Carbon nanocrosses: Synthesis, characterization and growth mechanism of a novel carbon nano-structure

Article abstract of DOI:10.1016/j.cplett.2012.03.105

We report the growth and structural information of a novel carbon-based nanostructure in the form of crosses with hollow interiors. The center of the cross has a diameter about 160-180 nm, and the four branches of the cross junction consisting of single-walled carbon nanotubes are uniformly tapered to form the pointed tips. These conical branches of the nanocrosses are composed of coaxial cylindrical graphene sheets, a continuous shortening of the graphene layers in the outer surface along the axial direction. A liquid droplet dispersion theory is proposed to explain the growth of this new carbon nanostructure.

Full text of DOI:10.1016/j.cplett.2012.03.105

Chemical Physics Letters 536 (2012) 113–117
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Carbon nanocrosses: Synthesis, characterization and growth mechanism of a
novel carbon nano-structure
⇑
Gui-Ping Dai, Shuguang Deng
Department of Chemical Engineering, New Mexico State University, Las Cruces, NM 88003, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the growth and structural information of a novel carbon-based nanostructure in the form of
crosses with hollow interiors. The center of the cross has a diameter about 160–180 nm, and the four
branches of the cross junction consisting of single-walled carbon nanotubes are uniformly tapered to
form the pointed tips. These conical branches of the nanocrosses are composed of coaxial cylindrical
graphene sheets, a continuous shortening of the graphene layers in the outer surface along the axial
direction. A liquid droplet dispersion theory is proposed to explain the growth of this new carbon
nanostructure.
Received 28 February 2012
In final form 28 March 2012
Available online 4 April 2012
Ó 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Nanoscience promises a future of novel devices and technologies
synthesize of carbon nanostructured materials for gas inlet 1 (Argon
and hydrogen), gas inlet 2 (Thiophene vapor with carrier gases), and
gas inlet 3 (Mixture solution vapor of benzene and ferrocene with
carrier gases), respectively. The reaction temperature was main-
tained at 1050–1150 °C and the growth of carbon nanostructured
materials was typically carried out with a time of 20 min. The carbon
nanostructured materials were synthesized from the CVD furnace
and collected on a silicon substrate, which were placed at the end
of the reactor tube. All chemicals (benzene, ferrocene, and thio-
phene) were purchased from Alfa Aesar. The gases (Argon and
hydrogen) were used in the purity of 99.99%.
that harness the atomic or molecular structure of matter. An intrigu-
ing manifestation of this paradigm is in the exploiting novel proper-
ties of carbon at the nanoscale with many interesting amorphous
and crystalline phases, which each shows a variety of one-, two-,
and three-dimensional morphologies. Carbon nanostructures with
morphologies of branch [1–2], star [3], helix [4], pipette [5], cone
[
6], bead [7], bud [8], and pearl [9] have been widely investigated.
Here, we first report another novel nanostructure of carbon
named as ‘nanocross’, characterized by hollow quad-conical struc-
tures, whose hollowness crosses throughout the length of the
structure.
3. Results and discussion
At the end of the deposition experiment, most regions of the sil-
icon substrate are coated with a web-like, silver-black, very light
and very thin, half-transparent, but free-standing film composed
of SWNTs. In this film, several novel nanostructures that are about
500–900 nm in length and 160–180 nm in diameter of the crossed
sections emerge. These nanocrosses have a pointed tip (in the
shape of a SWNT or twin SWNTs), while the length of SWNTs pro-
truding from the tips is in the range from tens of nanometers to
micrometers. We also observed a Y-shaped carbon nanostructure
(in the shape of spindle) named ‘carbon nanospindle’ (CNS) in
the film.
Transmission electron microscopy (TEM) was used for detailed
structural investigation. These nanocrosses were analyzed in a Jeol
model 2100-F TEM. As shown in Figure 2, the carbon nanostruc-
tures have typical cross morphology. The carbon nanocross (CNC)
is composed of quad-conical carbon structures jointed together.
The crossing point with hollow interiors is identified where two
SWNTs or twin SWNTs are lying perpendicular to each other. No
2
. Experimental
The experiments were carried out using a chemical vapor depo-
sition method (CVD) system with Ar and H
gases (Ar:H = 3:5), benzene as the carbon feedstock, ferrocene as
the catalyst precursor, and thiophene as the growth promoter. The
schematic of this system is shown in Figure 1. The ferrocene was dis-
2
as the reaction carrier
2
À2
solved in the benzene with a concentration of about 1.6 Â 10
g
Fe(C /ml C
5 5
H )
2
6
H
6
. Thiophene was carried into the reactor by the
carrier gases to enhance the growth of carbon nanostructured mate-
rials, the amount of which was precisely controlled by the temper-
ature of thiophene solution (around 27 °C) and the flow rate of Ar
and H
2
. The thiophene vapor was decomposed into sulfur at a high
temperature. Typical flow rates of 60, 1, and 50 sccm were used to
⇑
Corresponding author. Fax: +1 575 646 7706.
E-mail address: sdeng@nmsu.edu (S. Deng).
0
009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cplett.2012.03.105

