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F E AT U R E A R T I C L E
Department of Electrical and Computer Engineering, National University of Singapore,
4 Engineering Drive 3, Singapore 117576. E-mail: [email protected]
b
Data Storage Institute, 5 Engineering Drive 1, Singapore 117608
c
Department of Physics, National University of Singapore, 10 Kent Ridge Crescent,
Singapore 119260
Journal of
a
Materials
Chemistry
Yihong Wu,*a,b Bingjun Yang,b,c Baoyu Zong,b Han Sun,c Zexiang Shenc and
Yuanping Fengc
www.rsc.org/materials
Carbon nanowalls and related materials
Received 23rd September 2003, Accepted 15th December 2003
First published as an Advance Article on the web 22nd January 2004
Size, dimensionality, and shape play important roles in
determining the properties of nanomaterials. So far,
most of the nanomaterial researches have been focused
on zero-dimensional nanoparticles/nanodots and onedimensional nanowires/nanorods/nanotubes, but very few
studies have been carried out on two-dimensional nanosheets. Starting from carbon, recently we have succeeded
in growing a class of nanostructured two-dimensional
materials either in the pure forms or in the form of
composites with carbon. In this paper, we will first
briefly discuss various types of two-dimensional systems
and then focus on the formation mechanism of carbon
nanowalls and their field-emission and electron transport
properties. The use of carbon nanowalls as templates for
the formation of other types of nanomaterials will also
be discussed.
1 Introduction
DOI: 10.1039/b311682d
The property of a bulk material is largely determined by the
types of the constituent chemical elements and the nature of the
chemical bonds that ‘hold’ the atoms and molecules together
to form the material. However, this ‘conventional wisdom’ no
longer holds in the nanometer regime in which, in addition
to the chemical bonds, the size, dimensionality, and shape
also play important roles in determining the properties of the
Yihong Wu received his PhD from Kyoto University, Japan, in
1991 for his work on low-dimensional wide-bandgap semiconductors and their applications in blue-green lasers. From 1991 to
1996, he had been a research scientist with the Center for
Optoelectronics, National University of Singapore, a Senior
Research Engineer and deputy manager at the Panasonic
Singapore Laboratories, and a faculty member at Tohoku
University, Sendai, Japan. He re-joined the National University of Singapore in 1996 as
a lecturer and was promoted
to a senior lecturer in 1998
and an associate professor in
1999. He initiated a Nano
Spintronics group at the Data
Storage Institute in 1998 and
since then he has also been
serving as the group manager.
His current research interests
include spintronic sensors, magnetic random access memory,
Yihong Wu
and nanomaterials and devices.
materials, especially the electronic, magnetic, and optical
properties. The size matters when it becomes comparable to
the length scale of a certain physical phenomenon that is
concerned. Depending on their relative sizes in different spatial
directions, materials can be divided into categories of different
dimensionality ranging from three-dimensional (3D) to twodimensional (2D), one-dimensional (1D) and zero-dimensional
(0D). The above classification is introduced for materials with
simple shapes such as slabs/sheets (2D), wires/tubes/rods (1D),
and cubes/spheres (0D). With the rapid advance in nanotechnology, it is now also possible to produce materials with
a variety of shapes and asymmetry; this will add more degrees
of freedom to the nanomaterials that can be used to alter their
properties.
Although the recent work on nanomaterials has been
focused on the 0D and 1D systems,1–8 it is in the 2D system
where the top-down approach of nanotechnology has been
developed.9–16 This has led to the discovery of the quantum
Hall effect17,18 and the creation of new devices such as high
electron mobility transistors,19 intersubband infrared detectors
and quantum cascade lasers in semiconductor systems,20 and
the discovery of giant magnetoresistance (GMR)21,22 and
invention of spin-valves in metallic systems.23 The work on the
2D systems also became the foundation for subsequent work
on 1D and 0D systems.24,25
The above-mentioned 2D systems are obtained by the
so-called top-down appraoch. Most of these 2D systems were
realized in laminar structures of semiconductors, insulators,
and metals. In addition to these artificially structured 2D
systems, there are also many naturally formed 2D systems such
as graphite, MgB2, transition metal dichalcogenides, intercalation compounds of graphite, high-T c superconductors, and
many others.26–29 The common feature of these materials is
that the electrical conduction is highly anisotropic, with a low
reistance in the layer plane but a high resistance or being
insulating perpendicular to the layers. It is worth noting that
most of the layered compounds are also good superconductors.
