22B05
Optical Properties of High Purity Nitrogen Atom Endohedral Fullerene
Synthesized by Controlled Plasma Ion Irradiation
プラズマイオン制御照射により合成された
高純度窒素内包フラーレンの光学的特性
S. C. Cho, T. Kaneko, and R. Hatakeyama
趙 順天, 金子 俊郎，畠山 力三
Department of Electronic Engineering, Tohoku University,
6-6-05 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan.
東北大学 大学院工学研究科 電子工学専攻 〒980-8579 仙台市青葉区荒巻字青葉6-6-05
An optical investigation has been carried out on a purified nitrogen atom endohedral fullerene (N@C 60)
synthesized by controlling nitrogen ion irradiation energy to the fullerene (C 60). We have realized thehighest as-synthesized purity of 0.56 % in the world. The samples are purified up to 40 % using highperformance liquid chromatography (HPLC) and characterized by a laser-desorption time-of-flight mass
spectrometer (TOF-MS) and ultraviolet-visible (UV-Vis) absorption spectroscopy. In TOF-MS, the peak at
the mass number 734 indicating N@C60 is clearly observed. Furthermore, the UV/Vis absorption spectrumof high purity N@C60 is significantly different from that of the pure C60, i.e., a new peak at 345 nm appears
for N@C60.

1. Introduction
Fullerene-based materials are extremelyinteresting
andpromisingforpossible
applications in nanotechnology. Especially,
endohedral fullerenes have attracted great interestin their physical and/or chemical properties suchas pseudoatom behavior, magnetism, nonlinear
optical behaviors, and superconductivity [1]. For
example, it has been shown that N@C 60 could
have some unique advantages in isolating theatom from its environment, thereby providing abuilding block for the qubits of the quantum
computer [2]. However, one of the problems ofprobing the properties of these new materials isthe difficulty in producing them in large enoughamounts. The production rate of N@C60 is quite
low compared with the ordinary fullerenes, whichyield only limited quantities (e.g., only a few
nanograms) of a pure product after laborious
isolation procedures due to the extreme
difficulties in producing large amount quantities
and isolating pure samples. Although N@C 60 has
been studied over the last decade, the purity isstill extremely low at present (10 -3-10-2 %).
Therefore, the high yield synthesis of N@C 60
using plasma system is required in order toprogress the development of promising quantumdevices.
Here, we present a simple method for thesynthesisof high-yield N@C60 by aradio-frequency (RF) plasma. Also, using
multi-step
highperformanceliquidchromatography (HPLC), N@C 60 is purified andcharacterized using ultraviolet-visible (UV-Vis)absorption spectroscopy and matrix-assisted
time-of-flight mass spectrometer (MALDITOF-MS) analysis.
2. Experimental results and discussion
N@C60 can be synthesized by nitrogen ion
bombardment to C60. Experiments on the N@C60
synthesis are performed under the conditions asfollows: applied RF power PRF = 500 W, nitrogengas pressure PN2 = 25 Pa, grid bias voltage Vg = -90
V, substrate bias voltage Vsub = -90 V and synthesistime = 60 min. After the synthesis, the sample isdissolved in toluene and filtered through a 0.2 µm
membrane before the HPLC purification process.
The design and operation of the RF-plasma system
is reported in detail in our previous literature. [3]Fig. 1. Dependence of purity and C60 sublimation rateon Tov at Vg = -90 V, Vsub= -90 V and PRF= 500 W.

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Intensity (arb. units)

Pristine C60
C60

Intensity (arb. units)

Mass / Charge
C60

N@C60

Purity = 5 %Intensity (Normalization)

Figure 1 shows the purity and C60 sublimation
rate (Rsubli) as a function of oven temperature (Tov).
It is found that the purity increases with an increasein Tov and the highest purity of 0.56% is realized
although Rsubli is almost the same from 800 to 1000
oC. The increase in the purity is considered to becaused by sublimation of C60 with dispersed formand expansion rate increases as Tov increases.
Figure 2 shows a mass spectrum of the sample
with purity of 40% after condensation and pristine
C60. In both cases, the mass/charge values yielding
spectrum intensity peaks are observed in the range
from 720 to 724 owing to the existence of carbon
isotopes in C60. However, in the case of N@C60
with purity of 40%, the peak at 734 indicating
N@C60 is clearly observed.
Figure 3 shows the UV/Vis absorption spectra ofdifferent N@C60 purity in the range of 300-600 nm.
It is obviously recognized that there is a significant
difference in the peak at 345 nm between the(N@C60 with purity of 20 % and 40 %) enrichedsamples and N@C60 with purity of 5 %. The peaks
at 345 nm gradually increase, as purity increases.
Compared to the case of the N@C60 with purity of5 %, the peak becomes broad in the N@C60 withpurity of 20 % and 40 %. In addition, the C60
absorption peak at 406 nm is greatly diminished in
the N@C60 with purity of 20 % and 40 % [4], whichsuggests that the absorption spectrum of N@C60 isdifferent as purity increases.

300

Purity = 20 %Purity = 40 %

400

500

600

Wavelength (nm)Fig. 3. The UV/Vis absorption spectra of theN@C60 with purity of 5 %, 20 % and 40 %,respectively.

4. Summary
We have presented a simple method for the highpurity N@C60 synthesis using an RF discharge withexternal control of DC bias voltages. It is found thatthe purity increases with an increase in Tov and thehighest purity of 0.56% is realized. Furthermore,
the N@C60 sample enriched using HPLC sample ischaracterized by MALDI TOF-MS and UV/Visabsorption spectroscopy. In MALDI TOF-MS, thepeak at the mass number 734 indicating N@C60 isclearly observed. In addition, the UV region of theabsorption spectrum of the high purity N@C60 is
significantly different from the low purity N@C60 at
the wavelength of 345 nm.
Acknowledgments
This work was partly carried out at theEvaluation and Analysis Center, Research Instituteof Electrical Communication, Tohoku University.
This work was supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Culture, Sport, Science and Technology, Japan.
References

Mass / Charge

Fig. 2. Mass spectrum of solution after solvent
extraction obtained by MALDI TOF-MS analysis.
The purity of N@C60/(C60+N@C60) in the solution
is about 40%, which is enhanced by HPLC.

[1] A. I. Wabnig,, K. Porfyrakis, G. A. D. Briggs, and B.
Sundqvist: Phys. Status Solidi B. 246 (2009) 2767.
[2] W. Harneit: Phys. Rev. A 65 (2002) 032322.
[3] S. Miyanaga, T. Kaneko, H. Ishida, and R.
Hatakeyama: Thin Solid Films 518 (2010) 3509.
[4] M. Kanai, K. Porfyrakis, G. A. D. Briggs and T. J. S.
Dennis: Chem. Comm. 210 (2004) 210.

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