C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7

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Optimizing substrate surface and catalyst conditions for highyield chemical vapor deposition grown epitaxially alignedsingle-walled carbon nanotubes

Imad Ibrahim a,b,*, Alicja Bachmatiuk a, Felix Borrnert a, Jan Bluher b, Ulrike Wolff a,Jamie H. Warner d, Bernd Buchner a, Gianaurelio Cuniberti b,c, Mark H. Rummeli a,c

a IFW-Dresden e.V., PF 270116, 01171 Dresden, Germanyb Institute for Materials Science and Max Bergmann Center of Biomaterials, Technische Universitat Dresden, D-01062 Dresden, Germanyc Technische Universitat Dresden, D-01062 Dresden, Germanyd Department of Materials, University of Oxford, Parks Rd., Oxford OX1 3PH, United Kingdom

A R T I C L E I N F O

Article history:

Received 11 April 2011

Accepted 12 July 2011

Available online 22 July 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.07.020

* Corresponding author at: Institute for MateD-01062 Dresden, Germany. Fax: +49 0351 46

E-mail address: imad.ibrahim@nano.tu-d

A B S T R A C T

Single-crystal stable-temperature (ST)-cut quartz substrates, which have a (0 1 1 1) crystal-

lographic plane with their surface normal lying close to 38� from the y axis ([0 1 0]), were

annealed in air prior to use as a support for aligned carbon nanotube growth by chemical

vapor deposition. Very smooth substrate surfaces were obtained with annealing times in

the vicinity of 15 h at a temperature of 750 �C. These smooth surfaces are ideal for the

growth of horizontally aligned SWCNTs with high spatial density, while less dense

SWCNTs were obtained with less smooth surfaces. Under optimized growth conditions,

only SWCNT are observed and they can grow to lengths in excess of 100 lm. Our findings

suggest structural defects interfere with the growth process. A binary Fe/Co catalyst was

employed to grow the nanotubes. No obvious dependence on the Fe:Co ratio is observed.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Single-walled carbon nanotubes (SWCNTs) are considered to

be a potential material for next-generation nano-electronics

because of their physical and electrical properties [1,2]. Their

potential as key components for devices has already been

proven in a variety of systems including, field-effect transis-

tors [3], logic circuits [4], biosensors [5], environmental and

medical sensors [6], photo-detector and electrodes in electro-

chemistry [7,8]. If SWCNTs are to be used for large-scale elec-

tronics it is essential to control their spatial position,

orientation, yield and electronic type [9]. Moreover, the con-

trolled synthesis of arrays of well-aligned (horizontal) dense

SWCNTs on substrates is also important in this sense [10].

er Ltd. All rights reserved

rials Science and Max Be59 313.resden.de (I. Ibrahim).

Among the different CNT synthesis techniques, the chemical

vapor deposition (CVD) method has been shown to be the

most promising for vertically or horizontally aligned SWCNTs

on single crystal substrates [11–15]. CVD is not only a versatile

synthesis route but offers the possibility of easy scaling-up

[16]. Many investigations have explored different approaches

to grow and control oriented SWCNTs horizontally aligned on

substrates. These approaches include the use of low gaseous

fluxes [17,18], electric fields [19–21], and single crystal sub-

strates (e.g. quartz and sapphire) with specific surface orien-

tations [22–26].

An important goal behind many of these studies is to

maximize the density of the grown SWCNTs. Some examples

are: to pattern the catalyst nanoparticles on the substrate in

.

rgmann Center of Biomaterials, Technische Universitat Dresden,

5030 C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7

well-defined areas, successive CVD processes in which new

catalyst nanoparticles are added in each single process [27–

29], adding sulfur to the reaction [30], or using different metal

catalyst nanoparticles or mixtures of metals [31,32]. Thermal

annealing of the substrates prior the CVD process is an often

implemented step to improve yield. Nevertheless, the role of

the annealing step has not been fully investigated [33]. Rut-

kowska et al. [34] found that the best conditions for thermal

annealing in order to improve the alignment of the grown

SWCNTs on ST-cut quartz substrates to be 30 min at 950 �C.