Regardless of whether it is an artificial laminar structure or a
naturally formed layered coumpound, the 2D system formed in
such a fashion is not a perfect 2D system in the sense that each
2D layer still has to interact with the adjacent layers either
chemically or electronically. An ideal 2D system would be such
that it consists of only a single nano-sheet without any
electronic or chemical interactions with other types of materials
or layers. Fig. 1 illustrates the different types of 2D systems.
The 2D nano-sheets would be very useful not only for
fundamental physics studies but also for practical applications
due to their large surface-to-volume ratio. The nanobelts or
nanoribbons might be considered as one type of such material
This journal is ß The Royal Society of Chemistry 2004
J. Mater. Chem., 2004, 14, 469–477
469
form the tubular structure. This makes the nanotubes very
attractive for future applications in electronics,40–42 optoelectronics,43 and sensors.44,45 The large strength of the nanotubes
also makes them an ideal building block for nanomechanical
devices.46 In contrast to the closed boundary structure of the
fullerenes and nanotubes, the 2D graphite sheets are characterized by an open boundary. Theoretical studies have shown
that this may bring about nanocarbons’ unique transport and
magnetic properties.47–51 Of particular interest is the theoretical
predication of ferromagnetism and superconducting instability
in 2D graphite sheets and possible existence of unique transport
properties. In addition, the 2D nano-sheets also have a surface
area which is theoretically twice that of the closed boundary
structures, making them more attractive for chemical and
biosensor applications. The sharp edges are also promising for
field-emission applications.
2.2 Carbon nanowalls
Fig. 1 Schematic illustration of materials of different dimensionality
ranging from 0D to 3D and the different types of 2D systems.
system, depending upon the width-to-thickness ratio and the
physcal length scale.30,31 However, considering the structual
characteristics of layered compounds, the ideal 2D system would
be the one that is formed by ‘peeling’ off the material from a
layered compound in a layer-by-layer fashion. Such kinds of
materials might have already existed in nature from ancient
times, but most of them are yet to be discovered and explored
with well-defined objectives. In fact, most of the nanometersized 2D sheets will be more stable in tubular shapes than in flat
sheets. However, the 2D nano-sheets may be stable when they
form self-supported network structures. Recently we have
succeeded in growing such kinds of materials in the carbon
system while we were working on the carbon nanotubes.32
After the sucessful growth of 2D carbons, we have extended the
technique to the growth of other types of 2D materials, either in
the pure form or in the form of composites with carbon.33 As
expected, most of them are in network structures. In the
following sections, we will discuss the growth mechanism, fieldemission and electron transport properties of the 2D nanocarbons. The use of the carbon nanowalls as templates to grow
other types of 2D materials will also be discussed.
In spite of the unique properties of 2D nanocarbons predicted
by theory, they are yet to be verified experimentally. This is
mainly due to the unavailability of such kinds of samples. One
of the possible reasons is that nanometer-sized 2D carbons are
not stable and tend to form tubular or cage structures. Recently
we have succeeded in the growth of well-aligned 2D carbons
(dubbed carbon nanowalls) on various substrates using microwave plasma enhanced chemical vapor deposition (MPECVD).
The 2D carbons form a self-supported network structure which
enhances their stability. As has been discussed elsewhere, the
growth of carbon nanowalls was found during the growth of
carbon nanotubes.32 Fig. 2(a) shows a typical SEM image of
the carbon nanowalls. The distribution of the nanowalls is
remarkably uniform over the whole substrate surface area that
is typically 1 cm 6 1 cm. Fig. 2(b) shows some of the nanowalls
peeled off from the substrate and laid down on top of the
nanowall samples. The nanowalls grow very fast within the first
1–2 minutes and nearly stop growing after they reach a height
of about 2 mm. The width is in the range of 0.1–2 mm; it
increases with decreasing the nanowall density. The thickness
of the nanowalls is typically in the range of one to several
nanometers, as shown by the HRTEM images in Fig. 2(c) and
(d). Note that the HRTEM image at the center portion of
Fig. 2(c) was taken from a pile of carbon nano-sheets. The
thickness of the nanowall can be estimated from the HRTEM
image of a single piece of nano-sheet, as indicated by the arrow
2 Growth of carbon nanowalls
2.1 2D nano-sheets versus 1D nanotubes
With the discovery of fullerenes and carbon nanotubes,34,35 a
great deal of effort has been devoted to the development of
similar types of nanostructures made up of other materials such
as WS2, MoS2, NbS2, BN, BC2N, BC3, NiCl2, and CN, etc.36–39
The common feature of these materials is that they have a layered
structure in the bulk form. When the size of these materials in one
direction decreases to a few or tens of monolayers, the material
becomes a thin sheet which is usually unstable and tends to
curve and roll up in either one or more directions; this will lead
to the formation of various types of nanostructures such as
fullerenes, cages, cones, and tubes. The unique shape and
symmetry of these nanostructures give rise to unique mechanical, electronic, magnetic, and optoelectronic properties which
their bulk counterparts lack. Take the carbon nanotube as an
example: it can be either a metal or a semiconductor, depending
on the direction along which the graphene sheet is rolled up to
470
J. Mater. Chem., 2004, 14, 469–477
Fig. 2 SEM [(a) and (b)] and HRTEM [(c) and (d)] images of carbon
nanowalls. Scale bars: (a) 100 nm, (b) 1 mm, (c) and (d) 5 nm. (a) was
taken at a tilt angle of 25u.
