REVIEW ARTICLE

Low-temperature plasmas in carbon nanostructure synthesis

Igor Levchenkoa)

Plasma Nanoscience Laboratories, CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, NewSouth Wales 2070, Australia and Plasma Nanoscience@Complex Systems, School of Physics, The Universityof Sydney, Sydney, New South Wales 2006, Australia

Michael KeidarDepartment of Mechanical and Aerospace Engineering, The George Washington University, Washington, DC20052

Shuyan XuPlasma Sources and Applications Centre, NIE, and Institute of Advanced Studies, Nanyang TechnologicalUniversity, 637616 Singapore

Holger KerstenInstitute of Experimental and Applied Physics, University Kiel, D-24098 Kiel, Germany

Kostya (Ken) OstrikovPlasma Nanoscience Laboratories, CSIRO Materials Science and Engineering, P.O. Box 218, Lindfield, NewSouth Wales 2070, Australia and Plasma Nanoscience@Complex Systems, School of Physics, The Universityof Sydney, Sydney, New South Wales 2006, Australia

(Received 4 July 2013; accepted 3 September 2013; published 20 September 2013)

Plasma-based techniques offer many unique possibilities for the synthesis of various nanostructures

both on the surface and in the plasma bulk. In contrast to the conventional chemical vapor

deposition and some other techniques, plasma-based processes ensure high level of controllability,

good quality of the produced nanomaterials, and reduced environmental risk. In this work, the

authors briefly review the unique features of the plasma-enhanced chemical vapor deposition

approaches, namely, the techniques based on inductively coupled, microwave, and arc discharges.

Specifically, the authors consider the plasmas with the ion/electron density ranging from 1010 to

1014 cm�3, electron energy in the discharge up to �10 eV, and the operating pressure ranging from1 to 104 Pa (up to 105 Pa for the atmospheric-pressure arc discharges). The operating frequencies of

the discharges considered range from 460 kHz for the inductively coupled plasmas, and up to

2.45 GHz for the microwave plasmas. The features of the direct-current arc discharges are also

examined. The authors also discuss the principles of operation of these systems, as well as the

effects of the key plasma parameters on the conditions of nucleation and growth of the carbon

nanostructures, mainly carbon nanotubes and graphene. Advantages and disadvantages of these

plasma systems are considered. Future trends in the development of these plasma-based systems

are also discussed. [http://dx.doi.org/10.1116/1.4821635]

I. INTRODUCTION

Carbon nanostructures and nanomaterials such as gra-

phene1,2 and especially one-dimensional nanotubes,3,4 nano-

cones,5,6 nanorods,7,8 nanotips,9,10 nanowires,11,12 etc., feature

many unique mechanical, optical, magnetic, and electrical

properties, which offer superior opportunities for the modern

applications ranging from photovoltaic cells,13,14 energy stor-

age and conversation devices,15,16 memory devices,17,18 nano-

electronics19,20 and supercapacitors21,22 to nanomechanical

and biomedical devices,23–25 gas and biosensors,26,27 electron-

emitting panels,28,29 polymer–carbon reinforced composites,30

etc. Presently, carbon nanostructures and nanomaterials are

commonly fabricated using catalyzed chemical vapor

deposition (CVD) technique.31–33 This method is simple, but

a significant lack of controllability in the shape, size, density,

and other characteristics of the nanostructures produced by

the CVD motivates researchers to look for the alternative

techniques. Besides, the structure of the carbon nanomaterials,

such as the level of structural defects, vacancies, and impur-

ities, is often beyond the limits required for the effective use

of the nanostructures in the functional devices.

Presently, plasma-enhanced CVD (PECVD) is a subject of

a strong interest from academic and industry sectors because

of its capacity to produce the carbon nanostructures of high

quality in a controllable environment.34–36 Owing to a high

level of controllability intrinsic to the plasma-based nanofabri-

cation,37,38 the catalytic and catalyst-free PECVD is a tech-

nique particularly promising for the fabrication of various

carbon nanostructures including nanotubes and graphene.39,40a)Author to whom correspondence should be addressed; electronic mail:

Igor.Levchenko@csiro.au

050801-1 J. Vac. Sci. Technol. B 31(5), Sep/Oct 2013 2166-2746/2013/31(5)/050801/16/$30.00 050801-1

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http://dx.doi.org/10.1116/1.4821635http://dx.doi.org/10.1116/1.4821635mailto:Igor.Levchenko@csiro.auhttp://crossmark.crossref.org/dialog/?doi=10.1116/1.4821635&domain=pdf&date_stamp=2013-09-20

Plasmas are commonly perceived as the “fourth state of

the matter.” The plasmas represent a fully or partially ion-

ized gas, i.e., ions, electrons, and neutral species.

Importantly, the densities of the positive and negative par-

ticles in the plasma are equal, i.e., the plasma is a quasineu-

tral medium. One more important feature of the plasma is

the interaction among the charged particles leading to the

collective behavior of the whole ensemble. In technological

applications, mainly a low-temperature plasma is used, with

the ion energy not exceeding one hundred electron-volts (of

the order of one million Kelvin). A high-temperature plasma

with the ion energy exceeding 100 eV is mainly a subject of

the controlled fusion research and is thus outside the scope

of this review. Another way to classify the plasma is to com-

pare the ion thermal energy ei with the energy of electronsee. The plasma is termed “equilibrium” if ei � ee, and“nonequilibrium,” if ei� ee. Both equilibrium and nonequi-librium low-temperature plasmas are used in industry and

will be discussed here.

An industry-scale production of the carbon nanostructures

such as nanotubes and graphenes is another problem where

the use of the low-temperature plasmas may be a possible so-

lution.41,42 Indeed, the production of carbon nanotubes and

graphene requires specific conditions such as sufficiently

high energy of the particles supplied to the growth zone, bal-

anced influx of carbon material to the growing nanostructure,

and a specially prepared (fragmented and activated) cata-

lyst.43,44 In the conventional CVD process, these conditions

are ensured by the use of very high (typically about

800–1200 �C) process temperature. In the plasma-basedprocesses, these conditions can be easily met by the proper

selection of the process parameters such as density, electron

temperature, and surface potential. The requirement of the

high plasma density makes the arc plasma especially promis-

ing for the large-scale production of the carbon

nanomaterials.45

One more important point is the environment-benign na-

ture of the plasma-based techniques. In fact, many plasma-

based technological processes are conducted in a confined

reactor chamber where a controlled atmosphere is used (Fig.

1). In vacuum-based methods, air cannot be used as a pro-

cess environment due to too high oxygen content, and thus,

the controlled gas mixture is created and maintained in the

chamber during treatment, and then exhausted through the

filters, in a controlled manner. Thus, the human respiratory

tract remains isolated from the harmful volatile compounds,

and exposure to toxic gas or liquid chemicals is reduced.

Also, humans remain isolated from any expose to the nano-

materials which are kept in the reactor chamber during the

growth process.46

In our paper, we review the capability of the plasmas pro-

duced by the inductively coupled (ICP), microwave (MWP),

and arc discharge systems for the fabrication of various car-

bon nanostructures, mainly carbon nanotubes and graphenes.

We will discuss some of the principles of operation of these

systems, as well as the key plasma parameters and their

effects on the conditions of nucleation and growth of carbon

nanostructures. Advantages and disadvantages of these

plasma systems will be considered. Future trends in the de-

velopment of these plasma-based systems will also be

addressed.47,48

This review is mainly based on the results obtained in col-

laborative efforts of researchers from CSIRO (Australia),

George Washington University (USA), the University of

Kiel (Germany), and Nanyang Technological University,

Singapore. Specializing in different plasma systems and

processes, these research groups developed several practical

techniques through collaborative efforts. The demonstration

of the usefulness of international collaborations is one of the

aims of this work. Along with this, a large body of the results

obtained by other researchers is discussed.

