Raman spectroscopy relies on inelastic scattering of monochromatic light, usually from a laser in the visible, near infrared, or ultraviolet (UV) range. The laser light interacts with phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the vibrational modes in the system. A sample is illuminated with a laser beam. Scattered light from the illuminated spot is collected with a lens and sent through a monochromator. The elastically scattered radiation at the wavelength corresponding to the laser line - Rayleigh scattering - is filtered out by either a notch filter, edge pass filter, or a band pass filter, while the rest of the collected light is dispersed onto a detector. Spontaneous Raman scattering is weak, and as a result the main difficulty of Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh scattered laser light. Historically, Raman spectrometers used holographic gratings and multiple dispersion stages to achieve a high degree of laser rejection. Modern laser spectrometers employ notch or edge filters for laser rejection and CCD detectors. Photo below shows Raman Spectroscopy Laboratory – part of the Phonon Optimized Engineered Materials (POEM) Center at UC Riverside.
Professor Balandin developed an unconventional application for Raman spectroscopy – measuring the thermal conductivity of graphene and other atomically thin materials. The first experimental study of heat conduction in graphene was made possible by developing an optothermal Raman technique (see Figure below). The heating power ΔP was provided with laser light focused on a suspended graphene layer connected to heat sinks at its ends (e.g. Figure shows FLG with n=2 of rectangular shape suspended across a 3-μm-wide trench in Si wafer). Temperature rise ΔT in response to ΔP was determined with a micro-Raman spectrometer. The G peak in graphene’s Raman spectrum exhibits strong temperature dependence. Figure shows the temperature shift in bilayer graphene. The inset shows that the optical absorption in graphene is a function of the light wavelength due to many-body effects. The calibration of the spectral position of G peak with temperature was performed by changing the sample temperature while using very low laser power to avoid local heating. The calibration curve ωG(T) allows one to convert a Raman spectrometer into an “optical thermometer”. During the thermal conductivity measurements, the suspended graphene layer is heated by increasing laser power. Local temperature rise in graphene is determined as ΔT=ΔωG/ξG, where ξG is the temperature coefficient of G peak. The amount of heat dissipated in graphene can be determined either via measuring the integrated Raman intensity of G peak, as in the original experiments, or by a detector placed under the graphene layer, as in the follow up experiments. Since optical absorption in graphene depends on the light wavelength and can be affected by strain, defects, contaminations and near-field effects for graphene flakes suspended over the trenches it is essential to measure absorption under the conditions of the experiment.
A correlation between ΔT and ΔP for graphene samples with a given geometry gives the thermal conductivity value via solution of the heat diffusion equation. Large sizes of graphene layers ensure the diffusive transport regime. The suspended portion of graphene is essential for determining ΔP, forming 2D heat front propagating toward the heat sinks, and reducing thermal coupling to the substrate. The method allows one to monitor temperature of Si and SiO2 layer near the trench with suspended graphene from the shift in the position of Si and SiO2 Raman peaks. This can be used to determine the thermal coupling of graphene to SiO2 insulating layer. The optothermal Raman technique for measuring the thermal conductivity of graphene is a direct steady-state method. It has recently been extended to other suspended 2D materials or films, e.g. graphene films, made of materials with pronounced temperature-dependent Raman signatures. More information about Balandin Group optothermal Raman technique can be found in the reviews: A. A. Balandin, “Thermal properties of graphene and nanostructured carbon materials,” Nature Materials (2011) and D. L. Nika and A. A. Balandin, “Phonons and thermal transport in graphene and graphene-based materials,” Reports on Progress in Physics (2017).