in the top right-hand corner of Fig. 2(c). Both the SEM and
HRTEM observations show that there are two different types
of nanowalls, one an open boundary nanographite sheet
[Fig. 2(c)], and the other one more like a flattened tube with an
empty interior [Fig. 2(d)]; the former predominates.
2.3 Effect of gas flow rate
The hydrogen-to-methane flow rate ratio was found to cause
rather drastic changes to the morphology of the nanocarbon
films. Fig. 3 shows the morphology of the carbon films grown
on Au (ca. 20 nm) coated Si substrates at different H2/CH4 flow
rate ratios. The growth pressure was 1 Torr. MPECVD is a
well-known technique for growing diamond films at a H2/CH4
flow rate ratio of 100. As the growth temperature in this work is
about 650–700 uC, the high H2/CH4 flow rate ratio hardly led
to any observable growth of carbon within a short period of
5 min. As the flow rate ratio is decreased to 30, some columnar
structure of amorphous carbon forms. A further decrease of
the gas flow rate ratio leads to the formation of a mixture
of fibers/tubes and 2D nanographite sheets. A pure form of
carbon nanowalls forms when the gas flow rate ratio is in the
range of 4–8. The amorphous carbon forms again when the
ratio is too low. Based on these, in all the experiments to be
discussed below, the H2/CH4 flow rate ratio has been fixed at 4.
2.4 Effect of electrical field
In the early stages of our work on carbon nanowalls, we found
that the key to growing the nanowalls instead of nanotubes was
the emergence of a strong lateral field caused by the nonuniform charging of the catalyst islands.32 However, in subsequent experiments, it was found that the nanowalls could
also grow on substrates without any catalysts. This seemed to
cast some doubt on our initial findings and prompted us to
perform more experiments under a controlled environment for
investigation of the electric field influence. In order to find
unambiguous evidence on the effect of the electrical field, we
have performed the following: (i) to use surface plasmons (SPs)
Fig. 3 SEM images of carbon grown at different H2/CH4 flow rate
ratios: (a) 30, (b) 15, (c) 10, (d) 6, (e) 4, (f) 1. Scale bars: (a), (b), (d), and
(f) 1 mm; (c) and (e) 100 nm.
to excite localized electrical field, (ii) to use large metal droplets
as the catalysts, and (iii) to create sharp features on the
substrate surface. In what follows we first discuss the effect of
the electrical field generated by the surface plasmon, followed
by other techniques.
The details on how to create SPs on the substrate surface
during the growth of carbon nanowalls have been reported
elsewhere.52 The surface plasmon was known to cause large
enhancement of local electrical fields53 and thus it was expected
that the presence of a localized surface plasmon would affect
the growth of carbon nanowalls locally. As expected, the
influence of the surface plasmon on the growth of carbon
nanowalls was a rather drastic one. The influence of the surface
plasmon on the macro-scale has been discussed in detail in
ref. 52; here we only focus on the microscopic effects of the
plasmon-induced electrical field on the orientation of the
carbon nanowalls. Fig. 4 summarizes the unique carbon nanowall patterns that have been observed due to the presence of
surface plasmons. The circular region consists of an outer ring
with denser nanowalls and a flower-like nanowall structure at
the center. The latter consists of one to several poles and the
number of poles increases with the density of the nanowalls
surrounding the circular region, so is the size of the whole
region (see Fig. 4a–h). However, the occupation ratio of the
outer ring in the whole circular region decreases when the
number of poles increases. Fig. 4i shows an enlarged view of
the boundary between the circular region and the region
surrounding it. For clarity, the sample was tilted by 20u when
taking this picture. It shows clearly that the nanowalls orient
randomly outside the circular region, while they align well
along the circumference direction in the rim region and change
the direction by almost 90u when they move further to the
central region. It demonstrates clearly how the carbon nanowalls change their orientation in an extremely localized region.