1
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G.-P. Dai, S. Deng / Chemical Physics Letters 536 (2012) 113–117
metal crystal nodes or catalyst materials were observed in the TEM
of the CNCs, demonstrating that the crosses are composed of pure
carbon. The nature of the sharp tips consisting of the SWNT is
clearly demonstrated in high-resolution TEM images (Figure 3).
High-resolution micrographs reveal that CNCs are crystals of
graphite and all of quad-conical branches in a CNC are made of
coaxial tubular graphene sheets (Figure 3A). The conical structure
is caused by a gradual shortening of the length of the graphene
sheets in the outer surface along the axial direction at quad-ends,
and thus resulting in a high density of edge-plane sites on their
surfaces. The diameter of the hollow SWNT inside the conical
branch is around 4.1 nm (Figure 3A), while the diameter of hollow-
ness consisting of twin SWNTs is around 3 nm (Figure 3B), and the
nature of twin SWNTs in the center part of the branch is also
clearly exhibited in Figure 3C. This suggests that the diameters
are different between these crossed SWNTs as the bases forming
the CNCs. To further assess the crystalline feature of the CNCs,
we performed a Resonant Raman Spectroscopy (Renishaw Invia
Raman microscope) study of the CNCs by using a 632.8 nm
Figure 1. Schematic figure of the CVD system.
(
1.96 eV) excitation. Raman spectrum (Figure 3D) exhibit resonant
À1
radial breathing mode (RBM) peaks in the range of 120–350 cm
(
À1
characteristic of SWNTs), a disorder band (D-band) at 1322 cm
,
À1
and a sharp tangential graphite band (G-band) at 1589 cm which
reveals a high degree of graphitization of the CNCs. The D-band is
related to the finite crystalline size or defects in crystallites; in this
case, its contribution probably comes from the graphene edges in
the CNCs (Figure 3A–C). Conical carbon morphologies essentially
consisting of central carbon nanotube surrounded by helical graph-
ene sheets, known as either carbon nanopipettes [5] or tubular
graphite cones [6] have been experimentally observed, and multi-
branched junctions of carbon nanotubes via metal particles [10]
and X-shaped carbon nanotube junctions [11–13] have also been
reported. However, a large number of ordered carbon layers with
tapered structures at the quad-end grew on the core crossed
SWNTs and surprisingly forming the structures of carbon nano-
crosses never reported previously.
To better understand the nature of the base structure formed by
the crossing SWNTs, the schematic diagrams for a cross by two
individual SWNTs and a cross by a single SWNT and twin SWNTs
are illustrated in Figures 4 and 5, respectively. The Red SWNT does
not directly cross through the center rolled graphene layer of the
blue SWNT by atom reconstruction. They spatially cross each other
and contact by weak Van der Waals forces. The crossing features
can be more clearly seen in Figures 4 and 5 from A to C respectively
by different angles of view.
The growth mechanism of various carbon structures at different
morphologies is an interesting subject [5,11–12,14]. The formation
of the conical carbon nanopipettes was proposed by selectively
etching and simultaneous growth with a different aspect ratio
along the length of the platinum substrate. The explanation for
forming tubular graphite cones was given on the basis of a catalytic
mediated growth with single crystalline nanotips in which the car-
bon spherules enhance the cone nucleation in the presence of high-
temperature plasma annealing. In these cases, the X-shaped carbon
nanotube junction, created by welding of the CNTs, is a product of
post-treatment. Electron beam irradiation and ion beam irradiation
were responsible for the creation of the X-shaped CNT junctions at
a high temperature. None of the above crystals have a structure
similar to the one described in this work. Hence, none of the above
mechanisms could explain the formation of such a specific CNC
structure. We have observed that the CNC is a material of pure car-
bon, and the CNC is a crystal of graphite. From this microscopic evi-
dence, we construct a probable nanocross formation scenario.
The temperature in the isothermal zone of the CVD reactor can
reach as high as 1150 °C. The high temperature and the peculiar
cross shape with quad-conical ends make us assume that the
Figure 2. TEM images of carbon nanocrosses with hollow interiors. (A) The
horizontal bi-conical structure has a length of about 727 nm while the vertical bi-
conical structure has a length of about 863 nm, and the distances of two opposite
junction points in the stem are about 163.6 and 181.8 nm, respectively. (B) The two
bi-conical structures are in the range of about 560 and 788 nm, respectively, while
the distances for two opposite junction points in the stem are about 160 and about
1
76 nm, respectively.

