onal cross sections and some adjacent walls have a V-

shape.

Acknowledgement

This work was carried out with financial support from MOST

of China (2011CB932604).

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Fig. 5 – SEM images of P2 and P3 grown at 710 �C after the treatment with HCl aqueous solution.

Plasma-enabled, catalyst-free growth of carbon nanotubeson mechanically-written Si features with arbitrary shape

Shailesh Kumar a,b,c, Igor Levchenko a,b, Kostya (Ken) Ostrikov a,b,*,James A McLaughlin c

a Plasma Nanoscience Centre Australia (PNCA), CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, New South Wales 2070,

Australiab Plasma Nanoscience, School of Physics, University of Sydney, New South Wales 2006, Australiac Nanotechnology and Integrated BioEngineering Centre, University of Ulster at Jordanstown, Newtownabbey, N. Ireland BT37 0QB, UK

A R T I C L E I N F O

Article history:

Received 24 February 2011

Accepted 21 July 2011

Available online 29 July 2011

A B S T R A C T

Simple, rapid, catalyst-free synthesis of complex patterns of long, vertically aligned multi-

walled carbon nanotubes, strictly confined within mechanically-written features on a

Si(100) surface is reported. It is shown that dense arrays of the nanotubes can nucleate

and fully fill the features when the low-temperature microwave plasma is in a direct con-

tact with the surface. This eliminates additional nanofabrication steps and inevitable con-

tact losses in applications associated with carbon nanotube patterns.

� 2011 Elsevier Ltd. All rights reserved.

Available online 18 August 2011

0008-6223/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2011.07.060

* Corresponding author at: Plasma Nanoscience Centre Australia (PNCA), CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield,New South Wales 2070, Australia. Fax: +61 2 9413 7200.

E-mail address: Kostya.Ostrikov@csiro.au (Kostya (Ken) Ostrikov).

C A R B O N 5 0 ( 2 0 1 2 ) 3 2 1 – 3 4 1 325

the plasma relative to the substrate were used, see Fig. 1c

and d. We did not observe the nanotube nucleation in a neu-

tral gas environment, on either smooth or patterned surfaces.

Likewise, no CNT growth was found on both smooth and pat-

terned surfaces in the experiment with the remote plasma,

and only the process conducted in the plasma in direct con-

tact with the surface features produced very dense patterns

made of arrays of long (up to several hundred lm) verti-

cally-aligned nanotubes. The CNTs were produced at very

high growth rates, not common to metal catalyst-free pro-

cesses. Fig. 1e shows the SEM micrograph of a complex CNT

pattern made of a dot written directly on a line feature. We

emphasize that the nanotubes were strictly confined to the

feature patterns and did not grow anywhere outside these

patterns.

Fig. 2a illustrates a process of writing mechanical patterns

on the Si surface. Several SEM images of nanotube arrays

grown on mechanically-written patterns on the Si surface

are shown in Fig. 2b–e. These images clearly show that the

longest nanotubes reach several hundred microns in length.

The array density was also very high, up to �1000 nanotubes

per lm2. Moreover, the wafer surface between the features

has always been completely free of nanotubes. These very

Fig. 1 – Experimental setup, schematic of the process, and

an SEM micrograph of a representative pattern of CNT

arrays. (a) Microwave plasma reactor. (b) Photograph of the

plasma above the substrate. (c) Three typical process

environments: contact plasma, remote plasma, and neutral

gas CVD. (d) Complete experiment matrix, which shows the

6 possible combinations of the substrate conditions (with

and without mechanically written features), and the process

environment (neutral gas, remote plasma, and contact-

plasma CVD). Note that among all possible 6 combinations

tested, only the contact-plasma CVD produced the

nanotubes directly on the Si substrate. (e) Carbon nanotubes

on a linear feature with a dot.

