Liquid gallium columns sheathed with carbon: Bulk synthesis and manipulation

Article abstract of DOI:10.1021/jp050826t

It is impossible to fabricate isolated gallium nanomaterials due to the low melting point of Ga (29.8?°C) and its high reactivity. We report the bulk synthesis of uniform liquid Ga columns encapsulated into carbon nanotubes through high-temperature chemical reaction between Ga and CH₄. The diameter of filled Ga liquid columns is a??25 nm, and their length is up to several micrometers. The thickness of the carbon sheaths is a??6 nm. Simultaneous condensation of a Ga vapor and carbon clusters results in the generation of Ga-filled carbon nanotubes. A convergent 300 kV electron beam generated in a field emission high-resolution electron microscope is demonstrated to be a powerful tool for delicate manipulation of the liquid Ga nanocolumns: they can be gently joined, cut, and sealed within carbon nanotubes. The self-organization of a carbon sheath during the electron-beam irradiation is discussed. The electron-beam irradiation may also become a decent tool for Ga-filled carbon nanotube thermometer calibration. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp050826t

11580
J. Phys. Chem. B 2005, 109, 11580-11584
Liquid Gallium Columns Sheathed with Carbon: Bulk Synthesis and Manipulation
,†
†
‡
†
§
Jinhua Zhan,* Yoshio Bando, Junqing Hu, Dmitri Golberg, and Haruyuki Nakanishi
AdVanced Materials Laboratory and International Center for Young Scientists (ICYS), National Institute for
Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, and Toyota Motor Corporation, 1200 Mishuku,
Susono, Shizuoka 410-1193, Japan
ReceiVed: February 17, 2005; In Final Form: April 20, 2005
It is impossible to fabricate isolated gallium nanomaterials due to the low melting point of Ga (29.8 °C) and
its high reactivity. We report the bulk synthesis of uniform liquid Ga columns encapsulated into carbon
nanotubes through high-temperature chemical reaction between Ga and CH . The diameter of filled Ga liquid
4
columns is ∼25 nm, and their length is up to several micrometers. The thickness of the carbon sheaths is ∼6
nm. Simultaneous condensation of a Ga vapor and carbon clusters results in the generation of Ga-filled carbon
nanotubes. A convergent 300 kV electron beam generated in a field emission high-resolution electron
microscope is demonstrated to be a powerful tool for delicate manipulation of the liquid Ga nanocolumns:
they can be gently joined, cut, and sealed within carbon nanotubes. The self-organization of a carbon sheath
during the electron-beam irradiation is discussed. The electron-beam irradiation may also become a decent
tool for Ga-filled carbon nanotube thermometer calibration.
I. Introduction
mometer due to its temperature-dependent expansion and
1
3,14
contraction within a range of -20 to +500 °C.
Although
One-dimensional (1D) nanomaterials have attracted consider-
able attention due to their potential applications as building
blocks in nanoscale circuits and optoelectronic devices. Nano-
Ga was used to mediate the growth of multiwalled carbon
nanotubes, the continuous Ga filling failed due to the open tube
1
1
5
ends. Herein we demonstrate an effective approach for the
bulk synthesis of liquid Ga nanocolumns sheathed with carbon
in an induction furnace. Ga originates from the sublimation and
decomposition of GaN, whereas carbon comes from the
decomposition of methane (CH4) at a high temperature. A
nanosized electron beam generated within a 300 kV field
emission transmission electron microscope is demonstrated to
be a powerful tool for the delicate manipulation of the
nanostructures due to local atom collisions with high-energy
tube filling with foreign elements, compounds, and/or substances
2
-9
is a relatively new and exciting field of nanomaterial research.
D nanomagnets, nanocables, and other functional nanomaterials
1
may be synthesized by taking advantage of the tiny nanotube
channel dimensions and related confinement effects, and
functional properties of guest species encapsulated within, e.g.,
ferromagnetism, piezoelectricity, electro- and thermoconduc-
tivity, etc. Metallic gallium melts at nearly room temperature
and has one of the widest liquid ranges among all metals.
Gallium possesses unusual crystallization behavior.10 Similarly
to water, metallic Ga expands as it freezes. Ga displays a rather
complex phase diagram exhibiting many stable and metastable
phases. At ambient pressure, the R-Ga(I) phase is orthorhombic
with eight atoms in the unit cell of a Cmca space group lattice.
In this structure a Ga atom is bonded to the nearest neighboring
Ga atom in the manner of a diatomic molecule. With respect to
the second closest neighbors located at distances 10-13% longer
than that of the first coordination sphere, the atom retains
metallic properties. It is suggested that covalent and metallic
bonds coexist in the Ga metal. In addition, a number of
1
6-21
electrons.
