Abstract
A generalization of the Raman scattering (RS) spectrum, the Raman excitation map (REM) is a hyperspectral two-dimensional (2D) data set encoding vibrational spectra, electronic spectra and their coupling. Despite the great potential of REM for optical sensing and characterization with remarkable sensitivity and selectivity, the difficulty of obtaining maps and the length of time required to acquire them has been practically limiting. Here we show, with a simple setup using current optical equipment, that maps can be obtained much more rapidly than before (~ms to ~100 s now vs. ~1000 s to hours before) over a broad excitation range (here ~100 nm is demonstrated, with larger ranges straightforward to obtain), thus taking better advantage of scattering resonance. We obtain maps from different forms of carbon: graphite, graphene, purified single walled carbon nanotubes (SWCNTs) and chirality enriched SWCNTs. The relative speed and simplicity of the technique make REM a practical and sensitive tool for chemical analysis and materials characterization.
Highlights
Raman scattering (RS) spectra are one dimensional (1D) plots of intensity versus wavelength shift providing a fingerprint for chemical analysis1, widely applied to nanocarbons3, and many other sample types
For single walled carbon nanotubes (SWCNTs), the precise chemical structure is linked to the optical spectrum by the Kataura9 plot, with which resonant RS (RRS) can be used to identify nanotube species10
It is technically difficult and costly to span a large range of excitation wavelengths for RS with conventional monochromatic lasers which are ordinarily wavelength tuned for Raman excitation map (REM)
Summary
A generalization of the Raman scattering (RS) spectrum, the Raman excitation map (REM) is a hyperspectral two-dimensional (2D) data set encoding vibrational spectra, electronic spectra and their coupling. Plotting the intensity versus Raman shift and λ makes a REM, with horizontal slices that are RS spectra and vertical slices that are REPs. Ordinarily, to obtain a REM, many laser wavelengths are taken sequentially with a tunable laser to build up a two-dimensional (2D) map. Recent photoluminescence (PL) excitation experiments use such an approach16,17 Challenges for using such methods for RS as opposed to PL include the weak signal (~10−6 vs ~10−1 efficiency for PL18), the strong, unwanted Rayleigh background, the need for higher (~10×) spectral resolution, the potential for complications due to laser heating, and the more complicated data processing that is required. Unlike conventional RRS, an entire range of excitation wavelengths is used simultaneously
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