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Materials Science in Semiconductor Processing 38 (2015) 113–118
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing
journal homepage: www.elsevier.com/locate/mssp
Synthesis of vertically aligned flower-like morphologies
of BNNTs with the help of nucleation sites in Co–Ni alloy
Pervaiz Ahmad, Mayeen Uddin Khandaker n, Yusoff Mohd Amin
Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
PACS:
81.07.De
Keywords:
h-BN
BNNTs
Vapor deposition
Vertically aligned
abstract
A Co–Ni alloy deposited at the top of Si substrate produces nucleation sites when etches
with ammonia. The as-produced nucleation sites are used as a pattern to grow flower-like
morphologies of vertically aligned Boron nitride nanotubes (BNNTs) in the present study.
HR-TEM micrographs show black-blur like morphology on the outer part of the BNNTs.
The sample contains BNNTs with and without internal bamboo like structures. XPS survey
shows B 1s and N 1s peaks at 191 eV and 398 eV that indicate the presence of Boron
and Nitrogen components in the synthesized sample. Raman spectrum shows a major
peak at 1370 cm 1 that corresponds to E2g mode of h-BN. The as-synthesized BNNTs can
be used for its potential applications without any further purification.
& 2015 Elsevier Ltd. All rights reserved.
1. Introduction
BNNTs are one of the basic building blocks in nanotechnology. Their diameter independent electronic properties
have covered the deficiencies of carbon nanotubes (CNTs)
for different applications in the field of microelectronic
mechanical systems (MEMs) [1]. The properties of BNNTs
are almost similar to CNTs, however, CNTs can be conductor
or semiconductor depends on the chirality or helicity,
whereas BNNTs are wide band gap semiconductor independent of helicity [2–4]. The excellent electrical and mechanical
properties of the BNNTs have made it a very important
material for different applications in the scientific world of
nanotechnology. It has been successfully explored for its
potential applications in the field of engineering ceramics
and polymeric composites [5]. The possible role of BNNTs as
an insulating protective shield has also been observed in the
development of nanocables from semiconductor nanowires
[6–12]. It is experimentally observed that the superplasticity
of engineering ceramics increases to a great extent with the
addition of BNNTs [13]. Due to the bipolar nature of B–N
n
Corresponding author. Tel.: þ60 1115402880; fax: þ 60 379674146.
E-mail address: [email protected] (M.U. Khandaker).
http://dx.doi.org/10.1016/j.mssp.2015.04.017
1369-8001/& 2015 Elsevier Ltd. All rights reserved.
bond, BNNTs showed stronger adsorption of hydrogen.
Therefore, they are considered a very important material
for hydrogen storage applications [14–18]. Furthermore,
BNNTs can also be used for changing optical properties of
materials in the systems [19,20].
The potential use of BNNTs for any of the above
applications depends on its purity, size, morphology and
alignment. Purity of the BNNTs is one of the most important
factors for its use in any of its potential application. Therefore, the main target of the earlier researchers was not only
the quantity but also the quality of the final product [21].
After it has been proposed that vertically aligned BNNTs can
be used for its potential applications without further
purification, researchers have tried to obtain this format
of the BNNTs. However, in this regard no appreciable
success has been achieved as compare to its structure
counterpart CNTs. In this regard, some of the earlier work
seems to be no more than a claim. In which, vertically
aligned BNNTs bundles have claimed via plasma enhanced
pulse laser deposition (PE-PLD) at a lower temperature of
600 1C [22]. The reported SEM morphology of the asclaimed BNNTs bundle is seem to be no more than an
upward blur from a point-like structure on the substrate. In
spite of high magnification SEM analysis, the vertical
aligned morphology was so un-cleared that a separate
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P. Ahmad et al. / Materials Science in Semiconductor Processing 38 (2015) 113–118
sketch was drawn and shown with SEM micrographs to
indicate the vertical alignment [22]. The reported BNNTs
had not only un-cleared morphology (via SEM) but also had
a shorter length in the range of a few hundred's nanometers. Furthermore, the experimental set up utilized in
the synthesis technique (PE-PLD) is so complex and expensive to be hardly utilized by new researchers with limited
budget or research funding. Therefore, until now, almost no
further progress has been shown via purely this technique
in the synthesis of aligned BNNTs.
Pattern growth of BNNTs was obtained via combination
of pulse laser deposition (PLD) and thermal CVD technique.
