One of the authors (S.T.) is grateful to the financial support by a Research Fellowship of the Japan Society for the Promotion of Science (JSPS) for Young Scientist. This work is founded by a Grant-in Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture, Japan, and a Grant for the Future Program from the JSPS (JSPS–RFTF–98P–01203).
The carbon onion is a novel carbon nanoparticle consisting vacuum. As the annealing temperature increases, the transof concentric curved graphene sheets. The physical proper- formation of diamond nanoparticles into spherical carbon ties of this new member of fullerene-ralated materials are onions proceeds form the surface to the center . Our very much attractive. In particular, the optical properties previous TEM observation  revealed that spherical onions of carbon onions are of great interest, because they are 5 nm in diameter are formed at about 1700C. At higher possible candidates of the interstellar dust, where they could temeratures above 1900C, further progress of graphitization contribute to a strong extinction hump centered at 217.5 nm leads to the formation of polyhedral carbon onions with (4.6 µm-1) in the ultraviolet-visible (UV-Vis.) region.
facets . In the present study, diamond nanoparticles were For UV-Vis. absorption properties of the onions, several annealed at the temperatures ranging from 900 to 2100C.
theoretical analyses have been reported [1,2]. Nevertheless, The samples were ultrasonically dispersed in distilled water owing mainly to the small quantity of material available for (about 0.2 mg/cc), and then put into a synthesized quarts macroscopic experiments, the absorption properties based cell. UV-Vis. transmission spectra in the wavelength (µm) on the laboratory experiments remain poorly understood .
from 0.5 to 0.2 µm were recorded with a double-beam type In 1994, Kuznetsov et al. succeeded in preparing carbon spectrometer. The transmittance (T ) was converted into the onions in large quantity by annealing diamond nanoparticles extinction (E) by using an equation, E = -lg T.
about 5 nm in diameter . Such diamond nanoparticles Fig. 1 shows UV-Vis. extinction spectra for all the samples.
would be synthesized also in the external atmosphere of The horizontal axis is converted into the wavenumber stars by chemical vapor decomposition of light hydrocarbon molecules; carbon onions are very much likely to be generated in the interstellar space by the transformation of diamond nanoparticles under an appropriate heating process . In relation with this astrophysical context, laboratory data of the UV-Vis. absorption for carbon onions prepared from diamond nanoparticles is highly desired.
Recently, we have prepared spherical and polyhedral carbon onions by annealing diamond nanoparticles in vacuum, and studied them by transmission electron microscopy (TEM), electron energy-loss spectroscopy, Raman spectroscopy, and electron spin resonance (ESR)[6,7]. In the present contribution, we first report the UV-Vis. absorption spectra of these onions. Theoretical calculations for the spherical and polyhedral carbon onions are also carried out to explain the experimental spectra.
1. Experimental results The detailed sample preparation procedure can be found in elsewhere . Briefly, we prepared carbon onions Figure 1. Annealing temperature dependence of normalized by annealing diamond nanoparticles 5 nm in diameter in UV-Vis. extinction spectra for carbon onions.
4 434 S. Tomita, S. Hayashi, Y. Tsukuda, M. Fujii (1/ µm-1). The extinction is normalized by a following equation, (E() - E(=0.55))/(E(=0.44) - E(=0.55)), where E() is the extinction at the wavelength. A spectrum for diamond nanoparticles denoted by ”nc-D” increases monotonically to the ultraviolet region. The rising extinction continuum cannot be explained by the intrinsic absorption by diamond nanoparticles, and is likely to be caused by the aggregation of diamond nanoparticles. Further discussion on the spectrum for initial diamonds is beyond the scope of this contribution and will be reported in a later papers.
As the annealing temperature increases, the extinction at higher wavenumbers decreases. In addition, the sample annealed at 1100C shows a broad peak at about 3.8 µm-1.
