-wave plate Sample Polarization control in TERS I. The tip modifies the polarization state of the incident and scattered radiation. II. The far field signal can be reduced by using an analyzer. Polarization control may become an important parameter in the TERS experiment

8. Polarizer Analyzer Sample orientation S Calculation of the scattered intensity in the far field Raman intensity Rj: Raman tensor of j-phonon ei : incident polarization state (polarizer P) es : scattered polarization state (analyzer A) The scattered intensity depends on the polarization states ei, es as well as on the sample orientation S

9. Far field: experimental verification Calculation of the scattered intensity in the far field (100) c-Si (111) c-Si P || A P ┴ A Intensity (arb. un.) Intensity (arb. un.) Sample orientation S (deg) Sample orientation S (deg) The sample orientation (or azimuth) S modulates the scattered intensity.

10. Calculation of the scattered intensity in the near field (the tip-enhancement tensor) Far field (tip withdrawn): R (R : Raman polarizability tensor) Near field (light-tip intaction): R’ =ATR A (A : tip-enhancement tensor) Total field (tip in contact): Far field + Near field The tip tensor A transforms the sample Raman tensor R to an effective  scattering tensor R’ a and b : tip-dependent TERS parameters The tip-enhancement tensor A describes the field enhancing and polarization properties of the tip R. Ossikovski, Q. Nguyen and G. Picardi, Phys. Rev. B 75, 045412 (2007)

11. NH4F etched Si (111) NC-AFM STM Ut = -1.0 V It = 100 pA Au tip 750 nm Optical microscope Ut = -1.5 V It = 50 pA Pt/Ir tip 10 mm (b) Etch-pit initiation by dissolved oxygen on terraces of Si (111) C.P. Wade and C.E.D. Chidsey Appl. Phys. Lett. 71 (12) 1997

12. TERS on Si(111) with polarization control (I) Raman intensity of the 1st order Si phonon peak ( 521 cm-1 ) tip down tip up Tip #1 Analyzer fixed at 90° Tip #2 q = 20° q = 71° ( p ) ( s ) ( p ) ( s ) a : b = 1.6 : 1 a : b = 5.5 : 1 G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Appl. Spectr. 61 (12), 2007

13. Oblique incidence: p - polarization or s - polarization? Tip #2 BCB on Au (111) No analyzer (with p-pol ) (with s-pol ) Higher intensity with incident (p) polarization but strong TERS also with incident (s) light G. Picardi, Q. Nguyen, J. Schreiber and R. Ossikovski, Appl. Spectr. 61 (12), 2007

14. Influence of the incident polarization in TERS ( I ) ( p )– polarization (in-plane) of the incident electric field EFE up to 35 ( s )– polarization (out-of-plane) of the incident electric field EFE up to 10 ‘ Finite Element simulations of TERS ’ ‘ Understanding TERS ’ A. L. Demming, F. Festy and D. Richards J. Chem. Phys. 122, 1847 (2005) A. Downes, D. Salter and A. Elfick J. Phys. Chem. B 110, 6692 (2006)

15. Influence of the incident polarization in TERS ( II ) Overall higher field enhancement with p– polarized excitation, but field enhancement also with s– polarized ligth. 1. With p– polarized light illumination also enhancement of the field component in the substrate plane (i.e. out of the plane of incidence). 2. With s– polarized light illumination also enhancement of the field component normal to the substrate plane (i.e. in the plane of incidence). 2 bis. Near-field Raman spectra of SWCNT measured under p- and s- polarization conditions. ‘Under p- pol or s- pol light illumination the charge is differently concentrated.’ Polarization measuraments in TERS applied to SWCNT Y. Saito, H. Hayazawa, H. Kataura, T. Murakami, T. Tsukagoshi, Y. Inouye and S. Kawata Chem. Phys. Lett. 410, 136 (2005) Cross polarization effect (depolarized enhancement) 3. Imaging should be (?) different. S. Foteinopoulou, J. P. Vigneron and C. Vandenbem Opt. Expr. 15, 4253 (2007)

16. Far field artifacts when TERS probing bulk samples ‘ The (tip in) contact signal may include a component that is unrelated to the plasmon resonance enhancement due to reflection and scattering from the tip … leading to additional unlocalized Raman signal.’ N. Lee, R. D. Hartschuh, D. Methani, …and A. P. Sokolov , J. Raman Spectrosc. 38, 789 (2007). ‘ … significant enhancement of Raman scattering from silicon substrates can be achieved without a field enhancement effect by plasmon resonces. Pure laser deflection and near field scattering cause similar effects which are difficult to distinguish.’ C. Georgi, M. Hecker and E. Zschech, Appl. Phys. Lett. 90, 171102 (2007).

17. ACKNOWLEDGEMENTS Quang Nguyen Razvigor Ossikovski Bernard Drevillon (LPICM director)