Sensing carbon allotropes in protective coatings on optical fibers with far‐ and near‐field Raman spectroscopy and microscopy

Abstract

Hermetic carbon coatings of different thicknesses ranging from 1 to 100 nm on a series of optical fibers are characterized with confocal Raman spectroscopy and atomic force microscopy. Standard far‐field Raman spectra in the region of 1,000–2,000 cm−1 were fitted and well reproduced using 5 individual components assigned to different species of graphite‐like and disordered phases. Based on the fitting, a new spectroscopic indicator was proposed: G*D*true/Δ, where G* and D* and Δ are intensities at the maxima of the graphite‐like G peak and the disorder‐induced D peak and the dip between them, respectively. This indicator provides a measure of soot fraction in carbonaceous materials. A tip‐enhanced Raman scattering (TERS) technique confirmed the complex structure of the first‐order Raman D band assigned to disordered graphitic lattice vibrations. In addition, the TERS method provided convincing evidence for the formation of carbon allotropes, such as carbon nanotubes in the coatings. Possible effects of allotropes on hydrogen and water permeation through carbon coating are discussed in the view of the obtained results.

Spectrally Resolving Coherent TERS Spectroscopy of Electrically Biased Carbon-Coated Fibers

“…This magnitude can be determined through the G / D intensity ratio, where D being a net defect-induced disordered peak at 1320–1380 cm –1 , and the information on its intensity also allows estimating the average crystal size . As has been shown in our previous paper, the G / D ratio grows with the coating thickness until the latter achieves the magnitude of 30–60 nm. The G / D ratio was found to reach a maximum value and eventually decreased due to a heaped pile of randomly oriented sp 2 /sp 3 fragments.…”

“…For getting an access to spectral information on certain phonon modes a multipeak fitting analysis can be utilized. 16,20,33,34 The highly overlapped bands were decomposed to elementary peaks by using a regularized least-squares method (Figure S3). 35 From these net bands, an average size of quasi-graphitic crystals can be estimated using a Tuinstra−Koenig relation: 19,36…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XII

“…In addition, the TERS method provided convincing evidence for the formation of carbon allotropes, such as carbon nanotubes in the coatings. Possible effects of allotropes on hydrogen and water permeation through carbon coating were discussed in the view of their results . Liu et al studied the number‐of‐layer resolved (Mo, W)‐(S 2 , Se 2 ) phonon relaxation.…”

“…Possible effects of allotropes on hydrogen and water permeation through carbon coating were discussed in the view of their results. [80] Liu et al studied the number-of-layer resolved (Mo, W)-(S 2 , Se 2 ) phonon relaxation. They unified the number-of-layer dependence of the A 1g and the E1 2g phonon frequencies of layered MX 2 (M = Mo and W; X = S and Se) semiconductors with derivation of quantitative information on the bond relaxation dynamics and clarification of their physical origins.…”

Water-Anchored Edge Defects in Amorphous Carbon Probed with Thermal- and Electroassisted Raman Spectroscopy and Nanoscopy

“…Despite the fact that far-field Raman spectra of carbon materials are well studied, the intrinsic content of a broad Raman band within the spectral range of 1000–2000 cm –1 is still under debate. Depending on the structure of the carbonaceous material, a multipeak fitting analysis allows one to decompose a broad Raman spectrum into four, , five − or even six, elementary peaks. Following Occam’s razor principle, the use of additional a priori information on a sought solution must be justified by the nature rather than a desire to fit Raman spectra with a minimal total discrepancy.…”