We particularly demonstrate that liquid Ga
nanocolumns can be effectively joined, cut, and sealed using a
convergent electron beam.
II. Experimental Section
Liquid Ga columns sheathed with carbon were synthesized
in a vertical induction furnace. The furnace consisted of a fused-
quartz tube and an induction-heated cylinder made of high-purity
graphite coated with a carbon fiber thermoinsulating layer, as
illustrated in Figure 1 in the Supporting Information. The system
had the inlet and outlet pipes made of graphite on its top and
base, respectively. A graphite crucible, containing 0.5-2 g of
gallium nitride powder was placed at the center cylinder zone.
1
1
metastable phases including â, γ, and δ have been identified.
At low temperature Ga metal becomes a strong-coupling
superconductor with a well-defined superconducting transition
-
3
After evacuation of the quartz tube to ∼10 Pa, pure N and
2
1
2
temperature, as confirmed by tunneling experiments.
CH flows were set within the carbon cylinder at a constant
4
Due to the low melting point (29.8 °C) and high reactivity,
the preparation of isolated metallic Ga nanomaterials is a
difficult task. Recently, it has been shown that Ga encapsulated
in carbon nanotubes has a potential application as a nanother-
rate of 500-3000 and 5-20 sccm, respectively. The top inlet
N2 flow was fixed at a constant rate of 2000 sccm. The furnace
was heated to and kept at 1300 °C for 30 min. After the reaction
was terminated and the furnace cooled to room temperature, a
gray product was collected from the outlet carbon pipe in the
induction-heated cylinder.
*
To whom correspondence should be addressed. Fax: 81-29-851-6280.
E-mail: ZHAN.Jinhua@nims.go.jp.
The morphology of the product was examined using a JSM-
6700F scanning electron microscope operated at 10 kV. The
product was ultrasonically dispersed in ethanol and transferred
†
Advanced Materials Laboratory, National Institute for Materials Science.
ICYS, National Insitute for Materials Science.
Toyota Motor Corp.
‡
§
1
0.1021/jp050826t CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/21/2005

Liquid Gallium Columns Sheathed with Carbon
J. Phys. Chem. B, Vol. 109, No. 23, 2005 11581
Figure 1. (a) SEM image of the as-obtained liquid gallium columns
sheathed with carbon. (b) High-magnification SEM image displaying
the tips of the nanostructures.
to a carbon-coated copper grid for transmission electron
microscopy (TEM) observations. A field emission JEM-3000F
high-resolution electron microscope operated at 300 kV and
equipped with an energy-dispersive X-ray spectrometer was used
to perform the microanalysis. Electron irradiation experiments
were carried out with electron beams 5-100 nm in diameter.
A 300 kV voltage was used throughout the experiments.
Figure 2. (a) TEM image of Ga nanocolumns sheathed with C. (b)
EDS spectrum recorded from a composite nanostructure. (c) Selected-
area electron diffraction (SAED) pattern taken from an individual
nanostructure. (d, f) TEM images displaying the opposite ends of a
given nanostructure.
III. Results and Discussion
A representative scanning electron microscopy (SEM) image
of a sample is shown in Figure 1a, which indicated the product
is composed of wirelike nanostructures with a length of up to
several micrometers. High-magnification SEM analysis reveals
that the wirelike nanostructures have core-sheath structures.
Figure 1b shows the end of a core-sheath structure. The
synthetic temperature seems to play an important role in the
generation of the Ga-C core-sheath structure. When the
synthetic temperature was reduced to 1150 °C, some curly
nanostructures were produced (see Figure 2 in the Supporting
Information). Detailed structural and chemical analysis of
individual nanostructures was carried out using TEM, high-
resolution (HR) TEM, electron diffraction (ED), and energy-
dispersive X-ray spectroscopy (EDS). Figure 2a shows a TEM
image of a product. The image displays uniform core-sheath
structures. The diameter of a metallic Ga core is approximately
Figure 3. Growth scenario for the formation of liquid gallium columns
sheathed with carbon: (I) vaporization of gallium, (II) condensation
of gallium vapor to form gallium droplets, (III) nucleation of carbon
nanotubes, (IV) growth of carbon nanotubes and in situ filling with
gallium, (V) sealing of carbon nanotubes.
through the induction furnace at high temperature, it decomposes
to produce carbon clusters. Ga droplets induce the anisotropic
growth of carbon nanotubes; simultaneously the nanotube
cavities are filled with liquid Ga. The simultaneous generation
of a Ga vapor and carbon clusters promotes the uniform filling
of carbon nanotubes. The growth process is shown in Figure 3.
