How To Perform Raman Spectroscopy

Transcription

1 Spettroscopia Raman in campo prossimo Salvatore Patanè (1) and Pietro G. Gucciardi (2) (1) Dip. di Fisica della Materia e Tecnologie Fisiche Avanzate, Università di Messina, Salita Sperone 31, Messina, Italy. (2) CNR Istituto per i Processi Chimico-Fisici, sez. Messina, Via La Farina 237, I Messina, Italy.

2 Outlook Raman Spectroscopy: probing the morphological and chemical properties. Near-Field Optics: from Micro- to Nano- Raman spectroscopy Aperture SNOM: Raman imaging with 100 nm resolution Apertureless SNOM: Raman imaging down to the 10 nm scale.

3 Raman Spectroscopy MONOCHROMATIC 99.99% RADIATION Two kinds of scattering encountered: TRANSPARENT DUST-FREE SOLID, LIQUID or GAS Rayleigh (1 in every 10,000) same frequency Raman (1 in every 10,000,000) different frequencies

4 Rayleigh Vs Stokes emission

5 Conservation rules Ω (cm-1 ) hω L = hω S + hω phonon hk L = hk S + hq phonon 300 Raman Brillouin q phonon 4π 500 nm π 0.5 nm

6 What can be measured Carbon SW Nanotubes on SIlicon Peaks energy: Chemical properties Raman Spectrum Si Energy of the RBM: Structural properties (diameter of the CNT) CNT The G band: Electronic and symmetry properties (HOPG, MWNT, metallic, semiconductor) The D and G bands: morphological properties (defect characterization)

7 Coherent Anti-Stokes Raman Spectroscopy -CARS is a third-order coherent nonlinear Raman spectroscopy technique. -CARS uses three incident fields: a pump field, a Stokes field, and a probe field. -If the pump-probe frequency difference matches a specific Raman-active vibrational mode, an anti-stokes Raman signal is resonantly generated. - CARS microscopy provides three-dimensional imaging capability at a submicron scale.

8 MicroRaman Spatial resolution is diffraction-limited to ~ 250 nm Raman spectra of a PS - PMMA/PEP blend Witec CRM

9 Nanotechnology In recent general usage, any technology related to features of nanometer scale: thin films, fine particles, chemical synthesis, advanced lithography, etc. A technology based on the ability to build and look at structures of complex, atomic specifications. The Foresight Institute

10 Scanning Near-Field Optical Microscopy SNOM is an optical microscopy technique. It provides nanometer scale ( nm) spatial resolution. The same contrast mechanisms of confocal microscopy are available: fluorescence, raman, elastic scattering, polarimetry.

11 Raman at the Nanometer scale Motivations: Chemical Imaging. Morphological Imaging. Nanometer scale spatial resolution Added value: simultaneous sample topography and elastic scattering images. Difficulties: Low efficiency of the Raman scattering. Low throughput of the SNOM Fiber probes. Very long acquisition times for Imaging purposes. High mechanical and thermal stability are required.

12 Aperture-SNOM setup The sample is illuminated by a nanoscopic light source located close to the surface (10 nm). The probe is raster scanned. The resolution is limited by the source diameter

13 Aperture-NanoRaman setup Works in reflection mode (any sample) Collection: Nikon 50X objective, NA 0.5, high WD. Notch Filter: Rejection Ratio ~ Spectrometer: Triax 190, single grating, 1200 lines/mm, 190 mm focal. Detector: PMT in photon counting regime, or ICCD. Gucciardi et al., PCCP 4, 2747 (2002)

14 Aperture SNOM- NanoRaman Jahncke et al., APL 67, 2483 (1995) Webster et al., APL 72, 1478 (1998) Dekert et al., Anal. Chem. 70, 2646 (1998) Sample: Rb-doped KTP Scan points: (step 100 nm) Acquisition time: ~ 10h Estimated resolution: 250 nm (the aperture) Sample: Scratch on silicon Scan points: Acquisition time: > 9h Estimated resolution: submicron Sample: Dye-labeled DNA Scan points: (step 100 nm) Acquisition time: > 6h Estimated resolution: 100 nm

15 NanoRaman on TCNQ TCNQ-based complexes are used as dopants in organic opto-electronics High Raman Efficiency. The organometallic salt complexes can be discriminated based on the Raman shift of the vibrational peaks. Imaging at a fixed energy provides chemical contrast

16 NanoRaman imaging of TCNQ Topography Raman 1445 cm -1 Gucciardi et al., J. Microsc. 209, 228 (2003)

17 Resolution assessment TOPOGRAPHY RAMAN 1445 cm -1 Scan width: µm 2 Raman imaging confirms the spectral information on the chemical nature of the bumps. Raman (cts/s) nm nm 146 nm Position (µ m) Gucciardi et al., Appl. Opt. 42, 2724 (2003)

18 Limitations of aperture-snom NanoRaman Low light throughput Low signal to noise ratio Limited amount of excitation power Needs very efficient samples for imaging in reasonable times.

