Surface-enhanced Raman scattering on nanodiamond-derived carbon onions

Annealing nanodiamonds (ND) at high temperatures up to 1700 °C is a common method to synthesize carbon onions. The transformation from NDs to carbon onions is particularly interesting because of carbon onions' potential in the field of tribology and their application in ultra-charge/discharge devices. In this paper, a novel surface-enhanced Raman scattering technique that involves coating the sample with nanoscopic gold particles is proposed to characterize the NDs after different annealing treatments. Conventional Raman and surface-enhanced Raman spectra were obtained, and the changes of peak parameters as the function of annealing temperature were evaluated. It was found that the widths of the D and the G peaks decreased with increasing annealing temperature, reflecting an improved order in the sp²-hybridized carbon during the transformation from NDs to carbon onions. After annealing at 1700 °C, the sp²‑carbon was highly ordered, indicating desirable electrical conductivity and lubricity. With increasing annealing temperature, the D peak showed a blue shift of almost 30 cm⁻¹, while the G peak merely shifted by 5 cm⁻¹. For annealing temperatures above 1100 °C, an increase of intensity ratio I_D/I_G was observed. Compared to the uncoated area, red shifts of 0.5–2 cm⁻¹ and of 5–9 cm⁻¹ for the G and D peaks, respectively, were detected for the gold-coated area, which was due to the coupling of the plasmons and the phonons of the samples.