optimizing substrate surface and catalyst …...most promising for vertically or horizontally...
TRANSCRIPT
C A R B O N 4 9 ( 2 0 1 1 ) 5 0 2 9 – 5 0 3 7
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ava i lab le a t wwwjournal homepage: www.elsevier .com/ locate /carbon
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: [email protected]
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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