In contrast, Xiao et al. [35] reported optimum conditions for

the same type of substrates of 8 h at 875 �C. Other differing

annealing conditions are also reported [10,36].

In this study we systematically investigate the effect of the

annealing step on the morphology of the used ST-cut quartz

substrates and find it affects the smoothness of the surface.

We show here that thermal annealing dramatically enhances

the density and length of the as-grown and well-aligned

tubes. In addition, this dependency is shown to affect the size

distribution of the catalyst nanoparticles which in turn af-

fects their propensity to nucleate SWCNT. The catalyst mix

(Fe:Co) is shown to be unimportant parameter for the high

yield synthesis of horizontally aligned SWCNTs on ST-cut

quartz.

2. Experimental

Mixtures of ferrocene (Sigma Aldrich, P 98%) and cobalt (II)

phthalocyanin (Fluka Chemie, > 97%) were used as the cata-

lyst source with different molar ratios (1:1, 1:2 and 2:1). The

mixtures then were dissolved in ethanol (Merck, P99.5%) to

prepare a solution with a concentration of 0.01% catalyst

source which was drop coated on the support surface. ST-

cut quartz substrates prepared from ST-cut single crystal

quartz wafer (10 cm diameter, 0.5 mm thickness, angle cut

38�00 0, seeded, single side polished from Hoffman Materials,

LLC) were used as support. The substrates were subjected to

different annealing times in air at 750 �C in a 1 in. diameter

horizontal quartz tube furnace prior to drop coating the cata-

lyst source. Once coated, the substrate plus coating was an-

nealed in H2 (H2 = 99.9%) at 950 �C with a flow of 0.039 LPM

to decompose the ferrocene and cobalt (II) Phthalocyanin

yielding Fe:Co nanoparticles (catalyst particles) [37]. Thereaf-

ter, the optimized CVD reaction was conducted as follows: the

hydrogen flow rate was reduced to 0.013 LPM and methane

was introduced at a flow rate of 1.12 LPM for 15 min. Finally,

the gas flows were turned off and the reactor evacuated with

a membrane pump (ca. 1 mbar) while the oven cooled down to

room temperature. Separate experiments to characterize the

catalyst particles prior to CVD growth were conducted by

removing substrates after the H2 preatreatment (after first

cooling down in Ar). The substrates, catalyst particles and

as-produced SWCNTs were characterized in terms of their

morphology, yield, length, diameter, alignment and homoge-

neity with atomic force microscopy (Digital Instruments Vee-

co, NanoScope IIIa), scanning electron microscopy (FEI, NOVA

NanoSEM 200, with typical acceleration voltage of 3 kV), and

transmission electron microscopy (FEI, Tecnai T20, operating

at 200 kV). The electronic properties and quality of the

SWCNTs were also characterized using Raman spectroscopy

(Thermo Scientific, DXR Smart Raman) with excitation laser

wavelengths of 780 nm, 633 nm and 532 nm.

The as-grown SWCNTs were transferred from the original

ST-cut quartz substrate to target Si/SiO2 substrates as well as

onto standard Cu TEM grids using a transfer route similar to

that described elsewhere [38].

3. Results

3.1. The effect of Thermal annealing on the ST-cut quartzsurface roughness

The effect of thermal annealing on the surface morphology of

ST-cut quartz substrates in air was systematically investi-

gated. Annealing periods between 10 min and 48 h with tem-

peratures from 700 to 800 �C in air were explored. Fig. 1 shows

typical examples of tapping mode AFM topography images

from various surfaces after different annealing periods. From

the AFM images one can observe that the surface morphology

changes with annealing period exhibiting a lot of surface

structure initially which at first lessens with annealing (e.g.

panel c) and then increases with extended annealing times

(e.g. panel e).

To better investigate the surface roughness one can look at

the height profiles from cuts on the AFM images. Fig. 2 panel a

shows typical line profiles from samples annealed at 750 �Cfor different periods. The changes in the surface roughness

are clearly observable, and confirm that the annealing process

first smoothens the surface, but that with longer annealing

periods the surface becomes rough again. Panel b shows the

average surface roughness for all the samples and shows this

trend in more detail. It shows the smoothest surface is ob-

tained with an annealing time of 900 min (15 h).