326 C A R B O N 5 0 ( 2 0 1 2 ) 3 2 1 – 3 4 1

Using metal catalyst has long been considered essential

for the nucleation and growth of surface-supported carbon

nanotubes (CNTs) [1,2]. Only very recently, the possibility of

CNT growth using non-metallic (e.g., oxide [3] and SiC [4]) cat-

alysts or artificially created carbon-enriched surface layers [5]

has been demonstrated. However, successful integration of

carbon nanostructures into Si-based nanodevice platforms

requires catalyst-free growth, as the catalyst nanoparticles

introduce contact losses, and their catalytic activity is very

difficult to control during the growth [6]. Furthermore, in

many applications in microfluidics, biological and molecular

filters, electronic, sensor, and energy conversion nanodevices,

the CNTs need to be arranged in specific complex patterns

[7,8]. These patterns need to contain the basic features (e.g.,

lines and dots) written using simple procedures and fully

filled with dense arrays of high-quality, straight, yet separated

nanotubes. In this paper, we report on a completely metal or

oxide catalyst-free plasma-based approach for the direct and

rapid growth of dense arrays of long vertically-aligned multi-

walled carbon nanotubes arranged into complex patterns

made of various combinations of basic features on a Si(100)

surface written using simple mechanical techniques. The pro-

cess was conducted in a plasma environment [9,10] produced

by a microwave discharge which typically generates the low-

temperature plasmas at the discharge power below 1 kW [11].

Our process starts from mechanical writing (scribing) a

pattern of arbitrary features on pre-treated Si(100) wafers. Be-

fore and after the mechanical feature writing, the Si(100) sub-

strates were cleaned in an aqueous solution of hydrofluoric

acid for 2 min to remove any possible contaminations (such

as oil traces which could decompose to free carbon at ele-

vated temperatures) from the substrate surface. A piece of an-

other silicon wafer cleaned in the same way as the substrate,

or a diamond scriber were used to produce the growth pat-

terns by a simple arbitrary mechanical writing, i.e., by making

linear scratches or dot punctures on the Si wafer surface. The

results were the same in both cases, i.e., when scratching the

surface by Si or a diamond scriber. The procedure for prepara-

tion of the substrates did not involve any possibility of exter-

nal metallic contaminations on the substrate surface.

After the preparation, the substrates were loaded into an

ASTeX model 5200 chemical vapour deposition (CVD) reactor,

which was very carefully conditioned to remove any residue

contamination. The samples were heated to at least 800 �Cto remove any oxide that could have formed during the sam-

ple loading [12]. After loading the substrates into the reactor

chamber, N2 gas was supplied into the chamber at the pres-

sure of 7 Torr to ignite and sustain the discharge at the total

power of 200 W. Then, a mixture of CH4 and 60% of N2 gases

were supplied at 20 Torr, and the discharge power was in-

creased to 700 W (power density of approximately 1.49 W/

cm3). During the process, the microwave plasma was in a di-

rect contact with the substrate. During the plasma exposure,

no external heating source was used, and the substrate tem-

perature (�850 �C) was maintained merely due to the plasma

heating. The features were exposed to a microwave plasma

for 3–5 min. A photograph of the reactor and the plasma dis-

charge is shown in Fig. 1a and b.

The six different experimental combinations with respect

to the reactive plasma/gas environments and the location of

dense arrays were formed in a very fast process. Indeed, the

growth rates of up to 50 lm/min observed in our experiments

have not been reported previously in the absence of metal

catalyst. It is also noteworthy that the length of CNTs across

the growth features is not uniform. Near the edge of a dot fea-

ture, the nanotubes grow upright and then bend towards the

substrate surface, while in the central part of the feature, they

always remain vertically aligned and grow longer. Fig. 2d and

e show a linear array of CNTs, at a lower (d) and higher (e)

magnifications. Such dense linear arrays of nanotubes could

be produced over the entire sample which typically was 10–

15 mm across. Here we stress that no matter how close the

written features were from one another, the Si surface be-

tween them was completely free of the nanotubes. For exam-

ples, the distance between the edges of the two dot features

shown in Fig. 2a was less than 10% of the dot size; neverthe-

less, no CNT growth was observed between them. This is why

the density of the mechanically written features filled with

dense nanotube arrays is only limited by the spatial resolu-

tion of the writing technique. We also emphasize that no

CNT growth was achieved in thermal CVD and remote plasma

processes in a reasonably broad range of process parameters.

