Carbon Onions: Carriers of the 217.5 nm Interstellar Absorption Feature

Ultraviolet-visible absorption measurements of high purity and well separated carbon onion samples are reported. The results show that, after purification, absorption features from carbon onions match well with the interstellar UV spectrum. The measurements show that the absorption peak position remains constant at $4.55\pm 0.1\mu {\mathrm{m}}^{\mathrm{-}\mathrm{1}}$, and the width varies from $1.2–1.6\mu {\mathrm{m}}^{\mathrm{-}\mathrm{1}}$, a key feature of the interstellar spectrum. The similarities between the experimental and observed absorption spectra indicate that carbon onions are very strong candidates for the origin of the UV interstellar absorption peak at $4.6\mu {\mathrm{m}}^{\mathrm{-}\mathrm{1}}$.

Figure 1

(a) SEM image of dispersed carbon onion thin films on highly conducting silicon chip ($\mathrm{\text{scale bar}}=300\mathrm{n}\mathrm{m}$), obtained using Hitachi S800-FR SEM operated at 20 kV. The high-resolution transmission electron microscopy (HREM) was performed on a JEOL 2010FX microscope operated at 200 kV. The TEM specimens were prepared by sprinkling the unpurified material onto a holey carbon grid. (b) A low magnification TEM image of carbon onions ($\mathrm{\text{scale bar}}=20\mathrm{n}\mathrm{m}$). Enlarged high-resolution images of (c) $\sim 17\mathrm{n}\mathrm{m}$ ($\mathrm{\text{scale bar}}=10\mathrm{n}\mathrm{m}$) and (d) $\sim 5\mathrm{n}\mathrm{m}$ ($\mathrm{\text{scale bar}}=2.7\mathrm{n}\mathrm{m}$) diameter carbon onions.

Figure 2

Absorption spectra of different carbonaceous materials as well as the interstellar absorption spectrum. The vertical line represents $4.6\mu {\mathrm{m}}^{\mathrm{-}\mathrm{1}}$. Curve ($a$) is a spectrum from polyhedral graphite powder (Sigma Aldrich, $\mathrm{\text{particle size}}=1–2\mu \mathrm{m}$). Curve ($b$) is a typical spectrum of carbon soot scraped from graphite electrodes. Curve ($c$) is the interstellar absorption feature taken from Ref. 24. Curve ($d$) is a typical spectrum obtained from our carbon onion thin films prior to annealing and compares well with other reports in the literature [7]. Curve ($e$) is an absorption spectrum from carbon onions with a width of $1.5\mu {\mathrm{m}}^{\mathrm{-}\mathrm{1}}$, correlating well with the astronomical feature. Curve ($f$) is an absorption spectrum from carbon onions with a width of $1.2\mu {\mathrm{m}}^{\mathrm{-}\mathrm{1}}$, in excellent agreement with the interstellar curve. The red curves in ($c$) ($\mathrm{\text{peak position}}=4.6{\mathrm{\mu }\mathrm{m}}^{-1}$), ($d$) ($\mathrm{\text{peak position}}=4.0{\mathrm{\mu }\mathrm{m}}^{-1}$), ($e$) ($\mathrm{\text{peak position}}=4.55{\mathrm{\mu }\mathrm{m}}^{-1}$), and ($f$) ($\mathrm{\text{peak position}}=4.6{\mathrm{\mu }\mathrm{m}}^{-1}$) represent the Drude function [24], used to obtain the peak position and width of the measured data.

Figure 3

Raman spectra of carbon onion thin films prior to annealing and subsequent to annealing at $600°\mathrm{C}$ in air for 30 min. The Raman was carried out using a Renishaw Micro Raman RM 1000 system with a spectral resolution of $4–6\mathrm{c}{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$. A 514.5 nm Ar ion laser was used as the excitation source.

Figure 4

AFM images of onion particles subsequent to annealing. A Digital Instrument’s Nanoscope AFM was used in tapping mode at 0.3 Hz. The image is 512 pixels by 512 pixels. Most of the small particles are 20 nm in height and diameter and evenly distributed throughout the sample. The particles in the image appear pyramidal because their overall dimension is smaller than the AFM tip diameter. The scale bar is $1\mu \mathrm{m}$.

Figure 5

(a) Low loss region of EELS for (i) polycrystalline graphite and (ii) carbon onions. The EELS was performed on a Phillips CM30 TEM operated at 100 kV equipped with a Gatan 666 spectrometer with a collection semiangle of 0.5 mrad. The energy resolution was 0.7 eV. The zero loss peak has been removed for clarity. (b) Enlarged $\pi$-plasmon energy region shown in greater detail. The $\pi$ plasmon of (iii) graphite is centered at 6.6 eV while the maximum of the (iv) carbon onion $\pi$ plasmon is located at 5.7 eV ($4.6\mathrm{\mu }{\mathrm{m}}^{\mathrm{-}\mathrm{1}}$), correlating to the interstellar absorption maximum. The data shown here are an average of 15 measurements.