This could hardly be possible without the existence of strongly
localized electric fields induced by the surface plasmon.
The patterns shown in panels (a)–(h) resemble well the
electric field distribution of multiple pole SP predicted by Mie’s
theory with the number of poles increasing from (a) to (h).54 It
is interesting to note that patterns with both odd and even
numbers of poles were observed, though Mie’s theory for a
metallic sphere only predicts patterns with an even number of
poles. This was probably caused by the uneven shape of the Au
particles, which has been confirmed by the SEM observation of
Au particles formed on bare substrates. The size of the pattern
increases with the number of poles: it is ca. 8 mm for the dipole
Fig. 4 SEM images of carbon nanowall patterns formed by the
electrical field of surface plasmons with different numbers of poles. (i) is
the enlarged image of portion (A) in (h). Scale bars: 1 mm.
J. Mater. Chem., 2004, 14, 469–477
471
pattern shown in panel (a) and 20 mm for the multiple pole
patterns shown in (g) and (h). Assuming that the surface
plasmon emission travels at the same speed of light in vacuum,
it gives lifetimes of ca. 25 and 70 fs, respectively. These values
agree well with the reported lifetimes for surface plasmons
reported in the literature.
Although the theory for calculating the electrical field
distribution of SPs has existed for about a century since the
pioneering work of Mie, it has been difficult to observe the field
distribution experimentally due to the strong localization and
short lifetime of such fields.55–57 In addition to gaining an
insight into the growth mechanism of the carbon nanowalls,
this work has also successfully ‘fingerprinted’ the electric field
of the SP at nanometer-scale accuracy. This is a remarkable
result because it is the only technique reported so far which can
detect the electrical field instead of the intensity of the SPs.
The effect of the electrical field can also be seen clearly in the
region where the two surface plasmons interfere constructively.
The interaction occurs through the coherent addition of the
electric fields of the two SPs. As a result, the electric field
distribution of both SPs inside the interaction region was
mutually perturbed, while that of the larger SP outside the
interaction region remains almost intact. Fig. 5 shows one such
example in which a smaller plasmon (B) is contained within a
larger plasmon (A). In the case of an isolated SP, the nanowalls
just next to the rim region are oriented in the radial direction of
the circular pattern. This is still true along line BE in Fig. 5
where the radial directions of the two circular patterns coincide. On the other hand, at the cross point of AD and BC, and
AF and BG, the nanowalls are oriented in the direction which
almost bisects the two radial directions. This implies that the
nanowalls are orientated in the total electrical field direction
which is the vector sum of the electrical fields from the two SPs.
In the above experiment, we used Au nanoparticles to excite
surface plasmons which in turn generate localized fields to alter
the growth of carbon nanowalls. Another natural way to create
localized field distributions is to put large metallic particles on
the substrate surface which would generate unique field distributions surrounding the particles. We have chosen gallium
for the metallic particles because of its low meting point. Fig. 6
shows the SEM images of several types of nanowall pattern
formed surrounding the gallium droplets. As it is shown in
panel (a), the nanowalls started to grow on the surface of a
round gallium droplet in almost the radial direction. However,
once the surface stress becomes too high due to the temperature
rise or weakening of the surface tension due to the incorporation of carbon into the surface layer, the large gallium droplets
break from the top to form disk-like shapes like the ones shown
in panels (b) and (e). In this case, the orientation of the nanowalls changes from the radial direction in the proximity of the
droplet surface to the circumference direction next to it and
then finally back to the radial direction. It seems that the dynamic field distribution is not only determined by the topography
of the gallium droplet but also by the nanowalls grown on
its surfaces. The final orientation of the nanowalls will be
Fig. 5 SEM image of the carbon nanowalls grown in the area where
two surface plasmons interfere constructively. Scale bar: 1 mm.
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J. Mater. Chem., 2004, 14, 469–477
Fig. 6 SEM images of carbon nanowalls formed surrounding the Ga
droplets. Scale bars: (a)–(c) and (e) 1 mm; (d) 100 nm.
determined self-consistently. For small droplets, however, the
nanowalls have a spiral structure along the droplet surface
[(c) and (d)]. These results suggest again that the electrical field
plays an important role in determining the orientation of the
carbon nanowalls.
