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Journal of Science and Engineering
Vol. 2 (2), 2013, 81- 86
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An analogy between the characteristics of Carbon Nanostructures
and those of Boron Nitride
Hamidreza Akbarzadegan
Sama technical and vocational college, Islamic Azad University, Arak Branch, Arak, Iran
Abstract
Received: 06 Apr 2013
Accepted: 15 May 2013
Keywords:
Carbon Nano-Tubes
Boron Nitride Nano-Tubes
Hexagonal Structure
Cubic Structure
Nowadays, studying on carbon nano- Structures is widely being done, and due to unique
characteristics, is highly regarded by researchers. But using carbon nano- Structures in
some cases has its limits. In this paper we study another form of nano-Structures, namely
the boron nitride nano- Structures. First, define and identify characteristics of boron nitride
nano- Structures then compare the properties of carbon nano- Structures with born nitride
nano- Structures.
1. INTRODUCTION
Boron nitride is a chemical compound with chemical formula BN,
consisting of equal numbers of boron and nitrogen atoms. BN is
isoelectronic to a similarly structured carbon lattice and thus exists in
various crystalline forms. The hexagonal form corresponding to graphite is
the most stable and softest among BN polymorphs, and is therefore used as
a lubricant and an additive to cosmetic products. The cubic (sphalerite
structure) variety analogous to diamond is called c-BN. Its hardness is
inferior only to diamond, but its thermal and chemical stability is superior.
The rare wurtzite BN modification is similar to lonsdaleite and may even be
harder than the cubic form.
Boron nitride is not found in nature and is therefore produced
synthetically from boric acid or boron trioxide. The initial product is
amorphous BN powder, which is converted to crystalline h-BN by heating
in nitrogen flow at temperatures above 1500 °C. c-BN is made by annealing
h-BN powder at higher temperatures, under pressures above 5 GPa.
Contrary to diamond, larger c-BN pellets can be produced by fusing
(sintering) relatively cheap c-BN powders. As a result, c-BN is widely used
in mechanical applications. Because of excellent thermal and chemical
stability, boron nitride ceramics are traditionally used as parts of
high-temperature equipment. Boron nitride has a great potential in
nanotechnology. Nanotubes of BN can be produced that have a structure
similar to that of carbon nanotubes, i.e. graphene (or BN) sheets rolled on
themselves, however the properties are very different: whereas carbon
nanotubes can be metallic or semiconducting depending on the rolling
direction and radius, a BN nanotube is an electrical insulator with a wide
bandgap of ~5.5 eV (same as in diamond), which is almost independent of
tube chirality and morphology.
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82 | P a g e H. Akbarzadegan
Similar to other BN forms, BN nanotubes are more thermally and chemically stable than carbon nanotubes
which favors them for some applications.
2. MATERIALS AND METHODS
2.1. Structure
Boron nitride has been produced in an amorphous (a-BN) and crystalline forms. The most stable
crystalline form is the hexagonal one, also called h-BN, α-BN, or g-BN (graphitic BN). It has a layered
structure similar to graphite.
Within each layer, boron and nitrogen atoms are bound by strong covalent bonds, whereas the layers are
held together by weak van der Waals forces. The interlayer "registry" of these sheets differs, however, from
the pattern seen for graphite, because the atoms are eclipsed, with boron atoms lying over and above
nitrogen atoms. This registry reflects the polarity of the B-N bonds. Still, h-BN and graphite are very close
neighbors and even the BC6N hybrids have been synthesized where carbon substitutes for some B and N
atoms.[1]
As diamond is less stable than graphite, cubic BN is less stable than h-BN, but the conversion rate
between those forms is negligible at room temperature (again like diamond). The cubic form has the
sphalerite crystal structure, the same as that of diamond, and is also called β-BN or c-BN. The wurtzite BN
form (w-BN) has the same structure as lonsdaleite, a rare hexagonal polymorph of carbon. In both c-BN and
w-BN boron and nitrogen atoms are grouped into tetrahedra, but the angles between neighboring tetrahedra
are different.[2]
Figure 1. various structures of boron nitride
2.2. Thermal Stability
Hexagonal and cubic (and probably w-BN) BN show remarkable chemical and thermal stabilities. For
example, h-BN is stable to decomposition in temperatures up to 1000 °C in air, 1400 °C in vacuum, and
2800 °C in an inert atmosphere. The reactivity of h-BN and c-BN is relatively similar.
Thermal stability of c-BN can be summarized as follows:[3]
• In air or oxygen: B2O3 protective layer prevents further oxidation to ~1300 °C; no conversion to
hexagonal form at 1400 °C.
• In nitrogen: some conversion to h-BN at 1525 °C after 12 h.
• In vacuum (10−5 Pa): conversion to h-BN at 1550–1600 °C.