G.-P. Dai, S. Deng / Chemical Physics Letters 536 (2012) 113–117
115
1589
D
189
290
1322
0
200 400 600 800 1000 1200 1400 1600 1800 2000
-
1
Raman Shift (cm )
Figure 3. TEM analysis of the tips of carbon nanocrosses. (A) HRTEM image of a tip clearly showing a SWNT protruding from the tip and a gradual shortening of the length of
the graphene sheets in the outer surface along the axial direction, which is an enlargement of the blue boxed area in Figure 2B. (B and C) TEM images showing an enlargement
of the red boxed area in Figure 2B. (D) Raman spectrum of the CNCs by using an excitation of 632.8 nm (Elaser = 1.96 eV). (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Figure 4. A model of a nanocross, which a red SWNT is crossed by another blue
SWNT. The structure can be more clearly seen from different angles of view (A–C).
Figure 5. A model of a nanocross, which a blue SWNT is crossed by other two red
SWNTs. The structure can be more clearly seen from different angles of view (A–C).
(For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
(
For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)

1
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G.-P. Dai, S. Deng / Chemical Physics Letters 536 (2012) 113–117
Figure 6. Schematic diagram of the growth model of carbon nanocrosses: (A) Crossed SWNTs grown by VLS mechanism. (B) High condensed liquid carbon droplet depositing
to the crossed SWNTs. (C and D) The formation of carbon nanocrosses by the liquid droplet spreading out on the crossed SWNTs as a result of Rayleigh instability.
Figure 7. TEM images of a Y-shaped carbon nanospindle. (A) TEM image of the carbon nanospindle with clearly Y-shaped hollow interiors. (B–D) TEM images of three
branches, which are an enlargement of the boxed area in (A) with yellow, red, and blue, respectively. (For interpretation of the references to color in this figure legend, the
reader is referred to the web version of this article.)
growth mode can follow two steps nucleation processes during the
carbon deposition process. Initially, the SWNTs grow from hydro-
carbon gas in the hot-zone of the furnace. The diameter of SWNTs
critically depends on the size of localized liquefied geometrical
shape of the catalyst particle (iron). The formation of SWNTs fol-
lows the well-known vapor–liquid–solid (VLS) growth mechanism
hydrocarbon vapor decomposes to form a highly condensed liquid
carbon droplet. The droplet shape is not known while its size
should be related to that of the CNC, although it also is not known
that how many droplets are responsible for the formation of a CNC
by deposition onto the stochastically crossing SWNTs already
grown. The droplet’s deposition onto the crossing SWNTs
(Figure 6A–C) is affected not only by the probability in which the
[
15] in this step. At high temperature, mixture solution

