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Developing Optical Techniques for Analysis of Materials at the Nanoscale. Alexei Sokolov. Nanoscale. Approaching nanometer length scale leads to many qualitative changes in material properties:
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.
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).
CD £ 100 nm
There is a need for analysis of mechanical properties of materials in these nanostructures.
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.
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
Q|| = 22.6 mm-1 (q = 67º)
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.
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.
The frequency of the Rayleigh mode (depends on a shear modulus) appears to be independent of the structure size down to 80 nm.
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.
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
Rod shaped hot particle
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 IE4, 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.
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.
Image of an AFM tip through the Raman microscope
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.
Tip coated by silver clusters provides strong enhancement of the Raman signal.
Bringing uncoated tip into the laser beam leads to a decrease of the Raman signal due to a shadowing effect.
A gold coated tip has been used for measurements of enhancement factor in two CdS films with thicknesses 50 nm and 10 nm.
Enhancement ~2.5 times
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.
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.
Dr. M. Foster
Dr. A. Kisliuk
Dr. A.P. Mahorowala
Dr. C. Soles
Dr. W.L. Wu
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
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.
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.
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
Elastic Properties in Nanostructures
H. Namatsu et al.
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Fs = f (, R, )
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