After the reaction was terminated, the furnace cooled from
1300 °C to room temperature over 30 min. It is suggested that
rapid cooling may cause the discontinuity of encapsulated Ga
columns. Figure 4a shows a TEM image of a Ga column having
a gap of approximately 70 nm between its two segments. The
incident electron energy (Ep ) of a beam can be determined from
2
5 nm, whereas its sheath is ∼6 nm thick. The nanostructure
length can reach several micrometers. EDS analyses conducted
on the nanocables using a nanoprobe display that the core and
a sheath are composed of Ga and C, respectively. A representa-
tive EDS spectrum is depicted in Figure 2b. A selected-area
electron diffraction pattern (Figure 2c), taken on a Ga-C core-
sheath nanostructure, exhibits a pair of weak (002) diffraction
arcs and additional diffraction rings, which suggests that the
carbon sheaths are poorly ordered, as compared to standard
carbon nanotubes made up of cylindrically wrapped graphene
layers. The Ga cores are in a liquid state. Both ends of the Ga-C
core-sheath composite nanostructures are enclosed with carbon,
as revealed by Figure 2d,e. A cavity in the vicinity of one
nanotube end is formed due to Ga contraction confined with a
carbon nanotube when the whole structure cools from high to
room temperature.
2
2 1/2
the equation Ep ) mc (1/(1 - â ) - 1), where m is the rest
mass of an electron, c is the speed of light, V is the velocity of
2
2
an electron, and â ) V/c. When a high-energy electron passes
through a specimen, its energy may partially be transferred to
the target and enhance the movement of atoms. Consequently,
this energy may cause the local temperature rise or the
23,24
transformation of crystal structures.
When an electron beam
During the synthetic process, GaN sublimes and decomposes
at high temperature (approximately 1300 °C) to produce a Ga
vapor. This vapor condenses at a low temperature (approxi-
mately 800 °C) to form Ga droplets. When a CH4 gas flows
is focused within a spot of ∼100 nm in diameter and is moved
up and down along the bottom fraction of a Ga column (Figure
4a), the column segment continuously expands due to beam-
induced heating. Finally, the bottom segment joins the top one.

11582 J. Phys. Chem. B, Vol. 109, No. 23, 2005
Zhan et al.
Figure 4. Consecutive TEM images showing joining of two discrete
liquid Ga segments under electron-beam irradiation: (a) 0 min, (b) 2
min, (c) 5 min, (d) 8 min, (e) 10 min. The circle marks the electron
beam. The scale bar is equal to 50 nm.
Figure 6. HRTEM images displaying the structural changes of a Ga-C
composite nanostructure under electron-beam irradiation, in accord with
Figure 5b-f. End of the lower Ga-C segment after (a) 0 min, (b) 2
min, (c) 4 min, (d) 7 min, and (e) 9 min. (f) End of the upper segment
after 9 min.
electron beam is then moved across the tube back and forth.
After intense irradiation over 2 min (Figure 5c), the irradiated
region becomes thinner. The image contrast in this irradiation
region becomes lighter, suggesting that metallic Ga is partially
removed. A further 2 min of irradiation can completely remove
Ga from the irradiated region (Figure 5d). The nanostructure is
finally separated into two discrete parts (Figure 5e,f). Detailed
structural transitions caused by the electron irradiation were
studied using HRTEM. Parts a-f of Figure 6show the HRTEM
images taken from the irradiated regions during consecutive
irradiation stages, corresponding to Figure 5b-f. As suggested,
on the basis of the electron diffraction patterns (Figure 2c), a
carbon sheath does not possess a well-structured graphitic
network. In fact, the HRTEM image of a sheath does not display
a well-defined layered structure peculiar to carbon nanotubes
produced via arc discharge or chemical vapor deposition with
Figure 5. TEM images depicting the morphological changes of a
Ga-C composite structure when it is irradiated with a scanning electron
beam: (a) the white dot shows the electron beam position, (b) 0 min,
(c) 2 min, (d) 4 min, (e) 7 min, (f) 9 min.
Figure 4 displays the consecutive stages of the Ga expansion.
-4
Owing to the considerable expansion coefficient (1.015 × 10 /
2
5
T) and good thermal conductivity of liquid Ga, notable
expansion of the Ga column can be observed under electron-
beam irradiation. A wetting angle between the liquid Ga and
carbon sheath slightly changes during irradiation. This change
may also be caused by a thermal effect of the irradiation. Our
estimates show that a specimen can typically be heated to
approximately 100 °C during the irradiation using the present
irradiation dose,24 which is far above the melting point of
metallic Ga (29.8 °C). The top Ga segment remains unchanged
throughout Figure 4. This implies that the thermal conductivity
of the carbon sheath is negligible in contrast to that of metallic
Ga. Thus, the thermal effect of an electron beam only comes
into effect within the irradiated region. On the other hand, the
expansion of the lower Ga segment is an indicator of the
sufficient electron-beam-induced heating.