At 1700C, the broad peak is more pronounced and slightly shifted to a higher wavenumber. Note here that spherical onions were observed by TEM at this temperature. With further increasing the annealing temperature, an additional peak at about 4.6 µm-1 emerges; a spectrum at 2100C shows two peaks. The appearance of double peaks is believed to be due to the formation of polyhedral onions.
Figure 2. Calculated spectra for the defective spherical onions 2. Theoretical considerations with external radius R = 2.5 nm in water. Inner core radii r are 2 (a), 1 (b), 0.5 (c), and 0.35 nm (d). The cores are filled with 2.1. S p h e r i c a l o n i o n s. A broad extinction hump at diamond for curves a–c, while vacuum for curve d.
3.8 µm-1 observed for the spectra from 1100 to 1700Cis due to spherical carbon onions with diamond or hollow core.
The absorption spectra for such onions have already been calculated by Henrard et at. . In their model, by using Our previous ESR studies for the spherical onions  the spherical coordinates (unit vectors,, and ), the revealed that the graphite shell contains a number of defects dielectric tensor of the graphitic multishell can be expressed such as dangling bonds. The dielectric function of such as defective graphite shells should be different from that of = () =pp()( + ) +pl(), (1) bulk graphite. In order to take the effects of defects into our consideration, the dielectric function of defective graphite where pp() and pl() are the in-plane and out-of-plane shells was assumed to be a mixture of those of bulk graphite dielectric functions of graphite, respectively. The redii of (pp or pl) and amorphous carbon (ac). The in-plain the external graphitic shell and inner core are denoted by R ( pp) and out-of-plain ( pl) dielectric functions of defective and r, respectively (see inset in Fig. 2). The core, which is graphite shell are respectively assumed to be filled with diamond or vacuum, is described by an isotropic dielectric function of 1. Considering an onion placed in pp = cpp +(1 - c)ac, pl = cpl +(1 - c)ac, (4) a homogeneous medium with a dielectric function m, the multipolar polarizability of order l is expressed as where c is the concentration of the graphite component in the shell. We can calculate the absorption cross section of () =40R2l+the defective spherical onion from Eq. (1), in which pp and m[(plu--1l)(plu+-ml)-l(plu+ -1l)(plu- -ml)] pl are replaced by pp and pl. In all the calculation, we, (2) (l1 - plu+)[plu- + m(l + 1)]l used the dielectric data of graphite tabulated by Draine and - (l1 - plu-)[plu+ + m(l + 1)] Lee , that of diamond by Philipp and Taft , and that of amorphous carbon by Michel et al. .
where l = (r/R)u+-u-, and u± = -0.Fig. 2 shows the absorption cross section per particle vol± 0.25 + l(l + 1)pp/pl. At the non-retarded limit, ume (R3) for several isolated onions. The outer radius (R) the electric field of a plane-wave electromagnetic radiation is 2.5 nm and the inner radii (r) are 2 (curve a), 1 (curve b), with frequency only induces an electric dipole (l + 1) 0.5 (curve c), and 0.35 nm (curve d), respectively. For which absorbs energy from the wave with the cross section curve a–c, the inner cores are filled with diamond, while 4 with vacuum for curve d. The surrounding medium was () = Im 1(). (3) assumed to be water (m = 1.777). The absorption c peak due to the surface plasmon in the spherical onion Физика твердого тела, 2002, том 44, вып. Ultraviolet-visible absorption spectroscopy of carbon onions graphite ellipsoids which are randomly oriented in water.