II. INDUCTIVELY COUPLED AND MICROWAVEPLASMA SYSTEMS

A. Principle of operation and plasma parameters

The chemical vapor deposition is a process widely used

for the fabrication of various surface-bound nanomaterials

such as nanotubes, nanorods, nanocones, nanotips, and nano-

wires. In this process, a solid material is deposited onto a

heated substrate from gaseous precursors. Chemical reac-

tions in the gas environment are used to synthesize the target

nanomaterial on the surface. In many cases, CVD technique

involves a complex chain of the processes including dissoci-

ation of the carbon-containing precursors (such as methane,

acetylene, ethylene, ethanol, carbon dioxide, etc.), formation

of larger molecules in the gas environment and on the surfa-

ces of the substrate and catalyst, diffusion of various species

over the substrate and catalyst surfaces, saturation of the cat-

alyst,49,50 nucleation of the nanostructures on the cata-

lyst51,52 or directly on the substrate surface.53 As a result,

growth of the pattern of nanostructures occurs due the mate-

rial flux directly from the gas phase or by the surface diffu-

sion,54 and further incorporation of atoms into the growing

structure.55

FIG. 1. (Color online) Plasma-based technological nanofabrication is con-

ducted in a confined reactor chamber. The worker breathing zone remains

isolated from the harmful volatile compounds, and no toxic gas or liquid

chemicals can be exposed to humans. No radiation is present outside the

chamber, no harmful gas leakage and harmful by-products. Reprinted with

permission from Han et al., J. Phys. D: Appl. Phys. 44, 174019 (2011).Copyright 2011, IOP Publishing Ltd.

050801-2 Levchenko et al.: Low-temperature plasmas in carbon nanostructure synthesis 050801-2

J. Vac. Sci. Technol. B, Vol. 31, No. 5, Sep/Oct 2013

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Due to many drawbacks of the CVD technique such as

low controllability with respect to the nanostructure proper-

ties, position, size, and shape, as well as low growth rate and

need for a very high surface temperature, the plasma-based

CVD process was proposed. This process relies on the

plasma environment as an extra dimension to control

the processes related to the nucleation and growth of the

surface-bound nanostructures. The plasmas differ from neu-

tral gases by the presence of free electrons and various ion-

ized species which in turn could be in various excited

states.56 As a result, the plasma may be affected by an exter-

nal electric or magnetic field, and hence, the ion and electron

fluxes may be effectively controlled. Indeed, numerous stud-

ies on the plasma-based fabrication of various nanostructures

have demonstrated unprecedented level of control over the

material fluxes, heating, and eventually structure and proper-

ties of the nanostructures grown on the plasma-immersed

surfaces.57 Among others, the possibility to grow large pat-

terns of the vertically aligned nanostructures is one of the

major advantages of PECVD, as compared to the conven-

tional CVD technique.58 Other benefits of the plasma envi-

ronment include, but not limited to, the lower temperatures

required for nucleation and growth of the nanostructures, as

well as much higher growth rates.

The low-frequency inductively coupled plasma reac-

tors59,60 are successfully used for the fabrication of various

surface-bound nanostructures such as nanorods,61 nano-

cones,36 nanowires,62 graphene,63 and large patterns of nano-

dots.64 Schematics of the two typical ICP reactors are shown

in Fig. 2. The most frequently used reactors consist of a vac-

uum chamber and a planar inductive coil installed on the

upper lid of the chamber, as shown in Fig. 2(a), or a coil in-

stalled over the confinement tube [Fig. 2(b)]. Also, various

multispiral configurations may be used.59

In contrast to the capacitively coupled plasmas, the induc-

tively coupled discharge is sustained by the alternating radio

frequency electromagnetic field generated by the induction

coil installed on the dielectric lid or over the dielectric con-

finement tube. An alternating electromagnetic field is used

for the heating and electron-impact ionization of the gas,

eventually sustaining the steady discharge. The ICP operated

in the H-mode typically features a much higher plasma den-

sity, as compared to capacitively coupled plasmas.59

The microwave plasma reactors used in the nanofabrica-

tion operate at typical microwave frequencies such as 2.45

GHz. The schematic of the MWP setup is similar to that of

ICP.53 The reactor chamber is connected to the microwave

generator (magnetron) with the help of a waveguide, and

thus, the microwave energy is supplied through the quartz

window to the chamber where the discharge is sustained.

Both ICP and MWP reactors are usually equipped with

the vacuum and gas supply systems capable of maintaining

the required pressure and gas composition in the reaction

chamber. To increase the energy of ions supplied to the

growing nanostructures, a negative biasing potential can be

applied to the substrate holder. Also, the reactors are usually

equipped with a diagnostic system consisting of various

types of probes, to enable measurements of the most impor-

tant parameters such as the electron energy and the plasma

density. The gas pressure in the chamber is usually moni-

tored with the help of Pirani and Penning gauges, as well as

capacitance manometers.

It should be also stressed that the surface temperature is

one of the key parameters in nanofabrication.65,66 In the

plasma-based systems, the surface temperature can be con-

trolled even without external heating, by regulating the

energy flux from the plasma.67 Some typical characteristics

of the ICP and MWP reactors are summarized in Table I.

FIG. 2. (Color online) (a) Schematic of the ICP reactor with planar coil on top of the vacuum chamber. (b) Schematic of the ICP reactor with the coil installed

over the confinement chamber. (c) Photo of the discharge in the confinement chamber of the setup shown in (b).

050801-3 Levchenko et al.: Low-temperature plasmas in carbon nanostructure synthesis 050801-3

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B. Growth of vertical carbon nanostructures in ICP

Owing to high plasma density and ion energy, the ICP

reactors are effective for the growth of various vertically

aligned carbon nanostructures, such as carbon nanocones.

An example of such nanostructures successfully grown in

the ICP reactor with the planar inductive coil is shown in

Figs. 3 and 4.36 The nanocones were grown on a lightly

doped n-Si(100) substrate, which was coated with a nickel

catalyst (layer of 30 nm thick deposited by the magnetron

sputtering) before treatment in the ICP reactor. The catalyst-

coated specimens were installed in the reactor chamber on

the sample holder, and first pretreated by the discharge

ignited in argon for 30 min. Next, a discharge in a mixture of

argon and hydrogen was used to prepare the catalyst layer

for nanostructure nucleation. Specifically, during this stage,

the metal catalyst layer was fragmented into separate islands,

suitable for the nucleation of separate nanocones. After pre-

paring catalytically active metal islands, methane was sup-

plied to the reactor chamber, and the growth of nanocones

started. During the growth, no external heating was used and

the growing nanostructures were heated by the plasma flux,

which was strong enough to maintain the surface tempera-

ture of about 500 �C when the biasing potential of 300 V wasapplied.36 During the process, the plasma with the density of

1012 cm�3 was sustained, when the radio-frequency power of

0.1 W� cm�1 was applied to the upper coil.As it can be seen in Fig. 3, dense patterns of the carbon

nanocones with the height of 300–800 nm were grown. For

the short deposition time (5 min), very nonuniform pattern

was formed, whereas after 10 min into the process, the uni-

formity was significantly improved, and quite uniform

patterns eventually formed after 20 min of deposition [Fig.

