C A R B O N 4 8 ( 2 0 1 0 ) 3 9 6 4 – 3 9 7 3 3971

Single-walled carbon nanotube growth from liquid galliumand indium

Rahul Rao, Kurt G. Eyink, Benji Maruyama *

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH 45433, United States

A R T I C L E I N F O

Article history:

Received 18 May 2010

Accepted 29 June 2010

Available online 3 July 2010

A B S T R A C T

We present the first demonstration of single-walled carbon nanotube growth from liquid

gallium and indium catalysts. The nanotubes were grown via thermal chemical vapor

deposition from 1 to 3 nm films of gallium and indium, which dissociate into liquid drop-

lets on silicon substrates at high temperatures. The nanotubes were characterized by

Raman spectroscopy and atomic force microscopy and are found to have diameters

between 1 and 2 nm.

Published by Elsevier Ltd.

Catalytic chemical vapor deposition (CVD) has become the

most common method for growing carbon nanotubes (CNTs)

due to its versatility and scope for scale-up in production.

Using this method, both single-walled nanotubes (SWNTs)

and multi-walled nanotubes (MWNTs) can be synthesized

by varying experimental parameters such as pressure, tem-

perature, catalyst type, size and composition, and carbon

feedstock. To date, in addition to the traditional catalysts

based on Fe, Co and Ni, SWNTs and MWNTs have been syn-

thesized with an ever increasing number of materials includ-

ing metals [1–6], semiconductors [7], oxides [8,9] and carbides

[10]. Understanding the interactions between the catalyst par-

ticle and carbon is of crucial importance for controlling the

growth and termination of SWNTs [11]. For example, precise

knowledge of the physical state (solid or liquid) of the catalyst

at high temperatures could help in designing catalysts and

supports in order to achieve controlled growth of carbon

nanotubes. Metals such as gallium and indium are attractive

in this regard due to their low melting point, which ensures

their liquid state at typical CVD growth temperatures. More-

over, the low magnetic susceptibility of gallium and indium

make them potentially useful for magnetic studies on

SWNTs, where the influence of ferromagnetic catalyst parti-

cles would otherwise complicate experimental observations.

Researchers have previously demonstrated growth of large

diameter MWNTs from gallium oxide [12] and gallium nitride

[13], as well as conical and tapered nanotube geometries from

large gallium droplets [14]. In this letter we report for the first

time growth of SWNTs from gallium and indium. The nano-

tubes were grown from thin films (1–3 nm) of gallium and in-

dium on silicon substrates via decomposition of methane or

ethylene in a thermal CVD process (for experimental details

please see Supporting information). Fig. 1a and b shows

0008-6223/$ - see front matter Published by Elsevier Ltd.doi:10.1016/j.carbon.2010.06.065

* Corresponding author.E-mail address: Benji.Maruyama@wpafb.af.mil (B. Maruyama).

SEM images of the nanotubes grown from 3 nm gallium and

indium films, respectively. Catalyst particles at the tips of

the nanotubes were observed with both gallium- and in-

dium-catalyzed CNTs, indicating a tip-growth mechanism. A

tapping mode AFM image of a CNT with an attached gallium

nanoparticle is shown in Fig. 1c. Line scans along the nano-

tube and catalyst particle indicate their heights to be approx-

imately 1 nm and 2.5 nm, respectively, thus confirming the

presence of SWNTs. The deposited gallium film thickness

for the sample shown in Fig. 1c was 1 nm. A uniform distribu-

tion of nanoparticles on the substrate can also be observed in

the AFM image. Gallium was consistently observed to grow

more nanotubes compared to indium over the entire range

of experiments performed in this study. In addition, the initial

film thickness that yielded most growth of SWNTs was 1 nm,

while both MWNTs and SWNTs were grown from the 3 nm

films (see Supporting information Fig. 1 for TEM image of a

MWNT). The optimum synthesis temperature for methane

as the carbon source ranged between 900 and 950 �C while

ethylene produced SWNTs at 850 �C and a mixture of SWNTs

and MWNTs below 850 �C. Nanotube growth was very ineffi-

cient at lower temperatures indicating lack of thermal energy

for decomposition of hydrocarbon feedstock. This suggests

that a more efficient way to crack the hydrocarbon could lead

to increased yield of SWNTs.