3.2. Catalyst nanoparticle characterization

We first investigated if any relationship existed between the

annealing of the substrates surface roughness and the size

distribution of binary catalyst systems for different iron and

cobalt ratios. This was accomplished by measuring the height

(AFM) of the catalyst particles prepared on the substrates after

they were annealed. Overall, the particles range between 2 and

18 nm. A more detailed study is presented in Fig. 5 panels a, b

and c, in which the height distributions for Fe:Co = 1:1, 1:2 and

2:1 are presented for substrates subjected to different anneal-

ing times, respectively. Generally, regardless of catalyst mix,

the diameter distribution flattens with increasing annealing

time. This is probably related to the surface roughness [39].

This is further supported by SEM and AFM studies which re-

vealed fewer catalyst particles resided on substrates annealed

for long times as compared to those annealed for short peri-

ods. Given the same amount of catalyst material was placed

on all substrates initially, this intuitively suggests larger parti-

cles form on substrates annealed for longer times.

For samples annealed up to 15 h the surface roughness is

composed of two types of pits; broad and deep pits, and narrow

and shallow pits. The narrow and shallow pits tend be super-

imposed on the broad pits. As one anneals up to 15 h the broad

pits disappear. However, above 15 h annealing, the narrow pits

start to disappear and broad pits form. These pits changes are

Fig. 2 – Surface roughness of the thermally annealed

substrates: (a) typical height profiles for ST-cut quartz

annealed in air at 750 �C for different periods and (b) time

dependence of the roughness of annealed ST-cut quartz

substrates surfaces (curve is a guide to the eye).

Fig. 1 – Morphological characterization of the thermally annealed substrates: tapping mode AFM topography images of ST-cut

single crystal quartz substrates annealed in air at 750 �C for (a) 5 h, (b) 10 h, (c) 15 h, (d) 20 h and (e) 24 h (height scale shown to

the right is the same for all images).

C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7 5031

concomitant with the observed catalyst particle diameter dis-

tribution changes. In panel d the mode diameter and the full

width at half maximum (FWHM) from a Gaussian fit presented

as the error bars are shown. It shows the mode diameter in-

creases with annealing time and the FWHM shows a broaden-

ing of the diameter distribution with annealing time. There is

almost no difference between the three catalyst ratios ex-

plored. This is in agreement with studies by Liu et al. [32].

3.3. As-grown CNT characterization

In order to investigate the relationship between the surface

roughness and the yield of horizontally aligned SWCNTs.

The various annealed substrates were used as supports in

the CVD process using a Fe:Co binary catalyst (1:2). SEM obser-

vations of the samples showed varying densities of aligned

SWCNTs on the surfaces after the CVD reaction. Their align-

ment follows the x-direction ([1 0 0]). Representative exam-

ples are provided in Fig. 3. The data indicate one can tune

the tubes density with annealing time. Detailed studies of

the SEM images allowed us to determine the number of tubes

per unit area on the samples. In addition this was performed

for different Fe:Co catalyst ratios (1:2, 1:1 and 2:1). The data is

plotted in Fig. 4. The profile is almost the direct inverse of

Fig. 2, in other words there is a direct correlation between

the aligned SWCNTs density per unit area (yield) and the de-

gree of substrate surface smoothness. Moreover, there is no

observable difference between the different catalyst mix-

tures. The optimum yield in this study is obtained with an

annealing time of 900 min (15 h) which corresponds also to

the smoothest surface (Fig. 2b).

As mentioned above, the Fe:Co ratio does not affect the

yield of aligned SWCNTs, e.g. Fig. 5. This is also observable

in Fig. 6 (top row) in which typical SEM micrographs of aligned

SWCNTs grown using each catalyst ratio on ST-cut quartz

substrates annealed for 15 h at 750 �C are presented. In addi-

tion, Raman spectroscopy and AFM investigations indicate

the Fe:Co catalyst ratio does not affect the resultant diameter

of SWCNTs. Raman spectroscopy is a powerful technique to

analyze carbon nanotubes through the G mode (tangential

phonon modes), the D band (disorder-induced feature) and

the well-known radial breathing modes (RBM).