PLD was used to deposit a uniform thin film of MgO on Si
substrate and thermal CVD technique was used to grow
BNNTs from B, MgO and FeO as precursors. Though BNNTs
were grown in a particular pattern, however, they were
only partially vertically aligned and were not strong enough
to stand against a slight mechanical compression [23].
In our previous study [24], we have successfully grown
vertically aligned BNNTs on Si substrate by combining the
logic of vertically aligned CNTs [25] and Pattern growth of
BNNTs [23]. In that technique, a uniform thin film of 1.5 mm
was first deposited at the shining surface of Si substrate.
B, MgO and γ-Fe2O3 were used as precursors in a temperature of 1000–1200 1C. Vertically aligned BNNTs were grown
on the alumina deposited Si substrate in the presence of
Argon gas flow as a reaction atmosphere. Though, BNNTs
were synthesized in vertically aligned format, however, the
diameter of the as synthesized BNNTs was highly nonuniform in the range of below 100 to 580 nm. The size
and density of nucleation sites formed in the alumina
deposited thin film on the substrate due to ammonia
etching were thought to be the main factors responsible
for this reason (irregular diameter). Therefore, it has been
tried to search for such a type of catalysts or their alloys (to
be deposited at the top of Si substrate) which have high
etching rate with ammonia to produce nucleation sites with
higher density. In this regard the previous work on vertically aligned CNTs [25] was further searched and Co–Ni
alloy is found to be the most suitable materials to help in
the synthesis of vertically aligned BNNTs. Thus, a thin layer
of Co–Ni alloy is deposited at the shining surface of Si
substrate. During the NH3 etching, the nucleation sites
produced in the deposited alloy is followed by the BNNTs
as a pattern to grow in the vertical aligned format. The
detail methodology, obtained results and growth mechanism are fully described in the coming sections.
2. Experimental details
A sample of vertically aligned flower-like morphologies
of BNNTs is synthesized on Si substrate coated with a thin
Fig. 1. FESEM micrographs of vertically aligned flower-like morphologies of BNNTs. (a) Lower magnification top view of the BNNTs. (b) Higher
magnification top view shows BNNTs below the flower-like morphologies. (c) Higher magnification FESEM micrograph shows vertically aligned BNNTs
wrapped up in flower-like morphologies. (d) Separate higher magnification micrograph of flower-like morphologies.
P. Ahmad et al. / Materials Science in Semiconductor Processing 38 (2015) 113–118
115
Fig. 2. Cross-sectional FESEM view of vertically aligned flower-like morphologies of BNNTs.
layer of Co–Ni alloy. A mixer (200 mg) of Boron, MgO, and
γ-Fe2O3 powder in a 2:1:1 weight ratio is used as a
precursor. These precursors are put in alumina boat. The
boat is covered with a few Si substrates and placed inside
one end closed quartz tube. The quartz tube is then pushed
into the horizontal quartz tube chamber of the furnace in
such a way that the opened end of the tube is toward the
gas inlet. The furnace is tightly sealed to prevent the
effects of outside environment on the experimental parameters inside the furnace. Before the experiment, argon
(Ar) gas is passed through the system to remove air and
dust particles and to create an inert atmosphere. At the
same time, the precursors are heated up to 1000 1C in the
presence of Argon gas flow. At 1000 1C, argon flow is
stopped, and NH3 gas is introduced into the system at a
flow rate of 100–200 sccm. In the presence of NH3 flow,
the system is further heated up to 1200 1C and kept for
1-h. After 1-h, NH3 flow is stopped, and the system is
allowed to cool down to room temperature in the presence
of Ar gas. At room temperature white color BNNTs are
found deposited on Si substrate with good quality of
adhesion and on the inner walls of alumina boat.
3. Results and discussion
Fig. 1(a) shows the low magnification FESEM micrograph
(top view) of the BNNTs synthesized in the present study.
The top view shows flower-like morphologies with some
empty spaces in-between. The flower-like morphologies
can clearly be viewed in high magnification FESEM micrograph as shown in Fig. 1(b). The higher magnification
micrograph not only gives a clear view of the flower-like
morphologies but also of BNNTs in the empty space. The
BNNTs in the empty spaces are indicated with the help of
white circles in Fig. 1(b). The observation showed that most
of the flower-like morphologies lied at the top surface of
the BNNTs. Due to these morphologies BNNTs cannot be
clearly identified in the current micrograph. Therefore, it
was strongly felt to separately analyze both BNNTs and the
flower-like morphologies in higher magnification. The
results thus obtained are shown in Fig. 1(c) and (d).
