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Nano-Raman Spectroscopy

Responsible : Razvigor Ossikovski

Tip Enhanced Raman Spectroscopy (TERS) combines an atomic force microscope (AFM) and a high resolution Raman spectrometer, optomechanically coupled with each other.
The sample is analysed in backscattering, a convenient configuration for all kinds of samples.
The AFM apertureless tip probe in the focal region of the laser provides the two essential advantages of TERS over conventional Raman spectroscopy:

  • Intensity enhancement, by up to six orders of magnitude in most favorable cases, of the Raman spectrum of the analyzed compound.
  • Spatial resolution, in the nanometer range (thus really “sub-wavelength”), this resolution being defined by the radius of curvature of the AFM tip and not by the size of the focal spot as would be the case in ordinary Raman spectroscopy.

Instrumental developments
Polarization Control
- Between 2008 and 2010 we worked on the the implementation of the polarization control for both the incident (pump) and scattered (Raman) beams, including the delicate task of polarimetric calibration of the instrument.
- This issue is of paramount importance for the detailed modeling of the Raman measurements and the extension of the applications of the technique [A. Frigout, et al, Eur. Phys. J. WebConf. 5, 06002].
- The custom made polarization controllers comprise each a linear polarizer and a motorized retardation plate.

Schematics of the polarimetric Raman setup. Solid line: emission path; dashed line: detection path. Ls: laser source; P: polarizer; WP: quarter-wave plate; EF: edge filter; O: objective; D: detector (and grating); S: sample; 

 -In order to study the polarization properties of the Raman signal using our instrumentation, we have developed a phenomenological model of Raman tensor describing the interaction between the tip and the incident and diffused field.
-This tensor fully characterizes the sample polarimetric properties. In addition, we have developed a methodology to determine the stress magnitude in silicon semiconductors.
 - This method successfully applied to cubic crystal structures of well-known orientation, could also be extended to other symmetries; this way the stress tensor in a Si/SiGe system used in microelectronics industry, could be experimentally deduced.

Operating mode of the AFM to enhance TERS
- Another instrumental issue of paramount importance for TERS efficiency is the operating mode of the AFM, as well as the kind of tip actually used.
- The tunneling-based AFM, also called STM, features a very large TERS enhancement factors (see figure below) on many samples, together with excellent reproducibility of the tip fabrication process, based on electrochemical etching of a gold wire.

STM-TERS enhancement of the Raman spectrum of a self-assembled monolayer of azobenzene-thiol on gold. Red curve : Near field spectrum taken with the tip in STM mode. Blue curve far-field spectrum, taken with the tip removed.

- However, the STM-TERS is limited to conducting samples.
- To circumvent this rather serious shortcoming we implemented a shear force (SF) operating mode for the AFM, which can be used with any sample.
- The first SF results,obtained on organic molecules, were very encouraging. Moreover, a clever combination of both AFM and SF results in a “multimodal” approach which has been recently patented.
 -However, in its current implementation, the SF mode is still far from having realized its full potential.

Photoluminiscence (PL) Detection
- The implementation of PL detection has been another important experimental achievement of the period.
- This “option indeed allows to characterize seminconducting structures based on heavily doped GaAs, in addition to Si and Ge, which are directly accessible to standard Raman spectroscopy.