Further characterization of the nanotube structure was

performed with a high-resolution transmission electron

microscopy (HRTEM), as well as selected area electron diffrac-

tion (SAED) and Raman spectroscopy, see Fig. 3. According to

the TEM images, the diameters of the nanotubes were in the

range of 15. . .50 nm, with up to 20 walls. The distance be-

tween graphitic sheets was �0.34 nm (see Fig. 3b). The Raman

spectrum of as-grown nanotubes obtained at a room temper-

ature (Fig. 3c) shows that the CNT structure exhibits a broad-

band peak at 1588 cm�1, which is the characteristic of the in-

plane C–C stretching E2g mode of the hexagonal graphitic

sheets. The well-defined shape of the G-peak confirms the

presence of sp2-hybridized carbon network within the nano-

tube walls. The electron diffraction TEM pattern shown in

Fig. 3f reveals a typical diffraction ring pattern expected for

a multi-walled carbon nanotube.

High-resolution TEM images clearly show that the catalyst

particles of any nature are absent at the closed-end tip of the

CNTs. Besides, we have investigated the wafers by the X-ray

photoelectron spectroscopy using a Cu Ka X-ray radiation

(k = 1.540 A) source. No traces of any catalyst metal were

found (see the spectra in the electronic Supplementary mate-

rial) and the nanotube caps were closed. This reveals that the

nanotubes were developing from some surface-supported

features/structures at the nanotube bases. The TEM analysis

also did not reveal any native substrate particles attached to

Fig. 2 – Scheme of patterns writing and SEM micrographs of

CNTs grown on the features of different shapes and sizes

written on Si(1 0 0). (a) Process of writing linear and dot

patterns on a Si substrate. (b) Top-view of CNTs on two

closely-positioned dot features. (c) Tilted view of a high-

density nanotube array on a dot feature. In the central area,

the nanotubes can be a few hundred microns long. (d and e)

A dense array of vertically aligned nanotubes on a linear

feature, at different magnification.

Fig. 3 – TEM and Raman analysis of CNTs. (a and b) High-

resolution TEM images of the carbon nanotube walls. The

inter-plane distance is �0.34 nm. High-resolution TEM

image shows the absence of any catalyst particles inside the

nanotubes. (c) Raman spectrum of as-grown nanotubes. (d

and e) Low-resolution TEM images of the carbon nanotubes.

(f) SAED TEM pattern showing a typical diffraction ring

structure for multiwalled CNTs.

C A R B O N 5 0 ( 2 0 1 2 ) 3 2 1 – 3 4 1 327

the CNTs after being peeled off the substrate. Fig. 3d illus-

trates the fragment of the nanotube internal structure at a

lower magnification, with no external particles detected along

the entire CNT length; this clearly supports the scenario of

the nanotube growth supported by the small nano-elements

of the Si surface, which are intrinsic parts of the wafer and al-

ways remain on the surface.

To describe the nucleation and growth of carbon nano-

tubes directly on mechanically-written surface features ex-

posed to low-temperature microwave plasmas, we have

developed a growth scenario that explains the nanotube for-

mation in terms of localized heating of the nano-elements

of Si surface. In the absence of any catalyst particles on the

silicon surface and on the CNT tips, it is reasonable to assume

that the nanotube growth is supported by the small (nanome-

ter-sized) surface structures. Since no nanotube growth was

observed anywhere outside the written features, it is thus

likely that the nano-elements of the surface morphology in-

side of the as-produced features, termed nano-hillocks in this

work, play an important role in the nanotube formation.