In addition to the use of SPs and large metal droplets, we
have also tried to modify the morphology of the substrate
surface so as to alter the electrical field distributions at localized
areas. One of the approaches that we have tried was to use
anisotropic etching to create pyramid-like structures on the
Si substrate [Fig. 7(a)]. Owing to the sharp features of the
pyramids, the electrical field was expected to be enhanced at
the locations of the pyramids which would thus affect the
growth of the carbon nanowalls. As shown in Fig. 7(b)–(d), the
size of the nanowalls grown on top of the pyramids is
Fig. 7 SEM images of (a) pyramid structures formed on a Si
substrate using anisotropic etching, and (b)–(d) carbon nanowalls
grown on the Si substrate with the pyramid structures. Scale bars:
(a) and (b) 1 mm; (c) and (d) 100 nm.
apparently larger than that of the nanowalls formed in other
places. The directionality of the nanowalls grown on top of the
pyramids was also improved as compared with those grown in
the flat surface regions.
2.5 Possible growth mechanism
Although further studies are required to understand the growth
mechanism of the carbon nanowalls, the above results suggest
that the electrical field is the most important factor, at least
in the particular experimental set up used in this study. The
electrical field comes from both the DC bias and the nonuniform charging up of the substrate; the latter can be altered
by the existence of metallic particles and other types of sharp
features. When the metallic particles act as both a catalyst and
a field modulator, both nanotubes and nanowalls can be
formed depending on the strength of the lateral electrical field
and other growth parameters such as the gas flow rate, temperature, and pressure. However, when non-catalytic particles
or bare substrates are used, it is more likely that carbon
nanowalls will be formed. To shed some light on how the
nanowall was formed, we show in Fig. 8 the SEM images of the
nanowalls at different growth stages on the Si substrate coated
with a 20-nm-thick gold layer. The images were taken from
different portions of the same substrate which was placed on
the substrate holder in such a way that the growth rate would
vary from one side to the other due to the offset from the
optimum position. During the pre-heating stage, the Au film
becomes isolated nanoparticles. As can been seen from
Fig. 8(a), at the very beginning of the growth, carbon forms
surrounding the Au particles; but instead of forming a closed
tubular structure, they expanded in the lateral directions and
finally became connected with one another to form larger walllike structures. As shown in panel (e), at the initial stage, the
walls are not single pieces of graphite but are made up of
individual half-opened tubules. With the progress of the growth,
the individual half-opened tubules expand in the lateral direction to form single pieces of nanowalls [from panel (b) to (d)].
This observation is consistent with our previous observations
on the growth of carbon nanowalls using transition metals as
the catalysts,32 which implies that the existence of a strong
lateral field is the key to growing carbon nanowalls in our case.
on 1 cm 6 1 cm Cu substrates. The field-emission measurements were carried out in a vacuum chamber at a base pressure
of 1 6 1025 Torr. A polished copper (1 cm 6 1 cm) anode was
positioned 20–200 mm away from the nanowall cathode to
collect the emitted electrons from the latter. As the base
pressure of our measurement chamber is relatively high, the
measurements were carried out at different temperatures so as
to investigate the effect of absorbates on the nanowall surfaces.
The turn-on electrical field usually increases with decreasing
the anode–cathode distance. It is typically in the range of
1–1.5 V mm21 at an emission current density of 10 mA cm22,
though the value can be much smaller for some ‘best case’
samples. Fig. 9 shows the emission current densities as a function of the applied electric field for one of such samples
obtained at temperatures of 20, 200, 300 and 400 uC, and at an
anode–cathode distance of 50 mm. The experiments were
carried at 20 uC first, and then repeated at different temperatures after the substrate was heated up using a resistive
heater and stabilized at each temperature setting point. The
current densities were 0.19 and 9.53 mA cm22 at electrical fields
of 0.32 and 0.62 V mm21, respectively. The turn-on electric field
decreased to 0.26, 0.2 and 0.16 V/mm with the temperature
increasing to 200, 300 and 400 uC, respectively. The highest
emission current density obtained was 17.6 mA cm22 at an
applied electric field of 0.32 V mm21 at 400 uC. A further
increase of the applied electric field resulted in arcing, which
prevented us from evaluating the highest possible current
density. This is in part caused by the high base pressure of our
particular measurement chamber.
