Developing Optical Techniques for Analysis of Materials at the Nanoscale

1. Developing Optical Techniques for Analysis of Materials at the Nanoscale Alexei Sokolov

2. Nanoscale • Approaching nanometer length scale leads to many qualitative changes in material properties: • When material scales approach the size of electronic excitations (e.g. exciton radius) qualitative change in optical properties occur (e.g. Quantum Dots); • When scales of elements approach the size of polymer molecules (e.g. end-to-end distance or Rg) many thermodynamic properties deviate from their bulk values. For example, Tg of thin polymer films depends on their thickness. Developments of nanotechnology require experimental methods for analysis of structure, chemical composition and properties of materials at the nanoscale.

3. Submicron Lithography 100 nm features using l = 193 nm lithography Microelectronic technology demands fabrication of increasingly smaller features from polymeric resists. However, polymeric structures collapse at small dimensions (present ~100 nm). elastic/plastic deformation mat’l failure CD £ 100 nm There is a need for analysis of mechanical properties of materials in these nanostructures. interfacial decohesion

4. Brillouin Light Scattering The idea of the Brillouin spectroscopy: • Light scatters on thermal phonons. Scattered light changes frequency according to the phonon energy. • This frequency change (n) depends upon phonon velocity (V), modulus, and density. Thus one can estimate mechanical modulus of the material from analysis of the scattered light spectra.

5. Lithographic Structures from IBM 90 nm A 130 nm B 180 nm C We analyzed gratings produced in a polymeric photoresist. Gratings have various structure thicknesses (between 180 and 80 nm) and height ~300 nm. Structures with sizes below 80 nm collapse. SEM images of the gratings Multiple orders of X-ray diffraction suggest high periodicity of the gratings

6. Brillouin Spectra of the Gratings High-angle measurements Q|| = 22.6 mm-1 (q = 67º) Low-angle measurements Q|| = 3.4 mm-1 (q = 8º) No shift is observed for both the bulk mode (LGM) and the Rayleigh surface mode. This suggests no change in bulk or shear modulus down to size of structures ~80 nm. New modes appear in the spectra of the gratings that may contain additional information about moduli.

7. Rayleigh velocity: Vr=2pn/QII Changing Q, we probe vibrations with different l~2p/Q Dispersion of the new modes appears to be different from the dispersion of the Rayleigh modes and similar to a dispersion in a free-standing film. This result suggests that the new modes are surface modes on sides of the structures. Rayleigh mode New mode Si substrate The frequency of the Rayleigh mode (depends on a shear modulus) appears to be independent of the structure size down to 80 nm.

8. PBT PE Example of a Raman spectrum of a polymer blend (PE-red/PBT-blue). By selecting characteristic Raman modes, mapping of the chemical composition is obtained. However, Raman spectroscopy has two strong limitations: (i) limited lateral resolution (~l~500 nm); (ii) weak signal, decreases with volume. Scanning Nano-Raman Spectroscopy Analysis of chemical composition, structure, stresses and conformational states with nanometer lateral resolution is crucial for progress in nanoscience and nanotechnology. Raman spectroscopy is a non-destructive method that provides such information. It is based on analysis of vibrational spectra that are “fingerprints” of molecules and their structures.

9. The idea to combine unique lateral resolution of SPM with the power of Raman spectroscopy was first proposed in 1985 [Wessel, J. J. Opt. Soc. Am. B 1985, 2, 1538]. The idea is based on use of the plasmon resonance of a metallic particle to provide significant enhancement of the Raman signal strongly localized in the vicinity of the particle. Modern scanning probe microscopy (AFM, STM, etc.) provides exceptional lateral resolution, ~1 nm. In most cases, however, only surface topography, hardness or conductivity can be obtained from these measurements. Chemical composition, conformational states and stresses are not accessible. An AFM image of a polymer surface

10. E n h a n c e Rod shaped hot particle m e n t 57 Moskovits , M. Rev. Mod. Phys. , , 783. (1985) Faceted hot particle Observation of hot spots with enhancement up to ~1012-1014 [Nie, S.; Emory, S.R. Science 275, 1102 (1997)]. Cluster of particles Surface Enhanced Raman Scattering Plasmon resonance has been used in surface enhanced Raman spectroscopy (SERS) for many decades. Because the Raman signal is proportional to IE4, giant enhancement of the Raman signal (up to ~1012-1014 times) has been reported [Nie, S.; Emory, S.R. Science 275, 1102 (1997)]. Experimentally reported values of enhancement are even higher than theoretical predictions. Theoretical calculations of the enhancement factor for silver particles of different size. It has been demonstrated that only a few spots, so called “hot spots”, provide giant enhancement of the Raman signal. Understanding the nature of the hot spots remains a challenge.

