<s>
A	O
device	O
generating	O
linear	O
or	O
rotational	O
motion	O
using	O
carbon	O
nanotube(s )	O
as	O
the	O
primary	O
component	O
,	O
is	O
termed	O
a	O
nanotube	B-Algorithm
nanomotor	I-Algorithm
.	O
</s>
<s>
Nature	O
already	O
has	O
some	O
of	O
the	O
most	O
efficient	O
and	O
powerful	O
kinds	O
of	O
nanomotors	B-Algorithm
.	O
</s>
<s>
Some	O
of	O
these	O
natural	O
biological	O
nanomotors	B-Algorithm
have	O
been	O
re-engineered	O
to	O
serve	O
desired	O
purposes	O
.	O
</s>
<s>
However	O
,	O
such	O
biological	O
nanomotors	B-Algorithm
are	O
designed	O
to	O
work	O
in	O
specific	O
environmental	O
conditions	O
(	O
pH	O
,	O
liquid	O
medium	O
,	O
sources	O
of	O
energy	O
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
Laboratory-made	O
nanotube	B-Algorithm
nanomotors	I-Algorithm
on	O
the	O
other	O
hand	O
are	O
significantly	O
more	O
robust	O
and	O
can	O
operate	O
in	O
diverse	O
environments	O
including	O
varied	O
frequency	O
,	O
temperature	O
,	O
mediums	O
and	O
chemical	O
environments	O
.	O
</s>
<s>
The	O
vast	O
differences	O
in	O
the	O
dominant	O
forces	O
and	O
criteria	O
between	O
macroscale	O
and	O
micro/nanoscale	O
offer	O
new	O
avenues	O
to	O
construct	O
tailor-made	O
nanomotors	B-Algorithm
.	O
</s>
<s>
The	O
various	O
beneficial	O
properties	O
of	O
carbon	O
nanotubes	O
makes	O
them	O
the	O
most	O
attractive	O
material	O
to	O
base	O
such	O
nanomotors	B-Algorithm
on	O
.	O
</s>
<s>
Just	O
fifteen	O
years	O
after	O
making	O
the	O
world	O
's	O
first	O
micrometer-sized	O
motor	O
,	O
Alex	O
Zettl	O
led	O
his	O
group	O
at	O
University	O
of	O
California	O
at	O
Berkeley	O
to	O
construct	O
the	O
first	O
nanotube	B-Algorithm
nanomotor	I-Algorithm
in	O
2003	O
.	O
</s>
<s>
Despite	O
the	O
scaling	O
nature	O
of	O
the	O
electrostatic	O
force	O
it	O
is	O
one	O
of	O
the	O
major	O
mechanisms	O
of	O
sensing	O
and	O
actuation	O
in	O
the	O
field	O
of	O
microelectromechanical	B-Architecture
systems	I-Architecture
(	O
MEMS	B-Architecture
)	O
and	O
is	O
the	O
backbone	O
for	O
the	O
working	O
mechanism	O
of	O
the	O
first	O
NEMS	O
nanomotor	B-Algorithm
.	O
</s>
<s>
The	O
quadratic	O
scaling	O
is	O
alleviated	O
by	O
increasing	O
the	O
number	O
of	O
units	O
generating	O
the	O
electrostatic	O
force	O
as	O
seen	O
in	O
comb	B-Algorithm
drives	I-Algorithm
in	O
many	O
MEMS	B-Architecture
devices	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
nanotube	B-Algorithm
nanomotors	I-Algorithm
however	O
,	O
the	O
intershell	O
friction	O
in	O
the	O
multi-walled	O
nanotubes	O
(	O
MWNT	O
)	O
is	O
remarkably	O
small	O
.	O
</s>
<s>
The	O
sudden	O
rise	O
in	O
sliding	O
velocity	O
is	O
due	O
to	O
a	O
resonance	B-Application
condition	O
between	O
a	O
frequency	O
that	O
is	O
dependent	O
on	O
the	O
inter-tube	O
corrugation	O
period	O
and	O
particular	O
phonon	B-Language
frequencies	O
of	O
the	O
outer	O
tube	O
which	O
happen	O
to	O
possess	O
a	O
group	O
velocity	O
approximately	O
equal	O
to	O
the	O
sliding	O
velocity	O
.	O
</s>
<s>