Fig. 9(b) shows the corresponding Fowler–Nordheim (F–N)
plots of the field-emission curves shown in Fig. 9(a). The fieldemission characteristics can be analyzed using the F–N equation58
!
3/2
V2
{Bw
,
(1)
I! 2 exp
d
bV =d
where I is the current in Amps, V is the applied voltage in
3 Properties of carbon nanowalls
3.1 Field-emission
The unique morphology and geometric shape of the nanowalls make them promising for field-emission applications. To
this end, a series of experiments have been carried out to
investigate the field-emission characteristics of the nanowalls.
The nanowalls for the field-emission experiments were grown
Fig. 8 SEM images of carbon nanowalls grown on a Si substrate at
different stages of growth using Au as the catalyst. Scale bars: 100 nm.
Fig. 9 (a) Emission current density as a function of the electrical field
at different temperatures for carbon nanowalls, and (b) the corresponding F–N plots of the curves in (a).
J. Mater. Chem., 2004, 14, 469–477
473
units of Volts, B is a constant given by 6.8 6 107, w is the
emitter work function in eV, d is the spacing between the anode
and the cathode in units of cm, and b is the geometric
enhancement factor. As can be seen in Fig. 9(b), the plots of
ln(I/V 2) versus 1/V yielded a straight line which confirmed that
the current resulted from field-emission. This is particularly
true for the measurements performed at 300 and 400 uC.
However, the agreement is not good in the low-temperature
curves for which we need to divide the curves into two straight
line segments so as to fit the F–N equation. This suggests that
there exist two energy barriers with different heights at lower
temperatures, which could be caused by the absorbates on the
nanowall surfaces. As the absorbate-induced work function
change is proportional to the effective field at the absorbate, it
is not difficult to understand that a higher barrier height was
observed at the higher field portion of the F–N plot. Owing
to desorption of the absorbates, the higher barrier portion
disappears at higher temperatures. From these plots, we could
obtain the slope which is equal to Bw3/2d/b, from where the field
enhancement factor b could be estimated. Using the work
function w of 5 eV, we calculated that the field enhancement
factor b was 31 400, 46 200, 54 800 and 62 900 for the experiments performed at 20, 200, 300 and 400 uC, respectively. The
temperature dependence is difficult to be understood because
the enhancement factor is supposed to be only dependent upon
the geometrical shape of the emitter and distance between the
cathode and anode. One may argue that this could be due to the
decrease of the work function with the increase of temperature.
But, if we extend the straight lines of the F–N plot in Fig. 9(b)
to the left side until they intersect with the longitudinal axis, we
found that the four lines intersect at the same point. Note that
in this case, we used the lower field portions of the curves for
temperatures at 20 and 200 uC. As the intersection point is
dependent upon the work function only, it suggests that the
increase of b is not due to the decrease of w. In addition to the
temperature-dependence, the value of b at low temperature is
also one order of magnitude higher than those reported for
carbon nanotubes at similar value of cathode–anode spacing.
Further experiments are needed to find out the underlying
mechanism for the large enhancement factor. In addition to the
temperature-dependent field-emission measurements, experiments were also carried out to study the effects of N2, H2, O2
and CH4 gases on the field-emission properties of carbon
nanowalls at room temperature. The results will be published
elsewhere. Although we have in general obtained favorable
results for the carbon nanowalls, it is still too early to say that
the nanowalls are better than the nanotubes for field-emission
applications.59–63 Further comparative experiments need to be
carried out under similar conditions so as to find out the real
differences in field-emission between the two types of nanocarbon materials.64
dependence of the resistance in the temperature range from 2 to
300 K which is shown in the inset of Fig. 10. As can be seen
from the figure, the resistance decreases with decreasing the
temperature from 300 to about 106 K, and then shows an
upturn at lower temperatures. The temperature (T) dependence
of the resistance (R) can be fitted very well using the following
equation:65
{127
15:1
R~0:0018Tz2:5 exp
z10:8 exp
(2)
T
Tz34:5
in which the first term represents the metallic contribution, the
second term is due to the quasi-1D characteristic of the
network structure, and the last term accounts for the hopping/
tunnelling resistance of the junctions. The units of R and T are
in V and K, respectively. The junction resistance dominates the
total resistance in the entire temperature range, in particular, at
low temperatures. Hopping is thermally activated at high
temperature, but at low temperature tunnelling is dominant.
