1
Carbon
nanotube
-
sharp
tips and
carbon nanotube
-
soldering
i
ron
s
A. Misra
1
, C. Daraio
1,2
1
Graduate Aeronautical Laboratories (GALCIT),
2
Applied Physics
California Institute of Technology, Pasadena, CA, 91125
We report o
n
the
nano
-
electr
on beam assisted
fabrication of atomically sharp iron
-
based
tips and on the creation of a
nano
-
soldering iron
for nano
-
interconnects
using
Fe
-
filled
multiwalled carbon nanotubes (MWCNTs). H
igh energy electron beam machining has
been prove
n a powerful tool to modify
desired
nanostructures for technological
applications
(
1
-
4
)
and
to form molecular junctions
and interconnections
between
carbon
nanotubes
(
2,
5
)
.
R
ecent
studies (
5
)
showed the high
degree
of complexity in
the creation of
direct
interconnections between
multi
walle
d and CNTs
having
dissimilar
diameters
.
Our
technique
allows for
carving a MWCNT into a nanosoldering iron that was demonstrated
capable of
joining
two
separated
halves of a tube.
This approach
could easily
be
extended
to the interconnection of two largely
dissimilar CNTs, between
a CNT and a nanowire
or
between two nanowires.
The
development of the next generation of miniaturized electronic systems
demands
the
integration of nanoelectronic components creating reliable mechanical and electrical contacts.
Despite many years of carbon
nanotubes
research, the electrical interconnection of individual
nanotubes and nanowires remains a tedious task.
At the same time, the development of scanning
probe techniques and magnetic recording media require an ever decrea
sing tip size of ultrasharp
2
magnetic read
-
write heads (
6
).
Recently,
focused electron b
eam
s
have been
reported
for the
machining
of
both single walled
(SW
C
NTs)
and multiwalled
(MW
C
NTs)
nanotubes, for cutting
them or
for
forming
junctions at
the
atomic le
ve
l
(the latter being limited so far to only small and
similar diameter tubes)
(
5
).
Along with
the well known electrical, mechanical and thermal
properties of
the
carbon nanotubes
,
the presence of a
metal filling inside
their core has been
suggested to enhan
ce their applicability in
magnetic recording
media
,
new
electronic
devices
and
to
reinforce
the
material durability
(
7,8
)
.
Here
we
report
two interesting phenomena
of
nano
electron beam assisted
machining of iron
-
filled multiwalled carbon nanotubes (MWCNTs)
:
the fabrication of atomically sharp magnetic tips and the
direct interconnection
of two
large
diameter
MWCNTs
.
Previous attempts
to
solder
CNT
s
had been perfor
med
through
metal
(
5
)
and am
orphous carbon
(
2
)
deposition
at the tips
,
but
the results showed
l
ittle o
r
questionable
success
.
The Fe
-
filled
carbon nanotubes
(CNTs)
considered for this study were produced by
dissolution of a catalyst
source
and
a
carbon source
in
a two
-
stage
thermal
chemical vapor
deposition
system. This system consists of
a
30 mm
diameter, and 1000 mm long quartz tube
inserted in a tube furnace
having 200 mm preheating
(at
80
0
C
)
and 500 mm heating
(at
825
0
C
)
zone. Si was used as
growth
substrate
while a
mixture of Fe
-
catalyst (ferrocene) and carbon
source (toluene) (0.02 g/ml) w
ere injected into the preheating zone at the rate of 5
ml/15 min. A
flow of 100 SCCM of argon gas was maintained as carrier for
the
solution into
the
heating zone.
The
grown MWCNTs had
outer diameter
s
rang
ing
from 20
-
100 nm and t
heir length was ~100
μm. Fo
r
the
transmission electron microscope
analysis
and machining, a
small amount of
the
grown material
was scratched from
the
substrate
,
dispersed in iso
-
propyl alcohol and deposited
on a holey carbon coated TEM grid. Electron irradiation for manipulation, et
ching and imaging
3
was obtained in a
TEM
(
FEI Technai F
-
30 UT) with a field emission gun operating at an
acceleration voltage 300 kV
at room temperature
(
with no heating stage attached to the specimen
holder
).