3(c)]. Along with the uniformity, the shape of the nanocones

also changed and the nanostructures became much thinner.

The Raman characterization technique has shown that the ra-

tio of G and D bands increased with the deposition time, i.e.,

the crystalline structure was improved and the amount of

amorphous carbon decreased with the process time.36 The

XRD technique has also evidenced the best crystalline struc-

ture of the samples grown for the longer time. Thus, the

plasma-related processes can significantly enhance the qual-

ity of the synthesized nanostructures.

C. Catalyst-free growth of vertical carbon nanotubesin microwave plasma

Microwave plasmas can be successfully used for the

growth of large arrays of vertically aligned carbon nano-

tubes.53 Normally, the plasma-based fabrication of carbon

nanomaterials requires metal catalyst to initiate the nuclea-

tion and sustain the nanostructure growth. Fabrication of

metal-free nanotubes not containing any metal particles rep-

resents a strong interest for the nanoelectronic applications.

The nucleation and growth of carbon nanomaterials can be

achieved without a metal catalyst, but this is done by a com-

plex postprocessing,68 which leads to the degradation of the

nanotube properties and damage to the substrate.69

Therefore, the metal-free fabrication of the carbon nanotubes

is not practical for the industrial-scale production.70 It should

be noted that the carbon nanomaterials could be nucleated

on nonmetallic catalysts (e.g., on semiconductor nanopar-

ticles), but only low-quality structures could be produced in

such process.71,72 The nanotubes catalyzed and grown with-

out metal catalyst are usually not vertically aligned and have

irregular pattern structure. Nevertheless, the microwave

plasma technique enables an efficient catalyst-free nuclea-

tion and growth of dense, vertically aligned arrays of long

nanotubes on silicon wafers.53

In the experiments on catalyst-free fabrication of carbon

nanotubes,53 several different experimental variations were

used with respect to the plasma/gas environments, plasma

location relative to the substrate, and presence of the special

notch and dot patterns on the surface. Specifically, the fol-

lowing configurations were used: gas contacting the surface

with/without any written features; remote plasma and

TABLE I. Typical parameters of ICP and MWP reactors used for the

nanofabrication.

Parameter ICP MWP

Discharge power range (kW) 0.01–10 0.1–10

Plasma density (cm�3) 1010–1013 1010–1011

Electron energy in discharge (eV) 1–5 5–10

Operating pressure range (Pa) 1–1000 10–10 000

Typical frequency 0.46–13.56 MHz 2.45 GHz

Power density (W� cm�3) 0.01–0.5 1–2Substrate temperature range ( �C) 100–500 100–800

FIG. 3. SEM images of carbon nanocones grown in ICP reactor for different growth times: (a) 5 min; (b) 10 min; and (c) 20 min. Reprinted with permission

from Tsakadze et al., Carbon 45, 2022 (2007). Copyright 2007, Elsevier Ltd.

050801-4 Levchenko et al.: Low-temperature plasmas in carbon nanostructure synthesis 050801-4

J. Vac. Sci. Technol. B, Vol. 31, No. 5, Sep/Oct 2013

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surface with/without written features, and plasma contacting

the surface with/without written features. The nanotube

nucleation in the neutral gas environment and in the

remotely located plasma was not observed for any surfaces,

and only the plasma contacting the mechanically patterned

surface (parallel linear notches and spots were mechanically

written on silicon) has produced the nanotubes. The micro-

wave discharge was sustained for 3–5 min in a mixture of

methane and nitrogen at a pressure of typically 10 Torr, and

the surface temperature was maintained in the range of

700–800 �C by the plasma heating. These arrays wereformed in a very fast process with the growth rates up to

�50 lm/min. These growth rates are much higher comparedto the processes conducted in the absence of metal catalyst.

Figure 5 illustrates the fabricated nanotubes. Scanning

electron microscopy (SEM) has shown that a high-density

forest of highly aligned nanotubes precisely replicated the

notch pattern, including complex patterns consisting of lin-

ear notches and spots applied directly over the notch. The

transmission electron microscopy (TEM) and Raman charac-

terizations of the nanotube structure have clearly shown the

absence of catalyst particle at the closed end tip of the nano-

tubes, thus revealing the base-led growth mode.73,74 The

nanotubes reached 80 nm in diameter.

The mechanism of the nanostructure growth without

metal catalyst particles was explained based on the key role

of nanoscale features on the surface.53 The nanoscale fea-

tures were created by mechanical writing of the pattern.

Since the carbon solubility in silicon is low75 and very high

growth rate was observed, one can assume that the bulk dif-

fusion of carbon in silicon did not play a major role, and a

vapor–liquid–solid mechanism was not involved. Initially,

the tip of a Si nanoscale feature was heated up by the

plasma. Then, the heated tip reshapes76 due to the rearrange-

ment of material. Next, the carbon atoms form closed chains

in the steps on the silicon surface. Finally, the nanotubes nu-

cleate from these chains and start growing. When the nano-

tube reaches the maximum length, the tip of nanotube

closes. Hence, in this process, the nucleation of carbon nano-

tubes occurs by the minimization of surface energy at the

steps formed on silicon surface,77 where the adsorbed atoms

can be considered as “partially dissolved.” Since the silicon

surface is covered with a large number of nanoscale surface

features, the resulting array of long multiwall nanotubes is

very dense and formed only on the mechanically patterned

areas.

The plasma-related effects are essential for the above pro-

cess since the plasma-related heating and high rate of mate-

rial delivery are of the primary importance here. Therefore,

the catalyst-free, dense arrays of long vertically aligned mul-

tiwall nanotubes can be grown using a low-temperature

microwave discharge on the silicon wafers where the arbi-

trary pattern was mechanically applied. Schematic of the

nanotube growth on the surface features is shown in Fig. 6.

Note that the nanotube patterns fabricated using micro-

wave plasmas feature a very high density. In contrast, the

use of neutral gas-based CVD process results in the forma-

tion of nanotube patterns of a significantly lower density, as

evidenced by SEM images shown in Fig. 7(a). After post-

treatment in acetone, such patterns collapse by the action of

capillary forces [Figs. 6(b) and 6(c)].78

FIG. 4. SEM (a), (b) and TEM (inset) images of single-crystalline carbon

nanocones grown in ICP reactor. Reprinted with permission from

Levchenko et al., Appl. Phys. Lett. 91, 113115 (2007). Copyright 2007,American Institute of Physics.

FIG. 5. (Color online) Scheme of patterns writing and SEM micrographs of

nanotubes grown on the features of different shapes and sizes. (a) Process of

writing linear and dot patterns on a Si substrate. (b) Top-view of nanotubes

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), (e) Dense array of vertically aligned

nanotubes on a linear feature, at different magnification. Reprinted with

permission from Kumar et al., Carbon 50, 325 (2011). Copyright 2011,Elsevier Ltd.

050801-5 Levchenko et al.: Low-temperature plasmas in carbon nanostructure synthesis 050801-5

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D. Controllable growth of graphene flakes in ICP

Graphene was discovered recently and now it attracts

great attention of the researchers due to exceptional mechan-

ical, electrical, optical, and other properties. Graphene can

be a very promising material for many applications in nanoe-

lectronics, energy conversion, sensing, various nanodevices,

supercapacitors, nanoelectromechanical systems, and many

others.79,80

Many applications require graphene nanosheets in the

form of flakes consisting of a few graphene monolayers.