Raman spectra from the samples show a strong and nar-

row G band and weak D band which is typical for SWCNTs.

The RBM mode can be used to investigate the nanotube diam-

eter (dt [nm]) through its frequency (xRBM [cm�1]) using the fol-

lowing relationship: [40]

xRBM ¼ a=dt; where a ¼ 248 ½cm�1 nm�

Fig. 3 – Size of the formed catalyst on annealed substrates: height distributions of Fe:Co nanoparticles spread over ST-cut

quartz substrates annealed at 750 �C for different periods using different Fe:Co ratios (a) 1:1, (b) 1:2, (c) 2:1 and (d) trend of

nanoparticles size as a function of substrate annealing time (height of nanoparticles were measured with AFM. Number of

counted nanoparticles is 200 for each measurement. Note: the line is a guide to the eye).

5032 C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7

The diameter of the as-produced SWCNTs calculated from

the RBM modes measured using the excitation wavelengths

(780 nm, 633 nm and 532 nm) range from 1.33 to 1.52 nm

regardless of the catalyst ratio (e.g. Fig. 6 lower row). However,

one should bear in mind Raman spectroscopy in this case has

limitations to accurately determine the diameter distribution,

not only because of the limited number of excitation lasers

used, but also overlapping signals from the substrate make

identification more complicated. Hence supporting tech-

niques were implemented. AFM studies showed diameters

ranging from 0.8 to 1.7 nm (see Supplementary information

Fig. S2). The larger diameters measured in AFM could arise

from either bundles of SWCNTs and/or multi-walled NT.[2]

In order to investigate this point in greater detail TEM is an

ideal tool.

3.4. Transferred SWCNTs

Preparing samples for TEM analysis is, however, not straight

forward. Samples can be prepared using transfer techniques.

In addition, the transfer of horizontally aligned SWCNTs to

silicon substrates is an essential step when using them to fab-

ricate molecular electronic devices [41]. In this study we used

a transfer route similar to one described by Tabata et al. [38].

Fig. 7 (top row) shows SEM and AFM micrographs and corre-

sponding Raman spectrum of high-yield as-produced hori-

zontally aligned SWCNTs on ST-quartz (15 h annealing). The

lower row of Fig. 7 shows the SWCNTs after having been

transferred onto a silicon substrate. The SEM and AFM micro-

graphs confirm the transfer process does not affect the den-

sity or the alignment of the SWCNTs. The Raman spectra

Fig. 4 – Yield of the grown SWCNTs on annealed substrates: SEM images showing different yields of SWCNTs grown on ST-

cut quartz substrates annealed at 750 �C: (a) 10 min, (b) 15 h, (c) 48 h (black arrows indicate the x-direction ([1 0 0]) of the ST-cut

quartz substrate).

Fig. 5 – Yield of grown SWCNTs from different catalyst

mixtures on thermally annealed substrates: number of

SWCNTs per unit area dependence on annealing time of ST-

cut quartz substrates annealed at 750 �C (curve is a guide to

the eye).

C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7 5033

show that the transfer procedure does not reduce the quality

of the samples since the G band is not broadened and no D

band is measurable. These findings are similar to those found

by Ding et al. [42].

The same transfer procedure was used to transfer the

SWCNTs onto standard Cu and lacey carbon TEM grids.

Fig. 8 shows representative images of the obtained tubes.

Only individual SWCNTs were observed. The lengths of the

tubes ranged from a few microns to over 100 lm. Their diam-

eter ranged between 0.8 nm and 1.7 nm. The data show our

synthesis route is well-suited for homogeneous high yield

horizontally-aligned SWCNTs formation.