G.-P. Dai, S. Deng / Chemical Physics Letters 536 (2012) 113–117
117
liquid carbon droplets meet the crossing SWNTs, and also by the
physical appearance related to the wetting of the nanotube surface
by a liquid. The liquid droplet spread out by making menisci as a
result of Rayleigh’s instability [16] when it deposits on the crossing
SWNTs (Figure 6D). This meniscus can be clearly seen at the tip
interfaces (Figure 3A–B), which confirm the second process. Final-
ly, because the temperature of the liquid carbon droplet is the
same as that of the SWNTs in the isothermal zone of the reactor,
as a result, the liquid carbon transforms to a higher density
graphitic structure, thus forming the crystal CNCs. The second-
growth process is schematically depicted in Figure 6.
If the liquid droplet model is true, then we should see other
novel carbon nanostructures due to a lot of liquid carbon droplets
deposition to the SWNTs already formed, in fact, a novel carbon
nanostructure that we call ‘carbon nanospindle chains’ have been
synthesized [17]. Here, an excellent confirmation of two nucleation
processes contributing to the growth of the CNCs is shown in
Figure 7. The Y-shaped carbon nanostructure that we call ‘Y-
shaped carbon nanospindle’ (CNS) has three different tips
DWNTs, and only SWNTs bundles with a controllable SWNT
number respectively, and also it is possible to control the geometry
of multi-branched junctions of CNTs as we want.
The CNCs represent a new morphological form for carbon that
the quad-branch with a high density of edge-plane sites on their
surface are tapered to pointed tips consisting of the SWNT or twin
SWNTs. Electrical switches and logic devices have been reported in
Y-junction nanotubes without the need for an external gate [19],
the CNCs will be a more ideal candidate for application in car-
bon-based integrated logic circuits due to more functionalities of
the quad-branch. The CNCs will also be very promising geometries
for biological applications as nanoscale multifunctional carbon
probes or pipettes [5,20] due to the conical feature with indepen-
dent nanochannels.
4. Conclusions
In summary, we wish to emphasize that this is the first time
that such a novel nanoscale carbon hybrid material, nanocrosses,
consisting of conical quad-branch, has been experimentally dem-
onstrated. Due to the unique morphology, the carbon nanocrosses
can be a fertile ground for future research in making an overall
carbon-based nanoelectronic architecture.
(Figure 7A), each tip is composed of SWNT in different quantities.
Figure 7B shows that the nanotube protruding from the upper
branch of the Y-shaped CNS is a SWNT with a diameter of about
2
.5 nm. The nanotubes consisting of at least three SWNTs protrud-
ing from the left down tip of the Y-shaped CNS are clearly demon-
strated in Figure 7C, the SWNTs bundle have a nominal diameter of
about 4 nm, and it is obvious that the structure nature of the Y-
shaped CNS is a crystal of graphite. Although the third branch is
unclear due to the movement under the high-energy electron
beam in HTREM, we can clearly see at least two SWNTs protruding
from the tip (Figure 7D). Looking carefully at the center part of the
CNS, we observe that a SWNT directly passes through the two
branches of the Y-shaped CNS, other SWNTs do not show any signs
of interrupt or damage but show a sign of bend. The fact indicates
that the number of SWNT in the left down tip is the sum of SWNT
number from the other two branches. Thus, the formation of the Y-
shaped CNS is caused by the liquid carbon droplet deposition onto
the center part of a Y shape nanotube bundle already formed that is
composed of at least three SWNTs, where a SWNT directly lies in
the substrate, and other SWNTs become bent in the substrate,
and thus resulting in a Y shape. Although Y-junction CNTs have
been reported [2,18–19] before, they were composed of MWNTs
without any conical structures, and therefore, their micro-
structure is completely different from those of CNS. To describe
accurately the growth of these novel carbon nanostructures will
require much more refined considerations, for examples, the liquid
droplet native features including size, number, and chemical com-
position (attributing to the droplet viscosity and surface tension)
and experimental parameters including gas phase composition,
temperature, and flow hydrodynamics, which both play important
roles in the formation of the CNCs. Here, we primarily give infor-
mation on the steps in which the CNCs hypothetically follow to
grow. If this can be true in practice, it is possible that the CNCs
can be tailored with the crossed nanotubes of only SWNTs, only
Acknowledgments
The authors are thankful to Jean-Philippe Masse of Centre de
caractérisation microscopique des matériaux, École Polytechnique
de Montréal for his assistances with HRTEM measurements. This
work was partially supported by US Department of Energy
(DE-FC36-08GO88008).
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