These nanosheets should be separated and securely attached

to the surface of the substrate suitable for the specific appli-

cation (e.g., doped silicon). Besides, it is highly desirable to

precisely control the position and orientation of the graphene

nanosheets during the fabrication.81–85 The vertically aligned

graphene nanosheets expose open edges which ensure many

unique features, related to the high reactivity of the reactive

edges of graphene monolayers, which can enhance electric

field, attach atoms and biological species.86–88 The known

fabrication techniques capable of producing high-quality

graphenes (such as chemical solution methods,89 arc dis-

charges,90 and CVD) could not ensure precise control of the

graphene alignment and orientation. None of these techni-

ques is able to produce high-quality vertically aligned gra-

phenes on the wafer surface. Instead, only unbound (like in

arc discharges) or flat-oriented (by chemical methods) gra-

phenes can be produced.

The low-temperature ICP plasmas can be useful for the

fabrication of large arrays of well-separated graphene nano-

sheets, vertically aligned and bound on the wafer surface.

Moreover, the plasma-based process demonstrates an excel-

lent controllability of the graphene orientation and place-

ment on the surface. In the typical experiments, the plasma-

produced nanosheets were perfectly oriented along the

substrate center–substrate edge direction in the pretty wide

(as compared with the nanosheet size) substrate edge zone.

These nanosheets rested on the silicon nanograss (a dense

array of nanoscaled silicon cones on the silicon surface91),

fabricated from the silicon substrate in a separate plasma-

based process.92 SEM, TEM, and Raman techniques were

used to characterize the produced nanosheets. A perfect

alignment, orientation, and the few-layer internal structure

of the nanosheets were confirmed. These perfect nanostruc-

tures can be used for the development of advanced nanodevi-

ces that require graphene directly integrated into a Si-based

nanodevice platform.

The fabrication process involved two main stages. At the

first stage, silicon nanograss was fabricated on the silicon

substrate by processing the wafers for 120 min in a radio-

frequency ICP discharge93 ignited in H2 gas at a pressure of

5.0 Pa. The discharge was sustained by supplying

500–600 W RF power to the chamber. The substrate temper-

ature was maintained in the range of 350–400 �C, and thewafer was biased negatively by applying the �250–300 Vpotential relative to the grounded chamber walls. The nano-

grass consisted of sharp self-organized Si nanoconical struc-

tures, typically of about 1 lm in height, with the basediameter up to 100 nm. The produced pattern was fairly

dense and uniform (see more details on the nanograss struc-

ture and characterization elsewhere.92) Then, the nanograss

was used as a catalyst-free platform for the fabrication of

self-organized vertically aligned graphene nanosheets. The

wafer surface was activated for 3 min using radio-frequency

ICP plasmas ignited in nitrogen (gas pressure of 4 Pa and RF

power of 400 W). The growth process was initiated by

FIG. 6. (Color online) Plausible mechanism of nucleation and growth of mul-

tiwalled carbon nanotubes on a silicon nanohillock in the plasma. (a) Si

nanohillock is locally heated by the plasma. (b) Carbon atoms form chains

on the heated Si nanohillock. (c) Carbon chains close, nanotube walls nucle-

ate. (d) Nanotubewalls grow in the open-end mode and eventually close.

Reprinted with permission from Kumar et al., Carbon 50, 325 (2011).Copyright 2011, Elsevier Ltd.

FIG. 7. SEM images of the three-dimensional structures formed from CVD-grown carbon nanotube patterns and postprocessing with acetone, produced in

Plasma Nanoscience Labs at CSIRO (Yick et al., unpublished). (a) Side view; (b), (c) Top view.

050801-6 Levchenko et al.: Low-temperature plasmas in carbon nanostructure synthesis 050801-6

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adding methane to the discharge. During the growth, a dis-

charge power increased to 800–1000 W was used, along with

the higher (up to 5–10 Pa) gas pressure and surface bias of

�50 V. The wafer was heated by the plasma irradiation only,and the substrate temperature did not exceed 500 �C. After10 min into the process, well-aligned vertical graphene nano-

sheets (VGNS) were fabricated (Fig. 8).

Thus, the low-temperature ICP and microwave plasmas

are efficient tools for the fabrication of various surface-

bound nanostructures such as carbon nanotubes, nanocones,

and graphene flakes. Notably, some very important features

not possible in the gas-based fabrication techniques were

achieved in plasma-based processes, namely, an efficient

catalyst-free growth of carbon nanotubes, fabrication of the

complex structures of nanotubes on the mechanically written

patterns, and highly controllable fabrication of the few-layer

vertically aligned graphene nanosheets.

E. Synthesis of carbon nanoparticlesin radio-frequency plasma

Along with other plasma-grown carbon nanostructures, the

nanoparticles grown directly in the volume of process cham-

ber are of a particular interest due to possible precise deposi-

tion onto specific locations using custom-shaped electric

fields immediately after nucleation and growth in the gas

phase.94 The plasma technique allows efficient nucleation and

growth of the carbon nanoparticles from carbon-containing

gases.95,96 In typical experiments,97 a specially designed setup

was used in which an asymmetric radio-frequency discharge

at 13.56 MHz is sustained. Argon and acetylene are typical

gases that are used as a background gas and a reactive precur-

sor, respectively. The power of the discharge may be varied in

a broad range, reaching 50 W. Under these conditions, the

plasma density reaches �109 cm�3.The cloud of nucleated carbon nanoparticles can be made

visible by illuminating them with a laser beam, as shown in

Fig. 9(a). Furthermore, a void (dust-free region) can be

observed between the two clouds. These plasma-grown nano-

particles precipitate to the lower electrode after acquiring an

excess mass precluding them from levitation. Finally, they

can be collected on the electrode surface. Figure 9(b) is an

SEM image of nanoparticles found on the electrode surface.

The nucleation and growth of nanoparticles in low-

temperature plasma proceeds via the following mecha-

nism.98 After the initial phase, the nuclei of a few nanometer

size move in the plasma, collide, and coagulate.99 Finally,

they form larger particles with the size of several tens of

nanometers.100,101 The density of such particles typically

reach (1–5)� 103 cm�3 (up to 104 cm�3 for higher dischargepowers). Since the larger negatively charged nanoparticles

have larger collision cross-sections, they also accumulate

neutral radicals and positive ions, and thus grow up to sev-

eral hundred nanometers. The size distribution of the nano-

particles tends to be monodisperse.100,102 It should be also

mentioned that the density of nanoparticles can reach

FIG. 8. (Color online) Different orientation of the VGNSs (SEM images)

grown near the edges, corners, and in the central area of the silicon substrate

covered with nanograss (optical image in the center). The VGNSs grown in

the central area of the substrate (lower right panel) show a random orienta-

tion. The graphenes grown near the substrate edge align in the edge-center

direction. Reprinted with permission from Kumar et al., Appl. Phys. Lett.100, 053115 (2012). Copyright 2012, American Institute of Physics.

FIG. 9. (Color online) (a) Photo of illuminated clouds of nanoparticles in the

plasma. The region free of nanoparticles is clearly visible; (b) Typical car-

bon nanoparticles collected on the electrode surface (SEM image). The scale

bar in the inset is 200 nm. Reprinted with permission from Hundt et al., J.Appl. Phys. 109, 123305 (2011). Copyright 2011, American Institute ofPhysics.