The field emission gun
together with the probe
forming lenses of the microscope is
capable of producing a nanometer
-
sized electron beam with current density of the order ~1.3x10
3
A/cm
2
. Irradiation was carried out at beam current densities varying from 1.3x10
3
A/cm
2
to
7.5x10
2
A/cm
2
on different diame
ter multiwalled carbon nanotubes. Sequences of TEM images
were recorded
using ORCA
-
ER camera with 1280 x 1024 pixel format in a fixed bottom mount
configuration and this camera uses a Hamamatsu DCAM supported board for acquisition.
We first describe the c
reation of atomically sharp magnetic tips
(
a
schematic diagram of
the processing and resultant structures is
reported
in
Fig. 1A
-
D
)
and later
the synthesis
and
operation
of a CNT
-
soldering iron
(Fig. 1E
-
F
)
.
It was shown that u
nder controlled irradiation
wi
th a highly focused electron beam
,
Fe
-
filled MWCNTs
can be cut at selected locations
(
9
)
.
Using similar experimental conditions, we
s
lice
a MWCNT
in two parts
(Fig. 1B
) and expose
the
Fe
nanorods
encapsulated inside the
core of the
tube to a continuous e
-
b
eam
,
centered in close
proximity of the cut
.
This
high energy
irradiation
causes
an increase of the tube’s internal
pressure, localized joule heating and related electro
-
migration
(
5
)
,
that lead to a
modification of
the
Fe
nanorods
’s
properties
(
10
)
,
and
t
o
surface reconstruction
(
11
)
at the incised
rim
.
We report
TEM
based observations
of the described steps for the synthesis of an atomically sharp Fe tip in
Fig. 2.
The MWCNT studied had
inner and outer diameters of
9
nm and 32
nm respectively
(Fig.
2A
)
. E
lectron
beam cutting
was achieved
dragging
a nano e
-
beam (
spot size
8
nm
in diameter)
with a current density of
1.3x10
3
A/cm
2
along the CNT diameter.
Fig
ure
2B
shows the
process of
formation of
the opening
in the MWCNT
along the focused
e
-
beam path.
After
few seconds of
the cutting, the
surface re
construction at
the
unstable
open ends
of
the
carbon nanotube
walls
as
4
well
as o
n
the surface of the
Fe
-
nanorod
was
evident, as shown in Fig 2
B and C
(and pointed by
the arrow)
.
It is clear that
due to
a
strong te
ndency towards
the
reduction of
the
surface
energy
(
1
2
)
,
t
he
capping
of the open
wall
ends occur
s
with
the formation
of
stable closed
structure
by
cross linking between adjacent
graphene
planes (
inter
-
wall
s
interaction
) and the
generation
/migration
of vaca
ncy
-
interstitials
pairs
(
1
3
)
due to
the
structural damage
.
The
dangling
atoms at the
edge of the cut
combined with the localized high temperature
may
cause
the formation of
carbon dimer
C
2
units
(
1
4
,
1
5
)
which
after recombination
form pentagon
al
rings, the
most essential building
blocks
in
the
form
ation of
curved
structures
.
This
phenomenon
results in the
formation of
semi
-
fullerene
-
like cap
s interlinking the walls
and
healing the incised
surface
. As a result the Fe nanorod near the incision remains
trapp
e
d between
the
graphene caps
on one side
and
the CNT’s
walls
on the other.
T
he self compression
and dynamic behavior of
encapsulated material inside graphitic networks
such as
carbon
onion
or
multiwalled
carbon
nanotubes
was the
subject
of recent investigat
ions
(
10, 16
)
.
It was shown that
under electron
beam irradiation locally high internal pressure and temperature caused phase modification and
shape changes
of
the
metallic
particles trapped
inside
the
core
.
Upon continuous electron
irradiation, the damage
and reconstruction of
the outermost
carbon
lattice
further increased
the
pressure within the graphene cells
, enhancing such effects
.
Similarly, in our experiments
it is
evident that
i
mmediately after
the reconstruction of
the
sliced walls’
ed
ges
into a cl
osed cell
structure
(Fig. 2A
)
,
the
CNT
starts behaving as
a
high pressure cell
(
10
)
for
the
encapsulated
Fe
nanorod.
Because of the high pressure and temperature,
Fe
is pushed
out from
the nanotube’s
core and
local
recrystallization phenomena occur.
At the
same time, a continuous etching of the
outer
carbon
walls occurs, causing the incised nanotube’s edge to assume a pointed pencil
-
like
shape.