Fig. 1(c) shows high magnification FESEM micrograph of
the BNNTs. The micrograph shows that all the BNNTs are
covered or wrapped in cotton-like morphologies. These
morphologies seem like the growth species that were still
in process to become the part of the BNNTs in the form of hBN layers, however, due to incomplete growth it remains
stuck with BNNTs and appeared in the current shape. Most
of the BNNTs seemed to be vertically aligned. These aligned
BNNTs can be used for its potential applications without
further purification [26]. Some flower-like morphologies
can also be found in Fig. 1(c) indicated by white circles.
These flower-like morphologies are separately analyzed and
shown in higher magnification in Fig. 1(d). It seems that the
flower-like morphologies are under-developed BNNTs. In
other words, h-BN species combine and make layers tubular
structures of nanoscale h-BN. The mechanism thus developed during the growth of BNNTs is described at the end of
the paper and schematically shown via a sketch in Fig. 6.
FESEM is also employed to take a cross-sectional view
of the as-synthesized flower-like morphologies of BNNTs
to further verify its vertically aligned format. For this
purpose, a small portion of the BNNTs on the substrate is
carefully scratched from one side with the help of a sharp
blade and analyzed with FESEM. The as-obtained FESEM
micrograph is shown in Fig. 2. The figure shows vertically
aligned BNNTs. No flower-like morphologies can be seen
or observed in the current micrograph. Since, it is a crosssectional view in which mostly the lower part of the
BNNTs is observed, therefore, the undeveloped or flowerlike structures at the top cannot be seen here. Most of the
aligned BNNTs are straight, however, some of them can
also be found with a bit curve parts. Some of the BNNTs are
bent after their growth to a certain point. These bent or
curved parts suggest that, after grown to a certain point,
BNNTs need support to remain vertically align. This support can only be provided with the nearby BNNTs, which
in other words depend on the density of nucleation sites
i.e. higher the density of nucleation sites, the more align
BNNTs will grow [25]. Along with BNNTs some broken
species can also be found in the current micrograph which
might have formed or included due to scratching of
sample with blade.
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P. Ahmad et al. / Materials Science in Semiconductor Processing 38 (2015) 113–118
Fig. 3. TEM analysis of flower-like BNNTs sample. (a) Low and (b) high resolution TEM images show BNNT internal bamboo like structure. (c) Low and
(d) high resolution TEM images of another BNNT without bamboo like structure. (e) Higher resolution TEM image shows lattice fringes on BNNT surface.
(f) BNNT has interlayer spacing of 0.34 nm.
Fig. 3 shows transmission electron microscopy (TEM)
micrographs of the synthesized BNNTs in low and high
resolution. Fig. 3(a) shows the low resolution TEM micrograph of an individual BNNT characterized from the present
sample. The micrograph shows a black-blur like morphology
on the outer part of the BNNT. The internal part of the tube is
found to contain an irregular bamboo-like structure. Due to
this irregular shape of the bamboo-like structure, the wallthickness or external diameter of the BNNT varies in the
range of 40–70 nm, whereas the total diameter is found to be
128 nm. A high resolution TEM micrograph of the same
BNNT is also obtained (and shown in Fig. 3(b)) to further
clarify its internal and external morphology. The micrograph
shows a clear view of the BNNT internal bamboo like
structure with a variable diameter [21,27]. The black-blur
like structure can still be seen or viewed at this resolution.
Fig. 3(c) shows low resolution TEM micrograph of another
BNNT from the same sample. No internal bamboo like
structure can be seen in the current BNNT. The tube has
almost uniform wall-thickness and an internal diameter of
34 nm and total diameter of 100 nm. The black-blur
like morphologies can also be found attached with BNNT.