2.3. Chemical stability
Boron nitride is insoluble in usual acids, but is soluble in alkaline molten salts and nitrides, such as LiOH,
KOH, NaOH-Na2CO3, NaNO3, Li3N, Mg3N2, Sr3N2, Ba3N2 or Li3BN2, which are therefore used to etch
BN.[3]
2.4. Thermal conductivity
The theoretical thermal conductivity of hexagonal Boron nitride nanoribbons (BNNRs) can approach
1700–2000 W/(m·K), which has the same order of magnitude as the experimental measured value for
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P a g e | 83
graphene, and can be comparable to the theoretical calculations for graphene nanoribbons.[4][5] Moreover,
the thermal transport in the BNNRs is anisotropic. The thermal conductivity of zigzag-edged BNNRs is about
20% larger than that of armchair-edged nanoribbons at room temperature.[6]
3. PROPERTIES
Carbon nanotubes have received considerable research interests because of their remarkable electronic,
mechanical, and thermal properties. Single-walled carbon nanotubes (CNTs) are typically 1 nm in diameter
and can be either metallic or semiconducting depending on their helicity[7]. So far purification or controlled
synthesis of CNTs with selected helicity have not been achieved. This has made electronic device making with
CNTs difficult. Thus, researchers have looked to other nanoscale materials such as semiconducting nanowires
[8] in order to satisfy the need for smaller circuit components. We have concentrated our efforts on boron
nitride nanotubes (BNNTs), which have been predicted [9] to be purely wide band gap semiconducting
nanotubes.
Figure 2. Structures of parent materials. a) Graphite[10]. b) Boron nitride[11]
Boron nitride nanotubes (BNNTs) can be thought of as a tube rolled from a hexagonal sheet of boron
nitride. BNNTs were first synthesized in laboratory in 1995[12]. Fig. 3[13] shows transmission electron
microscopy (TEM) of single to multi-wall BNNTs with six walls. When electron diffractions on individual
tubes were performed using in-situ TEM, the tubes were found to be well crystallized[14].
Figure 3. Transmission electron micrograph of a single to six-walled boron nitride nanotubes: left to right in succession
Unlike carbon nanotubes, whose electronic properties can be either metallic or semiconducting depending
on their chemical structure, BNNTs are expected to have over 4 eV band gaps for observed diameter ranges of
over 1 nm. The calculated band structure for a (4,4) BNNT is shown on Fig. 4. This particular tube has an
indirect gap. The band structure is quite similar to that of a single layer of h-BN, but is with one notable
difference. At the bottom of the conduction band, near Γ point, there is a band, which has energy dispersion
close to the free electron gas. This state has been named “nearly free electron” (NFE) state. Although one
would naively anticipate the electronic states at the bottom of the conduction band to reside mostly on the
boron atomic sites, the NFE state exists approximates 2 Å inside of the nanotube.
84 | P a g e H. Akbarzadegan
Figure 4. Theoretical band structure of a (4,4) BNNT. Highlighted band is the nearly freeelectron state[9]
When imaging BNNTs with sample biases slightly above threshold voltages, intramolecular features are
sometimes revealed. Fig. 6 shows selected STM images of tubes showing such resolutions. Resolved features
are triangular lattice having consistent lattice spacing with bulk h-BN. We believe that a triangular lattice is
imaged instead of a hexagonal lattice because of charge transfer between boron and nitrogen atoms. Chiral
angles in BNNTs are found to be randomly distributed. Surprisingly, these high-resolution images can be
obtained with sample biases ranging from 4 to 5 volts. We are in progress of understanding the nature of
tunneling at such high sample biases.
Table 1, [15,16,17,18,19] compares the properties of CNTs and BNNTs. We have already discussed the
differences in electronic properties above.
Table 1. Comparison of properties of carbon nanotubes and boron nitride nanotubes
Mechanical properties, measured using Young’s modulus, are similar [15,16] for these materials; they are
both ideal for mechanical applications. Thermal conductivity, which is measured to be extremely high value
for CNTs[17], is also expected to be as high for BNNTs whose parent material h-BN has exceptional ab-plane
thermal conductivity[18]. Chemical resistance is better for BNNTs, which are able to survive in air up to much
higher temperature[19]. Over all, BNNTs have properties as desirable for application as carbon nanotubes if
not more so. Thus, there is a dire need for experimental investigation of electronic properties of BNNTs.
Because of their size scale, scanning tunneling microscope (STM) is the most ideal instrument for the
investigation. When an STM is used to image a large band gap material adsorbed on a metallic surface, one
would expect a large range of sample biases in which the material would not be completely visible[20]. Only
above a threshold voltage, determined by the band gap and the location of the material’s Fermi energy with
respect to the Fermi energy of the substrate, should the large band gap material reveal itself. BNNTs behave as
expected while we are imaging. Fig. 5a shows an example BNNT imaged at above its threshold voltage; the
tube shows its tubular geometry.
Below the threshold voltage, the tube appears with a “hollow core” structure as seen on Fig. 5b. Threshold
voltages vary from tube to tube, but it is usually about - 4 volts. This is consistent with BNNTs having large
band gaps. Similar behavior is seen with positive sample biases.
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FIGURE 5. An example BNNT imaged at a) –7 volt and b) –4 volt sample biases. Tunneling current used for
imaging was both 0.5 nA and the images are 300 Å squares.
Because BNNTs are expected to have large band gaps and to be nearly insulating, it is surprising that we
are able to image intramolecular resolution using an STM. We are currently in process of understanding the
nature of tunneling into BNNTs. Using scanning tunneling microscopy, we hope to be able to test the
theoretical band structure of these tubes calculated nearly a decade ago.
4. CONCLUSIONS
We examined properties of boron nitride nanostructures and contrast them to those of carbon
nanostructures. Boron nitride nanostructures expected to be as desirable for application as carbon
nanostructures. Although boron nitride nanostructures are wide band gap semiconductors and electrically
nearly insulating, scanning tunneling microscopy can be used to image and characterize them
.
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Please cite this article as: H. Akbarzadegan, (2013), An analogy between the characteristics of Carbon Nanostructures and those of Boron Nitride, Journal
of Science and Engineering, Vol. 2(2), 81-86.