19 Electromagnetic Field enhancement Energy density Vs Polarization Surface Charge Vs Polarization

20 Sensors: Tungsten Tips Tungsten tips are obtained by electrochemical etching of a 100- µm tungsten wire The tip shape strongly depends on: immersion depth, edge wire shape, wire diameter, electrolyte concentration, voltage applied Tips of apical radius ranging from 10 nm to 100 nm are commonly produced. Subsequently the tips can be coated with gold or silver by using metal evaporation techniques. To assure the coating homogeneity the tip is rotating during the deposition. A.J. Melmed, J. Vac. Sci. Tech.B. 9(2) p. 601 (1991).

21 Sensors: Gold Tips (collab. with Cambridge Univ.) Gold tips are produced by electrochemical etching in a HCl-Ethanol 50-50% solution, V DC = 2.4 V SEM Pictures B. Ren et al., Rev. Sci. Instrum. 75, 837 (2004)

22 Tip-enhanced Raman Spectroscopy (TERS) An STM tip is brought into tunneling position and forms an optical cavity with the surface. When the cavity is placed in the focus of a laser beam the e.m. field excites localized surface plasmons. Therefore the sample under the tip is illuminated by this enhanced and localized field that produces a TERS effect. TERS effect is observed for CN- species adsorbed on Au(111) surface using a gold tip and a 5mW HeNe laser source. Theoretical calculation claim the Raman signal to increases up to 12 orders of magnitude.. Experimental results show a Raman increasing ranging between 1.4 and 40! Pettinger B. et. al., Phys. Rev. Lett. 92 (9), (2004)

23 TERS II Malachite green isocthiocyanate, (MGITC) excited by a 632.8nm He-Ne laser line, shows an intrinic resonant Raman scattering. At the same time the fluorescence is almost completely quenched by the metal surface. The choice of the tip material plays a fondamental role in the exciting the localized surface plasmons. Acquisition time is usually a critical parameter. Long acquisition time and high laser power means bleaching of the matherials and usually lead in observe amorphous carbon Raman spectra.. Pettinger B. et. al., Phys. Rev. Lett. 92 (9), (2004)

24 Near Field Raman Microscopy of single walled Carbon Nanotubes This setup is based on an inverted microscope. The optical path is freezed and the transparent sample is scanned to image the Raman signal. Tips are produced by electrochemical etching. Sharpening is achieved by Focused Ion Beam milling (FIB) To obtain a strong field enhancement the electric field needs to be polarized along the tip axis. In this scheme the tip is displaced from the center of the beam in one of the two longitudinal field lobes of the strongly focussed gaussian beam. Silver tip obtained by means of a chemical etching and a FIB post processing. Hartschuh et al. Phys. Rev Lett. 90 (9) (2003)

25 NanoRaman on SWCNTs Imaging simultaneously the topography and the Raman signal of SWCNTs on glass substrate, it is observed that the optical resolution is better than the topographic ones. This is a clear near-field Raman origin. Hartschuh et al. Phys. Rev Lett. 90 (9) (2003) The presence of vertical and horizontal oriented nanotubes indicates the near field polarization is radially symmetric with respect to the tip axis. Some keywords: A smart light collection A sharp tip.

26 Micro- Vs Nano- Raman Imaging Raman scattering images of a single SWNT deposited on a glass coverslip. The contrast reflects the local intensity of the G G band (2640 cm - 1 ). Confocal SNOM Raman Spectra FWHM: 275 nm FWHM: 14 nm Anderson et al., JACS 127, 2533 (2005)

27 CARS The system is based on a standard AFM working in contact mode and employing a silicon cantilever tip coated with a silver film. The method was able to mage a DNA network of poly(da-dt)-poly(da-dt) with a lateral resolution well below the diffraction limit. The average powers of the w1 and w2 beams were 45 and 23 mw at the 800kHz repetition rate. Ichimura T et al. Phys. Rev. Lett, 92 (22), (2004)

28 Our Experimental setup The gold plated tungsten tip then glued to one arm of a tuning fork. The laser beam is focussed at the tip apex and the scattered light is collected by the same long working distance objective. Finally the light passes through a notch filter and coupled by means of an optical fiber to the spectrometer.

29 Interfacing with a commercial Raman spectrometer Collaboration with EDM, Cambridge University

30 Preliminary results on Field- Enhancement Normalized Raman Emission Far-Field Near-Field Raman shift (cm -1 ) Patanè et al., Riv. Nuovo Cimento 27, 1 (2004) ta-c:h thin film deposited on SIlicon

31 Conclusions and Outlook SNOM provides eyes for the nanoworld. NanoRaman imaging exploiting TERS is capable of resolutions in the 10 nm range. Coherent Anti-Stokes Raman scattering can be used for selective specimen recognition at the nanometer scale. We are working to develop new methods for better signal-to-background discrimination.