1 / 45

Raman Spectroscopy of Nanostructures

Raman Spectroscopy of Nanostructures. EE 235 16 March 2009. Raman studies of nanostructures. Multiple meanings! Study of the nanostructures themselves Study of a chemical analyte on top of nanostructures. Outline. General principles Light scattering

mariel
Download Presentation

Raman Spectroscopy of Nanostructures

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Raman Spectroscopy of Nanostructures EE 235 16 March 2009

  2. Raman studies of nanostructures Multiple meanings! • Study of the nanostructures themselves • Study of a chemical analyte on top of nanostructures

  3. Outline • General principles • Light scattering • Electronic and vibrational energy levels • Experimental implementation • Raman scattering in crystals • Resonant enhancement • Signal enhancement • Resonant enhancement • Surface enhancement (SERS)

  4. Outline • General principles • Light scattering • Electronic and vibrational energy levels • Experimental implementation • Raman scattering in crystals • Resonant enhancement • Signal enhancement • Resonant enhancement • Surface enhancement (SERS)

  5. Optical options l  l0 • Raman scattering: Inelastic light scattering Incident light l0 Absorbed Transmitted l0 l0 Reflected Elastic l0 Scattered l  l0 Inelastic

  6. Light scattering Rayleigh: Elastic scattering Raman: Inelastic scattering www.phy.cam.ac.uk/history/historypictures/LordRayleigh.jpg www.iisc.ernet.in/images/CVRaman.gif

  7. Elastic scattering Electronic excited state E=E0cos(wit) + Induced polarization P =  E (: polarizability) - + “Virtual state” - r0 r Dipole re-radiates at same frequency wi Electronic ground state

  8. - + - + - + Inelastic scattering Vibration changes polarizability: r0 a0 m1 m2 Periodic displacement a|Qmax wvib a|Qmin Frequency of scattered light differs from the incident light by an amount corresponding to a normal mode of the mechanical oscillator

  9. Classical scattering model Induced polarizability change from vibrations (first order) Taylor expand Rayleigh term: Raman term: Stokes Anti-Stokes

  10. O C O Energy scales • Visible light (red), 620 nm ↔ 2 eV ↔ 484 THz ↔ 16130 cm-1 • CO2 symmetric mode, 1335 cm-1↔ 41 THz ↔ 0.165 eV ↔ 7.5 mm Vibrational energies << Optical energies

  11. Scattering processes Energy Electronic excited state Raman Rayleigh Vibrational levels Electronic ground state FIR- MIR Stokes Anti-Stokes

  12. Scattering spectroscopy • Measure the wavelength-shifted scattered light as a probe of vibrational energies • Known characteristic bond frequencies

  13. Characteristic frequencies cm-1

  14. Scattering spectroscopy • Measure the wavelength-shifted scattered light as a probe of vibrational energies • Known characteristic bond frequencies • “Chemical fingerprinting”: every molecule has a unique vibrational spectrum

  15. Chemical fingerprinting • IR absorption and Raman scattering complementary • Same transitions, different selection rules - all has to do with symmetry cm-1

  16. Scattering spectroscopy • “Chemical fingerprinting”: every molecule has a unique vibrational spectrum • Check the aptly-named Journal of Raman spectroscopy: • Chemical analysis tool! http://www3.interscience.wiley.com/journal/3420/home

  17. Outline • General principles • Light scattering • Electronic and vibrational energy levels • Experimental implementation • Raman in crystals • Resonant enhancement • Signal enhancement • Resonant enhancement • Surface enhancement (SERS)

  18. In principle, any laser wavelength shorter (i.e. higher energy) than the vibrational mode of interest will work as a source Scattering processes Energy Electronic excited state Raman Rayleigh Vibrational levels Electronic ground state FIR- MIR Stokes Anti-Stokes

  19. O C O Implementation issues • Weak Raman-scattered light close to strong source laser + Rayleigh-scattered light • Need high rejection of unwanted “carrier frequency” light very close to Raman-shifted signal light 2 eV Intensity 0.165 eV eV

  20. Spectrometer Implementation issues • Need: very clean source line • Very sharp edge discrimination Dispersive stage Sample Filter Detector Narrow excitation source

  21. Spectrometer3 Spectrometer1 Spectrometer2 Raman instrumentation • Multi-stage spectrographs Disperse Recompress Disperse Input Detector • Excellent stray light rejection, can measure very close to laser line (few cm-1) • Large and expensive

  22. Spectrometer Detector Raman instrumentation • Single spectrometer, strong filters • Longpass filter in collection path • Line-pass filter (shortpass for tunable lasers) for laser line clean-up in excitation path • Final resolution depends on spectrograph and detector; filter limitation ~tens of cm-1 Input Long-pass filter to transmit only Stokes-scattered light semrock LP-633RE

  23. micro-Raman spectroscopy • Spatial imaging down to diffraction limit of excitation wavelength used • Confocal setup - small sample volume probed • Vibrational map of sample JobinYvon

  24. Outline • General principles • Light scattering • Electronic and vibrational energy levels • Experimental implementation • Raman scattering in crystals • Resonant enhancement • Signal enhancement • Resonant enhancement • Surface enhancement (SERS)

