Near-Field Raman imaging of morphological and chemical defects in organic crystals with Sub-Diffraction resolution

1. Near-Field Raman imaging of morphological and chemical defects in organic crystals with Sub-Diffraction resolution P. G. Gucciardi, S. Trusso, C. Vasi Istituto per i Processi Chimico-Fisici, sez. MESSINA, CNR, Via La Farina 237, I-98123 MESSINA, Italy S. Patanè I.N.F.M., Dipartimento di Fisica della Materia e Tecnologie Fisiche Avanzate,Università di Messina, Salita Sperone 31, I-98166 Messina, Italy. M. Allegrini I.N.F.M., Dipartimento di Fisica, Università di Pisa, Via F. Buonarroti 2, I-56127 Pisa, Italy.

2. Outline Motivations: Difficulties: • Chemical Imaging. • Stress Imaging. • High spatial resolution: 100 nm. • Added value: simultaneous sample topography and elastic scattering images. • 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. • Investigated Samples: • Tetracyanoquinodimethane (TCNQ) crystal showing surface defects. • Localized Cu-TCNQ complexes embedded in a TCNQ thin film.

3. NanoRaman Imaging Experiments Review 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: 4040 (step 100 nm) Acquisition time: ~ 10h Estimated resolution: 250 nm (the aperture) Sample: Dye-labeled DNA Scan points: 2020 (step 100 nm) Acquisition time: > 6h using SERS Estimated resolution: 100 nm Sample: Scratch on silicon Scan points: 2621 (step 154 nm) Acquisition time: > 9h Estimated resolution: sub-micron

4. 7,7’,8,8’ Tetracyanoquino-dimethane (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. C-H Bending 1196 cm-1 C-CN Stretch 2225 cm -1  2208 cm -1 C=C ring stretch 1620 cm-1 C=C wing Stretch:1445 cm -1  1380 cm -1

5. Experimental setup • Excitation: Ar++ laser line 514.5 nm. • Collection: Nikon 50X objective, NA 0.5, WD 10.6 mm. • Notch Filter: Rejection Ratio ~ 10-6. • Spectrometer: Triax 190, single grating, 1200 lines/mm, 190 mm focal. • Detector: PMT in photon counting regime, or ICCD. • Shear-Force: tuning-fork with etherodyne detection. • Signals: Topography, Elastic, Raman. • Modes: Illumination or Collection.

6. TCNQ NanoRaman Spectra

7. Imaging of defects in TCNQ crystals • Surface defects are visible in the topography map. • The nano Raman analysis evidence a corresponding scattering modulation. Topography NanoRaman @ 1445 cm-1 MicroRaman @ 1445 cm-1

8. Sub-diffraction Imaging of defects NanoRaman @ 2230 cm-1 NanoRaman @ 1445 cm-1 Resolution is better than 120 nm

9. Another sample: a CuTCNQ thin film • A thin TCNQ film (yellow) was deposited on a KBr substrate in vacuum conditions. • The sample was kept into contact with Cu powders giving rise to localized spots of Cu-TCNQ (blue) organometallic compounds. • Areas in which the film is scratched out evidence the presence of the substrate (white). Optical Microphotograph TCNQ Scratch Cu-TCNQ

10. Our Target: • Localization of: • TCNQ. • CuTCNQ Local spots. • Scratches evidencing the KBr substrate.

11. Localization of damaged areas by SNOM Localization by Reflectivity Topography Reflectivity • Scratched areas can be localized through the analysis of the surface topography. • The elastic scattering signal is locally enhanced because of the higher reflectivity of the KBr substrate. Localization by Raman Scattering Topography Raman @ 1445 cm-1 • Two holes appear in the topography. • A vanishing Raman activity is found therein. • Lateral resolution: ~ 300 nm.

12. Localization of CuTCNQ by SNOM Topography Elastic Scattering Topography shows no features  NO SCRATCHES. The stronger absorption of the CuTCNQ is evident in the elastic scattering map. • Only 100 ms of integration time are required to get a Raman spectrum of TCNQ. • The CuTCNQ shows a Raman activity strongly reduced. Integration time 5 s. The Raman map at 1445 cm-1 (Tint= 100 ms per point) shows the presence of areas of depleted intensities which can be attributed to CuTCNQ. Raman Spectra Raman Map @ 1445 cm-1

13. Sub-diffraction Raman Imaging RAMAN Map @1445 cm -1 Raman Map Elastic Scattering • Integration time: 100 ms per point. Total image acquisition time: ~ 1 hour. • The dark clusters can be attributed to the presence of CuTCNQ complexes localized on sub-micron length scales. • The line profile allows to assess a lateral resolution better than 200 nm.

14. CuTCNQ TOPOGRAPHY Scan width: 10 × 10 m2 The bumps turn out to be TCNQ-rich zones.

15. Resolution assessment in NanoRaman on Cu-TCNQ TOPOGRAPHY ELASTIC RAMAN 1445 cm -1

16. Resolution assessment in NanoRaman on Cu-TCNQ TOPOGRAPHY ELASTIC SCATTERING RAMAN 1445 cm -1 • Scan width: 2.5 × 2.5 m2 • Raman imaging confirms the spectral information on the chemical nature of the bumps. A resolution better than 150 nm can be assessed.

17. Conclusions • NanoRaman imaging has been demonstrated on organic materials, within reasonable acquisition times. • Sub-diffraction resolution has been achieved. • We have not taken advantage of field enhancement effects • Topography, Elastic and Raman scattering signals can be acquired simultanously. • A class of materials suitable for NanoRaman investigations has been identified.