Also shown in the inset of Fig. 10 is the rate of change of
resistance with temperature (dR/dT). The resistance continues
to increase with decreasing temperature, reaching the first local
maximum at about 4.2 K; after a local minimum is reached at
about 3.6 K, it increases again until the temperature decreases
to 3 K below which the resistance decreases monotonically
until reaching 2 K, which is the lowest temperature of the
SQUID setup used in this experiment. When a magnetic field is
applied in the direction perpendicular to the substrate surface,
i.e. almost along the surface of the carbon nano-sheets, both
local maxima of the curve shift to lower temperatures. The
resistance, in general, also increases with the magnetic field. It is
interesting to note that the resistance is not affected by the
applied field at temperatures higher than 7 K. The result may
indicate that superconducting instability is developed below
7 K in the 2D carbon nano-sheets, though we don’t exclude
other possibilities. The detailed mechanism responsible for the
temperature-dependent behaviour of the resistance will be discussed elsewhere in combination with the magnetic measurement data.
Fig. 11(a) shows the magnetoresistance curves at different
temperatures in which the average background signals have
been removed by first fitting the curves with polynomials and
3.2 Electron transport properties
The ideal graphite is a semimetal because of its zero bandgap
and vanished density of states at the Fermi level. However, the
situation changes for nanometer-sized graphene sheets due to
the edge and surface effect. This may bring about the nanocarbons’ unique magnetic and transport properties. In spite of
the theoretical efforts, to the best of our knowledge there has
been no report on the experimental study of the transport and
magnetic properties of 2D nanographite sheets, probably due
to the unavailability of such kinds of materials. The successful
growth of the carbon nanowalls without any catalysts now
allows us to carry out transport and magnetic property studies
of 2D carbons. Owing to the space limitations, here we only
present briefly the results on electron transport.
Fig. 10 shows the temperature dependence of the resistance
in the temperature range from 2 to 10 K at applied magnetic
fields of 0 and 400 Oe. We first discuss the temperature
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J. Mater. Chem., 2004, 14, 469–477
Fig. 10 Temperature dependence of the resistance of the carbon
nanowalls at low temperature at zero-field and a field of 400 Oe. Inset is
the temperature dependence of the resistance over a wider temperature
range. Also shown is the first derivative of the resistance with respect to
the temperature.
Fig. 12 SEM images of Au formed on the carbon nanowall templates
at different nominal thicknesses: (a) and (b) 20 nm; (c) and (d) 30 nm;
(e) and (f) 100 nm. Scale bars: (a)–(d) and (f) 100 nm; (e) 1 mm.
Fig. 11 Magnetoresistance curves of the carbon nanowalls measured
at different temperatures (a), and enlarged portion of the curve at
4.31 K (b). The inset of (b) is the Fourier transform spectrum of the
entire curve at 4.31 K shown in (a).
then subtracting the fitted data from the original data. For the
sake of clarity, the curves are displaced along the vertical axis.
The resistance oscillates strongly with the external field which is
applied perpendicular to the substrate surface. The oscillation
sets in at about 7 K and its amplitude increases by more than
three orders of magnitude when the temperature is decreased
from 6 to 2 K. The oscillations are, in general, quasi-periodic;
however, the periodicity improves with temperature. To
illustrate this trend, a portion of the magnetoresistance curve
at 4.31 K is shown in Fig. 11(b) with the inset being the Fourier
transform spectrum in which three peaks corresponding to
different periodicities have been observed. Detailed studies are
being carried out to reveal the physics behind the oscillatory
phenomenon.
4 Carbon nanowalls as a template for growing other
types of materials
The novel surface morphology of the carbon nanowalls makes
it an ideal template for synthesizing mesoporous materials with
high surface areas. One of the possible applications of the
carbon nanowalls is in batteries. To this end, we have tried to
fabricate composites comprised of carbon and magnetic nanoparticles. This has been done from two different approaches. In
the first approach, the nanowalls were used as the adhesive
bases to absorb nanoparticles available commercially, whereas
in the second approach the nanowalls were used as templates to
deposit the nanoparticles using electrochemistry. The details
can be found in refs. 33 and 66. In addition to electroplating,
we have also employed evaporation to deposit Au and Cu on
the carbon nanowalls. Fig. 12 shows the typical SEM images of
Au films formed on the nanowall surfaces: (a) and (b), for a
nominal thickness of 20 nm; (c) and (d), for a nominal thickness
of 30 nm; and (e) and (f) for a nominal thickness of 100 nm. The
Au film is in a particulate form when the nominal thickness is
20 and 30 nm, while it becomes a continuous layer when the
nominal thickness reaches 100 nm. It is interesting to see from
panel (a) that the nanoparticles formed on the carbon nanowalls are smaller than those formed on the bare substrate just
next to the nanowalls. Fig. 13 shows the SEM images of Cu
formed on the nanowalls with a nominal thickness of 30 nm.