The	O
first	O
nanomotor	B-Algorithm
can	O
be	O
thought	O
of	O
as	O
a	O
scaled	O
down	O
version	O
of	O
a	O
comparable	O
microelectromechanical	B-Architecture
systems	I-Architecture
(	O
MEMS	B-Architecture
)	O
motor	O
.	O
</s>
<s>
The	O
MWNT	O
at	O
this	O
point	O
displays	O
a	O
very	O
high	O
torsional	O
spring	O
constant	O
(	O
10−15	O
to	O
10−13	O
N	O
m	O
with	O
resonant	B-Application
frequencies	I-Application
in	O
the	O
tens	O
of	O
megahertz	O
)	O
,	O
hence	O
,	O
preventing	O
large	O
angular	O
displacements	O
.	O
</s>
<s>
Conventional	O
methods	O
like	O
chemical	B-Algorithm
vapor	I-Algorithm
deposition	I-Algorithm
(	O
CVD	O
)	O
allow	O
the	O
exact	O
placement	O
of	O
nanotubes	O
by	O
growing	O
them	O
directly	O
on	O
the	O
substrate	O
.	O
</s>
<s>
A	O
potential	O
of	O
around	O
18V	O
is	O
applied	O
across	O
two	O
graphite	B-Application
electrodes	O
(	O
diameters	O
of	O
the	O
anode	O
and	O
cathode	O
are	O
6mm	O
and	O
9mm	O
)	O
separated	O
by	O
a	O
short	O
distance	O
of	O
usually	O
1	O
–	O
4mm	O
within	O
this	O
chamber	O
.	O
</s>
<s>
The	O
total	O
yield	O
of	O
nanotubes	O
as	O
a	O
proportion	O
of	O
starting	O
graphitic	O
material	O
peaks	O
at	O
a	O
pressure	O
of	O
500	O
torr	O
at	O
which	O
point	O
75%	O
of	O
graphite	B-Application
rod	O
consumed	O
is	O
converted	O
to	O
nanotubes	O
.	O
</s>
<s>
There	O
are	O
several	O
advantages	O
of	O
choosing	O
this	O
method	O
over	O
the	O
other	O
techniques	O
such	O
as	O
laser	O
ablation	O
and	O
chemical	B-Algorithm
vapor	I-Algorithm
deposition	I-Algorithm
such	O
as	O
fewer	O
structural	O
defects	O
(	O
due	O
to	O
high	O
growth	O
temperature	O
)	O
,	O
better	O
electrical	O
,	O
mechanical	O
and	O
thermal	O
properties	O
,	O
high	O
production	O
rates	O
(	O
several	O
hundred	O
mg	O
in	O
ten	O
minutes	O
)	O
,	O
etc	O
.	O
</s>
<s>
The	O
energy	O
of	O
such	O
barriers	O
could	O
be	O
estimated	O
from	O
the	O
temperature	O
in	O
the	O
nanotube	O
,	O
a	O
lower	O
bound	O
for	O
which	O
can	O
be	O
estimated	O
as	O
the	O
melting	O
temperature	O
of	O
gold	O
(	O
1300K	O
)	O
by	O
noting	O
that	O
the	O
gold	O
plate	O
melts	O
(	O
Figure	O
2.4	O
)	O
to	O
form	O
a	O
spherical	O
structure	O
as	O
current	O
is	O
passed	O
through	O
the	O
nanomotor	B-Algorithm
.	O
</s>
<s>
Although	O
the	O
current	O
model	O
excites	O
multiple	O
phonon	B-Language
modes	O
,	O
selective	O
phonon	B-Language
mode	O
excitation	O
would	O
enable	O
lowering	O
the	O
phonon	B-Language
bath	O
temperature	O
.	O
</s>
<s>
As	O
figure	O
3.1	O
shows	O
,	O
the	O
nanomotor	B-Algorithm
consists	O
of	O
a	O
double-walled	O
CNT	O
(	O
DWNT	O
)	O
formed	O
from	O
an	O
achiral	O
(	O
18	O
,	O
0	O
)	O
outer	O
tube	O
clamped	O
to	O
external	O
gold	O
electrodes	O
and	O
a	O
narrower	O
chiral	O
(	O
6	O
,	O
4	O
)	O
inner	O
tube	O
.	O
</s>
<s>
Conventional	O
nanotube	B-Algorithm
nanomotors	I-Algorithm
make	O
use	O
of	O
static	O
forces	O
that	O
include	O
elastic	O
,	O
electrostatic	O
,	O
friction	O
and	O
van	O
der	O
Waals	O
forces	O
.	O
</s>