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�107 cm�3 in the pulsed symmetric Ar-C2H2 capacitivelycoupled discharges; in this case, the charged nanoparticles

can significantly affect the plasma properties.103

F. Growth of nanostructures on plasma-exposedsurfaces—theoretical examination

When examining the growth of nanostructures on the

plasma-exposed surface, the two main groups of the physical

processes should be distinguished, namely, surface processes

and plasma processes. While the surface processes are mainly

responsible for the formation of the internal structure of the

nanostructures and the morphology on nanostructure patterns,

the plasma processes are mainly responsible for the material

delivery to the substrate surface and surfaces of the growing

nanostructures. Nevertheless, these groups of the processes

are tightly interrelated and should be examined in the frame-

work of the integrated model. The most important processes

occurring on the plasma-exposed surfaces are the surface dif-

fusion between growing nanostructures, evaporation of the

adsorbed particles from the wafer and nanostructure surfaces

to the space and 2D vapor, and attachment of the particles to

the growing nanostructures.104 The aim of the surface model

is to calculate the balance of material fluxes on the surface of

the nanostructures, rate of the nanostructure growth and, even-

tually, an internal structure of the nanostructures.

The material balance on the individual nanostructure is

wi ¼ wþP þ wþS � w�E � w�N ; (1)

where wi is the total flux of material to the individual nano-structure, wþP is the flux of material to the nanostructure fromthe plasma, wþS is the flux of material adsorbed on the surfaceto an individual nanostructure, w�E is the evaporation from thenanostructure, and w�N is the flux of material evaporated fromthe nanostructure to the two-dimensional vapor. A similar

relation may be written for the energy balance, taking into

account various energy contributions such as radiation, power

transfer by electrons, ions and neutral species, and other

energy sources including possible exothermic and endother-

mic reactions on the surface.67,105 Then, the flux to the surface

of the individual nanostructure from the plasma wþP is

wþP ¼ðS

jSNdsþ SNwan; (2)

where jSN is the ion flux from plasma to the nanostructure, SNis the area of the nanostructure surface, and wan is the flux ofneutral particles to the nanostructure from the plasma. The

flux wþS can be obtained by solving the diffusion equation forthe density of atoms g(x, y, t) adsorbed on the substrate50,104

@g@t¼ D @

2g@x2þ @

2g@y2

!þ win � wvpi; (3)

where win is the total flux of atoms and ions to the substratefrom the plasma, wvpi is the flux of evaporation, and D is thesurface diffusion coefficient. The evaporation flux wvpi is

50

wvpi ¼ ð1=k2aÞ �0 expð�eSe=kTÞ; (4)

where eSe is the energy of evaporation from the surface, T isthe surface temperature, ka is the lattice constant, �0 is thefrequency of lattice atom oscillations, and k is theBoltzmann’s constant.

The total flux to the substrate from the plasma, win, is

win ¼ ð1=k2aÞðS

jSdsþ wa; (5)

where jS is the density of the ion flux to the substrate, and wais the flux of neutral species to the substrate from the

plasma.

Hence, the total flux of adsorbed atoms to the individual

nanostructure can be calculated as50

wþS ¼ �Lm

k3aqmD@g@x

� �; (6)

where L is the perimeter of nanostructure, m is the adatommass, and qm is the nanostructure material density. The sur-face diffusion coefficient can be calculated as

D ¼ k2a�04

expð�ed=kTÞ; (7)

where ed is the surface diffusion activation energy.The flux of atoms evaporating from the surface of an indi-

vidual nanostructure w�E into the 3D vapor is calculated as

w�E ¼ ðSi=kaÞ �0 expð�ea=kTÞ; (8)

where Si is the surface area of an individual nanostructure, andea is the evaporation energy. The two-dimensional flux w

�N from

the individual nanostructure is calculated similarly to Eq. (8).

To describe the growth dynamics of various nanostruc-

tures, different forms of the growth equations should be

used. The nanocones may be described with the following

set of equations:36

ð@V=@rÞ dr ¼ k3aðwþS � w�N Þ dt (9)

and

ð@V=@hÞ dh ¼ k3aðwþP � w�E Þ dt; (10)

where V is the volume, r is the base radius, and h is theheight of the nanocone.

The most important feature of the plasma–surface inter-

actions influencing the kinetics of material supply to the

plasma-immersed body is the formation of a sheath above

the surface.106 The ions enter the sheath with the Bohm ve-

locity Vb¼ (kTe/mi)1/2 (assuming that the sheath is colli-sionless), where Te is the electron temperature and mi isthe ion mass. In the most common case of the thick sheath

(for kTe � US, where US is the surface potential), thesheath width can be estimated as kS � kDð2US=kTeÞ, where

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kD ¼ ðe0kTe=neÞ1 2=

is the Debye length, e is the electroncharge, e0 is the dielectric constant, and n is the plasmadensity.

The electric field in the vicinity of the nanostructure is

very strong due to very large surface curvature. Thus, the

density of the electric charges in the plasma–surface sheath

can be neglected, and the electric field can be found from the

Laplace equation Du¼ 0, with the boundary condition ofequipotentiality over the entire conductive surface

/ðx; y; zÞjSURF ¼ US: (11)

Note that we are considering mainly carbon nanostruc-

tures, so the surface can be assumed conductive in most

cases, e.g., related to graphitic carbons.

Thus, the model describes material supply through the

plasma–surface sheath to the wafer surface, and further ma-

terial supply to the individual nanostructures. As it can be

seen from the above analysis, the nanostructure growth on

the plasma-exposed surface is determined by many process

parameters including the plasma density, electron tempera-

ture, surface temperature, surface diffusion energy, and

some other parameters. On one hand, this enables many

interesting possibilities for the growth control by managing

material supply to the wafer surface and each nanostructure.

On the other hand, this makes the control very complicated.

Also, the presence of the surface–plasma sheath limits the

rate of material supply. Nevertheless, it well exceeds that of

the neutral gas-based process, and thus the growth rate of the

plasma-fabricated nanostructures much exceeds the growth

rates observed in the neutral gas-based process. However,

the sheath makes it possible to precisely control (via the

plasma parameters) the material supply to the surface, and

thus to form a defect-free crystalline structure of the nano-

structures. This is especially important during formation of

carbon nanotubes and graphenes, which should in some

cases exhibit a perfect structure. In general, the energetic

ions can indeed produce defects in the crystalline structure.

Nevertheless, in the typical low-temperature plasma proc-

esses of interest here, the probability of generation of

ion-induced defects is not high. Indeed, the characteristic

energies of the onset of defect formation in carbon nanotubes

and graphenes are quite high and reach �10 eV for thehighly crystalline structures.107 This energy is comparable

with the typical cross-sheath potential in high-density low-

temperature plasma processes, e.g., inductively coupled plas-

mas.55 On the other hand, the ions bombarding the growing

structure on the surface lose their energy for many processes,

such as activation and removal of hydrogen usually terminat-

ing the dangling hydrogen bonds, heating the surface, acti-

vating atoms of the crystalline structure, and others. As a

result, most of the impinging ions are not able to produce

significant defects. Instead, the accelerated ions help activat-

ing the growing surface and to eventually produce the highly

crystalline structure. Indeed, highly crystalline carbon struc-

tures are effectively produced in the plasma.37

III. ARC DISCHARGE SYSTEMS

A. Principle of operation and plasma parameters

The arc discharge in vacuum or controllable gas atmos-

phere is one of the most promising techniques for the large-

scale production of various nanostructures with minimal

structural defects. This method combines a large yield with

relatively high controllability.108,109

A typical setup for the arc-assisted nanofabrication is

shown in Fig. 10. In this system, the temperature of the an-

ode surface may reach 3000–3500 K.110 An intense erosion

of electrode material in the discharge spots produces dense

thermally equilibrium plasma.111 In the discharge core, the

gas temperature may reach 5000 K. Therefore, the nanostruc-

tures growing in the plasma can be effectively heated up to

the temperatures required for the efficient crystal growth.