Comparing with Au, the Cu particles are even smaller on the
side walls. The other feature is that continuous Cu wires are
formed on the top edges of the nanowalls. The above results
demonstrate that, compared with the nanotubes, the nanowalls
are more suitable for functionalizing purposes which makes
them promising for chemical and biological applications. The
nanowalls were also found to be good templates for fabricating
nanocrystals of certain metals such as zinc. Fig. 14 shows the
SEM images of Zn nanocrystals formed by molecular beam
epitaxy at room temperature [(a) and (b)] and by electrodeposition (c), though the similar types of nanocrystals could
not be grown on a flat substrate under similar conditions. The
Fig. 13 SEM images of Cu deposited on the carbon nanowall
templates at a nominal thickness of 30 nm. Scale bars: 100 nm.
J. Mater. Chem., 2004, 14, 469–477
475
Fig. 14 SEM images of the Zn nanocrystals grown on the carbon
nanowall templates by: (a) and (b) molecular beam epitaxy; and
(c) electroplating. Scale bars: 100 nm.
carbon nanowalls have also been employed as templates for
depositing a variety of oxides which include ZnO, TiO2, SiOx,
and AlOx.33
5 Other kinds of 2D nanomaterials
In addition to carbon, we have also tried to form 2D oxides of
transition metals. The general technique that we have used was
to deposit metal films of appropriate thickness by electrodeposition and subsequently to anneal the sample in air for
several hours. The end product ranges from 0D nanoparticles
to 1D nanowires and 2D nano-sheets, depending on the
starting materials and the anneal temperature. The 2D nanosheets were found to form from electrodeposited iron and
cobalt in the temperature range of 350–500 uC. X-Ray diffraction measurements have confirmed that the 2D nano-sheets
are Fe2O3.
In addition to the growth of nanoparticles and films on the
carbon nanowalls, it would also be good if one can grow other
types of 2D nanostructures on top of the edges of the carbon
nanowalls. Considering the sharp edges of the carbon nanowalls, a natural way to do this is to use electroplating because
of the high current density at the edges of the nanowalls.
However, as discussed above, the deposition of magnetic
materials does not necessarily only originate from the edges but
also from other high current density points on the nanowalls,
and in most cases nanoparticles or continuous films are formed.
Is it possible to form 2D materials following the morphology of
the carbon nanowalls? We believe that there is a possibility
there and it is only a matter of how to identify the right
materials. As one of the examples, Fig. 15 shows 2D selenium
grown on top of the edges of carbon nanowalls using pulsed
electrodeposition. The solution used is 3CdSO4:8H2O (61.6 g),
H2SO4 (19.6 g), SeO2 (0.062 g), and water (800 ml). Fig. 15(a)
shows the top view, and Fig. 15(b) shows the cross-sectional
view. Although some small particles are formed inside the
carbon nanowalls, most of the selenium is deposited as 2D
nano-sheets on top of the carbon nanowall edges. The Raman
spectrum is dominated by a sharp peak at 256.92 cm21 which
suggests that the as-deposited material is amorphous selenium.
These types of 2D heterostructures could be useful in increasing
the electron–hole separation efficiency when being applied to
solar cells. To this end, further studies are needed to identify
the right materials so as to form pn junctions. Recently Ng
et al.67 have succeeded in the growth of ZnO nanowalls based
on the vapor–liquid–solid mechanism. In this case, the growth
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J. Mater. Chem., 2004, 14, 469–477
Fig. 15 SEM images of the heterostructure of 2D carbon and 2D
selenium: (a) top view; and (b) cross-sectional view. Scale bars: 1 mm.
of nanowalls is attributed to the formation of network-like
catalytic structures on the substrate through controlling the
thickness of the catalyst.
6 Conclusions
Although the artificially structured laminar-type of 2D systems
and the naturally formed layered materials have been studied
intensively for both fundamental interests and applications, the
work on ideal 2D systems formed from the bottom-up
technique is still very limited. In this paper, we have briefly
reviewed the different types of 2D systems and introduced our
recent work on 2D nanocarbons. Further development in this
field is expected due to their rich physics and potential
applications.
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