To further investigate
the effects of electron irradiation, w
e
investigat
ed
the
time
5
evolution
of
the
encapsulate
d Fe
nanorod
structure during the
beam exposure
o
f
one side o
f the
cut (upper edge of Fig. 2B
)
.
The Fe nanorod protrusion and the tip formation
are
shown in
Fig
.
2C
-
D
after
~
17
min
of
e
-
beam
irradiation
.
In addition to
the
outward extrusion of the nanorod
towards the
pressure gradient
,
the e
-
beam
interaction
also
induces
melting and re
-
crystallization
on
the
exposed
metal
surface
.
This is made possible by
self diffusion
processes cause
d by
the
small
-
size
effect
(
1
7
,
1
8
)
that
reduce
s
the melting temperature
o
f the nanorod
. The higher
interfacial free energy per unit area increases the tendency of melting at the surface at a
temperature below the thermodynamic equilibrium melting temperature.
Recently, the
phenomenon of liquid
-
state surface faceting as a precu
rsor to surface induced crystallization was
being observed in metal
–
alloy system
(
4
)
.
This
was considered as
one of the significant
hallmarks of the crystalline state
, as s
table facets with low specific surface free energy determine
the equilibrium shape o
f solid particle
s
.
In our experiments we
observe the equilibrium
faceting
of Fe after recrystallization
into a hex
agonal shape
(Fig.
2D
)
. Th
is enables the formation of
an
atomically sharp tip whose outer diameter
is
~
7 nm
, and
the vertex size of
<1
nm.
The
complete
formation of
the
re
crystallize
d
probe after
~
17
min
of e
-
beam
irradiation is shown in Fig. 2E
.
Because of the absence of
a
heating stage
in our system,
we could not preserve the
initial parallel
walls
-
tube structure
,
as the low defect mobility at
room temperature prevents reconstruction and
leads to rapid destruction of the graphite lattice
.
To ensure the graphite lattice reconstruction t
he
specimen should
be maintained at a temperature of
>
300
0
C
(
1
9
)
.
T
he melting and recrystallization
effects d
escribed above for the formation of the sharp
tips suggest the possibility of utilizing
the same
structures for nano
-
soldering and interconnect
s,
simply by prolonging the e
-
beam exposure of the tips to enable additional ejection of the melt
metal
.
Such tip
s could then be used as nanosoldering irons if places in proximity of one or two
6
other
elements to be connected.
T
o
explore
the effective soldering ability
of such probes
, w
e
performed a systematic inve
s
tigation
on
various
Fe
-
filled MW
CNTs
of different
inn
er and outer
diameter
s
(20 and 60
nm) by using
the same condition of
electron
irradiation
. Similar processing
steps were followed for the
cut
ting
(Fig.
3A
)
and etching
(Fig. 3B and 3C
)
of the tube
s
, with
variable exposure time dependent on the different tu
be’s
diameter
s
.
It was demonstrated that t
he
prolonged local electron irradiation on both sides of the incision causes an outward flow of the
melted Fe
-
nanorod
s
that “grow” towards each other until their successful
final
soldering
.
The
TEM
image
reported
in
Fig. 3C
shows a
complete
merging and “healing”
of two
previously
separated
halves
of the CNT
after
~30 min
of e
-
beam exposure
.
From these results it is clear that
time for cutting and soldering
of different nanotubes and/or nanowires
depends on the
oute
r
diameter and
number of walls composing the
nanotubes.
A higher magnification image of the
soldered zone is shown in
Fig. 3D
. The aggregation and recrystallization of
Fe
at the junction
is
noticeable, and it is evident that surface
modification phenomena
took place all over the
nanowire’s area exposed to the e
-
beam
.
Remarkably
,
two distinct features were observed in the
soldering process
between
the
two Fe nanowires: the
outward flow of the metal
due to
the
pressure gradient created by the e
-
beam energy
,
a
nd
its
surface melting and re
-
crystall
i
zation
forming at the
soldered zone
a polycrystalline
junction
.
It would be interesting to study
systematically the electrical properties of such junctions under variable irradiation condition
and
different
soldering
materials
.
To
further
characterize the
interconnection
-
s
tructure evolution, we
rec
or
ded electron diffraction pattern
s
(
see
inset
of F
ig.
