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Near-field Optical Imaging of Carbon Nanotubes

Near-field Optical Imaging of Carbon Nanotubes. Achim Hartschuh, Huihong Qian Tobias Gokus, Department Chemie und Biochemie and CeNS, Universität München Neil Anderson, Lukas Novotny The Institute of Optics, University of Rochester, Rochester, NY.

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Near-field Optical Imaging of Carbon Nanotubes

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  1. Near-field Optical Imaging of Carbon Nanotubes Achim Hartschuh, Huihong Qian Tobias Gokus, Department Chemie und Biochemie and CeNS, Universität München Neil Anderson, Lukas Novotny The Institute of Optics, University of Rochester, Rochester, NY Achim Hartschuh, Nano-Optics München

  2. Why High Spatial Resolution Near-field Optics? • Why Optics? • electronic and vibronic energies • DOS • excited state dynamics.... Achim Hartschuh, Nano-Optics @ LMU

  3. Why Near-field Optics? Achim Hartschuh, Nano-Optics @ LMU

  4. Why Near-field Optics? Achim Hartschuh, Nano-Optics @ LMU

  5. spatial resolution is limited by diffraction l/2 Why Near-field Optics? Achim Hartschuh, Nano-Optics @ LMU

  6. Uncertainty relation:  Diffraction Limit Abbé, Arch. Mikrosk. Anat. 9, 413 (1873) Achim Hartschuh, Nano-Optics @ LMU

  7. Tip-Enhanced Spectroscopy Wessel, JOSA B 2, 1538 (1985) Achim Hartschuh, Nano-Optics @ LMU

  8. Tip-Enhanced Spectroscopy laser illuminated metal tip Novotny et al. PRL 79, 645 (1997) Theory: Enhanced electric field confined to tip apex Mechanism: Lightning rod and antenna effect, plasmon resonances Achim Hartschuh, Nano-Optics @ LMU

  9. Tip-Enhanced Spectroscopy SEM micrograph diameter = 22 nm • enhanced electric field confined within 20 nm ? ……………….Optical imaging with 20 nm resolution?! ……………….Signal enhancement !? Achim Hartschuh, Nano-Optics @ LMU

  10. + Tip-sample distance control a sharp metal tip is held at constant height (~2nm) above the sample using a tuning-fork feedback mechanism K. Karrai et al., APL 66, 1842 (1995) 2 nm Topography of the sample Experimental Setup Confocal microscope Optical Images and Spectra Achim Hartschuh, Nano-Optics @ LMU

  11. Tip-enhanced Spectroscopy (Examples…………) Raman scattering S.M. Stoeckle et al., Chem. Phys. Lett. 318, 131 (2000) N. Hayazawa et al., Chem. Phys. Lett. 335, 369 (2001) A. Hartschuh et al. Phys. Rev. Lett. 90, 95503 (2003) B. Pettinger et al., Phys. Rev. Lett. 92, 96101 (2004) N. Anderson et al., J. Am. Chem. Soc. 127, 2533 (2005) C.C. Neacsu et al., Phys. Rev. B 73, 193406 (2006) D. Methani, N. Lee, R. D. Hartschuh et al. J. Raman Spectrosc. 36, 1068 (2005) ................................. Two-photon excited fluorescence E.J. Sánchez et al., Phys. Rev. Lett. 82, 4014 (1999) H.G. Frey et al., Phys. Rev.Lett. 81, 5030 (2004) J. Farahani et al. Phys. Rev. Lett. 95, 017402 (2005) A. Hartschuh et al. Nano Lett. 5, 2310 (2005) Fluorescence Achim Hartschuh, Nano-Optics @ LMU

  12. Single-Walled Carbon Nanotubes (SWCNT) single-walled carbon nanotube diameter ~ 1-2 nm Graphene sheet + roll up model length up to mm structure (n,m) determines properties: (n-m) mod 3 =0 metallic else semiconducting from Shigeo Maruyama Achim Hartschuh, Nano-Optics @ LMU

  13. Near-field Raman Imaging of SWCNTs Raman image (G’ band) Topography Hartschuh et al. PRL 90, 95503 (2003) 500 nm 500 nm only SWCNT detected in optical image chemically specific optical contrast with 25 nm resolution enhanced field confined to tip Achim Hartschuh, Nano-Optics @ LMU

  14. G RBM at 199 cm-1 diam = 1.2 nm structure (n,m)(14,2) metallic SWCNT RBM Near-field Raman Spectroscopy Raman image (G band) Topography image height: 0 - 1.9 nm Achim Hartschuh, Nano-Optics @ LMU

  15. Resolution enhancement Farfield Near-field no tip same area with tip Achim Hartschuh, Nano-Optics @ LMU

  16. Signal Enhancement Achim Hartschuh, Nano-Optics @ LMU

  17. Signal Enhancement tip-enhanced signal > signal * 2500 Hartschuh et al. Phil. Trans. R. Soc. Lond A, 362 (2004) Achim Hartschuh, Nano-Optics @ LMU