Comparing the carbon solubility in silicon and in metals typ-

ically used for catalyzing the nanotube growth (such as Fe, Co,

Ni, etc.) we conclude that the carbon solubility in Si is too low

(10�3 %) [13] and thus cannot explain the achieved very high

growth rates (up to 50 lm/min) by carbon diffusion in silicon

bulk. One can thus conclude that the nanotubes were grown

at such a high rate predominantly via the surface diffusion

of carbon, without involving the very slow bulk-Si diffusion.

Based on this fact, we propose the following mechanism,

namely, the reshaping-enhanced surface growth mode. At

the first stage of the growth, a silicon nano-hillock is locally

(mainly at the top) heated up by the plasma, mostly due to

the increased ion current density to the nano-sized hillock

which is caused by the stronger electric field near the tops

of nanostructures on the biased surface [10] (see Fig. 4). After

that, the silicon nano-hillock starts reshaping through the

surface reconstruction that involves the formation of stepped

silicon surfaces (terraces), where carbon chains can nucleate.

A quite similar nanoparticle reshaping process was previ-

ously reported for metal-catalyst-based carbon nanostructure

growth [14]. During this surface reconstruction, carbon spe-

cies delivered from the plasma diffuse and attach to the indi-

vidual steps. This in turn minimizes the surface energy of the

hillock and prevents it from further reshaping. Eventually,

carbon atoms form closed chains at multiple individual steps

which act as the nuclei for the nanotube walls. This leads to

the CNT growth in the multi-wall mode. As a result, this pro-

cess leads to the formation of very dense arrays of very long

multiwalled nanotubes on the arbitrary written features on

the Si surface. This proposed nucleation and growth mecha-

nism is partially justified by the absence of any metal catalyst

particles (as demonstrated by the TEM and XPS data). Further

studies are required to reveal the details of the nucleation and

growth dynamics of the carbon nanotubes.

In summary, our simple and environmentally-benign plas-

ma-enabled approach demonstrates the possibility for highly-

reproducible, catalyst-free, fast, cost- and energy-efficient

growth of arrays of long vertically-aligned CNTs that fully fill

arbitrarily written micro-patterns on a Si surface. This in turn

offers tantalizing prospects for the low-contact-loss integra-

tion of carbon nanostructures into a Si-based nanodevice

platform for the future energy conversion, nano- and opto-

electronic, biomedical, sensor, and environmental

applications.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2011.07.060.

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328 C A R B O N 5 0 ( 2 0 1 2 ) 3 2 1 – 3 4 1

Nucleation and growth of single crystal graphene onhexagonal boron nitride

Shujie Tang a, Guqiao Ding a, Xiaoming Xie a,*, Ji Chen a, Chen Wang a, Xuli Ding a,Fuqiang Huang b, Wei Lu c, Mianheng Jiang a

a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology,

Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, PR Chinab CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050,

PR Chinac i-LAB Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215125, PR China

A R T I C L E I N F O

Article history:

Received 25 June 2011

Accepted 27 July 2011

Available online 29 July 2011

A B S T R A C T

Direct graphene growth was demonstrated on exfoliated hexagonal boron nitride (h-BN)

single crystal flakes by low pressure CVD. The size of the hexagonal single crystal graphene

domain increases with deposition time, with maximum size of �270 nm. Most domains

were found to nucleate at screw dislocation sites, and a step-flow growth mechanism

was observed at atomic steps on the h-BN surface. Understanding the nucleation and

growth mechanisms is an important step towards the synthesis of large single crystal

graphene on h-BN substrates.

� 2011 Elsevier Ltd. All rights reserved.

Among many of the unprecedented properties of graph-

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Available online 18 August 2011

0008-6223/$ - see front matter � 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2011.07.062

* Corresponding author: Fax: +86 21 52419909.E-mail address: xmxie@mail.sim.ac.cn (X. Xie) .

C A R B O N 5 0 ( 2 0 1 2 ) 3 2 1 – 3 4 1 329