The application of an external magnetic field (e.g., by instal-

lation of the additional magnetic coil or permanent magnet

in the discharge zone, see Fig. 10) significantly increases the

plasma density and temperature.114 In this process called

magnetically enhanced synthesis, the plasma density is

increased by the two effects: first, by magnetic confinement

that restricts the plasma in the area close to the electrodes,

and second, by magnetizing electrons and thus creating con-

ditions for more effective ionization of the neutral gas

atoms.112 In turn, the plasma temperature increases in the

magnetic field due to stronger electric field in the magne-

tized plasma. This results in the increased heating of the

nanostructures in the plasma, and hence, an increased flux of

material to the surface. As a result, fast nucleation and

growth of the nanostructures is provided. Typical character-

istics of the arc discharge are summarized in Table II.

FIG. 10. (Color online) Schematic of the experimental setup and photo of the

discharge (inset). Graphene flake-containing material was collected from the

magnet surface (collection area), close to the discharge. A simulated two-

dimensional map of the magnetic field is shown on the background of the

discharge. Reprinted with permission from Levchenko et al., Carbon 48,4556 (2010). Copyright 2010, Elsevier Ltd.

TABLE II. Typical parameters of arc plasmas used for the nanofabrication.

Discharge power range (kW) 0.1–10

Plasma density (cm�3) 1011–1014

Electron energy in discharge (eV) 0.5–1.0

Discharge voltage (V) 30–100

Discharge current (A) 50–100

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B. Graphene production in arc discharge

In a typical arc-based process,113 the graphene flakes are

produced using carbon anode and cathode installed in the non-

magnetic vacuum chamber. The anode is a rod with a hole in

the center. The hole is filled with a mixture of graphitic car-

bon and catalyst (for example, Y-Ni powders in 1:4 ratio)

with the average particle size up to 1 mm. A discharge-

enhancing cube-shaped permanent magnet installed in the

chamber creates a nonuniform magnetic field of about 1 kG in

the interelectrode gap. An interelectrode gap may be varied in

the range of 1–10 mm. A typical topography of the magnetic

field is shown in Fig. 10. The process is conducted in a helium

gas at a pressure of 200–500 Torr.114 More details on the pro-

cess and setup design can be found elsewhere.113

Since the gas temperature in the magnetically enhanced

plasma may reach 5000 K, an intense evaporation of the

metal catalyst particles emitted to the plasma from the elec-

trode is sustained.115 In the remote zones of the discharge,

the gas temperature falls down, the metal catalyst renu-

cleates into small (several nm in diameter) particles, which

slowly grow in the plasma. When the temperature falls

below 1500 K, nucleation and growth of the graphene on the

small newly nucleated catalyst particles starts.

Graphene can be also produced in the catalyst-free arc dis-

charge process. In this case, a hydrogen-containing environment

should be used to terminate dangling bonds and thus prevent the

graphene sheets from rolling into nanotubes. Hydrogen can

effectively reduce oxides and carbides in the plasma, so the pro-

cess conducted in the hydrogen-enriched environment may

result in graphene doping with some dopants. On the other hand,

the catalyzed process in a noble gas environment makes the tech-

nique highly flexible with respect to any graphene compositions.

The deposition of the graphene nucleated and grown in

the arc discharge-based process can be controlled by using a

specially selected metal catalyst. In the typical experiments,

a mixture of the two transition metals was used. Yttrium,

which is a paramagnetic metal capable to easily form car-

bides and thus enable quick and efficient nucleation of gra-

phene, and nickel, which is a ferromagnetic metal with the

Curie temperature of about 350 �C, were used as cata-lysts.112,113 The metal catalyst nanoparticles are nonmag-

netic in the hot zone of the discharge. Then, the catalyst

temperature decreases below the Curie point outside of the

hot discharge zone, the catalyst particles become ferromag-

netic and change the trajectory in the external magnetic field.

Finally, these particles with the graphene flakes attached to

them deposit onto the specific surfaces of the collecting mag-

net. Therefore, a custom-designed catalyst composition led

to the graphene deposition onto the magnet surface.108,115

Figure 11 illustrates the results of characterization of the

typical graphene flakes collected from the surfaces of the

collection magnet. The size of the flakes reaches 2000 nm,

FIG. 11. (Color online) SEM, TEM, AFM, and Raman characterization of the pristine [(a) and (b)] and processed (c)–(g) samples collected from the magnet

surfaces. Representative SEM images of the carbon nanoparticles containing graphene flakes [(a) and (b)]; SAED pattern of the carbon material (c); TEM

image of the folding graphene layers (d); 2D [(e) and (f)] and 3D (g) AFM images of two graphene flake samples. Three profiles of the AFM image illustrate

the flake structure containing several graphene layers (total thickness 2–3 nm); Representative Raman spectra of the graphene flake samples (h). G-peak at

�1582 cm1 and symmetrical 2D-peak at �2650 cm1 demonstrate the presence of a relatively small number of graphene layers in the flakes. Reprinted withpermission from Levchenko et al., Carbon 48, 4556 (2010). Copyright 2010, Elsevier Ltd.

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and the flakes consist of typically up to 10 layers. Using the

atomic force microscopy (AFM), the flakes of about one

micron with a fairly uniform height in the 1–5 nm range

(Fig. 11), i.e., of 3–15 graphene layers were revealed. The

Raman characterization of the collected flakes has demon-

strated the presence of a D-peak at around 1325 cm�1, which

indicates relatively small amount of the structural defects in

sp2 bonds. The G-peak at 1582 cm�1 and highly symmetrical2D-peak at 2650 cm�1 evidence the occurrence of a rela-

tively small number of graphene layers.

The production yield of the graphene flakes in the arc dis-

charge can be estimated by the anode ablation rate reaching

(3–5)� 10�3 kg�min�1.108 Besides, the higher productionyield can be reached by increasing the arc discharge current,

since the carbon supply rate increases linearly with the cur-

rent.110 Therefore, the graphene production can be scaled up

to 1� 10�3 kg� h�1m�2 when the arc current of 100–150 Ais used.

C. Synthesis and separation of graphene and carbonnanotubes in arc discharge

The arc discharge plasma can be also used for the large-

scale production and simultaneous separation of the carbon

nanotubes and graphene flakes.116 In this case, a specially

designed discharge enhancing/separation magnetic unit

(DESMU) was used to collect the carbon nanostructures.

Also, the nanostructures were collected from the chamber

walls. In Fig. 12, one can see the schematic of the setup,

photographs of the discharge, as well as topography of the

magnetic field produced by DESMU and SEM images of the

typical carbon nanomaterials—ropes of nanotubes and gra-

phene fragments. The nanotubes and graphene flakes were

deposited on the magnet surfaces only, whereas lacey carbon

was found only on the chamber walls.

The size of the graphene flakes grown in such process

reaches 2500 nm, with the crystallographic faceting clearly

visible on some of the flakes. The flakes are usually covered

with amorphous carbon. The typical size of the metal par-

ticles found in the TEM images reached 10 nm. SEM charac-

terization of the deposits was used to roughly estimate the

production rate, which reaches 1� 10�4 m2� h�1 of gra-phene for the discharge current of 50 A, or 0.02 cm2 h�1A�1.

The numerous multiwalled carbon nanotubes were also

found in samples collected elsewhere on the chamber walls.