  18. Distance Dependence Enhanced Raman scattering signal ~ d-12 d tip-enhancement is near-field effect => tip has to be close to sample Achim Hartschuh, Nano-Optics @ LMU

  19. Near-field Raman Spectroscopy N. Anderson, unpub. RBM G 30 nm steps wRBM=251 cm-1  (10,3) (semiconducting) wRBM=191 cm-1  (12,6) (metallic) • intramolecular junction (IMJ) pn-junction Achim Hartschuh, Nano-Optics @ LMU

  20. Simultaneous Raman and PL Spectroscopy (7,5) (8,3) (9,1) (6,4) Photoluminescence Emission Raman Raman Excitation at 633 nm  Emission and Raman signals are spectrally isolated A. Hartschuh et al. Science 301, 1394 (2003) Achim Hartschuh, Nano-Optics @ LMU

  21. Topography Raman (G-band) Photoluminescence •  correlate Raman and photoluminescence • properties • length of emissive segment ~ 70 nm • changes in chirality (n,m)? • coupling to substrate? Near-field optical imaging of SWCNTs Farfield Raman image DNA-wrapped SWCNTs on mica Achim Hartschuh, Nano-Optics @ LMU

  22. Localized PL-Emisssion on Glass Photoluminescence Raman scattering SWCNTs on glass • Emission spatially • confined to within • 10 – 20 nm • Localized excited states • Bound excitons? • role of defects? • substrate? Photoluminescence Achim Hartschuh, Nano-Optics @ LMU

  23. Near-field PL-Spectroscopy Topography Photoluminescence PL spectra: 30 nm steps between spectra 1 2 3 4 5 6  Emission energy can vary on the nanoscale (caused by changes in local dielectric environment e) A. Hartschuh et al. Nano Lett. 5, 2310 (2005) Achim Hartschuh, Nano-Optics @ LMU

  24. Tip-enhanced Microscopy Spatial resolution < 15 nm Signal amplification Tip as nanoscale „light source“ Achim Hartschuh, Nano-Optics @ LMU

  25. kex: enhanced excitation field • SPL~ (Elocal / E0)2=f2 • krad: Purcell-effect krad Q = krad + knonrad Q is increased (Q0~10-4) cycling rate is increased SPL~ f2 Q/Q0 • knonrad: dissipative energy transfer to metal quenching of PL kex PL Enhancement depends on Q0! krad knonrad Signal Enhancement Raman scattering Photoluminescene Enhancement of incident field and scattered field SRaman ~ (Elocal / E0)2 (Elocal / E0) 2=f4 local field at tip field without tip Ein Eout Achim Hartschuh, Nano-Optics @ LMU

  26. 500 nm 500 nm Signal Enhancement Raman enhancement PL enhancement No far-field PL < 200 counts/s Near-field PL ~17000 counts/s PL-enhancement >17000 / 200 ~ 85 Far-field Raman ~ 2000 counts/s Near-field Raman ~ 4000 counts/s Raman-enhancement ~ 6000 / 2000 ~3 SPL~ f2 Q/Q0 SRaman ~ f4  PL quantum yield is increased by tip (SEF) Achim Hartschuh, Nano-Optics @ LMU

  27. Signal Enhancement (First data) d knon-rad kex krad  PL quenching for very small distances  optimum distance for PL enhancement Achim Hartschuh, Nano-Optics @ LMU

  28. Signal Enhancement P. Anger et al. (First data) d knon-rad kex krad Achim Hartschuh, Nano-Optics @ LMU

  29. tip-nanotube-distance Signal Enhancement Raman scattering Topography Photoluminescence 290nm 290nm 290nm DNA-wrapped SWCNTs Raman scattering Photoluminescence (schematic) Achim Hartschuh, Nano-Optics @ LMU

  30. High resolution provided by evanescent fields that have higher k-vectors: Nanotubes: Near-field Interactions Uncertainty relation: Diffraction limit for propagating waves: k-vectors of tip enhanced fields extend through BZ !  selection rules for optical transitions? Achim Hartschuh, Nano-Optics @ LMU

  31. Summary High-resolution optical microscopy of carbon nanotubes using a sharp laser-illuminated metal tip • PL and Raman spectroscoy and imaging • Spatial resolution < 15 nm • Signal enhancement Results • Resolved RBM variations (IMJ) on the nanoscale • Non-uniform emission energies that result from local variations of dielectric environment • Strongly confined emission signals  bound excitons? • PL-Quantum yield can be enhanced by metal tip Achim Hartschuh, Nano-Optics @ LMU

  32. PC Tuebingen: Alfred J. Meixner Mathias Steiner Antonio Virgilio Failla Thanks to PC Siegen: Gregor Schulte Funding: DFG, Cm Siegen, NSF, FCI Achim Hartschuh, Nano-Optics München

  33. Acknowledgement PC Tuebingen: Alfred J. Meixner Mathias Steiner Antonio Virgilio Failla PC Siegen: Gregor Schulte Funding: DFG, Cm Siegen, NSF, FCI Achim Hartschuh, Nano-Optics @ LMU

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