High stability Raman spectroscopy systems for nanoscale characterization
As technology continues its unrelenting march to the nanoscale, so must the techniques that are used for characterization. One technique that has adapted and persevered through time is Raman spectroscopy – a vibrational spectroscopy that can nondestructively identify molecules by their unique Raman signatures or "fingerprint" spectrum. Aside from molecular identification, Raman spectroscopy is particularly useful in determining the strain and the temperature of a probed sample. In this talk, I will discuss the various Raman spectroscopy systems that we have developed and implemented in RIKEN to study strained silicon, carbon nanotubes, and graphene – materials used in transistor technology. The talk is mainly divided into two parts. The first part focuses on the direct determination of anisotropic stress in strained silicon nanowires through the detection of transverse optical (TO) phonon modes as compared to the detection of longitudinal optical (LO) phonon modes. These measurements were made possible through the development of a Raman microscope that enables precise and stable polarization control using a stable tightly focusing system. The second part of my talk will be about temperature and strain characterization at the nanoscale through the use near-field scanning optical microscopy (NSOM) techniques, namely tip-enhanced terahertz Raman spectroscopy (TE-THzRS) and tip-enhanced Raman spectroscopy (TERS), respectively. Using TE-THzRS, the temperature of a metallic carbon nanotube was extracted through the detection of ultra-low frequency Raman (less than 200 cm-1, also called terahertz Raman) and the behavior of temperature with respect to changing laser power. Using TERS, the strain distribution in graphene was studied with sub-nanometer resolution due to the highly confined light at the tip end. I will also discuss how our TERS systems operate at ambient conditions and the phenomena that occur due to the high confinement of light.