11. Concept of the Spectrometer The idea is to design an AFM tip that will provide enhancement (~106-108 times) of the Raman signal at the apex. Controlling the position of the tip apex with the AFM, we expect to get mapping of the samples with lateral resolution ~10-50 nm. Sketch of the proposed Scanning Nano-Raman Spectrometer. Achieving enhancement of the signal ~1012 will open possibility for single molecular detection.

12. Objective of the Raman system and AFM head 2 mm Image of an AFM tip through the Raman microscope Nano-Raman Equipment Equipment for the scanning nano-Raman system (includes LabRam HR spectrometer, Quesant Q-Scope AFM, Coherent Ar and Kr lasers) is in place and working. The main challenge at present is development of the tip with strong enhancement of the Raman signal.

13. Silver Coated Tip 50 nm 50 nm Tip down Tip up Tip coated by silver clusters provides strong enhancement of the Raman signal. Uncoated Tip Tip up Tip down Bringing uncoated tip into the laser beam leads to a decrease of the Raman signal due to a shadowing effect.

14. A gold coated tip has been used for measurements of enhancement factor in two CdS films with thicknesses 50 nm and 10 nm. 10 nm, Enhancement ~2.5 times 50 nm, Enhancement ~2 times Stronger enhancement in the case of a thinner film is ascribed to localization of the plasmon enhanced field. Using the difference in the enhancement factor, we estimated the localization of the enhanced Raman signal to be ~30 nm. The maximum enhancement is achieved when polarization of the incident light is parallel to the tip axis. This result agrees with theoretical expectations.

15. Looking ahead • In addition to development of SNRS, we plan to extend the research towards other topics: • Plasmon nano-optics for various photonic applications • - Design of highly efficient surfaces for SERS use as bio- and chemical sensors. Achieving the enhancement factors ~1012-1014 opens unique possibilities for single molecular detection or detection of extremely low quantities of materials. That can be used in bio-medical research and in highly sensitive detectors of chemical and biological agents. Developments of tips with higher enhancement factors (at present ~104, however, factors ~106-108 are feasible). Developments of scanning capabilities and analysis of lateral resolution. Application of the technique to analysis of materials for photonic and micro-electronic applications, to biological samples.

16. Acknowledgments Dr. M. Foster Dr. A. Kisliuk D. Mehtani R. Hartschuh N.H. Lee Y. Ding S. Roberts Dr. A.P. Mahorowala Dr. C. Soles Dr. W.L. Wu Dr. R.L.Jones Dr. T.J. Hu Dr. J. Maguire Dr. R. Vaia Funding:Air Force Research Laboratory National Science Foundation National Institute of Standards & Technology Ohio Board of Regents

17. An alternative approach is use of aperture-less near-field optics. It is a rather new direction based on plasmon resonance enhancement of electric field near surfaces of particular metals (Ag, Au, etc.). A small metal particle at the apex of an SPM tip provides significant and very local enhancement of the electric field of light. Near-Field Optics Traditional approach to overcome diffraction limits is aperture-limited near-field optics. Lateral resolution down to ~100 nm has been reported. However, the optical signal is suppressed ~102-104 times because of a low transmission of the near-field tip. This approach works well for fluorescence or optical absorption measurements.

18. SERS on silver colloids Enhancement of the Raman signal on evaporated silver films and on silver colloids with sizes of particles from ~20 nm up to ~100 nm has been analyzed. A maximum enhancement on the order of ~105 has been achieved on clusters of silver particles. Raman spectra of Rhodamine 6G (R6G) on Ag, l=514 nm SEM image of silver colloids Optical image of silver colloids

19. Fs R 160 nm 220 nm  rinse liquid Elastic Properties in Nanostructures e-beam etched Si nanolines H. Namatsu et al. Appl Phys Lett, 66 (1995) 2655. resist structures Fs = f (, R, ) T. Tanaka et al. Jpn J Appl Phys 32 (1993) 6059.