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Tunable Low Frequency Raman Spectroscopy

Scientists use a tunable low-frequency Raman system at CNMS to characterize new emerging two-dimensional (2D) materials.  This system allows one to study very low frequency vibrations in layered 2D materials that provides valuable information about their crystal structure and phases. Understanding phases and stacking configurations of 2D layers is crucial for accelerating the development of new artificially structured materials for electronic and optoelectronic applications.


Tunable low-frequency Raman spectroscopy of carbon nanotubes. Previously unexplored low-frequency Raman modes of single-wall carbon nanotubes SWNTs have been revealed. Gold deposited onto a randomly-oriented top layer of vertically aligned carbon nanotube arrays was used to induce surface-enhanced Raman scattering SERS “hot spots” on large-diameter SWNTs with SERS intensities up to 1900 times greater than normal Raman intensities from undecorated arrays. The linewidths of the resonances down to 0.3 cm-1 are ten times narrower than previously measured for individual SWNTs. Pairs of intense sharp resonances with identical excitation profiles were found and tentatively interpreted as the low-energy longitudinal optical and radial breathing modes of the same nanotube. SERS lines in the region of 14–30 cm−1 were tentatively assigned to the ring modes of SWNTs in agreement with existing theories.

Anomalous interlayer vibrations in strongly coupled layered PdSe2. In this work, we show unusual effects of strong interlayer coupling on low-frequency (LF) Raman scattering in exfoliated PdSe2 crystals with different number of layers. Unlike many other layered materials, it is found that the measured frequencies of the breathing modes cannot be simply described by a conventional linear chain model (LCM) that treats each layer as a single rigid object. By using first-principles calculations, we show that strong deviations from layer rigidity can occur for the LF breathing vibrations of PdSe2, which accounts for the observed disagreement with the conventional LCM. The layer non-rigidity and strong interlayer coupling could also explain the unusual strong intensities of the LF breathing modes that are comparable with those of the high-frequency Raman modes. These strong intensities allowed us to use a set of the measured LF Raman lines as unique fingerprints for a precise assignment of the layer numbers. The assignment of the layer numbers was further confirmed using second harmonic generation that appeared only in the noncentrosymmetric even-layer PdSe2 crystals. This work thus demonstrates a simple and fast approach for the determination of the number of layers in 2D materials with strong interlayer coupling and non-rigid interlayer vibrations.


Lasers for Raman excitation:

  • Verdi 18 (Coherent), 532nm, 18 W maximum, cw
  • Mira 900 (Coherent), 5 ps or cw, 76MHz, 2W, nJ/pulse, 700-1000nm)
  • Mira OPO (Coherent), 1050-1600 nm, 2w: 525-800 nm
  • SHG/THG (Coherent), 2w: 350-500 nm; 3w: 250-300 nm
  • HeNe laser (Coherent), 35 mW
  • CW 532nm laser (Excelsior, Spectra-Physics), 100 mW
  • CW 532nm laser (Ondax), 50 mW

Triple spectrograph (JY specifications)

  • Focal length: 640 mm all stages
  • Aperture: f/7.5
  • Gratings: 1800, 2400, 300 gr/mm