  25. [111] c-axis: [0001] Raman scattering in crystals • Vibrational modes in crystals: phonons • Symmetry considerations = entire field of physics! Wurtzite: hexagonal crystal e.g. II-VI and III-Nitrides semiconductors: ZnO, ZnS, CdSe, GaN, etc. Zincblende: cubic crystal e.g. III-V semiconductors: usual for III-Arsenides and III-Antimonides, sometimes also III-Nitrides

  26. [0001] c-axis || c ┴c ┴c ┴c Raman scattering in crystals • Wurtzite crystal: hexagonal structure • Anisotropic crystal: Material properties like refractive index, effective masses depend on direction relative to c-axis N Ga N “optical axis” Ga • Restoring forces for crystal vibrations also have c-axis dependence

  27. Wurtzite crystal modes c-axis Displacement of atoms || c-axis: A and B modes Displacement of atoms ┴c-axis: E-modes N Ga E1 A1 E2(2) E2(1) B1(1) B1(2) Polar (TO - LO splitting) Polar (TO - LO splitting) Non-polar Non-polar

  28. Wurtzite crystal modes c-axis N Ga E1 A1 E2(2) E2(1) B1(1) B1(2) • Vibrational mode measured depends on form of tensor for a given measurement geometry

  29. N Ga Raman mode polarization • Vibrational mode measured depends on form of tensor for a given measurement geometry c-axis || z “A1“ Ei || z ki ks Es || z

  30. N Ga Raman mode polarization • Vibrational mode measured depends on form of tensor for a given measurement geometry • Cross-polarized measurement gives no mode at this “A1” frequency c-axis || z “A1“ Ei || z ki ks Es || z Es ┴z

  31. N Ga Raman mode polarization • Different mode, different intensity growth axis is not c-axis “A1“ Ei || z ki ks Es ┴z

  32. Summary: Raman in crystals • Measurement geometry / polarizations determine which modes can be Raman-scattered • This example was looking at how one particular mode (A1) would change • 1) Knowing the nanowire direction but under different measurement geometries • 2) Not knowing crystal orientation: use mode selection to determine • Non-destructive structural characterization tool for nanostructures! For example:

  33. Outline • General principles • Light scattering • Electronic and vibrational energy levels • Experimental implementation • Raman scattering in crystals • Resonant enhancement • Signal enhancement • Resonant Raman - good or bad? • Surface-enhancement (SERS)

  34. Electronic resonance Energy Electronic excited state Resonant Raman: effective cross-section increased ~10-105 from non-resonant case (comparable to Rayleigh scattering!) UV / VIS / NIR Vibrational levels Electronic ground state FIR- MIR Stokes

  35. Resonant Raman - GaAs nanocrystals • NIR used as excitation source, close to PL spectrum T = 1.2 K Excitation laser LO phonon Photoluminescence Raman-scattered phonon: follows laser energy 2-phonon process PL: no change in energy if laser wavelength changes

  36. Resonant Raman - GaAs nanocrystals • Track intensity of single vibrational mode over laser excitation tuning range of electronic resonance Outgoing resonance: Scattered phonon energy matches electronic transition Intensity of 294 cm-1 GaAs line Incoming resonance: Laser energy = electronic transition

  37. Resonant Raman • Why not do this all the time? • Energy of electronic resonance distinct from scattering process, does not follow laser (e.g. semiconductor bandgap) •  Excitation source must match electronic resonance • Much easier to match with tunable lasers (not readily available in the UV/VIS) • Could enhance a few particular lines, overwhelm others?

  38. Resonant Raman • Entirely sample-dependent. Chemical analysis: fluorescence always a bad word! end up choosing laser wavelength to avoid fluorescence

  39. Outline • General principles • Light scattering • Electronic and vibrational energy levels • Experimental implementation • Raman scattering in crystals • Resonant enhancement • Signal enhancement • Resonant enhancement • Surface enhancement (SERS)

  40. 1974: Pyridine on rough silver surface • Wet experiment; silver electrode ended up etched • No great claims of enhancement; were interested in type of bond to Ag surface

  41. 1974: Pyridine on Ag surfaces • Authors focus on middle peak to propose bond model • Intensity increases more than can be accounted for with surface roughening Increasing voltage on Ag electrode

  42. 1977: Pyridine on Ag surfaces • Proposed EM mechanism related to “hot spots” on metal surface

  43. Skip forward 30 years… • How to engineer “hot spots”? • Very active area of research • Increased Raman signal by >107 via EM field enhancements • Much lower excitation laser power requirements • Roughened metal surfaces, sharp-tipped metallic structures, bow-tie substrates • Also: “Tip-enhanced Raman scattering” (TERS) using NSOM • Jeff Neaton (LBL) - next week, Raman/SERS theory • Luke Lee (BioE) - 2 weeks, SERS experimental work

  44. r0 Mid- to far-infrared Energy scales E n = 2 Electronic level n = 1 r Vibrational levels Rotational levels (microwave) r0

  45. Energy scales Acoustic Radio Micro FIR NIR VIS UV X-ray g-ray Nuclear spins (NMR) Electron spins (ESR), rotational levels Core electron levels Vibrational levels Electronic levels

More Related