For the discussion of optical properties of the system, we can adopt a framework of average dielectric function for anisotropic ellipsoids developed by Hayashi et at. . For an ellipsoidal particle,,, and axes are set as shown in the inset in Fig. 3. Dielectric functions along each axis are expressed by,, and. Introducing depolarization factors (Lj j =,, ) along j axis and a filling factor ( f ), the average dielectric function can be written as 3(1 - f )(m - 1) + f ( + + ) av = 1 +, (5) 3(1 - f ) + f ( + + ) where j -j = 1 + Lj - 1, j =,,, (6) m Lj = 1, (7) Figure 3. Calculated spectrum for rotational graphite ellipj soids randomly dispersed in water. Depolarization factor L = L = 0.45, L = 0.1, and filling factor f = 0.3.
j =(j - 1)j, j =,,. (8) Here the absorption coefficient is directly given by can be seen at about 3.7 µm-1. With increasing c and decreasing r, i. e., as the transformation from a diamond = Im av. (9) c nanoparticle into a defective spherical onion proceeds, the peak shifts to higher wavenumbers. This shift qualitatively In the present study, axis of the ellipsoid is set parallel to agrees with the experimental results. However, the peak the graphite c axis. Therefore, corresponds to pl, while of calculated spectra is located at higher wavenumber and to pp. For depolarization factors, by assuming a than that of experimental spectrum. A defective spherical rotational ellipsoid, we set L = L, and 2L +L = 1. Fig. onion without diamond core (curve d) shows the peak shows a calculated absorption coefficient for the graphite at 4.3 µm-1, while corresponding experimental spectrum ellipsoid in water. L is 0.1, L and L are 0.45, and f is 0.3.
(1700C) at 3.9 µm-1. The wavenumber misfit between The graphite ellipsoids that we considered here successfully calculated and experimental spectra is thought to be caused reproduce an absorption spectrum with two peaks. These by the aggregation effect . The aggregation of the two absorption peaks at about 4.0 and 4.6 µm-1 are induced onions by van der Waals forces is likely to subsist in water by surface plasmons along and axes of ellipsoid.
suspension, because the applied ultrasonic dispersion seems We have studied the optical properties of spherical and to be insufficient to break the adhesion between the particles.
polyhedral carbon onions in relation with the interstellar dust The detailed theoretical consideration for the aggregated particles. The laboratory absorption spectroscopy indicated onions is now underway in our group.
that spherical onions in water show a broad extinction peak We also simulated an absorption spectrum of a defective at about 3.8 µm-1. The theoretical calculation suggested spherical onion in vacuum (m = 1). Although not that the peak is due to the surface plasmon in the defective shown here, the calculated spectrum shows a peak at about spherical onion. Furthermore, the calculated spectrum for 4.6 µm-1, and just fits the interstellar extinction spectrum.
the defective onion in vacuum can reproduce the interstelThis strongly suggests that a defective spherical onion is a lar extinction spectrum. This suggests that the defective likely candidate for the interstellar dust, which shows an spherical onion is a strong candidate for the interstellar extinction hump centered at 4.6 µm-1.
dust, which shows an extinction hump centered at 4.6 µm-1.
2.2. P o l y h e d r a l o n i o n s. The appearance of two Experimental spectra for polyhedral onions showed two absorption peaks above 1900C is apparently attributed extinction peaks at 3.9 and 4.6 µm-1. Approximating the to the formation of polyhedral carbon onions with facets. facet of the polyhedral onion as a graphite ellipsoid and We assume that a polyhedral onion is comprised of planar applying a framework of the average dielectric function, graphitic nanocrystals, and a nanocrystal can be treated as an we succeeded in reproducing the absorption spectrum with anisotropic graphite ellipsoid. Polyhedral onions dispersed in double peaks. Both peaks originate from the surface water are thus modeled as a system consisting of anisotropic plasmons in the facet part of the polyhedral onion.
4 Физика твердого тела, 2002, том 44, вып. 436 S. Tomita, S. Hayashi, Y. Tsukuda, M. Fujii References  A.A. Lucas, L. Henrard, Ph. Lambin. Phys. Rev. B49, (1994).
 L. Henrard, Ph. Lambin, A.A. Lucas. Astrophys. J. 487, (1997).
 W.A. de Heer, D. Ugarte. Chem. Phys. Lett. 207, 480 (1993).
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