2
7,28
transition-metal nanoparticles as catalysts.
After 2 min of
irradiation, the layer crystallinity is improved, while the carbon
sheath shrinks (Figure 6b). This result implies that the growth
defects, e.g., undulating, dangling carbon sheets, dislocation
edges, and local irregularities, are annealed in situ after a few
minutes of electron irradiation with 300 kV electrons. In some
respects, an intense irradiation resembles a high-temperature
treatment, allowing interstitials and vacancies to aggregate into
1
7,29,30
clusters and reduce their free energy.
Further electron
irradiation causes the complete shrinkage of the carbon sheath,
and a nested carbon nanostructure forms, as indicated by the
arrow in Figure 6c. A longer irradiation creates a notable cleft
within the composite nanostructure. Moreover, an elliptical
carbon “onion” of ∼15 nm diameter is generated. Figure 6d
depicts an elliptical C onion connected to the structure top
segment through an 8 nm wide graphitic neck. This carbon neck
is further completely peeled off by electron-beam irradiation,
thus resulting in the creation of two separated composite
nanostructures. A spherical nested carbon structure of ∼10 nm
diameter is left on the top right corner of the bottom segment
(Figure 6e). The top segment terminates with a sharp tip, as
shown in Figure 6f. Figure 6 clearly demonstrates that the
encapsulated liquid metal does not hinder the reconstruction of
the carbon sheath under intense electron-beam irradiation. Once
the C sheath shrinks, the liquid Ga column is ready to recede
When an electron beam strikes a specimen, the inelastic and
energy-loss electrons are generated with abundant signals. These
include X-rays and secondary electrons useful for the material
analysis.26 If the beam intensity is above the threshold, it may
cause the damage of a target. The beam damage of a graphitic
carbon and metals is generally dominated by direct displacement
of atoms from a specimen, namely, knock-on displacement. This
creates point defects. Apart from beam-induced heating, we also
demonstrate the possibility to cut carbon-sheathed Ga nano-
columns by high-energy electrons. Figure 5 displays this process
in detail. The beam is ∼5 nm in diameter, as marked with the
white dot on the initial TEM image in Figure 5a. The convergent

Liquid Gallium Columns Sheathed with Carbon
J. Phys. Chem. B, Vol. 109, No. 23, 2005 11583
indicated by the white arrow demonstrates a separation site.
After separation of the filled and unfilled tubular parts, an onion-
like tip end is formed on a 1D Ga-C composite nanostructure.
This carbon onion may prevent further liquid Ga spilling from
the tube.
IV. Conclusion
In conculsion, uniform liquid-Ga-filled carbon nanotubes have
been generated during a high-temperature chemical synthesis
utilizing CH4 and GaN reagents in an induction furnace. The
diameter of filled Ga liquid columns is ∼25 nm, and their length
is up to several micrometers. The thickness of the carbon sheaths
is ∼6 nm. Electron-beam irradiation inside an electron micro-
scope has been documented to be a powerful tool for delicate
manipulation of the prepared one-dimensional nanostructures.
We thoroughly demonstrate that the 1D Ga-C composite
nanostructures can be joined, cut, and sealed using electron-
beam irradiation. Since the electron-beam irradiation can cause
local structural transformations within the carbon sheath, it may
be possible to calibrate Ga-filled carbon nanotubes as thermom-
eters by producing marks on their periphery. The bulk synthesis
and manipulation of 1D Ga nanostructures are envisaged to open
further prospects for studies on their unusual physical properties,
i.e., electrical and thermal, and potential applications.
Figure 7. TEM images illustrating the morphological changes of the
C sheath of a Ga-C composite nanostructure under electron-beam
irradiation: (a) 0 min, (b) 2 min, (c) 4 min, (d) 8 min.
Acknowledgment. We thank Dr. Yoichiro Uemura and Keiji
Kurashima for technical support. This research was conducted
in the frame of the “Nanoscale Materials” project tenable at
the National Institute for Materials Science, Tsukuba, Japan.
Supporting Information Available: Schematic diagram of
a vertical induction furnace used for the synthesis, SEM image
of products obtained at 1150 °C, TEM image displaying the
bending of a C sheath under a constant 2 min electron-beam
irradiation at one nanostructure side, consecutive TEM images
showing two discrete liquid Ga segments under heating, and
TEM image showing a thermometer-like structure. This material
is available free of charge via the Internet at http://pubs.acs.org.
Figure 8. HRTEM images showing the structural changes of the C
sheath of a Ga-C composite nanostructure under electron-beam
irradiation. The images correspond to TEM images in parts a-d of
Figure 7: (a) 0 min, (b) 2 min, (c) 4 min, (d) 8 min. (e) An enlarged
view of Figure 6c indicating a separation site between the two fractions.
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