3D
)
from the
re
-
crystalliz
ed area, after
completing the
tube’s re
connection
.
We found scattered low intensity spots fro
m polycrystalline
iron
and the presence of
diffused rings typical of disordered
carbon
.
A low magnification image
of the final stage of the soldering process is shown in
Fig.
3
E
.
7
We analyzed the Fe nanorod “growth” rate in terms of volume of metal expose
d outside
its original CNT
-
enclosure as a function of irradiation time
on
two samples
differing in inner and
outer diameters (
Fig
. 4 A and
B
)
.
The plots
show
in all cases
a rather nonlinear behavior, likely
related to two leading phenomena: e
-
beam
etching
of the CNT’s wall in the initial phase, and
later
melting, flowing and recrystallization of the metal.
From these results it is clear that t
he
amount of time
necessary
for nanoso
ldering depends on the
geometry of the samples.
Through this work we present
t
he first experimental demonstration
of
the use of nano
-
electron beam engineering of MWCNTs as an advantageous tool for the creation of atomically
sharp Fe
-
based tips. In addition, we report the creation of
a
carbon nanotube
soldering iron
and
proved its ef
fectiveness in the connection of two MWCNTs.
The same approach can be use for
soldering a variety of different nanostructures.
This system represents a
viable tool for
interconnecting
nano
wires
,
for example in creating asymmetric heterostructures and
heter
ojunctions
(
20
)
.
Although far from being completely characterized, this work
adds a new
functionality in nanoelectromechanical
systems and integrated circuitry
that might dramatically
change the engineering of
nanoelectronic devices
.
8
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Acknowledgements
C.D. wishes to acknowledge the support of this work by Caltech start
-
up
funds, A.M. acknowledges support by the Moore Fellowship
.
This work benefited from use of
the Caltech KNI and Mat
Sci TEM facilities supported by the MRSEC Program of the National
Science Foundation under Award Number DMR
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0520565
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helpful discussions.
Correspondence
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daraio@caltech.edu
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10
Figure 1.
Processing steps for the formation of an atomically sharp
Fe tip
and of the
soldering
between two
CNTs
-
halves
under electron beam irradiation.
A
,
As
-
grown Fe
-
encapsulated
MWCNT
.
B
,
E
lectron
-
beam
as
sisted c
utting of a gap.
C
-
D
,
Growth of
the metal
tip.
E
-
F
,
M
elting
, flowing and
final
soldering
back together of the CNT’s halves
.
Figure 2.
TEM snapshots
showing
the melting, flowing and recrystallization
during the
form
ation of
atomically sharp
Fe tip
after
300
k
V
electron beam irradiation.
A
,
TEM image
showing the pristine
CNT
with Fe nanorod
(catalyst)
encapsulated
in
its
inner core.
B
,
Cutting
of
a
10
nm
gap
across
the tube
by moving a
highly
focused
electron beam
. The
area marked by
a
white circle
underlines
the
exposure
of Fe nanorod
.
C
,
High magnifica
tion image
of the
MWNTs
walls
reconstruction
into closed
fullerene
-
like
cap
s (
marked by
the
arrow
)
.
D
,
C
omplete
formation of
the sharp
Fe
-
tip
with
hexagonal geometry
after
17
min
.
E
,
MWCNT
-
supported
sharp
nanosoldering iron.
Figure 3.
TEM images showin
g
the n
anosoldering of
a cut Fe
-
filled
MWCNT
.
A
,
Initial
incision and gap
-
opening
.
B
,
Etching of the carbon walls surrounding the incision by e
-
beam
irradiation (and initial exposure of the nanowire).
C
,
Soldering back together of the
Fe nanorod
s
after
etching,
extrusion and surface
re
-
growth
.
D
, High resolution image of
the
soldered
area
showing
polycrystalline Fe. Inset shows
a
diffraction pattern recorded
in the area
.
E
,
Low
magnification image showi
ng the completed soldering process
.
Figure
4.
Growth rate of the Fe nanorod outside of its original CNT enclosure expressed as
exposed volume (nm
3
) as a fu
nction of time (minutes) for
A
,
9
inner
/32 nm
outer diameter tube
and
B
,
for the nanosoldering pro
cess described in Fig. 3.
11
Figure.1
a
b c
e
f
d
12
Figure. 2
13
Figure.3
14
Figure.4
a
b
a
b