D. Growth of nanostructures in arc plasmas—theoretical examination

Now we will discuss the probable mechanism of the gra-

phene flakes and carbon nanotubes growth of in the plasma of

the magnetic field-enhanced arc. The theoretical and experi-

mental studies have proven the following scenario for the for-

mation of large multilayered flakes in the plasma. At the first

stage, a single layer graphene flake nucleates on the metal cat-

alyst particle and grows to a larger radius without formation

of any additional layers. At the second stage, the single layer

graphene flake reaches a critical size and a new layer nucle-

ates, thus forming a two-layer flake. Next, the base layer and

a newly nucleated second graphene layer grow together and

after some time a new (third) graphene layer is formed on the

top. This process stops when the flake is deposited on the col-

lection surface. It is quite evident that the main condition for

the formation of a large graphene flake consisting of a few

graphene layers is the low density of carbon atoms adsorbed

on the flake during the growth in the plasma. When the atom

density on the flake is low, the deposited carbon atom evapo-

rates from the flake growing in the plasma before another

atom deposits onto the flake. As a result, no nucleation of a

new layer is possible, and the graphene flake grows as a

single-layer fragment. If the flux of carbon atoms from the

plasma to the surface of the graphene flake exceeds the flux of

carbon evaporation, the carbon adatoms are accumulated on

the flake surface, the density of adatoms increases and a new

graphene layer nucleates. The critical graphene flake size can

be estimated by taking into account the main processes on the

graphene flake growing in the plasma. Specifically, the carbon

flux to the flake from the plasma, carbon flux from the flake

by surface diffusion to the flake edges, carbon evaporation

from the flake to the plasma bulk, and removal of the carbon

adatoms by direct impact of the gas molecules deposited onto

the flake from the plasma should be considered.

The flux of carbon material to the surface of a single-

layer graphene flake from the plasma can be estimated as

WS ¼ VincS; (12)

where Vi is the ion velocity, nc is the density of carbon ionsin plasma, and S is the graphene flake surface area.Assuming S¼ pR2 and the ion velocity equals to the Bohmvelocity VB¼ (kTe/mi)

1=2, where R is the graphene flake ra-dius, Te is the electron temperature, mi is the carbon ionmass, and nc¼ np where np is the plasma density, one canobtain

WS ¼ pR2npffiffiffiffiffiffiffiffiffiffiffiffiffiffikTe=mi

p: (13)

Next, the flux of carbon adatoms to the flake edges, We,can be estimated using the random walk distance relation

R2¼DS t, where DS is the surface diffusion coefficient, and tis the characteristic diffusion time. The surface diffusion

coefficient can be written as

DS ¼ ½k2a�0=4� � expð�ed=kTÞ; (14)

where ed is the surface diffusion activation energy, and T isthe graphene flake surface temperature. Combining the two

above described formulas one can obtain the expression for

the characteristic time of adatom diffusion on the graphene

flake

t ¼ ½4R2=k2a�0� � expðed=kTÞ: (15)

Since the flux of carbon adatoms to the flake edges is

�esc¼ 1/t, one can obtain

We ¼ ½k2a�0=4R2� � expð�ed=kTÞ: (16)

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Substituting an expression for the lattice oscillation fre-

quency �0¼ 2kT /h, where h is the Planck’s constant, onecan finally obtain an estimate for the We as

We ¼ ½kTk2=2hR2� � expð�ed=kTÞ: (17)

The carbon adatoms are also removed from the graphene

flake by the impact of gas molecules depositing from plasma

with an energy exceeding the carbon evaporation energy. The

frequency of this process is �C¼V0 n0 k2� exp(�ea/kT0), whereV0¼ (2kT0/m)

1=2 is the gas atom velocity, m is gas atom mass, eais the surface evaporation energy, n0¼P0/(kT0), P0 and T0 arethe gas density, pressure, and temperature. Thus, one can obtain

Wi ¼ k2P0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2=mkT0

p� expð�ea=kT0Þ: (18)

The most important parameter of the graphene flake, the

critical radius, can be calculated by the balance between the

four main fluxes, i.e., WS¼WP þWe þWi, where WP is theevaporation. It can be concluded from the above model that

the graphene flake temperature, plasma density, and the elec-

tron temperature in the plasma are the key parameters which

specify the size of a single layer graphene flake. The calcula-

tions made for the typical arc plasma density of 1019 m�3 and

the two typical electron temperatures of 0.1 and 0.5 eV have

shown that the above model reasonably predicts the size of the

graphene flakes grown in the arc discharge-based process.108

FIG. 12. (Color online) Experimental setup, photo of the plasma reactor and discharge, and SEM micrographs of representative graphene flakes. (a), (b)

Representative SEM images of the carbon deposit collected from different collection areas. Ropes of carbon nanotubes found on the top and side surfaces of

the DESMU, in the areas close to the discharge; graphene layers found on the top and side surfaces of the DESMU, in the areas remote from the discharge. An

effective separation of the two different carbon nanostructures was ensured. (c) Schematic of the experimental setup. (d) Photograph of the experimental setup.

(e) Schematic of the mutual position of the cube-shaped magnet, anode and cathode, and the computed 2D map of the magnetic field (field strength of 1.2 kG

in the discharge gap was optimized for the highest yield of both graphene and nanotubes. (f) Consecutive photographs of the discharge development in the

nonuniform magnetic field. At the first moments after ignition (t1), the discharge is localized between the cathode and anode. Later (t2, t3, and t4), plasma

plume becomes unsymmetrical and pulls (due to the interaction with magnetic field) toward the magnet of the discharge enhancing/separation magnetic unit.

Reprinted with permission from Volotskova et al., Nanoscale 2, 2281 (2010). Copyright 2010, The Royal Society of Chemistry.

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Similar dependencies can be obtained for the growth of the

carbon nanotubes in arc discharge plasmas, assuming that the

nanotubes grow on the partially molten metal catalyst particle

supplied to the plasma from the ablated electrode (Fig. 13).

The metal particle immersed in the plasma is a subject of the

significant heating and ablation, which reduce the particle size

and eventually lead to the formation of an external liquid

shell. The carbon atoms deposit on the surface of the particle,

diffuse through the liquid shell, and eventually incorporate

into the nanotube. The ion flux at the sheath border–nanotube

surface supplies carbon atoms to the nanotube. Upon recombi-

nation, carbon adatoms migrate about the nanotube surface

and eventually reach the molten catalyst shell or re-evaporate

to the plasma bulk. Presently, the two growth scenarios are

accepted: the vapor–liquid-solid117,118 and solid–liquid–-

solid.119 Both scenarios include the diffusion of the carbon

atoms in the molten shell, and thus the carbon supply to the

external catalyst surface is a decisive factor that determines

the growth kinetics of the nanostructure. To evaluate the car-

bon supply to the catalyst, one should use a diffusion model

described in the Sec. II F devoted to the growth of carbon

nanostructures on the surface.

To understand the experimental fact of the nanotube length

increase in the magnetically enhanced discharge, one should

take into account that at higher plasma densities (up to for

5� 1018 m�3) the sheath around the nanotube is narrow, andthe ion focusing becomes much stronger [Fig. 13(b)]. The

number of ions deposited directly on the catalyst surface

increases, and the adatoms should pass a longer distance along

the nanotube. As a result, the re-evaporation loss decreases,

and the length increases.113 At lower plasma density, the

sheath between the plasma and surface of the nanotube is

large, and the ion flux is distributed nearly uniformly on the

surface. The catalyst immersed in the plasma adsorbs some

fraction of the total ion flux deposited on the nanotube, and

the rest atoms re-evaporate from the nanotube surface.