This BNNT is also viewed in high resolution and shown in
Fig. 3(d). The high resolution micrograph confirmed the
structure of the BNNT illustrated in Fig. 3(c) with black-blur
like morphologies and without internal bamboo like structure. Its external part is separately shown in further higher
resolution in Fig. 3(e). At this resolution lattice fringes can
P. Ahmad et al. / Materials Science in Semiconductor Processing 38 (2015) 113–118
be seen on the BNNT surface. These lattice fringes are further
clarified in Fig. 3(f), which shows that the BNNT has multilayers structure with an interlayer spacing of 0.34 nm. This
interlayer spacing is the characteristics of d(002) spacing of hBN [28] and its highly crystalline nature. This crystalline
nature of the BNNT is found to be of great interest in the
development of a solid state neutron detector [29].
The elemental composition of the synthesized BNNTs is
analyzed with the help of X-ray photoelectron spectroscopy (XPS). The as-obtained XPS survey is shown in Fig. 4.
XPS survey shows B 1s and N 1s peaks at 191 eV and
398 eV that confirmed B and N elemental composition of
the synthesized BNNTs [30,31]. The smaller intensity O 1s
peak at 533 eV shows content of oxygen which may either
be due to as-used Si substrate, B2O3 or B(OH)3 [32].
The composition and phase of the BNNTs are find out with
the help of Raman spectroscopy. The as-obtained Raman
spectrum is shown in Fig. 5. Raman spectrum shows a major
peak at 1370 cm 1 that corresponds to E2g mode of h-BN [33].
Along with the major peak, a smaller intensity peak is also
detected in the Raman spectrum at 1126.33 cm 1 for the
presence of B(OH)3, which might have formed by the spontaneous reaction of boron and B2O3 (left during the synthesis of
BNNTs) with moisture and oxygen in the air due to laser
interaction [34].
117
The mechanism for the synthesis of vertically aligned
flower-like morphologies of BNNTs in the present study is
described via a sketch shown in Fig. 6. The precursors at
higher temperature ( 1000 1C) produce growth species
(B2O2). When NH3 is introduced in to the system at
1000 1C, it plays a dual role [24]. On one side it acts as
an etching agent and on the other side provides N2 for the
formation of h-BN species. As an etching agent it etches
away the Ni particles from the deposited Co–Ni alloy
(because Ni has higher etching rate as compare to Co) on
the Si substrate and produces nucleation sites in the Co
particles. The density of the nucleation sites is dependent
on the etching rate of NH3 i.e. the higher the etching rate
the more nucleation sites will be prodeced [25].
The N2 from decomposed NH3 reacts with already
formed B2O2 (from the reaction of B and metal oxides
precursors) and synthesize h-BN species in the form of
vapors. These vapors are deposited on the substrate and
grow in the tubular structure following the nucleation
sites as pattern. The density of nucleation sites and
starting diameter of the tubes are playing a key role in
its vertical align format [25]. Because of this, the nearby
BNNTs are acting as a support for each other. The increasing length of the tubes causes a gradual decrease in the
diameter which in other words effect the alignment of the
BNNTs [24].
4. Conclusions
Fig. 4. XPS survey shows B 1s and N 1s peaks at 191 eV and 398 eV
that indicate h-BN nature of the BNNTs. The O 1s peak at 533 eV shows
content of oxygen which may either be due to substrate, B2O3 or B (OH) 3.
Synthesis of vertically aligned BNNTs with the help of
Co–Ni alloy deposited at the top of Si substrate showed
that the substrate's nature and the types of catalysts has a
key role in controlling the size, morphology and alignment
of BNNTs. The alignment of the BNNTs during the synthesis is found to depend on their support to each other,
which in other word depends on the density of nucleation
sites and diameter of the tubes. Flower-like and black-blur
like morphologies observed during FESEM and HR-TEM
microscopies indicates continues layer growth of the
BNNTs. Changes in the experimental parameters like:
separate NH3 etching of the catalysts deposited substrate,
temperature, growth duration and annealing of precursors
are further be helpful to grow high quality vertically align
BNNTs. The synthesized BNNTs are found to be of great
Fig. 5. Raman spectrum shows a major peak at 1370 (cm 1) that corresponds to E2g mode of h-BN.
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P. Ahmad et al. / Materials Science in Semiconductor Processing 38 (2015) 113–118
Fig. 6. A sketch of the growth mechanism for the BNNTs synthesized in the present study.
interest for its potential application in the field of biomedical, microelectronic mechanical system, targeted drug
delivery and solid state neutron detector.
Acknowledgments
We are extremely grateful to University of Malaya,
50603 Kuala Lumpur Malaysia, Project number: RG37515AFR, for providing funds and facilities for our
research work.
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