Origin of the1150−cm−1Raman mode in nanocrystalline diamond

The peak near 1150 ${\mathrm{cm}}^{\mathrm{-}1}$ in the visible Raman spectra of poor quality chemical-vapor-deposited diamond is often used as the signature of nanocrystalline diamond. We argue that this peak should not be assigned to nanocrystalline diamond or other ${\mathrm{sp}}^3$-bonded phases. Its wave number disperses with excitation energy, its intensity decreases with increasing excitation energy, and it is always accompanied by another peak near 1450 ${\mathrm{cm}}^{\mathrm{-}1}$, which acts similarly. This behavior is that expected for ${\mathrm{sp}}^2$-bonded configurations, with their smaller band gap. The peaks are assigned to transpolyacetylene segments at grain boundaries and surfaces.