IV. CONCLUDING REMARKS

A. Unique features of the plasma environment

As compared with the conventional neutral gas-based

process environment, the plasma-based process offers sev-

eral features important for the nanofabrication. The principal

advantage is very high (and highly controllable) energy den-

sity in the process area, which allows effective control over

the energy and material supply to the nanostructures.

Moreover, high energy density ensures unique possibilities

for the bulk synthesis of various precursors, which can be

used for the further deposition onto the nanostructures grow-

ing on the surfaces, and for the nanostructure nucleation and

growth in the plasma bulk. Normally, an external heating of

the growth surfaces can be avoided in the plasma-based pro-

cess, thus keeping the substrate material intact.

One more significant feature of the plasma-based tech-

nique is the involvement of the charged particles (ions and

electrons) and strong electric field in the process. As a result,

proper selection of the process parameters such as the

plasma density, electron temperature, and surface potential

could be a powerful tool for the precise control of the mate-

rial deposition onto nanostructures. Magnetic fields could

also be used for controlling the process parameters (plasma

density and temperature) and trajectories of the growing

nanostructures moving in the plasma bulk. These ways of

control are impossible in the neutral gas-based techniques.

A very high plasma density in the arc discharge makes it

very promising for the large scale, bulk production of vari-

ous nanoparticles and nanomaterials, including highly

demanded nanotubes and graphene. In contrast, neutral proc-

esses are usually based on the surface nucleation and growth,

which are substantially two-dimensional and hence, exclude

most of the process environment from participation in the

nanoscale synthesis and processing.

One more important feature of the plasma-based tech-

nique is the possibility to selectively produce specific carbon

allotropes during the growth. Indeed, several carbon allotro-

pies are important for various applications. In some special

cases, a complex combined platform involving two or more

allotropes should be created in a single process (e.g., amor-

phous carbon and dense pattern of carbon nanotubes for the

immobilization and protection of the biocatalysts).120 A low-

temperature plasma-based process provides unique possibil-

ities to fabricate such sophisticated platforms. Moreover, a

possibility to fabricate and separate several carbon allotropes

(e.g., carbon nanotubes and graphenes) was discussed above.

It should be noted that it is not easy to specify the key pa-

rameter for every specific carbon allotropy to emerge in any

FIG. 13. (Color online) (a) Growth of single-walled nanotube (SWNT) on

molten metal catalyst particle in a plasma. Carbon flux from the plasma is

nonuniformly distributed about nanotube and catalyst particle surface.

Carbon adatoms diffuse to the catalyst end, and then incorporate into nano-

tube structure through the molten catalyst shell. The nanotube diameter is

determined by the size of the molten catalyst core. Reprinted with permis-

sion from Keidar et al., Appl. Phys. Lett. 92, 043129 (2008). Copyright2008, American Institute of Physics. (b) Distribution of carbon ion flux on

nanotube surface with the plasma density and nanotube length as parame-

ters. Reprinted with permission from Keidar et al., J. Appl. Phys. 103,094318 (2008). Copyright 2008, American Institute of Physics.

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particular system. Nevertheless, the plasma environment

allows precise control over the most important parameters

such as the gas and surface temperatures and the rates of ma-

terial delivery and removal, and eventually, to adjust the syn-

thesis process for the specific allotrope. In particular, high-

purity graphenes were produced by controlling the plasma

parameters such as the electron temperature (e.g., raising it

up to �2 eV) using an external magnetic field and henceincreasing both the surface temperature and the material

delivery rate; the same setup without magnetic field was ca-

pable to produce only the carbon nanotubes.108 The process

conducted at lower discharge current and low electron tem-

perature produces only amorphous carbon.

B. Surface-based versus bulk growth of thenanostructures in plasmas

Along with the common features such as a high energy

density and involvement of charged species and electric fields,

various kinds of plasmas offer different parameters and hence

are better suited for either surface-based (inductively coupled

and microwave discharges) or bulk (arc discharge) growth of

the nanostructures. The main differences in the ICP/MWP

and arc plasmas are the operating pressure and plasma density

(see Tables I and II). The density of arc plasmas may reach

�1014 cm�3 versus �1012–1013 cm�3, which is common forICP discharges, and the highest operating pressure for the arc

discharge reaches 1 atm and even higher for some arc types.

As a result, quite different conditions for the nucleation and

growth are present in ICP/MWP and arc plasmas. Relatively

low material flux in ICP/MWP systems is a provision for their

preferential use in the surface-based techniques where accu-

mulation of the material on the substrate surface is a prerequi-

site for the nucleation and the following growth of

nanostructures. In this case, the growth is due to material

fluxes delivered directly from the plasma and also from the

surface accumulating all material supplied from the plasma-

surface sheath. Other applications of the ICP/MW plasmas

involve treatment of the nanostructures, modification of their

crystalline structures, and postprocessing of the surface-bound

nanostructures for their activation, etc.

In contrast, the arc plasmas appear to be promising for the

nucleation and growth of the nanostructures in the bulk. In this

case, material is supplied from the plasma and accumulated

only on the surface of the growing nanostructures (no material

is supplied from the plasma-immersed substrate by surface dif-

fusion), and thus, the high density of the plasma is the key rea-

son why the bulk nanosynthesis in arc discharge is effective.

Along with this, arc plasmas can also be used for the fabrica-

tion of nanostructured and hard films and coatings, e.g., nano-

crystalline diamond. Moreover, process control using magnetic

field is one more feature of the arc discharge, which makes it

possible to simultaneously fabricate and separate different

nanostructures directly in the process environment.

C. Plasma benefits for other nanomaterials

Though this review paper is mainly focused on the fabrica-

tion of carbon-based nanostructures in the low-temperature

plasma environment, the plasma benefits for the fabrication of

other (noncarbonous) nanomaterials are also worth of mention-

ing. Among others, the ability of plasma-based processes to pro-

duce highly crystalline materials such as single-crystal silicon

nanostructures121 and single-crystalline SiC/AlSiC core-shell

nanostructures122,123 should be stressed. Very high rate of

energy supply to the growth area and highly controllable mate-

rial supply ensure the formation of the surface-based nanostruc-

tures of the highest quality with the high level of

morphodimensional selectivity.124,125 The electric-field driven

self-organization on solid surfaces is one more unique feature of

the plasma-based processes.64,126,127 In particular, the material

fluxes on the surface, which are the main driving force of the

self-organization of surface-based nanostructures such as quan-

tum dots and nanocones, can be effectively controlled by the

electric field and eventually, by the plasma parameters.128–130

D. Future trends in the development of plasma-basedsystems

Analysis of the plasma-based techniques for nanoscale

synthesis demonstrates that the plasma has several features

as a nanofabrication environment such as good controllabil-

ity and high energy and material densities in the process vol-

ume. Owing to these features, many unique nanostructures

have been synthesized. It is quite natural to suppose that the

controllability and high output of the plasma-based systems

can be further improved. Specifically, the following trends in

the development of the plasma-based systems for nanofabri-

cation should be noted:

1) Enhancing controllability of the existing systems by the

development of new control tools, instruments, and meth-

ods enabling sophisticated management of power and

particle fluxes to the substrates and nanostructures;

2) Development of new systems ensuring even higher level

of controllability, especially in managing the precise dep-

osition of the fabricated nanostructures onto surfaces, as

well as plasma–chemical synthesis of new compounds.

As an outlook it should be pointed out that one of the

most critical areas where research is needed is basic under-

standing of the plasma-based technique by utilizing

advanced experimental techniques and numerical

simulations.131,132

ACKNOWLEDGMENTS

This work was partially supported by CSIRO’s OCE

Science Leadership Research Program, CSIRO Sensors and

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