Tunable Low Frequency Raman Spectroscopy

Tunable low-frequency Raman system is used to characterize structures and phases of different materials including emerging two-dimensional (2D) materials.  This system allows studying 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.

Specifications/Capabilities/Extras

• Tunable cw laser excitation from 450 nm to 650 nm (C-WAVE, Hubner).
• Low-frequency Raman measurements (< 50 cm^-1) with spectral resolution of ∼0.7 cm⁻¹
• Tunable high-frequency (>100cm^-1) Raman spectroscopy – picosecond laser excitation: 
  • Mira 900 with Verdi 18 pump: 5ps or cw, 76MHz, 2W, nJ/pulse, 700-1000nm (Coherent)
  • Mira OPO: 525-800nm; SHG/THG (350-500nm/250-300nm)
• Polarized Raman measurements
• Resonance Raman spectroscopy: measurements of Raman Excitation Profiles (REPs)
• Low and high temperature measurements 

Details

Raman spectroscopy gives very useful structural information based on measurements of inelastically scattered light by phonons and is routinely used for characterization of 2D and other materials. Tuning the excitation wavelength is beneficial not only for improving the scattering efficiency using resonant Raman scattering, but also for studying excitonic states and their interactions with phonons. Intensity variations of a specific phonon mode measured during tuning of excitation wavelengths give a Raman excitation profile (REP) that contains information about the exciton band positions, lifetimes, and strength of the exciton-phonon couplings. 

Science Overview

For low-frequency Raman measurements (< 50 cm^-1) a triple spectrometer based micro-Raman setup (T64000, Jobin-Yvon) is used that is equipped with three 1800 grooves per millimeter gratings (spectral resolution ∼0.7 cm⁻¹, FWHM) and a liquid nitrogen cooled CCD (Symphony, Horiba JY). A linearly polarized laser beam is focused onto a sample to a ∼1 μm spot using a microscope objective with the variable laser power on the samples. The polarized Raman measurements can be conducted by inserting a polarizer in the scattered beam and a half-wave plate to rotate the scattered light polarization to optimize the grating’s response. The tunable excitation can be achieved by using a Ti:Sapphire-based oscillator (Coherent Mira 900) pumped by a CW Nd:YVO₄ laser (Coherent Verdi V-18). When coupled to second- and third-harmonic generator crystals (Coherent model 5-050), or a periodically poled OPO (Coherent APE), the laser is tunable from 250 – 1700 nm in ps mode. It may be run in ps or cw mode and provides nJ/ps-pulse at 76 MHz. In the cw mode the spectral tunability range is (700-1000 nm). Other discrete Raman excitation lasers (405, 532, 633 and 785nm) are also available. When coupled to a confocal microscope with XYZ control and a high-resolution monochromator, it may be used to perform fluorescence measurements or Raman spectroscopy at any wavelength. This laser may also be used for ps time domain spectroscopy. A Coherent Model 9200 pulse picker is available to use with this system to change the repetition rate of the laser. The system is equipped with a liquid He-cryostat (MicrostatHiResII, Oxford Instruments) with a temperature controller (MercuryiT, Oxford Instruments) that allowed precise temperature control from 3.6 to 300 K. A high-temperature microscope stage (25-1500°C, TS 1500, Linkam) is also available.

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.

Specifications

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