Further verification of the structure of the nanotubes was

obtained by micro-Raman spectroscopy (Fig. 2). Spectra were

collected from various spots on the samples grown from

1 nm gallium and indium films using a laser excitation wave-

length of 633 nm. The presence of SWNTs is indicated by the

G-peak around 1592 cm�1 exhibiting the typical split G+/G�

structure [15]. The G band from the Ga-SWNTs (bottom trace

in Fig. 2) also shows some features below the G� peak at

�1567 cm�1. These features arise from other tangential

modes that typically appear when the SWNT alignment on

the substrate is polarized with respect to the laser [16]. In

addition, an almost negligible peak in the disorder-induced

D-band region at �1300 cm�1 indicates high quality of the

SWNTs. Several peaks between 125 and 250 cm�1 are ob-

served in the low frequency radial breathing mode (RBM) re-

gion (inset in Fig. 2). Using the relation between SWNT

diameter and RBM frequency established for isolated SWNTs

on silicon, xRBM = 248/dt [15], the SWNT diameters are found

to be between 1 and 1.9 nm, consistent with the AFM height

measurements.

To the best of our knowledge there is no report of a carbon–

gallium phase diagram and several reports in the literature

suggest that carbon has negligible solubility in gallium and in-

dium [13,14]. In addition, these metals are not known for their

ability to catalytically crack hydrocarbons. However, the re-

sults in this study indicate that gallium and indium nanopar-

ticles of the right size can ‘‘seed’’ SWNT or MWNT growth.

Gallium and indium exhibit poor wetting with silicon [17],

which results in the dissociation of the films into nanoparti-

cles during deposition or upon heating. This was observed

through SEM analysis on substrates before growth (see Figs.

2 and 3 in Supporting information). The average gallium and

indium nanoparticle size was concomitant with the deposi-

tion film thickness. Both metals also readily form oxides and

reduction to their metallic form at growth temperature was

necessary prior to SWNT growth. Nanotube growth was not

observed to take place without hydrogen pre-treatment for

�10 min at growth temperature. It has been suggested for iron

catalyst that SWNT growth is favored by the liquid phase over

solid at temperatures above the eutectic point for iron and car-

bon [18]. In the present case, the low melting points of bulk

gallium (29.7 �C) and indium (156.6 �C) combined with the fact

that the growth temperatures are several hundred degrees

above the melting temperatures imply that the catalyst must

be in the liquid state during growth. Furthermore, poor wet-

ting of the droplets with silicon ensure that they maintain a

spherical surface exposed to the hydrocarbon and are able to

template 1-D growth. SWNT nucleation then likely occurs

via surface diffusion of carbon atoms around the gallium

and indium droplets, followed by tip growth. In some initial

un-optimized experiments the gallium and indium droplets

were observed to form graphitic islands, with some regions

containing CNTs that appeared to grow vertically from highly

mobile catalysts via tip growth. The mobility of the catalysts

was further observed in some substrates that had scratches

towards the edge where most of the catalyst droplets migrated

and seeded dense CNT growth (see Supporting information,

Fig. 4). This offers a unique opportunity to utilize the high

mobility of gallium and indium combined with the right sup-

port to obtain directional and site-specific growth of SWNTs

and MWNTs for various applications.

In summary, it was successfully demonstrated that 1–3 nm

films of gallium and indium are capable of growing high qual-

ity SWNTs. The present study also illustrates the feasibility of

nanotube growth from liquid metal catalysts that exhibit very

low solubility for carbon. Current efforts are underway to

study the interaction of gallium, indium and other low melt-

ing metals with various substrates to achieve controlled

growth of dense and directional SWNTs.

Acknowledgments

The authors gratefully acknowledge the National Research

Council postdoctoral fellowship and funding from AFOSR.

Fig. 2 – Micro-Raman spectra collected from SWNTs grown

with (top) indium, and (bottom) gallium showing the D and

G bands. The inset shows the corresponding radial

breathing modes from the SWNTs. The peak at �300 cm�1

(asterisk) in the inset is from the silicon substrate.

Fig. 1 – SEM images of CNTs grown from (a) gallium, and (b)

indium nanoparticles formed from 3 nm films. Scale bars

are 500 nm, (c) tapping mode AFM image (2.6 · 2.6 lm2) of a

SWNT grown from a gallium nanoparticle formed from a

1 nm film with (d) height measurements (indicated by

arrows on the AFM image) on the SWNT and catalyst

particle. The height scale is 10 nm.

3972 C A R B O N 4 8 ( 2 0 1 0 ) 3 9 6 4 – 3 9 7 3

Appendix A. Supplementary material

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2010.06.065.

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