S7-1

Investigation of Fullerenes and Carbon Nanotubes
Based on Plasma Science
プラズマ科学視点のフラーレン・ナノチューブ研究
Rikizo Hatakeyama
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

A special remark is focused from the viewpoint of plasma science on challenging researches offullerenes and carbon nanotubes among a variety of carbon allotropes. Their inner
nano-spaces are controlled via plasma-ion acceleration/deceleration processes in the gaseousand solution states, creating novel atom/molecule encapsulated fullerenes and carbon
nanotubes. For further advance of the nanoscopic plasma process individually and verticallyaligned single-walled carbon nanotubes are synthesized by plasma chemical vapor deposition.

1. Introduction
Since carbon allotropes take on a diversityof structures and properties, carbon-basedmaterials have attracted much interest of not
only general materials but also plasma
scientists from the viewpoint of basic scienceand practical materials development. The
carbon chemical bond is characterized by sand p hybridized orbital: carbyne with sp
hybridized orbital, graphite with sp2
hybridized orbital, and diamond with sp3
hybridized orbital are one-, two-, andthree-dimensionally stretched, respectively.
Diamond of transparency and insulator is
known to be the most hardest material, whilegraphite of black and conductor is one of themost softest crystals. On the other hand,recentlydiscovered
fullerenes
withsoccerball-likestructureandcarbonnanotubes with cylindrical structure, whichare zero- and one-dimensionally stretched,respectively, are expected to be candidates forkey materials on bottom-up nanotechnologyunderthesituationthat
top-down
nanotechnology prevailing mainly in Si
semiconductor industries approaches aninevitable limit of processing in nano scale.
2. Relation between Plasma and Nanocarbon
Plasma scientists have paid attention thefullerene and carbon nanotube in view of
their formation and synthesis directly
connected with self-organization process,because they were discovered using laser
vaporization and arc discharge methods,

where the plasma generation intrinsicallytook place. In this sense, thermal plasma
decomposition/depositionandplasma-enhanced chemical vapor deposition(plasma CVD) have been regarded as playing
some pioneering roles in the production of
new kinds of nanocarbons. When we recall
plasma technology on etching and surface
modification of materials in semiconductor
processes, on the other hand, external
structure control of nanocarbons such as
fullerene and carbon nanotube is hoped to berealized using extended plasma technology.
Based on the background described above,
we here discuss efficient creation andstructural control of novel nanocarbons using
nanoscopic plasma processes. Our discussion
is divided into two categories. One isconcerned with inner nano-space control andsurface modification of fullerenes and carbon
nanotubes
using
fully-ionizedandgas-discharge plasmas, and electrolyte
plasmas in the solution state. The other isconcerned with growth control of pristine
carbon nanotubes by plasma CVD method.
3. Creation of Novel Structured Fullerenes
The encapsulation of atoms inside hollow
cages of fullerenes is one of the fascinatingtasks since their discovery. It is urgently
required to develop a new method allowing
the production of atom encapsulated
fullerenes in large quantities. Concerning
endohedral metallofullerenes, we have foundthat using alkali-fullerene plasmas, whichconsist of positive alkali-metal ions (A+) andnegative fullerene ions (C60−), is very effective
and the formation rate of alkali-metal
encapsulated fullerenes A@C60 is inversely
proportional to the diameter of thealkali-metal ions. This means that coulombinteractions,originatingfromtheacceleration and deceleration of A+ and C60−
ions, have a significant effect on the atom
encapsulation in C60 cages .
We also challenge to develop a novel
method for the efficient production of N@C60,
potential applications of which have been
proposed, using plasma confinement andheating technology .
4. Nano-Space Control of Carbon Nanotubes
Plasma-ion irradiation method has been
adopted in order to control the inner
nano-spaceofsingle-(SWNTs),double-(DWNTs), and multi-walled (MWNTs)
carbon nanotubes and resultantly develop
novel
functional
carbon-basednano-materials. When positive or negative
bias-voltages are applied to a substrate
covered with nanotubes and immersed in the
different-polarity ion plasmas, accelerated
C60− or A+ ions are irradiated to those carbon
nanotubes through collisionless plasma
sheaths in front of the substrate.
It is found that drastic structuralmodifications such as severe bending ofcarbon nanotube bundles or tube tip
termination take place after the plasma ion
irradiation. In the case of alkali-metal
irradiation, the Cs encapsulation inside theSWNTs (Cs@SWNTs) is directly observed forthe first time and the configuration of which
is demonstrated to comprise three varieties.
In the case of C60− irradiation, on the otherhand, the SWNTs and DWNTs encapsulating
fullerene molecules are clearly observed.
Both the degree of the structuralmodification and the fullerene filling
efficiency are found to become enhanced in
the order MWNTs, DWNTs, SWNTs, and bedirectly proportional to the ion irradiationenergy and plasma density used. Finally,
when the bias-voltage with polarity change issequentially applied to the SWNTs, theSWNT encapsulating a junction of Cs atoms
on one side and C60 molecules on the other isfor the first time observed. It is noteworthy toemphasize our results open a new possibility
for the novel structured material synthesis
using plasma technology.
According to measurements of electronictransportproperties
usingFET
configurations, Cs@SWNTs exhibit n-typetransport behavior, while C60@SWNTs giverise to a p-type semiconductive property.
The
above-mentioned
plasma-ion
irradiation method is applied to electrolyte
micro-plasmas in solutions in order to form
DNA encapsulated SWNTs (DNA@SWNTs)
available for novel electronic devices. Since
DNA is ball-shaped and negatively charged
in the solution, AC and DC electric fields are
effective for stretching due to ponderomotive
force and transporting/accelerating towards
a substrate covered with open-ended SWNTs
in the electrolyte micro-plasma. Thus,
negative DNA ions are inserted into SWNTs,
forming DNA@SWNTs for the first time.
5. Growth Control of Single-Walled Nanotubes
In order to accurately and efficiently
perform inner nano-space control of carbon
nantoubes, pristine/empty SWNTs have to
be individually and vertically aligned.
Plasma CVD method has a variety ofadvantages in the production, but yields only
MWNTs. We attempt to produce SWNTs
using Plasma CVD by devising size anddistribution of catalysts, and plasma-ion
influx towards a growth substrate in areactive plasma. Actually, the SWNT
formation is for the first time realized using
plasma CVD. Furthermore, owing to thebenefits of plasma CVD, low temperaturesynthesis and the individual SWNTalignment control are also achieved.
6. Summary
The nanoscopic plasma processes based on
substrate bias method in the gaseous and solution
states yield inner nano-space control ofnanocarbons and creation of novel structured
fullerenes and carbon nanotubes encapsulating
atoms or molecules including DNA. Single-walled
nanotubes synthesized by plasma CVD could
advance such a plasma-nanotechnological research.
The author thanks T. Hirata, T. Kaneko, W.
Oohara, T. Okada, T. Kato, and T. Izumida for theircollaboration.