4. Discussion

a-Phase quartz, while stable at room temperature transforms

into b-phase quartz at around 574 �C. In the b-phase the

atoms preferentially align in the x-direction ([1 0 0]). This x-

direction alignment and the weak anisotropic van der Waals

interaction between SWCNTs and surface atoms are usually

argued to provide the alignment mechanism for aligned

SWCNTs on specific surfaces [43,34]. Some suggested step

sites on the quartz surface are responsible for the alignment

during growth [33]. Our data as we will show below, indicate

step sites are not responsible.

The initial application of an annealing process to the sub-

strate in essence provides energy for atoms to re-arrange

themselves toward a new energy minimum [44]. In addition,

some surface atoms can escape. This leads to the surface

becoming smoother. However, continuing the annealing pro-

cess a degree of disordering due to atom displacement occurs

[45]. Moreover, bond breakage can occur and this leads to

microcrack formation which in turn creates a rough surface

[46]. Hence, to obtain a smooth surface the optimum anneal-

ing time needs to be determined. In our case we find the

smoothest surface to occur after 15 h annealing at 750 �C.

XRD and Raman spectroscopic data show no significant

change to the bulk crystal structure (e.g. Fig. S1 in the Supple-

mentary information Fig. S2). We also studied the catalyst

particle size dependence on the substrate roughness. The

data point to a trend in which the catalysts mode size in-

creases with annealing time and that the diameter distribu-

tion broadens with annealing time. The mode and

broadening effects correlate with the size of the pits formed

on the substrate surface. However, a direct correlation with

the catalyst sizes with the resultant SWCNTs is less easy to

extract. Numerous studies point to a direct correlation be-

tween the diameter of SWCNTs and that of the catalyst parti-

cle [16,47,48]. In this study, the particles are significantly

larger, at least prior to synthesis. The measured particle sizes

are an over-estimate of their size when in the reaction since

they are measured in ambient air and so are oxidized. Oxida-

tion can expand them by as much as 32% [49]. None-the-less,

this would still leave particles too large to nucleate the forma-

tion of SWCNTs. We suspect that the particles, upon heating,

melt and break up into smaller particles much as thin films

do [50–53]. This process will be hindered on rough surfaces

since the molten catalyst material becomes trapped within

valleys or pits and cannot easily break up. This will reduce

the likelihood of nucleating a SWCNT and hence the yield of

SWCNTs will drop on rough surfaces, exactly as we observe.

Fig. 6 – SEM and Raman characterization of the grown SWCNTs: (top row) SEM images and (bottom row) Raman signal

collected with excitation wavelength of 780 nm of horizontally aligned SWCNTs grown on ST-cut quartz annealed at 750 �Cfor 15 h using different Fe:Co catalyst ratios: (a) 1:1, (b) 1:2 and (c) 2:1, (insets) zoomed Raman signal in the range of RBM of

SWCNTs (red curves are for plain ST-cut quartz substrates, and the green ones are for the grown SWCNTs on ST-cut quartz

substrates). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this

article.)

Fig. 7 – As-grown and transferred SWCNTs: (a)–(c) SEM, AFM images and Raman spectroscopy respectively of SWCNTs as

grown on ST-cut quartz substrate annealed at 750 �C for 15 h using Fe:Co catalyst mixture of ratio 1:2. (d)–(e) SEM, AFM images

and Raman spectra respectively of transferred SWCNTs to silicon substrate. (Insets) Zoomed Raman signal in the range of

RBM of SWCNTs (White arrows indicate the x-direction of the ST-cut quartz substrate, Raman signals were collected with

excitation wavelength of 780 nm).

5034 C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7

With regards ternary catalysts, the ratio of the metals used

can affect the yield [54]. In this study there is no observable

dependence with the Fe:Co ratio in agreement with studies

by Liu et al. [32].

In this work we also find that we obtain the highest length

and yield of aligned SWCNTs when the substrate is at its most

smooth. With the optimal annealing (15 h) the tubes are mostly

longer than 100 lm. It is worth noting other reports indicate

even longer lengths can be accomplished. This is highlighted

by data from our work and from Refs. [33,34] presented in Table

1. As can be seen in Table 1, for equivalent cuts different angles

can exist [34] as well as different cuts. [33] In addition different

Table 1 – Growth parameters used in this work compared to those used in other reports.

Parameter This work Ref. [34] Ref. [33]

Synthesis method CVD CVD CVDCatalyst Fe, Co (mixtures with

different ratios)Ferritin Ferritin

Substrate ST cut quartz ST cut quartz AT-cut quartzAngle form the y axis 38� 42� –Annealing conditions 750 �C, 900 min, in air,

heating rate (100 �C/min),slow cooling (<5 �C/min).(optimized)

950 �C, 30 min, in air, heatingrate (10 �C/min)

900 �C, 7 h

Carbon source Methane (1.12 LPM) Methane MethaneOther used gases H2 (0.013 LPM) H2 H2

Growth temperature 950 �C 875 �C 900 �CGrowth time 15 min Up to 30 min 10 minHeating and coolingrates in growth

Heating (100 �C/min) in H2

environment, slow cooling(<5 �C/min)

Heated to 700 �C in 10 min,then till 875 �C over thefollowing 10 min

Heating rate (�C/min),cooling rate (<5 �C/min)

Tubes density (lm�1) 1.1 4.3 >10Tubes length >100 lm (optimized) Up to 400 lm 100 lm

Fig. 8 – TEM characterization of the grown SWCNTs: TEM micrographs of SWCNTs grown on ST-cut quartz substrates

annealed using optimum conditions and then transferred to (a) and (b) copper grids and (c) lacey grid.

C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7 5035

catalysts, and synthesis parameters (e.g. temperature and flow

rates) can be used. All these factors may also play a role in the

ultimate length and density of eptaxially grown SWCNT. In the

specific case of our study in which systematic pre-anealing

periods are investigatedwe find that for shorter annealing peri-

ods (<15 h) the tube lengths vary between 20 and 60 lm while

for periods above 15 h the tube lengths from a few lm to ca.

40 lm (see Fig. S3 in supporting information). The alignment

occurs in the x-direction ([1 0 0]). The data indicate that the

thermal treatment of the ST-cut quartz substrates prior to

the CVD process dramatically affects the length of the grown

tubes on those annealed substrates. In addition, we find no evi-

dence of step sites on these smooth surfaces which suggests

step sites are not important for the alignment process and sup-

ports the argument for weak van der Waals interactions be-

tween the SWCNT and preferentially aligned surface atoms

in b-phase quartz directing growth. The data also show that

rough surfaces block the SWCNTs growth similar to other stud-

ies in which trenches were shown to hinder growth [34].

5. Conclusion

Investigations on the influence of the pre-annealing of ST-cut

quartz substrates on the yield and length of epitaxially aligned

SWCNTs were carried out. The findings point to the preferen-

tially aligned surface structure of b-phase quartz and the weak

anisotropic van der Waals interaction with SWCNTs providing

the guidance mechanism. Structural defects in the surface

interfere with the guidance process, probably by either block-

ing further growth or altering the growth direction. This is fur-

ther evidenced by the ability to obtain tubes in excess of 100 lm

when using substrates treated with optimized annealing

(smooth surface). In addition, the surface roughness can hin-

der catalyst break-up into small particles suitable for nucleat-

ing SWCNTs. This effect is minimized on smooth surfaces

increasing the yield of SWCNTs. The catalyst (Fe:Co) ratio does

not influence the yield. Finally, unlike other techniques which

include at least a few multi-walled carbon nanotubes, when

using our optimized substrates only SWCNTs are found.

Acknowledgements

II thanks the DAAD (A/07/80841), A.B. thanks the Alexander

von Humboldt Foundation. F.B. thanks DFG (RU 1540/8-1).

M.H.R. thanks the EU (ECEMP) and the Freistaat Sachsen.

The authors would like to thank Agnieszka Rutkowska and

Jens Kunstmann for helpful discussions.

5036 C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7

Appendix A. Supplementary data

Raman spectroscopic data, particle size distributions from var-

ious annealed substrates and SWCNT diameter distributions.

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2011.07.020.

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